Abstract
A facile, previously unexplored, method to synthesize bimetallic Pt-Au nanowires (20 nm diameter x 120-170 nm long) on mesoporous FSM-16 (2.7 nm) was fabricated by co-impregnation of H2PtCl6 with HAuCl4 followed by evacuation at 300 K and finally exposure to the CO/H2O gas mixture (60:5 Torr) at 323 K for 1.0 h. On the other hand, spherical monometallic nanoparticles of pure Pt (7.0 nm diameter) and Au (7-26 nm diameter) were synthesized as well, by impregnation, at the same reaction conditions. The catalysts were characterized by in situ FTIR spectroscopy, UV-vis absorption spectroscopy, TEM. TPR and TPCOR. The catalytic activities toward the water-gas shift reaction (WGSR) were also examined under atmospheric pressure and at the margin of 323-373 K. The optical absorption spectra showed a remarkable shift and broadening of Pt-Au surface Plasmon resonance band at 515 nm apart from those of individual analogue emphasizing bimetallic formation. Results from in situ FTIR spectroscopy indicated that incorporation of Au assisted and stabilized the formation of carbonyl clusters of Pt-Au-CO (2084 cm(-1)) and Pt-CO (1888 cm(-1)) inside the host FSM-16. The Pt-Au carbonyl clusters built up at the moment of vanishing the linear carbonyl band of the charged Au (Au+-CO, 2186 cm(-1)) along with a concomitant increase in the reduced gold (Au-0-CO, 2124 cm(-1)) species. TPR profiles showed that the H-2 consumed was higher for Pt/FSM-16 than for Pt-Au/FSM-16 verifying the facile reduction of Pt moieties after addition of Au. The CO adsorption peak maximum, in TPCOR, for Pt/FSM-16 occurred at higher temperature than that of Pt-Au/FSM-16, which exhibited higher amounts of CO2 produced. The relative decrease in CO bindings on bimetallic surface was responsible for increasing the CO oxidation activity mainly through an association mechanism. Accordingly, the activity of Pt-Au/FSM-16 towards WGS showed a marked increase (8-23 times) compared with those of monometallics emphasizing the dependence of this reaction on the electronic defects of the nanowires. A straightforward reduction mechanism was deduced for Pt-Au alloy formation in view of the results obtained. (C) 2010 Elsevier Inc. All rights reserved.