Potential-Rate Correlations of Supported Palladium-Based Catalysts for Aqueous Formic Acid Dehydrogenation.

Aqueous formic acid dehydrogenation (FAD) is a crucial process for hydrogen production, as hydrogen is a clean energy carrier. During this process, formic acid converts into hydrogen and carbon dioxide over a catalyst. Pd-based catalysts have exhibited significant potential in FAD due to their high activity and selectivity. In this study, we investigated aqueous thermal FAD in a mixture of formic acid and sodium formate using electrochemical open-circuit potential (OCP) measurement by loading the catalysts onto a conductive substrate as a working electrode. By varying the reaction conditions such as the concentration of reactants and modifying Pd with Ag, different FAD rates were obtained. Consequently, we revealed the correlation between the catalyst OCP and FAD rate; superior FAD rates reflected a more negative catalyst OCP. Furthermore, deactivation was observed across all catalysts during FAD, with a concurrent increase in catalyst OCP. Interestingly, we found that the logarithm of the FAD rate showed a linear correlation with the OCP of the catalyst during the decay phase, which we quantitatively explained based on the reaction mechanism. This study presents a new discovery that bridges thermal and electrocatalysis.

Electron Storage in Monolayer Tungstate Nanosheets Produced via a Scalable Exfoliation Method.

Inorganic nanosheet materials with atomic thinness have been widely studied as (photo)catalytic materials due to their unique electronic states and surface structures. One scalable and reproducible method of producing monolayer nanosheets is a top-down approach based on the exfoliation of layered parent compounds using an alkylammonium solution as a surfactant. However, H2W2O7 layered tungstates dissolve in basic aqueous solutions, making them unsuitable for the exfoliation process. This work proposes a scalable method to obtain monolayer WO3 nanosheets with a very high external field responsiveness. This work shows that H2W2O7 topochemically swells in a concentrated octylamine (C8N17NH2) aqueous solution with a concentration above the solubility of octylamine in water. Water was added for exfoliation of the liquid crystalline phase into isolated W2O72- nanosheets with octylammonium (C8N17NH3+) protection. Crystalline WO3 nanosheets on the n-Si substrate obtained with calcination exhibited electron richness in the conduction band due to static electron transfer at the interface.

Direct microwave energy input on a single cation for outstanding selective catalysis.

Microwave (MW)-driven catalytic systems are attracting attention not only as an aggressive electrification strategy of the chemical industry but also as creating a unique catalytic reaction field that conventional equilibrium heating cannot achieve. This study unlocked direct and selective heating of single alkali metal cations in the pores of aluminosilicate zeolites under MW. Selectively heated Cs+ cations in FAU zeolite exhibited selective CH4 combustion performance, that is, COx generation at the heated Cs+ cations selectively occurred while side reactions in the low-temperature gas phase were suppressed. The Cs-O pair distribution function revealed by synchrotron-based in situ x-ray total scattering gave us direct evidence of peculiar displacement induced by MW, which was consistent with the results of molecular dynamics simulation mimicking MW heating. The concept of selective monoatomic heating by MW is expected to open a next stage in "microwave catalysis" science by providing physicochemical insights into "microwave effects."

Physicochemical insights into semiconductor properties of a semitransparent tantalum nitride photoanode for solar water splitting.

The self-conductivity of tantalum nitride (Ta3N5) thin film-based semitransparent photoanodes was found to promote the current originating from the photoelectrochemical oxygen evolution reaction (PEC OER) without a conducting substrate. With surface modification by the NiFeOx-electrocatalyst, an optimized Ta3N5 thin film fabricated directly on a transparent insulating quartz substrate generated a photocurrent density of ∼5.9 ± 0.1 mA cm-2 at 1.23 V vs. the reversible hydrogen electrode under simulated AM 1.5G solar illumination. The correlation between the PEC OER performance of NiFeOx-modified Ta3N5 photoanodes and the electrical properties of Ta3N5 thin films was investigated based on the Hall effect measurements. By changing the nitridation conditions, these properties can be tuned so that the higher the Hall mobility (0.2 to 1.7 cm2 V-1 s-1) and the lower the carrier concentration (1020 to 1019 cm-3). The surface chemical states of Ta3N5 thin films were investigated using X-ray photoelectron spectroscopy as a means of evaluating surface oxygen impurities and nitrogen vacancies, which may correlate with the PEC OER performance and the electrical properties of the material.

Quantitative Description of Bubble Formation in Response to Electrolyte Engineering.

The green hydrogen economy is expected to play a crucial role in carbon neutrality, but industrial-scale water electrolysis requires improvements in efficiency, operation costs, and capital costs before broad deployment. Electrolysis operates at a high current density and involves the substantial formation of gaseous products from the electrode surfaces to the electrolyte, which may lead to additional resistance and a resulting loss of efficiency. A detailed clarification of the bubble departure phenomena against the electrode surface and the surrounding electrolytes is needed to further control bubbles in a water electrolyzer. This study clarifies how electrolyte properties affect the measured bubble detachment sizes from the comparisons with analytical expressions and dynamic simulations. Bubble behavior in various electrolyte solutions and operating conditions was described using various physical parameters. A quantitative relationship was then established to connect electrolyte properties and bubble departure diameters, which can help regulate the bubble management through electrolyte engineering.

Photocatalytic formation of a gas permeable layer selectively deposited on supported metal nanoparticles for sintering-resistant thermal catalysis.

Nanoparticle aggregation of supported metal catalysts at high temperatures is a serious problem that causes a drop in catalytic performance. This study investigates the protection of metal nanoparticles from sintering by selectively forming nanoscale SiO2 shells on Pd supported on TiO2 by ultraviolet (UV) light irradiation. The proton-coupled reduction reaction increases the local pH around Pd nanoparticles, resulting in hydrolysis of tetraethoxyorthosilicate (TEOS) in only the vicinity of the metal. An apparent quantum efficiency of only 0.6% is obtained for the Pd/TiO2 catalyst in H2 evolution from ethanol-containing water under 370 nm excitation light. Therefore, the pH of raw slurry solution should be precisely controlled to that slightly below the threshold value for the TEOS hydrolysis reaction before the photodeposition. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) clearly show that the particle size of the Pd nanoparticles (∼40 nm) with the SiO2 shell (∼20 nm) was almost unchanged by the high-temperature treatment at 900 °C in air, suggesting that the SiO2 shell prevented thermal aggregation of Pd nanoparticles. The Pd/TiO2 without SiO2 shell decoration exhibited a drop in the number of active sites, which was likely due to aggregation of the Pd catalysts. However, the number of active sites on the Pd@SiO2/TiO2 catalyst was maintained even after the catalyst was calcined at 900 °C. Consequently, the Pd@SiO2/TiO2 catalyst maintained its catalytic performance for simulated exhaust gas purification even after treatment at 900 °C. This study presents a methodology to produce sintering-tolerant supported metal nanoparticles using the photocatalytic gas permeable layer fabrication method.

High Current Density Oxygen Evolution in Carbonate Buffered Solution Achieved by Active Site Densification and Electrolyte Engineering.

High current density reaching 1 A cm-2 for efficient oxygen evolution reaction (OER) was demonstrated by interactively optimizing electrolyte and electrode at non-extreme pH levels. Careful electrolyte assessment revealed that the state-of-the-art nickel-iron oxide electrocatalyst in alkaline solution maintained its high OER performance with a small Tafel slope in K-carbonate solution at pH 10.5 at 353 K. The OER performance was improved when Cu or Au was introduced into the FeOx -modified nanostructured Ni electrode as the third element during the preparation of electrode by electrodeposition. The resultant OER achieved 1 A cm-2 at 1.53 V vs. reversible hydrogen electrode (RHE) stably for 90 h, comparable to those in extreme alkaline conditions. Constant Tafel slopes, apparent activation energy, and the same signatures from operando X-ray absorption spectroscopy among these samples suggested that this improvement seems solely correlated with enhanced electrochemical surface area caused by adding the third element.

Exploring the Photocorrosion Mechanism of a Photocatalyst.

Photoelectrochemical (PEC) water splitting using Ta3N5 anodes shows a high solar-to-hydrogen (STH) efficiency approaching 10%. However, the long-term stability of gas evolution should be improved for the commercial utilization of PEC water-splitting technology. Herein, we examined the photocurrent degradation of photoanodes prepared by uniformly loading a NiFeOx cocatalyst onto a Ta3N5 semiconductor. Although spectroscopic analysis showed that the degradation was attributable to the formation of an oxide layer, several oxide growth kinetic laws and mechanisms are known. We theoretically derived the photocurrent kinetic laws instead of the oxide growth kinetic laws by generalizing the Cabrera-Mott oxidation theory of metal oxidation in air to apply it to photocorrosion. The measured photocurrent kinetics are fully consistent with the theoretical kinetic laws. We show that ion drift due to charging of the oxide layer limits oxide growth even though uniform cocatalyst loading is designed to prevent self-oxidation of Ta3N5.

A hygroscopic nano-membrane coating achieves efficient vapor-fed photocatalytic water splitting.

Efficient water vapor splitting opens a new strategy to develop scalable and corrosion-free solar-energy-harvesting systems. This study demonstrates highly efficient overall water splitting under vapor feeding using Al-doped SrTiO3 (SrTiO3:Al)-based photocatalyst decorated homogeneously with nano-membrane TiOx or TaOx thin layers (<3 nm). Here, we show the hygroscopic nature of the metal (hydr)oxide layer provides liquid water reaction environment under vapor, thus achieving an AQY of 54 ± 4%, which is comparable to a liquid reaction. TiOx coated, CoOOH/Rh loaded SrTiO3:Al photocatalyst works for over 100 h, under high pressure (0.3 MPa), and with no problems using simulated seawater as the water vapor supply source. This vapor feeding concept is innovative as a high-pressure-tolerant photoreactor and may have value for large-scale applications. It allows uniform distribution of the water reactant into the reactor system without the potential risk of removing photocatalyst powders and eluting some dissolved ions from the reactor.

Surface coverage control for dramatic enhancement of thermal CO oxidation by precise potential tuning of metal supported catalysts.

Chemical heterogeneous catalysis maximizes performance by controlling the interactions between the catalyst and the substrates. Steady-state catalytic rates depend on the heat of adsorption and the resultant coverage of adsorbates, which in turn reflects the electronic structure of the heterogeneous catalyst surfaces. This study aims to free the surface from high coverage of a kind of substance by externally controlling the electrochemical potential of the catalysts for improved thermal-catalytic rates. We employed aqueous CO oxidation at 295 K as a model reaction, where strong binding of chemisorbed CO (CO*) to the metal surfaces and its high coverage led to inhibition of O2 accessing the surface site. Based on the establishment of coverage-potential-performance correlation, our potential-controlling experiments used an electrochemical configuration to identify the appropriate potentials of Pt/C catalysts that can drastically enhance the CO2 formation rate through the thermal reaction pathway. An anodic potential was applied to suppress the high coverage of chemisorbed CO; consequently, the catalytic testing recorded a 5-fold increase in thermal CO2 formation compared to the open-circuit counterpart with a faradaic efficiency (FE) of over 400%. In situ infrared spectroscopy corroborates the potential-coverage correlation, where the suppression of high CO* coverage due to pinning the catalyst potential triggered the enhancement of thermal-catalytic contribution to CO2 formation. Our extended study employing other metal catalysts also exhibited FEs exceeding unity. This work establishes a universal methodology of electrochemical tools for thermal catalysis to precisely tune the electrochemical potential of solids and achieve green and innovative reactions.

Electrolyte Engineering for Oxygen Evolution Reaction Over Non-Noble Metal Electrodes Achieving High Current Density in the Presence of Chloride Ion.

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- ), cause corrosion of the electrode material, and its oxidation competes with the anodic oxygen evolution reaction (OER). By carefully tuning electrode substrate and electrolyte solutions, the CoFeOx Hy /Ti electrode with high double-layer capacitance actively and stably electro-catalyzed the OER in potassium borate solutions at pH 9.2 in the presence of 0.5 mol kg-1 Cl- . The electrode possesses an active site motif composed of either a Co- or Fe-domain and benefits from an enlarged surface area. Selective OER was demonstrated in Cl- -containing electrolyte solutions at an elevated reaction temperature, stably achieving 500 mA cm-2 at a mere potential of 1.67 V vs. reversible hydrogen electrode (RHE) at 353 K for multiple on-off and long-term testing processes with a faradaic efficiency of unity toward the OER.

Synthesis of Metal Chalcogenide Semiconductors by Thermal Decomposition of Organosulfur and Organoselenium Compounds.

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. CdSx Se1-x solid solution was successfully synthesized under molten thiourea as the reactive reaction medium at relatively low temperatures, even at 180 °C, with residual melamine derivatives in the solid phase. The luminescence properties of CdSx Se1-x and the products in the gas and solid phases were investigated. Optimization of the synthesis conditions for solid solutions of CdSx Se1-x and the role of organic compounds in the formation of metal chalcogenides are discussed.

Gas Crossover Regulation by Porosity-Controlled Glass Sheet Achieves Pure Hydrogen Production by Buffered Water Electrolysis at Neutral pH.

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 O2 by regulating their diffusion flux. In addition, a hydrophilized mechanically stable glass sheet was found to block the permeation of gas bubbles, further enhancing the purity of evolved gas from water electrolysis. This sheet had a lower resistance than conventional diaphragms such as Zirfon due to its high porosity and small thickness. A saturated K-phosphate solution at pH 7.2 was used as an electrolyte together with the hydrophilized glass sheet as a gas-separator. This led to a near-neutral pH water electrolysis with 100 mA cm-2 at a total cell voltage of 1.56 V with 99.9 % purity of produced H2 .

Noncatalytic Oxidative Coupling of Methane (OCM): Gas-Phase Reactions in a Jet Stirred Reactor (JSR).

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.

Maximizing Oxygen Evolution Performance on a Transparent NiFeOx/Ta3N5 Photoelectrode Fabricated on an Insulator.

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.

Delivering the Full Potential of Oxygen Evolving Electrocatalyst by Conditioning Electrolytes at Near-Neutral pH.

This study reports on the impact of identity and compositions of buffer ions on oxygen evolution reaction (OER) performance at a wide range of pH levels using a model IrOx electrocatalyst. Rigorous microkinetic analysis employing kinetic isotope effects, Tafel analysis, and temperature dependence measurement was conducted to establish rate expression isolated from the diffusion contribution of buffer ions and solution resistance. It was found that the OER kinetics was facile with OH- oxidation compared to H2 O, the results of which were highlighted by mitigating over 200 mV overpotential in the presence of buffer to reach 10 mA cm-2 . This improvement was ascribed to the involvement of the kinetics of the local OH- supply by the buffering action. Further digesting the kinetic data at various buffer pKa and the solution bulk pH disclosed a trade-off between the exchange current density and the Tafel slope, indicating that the optimal electrolyte condition can be chosen at a different range of current density. This study provides a quantitative guideline for electrolyte engineering to maximize the intrinsic OER performance that electrocatalyst possesses especially at near-neutral pH.

Water Electrolysis in Saturated Phosphate Buffer at Neutral pH.

Hydrogen production from renewable energy and ubiquitous water has a potential to achieve sustainability, although current water electrolyzers cannot compete economically with the fossil fuel-based technology. Here, we evaluate water electrolysis at pH 7 that is milder than acidic and alkaline pH counterparts and may overcome this issue. The physicochemical properties of concentrated buffer electrolytes were assessed at various temperatures and molalities for quantitative determination of losses associated with mass-transport during the water electrolysis. Subsequently, in saturated K-phosphate solutions at 80 °C and 100 °C that were found to be optimal to minimize the losses originating from mass-transport at the neutral pH, the water electrolysis performance over model electrodes of IrOx and Pt as an anode and a cathode, respectively, was reasonably comparable with those of the extreme pH. Remarkably, this concentrated buffer solution also achieved enhanced stability, adding another merit of this electrolyte for water electrolysis.

Author Correction: Insight on Tafel slopes from a microkinetic analysis of aqueous electrocatalysis for energy conversion.

A correction has been published and is appended to both the HTML and PDF versions of this paper. The error has not been fixed in the paper.

Efficient Water Oxidation Using Ta3 N5 Thin Film Photoelectrodes Prepared on Insulating Transparent Substrates.

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.

Determination and perturbation of the electronic potentials of solid catalysts for innovative catalysis.

Concerns about energy and the environment are motivating a reexamination of catalytic processes, aiming to achieve more efficient and improved catalysis compatible with sustainability. Designing an active site for such heterogeneous catalytic processes remains a challenge leading to a next level breakthrough. Herein, we discuss a fundamental aspect of heterogeneous catalysis: the chemical potential of electrons in solid catalysts during thermal catalysis, which directly reflects the consequent catalytic reaction rate. The use of electrochemical tools during thermal catalysis allows for the quantitative determination of the ill-defined chemical potentials of solids in operando, whereby the potential-rate relationship can be established. Furthermore, the electrochemical means can also introduce the direct perturbation of catalyst potentials, in turn, perturbing the coverage of adsorbates functioning as poison, promoters, or reactants. We collect selected publications on these aspects, and provide a viewpoint bridging the fields of thermal- and electro-catalysis.