Reports: DNI651052-DNI6: Theory of Heterogeneous Oxidation of Carbon Monoxide by Sub-nano Surface-Deposits from Electronic Structure to Catalysis

Anastassia N. Alexandrova, PhD, University of California (Los Angeles)

Small clusters of transition metals deposited on supporting surfaces are among the most promising but least understood catalytic materials. Their catalytic properties depend unpredictably and nonlinearly on cluster size and composition, and cannot be extrapolated from those of the extended surfaces or lager nanoparticles. We use and develop a full array of multi-scale modeling techniques, ranging from electronic structure to statistical mechanics, to gain insight into these dependencies, and enable rational design of highly active surface-deposited clusters. Here are our accomplishments so far: 1) We assessed structure, stability, and mobility of selected small Pd clusters on titania surfaces. We discovered that these cluster preferentially bind to stoichiometric surfaces, and they are rather mobile on it. O-vacancies serve as migration stoppers, and also facilitate cluster dissociation. 2) We developed a Monte Carlo method for simulating the process of sintering of deposited clusters via the mechanism of Ostwald ripening. We are now able to predict cluster size-distribution over a range of temperatures in agreement with experimental data. 3) We realize electronic reasons for preferred structures of deposited clusters, and develop a predictive qualitative theory of chemical bonding for surface-deposited clusters, to have an insight into their shapes, stabilities, and properties. In particular, we found that clusters change shape upon deposition, from 3D in the gas phase to 2D. Matching with surface O atoms for partially covalent bonding, and additional acquisition of the aromatic character of chemical bonding drives the transition. This is the first time aromaticity was discovered in surface-deposited clusters. 4) We study the activity of these clusters toward the reaction of CO oxidation as a function of cluster size. We now recognize that the key to the catalysis in the parallel experiments by Prof. Anderson (U of U) is the activation of oxygen, which is done better by clusters that permit multi-dentate binding of oxygen. Additionally, the process can be facilitate by the nearby O-vacancies that dissociate O2. This work is in progress. 5) Our insight into the electronic structure of these systems allowed us to rationally choose Au as a low-concentration dopant for deposited Pd clusters, for more efficient catalysis. The smallest such cluster, AuPd4 on titania is more efficient at CO oxidation than Pd5 on titatnia. The effect is due to smaller affinity to O2, which therefore cannot poison the catalyst, as in the case of pure Pd clusters. 6) In parallel with this effort, we also pay close attention to gas phase clusters and new aspects of chemical bonding in them. For example, we discovered that hybridization of atomic orbitals can be present in all-metal clusters and impact their shapes. Furthermore, the effect of hybridization on shape is opposite of that of delocalized (aromatic) bonding. Therefore, the two effects constitute the “knobs” of cluster design. 7) We used this discovered mechanism to design a new cluster containing a tetracoordinated square planar Si atom, for example. As a project of opportunity, along these lines, we discovered a new molecular motor, the B13+ cluster. It possesses a dual ring structure, and the inner ring can rotate with respect to the outer ring when the cluster is exposed to the circularly polarized light in the THz range. 8) We also presently develop new methods for: (i) The unbiased search for the global minima of deposited clusters (ii) Statistical mechanical modeling of catalytic solid/gas interfaces at high temperatures and pressures. This is a multi-thread QM/MM Monte Carlo method operating on Argonne supercomputer, utilizing as many as 10,000 cores at once. It is presently available in beta-version, and should be in full production by the end of the year 2012. (iii) Machine-learning guided Wannier Functions method for the analysis of electron transport pathways in surface-deposited clusters in catalysis 9) The current applied focus is on larger clusters of Pd and Pd doped with Au and Cu deposited on titania for CO oxidation.