Interfacial chemistry on transition metals in gaseous and electrochemical environments as probed by surface enhanced Raman spectroscopy
Abstract
Surface chemistry related to heterogeneous catalysis and oxide formation on transition metals (platinum-group metals and copper) in gaseous and electrochemical environments has been investigated in situ by surface-enhanced Raman spectroscopy (SERS). This technique features the molecular specificity of Raman scattering so to yield distinct vibrational evidence of chemical species, also providing monolayer sensitivity to probe the temperature- and potential-dependent surface speciation without bulk-phase spectral interference even under ambient liquid and gaseous conditions. An “overlayer strategy” has been utilized to induce SERS activity on Pt-group transition metals by electrodepositing them as ultrathin films on roughened gold substrates. The nature of reactant adsorption and substrate oxidation during heterogeneous reactions, specifically NO/CO reaction and methanol oxidation by oxygen on various transition metals, were revealed by SERS, elucidating surface chemical factors controlling the catalyst-dependent reaction rates and selectivities as measured by mass spectrometry. Surface reduction/oxidation reactions in metal-ambient pressure gaseous interfaces were also explored and compared with corresponding electrochemical SERS results. The occurrence of electrochemical half reactions in the former environments can control the adsorptive chemistry; these concepts were applied especially to elucidate copper surface oxidation in ambient air. The thermal oxidations of various Pt-group transition metals by oxygen were examined and compared to electrochemical oxidations in acidic aqueous media. The intrinsically more facile oxidations deduced to occur via the latter type of pathway are suggested to be due at least partly to solvent stabilization of displaced metal adatoms, so that the kinetic barrier for place-exchange mechanism is lowered. The utilization of SERS to study surface chemistry related to the processing of transition metal films in the microelectronics industry is also proposed and examined in preliminary form.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering
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