Understanding water-gas shift reaction mechanisms at palladium–ceria interfaces using in situ SERS coupled with online mass spectrometry†

Di-Ye Wei^a, Ge Zhang^a, Hong-Jia Wang^a, Qing-Na Zheng^a, Jing-Hua Tian^b, Hua Zhang *^a and Jian-Feng Li*^abc
aCollege of Materials, State Key Laboratory of Physical Chemistry of Solid Surfaces, MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, iChEM, Fujian Key Laboratory of Advanced Materials, College of Energy, Xiamen University, Xiamen, 361005, China. E-mail: li@xmu.edu.cn; zhanghua@xmu.edu.cn
^bInnovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen 361102, China
^cCollege of Chemistry, Chemical Engineering and Environment, Minnan Normal University, Zhangzhou 363000, China

First published on 6th August 2024

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Hua Zhang

Hua Zhang obtained his bachelor's and PhD degrees from the Department of Chemical and Biochemical Engineering at Xiamen University in 2010 and 2015, respectively. He worked as a postdoctoral researcher at the Department of Chemistry in the same university from 2015 to 2018. He is now a full professor in the College of Materials at Xiamen University. His research interests include heterogeneous catalysis, energy materials, surface-enhanced Raman spectroscopy, and in situ spectroscopic characterization for catalysis.

Introduction

The water-gas shift reaction (WGSR) refers to the process in which CO reacts with H₂O to generate CO₂ and H₂, and it is of great significance in applications such as hydrogen production and the elimination of CO in automobile exhaust.^1–3 The WGSR is an exothermic reaction that is thermodynamically favourable at low temperatures, but it is usually necessary to increase the temperature to accelerate the rate of reaction, which is energy-consuming and environmentally unfriendly. Therefore, the development of WGSR catalysts at low temperatures has very important industrial value.^4–7 Metal-oxide supported catalysts are the most classic and widely used WGSR catalysts, and the metal-oxide interface is often considered as the active centre of this reaction.^8–11 Understanding the reaction process and reaction mechanism at these interfaces is the key to the design and preparation of high-efficiency WGSR catalysts.¹²

With the development of diverse characterization methods, a lot of studies have been devoted to the mechanism study of the WGSR, two of which are most widely accepted: the redox mechanism^13–17 and the associative mechanism.^18–21 Mavrikakis et al. theoretically calculated the energy barriers on the Cu(111) surface during the WGSR, which showed the associative mechanism dominating with *COOH as the intermediate.⁷ Jia et al. found a CeO₂/Cu catalyst followed the dual mechanism and Ce–O was very active in reacting with CO to form oxygen vacancies.¹⁵ Pérez et al. found the WGSR has a greater tendency to follow an associative mechanism on CeO₂/Au(111), and proposed that Ce³⁺ was an effective dissociation site for water.¹¹ Ma et al. discovered unconventional WGSR routes on Au/MoC and Pt/MoC, leading to the significant generation of H₂ detected even at room temperature.^22,23 Although it has been extensively studied, the reaction mechanism of the WGSR is still controversial. This is because the reaction system is too complex, and it is difficult to directly detect the reaction species and products on the surface in real-time.

Rare earth oxide materials have attracted increasing interest in the WGSR, as they display excellent catalytic properties which can be further tuned by rational material design, orbital coupling engineering, and composite regulation.^24–28 For instance, cerium oxide, one of the most typical rare earth oxides, has always been widely investigated in the WGSR due to its variable valence state and rich oxygen release and storage capacity.^29–31 Fu et al. used infrared spectroscopy to determine that the bridge hydroxyl groups on the oxygen vacancies of ceria were the key active species to this reaction.³² Odriozola et al. used in situ X-ray absorption spectroscopy and found that both Au and Pt supported on cerium oxide were in an unoxidized state during the WGSR.³³ Huang et al. found the treatment of cerium oxide with H₂ at room temperature could increase the content of Ce³⁺, due to the dissociation of H₂ on the surface and the generation of Ce–H.³⁴ Wu et al. used neutron scattering to determine the distribution range of bulk and surface Ce–H.³⁵ These studies show that surface cerium oxide has unique catalytic effects and deepen our understanding of the WGSR process. However, the nature of active species on surfaces remains unknown. Therefore, it is particularly important to capture reaction intermediates and reactants at the same time to reveal the reaction mechanism.

Raman spectroscopy can provide fingerprint information on species adsorbed on surfaces. However, the low sensitivity of normal Raman spectroscopy limits its application in surface and interface analysis. The “borrowing” surface-enhanced Raman spectroscopy (SERS) overcomes the problem of low sensitivity of traditional Raman spectroscopy.^36,37 By using the plasmon core as a signal amplifier, the Raman signal of interfacial species on the catalytic shell can be enhanced significantly, making in situ monitoring of the reaction process possible.^38,39 It can not only provide signals of surface adsorbed species, but also shows the dynamic change process of the catalyst structure during the reaction process. Through reasonable design, the detection sensitivity can even reach the single-molecule level, and the model catalytic reaction on single-atom catalysts can also be studied.^40,41 Meanwhile, mass spectrometry (MS) can perform real-time online analysis of reaction products. Combining MS with SERS can help us better understand the catalytic reaction process.

Herein, we developed a “borrowing strategy” by depositing CeO₂ onto the surface of the plasmon metal Au core and modifying Pd through a seed-mediated growth method to prepare the Au@CeO₂–Pd core–shell satellite nanostructure. Through the combination of in situ SERS and online MS, simultaneous monitoring of intermediate species on the surface and reaction products is achieved. It is found that there are two main pathways for the water-gas shift reaction on the Pd–CeO₂ interfaces. CO adsorbed on Pd can either react with the oxygen species formed via water dissociation (the associative mechanism) or the lattice oxygen in CeO₂ (the redox mechanism), with the former having a higher activity. These findings reveal the important role of the Pd–CeO₂ interface in the WGSR at a molecular level.

Experimental section

Chemicals and reagents

Tetrachloroauric(III) acid tetrahydrate (HAuCl₄·4H₂O), sodium citrate (99.5%), palladium chloride (PdCl₂), hydrochloric acid (HCl ∼36–38%), ascorbic acid (99.7%), hexadecyl trimethyl ammonium bromide (CTAB, 99%), ethylene diamine tetraacetic acid (EDTA, 99.5%), ammonium hydroxide (NH₃·H₂O), and cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) were all purchased from Sinopharm Chemical Reagent Co., Ltd or Sinopharm Chemical Reagent Co., Ltd. Silica was purchased from Aladdin. N₂ (99.999%), CO (99.999%), and H₂ (99.999%) were purchased from Xinhang Gas. Milli-Q water (∼18.2 MΩ cm) was used throughout the study.

Synthesis of the core–shell-satellite nanostructures

Catalytic performance tests

SiO₂ was dispersed in 10 mL water, then Au@Pd or Au@CeO₂–Pd was slowly added to the dispersion and stirred. After leaving overnight, the sediments were obtained by centrifugation at 3000 rpm, and then dried at 80 °C overnight to obtain the Au@Pd/SiO₂ or Au@CeO₂–Pd/SiO₂ catalyst. The total Pd loading is 0.5%. 30 mg of the as-prepared Au@Pd/SiO₂ or Au@CeO₂–Pd/SiO₂ catalyst was put into a quartz tube in a fixed bed reactor. The catalyst was pretreated at 20 °C under H₂ conditions for 2 h and the feed gas consisted of 1% CO, 2% H₂O, and N₂ balance. The weight hourly space velocity was 20000 mL g⁻¹ h⁻¹. The reactants and products were analyzed by gas chromatography.

Characterization

The morphology was characterized and elemental mapping performed by using FEI Tecnai F30, F200, and JEOL 2100 microscopes. The Raman experiments were done on an EC-Raman (Xiamen SHINs Technology Co., Ltd.) or a Jobin-Yvon Horiba XplorA confocal Raman system. The MS was an OmniStar GSD 350 (Pfeiffer Vacuum) with a heatable capillary hose up to 200 °C and a mass range from 1 to 300 u. The X-ray photoelectron spectroscopy (XPS) data were collected on an Escalab Xi+ from Thermo Fisher.

Results and discussion

In order to employ in situ SERS to explore WGSR mechanisms on Pd–CeO₂ interfaces, CeO₂ shells are first deposited on Au cores using a seed growth method, and then Pd is modified to obtain the Au@CeO₂–Pd core–shell satellite structure. This structure has rich Pd–CeO₂ interfaces, and through further combination with online MS, the simultaneous detection of intermediate species and reaction products on the catalyst surface is achieved (Fig. 1a). To determine its fine structure, TEM and elemental maps are used to characterize its morphology and element distribution (Fig. 1b and S1†). It can be seen that cerium oxide is decorated on the gold NPs in a non-uniform island-like deposition manner, while Pd is on the outermost layer. The elemental maps clearly show the distribution of gold, palladium, and cerium, proving the successful preparation of the core–shell satellite structure. We further employed HR-TEM to characterize Au@CeO₂–Pd. As shown in Fig. S2,† we can clearly see that CeO₂ and Pd exist in the form of small particles on the surface of the Au core. Meanwhile, no diffraction peaks of CeO₂ are observed in the XRD as shown in Fig. S3.† This may be because the coated CeO₂ is highly dispersed on the Au surfaces and the amount of CeO₂ on the Au surfaces is very small. Based on these results, it can be concluded that the Au@CeO₂–Pd core–shell nanostructures are successfully constructed. The size of the Au core is greater than 50 nm, thus it is inert in catalysis and is mainly used to amplify the Raman signals of species adsorbed at the Pd–CeO₂ interfaces.

Isocyanobenzene (PIC) is further used as a SERS probe molecule to reveal the surface electronic structure of Au@CeO₂–Pd, as the frequency of CN stretching vibration is very sensitive to the change in the local electronic environment, which can be used to characterize the electron transfer process at the catalyst surface. As shown in Fig. 1c, the Raman peak of the PIC molecule on Au is at ∼2190 cm⁻¹, which disappears on Au@Pd, indicating the gold core is fully covered by Pd.^45,46 At the same time, the signal of bridge adsorption of PIC on Pd can be observed at around ∼2040 cm⁻¹. When a rich Pd–CeO₂ interface is formed, the CN stretching band of PIC adsorbed on Au@CeO₂–Pd red-shifts to ∼2030 cm⁻¹. This indicates that the electrons on CeO₂ may be partially transferred to Pd, which will enhance the d–π* back donation and weaken the CN bond energy, resulting in a red shift of the Raman peak.^46,47

Further catalytic performance tests show that the catalytic activity of Au@CeO₂–Pd is significantly improved compared to Au@Pd (Fig. 1d and S4†), proving the pivotal role of the metal-oxide interface in the WGSR. Meanwhile, XPS is applied to analyze the electron transfer of the samples (Fig. 1e and f). There is 34.6% Pd²⁺ in Au@Pd, while in Au@CeO₂–Pd, the Pd²⁺ content drops to 20.0%. The Ce³⁺ content increases from 22.7% in Au@CeO₂ to 32.9% in Au@CeO₂–Pd. These results are well consistent with the SERS results using PIC as the probe molecule, and demonstrate CeO₂ may donate electrons to Pd. These electronic interactions may contribute to the improved WGSR performance at the Pd–CeO₂ interface.

To clarify the reaction process, in situ SERS, combined with online MS, is applied for the study of the reaction mechanism. First, the process of the WGSR on Au@Pd is studied as shown in Fig. 2a–c under the reaction conditions of CO + H₂O. Obvious Raman signals can be observed at ∼364 and 1945 cm⁻¹, which can be assigned to the stretching vibration of Pd–C and CO caused by bridge adsorbed CO on Pd.⁴⁸ We further confirm these assignments through the ¹³CO isotopic labelling experiment (Fig. S5†), where these two peaks have shifted to around ∼354 and 1910 cm⁻¹, respectively, indicating Pd is the main site for CO adsorption. As the temperature gradually increases, the intensities of Pd–C and CO gradually decrease, showing the CO adsorbed on Pd gradually reacts with H₂O. When the temperature reaches ∼220 °C, the obvious signal of product CO₂ (m/z = 44) can be observed in MS. Meanwhile we speculate that Pd may be gradually reduced to a metallic state by CO under the reaction atmosphere since no other intermediate is observed. Then we further conduct a control experiment (Fig. S6†): first CO is introduced, then the feed gas is switched to water and the temperature is raised. It can be seen the absorbed CO is more active towards H₂O, and the CO₂ signal in MS can be captured around ∼160 °C.

Catalytic performance tests have shown the formation of the Pd–CeO₂ metal-oxide interfaces can effectively promote the WGSR. We then carry out the in situ Raman and online MS test on Au@CeO₂–Pd under the conditions of CO and H₂O co-feeding (Fig. 2d–f), and typical Raman signals can be observed at around ∼370, 450 and 1910 cm⁻¹. Compared with Au@Pd, the peak of CO moves to a lower wavenumber region, while Pd–C shifts to a higher region. This shows that the Pd–CeO₂ interface can effectively promote the adsorption and activation of CO, strengthen the Pd–C bond and weaken the CO bond, making it easier for it to participate in the subsequent reaction steps. The MS results show that the CO₂ begins to increase at ∼180 °C. The Raman signal around 450 cm⁻¹ can be attributed to Ce–O,⁴⁹ which shows a trend of first falling and then rising. Ce–O is relatively weak at the beginning because the metal oxide shell is poorly crystallized. As the temperature gradually increases, the intensity of Ce–O begins to gradually rise which is almost completely consistent with CO₂ signals in MS, showing the important role of Ce–O in the WGSR.

There may be two reaction paths on the interface: adsorbed CO can react with oxygen species from H₂O or with lattice Ce–O. In order to distinguish the differences between the two paths, further control experiments were conducted. First, the temperature rises under only CO conditions, and the appearance of CO₂ is not observed until ∼240 °C (Fig. 3a–c). Because there is no water present, the generation of CO₂ may be due to the reaction between CO and surface Ce–O. However, the signal of CO₂ can be observed at ∼180 °C in CO + H₂O (Fig. 2e). These results indicate the direct reaction between Ce–O and CO requires a higher temperature, thus hindering the WGSR via the redox mechanism. Meanwhile, in the absence of water, Ce–O is not generated until 300 °C. Another control experiment is carried out under the condition of first adsorbing CO and then switching to water (Fig. 3d–f). It can be seen that the CO signal is obviously attenuated, and the decrease in its intensity occurs at a lower temperature. Combined with the change in the CO₂ signal in MS, CO₂ signals can be observed at ∼140 °C, indicating the CO adsorbed on the Pd surface reacts with water. The Ce–O signal begins to increase rapidly at ∼180 °C which confirms that water molecules can quickly activate and transform to Ce–O.

To further distinguish the role played by water molecules and lattice oxygen Ce–O in the WGSR, an H₂¹⁸O substitution experiment is performed to study the process of the WGSR on the Pd–CeO₂ interface. The Au@Pd–CeO₂ sample was first treated with H₂¹⁸O, to allow the exchange of ¹⁸O with the lattice oxygen of ceria, which can be verified through the peak of Ce–¹⁸O around 435 cm⁻¹. At the same time, the atmosphere is switched to CO + H₂¹⁶O and the temperature is raised. It can be seen that the peak intensities of surface Pd–C and CO species show a rapid downward trend, while the peak location of Ce–O gradually shifts from 435 cm⁻¹ to ∼455 cm⁻¹ (Fig. 4a and S7†). As for MS (Fig. 4b and c), the MS signal of 44 for C¹⁶O₂ originates from the reaction of CO with another reactant, H₂O, and its intensity increases significantly at around 180 °C, which is basically consistent with our previous spectroscopic results (Fig. 2d). However, the MS signal of 46 or 48, assigned to C¹⁸O¹⁶O or C¹⁸O₂, is generated by the reduction of Ce–¹⁸O by CO, which requires a higher temperature of around 220 °C (Fig. 4c). Therefore, it can be concluded that the WGSR proceeds more efficiently via the reaction between adsorbed CO and oxygen species directly from water, while the kinetics for the CO reaction with the lattice Ce–O species is slower.

It is well known that the activation and reaction of water molecules at the metal-oxide interface are essential for the WGSR, but the specific activation and reaction sites are not yet clear. Therefore, XPS was conducted on Au@Pd–CeO₂ after the WGSR (Fig. S8†). The typical Au 4d peak can be seen since the oxide shell is thin. Pd was reduced after the reaction by CO, which was also consistent with the results of the in situ Raman test that no more oxide or hydroxyl was found on the Pd surface (Fig. 2a). Ce 3d shows that the content of Ce³⁺ gradually decreases from 32.9% to 14.7%, which is also consistent with the Raman spectra of Ce–O peak generation. This shows that adjacent Pd and Ce³⁺ at Pd–CeO₂ interfaces may be the active sites for the WGSR and the abundant oxygen vacancies can provide sufficient sites for the adsorption, activation and reaction of water molecules during the WGSR, which can improve the efficiency of catalytic reactions.¹¹

Based on the above findings, we propose the WGSR may proceed via two different reaction pathways at the Pd–CeO₂ interface, i.e., the redox or associative mechanism. For the associative mechanism, CO first reacts with oxygen species that are generated via the dissociation of H₂O at the Ce³⁺ sites, leading to the formation of formate or carbonate species. These species would further decompose to CO₂ and H₂ (Fig. 4d). For the redox mechanism, CO adsorbed on Pd directly reacts with the lattice Ce–O species nearby to produce CO₂ and an oxygen vacancy, and then H₂O is dissociated at the oxygen vacancy to generate H₂, thus completing the catalytic cycle (Fig. 4e). Our in situ SERS coupled with online mass spectrometry studies demonstrate that the associative mechanism occurs at a much lower temperature than that for the redox mechanism. Moreover, compared with pure Pd surfaces, water can be efficiently dissociated at the Ce³⁺ sites at the Pd–CeO₂ interfaces, thus leading to a much better performance.

Conclusions

In summary, by further developing the “borrowing” SERS strategy, Au@CeO₂–Pd core–shell satellite structure nanoparticles were prepared, and SERS was used to systematically study the reaction process of the WGSR at Pd–CeO₂ interfaces. Through the combination of in situ SERS and online MS, simultaneous real-time detection of surface intermediate species and reaction products has been achieved. The activity and reaction of the active oxygen species Ce–O at the Pd–CeO₂ interface in the WGSR were studied in real time. There are two main paths for the WGSR at the Pd–CeO₂ interfaces: CO adsorbed on Pd can either react with oxygen species formed by water dissociation on oxygen vacancies via the associative mechanism or with Ce–O via the redox mechanism, and the former has a higher activity. This work provides detailed fundamental insights for the mechanism of the WGSR at metal-oxide interfaces and would promote the design of more efficient WGSR catalysts.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Di-Ye Wei: conceptualization, validation, formal analysis, investigation, data curation, writing – original draft. Ge Zhang: formal analysis, investigation, data curation. Hong-Jia Wang: formal analysis, writing – review & editing. Qing-Na Zheng: formal analysis, writing – review & editing. Jing-Hua Tian: formal analysis, writing – review & editing. Hua Zhang: conceptualization, formal analysis, resources, writing – review & editing, supervision, project administration. Jian-Feng Li: conceptualization, formal analysis, resources, writing – review & editing, supervision, project administration.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2023YFE0120300), the National Natural Science Foundation of China (22122205, 22361132532, 22302163, 52171222, and T2293692), Natural Science Foundation of Fujian Province of China (2021J06001), and “111” Project (B17027).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02918f

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