Katarzyna Świrk

Maria Elena Gálvez

Monika Motak

Teresa Grzybek

Magnus Rønning
email: magnus.ronning@ntnu.no

Patrick Da Costa
email: patrick.da_costa@upmc.fr

Yttrium promoted Ni-based double-layered hydroxides for dry methane reforming

Keywords: dry reforming of methane, carbon dioxide, nickel, yttrium, double-layered hydroxides

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Introduction

Dry methane reforming (DRM), also called CO 2 reforming, is considered one of the promising processes for chemical CO 2 utilization [START_REF] Olajire | Valorization of greenhouse carbon dioxide emissions into value-added products by catalytic processes[END_REF][START_REF] Schakel | Assessing the technoenvironmental performance of CO 2 utilization via dry reforming of methane for the production of dimethyl ether[END_REF][START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF][4][START_REF] Nair | Nanocast LaNiO 3 perovskites as precursors for the preparation of coke-resistant dry reforming catalysts[END_REF][START_REF] Margossian | Supported bimetallic NiFe nanoparticles through colloid synthesis for improved dry reforming performance[END_REF]. DRM leads to synthesis gas, a mixture of hydrogen and carbon monoxide, that is currently used as feedstock in liquid fuel production processes, such as Fischer-Tropsch or methanol synthesis [START_REF] Xie | One-pot synthesis of supported, nanocrystalline nickel manganese oxide for dry reforming of methane[END_REF]. Nowadays, syngas is traditionally produced through steam methane reforming (SMR) [START_REF] Nair | Nanocast LaNiO 3 perovskites as precursors for the preparation of coke-resistant dry reforming catalysts[END_REF]. Still, SMR yields a syngas with a far too high H 2 /CO ratio and needs to be consequently pre-treated when used in liquid fuel synthesis. For instance, the production of dimethyl ether requires H 2 /CO ratio close to unity [START_REF] Schakel | Assessing the technoenvironmental performance of CO 2 utilization via dry reforming of methane for the production of dimethyl ether[END_REF]. This value can be easily obtained through dry methane reforming. In spite of this, DRM has not yet been fully launched on industrial scale because of its large demand for energy and the lack of active, selective and stable catalysts [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF].

Ni-based catalysts seem to be good candidates for DRM due to their availability, low cost and good catalytic performance [START_REF] Fan | Utilization of greenhouse gases through dry reforming: Screening of nickel-based bimetallic catalysts and kinetic studies[END_REF][START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF]. However, the main drawback for their application is susceptibility to carbon formation leading to fast deactivation [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF]4,[START_REF] Margossian | Supported bimetallic NiFe nanoparticles through colloid synthesis for improved dry reforming performance[END_REF][START_REF] Charisiou | Investigating the correlation between deactivation and the carbon deposited on the surface of Ni/Al 2 O 3 and Ni/La 2 O 3 -Al 2 O 3 catalysts during the biogas reforming reaction[END_REF]. The methods proposed to improve stability of Ni-based catalysts, include, among others, the addition of a second metal and/or a promoter, or the choice of an appropriate support. The proposed second metal may be either a noble metal (Pt, Ru, Rh, or Ir) or a transition metal (e.g. Co or Zn) [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF][START_REF] Tsyganok | Dry reforming of methane over supported noble metals: A novel approach to preparing catalysts[END_REF][START_REF] Rostrup-Nielsen | Aspects of CO 2 -reforming of methane[END_REF][START_REF] Hei | CO 2 -reforming of methane on transition metal surfaces[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF].

Ni/Al 2 O 3 catalysts have been widely studied in DRM, yielding satisfactory catalytic activity due to the textural properties of alumina [START_REF] Kawi | Progress in synthesis of highly active and stable nickel-based catalysts for carbon dioxide reforming of methane[END_REF]. However, severe deactivation attributed to carbon formation and/or sintering of the active phase was observed [4] [START_REF] Muraza | A review on coke management during dry reforming of methane[END_REF]. On the other hand, the presence of a NiO-MgO solid solution is known to enhance resistance of nickel particles towards sintering and to inhibit carbon formation [START_REF] Hu | The characterization of a highly effective NiO/MgO solid solution catalyst in the CO 2 reforming of CH 4[END_REF]. Additionally, with the increase of MgO loading, higher coking resistance was reported for Ni-MgO-Al 2 O 3 catalysts [START_REF] Min | Carbon dioxide reforming of methane on Ni-MgO-Al 2 O 3 catalysts prepared by sol-gel method: Effects of Mg/Al ratios[END_REF]. Double-layered hydroxides (DLHs) can be the source of prospective DRM catalyst supports, due to their layered structure and possible di-and/or tri-valent cations introduction into the layers, such as Mg 2+ , Ni 2+ , Co 2+ , Zn 2+ and/or Fe 3+ , Cr 3+ , Al 3+ . Interlayer anions, e.g. water molecules and/or carbonates, compensate the positive charge and they may also be exchanged for catalytically active components [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF][START_REF] Abdelsadek | In-situ hydrogasification/regeneration of NiAlhydrotalcite derived catalyst in the reaction of CO 2 reforming of methane: A versatile approach to catalyst recycling[END_REF][START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation, properties and applications[END_REF][START_REF] Tsyganok | Mg-Al layered double hydroxide intercalated with [Ni(EDTA)] 2chelate as a precursor for an efficient catalyst of methane reforming with carbon dioxide[END_REF][START_REF] Yu | Large-scale synthesis of carbon nanofibers on Ni-Fe-Al hydrotalcite derived catalysts. I. Preparation and characterization of the Ni-Fe-Al hydrotalcites and their derived catalysts[END_REF][START_REF] Kim | Effect of particle size and local disorder on specific surface area of layered double hydroxides upon calcination-reconstruction[END_REF]. Moreover, acid-base properties can be tailored, as reported e.g. for calcined NiMgAl-hydrotalcites by Pavel et al. [START_REF] Pavel | Acido-basic and catalytic properties of transition-metal containing Mg-Al hydrotalcites and their corresponding mixed oxides[END_REF] Basic properties, memory effect, and ability to exchange anions make them good catalysts and adsorbents [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF] [START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation, properties and applications[END_REF]. Ni-containing double layered hydroxides were found to be active in the DRM [START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF][START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF][START_REF] Zhu | Effect of Mg/Al ratio of NiMgAl mixed oxide catalyst derived from hydrotalcite for carbon dioxide reforming of methane[END_REF][START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF][START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF][START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Ni-Al hydrotalcitelike material as the catalyst precursors for the dry reforming of methane at low temperature[END_REF][START_REF] Bhattacharyya | CO 2 reforming of methane to syngas I: Evaluation of hydrotalcite clay-derived catalysts[END_REF][START_REF] Mette | High-temperature stable Ni nanoparticles for the dry reforming of methane[END_REF]. After appropriate thermal treatment DLHs form nano-oxides whose strong interaction hinders Ni from sintering at high temperatures required for DRM [START_REF] Mette | High-temperature stable Ni nanoparticles for the dry reforming of methane[END_REF]. It was also found that the stability of Ni-loaded hydrotalcite catalysts in DRM was enhanced by the addition of an appropriate promoter. Ce was reported to promote oxidation of carbon deposits through the Boudouard reaction [START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Bereketidou | Biogas reforming for syngas production over nickel supported on ceria-alumina catalysts[END_REF], while Zr inhibited reactions leading to the formation of carbon deposits [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF] [START_REF] Dębek | Low temperature dry methane reforming over Ce, Zr and CeZr promoted Ni-Mg-Al hydrotalcite-derived catalysts[END_REF]. Lanthanum promotion contributed to carbon gasification which was assigned to the formation of La 2 O 2 CO 3 [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF].

Nevertheless, novel catalysts with enhanced selectivity and stability for DRM need to be developed, and promotion with metals may highly contribute to the improvement of catalytic properties. Yttrium is a rare-earth element that has recently attracted large interest for catalytic applications. As reported by Francis and Whitlow [START_REF] Francis | The effect of yttrium on the high temperature oxidation resistance of some Fe-Cr base alloys in carbon dioxide[END_REF] yttrium oxides show high temperature oxidation resistance in the presence of carbon dioxide. Additionally, yttrium oxide has been studied as a support for DRM catalysts [START_REF] Chen | Yttria-stabilized zirconia supported platinum catalysts (Pt /YSZs) for CH 4 /CO 2 reforming[END_REF][START_REF] Bellido | Effect of the Y 2 O 3 -ZrO 2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF]. However, there are only a few literature reports describing the role of yttrium as a possible promoter in the DRM reaction [START_REF] Li | Methane reforming with CO 2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol-gel process[END_REF][START_REF] Wu | Effect of yttrium and praseodymium on properties of Ce 0.75 Zr 0.25 O 2 solid solution for CH 4 -CO 2 reforming[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF][START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF]. Li et al. [START_REF] Li | Methane reforming with CO 2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol-gel process[END_REF] observed high resistance towards carbon deposition for Y-promoted Ni-SBA-15 catalysts, which was attributed to high dispersion of yttrium with nickel nanoparticles, and the ability of yttrium to create oxygen vacancies. Similar properties were also observed in Y-Ce 0.75 Zr 0.25 O 2 catalysts [START_REF] Wu | Effect of yttrium and praseodymium on properties of Ce 0.75 Zr 0.25 O 2 solid solution for CH 4 -CO 2 reforming[END_REF]. Taherian et al. [START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF] reported a positive influence on controlling the nickel size and its dispersion into the mesopores of Ni/SBA-15 catalyst after modification with Y 2 O 3 . Moreover, in our previous study Ni-Y/KIT-6 catalyst exhibited better dispersion of nano-sized Ni particles inside the pores of the support as compared to Ni/KIT-6 material [START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. Also, the formation of coke was lower for Ymodified catalyst in the initial stability test at 700 °C. Furthermore, B. Li et al. [START_REF] Li | Alumina supported Ni and Co catalysts modified by Y 2 O 3 via different impregnation strategies: Comparative analysis on structural properties and catalytic performance in methane reforming with CO 2[END_REF] studied Ni/Y/Al 2 O 3 catalyst with yttrium introduced via different types of impregnation, and observed improved stability of the modified samples. However, in a recent study by Huang et al. [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF] the impregnated NiO-Y 2 O 3 -Al 2 O 3 mesoporous catalysts were reported to be less stable than the samples prepared with a one-pot evaporation-induced self-assembly method. The former resulted in the creation of loosely attached Ni particles on the support, which was the reason for fast deactivation of the catalyst due to graphitic carbon formation. The second method revealed the presence of Ni particles strongly anchored to the Al 2 O 3 support. This resulted in an improvement of catalytic activity and stability. Additionally, the best results were obtained for a catalyst containing ca. 2 wt % of yttrium, which was attributed to the high surface area, small nickel particles and very high nickel dispersion.

To the best of our knowledge, no studies concerning the influence of yttrium on Ni-Mg-Al double-layered hydroxides have been reported in the literature. Only a few studies have dealt with Mg-Al hydrotalcites modified with Y for expoxidation [START_REF] Pavel | The activity of yttrium-modified Mg,Al hydrotalcites in the epoxidation of styrene with hydrogen peroxide[END_REF], cyanoethylation [START_REF] Angelescu | Cyanoethylation of ethanol on Mg-Al hydrotalcites promoted by Y 3+ and La 3+[END_REF] and aldol condensation [START_REF] Wang | Reconstructed La-, Y-, Ce-modified MgAlhydrotalcite as a solid base catalyst for aldol condensation: Investigation of water tolerance[END_REF].

The aim of this work was to prepare novel double-layered hydroxide-derived catalysts containing yttrium (0.6 and 1.5 wt %), and to examine the influence of Y promotion on the final physico-chemical properties of the resulting catalysts, their structure and performance in DRM in comparison to the corresponding thermodynamic equilibrium.

Thermodynamic equilibrium analysis

Minimization of Gibbs free energy

The Gibbs free energy analysis was used to study the equilibrium of the system containing several species. The total Gibbs free energy is expressed as [START_REF] Cao | Tuning dry reforming of methane for F-T syntheses: A thermodynamic approach[END_REF][START_REF] Chen | Thermodynamic analysis of the partial oxidation of coke oven gas for indirect reduction of iron oxides in a blast furnace[END_REF][START_REF] Challiwala | A combined thermo-kinetic analysis of various methane reforming technologies: Comparison with dry reforming[END_REF]:

(1)
Where the G i 0 is standard Gibbs free energy of formation of species i, R stands for molar gas constant, T is temperature of the system, and a number of moles is assigned to n i of species i.

Standard-state is expressed as 25 ºC and 1 atm, while f i 0 is a fugacity at this state of species i, and f i refers to operating state. In view of equation [START_REF] Olajire | Valorization of greenhouse carbon dioxide emissions into value-added products by catalytic processes[END_REF], n i moles of species has to fulfill the following relation:

(

Where the number of element j expressed in grams or atoms of molecule i is assigned to a ji . b ji is a total number of moles of element j in the feed. Additionally, the following equality: 1≤j≤M, where M is the number of elements in the mixture, has to be fulfilled.

Calculation method

Thermodynamic calculations were carried out by HSC Chemistry 5 software. The minimization of Gibbs free energy method was used. The influence of temperature and feed gas composition

ú ú û ù ê ê ë é ÷ ÷ ø ö ç ç è ae + = å = 0 0 1 ln f f RT G n G i i N i i total å = = N i j ji b n a 1
were examined in an isobaric system in order to calculate the equilibrium amount of CO For the calculations, the chemical reactions in methane reforming processes were considered (Table 1) [START_REF] Cao | Tuning dry reforming of methane for F-T syntheses: A thermodynamic approach[END_REF][START_REF] Freitas | Thermodynamic analysis of methane reforming with CO 2 , CO 2 +H 2 O, CO 2 +O 2 and CO 2 +air for hydrogen and synthesis gas production[END_REF][START_REF] Nikoo | Thermodynamic analysis of carbon dioxide reforming of methane in view of solid carbon formation[END_REF]. The following reactants were taken into account: Ar (g) , CH 4(g) , CO 2(g) , CO (g) , H 2(g) , H 2 O (g) , C (s) , C 2 H 2(g) , C 2 H 4(g) , C 2 H 6(g) , CH 3 OCH 3(g) , CH 3 OH (g) , HCOOH (g) , while the substrates were assumed as: CH 4(g) , CO 2(g) and Ar (g) . All calculations were performed with constant pressure of 1 bar.

Equilibrium plots

Fig. 1 presents equilibrium amount of reactants as a function of temperature for dry methane reforming (CO 2 /CH 4 /Ar=1/1/8, p=1 bar). The results are in good agreement with other authors' work [START_REF] Pakhare | A review of dry (CO 2 ) reforming of methane over noble metal catalysts[END_REF][START_REF] Abasaeed | Catalytic performance of CeO 2 and ZrO 2 supported Co catalysts for hydrogen production via dry reforming of methane[END_REF]. Due to the endothermic nature of the process, and accompanied by parallel reactions, a large variation in the quantity of components while temperature increase (from 0 to 1000 ºC) is observed. In the middle of the temperature range (at 500 ºC) a large amount of carbon is formed, together with water, and greater amount of H 2 with respect to CO. The CO 2 equilibrium reaches a maximum peak at 500 ºC, which refers to the lowest conversion of this gas in the entire temperature range (0-1000 ºC) (Fig. 2). This suggests a very likely occurrence of other parallel reactions than just the dry reforming. Above this temperature (ca. 600 ºC and more), a fast decrease in carbon and water formation is observed. In addition, H 2 and CO, which represent a sufficient value of molar ratio (around 1) for further fuel production (Fig. 2), are observed. It is consistent that DRM likely occurs at very high temperature (<700 ºC), although from the economical point of view it would be desirable to run the process at slightly lower temperature with a very active and stable catalyst.

Experimental

Catalysts synthesis

The double-layered hydroxides were synthesized by a co-precipitation method using aqueous solutions of Mg(NO 3 ) 2 •6H 2 O (Sigma Aldrich, 99% pure), Ni(NO 3 ) 2 •6H 2 O (Sigma Aldrich, 98.5% pure), Al(NO 3 ) 3 •9H 2 O (Fluka, 98% pure) and Y(NO 3 ) 3 •6H 2 O (Aldrich, 99.8% pure). The molar ratios of Ni 2+ /Mg 2+ and M 2+ /M 3+ were adjusted to 0.33 and ca. 3.0, respectively. Catalysts preparation aimed at the following content of yttrium in the individual samples: 0.6 and 2.0 wt %.

As described further in the text the introduced amounts of yttrium were 0.6 and 1.5 wt %, and thus the samples were designated HTNi-Y0.6 and HTNi-Y1.5. The mixture of the salts was added dropwise to the 0.05 M sodium carbonate solution (AnalaR NORMAPUR, 99.9% pure), which was vigorously mixed and heated to 65 ºC. The pH during the co-precipitation was kept at 10±0.2 by using 2 M solution of NaOH. The mixture was allowed to react for 24 h. Then the slurry was filtered, washed with distilled water and dried overnight at 80 ºC. The catalysts were subsequently calcined in air at 550 ºC for 5 h.

Catalysts characterization

Elemental composition, structural parameters, and textural properties of catalysts

The elemental composition analysis was performed through Wavelength Dispersive X-Ray Fluorescence (WDXRF) in a Supermini200 analyzer. The analysis was carried out under vacuum at 36.5 °C. A proportional counter detector (PC) was used during experiments with the presence of P-10 gas (mixture of 10% CH 4 /Ar) with flow of 24.7 cm 3 /min. Before each experiment a pulse height analyzer (PHA) was adjusted to calibrate the detector. Samples were diluted in boric acid with a ratio of 1:40, and subsequently pelletized under pressure of 10 bar. The pellet was inserted between the sample retainer and sample holder and covered with 6 µm polypropylene film.

The structure and phase composition of the catalysts were examined by X-ray diffraction (PANalytical-Empyrean diffractometer, equipped with CuKα of λ=0.15406 nm radiation source).

The average crystallite size of Ni was determined from the Scherrer equation (K shape factor= 0.9) [START_REF] Kovanda | Crystallization of synthetic hydrotalcite under hydrothermal conditions[END_REF].

Textural properties were derived from low temperature N 2 adsorption measurements at -195 ºC using a Micromeritics TriStar II 3020. Prior to the test, 100 mg of calcined sample was outgassed at 110 ºC for 3 h. The Brunauer-Emmett-Teller (BET) method was used to calculate specific surface area, while pore diameter and pore volume were obtained from the Barrett-Joyner-Halenda (BJH) method.

Reducibility, basicity, Ni dispersion and crystallite size

The reducibility of calcined samples was determined by temperature-programmed reduction (TPR-H 2 ) measurements, which were carried out in a BEL Japan BELCAT-M, equipped with a thermal conductivity detector (TCD). 60 mg of each catalyst was first degassed in helium at 100 ºC for 2 h, and then reduced with 5% H 2 /Ar mixture in the temperature range from 100 to 900 ºC with a heating rate of 7.5 ºC/min. The basic properties of the samples were examined by temperature programmed desorption of carbon dioxide. The TPD-CO 2 measurement was performed just after the TPR-H 2 measurement, using the same apparatus. CO 2 was adsorbed from a mixture of 10% CO 2 in He. Subsequently, the weakly physically adsorbed carbon dioxide was desorbed in a flow of He for 15 min. Finally, the samples were heated from 80 ºC to 800 ºC in helium (10 ºC/min). In order to determine basicity, the obtained peaks were fitted by Gaussian contributions. The distribution of weak, middle and strong basic sites, was calculated based on the area of the fitted peaks.

The H 2 chemisorption was performed to determine nickel dispersion on reduced samples. The results were obtained using a Micrometrics ASAP 2020 instrument. Prior to the analysis, 200 mg of a calcined sample was evacuated in the He flow at 40 ºC for 1 h. Then, the reduction step occurred for the next hour in pure H 2 at 900 ºC. Afterwards, the second evacuation step was carried out under the same conditions as the first time. Finally, an analysis was performed in the flow of pure H 2 at 40 ºC.

Stability of the catalysts and carbon formation

The TGA/DSC-MS (STA 449C Jupiter TGA/DSC and Netzsch Aërlos QMS 403 MS) tests were performed on spent catalysts in order to calculate the mass loss and determine the thermal character of the reactions taking place during catalysts decomposition conducted in air with a heating rate of 10 ºC/min. 8 mg of each sample was studied as a function of temperature (from room temperature to 900 ºC). During mass spectroscopy analysis, the following gases were registered; m/e: 44 (CO 2 ) and 18 (H 2 O).

The micrographs obtained from Transmission Electron Microscopy (TEM, JOEL JEM-100XCII)

were used for the determination of Ni crystallite size distribution, and the type of formed carbon in the spent catalysts. The samples were prepared by dropwise addition of a colloidal solution in ethanol onto a copper grid covered with amorphous carbon film. 400 particles were measured in the ImageJ software in order to obtain the average Ni particle size in the spent catalysts.

Catalytic performance in dry reforming of methane

The catalytic tests were carried out in a fixed-bed flow reactor. A thermocouple type K was used to control the temperature, which was in contact with catalytic bed, protected by quartz shield.

Before the catalytic test, the samples were reduced in situ at 900 ºC for 1 h with a 5% H 2 /Ar gas mixture (heating rate 10 °C/min, flow 50 ml/min flow). Then the tests were performed in the temperature range of 850-600 ºC for every 50 ºC and cooling time of 15 min between the steps.

The duration of the measurement at each temperature was 30 minutes. Within this time steady-state was obtained. The composition of the inlet gas mixture was CH 4 /CO 2 /Ar=1/1/8 at GHSV= 20,000 h -1 . The total flow was 100 cm 3 /min. The outlet gases were analyzed by a gas chromatograph ( 490Varian Micro-GC). Time-on-stream experiments were carried out in order to check the stability of the prepared catalysts. The samples were reduced in situ with the mixture of 5% H 2 /Ar for 1 h. Then dry reforming was carried out at 700 ºC for 5 h with the same gas feed composition and gas hourly space velocity as above.

The relative change of CH 4 and CO 2 conversions after 5 and 10 h tests were calculated by using the following formula (3):

∆ = $ % &$ ' $ % • 100% (3)
Where ∆ is relative conversion change, x is conversion of CH 4 or CO 2 expressed in percentage, and subscripts stand for i -initial, h -after 5 or 10 hours.

The catalytic activity was also expressed as turnover frequency (TOF). The TOF values were calculated from equation ( 4), which represents the reaction rate per nickel active site.

,-. = &/•0 1233•4•5 (4)
of nickel, and D is the dispersion obtained from H 2 chemisorption. Table 2 shows the elemental composition of the calcined catalysts, as obtained by XRF. All samples have nickel content between 14 and 20 wt %. The yttrium loading was the same as the nominal loading for HTNi-Y0.6, but not for the HTNi-Y1.5 sample. This catalyst contained 1.5 wt % of Y, instead of 2.0 wt % assumed during the preparation procedure. This suggests that only in case of lower Y contents, Y could be fully introduced into the structure of the double-layered hydroxides. The obtained M 2+ /M 3+ atomic ratios were similar to the nominal value of 3.0 in all catalysts; however, the ratios of Ni 2+ /Mg 2+ were close to 0.33 only in HTNi and HTNi-Y0.6

samples. The catalyst with 1.5 wt % of yttrium showed the value lower than 0.33, i.e. 0.18, indicating that Mg 2+ ions were substituted only by a part of the assumed number of Ni 2+ cations.

On the other hand, the lower nickel content for this sample may also arise from possible substitution of Ni 2+ by Y 3+ , as suggested by J.F. Li et al. [START_REF] Li | Y 2 O 3 -promoted NiO/SBA-15 catalysts highly active for CO 2 /CH 4 reforming[END_REF] for yttrium promoted NiO/SBA-15 catalysts. In the latter case, this ion exchange contributed to the formation of oxygen vacancies, resulting in the elimination of carbon deposits.

The values for the lattice parameter a and unit cell parameter c (for uncalcined materials) are

shown in Table 2. The obtained values of c, which refers to the triple distance between brucite-like layers, remained practically unchanged after Y-modification as compared to non-modified material. Pavel et al. [START_REF] Pavel | The activity of yttrium-modified Mg,Al hydrotalcites in the epoxidation of styrene with hydrogen peroxide[END_REF] found an increase of c parameter suggesting Y aggregation on the external surface, which is not the case in our study. The c' parameter, equal to one brucite-like sheet and one interlayer (c´=c/3), is 7.8 Å and constant for our HTNi-Y catalysts, indicating the presence of CO 3 2-anions in the samples (7.65 Å) [START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation, properties and applications[END_REF]. Finally, the lattice parameter a is also stable for all catalysts (3.06 Å), although García-García et al. [START_REF] García-García | Chromium and yttrium-doped magnesium aluminum oxides prepared from layered double hydroxides[END_REF] assumed that the introduction of yttrium into DLHs layers, considering its ionic radius bigger than that of Mg and Al (Y 3+ =1.04 Å, Mg 3+ =0.86 Å, Al 3+ =0.675 Å) [START_REF] Fernández | New hydrotalcite-like compounds containing yttrium[END_REF], may cause lattice distortions, i.e. changes in the lattice parameter a. On the other hand, Fernández et al. [START_REF] Fernández | New hydrotalcite-like compounds containing yttrium[END_REF], reported that only yttrium loading higher than 4 wt %, was a possible reason for distortions in the DLHs layers, recognised as very low crystallinity observed in XRD patterns. As the amount of Y introduced into our catalysts was much lower, the stability of the lattice parameter a does not contradict the possibility of introduction of Y into the structure of double-layered hydroxides.

BET analysis for calcined samples revealed specific surface area values from 120 to 192 m 2 /g, where the highest S BET was observed for HTNi-Y1.5, and the lowest for the non-modified catalyst (Table 2). As reported by Kim et al. [START_REF] Kim | Effect of particle size and local disorder on specific surface area of layered double hydroxides upon calcination-reconstruction[END_REF] a partial crystal deformation of double-layered hydroxides occurs upon calcination, in which octahedral Al(OH) 6 migrates to tetrahedral AlO 4 resulting in the enhancement of S BET . In our case, the increase in specific surface area can be linked with the creation of narrower pores in Y-modified materials. The total pore volume remained unchanged after modification with yttrium. However, pore diameters decreased as compared to non-modified HTNi catalyst. It shows that Y-promotion positively influences the textural properties of Ni-based catalyst, since a larger number of smaller pores are formed, and higher specific surface area is obtained. Similar effect was observed by Huang et al. [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF], who reported that the modification of NiO-Al 2 O 3 supports with less than 3 wt % of yttrium increased the S BET. This was further observed as beneficial for catalytic performance. to NiO weakly-bonded to the double-layered hydroxides structure and nickel oxides present in the mixed oxides structure of double-layered hydroxides, respectively [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF][START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Broda | Sorbentenhanced methane reforming over a Ni-Ca-based, bifunctional catalyst sorbent[END_REF]. The maximum temperature of the latter peak (T max ) showed a shift to higher temperature after the yttrium addition, indicating stronger interaction of Ni species with the support as compared to HTNi catalyst. For 1.5 wt % of Y, the shift in reducibility was less pronounced, as reflected in the lower temperature of the maximum (858 ºC), as well as the presence of some weakly bonded NiO (minor maximum at 327 ºC). The higher T max , together with the presence of NiO, was observed before by Huang et al. [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF] for NiO-Al 2 O 3 materials modified with Y 2 O 3 . Dependence of T max similar to that observed for our samples, was also reported in the study of Dębek et al. [START_REF] Dębek | Promotion effect of zirconia on Mg(Ni,Al)O mixed oxides derived from hydrotalcites in CO 2 methane reforming[END_REF] for Zr-modified double-layered hydroxides. Lower amount of added zirconia resulted in a decrease of reducibility, in contrast to modification with higher ZrO 2 loading for which a reducibility increase was observed. The authors ascribed it to the location of ZrO 2 in the structure of hydrotalcite or on the outside. Higher loading of zirconia led to its deposition outside the periclase structure, i.e. on the surface, resulting in weaker interactions with Ni, and presence of weakly bonded NiO, as it is observed in our study.

Hydrogen consumption for the reduced samples is presented in Fig. 3. The highest consumption of 0.209 mmol H 2 /g was observed for non-modified HTNi catalyst. After promotion with yttrium, the values of hydrogen consumption decreased to 0.145 mmol H 2 /g cat and 0.165 mmol H 2 /g for HTNi-Y0.6 and HTNi-Y1.5 catalysts, respectively. The somewhat higher H 2 uptake in the HTNi sample, together with lower maximum temperature of reduction as compared to yttrium-containing catalysts, may be due to the weaker interaction between Ni and the support, as well as the presence of NiO loosely attached to the surface for the former sample.

Basicity of the reduced samples is shown in Fig. 4. Three types of basic sites could be distinguished with maxima at (i) 135-142 ºC, (ii) 203-218 ºC and (iii) 316-367 ºC, respectively.

They are ascribed to weak (Brønsted basic sides, such as hydroxyl groups), medium (Lewis acidbase sites), and strong basic sites (Lewis basic sites associated with oxygen anions) [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF]. The data in Table 3 show that total basicity was highest for the non-modified Ni-hydrotalcite-derived catalyst, and similar in value to a comparable HT-25Ni catalyst studied by Dębek et al. [START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF], which was found to have optimal basicity for DRM. The addition of Y modified both the number and the distribution of basic sites. The total basicity decreased after yttrium addition, showing the lowest value of 55.8 µmol/g for HTNi-Y0.6, and slightly higher for HTNi-Y1.5 sample, i.e. 77.1 µmol/g.

The amount of each type of sites (weak, medium and strong) in the HTNi-Y materials studied in this work also decreased compared to HTNi. For the HTNi-Y samples the site distribution shifted towards medium and weak sites, at the expense of strong sites; expressed as a higher sum of weak and medium sites for HTNi-Y0.6 and HTNi-Y1.5 catalysts (79.6 and 61.6%, respectively) compared to the sum calculated for HTNi (56.6%). This may have positively influenced DRM catalytic performance, which will be discussed further.

Fig. 5 shows XRD patterns of the reduced catalysts, where characteristic reflections for metallic nickel (Ni 0 ) (2ϴ ca. 44.5º, 53º, 76.5º) corresponding to crystal planes of (111), ( 200), (220) and periclase mixed oxides (2ϴ ca. 36.7º, 43º, 62.5º) are observed. [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF][START_REF] Shang | Enhanced activity of supported Ni catalysts promoted by Pt for rapid reduction of aromatic nitro compounds[END_REF] Table 4 compares the size of Ni particles, calculated from the 2ϴ diffraction peak at ca. 53º using the Scherrer equation.

It may be seen that the yttrium addition results in a decrease of nickel crystallites from ca. 8 nm for the unpromoted HTNi to ca. 6 nm for HTNi-Y (Table 4). A similar effect was observed for the Ni-Y-Al 2 O 3 samples and better performance in DRM was attributed to the formation of these smaller

Ni particles [START_REF] Li | Alumina supported Ni and Co catalysts modified by Y 2 O 3 via different impregnation strategies: Comparative analysis on structural properties and catalytic performance in methane reforming with CO 2[END_REF][START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF].

Fig. 6 shows TEM micrographs for the reduced samples, where also smaller particle sizes were registered for Y-promoted catalysts. The average particle size varies from 27 nm for the unpromoted HTNi, to 16 nm and 14 nm for HTNi-Y0.6 and HTNi-Y1.5, respectively (Table 4).

The reason for obtaining higher values from TEM analysis, as compared to XRD, may lie in differences in measurement techniques. By TEM, the particle size of all nickel aggregates is measured, irrespective if they are crystallite or amorphous. On the other hand, XRD data gives information only on nickel particles with high crystallinity. The nickel crystallite diameters were also calculated based on H 2 chemisorption data, assuming uniform spherical particles. The values varied from 11, to 5 and 5 nm for HTNi, HTNi-Y0.6 and HTNi-Y1.5, respectively (Table 4), and are in good agreement with values calculated from XRD.

The Ni crystallite size, calculated through these three characterization techniques, clearly show a decrease in Ni particle size with Y addition ( ). The addition of Y significantly improves both conversions, and a correlation between the increase of catalytic activity and the increasing Y content can be observed. Table 5 presents the catalytic activity results for the tested catalysts. The values of CH 4 conversion at 700 ºC obtained for HTNi, HTNi-Y0.6 and HTNi-Y1.5 catalysts were 66%, 74% and 76.2%, respectively. For CO 2 conversion, the following results were obtained: 76%, 78% and 80.8% for respectively HTNi, HTNi-Y0.6 and HTNi-Y1.5 catalysts. The same activity sequence was observed at 750 ºC (Table 5). Within this series of double-layered hydroxide-derived catalysts, the one promoted with 1.5 wt % Y is the most efficient in the whole temperature range during the experiment (Fig. 7A,7B). The similar trend was observed in H 2 /CO molar ratio (Fig. 7C), where higher values were obtained for Y-modified samples than for HTNi. However, for the promoted samples, the differences in the produced H 2 /CO ratios were significant only at the lowest studied temperature of 600 ºC. Most probably, other side reactions started to be more dominant than the dry reforming reaction, resulting in a slowly increasing fraction of CO compared to H 2 . [START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF] Table 5 shows additionally turnover frequencies (TOF) calculated at 700 and 750 ºC. Comparing

HTNi with HTNi-Y1.5 we can observe higher value of TOF for the latter sample, which is in agreement with the better nickel dispersion (19.8% versus 9.8% for Y-free catalyst). However, considering the results of Ni dispersion and the turnover frequencies calculations, the performance differences between HTNi and HTNi-Y0.6 cannot be easily explained. Most probably, the promotion with 0.6 wt % of yttrium is not enough to change the properties of the catalyst.

The stability experiments were carried out at 700 ºC and, as given in Table 6, showed initial CH 4 conversions of 74.0%, 73.9%, and 74.4% respectively for HTNi, HTNi-Y0.6 and HTNi-Y1.5.

Initial CO 2 conversion increased for the higher loading of yttrium in the tested samples: 75.7% for HTNi < 78.1% for HTNi-Y0.6 < 78.6% for HTNi-Y1.5. All samples showed relatively stable conversion values after 5 hours (300 min) (Fig. 8A-C). However, the relative changes in CH 4 and CO 2 conversions were the lowest for HTNi-Y1.5 catalyst, indicating that this was the most stable among the studied catalysts (cp. Table 6). Moreover, after 10 hours (600 min) of DRM test (Fig. 8A-C), the following relative change in CH4 conversion was obtained: 4.8, 4.9 and 1.0 for HTNi, HTNi-Y0.6 and HTNi-Y1.5 catalysts, respectively (Table 6). A similar trend could be observed for relative change in CO2 results, i.e. 5.5, 6.6, 1.5 for HTNi, HTNi-Y0.6 and HTNi-Y1.5 catalysts, respectively. This confirms enhanced stability of HTNi-Y1.5 compared to the other tested catalysts.

Similarly, as observed for activity tests, the highest conversions were observed for HTNi-Y1.5.

H 2 /CO molar ratio was also highest for the 1.5 wt % Y-promoted catalyst (Fig. 8C). Some differences may be observed in the registered H 2 /CO ratio for HTNi and HTNi-Y0.6 catalysts, where the values measured in the activity test and stability tests are compared. These dissimilarities may come from the methodology of the experiments, due to a possible slight initial deactivation of the catalysts, which were examined in the activity tests at the temperature higher than 700 ºC.

The positive effect of the Y promotion was also observed for Ni,Y-Al 2 O 3 [START_REF] Li | Alumina supported Ni and Co catalysts modified by Y 2 O 3 via different impregnation strategies: Comparative analysis on structural properties and catalytic performance in methane reforming with CO 2[END_REF], Ni-Y/SBA-15 [START_REF] Li | Methane reforming with CO 2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol-gel process[END_REF][START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF], Ni-Y/KIT-6 [START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF], NiO-Y 2 O 3 -Al 2 O 3 [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF] and Y-Ce 0.75 Zr 0.25 O 2 [START_REF] Wu | Effect of yttrium and praseodymium on properties of Ce 0.75 Zr 0.25 O 2 solid solution for CH 4 -CO 2 reforming[END_REF]. Also, CO 2 conversion was reported to be slightly higher than that of CH 4 , and the values of H 2 /CO ratio lower than 1, which was ascribed to occurrence of the reverse water gas-shift side reaction (CO 2 +H 2 =CO+H 2 O) [START_REF] Nair | Nanocast LaNiO 3 perovskites as precursors for the preparation of coke-resistant dry reforming catalysts[END_REF][START_REF] Xie | One-pot synthesis of supported, nanocrystalline nickel manganese oxide for dry reforming of methane[END_REF][START_REF] Charisiou | Investigating the correlation between deactivation and the carbon deposited on the surface of Ni/Al 2 O 3 and Ni/La 2 O 3 -Al 2 O 3 catalysts during the biogas reforming reaction[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF][START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF][START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF]. Moreover, other side reactions such as gasification of carbon deposits and the Boudouard reaction could have taken place [START_REF] Charisiou | Investigating the correlation between deactivation and the carbon deposited on the surface of Ni/Al 2 O 3 and Ni/La 2 O 3 -Al 2 O 3 catalysts during the biogas reforming reaction[END_REF][START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF]. This agrees with catalytic results in this study (CO 2 conversion slightly higher than CH 4 conversion, and H 2 /CO molar ratio less than 1).

However, Li et al. [START_REF] Li | Methane reforming with CO 2 using nickel catalysts supported on yttria-doped SBA-15 mesoporous materials via sol-gel process[END_REF] observed in SBA-15 supported catalysts with Y/Si ratio of 0.02, 0.06 and 0.08, CH 4 conversion higher than that of CO 2 , and ascribed it to methane decomposition. No such result was registered in our study, which shows possible enhanced stability of the Y modified double-layered hydroxides. As reported by Dębek et al. [START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF] the methane decomposition (which causes catalyst deactivation) was less pronounced for smaller Ni particles. Thus, the fact that Y promotion results in smaller crystallites may play an important role in suppression of this unwanted reaction.

The yttrium modification resulted also in better dispersion of metallic Ni, enhanced specific surface area, as well as in increased share of weak and medium basic sites compared to strong ones.

The better dispersion of nickel is of importance for catalytic performance, as better distribution of smaller Ni particles makes them more accessible to CH 4 in DRM process. [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF]. The increased specific surface area is another positive effect. As reported by Huang et al. [START_REF] Huang | Highly stable mesoporous NiO-Y 2 O 3 -Al 2 O 3 catalysts for CO 2 reforming of methane: effect of Ni embedding and Y 2 O 3 promotion[END_REF] the textural properties of NiO-Y 2 O 3 -Al 2 O 3 materials were linked with the behavior in CO 2 reforming reaction, with good catalytic results for enhanced S BET . On the other hand, the weak and medium basic sites may be also of advantage for DRM reaction rather than the strong sites, as suggested by Liu et al. [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. Dębek et al. [START_REF] Dębek | Methane dry reforming over hydrotalcite-derived Ni-Mg-Al mixed oxides: the influence of Ni content on catalytic activity, selectivity and stability[END_REF] found that enhanced basicity may hinder carbon formation and/or contribute to oxidation of formed coke. Additionally, the presence of the strongest basic sites may have a negative effect because of too strong CO 2 adsorption, which results in carbon formation [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF].

20

The mechanism of DRM can be shorty described as (i) decomposition of methane on Ni active site and (ii) dissociative adsorption of carbon dioxide on the metal surface and metal-surface interface [START_REF] Aramouni | Catalyst design for dry reforming of methane: Analysis review[END_REF][START_REF] Seo | Recent scientific progress on developing supported Ni catalysts for dry (CO 2 ) reforming of methane[END_REF]. During the first step, a carbon on the Ni surface is formed as a product of CH 4 decomposition (CH 4 = C + 2H 2 ). The carbon atom can be oxidized by atomic oxygen derived from CO 2 . It was reported that doping with Y 3+ improved the rate of carbon removal due to oxygen vacancies, which persuaded oxygen radicals from CO 2 to react with coke. [START_REF] Bellido | Effect of the Y 2 O 3 -ZrO 2 support composition on nickel catalyst evaluated in dry reforming of methane[END_REF][START_REF] Taherian | A comparative study of ZrO 2 , Y 2 O 3 and Sm 2 O 3 promoted Ni/SBA-15 catalysts for evaluation of CO 2 /methane reforming performance[END_REF][START_REF] Li | Y 2 O 3 -promoted NiO/SBA-15 catalysts highly active for CO 2 /CH 4 reforming[END_REF][START_REF] Daneshmand-Jahromi | Hydrogen Production from cyclic chemical looping steam methane reforming over yttrium promoted Ni/SBA-16 oxygen carrier[END_REF][START_REF] Lytkina | The influence of the support composition and structure (M Х Zr 1-X O 2-d ) of bimetallic catalysts on the activity in methanol steam reforming[END_REF] Thus, a possible removal of carbon could have taken place according to the reverse Boudouard reaction

(CO 2 + C = 2CO
). [START_REF] Li | Y 2 O 3 -promoted NiO/SBA-15 catalysts highly active for CO 2 /CH 4 reforming[END_REF] The latter may be also linked with possible re-dispersion of the Ni particles (described further in the text). When the excess amount of oxygen is in close proximity to the Ni 0 particles, it will lead to formation of NiO, further reduced again to Ni 0 due to H 2 presence in the system. [START_REF] Seo | Recent scientific progress on developing supported Ni catalysts for dry (CO 2 ) reforming of methane[END_REF] 4.3. Characterization of spent catalysts Fig. 9 shows TPR-H 2 profiles for catalysts after dry methane reforming tests. In contrast to the curves shown in Fig. 5, broad peak at around 850 ºC disappeared, and only the low temperature peak was registered. This shows that nickel was not kept totally in reduced state throughout the dry reforming test. Some Ni species were oxidized during DRM, since peaks at ca. 300 ºC and 500 ºC arise from bulk NiO [START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF][START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF]. On the other hand, no reflections of NiO were observed in the XRD patterns for the reduced and spent catalysts (Fig. 5 and Fig. 9), indicating that the size of NiO particles was below the XRD detection level due to e.g. their low crystallinity or high dispersion.

H 2 consumption has been calculated for the spent catalysts, shown in Fig. 9. The obtained results are approximately four times lower compared to the values found for calcined samples, i.e. 0.036 mmol H 2 /g cat , 0.047 mmol H 2 /g cat and 0.042 mmol H 2 /g cat for HTNi, HTNi-Y0.6 and HTNi-Y1.5, respectively. The presence of reduction peaks in the spent materials shows that during DRM a redox mechanism may have taken place, in which Ni particles were continuously oxidized and reduced.

TGA/DSC-MS was carried out on the catalysts after the activity test. Fig. 10 presents the curves of mass loss. The registered mass loss was: HTNi (81.9%), HTNi-Y0.6 (80.4%), and HTNi-Y1.5

(75.8%). The Y0.6 catalyst showed behavior different from HTNi and HTNi-Y1.5. For the former, the mass decline starts at around 500 ºC in comparison to ca. 350 ºC observed for other catalysts.

Moreover, a small weight increase may be observed at 300 ºC for all samples, which, as reported by Tsyganok et al. [START_REF] Tsyganok | Dry reforming of methane over catalysts derived from nickel-containing Mg-Al layered double hydroxides[END_REF], arises from Ni 0 oxidation to NiO. The registered MS data revealed that mass losses were connected to water desorption and CO 2 formation [START_REF] Tsyganok | Dry reforming of methane over catalysts derived from nickel-containing Mg-Al layered double hydroxides[END_REF]. According to DSC signals both processes had exothermic character. The carbon dioxide formation occurs due to the oxidation of carbonaceous species, which have been formed during dry reforming of methane [START_REF] Tsyganok | Dry reforming of methane over catalysts derived from nickel-containing Mg-Al layered double hydroxides[END_REF].

Fig. 11 shows XRD patterns for the spent catalysts, with typical reflections for metallic nickel (Ni 0 ) and periclase mixed oxides [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF][START_REF] Dębek | Nicontaining Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Świrk | Syngas production from dry methane reforming over yttrium-promoted nickel-KIT-6 catalysts[END_REF][START_REF] Shang | Enhanced activity of supported Ni catalysts promoted by Pt for rapid reduction of aromatic nitro compounds[END_REF]. These reflections were also observed in the XRD diffractograms for the reduced samples (Fig. 5). However, the XRD patterns for the reduced and spent samples differ in intensities of reflections. Two reflections in the spent catalysts, at ca. 2ϴ= 43-46º overlap, forming one broad diffraction peak. Thus, after reaction, Ni crystallites size was found to be 5 and 3 nm for unpromoted and Y-promoted DLHs, respectively.

TEM micrograph analysis revealed that most of the observed carbon deposits are similar to carbon nanotubes (Fig. 12A-C). The observed type of carbon is in line with the temperature of its removal (Fig. 10), showing a peak centered at around 450 ºC [START_REF] Souza | Dry reforming of methane at moderate temperatures over modified Co-Al Co-precipitated catalysts[END_REF]. On the right side of the micrographs, the distribution of Ni particles is presented. For HTNi spent catalyst nickel particles were detected in the range of 4-22 nm, with the highest frequency for the size of 10-12 nm. After yttrium modification, the range significantly changed to 5-6 nm and 4-5 nm for HTNi-Y0.6 and HTNi-Y1.5, respectively. Thus, re-dispersion observed in our study may be directly related to the mechanism of the dry reforming reaction. Considering CO 2 as a source of oxygen, once it is adsorbed on the catalyst surface, it may be involved in the oxidation of Ni 0 sites to NiO (cp. Figure 3, cp. Figure 9). By The re-dispersion of the metallic nickel during DRM was observed, since CO 2 may be considered as a source of oxygen and be involved in the oxidation of Ni 0 sites to NiO. 2) calculated from XRD patterns of uncalcined catalysts, from the position of the three first reflections c=d (003) + 2d (006) + 3d (009) , as suggested by Liu et al. [START_REF] Liu | Lapromoted Ni-hydrotalcite-derived catalysts for dry reforming of methane at low temperatures[END_REF] 3) specific surface areas calculated from the BET equation 1) based on the Scherrer equation, from the width at half-maximum of the XRD reflections at 2ϴ ca. 53º

4. Results and discussion 4 . 1 .

 41 Characterization of the catalysts before dry methane reforming 4.1.1. Elemental composition, structural parameters, and textural properties

4. 1 . 2 .

 12 Fig. 3 presents TPR-H 2 profiles for Y-modified Ni/Mg/Al double-layered hydroxides. The

Fig. 7A

  Fig. 7A and 7B present the CH 4 and CO 2 conversions for DRM as a function of temperature. The

  XRD and TEM results, indicating the presence of smaller Ni crystallites for the spent materials, suggest re-dispersion of Ni. Such effect was observed before in other studies of double-layered hydroxides. Zr promoted hydrotalcites showed lower nickel particle size after dry reforming of methane in the study performed by Dębek et al.[START_REF] Dębek | Influence of Ce/Zr molar ratio on catalytic performance of hydrotalcite-derived catalysts at low temperature CO 2 methane reforming[END_REF] Also, Ru-modified Ni/Mg/Al hydrotalcite, tested in propane reforming, showed ability of reversible reduction and oxidation between metallic nickel and nickel cations[START_REF] Li | Sustainable Rudoped Ni catalyst derived from hydrotalcite in propane reforming[END_REF]. The authors observed sintering of Ni particles after the treatment with a mixture of H 2 /H 2 O/N 2 , followed by re-dispersion due to a partial oxidation step.Although the decrease in crystallite size is rather unusual, the observation is supported by the research of Nakayama et al.[START_REF] Nakayama | Dispersion of nickel particles supported on alumina and silica in oxygen and hydrogen[END_REF]. The authors studied re-dispersion of Ni particles on supported catalysts (Al 2 O 3 , SiO 2 ) during H 2 reduction and treatment with oxygen. The scanning microscopy analysis showed sintering of the particles during long time-on-stream experiments in the presence of hydrogen. After testing the materials alternately in H 2 reduction and oxidation atmosphere, a redispersion of these particles took place.

adsorption of H 2 ,

 2 CO or perhaps H-species originating from methane molecules activated on the catalyst surface, the Ni-sites are subsequently reduced and thus ready to participate in the DRM reaction. The oxidation of Ni sites may lead to the regeneration of mixed NiMg oxides that, in the presence of the reducing atmosphere generated under reaction conditions, might yield Ni 0 species of smaller crystallite size than those present on the catalyst upon reduction pre-treatment prior to the catalytic tests. The balance between Ni 0 and NiO in DRM remains a possible hypothesis explaining re-dispersion observed in XRD and TEM analysis.5. ConclusionsNi-containing Mg/Al double-layered hydroxides (DLHs) were promoted with Y. The catalysts were tested in dry reforming of methane in the temperature range of 850-600 ºC, and in 10 h stability tests at 700 ºC.The promotion with yttrium did not result in a separate crystalline phase formation but led to a decrease in the size of Ni crystallites as compared to non-modified Ni-DLHs. Also, a decrease of the reducibility of Ni strongly bonded in a periclase structure and stabilization of weakly bonded NiO in the HTNi-Y1.5 were observed. The total basicity of the catalyst also decreased, and simultaneously modified the distribution of basic sites, giving an increase in medium and weak basic sites (as a sum of two values) at the expense of strong basic sites.The best catalytic performance was observed for Ni-based double-layered hydroxides modified with 1.5 wt % of Y. This, as compared to non-modified Ni-double layered hydroxide, can be ascribed to: (i) smaller Ni crystallite size, (ii) higher specific surface area, and (iii) increased fraction of medium basic sites. Carbon formation was minimized through the control of the Ni crystallite size. Still, some carbon fiber structures could be found, most of them resembling carbon nanotubes and not fishbone-like structures. Among all tested materials in 10 h experiments, the catalyst modified with 1.5 wt % yttrium revealed the highest and the most stable values in terms of CH 4 and CO 2 reaction rates, and the molar ratio of H 2 /CO. For the latter, the results were always lower than 1.0, which indicates the occurrence of side reactions. The smaller Ni crystallite sizes for Y-modified catalysts led to suppression of methane decomposition, thus increasing the stability. Moreover, possible substitution of Ni 2+ ions by Y 3+ , observed in the HTNi-Y1.5 catalyst, could have contributed to oxygen vacancies formation, further leading to the gasification of carbon formed upon DRM.

Figure captions:Fig. 1 .

 1 Figure captions:

Fig. 2 .

 2 Fig. 2. Equilibrium conversions of CH 4 , CO 2 , and H 2 /CO molar ratio in the dry reforming of

Fig. 3 .

 3 Fig. 3. Temperature-programmed reduction (TPR-H 2 ) profiles for the reduced samples.

Fig. 4 .

 4 Fig. 4. Temperature-programmed desorption (TPD-CO 2 ) profiles for the reduced samples.

Fig. 5 .Fig. 6 .

 56 Fig. 5. XRD patterns for reduced samples HTNi, HTNi-Y0.6 and HTNi-Y1.5.

Fig. 7 .Fig. 8 .

 78 Fig. 7. Activity results of HTNi-Y catalysts in the temperature range of 850 to 600 ºC: CH 4

Fig. 9 .

 9 Fig. 9. Temperature-programmed reduction (TPR-H 2 ) profiles for the spent catalysts.

Fig. 10 .

 10 Fig. 10. TGA/DSC-MS curves of spent HTNi-Y catalysts. Decomposition of the samples in air

Fig. 11 .

 11 Fig. 11. XRD patterns for spent samples HTNi, HTNi-Y0.6 and HTNi-Y1.5.

Fig. 1 .

 1 Fig. 1. Equilibrium amounts for gaseous components and coke in the dry reforming of methane

Fig. 2 .

 2 Fig. 2. Equilibrium conversions of CH 4 , CO 2 , and H 2 /CO molar ratio in the dry reforming of

Fig. 3 . 40 Fig. 4 .

 3404 Fig. 3. Temperature-programmed reduction (TPR-H 2 ) profiles for the reduced samples.

Fig. 5 .Fig. 6 .

 56 Fig. 5. XRD patterns for reduced samples HTNi, HTNi-Y0.6 and HTNi-Y1.5.

Fig. 7 .Fig. 8 .Fig. 9 .

 789 Fig. 7. Activity results of HTNi-Y catalysts in the temperature range of 850 to 600 ºC: CH 4

Fig. 10 .

 10 Fig. 10. TGA/DSC-MS curves of spent HTNi-Y catalysts. Decomposition of the samples in air

Fig. 11 .

 11 Fig. 11. XRD patterns for spent samples HTNi, HTNi-Y0.6 and HTNi-Y1.5.

Fig. 12 .

 12 Fig. 12. TEM images of HTNi spent (A), HTNi-Y0.6 spent (B), HTNi-1.5 spent (C), and

2 )

 2 average diameter of Ni particles obtained from TEM image 3) calculated from d = 971/(%D) based on the assumption of spherical crystallites of uniform size Catalysts d Ni from XRD 1)

  2, CH 4 , CO, H 2 , H 2 O, C, and equilibrium conversions of CO 2 , CH 4 , and H 2 /CO molar ratio.

Table 4

 4

	The dispersion of metallic nickel calculated from H 2 chemisorption is shown in Table 5. A
	positive influence of yttrium promotion resulting in higher Ni dispersion may be clearly seen, as
	the dispersion values are 8.9%, 17.7% and 19.8% for HTNi, HTNi-Y0.6 and HTNi-Y1.5,
	respectively. As reported by Huang et al. [41], better dispersion of nickel benefits catalytic

). The accuracy of these techniques was examined by Mustard et al.

[START_REF] Mustard | Determination of metal crystallite supported size and morphology supported nickel catalysts[END_REF]

. The authors described the application of TEM, H 2 adsorption and XRD for the determination of Ni crystallite size in supported catalysts, such as Ni/SiO 2 , Ni/Al 2 O 3 and Ni/TiO 2 . As stated by the authors, the precision of each technique depends on Ni loading and its dispersion, as well as the textural properties of used support.

Table 1 .

 1 The list of reactions of dry methane reforming process[START_REF] Pavel | The activity of yttrium-modified Mg,Al hydrotalcites in the epoxidation of styrene with hydrogen peroxide[END_REF][START_REF] Cao | Tuning dry reforming of methane for F-T syntheses: A thermodynamic approach[END_REF][START_REF] Chen | Thermodynamic analysis of the partial oxidation of coke oven gas for indirect reduction of iron oxides in a blast furnace[END_REF].

	Number Name	Reaction	ΔH θ [kJ/mol]
	1	Dry reforming of CH 4	CH 4 + CO 2 = 2CO + 2H 2	247
	2	Reverse water gas shift	CO2 + H2 = H2O + CO	41
	3	Methanation of CO2	CO2 + 4H2 = CH4 + 2H2O	-165
	4	Methanation of CO	CO + 3H2 = CH4 + H2O	-206
	5	Hydrogenation of CO 2	CO2 + H2 = HCOOH	15
	6	Hydrogenation of CO2	CO2 + 3H2 = CH3OH + H2O	-49
	7	Hydrogenation of CO	CO + 2H2 = CH3OH	-91
	8	Dehydration of CH3OH to	2CH3OH = CH3OCH3 + H2O	-37
		CH3OCH3
	9	Reforming of CH3OCH3	CH3OCH3 + H2O = 2CO + 4H2	205
	10	Reforming of CH3OCH3	CH3OCH3 + CO2 = 3CO + 3H2	258
	11	Oxidative coupling of CH4 2CH4 + CO2 = C2H6 + CO + H2O	106
	12	Reforming of C2H6	C2H6 + 2CO2 = 4CO + 3H2	430
	13	Oxidative coupling of CH4 2CH4 + 2CO2 = C2H4 + 2CO + 2H2O 284
	14	Reforming of C2H4	C2H4 + 2CO2 = 4CO + 2H2	290
	Reactions of coke
	15	Methane decomposition	CH 4 = C (s) + 2H 2	75
	16	Bouduard reaction	2CO = C (s) + CO 2	-172
	17	Hydrogenation of CO2	CO 2 + 2H 2 = C (s) + 2H 2 O	-90
	18	Hydrogenation of CO	CO + H 2 = C + H 2 O	-131
	19	Steam on C	C(s) + H2O = CO + H2	131

Table 2 .

 2 Elemental composition (XRF), structural parameters (XRD), and textural properties (BET analysis) of double-layered hydroxides modified with nickel and yttrium. Nominal values are presented in brackets. ) calculated from the (110) spacing a= 2d (110) as suggested by Cavani et al.[START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation, properties and applications[END_REF]

								Structural
								parameters	Textural properties
		Elemental composition (calcined samples)
								(uncalcined	(calcined samples)
	Catalyst							samples)
		Mg 2+ [wt %]	Al 3+ [wt %]	Ni 2+ [wt %]	Y 3+ [wt %]	Ni 2+ /Mg 2+ atomic ratio	M 2+ /M 3+ atomic ratio	a [Å] 1) c [Å] 2)	S BET [m 2 /g] 3)	V p [cm 3 /g] 4)	d p [nm] 5)
						0.29	3.6
	HTNi	30	12	20	-			3.06	23.45	120	0.6	19
						(0.33)	(3.0)
					0.6	0.28	3.5
	HTNi-Y0.6 29	12	19				3.06	23.37	162	0.5	11
					(0.6)	(0.33)	(3.0)
					1.5	0.18	3.4
	HTNi-Y1.5 32	12	14				3.06	23.42	192	0.6	14
					(2.0)	(0.33)	(3.0)

1

Fig. 12. TEM images of HTNi spent (A), HTNi-Y0.6 spent (B), HTNi-1.5 spent (C), and histograms of Ni particle size distribution.

process (CH 4 /CO 2 /Ar=1/1/8, GHSV= 20,000 h -1 ). The solid line represents the thermodynamic equilibrium limit.

[START_REF] Olajire | Valorization of greenhouse carbon dioxide emissions into value-added products by catalytic processes[END_REF] Based on equation[START_REF] Dębek | A short review on the catalytic activity of hydrotalcite-derived materials for dry reforming of methane[END_REF]

Acknowledgments K. Świrk acknowledges the French Embassy in Poland and InnoEnergy PhD School for the financial support during her co-tutelle doctoral studies at Sorbonne Université. K. Świrk would also like to express her gratitude to the KinCat Catalysis Group at NTNU and the Research Council of Norway for the possibility to conduct some of her research during an Erasmus+ traineeship. The authors would like to acknowledge Sandra Casale (Sorbonne Université) for TEM expertise. Part of the work was supported by AGH grant 11.11.210.373.

1)

Calculated basing on activity test at appropriate temperature 2) Based on the equation ( 4)