Electron spin resonance study of active centers in nickel-tungsten sulfide hydrogenation catalysts

Electron spin resonance absorptions have been found in nickel-tungsten sulfide and tungsten disulfide hydrogenation catalysts. One of these absorptions is identified as being due to surface tungsten ions in a coordination of low symmetry. The intensity of this ESR signal decreases with increasing sulfur content of the catalyst or by adsorption of cyclohexene or benzene on its surface. For a wide variety of tungsten-disulfide based catalysts the hydrogenation activity for benzene is proportional to the intensity of this ESR absorption. It is concluded that the active centers in these catalysts are tungsten ions, probably in the trivalent state.