Catalysts surface poisoning by arsenic; mechanism of triphenylarsine interaction with alumina supported nickel

Hydrogenolysis of triphenylarsine (AsPh₃) on alumina supported nickel (Ni/Al₂O₃) has been studied as model reaction for metallic catalyst poisoning. The hydrogenolysis of AsPh₃ on Ni/Al₂O₃ occurs at temperature ranging from 303 to 443 K under 12 bars of hydrogen and in n-heptane solution. It has been followed by kinetics analysis of the AsPh₃ consumption and Benzene and cylohexane evolution as well as XRD measurements of the metallic and intermetallic phase(s). At 303 K, the reaction proceeds selectively on the nickel surface, by stepwise hydrogenolysis of the As-Ph bonds, with final formation of partially decomposed Ni_s[AsPh_x]_y fragment grafted on the nickel surface. The chemical structure of the fragment (x value) does not depend on the metallic particle size, while the coverage of the nickel surface (y value) strongly decreases when the particle size of nickel increases. Increasing the temperature leads to complete hydrogenolysis of the As-Ph bonds and formation of a well characterized NiAs alloy (XRD). The mechanism of alloy formation is complex: Further hydrogenolysis of the Ni_s[AsPh_x]_y fragments leads progressively to the formation of arsenic «adatoms» which migrate quickly inside the nickel particles to form amorphous intermetallic phases (XRD) such as NiAs_0.45. The reaction then proceeds slowly up to the formation of the crystalline NiAs nickeline phase (XRD). The rate of benzene hydrogenation on the various solids obtained by reaction of AsPh₃ on Ni/Al₂O₃ was used as catalyst performance evaluation. It was found that benzene hydrogenation is already completely inhibited when the NiAs_0.45 intermetallic phase is formed.