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Abstract: We used a combination of experimental spectroscopies, density functional theory calculations, and CO2 hydrogenation studies to investigate the effects of modifying single-atom Rh1/TiO2 catalysts with functionalized phosphonic acid monolayers. We found that the deposition of specific amine-functionalized ligands resulted in an ～8× increase in site-specific CO2 reduction turnover frequency at 150 °C and a ～ 2× increase at 250 °C. On-stream stability also improved following ligand deposition. The effect of the modifier on reactivity was highly sensitive to the proximity of the amine functional group to the surface, which was controlled by adjusting the length of the phosphonic acid tail. Furthermore, deposition of alkyl phosphonic acids without an amine functional group resulted in blocked CO2 adsorption and a near-complete loss of catalytic activity. Infrared spectroscopy studies suggested that the amine group provided binding sites for CO2 that enabled hydrogenation when the amine was positioned near a Rh1 site. Phosphonic acid-modified catalysts also exhibited high selectivity to CO over the series product methane; the selectivity effect was traced to modification of the Rh1 sites to favor CO desorption. Phosphonic acid deposition resulted in 80–90% loss of accessible Rh1 sites, likely due to blocking by tail groups. However, even with the loss of sites, under low-temperature reaction conditions, the rates of CO2 hydrogenation were improved with the coatings, indicating that the remaining sites are highly efficient. Organic functionalization of the supports for atomically dispersed catalysts offers the opportunity to precisely control the positioning of functional groups in the vicinity of a well-defined active site, potentially enabling an additional level of control over active site design.

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Abstract: Heterogeneous catalysts are preferred over other catalytic systems for a wide variety of applications due to their high activity and reusability. Oxide-supported metal catalysts, typically consisting of transition metal nanoparticles anchored to a metal oxide carrier, are one of the most common classes of heterogeneous catalysts due to the high surface area of dispersed metal nanoparticles. Yet despite their frequent use, these catalysts still face considerable limitations such as low thermostability and the inability to independently tune interactions with different reactants. One promising strategy towards overcoming these limitations is the use of organic self-assembled monolayers to modify the surface and near-surface environment of heterogeneous catalysts. In this thesis, we investigate the effects of organic monolayers on the catalytic performance and surface properties of supported metal catalysts and develop broadly applicable approaches towards enhancing catalyst efficiency via rational catalyst design. When supported metal catalysts are exposed to elevated temperatures, metal adatoms on the oxide support become increasingly mobile, leading to metal nanoparticle sintering and loss of active sites. Organophosphonic acids were deposited onto the oxide support of Au/TiO2 and Pt/TiO2 catalysts and imaged with transmission electron spectroscopy. These monolayers were observed to prevent metal sintering at elevated temperatures while metal dispersion decreased significantly for unmodified catalysts. However, these ligands were also observed to block or alter active sites at the metal – support interface. This led to significantly suppressed CO oxidation rates, particularly on Au/TiO2, as well as improved resistance to the accumulation of surface carbonaceous species during acetelyene hydrogenation, increasing catalyst lifespan. Functionalization of organophospho_x005f_x0002_nic acid ligands to promote ligand – adsorbate interactions could lead to improved rates by adding bifunctionality at the metal – support interface. To further explore the effects of functionalized organic monolayers on adsorbate binding, ligand – adsorbate hydrogen bonding interactions were investigated with density functional the_x005f_x0002_ory quantum mechanical modelling. Molecular adsorption strengths were calculated on thiolate_x005f_x0002_modified fcc (111) surfaces. Ligand – adsorbate hydrogen bonds led to the preferred stabilization of hydroxyl-containing adsorbates, ‘shifting’ conventional linear scaling relations for molecular ad_x005f_x0002_sorption strengths. For hydrogen bonds sufficiently far from the metal surface, this increase in stabilization was solely dependent on the acidities of the hydrogen bonding functional groups, lead_x005f_x0002_ing to a constant shift in scaling across all metal surfaces. However, as the location of the hydrogen bond approached the surface, interactions with the metal strengthened, altering the slope of ad_x005f_x0002_sorption strength scaling. Additionally, conformational changes were found to offset the stabilizing effect of the ligands in sterically crowded systems. Single atom catalysts have received considerable attention due to their high metal dispersion and unique active sites. However these catalysts frequently suffer from poor thermostability due to Ostwald ripening, as well as low activity for certain reactions due to oxidic electronic states of the catalytic metal and an absence of ensemble sites. Organophosphonic acids were deposited onto Rh/TiO2 single atom catalysts to improve stability and activity during CO2 reduction to CO. Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) with CO probe molecules was used to verify the presence of Rh1 species and the absence of Rh nanoparticles following Rh/TiO2 synthesis and ligand deposition. Amine-functionalization of the ligand tails resulted in improved selectivity and specific activity towards CO, whereas alkyl ligands resulted in significant losses in activity due to blocked CO2 adsorption. However, the deposition of these organic monolayers also led to a significant loss in available active sites. Distance between the ligand’s terminal amine and phosphorus head group was also found to significantly affect both specific activity as well as site blocking.