Significance of This Article
Nowadays, environmental cleanup is one of the biggest issues in any human activities having an impact in terms of pollution and resulting in the production of harmful substances. In particular, this issue is a major source of concern in the chemical industry and related science and technology. In fact, in this field, specific “commercial” catalysts are used for environmental cleanup. Yet, these catalysts make large use of very rare and expensive elements such as platinum, rhodium, and palladium. Because of the scarcity and consequent high costs of these natural resources, import, export and handing of such materials have become a trans-national concern worldwide. Hence, production of effective catalysts replacing the ones making use of these rare elements with cheaper ones is becoming an increasingly urgent problem. In this scenario, ceria surfaces have been attracted a lot of interests from scientists and engineers because of their peculiar ability of forming and healing of oxygen vacancies promoting oxidation reactions. In the past two decades, several studies focused on copper doped ceria surfaces because of their catalytic activities for oxidation reactions, which turn out to be comparable or superior to that of Pt-based catalysts. Thus, this class of systems is nowadays regarded as a premier candidate for a cost-effective catalyst, replacing and supplanting conventional ones. For the reasons briefly summarized here, these ceria systems are the target of the work presented here.
Our present work elucidates and compares the electronic structures of both the defective CeO2 and the Cu/CeO2 surfaces using the most advanced computational tools. Specifically, we provide evidence for a peculiarity in the top of the valence bands of these systems, namely the highest occupied molecular orbital (HOMO), which turn out to be significantly different in the two systems. The gap state of the defective CeO2 surface, which has been extensively studied by the many theoretical and experimental researchers (Refs. 34—38 in our manuscript) disappears when a copper atom is introduced in the system because of the presence of an excess electron originating from the oxygen defect trapped by the d-state of copper rather than by 4f states of cerium atoms. The top of the HOMO acquires a 2p character from the oxygen atoms around the doped Cu site. We demonstrated that this rearrangement of the electronic structure enhances the reactivity of these oxygen atoms against nitrogen monoxide, realizing a Mars-van Krevelen reaction mechanism at the Cu/CeO2 surface which is enhanced by the doping of copper atoms. Thus, we have proposed an essential mechanism why the Cu doped ceria system exhibits high catalytic activities. We firmly believe that these results are prone to suggest a strategy to fabricate extended ceria based catalysts enhancing the catalytic reactivity by doping ceria surfaces with cheaper transition metal atoms, thus allowing for a significant cost reduction. Considering the high importance, broad industrial interest and social demand for the effective solution for air pollution, we assure that our results are of general interest to a wide readership covering a broad community from physicists to chemical engineers, and have a practical industrial and economic impact in the field of catalysis and nanoscience.