Atomically dispersed noble metal catalysts have drawn wide attention as candidates to replace supported metal clusters and metal nanoparticles. Atomic dispersion can offer unique chemical properties as well as maximum utilization of the expensive metals. Addition of a second metal has been found to help reduce the size of Pt ensembles in bimetallic clusters; however, the stabilization of isolated Pt atoms in small nests of nonprecious metal atoms remains challenging. We now report a novel strategy for the design, synthesis, and characterization of a zeolite-supported propane dehydrogenation catalyst that incorporates predominantly isolated Pt atoms stably bonded within nests of Zn atoms located within the nanoscale pores of dealuminated zeolite Beta. The catalyst is stable in long-term operation and exhibits high activity and high selectivity to propene. Atomic resolution images, bolstered by X-ray absorption spectra, demonstrate predominantly atomic dispersion of the Pt in the nests and, with complementary infrared and nuclear magnetic resonance spectra, determine a structural model of the nested Pt.
Methane’s abundance and low cost makes it an optimal raw material for chemical precursors and other energy dense fuels. Traditional methods of converting methane to methanol require large amounts of energy through thermal catalysis and high capital costs. However, electrochemical oxidation of methane is a cleaner and cheaper way to produce methanol, and a systematic study of an electrochemical cell could help assure selectivity over undesired products. Electrochemical oxidation of methane is an underdeveloped field, but electrochemical cells have been employed with high selectivity towards methanol. [1,2] Promising transition state metal-oxide catalysts have shown activity towards the selectivity of methanol over other by products, such as carbon dioxide, carbon monoxide, formic acid, formaldehyde. [2, 3] Further investigation of single site metal oxide catalysts is required to improve conversion and activity, but system optimization is also required. Reports of electrolyzer and fuel cell systems show varying assemblies, ranging from various solid and liquid electrolytes, temperatures, pressures, and current densities. [1-3] Few reports have been consistent on the operating current density and potential windows of such systems. Little work has been done on understanding how various components for these systems and understanding the phenomena of creating methanol in this partial oxidation pathway. In this study, we explore the activity, selectivity, efficiency of transition metal oxide catalyst in operation in an electrolyzer. The electrolyzer used was tested under a variety of relative humidity, operating temperatures, current densities, and membrane electrode assemblies. Through electrochemical and conductivity measurements, and gas phase analysis of the effluent, a better understanding of the partial electrochemical oxidation of methane to methanol is elucidated. The results of this study provide a systematic approach to this challenging problem and provide insights on new catalyst and electrochemistry pathways. References: Tomita, A., et. al., Direct Oxidation of Methane to Methanol at Low Temperature and Pressure in an Electrochemical Fuel Cell . Angewandte Chemie International Edition, 2008. 47 : p. 1462–1464. Lee, B., et. al., Direct oxidation of methane to methanol over proton conductor/metal mixed catalysts , Journal of Catalysis, 271( 2): p. 195-200. Rocha, R.S., et. al., Electrosynthesis of methanol from methane: The role of V2O5 in the reaction selectivity for methanol of a TiO2/RuO2/V2O5 gas diffusion electrode . Electrochimica Acta, 2013. 87: p. 606-610.
Abstract The 2020 Sturgis motorcycle rally resulted in widespread transmission of severe acute respiratory syndrome coronavirus 2 across the United States. At least 649 coronavirus disease 2019 cases were identified, including secondary and tertiary spread to close contacts. To limit transmission, persons attending events should be vaccinated or wear masks and practice physical distancing if unvaccinated. Persons with a known exposure should be managed according to their coronavirus disease 2019 vaccination or prior infection status and may include quarantine and coronavirus disease 2019 testing.
Electrolyte cation size is known to influence the electrochemical reduction of CO<sub>2</sub> over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pK<sub>a</sub> for cation hydrolysis decreases and is sufficiently low for hydrated K<sup>+</sup>, Rb<sup>+</sup>, and Cs<sup>+</sup> to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO<sub>2</sub>. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H<sub>2</sub> and CH<sub>4</sub>, and an increase in Faradaic efficiencies for CO, C<sub>2</sub>H<sub>4</sub>, and C<sub>2</sub>H<sub>5</sub>OH, in full agreement with experimental observations for CO<sub>2</sub> reduction over Ag and Cu.
This work considers the evaluation of density functional theory (DFT) when comparing against experimental observations of CO binding trends on the strong binding Pt(111) and intermediate binding Cu(111) and for weak binding Ag(111) and Au(111) surfaces important in electrocatalysis. By introducing thermal fluctuations using appropriate statistical mechanical NVT and NPT ensembles, we find that the RPBE and B97M-rV DFT functionals yield qualitatively better metal surface strain trends and CO enthalpies of binding for Cu(111) and Pt(111) than found at 0 K, thereby correcting the overbinding by 0.2 to 0.3 eV to yield better agreement with the enthalpies determined from experiment. The importance of dispersion effects are manifest for the weak CO binding Ag(111) and Au(111) surfaces at finite temperatures in which the RPBE functional does not bind CO at all, while the B97M-rV functional shows that the CO-metal interactions are a mixture of chemisorbed and physisorbed species with binding enthalpies that are within ∼0.05 eV of experiment. Across all M(111) surfaces, we show that the B97M-rV functional consistently predicts the correct <i>atop</i> site preference for all metals due to thermally induced surface distortions that preferentially favor the undercoordinated site. This study demonstrates the need to fully account for finite temperature fluctuations to make contact with the binding enthalpies from surface science experiments and electrocatalysis applications.
One of the main challenges towards achieving high efficiency solar fuel generators performing carbon dioxide reduction is the mass transport limitations in traditional aqueous systems. Typically, the current density for liquid phase carbon dioxide reduction is limited to approximately 10 mA/cm 2 due to concentration polarization near the cathode surface. 1 This can be overcome by introducing a gas diffusion electrode, which allows vapor phase carbon dioxide to be fed directly to the catalyst, significantly decreasing the diffusion length. Experiments have shown almost two orders of magnitude improvement in efficiency for vapor-fed devices compared to traditional aqueous systems. 2,3 This improvement is most likely due to a higher surface area in the porous electrode and an increased diffusivity of gas phase CO 2 . For this work, we have developed a multiphysics model for gas diffusion electrodes that simulates species transport in gas phase, liquid phase and solid phase, charge transport, and (electro)chemical reaction kinetics. The model will focus on understanding transport of species in and out of the catalyst layer, and investigate the effects of gas diffusion electrode properties such as thickness, porosity and hydrophobicity on cell performance. Acknowledgements This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. References Singh, M. R.; Clark, E. L.; Bell, A. T., Physical Chemistry Chemical Physics , 2015 , 17(29), 18924-36. Verma, S.; Lu, X.; Ma, S.; Masel, R. I.; Kenis, P. J. A., Physical Chemistry Chemical Physics , 2015 , 18, 7075-7084. Kim, B.; Hillman, F.; Ariyoshi, M.; Fujikawa, S.; Kenis, P. J. A., Journal of Power Sources , 2016 , 312, 192-198.
Chemical analysis of solid–liquid interfaces under electrochemical conditions has recently become feasible due to the development of new synchrotron radiation techniques. In this paper, we report the use of “tender” X-ray ambient-pressure X-ray photoelectron spectroscopy (APXPS) to characterize a thin film of Ni–Fe oxyhydroxide electrodeposited on Au as the working electrode at different applied potentials in 0.1 M KOH as the electrolyte. Our results show that the as-prepared 7 nm thick Ni–Fe (50% Fe) film contains Fe and Ni in both their metallic as well as oxidized states, and undergoes further oxidation when the sample is subjected to electrochemical oxidation–reduction cycles. Metallic Fe is oxidized to Fe<sup>3+</sup> and metallic Ni to Ni<sup>2+/3+</sup>. This work shows that it is possible to monitor the chemical nature of the Ni–Fe catalyst as a function of potential when the corresponding current densities are small. This allows for operando measurements just above the onset of OER; however, current densities as they are desired in photoelectrochemical devices (~1–10 mA cm<sup>–2</sup>) could not be achieved in this work, due to ohmic losses in the thin electrolyte film. We use a two-dimensional model to describe the spatial distribution of the electrochemical potential, current density, and pH as a function of the position above the electrolyte meniscus, to provide guidance toward enabling the acquisition of operando APXPS at high current density. Finally, the shifts in binding energy of water with applied potential predicted by the model are in good agreement with the experimental values.
A variety of experiments for the N<sub>2</sub>O decomposition over Fe-ZSM-5 catalysts have been simulated in the presence and absence of small amounts of nitric oxide and water vapor.
Polymer-electrolyte fuel cells and electrolyzers (PEFC&Es) have the potential to play a prominent role in green energy technologies including transportation, chemical manufacturing, and grid-scale energy storage. PEFC&Es have made significant advancements in recent years, largely due to improvements in the catalyst layers, and especially at the ionomer/catalyst interface. However, it is not yet definitively known how this solid-state environment impacts electrochemical kinetics, especially under various operating conditions (e.g., temperature, humidity, etc.). Additionally, it is challenging to probe local conditions at the catalyst/ionomer interface using traditional analytical techniques. In this study, we explore the influence and nature of proton activity in Nafion and 3M ionomers using a specialized microelectrode setup containing a 50 μm platinum microelectrode in a solid-state three-electrode cell for hydrogen oxidation and evolution (HOR&HER) reactions. Proton activity was calculated through open circuit voltage measurements, and was found to increase with increasing water content, mirroring trends in reaction performance. The effect of proton activity on the reactions' kinetics was investigated using semi-empirical fitting with the Butler-Volmer equation, which gives insight into the reaction rate order and possible mechanism for the reactions. This study demonstrates that microelectrodes can be used to probe solid-state kinetics and can also elucidate complex ion interactions within the ionomer at the catalyst/ionomer interface.
Abstract The electrochemical reduction of CO 2 is known to be influenced by the concentration and identity of the anionic species in the electrolyte; however, a full understanding of this phenomenon has not been developed. Here, we present the results of experimental and computational studies aimed at understanding the role of electrolyte anions on the reduction of CO 2 over Cu surfaces. Experimental studies were performed to show the effects of bicarbonate buffer concentration and the composition of other buffering anions on the partial currents of the major products formed by reduction of CO 2 over Cu. It was demonstrated that the composition and concentration of electrolyte anions has relatively little effect on the formation of CO, HCOO − , C 2 H 4 , and CH 3 CH 2 OH, but has a significant effect on the formation of H 2 and CH 4 . Continuum modeling was used to assess the effects of buffering anions on the pH at the electrode surface. The influence of pH on the activity of Cu for producing H 2 and CH 4 was also considered. Changes in the pH near the electrode surface were insufficient to explain the differences in activity and selectivity observed with changes in anion buffering capacity observed for the formation of H 2 and CH 4 . Therefore, it is proposed that these differences are the result of the ability of buffering anions to donate hydrogen directly to the electrode surface and in competition with water. The effectiveness of buffering anions to serve as hydrogen donors is found to increase with decreasing p K a of the buffering anion.
Light alkanes in shale gas are an attractive source of carbon for the production of alkenes and aromatics compared to petroleum-derived naphtha. Zinc-exchanged zeolite H-MFI (Zn/H-MFI) is active and selective for light alkane dehydrogenation and dehydroaromatization. In this study, Zn/H-MFI with varying Zn/Al ratios was prepared via solid-state ion exchange (SSIE) of ZnCl2and characterized by various methods. As-prepared Zn/H-MFI with Zn/Al ≤ 0.52 contains isolated [ZnCl]+and [ZnCl(HCl)]+species; Zn/H-MFI with higher Zn loadings also contains ZnAl2O4/ZnAl2O4-xCl2xnanoclusters. Postsynthetic treatment in He and subsequently in 2.5% H2in He at 773 K removes Cl and adsorbed HCl, resulting in the formation of [ZnH]+cations. Studies of C3H8dehydrogenation and cracking suggest that in the absence of cofed H2, [ZnH]+cations are transformed to bridging Zn2+cations, which exhibit higher C3H8dehydrogenation activity and selectivity relative to [ZnH]+cations. The kinetics of dehydrogenation and cracking over Zn/H-MFI were investigated as a function of Zn loading, C3H8partial pressure, and temperature. The turnover frequency for propane dehydrogenation and cracking increases with Zn loading, which we propose is due to localization of Zn2+cations either at increasingly distant pairs of Al atoms or at the β-site in the MFI framework. The selectivity to dehydrogenation over cracking over Zn2+is independent of C3H8partial pressure and temperature, consistent with dehydrogenation and cracking pathways that proceed via a common surface intermediate and have similar enthalpies of activation. The product distribution is thus determined by the entropy of activation for each pathway, which is less negative in the case of C3H8dehydrogenation.
