Structure sensitivity in the CO oxidation on rhodium: Effect of adsorbate coverages on oxidation kinetics on Rh(100) and Rh(111)

Abstract

Temperature-programmed reaction spectroscopy has been used to study the surface reaction between CO and O-atoms on Rh͑100͒ and Rh͑111͒ at a range of different adsorbate coverages. Comparison of the reaction on both surfaces in the low coverage regime, where the kinetics can be described by a straightforward Langmuir-Hinshelwood mechanism reveals that the CO oxidation is structure sensitive, with the rate constant being an order of magnitude higher on the Rh͑100͒ than on the Rh͑111͒ surface. As a consequence, the selectivity of the COϩO reaction to CO 2 is about 100% on Rh͑100͒, whereas on Rh͑111͒ the oxidation reaction competes with CO desorption. At low CO coverage, CO oxidation is an elementary step on Rh͑100͒ for a broad range of oxygen coverages. We report kinetic parameters E a ϭ103Ϯ5 kJ/mol and ϭ10 12.7Ϯ0.7 for O ϭ CO →0 on Rh͑100͒. The activation energy for CO oxidation on Rh͑100͒ decreases continuously with increasing O-coverage. At low coverage ( O Ͻ0.25 ML͒ we attribute this to destabilization of CO, leading to an increase in the CO 2 formation rate. At higher coverage ( O Ͼ0.25 ML͒ O-atoms become destabilized as well, as lateral interactions between O-atoms come into play at these coverages. The interactions result in a greatly enhanced rate of reaction at higher coverages.

First-principles extrapolation method for accurate CO adsorption energies on metal surfaces

“…The corrected energies agree with experimental site preference [28,29,30] with an energy difference of 0.163 eV. Likewise, our raw DFT data for Rh(111) is disparate with experimental site preference [31,32] while our corrected energies are in agreement. For Cu(111) the use of the correction gives the experimental site preference [24,33], though the corrected DFT results predict small (≤ 0.1 eV) differences between the top, bridge and hollow sites.…”

“…For each substrate, the site found to be preferred by experiment is marked with an asterisk (*). Experimental values for E chem are given in parentheses next to E corr GGA for Pt(111) [28,29,30], Rh(111) [31,32], Pd(111) [23,35], Cu(111) [24,33], Pt(100) [36,37,38], Rh(100) [39,40], Pd(100) [38,41], and Cu(100) [42,43] Site ∆Ε S-T (eV) …”

CO adsorption on metal surfaces: A hybrid functional study with plane-wave basis set

“…For Cu(111), the experimental values are in the range of −0.52 to −0.46 eV, 58,59 close to our calculated value (≈ −0.60 eV). For Rh(111), they are between −1.65 and −1.43 eV 60,61 . In this case, the theoretical values are too large by ∼ 0.40-0.60 eV and, worse, the hybrid functionals give a slight increase of the adsorption energy compared to the gradient corrected functional.…”

Quantification of lateral repulsion between coadsorbed CO and N on Rh(100) using temperature-programmed desorption, low-energy electron diffraction, and Monte Carlo simulations

“…We focus on the system COϩNO on Rh͑100͒, which is of interest for automotive exhaust catalysis 5,6 and for which a number of elementary kinetic parameters have recently been obtained. 2,7,8 The reason for selecting Rh͑100͒ instead of ͑111͒ is that the chemistry on the latter may be dominated by defects, whereas reactions on Rh͑100͒ are expected to reflect intrinsic chemistry of the ͑100͒ surface, as was observed for the elementary reaction COϩO to CO 2 . 8 To the best of our knowledge, there is hardly any quantitative information available on interactions between CO and coadsorbates on rhodium.…”