Adsorption and reaction of H2O and CO on oxidized and reduced Rh/CeOx(111) surfaces

doi:10.1016/S0039-6028(00)00332-0

Surface Science

Volume 457, Issues 1–2, 1 June 2000, Pages 51-62

https://doi.org/10.1016/S0039-6028(00)00332-0 Get rights and content

Abstract

The interaction of H₂O and CO with thin films of highly oriented CeO₂(111) with vapor deposited Rh was studied using soft X-ray photoemission spectroscopy and thermal desorption spectroscopy. Adsorbed surface states of CO and H₂O and desorption products were identified in relation with the ceria substrate oxidation state, adsorption/annealing temperature and the presence of Rh. Only molecularly adsorbed H₂O and CO were detected on Rh/oxidized ceria surfaces. Rh/reduced ceria surfaces are more reactive, and promote dissociation of both H₂O and CO. CO is preferentially adsorbed on the metal and H₂O on the ceria substrate. Two adsorbed states were identified following the low temperature water adsorption on Rh/reduced ceria, chemisorbed water and hydroxyls. In the absence of Rh, chemisorbed water desorbs below 300 K and hydroxyls decompose above 500 K to give H₂ as a desorption product. Rh promotes the dissociation of chemisorbed water and formation of hydrogen in the 200–350 K temperature range. It also lowers the temperature of hydrogen formation from hydroxyls. The interaction of CO and H₂O strongly depends on the ceria oxidation state. Reaction of atomic carbon (formed by CO dissociation) with oxygen from ceria is facilitated by the presence of coadsorbed water. On the other hand, hydrogen desorption is inhibited by the presence of coadsorbed CO.

Introduction

The role of ceria, as a component in automotive emission control catalysts, has been recognized in many catalytic processes [1]. Ceria acts as an oxygen storage component by storing and releasing oxygen depending on the stoichiometry of the gas phase [2], [3], [4], [5]. Ceria also promotes activity of precious metals (Pt, Pd, Rh) in the oxidation of CO [6], [7], through the ceria-mediated oxidation of CO adsorbed on the metal. Ceria is also a known water-gas-shift (WGS) catalyst [8], [9], [10]. The WGS reaction is important within the automotive exhaust emission control reactions because it contributes to CO removal, especially under fuel-rich operation [11].

It has been proposed that the WGS reaction on ceria-supported precious-metal catalysts occurs through a bifunctional mechanism [11]. In the first step, CO adsorbed on a precious metal is oxidized by ceria, and in the second step ceria is reoxidized by water to give hydrogen. A similar, ceria-mediated oxidation mechanism has been proposed for CO oxidation reaction in excess CO [12], [13]. The enhancement of reaction rates in both WGS and direct CO oxidation was related to the ability of the catalyst to use its oxygen storage capacity [11]. Growth of ceria crystallites was found to cause catalyst deactivation [11].

CO interaction with Rh supported on ceria surfaces is strongly structure-sensitive [14]. A fraction of CO adsorbed on Rh can be oxidized to CO₂ on polycrystalline ceria surfaces. However, the ability of single crystal ceria surfaces to donate oxygen for CO oxidation is much smaller [14]. In TPD studies, Stubenrauch and Vohs [15] showed that CO undergoes dissociation on Rh supported on reduced ceria surfaces. Two CO desorption states were found using isotopically labeled CO, a low temperature state that corresponds to desorption of molecular CO adsorbed on Rh, and a high temperature state formed by recombination of dissociated CO with oxygen from ceria. Both molecular and dissociated CO have been identified by C 1s spectra in soft X-ray photoemission studies which have shown that the degree of CO dissociation strongly depends on the degree of reduction of ceria [16].

Rh is a poor WGS catalyst [11], [17]. Rh adsorbs water weakly and does not promote water dissociation [18]. On the other hand, ceria can be easily oxidized by water to produce hydrogen [8]. Bunluesin et al. [11] found that WGS reaction rates on ceria supported Rh were higher than rates on alumina supported Rh and ceria alone. It is known that noble metals promote low-temperature reduction of ceria by hydrogen [1], [2], [3]. Similarly Otsuka et al. [8] reported that noble metals (Pt, Pd) enhance the rate of ceria oxidation by water.

In this work we have studied CO and H₂O interaction with highly oriented CeO₂ (111) films and Rh loaded ceria by temperature programmed desorption (TPD) and soft X-ray photoelectron spectroscopy (SXPS). Surface species formed upon CO and/or H₂O adsorption and subsequent annealing were monitored by the C 1s and O 1s photoemission spectra. Degree of oxidation of Ce and Rh was monitored by the Ce 4d and Rh 3d photoemission spectra, respectively. The effects of the adsorption temperature, Rh coverage and oxidation state of ceria were examined. Reaction of both CO and H₂O with ceria, as well as their interaction, strongly depends on the oxidation state of the ceria substrate.

Section snippets

Experimental

This work was performed in two separate ultrahigh vacuum (UHV) systems. The TPD experiments were conducted in a UHV chamber at ORNL with a working pressure of 2×10⁻¹⁰ Torr. This system contained quadrupole mass spectrometer, ion sputtering gun, and standard surface analysis techniques (low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy). The second system at the National Synchrotron Light Source at beamline X-1B was used for the SXPS studies using

Interaction of H₂O with oxidized and reduced ceria

Fig. 1 illustrates the TPD spectra following adsorption of H₂O on oxidized (Fig. 1a) and reduced (Fig. 1b) ceria surfaces at 100 K. The exposure was sufficient to saturate any chemisorbed state and initiate condensation. On the fully oxidized surface, water is the only desorption product as indicated by mass 18 desorption. Water is weakly held on oxidized ceria surfaces and it fully desorbs below 300 K. Both H₂O and H₂ are detected as desorption products following H₂O adsorption on reduced ceria

Discussion

Water interaction with ceria strongly depends on ceria oxidation state. Highly ordered oxidized CeO₂(111) surface adsorbs water only in a molecular form. Desorption occurs below 300 K. Only a low temperature H₂O desorption was reported for CeO₂ (001) surfaces [22]. In addition to multilayer and chemisorbed water, formation of hydroxyls at low temperature was reported for CeO₂ (001) surface [22]. These hydroxyls recombine and desorb as water at 275 K. In this work we did not find any indication of

Conclusions

The interaction and reaction of both water and carbon monoxide with cerium oxide films containing Rh strongly depends on the degree of reduction of the ceria support. Rh/oxidized ceria films adsorb H₂O and CO in a weakly bound molecular form. Chemisorbed and multilayer water were detected by X-ray photoemission on these surfaces. CO weakly adsorbs on ceria and in the presence of Rh is mainly adsorbed on the metal.

CO and water dissociate upon adsorption and annealing on Rh/reduced ceria

Acknowledgements

This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corporation under Contract No. DE-AC05-96OR22464, and in part by an appointment to the Oak Ridge National Laboratory Postdoctoral Research Associates Program administered jointly by the Oak Ridge Institute for Science and Education and Oak Ridge National Laboratory.

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Adsorption and reaction of H2O and CO on oxidized and reduced Rh/CeOx(111) surfaces

Abstract

Introduction

Section snippets

Experimental

Interaction of H2O with oxidized and reduced ceria

Discussion

Conclusions

Acknowledgements

J. Catal.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Appl. Catal. B: Environmental

J. Catal.

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Surf. Sci.

Surf. Sci.

J. Catal.

Surf. Sci.

Surf. Sci.

Surf. Sci.

Adsorption and reaction of H₂O and CO on oxidized and reduced Rh/CeO_x(111) surfaces

Interaction of H₂O with oxidized and reduced ceria