Dynamic Formation of Pdδ+-Fe2+Interface Promoting Reverse Water Gas Shift Reaction over Pd/FeOxCatalyst

ZHANG Dian-YuLIU FangDU Peng-FeiLI Meng-WeiWU Zhao-XuanFENG Yi-Bing ZHAO YangXU Xiao-YanZHANG Xin-XingLU Jun-LingYANG Bing＊,

(1Dalian National Laboratory for Clean Energy(DNL),Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian,Liaoning 116023,China)

(2University of Chinese Academy of Sciences,Beijing 100049,China)

(3Department of Chemical Physics,University of Science and Technology of China,Hefei 230026,China)

(4Shanghai Advanced Research Institute,Chinese Academy of Sciences,Shanghai 201210,China)

(5Department of Chemistry,the University of Chicago,Chicago,IL 60637,United States)

Abstract:We systematically investigated Pd/FeOxfor the reverse water gas shift(RWGS)reaction using a combination of ex situ and in situ characterizations,including transmission electron microscopy(TEM),Fourier-transform infrared(FTIR)spectroscopy,temperature-programmed desorption/reduction/oxidation(TPD/TPR/TPO),and X-ray photoelectron spectroscopy(XPS).A highly dispersed Pd/FeOxcatalyst was synthesized using Pd(acac)2as the precursor.The catalyst exhibited high activity,with CO2conversion of～29% and CO selectivity ogreater than 98% at 400℃,which are among the highest values in the literature.Moreover,Pd/SiO2and Pd-Fe/SiO2were further studied to determine the significant role of the Pd-FeOxinterface in promoting the RWGS reaction.Semi-in situ XPS revealed the dynamic formation of Pdδ+-Fe2+species at the Pd-FeOxinterface;the species acted as highly active sites for CO2dissociation.Our results also showed the formation of the Pdδ+-Fe2+interface during the RWGS reaction remarkably enhanced the activity and selectivity of the Pd-FeOxcatalyst for the reaction,benefiting CO2adsorption,C＝O dissociation,and CO desorption.

Keywords:palladium;carbon dioxide;interfaces;reaction mechanisms;heterogeneous catalysis

0 Introduction

Carbon dioxide reduction has received tremendous attention for its potential in solving the energy and environmental crises[1-3].The reverse water gas shift(RWGS)reaction is an effective route for syngas production via CO2hydrogenation[4-6].Numerous experimental studies have investigated the RWGS reaction,using Au/MoOx[7],Ru/Al2O3[8],Cu/FeOx[9],and other monometallic[10-12],bimetallic[13],and metal oxides[14]as catalysts.The reaction pathway can be summarized into two theories：the direct path and the indirect path.The direct path involves the direct CO2dissociation into COadand Oad,and then the COadis desorbed as CO[15].The indirect path involves the hydrogenation of CO2via formate or carbonate species as intermediates,and the CO2further decomposes into CO as the main product[3-4,16-17].The metal support interface also plays a vital role in the RWGS reaction[6-7,18]and affects the selectivity[9].To date,the reaction mechanism,including the active interface structure,is still under debate.

Iron oxide-supported palladium catalysts have been extensively studied for the RWGS reaction[19],water gas shift(WGS)reaction[20],and CO oxidation[21-23].The Pd-FeOxinterface has been found to be crucial in a variety of reactions,as it promotes oxygen vacancies and oxygen transportation[24-26].The higher reducibility of the FeOx-supported Pd species,which feature active interface structures,has also been extensively reported[20,26-28].However,these works focus solely on the metal-support interaction from a static point of view,determined viaexsitucharacterization[5].The dynamic evolution of the Pd-FeOxinterface during the reaction has not yet been clarified owing to the lack ofinsitutechniques.

In this study,Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts were systematically investigated forthe RWGS reaction.The catalytic performance for the RWGS reaction was evaluated,and a promoting role of the Pd-FeOxinterface was observed.Using a combination ofexsituandinsitucharacterizations,we identified a dynamic evolution of the Pd-FeOxinterface during reaction,in which interfacial Pdδ+-Fe2+species were formed as the active sites.The underlying reaction mechanism and the role of the Pd-FeOxinterface were further investigated using temperature-programmed desorption/reduction/oxidation (TPD/TPR/TPO) and Fourier-transform infrared(FTIR)spectroscopy.

1 Experimental

1.1 Catalysts preparation

The FeOxsample was prepared by the precipitation of Fe(NO3)3·9H2O in a NaOH solution.The Pd/FeOxcatalyst was prepared by the impregnation method,with 28.8 mg Pd(acac)2as the precursor and 200 mg FeOxand 60 mL toluene as the solvent in a flask[29].Moreover,Pd/SiO2was prepared by a wet impregnation method,using 148.5 mg Pd(NH3)4Cl2·H2O and 1 g SiO2.A GEMSTAR-6TM Benchtop atomic layer deposition(ALD)system was used to synthesize Pd-Fe/SiO2[30].Nitrogen(99.999%)was the carrier gas at 200 mL·min-1,and FeOxALD was performed at 150 ℃ by exposing the as-prepared Pd/SiO2to 20～25 cycles of ferrocene and oxygen(For more details,see in Supplementary Information).

1.2 Catalytic testing

Catalytic testing was performed in a continuousflow fixed-bed reactor with 40 mg of catalyst under atmospheric pressure,with a feed gas of 24% CO2and 72% H2(volume fraction,unless otherwise specified,all other gas contents in the paper are volume fractions),balanced with Ar.The total flow was 24 mL·min-1,which resulted in a weight hourly space velocity of 36 000 mL·h-1·gcat-1.After the gas phase was allowed to stabilize for 30 min to reach the steady-state for each reaction temperature,the outlet gas composition was analyzed by an on-line gas chromatograph(Agilent 7820A)equipped with a thermal conductivity detector and a flame ionization detector,and He was the carrier gas.

CO2conversion was calculated as follows：

Wheren0andn1stand for the amounts of CO2in feed gas and outlet gas.

CO selectivity was calculated as follows：

WherenCOstands for the amount of CO in outlet gas.

1.3 Catalysts characterization

X-ray diffraction(XRD,PAN analytical,X'Pert PRO X)was performed with a CuKαradiation source(λ=0.154 06 nm)in reflection mode.X-ray tube was operated at 40 kV and 40 mA.XRD diffraction range was set to 15°to 85°.

The catalysts were characterized via highresolution transmission electron microscopy(TEM),using a JEOL JEM-2100 microscope at 200 kV.The sample was dispersed by ethanol under ultrasonication.The samples were also analyzed via energy-dispersive X-ray spectroscopy(EDS),using the Aztec X-Max 80T EDS system(Oxford instrument,UK).

Hydrogen temperature-programmed reduction(TPR)was performed on a Micromeritics Auto Chem Ⅱ chemisorption analyzer.First,50 mg catalyst was loaded into a U-shape quartz reactor and purged with helium at 120℃for 2 h to remove physically adsorbed water and surface carbonates.After the catalyst was cooled to-50℃,the gas was switched to a flow of 10%(V/V)H2balanced with Ar,and the catalyst was heated to 500℃ at a ramping rate of 10℃·min-1.The amount of H2consumption was calculated with the H2peak area and calibration curve of the 10%(V/V)H2balanced with Ar standard gas.

Furthermore, CO2temperature-programmed desorption(TPD)experiments were performed on the AutoChemⅡ2920.The samples were placed in a quartz tube.Before the TPD experiment,all of the samples were pre-reduced by 10%(V/V)H2/Ar at 300℃for 30 min and then cooled down to room temperature(RT).Then the gas was switched to 10%(V/V)CO2/Ar at 50 mL·min-1for 1 h for the saturated adsorption of CO2.After N2was purged for 30 min,the sample was heated from 50 to 800℃at a ramping rate of 10℃·min-1,during which TPD data were obtained.

Cyclic H2-TPR and CO2temperature-programmed oxidation(TPO)experiments were performed on an OmniSTAR gas analysis system.First,20 mg catalyst was loaded into a U-shape quartz reactor and purged with helium at 150℃for 2 h to remove physically adsorbed water and surface carbonates.After the temperature was cooled to room temperature,the gas was switched to a flow of 5%(V/V)H2balanced with He,and the catalyst was heated to 400℃at a ramping rate of 10℃·min-1.Before the CO2-TPO experiment,the sample was purged again with He at 150℃for 2 h.After the temperature was cooled to room temperature,the gas was switched to a flow of 5%(V/V)CO2balanced with He,and the catalyst was heated to 400℃at a ramping rate of 10℃·min-1.

The FTIR spectra of CO adsorption were recorded in transmission mode on a Bruker Tensor 27 model.CO was introduced for 30 min at room temperature to reach saturated adsorption,followed by purging with N2to remove gaseous CO.

X-ray photoelectron spectroscopy(XPS)experiments were performed using Thermo Fisher ESCALAB 250Xi.A semi-insituexperiment was conducted by transferring the catalyst directly for XPS measurements after a certain treatment without exposing the catalyst to air,as reported elsewhere[31].The first set of experiments consisted of a reaction gas(24% CO2+72% H2+4%Ar,V/V)treatment for 30 min at room temperature and 400℃.The second set of experiments included three steps for each catalyst.First,the as-prepared catalyst was purged with N2at room temperature.Then,the sample was heated to 400℃under 10%(V/V)H2/Ar for 30 min.Finally,the gas was switched to 10%(V/V)CO2/Ar for 30 min.The sample was transferred back to the analysis chamber for XPS measurements in between each treatment.

2 Results and discussion

2.1 Catalysts characterization

The TEM images of as-prepared Pd-Fe/SiO2and the Pd/FeOxcatalysts are displayed in Fig.1a and 1b.The Pd-Fe/SiO2showed uniformly dispersed Pd nanoparticles(NPs)of～3 nm on amorphous SiO2.In contrast,Pd/FeOxdisplayed a highly polycrystalline FeOx,with no significant Pd particle present(see EDS mapping in Fig.S1).The EDS spectra identified the presence of Pd with a similar loading(Table S1)measured by inductively coupled plasma-optical emission spectrometry(Table S2).The results indicate that a system of highly dispersed Pd species,with a size likely below 2 nm,in as-prepared 3.6%(mass fraction)Pd/FeOxcatalysts was formed[32-33].

Fresh (as-prepared)and spent catalysts were examined via XRD,and the results are presented in Fig.1c.The fresh FeOxand the 3.6%(mass fraction)Pd/FeOxcatalysts before reaction showed a typical XRD pattern of Fe2O3,which was reduced to Fe3O4completely after the RWGS reaction.The absence of Pd and PdO XRD patterns in both the fresh and spent Pd/FeOxcatalyst indicates the presence of small Pd/PdOxnanoparticles(＜2 nm)or amorphous nature of Pd species in accordance with the TEM results[34].In the XRD patterns of the Pd/SiO2and Pd-Fe/SiO2,a broad band existed at 21.9°,which can be assigned to amorphous SiO2.The additional diffraction peaks at 40.4°,46.7°,and 68.4°are ascribed to Pd NPs.Moreover,no FeOxor Fe diffraction peaks were observed in Pd-Fe/SiO2,suggesting a very thin layer of Fe species on the ALD-synthesized Pd NPs.

In the H2-TPR results(Fig.1d),the blank FeOxsupport showed two reduction peaks：at 280 and 390℃,which are characteristic of the reduction of Fe2O3to Fe3O4[20].In the case of the Pd/FeOxcatalyst,the first peak(0℃)is ascribed to the reduction of PdO,and the latter two(150 and 250℃)are associated with the reduction of FeOx,which shift to lower temperatures.The presence of Pd thus facilitates the reduction of FeOx,likely via H2spillover effect[20,24].Compared with the H2-TPR results of Pd/SiO2and Pd-Fe/SiO2,the significantly enhanced H2consumption of Pd/FeOxat 0℃indicates a promoted reduction of the Pd-FeOxinterface,since the H2consumption was far more than the stoichiometric PdO amount(Table S3 and supple-mentary discussion).In addition,Pd-Fe/SiO2also showed a small reduction peak at 320℃,which corresponds to the reduction of ALD-synthesized FeOxon Pd NPs.

Fig.1 TEM images of Pd-Fe/SiO2(a)and Pd/FeOx(b),inserted scale bar is 10 nm;(c)XRD patterns of the fresh and spent catalysts;(d)H2-TPR results of Pd/FeOx,blank FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts

2.2 Catalytic tests

The catalytic test was performed in a fixed-bed reactor under atmospheric pressure,with a feed gas of 24% CO2and 72% H2(V/V),balanced with Ar.As shown in Fig.2a and 2b,the Pd/FeOxcatalyst exhibited a remarkably high activity for the RWGS reaction,with a CO2conversion of 10% at 300℃and 29% at 400℃,and a CO selectivity of over 98% at all reaction temperatures.In contrast,both the blank FeOxand Pd/SiO2catalysts were inactive,with CO2conversion below 5% at temperatures up to 400℃(see carbon balance data in Table S4).This strongly indicates that the Pd-FeOxinterface is highly required for the RWGS reaction.To further prove this point,a thin layer of FeOx(mass fraction of 1.1%)was added onto the inactive Pd/SiO2using ALD(the formed product was denoted as Pd-Fe/SiO2catalyst).The Pd-Fe/SiO2catalyst showed a dramatically enhanced CO2conversion,from 5% to 20%,at 400 ℃ (Fig.2c).Only a slight deactivation(＜4%)was observed,which was likely due to coking or mild sintering(Fig.S2 and S3).The enhanced activities of Pd/FeOxand Pd-Fe/SiO2compared with those of their counterparts(FeOxand Pd/SiO2)reveal the promoting role of the Pd-FeOxinterface as highly active sites for catalytic CO2hydrogenation.The long-term stability test of the Pd/FeOxcatalyst was performed at 400℃with time on stream up to 30 h,as shown in Fig.2d.The CO2conversion and CO selectivity remain stable at～29% and ～98% over the entire testing,suggesting an excellent stability of the catalyst.The water formation during reaction also shows a minor effect on the catalytic performance of Pd/FeOxcatalyst,with no significant deactivation in both CO2conversion and CO selectivity(Fig.S4).

Fig.2 Catalytic performance of all of the catalysts tested for RWGS：(a)CO2conversion;(b)CO selectivity;(c)Comparison of Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2at 400℃;(d)Long term stability of Pd/FeOx catalyst at 400℃with time on stream up to 30 h

Furthermore,the Pd-FeOxinterface also promoted CO selectivity.As shown in Fig.2b,the CO selectivity on the Pd/SiO2catalyst declined with increasing reaction temperature.The production of methane thus sug-gests a deep hydrogenation of CO on Pd/SiO2at elevated temperatures.In comparison,both Pd/FeOxand Pd-Fe/SiO2catalysts maintained a CO selectivity of over 98% at all temperatures,suggesting that the addition of FeOxcould prevent the further hydrogenation of CO to methane.Table 1 summarizes the catalytic performance of other literature-reported RWGS catalysts.The 3.6%(mass fraction)Pd/FeOxand 4.4%(mass fraction)Pd-Fe/SiO2catalysts in this work features a higher activity and CO selectivity than 1.5%(mass fraction)Rh/SrTiO3[35],1.67% (mass fraction)Pt/TiO2[10],3%(mass fraction)Pd/Fe3O4[19],and other metal carbide catalysts[13,36].

Table 1 Summary of the conditions and reactivities of various catalysts in this work and the literature

2.3 Dynamic formation of Pdδ+-Fe2+interface as determined by semi-in situ XPS

To identify the dynamic evolution of surface active sites,semi-insituXPS experiments were performed,which enabled transferring the catalyst directly for XPS measurements after certain treatment without exposing it to air(for details,see in supplementary methods).The Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts were stepwise heated in 10% H2and 10% CO2at 400℃,and the Pd3dcore-level evolution spectra are displayed in Fig.3a.Three characteristic chemical states of Pd species could be identified from the evolution spectra：337.2 eV[37],for oxidized Pd2+state;335.7 eV[38],for partially oxidized Pdδ+state;and 334.8 eV[38],for metallic Pd0state(Fig.S5～S8).For Pd/SiO2,the pristine catalyst showed a Pd3d5/2binding energy(BE)of 355.6 eV,corresponding to a Pdδ+state,which likely originated from the native oxide on the Pd NPs.In previous studies,a similar Pdδ+state was revealed by ambient-pressure XPS as a surface oxide and further assigned to Pd5O4stoichiometry[39-41].Reduction in H2at 400℃led to a prominent metallic Pd0of 334.8 eV,which re-oxidized to a Pdδ+state in the subsequent CO2treatment(400℃).Considering the chemical inertness of the SiO2support,we can conclude that Pd0was oxidized to Pdδ+by CO2at 400 ℃ ,strongly indicating a direct dissociation of CO2to CO on Pd NPs.The pristine Pd-Fe/SiO2and Pd/FeOxcatalysts showed a higher Pd3d5/2BE of 336.8 and 337.2 eV respectively,characteristic of PdO oxides[37].The similar redox properties of Pd-Fe/SiO2and Pd/FeOxunder H2and CO2treatments indicate the same reaction path of direct CO2dissociation for the RWGS reaction.Moreover,the addition of FeOxsignificantly promoted the enrichment of Pdδ+species in the reduced Pd/SiO2,Pd-Fe/SiO2,and Pd/FeOxcatalysts after H2treatment,as indicated by the gradual upshift of Pd3d5/2BE from metallic Pd0to Pdδ+(334.8,335.0,and 335.7 eV for Pd/SiO2,Pd-Fe/SiO2,and Pd/FeOx,respectively)with increasing FeOxcontent.The formation of Pdδ+species during the RWGS reaction was further verified by the treatment of Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts under the reaction gas of 24% CO2+72% H2+4% Ar(V/V)at 400℃.As shown in Fig.3b,Fig.S9,and Table S5,the Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts after reaction exhibited a prominent Pdδ+state with a Pd3d5/2peak of 335.7 eV,which further indicates the stability of the Pdδ+surface oxides during the RWGS reaction.

The Fe2pXPS core-level spectra are displayed in Fig.3c and Fig.S10～S11.In the stepwise H2/CO2heating experiments(Fig.3c),an evolution of the chemical state of iron oxides in Pd-Fe/SiO2occurred,resulting in a downshift of Fe2p3/2BE from 711.0 eV(Fe3+state)to 710.3 eV(a reduced Fe2+state)after H2reduction and an upshift back to 711.0 eV after CO2treatment.The dynamic formation of Fe2+species was thus facilitated by H2reduction,and the Fe2+species were further re-oxidized to Fe3+state by reacting with CO2.A similar oscillation of the Fe2p3/2core level was also observed on Pd/FeOx,but less prominent owing to the higher background signal from the bulk Fe2O3support in the Pd/FeOxcatalyst.This further indicates the surfaceenrichment of Fe2+species on the Pd/FeOxcatalyst,formed via hydrogen spillover to the periphery of the Pd NPs[42-43].After reaction gas treatment(Fig.S12),the Fe2p3/2of both Pd/FeOxand Pd-Fe/SiO2underwent a downshift from 710.9 to 710.4 eV,and the characteristic satellite peak for Fe2O3(718 eV)was absent[44],suggesting a reduction in iron oxide support,which agrees with our XRD results.Further reduction of the surface/bulk Fe species may require higher temperature or extended reaction time.

Fig.3 Semi-in situ XPS spectra：Pd3d core-level spectra of Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts during stepwise H2and CO2treatments(a)and after 400℃treatment in the reaction gas(b);(c)Fe2p core-level spectra of Pd/FeOx,Pd/SiO2,and Pd-Fe/SiO2catalysts during stepwise H2and CO2treatments;(d)Oscillation of Fe2+concentration and Pd valence state,deduced from Fe2p and Pd3d XPS results

Fig.3d displays the oscillation of Fe2+concentration and Pdvalence state deduced from Fe2pand Pd3dXPS results,as summarized in Tables S6 and S7.The enrichment of Pdδ+-Fe2+on both Pd-Fe/SiO2and Pd/FeOxwas identified under dynamic reduction by H2,so that the Pdδ+-Fe2+further transformed into Pd2+-Fe3+after reacting with CO2.Based on the higher activities of Pd-Fe/SiO2and Pd/FeOxcompared with that of Pd/SiO2,we can assign the dynamic Pdδ+-Fe2+interface as a highly active site for CO2dissociation during RWGS.

2.4 Promoting role ofPd-FeOxinterfacein adsorption/desorption process

Fig.4a displays the FTIR spectra of CO adsorption over spent catalysts.Two vibrational bands of CO stretching modes,at 2 086 and 1 965 cm-1,were observed on Pd/SiO2,which can be assigned to CO adsorption on multi-coordinated sites of Pd NPs[45-46].Moreover,the CO adsorption intensity dramatically diminished on Pd-Fe/SiO2and eventually became invisible on Pd/FeOx.The absence of CO adsorption on Pd/FeOxcan be ascribed to the higher oxidized Pdδ+/Pd2+state,which may also lead to encapsulation via strong metal-support interaction[47].The weakening of CO adsorption promoted by iron oxides facilitated the desorption of CO as a product and prevented the deep hydrogenation for methane formation.

Fig.4b presents the CO2-TPD spectra of reduced FeOx,Pd/SiO2,Pd/FeOx,and Pd-Fe/SiO2at temperatures up to 750℃.All of the tested catalysts were prereduced in 10% H2at 300℃.FeOxpresented only a subtle peak at 400℃,suggesting a very weak CO2adsorption on Fe2O3.In contrast,Pd/SiO2showed a desorption peak at～550℃,which can be ascribed to the molecular/reactive desorption of chemically adsorbed CO2on Pd NPs in a mixed Pd0/Pdδ+state,as identified by XPS.An additional desorption peak at higher temperatures(～670 ℃ )was observed on both Pd/FeOxand Pd-Fe/SiO2,reflecting an enhanced CO2adsorption owing to the Pdδ+-FeOxinterface,which will lead to the formation of carbonate species or other adsorbed intermediates[48].

Fig.4 CO and CO2adsorption：(a)FTIR spectra of CO adsorption over spent Pd/SiO2,Pd-Fe/SiO2,and Pd/FeOxcatalysts(All IR spectra were acquired at room temperature after evacuating gaseous CO);(b)CO2-TPD spectra of reduced FeOx,Pd/SiO2,Pd/FeOx,and Pd-Fe/SiO2at temperatures up to 750℃

2.5 Reaction mechanism

Fig.5 Cyclic H2-TPR/CO2-TPO experiments on Pd/SiO2(a)and Pd/FeOx(b)catalysts(Y-axis is the ion intensity of m/z=28 for CO);Schematic reaction path of CO2hydrogenation on Pd/SiO2(c)and Pd/FeOx(d)catalysts

The semi-insituXPS results demonstrated a dynamic evolution of Pd and Fe chemical states.To further reveal the reaction pathway,we performed cyclic H2-TPR and CO2-TPO on Pd/SiO2and Pd/FeOxcatalysts.In the case of the Pd/SiO2catalyst(Fig.5a),an indirect path was identified,as indicated by the pronounced CO production along the second H2-TPR cycle,via the hydrogenation of either adsorbed CO2or carbonate species on the metallic Pd surfaces[17,49].Meanwhile,the formation of a little amount of CO was also detected in the CO2-TPO cycle,suggesting the coexistence of direct CO2dissociation,likely on the remanence of Pdδ+species,as identified in the semi-in situXPS experiments.Moreover,the Pd/FeOx(Fig.5b)showed a remarkably enhanced CO formation in the CO2-TPO cycle(direct path),while the hydrogenation path(indirect path)was significantly inhibited,as no CO signal was observed in the second H2-TPR cycle.

Based on the dynamic evolution of the Pd/Fe chemical state based on the semi-insituXPS results,we can propose the overall reaction pathways and underlying mechanism for the RWGS reaction over Pd-Fe catalysts(Fig.5c and 5d).

(1)On Pd/SiO2,the majority of CO2is hydrogenated on the metallic Pd surface via an indirect path,leading to the formation of formate or bicarbonate intermediates,which are eventually decomposed to produce CO.Though less favorable,the direct path of CO2dissociation also coexists on Pd/SiO2,which is likely facilitated by the remanence of Pdδ+species near the SiO2interface.The enhanced CO adsorption on metallic Pd0favors the deep hydrogenation of CO,leading to undesired CH4as a byproduct[50].The low activity of Pd/SiO2is caused by the weak CO2adsorption,as indicated by the CO2-TPD experiments.

(2)On Pd/FeOxand Pd-Fe/SiO2,the Pd-FeOxinterface significantly enhances the CO2adsorption,resulting in a remarkably high CO2conversion compared with that on Pd/SiO2.The dynamic formation of Pdδ+-Fe2+species promoted CO2dissociation,likely following the Mars-van Krevelen mechanism,via active oxygen vacancies on the interface[8,18,43]：

The weakening of CO adsorption on the oxidized Pdδ+/Pd2+species enriched by iron oxide further enhances the CO selectivity(＞98%)by preventing the deep hydrogenation for CH4formation.

3 Conclusions

In this study,Pd/FeOx,Pd-Fe/SiO2,Pd/SiO2,and FeOxcatalysts were systematically investigated for the RWGS reaction.The Pd/FeOxand Pd-Fe/SiO2catalysts exhibited extraordinary activity for the RWGS reaction,with CO2conversions of 29% and 20% at 400℃,respectively,and remarkably high CO selectivity(over 98%)at all considered reaction temperatures.Thus,the Pd-FeOxinterface promoted the CO2hydrogenation to CO.Moreover,semi-insituXPS and cyclic TPR/TPO experiments identified the dynamic formation of Pdδ+-Fe2+species during the RWGS reaction;the species acted as highly active sites for direct CO2dissociation via oxygen vacancies enriched on the interface.The formation of Pdδ+-FeOxspecies was confirmed to play a significant role in the CO2adsorption and CO desorption,as they further enhanced the catalytic activity and selectivity of CO for the RWGS reaction.Our results provide new insights for the dynamic understanding of the metal-oxide interfaces during reactions and benefit the rational design of heterogeneous catalysts from a dynamic point of view.

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