Magnesia-Supported Nickel Catalysts: II. Surface Properties and Reactivity in Methane Steam Reforming

Abstract

Surface properties and reactivity of Ni/MgO catalysts, air-calcined at temperatures ( T C) in the range 400-800°C, have been evaluated by TEM, IR spectroscopy of adsorbed CO, and catalytic measurements in methane steam reforming. The effects of calcination and reduction temperature on the Ni particle size distribution (PSD) provide evidence of the singular structure properties of the Ni/MgO system. The "broad" PSD of samples calcined at T C ≤ 600°C well accounts for the structure model previously described (Arena, F., Horrell, B. A., Cocke. D. L., Parmaliana. A., and Giordano. N., J. Catal. 132, 58 (1991)). IR spectroscopy shows both the formation of mono- and polycarbonyls and the occurrence of the CO disproportionation reaction (2 CO ⇄ CO 2 + C). The higher reactivity towards CO of more dispersed systems has been attributed to the "defective nature" of small Ni crystallites. Catalytic measurements ( T R = 625°C: GHSV = 150,000 h −1) reveal the superior activity of the system calcined at T C = 400°C, notwithstanding that a comparable stability for the systems calcined up to 600°C has been experienced. At higher T C (>600°C) the catalytic pattern, both in terms of activity and resistance to coking, is negatively affected by the formation of the "bulk" NiO-MgO solid solution. The peculiar influence of the mean Ni particle size (20 ≤ d S ≤ 1400 Å) on the turnover frequency (TOF, s −1). resulting in a volcano-shape relationship with the maximum centred at 90-130 Å, has been explained by invoking a structure-sensitive character of the methane stream-reforming reaction on highly dispersed Ni/MgO catalysts.