(689d) Structure of the Highly Reduced CeO2{111} Surface and Its Interaction with Single Atom Rh

Although CeO2 is known for its oxygen storage capacity and is commonplace in catalytic converters, it is also used as a support for precious metals for reactions in harsh conditions. Its strong interaction with metals can stabilize metal nanoparticles and single precious metal atoms under these harsh conditions. For instance, CeO2 has been used as the support for Au in water gas shift[1] and Ni in methane reforming[2]. Despite the wide range of applications and reaction conditions, computational studies of these types of catalysts generally use idealized structures with little defects. Besides the environmental conditions, it is well-known that both metal dopants and adatoms can change the properties of the surface as well, thus making surface nonuniformity even more pronounced. Nolan[3] showed that when one Ni or Pd replaces Ce on the CeO2{111} surface, oxygen vacancy formation surrounding the atom is much more favorable. In this work, we performed DFT calculations on various highly defective CeO2{111} surfaces. We examined surface stability under various conditions, the interactions between types of defects, the effect of adding Rh to the surface, and the stability of single atom Rh active sites in these conditions.

To investigate these surface phenomena, we performed our DFT calculations using VASP[4] version 5.4.1.05. The PAW[6] method was used to describe the core electrons, and the one-electron functions were developed on a basis set of plane waves with a 450 eV energy cutoff. The exchange-correlation energy was calculated using the PBE[5] functional. Due to the self-interaction error of 4f electrons in Ce, a Hubbard like term was added through Dudarev’s approach[7] with an effective U value of 4.5 eV. A (3x3) CeO2{111} supercell with 3 trilayers, about 7 Å thick, was used for the calculations with the bottom trilayer fixed at bulk positions. Integration was performed on the Gamma point. The ab initio thermodynamics approach[8] was used to compare surface stability, and the equations developed by Mayernick and Janik[9] were used to compare site stability.

Through our calculations, we found that depending on the gas environment, the surface could be covered in vacancies, OH groups, or a mixture of both. The existence of OH groups and vacancies have been well-studied in literature[10,11], but not as much attention has been paid to the combination[12,13]. Under moderately reducing conditions, when the chemical potential of H2 is between -1.84 eV and -2.53 eV and chemical potential of water is between -1.57 eV and -2.37 eV, we predict the surface to contain a mixture of less than 0.5 monolayer of OH groups and 0.1 monolayer of subsurface vacancies. The two types of defects prefer to segregate. Under highly reducing conditions, when the hydrogen chemical potential is above -1.68 eV, the surface is dominated by either a monolayer of OH groups or a monolayer of surface vacancies. These predictions differ from with those of Lustemberg et al., who performed the same study but in a smaller cell and only considered surface vacancies in small quantities[13].

Regarding the interaction of the surfaces at various degrees of reduction, the nonreduced and slightly reduced surfaces could strongly bind Rh atoms[14]. But once the surface becomes highly reduced, Rh’s mobility is significantly enhanced. We compared the stability of adsorbed Rh on several reduced CeO2 surfaces and found that the stability trends are similar to that of the surface without Rh, except the stability of Rh anchored on moderately and slightly reduced surfaces are stronger because better binding sites are provided by these surfaces. Thus, the surface phase transition boundaries shifted to higher hydrogen and water chemical potentials. To summarize, we found the CeO2{111} surface, under moderately reducing conditions, is very sensitive to the environment and contain a variable mixture of vacancies and OH groups at equilibrium. The surface with a full monolayer of vacancies dominates the highly reducing gas environments. The strength of single atom Rh’s interaction with the surface depends on the degree of reduction.

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