(618g) (001)?-Fe2O3 and CeO2/Ag: Good Candidates for the Oxygen Reduction Reactions | AIChE

(618g) (001)?-Fe2O3 and CeO2/Ag: Good Candidates for the Oxygen Reduction Reactions

Authors 

Righi, G. - Presenter, University of Modena and Reggio-Emilia
Magri, R., University of Modena and Reggio-Emilia
Metal oxides have attracted more and more attention during the last years as materials suitable for applications in catalysis and are therefore objects of a search for new materials with an higher catalytic activity, and more suitable for green processes [1].

We have considered Iron and Cerium oxides: they are characterized by an high ability to exchange oxygen atoms during the reactions, and can, consequently, be considered for applications to energy storage and conversion, for example as electrodes in the fuel cells. On this issue a great challenge is to find long service life and cost effective catalysts to replace the common ones based on precious and expansive platinum [2].

In order to increase the reducibility, and, consequently, the catalytic activity of an oxide, the decrease of the energy required to create an oxygen vacancy is a key point.

Different approaches have been proposed to reduce this energy: i) to produce an oxide in form of nanocrystals or films, ii) to dope the oxide with a different atom, and iii) to deposit metal nanoparticles on the oxide surface [3]. The interface region between the metal and the oxide is chemically active: there is a charge transfer, which influences the reducibility of the entire system.

We report here studies of the redox properties of the (001) maghemite (γ-Fe2O3) surfaces, and of the reducibility of silver – cerium oxide interfaces with different surface orientations.

All the calculations have been performed in the density functional approach (DFT) as implemented in the Quantum Espresso (QE) code [4],[5]. In order to describe in a correct way the electronic properties of both these oxides we use the Hubbard correction (DFT+U). All the calculations are spin polarized, and the surfaces were modelled with the slab method.

Maghemite is the second most stable polymorph of iron oxides, and recently it has been asserted that the (001) maghemite surface may have an high catalytic activity [6]. Qui et al. [2] have shown experimentally that γ-Fe2O3 is a good candidate for oxygen reduction reactions, and for cost effective catalysts.

Maghemite, like magnetite, has a spinel crystal structure, however, to guarantee the neutrality of the structure, since all the octahedral and tetrahedral iron atoms of the bulk are in a trivalent state, the presence of cation vacancies is required. Due to the intricacy of the bulk structure, to our knowledge, this is the first ab-initio study concerning the reduction properties of the (001) maghemite surfaces where the iron vacancies are taken explicitly into account.

We have considered two different surface terminations, both iron and oxygen terminated, and have calculated the formation energy of different oxygen vacancies depending on the bonding configuration. We have found that the creation of oxygen vacancies causes a different variation of the magnetic and electronic properties of the two different surfaces.Comparison with the results obtained by Santos- Carballal et al. [7] on the (001) magnetite surface shows that the energy required to create an oxygen vacancy on the maghemite surface seems to be less than that required to create it on the magnetite one, enlightening the possible role of the cation vacancies on the catalytic activity of iron oxides.

As for the cerium oxide (CeO2) it is well known that cerium oxide is a good catalyst, due to the Cerium high capacity to change its oxidation state from Ce4+ to Ce3+ and vice versa, and to the strong interactions of the oxides with metals. Many theoretical and experimental results are present in the literature about the influence of single metal atoms, clusters and films on cerium oxide surfaces, especially on the (111) one. Instead, to our knowledge, only few works have studied thin film of ceria deposited on metal surfaces in the so-called inverse catalysis. Recently, Chen et al. [8] have shown the important role of the oxygen vacancies to explain the performance of CeO2/Cu catalyst.

Only one work addressed ceria on a silver surface [9], and, in particular, only the interaction between (111) layers on the Ag (111) surface was considered.

On the other hand, the (100) ceria surface has been found to be more reactive, so the presence of a metal substrate, could increase its catalytic properties.

To investigate the reduction properties of these systems (CeO2/Ag), and the role of the interface,we have considered different orientations of ceria monolayers on silver surfaces (111-100-110), and have calculated the changes in the formation energy of the oxygen vacancy on the surface and sub-surface due to the presence of the metal substrate.

The results show high charge transfers at the interface, and a decrease of the energy required to create an oxygen vacancy.

The results of our work are encouraging in the direction to exploit these oxide systems in the design of new more efficient catalysts.

References

[1] K. Zhao, H. Tang, B. Qiaio, L. Li, and J. Wang, ACS Catal, 5, 3528-3539 (2015)

[2] K. Qui, G. Chai, C. Jiang, M. Ling, J. Tang, and Z. Guo, ACS Catal, 6, 3558-3568 (2016)

[3] A. R. Puigdollers, P. Schlexer, S. Tosoni, and G. Pacchioni, ACS Catal, 7, 6493-6513 (2017)

[4] P. Giannozzi et al., J.Phys.:Condens.Matter 21, 395502 (2009)

[5] P Giannozzi et al. , J.Phys.:Condens.Matter 29, 465901 (2017)

[6] R. C. Baetzold and H. Yang, The Journ. Phys. Chem. B. 107, 14357-14364 (2003)

[7] D. Santos-Carballal, A. Roldan, R. Grau-Crespo, and N H de Leew, Phys. Chem. Chem. Phys., 16, 21082 (2014).

[8] S. Chen, L. Li, W. Hu, X. Huang, Q. Li, Y. Xu, Y. Zuo, G. Li, ACS Apppl. Mater. Interfaces, 7, 22999-23007 (2015)

[9] J. Graciani, P. A. B. Vidal, J. A. Rodriguez, and J. F. Sanz, The Journ. of Phys Chem C, 118, 26931-

26938 (2014)

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