Carbide supported Au catalysts for water gas shift reactions: a new

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Carbide supported Au catalysts for water gas shift reactions: a new territory for strong metal-support interaction effect Jinhu Dong, Qiang Fu, Zheng Jiang, Bingbao Mei, and Xinhe Bao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08246 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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Journal of the American Chemical Society

Carbide supported Au catalysts for water gas shift reactions: a new territory for strong metal-support interaction effect Jinhu Dong,a,b Qiang Fu,a,* Zheng Jiang,c Bingbao Mei,b,c Xinhe Baoa,* aState

Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China bUniversity

cShanghai

of Chinese Academy of Sciences, Beijing 100049, China

Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China

Corresponding authors: Qiang Fu ([email protected]) and Xinhe Bao ([email protected]) Abstract: Strong metal-support interaction (SMSI) has been regarded as one of the most important concepts in heterogeneous catalysis, which has been almost exclusively discussed in metal/oxide catalysts. Here, we show that gold/molybdenum carbide (Au/MoCx) catalysts are featured by highly dispersed Au overlayers, strong interfacial charge transfer between metal and support, and excellent activity in low temperature watergas-shift reaction (LT-WGSR), demonstrating the active SMSI state. Subsequent oxidation treatment results in strong aggregation of Au nanoparticles, weak interfacial electronic interaction, and poor LT-WGSR activity. The two interface states can be transformed into each other by alternative carbonization and oxidation treatments. This work reveals the active SMSI effect in metal/carbide catalysts induced by carbonization, which opens a new territory for this important concept. Keywords: Strong metal support interaction (SMSI); Carbide; Au catalysis; Water gas shift reaction;

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Introduction Strong Metal-Support Interaction (SMSI) has been extensively discussed to elaborate structure-activity relationship in supported metal catalysts.1-6 It has been shown that catalytic performance of metal catalysts can be strongly influenced by reducible oxide supports through electronic metal-support interaction (EMSI model)7-10 or decoration of metal surfaces by oxide supports (encapsulation model)11-13. The classical SMSI phenomena are widely observed for platinum group metals (PGMs) supported on reducible oxides such as TiO2, CeO2, and Fe2O3 upon high temperature reduction (HTR) but can be reversed by subsequent oxidation.3, 14 Mou and coworkers demonstrated that oxidation can induce the SMSI state in Au catalysts supported on ZnO nanorods, which is termed as OSMSI and opposite to the normal treatment conditions for SMSI.15 Christopher and coworkers observed that encapsulation overlayers form on oxide-supported Rh nanoparticles in CO2-H2 reaction environments, which have been attributed to adsorbatemediated SMSI state (A-SMSI).16 Recently, it has been shown that metals supported on phosphates17 and layered double hydroxides18 also exhibit the SMSI effects, in which SMSI occurs even without reduction and oxidation treatments. Au nanocatalysts show great potential in many industrial processes, which have been subjected to extensive investigations.19-21 For long time it has been well recognized that oxide-supported Au cannot manifest the SMSI effect due to its low work function and low surface energy.22-25 Goodman has highlighted that electron transfer indeed exists between Au and TiO2 upon reduction treatments which increases CO adsorption on Au.22 Recently, encapsulation of Au nanoparticles by partially reduced titania has been observed in Au/TiO2 catalysts evidencing the SMSI encapsulation state in supported Au catalysts.13 Moreover, SMSIs between Au and supports of ZnO, hydroxyapatite, and layered double oxides have been clearly demonstrated.15, 17-18 These new progresses suggest that the SMSI concept needs to be further elaborated to show its universality in catalytic systems beyond PGM/reducible-oxide catalysts and under treatment conditions other than HTR. 2


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Transition metal carbides (TMCx) exhibit noble-metal-like electronic properties, which are promising as catalytic materials or supports for active metal catalysts in many reactions.26-29 For example, Ma and coworkers recently showed that Pt atomically dispersed on -molybdenum carbide (α-MoC) enables low temperature production of H2 through aqueous-phase reforming of methanol and layered Au clusters supported on αMoC are successfully used for ultralow-temperature water gas shift reaction (LTWGSR).30-31 In many carbide-supported metal catalysts it has been well demonstrated that carbide supports can interact with metal catalysts strongly, for instance inducing charge polarization of interfacial Au atoms by TiC(001) surface,32-33 giving rise to raft-like Pt particles or embedded Pt structure when supported on Mo2C surface,34-35 stabilizing monolayer precious metals on tungsten carbides and molybdenum carbides,36-38 and forming Pt-Nb alloy between Pt and two-dimensional Niobium carbide.39 Moreover, the metal overlayers interacting strongly the carbide supports often present enhanced catalytic performance in reactions of hydrogenation, water electrolysis, CO2 reduction and WGSR. Therefore, there is a call for studies of SMSI effect in metal/carbide systems in order to extend this important concept to metals on supports other than oxides and to elucidate the nature of metal-carbide interactions. In this work, both Au catalyst and molybdenum carbide (MoCx) support have been chosen to construct the metal/carbide interface. Au catalysts are superior than PGMs in WGSR at low temperature (LT, < 200 oC)40 and TMCx catalysts are active in the reaction as well41. WGSR can be ideally used to probe interaction between Au and carbide. Our Au/MoCx catalysts were synthesized through one-step carbonization process. We found that Au remains highly-dispersed on the carbide support, interacts with the support via strong charge transfer, and more importantly shows excellent LT-WGSR activity. Oxidation treatment of the Au/MoCx catalysts leads to strong aggregation of Au nanoparticles and lower LT-WGSR activity. Our results confirm that the SMSI effect is active in metal catalysts supported on carbide and shows reversibility under alternative 3



oxidation and carbonization treatment conditions, which is analogous to the classical SMSI effect in metal/oxide catalysts reversed by oxidation and reduction treatments. The finding opens a new territory for the concept, which is of great significance for the new understanding of SMSI effect and effective modulation of carbide-supported metal catalysts.

Experimental Catalyst Preparation Au/MoO3 samples were synthesized through an impregnation-calcination process. 2.68 g ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) were dissolved into 5 mL deionized water, and then a certain amount of HAuCl4 aqueous solution (1 g HAuCl4·4H2O dispersed in 50 mL water) was added. Water was evaporated from the formed solution to produce solid powder, which was calcined in furnace at 550 °C for 4 h (denoted as Au/MoO3). Au/carbide catalysts were obtained by temperature program carbonization (TPC) treatment of the Au/MoO3 samples in 20% CH4/H2. The temperature ramping rate is 5 °C/min below 300 °C and 1 °C/min above 300 °C, respectively. It was kept at 700 °C for 2 h. All catalysts were passivated in 1% O2/Ar at room temperature overnight before exposing to air, which were denoted as Au/MoC1-x. The weight content of Au was controlled at 3.0 wt% if not specified. Catalysts with other Au loadings e.g. 5.1 wt% and 0.7 wt% were synthesized by the same process. Pure phase β-Mo2C was synthesized through one-step carbonization process, in which ammonium heptamolybdate was heated up to 700 °C in flowing 20% CH4/H2 gas and held at the temperature for 2 h. Pure phase α-MoC1-x was prepared through a two-step ammonization and carbonization process.42 Ammonium heptamolybdate was first treated in flowing NH3 at 700 °C to form Mo nitride. After cooling down to room temperature (RT) the gas was switched to 20% CH4/H2 gas and the sample was converted to α-MoC1-x using the same TPC process. All the Mo carbide catalysts were passivated in 1% O2/Ar 4


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before exposing to air. Instrument and Characterization X-ray diffraction (XRD) was performed using a PANalytical X'pert PPR diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å) at 40 kV and 40 mA. In-situ XRD experiments were operated with a Rigaku D/Max 2500 diffractometer equipped with a high temperature reaction cell (SRK 900, Anton Paar GmbH). The sample temperature was ramped up to 700 oC with a rate of 10 oC/min and the carbonization atmosphere is 20% CH4/H2. In situ XRD patterns were collected continuously with 2θ value ranged from 21 to 67o and scanning rate of 10o/min. Transmission electron microscopy (TEM) images were obtained on a Hitachi HT 7700 microscope operated at an acceleration voltage of 100 kV. High resolution TEM (HRTEM) images were collected on a JEM-2100 microscope operated at an accelerating voltage of 200 kV. The samples were analyzed using a scanning electron microscope (SEM, Quanta 200 FEG) under an accelerating voltage of 20 kV. Field emission SEM (FESEM) images were collected with a Hitachi S5500 microscope under an accelerating voltage of 30 kV. Scanningtransmission electron microscopy (STEM) and energy dispersive spectrum (EDX) element mapping images were recorded on a FEI Tecnai G2 microscope operated at an accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCALAB 250Xi Spectrometer with an Al Kα as the X-ray source. For in-situ XPS measurements samples were pretreated in H2 at 500 oC inside the XPS system. X-ray adsorption fine structure (XAFS) measurements at Au L3-edge in transmission mode were performed at BL14W1 in Shanghai Synchrotron Radiation Facility (SSRF).43 Samples were synthesized under each specific condition and passivated by diluted O2 carefully. Exsitu XAFS data were collected using a fixed-exit double-crystal Si (111) monochromator and the energy was calibrated using Au foil. The raw data analysis was performed using IFEFFIT software package according to the standard data analysis procedures.44 The 5



Fourier transformation of the k2-weighted Extended X-ray Absorption Fine Structure (EXAFS) oscillations, k2χ(k), from k space to R space was performed to obtain a radial distribution function. Catalyst performance evaluation WGSR performance was evaluated using a home-made fixed-bed micro-reactor, in which catalysts were loaded into a quartz tube with an inner diameter of 4 mm. The reactant was composed of 3% CO and 10% H2O (volume ratio), balanced with He. The outlet gas was analyzed online using an Agilent GC6890 chromatography equipped with Porapak Q column and 5A zeolite Column. Water was introduced into the reaction gases by an injection pump and vaporized by heated carrier gas. Before activity tests, each catalyst was pretreated in 20 % CH4/H2 at 550 °C to remove the surface molybdenum oxide layer. Total space velocity (SV) was 120000 mL per gram catalyst per hour (120000 mL/gcath), and the outlet gas was analyzed after reaction running at each temperature for 30 mins. For insitu WGSR activity experiments, the TPC process was carried out inside the micro-reactor, which means that the reactivity was tested following the carbonization without exposing to air. The reaction kinetic parameters were obtained under low CO conversion (< 15%) by increasing SV and diluting the catalyst with commercial SiC powder.

Results Electron microscopy (EM) images were used to show morphology of Au/MoO3 and Au/MoCx samples. TEM and SEM images from the Au/MoO3 sample are shown in Figures S1a and S1b, respectively. MoO3 supports have regular and rod-like morphology, and nanoparticles with diameter of 50 - 100 nm can be observed on the support surfaces. Au and Mo element mapping images in SEM displayed in Figures 1a-c (STEM images were shown in Figure S2) confirm that the big nanoparticles are supported Au, and most Au species in the Au/MoO3 sample are in the form of large aggregates. The sintering of Au nanoparticles supported on oxides upon calcination treatment is well expected considering 6


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weak interaction between Au and many oxides such as MoO3.45 Au/MoCx sample was obtained through the temperature programmed carbonization (TPC) treatment. The support morphology remains as well-defined rods, but more importantly no nanoparticles can be observed on the support surfaces in all EM images (Figures S3a, b, and Figure S4). However, EDX measurements in both SEM and TEM still show strong Au signals in all the samples, and both Au and Mo element mapping images suggest homogeneous distribution of Au element over the whole sample surfaces (Figures 1d-f and Figure S5). The morphology and element distribution results before and after the TPC process indicate that supported Au overlayers change from large aggregates to highly dispersed state.

Figure 1. Electron microscopy images of Au/MoO3 and Au/MoCx samples. (a) SEM, (b) Au and (c) Mo EDX element mapping images of the Au/MoO3 sample; (d) SEM, (e) Au and (f) Mo EDX element mapping images of the Au/MoCx sample. Phase structure and particle size of the Au/MoO3 and Au/MoCx samples were analyzed by XRD. As shown in Figure 2a all the strong diffraction peaks from the Au/MoO3 sample can be assigned to orthorhombic-MoO3 (PDF#35-0609) and cubic-Au (PDF#02-1095). Using Debye-Scherrer Equation grain size of Au nanoparticles is estimated to be 43 nm, 7



which is consistent with the results derived from TEM and SEM images. The TPC process has converted MoO3 to a mixture of α-MoC1-x (PDF# 01-089-2868) and β-Mo2C (PDF#350787). Meanwhile, no diffraction peaks from Au can be found in the pattern from the Au/MoCx sample. The enlarged XRD patterns are shown is Figure 2b, and the disappearance of Au (200) is notable. Again, this change can be attributed to high dispersion of Au species in the Au/MoCx sample, which is probably in the form of ultrathin layers or single atoms supported on the carbide surfaces.

Figure 2. Reversible structural transformation of supported Au upon cycled calcination and carbonization treatments. (a) ex-situ XRD patterns of Au/MoO3, Au/MoCx, Au/MoO3-recalcination, and Au/MoCx-recarbonization samples, respectively. (b) The detailed evolution of Au(200) diffraction peak from the different samples. FESEM images of (c) Au/MoO3, (d) Au/MoCx, and (e) Au/MoO3-recalcination samples. (f) Au/Mo atomic ratios calculated from XPS Au 4f and Mo 3d signals of the Au/MoO3 sample treated at different TPC-treatment temperatures. To further investigate morphology change of supported Au overlayers, the Au/MoCx sample was subject to calcination in air at 550 oC (denoted as Au/MoO3-recalcination) and then further treated by the second TPC process (denoted as Au/MoCx-recarbonization). XRD patterns from the above two samples (Figure 2a) show that diffraction peaks from both MoO3 and Au have reappeared in the Au/MoO3-recalcination sample, and the 8


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estimated Au particle size is 15 nm, which is smaller than the first Au/MoO3 sample. For the Au/MoCx-recarbonization sample only α-MoC1-x phase can be identified while the Au diffraction peaks disappear again, which indicate the carbonization-induced dispersion of Au overlayers once more. The enlarged patterns in Figure 2b illustrate the cyclic evolution of Au(200) diffraction peak with the treatments. Its appearance-disappearance is well consistent with the aggregation-dispersion of Au overlayers. Figures 2c-e display FESEM images from Au/MoO3, Au/MoCx, and Au/MoO3recalcination samples. It is clearly shown that the “disappeared” Au nanoparticles in the Au/MoCx sample have reappeared in the Au/MoO3-recalcination sample, which are due to calcination-driven aggregation of Au nanoparticles. XPS Au 4f and Mo 3d spectra were acquired from the Au/MoO3 samples which are subject to TPC-treatment at various temperatures and the calculated Au/Mo atomic ratios are given in Figure 2f. Au/Mo atomic ratio does not change significantly below 700 oC, while the twofold higher XPS Au/Mo ratios from the 700 oC-carbonized samples (namely Au/MoCx) unambiguously confirm highly dispersed Au overlayers supported on carbide surfaces. Overall, carbonization of Au/MoO3 induces the high dispersion of Au overlayers supported on Mo carbides, while calcination of Au/MoCx in air results in the strong aggregation of Au overlayers on MoO3. Au morphology can be reversibly manipulated by the cycled calcination-carbonization processes. The similar dispersion state of metal overlayers on MoCx support has been observed for other transition metals such as Fe, Ru, and Ag (Figure S6).

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Figure 3. Contour map of in-situ XRD patterns during the TPC treatment of Au/MoO3 sample, revealing the carbonization-induced structural evolution of Au and Mo oxide. The dominated phases of Mo species at each stage are listed on the left and standard Au diffraction peaks are displayed on the top. Standard diffraction peaks of Mo9O26, MoO2, HxMoOy, and MoOxCy phases are shown in Figure S7. To study structural evolution of Au overlayers and Mo-containing supports, in-situ XRD patterns were continuously collected during the TPC treatment of Au/MoO3 sample from 180 to 750 oC (Figure 3). At low temperature (< 350 oC), XRD patterns are dominated by MoO3 and partially reduced MoO3 phases (e.g. Mo9O26, PDF#12-0753).46 MoO3 phases convert to MoO2 (PDF#32-0671) and subsequently almost all Mo6+ species disappear above 350 oC. Owing to hydrogen spillover and hydrogen doping to the oxide, hydrogenated molybdenum oxides (HxMoOy, PDF#49-0652) may also exist between 350 and 450 oC, which are often observed under the treatment conditions in H2 atmosphere.4748

No decrease in Au diffraction peaks occurs until 550 oC, which is confirmed by ex-situ

XRD patterns in Figure S8. 10


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With the dissociation of CH4, the formed carbon atoms gradually diffuse into Mo oxide and a new phase - Molybdenum Oxycarbide (MoOxCy, PDF#17-0104) appears. This phase coexists with MoO2 from 350 to 630 oC. MoOxCy has the same cubic phase as Au and presents slightly different lattice parameters.49 Therefore, the main diffraction peaks of Au and MoOxCy phases mingle to broader peaks in the temperature between 350 and 630 oC, also seen in ex-situ XRD patterns in Figure S8. The TPC process of pure MoO3 is also monitored through in-situ XRD measurement, and the corresponding results are shown in Figure S9. For Au/MoO3 samples MoOxCy phase starts to form at 350 oC, but carbide phase doesn’t appear until 650 oC and no MoOxCy phase is recognized in MoO3 samples. The comparative studies indicate that the presence of transition metals facilitates diffusion of C atoms into the lattice of Mo oxides such that C-containing phases emerge at lower temperatures compared to the carbonization of pure MoO3. Considering the high lattice match between Au and MoOxCy phase, we suggest that the epitaxy structure at Au/MoOxCy interface is thermodynamically favored and this may induce wetting and dispersion of Au nanoparticles on the support. With the further increase of carbonization temperature more C atoms diffuse into the support, such that MoO2 structure disappears but MoOxCy phase becomes dominant around 600 oC (Figure 3 and Figure S8). Above 630 oC the diffraction peaks of MoOxCy get weakened, which is accompanied with rapid transformation to α-MoC1-x (Figure 3). Meanwhile, the Au diffraction peaks disappear with temperature higher than 600 oC, which can be confirmed by ex-situ XRD patterns in Figure S8. The fact that formation of Mo oxycarbide phases and dispersion of Au nanoparticles occur almost under the same conditions indicates that the MoOxCy intermediate phase plays a critical role in the Au dispersion during the carbonization process.

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Figure 4. Carbonization induced structural evolution of Au overlayers revealed by XAFS measurements. (a) Comparison of normalized XANES spectra at the Au L3-edge of different samples. (b) Fourier transforms of k2-weighted R-space EXAFS spectra of the various samples, which are subject to cycled calcination-carbonization treatments. Dashed lines correspond to the curve-fitting results. (c) Evolution of Au-Au coordination numbers (CNs) during TPC treatment of Au/MoO3 sample (various treatment temperatures are labeled). To follow the evolution of coordination environment of Au element, Au L3-edge XAFS spectra were collected from Au/MoO3 and Au/MoCx samples. As displayed in Figure 4a, adsorption edges from the Au/MoO3 and Au/MoCx samples are like that of Au foil. The results confirm the metallic Au state in both samples and exclude the state of Au compounds or single atomic Au in the Au/MoCx samples. The coordination environment of Au atoms was further investigated by analyzing EXAFS spectra (fitting results given in Table S1). As shown in Figure 4b, the signals around 2.5 Å are derived from the first shell of Au–Au bond and their intensities are correlated to the average Au-Au CNs. Au/MoO3 12


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sample has very similar intensity comparing with Au foil, and the fitted Au–Au CNs and bond distance of Au/MoO3 are 10.9 and 2.86 Å, respectively, confirming the bulk Au nanoparticles. For the Au/MoCx samples, the corresponding values change to 5.4 and 2.79 Å, in which the significant increase in Au-Au CNs and lattice shrinkage provide further evidence for highly dispersed Au state.8 In addition, the appearance of Au-Mo (CNs = 1.3) and Au-C (CNs = 0.4) bonds indicate the close contact between Au overlayer and carbide support. The bulk Au state and highly dispersed Au state are also observed in the Au/MoO3recalcination (Au–Au CNs = 11.1 and Au-Au bond distance of 2.86 Å) and Au/MoCxrecarbonization (Au–Au CNs = 3.7 and Au-Au bond distance of 2.78 Å) samples, respectively. The alternative variation of the Au-Au CNs reflects the reversible aggregation-dispersion evolution of Au overlayers upon cycled calcination and carbonization treatments. The evolution of Au-Au CNs at various TPC-treatment temperatures is shown in Figure 4c (see detailed fitting results in Table S1, R-space curves in Figure S10, and kspace curves in Figure S11). Au-Au CNs change from 10.9 to 5.4 with the treatment temperature increased up to 700 oC, and particularly Au-Au CNs start to decrease significantly from 570 oC. The above in-situ XRD results suggest that MoOxCy intermediate phase becomes dominant within this temperature regime. The change in AuAu CNs again also implies the key role of MoOxCy phase in the dispersion of Au particles. It is important to note that the Au-Au bond length has a slight shrink (Table S1) in Au/MoCx (2.79 Å) compared to that of Au foil (2.86 Å), which is also observed in other 2D Au nanosheets.8 We suggest that it originates from the lattice distortion in the highly dispersed Au overlayers. Analysis of Au-Au CN confirms the reversible aggregationdispersion process and the evolution of Au dispersion.

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Figure 5. Catalytic performance of the catalysts in LT-WGSR. (a) LT-WGSR activity from Au/MoCx, α-MoC1-x, β-Mo2C, and in-situ synthesized Au/MoCx samples. For ex-situ activity measurement, catalysts were pre-treated by 20% CH4/H2 at 823 K for 60 min. (b) Arrhenius plots of LT-WGSR rates of Au/MoCx and α-MoC1-x samples. CO conversions were controlled to be less than 15%. (c) LT-WGSR activity from the various catalysts upon the cycled calcination-carbonization treatments. Various catalysts with 3 wt% Au loading are subject to WGSR reaction, particularly at low temperatures (< 200 oC). Firstly, Au/MoCx, α-MoC1-x, and β-Mo2C catalysts were synthesized and tested for LT-WGSR between 100 and 220 oC under the same SV (120000 mL/gcat·h). As shown in Figure 5a, pure phase β-Mo2C catalyst shows little LT-WGSR reactivity within the tested temperature region. α-MoC1-x catalyst shows a little activity below 160 oC and has 60% CO conversion at 220 oC. In contrast, the Au/MoCx catalysts show 20% CO conversion at temperature as low as 100 oC, and the CO conversion reaches 100% at 160 oC. We also tested LT-WGSR activity over Au/MoCx catalysts which were prepared in the micro-reactor without exposing to air (denoted as in-situ Au/MoCx, see in 14


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Figure 5a), and even higher LT-WGSR activity (100% CO conversion at 140 oC) has been achieved. For the in-situ Au/MoCx catalyst, the specific activity per catalyst mass at 120 oC

is calculated to be 4.02  10-5 molCO/gcat·s, which is several times higher than many

noble metal catalysts supported on oxides and is much better than noble metal/carbide catalysts reported recently (see detail in Table S2). This performance is comparable to the most recent result on Au/α-MoC reported by Yao et al.30 The re-use of the catalyst for LTWGSR has been tested as shown in Figure S12. Though the used catalyst loses a portion of activity, but the performance could be completely recovered by activation. LT-WGSR reaction data from Au/MoCx catalysts with different Au loadings are shown in Figure S13. The lower Au loading (0.73 wt %) leads to a slight lower CO conversion but a higher specific activity per Au mass. Apparent reaction activation energies (ΔEaapp) were calculated through the Arrhenius plots of LT-WGSR reaction rates (Figure 5b). The Au/MoCx catalysts (ΔEaapp = 44.05 kJ/mol) show a lower activation energy compared to the α-MoC1-x catalyst (ΔEaapp = 56.89 kJ/mol). The measured ΔEaapp from the Au/MoCx catalysts is typical for Au catalysts in WSGR from previous studies.40 These results reveal that the Au/MoCx catalysts have a tremendous WGSR performance enhancement in comparison with pure Mo carbides. As discussed above, cycled calcination and carbonization treatments induce reversible aggregation-dispersion evolution of Au overlayers. The LT-WGSR performance was tested over catalysts subjected to different calcination and carbonization treatments. As shown in Figure 5c, Au/MoO3 catalyst exhibits little activity due to the large Au particle size, and then TPC process has enhanced its LT-WGSR activity. The strong aggregation of Au species induced by the second calcination treatment also quenches the activity, while the second carbonization treatment totally recovers the high LT-WGSR activity. The reversible structural changes of Au overlayers with the calcination-carbonization treatments result in the up-down oscillation in the catalytic performance. As shown in Figure 2 the in-situ Au/MoCx sample should consist of both α-MoC1-x and β-Mo2C phases 15



and the in-situ Au/MoCx-recarbonization sample mainly contains α-MoC1-x. The similar high activities from both samples suggest the critical role of highly dispersed Au overlayers in the reactions, which are all present in both catalysts.

Discussion Au nanoparticles often interact strongly with oxide supports such as Fe2O3,50 TiO2,13, 51

and CeO2,52 which mainly manifest by strong charge transfer at the metal-oxide

interfaces subjected to HTR. Recently, reversible encapsulation of Au nanoparticles by oxide supports has been observed over Au/oxide catalysts under cycled reduction and oxidation treatments, demonstrating the classical SMSI state.13, 17-18 Here, we have shown that carbide-supported Au catalysts can experience reversible aggregation-dispersion transformation under cycled calcination and carbonization treatments. Particularly, formation of carbide support is accompanied by spreading or wetting of Au onto the support surface and forming highly dispersed metal overlayers, which show high activity in LT-WGSR.

Figure 6. Electronic interaction between Au and carbide support revealed by in-situ XPS. (a) Au 4f and (b) C 1s spectra of Au/MoO3 sample carbonized at the indicated 16


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temperatures. To further investigate the interaction between Au and carbide support, in-situ XPS spectra were acquired from several samples (Figure 6) and the corresponding Mo 3d spectra were displayed in Figure S14. C 1s signals at 284.60 eV from surface contaminations are used to calibrate all binding energies (BEs) and the lower BE peaks at 283.20 eV are from C atoms in carbides.53 Au 4f7/2 binding energy of the Au/MoO3 sample is located at 83.80 eV, corresponding to Au0+ state of the Au nanoparticles. With the increasing carbonization temperature Au 4f7/2 BE shifts to a higher position. For instance, at the carbonization temperature of 500 oC Au 4f7/2 peak shifts to 84.00 eV, probably owe to interaction of Au with formed Mo oxycarbide (Figure S14). The Au/MoCx sample formed at 700 oC presents the peak position at 84.25 eV. The total positive Au 4f binding energy shift (+0.45 eV) of Au overlayers interfacing with MoCx support indicates strong charge transfer from Au to the carbide support, which could strengthen the Au-carbide support interaction.54 Such an interfacial phenomenon has been observed at metal/oxide interfaces.55 Ex-situ XPS spectra were also collected from various samples undergoing cyclic carbonization-calcination treatments (Figure S15). The charge transfer can still be verified in spite of the surface carbon contamination and oxidation of carbide. After the recalcination Au 4f peak shifts back to the original position. Therefore, interfacial charge transfer can be reversibly modulated by the treatments, which is characteristic for the classic SMSI effect.3, 8, 10, 13, 15 For WGSR catalyzed by Au/oxide catalysts, it is well accepted that CO molecules adsorb on Au surface, and the Au/oxide interface are responsible for activation of H2O.5658

In the Au/MoCx catalysts with SMSI state highly dispersed Au overlayers expose a large

number of surface Au atoms for CO adsorption and ensure high density of metal-support interface sites. The positively charged Au could weaken CO adsorption facilitating CO diffusion on the surface.15 Moreover, Mo carbide support is active for H2O activation to produce surface Mo oxycarbide species and hydroxyl groups.59-61 Then, hydroxyl groups 17



diffuse to the interface sites and react with the surface adsorbed CO molecules, producing the final products. Although CO and H2O can be activated on Mo carbide surfaces,41, 59 our results show that combining Au with MoCx has enhanced the reaction significantly. In fact, surface oxygen-containing groups on Mo carbide may not react with CO in time, which lead to over-oxidation of the carbide surface. In the SMSI state charge transfer to carbide helps to maintain the reduced state and keep the catalytic cycle continuing.

Figure 7. Schematic illustration of SMSI effect between Au overlayers and carbide supports. SMSI state in the Au/MoCx catalysts can be illustrated by Figure 7, which is featured by highly dispersed Au overlayers, strong charge transfer from Au to carbide, and excellent activity in LT-WGSR. Based on the above characterization results we suggest that the highly dispersed Au overlayers are in the form of ultrathin wetting layers, which is driven and stabilized by the strong interaction of Au with MoOxCy and MoCx phases. It is well known that one metal often forms ultrathin wetting layers on another metal surface, which is in contrast with metal nanoparticles on oxide surfaces.4-5 The noble-metal-like properties of carbides ensure that metal overlayers grown on carbides are more similar to epitaxy of one metal on another metal.34 Based on the same argument, the electronic interaction of metal overlayers with carbide support is facile. The rich Au-carbide interfaces supply dualfunction sites for WGSR and present high activity at low temperatures. The reversible structural transformation of Au overlayers between large aggregates 18


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and ultrathin wetting layers has been illustrated in Figure 7 as well. The reversible transformation of surface structures is one important feature of the classical SMSI state in metal/oxide catalysts, which is driven by cycled reduction and oxidation treatments. As we demonstrate in this work, the reversible structure transformation in metal/carbide catalysts needs to be driven by cycled carbonization and oxidation treatments. Our findings confirm that metal/carbide catalyst is a new territory for the SMSI effect.

Conclusion Highly-dispersed Au overlayers supported on MoCx have been synthesized using simple temperature programmed carbonization treatment of Au/MoO3 samples. The high dispersion of Au overlayers has been attributed to strong interaction between Au and Mo carbides, which behaves similarly as metal-metal interaction. Au overlayers show reversible aggregation-redistribution processes upon cycled calcination-carbonization treatments. SMSI state in Au/MoCx catalysts endows tremendous enhancement in LTWGSR activity. Our work indicates that the SMSI effect is applicable to other metal/carbide systems suggesting that SMSI can be used to design highly active metal/carbide catalysts. ASSOCIATED CONTENT Supporting Information. SEM, TEM, HRTEM, element mapping images, XRD, EXAFS, and XPS characterization data, as well as WGSR reaction data are included. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgement This work was financially supported by the National Natural Science Foundation of China (No. 21688102, No. 21573224, and No. 91545204), Ministry of Science and Technology of China (No. 2016YFA0200200 and No. 2017YFB0602205), Strategic 19



Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020000). The authors thank the fruitful discussions with Prof. Chuan Shi.

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Carbide supported Au catalysts for water gas shift reactions: a new

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