Toward tuning the surface functionalization of small ceria nanoparticles

Abstract

Understanding and controlling the performance of ceria nanoparticle (CNP) catalysts requires knowledge of the detailed structure and property of CNP surfaces and any attached functional groups. Here we report thermogravimetric analysis results showing that hydrothermally synthesized ∼30 nm CNPs are decorated with 12.9 hydroxyl groups per nm(2) of CNP surface. Quantum mechanical calculations of the density and distribution of bound surface groups imply a scaling relationship for surface group density that balances formal charges in the functionalized CNP system. Computational results for CNPs with only hydroxyl surface groups yield a predicted density of bound hydroxyl groups for ∼30 nm CNPs that is ∼33% higher than measured densities. Quantitative agreement between predicted and measured hydroxyl surface densities is achieved when calculations consider CNPs with both -OH and -Ox surface groups. For this more general treatment of CNP surface functionalizations, quantum mechanical calculations predict a range of stable surface group configurations that depend on the chemical potentials of O and H, and demonstrate the potential to tune CNP surface functionalizations by varying temperature and/or partial pressures of O2 and H2O.

Nanoceria: factors affecting its pro- and anti-oxidant properties

“…Hence stable nanoceria configurations balance the energy costs of anions from the environment against energy variations within the nanoceria itself, including effects of changes in oxidation state of Ce cations. 101 95 For cubic nanoceria structures considered in Ref. 74, DFT calculations predict stable configurations similar to those shown in Fig.…”

“…Table 4 Since water is ubiquitously present in environmental or biological milieu, the role of water on the surface structure and redox chemistry of nanoceria cannot be ignored. To address this issue, Traversa's group 100 have investigated the variation of surface structures for (111) ceria faces and Xing et al 95 have focused on the expected surface structural evolution for ceria nanoparticles in response to changing conditions in high water activity environments.…”

“…For example, it is possible to construct nanoparticles assuming they conserve stoichiometry or mass ratios, or that they conserve net charge and oxidation state. Recent quantum mechanical calculations of the stability of cubic nanoceria examined this issue in detail, considering the density and distribution of bound surface groups, and deriving scaling relationships for the stable surface concentration of these groups 95 . While the analysis of acid-base behaviour of surface hydroxyls by Nabavi 70 assumed that the surface atoms are separated from the bulk, quantum mechanical calculations permit charge to be distributed throughout the nanocrystal.…”

Metal-Free Low-Temperature Water–Gas Shift Catalysis over Small, Hydroxylated Ceria Nanoparticles

“…31 In addition, environmental conditions, including those relevant to the WGS reaction, that will maintain the hydroxylated surface configuration of catalyst CNPs have already been computed in a separate study. 44 In combination, these results demonstrate that the small, hydroxylated CNPs studied here could be directly employed as WGS catalysts and will exhibit at least two key advantages over existing hybrid metal/metal oxide WGS catalysts. First, small, hydroxylated CNPs do not require metal cocatalysts, reducing the cost, weight, and processing complexity of low-temperature WGS catalyst materials, and second, the density of catalytically active sites on hydroxylated CNPs (effectively, one active site for every surface lattice oxygen site) will be at least an order of magnitude higher than the density of active sites on catalyst materials that require the thermally induced formation of energetically unfavorable lattice oxygen vacancies.…”

“…In the case of metal-free WGS catalysis, though, the H − state is stabilized on sufficiently small CNPs (less than ∼10 nm) because such CNPs prefer to be covered with singly negative anionic species (e.g., -OH − or -O x − ). 44,49 We also note that previous mechanistic studies of the WGS reaction via associative processes have debated whether carboxyl or formate intermediates dominate the low-temperature WGS mechanism over metal/ceria hybrid catalysts. Here we show that, on OH-terminated CNPs, formation of carboxyl intermediates has the lowest activation energy and is the first step in the lowest-activation energy path for the overall WGS reaction.…”