Strong Electronic Interaction Enables Enhanced Solar-Driven CO2 Reduction into Selective CH4 on SrTiO3 with Photodeposited Pt2+ Sites.

Targeting selective CO2 photoreduction into CH4 remains a challenge due to the sluggish reaction kinetics and poor hydrogenation ability of the unstable intermediate. Here, the active Pt2+ sites were photodeposited on the SrTiO3 photocatalyst, which was well demonstrated to manipulate the CH4 product selectivity. The results showed that SrTiO3 mainly yielded the CO (6.98 µmol g-1) product with poor CH4 (0.17 µmol g-1). With the Pt2+ modification, 100% CH4 selectivity could be obtained with an optimized yield rate of 8.07 µmol g-1. The prominent enhancement resulted from the following roles: (1) the strong electronic interaction between the Pt2+ cocatalyst and SrTiO3 could prompt efficient separation of the photoelectron-hole pairs. (2) The Pt2+ sites were active to capture and activate inert CO2 into HCO3- and CO32- species and allowed fast *COOH formation with the lowered reaction barrier. (3) Compared with SrTiO3, the formed *CO species could be captured tightly on the Pt2+ cocatalyst surface for generating the *CH2 intermediate by the following electron-proton coupling reaction, thus leading to the CH4 product with 100% selectivity.

Stable CO2reduction under natural air on Ni-Sn hydroxide photocatalyst with dynamic renewable oxygen vacancies.

Advanced photocatalysts are highly desired to activate the photocatalytic CO2reduction reaction (CO2RR) with low concentration. Herein, the NiSn(OH)6with rich surface lattice hydroxyls was synthesized to boost the activity directly under the natural air. Results showed that terminal Ni-OH could serve as donors to feed protons and generate oxygen vacancies (VO), thus beneficial to convert the activated CO2(HCO3-) mainly into CO (5.60µmol g-1) in the atmosphere. It was flexible and widely applicable for a stable CO2RR from high pure to air level free of additionally adding H2O reactant, and higher than the traditional gas-liquid-solid (1.58µmol g-1) and gas-solid (4.07µmol g-1) reaction system both using high pure CO2and plenty of H2O. The strong hydrophilia by the rich surface hydroxyls allowed robust H2O molecule adsorption and dissociation at VOsites to achieve the Ni-OH regeneration, leading to a stable CO yield (11.61µmol g-1) with the enriched renewable VOregardless of the poor CO2and H2O in air. This work opens up new possibilities for the practical application of natural photosynthesis.

Thermal-Stabilized Protonated TiO2 for Heat-Accelerated Photoelectrochemical Water Splitting.

Enhancing the charge separation efficiency is a big challenge that limits the energy conversion efficiency of photoelectrochemical (PEC) water splitting. Surface states generated by protonation of TiO2 are the efficient charge separation passageways to massively accept or transfer the photogenerated electrons. However, a challenge is to avoid the deprotonation of a protonated TiO2 photoelectrode at the operation temperature. Here, we found that the terminal hydroxyl group (OHT) as surface states on the TiO2 surface generated via electrochemical protonation of TiO2 at 90 °C [90-TiO2-x-(OH)x] is thermally stable. As a result, the thermally enhanced photocurrent of the 90-TiO2-x-(OH)x electrode reached 1.05 mA cm-2 under 80 °C, and the stability was maintained up to 10 h with a slight photocurrent decrease of 3%. The thermally stable surface states as charge separation paths provide an effective method to couple the heat field with the PEC process via thermal-stimulating hopping of polarons.

Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids.

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, -7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (-7.1 to -6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, -0.50 to -0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<-7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes.

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion.

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T-1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

Iron 3D-Orbital Configuration Dependent Electron Transfer for Efficient Fenton-Like Catalysis.

Transition metals are excellent active sites to activate peroxymonosulfate (PMS) for water treatment, but the favorable electronic structures governing  reaction mechanism still remain elusive. Herein, the authors construct typical d-orbital configurations on iron octahedral (FeOh ) and tetrahedral (FeTd ) sites in spinel ZnFe2 O4 and FeAl2 O4 , respectively. ZnFe2 O4 (136.58 min-1 F-1 cm2 ) presented higher specific activity than FeAl2 O4 (97.47 min-1 F-1 cm2 ) for tetracycline removal by PMS activation. Considering orbital features of charge amount, spin state, and orbital arrangement by magnetic spectroscopic analysis, ZnFe2 O4 has a larger bond order to decompose PMS. Using this descriptor, high-spin FeOh is assumed to activate PMS mainly to produce nonradical reactive oxygen species (ROS) while high-spin FeTd prefers to induce radical species. This hypothesis is confirmed by the selective predominant ROS of 1 O2 on ZnFe2 O4 and O2 •- on FeAl2 O4 via quenching experiments. Electrochemical determinations reveal that FeOh has superior capability than FeTd for feasible valence transformation of iron cations and fast interfacial electron transfer. DFT calculations further suggest octahedral d-orbital configuration of ZnFe2 O4 is beneficial to enhancing Fe-O covalence for electron exchange. This work attempts to understand the d-orbital configuration-dependent PMS activation to design efficient catalysts.

Thermal suppression of charge disproportionation accelerates interface electron transfer of water electrolysis.

The sluggish electron transfer kinetics in electrode polarization driven oxygen evolution reaction (OER) result in big energy barriers of water electrolysis. Accelerating the electron transfer at the electrolyte/catalytic layer/catalyst bulk interfaces is an efficient way to improve electricity-to-hydrogen efficiency. Herein, the electron transfer at the Sr3Fe2O7@SrFeOOH bulk/catalytic layer interface is accelerated by heating to eliminate charge disproportionation from Fe4+ to Fe3+ and Fe5+ in Sr3Fe2O7, a physical effect to thermally stabilize high-spin Fe4+ (t2g3eg1), providing available orbitals as electron transfer channels without pairing energy. As a result of thermal-induced changes in electronic states via thermal comproportionation, a sudden increase in OER performances was achieved as heating to completely suppress charge disproportionation, breaking a linear Arrhenius relationship. The strategy of regulating electronic states by thermal field opens a broad avenue to overcome the electron transfer barriers in water splitting.

Nonredox trivalent nickel catalyzing nucleophilic electrooxidation of organics.

A thorough comprehension of the mechanism behind organic electrooxidation is crucial for the development of efficient energy conversion technology. Here, we find that trivalent nickel is capable of oxidizing organics through a nucleophilic attack and electron transfer via a nonredox process. This nonredox trivalent nickel exhibits exceptional kinetic efficiency in oxidizing organics that possess the highest occupied molecular orbital energy levels ranging from -7.4 to -6 eV (vs. Vacuum level) and the dual local softness values of nucleophilic atoms in nucleophilic functional groups, such as hydroxyls (methanol, ethanol, benzyl alcohol), carbonyls (formamide, urea, formaldehyde, glucose, and N-acetyl glucosamine), and aminos (benzylamine), ranging from -0.65 to -0.15. The rapid electrooxidation kinetics can be attributed to the isoenergetic channels created by the nucleophilic attack and the nonredox electron transfer via the unoccupied eg orbitals of trivalent nickel (t2g6eg1). Our findings are valuable in identifying kinetically fast organic electrooxidation on nonredox catalysts for efficient energy conversions.

Carbonized Polymer Dots/Bi/ß-Bi2O3 for Efficient Photosynthesis of H2O2 via Redox Dual Pathways.

A novel heterojunction photocatalyst of carbonized polymer dots (CPDs)/Bi/ß-Bi2O3 is successfully synthesized via a one-pot solvothermal method by adjusting the reaction temperature and time. As a solvent and carbon source, ethylene glycol not only supports the conversion of Bi3+ to ß-Bi2O3 but also undergoes its polymerization, cross-linking, and carbonization to produce CPDs. In addition, partial Bi3+ is reduced to Bi by ethylene glycol. As a result, the CPDs and Bi are deposited in situ on the surface of ß-Bi2O3 microspheres. There are four built-in electric fields in the CPDs/Bi/ß-Bi2O3 system, namely, the n-type semiconductor ß-Bi2O3/H2O interface, the p-type CPDs/H2O interface, the ohmic contact between Bi and ß-Bi2O3, and the Schottky junction between Bi and CPDs. Under the action of four built-in electric fields, the Z-type charge separation mechanism is formed. It promotes the effective separation of the photogenerated electron-hole and greatly improves the yield of H2O2. Under irradiation for 2 h, the H2O2 production is 1590 µmol g-1 h-1. The solar energy to H2O2 conversion efficiency is 0.11%.

Photothermal Effect- and Interfacial Chemical Bond-Modulated NiOx/Ta3N5 Heterojunction for Efficient CO2 Photoreduction.

Photothermal catalysis, which combines light promotion and thermal activation, is a promising approach for converting CO2 into fuels. However, the development of photothermal catalysts with effective light-to-heat conversion, strong charge transfer ability, and suitable active sites remains a challenge. Herein, the photothermal effect- and interfacial N-Ni/Ta-O bond-modulated heterostructure composed of oxygen vacancy-rich NiOx and Ta3N5 was rationally fabricated for efficient photothermal catalytic CO2 reduction. Beyond the charge separation capability conferred by the NiOx/Ta3N5 heterojunction, we observed that the N-Ni and Ta-O bonds linking NiOx and Ta3N5 form a spatial charge transfer channel, which enhances the interfacial electron transfer. Additionally, the presence of surface oxygen vacancies in NiOx induced nonradiative relaxation, resulting in a pronounced photothermal effect that locally heated the catalyst and accelerated the reaction kinetically. Leveraging these favorable factors, the NiOx/Ta3N5 hybrids exhibit remarkably elevated activity (≈32.3 µmol·g-1·h-1) in the conversion of CO2 to CH4 with near-unity selectivity, surpassing the performance of bare Ta3N5 by over 14 times. This study unveils the synergistic effect of photothermal and interfacial chemical bonds in the photothermal-photocatalytic heterojunction system, offering a novel approach to enhance the reaction kinetics of various catalysts.

Synergy Effect of the Enhanced Local Electric Field and Built-In Electric Field of CoS/Mo-Doped BiVO4 for Photoelectrochemical Water Oxidation.

Bismuth vanadate is a promising material for photoelectrochemical water oxidation. However, it suffers from a low quantum efficiency, poor stability, and slow water oxidation kinetics. Here, we developed a novel photoanode of CoS/Mo-BiVO4 with excellent photoelectrochemical water oxidation performance. It achieved a photocurrent density of 4.5 mA cm-2 at 1.23 V versus the reversible hydrogen electrode, ∼4 times that of BiVO4. The CoS/Mo-BiVO4 photoanode also exhibited good stability, and the photocurrent density generated by the CoS/Mo-BiVO4 photoanode did not significantly decrease after light irradiation for 2 h. Upon replacement of part of the V with Mo doping in BiVO4, the local electric field around the Mo-O bond was enhanced, thus promoting carrier separation in BiVO4. The CoS was deposited on the surface of Mo-BiVO4, forming a built-in electric field at the interface. Under the action of the bias electric field and the built-in electric field, the carriers of CoS/Mo-BiVO4 were efficiently separated in the direction of the inverse type II heterojunction. In addition, CoS improved the light absorption and charge injection efficiency of the CoS/Mo-BiVO4 photoanode.

Native frustrated Lewis pairs on core-shell In@InOxHy enhances CO2-to-formate conversion.

Strategies to efficiently activate CO2 by strongly inhibiting the competitive hydrogen evolution reaction process are highly desired for practical applications of the electrochemical CO2 reduction technique. Here, we assembled a core-shell In@InOxHy architecture on carbon black by one-step reduction of NaBH4 as a CO2-to-formate catalyst with high selectivity. The stable CO2-to-formate reaction originates from the creation of steritic frustrated Lewis pairs (FLPs) on the InOxHy shell with In-OVs (OVs, oxygen vacancies) Lewis acid, and In-OH Lewis base. During CO2 reduction, the electrochemically stable FLPs are capable of first capturing and stabilizing protons to protonate FLPs to In-H Lewis acid and In-OH2 Lewis base due to its strong steric electrostatic field; then, CO2 is captured and activated by the protonated FLPs to selectively produce formate. Our results demonstrated that FLPs can be created on the surface of oxyphilic single-metal catalysts efficient in accelerating CO2 reduction with high selectivity.

Flexomagnetic Effect Enhanced Ferromagnetism and Magnetoelectrochemistry in Freestanding High-Entropy Alloy Films.

Freestanding thin films of functional materials enable the tuning of properties via strain and strain gradients, broadening their applications. Here, a systematic approach is proposed to fabricate freestanding CrMnFeCoNi high-entropy alloy (HEA) thin films by pulsed laser deposition using expansion-contraction of NaCl substrates and weak van der Waals interaction of the interface, which form wrinkles with inhomogeneous strain gradients when transferred to a viscous handle. We demonstrate that the nonuniform gradients of external strain (flexomagnetic effect) can induce the partial structural phase transition from FCC to BCC in the wrinkled HEA film, resulting in a 10-fold increase in its room-temperature saturation magnetization compared with the unstrained flat HEA film. Furthermore, after applying an external magnetic field, due to the different electron transfer behavior caused by the electron scattering in wrinkled and flat HEA films, their electrocatalytic magnetic responses showed a diametrically opposite picture. Our work provides a promising strategy for tuning physical and chemical properties via complex strained geometries.

Electrochemically stable frustrated Lewis pairs on dual-metal hydroxides for electrocatalytic CO2 reduction.

The sluggish kinetics of CO2 activation and reduction severely limit the energy conversion efficiency of electrocatalytic CO2 reduction into fuels. Here, ZnSn(OH)6 with an alternating arrangement of Zn(OH)6 and Sn(OH)6 octahedral units and SrSn(OH)6 with an alternating arrangement of SrO6 and Sn(OH)6 octahedral units were adopted to check the effects of frustrated Lewis pairs (FLPs) on electrochemical CO2 reduction. The FLPs were in situ electrochemically reconstructed on ZnSn(OH)6 by reducing the electrochemically unstable Sn-OH to Sn-oxygen vacancies (Sn-OVs) as a Lewis acid site, which are able to create strong interactions with the adjacent electrochemically stable Zn-OH, a Lewis base site. Compared to SrSn(OH)6 without FLPs, the higher formate selectivity of ZnSn(OH)6 originates from the strong ability of FLPs to capture protons and activate CO2via the electrostatic field of FLPs triggering better electron transfer and strong orbital interactions under negative potentials. Our findings may guide the design of electrocatalysts for CO2 reduction with high catalytic performances.

Comprehensive treatment strategy for diesel truck exhaust.

At present, diesel vehicles still play an irreplaceable role in the traditional energy field in China. Diesel vehicle exhaust contains hydrocarbons, carbon monoxide, nitrogen oxides, and particulate matter, which can lead to haze weather, photochemical smog, and the greenhouse effect; endanger human health; and damage the ecological environment. In 2020, the number of motor vehicles in China reached 372 million, and the number of automobiles reached 281 million, of which 20.92 million are diesel vehicles, accounting for only 5.6% of the number of motor vehicles and 7.4% of the number of automobiles. Nevertheless, diesel vehicles emitted 88.8% of nitrogen oxides and 99% of particulate matter in total vehicle emissions. Diesel vehicles, especially diesel trucks, have become the top priority of motor vehicle pollution control. However, there are few reviews on the comprehensive treatment of diesel vehicle exhaust. This review provides an overview of exhaust gas composition, hazards, and treatment techniques. Phytoremediation, three-way catalytic conversion, rare earth catalytic degradation, and nanoscale TiO2 catalytic degradation are briefly described.

Influence of sliding orientation on tribological properties of hybrid PTFE/Kevlar fabric composites.

A fabric liner is an ideal self-lubricating material that has been widely used in self-lubricating spherical plain bearings. To investigate the influence of sliding orientation on the tribological properties of fabric liners, samples were prepared for different fiber orientations relative to the sliding direction and wear tests were conducted under normal loads of 25 N to 200 N. Composite-90° (sliding along Kevlar fibers) shows the best friction and wear properties under loads of 50 N and 100 N, while composite-0° (sliding along PTFE fibers) exhibits the best tribological properties when the load increases to 200 N. Due to the formation of a PTFE transfer film, the friction coefficient even decreases during the stable wear stage. Although the dry sliding condition is controlled by a transfer film formed on the friction interface, the contact geometry generated by the interlacing fibers as well as sliding direction continue to dominate the tribological properties under a light load. The combination of a suitable heavy load and sliding along the PTFE fiber is conducive to the formation of a stable and continuous transfer film of debris. Accordingly, the lowest friction coefficient of 0.105 and the lowest wear depth of 0.056 mm are achieved for composite-0° under 200 N.

Multifunctional Au/Hydroxide Interface toward Enhanced C-C Coupling for Solar-Driven CO2 Reduction into C2H6.

The high-grade C2+ products from CO2 photoreduction are limited by the kinetic bottleneck. Herein, a multifunctional Au/hydroxide interface was put forward to improve the C-C coupling. As a prototype, the synthesized Au/ZnSn(OH)6 tuned the CO generation and afforded about 50% electrons toward C2H6 selectivity. The prominent enhancement resulted from the following effects: (1) strong metal-support electronic interactions built an electric field at the interface of ZnSn(OH)6 nearby the Au nanoparticles, leading to fast transfer of electrons for the C-H and C-C bonding reactions. (2) The surface solid-state Sn-OH and Zn-OH lattice hydroxyls served as donors to feed rich H+ and oxygen vacancies (OVs) via hole-induced oxidation for the boosted C2H6 formation. (3) The synergetic OVs and Au sites allowed efficient e-/H+ to boost *CO hydrogenation toward *CH3 and *CH3*CH3 formation into the C2H6 product.

Thermally Accelerated Surface Polaron Hopping in Photoelectrochemical Water Splitting.

Electron-hole separation is a main challenge that limits the energy efficiency of photoelectrochemical water splitting for hydrogen fuel production. Surface polaron states with an energy level distribution near the conduction band are highly efficient charge separation passageways to massively accept or transfer the photogenerated electrons. Here, we found that the charge separation via surface polaron states could be further enhanced by heating (<100 °C) to accelerate the electron mobility of surface polaron states. As a result of heating from 30 to 70 °C, the saturated photocurrent increased about 34.5% under 1 sun and 18.3% under 10 suns from heat-induced increase in electron flux of surface polaron states. The heat-sensitive surface-state electron transfer provides a new heat-photoelectricity coupling mechanism to guide the design of new photoanodes that are available for complementary multienergy systems with high energy efficiency.

A Template Editing Strategy to Create Interlayer-Confined Active Species for Efficient and Durable Oxygen Evolution Reaction.

Substantial overpotentials and insufficient and unstable active sites of oxygen evolution reaction (OER) electrocatalysts limit their efficiency and stability in OER-related energy conversion and storage technologies. Here, a template editing strategy is proposed to graft highly active catalytic species onto highly conductive rigid frameworks to tackle this challenge. As a successful attempt, two types of NiO6 units of layered Ni BDC (BDC stands for 1,4-benzenedicarboxylic acid) metal organic frameworks are selectively edited by chemical etching-assisted electroxidation to create layered γ-NiOOH with intercalated Ni-O species. In such an interlayer-confined intercalated architecture, the large interlayer space with high ion permeability offers an ideal reaction region to sufficiently expose the OER active sites comprising high-density intercalated Ni-O species, which also suppresses the undesirable γ to ß phase transformation, thus exhibiting efficient and durable OER activity. As a result, water oxidation can occur at an extremely low overpotential of 130 mV and affords 1000 h stability at 100 mA cm-2 . The strategy conceptually shows the possibility of achieving stable homogeneous-like catalysis in heterogeneous catalysis.

Heat-Induced Magnetic Transition for Water Electrolysis on NiFeN@NiFeOOH Core-Shell Assembly.

The overpotentials of electrochemical oxygen evolution reaction (OER) inherently originate from high electron transfer barriers of the redox couple driven water oxidation. Here, we propose a heat-induced magnetic transition strategy to reduce the spin-related electron transfer barriers. Coupling heat into electrochemical OER on a ferro-antiferromagnetic core-shell NiFeN@NiFeOOH, the heat-induced ferro-to-paramagnetic transition for NiFeN core at 55 °C and antiferro-to-paramagnetic transition for NiFeOOH shell at 70 °C significantly accelerate and accordingly achieve a cascaded Ni2+/Ni3+ driven water oxidation reaction. In addition, paramagnetic Niδ+ (δ ≥ 3) in NiFeN@NiFeOOH can thermochemically react with water to produce oxygen. The heat-induced magnetic transition concomitantly triggers the electrochemical redox couple driven water oxidation and the thermochemical water oxidation due to that heat-induced paramagnetic spin reduces the barriers of electricity driving the spin flipping. Our findings offer new insights into constructing the heat-electricity coupling water splitting.