Weidong Zhang

Javier Díez-Ramírez

Paola Anguita

Claude Descorme

José Luís Valverde

Anne Giroir-Fendler
email: anne.giroir-fendler@ircelyon.univ-lyon1.fr

Anne Giroir

Nanocrystalline Co3O4 catalysts for toluene and propane oxidation: Effect of the precipitation agent

Keywords: Cobalt oxide, Toluene oxidation, Propane oxidation, Effect of precipitant, Reducibility

A set of cobalt oxide catalysts was prepared via precipitation using various precipitants and evaluated in the total oxidation of toluene and propane. A commercial Co3O4 catalyst was used as a reference. Various techniques were used to characterize the catalysts before and after testing. The results showed that the physicochemical properties and the catalytic performance of these cobalt oxide catalysts were greatly influenced by the nature of the precipitation agent used upon preparation. All catalysts exhibited a Co3O4 cubic spinel structure, with different crystallite sizes (27-41 nm) and specific surface areas (15-40 m 2 •g -1 ). The optimal catalyst, obtained via carbonate-precipitation and named Co-CO3, achieved 90% toluene and propane conversion at 256 and 226 °C, respectively. This Co-CO3 catalyst performed better than the commercial catalyst in the toluene oxidation and similarly to the commercial one in the propane oxidation. All catalysts maintained constant performance upon three consecutive catalytic cycles; however, they deactivated to a different extent upon longer-term stability tests depending on the molecule to be oxidized. In toluene oxidation, Co-CO3 exhibited good stability upon cycling but a poor long-term durability. To overcome this issue, a dual-bed catalytic system was proposed. In propane oxidation, Co-CO3 showed stable performance in both cases. The effects of propane and oxygen concentrations on the catalytic performance of Co-CO3 in propane oxidation were also investigated. A positive reaction order with respect to oxygen and the results of oxygen-free catalytic test revealed the dominant role of chemisorbed oxygen species in the oxidation of propane. The excellent performance of Co-CO3 was tentatively ascribed to the highly strained nanocrystalline structure, the high concentration in Co 2+ and the superior reducibility.

Introduction

Volatile organic compounds (VOCs) emitted from industrial and domestic sources are major air pollutants and contribute greatly to the formation of ozone and photochemical smog [START_REF] Zhang | Low-temperature catalysis for VOCs removal in technology and application: A state-of-the-art review[END_REF]. Moreover, some VOCs can cause diseases because of their toxicity [START_REF] Zhang | Low-temperature catalysis for VOCs removal in technology and application: A state-of-the-art review[END_REF].

Catalytic oxidation is a promising technique with high VOCs destruction efficiency and harmless products (CO2 and H2O) at relatively low temperature [START_REF] Huang | Low temperature catalytic oxidation of volatile organic compounds: a review[END_REF]. Noble metal-based catalyst (Pd, Pt, Au, etc.), which have been extensively studied in the past decades, show its limitation in price and lifetime. Thus, many researchers are developing less expensive non-noble metal (Co, Ce, Mn, Cu, Fe, Ni) and their mixed oxide catalysts for the total oxidation of VOCs [START_REF] Tang | Boosting catalytic propane oxidation over PGM-free Co 3 O 4 nanocrystal aggregates through chemical leaching: A comparative study with Pt and Pd based catalysts[END_REF][START_REF] Cai | Ni-Co-O solid solution dispersed nanocrystalline Co 3 O 4 as a highly active catalyst for low-temperature propane combustion[END_REF][START_REF] González-Prior | Catalytic removal of chlorinated compounds over ordered mesoporous cobalt oxides synthesised by hard-templating[END_REF][START_REF] Han | Preparation and high catalytic performance of Co 3 O 4 -MnO 2 for the combustion of o -xylene[END_REF][START_REF] Wang | CoMn x O y nanosheets with molecular-scale homogeneity: an excellent catalyst for toluene combustion[END_REF][START_REF] Luo | Insights into the high performance of Mn-Co oxides derived from metal-organic frameworks for total toluene oxidation[END_REF][START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF][START_REF] Ma | Indium-doped Co 3 O 4 nanorods for catalytic oxidation of CO and C 3 H 6 towards diesel exhaust[END_REF][START_REF] Salek | Low-temperature carbon monoxide and propane total oxidation by nanocrystalline cobalt oxides[END_REF].

Although mixed oxides have shown good activity for VOCs abatement, single nanocrystalline Co3O4 is more active for propane oxidation [START_REF] Tang | Boosting catalytic propane oxidation over PGM-free Co 3 O 4 nanocrystal aggregates through chemical leaching: A comparative study with Pt and Pd based catalysts[END_REF][START_REF] Salek | Low-temperature carbon monoxide and propane total oxidation by nanocrystalline cobalt oxides[END_REF]. It has been proposed that the unfilled d6 and d7 orbitals of Co 3+ and Co 2+ in Co3O4 are responsible for its high ability to activate C-H bonds, which is crucial in the oxidation of saturated hydrocarbons [START_REF] Liotta | Co3O4 nanocrystals and Co3O4-MOx binary oxides for CO, CH4 and VOC oxidation at low temperatures: a review[END_REF]. The activity of Co3O4 catalyst is determined by its crystalline size, morphology, reducibility and the amount of surface oxygen species [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF][START_REF] Liu | Mesoporous Co3O4-supported gold nanocatalysts: Highly active for the oxidation of carbon monoxide, benzene, toluene, and o-xylene[END_REF]. All these physicochemical properties of Co3O4 can be strongly affected by the preparation method. Several methods have been proposed for Co3O4 preparation: hard-template [START_REF] González-Prior | Catalytic removal of chlorinated compounds over ordered mesoporous cobalt oxides synthesised by hard-templating[END_REF][START_REF] Ren | 1D-Co 3 O 4 , 2D-Co 3 O 4 , 3D-Co 3 O 4 for catalytic oxidation of toluene[END_REF][START_REF] Du | Three-dimensional mesoporous manganese oxides and cobalt oxides: High-efficiency catalysts for the removal of toluene and carbon monoxide[END_REF][START_REF] Chen | Exotemplated copper, cobalt, iron, lanthanum and nickel oxides for catalytic oxidation of ethyl acetate[END_REF], hydrothermal [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF], citrate sol-gel [START_REF] Wang | Catalytic oxidation of vinyl chloride emissions over Co-Ce composite oxide catalysts[END_REF], combustion [START_REF] Baidya | Low-Temperature CO Oxidation over Combustion Made Fe-and Cr-Doped Co 3 O 4 Catalysts: Role of Dopant's Nature toward Achieving Superior Catalytic Activity and Stability[END_REF] and precipitation [START_REF] Zhang | Dispersion-precipitation synthesis of highly active nanosized Co 3 O 4 for catalytic oxidation of carbon monoxide and propane[END_REF]. Among these methods, precipitation is one of the most widely applied, using both simple chemicals (without surfactant) and set-ups, having advantages of low energy consumption and easy mass production [START_REF] Zheng | Complete combustion of methane over Co 3 O 4 catalysts: Influence of pH values[END_REF]. The precipitant used determines the type of catalyst precursor and in turn influences the structural properties as well as the catalytic performance of the final catalyst. Chromcáková et al. recently prepared different Co3O4 precursors by the precipitation of cobalt nitrate using NH3•H2O, NaOH and Na2CO3, and demonstrated the catalyst obtained from β-Co(OH)2 precursor was the most active in the decomposition of N2O [START_REF] Chromčáková | Effect of precursor synthesis on catalytic activity of Co3O4 in N2O decomposition[END_REF]. Fan et al. investigated the oxidation of formaldehyde over various precipitation derived Co3O4 catalysts and found that the KHCO3 precipitated material showed the best catalytic efficiency [START_REF] Fan | Low-temperature catalytic oxidation of formaldehyde over Co3O4 catalysts prepared using various precipitants[END_REF]. However, to the best of our knowledge, the effect of the precipitant on the catalytic performance of Co3O4 in typical VOC oxidation has not been reported yet.

There are many studies concerning toluene and propane oxidation on cobalt oxide-based catalysts. Regarding toluene oxidation, Dai and co-workers ascribed the excellent performance of porous Co3O4 nanorods to the combined effects of its large surface area, high adsorbed oxygen species and low-temperature reducibility [START_REF] Bai | Porous Co3O4 nanowires and nanorods: Highly active catalysts for the combustion of toluene[END_REF].

Liotta and co-workers [START_REF] Liotta | Catalytic removal of toluene over Co3O4-CeO 2 mixed oxide catalysts: Comparison with Pt/Al2O 3[END_REF] investigated toluene oxidation over Co3O4-CeO2 binary oxides and found that the lower reduction temperature led to the higher activity. Ye et al. demonstrated that surface adsorbed and subsurface oxygens contributed to the toluene oxidation activity of copper manganese oxide [START_REF] Ye | Influence of the preparation method on the activity of copper-manganese oxides for toluene total oxidation[END_REF]. Ren et al. synthesized 3D hierarchical cube-stacked Co3O4 nanocatalysts for toluene oxidation and attributed its high activity to the large surface area, the highly defective structure and the abundance in high valence Co ions [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF].

For propane oxidation, Liu et al. claimed that the amount of chemisorbed oxygen is the predominant factor leading to high activity in propane combustion [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF]. Salek et al. stressed the importance of porosity in propane combustion [START_REF] Salek | Low-temperature carbon monoxide and propane total oxidation by nanocrystalline cobalt oxides[END_REF]. Garcia et al. proposed that the high concentration in surface Co 2+ is responsible for the high intrinsic activity of meso-Co3O4 nanocrystals [START_REF] Garcia | Deep oxidation of volatile organic compounds using ordered cobalt oxides prepared by a nanocasting route[END_REF]. Solsona et al. found a direct relationship between reducibility and specific catalytic propane activity over iron oxide catalysts [START_REF] Solsona | Total oxidation of VOCs on mesoporous iron oxide catalysts: Soft chemistry route versus hard template method[END_REF]. Yao et al. compared the activity of four different Co3O4 catalysts exposing mainly {100}, {111}, {110} and {112} facets, respectively, and proved that the facile activation of C-H bonds on Co3O4 {111} planes is important for the excellent activity [START_REF] Yao | Facet-dependent activity of Co3O4 catalyst for C3H8 combustion[END_REF].

In this work, five different precipitants were selected to prepare precipitated cobalt precursors, which led to a number of Co3O4 catalysts after same calcination. All catalysts were physically and chemically characterized. Their catalytic performance was evaluated in the total oxidation of two different VOCs, toluene (a typical aromatic hydrocarbon) and propane (a typical short chain alkane). For comparison purposes, a commercial Co3O4 catalyst (Co-com, Alfa Aesar) was also studied. The aim of this work was to investigate the effect of different precipitants on the activity and stability of Co3O4 catalysts in order to identify the most suitable and promising catalyst for VOCs elimination.

Experimental

Catalyst preparation

Five Co3O4 catalysts were prepared by the precipitation method using several precipitants or precipitant precursors.

A 100-mL aqueous solution of precipitants such as H2C2O4•2H2O (0.22 M, Alfa Aesar), Na2CO3 (0.22 M, Sigma-Aldrich), NaOH (0.44 M, Chimie-plus Laboratory) or ammonium hydroxide (0.44 M, Chimie-plus Laboratory) was added to a 100-mL aqueous solution of Co(NO3)2•6H2O (0.2 M, Sigma-Aldrich) under continuous stirring for 1 h at room temperature. The obtained precipitate was separated by centrifugation and washed with distilled water several times to achieve a near-neutral pH washing liquor.

Regarding the use of urea as the precipitant precursor, 100 mL of urea (0.8 M, Sigma-Aldrich) solution was added to a 100 mL of Co(NO3)2•6H2O (0.2 M) solution under stirring. The mixture was then poured into a 250-mL Teflon-lined stainless-steel autoclave. The autoclave was sealed and placed in an oven at 80 °C for 24 h. After the autoclave naturally cooled down to room temperature, the precipitate was harvested by centrifugation and washed with distilled water.

All the washed solids were firstly dried at 80 °C over night and then calcined in a furnace under static air atmosphere according to the following sequence: 200 °C for 1 h and 500 °C for 1 h (2 °C min -1 ). To facilitate their description, the prepared catalysts were labeled as Co-C2O4, Co-CO3, Co-OH, Co-NH3 and Co-urea based on the precipitant used.

Catalyst characterization

Thermogravimetric and Differential Thermal Analysis (TG/DTA) was carried out over the catalyst precursors (80 °C dried overnight) from 25 °C to 925 °C (10 °C min -1 ) on a SETARAM Setsys Evolution 12 calorimeter, using 3-7 mg of samples, under flowing air (50 mL min -1 ). The reference was an empty 70-mL aluminum pan.

The catalyst composition was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Horiba Jobin Yvon). Before making the measurement, the cobalt oxides were dissolved in a mixture of inorganic acids (H2SO4 and HNO3).

Fourier Transform infrared (FTIR) spectra were recorded on a PerkinElmer FT-IR C92712 spectrometer in an attenuated total reflectance (ATR) mode with an instrument resolution of 1 cm -1 in the range of 4000 to 400 cm -1 .

Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D5005 diffractometer equipped with a Cu Kα radiation (λ = 0.154184 nm) and a graphite monochromator on the diffracted beam. Samples were scanned from 10° < 2θ < 80° with a step size of 0.02° and a counting time of 2 s per step. Phase identification was conducted by comparison with the joint committee on powder diffraction standards database cards (JCPDS). The crystallite size was further calculated according to the Scherrer equation:

= • •
where d is the average size of the ordered (crystalline) domains, which may be smaller or equal to the grain size, k (0.9) is the shape factor, λ (0.154184 nm) is the X-ray wavelength, β is the line broadening at half the maximum intensity (FWHM) in radians, and θ is the Bragg angle.

The FWHM (full width at half maximum) was calculated from the line broadening of six most intense XRD reflections of Co3O4 (111), (220), (311), (400), (511) and (440) crystal planes. Lattice constant (a) was computed from the following equation based on the (111), (220), (311), (400), (511) and (440) crystal planes of Co3O4:

= 2 • ℎ + +
where h, k, l are Miller indices of Co3O4.

XRD Density (ρXRD) was calculated by the following equation:

= 8 ⁄
where M is the molar mass of Co3O4, and NA is Avogadro constant.

Raman spectra were collected on a Horiba LabRam HR Spectrometer using A + laser beam of 514 nm wavelength for an excitation.

Textural properties were evaluated from the nitrogen adsorption-desorption isotherms determined at -196 °C with a Micromeritics TRISTAR II apparatus. Sample was previously degassed at 300 °C for 3 h under primary vacuum. The specific surface areas of the samples were determined by the standard Brunauer-Emmett-Teller (BET) procedure. The total pore volume and the pore size distribution were calculated using the Barrett-Joyner-Halenda (BJH) method.

Porosity was calculated using the classical formula:

= ! ! + 1 ⁄
where Vp is the total pore volume determined by the BJH method.

By assuming that all crystallites are spherical, the geometrical specific area (Sgeometrical) was calculated as follows:

# $%&'%()*+,-= 6 •
Temperature-programmed reduction experiments in hydrogen (H2-TPR) were performed in a commercial Micromeritics AutoChem 2950 HP unit with TCD detection.

In each test, samples (ca. 0.05 g) were placed into a U-shaped quartz tube and pretreated at 350 °C for 0.5 h under Ar flow (50 mL min -1 ). After cooling down, the gas was switched to a reducing gas mixture containing 5 vol.% H2 in Ar (50 mL min -1 ), and the temperature was ramped from 45 to 800 °C (10 °C min -1 ). The theoretical H2 consumption ratio of Co 3+ →Co 2+ to Co 2+ →Co for stoichiometric Co3O4 is 1/3.

Catalytic activity tests

150 mg of catalyst mixed with 500-700 mg of silicon carbide (SiC) in order to avoid any hot spot were used for the catalytic tests. The height of the catalytic bed was thus kept at 6 mm. The catalyst was immobilized over a quartz wool plug in a U-shaped reactor (220 mm in length and 4 mm in internal diameter).

Regarding the toluene oxidation tests, the reactant gas mixture, containing 1000 ppm toluene (generated by passing an air flow through a pure toluene-containing bottle that was submerged in an isothermal bath at 5 °C) and synthetic air (21 vol.% O2+79 vol.% N2), with a total flow of 100 mL min -1 , giving a weight hourly space velocity (WHSV) of 40,000 mL g -1 h -1 , was fed into the reactor. The reactor was heated from room temperature to 150 °C (5 °C min -1 ) and kept at the latter temperature for 0.5 h in order to stabilize the system. Then, the temperature was increased from 150 °C to 350 °C (2 °C min -1 ) and held at the latter temperature for 1 h. Next, the reactor was cooled down to 150 °C (2 °C min -1 ). The reaction rate (rC7H8, mol s -1 g -1 ) was calculated by the equation: rC7H8 = XC7H8•FC7H8/m, where XC7H8 is the toluene conversion, FC7H8 is the flow rate of toluene (mol s -1 ) in the inlet gas and m is the mass of the catalyst used (g). The normalized reaction rate (r'C7H8, mol s -1 m -2 ) was calculated by dividing the reaction rate by the specific surface area of the catalyst (SSA, m 2 g -1 ).

Regarding the propane oxidation, 100 mL min -1 of the reactant gas mixture (0.1 vol.% propane + 21 vol.% O2+79 vol.% He) was fed into the reactor at room temperature. The set temperature program was similar to that of toluene oxidation except that the reaction temperature range was 100-350 °C. Again, three consecutive heating-cooling catalytic cycles were performed to evaluate the catalytic stability, which was also evaluated by performing long term experiments at 195 °C for 24 h. Gas effluents were analyzed by a micro gas chromatograph (SRA % GC-R3000) coupled with a thermal conductivity detector. The propane conversion was calculated as follows:

X 0 G 2 3 4%6 = 7CO : ;<= > 7CO : ?@ 37C H D : E 100

where [CO2]out, [CO2]in and [C3H8] are the outlet and inlet CO2 concentrations, and the initial propane concentration, respectively. The carbon balance throughout the catalytic system was estimated to be 99.0 ± 1.0%. rC3H8 and r'C3H8 were calculated using the same equation as rC7H8 and r'C7H8.

To evaluate the effect of water in the reaction, 4.4 vol.% H2O stream was generated by passing the reactant gas flow through a water saturator at 34 °C. Regarding the effect of CO2, 10.0 vol.% of CO2 coming from a high-purity (99.99%) CO2 cylinder was introduced into the reactor via a mass flow controller.

For the oxidation test free of oxygen, the catalysts were pre-calcined at 350 °C for 2 h in air flow of 100 mL min -1 . After cooling down to room temperature, the gas line was switched to feed a propane/He mixture (0.1 vol.% propane + 99.9 vol.% He; 400 mL min -1 ). The oxidation test was performed from 25 °C to 350 °C with a heating rate of 2 °C min -1 at a WHSV of 160,000 mL h -1 g -1 .

Kinetic study

The low VOC concentration (1000 ppm) and low VOC conversions (1-10%) used in this work for the kinetic calculation should be beneficial for avoiding heat transfer effects. Besides, the linear velocity of the gas in the reactor (ca. 13 cm s -1 ) was high enough to hinder the effect of the external diffusion resistance while the size of the catalyst particles (ca. 200 μm) were supposed to be small enough to exclude internal diffusion resistance [START_REF] González-Prior | Oxidation of 1,2-dichloroethane over nanocube-shaped Co 3 O 4 catalysts[END_REF]. Furthermore, according to the literature [START_REF] Liu | Mesoporous Co3O4-supported gold nanocatalysts: Highly active for the oxidation of carbon monoxide, benzene, toluene, and o-xylene[END_REF], by-pass and axial dispersion effects can be neglected for a reactor/particle diameter ratio higher than 10 and a catalytic bed length/particle diameter ratio higher than 50. In the present work, these values were 20 and 300, respectively.

Similar to other studies [START_REF] Benard | Kinetics of the propene oxidation over a Pt / alumina catalyst[END_REF], the reaction rate was defined by a power-law equation:

r = k • 4 K G L 3 6 , • 4 M N 6 O
where k is the apparent reaction constant; PC3H8 and PO2 are the partial pressures in propane and oxygen in the feedstock, respectively; a and b are the partial reaction orders with respect to propane and oxygen, respectively. To determine the value of a, O2 concentration was set at 21 vol.% whereas the C3H8 concentration was ranged from 500 to 6000 ppm. To evaluate the value of b, C3H8 concentration was set at 1000 ppm whereas the O2 concentration was ranged from 2 vol.% to 40 vol.%.

Results and discussion

Decomposition of the catalyst precursors

TG/DTA analyses were carried out to elucidate the thermal decomposition of cobalt oxide precursors prepared from several precipitants after drying at 80 °C (Fig. 1). No apparent weight loss occurred in the temperature range from 500 °C to 900 °C, suggesting that Co3O4 was thermally stable after formation. On the other hand, there was an obvious weight loss between 900 °C and 920 °C, with an associated endothermic peak centered around 915 °C, due to the transformation of Co3O4 into CoO.

During the decomposition of the precursor yielding the Co-C2O4 catalyst, a 19.4 wt.% weight loss was observed in the range 100-200 °C, accompanied by an endothermic peak centered at ca. 176 °C, corresponding to the removal of crystal water linked in the dihydrate cobalt oxalate. This weight loss was really close to the theoretical value (20.3 wt.%) estimated from the stoichiometric loss of two water molecules in the cobalt oxalate dihydrate molecule, as shown in equation [START_REF] Zhang | Low-temperature catalysis for VOCs removal in technology and application: A state-of-the-art review[END_REF]. In addition, a significant weight loss (37.4 wt.%) occurred in the range 200-500 °C along with a sharp exothermic peak centered at 274 °C. It was attributed to the thermal decomposition of cobalt oxalate and the subsequent oxidation of cobalt. This value is similar to the theoretical value (34.4 wt.%), as evaluated from equation [START_REF] Huang | Low temperature catalytic oxidation of volatile organic compounds: a review[END_REF]. The difference could be attributed to the decomposition of residual NO3 -species that were adsorbed on the surface.

CoC2O4•2H2O → CoC2O4 + 2H2O (1) CoC2O4 + 2/3O2 → 1/3Co3O4 + 2CO2 (2)
The thermal decomposition of the precursors yielding Co-CO3 and Co-urea were very similar, with an overall weight loss of 28.4 and 26.4wt.%, respectively. Firstly, a small weight loss (1.9 and 1.2 wt.% weight loss for Co-CO3 and Co-urea, respectively) due to different dehydration processes appeared at ca. 150 °C. Secondly, a large weight loss in the range from 150 to 500 °C (26.5 and 25.1 wt.% weight loss for Co-CO3 and Co-urea, respectively) was attributed to the conversion of cobalt hydroxide carbonate into Co3O4. These weight losses were in good agreement with the theoretical value (24.2 wt.%) calculated from equation (3):

CoCO3•Co(OH)2 + 2/3 O2 → 2/3Co3O4 + CO2 + H2O (3)
Regarding the decomposition of the precursors yielding the Co-OH and Co-NH3 catalysts, although both of them were expected to be a cobalt hydroxide phase, different thermal decomposition processes were observed. The initial weight loss at 150 °C (1.3 and 2.2 wt.% weight loss for Co-OH and Co-NH3, respectively) was attributed to the loss of water physically adsorbed on the surface. In the temperature range from 150 °C to 500 °C, both samples were transformed into Co3O4, with a weight loss of 11.7 and 22.1 wt.%, respectively. According to equation ( 4), the theoretical weight loss should be ca. 13.6 wt.%. Therefore, while the precursor yielding Co-OH could be a pure cobalt hydroxide phase, the precursor yielding Co-NH3 might be a mixture of cobalt hydroxide and cobalt-ammine complex, which explains the much larger weight loss observed in the decomposition of Co-NH3

precursor.

Co(OH)2 + 1/6O2 → 1/3Co3O4 + H2O (4)
Consequently, based on all these TG results, a temperature of 500 °C was chosen to produce different Co3O4 samples with high crystallinity and thermal stability.

Assuming that all cobalt used in the preparation was converted into Co3O4, the theoretical Co3O4 yield was calculated to be 1.605 g. According to the harvested amount of product, the actual yields for Co-C2O4, Co-CO3, Co-OH, Co-NH3 and Co-urea equal to 86%, 88%, 75%, 84% and 45% respectively. Therefore, carbonate-precipitation can achieve the highest yield of product.

Fig. 2 shows the FTIR spectra of the cobalt precursors and the corresponding products. Looking at the Co-C2O4 precursor, a broad band centered at 3355 cm -1 was assigned to the hydration water [START_REF] Gabal | Effect of calcination temperature on Co(II) oxalate dihydrate-iron(II) oxalate dihydrate mixture: DTA-TG, XRD, Mössbauer, FT-IR and SEM studies (Part II)[END_REF][START_REF] Feng | Controlled synthesis of highly active mesoporous Co3O4polycrystals for low temperature CO oxidation[END_REF]. In addition, a band at 1620 cm -1 (asymmetric (O-C-O) stretching mode), together with two bands at 1360 and 1316 cm -1 (symmetric (O-C-O) stretching modes), confirmed the presence of bridging oxalate anions, with both pairs of oxygen ions coordinated to the cobalt ion [START_REF] Gabal | Effect of calcination temperature on Co(II) oxalate dihydrate-iron(II) oxalate dihydrate mixture: DTA-TG, XRD, Mössbauer, FT-IR and SEM studies (Part II)[END_REF]. The oxalate (O-C-O) bending vibration mode appeared at 825 cm -1 whereas the Co-O vibration mode was characterized by a band at 740 cm -1 [START_REF] Gabal | Effect of calcination temperature on Co(II) oxalate dihydrate-iron(II) oxalate dihydrate mixture: DTA-TG, XRD, Mössbauer, FT-IR and SEM studies (Part II)[END_REF]. Bands at 587 and 486 cm -1 were attributed to v(O-Co-O) modes [START_REF] Gabal | Effect of calcination temperature on Co(II) oxalate dihydrate-iron(II) oxalate dihydrate mixture: DTA-TG, XRD, Mössbauer, FT-IR and SEM studies (Part II)[END_REF]. All in all, the FTIR spectrum suggested the presence of a hydrated cobalt oxalate phase, which is in good agreement with the TG/DTA analyses.

In the case of the Co-CO3 and Co-urea precursors, the bands at 3495 (3492) cm -1

were attributed to the stretching vibration modes of the O-H groups in surface molecular water and the hydrogen-bonds between O-H groups. The bands at 3375 (3375) cm -1 were due to the O-H groups interacting with carbonate anions. The bands at 1534 (1504), 1345 (1357), 1067(1066), 835 (830), 770 (750) and 697 (692) cm -1 indicated the presence of carbonate anions, whereas the bands at 967 (958) and 659

(664) cm -1 suggested the existence of Co-OH [START_REF] Xu | Dimensional Control of Cobalt-hydroxide-carbonate Nanorods and Their Thermal Conversion to One-Dimensional Arrays of Co 3 O 4 Nanoparticles[END_REF][START_REF] Yan | Shape-controlled fabrication of the porous Co3O4 nanoflower clusters for efficient catalytic oxidation of gaseous toluene[END_REF][START_REF] Wang | Synthesis of 3D-Nanonet Hollow Structured Co 3 O 4 for High Capacity Supercapacitor[END_REF]. These characteristic bands confirmed the cobalt hydroxide carbonate structure of both precursors, which was also supported by the XRD characterization results (Fig. S1). The slight difference observed between the two samples could arise from the different x value in the Co(OH)x(CO3)0.5(2-x)•nH2O structure [START_REF] Xu | Dimensional Control of Cobalt-hydroxide-carbonate Nanorods and Their Thermal Conversion to One-Dimensional Arrays of Co 3 O 4 Nanoparticles[END_REF].

For the Co-OH precursor, a broad band ca. 1630 cm -1 was attributed to the bending vibration mode of water molecules [START_REF] Cheng | Influences of anion exchange and phase transformation on the supercapacitive properties of α-Co(OH)2[END_REF]; whereas the small band at 1420 and 1338 cm -1 could be characteristic of intercalated nitrates [START_REF] Zhao | Surfactant-assisted electrochemical deposition of α-cobalt hydroxide for supercapacitors[END_REF]. The bands in the domain 400-670 cm -1 could be assigned to the Co-O stretching vibrations and Co-OH bending vibrations of cobalt hydroxide [START_REF] Cheng | Influences of anion exchange and phase transformation on the supercapacitive properties of α-Co(OH)2[END_REF][START_REF] Gong | Facile synthesis and capacitive characteristics of Co(OH)2nanoflakes via a solid-reaction route at room temperature[END_REF]. In the case of the Co-NH3 precursor, the spectrum was similar to that of the Co-OH precursor. However, a higher amount of nitrate species seemed to be present in the Co-NH3 precursor and the v(Co-O) and δ(Co-OH) bands appeared to be shifted towards slightly lower wavenumbers.

After calcination at 500 °C, all cobalt precursors were converted into Co3O4, as clearly confirmed by the FTIR spectra. These spectra are similar to that of the commercial Co3O4 catalyst (Co-com, Alfa Aesar). Moreover, no impurity could be evidenced on their surface (Fig. S2). Two strong characteristic bands corresponding to the Co-O stretching vibrations were observed at around 656 and 550 cm -1 . The former could be assigned to the stretching vibration mode of tetrahedrally coordinated Co 2+ -O;

while the latter was associated with the stretching vibration mode of octahedrally coordinated Co 3+ -O [START_REF] Chromčáková | Effect of precursor synthesis on catalytic activity of Co3O4 in N2O decomposition[END_REF][START_REF] Zhu | Highly Active and Stable Co 3 O 4 /ZSM-5 Catalyst for Propane Oxidation: Effect of the Preparation Method[END_REF]. Based on the integral area of these two bands, an estimation of the surface Co 2+ /Co 3+ ratio could be measured (Table 1). Co 2+ was obviously more abundant on the surface of the Co-CO3 and Co-com catalysts compared with other samples. In general, a high amount of surface Co 2+ is a good indication of a high concentration in surface oxygen vacancies [START_REF] Zhang | Dispersion-precipitation synthesis of highly active nanosized Co 3 O 4 for catalytic oxidation of carbon monoxide and propane[END_REF][START_REF] Wang | Low-temperature catalytic oxidation of vinyl chloride over Ru modified Co 3 O 4 catalysts[END_REF], which is proposed to be a prerequisite to achieve high catalytic activity.

Structural and textural characterization

ICP-OES measurements revealed that the overall chemical composition of the different cobalt oxide catalysts was the same, regardless of the precipitant used upon preparation. The cobalt mass ratio was ca. 75 wt.% for all samples, which is close to the expected value for a pure Co3O4 spinel (73.4 wt.%).

The structural properties of the different cobalt oxide catalysts were characterized by XRD. Fig. 3 The average crystallite size (d), the lattice parameter (a), the microstrain and the XRD-derived density (ρXRD) of each sample, estimated using the MDI Jade 6.0 program based on six most distinct diffraction lines, are listed in Table 1. Among all the samples, Co-com and Co-CO3 showed the smallest average crystallite size, followed by Co-NH3, Co-OH and Co-urea, while Co-C2O4 presented the biggest crystallite size. In order to better compare their XRD profiles, the most intense diffraction peaks are featured in Fig. 3(b). The diffraction peaks of Co-CO3 appeared at higher diffraction angles, indicating a smaller d-spacing than other samples. Accordingly, the lattice parameter of Co-CO3 was the smallest and the XRD density was the largest. Moreover, a high microstrain value was observed for Co-CO3, suggesting a high degree of lattice distortion [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF], as does the commercial catalyst. It was earlier reported that lattice microstrains play a role in the formation of high amounts of O δ-species which can enhance the toluene and propane oxidation [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF]. Overall, Co3O4 nanocrystallites prepared upon carbonate-precipitation exhibited small crystallite size with a highly defective structure.

In order to get insight into the lattice distortion of the spinel catalyst, Raman spectra were recorded in the range of 100-800 cm -1 . As shown in Fig. 4, five peaks at ca. 190, 466, 512, 604 and 671 cm -1 were clearly observed for all catalysts, confirming the formation of spinel Co3O4 [START_REF] González-Prior | Oxidation of 1,2-dichloroethane over nanocube-shaped Co 3 O 4 catalysts[END_REF][START_REF] De Rivas | Synthesis, characterisation and catalytic performance of nanocrystalline Co3O4 for gas-phase chlorinated VOC abatement[END_REF]. The peak at 190 cm -1 was attributed to the tetrahedral sites (CoO4) with F 1 2g symmetry. The two medium-intensity peaks centered at around 466 and 512 cm -1 were associated with the Eg and F 2 2g symmetry modes, respectively. The weak peak at 604 cm -1 was due to the F 3 2g symmetry. The peak at 671 cm -1 was attributed to the octahedral sites (CoO6) with A1g symmetry. The peak position and the full width at half maximum (FWHM) of A1g varied as a function of precipitation agent (Table S1). The Co-CO3 catalyst exhibited the largest Raman red shift, along with the maximum FWHM, compared with other catalysts. This result demonstrated the highly defective structure of catalyst Co-CO3 and the presence of more lattice distortion in catalyst Co-CO3 in comparison with other catalysts [START_REF] González-Prior | Oxidation of 1,2-dichloroethane over nanocube-shaped Co 3 O 4 catalysts[END_REF][START_REF] De Rivas | Synthesis, characterisation and catalytic performance of nanocrystalline Co3O4 for gas-phase chlorinated VOC abatement[END_REF].

In addition, the textural properties of these catalysts were evaluated by N2 adsorption-desorption. N2 adsorption-desorption isotherms and BJH pore size distributions are shown in Fig. 5. The isotherms of all samples showed mixed characteristics of type II and IV (IUPAC classification). At low relative pressure (P/P0), limited N2 uptake was observed, indicating the presence of micropores in very small amount. The H3-type of hysteresis loops suggested the predominance of mesopores.

All isotherms exhibited a sharp uptake above 0.9, demonstrating the presence of macropores. The pore structure of the samples could be considered as slit-type, originating from an irregular assembly of particles with inter-particle voids. The pore sizes of the synthesized Co3O4 samples were mainly in the range 2-4 nm, whereas for Co-com the pore size was ca. 19-23 nm. The Co-CO3 and Co-urea samples exhibited a higher mesopore density than other precipitated samples, which could be due to the decomposition of carbonates, releasing CO2 as a possible pore-forming agent upon calcination.

The textural parameters of the different samples are summarized in Table 2.

Furthermore, the geometrical specific area (Sgeometrical) of each sample (assuming spherical oxide particles) was also calculated and listed in Table 2. Specific surface areas (SSA), total pore volumes (Vpore) and porosity were found to vary depending on the precipitant. SSA of all samples were somehow lower than the geometrical specific areas estimated from the crystallite size, indicating that part of the crystallites were agglomerated into larger particles [START_REF] Chromčáková | Effect of precursor synthesis on catalytic activity of Co3O4 in N2O decomposition[END_REF]. Among all samples, Co-CO3 showed the largest SSA (30 m 2 g -1 ), which is close to the geometrical one (34 m 2 g -1 ), indicating a much lower degree of agglomeration of the crystallites. The average crystallite size of Co-NH3 was smaller than that of Co-OH and Co-urea; while the SSA of Co-NH3 was lower than that of Co-OH and Co-urea, implying a much severe agglomeration of the crystallites in the case of Co-NH3. Co-CO3 and Co-urea exhibited much larger pore volumes than other catalysts, which could facilitate mass transfer upon reaction. As expected from the highest pore density, Co-com possessed the highest SSA and porosity.

Reducibility

H2-TPR experiments were carried out to investigate the reducibility of the different samples. The H2-TPR profiles are shown in Fig. 6, and the corresponding quantitative data is summarized in Table 3.

Firstly, a small peak centered ca. 85 °C was observed for all samples. According to previous studies [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF][START_REF] Song | Mesoporous Co 3 O 4 with Controlled Porosity: Inverse Micelle Synthesis and High-Performance Catalytic CO Oxidation at -60 °C[END_REF], this phenomenon might be explained by the reduction of surface oxygen species adsorbed on cobalt oxide oxygen vacancies.

In addition, as reported in the literature [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF][START_REF] Zhang | Dispersion-precipitation synthesis of highly active nanosized Co 3 O 4 for catalytic oxidation of carbon monoxide and propane[END_REF], two sequential peaks usually appear Co3O4 catalysts ranged from 14.4 to 16.9 mmol g -1 . Obviously, Co-CO3 was the most reducible catalyst, since the initial reduction temperature was the lowest. This could be related to the smaller crystallite size. Indeed, as reported elsewhere, the presence of structural defects in smaller crystallites could facilitate the reduction of cobalt oxide [START_REF] De Rivas | Synthesis, characterisation and catalytic performance of nanocrystalline Co3O4 for gas-phase chlorinated VOC abatement[END_REF][START_REF] Song | Mesoporous Co 3 O 4 with Controlled Porosity: Inverse Micelle Synthesis and High-Performance Catalytic CO Oxidation at -60 °C[END_REF].

Furthermore, it is well-known that VOC oxidation over Co3O4 catalysts mainly occurs below 300 °C. In order to better evaluate the reducibility of the catalysts that can affect the catalytic behavior, the H2 consumption below 300 °C was quantified (Table 3). Based on the quantitative result and Fig. 6(b), the low temperature reduction ability of the samples varied as follows: Co-CO3 > Co-com > Co-OH > Co-urea > Co-C2O4 > Co-NH3. A better reducibility is believed to favor the formation of more oxygen vacancies on the surface, which are ready available for the activation of gaseous oxygen and the generation of labile oxygen species [START_REF] Tang | Boosting catalytic propane oxidation over PGM-free Co 3 O 4 nanocrystal aggregates through chemical leaching: A comparative study with Pt and Pd based catalysts[END_REF][START_REF] Delimaris | VOC oxidation over MnOx-CeO2 catalysts prepared by a combustion method[END_REF]. Therefore, more active oxygen species should be present on the Co-CO3 catalyst.

Catalytic performance

The catalytic performance of all catalysts was evaluated for the total oxidation of toluene and propane. For each catalyst, three consecutive heating-cooling catalytic cycles were performed. This way, the catalytic cycling stability, both in toluene and propane oxidation, was evaluated. In this study, CO2 yield rather than VOCs conversion was used to establish and compare catalytic performance. Since these VOCs, especially toluene, can be adsorbed onto the surface of Co3O4 with high surface area, forming some intermediates at low temperatures that will be further oxidized to CO2 at high temperatures, which may result in overestimation of the catalytic performance.

Total oxidation of toluene

Regarding toluene oxidation, a hysteresis between the heating and cooling processes was detected (Fig. S3(a)), which may be due to changes in the surface cobalt oxidation state or differences in toluene and O coverage upon heating and cooling [START_REF] Chen | Achieving low temperature formaldehyde oxidation: A case study of NaBH 4 reduced cobalt oxide nanowires[END_REF][START_REF] Guan | Catalytically Active Rh Sub-Nanoclusters on TiO 2 for CO Oxidation at Cryogenic Temperatures[END_REF]. In some cases, CO2 yield exceeded 100% (Fig. S4(a)). This phenomenon was also observed by Imamura et al. [START_REF] Imamura | Effect of Sm on the catalytic activity of Co3O4in the oxidation of toluene[END_REF] and was attributed to carbonaceous species deposited on the surface of the catalysts at low temperature that were further burned at relatively high temperature. Chen and his co-workers found that the mineralization temperatures of toluene are much higher than those of toluene abatement over Mn-Fe oxide catalysts [START_REF] Chen | Hydrolysis driving redox reaction to synthesize Mn-Fe binary oxides as highly active catalysts for the removal of toluene[END_REF]. This way, some reaction intermediates could be accumulated and would be consumed at higher temperature than the temperature they were produced. Regarding these findings, the conversion-temperature profiles obtained upon cooling stage should be considered as a measurement of the performance of the catalysts. The results obtained for all catalysts upon the third catalytic run are shown in Fig. 7. The temperatures required for achieving 10, 50 or 90% toluene conversion during the cooling ramp are listed in Table S3. In all cases, CO2 production started from ca. 200 °C and full toluene conversion was achieved at temperatures below 300 °C, demonstrating the high activity of Co3O4 in toluene oxidation. During the reaction, the concentration of both CO and CO2 was simultaneously measured. The concentration of the CO produced during the whole process remained under 30 ppm except for the bumps caused by intermediates decomposition (Fig. S5(a)), which can be ignored. All in all, the selectivity of the catalysts towards CO2 and H2O should be considered to be excellent. Substantial different performance was clearly observed when different precipitants were use. Catalyst Co-CO3 showed the best catalytic performance (the T10, T50, and T90 were 223, 235, and 256 °C at WHSV = 40,000 mL g -1 h -1 in the third cooling run, respectively), followed by catalysts Co-com, Co-OH, Co-urea, Co-NH3

and Co-C2O4.

Although SSA is not always directly related to the catalytic activity, a higher surface area provides access to more active sites. To better compare the intrinsic activity of the samples, specific reaction rates r' (normalized by SSA) were calculated at 230, 235 and 240 °C. As shown in Table 4, catalyst Co-CO3 was the most active catalyst for toluene oxidation in terms of the specific parameter defined.

The Arrhenius plots for all catalysts are depicted in Fig. S6(a). Apparent activation energies (Ea) calculated from the slopes of the lines and apparent pre-exponential factors (A) calculated from their intercepts are listed in Table 4 (the corresponding toluene conversions to CO2 used in this case were between 1%-8%). The most active catalyst Co-CO3 showed in turn the highest Ea value and the highest ln A value among all catalysts. The pre-exponential factors increased as activation energies did. This unusual phenomenon can be explained by a compensation effect (Fig. S7).

The T10, T50 and T90 values of the three cycles obtained with all catalysts were almost the same, illustrating the good cycle-stability of the cobalt oxide catalysts (Table S3).

To further confirm the excellent cycle-stability of the as-prepared catalysts, 14 heatingcooling catalytic cycles were conducted over catalyst Co-CO3 presenting the best performance. As shown in Fig. S8, after 14 cycles, the light-off curves remained unchanged.

A long-term stability study for all catalysts is presented in Fig. 8. There was a loss of activity for all catalysts except for Co-CO3 in the initial two or three hours of time on stream. Then, the conversion remained stable or even bounced back a little. Catalyst

Co-com exhibited good long-term stability, being its stability curve wavy after the first 10 h, which could be caused by the continuous adsorption of toluene and desorption of CO2 on the catalyst surface. Similar oscillatory phenomenon was also observed by Genty and his co-workers [START_REF] Genty | Influence of CO addition on the toluene total oxidation over Co based mixed oxide catalysts[END_REF]. To better study the long-term stability of catalyst Co-CO3, two more experiments were conducted at a lower temperature, 245 °C. One started from the heating process, the other started from the cooling process. As displayed in Fig. S9, whatever the starting stage, the final conversions were stabilized at a similar value, ca. 10%, suggesting that catalyst Co-CO3 exhibited the highest degree of deactivation among the investigated catalysts. The deactivation of Co3O4 catalysts are generally caused by water vapor/reaction intermediates accumulation, coke deposition and sintering of Co3O4 crystals [START_REF] Chen | Exotemplated copper, cobalt, iron, lanthanum and nickel oxides for catalytic oxidation of ethyl acetate[END_REF]. As observed in Fig. S4(a), catalyst Co-CO3 presented the biggest CO2 bump during the heating process; namely, a large amount of carbonate species was easily accumulated on its surface at low conversion.

To check if this deactivation is reversible, the toluene gas was turned off after long hours of continuous test and kept for 1 h and then turned on again to continue the long-term stability test. It was observed that the activity of the regenerated catalyst was almost the same as that of the fresh one, confirming its reversibility. Moreover, the phase structure, crystalline size and SSA of Co-CO3 were well maintained after working for 88 h (Table S2). Based on the results above, accumulation of intermediates was proved to be the only reason for deactivation. During the continuous test at 245 °C, toluene was adsorbed and fleetly converted into intermediates over Co-CO3, but the reaction rate of intermediates to CO2 was low at this temperature. As a result, abundant intermediates would accumulate on the surface of the catalyst, causing a decrease of catalytic activity. As well known, a catalytic system for practical application sometimes needs to be run continuously without stopping. This is unfavorable for the Co-CO3 catalyst since it deactivated too much after a short time. To overcome this issue, a dual-line catalytic system was proposed (Fig. S10). Once the catalyst in one line is deactivated enough, the feed would be conducted to the other line. This way, the deactivated one would have time to restore its activity and the average toluene conversion should be kept high all the time. To study the feasibility of this proposal, a repeating toluene on and off experiment was performed for 90 h (Fig. 9). As observed, initial toluene conversion (93.5%) was completely recovered after 15 successive cycles.

On the other hand, the toluene conversion remained almost constant at 39.5% three hours after the injection of toluene. Therefore, it is possible to adjust and control the reaction rate without changing the temperature by using this dual-line catalytic system.

Total oxidation of propane

Unlike toluene oxidation, no obvious hysteresis was observed for propane oxidation (Fig. S3(b)) when heating and cooling processes were conducted, being the CO2 yield during the heating processes within 100% (Fig. S4(b)). This means that intermediates produced in propane oxidation are readily desorbed from the surface of Co3O4 at low temperature, avoiding any kind of carbon deposition. Fig. 10 shows the results for the third cooling cycle, and Table S4 lists the values of T10, T50, and T90

during the three cooling stages. All catalysts started producing CO2 at ca. 150 °C (50 °C lower than in toluene oxidation), and fully oxidized propane to CO2 at ca. Specific reaction rates r' (normalized by SSA) for propane oxidation were calculated at 140, 150 and 160 °C and listed in Table 5. Interestingly, catalyst Co-CO3 exhibited a specific reaction rate much higher than that of other catalysts. Regarding catalyst Co-com, despite the lower intrinsic activity, its larger SSA overcompensated its activity, thus exhibiting comparable performance as catalysts Co-CO3. Apparent activation energies (Ea) and apparent pre-exponential factors (A) were calculated from the Arrhenius plots (Fig. S6(b)) and listed in Table 5 (the corresponding propane conversions to CO2 used in this case were between 1%-8%). Contrary to the findings observed in the toluene oxidation study, catalyst Co-CO3 presented the lowest Ea and ln A values. The pre-exponential factors decreased as the activation energies did, due to the compensation effect (Fig. S7).

As seen in Table S4, no differences in T10, T50 and T90 values for the three cycles were observed for all catalysts, thus showing their good cycle-stability. Regarding the long-term stability, on-stream propane oxidation at 195 °C was conducted for each catalyst for ca. 24 h (Fig. 11). After 24 h, all catalysts showed a decline in propane conversion, which may be due to an induction period as observed elsewhere [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF]. The descending order in the catalytic activity is as follows: Co-OH (10.8%) > Co-com (9.8%) > Co-urea (7.3%) > Co-CO3 (5.0%) > Co-NH3 (4.6%) > Co-C2O4 (2.4%). The most inactive catalyst, Co-C2O4, showed the best long-term stability for propane oxidation. Catalysts Co-CO3 also exhibited a fairly good durability, while Co-com deactivated the most.

To verify the potential industrial applicability of catalyst Co-CO3, the influence of WHSV on the catalytic propane performance was investigated and showed in Fig. 12(a).

As expected, when WHSV increased (which means that contact time decreased) from 40,000 to 160,000 mL h -1 g -1 , the light-off curve of propane oxidation shifted to the higher temperature region. Anyway, 100% CO2 yield can still be obtained at 290 °C at a WHSV as high as 160,000 mL h -1 g -1 .

Although carbon dioxide and water vapor are the final products of propane oxidation, they may also present in real exhaust condition. This way, their effects on propane oxidation over Co-CO3 were examined. As shown in Fig. 12(b), the activity could be suppressed (with a T50 rise from 199 °C to 218 °C and 207 °C, respectively) when 4.4 vol.% H2O or 10 vol.% CO2 was introduced into the feedstock because of a shift of reaction equilibrium. After removing H2O or CO2 from the inlet, the activity of Co-CO3 was completely recovered. Compared with CO2, H2O had a more significantly adverse effect on the catalytic performance since the light-off profile shifted to much higher temperature. With the aim of further evaluating the effect of water vapor content on the catalytic performance of Co3O4, 2.2 vol.% or 8.8 vol.% water vapor was also fed in the process of propane oxidation. As shown in Fig. S11, increasing the water vapor content in the feed stream from 0 up to 4.4 vol.% led to a decrease of the propane oxidation performance of Co-CO3; further increase in water content in the feed to 8.8 vol.% allowed a partial recovering of the catalytic performance. The influence of CO2 or H2O on the long-term stability of Co-CO3 was also studied (Fig. S12). Firstly, propane conversion to CO2 was 76.5% at 214 °C.

After 15 h time on stream, it dropped slightly and kept constant at 73.5%. Then, 10 vol.% CO2 was fed in, causing an immediate declining of the conversion to 58.5%.

After that, the conversion was stabilized at 58.5% for 17.5 h. Once CO2 was cut off, the conversion ran back to initial level (73.5%) and remained unchanged for 15.8 h.

Next, 4.4 vol.% H2O vapor was introduced, causing a conversion reduction down to 34.0%, which was maintained for 11.9 h. Then, H2O flow entering the reactor was stopped, leading to an increase of conversion to ca. 55%, which was lower than that observed at the beginning of the experiment (76.5%). Finally, the catalyst was regenerated by heating the system at 350 °C for 1 h. After cooling it down to 214 °C, the conversion was 55%. These results showed that the CO2 deactivation is reversible while the H2O inhibition is only partially reversible in the long-term test.

Recently, Tang et al. synthesized a highly active Co3O4 catalyst by a chemical leaching method and found that its catalytic performance in propane oxidation was independent of both propane and oxygen concentrations [START_REF] Tang | Boosting catalytic propane oxidation over PGM-free Co 3 O 4 nanocrystal aggregates through chemical leaching: A comparative study with Pt and Pd based catalysts[END_REF]. In this work, the effects of propane and oxygen concentrations on the catalytic behavior of catalyst Co-CO3

were explored. As displayed in Fig. 13(a), the propane conversion at fixed O2 concentration (molar fraction equal to 0.21) decreases with increasing propane concentration from 500 to 6000 ppm. Conversely, when the C3H8 concentration was fixed at 1000 ppm the propane conversion increased with increasing O2 concentration from 2 vol.% to 40 vol.% (Fig. 13(b)). Fig. 14 shows the corresponding reaction rates computed at conversions lower than 90 %. It can be observed that the reaction rates increased accordingly with the increase of concentration of both C3H8 and O2. In addition, the reaction orders with respect to the partial pressures of C3H8 and O2 in a power-law rate equation were estimated and the corresponding results are presented in Fig. S13. The experimental reaction orders with respect to C3H8 were ca. 0.25 from 140 °C to 160 °C, and the ones with respect to O2 were ca. 0.22 for the same temperature range. These results suggest that the adsorption and activation of C3H8 or O2 on the Co-CO3 catalyst is facile, and the change of C3H8 concentration has a bit greater influence on the reaction rate than that of O2 concentration. The values of the reaction orders with respect to C3H8 or O2 reported elsewhere [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF][START_REF] Hu | Hydrothermal synthesis of NiCeO x nanosheets and its application to the total oxidation of propane[END_REF][START_REF] Choudhary | Kinetics of the complete combustion of dilute propane and toluene over iron-doped ZrO2catalyst[END_REF] are much higher than those computed in this work, which could be related to different reaction temperatures and reactant concentrations used in this work. Anyway, the positive-order dependence of the O2 partial pressure revealed that the oxygen species taking part in the oxidation of propane at low temperature were weakly bound oxygen species rather than lattice oxygen [START_REF] Liu | Dry citrate-precursor synthesized nanocrystalline cobalt oxide as highly active catalyst for total oxidation of propane[END_REF]. In addition, the total conversion of propane into CO2 at temperatures lower than 280 °C was reached in a wide range of C3H8 and O2 concentrations. In this sense, catalyst Co-CO3, the most active one, can be considered as a versatile and potential catalyst for practical applications.

In order to examine the role of the oxygen species present in the catalysts during propane oxidation, propane oxidation test was performed over catalysts Co-CO3 and Co-com in He flow. Fig. S14 shows the concentration of produced CO2 as a function of the temperature. The profiles suggest that even without the involvement of gaseous O2, CO2 can be generated from 150 °C to 250 °C. At such low temperatures, chemisorbed oxygen (O -) could play a role while lattice oxygen cannot act. Therefore, the participation of surface lattice oxygen in cobalt oxides should be responsible for the CO2 evolution.

Considering full conversion of propane was below 250 °C, chemisorbed oxygen must be more relevant than lattice oxygen (O 2-) for achieving a high activity over both catalysts, Co-CO3 and Co-com. Based on the above kinetic analysis and some studies reported in the literature, the rate-determining step of propane oxidation should be the surface reaction between chemisorbed oxygen and adsorbed propane, in which surface bound O -species extract hydrogen from the C-H bond and activate it [START_REF] Heynderickx | Kinetic modeling of the total oxidation of propane over CuO-CeO2/γ-Al2O3[END_REF].

Discussion

In this study, the nature of the precipitant used upon preparation of different Co3O4 catalysts was shown to influence the crystallite size, the specific surface area, the surficial cobalt valence as well as the reducibility of the final products, thus determining their activity. The catalytic performance of the Co-CO3 catalyst, derived from carbonate-precipitation, in both toluene and propane oxidation was superior to all other catalysts, including the commercial one.

The evolution of several parameters that may affect the catalytic oxidation of toluene and propane are summarized in Fig. 15. Co-CO3 appeared both highly strained and readily reducible, showing a relatively large surface area and an abundant amount of surficial Co 2+ species, thus favoring the formation of chemically absorbed O -species and resulting in an excellent ability to oxidize toluene and propane completely. Looking at the commercial catalyst, Co-com, the high microstrain value and the specific surface area contributed positively to propane oxidation, while a poorer reducibility negatively affected the catalytic performance in toluene oxidation.

In the case of Co-C2O4, Co-NH3 and Co-urea, the inferior reducibility and low concentration of Co 2+ species on the catalytic surface would explain their bad performance. Regarding Co-OH, it exhibited relatively good reducibility but low microstrain, thus showing a moderate catalytic performance. The reducibility of Co3O4 seemed to be a crucial factor, but not an exclusive one, that could determine the catalytic activity of Co3O4. It is generally accepted that the oxidation of VOCs over Co3O4 proceeds through a redox mechanism, including the reduction of Co3O4 (accompanied by the generation of surface oxygen vacancies) as the catalyst reacts with VOCs, and the re-oxidation of the reduced site through the consumption of gaseous oxygen [START_REF] Zhang | Dispersion-precipitation synthesis of highly active nanosized Co 3 O 4 for catalytic oxidation of carbon monoxide and propane[END_REF]. Therefore, the low-temperature reducibility of Co3O4 and the surface Co 2+ concentration that could influence the oxygen activation ability of Co3O4 are the two main factors governing the catalytic activity of Co3O4 in VOCs. This is confirmed by our experimental results.

Finally, a comparison between the catalytic performance of the Co-CO3 catalyst and that of catalysts recently reported in the literature is presented in Table S5. In the case of toluene oxidation, the performance of Co-CO3 was superior to CeO2-rods [START_REF] López | The prevalence of surface oxygen vacancies over the mobility of bulk oxygen in nanostructured ceria for the total toluene oxidation[END_REF] and OMSs [START_REF] Sihaib | Manganese oxide-based catalysts for toluene oxidation[END_REF], comparable to Co3O4-cubes [START_REF] Ren | Controllable synthesis of 3D hierarchical Co 3 O 4 nanocatalysts with various morphologies for the catalytic oxidation of toluene[END_REF] and δ-MnO2 [START_REF] Yang | Comparative study of α-, β-, γand δ-MnO2 on toluene oxidation: Oxygen vacancies and reaction intermediates[END_REF], and inferior to 3Mn1Ce [START_REF] Chen | Homogeneous introduction of CeO y into MnO x -based catalyst for oxidation of aromatic VOCs[END_REF], MOF-Mn1Co1 [START_REF] Luo | Insights into the high performance of Mn-Co oxides derived from metal-organic frameworks for total toluene oxidation[END_REF], CoCe-P [START_REF] Hu | Low Content of CoO x Supported on Nanocrystalline CeO 2 for Toluene Combustion: The Importance of Interfaces between Active Sites and Supports[END_REF], Co3Mn1-R [START_REF] Wang | CoMn x O y nanosheets with molecular-scale homogeneity: an excellent catalyst for toluene combustion[END_REF] MnO2/LaMnO3 [START_REF] Li | Enhanced low-temperature activity of LaMnO 3 for toluene oxidation: The effect of treatment with an acidic KMnO 4[END_REF],

Ce0.03MnOx [START_REF] Zhao | Synthesis of Ce a MnO x hollow microsphere with hierarchical structure and its excellent catalytic performance for toluene combustion[END_REF], and LaCoO3-1 [START_REF] Yang | Facile surface improvement method for LaCoO 3 for toluene oxidation[END_REF]. In propane oxidation, the Co-CO3 catalyst performed significantly better than Co1Ce1 [START_REF] Li | Correlation between the physicochemical properties and catalytic performances of micro/mesoporous CoCeO mixed oxides for propane combustion[END_REF], Co3O4-H [START_REF] Yao | Facet-dependent activity of Co3O4 catalyst for C3H8 combustion[END_REF], NiCeOx-4 [START_REF] Hu | Hydrothermal synthesis of NiCeO x nanosheets and its application to the total oxidation of propane[END_REF], La0.4-MnOx [START_REF] Xie | A highly-efficient La-MnOx catalyst for propane combustion: the promotional role of La and the effect of the preparation method[END_REF], Ir/Si350 [START_REF] Schick | Size-activity relationship of iridium particles supported on silica for the total oxidation of volatile organic compounds (VOCs)[END_REF], and Fe-D [START_REF] Solsona | Total oxidation of VOCs on mesoporous iron oxide catalysts: Soft chemistry route versus hard template method[END_REF], similarly to NiO-Co(0.3) [START_REF] Cai | Ni-Co-O solid solution dispersed nanocrystalline Co 3 O 4 as a highly active catalyst for low-temperature propane combustion[END_REF], Co2.3Mn0.7O4

[64], MnCo0.2 [START_REF] Chen | Cobalt-Doped MnO 2 Nanofibers for Enhanced Propane Oxidation[END_REF], and Co-DP [START_REF] Zhang | Dispersion-precipitation synthesis of highly active nanosized Co 3 O 4 for catalytic oxidation of carbon monoxide and propane[END_REF], and worse than Ru/CeO2-1 [START_REF] Hu | Total Oxidation of Propane over a Ru/CeO 2 Catalyst at Low Temperature[END_REF] and Ru/CeO2-R [START_REF] Wang | Ru/CeO 2 Catalyst with Optimized CeO 2 Support Morphology and Surface Facets for Propane Combustion[END_REF]. However, it should be pointed out that the measurement of the VOC conversion used in this work is actually based on the CO2 yield. Since the temperature required for completely converting toluene into CO2 was usually higher than the temperature when toluene disappeared from the outlet stream, due to the presence of toluene adsorption and reaction intermediates [START_REF] Chen | Hydrolysis driving redox reaction to synthesize Mn-Fe binary oxides as highly active catalysts for the removal of toluene[END_REF], the performance of our catalysts was possibly underestimated compared to the literature data.

The synthesis of catalyst Co-CO3, in this study was the simplest one, requiring neither harsh conditions nor the use of costly surfactants or noble metals, comparing with above catalysts reported in the literatures. Moreover, further optimization of the activity of the Co-CO3 catalyst, by adjusting the precipitation pH or the calcination conditions is possible and realistic, which makes this specific catalyst a potential candidate for practical applications.

Conclusions

In this study, several Co3O4 catalysts were prepared using a simple precipitation method and were evaluated in toluene and propane oxidation. For comparison purposes, a commercial Co3O4 catalyst was used as a reference. The Co3O4 catalyst prepared by carbonate-precipitation is the most promising catalyst. Large surface area, high lattice disorder, abundant surface Co 2+ and good reducibility are responsible for its superior catalytic performance. The stability of Co-CO3 upon cycling in both toluene and propane oxidation is excellent, with no change observed after three cycles.

The long-term stability of Co-CO3 is remarkable at high toluene conversion but poor at low toluene conversion. A dual-bed catalytic system was proposed to maintain the activity of Co-CO3 constant and high in toluene oxidation. For propane oxidation, the long-term durability of Co-CO3 is good. A compensation effect was found in the kinetics study of toluene and propane oxidation for the as-prepared Co3O4 catalysts.

Chemisorbed oxygen species are shown to play a significant role in the performance of Co-CO3 in propane oxidation, as demonstrated by the kinetic analysis and the oxygen-free catalytic tests.

Figure captions

Fig. 1. TG, DTG and DTA curves for the decomposition of the cobalt precursors at 10 °C min -1 in air atmosphere. a Specific surface area obtained using BET method and total pore volume obatined using BJH method.

b Geometrical specific area calculated using average crystallite size and XRD density by assuming that all crystallites are spherical.

c Porosity calculated using the classical formula: Porosity = Vpore/(Vpore + 1/ρXRD).

  upon reduction of Co3O4. The reduction of Co3O4 to CoO started from ca. 190 °C, with peaks in the range 190-330 °C. The subsequent reduction peak (315-430 °C), which is more intense and broader, is associated with the reduction of CoO to metallic Co. The ratio of the integral area for these two peaks was very close to the theoretical stoichiometric ratio (1/3) of well-defined Co3O4. The total theoretical H2 consumption for the reduction of well-defined Co2O3, Co3O4 and CoO is 18.1, 16.6 and 13.3 mmol g -1 , respectively. The total experimental H2 consumption measured for the different

270 °C ( 30

 30 °C lower than in toluene oxidation). In the whole temperature range, no CO was produced whereas trace amounts of propene (< 15 ppm, Fig.S5(b)) was detected at low temperatures, demonstrating the superior selectivity of the as-prepared Co3O4 catalysts for the total oxidation of propane. The catalytic performance ranked as follows: Co-com > Co-CO3 > Co-OH > Co-urea > Co-NH3 > Co-C2O4, this trend being similar to that observed for toluene oxidation. An exception was catalyst Co-com which performed better than catalyst Co-CO3 at the high temperature stage. The T10, T50 and T90 values of Co-CO3 in the third cooling run were 161, 199, and 226 °C, respectively.

Fig. 2 .

 2 Fig.2. FTIR spectra of (a) the cobalt precursors obtained after drying at 80 °C and (b) the cobalt oxides obtained after the calcination of the cobalt precursors at 500 °C.

Fig. 3 .

 3 Fig. 3. XRD patterns of the Co3O4 catalysts.

Fig. 4 .

 4 Fig. 4. Raman spectra of the Co3O4 catalysts.

Fig. 5 .

 5 Fig. 5. (a) N2 adsorption-desorption isotherms and (b) BJH pore size distribution curves of the Co3O4 catalysts.

Fig. 6 .

 6 Fig. 6. H2-TPR profiles of the Co3O4 catalysts.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Variation of the toluene conversion with the reaction temperature during the third consecutive cooling run over various Co3O4 catalysts.Fig. 8. Influence of time on stream on toluene oxidation over various Co3O4 catalysts starting from cooling ramp at 257 °C.

Fig. 9 .

 9 Fig. 9. Repeating toluene on and off oxidation test conducted at 248 °C for 90 h over catalyst Co-CO3.

Fig. 10 .Fig. 11 .Fig. 12 .

 101112 Fig. 10. Variation of the propane conversion with the reaction temperature during the third consecutive cooling run over various Co3O4 catalysts.Fig. 11. Influence of time on stream in propane oxidation at 195 °C during cooling ramp over various Co3O4 catalysts.Fig.12. Effect of (a) WHSV, (b) water vapor and carbon dioxide on propane oxidation over catalyst Co-CO3. Reaction conditions: 0.1 vol.% C3H8 + 21 vol.% O2 at a WHSV of 40,000 mL•h -1 •g -1 (cycle 1); introduction of 4.4% vol.% H2O (cycle 2); removal of 4.4% vol.% H2O (cycle 3); introduction of 10% vol.% CO2 (cycle 4); removal of 10% vol.% CO2 (cycle 5).

Fig. 13 .Fig. 14 .

 1314 Fig. 13. Effect of (a) propane concentration (molar fraction of oxygen equal to 0.21), (b) oxygen concentration (molar fraction of propene equal to 1000 ppm) on propane conversion to CO2 over catalyst Co-CO3.Fig. 14. Effect of (a) propane concentration (molar fraction of oxygen equal to 0.21), (b) oxygen concentration (molar fraction of propene equal to 1000 ppm) on propane oxidation reaction rate over catalyst Co-CO3.

Fig. 15 .Fig. 1 .Fig. 2 .

 1512 Fig. 15. The relationship between the catalytic reaction rate and several physical and chemical parameters.

Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .

 3456789 Fig. 3. XRD patterns of the Co3O4 catalysts.

Fig. 13 .Fig. 14 .

 1314 Fig. 13. Effect of (a) propane concentration (molar fraction of oxygen equal to 0.21), (b) oxygen concentration (molar fraction of propene equal to 1000 ppm) on propane conversion to CO2 over catalyst Co-CO3.

Fig. 15 .

 15 Fig.[START_REF] Du | Three-dimensional mesoporous manganese oxides and cobalt oxides: High-efficiency catalysts for the removal of toluene and carbon monoxide[END_REF]. The relationship between the catalytic reaction rate and several physical and chemical parameters.

  shows the XRD patterns of the cobalt oxide catalysts prepared from different precipitants and the Co-com catalyst. For all samples, intense diffraction lines

	were observed at 19.0, 31.3, 36.8, 44.8, 59.4 and 65.2°, corresponding to the (111),
	(220), (311), (400), (511) and (440) planes of the face-centered cubic spinel Co3O4
	phase (space group Fd3m (227), lattice constant a = 8.0840 Å, PDF # 74-2120). The
	presence of sharp and symmetric diffraction lines together with the absence of any extra

signal characteristic of an impurity indicated the good crystallinity and purity of the as-prepared catalysts. The result was in good agreement with the FTIR spectra analysis.

Table captions Table 1 .

 captions1 Chemical composition and structural properties of the Co3O4 catalysts.

Table 2 .

 2 Texture properties of the Co3O4 catalysts.

Table 3 .

 3 H2-TPR properties of the Co3O4 catalysts.

Table 4 .

 4 Specific reaction rates (r') at 230, 235 and 240 °C, apparent pre-exponential factors (A), apparent activation energies (Ea) and coefficient of determination (R 2 ) for toluene oxidation over various Co3O4 catalysts (reaction conditions: toluene concentration = 1000 ppm, O2 concentration = 21 vol.% and WHSV = 40,000 mL h -1 g -1 ).

Table 5 .

 5 Specific reaction rates (r') at 140, 150 and 160 °C, apparent pre-exponential factors (A), apparent activation energies (Ea) and coefficient of determination (R 2 ) for toluene oxidation over various Co3O4 catalysts (reaction conditions: propane concentration = 1000 ppm, O2 concentration = 21 vol.% and WHSV = 40,000 mL h -1 g -1 ).

Table 1 .

 1 Chemical composition and structural properties of the Co3O4 catalysts. Average crystallite size, lattice constant, microstrain and XRD density calculated from the XRD patterns.

	Catalysts	Co	d b (nm)	a b (Å)	Microstrain b	ρXRD b (g	Co 2+ /Co 3+c
		mass			(%)	cm -3 )
		ratio a
		(%)
	Co-C2O4 75.1	40.5±0.5 8.086±0.0004 0.24±0.003	6.05	0.245
	Co-CO3	74.7	29.4±0.3 8.084±0.0003 0.34±0.004	6.06	0.275
	Co-OH	75.5	37.4±0.5 8.084±0.0002 0.27±0.003	6.05	0.232
	Co-NH3	74.9	34.3±0.4 8.085±0.0004 0.32±0.003	6.05	0.232
	Co-urea	75.6	38.2±0.5 8.086±0.0004 0.26±0.003	6.05	0.220
	Co-com	74.1	26.9±0.3 8.087±0.0007 0.38±0.005	6.05	0.268

a Determined by ICP analysis. b c Estimated from the FTIR spectra.

Table 2 .

 2 Texture properties of the Co3O4 catalysts.

	Catalysts	SSA a (

m 2 g -1 ) Vpore a (cm 3 g -1 ) Sgeometrical b (m 2 g -1 ) Porosity c (%)

	Co-C2O4	15	0.043	24	21
	Co-CO3	30	0.082	34	33
	Co-OH	22	0.045	27	21
	Co-NH3	17	0.065	29	28
	Co-urea	21	0.092	26	36
	Co-com	40	0.197	37	54

Table 3 .

 3 H2-TPR properties of the Co3O4 catalysts. Calculated by integral of low and high temperature peak and normalized to 1/x for better comparison.

	Catalysts Total H2	Initial reduction	H2 consumption	Low/high peak area
		consumption	temperature	< 300 °C (mmol	ratio a
		(mmol g -1 )	(°C)	g -1 )
	Co-C2O4	16.9	195	2.41	1/3.8
	Co-CO3	16.0	175	3.01	1/3.1
	Co-OH	14.4	188	2.86	1/3.1
	Co-NH3	15.7	193	2.29	1/2.9
	Co-urea	14.8	192	2.57	1/2.9
	Co-com	15.4	180	2.98	1/3.2

a

Table 4 .

 4 Specific reaction rates (r') at 230, 235 and 240 °C, apparent pre-exponential factors (A), apparent activation energies (Ea) and coefficient of determination (R 2 ) for toluene oxidation over various Co3O4 catalysts (reaction conditions: toluene concentration = 1000 ppm, O2 concentration = 21 vol.% and WHSV = 40,000 mL h -1 g -1 ).

	Catalysts	r' (nmol m -2 s -1 )		lnA	Ea (kJ mol -1 )	R 2
		230 °C	235 °C	240 °C
	Co-C2O4	0.5	0.8	1.2	27±0.4	189±2	0.998
	Co-CO3	2.6	4.1	6.4	31±0.8	196±3	0.995
	Co-OH	1.0	1.5	2.1	20±0.5	156±2	0.996
	Co-NH3	0.7	1.1	1.6	22±0.4	170±2	0.998
	Co-urea	0.7	1.1	1.5	22±0.6	166±3	0.994
	Co-com	1.1	1.5	2.0	15±0.9	135±4	0.991

Table 5 .

 5 Specific reaction rates (r') at 140, 150 and 160 °C, apparent pre-exponential factors (A), apparent activation energies (Ea) and coefficient of determination (R 2 ) for toluene oxidation over various Co3O4 catalysts (reaction conditions: propane concentration = 1000 ppm, O2 concentration = 21 vol.% and WHSV = 40,000 mL h -1 g -1 ).

	Catalysts	r'(nmol m -2 s -1 )		lnA	Ea (kJ mol -1 )	R 2
		140 °C	150 °C	160 °C
	Co-C2O4	0.3	0.6	1.1	6.7±0.2	89±0.7	0.999
	Co-CO3	0.5	0.9	1.5	4.2±0.2	76±0.8	0.999
	Co-OH	0.3	0.5	0.9	7.1±0.5	89±1.9	0.999
	Co-NH3	0.3	0.5	1.0	6.7±0.3	89±1.1	0.999
	Co-urea	0.2	0.5	0.8	6.7±0.3	89±1.0	0.999
	Co-com	0.3	0.6	1.0	6.6±0.4	85±1.4	0.999

Acknowledgements

This work was financially supported by the University Claude Bernard Lyon 1 and CNRS. We gratefully acknowledge the China Scholarship Council of P.R. China for ZHANG Weidong's grant.


Lu Xi

Chao-Yi Jin

Hong-Wei Song

Xiao-Tong Wang

Li-Cun Li

Jean- Pascal Sutter
email: jean-pascal.sutter@lcc-toulouse.fr

Supramolecular heptanuclear Ln-Cu complexes involving nitronyl nitroxide biradicals: structure and magnetic behavior

Four novel heptanuclear Ln-Cu complexes with formula [Ln 2 Cu(hfac) 8 (NITPhTzbis) 2 ][LnCu(hfac) 5 (NITPhTzbis)] 2 (LnCu = YCu 1, TbCu 2, DyCu 3 and HoCu 4; hfac = hexafluoroacetylacetonate) were successfully constructed by employing the triazole functionalized nitronyl nitroxide biradical ligand NITPh-Tzbis (NITPh-Tzbis = 5-(1,2,4triazolyl)-1,3-bis(1′-oxyl-3′-oxido-4′,4′,5′,5′-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). These hetero-tri-spin complexes are composed of two biradical-bridged dinuclear [(LnCu(hfac) 5 (NITPhTzbis)] units and one trinuclear [Ln 2 Cu(hfac) 8 (NITPhTzbis) 2 ] unit which form a heptanuclear supramolecular structure through π-π interactions. Magnetic susceptibility investigations indicate that ferromagnetic exchange interactions dominate at low temperature for this supramolecular system which can be attributed to the Ln-nitroxide exchange and intramolecular NIT---NIT coupling mediated by the m-phenylene moiety. The DyCu derivative was found to exhibit slow magnetic relaxation behavior.

Introduction

The development of single-molecule magnets (SMMs) containing Ln ions has been one of the frontiers of research in the field of SMMs since the turn of the century [1][2][3][4][5] owing to the strong magnetic anisotropy and the large magnetic moments of the lanthanides ions. Employing this approach, some remarkable achievements have been obtained, [6][7][8][9] such as the [(Cp iPr5 )Dy(Cp * )] + complex with a record anisotropic energy barrier of 2217 K and a blocking temperature of 80 K, which is the best SMM reported to date. 10 However, some challenges remain to be mastered, like quantum tunneling of magnetization (QTM) caused by inherent characteristic of lanthanide ions, which brings about a rapid decay of magnetization at zero field. A promising approach is to implement a large exchange interaction with the rare earth centers in poly-spin complexes. [11][12][13][14][15][16][17] Nevertheless, achieving a strong exchange interaction with Ln ions is not obvious because the 4f magnetic orbitals are shielded by the more external 5s and 5p electron clouds. In this context, the use of paramagnetic radical ligands is an attractive strategy in which the direct exchange between lanthanide ion and coordinated radical could be achieved. 18 In 2011, Long et al 19 reported a N 2 3radical bridged Tb 2 complex exhibiting blocking temperature of 14 K, which demonstrated that better SMM properties can be achieved by introducing organic radicals in the Ln-based complexes. Murugesu et al 20 successfully obtained a Dy 4 complex by employing highly delocalized tetrazinyl radicals, which result a magnet-like behaviour with a large coercive field of 30 kOe in this complex. Most extreme situation, reported by Popov's group, 21 is found in dimetallofullerene compounds featuring a covalent lanthanide-lanthanide bond containing an unpaired electron acting as paramagnetic bridge between the Ln centers. These compounds exhibited remarkable magnetic relaxation behavior , as illustrated by Tb 2 @C 80 (CH 2 Ph) with a blocking temperature near 30 K. 22 Nitronyl nitroxide radicals (NIT), owing to their stability and ease of chemical modification, [START_REF] Amabilino | Magnetism: Molecules to Materials II: Molecule-Based Materials[END_REF][START_REF] Luneau | [END_REF] have been widely used for the construction of molecular nanomagnets. [25][26][27][28][29][30] While most attention focused on mono-nitronyl nitroxide radicals, poly-radical ligands provide new opportunities to obtain diverse spin topologies and interesting magnetic properties. [31][32][33] On the other hand, combining 3d ions and 4f ions within a molecular entity by the means of radical ligands has emerged as a promising approach for designing molecular magnetic materials in which the magnetic exchanges and the ligand field could be finely tuned. [34][35][36][37][38][39] Herein, a novel nitronyl nitroxide biradical ligand with a triazole group, i.e. NITPhTzbis (5-

Experimental Materials and physical measurements.

All solvents and other reagents (AR grade) for the experiments were commercially available and used as received without any further purification, and nitronyl nitroxide biradical NITPhTzbis was prepared according to literature procedure. 40,41 Elemental analysis was performed by a Perkin-Elmer 240 elemental analyzer. FT-IR data were tested using a Bruker-Vector 22 Spectrophotometer. Powder X-ray diffraction (PXRD) data of the as-prepared 1-4 was obtained on a Rigaku Ultima IV diffractometer at room temperature. Magnetic data were obtained on a Quantum Design MPMS 5S SQUID magnetometer and a PPMS-9 magnetometer. The magnetic measurements were performed on crystalline powder samples mixed with grease and hold in a gelatin capsule. The diamagnetic corrections for direct-current data were made for the constituent atoms using Pascal's constants. [START_REF] Boudreaux | Theory and applications of molecular paramagnetism[END_REF] The crystalline phase purities of four complexes are confirmed by PXRD (Figure S1).

Crystallography.

The structures of 1-3 were determined at 113 K on a Rigaku Saturn CCD diffractometer equipped with Mo-Kα radiation (λ = 0.71073 Å), while the data for 4 was collected at 150 K on an Oxford Diffraction Gemini E system (Mo-Kα, λ = 0.71073 Å). Absorption corrections were performed through multi-scan. All structures were solved with the SHELXS-2014 structure solution program [START_REF] Sheldrick | SHELXS-2014, Program for structure solution[END_REF] and refined anisotropically by using full-matrix least-squares based on F 2 with version of SHELXL-2014. [START_REF] Sheldrick | SHELXL-2014, Program for structure refinement[END_REF] All hydrogen atoms were geometrically placed. Some restraints, such as ISOR, DFIX, DELU, SADI, DANG, FLAT, RIGU and SIMU were employed on some of the disorderly C atoms and F atoms to ensure convergence of the refinement process. These data of 1-4 can be obtained freely with CCDC numbers 2142417-2142420 from The Cambridge Crystallographic Data Centre.

Synthesis of [Ln 2 Cu(hfac) 8 (NITPhTzbis) 2 ][LnCu(hfac) 5 (NITPhTzbis)] 2

Ln(hfac) 3 •2H 2 O (0.01 mmol) and Cu(hfac) 2 •2H 2 O (0.01 mmol) were suspended in 25 mL of dry n-heptane, which was kept under refluxing for 6 hours. Then, a dichloromethane solution containing NITPhTzbis (0.01 mmol) was slowly added to the above system and the obtained solution was kept heating for another 16 minutes. The resulting mixture was filtered after cooling and placed at room temperature to allow the slow evaporation of the solvent. Block-shaped dark blue crystals suitable for X-ray diffraction were obtained within 3 days.

R 1 = Σ(||F o | -|F c ||)/Σ|F o |, wR 2 = [Σw(|F o | 2 -|F c | 2 ) 2 /Σw(|F o | 2 ) 2 ] 1/2

Results and discussion

Description of the Crystal Structures

Complexes 1-4 were characterized by single-crystal X-ray diffraction, revealing that 1-4 are all isomorphous and crystallized in the triclinic system with the Pī space group. Crystallographic details and structural solutions are summarized in Table 1, and the selected bond lengths and angles are listed in Tables S1-S4. ORTEP drawing of all four structures are displayed in Figure S2. For simplicity, only the structure of complex 3 will be described in detail as a representative.

Complex 3 is composed of two dinuclear units [DyCu(hfac) 5 (NITPhTzbis)] (blue in Figure 1) and one trinuclear unit [Dy 2 Cu(hfac) 8 (NITPhTzbis) 2 ] (orange in Figure 1). In all units, the Dy III ion is coordinated by two neighboring nitroxide moieties of a NITPh-Tzbis ligand in a chelating manner and by three bidentate β-diketonate coligands (Dy1-O hfac : 2.321(9)-2.435(9) Å), forming a coordination polyhedron with biaugmented trigonal prism (C 2v ) geometry (Figure 2, SHAPE software, [START_REF] Casanova | [END_REF] Table S5). The Dy-O rad bond lengths (Dy-O rad : 2.359(9)-2.414(8) Å) are consistent with those reported in other Dy(hfac) 3 -nitronyl nitroxide biradical complexes. [START_REF] Bernot | [END_REF] Magnetic studies. The temperature-dependent molar magnetic susceptibility (χ M ) of complexes 1-4 were collected between 2 and 300 K under a dc field of 1000 Oe (Figure 4). The values of χ M T product at 300 K for complexes 1-4 are 4.46, 52.00, 61.70 and 60.97 cm 3 •K•mol -1 , respectively, which are close to the Curie contributions (4.13 cm 3 •mol -1 K for 1, 51.41 cm 3 •mol -1 K for 2, 60.81 cm 3 •mol -1 K for 3 and 60.41 cm 3 •mol -1 K for 4) expected for four Ln III ions, three Cu II ions (S = 1/2) and eight S = 1/2 radicals in the absence of magnetic exchange interactions. For YCu complex, the χ M T value gradually increased, reaching the maximum value of 6.56 cm 3 mol -1 K at 2 K, indicating a significant ferromagnetic coupling effect. The field dependence of the molar magnetization (M) for 1 was measured at 2-5 K in the field range of 0-50 kOe (Figure 4a inset). For the M-H curve at 2.0 K, it shows a sustained rise to reach to 10.18 Nβ at 50 kOe, which is basically consistent with theoretical saturation value of 11 Nβ. According to the crystal structure of 1, two kinds of magnetic exchanges might be anticipated.

One is the interaction between the two aminoxyl units belonging to the same ligand (J 1 , in Scheme 2) and this interaction may take place through Y III and/or the m-phenylene ring; the second is the magnetic interaction between the radicals and a Cu II ion transmitted through the benzene ring and the triazole ring. However, latter should be so weak that it can be ignored. Therefore, the magnetic behavior of 1 is best described as four [rad-Y-rad] units plus three isolated Cu II ions (Scheme 2).

To account for possible weak interactions within or between binuclear and trinuclear units, a mean-field parameter zJ′ was introduced. Modeling was performed with PHI software, 47 results are given based on the following Hamiltonian:

( )

1 1 2 ˆ2 r ra ad d H S S J = -
The simultaneous fitting of χ M T versus T and magnetization data (Figure 4a) for 1 resulted in J 1 = 12.49 cm -1 , zJ′ ′ = 0.020 cm -1 , g Cu = 2.25, g rad = 2.00 (fixed). The positive value of J 1 indicates that a ferromagnetic NIT---NIT exchange dominates in the system, which is in line with the reported results. 48,49 This ferromagnetic contribution can be attributed to the magnetic coupling between two NIT via m-phenylene ring that is anticipated to be ferromagnetic based on spin polarization, 33 whereas the magnetic exchange between two NO units through Y ion has been reported to be weakly antiferromagnetic. 50,51 The small positive zJ′ indicates a weak intermolecular ferromagnetic coupling likely resulting from through-space interactions between closely spaced radicals. Indeed, the opposite spin polarization between the oxygen atom of the NO group in NITPhTzbis and the sp 2 carbon atom of the adjacent NITPhTzbis (Figure 5) should lead to ferromagnetism according to the McConnells mechanism. 52,53 For complexes 2 and 3, the χ M T value shows a slight diminution as the temperature down to about 70 K for 2 and 50K for 3, followed by a rapid increase in the lower temperature domain and reaches the maximum value of 63.47 cm 3 •mol -1 K for 2 and 81.91 cm 3 •mol -1 K for 3. For 4, the χ M T value first gradually decreases to 54.54 cm 3 •mol -1 K and then increase quickly to 67.61 cm 3 •mol -1 K at 2 K. For these three complexes, the decrease of χ M T curve at high temperature mainly results from thermal depopulation of the Stark sublevels (i.e. the crystal field effect) of the Ln III ions, while the increases at low temperature region may results from the ferromagnetic interactions in the [NIT-Ln-NIT] units. 54,55 The molar magnetizations (M) of 2-4 were also measured 2.0-5.0 K with external fields(H) in the range of 0-50 kOe (Figure S5). At 2.0K, for three complexes, the intensity of magnetization increased steeply as the field increased to 10 kOe, then follow a gradual increase till reaching 28.94, 29.64 and 30.81Nβ at 50 kOe for 2-4, respectively. Besides, M(H/T) curves of 2-4 were not superimpose completely, suggesting the presence of significant magnetic anisotropy. 56

Dynamic Susceptibility Studies.

Alternating-current (ac) magnetic susceptibility measurements were performed for complexes 2-4 in zero dc field between 2 and 20 K to examine the spin dynamics. For 3, out-of phase signals (χ'') appeared (Figure S6), indicating a possible slow magnetic relaxation behavior, however, no peaks for χ'' were observed above 2 K, likely due to quantum tunnelling of magnetization (QTM).

When a static field was applied (400 Oe was found to be the optimal field, see Figure S7), QTM was suppressed effectively and both frequency and temperature-dependent out-of-phase (χ'') peaks were observed (Figure 6), evidencing slow magnetic relaxation behaviour for DyCu complex 3. Further insight in the relaxation dynamics of 3 was obtained by extracting relaxation times (τ) from χ''(ν) data using the generalized Debye model. 57,58 The analysis of the temperature dependence of τ with the Arrhenius equation τ = τ 0 exp (U eff /k B T) gave the energy barrier for the reversal of the magnetization U eff of 10.8 K with the pre-exponential factor τ 0 = 6.3 × 10 -7 s (Figure 7a). Furthermore, the Cole-Cole diagrams of 3 (Figure 7b) were also constructed and fitted by using a generalized Debye model and the obtained α values are in the range of 0.06-0.55, indicating a moderate distribution of relaxation time. For complex 2, only the onset of a weak outof-phase signal was observed whether in zero field or with applied field (Figure S8a) while, for compound 4, no out-of-phase signal was observed (Figure S8b). It is appealing to compare complex 3 with related biradical-based Dy complexes. By reviewing complexes with related isolated [NIT-Dy-NIT] tri-spin fragment (Table S6) [START_REF] Bernot | [END_REF][59][60][61] , those nitronyl nitroxide biradical Dy complexes exhibit magnetic relaxation behavior except for a MnDy complex 59 in which the local symmetry of the Dy ion belongs to D 2d . This means that the symmetry of the local crystal-field of Dy ion play crucial role on the magnetic relaxation. For these complexes, including 3, the values of effective energy barriers show some differences and this could be attributed to slightly different exchange interactions, the local symmetry of the Ln ion and supramolcular interactions.

Conclusion

Using a novel nitronyl nitroxide biradical functionalized by a triazole group (NITPhTzbis), four biradical-Ln-Cu complexes have been obtained. In these complexes, consist in a heptanuclear supramolecular structure formed dinuclear [LnCu(hfac) 5 (NITPhTzbis)] and trinuclear [Ln 2 Cu(hfac) 8 (NITPhTzbis) 2 ] units assembled through π---π interactions. Their magnetic

( 1 , 2 (LnCu = YCu 1 , 2 , 3 ,

 12123 Scheme 1. NITPhTzbis radical ligand.

[Y 2

 2 Cu(hfac) 8 (NITPhTzbis) 2 ][YCu(hfac) 5 (NITPhTzbis)] 2 (1). Yield 61%; Elemental analysis (%) calcd for C 178 H 138 Cu 3 Y 4 F 108 N 28 O 52 (M W = 6099.42): C, 35.08; H, 2.22; N, 6.43. Found: C, 35.15; H, 2.48; N, 6.55. FT-IR (cm -1 ): 1650 (s), 1505 (m), 1350 (m), 1251 (s), 1201 (s), 1138 (s), 993 (m), 870 (m), 797 (s), 659 (s), 587 (s), 544 (m).

[Tb 2

 2 Cu(hfac) 8 (NITPhTzbis) 2 ][TbCu(hfac) 5 (NITPhTzbis)] 2 (2

Figure 1 .

 1 Figure 1. Heptanuclear supramolecular structure of complex 3. Fluorine and hydrogen atoms are omitted for the sake of clarity.

  The N-O bond lengths of a NIT unit are unequivalent, it is slightly longer for the N-O involved in coordination to a metal ion. Torsion angles of Dy-O-N-C are 60.8(16)°, 65.4(15)° for Dy1 in dinuclear unit and 57.0(17)°, 62.0(17)° for Dy2 in trinuclear unit. The dihedral angles formed between the radical ON-C-NO plane and the benzene ring are 26.7(6)°, 25.0(5)° for dinuclear unit and 31.2(7)°, 28.0(3)° for trinuclear unit, while dihedral angles formed by the triazole ring and the benzene ring are 46.6(3)° and 28.5(4)°, respectively. Two different types of Cu II ions are found co-exist within the structure. The Cu1 center in dinuclear unit is five-coordinated, with one N atom from the triazole unit and four O atoms from two β-diketonate coligands. The coordination sphere exhibits a distorted pyramidal geometry. As a result of Jahn-Teller effect, one O atom of a β-diketonate coligand is located in the apical position, showing a significantly longer bond length (Cu1-O:2.219(9) Å) than those atoms in equatorial positions (Cu1-N: 1.988(9) Å; Cu1-O: 1.936(9)-1.972(9) Å). The Cu II -Dy III distance through phenyl and triazole rings in dinuclear is 10.312 Å. The Cu2 center in trinuclear unit sits in a distorted octahedron. The equatorial positions are occupied by two N atoms which come from the triazole moieties of two biradicals (Cu2-N: 1.990(9) Å) and two O atoms of β-diketonate coligands (Cu2-O: 2.083(9) Å). These coligands also provide another two O atoms coordinated to the Cu2 center in the apical positions with longer bond length (Cu2-O: 2.201(9) Å), which indicates a Jahn-Teller effect. The Cu II -Dy III distance through phenyl and triazole rings in trinuclear is 10.571 Å. Furthermore, the different coordination spheres of two kinds of Cu II centers may be responsible for the relatively large differences in dihedral angles formed by triazole rings and benzene rings.

Figure 2 .

 2 Figure 2. Coordination polyhedron of Dy III ions in (a) dinuclear and (b) trinuclear unit respectively.

Figure 3 .

 3 Figure 3. Packing arrangement formed by π-π interaction in DyCu complex. The dinuclear and trinuclear units are gripped together through the π-π interactions between the benzene rings of NITPhTzbis ligans(distance 3.507 Å, Figure 3) to form the heptanuclear supermolecule. In the heptanuclear cluster of 3, the Dy1•••Dy2, Dy1•••Dy2a and Dy1•••Dy1a distances are 9.823 Å, 18.328 Å, and 20.373 Å, respectively, and the distance between Cu1 and Cu2 is 7.741 Å. The shortest separation between the uncoordinated N-O groups is 4.043 Å. It should be noted that, for this supramolecule structure, there are short distances between radical moieties involving the uncoordinated NO group and the sp 2 carbon atoms of NIT (O24---C7: 3.776 Å; O10---C54: 3.135 Å) as revealed in Figure S3. The views of the crystal packing of all complexes are shown in Figure 3 and Figure S4.

Figure 4 .Scheme 2 .

 42 Figure 4. (a) χ M T vs. T curve for the YCu analogue, inset shows M vs. H curves in the field range 0-50 kOe and at the temperature range of 2-5 K (the solid line represents best fitted behavior by PHI). (b) χ M T vs. T curves for TbCu, DyCu and HoCu analogues.

Figure 5 .

 5 Figure 5. Spin interactions responsible for the ferromagnetic coupling based on McConnells mechanism.

Figure 6 .

 6 Figure 6. Temperature (left) and frequency dependence (right) of the in-phase and out-of-phase ac susceptibilities under applied dc field of 400 Oe for complex 3.

Figure 7 .

 7 Figure 7. Plot of lnτ versus T -1 (a) and Cole-Cole diagrams at different temperatures under a 400 Oe dc field (b) for 3 (the solid line stands for the fitting results).

Dy 2 Cu(hfac) 8 (NITPhTzbis) 2 ][DyCu(hfac) 5 (NITPhTzbis)] 2 (3).

  ). Yield 63%; Elemental analysis (%) calcd for C 178 H 134 Cu 3 Tb 4 F 108 N 28 O 52 (M W = 6375.42): C, 33.53; H, 2.12; N, 6.15. Found: C, 33.15; H, 2.48; N, 6.55. FT-IR (cm -1 ): 1649 (s), 1503 (m), 1351 (m), 1253 (s), 1200 (s), 1140 (s), 995 (m), 869 (m), 798 (s), 660 (s), 585 (s), 545 (m). Yield 59%; Elemental analysis (%) calcd for C 178 H 134 Cu 3 Dy 4 F 108 N 28 O 52 (M W = 6389.74): C, 33.46; H, 2.11; N, 6.14. Found: C,

	[33.75; H, 2.56; N, 6.38. FT-IR (cm -1 ): 1650 (s), 1503 (m), 1351 (m), 1252 (s), 1202 (s), 1139 (s),
	994 (m), 869 (m), 797 (s), 660 (s), 585 (s), 545 (m).
	[

Ho 2 Cu(hfac) 8 (NITPhTzbis) 2 ][HoCu(hfac) 5 (NITPhTzbis)] 2 (4).

  Yield 61%; Elemental analysis (%) calcd for C 178 H 134 Cu 3 Ho 4 F 108 N 28 O 52 (M W = 6099.42): C, 33.41; H, 2.11; N, 6.13. Found: C,

	33.11; H, 2.52; N, 6.22. FT-IR (cm -1 ): 1649 (s), 1504 (m), 1351 (m), 1252 (s), 1201 (s), 1140 (s),
	995 (m), 869 (m), 798 (s), 660 (s), 586 (s), 544 (m).

Table 1 .

 1 Crystallographic data and structure refinement details of complexes 1-4.

	Complex	1 (CuY)	2 (CuTb)	3 (CuDy)	4 (CuHo)
	Formula		C 178 H 134 Cu 3 Ln 4 F 108 N 28 O 52
	M(g•mol -1 )	6095.38	6375.42	6389.74	6399.46
	T(K)	113(2)	113(2)	113(2)	150(2)
	Crystal system	Triclinic	Triclinic	Triclinic	Triclinic
	Space group	Pī	Pī	Pī	Pī
	a(Å)	14.695(3)	14.696(3)	14.789(3)	14.6422(6)
	b(Å)	21.213(4)	21.242(4)	21.301(4)	21.1036(9)
	c(Å)	21.502(4)	21.576(4)	21.621(4)	21.4102(8)
	α(deg)	85.16(3)	85.11(3)	85.11(3)	85.124(3)
	β(deg)	71.34(3)	71.23(3)	71.25(3)	71.356(4)
	γ(deg)	85.75(3)	85.78(3)	85.82(3)	85.871(3)
	V(Å 3 )	6320(2)	6346(2)	6419(3)	6238.9(5)
	Z	1	1	1	1
	D calcd (g•cm -3 )	1.602	1.668	1.653	1.703
	Μ(mm -1 )	1.308	1.498	1.544	1.658
	θ(deg)	1.491-25.000	1.342-25.010	1.338-25.010	2.9828-25.010
	F(000)	3029	3133	3137	3141
	Reflnscollected	61433	61509	51809	52184
	Uniquereflns/R int 22233/0.1425	22348/0.0697	21888/0.0813	21988/0.0813
	GOF(F 2 )	1.060	1.059	1.066	1.040
	R 1 /wR 2 (I>2σ(I))	0.1204/0.3026	0.0820/0.2224	0.0980/0.2488	0.0958/0.2410
	R 1 /wR 2 (all data)	0.2169/0.3768	0.1093/0.2531	0.1266/0.2754	0.1340/0.2679

behaviors are governed by intramolecular Ln-NO (Ln = Tb, Dy, Ho) and intermolecular ferromagnetic interactions. Interestingly, CuDy derivative shows magnetic relaxation behavior. This work demonstrates that functional nitronyl nitroxide biradicals are appealing building blocks to construct SMMs with lanthanides and their magnetic properties could be tuned by different functional substituents via ligating transition metal ions.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2018YFA0306002) and the National Natural Science Foundation of China (No. 21773122).

Conflicts of interest

There are no conflicts of interest to declare.

Author contributions

Lu Xi: investigation, fitting the magnetic data; writing -original draft; Chao-Yi Jin: investigation;

Hong-Wei Song: data analysis; Xiao-Tong Wang: data analysis; Licun Li: Supervision, Writing -review and editing, Funding acquisition; Jean-Pascal Sutter: Magnetic measurements, Writingreview and editing. All authors have given approval to the final version of the manuscript.