Mechanistic insights and catalytic enhancement of phenolic wastewater supercritical water gasification: A combined experiment and density functional theory study.

Supercritical water gasification technology provides a favorable technology to achieve pollution elimination and resource utilization of phenolic wastewater. In this study, the reaction mechanism of phenolic wastewater supercritical water gasification was investigated using a combination of experimental and computational methods. Five reaction channels were identified to elucidate the underlying pathway of phenol decomposition. Importantly, the rate-determining step was found to be the dearomatization reaction. By integrating computational and experimental analyses, it was found that phenol decomposition via the path with the lowest energy barrier generates cyclopentadiene, featuring a dearomatization barrier of 70.97 kcal/mol. Additionally, supercritical water plays a catalytic role in the dearomatization process by facilitating proton transfer. Based on the obtained reaction pathway, alkali salts (Na2CO3 and K2CO3) are employed as a catalyst to diminish the energy barrier of the rate-determining step to 40.00 kcal/mol and 37.14 kcal/mol. Alkali salts catalysis significantly improved carbon conversion and pollutant removal from phenolic wastewater, increasing CGE from 58.44% to 93.55% and COD removal efficiency from 94.11% to 99.79%. Overall, this study provides a comprehensive understanding of the decomposition mechanism of phenolic wastewater in supercritical water.

dz2 Band Links Frontier Orbitals and Charge Carrier Dynamics of Single-Atom Cocatalyst-Aided Photocatalytic H2 Production.

The Cu single-atom catalyst (SAC) supported on TiO2 exhibits outstanding efficacy in photocatalytic hydrogen evolution. The precise operational mechanism remains a subject of ongoing debate. The focus resides with the interplay linking heightened catalytic activity, dynamic valence state alterations of Cu atoms, and their hybridization with H2O orbitals, manifested in catalyst color changes. Taking anatase TiO2 (101) as a prototypical surface, we perform ab initio quantum dynamics simulation to reveal that the high activity of the Cu-SAC is due to the quasi-planar coordination structure of the Cu atom after H2O adsorption, allowing it to trap photoexcited hot electrons and inject them into the hybridized orbital between Cu and H2O. The observed alterations in the valence state and the coloration can be attributed to the H atom released during H2O dissociation and adsorbed onto the lattice O atom neighboring the Cu-SAC. Notably, this adsorption of H atoms puts the Cu-SAC into an inert state, as opposed to an activating effect reported previously. Our work clarifies the relationship between the high photocatalytic activity and the local dynamic atomic coordination structure, providing atomistic insights into the structural changes occurring during photocatalytic reactions on SACs.

Structurally Ordered PtNi Intermetallic Nanoparticles as Efficient and Stable Cathode Catalysts for Proton Exchange Membrane Fuel Cells.

Exploring oxygen reduction reaction (ORR) catalysts with superior electrochemical performance and long-term stability is crucial to the development of proton exchange membrane fuel cells (PEMFCs). In this work, graphited carbon with high specific surface area was obtained under relatively low temperature using Ni catalyst, then ordered nanoparticles (NPs) PtNi catalysts attaching to graphited carbon were synthesized via polyol reduction and thermal treatment. Benefiting from graphitized carbon support and appropriate order degree, PtNi/GC-700 NPs catalyst exhibits excellent electrocatalytic ORR performance with specific and mass activities as high as 2.8-fold and 3.7-fold of the commercial Pt/C catalyst, respectively. Besides, the as-prepared PtNi/GC-700 catalyst exhibits superior stability with negligible degradation after 10000 potential cycles, due to its ordered chemical structure. The work described herein highlights the potential of structurally ordered electrocatalysts for efficient and durable fuel cell cathodic catalysts.

Influence of subatmospheric pressure on bubble evolution on the TiO2 photoelectrode surface.

The increase of reaction resistance caused by bubble nucleation and long-time growth on the surface of the photoelectrode is an important factor that leads to the low efficiency of photoelectrochemical water splitting. In this study, we adopted an electrochemical workstation synchronous with a high-speed microscopic camera system to achieve in situ observation of oxygen bubble behavior on the surface of TiO2 and to study the internal relationship between the geometric parameters of oxygen bubbles and photocurrent fluctuations under different pressures and laser powers. The results indicate that with the decrease of pressure, the photocurrent decreases gradually and the bubble departure diameter increases gradually. In addition, the nucleation waiting stage and the growth stage of bubbles are both shortened. However, the difference between the average photocurrents corresponding to the moment of bubble nucleation and the stable growth stage hardly changes with the pressure. The production rate of gas mass reaches a peak near 80 kPa. In addition, a force balance model suitable for different pressures is constructed. It is found that as the pressure decreases from 97 kPa to 40 kPa, the proportion of the thermal Marangoni force in the Marangoni force decreases from 29.4% to 21.3%, while the proportion of the concentration Marangoni force increases from 70.6% to 78.7%, indicating that the concentration Marangoni force is the main factor affecting the bubble departure diameter under subatmospheric pressure conditions.

Growth characteristics and the mass transfer mechanism of single bubble on a photoelectrode at different electrolyte concentrations.

In the photoelectrochemical water splitting reaction, the bubble attached to the working electrode is an essential factor affecting the reaction resistance, current density and gas-liquid mass transfer. An experimental measurement system based on an electrochemical workstation synchronously coupled with a high-speed microscopic camera was proposed and used to systematically study the growth kinetics and mass transfer mechanism of single oxygen bubbles at different electrolyte concentrations (Na2SO4, 0.1-2.0 M) on the TiO2 photoanode surface. Under constant voltage and constant current control conditions, when the electrolyte concentration increases, the bubble detachment diameter and the bubble detachment frequency gradually decrease. The bubble coverage equation expressed in terms of gas evolution efficiency is proposed and is associated with the photocurrent and bubble radius. The average bubble coverage and average gas evolution efficiency decrease when the electrolyte concentration is increased. According to the Sherwood dimensionless number, various mass transfer coefficients during bubble growth were calculated. The results show that the average total mass transfer coefficient is positively correlated with the change trend of the electrolyte concentration, and the mass transfer coefficient of single-phase natural convection is one order of magnitude larger than the mass transfer coefficient of bubble-induced convection. Finally, a conclusion on the transient mass transfer process in the bubble evolution process was obtained, that is, the mass transfer coefficient of single-phase natural convection and the total mass transfer coefficient remain high during the first growth stage, and gradually decrease during the second growth stage. Therefore, regulating the electrolyte concentration can effectively promote the gas-liquid mass transfer in the photoelectrochemical water splitting reaction.

Improved Carrier Lifetime in BiVO4 by Spin Protection.

Mechanistic understanding of the effect bulk defects have on carrier dynamics at the quantum level is crucial to suppress associated midgap mediated charge recombination in semiconductors yet many questions remain unexplored. Here, by employing ab initio quantum dynamics simulation and taking BiVO4 with oxygen vacancies (Ov) as a model system we demonstrate a spin protection mechanism for suppressed charge recombination. The carrier lifetime is significantly improved in the high spin defect system. The lifetime can be optimized by tuning the Ov concentration to minimize the nonradiative relaxation. Our work addresses literature ambiguities and contradictions about the role of bulk Ov in charge recombination and provides a route for defect engineering of semiconductors with enhanced carrier dynamics.

Experimental study on alkali catalytic gasification of oily sludge in supercritical water with a continuous reactor.

Realizing the harmless resource utilization of oily sludge is urgent for petroleum industry and of great significance for environmental management. The treatment of oily sludge was investigated using supercritical water gasification (SCWG) with a continuous fluidized bed reactor. The effect of operating parameters on gasification efficiency and gas yield without catalyst was tested, and then the influences of catalyst type (K2CO3 and Na2CO3) and concentrations (1-8 wt%) were systematically studied. The results indicated that a medium mass flow ratio and low feedstock concentration were beneficial for gas production. Alkali catalyst improved carbon gasification efficiency (CE) prominently, and Na2CO3 showed better performance due to its better stability. A maximum CE of 95.87% was achieved when 5 wt% Na2CO3 was added at 650 °C, 23 MPa with 5 wt% oily sludge concentration. Besides, according to XRD patterns of solid residues, Na2CO3 was more stable than K2CO3 during SCWG. SEM-EDX results also revealed that more K was migrated into solid residues than Na. The analysis of pore structure demonstrated that alkali catalyst promoted the evolution of pore structure, resulting in higher specific surface areas and total pore volumes. Na2CO3 has a more substantial destructive effect on solid matrix, causing the matrix structure to collapse and inhibiting pore structure development. The FTIR spectra of solid products exhibited a lower content of carbohydrates and aromatic structures than the initial oily sludge. NH4-N results demonstrated that SCWG was a potential green treatment process for oily sludge. This work can not only give an insight into the reaction mechanism of alkali catalytic gasification of oily sludge, but also help to guide the optimal design of reactor and the regulation of operating parameters.

Solar Hydrocarbons: Single-Step, Atmospheric-Pressure Synthesis of C2 -C4 Alkanes and Alkenes from CO2.

Cobalt ferrite (CoFe2 O4 ) spinel has been found to produce C2 -C4 hydrocarbons in a single-step, ambient-pressure, photocatalytic hydrogenation of CO2 with a rate of 1.1 mmol g-1 h-1 , selectivity of 29.8 % and conversion yield of 12.9 %. On stream the CoFe2 O4 reconstructs to a CoFe-CoFe2 O4 alloy-spinel nanocomposite which facilitates the light-assisted transformation of CO2 to CO and hydrogenation of the CO to C2 -C4 hydrocarbons. Promising results obtained from a laboratory demonstrator bode well for the development of a solar hydrocarbon pilot refinery.

A Semiconductor-Mediator-Catalyst Artificial Photosynthetic System for Photoelectrochemical Water Oxidation.

In fabricating an artificial photosynthesis (AP) electrode for water oxidation, we have devised a semiconductor-mediator-catalyst structure that mimics photosystem II (PSII). It is based on a surface layer of vertically grown nanorods of Fe2 O3 on fluorine doped tin oxide (FTO) electrodes with a carbazole mediator base and a Ru(II) carbene complex on a nanolayer of TiO2 as a water oxidation co-catalyst. The resulting hybrid assembly, FTO|Fe2 O3 |-carbazole|TiO2 |-Ru(carbene), demonstrates an enhanced photoelectrochemical (PEC) water oxidation performance compared to an electrode without the added carbaozle base with an increase in photocurrent density of 2.2-fold at 0.95 V vs. NHE and a negatively shifted onset potential of 500 mV. The enhanced PEC performance is attributable to carbazole mediator accelerated interfacial hole transfer from Fe2 O3 to the Ru(II) carbene co-catalyst, with an improved effective surface area for the water oxidation reaction and reduced charge transfer resistance.

Atomically Dispersed Janus Nickel Sites on Red Phosphorus for Photocatalytic Overall Water Splitting.

Single-atom nickel catalysts hold great promise for photocatalytic water splitting due to their plentiful active sites and cost-effectiveness. Herein, we adopt a reactive-group guided strategy to prepare atomically dispersed nickel catalysts on red phosphorus. The hydrothermal treatment of red phosphorus leads to the formation of P-H and P-OH groups, which behave as the reactive functionalities to generate the dual structure of single-atom P-Ni and P-O-Ni catalytic sites. The produced single-atom sites provide two different functions: P-Ni for water reduction and P-O-Ni for water oxidation. Benefitting from this specific Janus structure, Ni-red phosphorus shows an elevated hydrogen evolution rate compared to Ni nanoparticle-modified red phosphorus under visible-light irradiation. The hydrogen evolution rate was additionally enhanced with increased reaction temperature, reaching 91.51 µmol h-1 at 70 °C, corresponding to an apparent quantum efficiency of 8.9 % at 420 nm excitation wavelength.

First-Principles Investigation of ß-FeOOH for Hydrogen Evolution: Identifying Reactive Sites and Boosting Surface Reactions.

Akaganeite (ß-FeOOH) is a widely investigated candidate for photo(electro)catalysis, such as water splitting. Nevertheless, insights into understanding the surface reaction between water and ß-FeOOH, in particular, the hydrogen evolution reaction (HER), are still insufficient. Herein, a set of first-principles calculations on pristine ß-FeOOH and halogen-substituted ß-FeOOH are applied to evaluate the HER performance through the computational hydrogen electrode model. The results show that the HER on ß-FeOOH tends to occur at Fe sites on the (010) surface, and palladium and nickel are found to serve as excellent co-catalysts to boost the HER process, due to the remarkably reduced free energy change of hydrogen adsorption upon loading on the surface of ß-FeOOH, demonstrating great potential for efficient water splitting.

Growth of NiMn layered double hydroxides on nanopyramidal BiVO4 photoanode for enhanced photoelectrochemical performance.

Photoelectrochemical water oxidation for hydrogen generation via utilizing sunlight is considered a very promising pathway for generating sustainable energy in an environmental manner. Here, a composite photoanode, consisting of nanopyramidal BiVO4 arrays and one layered double hydroxide (NiMn-LDH) was designed and fabricated via a facile route. The obtained BiVO4/NiMn-LDH composite photoelectrode presented a significant enhancement in the photoelectrochemical (PEC) current density, conversion efficiency and stability for solar water oxidation. With 2D NiMn-LDH decoration, an obvious cathodic shift of ∼480 mV in the onset potential can be observed, and more than two times enhancement in photocurrent performance is achieved. The improvement in photoelectrochemical activity for BiVO4/NiMn-LDH composite photoanode can be attributed to the enhanced water-oxidation kinetics leading to the efficient separation, transfer and collection of charge carriers at the photoanode/electrolyte interface. The result demonstrates NiMn-LDH represents one of the active oxygen evolution catalysts (OECs) to improve the PEC activity of metal oxide photoanode.

On factors limiting the performance of photoelectrochemical CO2 reduction.

The photoelectrochemical CO2 reduction reaction (PEC-CO2RR) is a promising artificial photosynthetic system for storing solar energy as the energy of chemical bonds and stabilizing the atmospheric CO2 level. An applicable PEC-CO2RR is expected to have broad light absorption, high selectivity to a single product, and high solar to fuel efficiency. However, the PEC-CO2RR still faces challenges from complex reaction pathways, obstructed mass transfer, and large photovoltage requirements. The goal of this perspective is to point out some of the limitations of PEC-CO2RR to a practical application. In brief, we discuss the basic concepts of PEC-CO2RR and summarize state-of-the-art progress. Moreover, we highlight the remaining challenges to both science and engineering and propose the key steps in developing a fully functional PEC-CO2RR system. Finally, an ideal PEC-CO2RR system is proposed for future studies, which is essentially wireless and combines the advantages of minimized polarization loss and broad light absorption.

Controlled Deposition of ZnS Nanoparticles on CuS Nanoplates for Visible-Light-Driven Photocatalytic H2 Production.

With giving property to guide and control charge carries at the nanoscale, fabrication of heterostructure photocatalyst with desirable spatial distribution has been significantly valued. In this study, by using CuS nanoplates as seeds, CuS@ZnS core-shell heterojunction photocatalysts with diverse morphologies were developed via controlled synthesis kinetics. Kinetic control was completed through manipulation of the injection rate of Zn2+ precursor with a syringe pump as well as the reaction temperature. It is found that the growth is determined by the deposition rate relative to the diffusion rate of the ZnS growth monomers. Specifically, at a high injection rate and a relatively low reaction temperature, ZnS monomers on the surface of the CuS nanoplate will be in a localized manner and tend to form island nanoparticles. On the contrary, when surface diffusion is facilitated at a lower injection rate and a higher reaction temperature, the morphology of the ZnS nanocrystals can be switched to flat ZnS layers covering the surface of CuS. In addition, the heterostructures have found with shape-dependent photocatalytic performance toward H2 evolution under visiblelight irradiation. The CuS@ZnS core-shell composites that possess ZnS islands exhibit the highest photocatalytic activity. The corresponding H2 generation rate reaches 6.3 µmol h-1 g-1, which is 37.9 times of that for CuS@ZnS core-shell nanostructure with flat surface. This work thus provides a powerful means for the rational design and synthesis of heterojunctions with spatially controlled distribution of the component.

Vapor-Phase Epitaxial Growth of Aligned Nanowire Networks of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I).

With the intense interest in inorganic cesium lead halide perovskites and their nanostructures for optoelectronic applications, high-quality crystalline nanomaterials with controllable morphologies and growth directions are desirable. Here, we report a vapor-phase epitaxial growth of horizontal single-crystal CsPbX3 (X = Cl, Br, I) nanowires (NWs) and microwires (MWs) with controlled crystallographic orientations on the (001) plane of phlogopite and muscovite mica. Moreover, single NWs, Y-shaped branches, interconnected NW or MW networks with 6-fold symmetry, and, eventually, highly dense epitaxial network of CsPbBr3 with nearly continuous coverage were controllably obtained by varying the growth time. Detailed structural study revealed that the CsPbBr3 wires grow along the [001] directions and have the (100) facets exposed. The incommensurate heteroepitaxial lattice match between the CsPbBr3 and mica crystal structures and the growth mechanism of these horizontal wires due to asymmetric lattice mismatch were proposed. Furthermore, the photoluminescence waveguiding and good performance from the photodetector device fabricated with these CsPbBr3 networks demonstrated that these well-connected CsPbBr3 NWs could serve as straightforward platforms for fundamental studies and optoelectronic applications.

Control of Charge Carriers Trapping and Relaxation in Hematite by Oxygen Vacancy Charge: Ab Initio Non-adiabatic Molecular Dynamics.

Ultrafast charge recombination in hematite (α-Fe2O3) severely limits its applications in solar energy conversion and utilization, for instance, in photoelectrochemical water splitting. We report the first time-domain ab initio study of charge relaxation dynamics in α-Fe2O3 with and without the oxygen vacancy (Ov) defect, using non-adiabatic molecular dynamics implemented within time-dependent density functional theory. The simulations show that the hole trapping is the rate-limiting step in the electron-hole recombination process for both neutral and ionized Ov systems. The electron trapping is fast, and the trapped electron are relatively long-lived. A similar asymmetry is found for the relaxation of free charge carriers: relaxation of photoholes in the valence band is slower than relaxation of photoelectrons in the conduction band. The slower dynamics of holes offers an advantage to water oxidation at α-Fe2O3 photoanodes. Notably, the neutral Ov defect accelerates significantly the charge recombination rate, by about a factor of 30 compared to the ideal lattice, due to the stronger electron-vibrational coupling at the defect. However, the recombination rate in the ionized Ov defect is decreased by a factor of 10 with respect to the neutral defect, likely due to expansion of the local iron shell around the Ov site. The Ov defect ionization in α-Fe2O3 photoanodes is important for increasing both electrical conductivity and charge carrier lifetimes. The simulations reproduce well the time scales for the hot carrier cooling, trapping and recombination available from transient spectroscopy experiments, and suggest two alternative mechanisms for the Ov-assisted electron-hole recombination. The study provides a detailed atomistic understanding of carrier dynamics in hematite, and rationalizes the experimentally reported activation of α-Fe2O3 photoanodes by incorporation of Ov defects.

Single-Crystal Thin Films of Cesium Lead Bromide Perovskite Epitaxially Grown on Metal Oxide Perovskite (SrTiO3).

High-quality metal halide perovskite single crystals have low defect densities and excellent photophysical properties, yet thin films are the most sought after material geometry for optoelectronic devices. Perovskite single-crystal thin films (SCTFs) would be highly desirable for high-performance devices, but their growth remains challenging, particularly for inorganic metal halide perovskites. Herein, we report the facile vapor-phase epitaxial growth of cesium lead bromide perovskite (CsPbBr3) continuous SCTFs with controllable micrometer thickness, as well as nanoplate arrays, on traditional oxide perovskite SrTiO3(100) substrates. Heteroepitaxial single-crystal growth is enabled by the serendipitous incommensurate lattice match between these two perovskites, and overcoming the limitation of island-forming Volmer-Weber crystal growth is critical for growing large-area continuous thin films. Time-resolved photoluminescence, transient reflection spectroscopy, and electrical transport measurements show that the CsPbBr3 epitaxial thin film has a slow charge carrier recombination rate, low surface recombination velocity (104 cm s-1), and low defect density of 1012 cm-3, which are comparable to those of CsPbBr3 single crystals. This work suggests a general approach using oxide perovskites as substrates for heteroepitaxial growth of halide perovskites. The high-quality halide perovskite SCTFs epitaxially integrated with multifunctional oxide perovskites could open up opportunities for a variety of high-performance optoelectronics devices.

WO3/g-C3N4 composites: one-pot preparation and enhanced photocatalytic H2 production under visible-light irradiation.

A series of WO3/g-C3N4 composites with different WO3 contents were prepared via a facile one-pot pyrolysis method, and showed notably enhanced visible-light-driven photocatalytic H2-evolution activities, with the highest rate of 400 µmol h-1 gcat-1 that was 15.0 times of that for pristine g-C3N4. Contents and sizes of WO3 crystallites in the composites were easily adjusted by changing the molar ratios of (NH4)2WS4 to C3H6N6 in the feed reagents, thereby successfully optimizing the Z-scheme system constructed by WO3 and g-C3N4 and thus effectively reducing the recombination of photogenerated charge carriers in g-C3N4. Moreover, pore volumes and surface areas of the composites were gradually enlarged by introducing WO3 into g-C3N4 via the one-pot preparation strategy, therefore promoting the redox reactions to evolve H2. This work presented an effective route to simultaneously optimize the phase compositions and textural structures of photocatalysts for enhanced H2 evolution.

Enhanced photoelectrochemical water oxidation of bismuth vanadate via a combined strategy of W doping and surface RGO modification.

Nanoporous bismuth vanadate is modified simultaneously via tungsten doping and graphene surface modification for use as an efficient photoanode. The modified films were prepared on a FTO substrate by a drop-cast method followed by photoreduction of graphene oxide. SEM, XRD, Raman and XPS characterization was conducted to confirm the incorporation of tungsten and reduced graphene oxide (RGO), and to look into their influences on the structure and performance of BiVO4. Electrochemical impedance spectroscopy analysis clearly revealed enhanced carrier density and improved electronic conductivity, which are beneficial for the enhancement of PEC performance in comparison to either individually doped or RGO modified BiVO4. Our results indicated that the enhanced PEC performance can be attributed to the synergistic effect of bulk doping and surface modification that facilitates electron and hole transport and transfer in the bulk and at the semiconductor-electrolyte interface.

Understanding divergent behaviors in the photocatalytic hydrogen evolution reaction on CdS and ZnS: a DFT based study.

It has been a long time that divergent behaviors were observed in many photocatalytic hydrogen evolution reactions (HER) on CdS and ZnS although the two photocatalysts have similar compositions and structures. For example, CdS itself is inactive and loading of cocatalysts is indispensable to achieve high efficiency of hydrogen evolution, but the reverse is true for ZnS. The underlying reasons are still unclear to date. The Volmer reaction of HER on catalysts is H(+) + e(-) + * → H*, and its free energy (ΔGH* = ΔEH* + ΔEZPE - TΔS + eU; the adsorption energy, zero-point energy, entropy and potential energy are on the right side) is a good theoretical descriptor of the electrocatalytic HER activity from the electrocatalytic HER theory. In this paper, we firstly determined the most stable CdS and ZnS(110) termination under the conditions of photocatalytic HER, i.e., pure (110), by calculating the free energies of three reactions related to H2O dissociation on (110). Then we rationalized these behaviors by calculating the free energy of H* adsorption on pure and Pt loaded CdS and ZnS(110) at different pH. The performance of photocatalytic HER on CdS and ZnS was found to be determined jointly by the free energy of H* adsorption and the conduction band minimum (CBM) of the photocatalysts. On pure (110) with large ΔGH*, the photocatalytic HER is favored on ZnS due to its higher CBM; on Pt loaded (110) with small ΔGH*, the photocatalytic HER is favored on CdS due to its lower CBM. These results well explained the divergent behaviors observed in the photocatalytic HER on CdS and ZnS.