Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.
Methane is a relatively inexpensive and abundant resource and its partial transformation to chemicals and chemical fuels presents attractive yet challenging pathways for its utilization. Conventional synthesis for methanol involve a multistep process involving steam reforming of methane with subsequent catalytic reactions, which require high energy input and run at high cost [1]. Alternatively, methane oxidation over catalyst surfaces in an electrochemical cell is a promising single-step approach to achieve a direct conversion of methane to methanol at lower temperatures and low cost [2]. Previous studies have demonstrated that methane can be electrochemically converted into methanol with high selectivity but with low overall conversion efficiency. Increasing the rate of methane conversion can be achieved by systematic improvement of the electrochemical cell design along with the concurrent development of new efficient catalysts materials. Methane oxidation is energetically challenging process and dual role of the catalysts involve first, activation of the relatively inert C-H bond enabling the oxidative hydroxylation, and methanol formation. Second, the effective catalysts should simultaneously inhibit the methanol oxidation, which proceeds with a much lower energy barrier and can result in the formation of formaldehyde, formic acid, carbon monoxide, and carbon dioxide. To date, a variety of the catalysts, among different supported metals (Pd, Ru, Au, Ag) and metal oxides (V 2 O 5 , Fe 2 O 3 , CoO, Mn 2 O 3 , MoO 3 , CrO), have been tested in electrochemical cell and shown promise for the direct oxidation of methane [2]. However, further systematic studies are essential for understanding the mechanism for methane oxidation, enabling catalysts rational design. On the other hand, bioinspired supported binuclear metal catalysts show potential for high selectivity and conversion [3], but have not yet been explored for electrochemical methane conversion. In this study, first-row transition metal oxides as well as single and binuclear catalysts, supported on well-defined crystalline 2D materials including carbides, oxides, and nitrides will be presented. The catalysts are synthesized with wet-chemical synthesis routes and subsequently fabricated in membrane electrode assembly for testing their activity, methanol selectivity, and conversion efficiency in an electrochemical fuel-cell-type reactor. In-depth structural and chemical analyses of catalysts using a combination of various transmission electron microscopy techniques, complimented with spectroscopy analyses is used to establish structure-property relationship. These insights will provide valuable basis for a scientific-guided approach toward the optimization of the known, and the identification of the new metal oxide and single-site supported catalysts for this challenging process. Ravi, M., et al, Angew. Chemie Int. Ed., 2017 , 56 , 26464. Tomita, A., et al, Angew. Chemie Int. Ed., 2008 , 47 , 1462. Starokon, E. V., Phys Chem. C. , 2011 , 115 , 2155.
Because of their relatively low cost, ethane and propane derived from shale gas are the currently preferred feedstocks for the production of aromatics. Ga-exchanged H-MFI zeolite (Ga/H-MFI) exhibits high activity and selectivity for light alkane dehydroaromatization, a process that involves alkane dehydrogenation to form alkenes, followed by alkene oligomerization and cyclization, and further dehydrogenation of the resulting products. Recent work has shown (Phadke et al. Characterization of Isolated Ga3+ Cations in Ga/H-MFI Prepared by Vapor-Phase Exchange of H-MFI Zeolite with GaCl3. ACS Catal. 2018, 8, 6106–6126; Phadke et al. Mechanism and Kinetics of Propane Dehydrogenation and Cracking over Ga/H-MFI Prepared via Vapor-Phase Exchange of H-MFI with GaCl3. J. Am. Chem. Soc. 2019, 141, 1614–1627; Phadke et al. Mechanism and Kinetics of Light Alkane Dehydrogenation and Cracking over Isolated Ga Species in Ga/H-MFI. ACS Catal. 2021, 11, 2062–2075; Mansoor et al. ACS Catal. 2018, 8, 2146–6162) that Ga3+ ([GaH]2+ and [Ga(H)2]+) sites catalyze the initial dehydrogenation of alkanes; however, the role of Ga3+ sites in the subsequent alkene oligomerization step requires clarification. In this work, the kinetics of ethene oligomerization over Ga/H-MFI were investigated as a function of Ga loading, feed space time, temperature, and ethene partial pressure. The presence of Ga3+ sites gives rise to enhanced rates of ethene oligomerization and higher selectivities to butene and hexene relative to H-MFI. However, selective titration of Brønsted acid sites with NH3 reveals that, in the absence of Brønsted acid sites, [GaH]2+ and [Ga(H)2]+ cations do not contribute appreciably to the higher activity of Ga/H-MFI. Similarly, in situ Fourier-transform infrared spectroscopy shows that the reaction pathway for ethene oligomerization over Ga/H-MFI involves the same intermediates as that over H-MFI. The higher ethene oligomerization activity and selectivity to even-carbon-numbered alkenes of Ga/H-MFI stems from cooperative effects between Ga3+ sites and Brønsted acid protons.
We assess the accuracy of popular nonempirical GGAs (PBE, PBEsol, RPBE) and meta-GGAs (TPSS, revTPSS, and SCAN) for describing chemisorption reactions at metal surfaces. Except for RPBE, all the functionals tend to overbind the adsorbate significantly. We then propose a nonempirical meta-GGA, denoted as RTPSS, that is based on RPBE in the same way that TPSS is based on PBE. The RTPSS functional remedies the overbinding problem and improves the description of chemisorption energies. As an example of an application of RTPSS, we study the adsorption of CO on Cu surfaces (a notably difficult problem for semilocal functionals) and find that RTPSS is the only tested functional that predicts accurate chemisorption energies and the preferred adsorption site of CO. Although RTPSS gives an accurate description of chemisorption, nonlocal correlation may be necessary to describe physisorption if long-range van der Waals interactions are involved (however, this is true for semilocal functionals in general). We suggest that RTPSS can be a useful meta-GGA for studying chemisorption processes and mechanisms of heterogeneous catalysis.
Abstract Growing concern with the effects of CO 2 emissions due to the combustion of petroleum‐based transportation fuels has motivated the search for means to increase engine efficiency. The discovery of ethers with low viscosity presents an important opportunity to improve engine efficiency and fuel economy. We show here a strategy for the catalytic synthesis of such ethers by reductive etherification/O‐alkylation of alcohols using building blocks that can be sourced from biomass. We find that long‐chain branched ethers have several properties that make them superior lubricants to the mineral oil and synthetic base oils used today. These ethers provide a class of potentially renewable alternatives to conventional lubricants produced from petroleum and may contribute to the reduction of greenhouse gases associated with vehicle emissions.
Read moreThis project aimed to develop fundamental understanding of the chemistry of NO adsorption and reaction in Pd/zeolites so as to facilitate the rational design of passive NOx adsorber catalysts. The approach adopted combined both experimental and computational methods, which together allow a deeper understanding of the governing chemistry than the use of either method alone. The workflow began with Pd/H-CHA and Pd/H-BEA catalyst synthesis and characterization, in which the Si/Al ratio and Al siting were systematically varied. This was followed by catalyst evaluation using temperature-programed adsorption/desorption methods, as well as in situ spectroscopic measurements to probe the chemistry of NO adsorption. In parallel, the adsorption of NO and other relevant species (H<sub>2</sub>O, CO, HCs) was studied by means of quantum chemical calculations in order to rationalize the experimental data and provide additional insights. Catalyst aging studies were also performed with the aim of elucidating the mechanism of catalyst degradation. Finally, the insights gained in this project were applied to the preparation of an optimized HC/NOx adsorber catalyst, the performance of which was studied using exhaust gas from an engine dynamometer.
Read moreStructural and chemical disorder are part of almost any engineering material. Their detailed characterization with theoretical and experimental approaches therefore forms the cornerstone of modern materials science and engineering. To assess the state of this field, the Workshop on Thermodynamic and Structural Properties of Alloy Materials was held on 20-24 June 1999 at the Sonesta Hotel in Oranjestad, Aruba. The workshop brought together experimentalists and theorists in the fields of metals, oxides and semiconductors. Particular emphasis was placed on efforts to transfer the success of first-principles modelling of configurational disorder to topological disorder, as occurs in metallic glasses and liquids. Fittingly, the workshop was dedicated to Professor de Fontaine of the University of California at Berkeley, whose influence on the field of order-disorder reactions and first-principles phase diagram calculations has been of key importance to the development of the subject.
Read moreBipolar membranes (BPMs), which have long seen usage in electrodialysis reactors for the generation of acid and base, have recently demonstrated potential to become critical components in electrochemical synthesis devices. Because they can operate under large pH gradients, BPMs enable favorable environments for electrocatalysis at the individual electrodes. Critical to the implementation of BPMs in these devices is understanding the kinetics of water dissociation that occurs within the BPM junction as well as the co- and counter-ion crossover through the BPM, which both present significant obstacles to developing efficient and stable BPM-devices for electrosynthesis applications. Prior work has modeled ion transport in bipolar membranes in neutral salt solutions for electrodialysis. However, no model exists for the BPM under the harsh applied pH gradients that would be present in electrosynthesis, and there is significant need to explore the effects of the internal hydration on the lifetime and performance of BPMs in such environments. Additionally, a mechanistic understanding of water dissociation catalysis will be required to develop interfacial catalysts that enable the high current density operation required for scalable electrosynthesis of fuels. In this talk, we discuss modeling methodologies and physics inherent in BPMs and present our recent model of ion transport and water dissociation catalysis in BPMs across the pH scale. Specifically, we simulate multi-ion transport for a BPM with various electrolyte combinations on each side of the membrane, demonstrating the significance of co- and counter-ion crossover in BPMs operating under harsh pH gradients. We then investigate effects caused by hydration gradients that occur due to internal ion-exchange and examine potential methods for improving performance and mitigating crossover. Finally, we examine the impact of the interfacial water dissociation catalyst and perform sensitivity analysis on the key properties (catalyst point of zero charge and pK a ) that dictate catalyst performance. These results provide information that is critical to developing a comprehensive understanding of multi-component phenomena in BPMs and to informing the design and implementation of BPMs in next-generation devices for the numerous electrosynthesis chemistries that benefit from operation under an applied pH gradient.
Read moreSignificance Chemical storage of solar energy can be achieved by electrochemical reduction of CO 2 to CO and H 2 , and subsequent conversion of this mixture to fuels. Identifying optimal conditions for electrochemical cell operation requires knowledge of the CO 2 reduction mechanism and the influence of all factors controlling cell performance. We report a multiscale model for predicting the current densities for H 2 and CO formation from first principles. Our approach brings together a quantum-chemical analysis of the reaction pathway, a microkinetic model of the reaction dynamics, and a continuum model for mass transport of all species through the electrolyte. This model is essential for identifying a physically correct representation of product current densities dependence on the cell voltage and CO 2 partial pressure.
Read moreAtomically 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.
Read moreMembrane-electrode assemblies utilize ionomer-coated electrocatalysts to achieve facile ion transport. Consequently, isolation of intrinsic catalyst kinetics from measured polarization curves is challenging, as the properties of the catalyst and ionomer both affect the measurements. Here, we employ a Pt microelectrode coated by a thin perfluorosulfonic acid (PFSA) layer to measure polarization curves for the hydrogen oxidation reaction/hydrogen evolution reaction (HER/HOR). Intrinsic electrode kinetics are isolated by theoretical analysis of the local catalyst microenvironment, accounting for mass transport and thermodynamics. The observed enhancements in HER and HOR rates with increasing relative humidity (RH) at the working electrode are attributable to two competing factors: the decrease in activity of H+ in the ionomer and the dominant decrease in the water reorganization energy in the Marcus–Hush–Chidsey representation of HER/HOR kinetics. The increase in intrinsic rate with increasing RH is attributed to increased H+ transfer dynamics resulting from reduced confinement of water within the subnanometer water layer between the catalyst and ionomer as RH increases.
Read moreAbstract Synthesis of a pentasil‐type zeolite with ultra‐small few‐unit‐cell crystalline domains, which we call FDP (few‐unit‐cell crystalline domain pentasil), is reported. FDP is made using bis‐1,5(tributyl ammonium) pentamethylene cations as structure directing agent (SDA). This di‐quaternary ammonium SDA combines butyl ammonium, in place of the one commonly used for MFI synthesis, propyl ammonium, and a five‐carbon nitrogen‐connecting chain, in place of the six‐carbon connecting chain SDAs that are known to fit well within the MFI pores. X‐ray diffraction analysis and electron microscopy imaging of FDP indicate ca. 10 nm crystalline domains organized in hierarchical micro‐/meso‐porous aggregates exhibiting mesoscopic order with an aggregate particle size up to ca. 5 μm. Al and Sn can be incorporated into the FDP zeolite framework to produce active and selective methanol‐to‐hydrocarbon and glucose isomerization catalysts, respectively.
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