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Abstract

As-prepared amorphous Ni-P alloys are known to be rather inactive for the hydrogenation of hydrocarbons and carbon monoxide. The active state is generated usually after cyclic treatments with O₂ and H₂ at temperatures far below the bulk recrystallization temperature. X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and ion scattering spectroscopy (ISS) were applied to monitor the chemical changes of an amorphous Ni₈₀P₂₀ alloy under conditions close to those used for pretreating technical catalysts. Exposure to O₂ at 403 K resulted in a mixed oxide layer with a thickness of about 3–5 monolayers containing Ni³⁺ ions and P in the formal oxidation state +5. After reduction in H₂ and subsequent O₂ exposure at 513 K, the surface region involving at least 30 monolayers was found to be oxidized to NiO. Both XPS and ISS measurements demonstrated the absence of phosphorus in the oxide layer. After rereduction at 553 K the surface region was observed to consist of Ni₃P which remained unchanged when heating beyond the recrystallization temperature of about 650 K. The active state generated during the final rereduction of the Ni-P catalyst below the bulk recrystallization temperature is therefore formed by the diffusion of P into the Ni overlayer yielding crystalline Ni₃P. The increase in catalytic activity of the initially amorphous catalyst is assumed to be due to the surface restructuring during the redox pretreatment.