Dissociative Adsorption Of CO Research Articles

Coverage-Dependent CO Adsorption and Dissociation Mechanisms on Iron Surfaces from DFT Computations

CO adsorption structures and energetics on the iron (100), (110), (111), (210), (211), and (310) surfaces from the lowest coverage up to saturation have been computed using spin-polarized density functional theory and ab initio thermodynamics. It is found that different adsorption configurations on each of these surfaces at high coverage can coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. Only molecular CO adsorption is possible at very high coverage and only dissociative CO adsorption at very low coverage, whereas mixed molecular and dissociative CO adsorption becomes possible at medium coverage. The computed stable adsorption configurations and the respective C–O and Fe–C stretching frequencies as well as desorption temperatures on the (100), (110), and (111) surfaces agree very well with the available experimental data. Such agreements between theory and experiment validate our co...

Promotion Effects of Nitrogen Doping into Carbon Nanotubes on Supported Iron Fischer–Tropsch Catalysts for Lower Olefins

Fischer–Tropsch synthesis (FTS) is a classical topic of great significance because of the approach of post-petroleum times. For decades, people have attempted to develop iron-based FTS catalysts with high selectivity for lower olefins. By means of the anchoring effect and the intrinsic basicity of nitrogen-doped carbon nanotubes (NCNTs), iron nanoparticles were conveniently immobilized on NCNTs without surface premodification. The so-constructed Fe/NCNTs catalyst presents superb catalytic performance in FTS with high selectivity for lower olefins of up to 46.7% as well as high activity and stability. The excellent performance is well-correlated with enhanced dissociative CO adsorption, inhibition of secondary hydrogenation of lower olefins, and promoted formation of the active phase of χ-Fe5C2. All of these merits result from participation of the nitrogen, as revealed by our experimental characterization. These results may lead to a new strategy for exploring advanced FTS catalysts with abundant N-doped c...

Composition, structure, and sorbability of the thermally treated water deironing precipitate with respect to carbon monoxide

This work deals with studying the composition, structure, and sorbability of the water deironing precipitate sample thermally treated at 850 °C (WDP-850) with respect to carbon monoxide . According to the data of X-ray fluorescence (XRF) spectroscopy, WDP-850 contains 44.14% of iron, 6.29% of silicon, and 34.78% of oxygen. The results of X-ray diffraction (XRD) analysis indicate the presence of Fe 2 O 3 , Fe 3 O 4 , and SiO 2 nanocrystallites (~ 11.4, 30.6, and 8.0%, respectively). The studies performed via the temperature-programmed desorption of CO show that WDP-850 can sorb carbon monoxide at 25 and 200 °C. The composition and structure of the precipitate sample obtained after CO adsorption at 200 °C (WDP-850-CO) have also been studied. According to XRD data, nearly 9.3% of the silicon carbide (SiC) phase with the moissanite's structure has appeared in the material. The SiC phase is supposed to result from the dissociative adsorption of CO on the surface of WDP-850. It has been concluded that WDP can be used as a base for the synthesis of functional materials, for example, CO sorbents . • The water deironing precipitate (WDP) contained Fe 2 O 3 , Fe 3 O 4 , and SiO 2 nanocrystallites. • The studied precipitate was established to be capable for sorption of CO. • SiC crystallites were identified in the sample of WDP treated with carbon monoxide.

Kinetic Models for Fischer-Tropsch Synthesis over a Ru-Promoted Co/Al2O3Catalyst in a Slurry Reactor

Increasing oil prices, strict environmental regulations and lack of sufficient growth in renewable energy sector have led to renewed interest in Fischer- Tropsch technology. Improved understanding of reaction mechanisms and development of detailed kinetic models for Fischer-Tropsch synthesis (FTS) would facilitate better design and optimization of all FTS reactor configurations. In this work, detailed kinetic models have been developed utilizing mechanistic approach. Experiments were conducted over 25% Co/0.27%Ru/Al2O3 (in parts by weight) catalyst in a 1L stirred tank slurry reactor over a wide range of conditions. Langmuir-Hinshelwood-Hougen-Watson type rate expressions were derived for the entire product spectrum. Models are based on the assumption that 1-olefins re-adsorb on active sites. Effective pressure of olefin (PCnH2n*) at the catalyst surface was assumed to vary exponentially with carbon number (according to Henry's law). The genetic algorithm followed by Levenberg-Marquardt method was used to estimate kinetic parameters for 13 models using a single set of process conditions (i.e. T = 220°C, P = 2.4 MPa, H2/CO feed ratio of 2.1, and gas space velocity of 6 NL/g-cat/h). Two models FT-6 and FT-8 showed carbon number dependent chain growth probability and olefin to paraffin ratios. The model predictions were in good agreement with experimental data. Model FT-6 is based on dissociative adsorption of CO, followed by Eley-Rideal reaction with molecular H2, while FT-8 follows dissociative adsorption of CO and H2, to form building block monomer CH2. For both of the models, chain growth takes place by alkyl mechanism and oE-olefins are formed by aB-hydride elimination reaction. Formation of paraffin occurs via single-site reaction with molecular hydrogen (FT-6) or via dual site reaction with adsorbed hydrogen (FT-8). Parameter estimation resulted in at least one negative parameter in both models. However, to assess the physical meaningfulness of results and the true values of kinetic parameters, one has to use data at multiple sets of process conditions. This work is in progress. Nevertheless, from a qualitative point of view initial results provide valuable insight into selection of reaction mechanisms and rate determining steps for future developments and refinements.

Kinetics of the Fischer-Tropsch Reaction Over Alumina Supported Cobalt Catalyst in a Slurry Reactor

Abstract Depleting oil reserves, environmental pressure, as well as abundant reserves of coal, natural gas and biomass, have all contributed to a revived interest in Fischer-Tropsch (F-T) technology for producing ultra-clean, virtually sulfurfree, transportation fuels and chemicals. F-T technology involves conversion of synthesis gas (i.e., a mixture of H2 and CO) to a wide spectrum of hydrocarbons. In this study, the kinetics of the Fischer Tropsch (FT) synthesis reaction over 0.27 % Ru 25 % Co/Al2O3 catalyst was studied in a 1L stirred tank slurry reactor (STSR). Supported cobalt catalysts and slurry reactors are used in commercial processes for natural gas conversion to liquid fuels (e.g. ORYX GTL in Qatar). With known kinetics one can size reactors and predict their performance as a function of process conditions. Experiments were conducted at reactor pressures of 1.4 MPa and 2.4 MPa, temperatures of 205°C and 220°C, H2/CO feed ratios of 1.4 and 2.1 and gas space velocities ranging from 2 to 15 NL/g-cat*h. Langmuir-Hinshelwood-Hougen-Watson (LHHW) type rate equations were derived on the basis of a set of reactions originating from carbide, Eley-Rideal and enolic pathways, and two empirical power laws from the literature were used to describe CO disappearance rates. Model rate laws were fitted to isothermal experimental rates using least-squares nonlinear regression to obtain model parameter values. Physical and statistical tests were used to discriminate between rival models. Optimisations were performed by first applying bounds to obtain realistic values of the parameters and then assumptions were made regarding the degree of adsorption for some species. Finally, nonlinear regression of model rate laws using non-isothermal experimental rates was also performed by applying Arrhenius law-based constraints to obtain physically meaningful results. Goodness of fit for the most physically significant models were compared using qualitative (parity curves) and quantitative (mean absolute relative residuals (MARR), R-square and F-test) analysis. It was found that the model based on carbide mechanism involving dissociative CO and hydrogen adsorption (M1) and the model based on hydrogen-assisted dissociative adsorption of CO followed by hydrogenation of dissociatively adsorbed CO (M3) provided the best fit to the experimental data.

Mechanistic aspects of the selective methanation of CO over Ru/TiO 2 catalyst

The mechanism of selective methanation of CO/CO 2 over 5%Ru/TiO 2 catalyst is investigated employing in situ FTIR spectroscopy (DRIFTS) and transient mass spectrometry techniques. It is shown that interaction of the prereduced catalyst with the reaction mixtures results in the development of various Ru-bonded carbonyl species on reduced and partially oxidized sites as well as on sites located at the metal-support interface. The nature and population of these species depend strongly on feed composition and reaction temperature. Results of the present study confirm our previous suggestion that the mechanism of CO methanation includes both dissociative and associative reaction pathways. The former, which dominates at lower reaction temperatures, involves hydrogenation of surface carbon produced by dissociative adsorption of CO, whereas the latter involves hydrogenation of CO species adsorbed at the metal-support interface. At low temperatures, typically <250 °C, dissociation of CO results in accumulation of adsorbed oxygen species which cannot be removed from the catalyst surface, rendering it inactive. Catalytic activity is restored at higher temperatures, where partially oxidized sites are reduced efficiently by adsorbed hydrogen atoms. The associative reaction pathway is the only one which is operable under conditions of CO 2 methanation and proceeds via intermediate formation of carbonyl species at the metal-support interface, produced by the RWGS reaction. Selective methanation of CO in CO/CO 2 mixtures occurs efficiently under conditions where the dissociative reaction pathway is operable and the associative reaction pathway is completely suppressed.

The influence of the potassium promoter on the kinetics and thermodynamics of CO adsorption on a bulk iron catalyst applied in Fischer–Tropsch synthesis: a quantitative adsorption calorimetry, temperature-programmed desorption, and surface hydrogenation study

The adsorption of carbon monoxide on an either unpromoted or potassium-promoted bulk iron catalyst was investigated at 303 K and 613 K by means of pulse chemisorption, adsorption calorimetry, temperature-programmed desorption and temperature-programmed surface reaction in hydrogen. CO was found to adsorb mainly molecularly in the absence of H(2) at 303 K, whereas the presence of H(2) induced CO dissociation at higher temperatures leading to the formation of CH(4) and H(2)O. The hydrogenation of atomic oxygen chemisorbed on metallic iron was found to occur faster than the hydrogenation of atomically adsorbed carbon. At 613 K CO adsorption occurred only dissociatively followed by recombinative CO(2) formation according to C(ads) + 2O(ads)→ CO(2(g)). The presence of the potassium promoter on the catalyst surface led to an increasing strength of the Fe-C bond both at 303 K and 613 K: the initial differential heat of molecular CO adsorption on the pure iron catalyst at 303 K amounted to 102 kJ mol(-1), whereas it increased to 110 kJ mol(-1) on the potassium-promoted sample, and the initial differential heat of dissociative CO adsorption on the unpromoted iron catalyst at 613 K amounted to 165 kJ mol(-1), which increased to 225 kJ mol(-1) in the presence of potassium. The calorimetric CO adsorption experiments also reveal a change of the energetic distribution of the CO adsorption sites present on the catalyst surface induced by the potassium promoter, which was found to block a fraction of the CO adsorption sites.

Mechanistic Study of the Selective Methanation of CO over Ru/TiO2Catalyst: Identification of Active Surface Species and Reaction Pathways

Selective methanation of CO can be used as the final purification step of re-formate gas for the generation of hydrogen-rich gas streams suitable for PEM fuel cell applications. In the present study, the adsorption/desorption characteristics of reactants, the nature of active sites, and the mechanism of CO/CO2 hydrogenation reactions have been investigated over 5% Ru/TiO2 catalyst with the use of in situ FTIR and temperature-programmed techniques. Results show that adsorption of hydrogen and CO are activated processes. The amount of adsorbed hydrogen increases by a factor of 2 with the increase of adsorption temperature from 25 to 200 °C, mainly due to migration of hydrogen atoms from the metal to the support. Adsorbed CO species evolve during TPD in the form of CO2, which is produced via the Boudouard and water−gas shift (WGS) reactions. Adsorption of CO2 occurs via formation of formate and carbonate species associated with the support whereas in the presence of H2 part of these species are converted to Ru-bonded carbonyls via the RWGS reaction at the metal−support interface. Depending on the experimental conditions employed, interaction of CO-containing gas mixtures with the catalyst surface results in the development of a variety of bands in the ν(CO) region attributed to mono- and multicarbonyl species adsorbed on reduced ruthenium crystallites (Rux sites), partially oxidized ruthenium (Run+ sites), and the metal−support interface. Evidence is provided that methanation of CO occurs via two distinct reaction pathways. The first one, which dominates at lower reaction temperatures, involves hydrogenation of surface carbon produced by dissociative adsorption of CO, whereas the second involves hydrogenation of Rux−CO species. The latter pathway is the only one that is operable under conditions of CO2 methanation and proceeds with intermediate formation of Ru-bonded carbonyls at the metal−support interface via the RWGS reaction. Selective methanation of CO in CO/CO2 mixture occurs under conditions where conversion of CO2 is almost completely suppressed because of the kinetically faster hydrogenation of surface carbon, which is produced only in the presence of gas-phase CO.

ChemInform Abstract: Direct Synthesis of Methanol, Dimethyl Ether, and Paraffins from Syngas Over Pd/Zeolite Y Catalysts.

Abstract CO hydrogenation has been studied over Pd/NaY and Pd/HY catalysts prepared by ion exchange. By controlling the calcination program, Pd/NaY catalysts can be tuned to selectively produce either branched C4+ hydrocarbons or CH3OH and CH3OCH3. On Pd/HY catalysts, C2H6 and C3H8 predominate. Dissociative adsorption of CO over small Pd particles leads to formation of methane and an interstitial PdC0.13 compound, detected by XRD. Reduction with H2 converts PdC0.13 to Pd and methane. During CO hydrogenation catalysis, palladium particles grow, leading to local collapse of the zeolite lattice. Pd migration is also manifest from changes in the Pd/Si ratio detected by XPS. The reaction network reveals bifunctional catalysis: formation of CH3OH on Pd particles is followed by its conversion to CH30CH3 and hydrocarbons over acid sites. Readsorption of hydrocarbons on the Pd particles results in hydrogenation of unsaturated compounds and to changes in the activity, selectivity, and deactivation behavior of the catalysts.

Adsorption Properties of the Cu(115) Surface: Basic Interfaces

The interfaces: K/Cu(115) and CO/Cu(115) have been characterized using surface sensitive techniques, including low energy electron diffraction and photoelectron spectroscopy. K adatoms show tendency to occupy the sites close to the step edges. At low temperature (near 125 K), after completion of two layers, potassium grows in 3D islands (the Stranski-Krastanov mode). At higher temperature, e.g. at room temperature, potassium introduces reconstruction of the substrate even at low coverages. Calibration of the alkali coverage, up to completion of the first layer, using the work function changes curve has been confirmed as a very convenient and precise procedure. The adsorbed state of CO at 130 K has been identified by registration of core levels obtained by the use synchrotron radiation photoelectron spectroscopy. The characteristics of the main is and satellite peaks have been analyzed in context of substrate geometry and compared with the ones of other copper planes. There are no indications of dissociative adsorption of CO, only residual carbon and oxygen were found after adsorbate desorption around 220 K. CO molecules show a strong tendency to on top adsorption in sites far from the step edges of the Cu(115) surface.

Fischer−Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K = Rb = Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Hägg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Hägg carbide. A model containing Hägg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests.

Density Functional Studies of the Adsorption and Dissociation of CO2Molecule on Fe(111) Surface

Spin-polarized density functional theory calculation was carried out to characterize the adsorption and dissociation of CO(2) molecule on the Fe(111) surface. It was shown that the barriers for the stepwise CO(2) dissociation reaction, CO(2(g)) --> C(a) + 2O(a), are 21.73 kcal/mol (for OC-O bond activation) and 23.87 kcal/mol (for C-O bond activation), and the entire process is 35.73 kcal/mol exothermic. The rate constants for the dissociative adsorption of CO(2) have been predicted with variational RRKM theory, and the predicted rate constants, k(CO(2)) (in units of cm(3) molecule(-1) s(-1)), can be represented by the equations 2.12 x 10(-8)T(-0.842) exp(-0.258 kcal mol(-1)/RT) at T = 100-1000 K. To gain insights into high catalytic activity of the Fe(111) surface, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.

Density functional study of CO2 adsorption on Pu(100) surface

The adsorption of CO2 on Pu（100） surface has been studied with periodic slab model by revised Perdew-Burke-Ernzerh approximation within the framework of density functional theory. The optimized results of adsorption energies and geometrical structures show that the H-C4O4-type adsorption is optimum adsorption mode with adsorption energy of 1.48 eV. The atomic population and density of states analysis indicate that the interaction between Pu atom and CO2 molecule results mainly from strong electron transfer and weak overlap-hybridization between molecular orbital 2πμ of CO2 molecule and Pu5f, Pu6d and Pu7s orbitals of surface Pu atom. The calculated activation barrier and adsorption energy CO2→CO+O dissociative reaction are 0.66 and 2.65 eV, respectively, which indicates the dissociative adsorption of CO2 on Pu（100） surface is favorable under the certain heat activation condition. The comparison of O2，H2，CO and CO2 adsorption on Pu（100） surface indicates the adsorption strength follows the ordering: O2，CO，CO2，H2 and O2，CO2，CO，H2 at lower and higher temperature, respectively.

Density functional study of CO adsorption on Pu (100) surface

The adsorption of CO on Pu (100) surface were studied by the density functional theory (DFT). The calculation results show that the O-down adsorption is less stable than the C-down adsorption which is found to be a strong chemisorption. The stability of adsorption configuration of CO is hollow tilted &gt; hollow vertical &gt; bridge &gt; on-top. The interaction between Pu atom and CO molecule results mainly from the contribution of hybridized molecular orbital of CO molecule and hybridized orbital of surface Pu atom. A small activation barrier of 0.280eV is found for the dissociation of CO molecule in hollow tilted configuration, which indicates the dissociative adsorption of CO on Pu (100) surface at the lower temperature with dissociated C and O atoms favoring the hollow site of lowest energy.

Biogas to Syngas by Microwave-Assisted Dry Reforming in the Presence of Char

The aim of this paper is to study the reforming of CH4 with CO2 using as a catalyst a rich potassium char obtained from biomass pyrolysis. The reaction was carried out at 800 °C by means of two different methods of heating, microwave and electrical heating (MWH and EH, respectively). In addition, the individual reactions proposed for the dry reforming of methane, that is, (i) the decomposition of CH4 to form hydrogen and carbon and (ii) the dissociative adsorption of CO2 followed by reduction to give CO, were also studied with both heating methods at the same temperature. The results showed that MWH produces hot spots (microplasmas located inside the catalyst bed) that favor heterogeneous catalytic reactions. Thus, it was found that the conversion of CH4 and CO2 in the individual reactions was greater for MWH than for EH. An examination of the CH4 decomposition reaction proved the formation of coke deposits, which reduced catalytic activity and CH4 conversion. When a CH4/CO2 mixture was used, this problem...

Mechanistic study of the unusual catalytic properties of a new Ni [sbnd]Ce mixed oxide for the CO 2 reforming of methane

The origin of the exceptional properties of a Ni 10Ce 90 mixed oxide relative to a Ni/Al 2O 3 catalyst (i.e., immediate activity without catalyst prereduction, high resistance toward coking, high catalytic activity) and its catalytic deactivation for the CO 2 reforming of methane was investigated by XRD, XPS, pulse experimentation, and regeneration tests. Based on those results, a reaction mechanism is proposed. First, the catalytic property with no prereduction step relies on the reactivity of oxygen contained in the Ni phase. The rapid activation of Ni 10Ce 90 is based on the active interfacial oxygen in the Ni Ce boundary, which leads to the complete oxidation of methane and the creation of active sites for the subsequent reforming reaction, whereas a slow decomposition of NiAl 2O 4 spinel results in a long induction period of the Ni/Al 2O 3 catalyst. Second, the coking resistance as well as the catalytic activity rely on the reactivity of lattice oxygen in the ceria. The ceria lattice oxygen participates in the reforming reaction through migration, which leads to the formation of oxygen vacancies. Subsequently, the vacancies are completely supplemented by oxygen arising from the dissociative adsorption of CO 2. Finally, the stability of Ni 10Ce 90 relies on the balance between the rate of generation and supplement of oxygen vacancies. Unbalanced rates of these processes result in deactivation.

Oxygen adsorption and oxidation reactions on Au(2 1 1) surfaces: Exposures using O 2 at high pressures and ozone (O 3) in UHV

Nanosized gold particles supported on reducible metal oxides have been reported to show high catalytic activity toward CO oxidation at low temperature. This has generated great scientific and technological interest, and there have been many proposals to explain this unusual activity. One intriguing explanation that can be tested is that of Nørskov and coworkers [Catal. Lett. 64 (2000) 101] who suggested that the “unusually large catalytic activity of highly-dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au-support interface”. In particular, their calculations indicated that the Au(2 1 1) stepped surface would be much more reactive towards O 2 dissociative adsorption and CO adsorption than the Au(1 1 1) surface. We have now studied the adsorption of O 2 and O 3 (ozone) on an Au(2 1 1) stepped surface. We find that molecular oxygen (O 2) was not activated to dissociate and produce oxygen adatoms on the stepped Au(2 1 1) surface even under high-pressure (700 Torr) conditions with the sample at 300–450 K. Step sites do bind oxygen adatoms more tightly than do terrace sites, and this was probed by using temperature programmed desorption (TPD) of O 2 following ozone (O 3) exposures to produce oxygen adatoms up to a saturation coverage of θ O = 0.90 ML. In the low-coverage regime ( θ O ⩽ 0.15 ML), the O 2 TPD peak at 540 K, which does not shift with coverage, is attributed to oxygen adatoms that are bound at the steps on the Au(2 1 1) surface. At higher coverages, an additional lower temperature desorption peak that shifts from 515 to 530 K at saturation coverage is attributed to oxygen adsorbed on the (1 1 1) terrace sites of the Au(2 1 1) surface. Although the desorption kinetics are likely to be quite complex, a simple Redhead analysis gives an estimate of the desorption activation energy, E d, for the step-adsorbed oxygen of 34 kcal/mol and that for oxygen at the terraces near saturation coverage of 33 kcal/mol, values that are similar to others reported on Au surfaces. Low Energy Electron Diffraction (LEED) indicates an oxygen-induced step doubling on the Au(2 1 1) surface at low-coverages ( θ O = 0.08–0.17 ML) and extensive disruption of the 2D ordering at the surface for saturation coverages of oxygen ( θ O ⩾ 0.9 ML). Overall, our results indicate that unstrained step sites on Au(2 1 1) surfaces of dispersed Au nanoparticles do not account for the novel reactivity of supported Au catalysts for CO oxidation.

Study of CO Dissociative Adsorption over Pt and Rh Catalysts by Inverse Gas Chromatography

Carbon monoxide dissociative adsorption was studied over silica-supported platinum, rhodium and Pt-Rh alloy catalysts, by Reversed Flow-Gas Chromatography. Using appropriate mathematical analysis physicochemical quantities such as fractional catalytic conversions of CO to CO2, as well as rate constants for the adsorption, desorption and surface reaction, describing the dissociative adsorption of CO, were determined. From the variation of the above parameters against the nature of the studied catalysts (Rh content) useful conclusions concerning the mechanism of CO dissociative adsorption were extracted.

Palladium-based catalysts for the synthesis of alcohols

The present work intends to illustrate the effects of precursor and support composition on the structure and performance of Pd-based catalysts for the synthesis of methanol and higher oxygenated compounds. The power of the XRD technique and the reaction itself, as characterisation tools, was evidenced. The hydrogenation of carbon monoxide has been studied over palladium supported on ZnO, Zr(OH)4 and ZrO2 using PdCl2 and Pd(NO3)2 as salt precursors. Catalysts were characterised by XRD and Raman spectroscopy. The results show that activity and selectivity are strongly dependent on the salt precursor and on the acid–basic properties of the support. As seen from the CH3OH/DME ratio, the order of acidity of the supports is: Zr(OH)4>ZrO2>ZnO independently of the salt precursor used. The Pd(NO3)2/ZnO catalyst shown the highest methanol selectivity which was related to the presence of large particles of a PdZn alloy. A double bifunctionality on the catalysts for the production of higher oxygenated seems evident. On the one hand, an acid–base bifunctionality of the support seems to be needed for the formation, stabilisation, chain growth and further reactivity of intermediates such as formate, as well as for the dissociative adsorption of CO, and on the other hand, a metal-support bifunctionality, where the palladium metal particles would play the role of hydrogenating the intermediates and possibly adsorb non-dissociatively the CO.

The Mechanistic Study of Methane Reforming with Carbon Dioxide on Ni/α-Al2O3

The reactions of oxidized and reduced 6 wt % NiO/α-Al2O3 with H2, CH4, CO2, O2, and their mixtures are studied in flow and pulse regimes using a setup equipped with a differential scanning calorimeter DSC-111 and a system for chromatographic analysis. It is shown that treatment with hydrogen at 700°С results in the partial reduction of NiO to Ni. Methane practically does not react with oxidized Ni/α-Al2O3 but it does react actively with the reduced catalyst to form H2 and surface carbon. The latter is capable of reacting with lattice oxygen of Ni/α-Al2O3 (slowly) and with adsorbed oxygen (rapidly). Carbon dioxide also reacts with surface carbon to form CO (rapidly) and with metallic Ni to yield CO and NiO (slowly). Thus, the main route of methane reforming with carbon dioxide on Ni/α-Al2O3 is the dissociative adsorption of CH4 to form surface carbon and H2 and the reaction of this carbon with CO2 resulting in the formation of CO by the reverse Boudouard reaction. Side routes are the interaction of the products of methane chemisorption with catalyst oxygen and the dissociative adsorption of CO2 on metallic nickel. A competitive reaction of surface carbon with adsorbed oxygen results in a decrease in the CO2 conversion in methane reforming with carbon dioxide. Therefore, the presence of gaseous oxygen in the reacting mixture decelerates methane reforming (catalyst poisoning by oxygen).