Correlation between catalytic activity and support reducibility in the CO2reforming of methane over Pt/CexZr1-xO2 catalysts

We have investigated the relationship between the reducibility of the suppo rt and the catalytic activity of supported Pt on a series of catalysts supp orted on ceria-zirconia mixed oxides. The supports were prepared by co-prec ipitation with varying Ce/Zr ratios. X-ray diffraction analysis indicated t hat, depending on the Ce/Zr ratio, solid solutions of cubic CexZr1-xO2 can be formed. The reducibility of the supports was determined by X-ray photoel ectron spectroscopy (XPS). After reduction at 773 K in hydrogen the fractio n of reduced cerium (i.e. Ce3+) was found to vary with the Ce content, exhi biting a maximum at a composition Ce0.5Zr0.5O2 A good correlation was found between the reducibility and the catalytic act ivity. It was found that the conversion of methane and CO2 obtained on the different catalysts after 22 h on stream went through a maximum as a functi on of Ce content in the support and that maximum occurred at the compositio n that exhibited the maximum reducibility. The H-2/CO product ratio was als o a function of the support composition, also presenting a maximum for the Pt/Ce0.5Zr0.5O2 catalyst. The amount and nature of carbonaceous deposits were investigated by combini ng temperature-programmed oxidation (TPO) studies with (XPS). The TPO profi les of all the spent samples revealed two oxidation peaks, one in the low-t emperature region, 623-723 K, and the other in the high-temperature region, 873-973 K. The peak in the high-temperature region is dominant in the unpr omoted catalysts, while the peak at low temperature is more prominent in th e Pt/CexZr1-xO2 catalysts. XPS exhibits three types of carbon with differen t binding energies on the spent catalysts, two of them are two forms of cok e, and the third one is due to carbonates. However, all of the peaks decrea sed after an oxidation at intermediate temperatures (i.e. 723 K). Therefore , it appears that the different peaks observed in TPO are not due to differ ent forms of carbon, but rather to different locations on the catalyst surf ace. The amount of carbonates on the spent catalysts increased with the Ce content, but the correlation between carbonate concentration and activity w as not as good as that between reducibility of the support and activity. (C ) 2001 Elsevier Science B.V. All rights reserved.