CH4 Reaction Research Articles

Partial Oxidation of Methane to Methanol by Using Molecular O2 on Pd2+ Catalyst: An Insight from Theory

AbstractThe partial oxidation of methane to methanol using cationic Pd2 dimers is investigated by employing density functional theory (DFT) method. We used B3PW91 functional for geometry optimization, and frequency calculations of all species involved in [Pd2]++O2+2CH4 reaction. Furthermore, a density‐fitting triple zeta valence with single‐polarization (def2TZVP) is used in the calculation to determine the atomic orbitals of the atoms. We oxidized Pd2+ to [Pd2O2]+ using O2 and performed possible partial oxidation of methane to methanol on [Pd2O2]+ and [Pd2O]+ and explored various intermediates and transition states on the potential energy surface (PES) diagram. From Potential Energy Surface (PES) analysis, it is found that the [Pd2O2]+ in doublet spin state multiplicity (SM=2) following radical mechanism is the more preferred pathway for methane to methanol conversion.

Vibrational Mode-Specific Dynamics of the OH + C2H6 Reaction.

We investigate the effects of the initial vibrational excitations on the dynamics of the OH + C2H6 → H2O + C2H5 reaction using the quasi-classical trajectory method and a full-dimensional analytical ab initio potential energy surface. Excitation of the initial CH, CC, and OH stretching modes enhances, slightly inhibits, and does not affect the reactivity, respectively. Translational energy activates the early-barrier title reaction more efficiently than OH and CC stretching excitations, in accord with the Polanyi rules whereas CH stretching modes have similar or higher efficacy than translation, showing that these rules are not always valid in polyatomic processes. Scattering angle, initial attack angle, and product translational energy distributions show the dominance of direct stripping with increasing collision energy, side-on OH and isotropic C2H6 attack preferences, and substantial reactant-product translational energy transfer without any significant mode specificity. The reactant vibrational excitation energy of OH and C2H6 flows into the H2O and C2H5 product vibrations, respectively, whereas product rotations are not affected. The computed mode-specific H2O vibrational distributions show that initial OH excitation appears in the asymmetric stretching vibration of the H2O product and allow comparison with experiments.

High-Temperature Pyrolysis of N-Tetracosane Based on ReaxFF Molecular Dynamics Simulation.

In order to further understand the high-temperature reaction process and pyrolysis mechanism of hydrocarbon fuels, the high-temperature pyrolysis behavior of n-tetracosane (C24H50) was investigated in this paper via the reaction force field (ReaxFF) method-based molecular dynamics approach. There are two main types of initial reaction channels for n-heptane pyrolysis, C-C and C-H bond fission. At low temperatures, there is little difference in the percentage of the two reaction channels. With the temperature increase, C-C bond fission dominates, and a small amount of n-tetracosane is decomposed by reaction with intermediates. It is found that H radicals and CH3 radicals are widely present throughout the pyrolysis process, but the amount is little at the end of the pyrolysis. In addition, the distribution of the main products H2, CH4, and C2H4, and related reactions are investigated. The pyrolysis mechanism was constructed based on the generation of major products. The activation energy of C24H50 pyrolysis is 277.19 kJ/mol, obtained by kinetic analysis in the temperature range of 2400-3600 K.

Multifluid computational fluid dynamics simulation for sawdust gasification inside an industrial scale fluidized bed gasifier

Scarcity of fossil fuels, and to fulfill the energy demand of the increasing population, researchers need to shift their attention to the alternative of fossil fuels. It would be impractical and time-consuming to conduct extensive experiments on Fluidized Bed Gasifiers (FBG) of varying fuel types, sizes, geometries and operating conditions. The objective of the current research work is focused on numerical simulation of hydrodynamics behavior and sawdust gasification in an industrial scale FBG using the Eulerian-Eulerian multifluid model. For the accurate prediction of thermal and hydraulic performance (bed voidage, suspension density, pressure drop, heat transfer, and combustion kinetics), one should incorporate the correct parameters in the Computational Fluid Dynamics (CFD) simulation of a fluidized bed gasifier. It includes the multiphase gasification modeling using the Eulerian-Eulerian approach with an user defined heterogeneous reaction kinetics. The adopted CFD methodology is validated with experimental data and extended for the prediction of gasification characteristics of sawdust by incorporating eight heterogeneous (moisture release, volatile cracking, tar cracking, tar oxidation, char combustion, CO₂ gasification, steam gasification, methanation reaction) and five homogeneous oxidation of CO, CH₄, H₂, forward and backward water gas shift (WGS) reactions. In the result section, composition of gasification products is analyzed, along with the hydrodynamics of sawdust and sand phase, heat transfer between the gas, sand and sawdust, reaction rates of different homogeneous and heterogeneous reactions are being analyzed along the height of the domain.

OH(2Π) + C2H4 Reaction: A Combined Crossed Molecular Beam and Theoretical Study.

The reaction between the ground-state hydroxyl radical, OH(2Π), and ethylene, C2H4, has been investigated under single-collision conditions by the crossed molecular beam scattering technique with mass-spectrometric detection and time-of-flight analysis at the collision energy of 50.4 kJ/mol. Electronic structure calculations of the underlying potential energy surface (PES) and statistical Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of product branching fractions on the derived PES for the addition pathway have been performed. The theoretical results indicate a temperature-dependent competition between the anti-/syn-CH2CHOH (vinyl alcohol) + H, CH3CHO (acetaldehyde) + H, and H2CO (formaldehyde) + CH3 product channels. The yield of the H-abstraction channel could not be quantified with the employed methods. The RRKM results predict that under our experimental conditions, the anti- and syn-CH2CHOH + H product channels account for 38% (in similar amounts) of the addition mechanism yield, the H2CO + CH3 channel for ∼58%, while the CH3CHO + H channel is formed in negligible amount (<4%). The implications for combustion and astrochemical environments are discussed.

Molecular dynamics simulation on CH4 combustion in CO2/O2/N2 atmosphere subjected to electric field

ABSTRACT In this paper, the CH4 combustion characteristics in CO2/O2/N2 atmosphere subjected to electric field were studied by the molecular dynamics simulation method. Conventional and unique pathways were obtained. The evolution law of reactants and main products and the first reaction time of main intermediates under the influence of different electric field intensities were analysed. Results showed that the addition of external electric fields increased conventional responses and species diversity. The generation of new pathways induced the production of new species, and the further reactions of new species produced other new pathways, thus accelerating the generation of new pathways and species in the electric field. It was noteworthy that, the applied external electric field could advance the reaction start time and reinforce the combustion process, but its effect on the reaction rate was highly nonlinear. The CH4 reaction with CO2 was significantly intensified at E ≥ 105 V/m, while suppressed at E = 104 V/m. Furthermore, electric fields could promote the oxidation degree of CH4, especially at E = 106 V/m. The studies provide theoretical guidance for revealing the mechanism of methane combustion under a high concentration CO2 atmosphere and promoting the efficient capture of CO2.

Investigation of the reactions NCN + CH3, NCN + OH, and CH3 + OH behind shock waves

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. As both CH3 and OH radicals are present at high mole fraction level in the reaction zone, their radical-radical cross-reactions with NCN are of special interest, but no experimental rate constants are available. In contrast to the OH reaction, the reaction with CH3 is not considered in common detailed flame mechanisms as yet. It is shown in this study, however, that the NCN + CH3 reaction proceeds close to collision limit and therefore acts as one of the most important reactions to describe the fate of NCN in flames. NCN and OH concentration-time profiles were recorded behind shock waves. The thermal decomposition of cyanogen azide (NCN3) and tert‑Butyl hydroperoxide (TBHP) served as quantitative source of NCN and CH3/OH radicals. Representing a secondary reaction under the applied experimental conditions, the rate constant of the reaction CH3 + OH has been first determined from OH profiles with TBHP as precursor. With k(CH3 + OH) = 4.34 × 10–10 × (T / K)6.38 exp(+68.0 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 925 – 1800 K, p ≈ 393 mbar) it was found to be in good agreement with available literature results. Rate constants of the reaction NCN + CH3 were then determined from NCN profiles by shock-heating gas mixtures with TBHP and NCN3. According to previous as well as our own statistical rate theory calculations, the only relevant product channel is the formation of CH2NH + CN. In the temperature range 857 – 1817 K and at a pressure of p ≈ 314 mbar, a high value for the rate constant k(NCN + CH3) = 5.40 × 1013 exp(+1.77 kJ mol–1 / RT) cm3 mol–1 s–1 (±30%) was found. For future implementation into flame models, we recommend the rate constant expression k(NCN + CH3) = 8.44 × 1019 (T / K) –1.76 exp(–15.2 kJ mol–1 / RT) cm3 mol–1 s–1 (±40%, 800 – 4000 K), which is based on a theoretically assisted extrapolation of the experimental data. Finally, OH profiles in the presence of CH3 and NCN have been analyzed. In agreement with previous theoretical estimates, the reaction is indeed comparably slow and therefore only a rough upper limit of k(NCN + OH) < 8 × 1012 cm3mol–1s–1 (918 – 1808 K, p ≈ 415 mbar) could be inferred from the experiments.

Entropy Generation and Exergy Assessment of Methane–Nitrous Oxide Diffusion Flames in a Triple-Port Burner

A triple-port burner was used in this study, and a numerical simulation was employed to investigate the entropy generation rate of CH4–N2O diffusion flames at the R ratio = 1 . Here, R ratio refers to the ratio of the oxidizer flow velocity to the fuel flow velocity. In order to scrutinize the decomposition effect of N2O on entropy generation, an oxygen-enriched gas with the same nitrogen to oxygen ratio as N2O (N-to- O = 2 ) was used in CH4–N2–O2 diffusion flames. Besides, because the N2O could decompose into the oxygen-enriched gas, the oxygen-enriched effect was also studied by the CH4–air diffusion flames that were conducted in this research. The entropy generation rate comprises of three items in this study, including heat conduction, mass diffusion, and chemical reaction. As a result, the different reaction pathways would take part in the major reaction pathway in CH4–N2O diffusion flames, causing more entropy generation rate being produced through the more intense reactions in CH4–N2O diffusion flames. The irreversibility in CH4–air diffusion flames are dominated through heat conduction and chemical reaction, which is an identical result in CH4–N2–O2 diffusion flames. However, in CH4–N2O diffusion flames, chemical reactions dominated the irreversibility because of the more intense reaction caused by the thermal effect of N2O decomposition. As a result, the decomposition effect of N2O influences the availability of CH4–N2O diffusion flames.

The dual-active-site tandem catalyst containing Ru single atoms and Ni nanoparticles boosts CO2 methanation

Hydrogenation of CO2 into CH4 is an effective strategy for dealing with CO2-relevant environmental problems. Since the CO2 methanation reaction involves multiple electron transfers and various C1 intermediates, improving the reaction rate at each step is critical to accelerating the entire reaction. Here, we report a dual-active-site tandem catalyst (Ru1Ni/CeO2) composed of Ru single atoms (Ru1) and Ni nanoparticles, which can effectively convert CO2 to CH4, showing ∼90% CO2 conversion and ∼99% CH4 selectivity at 325 °C, much higher than those of the Ru1/CeO2 and Ni/CeO2 catalysts. Experimental and theoretical calculation results reveal that Ru1 is extremely active for converting CO2 to CO, while the Ni site is highly efficient for the subsequent sequential CO to CH4 reaction step. The coexistence of the Ru1 and Ni sites significantly boosts the overall reaction. This work offers a promising strategy for the rational design of efficient multisite tandem catalysts.

Exploring Synthesis Approaches of Co-based Catalysts for the Efficient Oxidation of CH4 and CO

AbstractCo-based catalysts were synthesized and studied as novel oxidation catalysts, exploring and optimizing the effect of synthesis method on the redox behavior, the oxygen storage ability and thus the catalytic performance of the derived Co3O4 materials in the complete CH4 and/or CO oxidation reactions. Thus, a series of Co-based catalysts were synthesized applying either the precipitation and/or the hydrothermal method, using different precipitating agents (Na2CO3, NH3, urea or NaOH), Co precursor salt (nitrate or acetate) and finally varying the Co/Na ratio. In addition, the reaction time (6 or 24 h aging) was also investigated for the hydrothermally prepared Co3O4. The best catalysts for the CH4 oxidation are the precipitated Co3O4, using cobalt acetate as precursor salt and NaOH as precipitating agent, presenting the highest surface areas and the lowest Co3O4 particle sizes. On the other side, hydrothermally prepared cobalt oxides reveal higher performance for CO oxidation, with Co3O4 prepared with cobalt acetate, NaOH and low aging time shown as the optimum materials. The best catalysts were further promoted with incorporation of Pd (0.5wt.%) and explored for both reactions. The addition of Pd enhanced the activity of Co3O4 for CH4 oxidation, while Pd did not improve any further the catalyst performance for CO oxidation, presenting thus the same activity with pure cobalt oxides.

A Density Functional Theory and Microkinetic Study of Acetylene Partial Oxidation on the Perfect and Defective Cu2O (111) Surface Models.

The catalytic removal of C2H2 by Cu2O was studied by investigating the adsorption and partial oxidation mechanism of C2H2 on both perfect (stoichiometric) and CuCUS-defective Cu2O (111) surface models using density functional theory calculations. The chemisorption of C2H2 on perfect and defective surface models needs to overcome the energy barrier of 0.70 and 0.81 eV at 0 K. The direct decomposition of C2H2 on both surface models is energy demanding with the energy barrier of 1.92 and 1.62 eV for the perfect and defective surface models, respectively. The H-abstractions of the chemisorbed C2H2 by a series of radicals including H, OH, HO2, CH3, O, and O2 following the Langmuir–Hinshelwood mechanism have been compared. On the perfect Cu2O (111) surface model, the activity order of the adsorbed radicals toward H-abstraction of C2H2 is: OH > O2 > HO2 > O > CH3 > H, while on the defective Cu2O (111) surface model, the activity follows the sequence: O > OH > O2 > HO2 > H > CH3. The CuCUS defect could remarkably facilitate the H-abstraction of C2H2 by O2. The partial oxidation of C2H2 on the Cu2O (111) surface model tends to proceed with the chemisorption process and the following H-abstraction process rather than the direct decomposition process. The reaction of C2H2 H-abstraction by O2 dictates the C2H2 overall reaction rate on the perfect Cu2O (111) surface model and the chemisorption of C2H2 is the rate-determining step on the defective Cu2O (111) surface model. The results of this work could benefit the understanding of the C2H2 reaction on the Cu2O (111) surface and future heterogeneous modeling.

Methane activation on metal oxide nanoparticles: spectroscopic identification of reaction mechanism

Currently, there is a lack of detailed knowledge about methane (CH4) reactions on metal oxide surfaces. This reaction involves in different processes such as chemical looping combustion and catalytic carbon dioxide (CO2) reforming of CH4. In this study, in situ diffuse reflectance infrared Fourier transform spectroscopy was used to investigate CH4 reaction on three transition metal oxides (CoO, CuO, and α-Fe2O3) which are commonly used in these processes at temperatures ranging from 100 °C to 700 °C. The results show that CH4 is adsorbed on the metal oxide surfaces as methoxy and formate, and then the methoxy group reacts with the metal oxide lattice oxygen forming bicarbonate. Under the effect of the used reaction temperature, the bicarbonate decomposes to CO2 and water vapor.

Bay capping via acetylene addition to polycyclic aromatic hydrocarbons: Mechanism and kinetics

The bay-capping mechanism on PAH armchair edges and the kinetics of acetylene addition to 6–6–5 and 5–6–5 bays have been explored by ab initio/RRKM-ME calculations. The bays on the edges were modeled by C21H11 and C20H9 radicals produced by H abstractions from 7H-benzo[c]cyclopenta[e]pyrene and dicyclopenta[cf]pyrene. The C20H9 + C2H2 reaction is shown to have a low entrance barrier and to rapidly form the capped product, indaceno[2,1,8,7-cdefg]pyrene, along with ethynyl substituted dicyclopenta[cf]pyrene at temperatures above 1400 K. The reactivity of C21H11 is shown to be governed by the location of the unpaired electron; the π radical R1 formed by H abstraction from the CH2 group in 7H-benzo[c]cyclopenta[e]pyrene reacts with C2H2 very slowly owing to a high entrance barrier, with the bay-capping rate constant approaching 10−16 cm3 molecule−1 s−1 only at temperatures above 2000 K. This result reaffirms that the growth of π aryl radicals via acetylene addition is inefficient and reflects the generally low reactivity of such radicals where the spin density is highly delocalized over the entire polyaromatic system. Alternatively, the σ C21H11 radical R2 produced by H abstraction from the five-membered ring at the bay rapidly reacts with C2H2 forming the bay-capped product, with the rate constant on the order of 10−12 cm3 molecule−1 s−1 at T ≥ 1500 K. Rate constants for the capping reactions at the 6–6–5 and 5–6–5 bays are compared with those at the 6–0–6, 6–6–6, and 6–5–6 bays. The site-specific bay-capping rate constants have been utilized in kMC simulations of the PAH growth and the results showed measurable differences when the 6–6–5 and 5–6–5 bay-capping reactions are taken into account, including an increase of the growth rate and the formation of closed-shell PAH and a rise of the number of embedded five-membered rings accompanied with a slight decrease of their overall amount.

Hydrocarbon chemistry in the atmosphere of a Warmer Exo-Titan

Hosting a ∼1.5 bar N2 atmosphere and reducing atmospheric composition, Titan has the energy sources needed to drive disequilibrium chemistry and hosts an aerosol layer which shields the surface from incident UV radiation. This world draws parallels to an early Earth-like world (although ∼200 K cooler), and the atmospheric chemistry may be capable of forming relevant prebiotic species. Exo-Titan worlds at close-in orbits host photochemistry relevant to habitability with rich hydrocarbon chemistry. We investigate the effect of stellar type of the host star, equilibrium temperature, incident radiation, and vertical transport efficiency on the production of higher-order hydrocarbons. We find a greater incident radiation (a closer orbit) increases the rate of methane photolysis as well as photolysis of hydrocarbons. A larger H2 abundance and warmer temperature increases the rate of the back reaction H2 + CH3 → CH4 + H, and the temperature dependence is so great that CH3 recycles back into CH4 instead of forming C2H6. A larger H2 abundance and warmer temperature also encourages interesting cycling between C2H2, C2H3, and C2H4via reactions with atomic H.

Looping upcycling SO2 into value-added H2S by fast-induced reduction process for heavy metals treatment in nonferrous smelting industry

Reversing the pollution to a value-added product can simultaneously alleviate environmental problems and achieve a circular economy. In nonferrous smelting industries, SO2 is hazardous pollution that should be disposed, but H2S is usually required for heavy metals (HMs) treatment. Herein, we developed a field SO2 looping upcycling technology for value-added H2S production to meet the HMs treatment. Results demonstrated that the H2 could accelerate the kinetic rate and decrease the energy barriers of the SO2 and CH4 reaction by a fast-induced reduction process (fast-IRP). Thus, the H2S production rate in fast-IRP was three times higher than the direct reaction between SO2 and CH4. Meanwhile, the T90 temperature (690 °C) was significantly lower than SO2 and CH4 reaction about 90 °C. Fast-IRP also can resist the SO2 poison effect and prevent sulfation, enhancing the activity, long-term stability (no visible efficiency loss in 50 h), and selectivity (nearly 100 %). Integrated the characterizations and DFT results, we found that the H2 would firstly attack the uncoordinated sulfur species with a hydrodesulfurization process to form the HS-/H2S intermediate species with a lower energy barrier (1.57 eV) than the CH4 (2.83 eV), which can facilely protect the Mo-terminated to avoid sulfation poison. Subsequently, the HS-/H2S* species would react with the adsorbed SO2 to Sx* to support the deep-reduction reaction for H2S production. This work proposes an actual SO2 field utilization route and also will enlighten an induced redox cycle in the sulfur reduction over the pre-sulfurized catalysts.

Unpaired Electron Engineering Enables Efficient and Selective Photocatalytic CO2 Reduction to CH4.

The photocatalytic CO2 reduction to CH4 reaction is a long process of proton-coupled charge transfer accompanied by various reaction intermediates. Achieving high CH4 selectivity with satisfactory conversion efficiency therefore remains rather challenging. Herein, we propose a novel strategy of unpaired electron engineering to break through such a demanding bottleneck. By taking TiO2 as a photocatalyst prototype, we prove that unpaired electrons stabilize the key intermediate of CH4 production, i.e., CHO*, via chemical bonding, which converts the endothermic step of CHO* formation to an exothermic process, thereby altering the reaction pathway to selectively produce CH4. Meanwhile, these unpaired electrons generate midgap states to restrict charge recombination by trapping free electrons. As an outcome, such an unpaired electron-engineered TiO2 achieves an electron-consumption rate as high as 28.3 μmol·g-1·h-1 (15.7-fold with respect to normal TiO2) with a 97% CH4 selectivity. This work demonstrates that electron regulation holds great promise in attaining efficient and selective heterogeneous photocatalytic conversion.

Synthesis of Heterogeneous Catalysts in Catalyst Informatics to Bridge Experiment and High-Throughput Calculation.

The coupling of high-throughput calculations with catalyst informatics is proposed as an alternative way to design heterogeneous catalysts. High-throughput first-principles calculations for the oxidative coupling of methane (OCM) reaction are designed and performed where 1972 catalyst surface planes for the CH4 to CH3 reaction are calculated. Several catalysts for the OCM reaction are designed based on key elements that are unveiled via data visualization and network analysis. Among the designed catalysts, several active catalysts such as CoAg/TiO2, Mg/BaO, and Ti/BaO are found to result in high C2 yield. Results illustrate that designing catalysts using high-throughput calculations is achievable in principle if appropriate trends and patterns within the data generated via high-throughput calculations are identified. Thus, high-throughput calculations in combination with catalyst informatics offer a potential alternative method for catalyst design.

Full-dimensional automated potential energy surface development and dynamics for the OH + C2H6 reaction.

We develop a full-dimensional analytical potential energy surface (PES) for the OH + C2H6 reaction using the Robosurfer program system, which automatically (1) selects geometries from quasi-classical trajectories, (2) performs ab initio computations using a coupled-cluster singles, doubles, and perturbative triples-F12/triple-zeta-quality composite method, (3) fits the energies utilizing the permutationally invariant monomial symmetrization approach, and (4) iteratively improves the PES via steps (1)-(3). Quasi-classical trajectory simulations on the new PES reveal that hydrogen abstraction leading to H2O + C2H5 dominates in the collision energy range of 10-50 kcal/mol. The abstraction cross sections increase and the dominant mechanism shifts from rebound (small impact parameters and backward scattering) to stripping (larger impact parameters and forward scattering) with increasing collision energy as opacity functions and scattering angle distributions indicate. The abstraction reaction clearly favors side-on OH attack over O-side and the least-preferred H-side approach, whereas C2H6 behaves like a spherical object with only slight C-C-perpendicular side-on preference. The collision energy efficiently flows into the relative translation of the products, whereas product internal energy distributions show only little collision energy dependence. H2O/C2H5 vibrational distributions slightly/significantly violate zero-point energy and are nearly independent of collision energy, whereas the rotational distributions clearly blue-shift as the collision energy increases.

LaFe0.8Co0.15Cu0.05O3 Supported on Silicalite-1 as a Durable Oxygen Carrier for Chemical Looping Reforming of CH4 Coupled with CO2 Reduction.

Chemical looping reforming of CH4 coupled with CO2 reduction is a novel technology for the utilization of CH4 and CO2. Here, we report a durable and outstanding LaFe0.8Co0.15Cu0.05O3/S-1 oxygen carrier at lower operating temperature to efficiently convert CH4 and utilize CO2. LaFe0.8Co0.15Cu0.05O3 showed a high CH4 reaction rate (7.0 × 10-7 mol·(g·s)-1), CO selectivity (84.2%), and CO yield (0.045 mol·g-1) at 800 °C. However, the reactivity of LaFe0.8Co0.15Cu0.05O3 reduced quickly with the redox cycles. The introduction of Silicalite-1 promoted the performance of the LaFe0.8Co0.15Cu0.05O3 perovskite oxygen carrier during the redox cycles. It can be attributed to the fact that under heat treatment, the LaFe0.8Co0.15Cu0.05O3 particles grew along the edge of Silicalite-1 and the LaFe0.8Co0.15Cu0.05O3 nanoparticles were homogeneously dispersed on the Silicalite-1 surface, which improved the thermal stability and reactivity of the oxygen carrier. In addition, the interface between Silicalite-1 and LaFe0.8Co0.15Cu0.05O3 nanoparticles also played important roles because the porous structure of Silicalite-1 could reduce the mass transfer restriction of the interface. In addition, Silicalite-1 also possessed high CH4 and CO2 adsorption selectivity, leading to higher reactivity.

Novel CeOx-modified In2O3 with stabilized Ce3+ states as a highly efficient photocatalyst for photoreduction of CO2 with CH4 or H2O

In this work, CeOx/In2O3 containing mass ratios of 0.1, 0.2 and 0.3 CeOx were prepared via a simple wet impregnation method and their photoactivity was evaluated for CO2 photoreduction with CH4 or H2O. HRTEM verified the formation of an oxide-oxide interface in CeOx/In2O3. XPS analysis revealed the presence of trivalent Ce3+ as the predominant species in CeOx/In2O3 that were formed due to the stabilization of Ce3+ states by In2O3. The presence of a higher content of Ce3+ reflected a strong interfacial contact between CeOx and In2O3. The optimized CeOx/In2O3 showed 4–7 times enhancement as compared to the unmodified In2O3 for CO2 photoreduction with CH4 or H2O reactions. The selectivity of CO and H2 were dependent on the reductant used. The selectivity of CO and H2 for CO2 photoreduction with CH4 remained at 33 % and 66 % after CeOx addition. Contrarily, the selectivity of H2 showed a gradual drop from 61 % to 18 % with an increase in CeOx loading when H2O was used as reductant. The improved activity of CeOx/In2O3 was because of (i) promoted charge transfer and separation due to strong interfacial contact and Type II band alignment, (ii) enhanced light absorption, and (iii) increased basic sites for CO2 adsorption.