Study Of CO Hydrogenation Research Articles

Transient In situ DRIFTS Investigation of CO2 Hydrogenation to Methanol over Unsupported CuGa Catalysts

AbstractGallium oxide‐based catalysts are promising candidates for CO2 hydrogenation to methanol, whilst mechanistic understanding of the catalytic systems is not yet fully understood. Here, transient in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies of CO2 hydrogenation over the unsupported CuGa and CuGaN (with N doping) catalysts were conducted. The findings show that N doping could possibly promote the formation of surface Cu+ sites on the interface between Cu and Ga2O3, and hence increasing the selectivity to methanol. However, N doping prohibited the reduction of Ga2O3 to oxygen‐deficient Ga2O3‐x, which is unfavorable for CO2 conversion. Additionally, during DRIFTS the steady‐state isotope transient kinetic analysis (SSITKA) was performed (under H2/D2 isotopic switching conditions). The SSITKA results confirm that the Cu+‐bound formates were likely the key intermediates for methanol synthesis over the catalyst, and the N doping could also weak the interaction between CO* species and the catalyst surface, and thus facilitating the selectivity towards methanol rather than CO. Findings of the work show that the partial nitridation of the metal oxides could be the strategy to promote methanol synthesis selectivity.

A study of CO2 hydrogenation over Ni-MgAlOx catalysts derived from hydrotalcite precursors

Ni/Mg/Al mixed oxides have been prepared by decomposing corresponding layered double hydroxides of the hydrotalcite family. XRD, FTIR and UV–vis–NIR analyses show that prepared materials are constituted by a rock-salt type Mg1−xNixO solid solution with Al3+ in tetrahedral interstices of the cube close packing of oxide anions. When activated at sufficiently high temperature, they convert into optimal size supported Ni metal catalysts which show very active and selective for CO2 methanation. Catalytic data show that these materials are more active at low temperature than Ni/γ-Al2O3 catalysts, with a comparable enhanced activity with respect to Ni/La2O3-Al2O3 ones. A role of basic oxides as activating components can be envisaged and related to the strength of the adsorption of CO2 on the “support” likely forming surface (bi)carbonates as active species. Activation energies and reactions orders have been calculated by flow reactor studies in differential reactor conditions. IR spectroscopy data show that carbon dioxide adsorbs on the reduced catalyst in the form of hydroxycarbonates, that convert at higher temperature in carbonates and strongly adsorbed linear and bridging carbonyl species on metallic nickel. The formation of surface CHx species is also evident.

Hydrogenation of CO2 over 15% Fe/SiO2 Catalyst under Sub- and Supercritical Conditions

The results of comparative studies of CO2hydrogenation over 15% Fe/SiO2catalyst under sub- and supercritical conditions are presented for the first time. The reaction was studied at 300–500 °C and atmospheric pressure in gas phase and at 95 atm under supercritical conditions. The molar H2: CO2ratio was 2 : 1. Under supercritical conditions, the selectivity to CO2decreased from 90–95 to 30–50 % at all temperatures, while the selectivity to hydrocarbons increased up to 60 %. The reaction under supercritical conditions, unlike gas-phase hydrogenation, produced alcohols. TG-DTG-DTA techniques were used to demonstrate 2.2-fold decrease in the quantity of carbon-like deposits in comparison to that in the gas-phase reaction. XRD studies revealed the formation of graphite-like species on the catalyst surface under gas-phase but not supercritical conditions. The developed process and catalyst for hydrogenation of CO2can be recommended to be further modified in order to improve the catalyst based on iron nanoparticles that is as expensive as 0.1–0.01 of the known catalysts for CO2hydrogenation.

Hydrogenation of CO to Methanol on Ni(110) through Subsurface Hydrogen.

We present a combined theoretical and experimental study of CO hydrogenation on a Ni(110) surface, including studies of the role of gas-phase atomic hydrogen, surface hydrogen, and subsurface hydrogen reacting with adsorbed CO. Reaction mechanisms leading both to methane and methanol are considered. In the reaction involving surface or subsurface hydrogen, we investigate four possible pathways, using density functional theory to characterize the relative energetics of each intermediate, including the importance of further hydrogenation versus C-O bond breaking, where the latter may lead to methane production. The most energetically favorable outcome is the production of methanol along a pathway involving the sequential hydrogenation of CO to a H3CO* intermediate, followed by a final hydrogenation to give methanol. In addition, we find that subsurface hydrogen noticeably alters reaction barriers, both passively and through the energy released by diffusion to the surface. Indeed, the effective reaction barriers are even lower than for CO methanolation on Cu(211) and Cu(111) than for Ni(110). In studies of gas-phase H atoms impinging on a CO-adsorbed Ni(110) surface, Born-Oppenheimer molecular dynamics simulations show that direct impact of H is unlikely to result in hydrogenation of CO. This means that Eley-Rideal or hot-atom mechanisms are not important; thus, thermal reactions involving subsurface hydrogen are the primary reaction mechanisms leading to methanol. Finally, we demonstrate experimentally for the first time the production of methanol and formaldehyde from CO hydrogenation on Ni(110) and confirm the role of subsurface hydrogen in the mechanism of this reaction.

H-atom addition and abstraction reactions in mixed CO, H2CO and CH3OH ices – an extended view on complex organic molecule formation

Complex organic molecules (COMs) have been observed not only in the hot cores surrounding low- and high- mass protostars, but also in cold dark clouds. Therefore, it is interesting to understand how such species can be formed without the presence of embedded energy sources. We present new laboratory experiments on the low-temperature solid state formation of three complex molecules: methyl formate (HC(O)OCH3), glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH), through recombination of free radicals formed via H-atom addition and abstraction reactions at different stages in the CO-H2CO-CH3OH hydrogenation network at 15 K. The experiments extend previous CO hydrogenation studies and aim at resembling the physical&chemical conditions typical of the CO freeze-out stage in dark molecular clouds, when H2CO and CH3OH form by recombination of accreting CO molecules and H-atoms on ice grains. We confirm that H2CO, once formed through CO hydrogenation, not only yields CH3OH through ongoing H-atom addition reactions, but is also subject to H-atom-induced abstraction reactions, yielding CO again. In a similar way, H2CO is also formed in abstraction reactions involving CH3OH. The dominant methanol H-atom abstraction product is expected to be CH2OH, while H-atom additions to H2CO should at least partially proceed through CH3O intermediate radicals. The occurrence of H-atom abstraction reactions in ice mantles leads to more reactive intermediates (HCO, CH3O and CH2OH) than previously thought, when assuming sequential H-atom addition reactions only. This enhances the probability to form COMs through radical-radical recombination without the need of UV photolysis or cosmic rays as external triggers.

The application of a supported palladium catalyst for the hydrogenation of aromatic nitriles

The use of a Pd/C catalyst in the liquid phase hydrogenation of various aromatic nitriles (benzonitrile, benzyl cyanide, 3-phenyl propionitrile and cinnamonitrile) has been studied in order to assess the effectiveness of this type of catalyst for this class of reaction. On modifying the nitrile substituent and upon introducing conjugation, varying degrees of conversion are observed. For benzyl cyanide and 3-phenylpropionitrile, incomplete mass balance profiles are linked to spill-over to the carbon support. In the case of benzonitrile hydrogenation, a hydrogenolytic step leads to a loss of selectivity to the primary amine to yield toluene with, ultimately, complete selectivity. Co-hydrogenation measurements on mixtures of benzonitrile and benzylamine indicate the presence of site-selective chemistry. Co-hydrogenation studies on mixtures of benzonitrile and benzyl cyanide highlight the competitive nature of the reaction system and, indirectly, establish a contribution from adsorbed imine species.

Combined quantum chemical and modeling study of CO hydrogenation on water ice

Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most e cient mechanisms in interstellar environments for the formation of H2CO and CH3OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores. Aims. We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters. Methods. The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules. Results. Our main result is that the interaction of CO and H2CO with the water cluster surfaces through H-bonds with the O atoms increases the C O polarization, thus weakening the C O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H2CO and CH3OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.

Study of syngas conversion to light olefins by statistical models

This paper presents the application of the statistical models in CO hydrogenation to light olefins studies. The effect of support content as a structural variable of a precipitated Fe–Co–Mn catalyst in conjunction with two reactor variables, temperature and total pressure was investigated via an Orthogonal Second-order Design. A quadratic polynomial was successfully fitted to the experimental data and the significance of regression coefficients was statistically investigated by t-student test. The results were also compared with simulated artificial neural network results that was constructed and trained based on data collected according to the experimental designs. The response surface model (RSM) simulated data were also employed to kinetic study of CO hydrogenation under condition of support 20%, temperature 340–380°C, pressure 3–7bar and a rate equation based on direct reaction of hydrogen from gas phase with molecularly adsorbed CO were proposed. The estimated activation energy under above conditions was 40kJ/mol.

A first-principles study of CO hydrogenation into methane on molybdenum carbides catalysts

The reaction mechanisms for the CO hydrogenation to produce CH4 on both fcc-Mo2C (100) and hcp-Mo2C (101) surfaces are investigated using density functional theory calculations with the periodic slab model. Through systematic calculations for the mechanisms of the CO hydrogenation on the two surfaces, we found that the reaction mechanisms are the same on both fcc and hcp Mo2C catalysts, that is, CO→HCO→H2CO→H2COH→CH2→CH3→CH4. The activation energy of the rate-determining step (CH3+H→CH4) on fcc-Mo2C (100) (0.84eV) is lower than that on hcp-Mo2C (101) (1.20eV), and that is why catalytic activity of fcc-Mo2C is higher than hcp-Mo2C for CO hydrogenation. Our calculated results are consistent with the experimental observations. The activity difference of these two surfaces mainly comes from the co-adsorption energy difference between initial state (IS) and transition state (TS), that is, the co-adsorption energy difference between IS and TS is −0.04eV on fcc Mo2C (100), while it is as high as 0.68eV on hcp Mo2C (101), and thus leading to the lower activation barrier for the reaction of CH3+H→CH4 on fcc-Mo2C (100) compared to that of hcp-Mo2C (101).

Kinetic study of CO hydrogenation on the MgO supported Fe–Co–Mn sol–gel catalyst

The kinetic of the Fischer–Tropsch synthesis over the MgO supported Fe–Co–Mn catalyst prepared using sol–gel procedure, was investigated in a fixed bed micro-reactor. Experimental conditions were varied as follow: reaction pressure 5–20bar, reaction temperature 220–250°C, H2/CO feed molar ratio of 0.67–2 and space velocity range of 2400–3600h−1. 18 models according to the Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equation were derived, and the reaction rate is fitted fairly well by one kinetic expressions based on LHWW mechanism. The kinetic parameters were estimated with non-linear regression method. The activation energy was obtained 110.9kJ/mol for the best-fitted model.

Kinetic study of CO hydrogenation over co-precipitated iron–nickel catalyst

The kinetic of the Fischer–Tropsch synthesis over a Fe–Ni/Al2O3 catalyst was investigated in a fixed bed micro reactor. Experimental conditions were varied as follow: reaction pressure 2–10bar, H2/CO feed ratio of 2/1 and space velocity of 96–450cm3(STP)/h/gramcatalyst at the temperature range 523–573K. On the basis of carbide-enol mechanism and Langmuir–Hinshelwood–Hougen–Watson (LHHW) type rate equations, seventeen kinetic expressions for CO consumption were tested and interaction between adsorption HCO and dissociated adsorption hydrogen as the controlling step gave the most plausible kinetic model. The activation energy was 46.5kJ/mole for optimal kinetic model.

Kinetics study of CO hydrogenation on a precipitated iron catalyst

The kinetics of the gas–solid Fischer–Tropsch synthesis over a precipitated Fe/Cu/La/SiO2 catalyst was studied in a well mixed, continuous spinning basket reactor. A wide range of synthesis gas conversions have been obtained by varying experimental conditions. Several Langmuir–Hinshelwood–Hougen–Watson type rate equations were derived based on detailed sets of possible reaction mechanisms originating from the carbide, enolic and combined enol/carbide mechanisms. Three models for the Fischer–Tropsch reaction rate were fitted to the experimental reaction rates. Kinetic parameters of models are determined using the genetic algorithm approach (GA), followed by the Levenberg–Marquardt (LM) method to make refined optimization, and are validated by means of statistical analysis. Simulations using the optimal kinetic models derived showed good agreement with experimental data.

Ab Initio Study of CO Hydrogenation to Oxygenates on Reduced Rh Terraces and Stepped Surfaces

Previously reported syngas conversion experiments on silica-supported Rh nanoparticles show that CO conversion and oxygenate selectivity vary as a function of nanoparticle size. Theoretical studies in the literature have examined the effect of steps on CO dissociation, but structure sensitivity for C1 and C2 oxygenates has not been systematically investigated. In this study, density functional theory-based reaction energetics and kinetics for C−H, C−C, C−O, and O−H bond formation on flat Rh(111) and stepped Rh(211) surfaces are reported and compared. Multiple paths for methanol and ethanol formation are considered to ascertain the lowest energy pathways. Nearly an identical methanol formation route via CO → CHO → CH2O → CH3O → CH3OH is found to be favored on both Rh terrace and (211) sites. CO insertion into CH2 is deduced to be the precursor for C2 oxygenate formation irrespective of site structure. Ethanol formation pathways, however, are determined to be markedly different on flat and stepped Rh surfac...

Effect of potassium addition on the product selectivity of Ru:Mn/alumina supported catalyst system for Fischer–Tropsch reaction

The catalytic behavior of a series of Ru/Al 2 O 3 catalysts promoted with various levels of Mn and K prepared by co-impregnation method were investigated in the CO hydrogenation reaction. Catalytic studies showed that the addition of Mn enhances the production of Fischer–Tropsch products and K promotion in particular enhances the production of unsaturated hydrocarbons while inhibiting the methanation reaction. The influence of K and Mn on the Fischer–Tropsch product selectivity in CO hydrogenation studies is discussed in terms of ensemble and electronic effect; and the production of new surface site Ru:Mn:K in accordance with our characterization results.

CO Hydrogenation on Ru-Promoted Co/MCM-41 Catalysts

A study of CO hydrogenation on MCM-41 and SiO 2-supported Ru-promoted Co catalysts has been performed using both global rate measurements and steady-state isotopic transient kinetic analysis (SSITKA). A significant increase in CO hydrogenation was observed when MCM-41 was used as the support in the order M1>M2>SiO 2 at methanation conditions, where M1 and M2 are small pore ( d p=3 nm) and larger pore ( d p=7 nm) MCM-41, respectively. TOF H based on H 2 chemisorption can be misleading since any suppression of hydrogen chemisorption, as probably occurred for CoRu/M1, results in larger values of TOF H being calculated. SSITKA results, on the other hand, provide a measure of the “true” intrinsic CO hydrogenation activity of the Co sites. SSITKA studies indicated that the higher CO hydrogenation rates for MCM-41-supported catalysts are due to their greater number of active sites, not to a change in the intrinsic site activity. N 2 physisorption and XRD results reveal that the structure of MCM-41 became less ordered and the surface area decreased after standard reduction and 24 h of FTS, probably due to an effect of water vapor produced during metal reduction and reaction. However, the Co surface was still accessible since there was no significant loss of activity with TOS. The extent of deactivation during the initial reaction period was similar for MCM-41- and SiO 2-supported CoRu catalysts. By providing high activity and unrestricted diffusion of FT reactants and products, CoRu/MCM-41 may be potentially useful for FTS as well as for other catalytic applications, although modified forms of MCM-41 will probably be required in order to increase its hydrothermal stability.

Hydrogenation of carbon dioxide catalyzed by ruthenium trimethylphosphine complexes — Effect of gas pressure and additives on rate in the liquid phase

Kinetic and mechanistic studies of CO2 hydrogenation were performed in liquid triethylamine and at subcritical CO2 pressures to avoid complications from phase behaviour that are observed under supercritical conditions. Kinetic measurements of the hydrogenation of CO2 to formic acid, catalyzed by RuCl(O2CMe)(PMe3)4, support a CO2 insertion mechanism. The reaction is first-order in both H2 and CO2 under most conditions. The rate is strongly dependent on the choice of additive, with methanol giving the greatest rates. Because only trace amounts of methanol are needed, the effect of the additive is believed to involve direct interactions with the catalyst rather than changes in the physical properties of the reaction medium. The optimized rates exceed 3500 h–1. Addition of an inert gas affects the rate of the reaction, probably via the phenomenon of gas expansion of the liquid phase.Key words: carbon dioxide, homogeneous catalysis, hydrogenation, formic acid, expanded liquids.

CO hydrogenation using supported iron carbonyl complexes

Carbon monoxide hydrogenation using catalysts prepared via thermal decomposition of [(η 5–C 5H 5)Fe(CO) 2M(CO) 5], M = Mn, Re supported on γ-alumina is described and discussed. The thermal decomposition of the complex is characterised using FTIR spectroscopy and thermal analysis and involves the formation of ferrocene in addition to small metal particles supported on γ-alumina. CO hydrogenation studies indicate that the addition of Mn decreases the yield of C 1 products (methane and methanol) and increases catalyst activity. In contrast, addition of Re enhances the hydrogenation activity of the catalyst and decreases the probability of chain propagation. Carbon monoxide hydrogenation over a series of catalysts prepared via the thermal decomposition of [(η 5–C 5H 5)Fe(CO) 2] 2–{μ-(CH 2) n }, n = 0, 3, 5 was also studied and the catalyst activity was observed to decrease with increasing n. The catalyst having n = 3 gave the lowest yield of C 1 products for these iron containing catalysts.

Effect of La3+Promotion of Co/SiO2on CO Hydrogenation

A study of CO hydrogenation on La3+-promoted Co/SiO2catalysts (La/Co atomic ratios of 0–0.75) has been performed using both global rate measurements and steady-state isotopic transient kinetic analysis (SSITKA). Aqueous impregnation (the means of adding La3+) of Co/SiO2led to a significant decrease in the number of intermediates leading to methane during CO hydrogenation, but it did not alter the nature of the active sites. In a previous characterization paper (19) La3+promotion of Co/SiO2was shown to moderate the strong Co–support interactions brought about by aqueous impregnation of the Co/SiO2and to enhance the reducibility of the Co oxide. La3+addition to the Co/SiO2catalyst was found to lead to a significant increase in the activity for Fischer–Tropsch synthesis (on a per gram Co basis). The SSITKA results showed that the concentration of active intermediates leading to methane increased with increase in La/Co ratio. However, the TOF, the pseudo-first-order rate constant, the reactivity distribution function, and the activation energy were not affected by La3+addition. Thus, it can be concluded that La3+did not change the nature of the active sites, just their concentration. This increase in the concentration of active sites follows exactly the increase in surface Co0. La3+promotion (La/Co≥0.1) also caused the chain growth probability and the olefin/paraffin ratio to increase significantly at 220°C and for H2/CO=2.

A high pressure, high temperature infrared study of CO hydrogenation over Rh/ZrO 2

A Rh/ZrO 2 catalyst has been studied in the hydrogenation of CO in an in situ IR cell operating at high temperature and pressure, and using FTIR to follow the formation of surface and gas phase species as a function of reaction rime. The reaction between CO and hydrogen was studied by varying the catalyst reduction temperature, reaction temperature, total pressure and hydrogen/CO ratio. Surface species identified were carbonyls on the rhodium surface, formates, carbonates and hydrocarbons. The relationship between adsorbed carbonate and formate is discussed in detail.

Monte Carlo study of CO hydrogenation on cobalt model catalysts

Useful information on catalytic reactions can be achieved using Monte Carlo simulations combined with experimental data from model catalysts. We present a comprehensive analysis of the simulation studies of CO hydrogenation on a hexagonal surface using a discrete computer model for the irreversible reaction kinetics with no interactions between the surface species but their mutual reactions. The simulation results are compared to experimental data from a cobalt foil model catalyst at 101 kPa and 525 K. As a result, the following microscopic picture of the reaction on the catalyst surface is extracted: the rate-limiting reaction step is the termination of the carbon chains (α-hydrogenation), hydrogen atoms occupy different adsorption sites from other reactants, and the diffusion of hydrogen along the surface is fast. The model is also used to address the relevance of the ensemble effects for CO dissociation and the surface sensitivity of the CO hydrogenation reaction. Our simulation results imply that these aspects have little effect on the rates of hydrocarbon formation.