Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoSx clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H4C4S desorbs molecularly at 190-400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (v = 1 x 10(13)/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H4C4S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H-2, H2S, and mostly alkynes are detected in the gas phase as decomposition products. H4C4S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H4C4S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H-2 and H2S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O-2. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H-2, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S-0, of C4H4S has been determined. At thermal impact energies (E-i = 0.04 eV), So for molecular adsorption amounts to 0.43 +/- 0.03 for a surface temperature of 200 K. S-0 increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S-0(T-s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S-0(T-s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C4H4S, which amounts to (79 +/- 2) kJ/mol and (52 +/- 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C4H4S than are smaller clusters. (C) 2010 Elsevier B.V. All rights reserved.