Oxidative addition of dihydrogen as the key step of the active center formation in the HDS sulfide bimetallic catalysts: ab initio MO/MP2 study

The electronic structure of Ni in the sulfide bimetallic species (SBMS), wh ich is the active component of the sulfide HDS catalysts, is studied with t he ab initio molecular orbital calculations. In the previous paper [I.I. Za kharov, A.N. Startsev, G.M. Zhidomirov, J. Mol. Catal. 119 (1997) 437], we have shown that the d(8) Ni(II) electronic state in the SBMS composition ca nnot be active in HDS reaction because of the lack of possibility to coordi nate S-containing molecule. Therefore, this paper deals with the study of t he possibility to stabilize d(6) electron configuration with the formal Ni( IV) oxidation state. With this in mind, the reaction of oxidative addition of dihydrogen to square-planar complex Ni(II)Cl-2(PH3)(2) has been studied, which allowed to predict a stabilization of the octahedral complex Ni(IV)H 2Cl2(PH3)(2) with d(6) configuration. This allows us to assume a possibilit y of an oxidative adsorption of dihydrogen to the Ni atom entering the SBMS composition. Ab initio calculations have shown that such type of oxidative addition is thermodynamically favorable resulting in stabilization of the Ni(IV) d(6) electronic state. Consequently, the dihydrogen molecule is assu med to dissociate on the Ni atom resulting in the formation of 'surface' H- s and 'occluded' H-o hydrogen, which is located under the Ni atom in the ce nter of the trigonal sulfur prism. The structure of the active centers is o ptimized and the stretching modes of the hydrogen atoms are calculated, whi ch appear to be close to the literature data. The H2S adsorption on the act ive center was also investigated and ii. was shown that the hydrogen disulf ide molecule benefits to stabilization of the active Ni(IV) d(6) state. The conclusion is drawn that the deciding factor in the formation of the activ e centers of sulfide HDS catalysts is the 'occluded' hydrogen. (C) 1999 Els evier Science B.V. All rights reserved.