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

世界各地的大多数化工过程都使用催化剂，这些催化剂可以促进化学品和燃料的高效生产，同时还可以降低能源成本和环境足迹。催化剂技术的进步反过来又取决于了解催化剂的性能是如何由其表面和反应分子的性质决定的。本项目侧重于发展对合成乙酸乙烯酯(VA)所涉及的催化反应的理解。VA是一种由乙烯和醋酸分子在昂贵的钯基催化剂上偶联而成的高价值化学品。它是许多聚合物和消费品的重要组成部分。然而，目前的VA制造工艺有几个限制。该项目通过研究旨在开发更有效的催化剂来解决这些限制，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生和高中生的教育和推广活动相结合。在实际条件下，许多催化反应的表面覆盖率很高。然而，在实验数据分析中严格纳入覆盖面的影响，以及在模拟模型中纳入，是具有挑战性的。因此，很少有研究侧重于设计在高覆盖条件下有效运行的催化剂。覆盖度在醋酸和乙烯氧化偶联合成VA过程中起着重要作用。在该反应中，醋酸酯物种的表面拥挤促进了C-O偶联，并以影响反应活性和选择性的方式抑制了解离步骤。对该反应的机理理解源于表面科学和密度泛函理论(DFT)，但基本上所有过去的工作都集中在负载型钯(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，容易长时间耗尽。这个项目的目的是：(I)了解(使用动力学和密度泛函理论)与稳态催化有关的机理方面，(Ii)评估分散在Au和Cu基质中的活性原子的侧向相互作用的非常不同的强度如何影响速率和选择性，以及(Iii)利用这些见解设计更有效的VA合成催化剂。该项目建立在研究人员与他所在机构的同事合作获得的单金属钯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，当稳态催化中的反应物压力改变时，覆盖层如何改变速率浸灰步骤。DFT研究将建立在这些见解的基础上，并以分散在铜和金基质上的一系列单原子为基础，探索(I)较小的晶格常数所施加的增强的横向相互作用如何促进C-O耦合，以及(Ii)覆盖率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，在VA合成催化中具有更好的活性、选择性和稳定性。研究中使用的数据生成、催化剂表征和分析方法-结合从VA合成中获得的见解-将延续到在吸附物种的表面覆盖率较高的条件下运行的其他催化反应。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.