Metal oxides (Al(2)O(3), SiO(2), MgO)-supported Au cluster catalysts prepared by colloid deposition method and well established Au/TiO(2) prepared by deposition-precipitation method were tested for the selective reduction of nitrocyclohexane into cyclohexanone oxime. The activity and selectivity depended strongly on the size of Au and support material. Au/Al(2)O(3) with smaller Au particle size (2.5 nm) was the most effective catalyst, exhibiting a high cyclohexanone oxime yield (86%) under mild condition (0.6 MPa H(2), 100 degrees C). To study the origin of support and size effects, in situ FTIR experiments for OH/D(2) exchange reaction and nitrobenzene adsorption combined with poisoning experiments using pyridine and acetic acid were carried out. It is suggested that cooperation of coordinatively unsaturated Au atoms and the acid-base pair site (Al(delta+)-O(delta-) site) plays an important role in H(2) dissociation step. Al(delta+)-O(delta-) site also acts as the adsorption site of the nitro group. (C) 2011 Elsevier B.V. All rights reserved.