First-principles Study On The Cu/CeO₂（110） Interface And Its Interaction With H₂S

The system of Cu supported on CeO2( denoted as Cu/CeO2) is getting considerable attention for its excellent ability to catalyze a number of reactions. Especially, it has been reported that Cu-CeO2 can be a potential alternative to Ni/YSZ, the traditional anode material for SOFC, which, however, has some problems in the pratical application. For instance, there are always certain impurity H2 S in the H2 fuel, which will degrade the catalytic activity of Ni/YSZ, i.e. the so-called sulfur poison. Scientists have made much efforts to resolve this problem, among which the searching for some other alternative materials has caused much attention. For example, the Cu/CeO2 system can be used to replace the Ni/YSZ system. However, the sulfur tolerance mechanism of Cu/CeO2 system is not yet clear. Although the interactions of isolated Cu and CeO2 system with H2 S have been studied extensively, the interaction of the complicated Cu/CeO2 with H2 S has not been reported.Density functional theory(DFT) calculations are employed to investigate the interfacial properties of the system with the CeO2(110) with the adsorption of Cu atom and Cu cluster, the interaction of H2 S with Cu/CeO2(110), as well as the influence of the temperature and pressure on the interaction, with the aim to shed light on the sulfur tolerant mechanism of Cu/CeO2(110). The work performed and results reached are as follows:1. The interfacial properties of the Cu/CeO2(110) are investigated by first-principles method based on density functional theory. It is found that: 1) the single Cu adatom prefer to be adsorbed on the oxygen bridge site; 2) the distorted tetrahedron structure of Cu4 cluster is the most stable cluster configuration on CeO2(110) surface; 3) the metal-introduced gap states are presented in the gap of CeO2(110), which are mainly from the adsorbed Cu(or Cu cluster), its neighbor O and the reduced cerium ion(s), indicating that the activity of CeO2(110) surface is improved by copper adsorption; 4) the adsorbed Cu adatom and Cux(x=1, 4) are oxidized to Cuxδ + by their neighbor Ce ion(s) with formation of the Ce3+ ion(s), which could be summarized as Cux/Ce4+ â†'Cuxδ+/Ce3+; 5) the adsorption of small clusters introduces more Ce3+ ions than does a single Cu adatom, indicating that more Cuδ +-Ce3 + catalytic active centers are formed.2. The interaction of H2 S with the Cu12/CeO2 system is investigated using the first-principles method. It is found that the formation energy of surface oxygen vacancies is lower than that of interface oxygen vacancies and the spillover of an oxygen ion from ceria to Cu strip is an exothermic process, suggesting that the oxygen ions in the substrate are extremely active. The dissociation of H2 S molecule leads to atomic S absorbed preferentially at the Cu strip on both unreduced and reduced Cu12/CeO2(110), instead of interacting with the ceria and diffusing into the ceria bulk, alleviating the deactivation of the ceria. On the other hand, the sulfur atom at the Cu strip could be removed by forming SO2 at suitable partial pressure of water as suggested by our thermodynamics prediction. Therefore the accumulation of sulfur at the Cu strip and the sulfur poisoning to the Cu/CeO2 system can be avoided.