Over the last 50 years, the reduction of automotive emissions has been a critical component of environmental legislation. The catalytic converter is one of the tools used by automotive engineers to reduce emissions: it facilitates the conversion of residual fuels and CO to CO2 and the reduction of NOx to N2 over the ‘three-way catalyst’ (TWC) containing a mix of precious metals, such as Pt, Rh, or Pd supported on high surface area γ-Al2O3. NOx reduction is accomplished by Rh, and while it achieves nearly complete conversion to N2 at steady state operating conditions, harmful emissions such as NH3¬ and N2O are produced during startup.
Previous studies identified a mix of Rh particles and atoms in typical TWC formulations, and others found that Rh atoms are responsible for NH3 production in the presence of NO, CO, and H2O (which is present in automotive exhaust). We aim to study aspects of the mechanism of NO reduction over atomically dispersed Rh/γ-Al2O3 catalysts to inform the design of catalysts and control systems that limit the production of harmful emissions. We focus on the dry reduction of NO by CO (no H2O), as this will serve as the base case for future studies including water and employ a combination of in situ, temperature programmed, and cryogenic infrared (IR) spectroscopy with UV photolysis in our study.
First, we found that the stable Rh dicarbonyl, Rh(CO)¬2, is the most abundant Rh species during the reaction in 0.5 kPa CO and 0.1 kPa NO (5000 ppm and 1000 ppm, respectively) at 205 °C, which models dry reaction conditions during startup of the catalytic converter. The abundance of Rh(CO)¬2 suggests that reactions involving this species initiate the catalytic cycle. Experimental and theoretical study of the influence of surface *OH (* denoting species adsorbed on γ-Al2O3 or Rh) on Rh(CO)¬2 found that high local *OH density on the support (in excess of ~10 *OH nm–2) near Rh(CO)¬2 lowers *CO vibrational frequency and energy barriers to *CO desorption while stabilizing a monocarbonyl intermediate, Rh(CO). At reaction conditions, it is likely that Rh(CO)¬2 in *OH dense regions of γ-Al2O3 are the most active species, and that Rh(CO)¬2 is directly coordinated to a *OH ligand shared with the support.
After *CO desorbs from Rh(CO)¬2, Rh was initially assumed to interact with gaseous NO to form a mixed Rh–(NO)x–CO intermediate during NO reduction, but this could not be verified in situ due to the presumed rapid reactivity of this intermediate and the abundance of Rh(CO)¬2 species. Therefore, we used photolysis to induce *CO desorption from Rh(CO)2 at cryogenic temperature in the presence of NO to form and kinetically trap what we believe is Rh(CO)(NO)2, which was highly reactive below –75 °C. The assignment of this species and its speculated participation in the reaction mechanism is supported by previous studies of NO reduction by CO by homogeneous Rh catalysts and by density functional theory (DFT) calculations presented in this work. Finally, we extended the boundary of Rh interactions with support species by identifying reactions between Rh(CO)¬2 and NO-derived species on γ-Al2O3 at cryogenic temperature which consume Rh(CO)¬2 and oxidize Rh.
In this work, we establish the kinetic relevance of Rh(CO)¬2 consumption to the NO-CO reaction over atomically dispersed Rh/γ-Al2O3 and identify three processes that could be involved: *CO desorption followed by formation of Rh(CO)(NO)2 and subsequent reaction, reaction of Rh(CO)¬2 with NO-derived surface species on γ-Al2O3, and a Rh redox cycle. We also demonstrate the feasibility of UV photolysis for the generation and observation of reactive intermediates. Finally, we propose further experiments to determine which of the three processes are relevant to NO reduction over atomically dispersed Rh/γ-Al2O3 at TWC conditions.