Robert J. Nielsen, Ph.D. - US grants

With this award from the Chemical Catalysis Program of the Chemistry Division, Professor William A. Goddard and colleague Robert Nielsen from the Departments of Chemistry and Chemical Engineering at the California Institute of Technology will develop and apply reactive force field (ReaxFF), through Monte Carlo (MC) and molecular dynamics (MD) simulations, to determine the in situ atomic scale structure of catalyst surfaces and heterogeneous interfaces in coordination with quantum mechanical (QM) studies of reactivity. The multiparadigm ReaxFF/MC/MD framework will be used to resolve the integral-occupation supercell structures of occupationally disordered multimetal oxides responsible for propane and propene ammoxidation. Most prominent among these are MoVNbTeO catalysts for ammoxidation of propene to acrylonitrile. Combining high temperature reactive dynamics trajectories with quantum-mechanical studies of key intermediates and transition states, the metallic elements involved in rate- and selectivity determining CH activation and carbon-heteroatom bond-forming steps will be identified. Separately, the chemical mechanism by which vanadyl pyrophosphate ((VO)2P2O7) stores oxygen atoms at its surface for use in the 14-electron oxidation of butane to maleic anhydride will be determined by simulating the annealing and calcination of large unit cell models of the vanadyl pyrophosphate and other high oxidation state V/P/O phases. New heterogeneous multimetallic catalysts for hydrocarbon functionalization will be posited through the optimal combination of the CH activation, radical trapping, ammonia activation and oxygen activation functions of existing catalysts. This computational framework has been validated on simpler bimetallic oxides, and will continue to be optimized. Developmental work will focus on (1) streamlining the fitting of force-field parameters for new combinations of elements against QM training sets and (2) extending the MC application of ReaxFF to grand canonical implementations.

Interfacial phenomena in catalysis and applications necessary for sustainable energy consumption occur at a regime too large for mature quantum mechanics-based simulations to play a predictive role: passivation and band tuning at semiconductor surfaces, grain boundaries in layered thermoelectric materials, semiconductor-metal and semiconductor organometallic solvent interfaces which facilitate charge transfer between photosensitive and catalytic components in photosynthetic or photovoltaic devices. The broader scientific impact of developing the ReaxFF/MC/MD approach to classical but reactive molecular dynamics simulations will be a tool for addressing a limiting factor in many heterogeneous catalysis and interfacial science applications: determining atomic-scale structures under operating conditions. Multimetal oxide catalysts are used to produce the commodity chemicals acrolein and acrylonitrile from propene, and even small improvements in catalytic performance (i.e., selectivity and activity) will have a substantial effect on energy consumption and yield. The economic impact of replacing propene with propane cannot be overstated. Since propene is generated from propane with an approximate cost of 10 ¢/lb, elimination of this step has the potential to save the US chemical industry several hundred million dollars per year. In addition, each quarter of Prof. Goddard's year long class is informed by progress in the group's current research. Applications which illustrate specific kinds of atomic interactions are discussed in the quarter of lectures in chemical physics; new and established computational techniques are taught in the quarter of lectures on methods; methods and software developed through the group's research are applied by students from experimental groups in the quarter of hands-on project work in students' respective fields.