In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn underpotential deposition (UPD) on Ni surface from 0.2 M HClO 4 solution containing 10 −3 M Sn 2+ with relation to the inhibition effect of Sn on aqueous corrosion of Ni. The periodical emersion method under potentiostatic polarization, using the surface-roughened Ni plate (surface roughness S r = 78.3) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Ni. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn K α1 fluorescence line. The Sn K-edge absorption near-edge structure (XANES) in the Sn-UPD potential region has revealed that the Sn-UPD layer on Ni is oxygenated. The extended X-ray absorption fine structure (EXAFS) analysis was performed with a two shell fit of the nearest neighbor Sn-Ni and Sn-O interactions, assuming that the uppermost Ni surface exposed to the solution is mainly oriented to the (111) plane. The results have indicated that Sn atoms are substituted like a surface alloy at face-center-cubic sites in the first Ni layer and further bonded with oxygen atoms. The strong inhibition effect of Sn on aqueous corrosion of Ni is ascribed to the bond between Sn and O atoms in addition to the bond between Sn and Ni atoms. The formation of metal monolayer on a foreign metal electrode at potentials more noble than the equilibrium potential of bulk electrodeposition is termed underpotential deposition (UPD). The UPD of Sn on Pt 1-4 has been investigated with relation to electrocatalysts for electro-oxidation of CO, 5 HCOOH 6 or methanol 7-10 while Sn-plating has been widely used in steel industry due to the corrosion prevention of iron and steels by Sn.11-14 Our recent study 15 has revealed that 10 −3 M Sn 2+ added in 0.2 M HClO 4 inhibits completely the anodic dissolution of pure Ni, suggesting the participation of Sn-UPD in corrosion inhibition of Ni. We reported previously that the addition of Pb 2+ in acidic perchlorate solution suppresses the anodic dissolution of Ni and the suppression of Ni corrosion is ascribed to the Pb-UPD. 16 Moreover, it was confirmed by in situ surface X-ray absorption spectroscopy (XAS) that the Pb species deposited on Ni are in metallic state. 17The following differences, however, are distinct between inhibition effects of Pb 2+ and Sn 2+ on anodic dissolution of Ni. In the case of Pb 2+ , although the Pb atoms deposited on Ni in the UPD potential region suppress the anodic dissolution of Ni, the anodic dissolution of Ni is re-activated due to anodic stripping of the deposited Pb atoms as the potential shifts to the noble direction and Ni is subsequently passivated. 16 In contrast in the case of Sn 2+ , no anodic dissolution of Ni is observed in the wide potential region exceeding the Sn-UPD region.15 The X-ray photoelectron spectroscopy (XPS) analysis has showed that in the case of Pb 2+ , any Pb species are not detected on the passive surface of Ni, 16 while in the case of Sn 2+ , Sn species are detected on the pa...

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A multiphoton dissociation (MPD) of volatile metal carbonyl was applied for pulse production of neutral transition metal dimers in a gas-phase kinetic study. Its feasibility was discussed in the reaction systems of Mo2 with small molecules (O2, CO, NO, H2, SF6, NH3, and C2H4). Some potential problems, such as disturbances to the dimer production mechanism from the reactant addition and to neutral reaction kinetics from ionic species inevitably produced in the MPD of metal carbonyls, were experimentally examined. The transient concentration of Mo2 (X 1Σg+, ν = 0) produced by 266-nm MPD of Mo(CO)6 was measured by a laser-induced fluorescence (LIF) at 518.60 nm (A 1Σu+, ν″ = 0 ← X 1Σg+, ν′ = 0). The pseudo-first order decay rates of Mo2 were found to increase with increasing the concentration of some reactive species (O2, NO, and NH3). Our results obtained from time-resolved measurements were found to be almost consistent with the observations of Lian et al. (J. Phys. Chem., 98, 11637 (1994)) who prepared Mo2 by laser vaporization and measured its depletion in a fast-flow reactor.