Low Temperature Infrared Study of Carbon Monoxide Adsorption on Rh/CeO2

Hicham Idriss,Jordi Llorca

doi:10.3390/catal9070598

Abstract

Fundamental studies of the interaction of adsorbates with metal oxides alone and on which a noble metal is deposited provide information needed for catalytic reactions. Rh/CeO2 is one of the textbook catalysts for many reactions including syngas conversion to ethanol, water gas shift reaction (WGSR), and ethanol steam reforming. In this work, the adsorption of CO is studied by infrared (IR) spectroscopy, over CeO2 and 0.6 at. % Rh/CeO2 at a temperature range of 90 to 300 K. CeO2 is in the form of nanoparticles with sizes between 5 and 10 nm and exposing predominantly {111} surface termination in addition to non-negligible fraction of the {100} termination, determined from high resolution transmission electron microscopy (HRTEM). The as prepared Rh/CeO2 contained metallic Rh as well Rh cations in higher oxidation states. At 90 K two IR bands were observed at 2183–2186 and 2161–2163 cm−1, with the former saturating first. The 2163 cm−1 peak was more sensitive to CO pressure than the 2186 cm−1. Heating resulted in the depopulation of the 2163 cm−1 before the 2186 cm−1 peak. The desorption energy computed, assuming a first-order desorption kinetic, was found to be 0.35 eV for the 2186 cm−1 and 0.30 for the 2163 cm−1 IR peak (+/−0.05 eV). The equilibrium constant at 90 K was computed equal to 1.83 and 1.33 Torr−1 for the 2183 and 2161 cm−1, respectively. CO adsorption at 90 K on Rh/CeO2 resulted (in addition to the bands on CeO2) in the appearance of a broad band in the 2110–2130 cm-1 region that contained two components at 2116 and 2126 cm−1. The high frequency of this species is most likely due to adsorption on Rh clusters with very small sizes. The desorption energy of this species was found to be equal to 0.55 eV (+/−0.05 eV). Heating the CO covered Rh/CeO2 surface accelerated the disappearance of CO species over CeO2 and resulted in the appearance of CO2 bands (at about 150 K) followed by carbonate species. At 300 K, the surface was mainly composed of carbonates.

Highlights

The interaction of CO with the surfaces of metal oxides and metal deposited on the metal oxides gives considerable information related to bonding, metal dispersion, and redox properties, among others [1,2,3,4]
Many Density Functional Theory (DFT) computation studies have been conducted to probe into the vibrational frequency modes and adsorption energies of different systems of CO on CeO2 surfaces, among other parameters [12,13,14]
Still there is a good agreement between theory and experiments on a few main aspects: (i) That CO vibrational frequency is further blue shifted with respect to gas phase CO on CeO2, (ii) that this vibrational frequency is further blue shifted on a reduced surface, when compared to that on a stoichiometric surface, (iii) that the adsorption energy is slightly higher on reduced CeO2 when compared to the stoichiometric one; while the adsorption energy changes with surface structure, the exact adsorption sites on reduced surfaces is still subject to further studies

Summary

Introduction

The interaction of CO with the surfaces of metal oxides and metal deposited on the metal oxides gives considerable information related to bonding, metal dispersion, and redox properties, among others [1,2,3,4]. Many Density Functional Theory (DFT) computation studies have been conducted to probe into the vibrational frequency modes and adsorption energies of different systems of CO on CeO2 surfaces, among other parameters [12,13,14]. On Rh deposited on CeO2 thin film, a band with a νCO frequency at 2100 cm−1 and attributed it to the symmetric stretch of a gem-dicarbonyl species [17] on small Rh-O particles by Reflection Absorption Infra Red Spectroscopy (RAIRS). As presented in this work, it was possible to see two peaks on Rh/CeO2 upon CO adsorption at a high coverage, in this region These were at 2126 and 2114 cm−1 with no evidence of bands due to an asymmetric stretch indicating that these species are most likely due to linearly adsorbed CO on small clusters of Rh cations and not due to a gem dicarbonyl species. We observed some transformation of adsorbed CO into carbonate species upon heating to temperatures above 120 K

Irreversible Adsorption

Reversible Adsorption

Temperature Effect

Experimental

Conclusions