Rhodium is one of the key components used in commercial three-way catalysts (TWCs) to remove NOx, CO and unburnt hydrocarbons. Rh is traditionally favored for NO to N2 reduction, when compared to other Pt-group metals. However, with Rh price volatility, a better understanding of the reaction mechanisms involving Rh is necessary to optimize Rh use in the TWC catalyst design. Here, we focus on the kinetics of NO reduction by CO on a highly dispersed Rh/Al2O3 catalyst. Although many mechanistic studies have been carried out to understand the reaction pathways of NO reduction on Rh, no agreement on the reaction mechanism has been reached. In addition, most of these studies investigated conditions in the absence of water and far from realistic conditions. We present NO reduction by CO kinetic data in the presence and absence of water under stoichiometric, excess NO and excess CO conditions. While in the absence of water the apparent activation energies for all conditions were similar, in the presence of water the calculated values differed when considering excess CO conditions. One of the main products formed under these conditions is NH3, whose influence at different temperatures on the reaction was investigated via kinetic and spectroscopic studies. Although the measured apparent reaction order in NH3 was 0, our data show that NH3 inhibits the reaction by lowering the rate of NO consumption as well as the rate of N2O formation. IR spectroscopy resolves this via characterization of the intermediates formed in the presence of NH3.

Influence of water and NH3 byproduct formation on NO reduction by CO on Rh/Al2O3

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