Generation of solar fuels through photo-catalytic reduction of CO 2 could simultaneously address both the need for sustainable and affordable energy sources and the necessity of reducing global CO 2 footprint. Discovering and designing new catalysts that have both high activity and selectivity for CO 2 reduction is the critical barrier to overcome in order for solar fuel generation processes to commercialize. So far, copper is the sole material known to efficiently catalyze conversion of CO 2 to considerable amounts of C2 hydrocarbon products over time. However, CO 2 reduction on Copper exhibits a high overpotential plus the added disadvantage of producing a mixture of several products including hydrogen. 1,2 Thus, it is of vital importance to understand the so far ambiguous and poorly understood catalytic CO 2 reduction mechanism on copper. In this work, a combination of various in-situ X-ray spectroscopy techniques are utilized to probe the electrode/electrolyte interface and provide insight into the reduction reaction.X-ray absorption spectroscopy (XAS) provides an element-specific probe of the conduction band via a core-level excitation into unoccupied electronic states and can reveal both oxidation state and details of the electronic structure. Operando Grazing incidence XAS at beamline 11-2 of SSRL is utilized to study the electrode surface at the metal K- and L-edges. A specially designed 3D printed flow cell maintainsa 300 micron liquid layer above the catalyst surface, enabling XAS characterization of the catalyst surface during electrochemistry in grazing incidence mode. The grazing incidence geometry provides a probe depth of about 5 nm into the catalyst, hence providing a highly surface sensitive spectroscopic tool. Preliminary measurements were conducted on the relatively inert surface of AuPd alloy catalyst. This is the first step in determining the feasibility of the study before examining the more complicated copper system. Distinct reversible shifts are observed in spectroscopic features with applied potential, which are believed to be due to H + intercalation into Pd phase. (Figure 1) Further, operando soft X-ray spectroscopy measurements at C-edge are conducted at beamline 8.0.1 of ALS to detect intermediate carbon species. Electron yield soft XAS data are collected at a depth of 1 nm above the electrode surface in a specially designed in-situ electrochemical flow cell 3 . Strong 1s to π* transition in C edge spectra provides fingerprint for different products. First principles DFT calculations are conducted to characterize features obtained through these XAS measurements. (Figure 2) Combination of these spectroscopic techniques with electrochemical and theoretical calculations will paint a comprehensive picture of the exact CO2RR mechanism on Cu surface, which then can be used to design next generation photo-catalysts. References: Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett., 1985, 1695–1698. K. P. Kuhl, E. R. Cave, D. N. Abram and T. F. Jaramillo, Energy Environ. Sci., 2012, 5, 7050-7059. J.J. Valesco-Velez, C.H. Wu, T.A. Pascal, L.F. Wan, J. Guo, D. Pendergast and M. Salmeron, Science, 2014, 346, 831-834. Figure 1

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.

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 N₂O decomposition over Fe-ZSM-5 catalysts have been simulated in the presence and absence of small amounts of nitric oxide and water vapor.

The objective of this study is to examine the mechanisms and kinetics of C2H6 dehydrogenation and n-C4H10 dehydrogenation and cracking over isolated Ga species in Ga/H-MFI and to compare these results to those reported previously for C3H8 dehydrogenation and cracking. C2H6 dehydrogenation is found to be catalyzed by both [GaH]2+ and [GaH2]+ cations at similar turnover frequencies. Rate measurements over Ga/H-MFI containing predominantly [GaH2]+ cations reveal that C2H6 dehydrogenation rates exhibit a Langmuir–Hinshelwood dependence on C2H6 partial pressure at elevated temperatures (>730 K), consistent with the involvement of chemisorbed [C2H5–GaH]+ species. The reaction kinetics suggest that C2H6 dehydrogenation proceeds via heterolytic C–H cleavage of adsorbed C2H6 by [GaH2]+ cations to form H2 and [C2H5–GaH]+ species, which further decompose via β-hydride elimination to form C2H4. By contrast, C4H10 dehydrogenation and both terminal and central cracking are catalyzed exclusively by [GaH]2+ cations. All three reactions exhibit a Langmuir–Hinshelwood dependence on C4H10 partial pressure and are inhibited by H2. Ratios of dehydrogenation to cracking (total) and terminal to central cracking are approximately independent of C4H10 partial pressure consistent with the involvement of a common C4H10-derived surface intermediate. The observed reaction kinetics are consistent with an alkyl-mediated mechanism occurring over [GaH]2+, analogous to that reported previously for C3H8 dehydrogenation/cracking over Ga/H-MFI (Phadke, N. M.; J. Am. Chem. Soc. 2019, 141, 1614−1627). The mechanism proceeds via facile, heterolytic dissociation of adsorbed C4H10 to form [C4H9–GaH]+–H+ cation pairs via methyl C–H-activated pathways. Dehydrogenation then proceeds via β-hydride elimination, respectively, forming butene, while terminal and central cracking proceed via C–H-activated H+ attack. Methylene activation was also considered but found to occur at a significantly lower rate. Theoretical analysis of the proposed reaction pathways leads to apparent activation enthalpies in good agreement with values extracted from the measured kinetics, thereby supporting the proposed pathways and the roles of [GaH]2+ and [GaH2]+ cations in the dehydrogenation and cracking of light alkanes on Ga/H-MFI.

An interest in the on-purpose production of 1,3-butadiene (1,3-BD) has grown, as a consequence of the decline in naphtha cracking for the production of ethene and propene, products that can now be produced economically by thermal dehydrogenation of ethane and propane contained in natural gas. In this study, the mechanism and kinetics of n-butane dehydrogenation to 1,3-BD are explored over atomically distributed Pt sites grafted onto dealuminated zeolite BEA (DeAlBEA) in the form of (Si-O-Zn)4-6Pt complexes. Reaction of n-butane dehydrogenation carried out at 823 K with 2.53 kPa n-butane/He and a weight-hourly space velocity (WHSV) of 14.5 h-1 produced 1,3-BD with a turnover frequency of 0.45 mol 1,3-BD (mol Pt)-1 s-1. Space-time studies and identification of the reaction intermediates suggest that n-butane first undergoes dehydrogenation primarily to 1-butene, which then rapidly isomerizes to produce an equilibrated mixture of 1-butene and 2-butene. 1-Butene then undergoes secondary dehydrogenation to produce 1,3-BD. We report, here, a detailed study of the kinetics of n-butane dehydrogenation to butenes and 1-butene dehydrogenation to 1,3-BD over isolated Pt sites. Both reactions exhibit a Langmuir-Hinshelwood dependence on n-butane and 1-butene partial pressures, respectively. Comparison of effective forward rate constants of n-butane dehydrogenation to butenes (k1f) and butene dehydrogenation to 1,3-BD (k2f) shows that the isolated Pt sites grafted onto DeAlBEA exhibit a very high activity for sequential dehydrogenation of n-butane to 1,3-BD relative to other Pt-based catalysts previously reported.

An artificial-photosynthesis device is a multicomponent system composed of various components including perhaps light absorbers, electrocatalysts, membranes or separators, and electrolytes in a specific system geometry. The overall solar-to-fuel conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as the design of the system. In this talk, we will cover recent modeling of various motifs of such artificial-synthesis systems. In particular, we will examine modeling and experimental results for particle-based devices for solar-hydrogen production as well as vapor-feed devices for solar water splitting and electrochemical carbon-dioxide reduction. Mathematical modeling is ideally suited to examine the various tradeoffs and determine design targets and feasibility. Z-scheme particle-suspension reactor designs consisting of freely suspended semiconductor particles in an electrolyte to drive solar water splitting could be cost-effective alternatives to produce renewable hydrogen. In this work, we develop a device-scale model to evaluate the effects of coupled light absorption, electrolyte species transport, and reaction kinetics on overall reactor performance and ability to sustain rector operation via diffusion. We also extend this work by numerically investigating particle-scale and -size effects on colloidal stability, light absorption and scattering, and charge-carrier transport across the semiconductor/cocatalyst/electrolyte interface. Within the Joint Center for Artificial Photosynthesis (JCAP), we utilize continuum-scale modeling of the various components in order to determine design tradeoffs of vapor-feed or gas-diffusion electrode systems. Such systems can provide routes towards optimizing local reaction conditions and overcoming inherent liquid-phase transport limitations for carbon-dioxide reduction as well as solar water splitting to produce hydrogen. For the latter, integrated architectures can be used that are more stable than those in liquid environments. Overall, the functioning of both vapor feed and particle systems will be explored.

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.

In order to understand the remarkable activity of α-Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> for selective oxidation and ammoxidation of propene, the propene activation ability of four molybdenum-based mixed metal oxides - Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>, PbMoO<sub>4</sub>, Bi<sub>2</sub>Pb<sub>5</sub>Mo<sub>8</sub>O<sub>32</sub>, and MoO<sub>3</sub> - was investigated using density functional theory. Propene activation is considered to occur via abstraction of a hydrogen atom from the methyl group of physisorbed propene by lattice oxygen. For each material, the apparent activation energy was estimated by summing the heat of adsorption of propene, the C-H bond dissociation energy, and the hydrogen attachment energy (HAE) for hydrogen addition to lattice oxygen; this sum provides a lower bound for the apparent activation energy. It was found that two structural features of oxide surfaces are essential to achieve low activation barriers: under-coordinated surface cation sites enable strong propene adsorption, and suitable 5- or 6-coordinate geometries at molybdenum result in favorable HAEs. The impact of molybdenum coordination on HAE was elucidated by carrying out a molecular orbital analysis using a cluster model of the molybdate unit. This effort revealed that, in 5- and 6-coordinate molybdates, oxygen donor atoms trans to molybdenyl oxo atoms destabilize the molybdate prior to H addition but stabilize the molybdate after H addition, thereby providing an HAE ~15 kcal/mol more favorable than that on 4-coordinate molybdate oxo atoms. Bi<sup>3+</sup> cations in Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> thus promote catalytic activity by providing both strong adsorption sites for propene and forcing molybdate into 5-coordinate geometries that lead to particularly favorable values of the HAE. (Graph Presented).

The electrochemical reduction of CO 2 reaction (CO 2 RR) offers an attractive means for converting the carbon content of CO 2 , released from stationary source or the atmosphere, into fuels and chemicals [1].Utilization of CO 2 captured from the atmosphere and energy provided by electricity sourced from wind or solar energy is particularly appealing because it enable sustainable production of carbon-containing products.The most attractive type of CO 2 electrolyzer for commercial application is a membrane electrode assembly (MEA), as shown in Fig. 1a [2,3].This device consists of an anion-exchange membrane (AEM) sandwiched between two gas diffusion electrodes (GDE), each of which comprises a gas diffusion layer (GDL) and a catalyst layer (CL).The cathode CL contains metal nanoparticles,