A number of reaction mechanisms have been proposed as being the rate determining step for the low temperature water–gas shift (WGS) reaction, CO + H2O ↔ H2 + CO2, over metal catalysts. Support for two main types of mechanism, the so-called redox mechanism (involving atomic O) and the associative mechanism (involving surface formate), exist in the literature. In the latter mechanism surface carbonates or carboxylates could be important intermediates. Depending on reaction conditions, both mechanisms may be operational.
Microkinetic modeling has predicted that the activity of copper-based catalysts for water-gas shift should be enhanced if the surface bonding energies of CO and O are increased. According to the d-band model this can be achieved by growing a strained monolayer of Cu on Pt(111). Contrary to investigations on well-defined Cu single crystals, carbonate was observed at 0.5 bar CO2 using PM-IRRAS. The carbonate species was subsequently identified in UHV by HREELS. TPD showed that dissociation into CO2 and atomic oxygen sets in around 460 K corresponding to onset of surface alloying. The relevance for WGS and methanol synthesis is discussed.
To date most surface science investigations on the kinetics of elementary-step surface reactions in the WGS have been performed under UHV conditions. Considering reports on an auto-catalytic role of water in water dissociation as well as formate decomposition, the investigations will be expanded to include in-situ studies of surface speciation in WGS on Cu surfaces as well as the influence of water-gas on the kinetics of elementary surface reactions.
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