H2 CO2 Research Articles

Facet-Dependent Photocatalytic Behavior of Fe-soc-MOF for Carbon Dioxide Reduction.

Exposing different facets on metal-organic frameworks (MOFs) is an efficient approach to regulate their photocatalytic performance for CO2 reduction. Herein, Fe-soc-MOFs exposed with different facets were successfully synthesized, and the morphologies of Fe-soc-MOF exposed with eight {111} facets (Fe-soc-O) and that exposed with eight {111} and six {100} crystal facets (Fe-soc-M) are first reported. Fe-soc-MOFs have facet-dependent active sites on their surface and correspondingly different catalytic performance for photocatalytic CO2 reduction. Fe-soc-O has the highest CO production of 1804 μmol g-1 h-1, while the Fe-soc-MOF exposed with six {100} facets (Fe-soc-C) has the best CO selectivity of 94.7%. Density functional theory (DFT) calculations demonstrate that the (111) facet has more favorable thermodynamic potential for CO2 reduction and H2 evolution compared with the (100) one, deriving from its facet-dependent active sites. This work shows that utilizing the facet-engineering strategy to regulate the active sites exposed on the surface of MOFs is feasible. The results display the relation between the facet of MOFs and the photocatalytic behavior for CO2 reduction.

Mechanism and Selectivity of Electrochemical Reduction of CO2 on Metalloporphyrin Catalysts from DFT Studies

Electrochemical reduction of CO2 to value-added chemicals has been hindered by poor product selectivity and competition from hydrogen evolution reactions. This study aims to unravel the origin of the product selectivity and competitive hydrogen evolution reaction on [MP]0 catalysts (M = Fe, Co, Rh and Ir; P is porphyrin ligand) by analyzing the mechanism of CO2 reduction and H2 formation based on the results of density functional theory calculations. Reduction of CO2 to CO and HCOO- proceeds via the formation of carboxylate adduct ([MP-COOH]0 and ([MP-COOH]-) and metal-hydride [MP-H]-, respectively. Competing proton reduction to gaseous hydrogen shares the [MP-H]- intermediate. Our results show that the pKa of [MP-H]0 can be used as an indicator of the CO or HCOO-/H2 preference. Furthermore, an ergoneutral pH has been determined and used to determine the minimum pH at which selective CO2 reduction to HCOO- becomes favorable over the H2 production. These analyses allow us to understand the product selectivity of CO2 reduction on [FeP]0, [CoP]0, [RhP]0 and [IrP]0; [FeP]0 and [CoP]0 are selective for CO whereas [RhP]0 and [IrP]0 are selective for HCOO- while suppressing H2 formation. These descriptors should be applicable to other catalysts in an aqueous medium.

Reduction disintegration behavior of vanadium titanomagnetite pellets in H2–CO–CO2–N2 mixtures

In this paper, the low–temperature disintegration behavior of vanadium titanomagnetite (VTM) pellets in H 2 –CO–CO 2 –N 2 mixtures was investigated. Thermodynamic calculations indicated that pseudobrookite (Fe 2 TiO 5 ) was reduced in the order Fe 2 TiO 5 → Fe 2 TiO 4 → FeTiO 3 → FeTi 2 O 5 , and the reduction potentials of H 2 and CO were mainly depended on the temperature. The experiments were carried out at temperatures of 400–700 °C with H 2 /(H 2 + CO) of 0–1. The experimental results revealed that disintegration mainly occurred over the temperature range of 500–600 °C, because hematite/pseudobrookite was reduced to magnetite/titanomagnetite resulted in the lattice expansion. A maximum RDI −3.15mm of 45.30% was obtained at 550 °C with a H 2 /(H 2 + CO) ratio of 0.26, and the corresponding reduction degree was 13.31%. Moreover, cracks were observed with a H 2 /(H 2 + CO) ratio of 0.20. A certain interaction between H 2 and CO reduction exists, owing to their different diffusion coefficients and reduction potentials. The reduction by individual CO gases tended to follow the topochemical reaction mode, whereas the non-topochemical reaction mode was dominant with H 2 –CO mixtures. • Thermodynamically, the reduction of pseudobrookite (Fe2TiO5) by H2 and CO is studied. • The disintegration behavior under different conditions is investigated. • The effect of different parameters on disintegration is obtained, and the reason is revealed. • The reaction modes named topochemical and non–topochemical under different gas mixtures are discussed.

Guest Editorial: Solar Energy Utilization

Guest Editorial: Solar Energy Utilization

Modulating the doping state of transition metal ions in ZnS for enhanced photocatalytic activity.

Transition metal ions (M = Ag+, Cu2+, Co2+, and Cr3+) are surface or homogeneously doped into ZnS via facile cation-exchange reaction, and while Ag+ and Cu2+ doping does not induce sulphur vacancies (Vs) or zinc vacancies (VZn), Co2+ and Cr3+ doping induces Vs. The surface doped catalysts exhibit greatly higher activity than the ZnS and homogenous doped catalysts for H2 evolution and CO2 reduction. The important role of the doping state on affecting the photo-absorption, carrier separation efficiency, and photoreaction kinetics has been systemically investigated and proposed. This work sheds light on the future design and fabrication of high-performance photocatalysts by element doping.

H2-rich syngas production from gasification involving kinetic modeling: RSM-utility optimization and techno-economic analysis.

In this research article, H2 rich syngas production is optimized using response surface methodology (RSM) and a utility concept involving chemical kinetic modeling considering eucalyptus wood sawdust (CH1.63O1.02) as gasification feedstock. By adding water gas shift reaction, the modified kinetic model is validated with lab scale experimental data (2.56 ≤ root mean square error ≤ 3.67). Four operating parameters (i.e., particle size "d p", temperature "T", steam to biomass ratio "SBR", and equivalence ratio "ER") of air-steam gasifier at three levels are used to frame the test cases. Single objective functions like H2 maximization and CO2 minimization are considered whereas for multi-objective function a utility parameter (80% H2 : 20% CO2) is considered. The regression coefficients (R H2 2 = 0.89, R CO2 2 = 0.98 and R U 2 = 0.90) obtained during the analysis of variance (ANOVA) confirm a close fitting of the quadratic model with the chemical kinetic model. ANOVA results indicate ER as the most influential parameter followed by T, SBR, and d p. RSM optimization gives H2|max = 51.75 vol%, CO2|min = 14.65 vol% and utility gives H2|opt. = 51.69 vol% (0.11%↓), CO2|opt. = 14.70 vol% (0.34%↑). The techno-economic analysis for a 200 m3 per day syngas production plant (at industrial scale) assured a payback period of 4.8 (∼5) years with a minimum profit margin of 142% when syngas selling price is set as 43 INR (0.52 USD) per kg.

Preferred catalysis distinctly determined by metals doped with nitrogen in three-dimensionally ordered porous carbon materials.

Three-dimensionally ordered nanoporous structures were generated in carbon materials doped with metals and nitrogen as catalytically active sites for electrochemical reactions. Free-base and metal phthalocyanines with a strategically designed molecular structure were used as carbon sources to obtain an ordered porous structure via homogeneous self-assembly with Fe3O4 nanoparticles as the pore template and the prevention of melting away during carbonization. The doping of Fe and nitrogen was achieved by a reaction between the free-base phthalocyanine and Fe3O4 through carbonization at 550 °C, while Co and Ni were doped using the corresponding metal phthalocyanines. The preference of these three types of ordered porous carbon materials for catalytic reactions was distinctly determined by the doped metals. Fe-N-doped carbon showed the highest activity for O2 reduction. Additional heat treatment at 800 °C enhanced this activity. CO2 reduction and H2 evolution were preferred by the Ni- and Co-N-doped carbon materials, respectively. A change in the template particle size was capable of controlling the pore size to enhance mass transfer and improve performance. The technique presented in this study enabled systematic metal doping and pore size control in the ordered porous structures of carbonaceous catalysts.

Surface anchored atomic cobalt-oxide species coupled with oxygen vacancies boost the CO-production yield of Pd nanoparticles

The high density of oxygen vacancies and adjacent Pd domains promote CO2 activation and H2 dissociation, respectively.

Main group elements in electrochemical hydrogen evolution and carbon dioxide reduction.

Main-group elements are renowned for their versatile reactivities in organometallic chemistry, including CO2 insertion and H2 activation. However, electrocatalysts comprising a main-group element active site have not yet been widely developed for activating CO2 or producing H2. Recently, research has focused on main-group element-based electrocatalysts that are active in redox systems related to fuel-forming reactions. These studies have determined that the catalytic performances of heavier main-group element-based electrocatalysts are often similar to those of transition-metal-based electrocatalysts. Our group has recently reported the scope of including the main-group elements in the design of molecular catalysts and explored their applications in redox catalysis, such as the generation of H2 upon coupling of two protons (H+) and two electrons (e-). This feature article summarizes our research efforts in developing molecular electrocatalysts comprising main-group elements at their active sites. Furthermore, we highlight their influence on the rate-determining step, thereby enhancing the reaction rate and product selectivity for multi-H+/multi-e- transfer catalysis. Particularly, we focus on the performance of our recently reported molecular Sn- or Sb-centered macrocycles for electrocatalytic H2 evolution reaction (HER) and on how their mechanisms resemble those of transition-metal-based electrocatalysts. Moreover, we discuss the CO2 reduction reaction (CO2RR), another promising fuel-forming reaction, and emphasize the recent progress in including the main-group elements in the CO2RR. Although the main-group elements are found at the active sites of the molecular catalysts and are embedded in the electrode materials for studying the HER, molecular catalysts bearing main-group elements are not commonly used for CO2RR. However, the main-group elements assist the CO2RR by acting as co-catalysts. For example, alkali and alkaline earth metal ions (e.g., Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Ba2+) are known for their Lewis acidities, which influence the thermodynamic landscape of the CO2RR and product selectivity. In contrast, the elements in groups 13, 14, and 15 are primarily used as dopants in the preparation of catalytic materials. Overall, this article identifies main-group element-based molecular electrocatalysts and materials for HER and CO2RR.

Inside Back Cover

This cover picture describes that MOF‐based artificial and semi‐artificial photosynthesis systems have been sustainably catalyzing the CO2 reduction, H2 evolution, and other chemical processes upon sun‐light irradiation. In such a way, our Earth was successfully protected from pollution and energy crisis due to the depletion of fossil fuels and excessive CO2 emission. More details are discussed in the article by Li et al. on page 335–354.image

Recent Advances on Multivariate MOFs for Photocatalytic CO2 Reduction and H2 Evolution (Adv. Sustainable Syst. 1/2023)

Solar Energy Conversion MTV-MOF-based photocatalyst is one of the significantly candidates in achieving solar-energy conversion. In article number 2200394, Falu Hu, Guowei Zhou and coworkers review the recent development of MTV-MOF-based photocatalysts, classed into mixed-metal MOF, mixed-ligand MOF, and mixed-metal and mixed-ligand MOF, which are used to achieve and regulate photocatalytic CO2 reduction or H2 evolution by introduction of multiple metals or different functional ligands.

Solar driven CO2 hydrogenation to HCOOH on (TiO2)n (n = 1-6) atomic clusters.

Artificial photosynthesis is a crucial reaction that addresses energy and environmental challenges by converting CO2 into fuels and value-added chemicals. However, efficient catalytic activity using earth-abundant materials can be challenging due to intrinsic limitations. Herein, we explore neutral (TiO2)n (n = 1-6) atomic clusters for CO2 hydrogenation via comprehensive ab initio calculations combined with time-dependent functional theory. Our results show that these (TiO2)n clusters exhibit outstanding thermodynamic stabilities and decent surficial activities for CO2 activation and H2 dissociation, both of which possess kinetic barriers down to 0-0.74 eV. We establish a relationship between the binding strength of *CO2 species and electron characterization for these (TiO2)n clusters. These clusters, which have a wide energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccpied molecular orbital (LUMO) that allows them to harvest the solar light in the ultraviolet regime, enabling efficient catalysis for driving the catalysis of CO2 conversion. They provide exclusive reaction channels and high selectivity for yielding HCOOH products via the carboxyl mechanism, involving the kinetic barrier of the limiting step of 0.74-1.25 eV. We also investigated the substrate effect on supported (TiO2)n clusters, with non-metallic substrates featuring inert surfaces serving as suitable options for anchoring (TiO2)n clusters while preserving their intrinsic activity and selectivity. These computational results have significant implications not only for meeting energy demands but also for mitigating carbon emissions by utilizing CO2 as an alternative feedstock rather than considering it solely as a greenhouse gas.

Modelling the CO streamers in the explosive ejection of Orion BN/KL region

ABSTRACT We present reactive gasdynamic, axisymmetric simulations of dense, high-velocity clumps for modelling the CO streamers observed in Orion BN/KL. We have considered 15 chemical species, a cooling function for atomic and molecular gas, and heating through cosmic rays. Our numerical simulations explore different ejection velocities, interstellar medium density configurations, and CO content. Using the CO density and temperature, we have calculated the CO (J = 2 → 1) emissivity, and have built CO maps and spatially resolved line profiles, allowing us to see the CO emitting regions of the streamers and to obtain position velocity diagrams to compare with observations. We find that in order to reproduce the images and line profiles of the BN/KL CO streamers and H2 fingers, we need to have clumps that first travel within a dense cloud core, and then emerge into a lower-density environment.

Interfacial intimacy and internal electric field modulated S-scheme Sv-ZnS/ZnIn2S4 photocatalyst for efficient H2 evolution and CO2 reduction

The construction of S-scheme heterojunctions is an effective approach to realize artificial photocatalytic processes. For the higher solar energy conversion efficiency, current research focuses on improving the interfacial intimacy and precisely modulating the strength of the internal electric field (IEF). To address this issue, we propose a novel MOF-based synthesis and derivation strategy. The heterojunction obtained by this strategy tends to form an intimate interface and a tunable IEF, which facilitates the transfer and separation of photogenerated carriers. Herein, a ZnS/ZnIn2S4 (ZIS) S-Scheme heterojunction containing sulfur vacancies (Sv) was successfully synthesized, and its good photocatalytic hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR) activity confirmed the feasibility of this strategy. The prepared Sv-ZnS/ZIS exhibits an apparent quantum yield of 19.8 ± 1.0 % at 420 nm and a hydrogen evolution rate of 2912.3 ± 185.9 μmol g−1h−1, which is 9.0 and 33.6 times higher than pure ZIS and Sv-ZnS, respectively. Furthermore, the yield of photoreduction CO2 to CO reaches 2075.7 ± 63.0 μmol g−1h−1 with a CO selectivity of 93.0 ± 0.8 %. This work provides new sights for the rational design and construction of S-scheme photocatalysts with sulfur vacancies for efficient photocatalysis.

Defect-Engineered Cu-Based Nanomaterials for Efficient CO2 Reduction over Ultrawide Potential Window

High conversion efficiency over a wide operating potential window is important for the practical application of CO2 reduction electrocatalysis, yet that remains a huge challenge in differentiating the competing CO2 reduction and H2 evolution. Here we introduce point defects (Sn doping) and planar defects (grain boundary) into the Cu substrate. This multidimensional defect integration strategy guides the fabrication of highly diluted SnCu polycrystal, which exhibits high Faradaic efficiencies (>95%) toward CO2 electroreduction over an ultrawide potential window (ΔE = 1.3 V). The theoretical study indicates that the introduction of Sn doping and grain boundary synergistically provides an optimized electronic effect, which helps suppress H2 evolution and promotes the hydrogenation of *CO2.

Turning photocatalytic H2 evolution into CO2 reduction of molecular nickel(II) complexes by using a redox–active bipyridine ligand

Two bis–NHC (NHC = N–heterocyclic carbene) Ni(II) complexes has been designed and synthesized as molecular catalysts in photocatalytic H2 evolution and CO2 reduction. The Ni(II) complex with a redox–innocent 1,2–bis(diphenylphosphaneyl)ethane ligand only generates H2 with a TON of 149 ± 5 and no product of CO2 reduction is found, while the other Ni(II) complex with a redox–active 4,4′–dimethyl–2,2′–bipyridine ligand can catalyze reduction of CO2 to CO with a TON of 41 ± 2 and 93.2% selectivity over H2 evolution. This finding shows a redox–active ligand is capable of boosting CO2 photoreduction of a molecular metal complex, which is helpful for designing molecular catalysts in CO2 photoreduction.

Low-temperature Cu/Zn/SBA-16 integrating carbon membrane reactor for hydrogen production and high CO conversion through water-gas shift reaction

The increasing demand for H2 energy has led to a great amount of research being conducted in a membrane reactor (MR), in which a membrane is applied during the water-gas shift (WGS) reaction. In this study, Cu/Zn/SBA-16 WGS catalysts and carbon molecular sieve (CMS) membranes were integrated into CMS MRs. To improve the CO conversion and H2 yield, C MRs were investigated, and different steam/CO (S/C) ratios were used to evaluate the conversion performance. In this study, a tubular CMS membrane was used as the membrane material for a MR. The as-prepared CMS membrane exhibited excellent selectivity of 185.64 for H2 and CO2 mixed gas, and an ideal H2 permeability of 9.7 × 10−9 mol m−2 s−1 Pa−1 when operated under low temperature/pressure conditions (300 °C/3 bar). The Cu/Zn/SBA-16 catalyst synthesized via coprecipitation was used in the WGS reaction. With a relatively low reaction temperature of 300 °C, 2500 h−1 gas hourly space velocity, and S/C equal to 1.5, the CO conversion efficiency of MR could reach up to 99%, and the recovery of H2 was approximately 76%. However, as the S/C increased to 2, the H2 recovery increased to 99%, whereas the CO conversion decreased to 89% because of the water vapor adsorbed on the active site. The hydrophobic Si/C-modified membrane was further synthesized and showed outstanding performance in CO conversion of over 99% with S/C equal to 2.

Hexagonal-borocarbonitride (h-BCN) based heterostructure photocatalyst for energy and environmental applications: A review

Formulation of heterojunction with remarkable high efficiency by utilizing solar light is promising to synchronously overcome energy and environmental crises. In this concern, hexagonal-borocarbonitride (h-BCN) based Z-schemes have proved potential candidates due to their spatially separated oxidation and reduction sites, robust light-harvesting ability, high charge pair migration and separation, and strong redox ability. H-BCN has emerged as a hotspot in the research field as a metal-free photocatalyst with a tunable bandgap range of 0–5.5 eV. The BCN photocatalyst displayed synergistic benefits of both graphene and boron nitride. Herein, the review demonstrates the current state-of-the-art in the Z-scheme photocatalytic application with a special emphasis on the predominant features of their photoactivity. Initially, fundamental aspects and various synthesis techniques are discussed, including thermal polymerization, template-assisted, and template-free methods. Afterward, the reaction mechanism of direct Z-scheme photocatalysts and indirect Z-scheme (all-solid-state) are highlighted. Moreover, the emerging Step-scheme (S-scheme) systems are briefly deliberated to understand the charge transfer pathway mechanism with an induced internal electric field. This review critically aims to comprehensively summarize the photo-redox applications of various h-BCN-based heterojunction photocatalysts including CO2 photoreduction, H2 evolution, and pollutants degradation. Finally, some challenges and future direction of h-BCN-based Z-scheme photocatalyst in environmental remediation are also proposed.

Photocatalytic CO2 -to-Syngas Evolution with Molecular Catalyst Metal-Organic Framework Nanozymes.

Syngas, a mixture of CO and H2 , is a high-priority intermediate for producing several commodity chemicals, e.g., ammonia, methanol, and synthetic hydrocarbon fuels. Accordingly, parallel sunlight-driven catalytic conversion of CO2 and protons to syngas is a key step toward a sustainable energy cycle. State-of-the-art catalytic systems and materials often fall short as application-oriented concurrent CO and H2 evolution requires challenging reaction conditions which can hamper stability, selectivity, and efficiency. Here a light-harvesting metal-organic framework hosting two molecular catalysts is engineered to yield colloidal, water-stable, versatile nanoreactors for photocatalytic syngas generation with highly controllable product ratios. In-depth fluorescence, X-ray, and microscopic studies paired with kinetic analysis show that the host delivers energy efficiently to active sites, conceptually yielding nanozymes. This unlocked sustained CO2 reduction and H2 evolution with benchmark turnover numbers and record incident photon conversions up to 36%, showcasing a highly active and durable all-in-one material toward application in solar energy-driven syngas generation.

Effect of bulk and surface composition of Ni + Ga intermetallic compound catalysts in propane steam/wet reforming: Origins of nearly ideal experimental product selectivity

In our prior studies, Ni + Ga IMCs were isolated as promising wet reforming catalysts that achieved nearly ideal product distributions of H2 and either CO or CO2 and exhibited excellent stability (reduced coke formation). A systematic understanding of IMC surface chemistry as a function of constituent element selection directed our studies to the Ni + Ga IMC compositional space. However, Ni + Ga IMC surface chemistry towards oxygen and in oxidation reaction steps remained to be understood. This study focuses upon exploring the origin of the performance of Ni + Ga IMCs in the wet reforming of propane with a specific focus on controlling the fate of atomic H and understanding the control of oxidation steps to dictate CO vs CO2 production. Additionally, a simple kinetic coupling parameter was developed to predict coke formation and catalyst stability with excellent agreement between predictions and experimentally observed stability.