Reaction of H2 and H2S with CoMoO4 and NiMoO4:: TPR, XANES, time-resolved XRD, and molecular-orbital studies

The combination of two metals in an oxide matrix can produce materials with novel physical and chemical properties. The reactivity of a series of cobalt and nickel molybdates (alpha-AMoO(4), beta-AMoO(4), and AMoO(4) nH(2)O; A = Co or Ni) toward H-2 and H2S was examined using temperature programmed reduction (TPR), synchrotron-based X-ray powder diffraction (XRD), and X-ray absorption near-edge spectroscopy (XANES). In general, the cobalt and nickel molybdates an more reactive toward H-2 and easier to reduce than pure molybdenum oxides: MoO2 < MoO3 < CoMoO4 < NiMoO4. The interaction of H-2 With surfaces of alpha-NiMoO4, alpha-CoMoO4, and alpha-MoO3 was investigated using ab initio SCF calculations and cluster models. The mixed-metal oxides are easier to reduce due to the combination of two factors. First, it is easier to adsorb and dissociate H-2 on Ni or Co sites than on Mo sites of an oxide. And second, as a result of differences in the strength of the metal-oxygen bonds, it is easier to remove oxygen as water from the nickel and cobalt molybdates than from MoO3 or MoO2. The extra reactivity that the Co and Ni atoms provide also makes the rate of sulfidation of the cobalt and nickel molybdates faster than that of purr molybdenum oxides. For the adsorption of H2S, HS, and S on alpha-NiMoO4 and alpha-MoO3 clusters, the results of ab initio SCF calculations show bigger bonding energies on the Ni sites than on the Mo sites. In these systems, the oxidation state of the Ni atoms is substantially lower (i.e., larger electron density) than that of the Mo atoms, favoring the formation of Ni --> SH and Ni --> S dative bonds. The behavior of the cobalt and nickel molybdates is a very good example of how one can enhance the chemical activity of an oxide (MoO3) by adding a second metal cation to the system. Results of time-resolved XRD and XANES indicate that the reduced AMoO(4) compounds can be regenerated by reaction with O-2 at high temperatures (350-450 degrees C). A similar procedure (S-a + O-2,O-gas --> SO2,gas) can be used to remove most of the sulfur from the sulfided oxides.