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 (ν = 1 × 1013/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, H2, 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 H2 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 O2. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H2, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S0, of C4H4S has been determined. At thermal impact energies (Ei = 0.04 eV), S0 for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S0 increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S0(Ts) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S0(Ts) 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.