WGSR rate-determining step

The steady-state concentration of [Rh(CO)2l2] which affects the carbonylation rate depends on whether the reduction or the oxidation process is rate-limiting in the WGSR catalytic cycle (Scheme 1). The CH3OAC concentration determines which reaction is the rate-determining step of the WGSR by influencing the hydriodic acid concentration in the catalyst solution. The CH3OAC concentration affects the equilibrium concentration of HI due to the equilibrium represented in eq. (10) [23]. 

At this low HI concentration, the rate-determining step of the WGSR is probably the oxidative addition of HI to [Rh(CO)2l2] to form [HRh(CO)2l3] (Scheme 1). The rate of oxidation of [Rh(CO)2l2) is very low. The [Rh(CO)2l2r complex is therefore the predominant species in solution which affords a maximum methanol carbonylation rate for each level of water concentration. Increasing the water concentration within the low-water regime, the rate-limiting step of the WGSR remains the oxidation of [Rh(CO)2l2] , so the CO2 production is still very low but the [Rh(CO)2l2] concentration increases since the rate of reduction of [Rh(CO)2l4] increases (Scheme 1). 

The process chemistry of the methanol carbonylation reaction is summarized in Scheme 1. This catalytic reaction scheme depicts the balanced relationship between the methanol carbonylation, the WGSR and the iodide cycles under both regimes of water concentration. Within the scope of methanol carbonylation in an aqueous/acetic acid medium, the overall reaction rate depends not only on the nature of the rate-determining step(s), but also on reaction conditions influencing the steady-state concentration of the active Rh species, [Rh(CO)2l2]. ... 

In situ characterization with X-ray absorption spectroscopy was used to determine the chemical state of the gold during the WGSR over an Au/ceria catalyst. The data indicated that cationic Au species present in fresh AuOyGeOg catalysts do not exist under reaction conditions and they could not be the active sites. The authors concluded that the rate-determining steps for the WGS at temperatures above 250 °C occur at the gold-ceria interface, with the active... 

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