Stability of Single-Atom Catalysts Under Aqueous Reductive Conditions

Date of Award

Spring 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Environmental Engineering (ENAS)

First Advisor

Kim, Jaehong

Abstract

Single-atom catalysts (SACs) have reinvigorated the potential of catalytic materials for environmental applications. Containing atomically dispersed metal atoms, SACs improve metal utilization compared to conventional nanoparticulate catalysts, thereby reducing the cost-barrier to implementation. However, SACs are difficult to stabilize due to the inherent high surface energy of single-atoms, resulting in a propensity for aggregation into clusters or nanoparticles. The combined effects of metal, coordination environment, and catalytic application significantly influence the extent of aggregation. While much effort has been made regarding SAC stability in gas-phase catalysis, less is known about aqueous-phase SAC stability, especially pertinent as we develop materials targeting water treatment. We first investigate the stability and performance of single-atom Pd on TiO2 for the selective dechlorination of 4 chlorophenol. The instability of Pd single-atoms and aggregation into clusters vastly enhanced dechlorination kinetics without diminishing carbon-chlorine bond selectivity. This work further evaluates various factors affecting the stability of Pd single-atoms, including atomic dispersion, coordination environment, and substrate properties. X-ray absorption spectroscopy (XAS) analysis using a novel in situ jetting/gas delivery system followed in real-time the dynamic transformation of single-atoms into amorphous clusters, presenting nuance to the relationship between catalytic activity, selectivity, and stability. Next, the examination of SAC stability is extended to electrocatalysis. We evaluated the aggregation behavior of a suite of SACs varied by metal identity (Fe, Co, Ni, and Cu) during electrocatalytic nitrate reduction using in situ XAS to probe stability as a function of applied reductive potential. The metal center had a significant influence on reconstruction, where under identical applied reductive potentials, SACs experienced varying levels of stability ranging from no discernible change to complete reduction into metallic nanoparticles, pointing to the key role of metal identity on SAC stability. We further our investigation of electrocatalytic SAC stability to address the role of coordination environment on the aggregation of Cu SACs. The aggregation behaviors of Cu single-atoms anchored to O , B , and N doped graphene were monitored using in situ XAS. We analyzed the transformation of Cu species using linear combination fitting to evaluate changes in composition over a range of applied potentials. Connecting the stability profiles to nitrate reduction performance reveals how non-metal dopants can both moderate SAC stability and control nitrate reduction activity. Lastly, we assess Cu SAC stability in comparison to conventional Cu nanoparticles through the lens of leaching to determine whether single atom aggregation results in enhanced metal loss. Monitoring Cu leaching using inductively coupled plasma mass spectrometry (ICP-MS) alongside time-resolved in situ XAS, we reveal the effects of applied potential, reaction time, and nitrate concentration on metal dissolution during electrocatalytic nitrate reduction. Taken together, we uncover the dual nature of SAC instability under reductive electrocatalysis to be governed by both aggregation and leaching. Much of current SAC research has focused on the pursuit of maximal catalytic performance with lesser regard to possible conformational changes that may occur during operation. This dissertation highlights the dynamic nature of single-atoms under both thermo- and electrocatalytic schemes relevant to water treatment applications. The intricate relationship between stability and performance underscores the vital role of detailed characterization to properly determine the true catalytic active species. Our findings reveal that SAC instability can, at times, enhance catalytic reactions, demonstrating the complexities surrounding SAC stability. Only by investigating SACs collectively through the lens of stability, activity, and selectivity can we discern the potential of SACs for aqueous pollutant remediation.

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