Engineering single atom catalysts for selective chemistry
Date
2023
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Today, approximately 90% of the chemicals produced globally go through a catalytic process at some stage of development, and catalysts contribute roughly 40% of the worldwide GDP. More specifically, the development of heterogeneous catalysts is of great interest as they are utilized for various chemistries and processes in the petrochemical, energy, pharmaceutical industries, and more. Their thermal stability (ability to operate under high temperatures and pressures) and ease of separation are the main advantages of heterogeneous catalysts, while the inability to perform selective chemistry is their main challenge. ☐ In the past two decades, a new class of catalysts has emerged with hopes of bridging the advantages of homogeneous and heterogeneous catalysis: atomically dispersed metal catalysts (a.k.a. single-atom catalysts) on metal oxide supports. Compared to their supported nanoparticle or nanocluster counterparts, uncharacteristically high catalytic activity and selectivity have been reported for various reactions. However, our understanding of their structure-activity relationship is still rather poor as ex-situ and in-situ characterizations reveal the surface's complex role and the active site's dynamic nature under reaction conditions. ☐ In this thesis, I report how computational catalysis, in collaboration with experiments, provides fundamental insights that can guide catalyst design for selective chemistry (i.e., ethylene hydroformylation and hydrodeoxygenation of furfuryl alcohol) on single-atom catalysts. In Chapters 2 and 3, we use density functional theory (DFT) and microkinetic modeling to establish the electronic properties of atomically dispersed Rh on Al2O3, in the absence of and in the presence of MOx (M = Re, W) promoters, and to determine how the changes in electronic properties of Rh govern the catalytic reactivity of ethylene hydroformylation. For Rh supported on Al2O3, we performed mechanistic studies and microkinetic analysis of an extensive hydroformylation and hydrogenation reaction network and proposed dominant catalytic pathways that were able to reproduce all the major trends in selectivity and kinetic parameters reported. Compared to Rh supported on Al2O3, an enhancement in hydroformylation selectivity and activity are observed on the Rh-MOx (M = Re, W) pair sites. Mechanistic studies revealed that the ReOx promoter in the immediate vicinity of Rh impedes the ethane catalytic pathway, ultimately promoting propanal formation. On the other hand, Rh-WOx pair sites facilitate high selectivity hydroformylation by the dynamic formation of the Rh-W bond upon catalyst activation via WOx reduction. ☐ In Chapter 4, we extend our understanding of the hydroformylation reaction on atomically dispersed Rh to investigate how the support properties impact catalytic activity. The catalytic activity of Rh is enhanced in the order MgO (lowest), Al2O3, CeO2, ZrO2, SiO2, and ZSM-5 (highest) for ethylene hydroformylation. This reactivity order with respect to the support tracks the increase in the CO asymmetric stretch of the Rh gem-dicarbonyl species. DFT investigation indicates that the increase in the CO stretch is directly related to decreased π back-donation from Rh due to interaction with the support oxide of different basicity. Lastly, DFT calculations suggest that the basic supports (e.g., MgO) retard the ethylene hydroformylation CO insertion step as they raise the π_CO^* orbital, which is important in relieving the anti-bonding interaction between the ethyl anion and the carbonyl. ☐ In Chapter 5, we demonstrated a new strategy to control C-O bond cleavage via tuning the facet-sensitive strong metal-support interaction (SMSI) between Ir and CeO2. DFT calculations and XPS analysis indicated that Ir encapsulation is favored on CeO2 (111) under reaction conditions, and oxygen vacancies form more readily on encapsulated Ir than on pristine ceria. Kinetic studies correlate the increase in oxygen vacancy concentration to higher C-O bond activation, which is consistent with the decrease in the computed oxygen vacancy formation energy. ☐ In summary, this dissertation showcases how the gaps in the structure-activity relationship of atomically dispersed metals can be closed and suggests new strategies for future catalyst design to achieve high selectivity on heterogeneous catalysis.
Description
Keywords
Computational catalysts, Density functional theory, Single atom catalyst, Catalytic reactivity, Ethylene hydroformylation