Publication Date

2024

Document Type

Dissertation/Thesis

First Advisor

Xu, Tao

Degree Name

Ph.D. (Doctor of Philosophy)

Legacy Department

Department of Chemistry and Biochemistry

Abstract

This dissertation presents a comprehensive study on the catalyst supports and the size of catalytic active sites. From a microscopic perspective, this research elucidates the implications of the nanoconfinement effects and the size effects of active sites in the electrocatalytic CO2 reduction process. It lays a solid foundation for the future development of superior catalysts.Electrochemical conversion of CO2 into value-added fuels and chemical feedstocks can serve future energy demands by storing renewable energy and reducing anthropogenic emissions of CO2. CO2 reduction reaction (CO2RR) to multi-carbon chemicals have generated growing interest in recent years, stimulated not only by the economic benefit of producing higher value chemicals, but also rich fundamental sciences associated with the new electrocatalysis. Despite having achieved high activity and selectivity of CO2RR towards C1 chemicals (CO, HCOOH, etc.), conversions to C2 (C2H4, C2H5OH, etc.) and C3(C3H7OH, etc.) products are still under intensive studies facing significant challenges. Unlike the formation of CO or formic acid that involve only two proton-electron pairs (PEPs), the numbers of PEPs escalate quickly during the CO2RR to C2 and C3 chemicals, in addition to C-C coupling required in the reactions. Large number of PEPs and C-C bond formation lead to multistep and convolute CO2 conversion reaction coordinates with many possible branching paths, rendering it difficult to achieve high single product selectivity, or Faradaic efficiency (FE). Specifically, we conducted a thorough investigation into a series of carbon-supported Sn (tin) electrocatalysts, encompassing a spectrum of Sn sizes ranging from single atoms and ultra-small clusters to nano-crystallites. This study reveals remarkable findings in terms of achieving high single-product FE and low onset potential for the conversion of CO2 to acetate (FE = 90% at -0.6 V), ethanol (FE = 92% at -0.4 V), and formate (FE = 91% at -0.6 V) using catalysts with varying dimensions of active sites. The mechanism underlying the CO2 conversion, attributable to these highly selective, size-modulated p-block element catalysts, is elucidated through a combination of structural characterization and computational modeling. This research not only contributes to the understanding of size-dependent catalytic behaviors but also advances the development of efficient electrocatalysts for CO2 reduction. Moreover, to promote high FE for long-chain carbon chemical formation, the catalytic center for CO2RR should provide strong retention for the reaction intermediates to complete the necessary multistep PEP transfers for the electrocatalytic reduction. Such robust interaction can be accomplished by two complementary factors in the catalysis: binding between the catalytic site and reactant intermediate and confinement within the microenvironment, respectively. In catalysis, stronger binding can be achieved through the ligation of reactant with the uncoordinated orbital of active sites, which is often found at the edge or the defect of the metal center. Single metal atom has the highest unsaturated orbitals, representing the ultimate case of the catalytic “defect” therefore highest binding energy with CO2 during the electrocatalytic reactions. The retention of CO2 during the multiple steps electrocatalytic conversion for long chain chemicals can be further enhanced by the confinement effect afforded by the catalyst support. For example, the micropores in the support can provide physisorption of CO2 through enhanced van der Waals (vdW) interaction. The heat of adsorption (ΔHads, adsorption enthalpy) by physisorption through non-polarized vdW force inside of micropore can reach over 20 kJ/mol. The nanoconfinement from the support retains CO2 and conversion intermediates inside micropores of the active center’s proximity, promoting catalytic reaction through enhanced encountering frequency. Non-polarized pore wall interaction will not induce charge redistribution in CO2, therefore will not alter the electrocatalytic reaction pathways. Another way of increasing CO2-support interaction is to functionalize the surface with nucleophile group such as amine to produce chemisorption of significantly higher ΔHads than physisorption. Several systems with CO2 chemisorption promoted by amine groups have been studied recently in an attempt to achieve combined direct CO2 capture from ambient air followed by electrochemical conversion to chemicals. These studies found that the amine-CO2 adducts could produce Zwitterion with negatively charged carbamate group, forming a Helmholtz layer to slow down the electron transfer from the catalyst, limiting most of the catalytic conversions to 2 e-transfer to form C1 chemicals (CO and formate). We applied Cu single-atom catalyst (SAC) over 3 commercial carbon supports of different micro-porosity distributions. The electrocatalysts achieved high CO2RR selectivity toward glycerol (C3H8O3), a C3 chemical, with FE of 89.9% at –0.7 V vs. reversible hydrogen electrode (RHE). In addition, we found that the conversion to glycerol was highly sensitive to the microporosity of the carbon support. As the pore size in the carbon support transitioned from majority of micropore to mesopore, the FEs also switched from mostly glycerol (C3) to ethanol (C2). Such changing trend also correlated well with the measured CO2 absorption enthalpy.

Extent

171 pages

Language

en

Publisher

Northern Illinois University

Rights Statement

In Copyright

Rights Statement 2

NIU theses are protected by copyright. They may be viewed from Huskie Commons for any purpose, but reproduction or distribution in any format is prohibited without the written permission of the authors.

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