Mesostructure-Induced Selectivity in CO2 Reduction Catalysis

Gold inverse opal (Au-IO) thin films are active for CO[subscript 2] reduction to CO with high efficiency at modest overpotentials and high selectivity relative to hydrogen evolution. The specific activity for hydrogen evolution diminishes by 10-fold with increasing porous film thickness, while CO evolution activity is largely unchanged. We demonstrate that the origin of hydrogen suppression in Au-IO films stems from the generation of diffusional gradients within the pores of the mesostructured electrode rather than changes in surface faceting or Au grain size. For electrodes with optimal mesoporosity, 99% selectivity for CO evolution can be obtained at overpotentials as low as 0.4 V. These results establish electrode mesostructuring as a complementary method for tuning selectivity in CO[subscript 2] -to-fuels catalysis

Impacts of dust deposition on dissolved trace metal concentrations (Mn, Al and Fe) during a mesocosm experiment

The deposition of atmospheric dust is the primary process supplying trace elements abundant in crustal rocks (e.g. Al, Mn and Fe) to the surface ocean. Upon deposition, the residence time in surface waters for each of these elements differs according to their chemical speciation and biological utilization. Presently, however, the chemical and physical processes occurring after atmospheric deposition are poorly constrained, principally because of the difficulty in following natural dust events in situ. In the present work we examined the temporal changes in the biogeochemistry of crustal metals (in particular Al, Mn and Fe) after an artificial dust deposition event. The experiment was contained inside trace metal clean mesocosms (0–12.5 m depths) deployed in the surface waters of the northwestern Mediterranean, close to the coast of Corsica within the frame of the DUNE project (a DUst experiment in a low Nutrient, low chlorophyll Ecosystem). Two consecutive artificial dust deposition events, each mimicking a wet deposition of 10 g m−2 of dust, were performed during the course of this DUNE-2 experiment. The changes in dissolved manganese (Mn), iron (Fe) and aluminum (Al) concentrations were followed immediately after the seeding with dust and over the following week. The Mn, Fe and Al inventories and loss or dissolution rates were determined. The evolution of the inventories after the two consecutive additions of dust showed distinct behaviors for dissolved Mn, Al and Fe. Even though the mixing conditions differed from one seeding to the other, Mn and Al showed clear increases directly after both seedings due to dissolution processes. Three days after the dust additions, Al concentrations decreased as a consequence of scavenging on sinking particles. Al appeared to be highly affected by the concentrations of biogenic particles, with an order of magnitude difference in its loss rates related to the increase of biomass after the addition of dust. In the case of dissolved Fe, it appears that the first dust addition resulted in a decrease as it was scavenged by sinking dust particles, whereas the second seeding induced dissolution of Fe from the dust particles due to the excess Fe binding ligand concentrations present at that time. This difference, which might be related to a change in Fe binding ligand concentration in the mesocosms, highlights the complex processes that control the solubility of Fe. Based on the inventories at the mesocosm scale, the estimations of the fractional solubility of metals from dust particles in seawater were 1.44 ± 0.19% and 0.91 ± 0.83% for Al and 41 ± 9% and 27 ± 19% for Mn for the first and the second dust addition. These values are in good agreement with laboratory-based estimates. For Fe no fractional solubility was obtained after the first seeding, but 0.12 ± 0.03% was estimated after the second seeding. Overall, the trace metal dataset presented here makes a significant contribution to enhancing our knowledge on the processes influencing trace metal release from Saharan dust and the subsequent processes of bio-uptake and scavenging in a low nutrient, low chlorophyll are

Tuning the Photoresponse of CuFeO2 with Mg Doping

The influence of semiconductor photoelectrodes’ carrier concentration on their photoresponse is a long hypothesized, but hitherto not quantitatively approached avenue to optimize materials for photoelectrochemical electrode applications. Here, I report tuning of CuFeO$_{2}$ photoelectrodes using Mg doping to demonstrate the effects of carrier concentration on the photoresponse by combining a variety of solid-state chemistry and photoelectrochemical methods. After synthesizing dense ($\ge$85% theoretical), pure, Mg-incorporated pellets (CuFe$_{1-x}$Mg$_{x}$O$_{2}$, x = 0, 0.0005, 0.005, 0.02), carrier type and concentrations were quantitatively assessed using the Hall Effect as well as the samples’ resistivities and majority carrier mobility. The same samples were used in a newly designed photoelectrochemical cell to measure their photoresponse. I observe that carrier concentration cannot be independently tuned. In the CuFe$_{1-x}$Mg$_{x}$O$_{2}$ system,optimization of the photoresponse by carrier concentration alone is not possible due to coupled electronic effects, changing the nature of the majority carrier type near the saturation limit (x$\ge$0.05). To this end, I also explore possible defect chemistries giving rise to varying electronic and photoresponses. I find that the photoresponse is directly correlated to the majority carrier mobility

Controlling kinetic branching in C0₂-to-fuels catalysis

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2018.Cataloged from PDF version of thesis.Includes bibliographical references.While electrified transition-metal surfaces mediate the synthesis of carbonaceous products from CO₂, these processes suffer efficiency losses due to the multitude of products accessible over a narrow potential range. Selective product formation requires knowledge and control over various branch points in the reaction pathway. In this work, we will present two specific branch points: (1) the requirements for selective activation of CO₂ over H+ to form the two-electron reduced CO product; and (2) the requirements for the accumulation of surface-bound CO species that can be reduced to higher order products beyond CO. Using model Au and Cu electrocatalysts, we uncover mechanistic insights into these branch points. We identify the differential proton-coupling requirements for CO₂ versus H+ activation on polycrystalline Au surfaces that establish a mechanistic basis for CO versus H₂ product selectivity. Electrokinetic data are consistent with a mechanism of CO production involving rate-limiting single electron transfer to CO₂ with concomitant adsorption to surface-active sites, followed by one electron, two proton transfer, and CO liberation from the surface. In contrast, the data suggest a H₂ evolution mechanism involving rate-limiting single electron transfer coupled with proton transfer from bicarbonate, hydronium, and/or carbonic acid to form adsorbed H species, followed by sequential one electron, one proton transfer or H recombination reactions. We elucidate the differential CO electrosorption dynamics on polycrystalline Au and Cu surfaces using temperature-dependent in-situ surface-enhanced infrared absorption spectroscopy, establishing a mechanistic basis for potential-dependent CO binding. On Au surfaces, we observe that reversible linearly-bonded CO electrosorption is a water substitution process, where bound CO species readily dissociate from the surface upon negative potential bias. Conversely, labile CO species accumulate upon negative potential bias on Cu surfaces via a charge displacement reaction with carbonate, providing a pool of reactant primed for further reduction to higher order products. The enthalpy and entropy of electrosorption are also quantified.by Anna Lydia Nakamura Wuttig.Ph. D

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

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 (&gt;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

An Accessible Approach to Preparing Water-Soluble Mn 2+ -Doped (CdSSe)ZnS (Core)Shell Nanocrystals for Ratiometric Temperature Sensing

A new synthetic scheme allowing structural modifications to temperature-sensitive and watersoluble D-penicillamine-passivated Mn2+-doped (CdSSe)ZnS (core)shell nanocrystals (MnQDs) was reported using air-stable chemicals. The temperature-dependent optical properties of the nanocrystals were tuned by changing their structure and composition—the ZnS shell thickness and the Mn2+-dopant concentration. Thick ZnS shells significantly reduce the interference of nonradiative transitions on ratiometric emission intensities. High dopant concentration affords consistent temperature sensitivity. In addition to the new base structure for quantum dot ratiometric temperature sensing via flexible, glovebox-free routes, the results also underscore the generalizability of the emission intensity ratio scheme for temperature sensing, originally proposed for rare earth-doped materials