Electronic properties of dispersed copper oxide entities interacting with ceria: near-ambient XPS analysis of the reduction under CO

Abstract

Catalysts combining copper and cerium oxides are an economically interesting alternative to noble metal catalysts for various processes involved during production-purification of hydrogen generated from hydrocarbons or biomass feedstocks. In particular, they show promising characteristics for processes involving CO oxidation like the water-gas shift reaction (WGS) or preferential oxidation of CO (CO-PROX). Their catalytic properties for such processes depend strongly on the characteristics of the interface formed between the two oxide components [1,2]. However, direct information on interfacial properties as well as about those of well dispersed copper oxide nanoparticles interacting with CeO2 in powder catalysts is not generally available due to the important difficulties for the characterization of such nanosized generally amorphous entities. The present contribution explores these issues by means of near-ambient XPS spectroscopy complemented with theoretical DFT analysis. Two samples of copper oxide dispersed on two different ceria supports (in the form of nanospheres ±NS- and nanocubes ±NC-, respectively), which allow comparison between well differentiated situations in terms of degree of dispersion and interfacial morphology, are employed for this purpose. Direct evidence of reduced interfacial copper entities, considered as active sites for CO oxidation in this type of catalysts [1,2], as well as significant details of the electronic properties of the dispersed partially reduced copper oxide entities are provided by detailed analysis of the results obtained. Details of the preparation and multitechnique (XRD, HREM, Raman, XPS, EPR, H2-TPR, SBET) characterization of the two catalysts can be found elsewhere [3]. The catalysts subjected to treatment under diluted CO up to 300 oC are examined by XPS in gas environment (100 Pa) obtained at ISISS station of the BESSY II synchrotron in Berlin. Figure 1 provides an example of the results obtained. New XPS Cu 2p (at 930.1 eV) and Auger L3M45M45 signals (at ca. 913 eV) specific of interfacial Cu+ entities are identified on the basis of theoretical analysis and consideration of respective amount and characteristics of the interfacial sites present in each sample [3]. The evolution of the chemical state of the various copper species as a function of the reduction temperature under CO will be shown while information from other spectral regions, providing interesting hints on the characteristics of the carbonyl species formed under CO, will be also presented.