Dissociative Adsorption Of CO Research Articles

Structural evolution of bimetallic Co-Cu catalysts in CO hydrogenation to higher alcohols at high pressure

Bimetallic Co-Cu catalysts are widely applied in higher alcohol synthesis (HAS), but the formation of the final active structure has not yet been fully clarified, especially for Co-rich catalysts. We investigated the structural evolution of a Co-Cu catalyst (Co:Cu = 2) from the hydrotalcite precursor containing additional Al3+ and Zn2+ to the final active state after 80 h under reaction conditions at 280 °C and 60 bar. The reconstruction of the bimetallic Co-Cu nanoparticles obtained by H2 reduction was induced by the feed gas consisting of an equimolar H2 and CO syngas mixture resulting in fast phase separation and sintering of metallic Cu0 and Co0 in the first 2 h time on stream (TOS) and a continuous carbidization of Co0 forming Co2C and its sintering until steady state was reached after 40 h TOS. An intergrowth of metallic Cu0 nanoparticles with Co2C nanoparticles was observed to occur under reaction conditions. The high selectivity to oxygenates amounting to 41% compared with 29% to hydrocarbons is ascribed to the multi-functional Co2C/Cu0 interface enabling dissociative CO adsorption, hydrogenation and CO insertion. The formation of hydrogenated carbon species (CxHy) originating from dissociative CO chemisorption is assumed to be favored by hydrogen spillover from Cu0 to Co2C. The adsorption sites for molecular CO provided by both Cu0 and Co2C facilitate its insertion into the CxHy intermediates thus leading to a higher selectivity to alcohols following the Anderson-Schulz-Flory distribution.

Ni-sepiolite and Ni-todorokite as efficient CO2 methanation catalysts: Mechanistic insight by operando DRIFTS

The hydrogenation of CO2 to methane was studied on Ni-sepiolite and Ni-todorokite catalysts. Catalytic testing and operando DRIFTS studies revealed the excellent performance of in particular todorokite-based catalysts. The catalytic activity is related to specific reaction pathways depending on the support material. Based on operando DRIFTS studies, different mechanisms are proposed for the methanation reaction. Over Ni-sepiolite catalysts, a dissociative adsorption of CO2 is observed. Linearly and bridged bonded CO on Ni0 was identified as intermediate. On Ni-todorokite this dissociative mechanism is accompanied by an associative one. Here, carbonate species were identified as additional intermediates, the formation of which is facilitated by the MnOx support. The ability of Ni-todorokite catalysts to activate CO2 via a dissociative as well as an associative mechanism explains their excellent catalytic performance. The influence of the applied preparation method on the catalytic performance was demonstrated for the Ni-sepiolite catalysts.

Magnetization Reversal and Domain Nucleation in Ultra-Thin Co/Re(0001) Capped by Graphitic C

By using X-ray magnetic circular dichroism-photoemission electron microscopy, we study the magnetic behavior of ultra-thin Co on Re(0001) covered by a single layer of graphitic carbon. The carbon overlayer is lithographically fabricated via electron-stimulated dissociative adsorption of CO and subsequent annealing to 650 K. Graphitization is found to induce perpendicular magnetic anisotropy in cobalt at a thickness in the range 4-5 atomic layers, where out-of-plane domains separated by chiral Neél type walls are observed. The graphitic overlayer efficiently protects Co from oxidation upon exposure to ambient conditions, leaving its magnetic structure completely unaffected. The magnetic response of the Co film to an external field is imaged in situ across the hysteresis loop, allowing a quantitative analysis of the field-dependent domain area and structure. Our observations pinpoint the influence of the step morphology on the local magnetic behavior. Moreover, below the saturation field, the domains with chiral walls evolve into nanometric-sized magnetic structures resembling skyrmionic bubbles for applied fields <;20 G.

Low temperature conversion of syngas to ethanol in liquid phase over Fe modified CuZn catalyst: The promoting effect of Fe

Fe modified CuZn catalysts were prepared and first used for the low temperature ethanol synthesis from syngas in liquid phase. Fe greatly enhanced the CO dissociative adsorption and promoted the formation of C2+ alcohols. The role of Fe was dependent on the catalyst preparation method. The co-precipitation method enhanced the interaction between Fe and other components, which could control the CO dissociative desorption to terminate the carbon chain growth of alcohols at ethanol. The wetness impregnation method benefited the dispersion of iron species and the synthesis of methanol and butanol. As a result, the co-precipitation method prepared Fe modified CuZn catalyst with spinel structure exhibited excellent catalytic performance for the low temperature syngas hydrogenation to ethanol in liquid phase with an ethanol selectivity of 40.9%.

Syngas-derived olefins over iron-based catalysts: Effects of basic properties of MgO nanocrystals

A series of Fe/MgO catalysts with well-defined exposed crystal planes were synthesized by impregnation, deposition-precipitation and ultrasonic impregnation methods. The catalysts were characterized by X-ray powder diffraction, high-resolution transmission electron microscopy, CO2 temperature-programmed desorption, H2 temperature-programmed reduction, X-ray spectroscopy and N2 adsorption-desorption isotherms. The characterization results indicate that the basicity of MgO supports strongly affect the catalytic performance of iron-based catalysts for Fischer-Tropsch synthesis. It is found that the strong basicity sites of MgO supports remain during the ultrasonic impregnation process. The intrinsic basicity of Fe/MgO catalysts enhances dissociative CO adsorption and promotes the olefin selectivity. In addition, the catalyst of iron particles on the (111) crystal planes of MgO nanosheets presents higher TOF value and olefins selectivity than that of the catalyst using the (100) crystal planes of MgO nanocubes as a support. The effect of basic properties of MgO nanocrystals facilitates CO chemisorption, suppressing H2 adsorption and olefin desorption on the corresponding Fe/MgO catalysts.

DFT study of the catalytic effect of Na on the gasification of carbon--CO2

A key question in the study of the CO2-gasification process is the mechanism of the Na-containing catalytically active center. In this study, density functional theory calculation was carried out to examine the catalytic role of Na in two graphite models (armchair-edge and zigzag-edge). Two Na-containing groups (CNa, CONa) were considered in each model. The activation energies for all reaction pathways indicated that Na showed catalytic activity in the dissociative adsorption of CO2 in both of the graphite models. However, in the subsequent desorption of CO, which is the rate-determining step, only the armchair-edge model indicated catalysis by Na. The molecular structures and reduced density gradient analysis showed that, during CO2 dissociative adsorption, the Na atom promoted breakage of the CO bond and weakened the conjugate structure at the aromatic nucleus to release CO.

Methanation of CO2 over alkali-promoted Ru/TiO2 catalysts: II. Effect of alkali additives on the reaction pathway

The effect of alkali (Li, Na, K, Cs) promotion of TiO2 on the CO2 hydrogenation pathway has been investigated over 0.5%Ru/TiO2 and 5%Ru/TiO2 catalysts, employing in situ FTIR spectroscopy (DRIFTS) and transient-mass spectrometry techniques. In the case of 0.5%Ru/TiO2 catalysts, characterized by well-dispersed Ru particles, specific activity and selectivity toward methane increases significantly with the addition of suitable amount of alkalis. The nature and population of Ru-bonded carbonyl species, formed on the surface of 0.5%Ru/TiO2 catalyst under CO2 hydrogenation conditions, varies significantly upon alkali promotion. The population of reactive Rux-CO species, which has been previously found to be direct methane precursors, increases in the presence of alkalis following a trend similar to that of specific activity. Results of DRIFT and mass spectrometry experiments provide evidences that alkali promotion enhances the dissociative adsorption of CO and hydrogenation of the so formed surface carbon to methane, which seems not to be operable over the unpromoted 0.5%Ru/TiO2 catalyst. In the case of 5%Ru/TiO2 catalysts, characterized by large Ru particles, both CO2 methanation activity and the nature/population of intermediate surface species seems not to be significantly affected by the presence of sodium. It is suggested that the structure sensitivity of the reaction predominates to the effect of alkali promotion and therefore, CO2 methanation activity cannot be further improved.

Mn-Fe nanoparticles on a reduced graphene oxide catalyst for enhanced olefin production from syngas in a slurry reactor.

Fe nanoparticles (NPs) supported on reduced graphene oxide (rGO) nano-sheets were promoted with Mn and used for the production of light olefins in Fischer–Tropsch reactions carried out in a slurry bed reactor (SBR). The prepared catalysts were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), transmission electron microscope (TEM), Raman spectroscopy, N2 physisorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopic (XPS) methods. Mn was shown to preferentially migrate to the Fe NP surface, forming a Mn-rich shell encapsulating a core rich in Fe. The Mn shell regulated the diffusion of molecules to and from the catalyst core, and preserved the metallic Fe phase by lowering magnetite formation and carburization, so decreasing water gas shift reaction (WGSR) activity and CO conversion, respectively. Furthermore, the Mn shell reduced H2 adsorption and increased CO dissociative adsorption which enhanced olefin selectivity by limiting hydrogenation reactions. Modification of the Mn shell thickness regulated the catalytic activity and olefin selectivity. Simultaneously the weak metal–support interaction further increased the migration ability owing to the utilization of a graphene-based support. Space velocities, pressures and operating temperatures were also tested in the reactor to further enhance light olefin production. A balanced Mn shell thickness produced with a Mn concentration of 16 mol Mn/100 mol Fe was found to give a good olefin yield of 19% with an olefin/paraffin (O/P) ratio of 0.77. Higher Mn concentrations shielded the active sites and reduced the conversion dramatically, causing a fall in olefin production. The optimum operating conditions were found to be 300 °C, 2 MPa and 4.2 L g−1 h−1 of 1 : 1 H2 : CO syngas flow; these gave the olefin yield of 19%.

Production of Higher Hydrocarbons from CO2 over Nanosized Iron Catalysts

AbstractThe effects of the particle size of a Fe/Cu/K catalyst on CO and CO2 hydrogenation reactions as well as the variation of crucial factors such as surface area and basicity, reduction, carburization, and catalytic behavior of precipitated Fe/Cu/K catalysts were evaluated. Hematite nanoparticle catalysts with various surface tensions were produced by homogeneous precipitation in alcohol/water solvents. The basicity of the K‐promoted iron catalyst was higher in iron catalysts with lower particle size. The increase in K‐basic sites at the surface of catalysts with smaller particle size was attributed to their higher surface areas. Elevation of catalyst basicity led to considerably stronger dissociative CO adsorption. Shifting the oxygen removal pattern to lower temperature was the consequence of faster nucleation of FeCx crystallites on promoted surface oxides. CO2 hydrogenation can occur in two distinct direct and indirect routes via the Fischer‐Tropsch mechanism.

Ga-promoted CO insertion and C–C coupling on Co catalysts for the synthesis of ethanol and higher alcohols from syngas

This work focuses on Ga promotion to Co sites in supported CoGa catalysts on CO insertion and C–C coupling in syngas conversion. The CoGa catalysts were developed in our group for syngas conversion; they possess uniform dispersion of CoGa particles and pseudo-homogeneous distribution of Co and Ga elements. The Ga atoms contiguous to Co atoms not only contributed to isolating Co centers but also donated electrons to neighboring Co sites. The isolated Co sites were responsible for linearly nondissociative CO adsorption, boosting the CO insertion step to afford alcohol products. Electronic donation from Ga to Co sites promoted dissociative CO adsorption, enhancing carbon-chain growth. The match of CO insertion and C–C coupling gave rise to both enhanced activity in the syngas conversion and impressive selectivity to ethanol and higher alcohols.

CO Oxidation over Nanostructured Ceria Supported Bimetallic Cu–Mn Oxides Catalysts: Effect of Cu/Mn Ratio and Calcination Temperature

Nanostructured CeO2 supported CuO–MnO2 (CuMn/Ce) catalysts were synthesized by wet co-impregnation method and compared with CuO/CeO2 and MnO2/CeO2 catalysts on CO oxidation. The former shows significantly higher catalytic activity toward CO oxidation than the bimetallic oxides catalysts with minimum T50. Dual synergistic effects in the interfaces of CuO–MnO2 and CuO–CeO2 shall be responsible for the higher oxidation performance of CuMn/Ce catalyst. As Cu/Mn mass ratio varies from 1/0.2 to 1/9, CO oxidation activities of the catalysts present unimodal distribution, and the catalyst with Cu/Mn = 1/1 shows the highest performance (T50 = 45.8 °C; Ea = 24.11 kJ mol−1). Cu acts as CO dissociative adsorption sites, and Mn–Ce supplies activated O species from gas molecule and lattice oxygen, which are both indispensable for improving utilization efficiency of the reactants in the dual redox processes. The catalytic performance of catalyst is also vitally correlated with calcination temperature (160–320 °C). The optimum calcination temperature of CuMn/Ce catalyst is 320 °C. Catalyst characterization results indicate that the pretreatment at 320 °C is decent to obtain desired metal oxides along with fine texture properties.

High coverage water adsorption on the CuO(111) surface

Water adsorption on the CuO(111) surface at different coverage has been systematically studied by spin-polarized density functional theory calculations (GGA+U) and atomic thermodynamics. Both molecular and dissociative adsorptions are studied. It is found that H2O molecule can chirally adsorb on CuO(111) surface which maybe play an important role in catalysis field. Molecular H2O adsorption is preferred for one H2O molecule, while mixed molecular and dissociative coadsorption is preferred on CuO(111) surface for two, three and four H2O molecules. Molecular and dissociative H2O adsorption can coexist at high temperature by using Boltzmann statistics. The phase diagram shows that only three surface configurations are stable thermodynamically: clean CuO(111) surface, two H2O and four H2O adsorption. The surface uncoordinated copper and oxygen atoms, and hydrogen bonding contribute to the adsorption energies. H2O adsorption mechanism has been analyzed by projected density of states (PDOS).

High coverage water adsorption on CuO(011) surface.

Spin-polarized density functional theory calculations (GGA+U) and atomic thermodynamics have been used to study the adsorption of water on the CuO(011) surface at different coverages. It was found that H2O molecular adsorption on CuO(011) surface is energetically favorable for one H2O molecule, but dissociative adsorption is preferred for two and three molecules, while a mixed molecular and dissociative coadsorption is favorable for four water molecules. The phase diagram of water adsorption on the CuO(011) surface shows that the adsorption of three and four water molecules is favorable thermodynamically. Different single-water adsorption states were analyzed by the Boltzmann model at different temperatures. The adsorption energy is contributed to by the surface uncoordinated copper and oxygen atoms, and by hydrogen chemical bonding. The energetic trends are related to the underlying electronic mechanisms.

Size-Specific, Dissociative Activation of Carbon Dioxide by Cobalt Cluster Anions

The reaction of cobalt cluster anions Con– (3 ≤ n ≤ 17) with CO2 was studied experimentally and theoretically to explore the size-specific activation mode of CO2 by Con–. Mass spectrometric measurements revealed that the reactivity depends strongly on cluster size: the reactivity emerges abruptly at n = 7, peaks at n = 8–10, and then gradually decreases with increasing n. Infrared multiple photon dissociation spectra of ConCO2– exhibit a single peak at ∼1870 cm–1, similarly to the previously reported spectra of ConCO–. Density functional theory calculations for Co7CO2– as an example revealed that the dissociative adsorption of CO2 into CO and O is energetically more favorable than nondissociative adsorption. The infrared spectra calculated for dissociated isomers Co7(CO)O– reproduced the experimental results, whereas those for nondissociated isomers Co7CO2– did not. The photoelectron spectra of ConCO2– were shifted dramatically toward higher energies relative to those of Con–, suggesting electron transfer...

Size and Promoter Effects in Supported Iron Fischer–Tropsch Catalysts: Insights from Experiment and Theory

The fundamentals of structure sensitivity and promoter effects in the Fischer–Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12CO to 13CO and H2 to D2, was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2–7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH4 response curves revealed the presence of parallel side-pools of reacting carbon. CHx coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CHx coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppress...

Transition metal promoted K/Mo2C as efficient catalysts for CO hydrogenation to higher alcohols

The transition metals strongly interact with molybdenum, forming mixed carbidic phases, that are likely responsible for the enhanced higher alcohol formation. These phases serve as active sites for molecular rather than dissociative adsorption of CO and offer a balance of non-dissociated and dissociated CO species on the surface.

Effect of sulfur on α-Al2O3-supported iron catalyst for Fischer–Tropsch synthesis

The effects of sulfur modification (S/Fe=0.001∼0.05, molar ratio) on the α-Al2O3-supported iron catalysts for the Fischer–Tropsch synthesis (FTS) were investigated. It was found that the presence of sulfur promoted the reduction of FeO to metallic Fe during the H2 reduction process, but decreased the rates of reduction, carburization and carbon deposition of the catalysts when CO is used as the reducing agent. Both the sulfide (S2−) and sulfate species (SO42-) are found in the SxFe/α-Al2O3 catalysts after H2 reduction and FTS reaction. The sulfide species in the H2-reduced SxFe/α-Al2O3 catalysts are located on the surface of metallic Fe particles. The intensity of H2 desorption peak around 500K is enhanced by a small amount of sulfur, while an excess of sulfur suppresses the H2 adsorption. The dissociative adsorption of CO on H2-reduced catalysts was also found to be inhibited by the presence of sulfur. The activity of the Fe/α-Al2O3 catalyst can be decreased significantly by the addition of sulfur due to the inhibition of CO dissociation and thus reduce the formation of iron carbide phases under FTS reaction conditions. Resistance to sulfur poisoning of the Fe/α-Al2O3 catalyst was found to improve with increasing the reaction temperature. The presence of sulfur also suppressed the formation of C5+ hydrocarbons and shifted the products to C2–C4 hydrocarbons. At the same time, the olefin to paraffin (O/P) ratio of the C2–C4 hydrocarbons decreased with increasing S/Fe molar ratio. These may have resulted from the increasing of the H/C ratio on the surface of sulfur modified catalyst under FTS conditions.

A first-principles study of CO dissociative adsorption on iron nanoparticles supported on doped graphene

Abstract We study Fe 13 nanoparticles supported on doped graphene and investigate the dissociative adsorption of CO on the nanoparticles using first-principle calculations. It is found that boron doping enhances the binding energy of Fe 13 on the graphene but nitrogen doping reduces it. We show that difference in the work-function and subsequently in the charge transfer causes such behavior in the binding energies. Calculated d -band width and d -band center are well correlated with the Fe binding energy, mostly because of the orbital hybridization effect. We also show that the dissociative adsorption of CO on the Fe-graphene substrate is strongly correlated with the d- band center, which is in turn modulated by the doping concentration.

High Alcohols Synthesis via Fischer–Tropsch Reaction at Cobalt Metal/Carbide Interface

Utilization of nonprecious transition metals for high alcohols synthesis is of a great importance in heterogeneous catalysis. We synthesized successfully cobalt metal-carbide (Co–Co2C) catalysts, which present remarkable activity and selectivity for high alpha-alcohols via the Fischer–Tropsch reaction. The formation of the stable cobalt carbide and the Co–Co2C interface are found to be essential for the observed reactivity. Density functional theory calculations show that Co2C is highly efficient for CO nondissociative adsorption, behaving as noble-metal-like, whereas the Co metal is highly active for CO dissociative adsorption and the subsequent carbon-chain growth. The interface between the cobalt metal and its carbide phase, as well as the dual sites available at the interface for facile CO insertion to hydrocarbon, could be used to rationalize the design of the nonprecious transition metal catalysts for the oxygenates in syngas conversion.

Adsorption and oxidation of carbon monoxide on Au and Ni nanoparticles deposited on Al2O3 by laser electrodispersion

Dissociative adsorption of CO on Au and Ni nanoparticles deposited on Al2O3 by laser electrodispersion (LED) was for the first time observed using diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The average particle size was 4.0 nm for the Au nanoparticles and 1.5–2.0 nm for the Ni nanoparticles. The process on alumina-supported Au nanoparticles of similar size, prepared by ion exchange proceeds with lower efficiency. The Ni/Al2O3 catalyst prepared by LED exhibits unusually high activity in the oxidation of CO at T > 600 K. When the catalyst was reused, the temperature of CO oxidation decreased to 450 K. The activity of the Au- and Ni-catalysts prepared by LED is higher than that of the catalysts of identical composition prepared by traditional methods, namely, ion exchange and impregnation. The DRIFT and X-ray photoelectron spectroscopy (XPS) data are used to analyze the structure of the catalysts prepared by LED that is associated with specific features of their adsorption and catalytic properties.