Sacrificial Adsorbate Strategy Achieved Strong MetalâSupport

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Sacrificial Adsorbate Strategy Achieved Strong Metal -Support Interaction of Stable Cu Nanocatalysts Xiuyun Wang, Yi Liu, Xuanbei Peng, Bingyu Lin, Yanning Cao, and Lilong Jiang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00049 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Energy Materials

Sacrificial Adsorbate Strategy Achieved Strong Metal−Support Interaction of Stable Cu Nanocatalysts Xiuyun Wang, Yi Liu, Xuanbei Peng, Bingyu Lin, Yanning Cao and Lilong Jiang* National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China. Corresponding Author E-mail: [email protected]; [email protected].

ABSTRACT: A new adsorbate-mediated strategy was developed to enhance metal-support interaction of Cu/CeO2, aiming to improve its catalytic activity and sintering resistance in water-gas shift (WGS) reaction. By treating Cu/CeO2 in a 20CO2:2H2 gas mixture for the formation of surface HCOn (n=2, 3), there was significant enhancement of interaction between CeO2 and Cu. The HCOn adsorbate was removed through calcination in an O2/Ar atmosphere at 400 °C for 6 h. The as-obtained Cu/CeO2 catalyst was compared with the untreated counterpart in WGS reaction. It was observed that CO conversion at 350 °C was 86% and 47%, respectively, over the two catalysts. The superiority of the former is attributed to the enhanced interaction between Cu and CeO2. In a run of 15 h at 400 °C, the treated catalyst showed no obvious sign of deactivation. KEYWORDS: WGS; Adsorbate; Metal-support interaction; Heterogeneous catalysis; Cu/CeO2 catalysts.

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1. INTRODUCTION In chemical industry, the water-gas shift (WGS) reaction is for the formation of H2.1-3 Precious-metal-based catalysts are promising due to their high activity at relatively low temperatures.4, 5 Their application, however, is not desirable because a high loading of the metals is required. Add to the fact that precious metals are expensive to purchase is the easy sintering of metal nanoparticles at high temperatures, and concerns such as these limit the precious-metal-based catalysts from commercial use. An alternative is to use cheap transition metal oxides such as, for example, those of Cu, Co and Ni.6-8 On the other hand, ceria is an interesting support material because it provides oxygen vacancies and Ce3+ sites for water dissociation.9,

10

Among the CeO2-support catalysts, Cu/CeO2 has been a research focus

because of its superior catalytic activity in WGS reaction. However, the practical application of Cu/CeO2 in WGS processes is somewhat limited. Because of high surface energy, Cu nanoparticles are thermodynamically unstable and tend to sinter at high temperatures and/or upon prolonged reaction. To date, many efforts have been devoted to improving its catalytic stability via controlling preparation conditions and/or adding catalyst promoters. For instance, Kubacka et al. have found that that the presence of tungsten in CuO/CeO2 catalyst results in the formation of W-Cu local entities and improvement of the ceria redox capabilities, thus a higher WGS catalytic stability at 300 oC compared with the parent one.10 Further efforts were put in to develop an effective strategy to stop the Cu nanoparticles from sintering in WGS reaction at high reaction temperatures (>300 o

C). Recently, several methods were developed to stabilize metal nanoparticles, such as

localizing the nanoparticles in the channels of nanotubes and porous materials.11,

12

In

particular, the facilitation of strong metal-support interaction (SMSI) between support and active metal is a facile way to achieve excellent catalytic activity and durability.13

Scheme 1. Illustration of adsorbate-mediated strategy for controlling Cu nanoparticles. 2 ACS Paragon Plus Environment

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Herein, we report a smart strategy to enhance the interaction between Cu and CeO2. The method is simple and facile: Cu/CeO2 catalysts were thermally treated in an atmosphere of 20CO2:2H2 at a suitable temperature for the formation of surface HCOn, and the HCOn remnant was removed via calcination in a O2/Ar atmosphere at 400 °C for 6 h. The precursors before calcination are herein denoted as Cu/CeO2@HCOn-T (T represents the temperature for thermal treatment, e.g. for the sample treated at 400 °C, the sample is denoted as Cu/CeO2@HCOn-400). Similarly, the corresponding catalysts obtained after calcination in O2/Ar atmosphere are denoted as Cu/CeO2@-SMSI-T (Scheme 1). More details for the preparation of the Cu/CeO2 and Cu/CeO2@-SMSI-T samples are given in Supporting Information. It was found that the presence of HCOn (n=2, 3) on Cu/CeO2 enhances Cu and CeO2 interaction. As a result, the reducibility and CO adsorption ability of Cu/CeO2 is promoted, and there is improvement in low-temperature catalytic activity as well as resistance against sintering at high temperatures in WGS reaction. 2. Experimental Section 2.1 Materials preparation The synthesis of the ceria has been described in detail recently. 1.7369g Ce(NO3)3·6H2O was added into the 70 mL of urea (6 M) aqueous solution, then transferred to the Teflon lined stainless steel autoclaves (100 mL), and heated at 120 °C for 24 h. Thereafter, the obtained 3 ACS Paragon Plus Environment


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precipitates were filtered and followed by drying at 80 °C for 12 h. Finally, the powders were calcined at 500 °C for 3 h, yielding CeO2. CeO2 support (1.0 g) in continuous stirring water (50 mL) was heated to 60 oC for 3 h, 10 wt%Cu(NO3)2.3H2O in H2O (30 mL) were added, the resultant mixtures were evaporated to dryness at 60 oC, dried at 100 oC for 12 h and calcined at 500 oC for 3 h. The obtained catalysts were labeled as Cu/CeO2. Then, Cu/CeO2 was treated with 20CO2:2H2 at 250 and 400 oC for 3 h, the gained samples were marked as Cu/CeO2@HCOn-250 and Cu/CeO2@HCOn-400, respectively. Finally, Cu/CeO2@SMSI-250 and Cu/CeO2@SMSI-400 were obtained via 20%O2/Ar calcination at 400 oC for 6 h. 2.2 Catalytic Activity Tests Before activity test, all of the samples were in situ reduced by 10%H2/Ar at 250 oC for 4 h. The catalytic activity of the catalysts for the WGS reaction was tested in a fixed bed reactor at atmospheric pressure from 200 to 400 oC at an interval of 50 oC. Typically, 0.5 mL of catalyst (20-40 mesh) was used and the space velocity was calculated to be 4500 h-1; feed gas was a model reformates containing 15 vol. % CO, 55 vol. % H2, 6 vol. % CO2 and balance with N2; the ratio of vapor to feed gas was maintained at 1:1. After passing through a condenser to remove the residual water, the outlet entered a gas chromatograph (Shimadzu, GC-8A) equipped with a thermal conductivity detector (TCD) to evaluate the concentration of CO. The CO conversion (XCO)was calculated according to the following equations:

X = C c− C *100% in

out

CO

[1]

out

2.3 Characterizations Powder X-ray diffraction (XRD) was performed on a Panalytical X’Pert Pro diffractometer using a Cu-Kα radiation at 40 kV and 40 mA. N2 physisorption measurement was performed on an ASAP 2020 apparatus; the sample was degassed in vacuo at 180 oC at least 6 h before each measurement. H2 temperature-programmed reduction (H2-TPR) was performed on 4 ACS Paragon Plus Environment

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AutoChem II 2920 equipped with a TCD detector, in which the sample was pretreated under Ar ﬂow (30 mL/min) at 400 oC for 0.5 h. After cooling to room temperature, the temperature was increased to 800 oC at 5 oC/min in a gas flow of 10 vol% H2/Ar (30 mL/min). CO temperature-programmed reduction (CO-TPR) was also performed on a Micromeritics Autochem II 2920 instrument equipped with a Hiden QIC-20 mass spectrometer. A 50 mg sample was pretreated at 300 °C for 1 h under an Ar stream and cooled to 50 °C. The analysis was performed in a mixture of 1vol % CO in Ar (30 mL/min) from 50 to 800 °C. The outlet products (CO2, CO, H2O) were measured instantaneously via a mass spectrometer (MS). FTIR was carried out on a Nicolet Nexus FT-IR spectrometer in the range of 700-4000 cm−1 with a resolution of 4 cm−1, in which the sample was grounded with KBr and pressed into thin wafer. CO-TPD quadruple mass spectroscopy (Q-MS) was used to evaluate evolving gases. The signals for CO (m/z = 28), CO2 (m/z=44) and H2O (m/z =18) were monitored by using a QIC20 bench top gas analysis system. X-ray photoelectron spectroscopy (XPS) analysis was performed on Physical Electronics Quantum 2000, equipped with a monochromatic Al-Kα source (Kα=1,486.6 eV) at 300 W under UHV. Catalyst charging during the measurement was compensated by an electron flood gun. Transmission Electron Microscope (TEM) and high-resolution transmission electron microscopy (HR-TEM) measurements were carried out on a JEM-2010 microscope operating at 200 kV in the mode of bright field with an energy dispersive X-ray spectroscopy (EDX) detector. The preparation of samples for analysis involved being ultrasonically dispersed in absolute ethanol and deposition on a carbon-Mo grid. Raman spectra of samples were collected at ambient condition on a Renishaw spectrometer. A laser beam (λ= 532 nm) was used for an excitation. Atomic force microscopy (AFM) imaging was done using Bruker Dimension ICON AFM. The AFM imaging was done by drop-casting the methanolic dispersions of the LDHs on silicon wafers followed by drying and incubation for 12 h.



3. RESULTS and DISCUSSION

Figure 1 AFM images of (A-B) Cu/CeO2 and (D-E) Cu/CeO2@HCOn-400; surface roughness profiles of (C) Cu/CeO2 and (F) Cu/CeO2@HCOn-400.

To visualize the effect of the 20CO2:2H2-treatment on the structure of Cu/CeO2, we performed atomic force microscopy (AFM) investigation on the Cu/CeO2 and Cu/CeO2@HCOn-400 samples. In comparison with Cu/CeO2 (Fig.1A&B), Cu/CeO2@HCOn400 has an amorphous overlay on the surface (Figs. 1C&D). The FT-IR spectra (Figure 2) of Cu/CeO2@HCOn-250 and Cu/CeO2@HCOn-400 indicate presence of formate (HCO2, 1536 cm−1) and bicarbonate-like species (HCO3, 1425 cm−1), revealing that the amorphous overlay is made up of HCO2 and HCO3.14 As determined from the roughness profiles (insets), the average surface roughness value (Sa) of Cu/CeO2 (Figure 1B) and Cu/CeO2@HCOn-400 (Figure 1D) is 1.07 and 1.44, respectively, suggesting that with the assembly of HCOn species on Cu/CeO2, there is surface roughening. The FT-IR spectrum of Cu/CeO2-SMSI-400 confirms the successful removal of HCOn adsorbates from Cu/CeO2@HCOn-400 using the calcination method (Figure 2).


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Figure 2 FT-IR spectra of Cu/CeO2, Cu/CeO2@HCOn-250, Cu/CeO2@HCOn-400 and Cu/CeO2-SMSI-400. Scheme 2. Diagram of the surface and bulk molecule structures of the Cu/CeO2 catalyst fresh and before the WGS reaction (Ce atom: yellow; O atom: Red; Cu atom: blue).



Figure 3 (A) CO conversion of as-prepared catalysts and (B) thermal stability of as-prepared catalysts at 400 oC. Before WGS reaction, Cu/CeO2, Cu/CeO2-SMSI-250 and Cu/CeO2-SMSI-400 catalysts were in situ reduced with 10%H2/Ar (Scheme 2). Figure 3A illustrates the catalytic activity of the catalysts in WGS reaction between 200 and 400 °C. For all the catalysts, CO conversion below 250 °C is poor due to kinetic limitation, and maximum activity is reached at 350 °C. Further increase of temperature would result in decline of CO conversion due to thermodynamic limitation. Clearly, CO conversions over Cu/CeO2-SMSI-400 and Cu/CeO2SMSI-250 are higher than that over Cu/CeO2, which could be ascribed to the stronger metalsupport interaction of the former two catalysts. For example, CO conversion over Cu/CeO2 is 47% at 350 °C, whereas that over Cu/CeO2-SMSI-400 is 86% at the same temperature. Note that the slightly higher WGS catalytic activity in as-prepared Cu/CeO2 compared to that in previously reported Cu/CeO2.10 Using the temperature at 90% CO conversion (T90) as an indicator for catalytic efficiency, the activity of the catalysts decreased in the order of: Cu/CeO2-SMSI-400 > Cu/CeO2-SMSI-250 > Cu/CeO2. It is worth pointing out that the T90 value of Cu/CeO2-SMSI-400 and Cu/CeO2-SMSI-250 is 315 °C and 340 °C, respectively (Table S1), while at 400 °C, the CO conversion is still below 90% over Cu/CeO2. Shown in Figure 3B is the stability test of Cu/CeO2 and Cu/CeO2-SMSI-400 in WGS reaction at 400 °C for 15 h. Over the Cu/CeO2 catalyst, there is a gradual drop of CO conversion from 88 to 74% whereas over Cu/CeO2-SMSI-400 the decline is insignificant. On the basis of the results, it is deduced that the Cu/CeO2-SMSI-400 catalyst is more robust in terms of resistance against sintering. As revealed in transmission electron microscopy (TEM) and HRTEM (Figure 4) analyses, the size of Cu nanoparticles in Cu/CeO2 become bigger as detected after the WGS reaction (Figure 4G). As for the used Cu/CeO2-SMSI-250 (Figure 4I) and Cu/CeO2-SMSI-400 (Figure 4K), there is little change of Cu particle size. The results indicate that thermal sintering of Cu nanoparticles has been efficiently suppressed in 8 ACS Paragon Plus Environment

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Cu/CeO2-SMSI-250 (Figure 4I) and Cu/CeO2-SMSI-400 (Figure 4K) during the WGS process.

Figure 4 TEM and HR-TEM images of (A, B) Cu/CeO2, (C, D) Cu/CeO2-SMSI-250, (E, F) Cu/CeO2-SMSI-400. TEM and HR-TEM images of (G, H) used Cu/CeO2, (I, J) used Cu/CeO2-SMSI-250 and (K, L) used Cu/CeO2-SMSI-400.

The effects of 20CO2:2H2 treatment on crystalline structures and texture properties of Cu/CeO2 were further studied by XRD, Raman spectroscopy and N2 adsorption. The XRD patterns (Figure S1A) indicate that all of the catalysts show the fluorite structure of ceria (JCPDS No. 00-043-1002), with the (11̅1) and (111) reflections of CuO (PDF 00-048-1548) being the most intense. The crystalline size calculated based on the (111) plane of cubic CeO2 (Figure S1B) in Cu/CeO2 is 23.98 nm, which is much larger than that of Cu/CeO2-SMSI-250 (16.66 nm) and Cu/CeO2-SMSI-400 (16.65 nm) (Table S1). Interestingly, the crystalline size of cubic CeO2 (Figure S1C) in Cu/CeO2 is increased to 52.09 nm after the WGS catalytic reaction, while that is slightly increased to 21.15 nm for used Cu/CeO2-SMSI-400 (Table S1). Additionally, large crystal sizes of CuO particles (36.5 nm) are obtained in Cu/CeO2, thus weakening the Cu/CeO2 synergetic interaction. After WGS reaction, these Cu nanoparticles 9 ACS Paragon Plus Environment


are inclined to form large Cu nanoparticles (110.1 nm), thus leading to lower WGS activity. Notably, smaller Cu crystal sizes (29.5 nm) can be obtained over used Cu/CeO2-SMSI-400 in comparison with used Cu/CeO2, thus leading to higher WGS activity. The results further suggest that Cu/CeO2-SMSI-400 exhibits better resistance to Cu and Ce nanoparticles sintering in WGS reaction than Cu/CeO2. To determine the crystal structure of the Cu catalysts in greater details, Raman spectroscopy using 532 nm laser for excitation was employed. The Cu catalysts show a Raman peak (Figure S2) at 441 cm−1 assignable to the F2g symmetrical stretching vibration mode of metaldoped CeO2 of fluorite structure.15 The Raman band can be attributed to the oxygen breathing frequency around the Ce4+ cations, and its position provides information concerning the introduction of foreign cations into the ceria lattice. The F2g peak of the Cu catalysts shifts toward lower value with respect to that of pure ceria (about 464 cm−1), which can be attributed to partial incorporation of Cu into the CeO2 lattice. There is no detection of Raman signals at 292, 340 and 626 cm−1 that are characteristics of CuO phases. The missing of the CuO signals could be due to the formation of Ce-Cu-O solid solutions,16 or the weak CuO signals were overshadowed by the strong CeO2 signals. According to N2 adsorption-desorption measurements (Figure S3), the as-prepared Cu samples are different in BET specific surface, showing a trend of: Cu/CeO2-SMSI-250 (49 m2/g) > Cu/CeO2-SMSI-400 (39 m2/g) > Cu/CeO2 (33 m2/g) (Table S1). Notably, the BET surface area of samples decreases after WGS reaction, which can be ascribed to the grown-up of nanoparticles. The BET surface area of Cu/CeO2-SMSI-400 and Cu/CeO2 is changed from 39 to 33 and 33 to 16 m2/g, respectively. The subtle decrease in the surface area of Cu/CeO2SMSI-400 further verifies that it exhibits better thermal stability in WGS reaction than Cu/CeO2. To investigate the effect of metal-support interaction on reducibility, H2-TPR experiments were conducted. Over Cu/CeO2, a weak peak at 133 °C and a strong one at 173°C ascribable to the reduction of surface Cu2O (as proved by XPS, vide infra) and bulk 10 ACS Paragon Plus Environment

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CuO (as proved by XRD), respectively, were observed, and the lower temperature of the former is considered to be a result of SMSI (Figure 5A).17

Figure 5 (A) H2-TPR profiles, (B) CO-TPR-MS profiles, (C) CO-TPD-MS profiles of asprepared catalysts, and (D) XPS Cu2p spectra of Cu/CeO2, Cu/CeO2-SMSI-250 and Cu/CeO2SMSI-400 after situ treated with 10%H2/Ar at 250 oC for 4 h. The total amount of hydrogen consumption (4.26 µmol/g) is higher than the theoretical value (2.86 µmol/g) for complete reduction of Cu2+ to Cu0, revealing that certain amount of ceria is also reduced. Notably, the reduction peak temperatures of Cu/CeO2-SMSI-250 and Cu/CeO2SMSI-400 are obviously lower than those of Cu/CeO2, indicating higher reducibility of the former two catalysts. Most importantly, the Cu/CeO2-SMSI-250 and Cu/CeO2-SMSI-400 catalysts show a reduction signal at 133 °C much larger than that of Cu/CeO2. The phenomena may be attributed to the fact that Cu/CeO2-SMSI-250 and Cu/CeO2-SMSI-400 are 11 ACS Paragon Plus Environment


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smaller in Cu crystalline size and higher in SMSI than Cu/CeO2. According to the H2-TPR results over the Cu-based catalysts, the main Cu oxidation state of Cu/CeO2, Cu/CeO2-SMSI250 and Cu/CeO2-SMSI-400 is Cu0, which has been confirmed by XPS (vide infra). I It is understandable because the catalysts were in situ reduced in 10%H2/Ar before WGS reaction. CO-TPR-MS was employed to identify the surface oxygen species as well as the reactivity of the catalysts towards CO interaction.18 Meanwhile, the activation of surface hydroxyls which are responsible for the onset of WGS reaction could also be identified.19,

20

As

illustrated in equation (1), CO2 is generated when adsorbed CO reacts with OH groups, CO (ads) + 2OH

CO2 (g) + H2 (g) + O (Latt)

[2]

The CO2 profiles of the CO-TPR experiments are depicted in Figure 5B. The lowtemperature (100−300 °C) evolution of CO2 is associated with the removal of surface hydroxyls. Note that the intensity of the low-temperature CO2 peak of Cu/CeO2-SMSI-400 is larger than that of Cu/CeO2, indicating greater amount of OH on the former (Eq. 1).21 According to CO-TPD-MS results (Figure 5C), CO desorption starts at 50 °C and maximizes at around 82 °C. The areas of the CO desorption peaks of Cu/CeO2-SMSI-250 and Cu/CeO2SMSI-400 are obviously higher than that of Cu/CeO2, confirming larger CO adsorption capacity of the former two catalysts, which is plausibly a result of enhanced SMSI. Figure S4 and Figure 5D show the results of surface analysis using X-ray photoelectron spectroscopy (XPS). According to the Cu2p spectra (Figure S4), the Cu species of fresh Cu/CeO2, Cu/CeO2-SMSI-250 and Cu/CeO2-SMSI-400 are mainly Cu+. Meanwhile, the Cu2p3/2 binding energy of Cu/CeO2 is 933.5 eV. It shifts to around 932.8 eV for Cu/CeO2SMSI-250 and Cu/CeO2-SMSI-400, implying electron enrichment on the Cu atoms. A transfer of electrons from support or oxygen vacancies to metal is a strong indication of improved interfacial contact between Cu and CeO2.22, 23 Interestingly, Cu2p XPS (Figure 5D) studies indicate that metallic Cu as the main Cu species after Cu/CeO2, Cu/CeO2-SMSI-250 and Cu/CeO2-SMSI-400 were situ treated with 10%H2/Ar at 250 oC for 4 h. 12 ACS Paragon Plus Environment

14b

For the Ce3d

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spectra (Figure S4), the peaks labeled U, U′′, U′′′, V, V′′ and V′′′ are characteristics of 3d104f0 electronic configuration corresponding to Ce4+, while the U′ and V′ peaks are indicative of the 3d104f1 electronic configuration of Ce3+.24 The existence of Ce3+ species could cause charge imbalance as well as formation of unsaturated chemical bonds on the surface, thereby giving rise to the enrichment of surface oxygen vacancies. Compared to Cu/CeO2, Cu/CeO2-SMSI250 and Cu/CeO2-SMSI-400 contain more Ce3+ species (Table S2), suggesting the occurrence of the copper oxidation step: Cu+ + Ce4+ → Cu2+ + Ce3+ after 20CO2:2H2 treatment. The deduction matches well with the synergetic mechanism for interaction enhancement between Cu and Ce oxides. Combined with catalytic activity results, it can be found that the presence of more Ce3+ species are beneficial to improve the catalytic activity. Additionally, the main Cu species in as-prepared catalysts are Cu0 and Cu+ due to the catalysts were in situ reduced with 10%H2/Ar at 250 oC for 4 h before the activity test. O1s spectra of the catalysts were also acquired to reveal the type of oxygen species involved in the redox mechanism (Figure S4).25 The peaks at 529.4-529.7 eV can be assigned to lattice oxygen (Olatt),26 while those at 531.4-531.6 eV to adsorbed oxygen species (Oads) as a result of O2 adsorption on oxygen vacancies.27 Compared to those of Cu/CeO2, the O 1s peaks of Cu/CeO2-SMSI-400 are slightly higher in binding energy. On the basis of the XPS and COTPD-MS results, it is inferred that the latter shows higher SMSI than the former. 4. CONCLUSIONS In summary, we have demonstrated a new route to achieve enhanced SMSI between Cu and CeO2. After the 20CO2:2H2 treatment, Cu/CeO2 is covered with HCOn species, and there is strengthening of metal-support interaction. The consequence is improved reducibility and CO adsorption performance of the Cu/CeO2 catalyst; and CO conversion was increased from 47% over Cu/CeO2 to 86% over Cu/CeO2-SMSI-400 at 350 °C. The performance of the Cu/CeO2-SMSI-400 catalyst was found stable in WGS reaction at 400 °C for 15 h. The 13 ACS Paragon Plus Environment


findings reveal a new avenue for the tuning of SMSI for reactivity enhancement of CeO2supported transition metal catalysts. SUPPORTING INFORMATION The Supporting Information (XRD, Raman, N2 physisorption and XPS) is available free of charge on the ACS Publications website. AUTHOR INFORMATION *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National key research and development program (2016YFC0203900, 2016YFC0203902), the National Natural Science Foundation of China (21703037, 21407025), and leading project of Fujian Province (2017H0049). REFERENCES 1. Miao, D.; Goldbach, A.; Xu, H. Platinum/Apatite Water-Gas Shift Catalysts, ACS Catal. 2016, 6, 775−783. 2. Schweitzer, N. M.; Schaidle, J. A.; Ezekoye, Q. K.; Pan, X.; Linic, S.; Thompson, L.T. High Activity Carbide Supported Catalysts for Water Gas Shift, J. Am. Chem. Soc. 2011, 133, 2378–2381. 3. Duke, A. S.; Xie, K.; Brandt, A. J.; Maddumapatabandi, T. D.; Ammal, S. C.; Heyden, A.; Monnier, J. R.; Chen, D. A. Understanding Active Sites in the Water–Gas Shift Reaction for

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