Developing High-Capacity Solid "Molecular Basket" Sorbents for Selective CO2 Capture and Separation.

ConspectusSince carbon-based energy continues to dominate (over 80%) the global primary energy supply, carbon dioxide capture, utilization, and sequestration (CCUS) is deemed to be a promising and viable option to mitigate greenhouse gas emissions and climate change, for which CO2 capture is critical to the overall success of CCUS. Although liquid amine scrubbing is a mature technology for carbon capture, it is energy-intensive and costly due to energy consumption in solvent heating and water evaporation apart from the energy needed to break amine-CO2 bonding. To address this challenge, Song's group developed a new design approach for adsorptive CO2 capture and separation, namely, "molecular basket" sorbents (MBS), without the need for dealing with solvent heating and water evaporation. The solid MBS consisting of polymeric amines (such as PEI) immobilized into nanoporous materials (such as SBA-15) possesses a high capacity for CO2 capture with high selectivity, fast kinetics, and good regenerability. Consequently, MBS can greatly reduce energy consumption and carbon capture cost. Conventional adsorbents such as zeolites, activated carbon, alumina, and silica have low adsorption capacities, and their use of CO2 adsorption requires prior removal of moisture and cooling of flue gas (∼35 °C). On the contrast, the CO2 sorption capacity of MBS can even be promoted by the presence of moisture/steam and reaches the best performance closer to flue gas temperature (∼75 °C). This Account presents an overview of our research progress in the material development and fundamental understanding of MBS for CO2 capture and the separation of CO2 from various gas streams. It begins with an illustration of the MBS concept, followed by efforts to improve the performance and pilot-scale demonstration of MBS for CO2 capture. With the systematic characterization of MBS by various ex situ and in situ techniques, a better understanding is developed for the CO2 sorption process mechanistically. Furthermore, this Account demonstrates how the fundamental understanding of the CO2 sorption mechanism promotes the further development of more robust and advanced sorbent materials with improved CO2 sorption capacity, kinetics of sorption and desorption, and cyclic stability. Finally, an outlook is provided for the future design and development of novel sorbent materials and the CO2 sorption process for various gas streams including flue gas, biogas, air, and hydrogen streams.

Engineering a Self-Grown TiO2 /Ti-MOF Heterojunction with Selectively Anchored High-Density Pt Single-Atomic Cocatalysts for Efficient Visible-Light-Driven Hydrogen Evolution.

A photocatalyst TiO2 /Ti-BPDC-Pt is developed with a self-grown TiO2 /Ti-metal-organic framework (MOF) heterojunction, i.e., TiO2 /Ti-BPDC, and selectively anchored high-density Pt single-atomic cocatalysts on Ti-BPDC for photocatalytic hydrogen evolution. This intimate heterojunction, growing from the surface pyrolytic reconstruction of Ti-BPDC, works in a direct Z-scheme, efficiently separating electrons and holes. Pt is selectively anchored on Ti-BPDC by ligands and is found in the form of single atoms with loading up to 1.8 wt %. The selective location of Pt is the electron-enriched domain of the heterojunction, which further enhances the utilization of the separated electrons. This tailored TiO2 /Ti-BPDC-Pt shows a significantly enhanced activity of 12.4 mmol g-1 h-1 compared to other TiO2 - or MOF-based catalysts. The structure-activity relationship further proves the balance of two simultaneously exposed domains of heterojunctions is critical to fulfilling this kind of catalyst.

Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis.

The utilization of fossil fuels has enabled an unprecedented era of prosperity and advancement of well-being for human society. However, the associated increase in anthropogenic carbon dioxide (CO2) emissions can negatively affect global temperatures and ocean acidity. Moreover, fossil fuels are a limited resource and their depletion will ultimately force one to seek alternative carbon sources to maintain a sustainable economy. Converting CO2 into value-added chemicals and fuels, using renewable energy, is one of the promising approaches in this regard. Major advances in energy-efficient CO2 conversion can potentially alleviate CO2 emissions, reduce the dependence on nonrenewable resources, and minimize the environmental impacts from the portions of fossil fuels displaced. Methanol (CH3OH) is an important chemical feedstock and can be used as a fuel for internal combustion engines and fuel cells, as well as a platform molecule for the production of chemicals and fuels. As one of the promising approaches, thermocatalytic CO2 hydrogenation to CH3OH via heterogeneous catalysis has attracted great attention in the past decades. Major progress has been made in the development of various catalysts including metals, metal oxides, and intermetallic compounds. In addition, efforts are also put forth to define catalyst structures in nanoscale by taking advantage of nanostructured materials, which enables the tuning of the catalyst composition and modulation of surface structures and potentially endows more promising catalytic performance in comparison to the bulk materials prepared by traditional methods. Despite these achievements, significant challenges still exist in developing robust catalysts with good catalytic performance and long-term stability. In this review, we will provide a comprehensive overview of the recent advances in this area, especially focusing on structure-activity relationship, as well as the importance of combining catalytic measurements, in situ characterization, and theoretical studies in understanding reaction mechanisms and identifying key descriptors for designing improved catalysts.

Defects Promote Ultrafast Charge Separation in Graphitic Carbon Nitride for Enhanced Visible-Light-Driven CO2 Reduction Activity.

Fundamental photocatalytic limitations of solar CO2 reduction remain due to low efficiency, serious charge recombination, and short lifetime of catalysts. Herein, two-dimensional graphitic carbon nitride nanosheets with nitrogen vacancies (g-C3 Nx ) located at both three-coordinate N atoms and uncondensed terminal NHx species were prepared by one-step tartaric acid-assistant thermal polymerization of dicyandiamide. Transient absorption spectra revealed that the defects in g-C3 N4 act as trapped states of charges to result in prolonged lifetimes of photoexcited charge carriers. Time-resolved photoluminescence spectroscopy revealed that the faster decay of charges is due to the decreased interlayer stacking distance in g-C3 Nx in favor of hopping transition and mobility of charge carriers to the surface of the material. Owing to the synergic virtues of strong visible-light absorption, large surface area, and efficient charge separation, the g-C3 Nx nanosheets with negligible loss after 15 h of photocatalysis exhibited a CO evolution rate of 56.9 µmol g-1 h-1 under visible-light irradiation, which is roughly eight times higher than that of pristine g-C3 N4 . This work presents the role of defects in modulating light absorption and charge separation, which opens an avenue to robust solar-energy conversion performance.

Preassembly Strategy To Fabricate Porous Hollow Carbonitride Spheres Inlaid with Single Cu-N3 Sites for Selective Oxidation of Benzene to Phenol.

Developing single-atom catalysts with porous micro-/nanostructures for high active-site accessibility is of great significance but still remains a challenge. Herein, we for the first time report a novel template-free preassembly strategy to fabricate porous hollow graphitic carbonitride spheres with single Cu atoms mounted via thermal polymerization of supramolecular preassemblies composed of a melamine-Cu complex and cyanuric acid. Atomically dispersed Cu-N3 moieties were unambiguously confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure spectroscopy. More importantly, this material exhibits outstanding catalytic performance for selective oxidation of benzene to phenol at room temperature, especially showing phenol selectivity (90.6 vs 64.2%) and stability much higher than those of the supported Cu nanoparticles alone, originating from the isolated unique Cu-N3 sites in the porous hollow structure. An 86% conversion of benzene, with an unexpectedly high phenol selectivity of 96.7% at 60 °C for 12 h, has been achieved, suggesting a great potential for practical applications. This work paves a new way to fabricate a variety of single-atom catalysts with diverse graphitic carbonitride architectures.

DFT insight into the effect of potassium on the adsorption, activation and dissociation of CO2 over Fe-based catalysts.

Catalytic conversion of CO2 including hydrogenation has attracted great attention as a method for chemical fixation of CO2 in combination with other techniques such as CO2 capture and storage. Potassium is a well-known promotor for many industrial catalytic processes such as in Fischer-Tropsch synthesis. In this work, we performed density functional theory (DFT) calculations to investigate the effect of potassium on the adsorption, activation, and dissociation of CO2 over Fe(100), Fe5C2(510) and Fe3O4(111) surfaces. The function of K was analyzed in terms of electronic interactions between co-adsorbed CO2 and K-surfaces which showed conspicuous promotion in the presence of K of the adsorption and activation of CO2. The adsorption strength of CO2 on these surfaces ranks as oct2-Fe3O4(111) > Fe(100) > Fe5C2(510). Generally, we observed a direct proportional correlation between the adsorption strength and the charges on the adsorbates. Adding K on the catalyst surface also reduces the kinetic barrier for CO2 dissociation. CO2 dissociation is more facile to occur on Fe(100) and Fe5C2(510) in the presence of K whereas the Fe3O4(111) surfaces impede CO2 dissociation regardless of the existence of K. Instead, a stable CO3- species is formed upon CO2 adsorption on Fe3O4(111) which will be directly hydrogenated when sufficient H* are available on the surface. Our results highlight the origin of the promotion effect of potassium and provide insight for the future design of K-promoted Fe-based catalysts for CO2 hydrogenation.

In-Plane Epitaxial Growth of Highly c-Oriented NH2 -MIL-125(Ti) Membranes with Superior H2 /CO2 Selectivity.

Preferred-orientation control has significant impact on the separation performance of MOF membranes. Under most conditions the preferred orientation of MOF membranes is dominated by the Van der Drift mechanism of evolutionary growth selection so that the obtained orientation may not be optimized for practical application. In this study, highly c-oriented NH2 -MIL-125 membranes were prepared on porous α-alumina substrates by combining oriented seeding and controlled in-plane epitaxial growth. Dynamic air-liquid interface-assisted self-assembly of c-oriented NH2 -MIL-125(Ti) seed monolayers, the use of layered TiS2 as the metal precursor, and single-mode microwave heating were crucial in ensuring the preferred c-orientation while simultaneously suppressing undesired twin growth. Owing to reduced grain boundary defects, the prepared c-oriented membranes showed an ideal H2 /CO2 selectivity of 24.8, which was 6.1 times higher than that of their randomly oriented counterparts under similar operating conditions.

Comparative Study of Molecular Basket Sorbents Consisting of Polyallylamine and Polyethylenimine Functionalized SBA-15 for CO2 Capture from Flue Gas.

Polyallylamine (PAA)-based molecular basket sorbents (MBS) have been studied for CO2 capture in comparison with polyethylenimine (PEI)-based MBS. The characterizations including N2 physisorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermogravimetric analysis (TGA) showed that PAA (Mn =15 000) is more rigid and has more steric hindrance inside SBA-15 pores than PEI owing mainly to its different polymer structure. The effects of temperature and PAA loading on the CO2 sorption capacity of PAA-based MBS have been examined by TGA by using 100 % CO2 gas stream and compared with PEI/SBA-15. It was found that the capacity of the PAA/SBA-15 sorbent increased with increasing temperature. The optimum capacity of 88 mgCO2 gsorb-1 was obtained at 140 °C for PAA(50)/SBA-15 whereas the optimum sorption temperature was 75 and 90 °C for PEI-I(50)/SBA-15 (PEI-I, Mn =423) and PEI-II(50)/SBA-15 (PEI-II, Mn =25 000), respectively. The capacity initially increased with the increase of PAA loading and then dropped at high amine contents, owing to the increased diffusion barrier. The highest CO2 capacity of 109 mgCO2 gsorb-1 was obtained at a PAA loading of 65 wt %, whereas the PAA(50)/SBA-15 sorbent gave the best amine efficiency of 0.23 molCO2 molN-1 . The effect of moisture was examined in a fixed-bed flow system with simulated flue gas containing 15 % CO2 and 4.5 % O2 in N2 . It was found that the presence of moisture significantly enhanced CO2 sorption over PAA(50)/SBA-15 and greatly improved its cyclic stability and regenerability. Compared with PEI/SBA-15, PAA/SBA-15 possesses a better thermal stability and higher resistance to oxidative degradation. However, the CO2 sorption rate over the PAA(50)/SBA-15 sorbent was much slower.

In Situ Etching-Hydrolysis Strategy To Construct an In-Plane ZnIn2S4/In(OH)3 Heterojunction with Enhanced CO2 Photoreduction Performance.

The in-plane heterojunctions with atomic-level thickness and chemical-bond-connected tight interfaces possess high carrier separation efficiency and fully exposed surface active sites, thus exhibiting exceptional photocatalytic performance. However, the construction of in-plane heterojunctions remains a significant challenge. Herein, we prepared an in-plane ZnIn2S4/In(OH)3 heterojunction (ZISOH) by partial conversion of ZnIn2S4 to In(OH)3 through the addition of H2O2. This in situ oxidation etching-hydrolysis approach enables the ZISOH heterojunction to not only preserve the original nanosheet morphology of ZnIn2S4 but also form an intimate interface. Moreover, generated In(OH)3 serves as an electron-accepting platform and also promotes the adsorption of CO2. As a result, the heterojunction exhibits a remarkably enhanced performance for photocatalytic CO2 reduction. The production rate and selectivity of CO reach 1760 µmol g-1 h-1 and 78%, respectively, significantly higher than those of ZnIn2S4 (842 µmol g-1 h-1 and 65%). This work puts forward a feasible and facile approach to construct in-plane heterojunctions to enhance the photocatalytic performance of two-dimensional metal sulfides.

Computational Design of Single-atom Modified Ti-MOFs for Photocatalytic CO2 Reduction to C1 Chemicals.

In this work, density functional theory (DFT) calculations were conducted to investigate a series of transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Ru, Rh, Pd, Ag, Hf, Ta, Os, Ir, and Pt) as single-atom components introduced into Ti-BPDC (BPDC=2,2'-bipyridine-5,5'-dicarboxylic acid) as catalysts (M/Ti-BPDC) for the photocatalytic reduction of CO2. The results show that Fe/Ti-BPDC is the most active candidate for CO2 reduction to HCOOH due to its small limiting potential (-0.40 V). Ag, Cr, Mn, Ru, Zr, Nb, Rh, and Cu modified Ti-BPDC are also active to HCOOH since their limiting potentials are moderate although the reaction mechanisms are different across these materials. Most of the studied catalysts show poor activity and selectivity to CO product because the stability of *COOH/*OCOH intermediates is significantly weaker than *OCHO/*HCOO species. The moderate binding strength of *CO on Pd/Ti-BPDC is responsible for its superior catalytic activity toward CH3OH generation. Electronic structural analysis was performed to uncover the origin of the activity trend for CO2 reduction to different products on M/Ti-BPDC. The calculation results indicate that the activity and selectivity of CO2 photoreduction can be effectively tuned by designing single-atom metal-based MOF catalysts.

Addressing the Carbonate Issue: Electrocatalysts for Acidic CO2 Reduction Reaction.

Electrochemical CO2 reduction reaction (CO2RR) powered by renewable energy provides a promising route to CO2 conversion and utilization. However, the widely used neutral/alkaline electrolyte consumes a large amount of CO2 to produce (bi)carbonate byproducts, leading to significant challenges at the device level, thereby impeding the further deployment of this reaction. Conducting CO2RR in acidic electrolytes offers a promising solution to address the "carbonate issue"; however, it presents inherent difficulties due to the competitive hydrogen evolution reaction, necessitating concerted efforts toward advanced catalyst and electrode designs to achieve high selectivity and activity. This review encompasses recent developments of acidic CO2RR, from mechanism elucidation to catalyst design and device engineering. This review begins by discussing the mechanistic understanding of the reaction pathway, laying the foundation for catalyst design in acidic CO2RR. Subsequently, an in-depth analysis of recent advancements in acidic CO2RR catalysts is provided, highlighting heterogeneous catalysts, surface immobilized molecular catalysts, and catalyst surface enhancement. Furthermore, the progress made in device-level applications is summarized, aiming to develop high-performance acidic CO2RR systems. Finally, the existing challenges and future directions in the design of acidic CO2RR catalysts are outlined, emphasizing the need for improved selectivity, activity, stability, and scalability.

Stabilizing Co2C with H2O and K promoter for CO2 hydrogenation to C2+ hydrocarbons.

The decomposition of cobalt carbide (Co2C) to metallic cobalt in CO2 hydrogenation results in a notable drop in the selectivity of valued C2+ products, and the stabilization of Co2C remains a grand challenge. Here, we report an in situ synthesized K-Co2C catalyst, and the selectivity of C2+ hydrocarbons in CO2 hydrogenation achieves 67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co2C in the reaction, while the stabilization of Co2C is dependent on the reaction atmosphere and the K promoter. During the carburization, the K promoter and H2O jointly assist in the formation of surface C* species via the carboxylate intermediate, while the adsorption of C* on CoO is enhanced by the K promoter. The lifetime of the K-Co2C is further prolonged from 35 hours to over 200 hours by co-feeding H2O. This work provides a fundamental understanding toward the role of H2O in Co2C chemistry, as well as the potential of extending its application in other reactions.

Machine learning-assisted crystal engineering of a zeolite.

It is shown that Machine Learning (ML) algorithms can usefully capture the effect of crystallization composition and conditions (inputs) on key microstructural characteristics (outputs) of faujasite type zeolites (structure types FAU, EMT, and their intergrowths), which are widely used zeolite catalysts and adsorbents. The utility of ML (in particular, Geometric Harmonics) toward learning input-output relationships of interest is demonstrated, and a comparison with Neural Networks and Gaussian Process Regression, as alternative approaches, is provided. Through ML, synthesis conditions were identified to enhance the Si/Al ratio of high purity FAU zeolite to the hitherto highest level (i.e., Si/Al = 3.5) achieved via direct (not seeded), and organic structure-directing-agent-free synthesis from sodium aluminosilicate sols. The analysis of the ML algorithms' results offers the insight that reduced Na2O content is key to formulating FAU materials with high Si/Al ratio. An acid catalyst prepared by partial ion exchange of the high-Si/Al-ratio FAU (Si/Al = 3.5) exhibits improved proton reactivity (as well as specific activity, per unit mass of catalyst) in propane cracking and dehydrogenation compared to the catalyst prepared from the previously reported highest Si/Al ratio (Si/Al = 2.8).

Ab initio thermodynamics examination of sulfur species present on Rh, Ni, and binary Rh-Ni surfaces under steam reforming reaction conditions.

The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO(x) (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO(x) adsorption under steam reforming conditions. The transition temperature from a clean surface (<(1)/(9) ML) to S adsorption was identified on Rh(111), Ni(111), Rh(1)Ni(2)(111), and Rh(2)Ni(1)(111) metals at various P(H(2))/P(H(2)S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with (1)/(3) ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh(1)Ni(2) is covered with (1)/(9) ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh(4) cluster and CeO(2)(111) surface. SO(x) surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.

A computational investigation of ring-shift isomerization of sym-octahydrophenanthrene to sym-octahydroanthracene catalyzed by acidic zeolites.

The ring-shift isomerization of sym-octahydrophenanthrene (sym-OHP) to sym-octahydroanthracene (sym-OHA) catalyzed by acidic zeolites (Mordenite (MOR) and Faujasite (FAU)) was investigated by the ONIOM(DFT:UFF) and DFT approaches. A "five-membered ring" mechanism through carbocation rearrangement via 1-2 migration was proved to be kinetically favored over a "six-membered ring" mechanism through Friedel-Crafts reactions. Computational studies based on the "five-membered ring" mechanism demonstrate that a decreasing Brønsted acid site strength from Al-H-MOR to Ga-H-MOR to B-H-MOR reduces the catalytic activity. The catalyst acid site strength would thereby impact the yield of sym-OHA. The isomerization barrier increases when using an Al-H-FAU catalyst that has a similar Brønsted acid site strength as Al-H-MOR but considerably bigger cages, indicating that apart from the desired density and strength of acid sites, optimal zeolite catalysts should have a pore size that better fits the intermediates and transition states. DFT calculations on Gibbs free energy were performed to evaluate the equilibrium ratios of sym-OHA to sym-OHP at specific reaction temperatures from 175 to 325 °C. The results indicate that reaction temperature has a moderate impact on the equilibrium yield of sym-OHA, whose formation is relatively favorable at a lower temperature under experimental conditions.

A solid molecular basket sorbent for CO2 capture from gas streams with low CO2 concentration under ambient conditions.

In this paper, a solid molecular basket sorbent, 50 wt% PEI/SBA-15, was studied for CO(2) capture from gas streams with low CO(2) concentration under ambient conditions. The sorbent was able to effectively and selectively capture CO(2) from a gas stream containing 1% CO(2) at 75 °C, with a breakthrough and saturation capacity of 63.1 and 66.7 mg g(-1), respectively, and a selectivity of 14 for CO(2)/CO and 185 for CO(2)/Ar. The sorption performance of the sorbent was influenced greatly by the operating temperature. The CO(2)-TPD study showed that the sorbent could be regenerated under mild conditions (50-110 °C) and was stable in the cyclic operations for at least 20 cycles. Furthermore, the possibility for CO(2) capture from air using the PEI/SBA-15 sorbent was studied by FTIR and proved by TPD. A capacity of 22.5 mg g(-1) was attained at 75 °C via a TPD method using a simulated air with 400 ppmv CO(2) in N(2).

Plastering Sponge with Nanocarbon-Containing Slurry to Construct Mechanically Robust Macroporous Monolithic Catalysts for Direct Dehydrogenation of Ethylbenzene.

Nanocarbons have shown great potential as a sustainable alternative to metal catalysts, but their powder form limits their industrial applications. The preparation of nanocarbon-based monolithic catalysts is a practical approach for overcoming the resulting pressure drop associated with their powder form. In our previous work, a ploycation-mediated approach was used to successfully prepare nanocarbon-containing monoliths. Unfortunately, because there are no macropores in the monolith, it needs to be crashed into millimeter-sized particles before application. Therefore, developing a facile method for preparing mechanically robust nanocarbon-based macroporous monolithic catalysts is vital but still challenging. Herein, evoked by swallows building their nests, we report an approach for successfully preparing a mechanically robust nanodiamond-based macroporous monolith catalyst by plastering melamine sponge (MS) with a slurry composed of nanodiamonds (NDs) and poly(imidazolium-methylene) chloride (PImM) followed by an annealing process. The macroporous monolith catalyst (ND/NCMS-NCPImM) containing NDs well dispersed in N-doped carbon is mechanically robust with enriched macroscopic pores. It exhibits outstanding catalysis toward ethylbenzene to styrene through a direct dehydrogenation reaction with a high styrene rate in a steady state (5.50 mmol g-1 h-1) and high styrene selectivity (99.5%). ND/NCMS-NCPImM shows much higher activity than powder ND by 1.9 fold. In addition, this work solves the significant problem of large pressure drop encountered with conventional powdered nanocarbon catalysts in the flow reactor. This work not only creates an excellent nanodiamond-based macroporous monolithic ethylbenzene direct dehydrogenation catalyst but also presents a promising avenue for preparing other macroporous monolithic catalysts for diverse transformations.

Metal-Organic Framework-Derived Tubular In2O3-C/CdIn2S4 Heterojunction for Efficient Solar-Driven CO2 Conversion.

Realizing high-efficiency solar-driven CO2 reduction to chemicals and fuels requires high-performance photocatalysts with high utilization efficiency of solar light, efficient charge separation and transfer, and robust adsorption capacity for CO2. In this work, a tubular In2O3-C/CdIn2S4 (IOC/CIS) ternary heterojunction with an intimate interfacial contact was fabricated by pyrolysis of In-MIL-68 and subsequent solvothermal synthesis of CdIn2S4. The construction of a heterojunction promotes the separation and transfer efficiency of photogenerated carriers. The introduction of carbon not only accelerates the interfacial charge migration but also enhances light absorption and CO2 adsorption. The resulting 5IOC/CIS sample presents a remarkably improved photocatalytic CO2 reduction activity with a CO generation rate of 2432 µmol g-1 h-1, much higher than that of the In2O3/CdIn2S4 (IO/CIS) heterojunction (1906 µmol g-1 h-1). Furthermore, the type II charge transfer mechanism of the heterojunction was confirmed by the electron paramagnetic resonance characterization. This work provides new insight into the design and preparation of a highly efficient hollow heterojunction for photocatalytic applications.

One-step direct conversion of methane to methanol with water in non-thermal plasma.

Achieving methane-to-methanol is challenging under mild conditions. In this study, methanol is synthesized by one-step direction conversion of CH4 with H2O at room temperature under atmospheric pressure in non-thermal plasma (NTP). This route is characterized by the use of methane and liquid water as the reactants, which enables the transfer of the methanol product to the liquid phase in time to inhibit its further decomposition and conversion. Therefore, the obtained product is free of carbon dioxide. The reaction products include gas and liquid-phase hydrocarbons, CO, CH3OH, and C2H5OH. The combination of plasma and semiconductor materials increases the production rate of methanol. In addition, the addition of Ar or He considerably increases the production rate and selectivity of methanol. The highest production rate of methanol and selectivity in liquid phase can reach 56.7 mmol gcat-1 h-1 and 93%, respectively. Compared with the absence of a catalyst and added gas, a more than 5-fold increase in the methanol production rate is achieved.