On the contribution of X-ray absorption spectroscopy to explore structure and activity relations of Pt/ZrO2 catalysts for CO2/CH4 reforming

Abstract

X-ray absorption spectroscopy (XAS) has proven to be a very useful technique in characterizing metal-based catalysts exposed to extreme operating conditions. The technique allows in situ evaluation of structural parameters (XAFS) and electronic properties (XANES). The elucidation of the nature and state of Pt-based catalysts in dry reforming of methane with carbon dioxide is presented as case study to show the contribution and potential of XAS to explore property/performance relationships for heterogeneous catalysts. Pt/ZrO2 is an active and stable catalyst for the reaction between CH4 and CO2 to synthesis gas (H2/CO). The activity and stability of the catalyst is strongly influenced by the catalyst pretreatment (calcination/reduction). The combination of hydrogen chemisorption, IR spectroscopy, XPS and XAS is shown to be suitable to track the changes of the state of the catalyst. In particular, it will be demonstrated, how XAFS helped to correctly attribute variations in the chemisorptive properties of Pt/ZrO2 after severe temperature treatment to partial and reversible decoration of the small Pt particles with fragments of the oxide support. In situ tracking of the reduction of the catalysts by XANES additionally helped to semiquantitatively assess the partial reduction of the ZrO2. Finally, XANES helped to demonstrate that CO2 exposure under these severe conditions did not lead to detectable levels of surface oxidation of Pt. Based on XANES, IR spectroscopy and kinetic measurements it is concluded that in dry reforming activation of methane occurs on Pt, while CO2 is activated on the support and the two entities react at the metal–support interface.