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Oxidative coupling of methane (OCM) is an exothermic reaction that results in the temperature rise towards the latter part of the reactor. This study reports the consequence of OCM product distribution after O2 depletion at high temperatures.

This paper presents a new methodology to produce a sintering-resistant exhaust gas purification catalyst with ultrathin SiO2 decoration on already supported Pd metal nanoparticles using a vacuum-free photocatalytic layer deposition method.

Photoelectrochemical (PEC) water splitting using Ta

A semitransparent Ta3N5 photoanode is designed for efficient and durable solar water splitting. The Ta3N5-CuInSe2 tandem device exhibits an initial and stabilized solar-to-hydrogen efficiency of ∼9% (highest for metal oxides/nitrides) and 4%, respectively.

The crystal structure and the resultant electronic structure of Ta-supplemented bismuth oxides were altered with the amount of Ta added. Their optoelectronic and redox properties were elucidated both experimentally and theoretically.

Microporous Si–Ta membrane fabrication via CVD is studied with emphasis on the formation of graded and defectless γ-Al2O3 intermediate layers using size-controlled boehmite sols. The membrane shows excellent H2 selectivity and hydrothermal stability.

Surface modifications of semiconductor photoelectrodes are effectively introduced to enhance charge separation and reaction kinetics during solar-driven water splitting.

Microkinetic analysis can establish the relationship between the atomic-level reaction mechanism and macroscopic observables, such as reaction rates, product selectivity, Tafel slope, reaction order, isotopic effect, and apparent activation energy,...

The field of heterogeneous photoredox catalysis has grown substantially and impacted organic synthesis because of the affordability and reusability of catalysts. This study reports radical trifluor...

Oxidative coupling of methane (OCM) is a promising technique for converting methane to higher hydrocarbons in a single reactor. Catalytic OCM is known to proceed via both gas-phase and surface chemical reactions. It is essential to first implement an accurate gas-phase model and then to further develop comprehensive homogeneous–heterogeneous OCM reaction networks. In this work, OCM gas-phase kinetics using a jet-stirred reactor are studied in the absence of a catalyst and simulated using a 0-D reactor model. Experiments were conducted in OCM-relevant operating conditions under various temperatures, residence times, and inlet CH4/O2 ratios. Simulations of different gas-phase models related to methane oxidation were implemented and compared against the experimental data. Quantities of interest (QoI) and rate of production analyses on hydrocarbon products were also performed to evaluate the models. The gas-phase models taken from catalytic reaction networks could not adequately describe the experimental gas-phase performances. NUIGMech1.1 was selected as the most comprehensive model to describe the OCM gas-phase kinetics; it is recommended for further use as the gas-phase model for constructing homogeneous–heterogeneous reaction networks.

Metal chalcogenides - because of their excellent optical and electrical properties - are important semiconductor materials for optical devices, such as solar cells, sensors, and photocatalysts. The challenges associated with metal chalcogenides are the complexity of the conventional synthesis methods and the stringent synthesis conditions. In this study, the synthesis conditions were simplified in a solvent-free synthesis method using cadmium precursor, thiourea and selenium to synthesize metal chalcogenides, such as CdS and CdSe, which have particularly suitable band gaps for the optical devices. CdS

Direct seawater electrolysis potentially simplifies the electrolysis process and leads to a decrease in the cost of green hydrogen production. However, impurities present in the seawater, especially chloride ions (Cl

A part of this work was supported by the JSPS KAKENHI (grant number 19KK0126) and King Abdullah University of Science and Technology (KAUST) AMPM research center under collaborative funding. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract no. DE-AC02-76SF00515.

A transparent Ta₃N₅ photoanode is a promising candidate for the front-side photoelectrode in a photoelectrochemical (PEC) cell with tandem configuration (tandem cell), which can potentially provide high solar-to-hydrogen (STH) energy conversion efficiency. This study focuses in particular on the semiconductor properties and interfacial design of transparent Ta₃N₅ photoanodes fabricated on insulating quartz substrates (Ta₃N₅/SiO₂), typically the geometric area of 1 × 1 cm² in contact with indium on its edge. This material utilizes the self-conductivity of Ta₃N₅ to make the PEC system operational, and the electrode would strongly reflect the intrinsic nature of Ta₃N₅ without a back contact that is commonly introduced. First, PEC measurements using acetonitrile (ACN)/H₂O mixed solution were made to elucidate the intrinsic photoresponse in the presence of tris(2,2′-bipyridine)ruthenium(II) bis(hexafluorophosphate) (Ru(bpy)₃(PF₆)₂) without water contact which avoids a multielectron-transfer oxygen evolution reaction (OER) and photoinduced self-oxidation. The potential difference between the onset potential of Ru²⁺ PEC oxidation by Ta₃N₅/SiO₂ and the redox potential of Ru²⁺/³⁺ in the nonaqueous environment was about 0.7 V. While a stable photoanodic response was observed for Ta₃N₅/SiO₂ in the nonaqueous phase, the addition of a small quantity of water into this nonaqueous system led to the immediate deactivation of Ta₃N₅/SiO₂ photoanode under illumination by self-photooxidation to form TaOₓ at the solid/water interface. In aqueous phase, flatband potentials estimated from Mott–Schottky analysis varied with solution pH (constant potential against reversible hydrogen electrode (RHE)). Photoelectrode modification by a transparent NiFeOₓ layer was attempted. The complete coverage of the Ta₃N₅ surface with transparent NiFeOₓ electrocatalysts, achieved by an optimized spin-coating protocol with controlled Ni–Fe precursors, allowed for the successful protection of Ta₃N₅ and demonstrated an extremely stable photocurrent for hours without any additional protective layers. The stability of the resultant NiFeOₓ/Ta₃N₅/SiO₂ was limited not by Ta₃N₅ but mainly by a NiFeOₓ electrocatalyst due to Fe dissolution with time.

Water electrolysis driven by renewable energy can produce clean hydrogen, but its efficiency remains low, in part because of slow kinetics at the anode for the oxygen evolution reaction (OER). Learning from the most active catalysts for the OER, iridium oxides, would be the key to the development and establishment of design guidelines for active and stable OER catalysts. This article reviews in situ or operando spectroscopic and advanced computational studies in the past decade concerning the OER over iridium oxide for both the oxidation of water molecules and hydroxide ions. By collectively reviewing the reported findings, we illustrate the plausible OER catalytic cycles including the dissolution of iridium during the reaction, which at the same time disclosed discrepancies in the proposed mechanisms. Such discrepancies are thought to originate from variations in the experimental conditions employed in those studies, calling for comprehensive and systematic in situ or operando studies in the future. Toward the end, we discuss a recent approach for improving the activity and stability of OER catalysts.

We report the oxidative coupling of methane (OCM) performance over Na2ZrO3 under various temperatures, CH4/O2 ratios, and reaction pressures. Sensitivity of CH4 conversion rate was investigated with respect to feed components CH4 and O2 pressures, as well as reaction products CO2 and H2O pressures. Kinetic data obtained under various conditions were fitted to establish an OCM reaction network to describe the product distribution based on the rate constants at each reaction step. Temperatures above 750 °C caused gradual decay in catalytic performance, likely due to Na loss associated with the transformation of Na2ZrO3 crystal phase to monoclinic ZrO2. This catalyst exhibited competitive inhibition by reversible CO2 adsorption and CO oxidation activity, both of which were necessary to include to construct an accurate kinetic model.

International audience; In this study, the influence of the degree of crystallization and thickness of films was correlated with the photoelectrochemical performance of SrTaO2N semiconductor films for O2 evolution reaction under visible light irradiation. Oxynitride films were deposited on various substrates using the sputtering in Ar + N2 reactive atmosphere from a home-made SrTaO2N target. Films with stoichiometric composition were obtained at a high temperature (TS =800 °C) with reduced bandgap. The different substrates led to diverse degrees of film crystallization, from weakly crystallized to fully c-axis oriented. The photoelectrochemical performance was improved by improving the film crystallinity and the thickness. For further improvement of the photoelectrochemical performance, the following three limiting factors are identified 1) low absorption coefficient, especially in the visible domain from 500 to 600 nm; 2) short lifetimes of excited charge carriers; and 3) permittivity with only moderate values lower than 10 in the visible-light domain.

Hydrogen is a promising alternative to fossil fuels that can reduce greenhouse gas emissions. Decoupled water electrolysis system using a reversible proton storage redox mediator, where the oxygen evolution reaction and hydrogen evolution reaction are separated in time and space, is an effective approach to produce hydrogen gas with high purity, high flexibility, and low cost. To realize a fast hydrogen production in such a system, a redox mediator capable of releasing proton rapidly is required. Herein, α-MoO3, with an ultrafast proton transfer property that can be explained by a dense hydrogen bond network in the lattice oxygen arrays of HxMoO3, was examined in this work as a high-rate redox mediator for fast hydrogen production in acidic electrolytes. The α-MoO3 redox mediator shows both a large capacity of 204 mAh g-1 and fast hydrogen production at a current rate of 1 A cm-2 (100 A g-1), outperforming most of the previously reported solid-state redox mediators.

Water electrolysis driven by electrical power generated from renewable energy sources will play a pivotal role in future sustainable societies, which requires adaptation of various reaction conditions as well as electrolyte identities. Regardless, the anodic half-reaction of the oxygen evolution reaction (OER) is considered a kinetic bottleneck. This study provides quantitative description of the OER kinetics based on rigorous microkinetic analyses including Tafel analysis, isotope effects and temperature dependence using an IrOx electrocatalyst in unbuffered solution at varying pH levels. The diffusional constraints of H+/OH− determine three distinctive kinetic regimes in the pH-potential-current relationships: below pH 5, between pH 5 and 10, and above pH 10 at appreciable current densities on the order of 1 mA cm−2. When shifting from alkaline to acidic solution, the complete consumption of local OH− near the electrode surface switches the OER proceeding as the oxidation of OH− to that of the water molecule at pH ~ 11 irrespective of the electrode identity. At pH 5–10, the diffusional constraints of H+ generated via oxidation reaction yield an environment with pH ~ 4 near the electrode surface even prior to the OER, resulting in a bulk pH-independent region for the OER performance. Under this unbuffered near-neutral-pH condition, the isotope effect was diminished for the OER catalysis, which is consistent with the rate-determining step (rds) being the sole electron-transfer step via the formation of O-O bonds, decoupled from proton transfer. This reaction mechanism is distinct from that under more acidic conditions (pH

The research reported in this study was supported by the King Abdullah University of Science and Technology. The authors acknowledge the support of SABIC in funding this research. Computational resources were provided primarily by the KAUST Supercomputing Laboratory and the Shaheen II supercomputer. Computational work was also partially performed using resources of the National Supercomputing Centre, Singapore (https://www.nscc.sg).

Photoelectrochemical (PEC) water splitting using visible-light-responsive photoelectrodes is the preferred approach to converting solar energy into hydrogen as a renewable energy source. A transparent Ta3 N5 photoanode embedded within a PEC cell having a tandem configuration is a promising configuration that may provide a high solar-to-hydrogen energy conversion efficiency. Ta3 N5 thin films are typically prepared by heating precursor films in an NH3 flow at high temperatures, which tends to degrade the transparent conductive layer, such that producing efficient Ta3 N5 transparent photoanodes is challenging. Herein, the direct preparation of transparent Ta3 N5 photoanodes on insulating quartz substrates was demonstrated without the insertion of a transparent conductive layer. The resulting devices generated a photocurrent of 6.0 mA cm-2 at 1.23 V vs. a reversible hydrogen electrode under simulated sunlight. This study provides a new strategy for the preparation of transparent photoelectrodes that mitigates current challenges.

High current density reaching 1 A cm

Near-neutral pH water electrolysis driven by renewable electricity can reduce the costs of clean hydrogen generation, but its low efficiency and gas crossover in industrially relevant conditions remain a challenge. Here, it was shown that electrolyte engineering could suppress the crossover of dissolved gases such as O

This study simulates a high-temperature reaction in a plug-flow reactor (PFR) for the aromatization of methane via oxidative coupling of methane (OCM) using a state-of-the-art gas-phase chemical ki...

Although the electrochemical hydrogen evolution reaction (HER) on a Pt electrode is among the most studied electrocatalytic reactions, its reaction mechanism and exchange current density are still ...

Selective CO2 reduction via photoelectrocatalysis is a creative approach to alleviating the energy crisis and environmental deterioration as well as to producing value-added chemicals. A considerable challenge is the development of electrocatalysts and photoelectrocatalytic systems to selectively and efficiently produce a target product. Here, we report an efficient and selective photocathode for converting CO2 to HCOOH in aqueous solution. A rational compositional screening strategy was first applied to pinpoint In0.4Bi0.6 from ternary In–Bi–Sn alloys as the most HCOOH-selective electrocatalyst composition among the electrocatalysts reported. A photocathode was then fabricated by coating the catalytic, protective, and conductive In0.4Bi0.6 alloy layer on a halide perovskite photovoltaic, effectively utilizing its low melting point eutectic molten state. The generated photovoltage assisted reduction of the overpotential by 680 mV while producing a stable current for nearly exclusive HCOOH production under...

NaLa1–xBixS2 solid solutions with tunable band gaps were synthesized, and their optoelectronic structures and photocatalytic performance were investigated via experimental and theoretical approaches. The solid solution powders with various La/Bi ratios were synthesized with Na2CO3, La2O3, and Bi2O3 as precursors and via sulfurization with flowing CS2 at 800 °C for 2 h. The Vegard’s law behavior of cell parameters showed a perfect Bi/La solid solution in the cubic NaLa1–xBixS2 with the associated linear variation of the lattice constants. On the contrary, the combination of diffuse reflectance ultraviolet–visible spectroscopy with density functional theory (DFT) calculations employing the HSE06 functional reveals a monotonic but nonlinear variation of the band gap of the solid solution. While consistent valence band maximum was obtained in NaLa1–xBixS2—consisting mainly of S 3p orbitals—the conduction band minimum was contributed by discrete Bi orbitals present at more positive potential than La. As a resu...