CAREER: Understanding Effects of Surface Coverage and Catalyst Composition on Vinyl Acetate Synthesis

全球大多数化学工艺都使用催化剂，促进化学品和燃料的高效生产，同时降低能源成本和环境足迹。反过来，催化剂技术的进步取决于理解催化剂的性能如何由其表面和反应分子的性质决定。本项目的重点是发展这种理解的催化反应中涉及的乙酸乙烯酯（VA）的合成。VA是一种高价值的化学品，通过乙烯和乙酸分子在昂贵的钯基催化剂上偶联而大量生产。它是许多聚合物和消费品的重要组成部分。然而，目前的VA制造工艺具有若干限制。 该项目通过旨在开发更有效的催化剂的研究来解决这些限制，这些催化剂由原子分散在廉价铜基质上的贵金属原子组成。 该项目将研究与针对初中和高中学生的教育和推广活动结合起来，在实际条件下，高表面覆盖率在许多催化反应中占主导地位。然而，在实验数据分析中严格纳入覆盖率的影响以及纳入模拟模型是具有挑战性的。 因此，很少有研究集中在催化剂的设计，在高覆盖率的条件下有效地运行。 覆盖度在乙酸和乙烯氧化偶联合成VA中起重要作用。在这个反应中，拥挤的表面与乙酸物种促进C-O耦合和抑制解离步骤的方式，影响反应性和选择性。对该反应的机理理解已经出现在表面科学和密度泛函理论（DFT）中，但基本上所有过去的工作都只集中在负载的钯（Pd）或钯-金（PdAu）催化剂上，这些催化剂在实际条件下不稳定，没有碱稳定剂，这些稳定剂往往会在长时间内耗尽。该项目的目的是，（i）理解（使用动力学和DFT）的稳态催化的机理方面，（ii）评估如何分散在Au与Cu矩阵的影响率和选择性的活性原子的横向相互作用的非常不同的强度，以及（iii）利用这些见解设计更有效的VA合成催化剂。该项目建立在由研究人员与其机构的同事合作获得的monoclonal Pd催化剂的初步实验和计算结果的基础上。 Pd的工作提供了重要的见解如何覆盖率施加变化时，反应物的压力变化，在稳态催化速率石灰步骤。 DFT研究将建立在这些见解和探针上，在分散在Cu和Au基质上的一系列单原子上，（i）由较小的Cu晶格常数施加的增强的横向相互作用如何促进C-O耦合，以及（ii）覆盖和组成的组合效应如何影响催化性能。这些结果将进一步指导单原子合金催化剂的制备和结构-功能表征，具有更好的活性，选择性和稳定性的VA合成催化潜力。 该研究中采用的数据生成、催化剂表征和分析方法--结合VA合成的结果见解--将延续到在吸附物种高表面覆盖率条件下运行的其他催化反应中。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

The majority of chemical processes worldwide utilize catalysts that promote the efficient manufacturing of chemicals and fuels, while also lowering energy costs and environmental footprint. Advances in catalyst technology depend, in turn, on understanding how a catalyst’s performance is determined by the properties of its surface and of the reacting molecules. This project focuses on developing such understanding for the catalytic reaction involved in the synthesis of vinyl acetate (VA). VA is a high-value chemical produced in large volumes by the coupling of ethylene and acetic acid molecules on expensive palladium-based catalysts. It is an important component of many polymers and consumer products. However, current VA manufacturing processes have several limitations. The project addresses those limitations through research aimed at developing more efficient catalysts consisting of atomically dispersed precious metal atoms on inexpensive copper matrices. The project integrates research with educational and outreach activities aimed at middle- and high-school students.High surface coverages prevail in many catalytic reactions at practical conditions. Rigorous incorporation of the effects of coverage in the analysis of experimental data, as well as incorporation in simulation models, is challenging, however. Thus, few studies exist focusing on the design of catalysts to operate efficiently under high-coverage conditions. Coverage plays an important role in VA synthesis via oxidative coupling of acetic acid and ethylene. In this reaction, crowding of the surface with acetate species promotes C-O coupling and inhibits dissociative steps in ways that influence both reactivity and selectivity. Mechanistic understanding of this reaction has emerged from surface science and density functional theory (DFT), but essentially all past work has focused solely on supported palladium (Pd) or palladium-gold (PdAu) catalysts that are unstable at practical conditions without alkali stabilizers that tend to deplete over long times. This project is aimed at, (i) understanding (using kinetics and DFT) the mechanistic aspects pertaining to steady-state catalysis, (ii) assessing how the very different strengths of lateral interactions for active atoms dispersed in an Au versus Cu matrix influence rate and selectivity, and (iii) leveraging such insights to design more efficient VA synthesis catalysts. The project builds on preliminary experimental and computational results for monometallic Pd catalysts obtained by the investigator in collaboration with colleagues at his institution. The Pd work has provided significant insights on how coverages impose changes in rate liming steps when reactant pressures change in steady-state catalysis. DFT studies will build on these insights and probe, on a range of single atoms dispersed on Cu and Au matrices, how (i) enhanced lateral interactions imposed by the smaller lattice constant of Cu can promote C-O coupling, and (ii) how combined effects of coverage and composition influence catalytic performance. These results will further guide the preparation and structural-functional characterization of single atom alloy catalysts with potential for better activity, selectivity, and stability in VA synthesis catalysis. The data generation, catalyst characterization, and analysis methods employed in the study – combined with the resulting insights from VA synthesis - will carry over to other catalytic reactions that operate under conditions of high surface coverage of adsorbed species.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.