Alexandre Carvalho

Vitaly V Ordomsky

Nilson R Marcilio

Andrei Y Khodakov
email: andrei.khodakov@univ-lille.fr

Number and intrinsic activity of cobalt surface sites in platinum promoted zeolite catalysts for carbon monoxide hydrogenation

Supported cobalt catalysts are the catalysts of choice for synthesis of paraffinic hydrocarbons from carbon monoxide and hydrogen. In this paper, the number and intrinsic activity of cobalt nanoparticles promoted with Pt localized in zeolites with different structure and pore diameter (ZSM-5, MOR and BEA) have been investigated by steady state isotopic kinetic analysis (SSITKA) under the conditions favouring higher methane selectivity. Promotion with Pt resulted in higher number and higher reactivity of surface intermediates leading to methane. The cobalt catalyst supported by MOR zeolite displayed the highest SSITKA reaction rate in carbon monoxide hydrogenation, followed by Co supported on BEA and in the last for Co supported on the zeolite ZSM-5. Good dispersion of cobalt nanoparticles inside and outside of the surface of MOR and BEA zeolites led to higher number of active sites. On the other hand, poor dispersion of cobalt nanoparticles on the external surface of ZSM-5 resulted in lowest intrinsic catalytic activity and lowest number of active sites. The intrinsic active of cobalt sites was however, almost unaffected by the inclusion of cobalt nanoparticles in the zeolite pores.

Introduction

Catalysts with supported cobalt nanoparticles are the catalysts of choice for low temperature Fischer-Tropsch (FT) synthesis, which produces middle distillates and waxes from syngas. The reaction occurs over supported metal nanoparticles, which provide active sites for carbon monoxide hydrogenation. The catalytic performance of cobalt catalysts in FT synthesis can be affected by several parameters such as promotion, cobalt nanoparticle size and cobalt localisation within the porous support.

It is known that the use of noble metals as promoters leads to a significant increase in FT catalytic activity 1 in both slurry and fixed bed reactors [2][3][4] . Pt as a promoter for cobalt FT catalysts supported on alumina and silica has been demonstrated a strong effect on the FT reaction rate 5,6 . Pt can produce several effects on the catalyst structure and performance 1 . Among them is better cobalt reducibility 7 , which is due to lower activation energy to form the cobalt metallic phase from cobalt oxide in the presence of Pt 5 .

The influence of the support on FT catalyst can be also important, once the support may significantly modify cobalt particle size, metal-support interactions, support acidity, porosity and mass transfer limitations 5,8 . Silica, alumina, titania and zeolites are common commercial supports for cobalt FT catalysts [9][10][11][12] . Syngas conversion over metal-zeolite catalysts involves bifunctional catalysis, where CO hydrogenation occurs on metal sites, while the zeolite could perform hydrocarbon cracking and isomerization under the conditions of FT synthesis 13,14 .

Transient kinetic methods including the steady state isotopic transient kinetic analysis (SSITKA) are powerful techniques for the kinetic investigation of heterogeneous catalytic reactions at molecular level.

SSITKA addresses measuring the transient response of isotopic labels in the reactor following an abrupt change (switch) in the isotopic composition of one of the reactants.

Only isotopic composition of the feed is changed, while chemical composition remains the same 15 . SSITKA can be used to determine the number and intrinsic activity of active sites, site heterogeneity Please do not adjust margins Please do not adjust margins and concentration of different types of adsorbed reaction intermediates, surface residence time, activity distributions and site coverage 12,16,17 . In previous reports [18][19][20][21][22][23][24][START_REF] Efstathiou | Characterization of Solid Materials and Heterogeneous Catalysts: From Structure to Surface Reactivity[END_REF] , SSITKA has been used to evaluate the effect of different promoters, support modification and cobalt particle size on the type, concentration and reactivity of the surface intermediates. For example, Rane [START_REF] Rane | [END_REF] The aim of this work is to evaluate the number of active sites and their intrinsic activity for CO hydrogenation over cobalt FT catalysts promoted with Pt and supported on MOR, BEA and ZSM-5 zeolites using SSITKA. 3D). A more significant decrease in the BET surface area and micropore volume of BEA compared to that of ZSM-5 and MOR could be assigned, therefore, to a much easier diffusion of Co species into the BEA zeolite pores during the impregnation 13 . This would result in a higher Co fraction located in the BEA micropores.

Results and discussion

The number of sites of CO adsorption in the cobalt zeolite catalysts was measured in this work by SSITKA. The SSITKA switch from 12 CO/He/Ne to 13 CO/He was realized (Figure S1, Electronic Supplementary Information-ESI) in order to determine the total number of surface sites in the catalysts containing 20% of cobalt and 0.1% of Pt supported on ZSM-5, MOR and BEA zeolites. Table 1 shows that the CO surface residence time (τ CO ) for the cobalt catalyst supported on silica was 5.2 s contrasting with an average residence time of 3.5 s for the cobalt counterparts supported on all zeolites. This proves stronger chemical interaction of CO with cobalt nanoparticles on silica support compared to the zeolite catalysts

The concentration of reversibly chemisorbed carbon monoxide (N CO ) on cobalt silica supported catalysts is also higher than that for Please do not adjust margins Please do not adjust margins the cobalt zeolite samples. The switch during the CO adsorption in the presence of hydrogen (from 12 CO/H 2 /He/Ne to 13 CO/H 2 /He) but in the absence of FT reaction was performed for the silica and BEA supported cobalt catalysts (Table 1). The concentrations of reversibly adsorbed carbon monoxide (N CO ) on both samples in helium and in the presence of hydrogen are almost the same. This suggests that at T = 100 °C there is non-competition between H 2 and CO for active sites. Hence, CO is a dominating specie on the surface of the amorphous silica and on the BEA zeolite. This observation is in agreement with previous report by Frøseth et al. 29 . However, Enger et al. 31 noticed a different trend for cobalt supported on different modified aluminas with and without promoters such as Zn, Re, Ni and Mg.

The authors have found that the co-adsorption of CO with H 2 prior to the reaction showed an apparent competitive H 2 -CO adsorption which resulted in a lower CO saturation coverage in hydrogen than with only CO feed.

The amount of reversibly adsorbed carbon monoxide (N CO ) measured by SSITKA is a function of cobalt dispersion. Among the cobalt zeolite catalysts, the highest number of CO adsorption sites was detected over the mordenite-based sample. Indeed, according to our previous results 13 , the Co/MOR and Co/BEA catalysts showed a significant dispersion in the pore volume after cobalt impregnation, which is related to cobalt localisation inside the zeolite micropores. The pore volume of Co/ZSM-5 is only slightly affected by impregnation. This suggests preferential location of cobalt particles on the zeolite outer surface. The nitrogen adsorption data are consistent with TEM images of the relevant catalysts 13 , which also suggest preferential localisation of cobalt on the outer surface of narrow pore ZSM-5 zeolite. • b Amount of CO adsorbed at 100 ͦ C, evaluated from SSITKA. c Amount of CO adsorbed at 100 ͦ C in presence of hydrogen, evaluated from SSITKA. Please do not adjust margins

Pt-promoter effects on the kinetic parameters of cobalt catalysts during FT synthesis

Please do not adjust margins

The Pt-promoted catalyst exhibits higher CO conversions with both H 2 /CO ratios in syngas (Figure 1a). Similar behaviour was noticed by Jacobs et al. 3 using the Pt and Ru promoters on the alumina, titania and silica supported cobalt catalysts. For the non-promoted catalyst at H 2 /CO ratio feed of 2, the CO conversion slightly decreases at the first hours of reaction. Considering the whole time on stream (around 13 h), the non-promoted catalyst lost 50% of the CO conversion, contrasting with the 30% decrease for the Pt-promoted cobalt catalyst. Increasing hydrogen partial pressure from 0.29 bar to 0.58 bar improved catalyst stability for both samples. In fact, the promoted Pt catalyst becomes completely stable with H2/CO=5 in syngas during 14 hours of reaction.

The non-promoted catalyst exhibits a slight decrease in CO conversion from 15% to 11%. The literature points out that the presence of noble metals has the ability to restrict carbon deposition and formation of cobalt support mixed oxides during FT synthesis 1,32 . The promoter effect over catalyst stability can be also associated to the type of support applied. As showed in Table 2, methane is the major product of the reaction under these conditions for both catalysts, with methane selectivity higher than 80%. and high chain growth probability, which causes an increase in the selectivity toward the C 5+ hydrocarbons and a decrease in the selectivity to methane 33,34 . Note that the lifetime of adsorbed CH x species slightly decreases after the promotion with Pt. The higher number of CH 4 intermediates (N CHx ) combined with their slightly shorter lifetime ( CHx ) in the promoted cobalt catalysts are responsible for 1.3 and 1.6 times higher TOF SSITKA for the Ptpromoted catalyst compared to the unpromoted counterpart in the ratio of H 2 /CO equal to 2 and 5, respectively. In addition, the both reaction rates also increased over the Pt-promoted catalyst. Hence, it is possible to conclude that promotion with Pt results in a significant increase in the activity. This effect is due to both the increase in the number of active sites and their intrinsic activity (TOF SSITKA ).

θ CO and θ CHx over the CoPt promoted catalysts were altered as function of the H 2 to CO ratio feed. θ CO is slightly higher than θ CHx at H 2 /CO ratio of 2 and at ratio feed of H 2 /CO equal to 5, θ CHx became higher than θ CO because at higher hydrogen partial pressure, more hydrogen molecules at the catalyst surface favoured the CO hydrogenation. Vasiliades et al. 27 noticed a significantly higher θ CO In addition, the authors observed lower total surface coverage than in the present work, which can be attributed to the longer time on stream applied in their study (32h against 3h of the present work).

At longer time on stream, the presence of inactive carbonaceous species and other deactivation phenomena can modify the surface kinetic parameters and increase the lifetime of intermediates ( CHx ) on the catalyst surface causing the decrease in the turnover frequency 12 Table 2 also demonstrates the effect of the H 2 /CO feed ratio. The increase in the H 2 /CO feed ratio influenced the catalyst activity for both cobalt catalysts. This may be correlated to the reaction condition applied, due to the influence of Pt over FT reaction rate be more noticeable at atmospheric pressure 6,34 . The lifetime of CH x intermediates leading to methane ( CHx ) is shorter at higher H 2 /CO ratio due to higher number of available hydrogen species and faster hydrogenation. Note that the rate constant determined by GC analysis at conventional steady state conditions presents similar values with the rate constant in the SSITKA experiments (Table 1).

Influence of the zeolite support on the intrinsic kinetic parameters of cobalt catalysts in CO hydrogenation

All cobalt based catalysts were reduced in hydrogen flow of 8 mL/min at 400 °C before the CO hydrogenation. Afterwards, the samples were cooled down to the reaction temperature of 250 °C.

The SSITKA measurements were done on all cobalt based catalysts at ambient pressure and after 3 h of reaction under different feed H 2 /CO ratios (2, 5 and 10) corresponding to hydrogen partial pressure of 0.29 bar, 0.58 bar and 0.83 bar, respectively. Carbon monoxide conversion and methane selectivity measured at quasisteady state reaction conditions as a function of the H 2 /CO feed ratio for each catalyst are displayed in Table 3. Table 3 also shows the SSITKA data for each catalyst as functions of the H 2 /CO feed ratio.

Under steady state conditions and in the range of hydrogen partial pressures, the CO conversion was higher over the cobalt catalysts supported on MOR, followed by BEA and ZSM-5. The ZSM-5 supported cobalt catalyst presented the smallest FT reaction rate.

Increasing the hydrogen partial pressure in the feed affects the CO conversion over all supported catalysts. However, while the same trend is observed among the zeolites, CoPt/MOR is the most active catalyst at all H 2 /CO ratio feed applied. Methane and water were the major products for all samples under applied conditions. The CO residence time (τ CO ) slightly decreases or even remains constant, while the CH x residence time significantly decreases as a function of H 2 partial pressure (Table 3). Please do not adjust margins Please do not adjust margins higher partial pressures of hydrogen. Table 3 also shows that higher The authors also concluded that the intrinsic site activity and concentration of surface intermediates for CO hydrogenation were strongly dependent on the hydrogen partial pressure.

The influence of the H 2 /CO feed ratio for each sample on the SSITKA reaction rate (R SSITKA ), number of adsorbed carbon monoxide (N CO )

and concentration of CH 4 intermediates (N CHx ) is displayed in Figure 3(a,b).

Note that the total number of active sites number of molecularly adsorbed carbon monoxide molecules (N CO ) decreases with increasing the H 2 /CO ratio for CoPt/MOR and CoPt/BEA, while the number of CH x intermediates increases. This occurred due to higher CO conversion (see Table 3) and site competition between CO and Please do not adjust margins Please do not adjust margins giving methane on their hydrogenation was observed on CoPt/MOR.

Note that the MOR zeolite showed the highest N CHx among all zeolites. Interestingly, for cobalt catalyst supported on ZSM-5, N CO and N CH4 are smallest among all samples. This is also consistent with low cobalt dispersion detected by characterization techniques in CoPt/ZSM-5 13 .Our results clearly suggest that higher cobalt dispersion obtained by insertion and confinement of cobalt nanoparticles into the micropores of the MOR and BEA zeolites leads to higher concentration of the CH x hydrogenation sites and higher FT reaction rate. This can possibly explain highest FT reaction rates observed over the catalyst supported by MOR (Figure 4).

Previous DFT modelling 37 suggests that carbon monoxide molecular adsorption occurs on cobalt nanoparticle terraces, while carbon monoxide dissociation and hydrogenation to methane may involve edges of cobalt nanocrystallites. Our results did not show any noticeable increase in the intrinsic reactivity of CH x intermediates over cobalt nanoparticles inside the zeolite micropores. It seems that the intrinsic activity of cobalt sites is not modified by their inclusion into the zeolite micropores.

The variations of N CO , N CHx and R SSITKA as functions of the zeolite supports are consistent with cobalt localization within porous structure. According to our earlier work 38 , an increase in the size of the zeolite micropore results in an increase in the amount of cobalt located inside of the pores. Since the channel diameters of BEA and MOR are larger than for ZSM-5, this leads to the fact that the micropores of ZSM-5 contain a smaller amount of cobalt available for the reaction. Most of cobalt is located on the zeolite outer surface of ZSM-5. Consequently, cobalt dispersion and amount of cobalt sites are smaller. This affects negatively the CO conversion and SSITKA kinetic parameters such as N CHx and N CO . The SSITKA experiments demonstrated larger number of CH x intermediates in the CoPt/MOR and CoPt/BEA catalysts with medium micropore size relative to the CoPt/ZSM-5 catalyst with smaller pore size.

Our previous report 13 showed rather different distribution of cobalt nanoparticles between outer surface and micropores of the zeolites. For example, in cobalt catalyst supported on ZSM-5, TEM

showed 13 A more uniform distribution of cobalt between outer surface and micropores of the CoPt/MOR and CoPt/BEA catalysts results in higher FT reaction rate. Importantly, a higher FT reaction rate measured at the quasi steady state conditions coincides with a higher fraction of CH x intermediates (Figures 3 and4) and not to the higher concentration of molecularly adsorbed carbon monoxide molecules (N CO ). This is consistent with previous reports 19 and indicates that CH x hydrogenation can be a kinetically relevant step of CO hydrogenation under these conditions. N CHx , that is a measure of the concentration of intermediates able to form hydrocarbons, seems to correlate to the overall activity of the cobalt zeolite catalysts at high pressure FT synthesis. Subramanian et al. 13 , using the same samples for FT synthesis under high pressure test (P=20 bar) also observed that catalytic performance was strongly affected by the type of zeolite. CoPt/MOR showed the highest CO conversion at low and high pressures among all catalysts studied.

Catalysis Science & Technology Accepted Manuscript

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Conclusion

This work elucidated the roles of promotion with Pt and localization of cobalt nanoparticles within BEA, MOR and ZSM-5 zeolites on the number and intrinsic activity of active sites for CO hydrogenation using SSITKA. Platinum promotion results in the increase in both the number of active sites for CH x hydrogenation and in their higher intrinsic activity (TOF SSITKA ).

In the cobalt zeolite catalysts promoted with platinum, the number of active sites is a function of cobalt distribution between the zeolite outer surface and zeolite micropores. Larger amounts of active sites and higher FT reaction rates are observed over the cobalt-mordenite catalyst with more uniform distribution of cobalt species within zeolite crystallites. The intrinsic activity of cobalt surface sites is almost unaffected by their localization in either on the zeolite outer surface or in the micropores.

The FT reaction rate at steady state conditions both under atmospheric and high pressures correlates with the amount of CH x surface intermediates. Furthermore, the rise of hydrogen partial pressure resulted in a decline of surface residence time and an increase in the concentration of surface CH x intermediates producing methane on hydrogenation.

Experimental Catalyst preparation and characterization

Cobalt catalysts supported on silica with and without platinum

SSITKA and steady state catalytic tests

In all the experiments, 40 mg of catalyst was mixed with 80 mg of SiC. The samples were loaded into a millimetric fixed bed reactor (d in = 2 mm) and reduced in pure H 2 flow of 8 cm 3 /min for 2 h at 400 °C with a 5 °C/min heating rate. In the CO adsorption study, after reduction, the sample were cooled down in the same rate ramp to 100 °C and switches from 12 CO/He/Ne to 13 CO/He were applied to determine the total number of active sites. To evaluate both the effect of support structure and hydrogen partial pressure, the samples were submitted to the reaction temperature of 250 °C.

The catalytic experiments were conducted at different ratios of H 2 /CO (2, 5 and 10) in syngas with the gas hourly space velocities (GHSV) varying from 6750 h -1 to 15 191 h -1 . Then, after 3 h of reaction with the catalysts at steady-state conditions, the switches from 12 CO/H 2 /He/Ne to 13 CO/H 2 /He were applied.

Carbon monoxide conversion and reaction selectivities were determined by analysing the reaction effluents with a Shimadzu 2014 gas-chromatograph equipped with a CP-PoraPLOT and a CTR-1 column, as well as FID and TCD detectors. Nitrogen was used as an internal standard for calculating carbon monoxide conversion. The SSITKA apparatus has been described in detail in our previous reports 12,39 . It is composed of two independent gas feed lines (Figure S2, ESI). The first line is dedicated to unlabelled compounds and a tracer (CO, H 2 , He, and Ne), and the second one to the isotope-labelled compounds ( 13 CO in this case). The experiment consists of a sudden replacement of one reactant by its labelled counterpart without any modification of the reaction total pressure, flow or reactant chemical composition, while an inert tracer is also

ARTICLE Catalysis Science & Technology 2 |

 2 Catalysis Science & Technology., 2019, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

  et al. found different concentrations of CO and CH x intermediates on the cobalt catalysts supported by ɣ, Φ-, δ-and α-Al 2 O 3 . Vasiliades 27 et al. uncovered that the coverage, turnover frequency (TOF) and CH x residence time slightly increased with increasing Co particle size (in the 7-10 nm range). The effect of the support on the SSITKA kinetic parameters was also observed by Kim Phan 28 et al. for cobalt catalysts supported on conventional and macroporous (MPS) alumina. The number of intermediates leading to methane (N CH4 ) on cobalt supported on MPS-Al 2 O 3 was higher (7.0 μmol/g) than that for Co/α-Al 2 O 3 (4.1 μmol/g), while the Re promotion led to the increase in the number of CH x intermediates to 11.3 μmol/g. This trend correlated with the reaction rate, which was higher for cobalt Repromoted supported on MPS-A l2 O 3 . The conclusions are in agreement with earlier results of Frøseth 29 et al. and Hanssen 30 et al. who observed an increase in the overall activity and in the concentration of active surface intermediates on the promotion with Pt and Re. Catalytic performance of cobalt zeolite catalysts often depends on the type of zeolite. This effect can be attributed either to the variation of the number of active sites or to the modification of intrinsic site activity. Measuring the size of metal nanoparticles, number of metal active sites in zeolites and their intrinsic activity is usually a difficult task; the metal particles in zeolite micropores are often very small (<1 nm) and hardly visible by most of characterization techniques such as conventional TEM. Conventional chemisorption experiments are habitually conducted with freshly activated catalysts and under the conditions very different from those in the catalytic tests. Most of conventional steady state kinetic techniques cannot separate these two rather different phenomena responsible for modification of catalytic performance. SSITKA has a number of advantages compared to conventional characterization and kinetic methods. It can provide independent information on both the number of active species and their intrinsic reactivity.

Figure 1 (

 1 Figure 1(a,b) shows CO conversion as a function of time on stream over non-promoted and Pt-promoted silica supported cobalt catalysts with syngas with the H 2 /CO ratio of 2 and 5.

Figure 1 . 4 |

 14 Figure 1. CO conversion as a function of time on stream for non-promoted and promoted cobalt catalysts at (a) Ratio H 2 /CO=2 and (b) Ratio H 2 /CO = 5. Reaction conditions: 250 °C, GHSV = 6750 h -1 and 9000 h -1 .

  than θ CHx over two commercial Co-based catalysts supported on carbon-cobalt and uncoated γ-Al 2 O 3 catalysts for the FT synthesis.

Figure 2

 2 Figure 2 displays the methane and inert gas (neon) transient response curves obtained during the switch of 12 CO/H 2 /He/Ne to 13 CO/H 2 /He at different H 2 /CO ratios. The transient curves help to understand the influence of H 2 to CO feed ratio on the catalytic activity, once faster methane transient responses were observed at

Figure 2 . 6 |

 26 Figure 2. Transient curves of inert (Ne) and intermediates leading to CH 4 from the switch of 12 CO/H 2 /He/Ne to 13 CO/H 2 /He in function of ratio feed of H 2 /CO (2, 5 and 10). Reaction condition: 250 °C, 1 atm, GHSV = 8 599 h -1 , 11 465 h -1 and 15 191 h -1 .

H 2 /

 2 CO ratio in syngas results in the decrease in the residence time of CH x intermediates, while the residence time of reversibly adsorbed CO is only slightly affected by syngas chemical composition. This observation is consistent with earlier results of Panpranot et al.35 who observed for amorphous and mesoporous silica (MCM-41), a consistent drop of τ CH4 with increasing hydrogen partial pressure. On the other hand, Frøseth et al.36 found that increasing H 2 /CO ratio slightly affects the surface residence time of CH x intermediates (τ CHx ).

  CH x chemisorbed intermediates. With the increase in the hydrogen partial pressure, the catalyst surface got enriched with hydrogen available to bond with the carbon atoms formed on the CO dissociation. For that reason, faster formation of CH x intermediates resulted in lower surface lifetime of the CH x intermediates (τCH x ) and in their faster hydrogenation to CH 4 . For the cobalt catalyst supported on ZSM-5 (microporous structure), N CO value remained constant (33 μmol/g for ratio 2; 32.5 μmol/g for ratio 5 and 33.6 μmol/g for ratio 10). Higher amount of CH x intermediates

Figure 3 .

 3 Figure 3. Number of CO sites (N CO ), number of CH x sites (N CHx ) and SSITKA reaction rate in ratio feed of H 2 /CO equal to 2 (a) and 5 (b). Reaction condition: 1 atm, 250 °C, GHSV = 8 599 h -1 and 11 465 h -1 .

  polycrystalline zeolite particles covered by large agglomerates of cobalt nanoparticles with sizes between 100-200 nm. Therefore, there is preferential cobalt localization on the external surface of ZSM-5. The TEM images of cobalt catalyst supported on MOR demonstrated a significantly higher incorporation of cobalt nanoparticles in the zeolite crystallites. Even though, some fraction of cobalt nanoparticles was also observed on the surface of the zeolite crystals. TEM images of cobalt catalyst on BEA displayed cobalt located inside of zeolite crystals. The low activity of cobalt catalyst supported on ZSM-5 was attributed to the preferential localization of cobalt on the outer surface of the zeolite and low concentration of active sites because of poor cobalt dispersion. The long lifetime of the CH x intermediates (τ CHx ) also contributed for the lower activity of CoPt/ZSM-5 zeolite.

Figure 4 .

 4 Figure 4. Correlation between FT reaction rates measured from steady state experiments and concentration of CH 4 intermediates evaluated from SSITKA. Reaction conditions: 1 atm, 250 °C, GHSV = 8 599 h -1 and ratio feed of H 2 /CO equal to 2.

(

  20 wt. % Co and 0.1 wt. %% Pt) have been synthesized by incipient wetness impregnation of commercial amorphous silica (CARIACT Q-10, Fuji Silysia) with aqueous solutions of cobalt nitrate (Co(NO 3 ) 2 •6H 2 O) and tetramine platinum nitrate (Pt(NH 3 ) 4 (NO 3 ) 2 ). The cobalt catalysts supported on zeolites (ZSM-5, BEA, MOR) have been synthesized by incipient wetness impregnation using the same precursors and weight contents of cobalt and platinum used for cobalt supported on silica. The ZSM-5, MOR and BEA zeolites with the Si/Al ratio of 13, 8 and 9 have been provided by Zeolyst. The catalysts were dried for 1 h at 100 °C and calcined at 450 °C in air flow of 10 mL/min for 7 h.

Measuring the number of cobalt surface sites by SSITKA

	All catalysts were activated in hydrogen flow of 8 mL/min at 400 °C
	with the temperature ramp of 5 °C/min. The Pt promoted cobalt
	catalysts supported on zeolites did not shown any visible
	deactivation during 12-14 h of the reaction. The reduction
	temperature of the cobalt supported on the zeolites was previously
	determined by Subramanian et al. 13 by temperature-programmed
	reduction with hydrogen (H 2 -TPR). The TPR profiles exhibited a
	single group of peaks around 350-400 °C assigned to the reduction
	of cobalt oxide species to metallic phases. Subramanian et al. 13 also
	observed a decrease in the pore volume for zeolite BEA of around
	50%, for MOR (around 20%) and for ZSM-5 (10%) after the
	introduction of about 20 wt. % of Co via impregnation. The surface
	area and pore volume of CoPt/ZSM-5 only slightly decreases after
	impregnation compared to other zeolites.
	The 3D pore structure of BEA (0.76×0.64 nm) is more open than
	that of MOR (0.65×0.7 nm; 1 D) and ZSM-5 (approximately 0.55 nm;

Table 1

 1

	Catalyst	Co size (nm) a	τ CO (s)	N CO b (µmol/g)	N CO c in H 2 presence (µmol/g)
	CoPt/SiO 2	17.3	5.2	90	90
	CoPt/ZSM-5	26.4	3.0	49.6	-
	CoPt/BEA	16.2	2.9	51.1	53
	CoPt/MOR	27.5	3.5	60.2	-

. Catalyst characterization and SSITKA CO adsorption data (switches between

12

CO/He/Ne and 13 CO/He).

a Co = 0.75 dCobalt oxide (nm). Determined by XRD analysis.

Table 2

 2

	also shows the number of active sites measured by
	SSITKA (N CHx ) and the SSITKA reaction rate (RSSITKA). The RSSITKA is
	obtained by the division of the amount of active intermediates by

its surface residence time. Note that the RSSITKA and the reaction rate that considers the amount of CO converted per gram of catalyst (Table

2

) display similar values. While the number of adsorbed carbon monoxide (N CO ) surface species is similar for both catalysts, cobalt Pt-promoted catalysts present larger amount of intermediates leading to methane (N CHx ) at both H 2 /CO ratios in comparison with the non-promoted samples. This result is interesting, since SSITKA studies have demonstrated, that higher coverage of CH x intermediates could lead to higher reaction rate

Table 2 .

 2 Catalytic performance of promoted Pt and unpromoted cobalt catalysts. Reaction conditions: 250 °C, 1 atm, GHSV = 6750 h -1 and 9000 h -1 . is the catalyst loaded. (Experimental error in the number of active sites equal to 6%).𝑁 𝑖 = Surface coverage. θ = N i /N total. , (i = CO, CHx). c TOF FSSITKA = surface coverage/surface residence time of CHx.

	Catalyst	Ratio feed H 2 /CO	X CO (%)	S CH4 (%)	 CO (s)	 CHx (s)	N CO a (µmol/g)	N CHx a (µmol/g)	θ CO b	θ CHx b	TOF SSITKA c 10 -2 (s -1 )	Rate d (µmol/g s)	R SSITKA e (µmol/g s)
		2	6	86		2.3	19.0	38	17	nd	nd	1.8	1.1	0.9
	Co/SiO 2
		5	11	92		2.0	13.0	31	24	nd	nd	3.6	1.9	1.8
		2	15	85		2.8	17.0	39	35	0.43	0.39	2.9	2.5	2.1
	CoPt/SiO 2
		5	21	93		1.8	11.5	23	38	0.26	0.42	4.7	3.6	3.3
	𝐹𝑖 𝑜𝑢𝑡 𝑊 𝑖
	d Rate determined by GC analysis (	𝑅 =	). 𝐶𝑂 𝑐𝑜𝑛𝑣𝑒𝑟𝑡𝑒𝑑 𝑊
	e Rate determined by SSITKA analysis (	𝑅 𝑆𝑆𝐼𝑇𝐾𝐴 =	). 𝑁 𝑖 𝜏 𝑖

a

, where W b

Table 3 .

 3 Catalytic performance and SSITKA results (switches between12 CO/H 2 /He/Ne and13 CO/H 2 /He) under different feed ratio of H 2 /CO. Reaction condition: 250 °C, 1 atm, GHSV = 8 599 h -1 , 11 465 h -1 and 15 191 h -1 .

	Catalyst	H 2 /CO	X CO (%)	S CH4 (%)	TOF SSITKA 10 -2 (s -1 )	τ CO (s)	τ CHx (s)
		2	11.0	78	2.5	3.1	16.9
	CoPt/MOR	5	17.0	82	4.1	2.5	12.0
		10	26.0	78	5.2	2.9	12.1
		2	4.6	71	1.3	2.0	19.0
	CoPt/ZSM-5	5	6.7	87	2.0	2.0	16.6
		10	10.0	90	2.7	2.4	12.4
		2	6.4	82	1.8	2.7	17.3
	CoPt/BEA	5	8.9	87	2.6	2.5	13.9
		10	16.0	75	3.7	2.3	12.1

  Cobalt catalysts supported on silica and on zeolites have shown different selectivities for the C 12+ hydrocarbons in comparison with the short-chain C 5 -C 12 paraffins at realistic FT reaction conditions13 .

	ARTICLE	Catalysis Science & Technology
		View Article Online
		DOI: 10.1039/C9CY02421B
	The CoPt/MOR catalyst demonstrated a somewhat lower activity in
	the isomerization of short-chain hydrocarbons probably due to the
	partial neutralization of zeolite Brönsted acid sites in the 1D
	channels of the MOR pores by cobalt cations. In ZSM-5, cobalt is
	mostly located on the external surface of the zeolite crystals.
	Therefore, FT synthesis occurs over cobalt sites on the zeolite
	external surface and that only short-chain hydrocarbons can diffuse
	and isomerize effectively inside the pores of ZSM-5 zeolite. Because
	of stronger acidity compared to other zeolites, CoPt/ZSM-5
	exhibited a higher activity towards the isomerization of C 5 -C 12
	hydrocarbons. The trend was different for larger molecules, where
	isomerization activity was hindered by hydrocarbon diffusion inside
	the narrow ZSM-5 zeolite pores.

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Published on 02 March 2020. Downloaded by Karolinska Institutet University Library on 3/3/2020 9:31:21 AM. View Article Online DOI: 10.1039/C9CY02421B

Acknowledgements

A. Carvalho is thankful to International Cooperation Program CAPES/COFECUB Foundation funded by CAPES -Brazilian Federal Agency for Support and Evaluation of Graduate Education within the Ministry of Education of Brazil for providing him a PhD stipend. The authors acknowledge financial support of the French National Research Agency (DirectSynBioFuel, Ref. ANR-15-CE06-0004 and NANO4-FUT, Ref. ANR-16-CE06-0013).

Catalysis Science & Technology Accepted Manuscript

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Conflicts of interest

There are no conflicts to declare.

Catalysis Science & Technology Accepted Manuscript

Published on 02 March 2020. Downloaded by Karolinska Institutet University Library on 3/3/2020 9:31:21 AM.

View Article Online DOI: 10.1039/C9CY02421B


Dr Xiang Yu

Dr Vitaly V Ordomsky
email: vitaly.ordomsky@univ-lille.fr

Dr Andrei Y Khodakov
email: andrei.khodakov@univ-lille.fr

oxides on BiVO4 facets for enhancement of CO2 photocatalytic reduction to hydrocarbons

The nanostructure of a semiconductor is a crucial parameter for efficient photocatalytic performance. Hereby, we have synthesized monoclinic bismuth vanadate (BiVO4) crystals with controlled ratio of {010} and {110} facets. The selective deposition of CuOx and CoOx over {010} and {110} facets has been performed using photo-reduction and photo-oxidation methods, respectively. It resulted in a highly efficient charge separation of photogenerated electrons and holes and enhancement of the photocatalytic reduction of CO2 by H2O into hydrocarbons, including CH4, C2H6 and C3H8. The controlled co-catalyst deposition over different semiconductor facets provides a strategy for the synthesis of hydrocarbons from CO2 and H2O by efficient charge separation.

Introduction

Carbon dioxide is widely recognized as a major greenhouse gas that causes global 10.1002/cctc.201901115 environmental problems [1] . Worldwide consumption of limited fossil fuels not only accelerates their depletion but also results in production of excessive atmospheric CO2. In this regard, transformation of CO2 to the fuels is a promising method for simultaneous solution of energy and environmental problems [2] . In 1979, Japanese researcher Fujishima and Honda reported [3] for the first time the photocatalytic reduction of CO2 into organic compounds using semiconductor catalysts such as SiC, GaP, and TiO2. Since then, numerous works have focused on the development of new semiconductor materials for reduction of CO2 [4][5][6][7] . Various semiconductors were proven to be effective in photocatalytic reduction of CO2, for example: TiO2 [8,9] , Ga2O3 [10,11] , Zn2GeO4 [12,13] , WO3 [14] , Bi2WO6 [15,16] and so on. The main products of photocatalytic reduction of CO2 by water are usually methane, higher hydrocarbons, methanol, ethanol, formaldehyde, formic acid or CO [7] .

Although a lot of progress has been made in this field, the activity of photocatalytic reduction of CO2 is still insufficient due to poor charge separation and low quantum efficiency.

In addition, photocatalytic transformation of CO2 is limited due to low adsorption over the catalyst and difficulties in the activation of the stable CO2 molecule.

In the photocatalytic reaction, the difference in the morphology of the catalyst often has a significant impact on the photocatalytic performance [17] . The difference in catalyst morphology is mainly reflected by the exposure of different crystal facets, or the proportion of exposed crystal facets. In recent years, more and more reports have addressed the effects of exposed crystals on photocatalytic properties. It is generally believed that photocatalysts exhibit better catalytic properties when more active crystal facets are exposed to reacting molecules [18,19] . In many classical photocatalytic semiconductors, it has been shown that controlling the exposure of a suitable crystal facets is an effective strategy to improve photocatalytic performance, for example: TiO2 [20,21] , BiVO4 [22][23][24] , Cu2O [25,26] and CeO2 [27] .

In photocatalytic reactions, the high recombination rate of photogenerated electrons and holes is a major factor limiting its photocatalytic performance. The recombination rate of photogenerated electrons and holes can be greatly reduced with increase of activity by deposition of co-catalysts over semiconductors. In general, the role of co-catalyst can be relevant to the following processes: (1) better charge separation, (2) higher activity and selectivity of CO2 reduction by suppression of side reactions and (3) enhanced stability of photocatalyst [28] .

Bismuth vanadate (BiVO4) is a promising visible light-driven semiconductor photocatalyst with low production cost, low toxicity, high stability and narrow band gap (2.4 eV) with good response to visible light excitation. BiVO4 has demonstrated as efficient catalyst for the reduction of CO2 into methanol and ethanol [29,30] . However, the photocatalytic efficiency of BiVO4 is low because of relatively low visible-light utilization and inefficient charge separation. The formation of Z-scheme photocatalytic systems that mimic natural photosynthesis with photocatalytic systems and electron mediator is a promising strategy to improve charge separation [31] . According to a few reports, the photocatalytic efficiency can be improved by Z-scheme on the BiVO4 catalyst [32][33][34] .

Herein, the separation of electrons and holes in bismuth vanadate (BiVO4) is improved by control of the semiconductor morphology. The photocatalytic reduction of CO2 under irradiation is significantly enhanced by selective photo-deposition of Cu and Co co-catalysts over different facets providing Z-scheme charge flow.

Experimental

Synthesis of monoclinic BiVO4 crystal with controllable exposed facets

Monoclinic BiVO4 crystal was synthesized by a hydrothermal method. In brief, the precursors NH4VO3 (0.015 mol) and Bi(NO3)3•5H2O (0.015 mol) were dissolved in an aqueous nitric acid solutions (2 M), and the pH value of the solution was adjusted to 2.0 with ammonia solution (14.84 M) under continuous stirring. An orange precipitate appears over time. After that, NaCl was added into the suspension and stirring was continued for 30 minutes.

After about 5 h aging, the suspension was transferred to a Teflon-lined stainless steel autoclave with a capacity of 100 mL and treated under hydrothermal conditions at 473 K for 24 h. After the autoclave was cooled down to room temperature, the yellow powdery sample was separated by filtration, washed with deionized water for several times, and then dried at 353 K in air for overnight. Finally, the sample was calcined at 773 K in air for 2 h. The samples were denoted as BiVO4-x M, and x represents the concentration of NaCl in the suspension.

Synthesis of monoclinic BiVO4 crystal with predominantly exposed {010} and {110} facets

Synthesis of BiVO4-010

The precursor Bi(NO3)3•5H2O (0.015 mol) was dissolved into an aqueous nitric acid solutions (4 M). And the precursors NH4VO3 (0.015 mol) was dissolved into another aqueous sodium hydroxide solution (2 M). Afterwards, sodium dodecyl benzene sulfonate (0.002 mol) was added to the above solutions separately. After stirring for 30 minutes, the above two solutions were mixed under continuous stirring. The pH value of mixed solutions was adjusted to 7.0 with sodium hydroxide solution (2 M). After about 1 h stirring, the mixed solution was transferred into Teflon-lined stainless steel autoclave and hydrothermally treated at 473 K for 1.5 h. The yellow powdery was recovered by filtration, washed with deionized water for several times, and then dried at 353 K in air for overnight. This sample was denoted as BiVO4-010.

Synthesis of BiVO4-110

The BiVO4-110 sample was synthesized using solid-liquid state reaction by mixing equal amount of Bi2O3 (0.005 mol) and V2O5 (0.005 mol) in an aqueous nitric acid solution (1 M).

The suspension was stirred for four days at room temperature. The yellow powdery was separated by filtration, washed with deionized water for several times and dried at 353 K in air for overnight. This sample was denoted as BiVO4-110.

Selective photo-deposition of metal oxides in monoclinic BiVO4 crystal

Photo-reduction and photo-oxidation procedures have been used for deposition of cocatalysts. 1.0 g BiVO4 powder and a calculated amount of metal precursors were added in 40 mL deionized water containing a sacrificial agent. After about 20 minutes ultrasonic dispersion, the aqueous suspension under irradiation with a 300 W Xe lamp for 5 h under continuous stirring with the protection of nitrogen. Then the suspension was filtered, washed with deionized water for several times and dried at 353 K in air for overnight. 5 wt% of metal in the form of metal oxides (WOx, FeOx, ZnOx, CuOx and CoOx) were deposited over BiVO4-0.20M by photo-reduction procedure using 20 wt% of methanol as sacrificial agent and (NH4)10(H2W12O42)•4H2O, Fe(NO3)3•9H2O, Zn(NO3)2•6H2O and Cu(NO3)2•3H2O as precursors, respectively. The obtained samples were denoted as WOx/BiVO4, FeOx/BiVO4, ZnOx/BiVO4 and CuOx/BiVO4.

Characterization.

The X-ray powder diffraction (XRD) experiments were conducted using a Bruker AXS D8 diffractometer with Cu Kα radiation (λ = 0.1538 nm). The XRD patterns were collected in the 5-80° (2θ) range. The diffuse reflectance UV-visible spectra of the catalysts were recorded on a Perkin-Elmer Lambda 650 S UV/VIS spectrometer equipped with an integrating sphere covered with BaSO4 as a reference.

The textural properties of the samples were studied by N2 physisorption on a Micromeritics Tristar 3020 apparatus.

The photoluminescence spectroscopy measurements were performed on a LabRam HR (Horiba scientific). For excitation, 532 nm radiation from a diode-pumped solid-state 300 µW laser was used. The spectrophotometer has an entrance slit of 100 µm, and is equipped with a 300 lines/mm -1 grating that permits to achieve a spectral resolution of 3.8 cm -1 per pixel. The luminescence light was detected with a CCD camera operating at 138 K.

The SEM measurements were carried out using JEOL scanning electron microscope with 30 Kv accelerating voltage.

The XPS spectra were registered using a Kratos Axis spectrometer, equipped with an aluminum monochromater for a 1486.6 eV source working at 120 W. All spectra were recorded under a vacuum of 10 -8 Torr and recalibrated afterwards with the binding energy of the Al 2p at 74.6 eV.

Photocatalytic tests

The photocatalytic reduction of CO2 in water was carried out in a homemade stainlesssteel batch reactor (~250 mL) with a quartz window on the top of the reactor. The light source was 400 W Xe lamp (Newport). The catalyst was placed on a quartz glass holder in the middle of reactor (Figure S1, SI). 15 ml of liquid water was added at the bottom of reactor. The reaction has been conducted at 323 K and 0.2 MPa of CO2. Before start of the photocatalytic reaction, the reactor was kept in the dark for 1 h to ensure an adsorption-desorption equilibrium between the photocatalyst and the reactants. Subsequently, the reactor was irradiated by a 400 W Xe lamp. The spectral range of the irradiation was selected using Hamamatsu optical filter. The photocatalytic reaction time was typically 7 h.

The gas phase reaction products (CO, CH4, C2H6 and C3H8) were analyzed by on-line gas chromatography (GC, PerkinElmer Clarus ® 580). Liquid products such as HCHO and CH3OH were analyzed by off-line GC analysis. The GC was equipped with a PoraBOND Q and ShinCarbon ST 100/120 columns with detection by flame ionization detector (FID) and a thermal conductivity detector (TCD).

Results and discussion

Monoclinic BiVO4 crystal with designable facets

Uniform truncated tetragonal bipyramidal morphology BiVO4 with designable exposed facets has been prepared using hydrothermal method with variation of NaCl content [24] . XRD measurements indicate that these samples exhibit a monoclinic structure [23,35] (Figure 1). The intensity of the (040) diffraction peak at 30.5 2θ depends on the concentration of NaCl.

Increase of NaCl concentration from 0.04 to 0.5 M leads to increase of intensity with subsequent rapid decrease when the concentration reaches 2 M, indicating that NaCl affects the morphology of BiVO4.

The SEM images show that the BiVO4 crystals were composed of uniformed single crystal with truncated tetragonal bipyramidal shape (Figure 2b-e) when the concentration of NaCl is between 0.04 -0.50 M. However, the BiVO4 crystals has an irregular morphology (Figure 2a,f) when NaCl was not included or if the concentration is too high. Interestingly, the morphology of monoclinic BiVO4 crystals changed gradually with the concentration of NaCl. Crystal model of monoclinic BiVO4 crystals with truncated tetragonal bipyramidal shape contains two {010} facets on the top and bottom and eight {110} facets on the edges (Figure 3). The fraction of {010} facets has been calculated by the crystal model equation.

The percentages of {010} facets can be calculated as follows [21,24] . Here θ is the theoretical value (66.07 o ) for the angle between the {010} and {110} facets of monoclinic BiVO4 crystal. Note that the fraction of {010} facets increases significantly with the increase of NaCl concentration from 26 % to 75 % (Table 1). BiVO4-010 and BiVO4-110 samples with predominantly {010} and {110} facets (Figure 1).

The UV-Vis diffuse reflectance spectra of the BiVO4 crystals with different concentration of NaCl is shown in Figure 4. The samples exhibit intense absorption in the ultraviolet region (< 400 nm) as well as visible light region (< 500 nm). The band gap energy for different morphology samples estimated using Tauc's plots of UV-Vis spectra is the same, being 2.3 eV (Table 1). Photoluminescence spectroscopy has been used to evaluate the recombination of holes and electrons of different BiVO4 catalysts. A luminescence band centered at 620 nm was observed for monoclinic BiVO4 crystals (Figure 5). It is generally accepted that photoluminescence is produced by the recombination of photogenerated carriers on a semiconductor, and therefore the higher intensity of the luminescent band means that the probability of photogenerated carrier recombination is higher [36] . For the series of BiVO4 samples, the intensity of the luminescence band decreases in the order of BiVO4-0.04M > BiVO4-0M > BiVO4-2.0M > BiVO4-0.50M > BiVO4-0.10M > BiVO4-0.20M, indicating that BiVO4-0.20M with 64 % of {010} facet has the lowest ability of electron-hole recombination.

Note that the irregular BiVO4 crystals show a higher ability of electron-hole recombination.

In other words, truncated tetragonal bipyramidal shape is good for electron-hole separation. 7). SEM images show that CuOx nanoparticles were selectively deposited onto the {010} facets of BiVO4 crystal (Figure 8b). Several oxide cocatalysts MnOx, PbO2 and CoOx have been deposited by photo-oxidation using NaIO3 as sacrificial agent on the surface of CuOx/BiVO4 catalyst. SEM analysis demonstrates that these oxides are located on the {110} facets of BiVO4 crystal (Figure 7,8c). To provide further insights into the copper and cobalt state in CoOx/CuOx/BiVO4 catalyst was characterized by XPS (Figure S2, SI). The binding energies of Cu 2p3/2 for the catalyst was about 932.7 eV, which could be attributed to either Cu 0 or Cu I [37][38][39] . The positions of the Cu L3VV Auger lines for these samples were centered at 916.7 eV, corresponding to that of Cu I in Cu2O [40]. [41][42][43] . activity (6.8 μmolg -1 h -1 ) has been observed over CuOx/BiVO4-0.20M with formation of broad range of hydrocarbons (methane, ethane and propane) and oxygenates (formaldehyde and methanol).

Catalytic performance

)

CO CH 4 C 2 H 6 C 3 H 8 HCHO CH 3 OH H 2 C u O x / B i V O 4 Z n O x / B i V O 4 F e O x / B i V O 4 W O x / B i V O 4 B i V O 4 -0 . 2 M Figure 9
. Effect of metal oxides on the performance of BiVO4-0.20M for photocatalytic reduction CO2 with H2O.

Further we have studied effect of CuOx deposition over BiVO4 catalyst with systematically changed fraction of {010} facets (Figure 10). As compared to BiVO4 with an irregular morphology, which was synthesized without addition of NaCl, CuOx/BiVO4 catalysts with a uniform truncated tetragonal bipyramidal morphology exhibited higher formation rate of hydrocarbon fuels. Interestingly, the photocatalytic performance depends on the fraction of {010} facets. The formation rate of hydrocarbons has bell shaped curve with maximum at the fraction of exposed {010} facets 64 % with formation rate 6.8 μmolg -1 h -1 .

CuOx/BiVO4-010 and CuOx/BiVO4-110 with predominantly {010} and {110} facets demonstrate lower formation rate of hydrocarbons in comparison with catalysts containing both facets.

Our characterizations clarified that the specific surface areas change slightly with an increase in the fraction of {010} facets. These cannot explain the change in the catalytic behaviours with an increase in the fraction of {010} facets. Furthermore, the bandgap energy values calculated from the UV-vis spectroscopy for this series of catalysts were the same, being 2.3 eV (Table 1). Thus, the observation that the BiVO4 single crystal with an 64% fraction of {010} and {110} facets shows the highest catalytic performance (Table 1) suggests that both facets play crucial roles in the photocatalytic reduction CO2.

) Recent studies have demonstrated that photogenerated electrons and holes in monoclinic BiVO4 crystals will migrate to the {010} and {110} facets respectively [23]. Li and coworker [44] evaluated the energy levels of {010} and {110} facets of BiVO4 crystals by DFT calculation method. They observed a slight difference in energy of the valence and conductive band between the {010} and {110} facets which suggested that the electron transfer from {110} to {010} facets is feasible thermodynamically. Thus, it can be expected that the coexistence of the {010} and {110} facets with the appropriate ratio will favor the separation of photogenerated electrons and holes. This order fits well with obtained photoluminescence measurement, confirming that the {010} and {110} facet coexistence and appropriate ratio facilitates the separation of electron-hole pairs. Furthermore, the order of the efficiency of electron-hole pairs separation is the same as that of the formation rate of hydrocarbon fuels for the photocatalytic reduction of CO2 with H2O (Figure 10). Therefore, we believe that the ability of electron-hole pairs separation is a key factor in determining the catalytic behavior of CuOx/BiVO4 with different fraction of {010} facets. 11, Figure S3, SI). This result indicates that the photocatalytic performance can be significantly enhanced when the reduction/oxidation co-catalysts were selectively deposited on the corresponding reduction/oxidation reaction facets, which can minimize electron-hole pairs recombination.

CO CH 4 C 2 H 6 C 3 H 8 HCHO CH 3 OH H 2 B i V O 4 a C u O x C u O x C u O x C u O x / B i V O 4 -1 1 0 / B i V O 4 -0 1 0 / B i V O 4 -0 . 0 4 M / B i V O
This assumption has been confirmed by the fact that intensity of photoluminescence band decreased significantly by loading CuOx and CoOx onto the BiVO4-0.20M crystal (Figure 5). CB of Cu2O are used for the reduction of CO2 produce hydrocarbon fuels, while the photogenerated holes in the VB of BiVO4 transfer to Co3O4 of oxidation co-catalyst, which used for the oxidation reaction of water (Figure 13).

Accepted Manuscript

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5

  wt% of metal in the form of metal oxides MnOx, PbO2 and CoOx have been deposited over CuOx/BiVO4 by photo-oxidation technique using NaIO3 (0.1 M) as the electron acceptor and Mn(NO3)2•xH2O, Pd(NO3)2•xH2O and Co(NO3)3•6H2O as precursor, respectively. The obtained samples were denoted as MnOx/CuOx/BiVO4, PbO2/CuOx/BiVO4 and CoOx/CuOx/BiVO4. For comparison, the reference sample CoOx/CuOx/BiVO4(imp) has been synthesized by impregnation of Cu(NO3)2•3H2O and Co(NO3)3•6H2O over BiVO4-0.20M with subsequent calcination at 673 K.

S010 = 2a 2 ,

 2 S110 = 8 * 0.5 * (a + b) * LDE, LDE = LCE -LCD, LCE = 0.5b / cosθ, LCD = 0.5a / cosθ, S110 = 2 * (b 2a 2 ) / cosθ, F010% = S110 / (S110 + S010) = 2a 2 / 2 * (b 2a 2 ) / cosθ = cosθ / (cosθ + b 2 / a 2 -1) 10.1002/cctc.201901115 Accepted Manuscript ChemCatChem This article is protected by copyright. All rights reserved.

Figure 1 .

 1 Figure 1. XRD patterns of the monoclinic BiVO4 samples with different concentration of NaCl

Figure 2 .ChemCatChemFigure 3 .

 23 Figure 2. SEM images of BiVO4 samples synthesized with different concentration of NaCl.

  protected by copyright. All rights reserved.

Figure 4 . 4 Figure 5 .

 445 Figure 4. UV-Vis spectra of BiVO4 samples synthesized with different concentration of NaCl.

Figure 7 .Figure 8 .

 78 Figure 7. Schematic of the synthesis of CuOx/CoOx/BiVO4 catalyst.

Figure S2 ,

 S2 SI represents the Co 2p XPS spectra of the catalyst. Co 2p3/2 binding energies at 781.5 eV, absence of the satellite structure and 2p3/2-2p1/2 spin-orbit splitting (ΔE) of 15.1 eV indicates on the presence of Co3O4 phase

Figure 9

 9 Figure 9 demonstrates the effect of photo-reductive deposition of different metal oxides

Figure 10 .

 10 Figure 10. Effect of the fraction of {010} facets on the photocatalytic performance of

Figure 11 .

 11 Figure 11. Effect of cocatalyst on the performance of CuOx/BiVO4-0.20M for

Figure 12 .

 12 Figure 12. Repeated uses of the 5 wt% CuOx/5 wt% CoOx /BiVO4-0.20M catalyst for

  The effect of close position of CB of BiVO4 and VB of Cu2O has been verified by deposition of other metal oxides (WOx, FeOx and ZnO) by photo-reduction technique with different band positions instead of CuOx (Figure9). The activity correlates well with band position on the Figure13. Thus, FeOx and WOx almost did not affect catalytic activity of pure BiVO4 due to low CB potential in comparison with the CO2/CH4 reduction potential. However, ZnO with higher reduction potential results in significant increase of the catalytic activity.

Figure 13 .

 13 Figure 13. (a) Comparison of the band structures of various semiconductor with respect to

hv (eV) BiVO 4 CuO x /BiVO 4 CuO x /CoO x /BiVO 4

		10.1002/cctc.201901115
	more electron-hole pairs under the same visible light illumination and then result in higher
	photocatalytic activity.
	(a)	BiVO 4
		CuO x /BiVO 4
		CuO x /CoO x /BiVO 4
	300 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 400 500 600  (nm) 0 Absorbance (a. u.) 4 (b) 2 (F(R)*hv) 1/2	Accepted Manuscript
	The direct band gaps of BiVO4, CuOx/ BiVO4 and CuOx/CoOx /BiVO4
	Figure 6 a. UV-Vis spectra of CuOx/CoOx /BiVO4-0.20M sample b. [F(R∞)hν] 1/2 versus hν
	samples are estimated to be 2.34, 2.30 and 2.16 eV. They are slightly smaller than the reported
		for CuOx/CoOx /BiVO4-0.20M sample

values of pure BiVO4. The decrease in band gap energy indicates that the nanocomposite has a much greater optical absorption region than pure BiVO4, which can be excited to produce ChemCatChem This article is protected by copyright. All rights reserved.

Table 1 .

 1 Fraction of {010} facets, specific surface areas, bandgap energies of BiVO4samples with different NaCl concentration.

		Fraction of	Surface area	Bandgap energy
	Sample	{010} facets (%)	(m 2 g -1 )	(eV)
	BiVO4-0M	/	1.3	2.3
	BiVO4-0.04M	26	2.2	2.3
	BiVO4-0.10M	44	2.8	2.3
	BiVO4-0.20M	64	2.9	2.3
	BiVO4-0.50M	75	2.6	2.3
	BiVO4-2.0M	/	1.4	2.3

ZnOx, WOx, FeOx and CuOx were loaded onto series of BiVO4 samples through photoreduction deposition technique (Figure

Table 2

 2 displays the photocatalytic performance data of CuOx/BiVO4 with different

	CuOx contents over BiVO4-0.20M. Note that the increase in Cu content from 1 to 5 wt%
	enhances the formation rate of hydrocarbons with further stabilization at 8 wt%. The surface
	area of BiVO4 is quite low and saturation of {010} facets takes place already at relatively low
	Cu content. Hydrogen production follows the same trend as hydrocarbon production and
	indicates on not full utilization of hydrogen for CO2 reduction during photocatalysis. For other
	catalysts hydrogen production was also related to production of hydrocarbons.

Table 2 .

 2 Effect of Cu content on the performance of CuOx/BiVO4-0.20M for

	ChemCatChem	10.1002/cctc.201901115
	photocatalytic reduction CO2 with H2O
	Product formation rate	Hydrocarbon
	Catalyst 1% CuOx/BiVO4 0.51 0.73 CO CH4 3% CuOx/BiVO4 0.82 2.5 5% CuOx/BiVO4 1.4 3.5 8% CuOx/BiVO4 1.3 3.3 The catalytic properties after deposition of co-catalysts by photo-oxidation over (μmol g -1 h -1 ) formation rate (μmol g -1 h -1 ) C2H6 C3H8 HCHO CH3OH H2 0 0 0 0 0 0.73 0.61 0.42 0.35 0.55 0.42 4.4 1.1 0.80 0.60 0.82 0.91 6.8 1.0 0.63 0.51 0.60 0.96 6.0 CuOx/BiVO4 have been studied by catalytic tests at the same conditions (Figure 11). The catalytic activity was gradually increasing in the row: CuOx/BiVO4<MnOx/CuOx/BiVO4 <PbO2/CuOx/BiVO4 <CoOx/CuOx/BiVO4. It is interesting to note that deposition of co-catalysts does not lead to significant change in the selectivity to hydrocarbons (Figure S3, SI). Methane was still responsible for about 40 % of carbon utilization with selectivity about 20 % to ethane and propane and rest to heavier hydrocarbons. Afterwards the catalytic performance	Accepted Manuscript
	of CoOx/CuOx/BiVO4 has been optimized by variation of CoOx content. The catalytic behavior
	is similar to those of Cu with increase of the catalytic activity till 5 wt% of CoOx and stable

performance at higher loading (Table

S1

, SI). Thus, the highest catalytic activity for CO2 reduction by water is achieved for the CuOx/CoOx/BiVO4 catalyst with selective photo-

Table 3

 3 shows that the formation rate of the CO2 reduction over CoOx/CuOx/BiVO4 catalyst under visible light range (382-600 nm) is only slightly lower compared to the results with light range before filtration (240-600 nm). The main advantage of BiVO4 as semiconductor in comparison with TiO2 is its ability to work under visible light irradiation

Table 3 .

 3 Performances of the 5 wt% CuOx/5 wt%CoOx /BiVO4-0.20 M catalyst for 10.1002/cctc.201901115

	Accepted Manuscript

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This article is protected by copyright. All rights reserved.

The valence band (VB) of Cu2O is very close to the conduction band (CB) of BiVO4.

This fact suggests a possibility of construction of the Z-scheme photocatalysis system for reduction of CO2 similar to the previous reports [34,[45][46][47] . The significant enhancement of photocatalytic CO2 reduction can be explained by Z-scheme charge-transfer mechanism (Figure 13).

In summary, when both Cu2O and BiVO4 were excited by the irradiation, the photogenerated electrons in the CB of BiVO4 recombine with the photogenerated holes in the This catalyst demonstrated the highest stable catalytic activity toward production of hydrocarbons even under visible light irradiation. The construction of Z-scheme for CO2 reduction by water using oxidation and reduction co-catalysts played a key role in enhancing of the efficiency of charge separation.

Keywords: photocatalysis; structural sensitivity; reduction of CO2; bismuth vanadate; Zscheme