CO Chemisorption Research Articles

Optimization of olefins' yield in Fischer-Tropsch synthesis using carbon nanotubes supported iron catalyst with potassium and molybdenum promoters

Light olefins' (C2-C4) production from syngas (H2 +CO) through Fischer-Tropsch synthesis (FTS) benefits from Fe-based catalysts supported on carbon nanotubes (CNTs). Two-level full factorial design was applied for K and/or Mo-promoted Fe/CNTs catalyst to investigate the effects of synthesis conditions including Mo/K mass ratio, ultrasonic time, and iron loading on light olefins' yield. These catalysts were characterized to perform an in-depth study of Mo/K binary promoter effects on Fe/CNTs catalyst in Fischer-Tropsch to light olefins. CO chemisorption and TEM revealed that molybdenum plays a significant role in metal dispersion, leaving structural defects on CNTs support. Additionally, H2-TPR confirmed that K as promoter facilitates reducibility of Fe/CNTs catalysts, which promoted CO conversion in FTS. Compared with the un-promoted Fe/CNTs catalysts, addition of molybdenum as a promoter increased light olefins' selectivity by 33.4%, while potassium led to an increase in CO conversion by 96.3%. The optimum formulation (0.5K5Mo10Fe/CNTs) obtained olefins' yield of 35.5%.

Structure-dependent activity and stability of Al2O3 supported Rh catalysts for the steam reforming of n-dodecane

Steam reforming of liquid hydrocarbon fuels is an appealing way for the production of hydrogen. In this work, the Rh/Al2O3 catalysts with nanorod (NR), nanofiber (NF) and sponge-shaped (SP) alumina supports were successfully designed for the steam reforming of n-dodecane as a surrogate compound for diesel/jet fuels. The catalysts before and after reaction were well characterized by using ICP, XRD, N2 adsorption, TEM, HAADF-STEM, H2-TPR, CO chemisorption, NH3-TPD, CO2-TPD, XPS, Al27 NMR and TG. The results confirmed that the dispersion and surface structure of Rh species is quite dependent on the enclosed various morphologies. Rh/Al2O3-NR possesses highly dispersed, uniform and accessible Rh particles with the highest percentage of surface electron deficient Rh0 active species, which due to the unique properties of Al2O3 nanorod including high crystallinity, relatively large alumina particle size, thermal stability, and large pore volume and size. As a consequent, Rh/Al2O3-NR catalyst exhibited superior catalytic activity towards steam reforming reactions and hydrogen production rate over other two catalysts. Especially, Rh/Al2O3-NR catalyst showed the highest hydrogen production rate of 87,600 mmol gfuel−1 gRh−1min−1 among any Rh-based catalysts and other noble metal-based catalysts to date. After long-term reaction, a significant deactivation occurred on Rh/Al2O3–NF and Rh/Al2O3-SP catalysts, due to aggregation and sintering of Rh metal particles, coke deposition and poor hydrothermal stability of nanofibrous structure. In contrast, the Rh/Al2O3-NR catalyst shows excellent reforming stability with negligible coke formation. No significantly sintering and aggregation of the Rh particles is observed after long-term reaction. Such great catalyst stability can be explained by the role of hydrothermal stable nanorod alumina support, which not only provides a unique environment for the stabilization of uniform and small-size Rh particles but also affords strong surface basic sites.

CO2 chemisorption and dissociation on flat and stepped transition metal surfaces

As a promising and practical way to decrease CO2 emissions, the conversion of CO2 to value-added chemicals has received significant recent attention. The activation of CO2 on catalyst surfaces might proceed via a chemisorption state with a bent CO2 configuration, in which substrate electrons are transferred into the antibonding orbital of the CO2 adsorbate. Based on density functional theory calculations, we present an extensive survey of CO2 chemisorption and dissociation on flat and stepped surfaces of several transition metals. The binding energy of chemisorbed CO2 is closely correlated with the extent of electron transfer from the metal to CO2, as evidenced by a linear relationship found between the CO2 adsorption energy and its Bader charge. Transition state scaling (TSS) correlations between binding energies of transition states and binding energies of either initial or final states are found to exist for the dissociation of the chemisorbed CO2 on flat and stepped surfaces, which can be used to predict the efficacies of the catalysts. Our results show that defect sites at stepped surfaces have a strong influence on CO2 chemical activation and dissociation.

Surface Coordination Decouples Hydrogenation Catalysis on Supported Metal Catalysts

Surface Coordination Decouples Hydrogenation Catalysis on Supported Metal Catalysts

Carbon Dioxide Capture at Nucleophilic Hydroxide Sites in Oxidation‐Resistant Cyclodextrin‐Based Metal–Organic Frameworks**

AbstractCarbon capture and sequestration (CCS) from industrial point sources and direct air capture are necessary to combat global climate change. A particular challenge faced by amine‐based sorbents—the current leading technology—is poor stability towards O2. Here, we demonstrate that CO2 chemisorption in γ‐cylodextrin‐based metal–organic frameworks (CD‐MOFs) occurs via HCO3− formation at nucleophilic OH− sites within the framework pores, rather than via previously proposed pathways. The new framework KHCO3 CD‐MOF possesses rapid and high‐capacity CO2 uptake, good thermal, oxidative, and cycling stabilities, and selective CO2 capture under mixed gas conditions. Because of its low cost and performance under realistic conditions, KHCO3 CD‐MOF is a promising new platform for CCS. More broadly, our work demonstrates that the encapsulation of reactive OH− sites within a porous framework represents a potentially general strategy for the design of oxidation‐resistant adsorbents for CO2 capture.

Carbon Dioxide Capture at Nucleophilic Hydroxide Sites in Oxidation-Resistant Cyclodextrin-Based Metal-Organic Frameworks.

Carbon capture and sequestration (CCS) from industrial point sources and direct air capture are necessary to combat global climate change. A particular challenge faced by amine-based sorbents-the current leading technology-is poor stability towards O2 . Here, we demonstrate that CO2 chemisorption in γ-cylodextrin-based metal-organic frameworks (CD-MOFs) occurs via HCO3 - formation at nucleophilic OH- sites within the framework pores, rather than via previously proposed pathways. The new framework KHCO3 CD-MOF possesses rapid and high-capacity CO2 uptake, good thermal, oxidative, and cycling stabilities, and selective CO2 capture under mixed gas conditions. Because of its low cost and performance under realistic conditions, KHCO3 CD-MOF is a promising new platform for CCS. More broadly, our work demonstrates that the encapsulation of reactive OH- sites within a porous framework represents a potentially general strategy for the design of oxidation-resistant adsorbents for CO2 capture.

Promoting low-temperature methanol production over mixed oxide supported Cu catalysts: Coupling ceria-promotion and photo-activation

Catalytic methanol production from CO2 hydrogenation at a lower temperature is limited, predominantly constricted by sluggish reaction kinetics. In this work, a ceria-modified Cu/ZnO/Al2O3 catalyst (CuZnCe-Al) is fabricated which delivers efficient low-temperature methanol production (yield= 822 g/kgCu/h and selectivity = 94% at 225 ºC and 20 bar) under light illumination. The influence of ceria loading on the morphology/microstructure, interfacial features, and surface chemistry of a Cu/ZnO-based catalyst is systematically assessed. Incorporating ceria (≤ 10 at%) into ZnO initially forms ZnO/CeOx interface which promotes both CO2 chemisorption and the formate-pathway which favours methanol production over CO. Increasing the ceria loading beyond 10 at% invoked a Cu/CeOx-dominated structure, leading to over-stabilisation of the surface reaction species and a decreased preference for methanol. The photo-enhancement factor observed for each CuZnxCey catalysts is largely independent of the ceria loading, implying that ceria inclusion imposed little influence on the Cu-ZnO interfacial characteristics.

Adsorption of Co(II) ions using Zr-Ca-Mg and Ti-Ca-Mg phosphates: adsorption modeling and mechanistic aspects.

The environmental pollution by toxic Co(II) ions had a negative impact on living organisms and water resources. The amorphous Zr-Ca-Mg and Ti-Ca-Mg phosphates with varied Zr and Ti content with the mesoporous structure (ABET = 19-232 m2/g, Vdes. = 0.075-0.370 cm3/g, Ddes. = 6.2-10.9nm) were synthesized. The effect of adsorbent chemical composition, the presence of competing ions (0.1-1.0M NaCl and 0.01-0.1M CaCl2 backgrounds), and pH (3.0-7.0) of aqueous solution on adsorption removal of Co(II) ions by Zr-Ca-Mg and Ti-Ca-Mg phosphates was studied. The highest adsorption capacity of Zr-Ca-Mg-1 and Ti-Ca-Mg-1 samples reached 253.3 and 212.8mg/g. The prepared adsorbents demonstrated high efficiency at pH in the range of 3.0-7.0 and the presence of 0.1-1.0M NaCl and seawater with a salinity of 35.0g/L backgrounds. The chemisorption and ion-exchange mechanisms of Co(II) ions removal for Zr-Ca-Mg and Ti-Ca-Mg phosphates were proposed. The adsorption isotherms were well fitted with Sips and Langmuir models that proved the heterogeneous nature of adsorption sites as well as assumed the monolayer adsorption that occurs at specific homogeneous sites within the adsorbent without any interaction between the adsorbed substances. The kinetic data was well described by the pseudo-second-order model that is suitable for chemisorption processes as liming adsorption stage. The presented results shown the prospects of developed adsorbents for the investigation of real wastewater treatment from heavy metal ions and liquid radioactive waste purification.

Mesoporous Silica SBA-15 Supported Pt–Ga Nanoalloys as an Active and Stable Catalyst for Propane Dehydrogenation

Propane dehydrogenation is one of the most admired technologies for propylene production. However, the development of an active and stable catalyst presents a significant challenge. In this study, Pt–Ga/SBA-15 catalysts were prepared via coimpregnation, followed by H2 reduction, and applied in propane dehydrogenation. The Pt–Ga/SBA-15 catalyst with a Ga/Pt molar ratio of 1 exhibited a high propane conversion of up to 60% and high selectivity toward propylene of 98.2% at 550 °C for 10 min, with a propane conversion of 51.6% and propylene selectivity of 99.5% after reaction for 25 h. The physicochemical properties characterized using N2 physisorption, XRD, TEM, XPS, CO chemisorption, and in situ CO–DRIFT spectroscopy reveal that the Pt–Ga alloy nanoparticles were formed on the SBA-15 support after direct reduction with H2. The study of coke formation via temperature-programmed oxidation showed that a large amount of coke was deposited over the support of spent Pt–Ga/SBA-15 catalyst, indicating coke mobility from the active sites to the supports and its contribution to catalyst stability. The activities of spent catalysts could be almost completely recovered via reduction regeneration, during which Pt–Ga alloy retains the original geometrical and electronic structure well. The dilution of Pt ensembles due to the addition of Ga, e.g., Pt–Ga alloy formation, plays an important role in enhancing the catalytic performance for propane dehydrogenation.

Anchoring Pd species over defective alumina to achieve high atomic utilization and tunable electronic structure for semi-hydrogenation of acetylene

Semi-hydrogenation of acetylene is one of the most promising methods to eliminate acetylene from ethylene/acetylene mixtures to avoid Ziegler−Natta polymerization catalysts being poisoned. Given that acetylene is always over-hydrogenated on conventional palladium-based catalyst, it is of great importance to facilitate the desorption of ethylene to enhance the ethylene selectivity. Herein, we report a simple method to anchor Pd by unsaturated penta-coordinated aluminum (Al3+penta) on Al2O3 (meso-Al2O3). Characterized by 27Al-NMR, XRD, HR-TEM, CO chemisorption and XPS, Pd/meso-Al2O3 catalyst had a highly dispersion and unique electronic property of Pd, thus exhibiting a high ethylene selectivity (~83%) while the acetylene conversion is > 99%. Besides, the agglomeration of Pd is suppressed by the metal-support interaction (MSI) between Pd and meso-Al2O3 support. Therefore, the preparation method of the catalyst is relatively simple and has certain commercial application prospects.

Elucidating the synergistic fabrication of dual embedded (χ-Fe5C2 + θ-Fe3C) carbide nanocomposites in Na-FeCa@AC/HZSM-5 integrated catalyst for syngas conversion to aromatics

Herein, an activated carbon (AC) modified Fe-oxide catalyst has been employed in Na-FeCa@AC/HZSM-5 integrated catalytic system for the direct conversion of syngas to aromatics (STA). In comparison to the conventionally impregnated Fe/C catalysts, the spatial confinement of highly dispersed Fe3O4 nanocrystals in a rigid geometry of defective graphite sheet by the efficient modulation of different (AC, Ca) promoters, calcination environment and the surface carbon reactions in sol-precipitation method, potentially deflected the sintering and agglomeration behavior of Fe nanoparticles. The meliorated higher extent of CO chemisorption on the tuned Fe surface and the different reduction/carburization schemes cultivated the high-purity (54% χ-Fe5C2 + 44% θ-Fe3C) carbide nanoparticles. The encapsulation of perforated carbon patches during carburization favored the stabilization of highly active carbide phase under reaction conditions, where the subsequent precipitation of carbon elements led to the further recruitment of multi-layer carbide films for the accelerated CO dissociation and the partial decomposition of olefinic intermediates. Since, the dually embedded terrace carbide islands were rather enriched from one phase to another (27.3%θ-Fe3C ⇌ 70.7%χ-Fe5C2) during the reaction, giving a simultaneously enhanced catalytic activity of 97% CO conversion and 60% aromatics selectivity while suppressing the net CO2 release (21%). The current study anticipates a promising scheme to formulize a highly active and stable iron-based catalyst to be integrated with HZSM-5 for STA process.

Acid-Base Surface-Driven NO-CO Conversion over Copper-Manganese-Based Metal Oxides.

Recently, tuning catalytic material has gained huge importance for enhancing the catalytic performance of reactions. The present work describes the influence of aluminum in copper-manganese composite oxide with respect to the NO-CO redox reaction. The Al3+ fabricated composite oxide showed the highest conversion in a lower temperature window. The nanocomposite metal oxide of Cu-Mn formed as a porous structure with aluminum and helped in establishing a more surface acidic/basic character on the catalyst. These acidic/basic sites are crucial in activating the NO and CO chemisorption for significant redox conversion.

Balancing CO chemisorption with hydrogen electrochemical adsorption on Pt alloy catalyst for improving direct CO reduction to formaldehyde

Direct CO reduction to formaldehyde (CORTF) provides an alternative, energy-efficient route for production of formaldehyde, a critical chemical commodity as well as promising hydrogen carrier. However, the conventional thermal catalysis approach is challenged with slow reaction kinetics, which can be attributed to imbalanced CO and hydrogen competitive chemisorption to catalyst, with stronger CO adsorption significantly inhibiting hydrogen adsorption that suppresses their reaction towards formaldehyde generation. We report a new, innovative CORTF mechanism to promote the reaction by utilizing hydrogen underpotential deposition to balance the species coverage on catalyst surface. Our study shows that, by separately tuning Pt alloy nanoparticle catalyst composition to control CO chemical adsorption favorability and tuning hydrogen underpotential deposition potential to control hydrogen electrochemical adsorption favorability, the formaldehyde production rate with Pt alloys (866 mg/gcat/h) is 300 times higher compared to conventional thermal catalysis method (2.6 mg/gcat/h) and 50 time higher than previously reported molybdenum phosphide (MoP, 16.3 mg/gcat/h) electrocatalyst. DFT simulations show reduced energy barriers for the rate-limiting steps by means of balancing hydrogen and CO adsorption, thus supporting this new mechanism to achieve more dramatically improved CORTF kinetics. This CORTF mechanism provides a new strategy towards efficient formaldehyde production.

A highly dispersed Ni3P/HZSM-5 catalyst for hydrodeoxygenation of phenolic compounds to cycloalkanes

A highly dispersed Ni3P/HZSM-5 catalyst was prepared by deposition-precipitation of Ni species followed by electroless plating and thermal treatment, and characterized by means of XRD, ICP, N2 physical adsorption, TEM, XPS, NH3-TPD, and CO chemisorption. Electroless plating reaction takes place in an aqueous solution under mild conditions, facilitating the high dispersion of the formed Ni3P particles. XRD measurements illustrated the lamellar nickel phyllosilicate was produced at the first step of deposition-precipitation, amorphous Ni-P was formed in the subsequent electroless plating, and highly dispersed Ni3P crystallites were generated in thermal treatment. TEM observation demonstrated that the average particle size of Ni3P crystallites in the prepared Ni3P/HZSM-5 catalyst was 4.3 nm. Bifunctional Ni3P/HZSM-5 was investigated in hydrodeoxygenation (HDO) of phenolic compounds, including phenol, m-cresol, and guaiacol. Ni3P/HZSM-5 exhibited high catalytic performance in HDO with enhanced hydrogenation and dehydration pathway.

Euler/Euler large eddy simulation of bubbly flow in bubble columns under CO2 chemisorption conditions

It has been generally recognised that the turbulent dispersion force plays an important role in interphase momentum transfer. However, the effect of the added mass stress on the momentum and mass transfer in bubble column bubbly flow has not been addressed appropriately. As the turbulent eddies in the surroundings of bubbles interact strongly with the rising bubbles in bubble column bubbly flow, such interactions will bring out the change of interfacial areas between the bubbles and carrier fluid, consequently leading to changes in the interfacial mass transfer. When employing large eddy simulation for modelling bubbly flow coupled with the chemisorption process, the SGS filtered velocity fluctuations of liquid phase can be interpreted as the turbulent eddies that continuously hit the surfaces of bubbles, causing bubble deformation and the variation of bubble interfacial areas, which give rise to the turbulent dispersion and added mass stress forces. The present study will demonstrate through Euler/Euler large-eddy simulations (LES) modelling that by considering the turbulent dispersion force (SGS-TDF) and added mass stress (SGS-AMS), the bubble dynamics and mass transfer under the chemisorption conditions can be better indicated, which leads to remarkable improvements in the prediction of bubble lateral dispersion and the interfacial mass transfer. The turbulent dispersion and added mass stress related to the spatially filtering were proposed with a modification on SGS eddy viscosity to reflect turbulent dispersion due to bubble-induced turbulence. A comprehensive assessment of the effects of these additional filtered stress terms on the time-averaged velocity and bubble volume fraction profiles, flow patterns, mass transfer and the pH variation during CO2 chemisorption and the turbulent kinetic energy and species concentration spectra was conducted.

Enhancing the catalytic activity and CO2 chemisorption ability of the perovskite cathode for soild oxide electrolysis cell through in situ Fe-Sn alloy nanoparticles

Herein, the Sn-doped perovskite oxide Sr1.95Fe1.4Sn0.1Mo0.5O6-δ (SFSnM), which exhibited in situ exsolved Fe-Sn alloy nanoparticles and controllable phase transformation, was synthesized to catalyze the reduction of CO2 through A-site deficiency regulation. The in situ exsolved Fe-Sn alloy nanoparticles were uniformly distributed on the surface of the SFSnM substrate after reduction, which significantly enhanced the catalytic activity and CO2 adsorption capacity of the SFSnM cathode. Moreover, a single cell with an FeSn@SFSnM cathode exhibited excellent CO2 electrolysis performance, achieving a current density of 3.269 A cm−2 and an Rp value of 0.145 Ω cm2 at 800 °C and 1.8 V. Additionally, no significant performance attenuation was observed during a long-term stability test (200 h), indicating the good stability of FeSn@SFSnM cathode. Overall, these results demonstrated that the designed FeSn@SFSnM cathode shows great potential for high-performance solid oxide electrolysis cells (SOECs).

Characterization and Hydroisomerization Performance of Mg‐Promoted, Pt/ZSM‐23‐Based Catalysts

AbstractThe effect of Mg on the acidic and metal functions in Pt/ZSM‐23 catalysts has been studied, using both pure zeolite (Z) and preshaped ZSM‐23/Al2O3 composite (ZA) and varying the content and method of Mg loading. N2 physisorption, CO chemisorption, H2/O2 titration, XRD, TEM, TPR, and FTIR were employed to characterize the phase composition, textural and acidic properties, platinum location and dispersion. The shaping procedure used left the acidity of the parent zeolite virtually intact, while Pt dispersion on the composite support was greatly increased. Despite the increased CPt/CH+ ratio, the Pt/ZA catalyst underperformed Pt/Z in the yield of branched isomers produced in n‐decane hydroconversion. Irrespective of the support, its impregnation with Mg allowed tuning the surface acidity and improving the isomerization selectivity above the level characteristic of unpromoted Pt/Z. However, the addition of Mg through ion‐exchange resulted in an opposite effect on the catalyst selectivity. Possible reasons of such a complex behavior are discussed.

Optimizing the oxide support composition in Pr-doped CeO2 towards highly active and selective Ni-based CO2 methanation catalysts

In this study, Ni catalysts supported on Pr-doped CeO2 are studied for the CO2 methanation reaction and the effect of Pr doping on the physicochemical properties and the catalytic performance is thoroughly evaluated. It is shown, that Pr3+ ions can substitute Ce4+ ones in the support lattice, thereby introducing a high population of oxygen vacancies, which act as active sites for CO2 chemisorption. Pr doping can also act to reduce the crystallite size of metallic Ni, thus promoting the active metal dispersion. Catalytic performance evaluation evidences the promoting effect of low Pr loadings (5 at% and 10 at%) towards a higher catalytic activity and lower CO2 activation energy. On the other hand, higher Pr contents negate the positive effects on the catalytic activity by decreasing the oxygen vacancy population, thereby creating a volcano-type trend towards an optimum amount of aliovalent substitution.

Bandgap Engineering and Oxygen Vacancies of NixV2O5+x (x = 1, 2, 3) for Efficient Visible Light‐Driven CO2 to CO with Nearly 100% Selectivity

It is difficult to design a new single‐component photocatalyst to simultaneously possess a bandgap small enough to absorb most of sunlight and strong redox ability to reduce CO2 into value‐added chemical fuels. Herein, bandgap engineering of nickel vanadate compounds (NixV2O5+x, x = 1, 2, 3) is rationally designed to overcome the above challenge. Through changing the Ni:V ratio, the bandgap and band edge positions of nickel vanadates can be regulated, enabling Ni2V2O7 and Ni3V2O8 to reduce CO2 in the presence of water under visible light irradiation that do not exist in NiV2O6. Ni 3d orbitals of Ni2V2O7 and Ni3V2O8 replace V 3d orbitals of NiV2O6 and hybridize with O 2p orbitals to form the valence band maximums, resulting in their negative shifts. Meanwhile, the relatively weaker effect of the crystal field in VO4 tetrahedron over Ni2V2O7 and Ni3V2O8 results in less V 3d split, thus making the conduction band edges to shift upward. In addition, higher concentration of oxygen vacancies over Ni2V2O7 can further enhance its photocatalytic activity for CO2 conversion into CO with nearly 100% selectivity by prolonging the lifetime of photogenerated carriers and improving the chemisorption of CO2.

Viability of Au/La2O3/HAP catalysts for the CO preferential oxidation reaction under reformate gas conditions

The viability of gold supported on lanthanum-modified HAP catalysts is investigated for CO preferential oxidation (PROX) in H2-rich stream. All samples comprise small nanoparticles (NPs) of Au (< 4 nm). Addition of La enhances the chemisorption of CO, whereas it lowers that of H2 and H2O. Moreover, lanthanum improves the reducibility of the catalysts and the mobility of oxygen. FTIR studies show that under CO oxidation conditions Au exists in two distinct forms on La-promoted samples, namely Auσ+ and Au+ species. The catalytic tests under different PROX conditions show an improvement of the performance with lanthanum addition. The observed improvement is linked to suitable chemical properties, whereas efficiency dependence on Au NP sizes is rather secondary. At 80 °C, La-rich catalysts completely eliminate CO and prove to be selective (66%) under realistic PROX conditions (in the presence of H2O and CO2). Moreover, they are exceptionally stable during an extended period (240 h).