EXAFS characterization of Ti/Al2O3 supports and Co/Ti/Al2O3 catalysts

The structure of Ti/Al₂O₃ supports (0–14 wt% Ti) and Co/Ti/Al₂O₃ catalysts (3 wt% Co) was examined by EXAFS. The results indicated that the Ti was present primarily as a highly dispersed surface phase. The Ti EXAFS results indicated that the Ti species were octahedrally coordinated. Evidence of Ti—Ti interactions was found for all loadings (2–14 wt% Ti) suggesting that the Ti surface species are present as small clusters of TiO₂.The Co EXAFS results showed evidence for several structurally different Co surface phases as a function of Ti loading. Evidence of a Co species interacting with the Ti surface phase was observed for the 3% Co/2% Ti-3%Co/6%Ti catalysts. At the highest loadings studied, 3%Co/8%Ti and 3%Co/14%Ti, evidence was found for a CoTiO₃-like phase.     

Adsorption and reaction studies on cobalt/alumina catalysts prepared by changing pH coprecipitation

Co/Al₂O₃ catalysts prepared by changing pH coprecipitation with Co loadings in the 8.7–36 wt.% range were analyzed by TSA, TPV, pore structure, XRD as well as CO, H₂, O₂ adsorption and CO hydrogenation. High O₂ uptake and reducibility coupled with low dispersion and constant MSA above 17 wt.% Co indicate large crystallites that are less exposed to H₂. CO hydrogenation per Co site decreases with increasing dispersion or decreasing metal loading.     

Effect of the Support in de-NOx HC-SCR Over Transition Metal Catalysts

Several catalysts based on transition metals (Cu, Co, Fe) and different supports (ZSM-5, activated carbon, Al₂O₃) have been tested by Temperature-Programmed Reaction (TPR) experiments for the selective catalytic reduction of NO_x with propene in the presence of excess oxygen, simulating lean-burn conditions. The activity order with respect to the metal was CuFe>Co for all supports used. ZSM-5 catalysts have a superior behavior over Al₂O₃, as observed for noble metal catalysts. Application of activated carbon as a support is not practical due to its consumption at the reaction temperatures. The selectivity to N₂ of the catalysts was also independent of the support, being higher than 95% in all the cases.     

Pt3Co Octapods as Superior Catalysts of CO2 Hydrogenation

As the electron transfer to CO₂ is a critical step in the activation of CO₂, it is of significant importance to engineer the electronic properties of CO₂ hydrogenation catalysts to enhance their activity. Herein, we prepared Pt₃Co nanocrystals with improved catalytic performance towards CO₂ hydrogenation to methanol. Pt₃Co octapods, Pt₃Co nanocubes, Pt octapods, and Pt nanocubes were tested, and the Pt₃Co octapods achieved the best catalytic activity. Both the presence of multiple sharp tips and charge transfer between Pt and Co enabled the accumulation of negative charges on the Pt atoms in the vertices of the Pt₃Co octapods. Moreover, infrared reflection absorption spectroscopy confirmed that the high negative charge density at the Pt atoms in the vertices of the Pt₃Co octapods promotes the activation of CO₂ and accordingly enhances the catalytic activity.     

CO2 fixation by hydrogenation over coprecipitated Co/Al2O3

CO₂ fixation by hydrogenation over coprecipitated 36 wt.% Co/Al₂O₃ has been studied under a range of reaction conditions to clarify the effects of reaction variables and to determine the kinetics and mechanism of the reaction. A comparison of the results with those reported for CO hydrogenation on the same catalyst indicates that, although product distributions of CO₂ and CO hydrogenation differ, the kinetics and mechanism are similar.     

负载型CuO/TiO2催化剂催化CO2加氢制甲醇

采用浸渍法制备了CuO/TiO_2负载型催化剂,并将其用于CO2加氢制甲醇反应。重点考察了铜的负载量对催化剂性能的影响,并对其物化性能和催化性能之间的关系进行了讨论。结果发现,随着铜负载量的增加,催化剂中金属铜的比表面先增加后减小,当铜的负载量为10%(质量百分数)时达到最大值。催化剂的表面碱性位数量随铜含量的增加持续减小,中等碱位和强碱位的强度下降。当铜的负载量不高于10%时,CO2的转化率与铜的比表面积呈线性关系。甲醇选择性与催化剂的表面碱位性质有关,过强的碱性位会降低甲醇选择性。     

负载型CuO/TiO2催化剂催化CO2加氢制甲醇

采用浸渍法制备了CuO/TiO₂负载型催化剂,并将其用于CO₂加氢制甲醇反应。重点考察了铜的负载量对催化剂性能的影响,并对其物化性能和催化性能之间的关系进行了讨论。结果发现,随着铜负载量的增加,催化剂中金属铜的比表面先增加后减小,当铜的负载量为10%（质量百分数）时达到最大值。催化剂的表面碱性位数量随铜含量的增加持续减小,中等碱位和强碱位的强度下降。当铜的负载量不高于10%时,CO₂的转化率与铜的比表面积呈线性关系。甲醇选择性与催化剂的表面碱位性质有关,过强的碱性位会降低甲醇选择性。     

Regulating Efficient and Selective Single-atom Catalysts for Electrocatalytic CO2 Reduction

Anchoring transition metal (TM) atoms on suitable substrates to form single-atom catalysts (SACs) is a novel approach to constructing electrocatalysts. Graphdiyne with sp−sp² hybridized carbon atoms and uniformly distributed pores have been considered as a potential carbon material for supporting metal atoms in a variety of catalytic processes. Herein, density functional theory (DFT) calculations were performed to study the single TM atom anchoring on graphdiyne (TM₁−GDY, TM=Sc, Ti, V, Cr, Mn, Co and Cu) as the catalysts for CO₂ reduction. After anchoring metal atoms on GDY, the catalytic activity of TM₁−GDY (TM=Mn, Co and Cu) for CO₂ reduction reaction (CO₂RR) are significantly improved comparing with the pristine GDY. Among the studied TM₁−GDY, Cu₁−GDY shows excellent electrocatalytic activity for CO₂ reduction for which the product is HCOOH and the limiting potential (U_L) is −0.16 V. Mn₁−GDY and Co₁−GDY exhibit superior catalytic selectivity for CO₂ reduction to CH₄ with U_L of −0.62 and −0.34 V, respectively. The hydrogen evolution reaction (HER) by TM₁−GDY (TM=Mn, Co and Cu) occurs on carbon atoms, while the active sites of CO₂RR are the transition metal atoms . The present work is expected to provide a solid theoretical basis for CO₂ conversion into valuable hydrocarbons.     

Evidence of Structure Sensitivity in the Fischer–Tropsch Reaction on Model Cobalt Nanoparticles by Time-Resolved Chemical Transient Kinetics

The Fischer–Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer–Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non-steady-state conditions. Co nanoparticles of 4.3 nm and 9.5 nm diameters were synthesized and tested under atmospheric pressure conditions and H₂/CO=2. Large differences in carbon coverage (Θ_C) were observed for the two catalysts: the 4.3 nm Co catalyst has a Θ_C less than one while the 9.5 nm Co catalyst supports a Θ_C greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B₅-B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes.     

Evidence of Structure Sensitivity in the Fischer–Tropsch Reaction on Model Cobalt Nanoparticles by Time‐Resolved Chemical Transient Kinetics

The Fischer–Tropsch process, or the catalytic hydrogenation of carbon monoxide (CO), produces long chain hydrocarbons and offers an alternative to the use of crude oil for chemical feedstocks. The observed size dependence of cobalt (Co) catalysts for the Fischer–Tropsch reaction was studied with colloidally prepared Co nanoparticles and a chemical transient kinetics reactor capable of measurements under non‐steady‐state conditions. Co nanoparticles of 4.3 nm and 9.5 nm diameters were synthesized and tested under atmospheric pressure conditions and H₂/CO=2. Large differences in carbon coverage (Θ_C) were observed for the two catalysts: the 4.3 nm Co catalyst has a Θ_C less than one while the 9.5 nm Co catalyst supports a Θ_C greater than two. The monomer units present on the surface during reaction are identified as single carbon species for both sizes of Co nanoparticles, and the major CO dissociation site is identified as the B₅‐B geometry. The difference in activity of Co nanoparticles was found to be a result of the structure sensitivity caused by the loss of these specific types of sites at smaller nanoparticle sizes.     

Effects of Titania on the Adsorptive and Catalytic Properties of Silica-Supported Cobalt Catalyst

A series of Ti-promoted (6 wt%) Co/SiO₂ catalysts with titania content of 0 to 10 wt% were sequentially prepared by incipient wetness impregnation, and characterized with X-ray diffraction, thermogravimetric analysis, chemisorption, temperature-programmed desorption and infrared spectroscopy. The influences of Ti addition and reduction temperature (400–700 °C) on the adsorptive behavior and the catalytic properties for CO hydrogenation were investigated. The presence of Ti decreases the adsorption capacity of the cobalt surface for H₂, but enhances activity per gram cobalt. In addition, the turnover frequency increases 2–4 times upon Ti addition at reduction temperatures of 400–700 °C. The promotion in activity is accompanied by an enhanced selectivity for higher hydrocarbons and olefins. These modifications can be rationalized by the creation of active sites for CO dissociation. The desorption of CO₂ at 100 °C during temperature-programmed desorption of CO indicates the formation of active sites for CO disproportionation. Infrared spectroscopy indicates an increase in the relative absorbance of 2060–2075 cm^?1 bands upon Ti addition, which are attributed to CO adsorbed on the defect sites of the cobalt surface. Therefore, the promotion effect of Ti may be directly related to the formation of defect sites on the cobalt surface induced by the decorated titania moieties.     

SiO2-TiO2 membranes by the sol-gel process

The use of membranes for gas separation represents an important alternative from the viewpoint of energy conservation in industrial separation processes. Polymeric Si-Ti sols prepared from titanium isopropoxide (Ti(OPrⁱ)₄) and tetraethoxysilane (TEOS) were used to deposit membranes on α-Al₂O₃ supports. Acetylacetone (2,4 pentanedione, acacH) and isoeugenol (2-methoxy-4-propenylphenol, isoH) were employed separately to chelate the Ti precursor in order to slow down the chemical reactivity, avoiding precipitation. The radial distribution functions (RDF) of the gels aging at room temperature were obtained. The xerogels were studied by Thermal Gravimetric (TGA) and Differential Thermal (DTA) Analysis in air. The Microporosity of the solids calcined at 773 K was determined by N₂-adsorption at 77 K. The membrane thickness was determined from SEM photographs. Preliminary permeance results of the supported membranes on commercial alumina support were obtained for He, N₂ and CO₂ in a single gas equipment. At 773 K the separation factors α(He/CO₂) and α(N₂/CO₂) for both membranes exceeds the theoretical Knudsen limit.     

微量Li助剂对Co/AC催化剂合成高碳醇性能的影响

采用等体积浸渍法制备了含微量Li 的15Co_xLi/AC 催化剂,考察了微量Li 助剂对15Co/AC催化剂上CO加氢合成高碳醇性能的影响. 采用X射线衍射、程序升温还原和程序升温表面反应技术对15Co_xLi/AC 催化剂进行了表征,结果表明,微量Li 的添加可以提高催化剂上CO加氢活性、生成C₅₊烃的选择性、合成醇的选择性以及高碳醇的分布. 这主要是由于微量Li 助剂与Co物种形成了弱相互作用,促进了催化剂Co物种的分散,形成较小Co晶粒,促进了Co₂C的形成.     

Ceria-Supported Cobalt Catalyst for Low-Temperature Methanation at Low Partial Pressures of CO2

The direct catalytic conversion of atmospheric CO₂ to valuable chemicals is a promising solution to avert negative consequences of rising CO₂ concentration. However, heterogeneous catalysts efficient at low partial pressures of CO₂ still need to be developed. Here, we explore Co/CeO₂ as a catalyst for the methanation of diluted CO₂ streams. This material displays an excellent performance at reaction temperatures as low as 175 °C and CO₂ partial pressures as low as 0.4 mbar (the atmospheric CO₂ concentration). To gain mechanistic understanding of this unusual activity, we employed in situ X-ray photoelectron spectroscopy and operando infrared spectroscopy. The higher surface concentration and reactivity of formates and carbonyls—key reaction intermediates—explain the superior activity of Co/CeO₂ as compared to a conventional Co/SiO₂ catalyst. This work emphasizes the catalytic role of the cobalt-ceria interface and will aid in developing more efficient CO₂ hydrogenation catalysts.     

双配体CuFe@MOFs材料为前驱体的催化剂的组分调控对CO2加氢制C2+醇性能的影响

采用水热法制备了以对苯二甲酸和对氨基苯甲酸为配体的双配体Fe基MOFs材料(MIL-88B(Fe)),在浸渍一定量Cu物种后经氮气气氛焙烧得到活性组分均匀分散的CuFe基催化剂。通过改变2种配体的比例调控催化剂表面Fe活性物种的价态分布,并考察了其用于固定床反应器上CO_2加氢制C_(2+)醇的催化性能,结合X射线衍射(XRD)、H_2程序升温还原(H2-TPR)、N_2吸附-脱附、扫描电子显微镜(SEM)、X射线光电子能谱(XPS)等表征结果发现,对苯二甲酸与对氨基苯甲酸物质的量之比为5∶2时,催化剂表面低价态铁原子占比为71.27%,催化剂展现最优的催化活性,CO_2转化率为8.80%,总醇选择性为31.52%,其中C_(2+)醇的物质的量分数达到94.70%。     

Efficient Ni-based catalysts for low-temperature reverse water-gas shift (RWGS) reaction

CO₂ hydrogenation for syngas can alleviate the pressure of un-controlled emissions of CO₂ and bring enormous economic benefits. Advantageous Ni-catalysts have good CO₂ hydrogenation activity and high CO selectivity merely over 700 °C. Herein, we introduced Cu into Ni catalysts, which were evaluated by H₂-TPR, XRD, BET, in-situ XPS and CO₂-TPD, and their CO₂ hydrogenation activity and CO selectivity were significantly affected by the Ni/Cu ratios, which was rationalized by the synergistic effect of bimetallic catalysts. In addition, the reduction temperatures of studied catalysts apparently affected the CO₂ hydrogenation, which were caused by the number and dispersion of the active species. It's found that the Ni₁Cu₁-400 had good stability, high CO selectivity (up to 90%), and fast formation rate (1.81×10⁻⁵ mol/g_cat/s) at 400 °C, which demonstrated a good potential as a superior catalyst for reverse water-gas shift (RWGS) reaction.     

Co–Cu–La catalysts for selective CO2 hydrogenation to higher hydrocarbons

Copper and lanthanum promoted cobalt catalysts for CO₂ hydrogenation to higher hydrocarbons are described. The catalysts were prepared by the self-propagating high-temperature synthesis followed by alkaline leaching. They are active in CO₂ hydrogenation at 200 °C under 10 bar pressure (CO₂ : H₂ = 1 : 3) with selectivity to C₂₊ alkanes up to 39%; no alkenes and alcohols are formed under these experimental conditions.     

Tuning product selectivity in CO2 hydrogenation over metal-based catalysts

Conversion of CO₂ into chemicals is a promising strategy for CO₂ utilization, but its intricate transformation pathways and insufficient product selectivity still pose challenges. Exploiting new catalysts for tuning product selectivity in CO₂ hydrogenation is important to improve the viability of this technology, where reverse water-gas shift (RWGS) and methanation as competitive reactions play key roles in controlling product selectivity in CO₂ hydrogenation. So far, a series of metal-based catalysts with adjustable strong metal–support interactions, metal surface structure, and local environment of active sites have been developed, significantly tuning the product selectivity in CO₂ hydrogenation. Herein, we describe the recent advances in the fundamental understanding of the two reactions in CO₂ hydrogenation, in terms of emerging new catalysts which regulate the catalytic structure and switch reaction pathways, where the strong metal–support interactions, metal surface structure, and local environment of the active sites are particularly discussed. They are expected to enable efficient catalyst design for minimizing the deep hydrogenation and controlling the reaction towards the RWGS reaction. Finally, the potential utilization of these strategies for improving the performance of industrial catalysts is examined.

A series of metal oxide, phosphate, alloy, and carbide-based catalysts for selective CO₂ hydrogenation are summarized, showing their abilities to switch CO₂ methanation to RWGS.     

Fischer–Tropsch Synthesis by Carbon Dioxide Hydrogenation on Fe-Based Catalysts

Impregnated and co-precipitated, promoted and unpromoted, bulk and supported iron catalysts were prepared, characterized, and subjected to hydrogenation of CO₂ at various pressures (1–2 MPa) and temperatures (573–673 K). Potassium, as an important promoter, enhanced the CO₂ uptake and selectivity towards olefins and long-chain hydrocarbons. Al₂O₃, when added as a structural promoter during co-precipitation, increased CO₂ conversion as well as selectivity to C₂₊ hydrocarbons. Among V, Cr, Mn and Zn promoters, Zn offered the highest selectivity to C₂–C₄ alkenes. The different episodes involved in the transformation of the catalyst before it reached steady-state were identified, on the co-precipitated catalyst. Using a biomass derived syngas (CO/CO₂/H₂), CO alone took part in hydrogenation. When enriched with H₂, CO₂ was also converted to hydrocarbons. The deactivation of impregnated Fe–K/Al₂O₃ catalyst was found to be due to carbon deposition, whereas that for the precipitated catalyst was due to increase in crystallinity of iron species. The suitability of SiO₂, TiO₂, Al₂O₃, HY and ion exchanged NaY as supports was examined for obtaining high activity and selectivity towards light olefins and C₂₊ hydrocarbons and found Al₂O₃ to be the best support. A comparative study with Co catalysts revealed the advantages of Fe catalysts for hydrocarbon production by F–T synthesis.     

Novel Co/graphene oxide and Co/nanoporous graphene catalysts for Fischer–Tropsch reaction

For the first time, nanoporous graphene and graphene oxide sheets have been synthesized and used as supports for preparation of Co/graphene-based catalysts to evaluate their efficiency in Fischer–Tropsch synthesis and for comparison with the performance of Co/Al₂O₃ to study the effects of the carbon supports on the reaction. Outstanding results were obtained compared with the alumina counterpart. Application of nanoporous graphene yielded heavier hydrocarbons compared with the Co/Al₂O₃ catalyst, possibly due to the high surface area and intrinsic properties of the carbon nanostructures as effective hydrogen carriers. Use of graphene oxide and nanoporous graphene supports also resulted in high CO₂ selectivity. However, the graphene-supported catalysts displayed lower C₁–C₄ hydrocarbon selectivity compared with the Al₂O₃ catalyst.