Tuning Heterogeneous Catalysis Using Interfacial Polarization

A plethora of emerging energy conversion technologies employ heterogeneous catalysis at solid-liquid interfaces, ranging from faradaic electrocatalysis and nonfaradaic thermochemical catalysis. Unlike for a gas-solid interface, charge transfer events at a solid-liquid interface can polarize the interface, resulting in a local environment at the active site that is radically distinct from the environment in the bulk liquid phase. Thus, unraveling the influence of electrical polarization on solid-liquid interface is a critical prerequisite for elucidating reactivity trends and for the rational design of new catalysis. Accordingly, this dissertation aims to understand (Part I) and control (Part II) the interfacial polarization effects during heterogeneous catalysis. Part I of this thesis establishes a quantitative correlation between the degree of interfacial polarization and the perturbation of the local interfacial microenvironments under the conditions of catalysis. In particular, we examine spontaneous and driven polarization mechanisms that give rise to interfacial electrostatic gradient (Chapter 2) and non-equilibrium concentration profiles (Chapter 3), respectively. Exploiting a surface-specific nonfaradaic reaction probe to sample the local activity of protons, which serve both as free ionic charge carriers and reactants/products of proton-coupled electron transfer reactions, we quantify interfacial electrostatic field strength and non-equilibrium pH gradient within molecular length scales from the catalytic surface. Leveraging the fundamental knowledge of interfacial polarization mechanisms gained in Part I, Part II of this thesis establishes a general mechanistic framework for exploiting interfacial polarization to mediate and promote thermochemical catalysis. Specifically, we demonstrate that thermochemical aerobic oxidation catalysis in water is mediated via spontaneous interfacial polarization induced by the coupling of constituent electrochemical half-reactions (Chapter 4). Additionally, exploiting driven-polarization to induce the non-equilibrium local pH swing, we show that Pd-catalyzed thermochemical CO₂ hydrogenation to formate can be dramatically promoted with modest electrical bias under mild reaction conditions (Chapter 5).Ph.D

Polarization-Induced Local pH Swing Promotes Pd-Catalyzed CO2 Hydrogenation

Electrochemical polarization can dramatically promote the rate of concurrent nonfaradaic catalytic reactions, but the mechanistic basis for these promotion effects at solid-liquid interfaces remains poorly understood. Herein, we establish a mechanistic framework for nonfaradaic promotion in aqueous media that operates via a local pH swing induced by a concurrent faradaic reaction. As a model system, we examined the kinetics of nonfaradaic Pd-catalyzed CO2 hydrogenation to formate and find that the reaction can be promoted by a combination of high alkalinity and high CO2 concentration. In bulk electrolyte, alkalinity and CO2 concentration are inversely correlated to each other as set by the CO2/bicarbonate equilibrium. We show that this impasse can be overcome by using electrical polarization to generate a nonequilibrium local environment that has both high alkalinity and high CO2 concentration. We find that this local pH swing promotes the rate of nonfaradaic CO2 hydrogenation to formate by nearly 3 orders of magnitude at modest potential bias. The work establishes a rigorous mechanistic model of nonfaradaic promotion in aqueous media and provides a basis for enhancing hydrogenation catalysis under mild conditions via electrical polarization.N

Tracking Electrical Fields at the Pt/H₂O Interface during Hydrogen Catalysis

We quantify changes in the magnitude of the interfacial electric field under the conditions of H₂/H⁺ catalysis at a Pt surface. We track the product distribution of a local pH-sensitive, surface-catalyzed nonfaradaic reaction, H₂ addition to cis-2-butene-1,4-diol to form n-butanol and 1,4-butanediol, to quantify the concentration of solvated H⁺ at a Pt surface that is constantly held at the reversible hydrogen electrode potential. By tracking the surface H⁺ concentration across a wide range of pH and ionic strengths, we directly quantify the magnitude of the electrostatic potential drop at the Pt/solution interface and establish that it increases by ∼60 mV per unit increase in pH. These results provide direct insight into the electric field environment at the Pt surface and highlight the dramatically amplified field existent under alkaline vs acidic conditions.Air Force Office of Scientific Research (Award FA9550-18-1-0420

Quantification of Interfacial pH Variation at Molecular Length Scales Using a Concurrent Non‐Faradaic Reaction

We quantified changes in interfacial pH local to the electrochemical double layer during electrocatalysis by using a concurrent non-faradaic probe reaction. In the absence of electrocatalysis, nanostructured Pt/C surfaces mediate the reaction of H-2 with cis-2-butene-1,4-diol to form a mixture of 1,4-butanediol and n-butanol with selectivity that is linearly dependent on the bulk solution pHvalue. We show that kinetic branching occurs from a common surface-bound intermediate, ensuring that this probe reaction is uniquely sensitive to the interfacial pHvalue within molecular length scales of the surface. We used the pH-dependent selectivity of this reaction to track changes in interfacial pH during concurrent hydrogen oxidation electrocatalysis and found that the local pHvalue can vary dramatically (>3units) relative to the bulk value even at modest current densities in well-buffered electrolytes. This study highlights the key role of interfacial pH variation in modulating inner-sphere electrocatalysis.Y

Electrolyte Competition Controls Surface Binding of CO Intermediates to CO2 Reduction Catalysts

Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO2 to fuels. Directed design of CO2RR electrocatalysts have centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. Yet, this approach has largely ignored the role of competitive electrolyte adsorption in defining the CO surface population relevant for catalysis. Using in situ infrared spectroelectrochemistry, we disclose the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Whereas reversible CO binding to Au surfaces is driven by substitution and reorientation of adsorbed water, CO binding to Cu surfaces requires the reductive displacement of adsorbed carbonate anions. The divergent role of electrolyte competition for CO adsorption on Au vs. Cu leads to a ~600 mV difference in the potential region where CO accumulates on the two surfaces. The contrasting CO adsorption stoichiometry on Au and Cu also explains their disparate reactivity: water adsorption drives CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption drives CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insight into how electrolyte constituents can serve as powerful design parameters for fine-tuning of CO surface populations and, thereby, CO2-to-fuels reactivity.<br /

Electrolyte Competition Controls Surface Binding of CO Intermediates to CO 2 Reduction Catalysts

Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO2 to fuels. The directed design of CO2RR electrocatalysts has centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. This approach has largely ignored the role of competitive electrolyte adsorption in defining the CO surface population relevant for catalysis. Using in situ infrared spectroelectrochemistry and voltammetry, we uncover the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Although reversible CO binding to Au surfaces is primarily driven by the adsorption processes associated with interfacial water, CO binding to Cu surfaces requires the reductive displacement of adsorbed carbonate anions. The divergent role of electrolyte competition for CO adsorption on Au versus Cu leads to a similar to 600 mV difference in the potential region where CO accumulates on the two surfaces. The contrasting CO adsorption stoichiometry on Au and Cu also explains their disparate reactivity: interfacial water adsorption contributes to CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption contributes to CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insights into how electrolyte constituents fine-tune CO surface populations and, thereby, CO2-to-fuel reactivity.Y

Orthogonal reactivity of acyl azides in C-H activation: Dichotomy between C-C and C-N amidations based on catalyst systems

The dual reactivity of acyl azides was utilized successfully in C-H activation by the choice of catalyst systems: while selective C-C amidation was achieved under thermal Rh catalysis, a Ru catalyst was found to mediate direct C-N amidation also highly selectively. Investigations of the mechanistic dichotomy between two catalytic systems are also presented. © 2014 American Chemical Society.153541sciescopu

Orthogonal Reactivity of Acyl Azides in C-H Activation: Dichotomy between C-C and C-N Amidations Based on Catalyst Systems

The dual reactivity of acyl azides was utilized successfully in C-H activation by the choice of catalyst systems: while selective C-C amidation was achieved under thermal Rh catalysis, a Ru catalyst was found to mediate direct C-N amidation also highly selectively. Investigations of the mechanistic dichotomy between two catalytic systems are also presented.N

Bicarbonate Is Not a General Acid in Au-Catalyzed CO₂ Electroreduction

We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO₂ reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO₂-saturated aqueous electrolytes.United States. Air Force Office of Scientific Research (Grant FA9550-15-1-0135

Ir(III)-Catalyzed Mild C-H Amidation of Arenes and Alkenes: An Efficient Usage of Acyl Azides as the Nitrogen Source

Reported herein is the development of the Ir(III)-catalyzed direct C-H amidation of arenes and alkenes using acyl azides as the nitrogen source. This procedure utilizes an in situ generated cationic half-sandwich iridium complex as a catalyst. The reaction takes place under very mild conditions, and a broad range of sp(2) C-H bonds of chelate group-containing arenes and olefins are smoothly amidated with acyl azides without the intervention of the Curtius rearrangement. Significantly, a wide range of reactants of aryl-, aliphatic-, and olefinic acyl azides were all efficiently amidated with high functional group tolerance. Using the developed approach, Z-enamides were readily accessed with a complete control of regio- and stereoselectivity. The developed direct amidation proceeds in the absence of external oxidants and releases molecular nitrogen as a single byproduct, thus offering an environmentally benign process with wide potential applications in organic synthesis and medicinal chemistry.N