The effect of NO and CO on the Rh(100) surface at room temperature and atmospheric pressure

Rhodium is used in automotive catalysis to reduce NO and CO emission by catalyzing the reduction of NO to N${}_2$ and the oxidation of CO to CO${}_2$. Rhodium nanoparticles in the catalyst are exposed to high pressures of NO and CO, which leads to disintegration and sintering of the catalyst. To design more stable catalysts, the effects of high pressures of NO and CO on rhodium must be understood. Therefore, we studied the Rh(100) surface, which is most active for NO reduction by CO, at atmospheric pressures of NO and CO with scanning tunneling microscopy. Atomistic thermodynamics, low-energy electron diffraction, and Auger electron spectroscopy were used to understand the behavior of adsorbates on the surface. We observe the formation of rhodium islands and roughening of the step edges at high CO pressures. Roughening does not occur at the same pressures of NO, and is also less severe when co-dosing NO and CO, even at identical CO partial pressures. Atomistic thermodynamics shows that NO likely inhibits CO adsorption by blocking adsorption sites, preventing carbonyl formation, and decreasing surface roughening.