Correlation between catalytic activity and support reducibility in the CO2 reforming of methane over Pt/CexZr1-xO2 catalysts

We have investigated the relationship between the reducibility of the support and the catalytic activity of supported Pt on a series of catalysts supported on ceria-zirconia mixed oxides. The supports were prepared by co-precipitation with varying Ce/Zr ratios. X-ray diffraction analysis indicated that, 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 photoelectron spectroscopy (XPS). After reduction at 773 K in hydrogen the fraction of reduced cerium (i.e. Ce3+) was found to vary with the Ce content, exhibiting a maximum at a composition Ce0.5Zr0.5O2 A good correlation was found between the reducibility and the catalytic activity. 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 function of Ce content in the support and that maximum occurred at the composition that exhibited the maximum reducibility. The H-2/CO product ratio was also 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 combining temperature-programmed oxidation (TPO) studies with (XPS). The TPO profiles of all the spent samples revealed two oxidation peaks, one in the low-temperature 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 unpromoted catalysts, while the peak at low temperature is more prominent in the Pt/CexZr1-xO2 catalysts. XPS exhibits three types of carbon with different binding energies on the spent catalysts, two of them are two forms of coke, and the third one is due to carbonates. However, all of the peaks decreased after an oxidation at intermediate temperatures (i.e. 723 K). Therefore, it appears that the different peaks observed in TPO are not due to different forms of carbon, but rather to different locations on the catalyst surface. The amount of carbonates on the spent catalysts increased with the Ce content, but the correlation between carbonate concentration and activity was not as good as that between reducibility of the support and activity. (C) 2001 Elsevier Science B.V. All rights reserved.