Enhancement by praseodymium addition of catalytic activity of nickel supported on cerium-zirconium oxide in methane steam reforming

The performances in methane steam reforming of Ni catalysts supported on (Ce_0.75Zr_0.25)O_2-y (CZ) and (Ce _0.75/_1.025Zr_0.25/_1.025-Pr _0.025/_1.025)O_2-y (CZP), which were fired at various temperatures before Ni deposition, were studied. The catalytic activities of the Ni/CZP catalysts were several per cent higher than those of the Ni/CZ catalysts in terms of both CH₄ conversion and H₂ yield. Distinct catalytic activities were observed, even after oxidation, for Ni/CZP with supports pre-fired at 1073 and 1273 K. These observations are discussed in terms of the structural characteristics of the catalysts. The X-ray diffraction profiles and Raman spectra indicated that CZP was a mixture of two phases, i.e., a Pr-rich λ phase and a Ce-rich cubic phase. The higher catalytic performances of the Ni/CZP catalysts than of the Ni/CZ catalysts and the retention of catalytic activity even after oxidation were attributed to a strained but coherent interface between the λ phase and the cubic phase, at which oxygen vacancies were easily formed. The Ni/CZ catalyst with the CZ support pre-fired at 1273K also showed distinct catalytic activity even after oxidation. The activity of Ni/CZ after oxidation was also attributable to an active interface between a Zr-rich tetragonal phase and a Ce-rich cubic phase, similar to the case of the Ni/CZP catalyst.