XPS and DRIFTS operando studies of an inverse CeO₂/CuO WGS catalyst: deactivating role of interfacial carbonates in redox activity

Abstract

An inverse CeO₂/CuO catalyst has been examined by XPS and DRIFTS spectroscopies under reactant mixtures relevant to the low temperature WGS process. It is shown that the presence of H₂O in the reactant mixture promotes catalyst reduction by facilitating the decomposition of interfacial carbonate species which hinder the redox processes leading to generation of the species catalytically active in WGS.

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The water-gas shift (WGS; CO + H₂O → CO₂ + H₂) is an important reaction for hydrogen production from hydrocarbons or biomass since it allows decreasing/upgrading the CO/H₂ concentrations in the H₂-rich stream resulting from reforming. Main challenges in this field are related to mobile application of fuel cells (PEMFC in particular) for which WGS catalysts based on Cu–ZnO industrially employed for stationary hydrogen production are not valid mainly because of their low tolerance to air and consequent loss of stability during start-up–shut-down cycles.¹ In this sense, ceria-based catalysts appear to show important potential for such mobile application.^1,2 Among them, formulations combining copper and ceria appear most interesting from an economical point of view in comparison with those based on noble metals.¹ In turn, inverse configurations of them (i.e. those in which ceria is supported on copper oxide instead of acting as a catalyst support) could apparently display enhanced performance with respect to the traditional direct configuration.^3,4 WGS catalysts based on copper–ceria combinations have been proposed to operate, at least at relatively high temperature above 573 K, through a redox mechanism by which CO and H₂O can act as a reductant and an oxidant of ceria, respectively, in the presence of active metallic copper.^4,5 However, little is known about the behaviour of this type of catalysts at lower temperature, most relevant to the catalyst start-up period.

In this context, an inverse CeO₂/CuO catalyst prepared by a microemulsion-based method (see ESI†) has been examined separately by XPS and DRIFTS (with online MS gas monitoring) under various gas mixtures relevant to the WGS reaction (see ESI†). As will be shown by the experiments here, under these conditions water adsorption acts favouring the progress of the reduction of the catalyst to reach its most active state, an effect that is associated with displacement of surface carbonate groups which block that reduction process. Fig. 1 displays the evolution of copper and cerium related features during the XPS experiments. The catalyst exhibits a partially reduced state for the two components under CO at 473 K. Thus, the binding energy (BE) value of the Cu 2p_3/2 photoelectron along with the practical absence of satellite peaks in the 940–945 eV zone and the kinetic energy observed for the corresponding Auger electron evidence that copper is mostly in a Cu⁺ state;⁶ in turn, the spectrum in the Ce 3d BE zone is characteristic of a partially reduced state in view of the presence of contributions from Ce³⁺ and Ce⁴⁺.^3,7 Unexpectedly, if a redox mechanism were assumed, subsequent introduction of H₂O in the reactant mixture at the same temperature does not produce oxidation of the catalyst but rather it increases the reduction degree of both catalysts components, according to the appearance of new copper species (ascribable to Cu⁰ in view of the features in the Cu L₃VV Auger electron spectrum and of the absence of the mentioned satellite peaks in the 940–945 eV Cu 2p BE zone),^3,6 as well as the small increase in the characteristic Ce³⁺ features of the Ce 3d spectrum. Finally, the reduction degree of the two components (particularly of copper for which metallic copper becomes predominant) increases upon increasing the temperature up to 523 K under the CO + H₂O reactant mixture, which is in line with previous XRD results on catalysts of this type.⁴ It must be noted for this analysis that the overall intensity of spectra in the Ce 3d and Cu 2p regions may change in principle as a consequence of the balance between the following factors: (i) nature of the reactant mixture; i.e. the presence of water induces a higher photoelectron screening leading to decrease in the intensity (compare spectra (a) and (b) in Fig. 1); (ii) temperature; since the total gas pressure is kept constant (as explained in ESI†), a temperature increase induces a decrease in the gas density and a consequent decrease in the photoelectron screening upon interaction with the gaseous atmosphere (compare spectra (b) and (c) in the Ce 3d region); and (iii) chemical state of the catalyst components; this affects basically copper (which apparently suffers the strongest redox change) since in principle the mean free path of photoelectrons is appreciably lower for metallic copper than for Cu₂O,⁸ this being probably the main factor producing the decrease in intensity in the Cu 2p region when comparing corresponding spectra (b) and (c) in Fig. 1. Taking such general aspects into account for the analysis of the redox changes undergone by cerium as a function of the treatment performed, it must be noted the increase of the v′ peak with respect to v′′ or v ones, revealing a gradual cerium reduction when going from spectrum (a) to (b) and then (c); it must be considered in this respect that the three mentioned Ce 3d peaks correspond to photoelectrons with close kinetic energy so that changes in attenuation by the gas phase, due to concentration changes in the latter, will be minimal (which would not be the case if one had to compare these peaks with peak u′′′ typically used for this type of redox analysis in spectra recorded under high vacuum conditions).⁷

Fig. 1 Cu 2p and Ce 3d XPS spectra, and corresponding Cu L₃VV Auger spectra (characteristic features corresponding to the different observed valence states of copper and cerium are indicated),^3,6,7 during treatments of CeO₂/CuO catalyst under CO at 473 K (a); subsequent H₂O addition to the reactant mixture at 473 K (b); and subsequent heating at 523 K under a CO + H₂O reactant mixture (c).

The redox changes observed by XPS are confirmed by Cu L edge XANES spectra taken in the same experiments (Fig. S3 in the ESI†). These show, besides a small Cu²⁺ contribution (detected by XANES due to the larger probing depth of the technique and evidencing the presence of CuO in deeper regions) which decreases with successive treatments, a major Cu⁺ feature in the sample under CO at 473 K and the emergence, after H₂O addition at the same temperature, of new features at positions typical of Cu metal arising from scattering of ejected electrons by Cu neighbours and developed further after heating at 523 K.^9,10

Thus, H₂O apparently acts as a reductant under the examined conditions. However, H₂O interaction with oxide surfaces is known to proceed generally as a pure acid–base process in principle not involving redox changes in the catalyst.¹¹ Even though a recent report points towards surface cerium reduction induced by H₂O dissociation over partially reduced ceria,¹² this must be understood at most as a consequence of charge redistribution (i.e. bulk → surface electron transfer) within the oxide in the presence of a hydroxyl surface layer. In any case, no such surface redox changes are expected over the copper component, according to previous studies on Cu₂O.¹³ One possibility to explain the observed results can be related to the fact that H₂ produced as a consequence of WGS reaction acts as a reductant under the employed conditions. However, CO is known to be a much more powerful reductant than H₂ over this type of catalysts,^14,15 so that this explanation would imply a dominance of kinetic over thermodynamic factors to determine the course of the reduction reaction, which would in any case be inconsistent with kinetic results.^14,15

Then, what could be the explanation for the observed reductions? Careful inspection of gases evolution during the XPS experiments (Fig. 2) shows an absence of correlation between the rates of CO consumption and CO₂ production occurring upon introduction of H₂O in the reactant mixture at 473 K. Thus, a sharp maximum in CO₂ production is observed about 2 min after H₂O introduction while CO shows an initial transient with much longer time constant. CO₂ production should occur simultaneously to CO consumption if it results from either catalysts reduction or WGS activity (this latter being evidenced to take place upon H₂O introduction by increases in H₂ and CO₂ contributions as well as apparent CO decrease; note that mismatch in H₂ production/CO consumption evolution profile shapes can account for CO employed for the observed catalyst reduction). Therefore, the fact that an abnormally higher CO₂ amount is produced immediately after H₂O introduction in the reactant mixture indicates that CO₂ desorption from species already present in the catalyst is taking place. This strongly points towards some type of carbonate species which becomes destabilized and desorbs as CO₂ in the presence of H₂O. Indeed, comparative analysis of the C 1s BE zone during such experiments apparently supports such hypothesis (see Fig. S4, ESI†).

Fig. 2 Gases evolution during XPS experiments shown in Fig. 1. Note the sample was treated under diluted CO up to 25 min at which time H₂O is introduced in the reactant mixture and finally the sample was heated up to 523 K under CO + H₂O mixture. The bottom graphic shows the detail of CO and CO₂ evolutions upon introduction of H₂O into the CO-containing reactant mixture at 473 K.

In order to confirm this process, the catalyst was examined by DRIFTS under similar conditions. Different bands due to hydroxyls, carbonate- or formate-type species as well as a Cu⁺-carbonyl are observed after treating the sample under CO at 473 K and subsequent introduction of H₂O into the reactant mixture at the same temperature (Fig. 3). Main changes produced upon H₂O introduction are related to an increase of a band in the 3400–3000 cm⁻¹ region, due to associated hydroxyl species (consistent with occurrence of H₂O dissociation and WGS activity, in accordance with gases evolution shown in ESI†, Fig. S5),^1,5 a decrease and slight red shift (from 2120 to 2116 cm⁻¹) of the Cu⁺-carbonyl band (the decrease is consistent with formation of metallic copper, in agreement with XPS results in Fig. 1, taking into account that metallic copper carbonyls are not expected to be stable at 473 K)¹⁶ and, in agreement with the mentioned hypothesis, a decrease of carbonate species (Fig. 3). According to the corresponding difference spectrum (Fig. 3), carbonates which become destabilized upon H₂O introduction are mainly those yielding bands at ca. 1475 and 1386 cm⁻¹ (accompanied by blue- and red-displaced shoulders, respectively), attributable to asymmetric and symmetric CO₃²⁻ stretching modes, respectively; this carbonate would be of monodentate type (as classically attributed;^17,18 note however that some controversy exists with respect to the specific coordination configuration of this type of species, attribution to polydentate species being probably more correct).^19–21 Additionally, a small amount of bidentate (or tridentate)²¹ carbonates appears also involved (bands at ca. 1600 and 1295 cm⁻¹).^19,20 In line with the occurrence of such carbonate decomposition and similarly to the observation made during the XPS experiments (Fig. 2), a small additional CO₂ desorption is produced in the DRIFTS experiments immediately after H₂O introduction (Fig. 4).

Fig. 3 Top: DRIFTS spectra of the CeO₂–CuO catalyst either under diluted CO or CO + H₂O mixture at 473 K. Middle: detail of the zone corresponding to C–O carbonate stretching modes. Bottom: difference spectrum between those shown in middle graphic.

Fig. 4 Evolutions of CO and CO₂ concentrations during DRIFTS experiments shown in Fig. 3 and difference between them.

The thus observed decomposition of the mentioned type of carbonates in the presence of H₂O is in agreement with earlier report by Shido and Iwasawa in a study of WGS processes over CeO₂ who pointed out that “the decomposition of the unidentate carbonates to CO₂ was promoted by coadsorbed water”.¹⁸ More recently, Hilaire et al. also addressed this issue in their study of a Pd/CeO₂ WGS catalyst.²² They proposed that reoxidation of the catalyst facilitates carbonate decomposition upon its interaction with water. In this sense, the main novelty here is then the simultaneous occurrence of catalyst reduction upon interaction with water, which indicates that such carbonates could be blocking the progress of catalyst reduction. Furthermore, such reduction apparently affects the two catalysts components, i.e. it appears as an interfacial process upon which the generation of the WGS active species, attributed in previous works to the combination of metallic copper and oxygen vacancies in partially reduced ceria,^3–5 takes place. This reinforces the idea of an important (deactivating) catalytic role of carbonate species in ceria-based catalysts for processes involved in hydrogen production like the WGS reaction or the preferential oxidation of CO, as noted in previous works on catalysts of this type or other ceria-based systems.^23–25 On the other hand, concerning the mechanism by which water destabilizes the mentioned carbonate species, it is tempting to propose, in view of results in Fig. 3, a displacement of carbonates by adsorbed water followed by partial dissociation of the latter to give H-bonded hydroxyls. The higher mobility of the protons involved (as compared to carbonates) could then facilitate the movement of electrons associated with surface reduction. It is not however feasible making an adequate quantification with the results obtained which could provide information in that sense. On the other hand, it must be noted that hydroxyl species could alternatively originate by other routes related to the WGS activity and not necessarily linked to such an exchange mechanism.¹ Further experiments are certainly required to get hints into such a mechanism.

Acknowledgements

This work was funded by Ministerio de Ciencia e Innovación (Plan Nacional Project CTQ2009-14527) and Comunidad de Madrid (Project DIVERCEL, Ref.: S2009/ENE-1475). The help provided by Dr M. Hävecker and the rest of the staff of the ISISS station in BESSY II during XPS measurements is gratefully acknowledged. A.L.C. and M.M. acknowledge PhD grants from the CSIC JAE program and Ministerio de Ciencia e Innovación FPI program, respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section; characterization results and general physico-chemical properties including WGS activity in a tubular catalytic reactor; C 1s XPS spectra; Cu L XANES spectra; gases evolution during operando DRIFTS experiments. See DOI: 10.1039/c2cy20399e

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