Computational Design of Novel Nanocatalysts and Nanomaterials Towards Carbon Neutral Energy
Chen, Zhongfang (Consejero)
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In this dissertation, the computational design of novel nanocatalysts and nanomaterials towards neutral energy, including Mn-modified Rh(100) for syngas-ethanol conversion, Feanchored graphene oxide and single metal atom supported on oxides for CO oxidation, the electronic and magnetic properties of MoO3-based nanomaterials and their application as cathode in lithium ion batteries, was systemically investigated. The demand for renewable energy keeps increasing as fossil fuels are being exhausted. Biomass formed by photosynthesis of carbon dioxide and water is one of the most promising alternative energy sources. Converting biomass to ethanol is a major way to extract the energy content in biomass. Rh is unique in its ability to convert syngas to ethanol with the help of promoters. We performed systematic first-principles computations to examine the catalytic performance of pure and Mn modified Rh(100) surfaces for ethanol formation from syngas. CO dissociation on the surface as well as CO insertion between the chemisorbed CH3 and the surface are the two key steps. The CO dissociation barrier on Mn monolayer modified Rh(100) surface is remarkably lowered by ~1.5 eV compared to that on Rh(100). Encouragingly, the reaction barrier of CO insertion into the chemisorbed CH3 group on Mn monolayer modified Rh(100) is 0.34 eV lower than that of methane formation. Thus the present work provides new mechanistic insight into the role of Mn promoters in improving Rh’s selectivity to convert syngas to ethanol. Low-temperature oxidation of CO is one of the most extensively studied reactions of heterogeneous catalysis, due to the impending demand of lowing emissions from automobiles, industrial processes, etc. The stability and catalytic capability for CO oxidation with O2 of Feanchored graphene oxide (GO) instead of the widely used noble-metal-based catalysts was studied. The high energy barrier of Fe atom diffusion on GO, and the strong binding strength of Fe anchored on GO exclude the metal clustering problem and enhance the stability of the Fe-GO system. The Fe-anchored GO exhibits good catalytic activity for CO oxidation via the favorable Eley-Rideal (ER) mechanism with a two-step route. The low-cost Fe-anchored GO system can be easily synthesized, and serve as a promising green catalyst for low-temperature CO oxidation. Following the satisfying performance of low-cost Fe-anchored GO on CO oxidation, and inspired by the recent experimental progress on the successful synthesis of highly dispersed metal atoms on supports, the single-metal-atom-based composites were further investigated for their stability and activity for CO oxidation. We systemically examined various single-atom catalysts M1/FeOx (M=Au, Rh, Pd, Co, Cu, Ru and Ti) by means of DFT computations, aiming at developing even more efficient and low-cost nanocatalysts for CO oxidation. Our computations identified five single-atom catalysts, namely the oxygen-defective Rh1/FeOx and Pd1/FeOx, Ru1/FeOx with or without oxygen vacancy, vacancy-free Ti1/FeOx and Co1/FeOx, which exhibit improved overall catalytic performance than the Pt1/FeOx [Nature Chem. 3, 634 (2011)] for the CO oxidation via Langmuir-Hinshelwood (L-H) mechanism. We further examined the systems with bimetal atoms anchored on the FeOx surface, and found that oxygendefective Rh1Pd1/FeOx and Ru1Rh1/FeOx also show higher catalytic activities than Pt1/FeOx or their monometallic counterparts. Moreover, another four different metal oxides (TiO2, Al2O3, CeO2 and MgO) were used to examine the support effect. These theoretical results provide new guidelines to design even more active and/or cost-effective heterogeneous catalysts for the CO oxidation. The explosive studies on graphene after its realization in 2004 have sparked new interests towards graphene-analogous materials. Layered a-MoO3, as one inorganic graphene, has been widely explored. Here we performed DFT computations to examine the electronic and magnetic properties of MoO3 two-dimensional (2D) nanosheet and its derived one-dimensional (1D) nanoribbons (NRs). The pristine 2D MoO3 sheet is a nonmagnetic semiconductor with an indirect band gap, but can be transformed to a magnetic metal by surface hydrogenating. Depending on the cutting pattern, the pristine 1D NRs can be indirect-band-gap nonmagnetic semiconductors, magnetic semiconductors or magnetic metals. The fully hydrogenated NRs are metallic, while the edge-passivated NRs possess the nonmagnetic semiconducting feature, but with narrower band gap values compared to the pristine NRs. Both the 2D monolayer MoO3 sheet and the 1D nanoribbons maintain the semiconducting behaviors when exerting axial strain. These findings provide a simple and effective route to tune the magnetic and electronic properties of MoO3 nanostructures in a wide range and also facilitate the design of MoO3-based nanodevices. Lithium-ion batteries are being actively pursued for transportation and stationary storage of renewable energies, and the success of lithium-ion technology for these large-cell applications is determined by the development of new electrode materials with superior electrochemical properties that can offer high energy and power at an affordable cost and acceptable safety. Based on the study of the electronic and magnetic properties of MoO3 2D nanosheet and its derived 1D NRs, we systematically investigated the adsorption and diffusion of Li on the 2D MoO3 nanosheets and 1D MoO3 nanoribbons, in comparison with MoO3 bulk. The Li mobility can be significantly facilitated in MoO3 nanosheets, meanwhile the binding energies are decreased but still much higher than the cohesive energy per Li in Li metal. Due to the presence of unique edge states, the NRs have a considerably enhanced binding interaction with Li atoms without sacrificing the Li mobility compared to MoO3 monolayer. Thus the MoO3 monolayer nanosheets and nanoribbons are promising as cathode materials of Li-ion battery with a high power density and fast charge/discharge rates.