Molecular insights into competitive adsorption of CO2/CH4 mixture in shale nanopores

In the present study, competitive adsorption behaviour of supercritical carbon dioxide and methane binary mixture in shale organic nanopores was investigated by using grand canonical Monte Carlo (GCMC) simulations. The model was firstly validated by comparing with experimental data and a satisfactory agreement was obtained. Then the effects of temperature (298–388 K), pressure (up to 60 MPa), pore size (1–4 nm) and moisture content (0–2.4 wt%) on competitive adsorption behaviour of the binary mixture were examined and discussed in depth. It is found that the adsorption capacity of carbon dioxide in shale organic nanopores is much higher than that of methane under various conditions. The mechanism of competitive adsorption was discussed in detail. In addition, the results show that a lower temperature is favorable to both the adsorption amount and selectivity of CO₂/CH₄ binary mixture in shale organic nanopores. However, an appropriate CO₂ injection pressure should be considered to take into account the CO₂ sequestration amount and the exploitation efficiency of shale gas. As for moisture content, different influences on CO₂/CH₄ adsorption selectivity have been observed at low and high moisture conditions. Therefore, different simulation technologies for shale gas production and CO₂ sequestration should be applied depending on the actual moisture conditions of the shale reservoirs. It is expected that the findings in this work could be helpful to estimate and enhance shale gas resource recovery and also evaluate CO₂ sequestration efficiency in shale reservoirs.

Competitive adsorption behaviour of CO₂/CH₄ mixture in shale slit nanopores under various geological conditions was explored by molecular simulations.     

Investigation on the transformation behaviours of Fe-bearing minerals of coal in O2/CO2 combustion atmosphere containing H2O

The transformation behaviors of Fe-bearing minerals in coals of Xinjiang (XJC) and Shenhua (SHC) were investigated in an O₂/CO₂ atmosphere containing H₂O in a drop-tube-furnace (DTF). The solid products were characterized using XRD, Mössbauer spectroscopy, particle size analyzer and SEM-EDX techniques. The results show that the change in the combustion atmosphere does not significantly alter the main phases of Fe-bearing minerals in the coal ashes, but does affect their relative contents. The ratio of Fe²⁺-glass to Fe³⁺-glass in the ashes produced from the O₂/CO₂ combustion atmosphere was significantly increased. During the XJC combustion and under different combustion conditions examined, the content of Fe-glass phases remained almost unaltered. However, in SHC samples, combustion under O₂/CO₂ atmosphere resulted in a higher amount of iron melting into Fe-glass phases and less amount of iron oxide formation. This could be attributed mainly to the presence of Fe-bearing minerals mostly included in nature in SHC samples, which more easily interacted with clays or other silicates inside coal-formed Fe-glass phases. Increasing the O₂ level of the O₂/CO₂ atmosphere during SHC combustion could promote the formation of iron oxides. In O₂/CO₂ atmosphere, with the same oxygen level, the replacement of 10% of CO₂ with H₂O promoted the formation of iron oxides, regardless of the occurrence form (included or excluded) of iron minerals in coal. Furthermore, the addition of steam resulted in an increase in the size of the particles in ash, resulting probably in a decrease in the deposition and slagging propensity of coal ash.

The ratio of Fe²⁺-glass to Fe³⁺-glass in ashes from O₂/CO₂ atmosphere is significantly increased. The iron oxides (hematite or magnetite) formation of included iron minerals may be delayed in O₂/CO₂. H₂O promotes iron oxides formation.     

A nanoprecursor method for successfully synthesizing clinoptilolite with high-crystallinity and resultant effects on CO2/CH4 selective adsorption

Nanoprecursors used as a structural promoter (SP) were prepared by a hydrothermal method and named sol-SP. After centrifugation, the supernatant and precipitate were denoted as solution-SP and solid-SP, respectively. The effect of the additive amount on the structures and properties of the synthesized clinoptilolite was investigated using various characterization techniques. The activation energies of crystallization kinetics during induction and growth periods were calculated. The results showed that the induction period is the control step during the synthesis of clinoptilolite, while additive sol-SP or solid-SP was beneficial to shorten the induction period and therefore enhance the formation of the crystal nucleus. When their pre-crystallization time was too long or the additive amount was too much, the impure phase (phillipsite) in the synthesized clinoptilolite was easily generated. Although the addition of solution-SP had no obvious effect on the induction period, it promoted the growth of crystals after nucleation. Finally, the adsorption performances for CO₂ and CH₄ were preliminarily assessed using synthetic clinoptilolite as the adsorbent, showing the promising application for the separation of CO₂/CH₄.

Nanoprecursors used as a structural promoter (SP) were prepared by a hydrothermal method and named sol-SP.     

CO2 photoreduction to CO/CH4 over Bi2W0.5Mo0.5O6 solid solution nanotubes under visible light

In this work, Bi₂W_0.5Mo_0.5O₆ solid solution nanotubes have been synthesized through a structure-directing hard template approach, which demonstrated greatly enhanced CO₂ photoreduction to CO/CH₄. The crystalline phase, components and morphologies of the as-prepared composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present design of Bi₂W_0.5Mo_0.5O₆ solid solution nanotubes leads to remarkably enhanced photocatalytic activities with a peak CO/CH₄ production rate of 6.55/3.75 mmol g⁻¹ h⁻¹ under visible light irradiation at room temperature, which was about 7 times that on pure Bi₂WO₆ and Bi₂MoO₆ nanotubes, respectively. Hollow nanotubular structures and synergistic electronic effects of various elements contribute to the enhanced visible light photocatalytic activity of Bi₂W_0.5Mo_0.5O₆ solid solution nanotubes.

In this work, Bi₂W_0.5Mo_0.5O₆ solid solution nanotubes have been synthesized through a structure-directing hard template approach, which demonstrated greatly enhanced CO₂ photoreduction to CO/CH₄.     

Measurement and modeling of the adsorption isotherms of CH4 and C2H6 on shale samples

CH₄ and C₂H₆ are two common components in shale gas. Adsorption isotherms of CH₄, C₂H₆, and their binary mixtures on shale samples are significant for understanding the fundamental mechanisms of shale gas storage and the recovery of shale resources from shale reservoirs. In this study, the thermogravimetric method is applied to obtain the adsorption isotherms of CH₄, C₂H₆ and their binary mixtures on two typical shale core samples. A simplified local density theory/Peng–Robinson equation of state (SLD-PR EOS) model is then applied to calculate the adsorption of CH₄ and C₂H₆ on shale, and the efficiency of the SLD-PR EOS model is thus evaluated. The results show that C₂H₆ exhibits a higher adsorption capacity than CH₄ on shale samples, indicating the greater affinity of C₂H₆ to organic shale. As the molar fraction of C₂H₆ increases in the CH₄/C₂H₆ mixtures, the adsorption capacity of the gas mixtures increases, indicating the preferential adsorption of C₂H₆ on shale. Based on the predicted results from the SLD-PR EOS model, a reasonable agreement has been achieved with the measured adsorption isotherms of CH₄ and C₂H₆, validating the reliability of the SLD-PR EOS model for predicting adsorption isotherms of CH₄ and C₂H₆ on shale samples. In addition, the SLD-PR EOS model is more accurate in predicting the adsorption of CH₄ on shale than that of C₂H₆. This study is expected to inspire a new strategy for predicting the adsorption of hydrocarbons on shale and to provide a basic understanding of competitive adsorption of gas mixtures in shale reservoirs.

CH₄ and C₂H₆ are two common components in shale gas.     

Synthesis of SrTiO3 submicron cubes with simultaneous and competitive photocatalytic activity for H2O splitting and CO2 reduction

Single crystalline strontium titanate (SrTiO₃) submicron cubes have been synthesized based on a molten salt method. The submicron cubes showed superior photocatalytic activity towards both water splitting and carbon dioxide reduction, in which methane (CH₄) and hydrogen (H₂) were simultaneously produced. The average production rate of methane up to 8 h is 4.39 μmol g⁻¹ h⁻¹ but drops to 0.46 μmol g⁻¹ h⁻¹. However, the average production rate of hydrogen is 14.52 before 8 h but then increases to 120.23 μmol g⁻¹ h⁻¹ after 8 h. The rate change of the two processes confirms the competition between the H₂O splitting and CO₂ reduction reactions. Band structure and surface characteristics of the SrTiO₃ submicron cubes were characterized by diffuse reflective UV-Vis spectroscopy, Mott–Schottky analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The results reveal that the simultaneous and competitive production of methane and hydrogen is due to a thermodynamics factor, as well as the competition between the adsorption of carbon dioxide and water molecules on the surface of the faceted SrTiO₃. This work demonstrates that SrTiO₃ photocatalysts are efficient in producing sustainable fuels via water splitting and carbon dioxide reduction reactions.

There is a clear competitive relationship between water splitting and photocatalytic reduction of carbon dioxide in the whole process of photocatalytic reduction of carbon dioxide with the prepared cubic SrTiO₃ as a photocatalyst.     

Introducing hydrophilic ultra-thin ZIF-L into mixed matrix membranes for CO2/CH4 separation

Mixed matrix membranes (MMMs) were developed by mixing hydrophilically modified two-dimensional (2D) imidazole framework (named as hZIF-L) flakes into a Pebax MH 1657 (Pebax) matrix, and designed to separate carbon dioxide/methane (CO₂/CH₄) mixtures. The hZIF-L flakes were important for increasing the effectiveness of the MMMs. First, the tannic acid (TA) etched hZIF-L flakes have a large number of microporous (1.8 nm) and two-dimensional anisotropic transport channels, which offered convenient gas transport channels and improved the permeability of CO₂. Second, the TA molecules provide the surface of the ZIF-L flakes with more hydrophilic functional groups such as carbonyl groups (C O) and hydroxyl groups (–OH), which could effectively prevent non-selective interfacial voids and filler agglomeration in the Pebax matrix, and also presented strong binding ability to water and CO₂ molecules. The satisfactory interface compatibility and affinity with the CO₂ molecule promoted its permeability, solubility, and selectivity. As a result, the MMMs exhibited the highest performance of gas separation with the hZIF-L flake weight content of 5%, at which the CO₂ permeability and CO₂/CH₄ selectivity were 502.44 barrer and 33.82 at 0.2 MPa and 25 °C, respectively.

Schematic diagram of CO₂ transfer in Pebax/hZIF-L mixed matrix membranes.     

Water splitting by MnFe2O4/Na2CO3 reversible redox reactions

Future energy systems must call upon clean and renewable sources capable of reducing associated CO₂ emissions. The present research opens new perspectives for renewable energy-based hydrogen production by water splitting using metal oxide oxidation/reduction reactants. An earlier multicriteria assessment defined top priorities, with MnFe₂O₄/Na₂CO₃/H₂O and Mn₃O₄/MnO/NaMnO₂/H₂O multistep redox cycles having the highest potential. The latter redox system was previously assessed and proven difficult to be conducted. The former redox system was hence experimentally investigated in the present research at the 0.5 to 250 g scale in isothermal thermogravimetry, an electrically heated furnace, and a concentrated solar reactor. Over 30 successive oxidation/reduction cycles were assessed, and the H₂ production efficiencies exceeded 98 % for the coprecipitated reactant after these multiple cycles. Tentative economics using a coprecipitated reactant revealed that 120 cycles are needed to achieve a 1 € per kg H₂ cost. Improving the cheaper ball-milled reactant could reduce costs by approximately 30 %. The initial results confirm that future research is important.

Investigating H₂ production by MnFe₂O₄/Na₂CO₃/H₂O redox cycles, using different reactants. Using the more efficient coprecipitated reactant, production costs will be ∼1€ per kg H₂, if 120 cycles are achieved. Improving the cheaper ball-milled reactant is recommended.     

Redox behavior of potassium doped and transition metal co-doped Ce0.75Zr0.25O2 for thermochemical H2O/CO2 splitting

CeO₂ slow redox kinetics as well as low oxygen exchange ability limit its application as a catalyst in solar thermochemical two-step cycles. In this study, Ce_0.75Zr_0.25O₂ catalysts doped with potassium or transition metals (Cu, Mn, Fe), as well as co-doped materials were synthesized. Samples were investigated by X-ray diffraction (XRD), N₂ sorption (BET), as well as by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) to gain insight into surface and bulk features, which were connected to redox properties assessed both in a thermogravimetric (TG) balance and in a fixed bed reactor. Obtained results revealed that doping as well as co-doping with non-reducible K cations promoted the increase of both surface and bulk oxygen vacancies. Accordingly, K-doped and Fe-K co-doped materials show the best redox performances evidencing the highest reduction degree, the largest H₂ amounts and the fastest kinetics, thus emerging as very interesting materials for solar thermochemical splitting cycles.

Potassium doped and co-doped ceria–zirconia show improved CO₂/H₂O splitting activity. This holds huge promise for the design of high performance systems for solar thermochemical splitting cycles allowing the production of solar fuels.     

Sm2O3/Co3O4 catalysts prepared by dealloying for low-temperature CO oxidation

A series of Co₃O₄ catalysts modified by Sm were prepared by a combined dealloying and calcination approach, and the catalytic activities were evaluated using CO catalytic oxidation. The Sm₂O₃/Co₃O₄ catalysts were composed of a large number of nanorods and nanosheets, and exhibited a three-dimensional supporting structure with pores. The experimental results revealed that the addition of a small amount of Sm into the precursor AlCo alloy led to a dealloyed sample with improved catalytic activity, and the dealloyed Al₉₀Co_9.5Sm_0.5 ribbons (0.5 Sm₂O₃/Co₃O₄) calcined at 300 °C showed the highest activity for CO oxidation with complete CO conversion at 135 °C, moreover, CO conversion almost no attenuation, even after 70 hours of catalytic oxidation, which is superior to that of Co₃O₄. The enhanced catalytic activity of the Sm₂O₃/Co₃O₄ catalyst can be attributed to the large specific surface area, more reactive oxygen species and Co³⁺ ion, as well as electronic interactions between Sm and Co.

A series of Co₃O₄ catalysts modified by Sm were prepared by a combined dealloying and calcination approach, and the catalytic activities were evaluated using CO catalytic oxidation.     

Inhibition of aquaporin-mediated CO2 diffusion and voltage-gated H+ channels by zinc does not alter rabbit lung CO2 and NO excretion

Aquaporins (AQs) increase cell membrane CO(2) diffusivity, and it has been proposed that they may serve as transmembrane channels for CO(2) and other small gas molecules. In addition, it has been hypothesized that voltage-gated H(+) channels located on the apical membrane of the alveolar epithelium contribute to CO(2) elimination by the lung. To test whether these membrane proteins contribute to CO(2) elimination in vivo, we measured CO(2) exchange in buffer- and blood-perfused rabbit lungs before and after addition of 0.5 mM ZnCl(2), an inhibitor of both AQ-mediated CO(2) diffusion and voltage-gated H(+) channels. For comparison, red cell and lung carbonic anhydrases (CAs) were inhibited by 0.1 mM methazolamide. ZnCl(2) had no effect on CO(2) exchange when inspired CO(2) was altered between 2% and 5% in 5-min intervals. Pulmonary vascular and airway resistances were not altered by ZnCl(2). In contrast, methazolamide inhibited CO(2) exchange by 30% in buffer-perfused lungs and by 65% in blood-perfused lungs. Exhaled NO concentrations were unaffected by ZnCl(2) or by CA inhibition. Lung capillary gas exchange modelling shows that under normal resting conditions it would be necessary to reduce the alveolar-capillary membrane CO(2) diffusion capacity by >90% to lower CO(2) elimination by 10%. Therefore we conclude that red cell and lung AQs and voltage-gated H(+) channels in the alveolar epithelium contribute minimally to normal physiological CO(2) elimination.     

NOx reduction by CO over ASC catalysts in a simulated rotary reactor: effect of CO2, H2O and SO2

The influence of CO₂, H₂O and SO₂ on the NO reduction by CO over Fe/Co activated semi-coke catalyst was investigated in a simulated rotary reactor. The results showed that, in the simulated rotary reactor, the influence of CO₂ and H₂O on the NO adsorption was significant at low temperatures, and the inhibition became weak when increasing the temperature. However, the NO adsorption efficiency could not be improved by increasing temperature after catalyst sulfur poisoning. The heavily inhibited NO adsorption process, which was due to the competitive adsorption and formation of the sulfate, resulted in a low NO reduction efficiency in the presence of CO₂, H₂O or SO₂. The in situ DRIFT study showed that the dominant effect of CO₂, H₂O and SO₂ on the NO adsorption was the inhibition of the free nitrate ions formation. In addition, the introduction of CO₂, H₂O and SO₂ could not change the route of NO reduction, but just reduced the degree of the NO + CO reduction.

The influence of CO₂, H₂O and SO₂ on the NO reduction by CO over Fe/Co activated semi-coke catalyst was investigated in a simulated rotary reactor.     

Revealing the different performance of Li4SiO4 and Ca2SiO4 for CO2 adsorption by density functional theory

To reveal the difference between Li₄SiO₄ and Ca₂SiO₄ in CO₂ adsorption performance, the CO₂ adsorption on Li₄SiO₄ (010) and Ca₂SiO₄ (100) surfaces was investigated using density functional theory (DFT) calculations. The results indicate that the bent configuration of the adsorbed CO₂ molecule parallel to the surface is the most thermodynamically favorable for both Li₄SiO₄ and Ca₂SiO₄ surfaces. The Li₄SiO₄ (010) surface has greater CO₂ adsorption energy (E_ads = −2.97 eV) than the Ca₂SiO₄ (100) surface (E_ads = −0.31 eV). A stronger covalent bond between the C atom of adsorbed CO₂ and an O_S atom on the Li₄SiO₄ (010) surface is formed, accompanied by more charge transfer from the surface to CO₂. Moreover, the Mulliken charge of O_S atoms on the Li₄SiO₄ (010) surface is more negative, and its p-band center is closer to the E_f, indicating O_S atoms on Li₄SiO₄ (010) are more active and prone to suffering electrophilic attack compared with the Ca₂SiO₄ (100) surface.

The Li₄SiO₄ (010) exhibits greater adsorption towards CO₂ than the Ca₂SiO₄ (100) with a stronger covalent bond and more charge transfer between the surface and CO₂.     

Employing a novel O3/H2O2 + BiPO4/UV synergy technique to deal with thiourea-containing photovoltaic wastewater

Photovoltaic wastewater contains a large amount of thiourea that cannot be directly treated by biological methods because of its biotoxicity. In this study, a novel O₃/H₂O₂ + BiPO₄/UV synergy technique was used as a pre-treatment process to degrade thiourea. The effects of H₂O₂ and catalyst loading were investigated, and the transformation pathway of thiourea was predicted based on the intermediates detected by UPLC-Vion-IMS-QToF. The synergy technique degraded 89.14% thiourea within only 30 min, and complete degradation occurred after 150 min. The TOC removal of O₃/H₂O₂ + BiPO₄/UV was 1.8, 1.5, and 1.9 times that of O₃/H₂O₂ and BiPO₄/UV/H₂O₂ single processes and O₃/H₂O₂ + UV process, respectively, which was due to the synergy between H₂O₂ residues and BiPO₄. In addition, thiourea was mainly degraded by ·OH into thiourea dioxide and melamine (polymerized by other intermediates) and then further degraded into biuret and methyl carbamate by the holes of BiPO₄, followed by complete mineralization into H₂O and CO₂. These results confirm that the O₃/H₂O₂ + BiPO₄/UV synergy technique is a promising option for the degradation of thiourea.

Photovoltaic wastewater contains a large amount of thiourea that cannot be directly treated by biological methods because of its biotoxicity.     

Effect of H2SO4/H2O2 pre-treatment on electrochemical properties of exfoliated graphite prepared by an electro-exfoliation method

The effect of pre-treating graphite sheets in a H₂SO₄/H₂O₂ solution before electro-exfoliation is reported. It was revealed that the volume fraction of H₂SO₄ to H₂O₂ during pre-treatment could control the degree of exfoliation of the resulting exfoliated graphite (EG). X-ray diffraction (XRD), Raman, and Fourier transform infrared (FTIR) spectroscopy analyses have suggested that EG produced by first pre-treating the graphite sheet in H₂SO₄/H₂O₂ solution with the H₂SO₄ : H₂O₂ volume fraction of 95 : 5 demonstrates the highest exfoliation degree. This sample also demonstrated excellent electrochemical properties with good electrical conductivity (36.22 S cm⁻¹) and relatively low charge transfer resistance (R_ct) of 21.35 Ω. This sample also showed the highest specific capacitance of all samples, i.e., 71.95 F g⁻¹ at 1 mV s⁻¹ when measured at a voltage range of −0.9 to 0 V. Further measurement at an extended potential window down to −1.4 V revealed the superior specific capacitance value of 150.69 F g⁻¹. The superior morphology characteristics and the excellent electrical properties of the obtained EG are several reasons behind its exceptional properties. The pre-treatment of graphite sheets in H₂SO₄/H₂O₂ solution allegedly leads to easier and faster exfoliation. The faster exfoliation is allegedly able to prevent massive oxidation, which frequently induces the formation of graphite/graphene oxide (GO) in a prolonged process. However, too large H₂O₂ volume fraction involved during pre-treatment seems to cause excessive expansion and frail structure of the graphite sheets, which leads to an early breakdown of the structure during electrochemical exfoliation and prohibits layer by layer exfoliation.

Early expansion of graphite after H₂SO₄/H₂O₂ pre-treatment and cyclic voltammograms of exfoliated graphite (EG) prepared with various volume fractions of H₂O₂.     

Synthesis of ordered Ca- and Li-doped mesoporous silicas for H2 and CO2 adsorption at ambient temperature and pressure

Ca- and Li-doped mesoporous silicas have been prepared successfully using cetyltrimethylammonium bromide (CTAB) surfactant in basic media. Sol–gel synthesis and hydrothermal treatment produced highly ordered mesoporous Ca and Li loaded silica particles. The MCM-41 type mesostructures, the porosity, the pore sizes as well as the surface area of Ca- and Li-silicas have been thoroughly investigated using small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and N₂ sorption analysis. Samples prepared with varying amounts of Li and Ca loading have been further analyzed using inductive coupled plasma-atomic emission spectroscopy (ICP-AES) and field-emission scanning electron microscopy (FESEM) with an energy dispersive spectral attachment (EDS), which confirmed quite a large amount of Ca while the amount of Li was not enough. Additionally, H₂ and CO₂ gas uptake studies of these metal-loaded silicas have been carried out using a thermogravimetric analyzer (TGA) at normal temperature (25 °C) and pressure (1 atm). H₂ uptake of up to 10 mmol g⁻¹ by Ca-doped silica was recorded. CO₂ and H₂ selectivity were tested with both pure metal-MCM-41 and amine loaded silica using pure N₂ gas and a mixed flow of CO₂/N₂ and H₂/N₂. The effect of temperature on CO₂ uptake was also studied using Ca-MCM-41 materials.

Mesoporous Ca- and Li-doped silica materials have been synthesized in a surfactant mediated sol–gel method and the materials showed significant H₂ uptake capabilities at ambient temperature and pressure.     

Heterogeneous photocatalytic performances of CO2 reduction based on the [Emim]BF4 + TEOA + H2O system

The photochemical reduction of CO₂ was studied in a 1-ethyl-3-methylimidazolium tetrafluoroborate, triethanolamine and water ([Emim]BF₄ + TEOA + H₂O) system under visible light irradiation. The integration of CdS and the Co–bpy complex, which acted as a photocatalyst and cocatalyst, respectively, was employed as an efficient catalytic system for the CO₂-to-CO conversion. The utilization of [Emim]BF₄ and water took advantage of their green properties. The amount of CO production showed that the test medium containing 10 vol% H₂O was favourable for the catalytic performance of the CO₂ reduction. In order to further study the factors that influenced the current system, the physical and spectroscopy properties were characterized by altering the composition ratio of the ingredients. Relevant parameters, including the viscosity, conductivity, solubility and coordination, were adjusted using the ratio of the H₂O/[Emim]BF₄ addition, resulting in a different catalytic performance. All of these attempts led to an optimal reaction condition for the CO₂ reduction process.

The photochemical reduction of CO₂ was studied in a 1-ethyl-3-methylimidazolium tetrafluoroborate, triethanolamine and water ([Emim]BF₄ + TEOA + H₂O) system under visible light irradiation.     

Selective CO2 reduction to HCOOH on a Pt/In2O3/g-C3N4 multifunctional visible-photocatalyst

Selective photocatalytic reduction of CO₂ has been regarded as one of the most amazing ways for re-using CO₂. However, its application is still limited by the low CO₂ conversion efficiency. This work developed a novel Pt/In₂O₃/g-C₃N₄ multifunctional catalyst, which exhibited high activity and selectivity to HCOOH during photocatalytic CO₂ reduction under visible light irradiation owing to the synergistic effect between photocatalyst, thermocatalyst, and heterojunctions. Both In₂O₃ and g-C₃N₄ acted as visible photocatalysts, in which porous g-C₃N₄ facilitated H₂ production from water splitting while the In₂O₃ nanosheets embedded in g-C₃N₄ pores favored CO₂ fixation and H adsorption onto the Lewis acid sites. Besides, the In₂O₃/g-C₃N₄ heterojunctions could efficiently inhibit the photoelectron–hole recombination, leading to enhanced quantum efficiency. The Pt could act as a co-catalyst in H₂ production from photocatalytic water splitting and also accelerated electron transfer to inhibit electron–hole recombination and generated a plasma effect. More importantly, the Pt could activate H atoms and CO₂ molecules toward the formation of HCOOH. At normal pressure and room temperature, the TON of HCOOH in CO₂ conversion was 63.1 μmol g⁻¹ h⁻¹ and could reach up to 736.3 μmol g⁻¹ h⁻¹ at 40 atm.

A multifunctional Pt/In₂O₃/g-C₃N₄ catalyst exhibited high activity and selectivity to HCOOH during CO₂ reduction owing to the synergy between visible-light harvesting, CO₂ activation, HER, and photoelectron–hole separation via heterojunctions.     

Theoretical study of the H/D isotope effect of CH4/CD4 adsorption on a Rh(111) surface using a combined plane wave and localized basis sets method

We analysed the H/D isotope effect of CH₄/CD₄ adsorption on a Rh(111) surface using our combined plane wave and localized basis sets method, that we proposed for the consideration of delocalized electrons on a surface and the quantum effect of protons (deuterons) in metal–molecule interactions. We observed that the adsorption distance and energy of CD₄ were larger and lower than those of CH₄, respectively. This is in reasonable agreement with the corresponding experimental results of cyclohexane adsorption. We clearly found that the trend of the H/D isotope effect in the geometrical and energetic difference was similar to that of the hydrogen-bonded systems.

Using our CPLB method, we elucidate that the adsorption distance and adsorption energy of CH₄ on the Rh(111) surface are shorter and larger than those of CD₄, which is in reasonable agreement with the corresponding H/D isotope trend in experiments.     

A windowed carbon nanotube membrane for CO2/CH4 gas mixture penetration separation: insights from theoretical calculation

A new class of species-permselective molecular sieves with functionalized nanowindows has been prepared by modifying the armchair single-walled carbon nanotubes (SWNTs) of a pillared graphene membrane, namely windowed carbon nanotube membrane. The mechanism and characteristics of the windowed carbon nanotube membrane for the selective separation of the CO₂/CH₄ gas mixture are comprehensively and deeply studied. Selective gas separation has a great dependence not only on the interaction of the gas adsorbing on the graphene membrane and inside the CNT channel but also with the energy barrier for the gas diffusing through the nanowindow. In all the functional nanowindows investigated, CH₄ is completely rejected by the N/F-modified nanowindows while maintaining extremely high CO₂ permeability. The CO₂ permeance of the nanowindows is as high as 10⁹ GPU. It emerged that these windowed carbon nanotube membranes are efficient species-selective molecular sieves possessing excellent CO₂/CH₄ selectivity and brilliant CO₂ capture capability.

Final snapshot of the CO₂/CH₄ gas mixture separating through the windowed carbon nanotube membrane.