Effect of Catalyst Preparation on Catalytic Activity: V. Chemical Structures on Nickel/Alumina Catalysts

Abstract

In situ X-ray photoelectron spectroscopy (XPS or ESCA) was used to identify the nickel species on Ni/Al 2O 3 catalyst surfaces as a function of nickel weight loading. The ESCA results show that the only nickel species on the reduced catalyst surface is NiAl 2O 4 for low weight loading catalysts (< 1 wt.%) whereas both nickel and NiAl 2O 4 are present on high weight loading catalysts. The carbon monoxide temperature-programmed reduction (CO-TPR) results, after reduction of the catalysts in hydrogen, confirm that each type of nickel species acts as a different reaction center for methane production. The high-temperature methane peak observed only from CO-TPR spectra of low weight loading catalysts is due to NiAl 2O 4; a low-temperature peak, which appears as the weight loading is increased, is due to nickel. Steady-state reaction kinetics for methane production yields activation energies that increase with increasing weight loading of the catalyst. The apparent activation energies for catalysts with a single methane peak in their CO-TPR spectra were normally distributed. The apparent activation energies for catalysts with two methane peaks in their CO-TPR spectra were also normally distributed when the method of the catalysts' preparation was considered in testing the statistical nature of the distribution. This study suggests that the spread in the distribution of the apparent activation energies can be related to the structure of the reaction centers on the catalyst surface. The catalysts with a single methane peak in their CO-TPR spectra have uniformly dispersed NiAl 2O 4 species and result in the lowest spread (±0.2 kcal/mol, 0.84 kJ/mol). Catalysts prepared by incipient wetness with two methane peaks in their CO-TPR spectra have more uniform and larger “particle-like” nickel present on the surface than catalysts prepared by wet impregnation; the result is a spread of ±1.0 kcal/mol (4.2 kJ/mol) in the normal distribution in apparent activation energies. Catalysts prepared by wet impregnation have smaller nickel particle sizes and greater heterogeneity in reactive centers; the spread in the distribution of activation energies of catalysts prepared by this method is ±2.6 kcal/mol (10.9 kJ/mol).