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Trinity College Dublin

Protecting the Environment

Modelling cerium dioxide environmental catalysts.

Dr Graeme W. Watson, School of Chemistry, Trinity College Dublin

Cerium dioxide (CeO2) is a catalyst with a wide range of potential application in the field of catalysts. Of particular relevance to the environment is its use as a component of the three way catalytic converter used on automobiles to reduce the emission of harmful gases [1]. In this catalyst carbon monoxide (CO) and unburnt hydrocarbons are oxidized to carbon dioxide (CO2) and water (H2O) while simultaneously nitrogen oxide (NO) and nitrogen dioxide (NO2) (collectively revered to as NOx) are reduced to nitrogen (N2). One of the problems of this process is the requirement to keep the oxidation and reduction in balance and modern engines have tight controls on the air intake of the engine to try to keep the oxidization and reduction in balance.

To help keep this balance the surface of the oxide must be able to be reduced and oxidized – that is the metal must be able to change its oxidation state to take in oxygen or release it as needed. A metal oxide can oxidize a molecule like carbon monoxide using oxygen from the solid and hence convert CO to CO2 leaving the metal oxide oxygen deficient. One of the key functions of CeO2 is the ability of cerium to change its oxidation state between Ce4+ and Ce3+ depending on the amount of oxygen available. The reduction of ceria results in the formation of oxygen vacancies and the release of oxygen for catalytic processes. [1].

$ CeO_2 \rightarrow CeO_{(2-x)} +\frac{1}{2} x O_2 $

This facile storage and release of oxygen for catalytic reactions is known as the oxygen storage capacity (OSC). Among the important reactions catalysed by ceria is the oxidation of CO to CO2 [2] with reduction of the surface [3], i.e. the formation of oxygen vacancies.

$ Ce(IV)-O –Ce(IV) + CO  \rightarrow Ce(III)-VAC-Ce(III)+ CO $

Oxidation of the surface (filling of the oxygen vacancies) can occur when the surface reacts with NOx resulting in its reduction and a catalytic cycle as shown in figure 1. Despite the importance of the oxidation of CO in environmental protection here has been little theoretical progress towards a comprehensive understanding of the factors driving the interaction of CO with ceria.

Density functional theory (DFT) is a method of calculating the electronic structure of materials based on the electron density. As such this method has found wide use in oxide modelling due to its relative speed and accuracy. However in this case standard DFT has some significant problems associated with unrealistic delocalisation of the Ce(lll) defect states resulting from surface reduction [4].

Figure 1: Schematic of the catalytic cycle involving oxidation of CO and CO2 with concomitant reduction of the ceria surface

       
Figure 2: Structure of absorded CO showing a) unreacted CO on the (111) surface, b) and c) formation of CO32- and subsequent reduction of the surface on the (110) and (100) surface respectively

We have utilized the DFT+U method to study defective ceria and the adsorption of CO onto the low index (111), (110) and (100) surfaces. On the (111) surface interaction with the CO molecule leads to weak adsorption, with almost not change the structure of the molecules. For the (110) and (100) surfaces, interaction with CO leads to formation of a surface carbonate (CO3)2- species, in which two oxygen atoms are pulled out of the surface. This results in spontaneous oxidation of the CO to form a carbonate (CO32-) and reduction of the surface to form two Ce(III) species (see figure 2).

$ CO + 2O^{2-} + 2Ce^{4+} \rightarrow CO_3^{2-} + 2 Ce^{3+} $

The formation of surface bound carbonate-like structures on the (110) and (100) surfaces and the accompanying surface distortions are consistent with the findings of [5], in which CO was found to be reactive only on exposed (110) or (100) faces of nanoparticles and nanorods of ceria, rather than on exposed (111) faces.

Analysis of the electronic structure for the (110) and (100) surfaces shows formation of the Ce 4f derived gap state between the valence band and the unoccupied Ce 4f states, which is a characteristic signature of reduced ceria (figure 3). The excess spin density (figure 2) and the partial charge density analysis of this state confirms that these Ce 4f states are localised on two Ce3+ ions at the surface. Further analysis of the geometry and electronic structure confirms the presence of a carbonate ion (CO3)2- at the surface. Thus, with strong chemisorption of CO on a stoichiometric ceria (110) or (100) surface, the molecule is oxidised and the ceria surface is reduced giving valuable information regarding catalysts optimisation.

Acknowledgements

This research was funded by the HEA PRTLI IITAC Programme and Science Foundation Ireland.

References

  1. Trovarelli, A. Catalysis by Ceria and Related Materials, Imperial College Press, London, UK, 2002
  2. Bedrane, S.; Descorme, C.; Duprez, D. Catalysis Today, 2002, 75, 401; Li, W.; Gracia, F. J.; Wolf, E. E. Catalysis Today, 2003, 81, 437; Tang, X.; Zhang, B.; Li, Y.; Xu, Y.; Xin Q.; Shen, W. Catalysis Today, 2004, 93-95, 183
  3. Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc. Faraday Trans 1 1989, 85, 929; Li, C.; Sakata, Y.; Arai, T.; Domen, K.; Maruya, K.; Onishi, T. J. Chem. Soc. Faraday Trans 1 1989, 85, 1451
  4. Nolan, M.; Parker S. C.; Watson, G. W. Surface Science, 2005, 595, 223
  5. Zhou, K.; Wang, X.; Sun, X.; Ping Q.; Li, Y. J. Cat., 2005, 229, 206 ; Aneggi, E.; Llorca, J.; Boaro M.; Trovarelli, A. J. Cat., 2005, 234, 88

 


Figure 3 Cerium partial electronic density of states showing the valence band (l), the Ce3+ states (ll) and the empty 4f states (lll).


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