An in situ study using anomalous wide angle X-ray scattering and X-ray absorption spectroscopy of the binary metal oxide catalytic system SnO2-ZnAl2O4 supported on alumina

A fine structural description of the local order around zinc and tin atoms of a binary metal oxide catalyst, namely SnO2-ZnAl2O4/Al2O3 which can be used as a DeNO(x) catalyst, is achieved through XAS (Xray absorption Spectroscopy) and AWAXS (Anomalous wide angle X-ray scattering). The analysis of the data was supported by ab initio calculations based on the multiple scattering processes for the XAS spectroscopy and ab initio calculations based on the Debye equation in the case of anomalous scattering. We found that the tetrahedral sites occupied by the zinc atoms are not completely filled and that part of the zinc atoms are engaged in a SnO2 like structure. Also, it seems that most of the tin atoms are engaged in tin dioxide clusters. For the set of in situ XAS experiments done at the K Zn edge and K Sn edge, no significant modification of the interatomic distances around each of the two metals versus the reactive gases are measured. Taking into account the previous results obtained on the monoxide metal supported catalyst ZnAl2O4/Al2O3, we can assume thus that only a dramatic lack of occupancy on the metal site favours an incursion of light atoms in the network. This structural property can explain in return the expansion of the crystallographic cell as well as a significant increased of the Debye-Waller factor associated to zinc-zinc pairs.