Infrared characterization of MoOx/ZrO2 catalysts: The reducibility of Mo(VI) species by thermal treatment in H-2

MoOx/ZrO2 samples prepared by different methods, were heated in O-2 and sub sequently reduced in H-2 at increasing temperature. On oxidized samples wit h Mo-content up to 5 atoms nm(-2), prepared by adsorption or impregnation, low and high nuclearity Mo-VI surface species were present, in relative amo unts depending only on the Mo-content. On oxidized samples with Mo-content > 5 atoms nm(-2) prepared by adsorption or impregnation and on all samples prepared by mechanical mixture, MoO3 and ZrMo2O8 were also revealed. FT-IR results showed that the sample behaviour under the reduction was controlled mainly by the nuclearity of the molybdenum species. From the analysis of t he IR v(Mo-VI,Mo-V,Mo-IV=O) modes we could evaluate the nature and the rela tive amounts of the different Mo species present at each reduction stage. A fraction of low nuclearity Mo-VI species was reduced to Mo-V, which we wer e unable to reduce further. In contrast, high nuclearity Mo-VI species gave Mo-IV after reduction at 623 K, and further reduced to Mo-0 at 773 K. At 7 73 K, MoO3 and ZrMo2O8 were almost completely reduced to Mo-0. To further d efine the nature of the Mo species formed by reduction, CO and NO were used as probe molecules. MoV was not capable of coordinating CO and it was exte nsively re-oxidized by NO, whereas Mo-IV gave dinitrosyl and polycarbonyl s pecies upon NO and CO adsorption, respectively. (C) 1999 Elsevier Science B .V. All rights reserved.