A TEMPERATURE-PROGRAMMED REACTION AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY OF COPRECIPITATED NA-MN-NI CATALYSTS

Temperature-programmed reaction and X-ray photoelectron spectroscopy studies have been employed to investigate coprecipitated Na-Mn-Ni catalysts which are active for higher oxygenate synthesis from syngas. The results were compared to N'/SiO2, a good methanation catalyst. XPS results revealed that the reduction of the air-exposed coprecipitated catalysts at 673 K led to the migration of Na2CO3 to the catalyst surface; CO hydrogenation at 573 K induced the migration of the Ni and Mn back to the catalyst surface. The changes in surface composition with reduction and reaction condtions gives rise to uncertainty in the estimation of the turnover frequency calculated from the number of Ni surface atoms determined by Hz chemisorption. It was observed by TPR experiments that oxygen from absorbed CO readily associates with the surface of the coprecipitated catalysts, possibly oxidizing the surface and causing the surface to restructure. Two pools of carbon are present on the surface of the catalyst during TPR experiments as evidenced by two methane peaks. The methanation peak temperatures for the coprecipitated Na-Mn-Ni catalysts were higher than that of Ni/SiO,; the CO dissociation activity of the coprecipitated catalysts is lower than that of Ni/SiO,. Lower CO dissociation and methanation activities results in a high selectivity for C2+-Oxygenates. © 1992.