XPS study of mixed cerium–terbium orthophosphate catalysts

Monazite structure‐type solid solutions of cerium and terbium orthophosphates Ce 1− x Tb x PO 4 , with 0 < x < 1, were prepared by coprecipitation followed by calcination at 800°C for 2 h. They were used as catalysts for the oxidative dehydrogenation of ethylbenzene. The compounds Ce 0.9 Tb 0.1 PO 4 and Ce 0.7 Tb 0.3 PO 4 , which give the maximum yield of styrene, were studied by XPS before and after catalysis. The binding energies were determined with carbon contamination as a reference (level 1s). Quantitative analysis was performed by using the limit compounds CePO 4 and TbPO 4 as standards. The surface composition before catalysis is slightly different from the bulk composition, whereas no significant variation of binding energies is observed. After catalysis, an important broadening of the peaks and a decrease of the (Ce + Tb)/P ratio appears. The decomposition of the P 2p and O 1s peaks allows us to show the formation of pyrophosphate at the catalyst surface. The formation of this phase during the catalytic reaction is connected to the mechanism of the surface reaction.