First Principles-Based Microkinetic Modeling of Ethanol from Syngas on Bimetallic Co-Pd Catalysts

In the future, the availability of reliable alternative fuels will be crucial for any country to become energy independent. One such alternative is ethanol as it can be used both as a fuel and as a fuel additive. Most of the ethanol produced in the world today is derived from biomass. The biomass feedstocks and fermentation broths used in ethanol production both contain high amounts of water and therefore, the energy efficiency of the process is lessened by product separation processes (azeotropic separation of water and ethanol) that are non-trivial and highly inefficient (due to the evaporation of water). An alternative route to produce ethanol, which negates the need for costly distillation processes, is via the catalytic conversion of syngas (CO and H2) generated from biomass. Syngas is a mixture of carbon monoxide and hydrogen, which results from the reforming of natural gas, as well as the gasification of coal, biomass, and solid wastes. In theory, syngas can be readily converted to ethanol using chemical catalysts, but to-date no high efficiency, low-cost catalyst has been found. In this work, sub-nanometer size, bimetallic cobalt-palladium particles are found to be active and selective catalysts for the desired reaction as the particles contain two metals having different CO dissociation capabilities. The reaction mechanism considered for this study includes forty-six reversible reactions, including Fischer-Tropsch reactions. We used Density Functional Theory (DFT) coupled with nudged elastic band methods to determine the activation barrier heights and enthalpy change with reactions for the full reaction pathway needed for ethanol production from syngas. To lessen the computational burden, linear Bronsted-Evans –Polanyi (BEP) relations, for association and dissociation reactions, are developed. A microkinetic model is built using the reaction information derived from combined DFT and BEP studies, which is used to examine if there is a synergistic effect between Co and Pd favoring the production of ethanol. Coverage dependent sticking coefficients are used to examine the effects of surface coverage on reactivity. It also incorporates diffusion of intermediate species between the sites. One of the first and important steps in the syngas to ethanol conversion process is carbon monoxide (CO) adsorption on the metal catalyst. Therefore, computational models were developed to help understand CO adsorption energetics as well as surface coverage effects on a Co7Pd6 catalyst. From these initial studies, we determined the adsorption energies of CO on both cobalt and palladium as a function of CO surface coverage (where the number of CO species on the catalyst surface was varied from 1 to 6). Further, we calculated the infrared spectra for adsorbed CO species and key bond lengths (metal–carbonyl carbon and adsorbed CO bond lengths) using DFT. Results from the DFT simulations compared favorably with experimental values. Separate microkinetic models results on Co, CoPd and Pd sites indicate that ethanol formation happens only on CoPd bimetallic sites indicating the synergetic effect of Co and Pd to make ethanol from syngas. A batch reactor is modeled and 24 ordinary differential equations are solved simultaneously to obtain time evolution of products and intermediates. The pathway for ethanol production is identified as: CO* →HCO*→CH2O*→CH3O*→CH3CO*→CH3CHO*→CH3CH2O*→CH3CH2OH. Further, the microkinetic model was modified to include diffusion reactions. Ratio of number of sites of cobalt, cobalt-palladium and palladium is altered to study CoxPdy catalysts of different cobalt and palladium ratios

Utilization of CO2 for syngas production by CH4 partial oxidation using a catalytic membrane reactor.

In this research, a synthetic flue gas mixture with added methane was used as the feed gas in the process of dry reforming with partial oxidation of methane using a laboratory scale catalytic membrane reactor to produce hydrogen and carbon monoxide that can present the starting point for methanol or ammonia synthesis and Fischer-Tropsch reactions. 0.5% wt% Rh catalyst was deposited on a γ-alumina support using rhodium (III) chloride precursor and incorporated into a shell and tube membrane reactor to measure the yield of synthesis gas (CO and H2) and conversion of CH4, O2 and CO2 respectively. These measurements were used to determine the reaction order and rate of CO2. The conversion of CO2 and CH4 were determined at different gas hourly space velocities. The reaction order was determined to be a first-order with respect to CO2. The rate of reaction for CO2 was found to follow an Arrhenius equation having an activation energy of 49.88 × 10−1 kJ mol−1. Experiments were conducted at 2.5, 5 and 8 ml h−1 g−1 gas hourly space velocities and it was observed that increasing the hourly gas velocities resulted in a higher CO2 and CH4 conversions while O2 conversion remained fairly constant. CO2 had a high conversion rate of 96% at 8 ml h−1 g−1. The synthesized catalytic membrane was characterized by Scanning Electron Microscopy (SEM) and the Energy Dispersive X-ray Analysis (EDXA) respectively. The micrographs showed the Rh particles deposited on the alumina support. Single gas permeation of CH4, CO2 and H2 through the alumina support showed that the permeance of H2 increased as the pressure was increased to 1 × 105 Pa. The order of gas permeance was H2 (2.00 g/mol) > CH4 (16.04 g/mol) > N2 (28.01 g/mol) > O2 (32 g/mol) > CO2 (44.00 g/mol) which is indicative of Knudsen flow mechanism. The novelty of the work lies in the combination of exothermic partial oxidation and endothermic CO2 and steam reforming in a single step in the membrane reactor to achieve near thermoneutrality while simultaneously consuming almost all the greenhouse gases in the feed gas stream

Homogeneous catalytic hydrogenation studies using transition metal complexes

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The Adsorption of Simple Molecules on Supported Rhodium Catalysts

The adsorption of a number of molecules relevant to the hydrogenation of carbon monoxide to organic oxygenate products over various supported rhodium catalysts has been investigated, under both static and pulsed flow conditions, using radioactive and stable isotope tracer techniques, temperature-programmed desorption and Fourier transform infra red spectoscopy. The catalysts used were 2%(w/w) and 5%(w/w) rhodium on silica, 2%(w/w) rhodium on alumina and 2%(w/w) rhodium on molybdenum trioxide. Temperature-programmed reduction techniques have also been used to characterise the catalysts. Determination of the [14-C]carbon monoxide and [14-C]carbon dioxide adsorption isotherms, under static conditions, showed that the amounts of each adsorbed were dependent on both the adsorbate and the support. Thus, each catalyst adsorbed considerably more carbon monoxide than carbon dioxide. However, whilst the carbon monoxide would undergo complete exchange at ambient temperature, no such exchange was found with adsorbed carbon dioxide. The average heat of adsorption of carbon monoxide was found to be support independent with a value of -93+/-6 kJ mol-1. The amounts of carbon monoxide adsorbed by each catalyst decreased in the order alumina > silica > molybdenum oxide. Exposure of [14-C]carbon monoxide precovered surfaces to either hydrogen or oxygen in the presence of gas phase [14-C]carbon monoxide resulted in an increase in surface radioactivity. Similar measurements with [14-C]carbon dioxide showed that the presence of hydrogen had little effect, the presence of oxygen dramatically reduced the amount of carbon dioxide adsorbed. Examination of the effects of preadsorbed carbon monoxide on the adsorption of carbon dioxide, and vice versa, showed that whilst preadsorbed carbon dioxide reduced the adsorption capacity of each catalyst for carbon monoxide, probably through a site-blocking mechanism, preadsorption of carbon monoxide increased the extent of adsorption carbon dioxide on both the silica- and molybdenum trioxide supported catalysts, but decreased the amount of carbon dioxide adsorbed on the alumina-supported catalyst. Pulsed flow adsorption measurements gave similar results to those obtained under static conditions, except that the amounts adsorbed were less due to the removal of weakly adsorbed species by the flow gas. Determinations of the amounts of carbon monoxide and carbon dioxide adsorbed at a variety of temperatures leads to the conclusion that adsorption of the dioxide is an activated process, whilst that of the monoxide proceeds with minumum activation energy. Adsorption of a 1:1 mixture of 13C16O and -12C18O showed that scrambling of the adsorbed species only occurred at elevated temperatures. Evidence is presented to show that this scrambling occurs by a concerted mechanism, rather than via dissociation, whilst the formation of small amounts of carbon dioxide at the elevated temperatures probably arises from a water gas shift reaction involving water retained by the catalyst following reduction and activation. A surprising feature to emerge from the temperature-programmed desorption studies was that whilst some carbon monoxide is still retained by the catalyst at 593K, this material would readily undergo isotopic exchange with gas phase carbon monoxide. FTIR studies of the adsorption of carbon monoxide on the rhodium-silica and rhodium-alumina catalysts showed the presence of bands ascribable to linear, bridged and gem adsorbed species on the reduced metal. An additional band ascribed to carbon monoxide adsorbed on a partially reduced rhodium site was also observed. No satisfactory measurements could be made, even in diffuse reflectance, with the rhodium-molybdenum trioxide catalysts, due to the extremely high absorbance of the catalyst samples. The effect of increasing temperature on the spectra of the adsorbed carbon monoxide was to increase the intensity of the bands due to the gem-adsorbed species, whilst the intensity of the linear band simultaneously decreased. This is taken to indicate that the presence of the adsorbed carbon monoxide causes a restructuring of the metal surface, and thereby an increase in the number of isolated rhodium sites on which the gem-dicarbonyl species are adsorbed, as the temperature is increased. The FTIR spectra of adsorbed carbon dioxide on rhodium-silica and rhodium-alumina revealed bands due to both surface carbonates and adsorbed carbon monoxide, indicating that at least some of the dioxide is dissociatively adsorbed on the surface. Investigations of the adsorption of methanol, ethanol and ethanal by infra red spectroscopy showed that, on both rhodium-silica and rhodium-alumina catalysts, decomposition to carbon monoxide occurred at ambient temperature. The extent of this decomposition, which was support dependent, was promoted by increase in temperature and by the presence of hydrogen. Bands due to the presence of surface hydrocarbonaceous species were also observed

Catalytic Reforming of Higher Hydrocarbon Fuels to Hydrogen: Process Investigations with Regard to Auxiliary Power Units

This thesis discusses the investigation of the catalytic partial oxidation on rhodium-coated honeycomb catalysts with respect to the conversion of a model surrogate fuel and commercial diesel fuel into hydrogen for the use in auxiliary power units. Furthermore, the influence of simulated tail-gas recycling was investigated

Catalysts design for higher alcohols synthesis by CO2 hydrogenation: Trends and future perspectives

Global warming due to the accumulation of atmospheric CO2 has received great attention in recent years. Hence, it is urgent to reduce CO2 emissions into the atmosphere and develop sustainable technologies for a circular carbon economy. In this regard, CO2 capture coupled with the conversion into chemicals and fuels provides a promising solution to reduce CO2 emissions as well as to store and utilize renewable energy. Among the many possible CO2 conversion pathways, CO2 hydrogenation to higher alcohols is considered an important strategy for the synthesis of carbon-based fuels and feedstock and holds great promise for the chemical industry. Thus, this review provides an overview of advances in CO2 hydrogenation to higher alcohols that have been achieved recently in terms of catalyst design, catalytic performance, and insight into the reaction mechanism under different experimental conditions. First, the limitations provided by reaction thermodynamics and the indispensability of catalysts for CO2 hydrogenation to higher alcohols are discussed. Then, four main categories of catalysts will be introduced and discussed (i.e. Rh-, Cu-, Mo-, and Co-based catalysts). Moreover, important factors significantly influencing the efficiency of the catalytic transformation such as alkali/alkaline earth metal promoters, transition metal promoters, catalyst supports, catalyst precursors, and reaction conditions, as well as the reaction mechanism are explained. Finally, the review discusses emerging methodologies yet to be explored and future directions to achieve a high efficiency for the hydrogenation of CO2 to higher alcohols

Kinetics, catalysis and mechanism of methane steam reforming

The search for an alternative clean and renewable energy source has become an urgent matter. One such energy-saving technology is a fuel cell; it uses fuel as the source of energy to produce electricity directly and the byproducts formed are not as voluminous and environmentally harmful. The conventional low temperature fuel cells use hydrogen as the fuel which is produced from conventional fuels via reforming. However, developing reformers for hydrocarbon fuels requires AN understanding of the fundamental mechanisms and kinetics studies. In this study, simple hydrocarbon fuel, namely methane, in external reforming or internal reforming within a solid oxide fuel cell has been studied because of its importance and with the hope that it will ultimately lead to an understanding of reforming of higher hydrocarbons, such as logistic fuels like JP-8. For this purpose, methane was used the starting point and building block for the progressive understanding of reforming of complex hydrocarbons. Methane steam reforming (MSR), CH4 + 2H2O = CO2 + 4H2 is, in fact, the most common method of producing commercial bulk hydrogen along with the hydrogen used in ammonia plants. United States alone produces 9 million tons of hydrogen per year. The overall MSR reaction CH4 + 2H2O = CO2 + 4H2 is in fact composed of two reactions, the water gas shift reaction, CO + H2O = CO2 + H2, which has recently been investigated by a former Ph.D. student in our group, Caitlin Callaghan. Here, the first reaction CH4 + H2O = CO + 3H2, i.e., methane reforming, is analyzed using a reaction route network approach to obtain the overall methane steam reforming network and kinetics. Kinetics providing detailed information of elementary reaction steps for this system, namely micro-kinetics, has not yet been fully addressed. Employing the theory of Reaction Route Network Theory, recently developed by Fishtik and Datta, and using the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method of Shustorovich to predict elementary step kinetics coupled with transition-state theory, a detailed microkinetic model of steam and dry reforming of methane has been developed for Rh(111) and Ni(111) in this thesis. While there is extensive literature on it, the standard reference on the mechanism and kinetics of MSR is that of Xu and Froment, who proposed a 13 step mechanism. Based on the assumption of rate limiting steps for these overall reactions, Xu and Froment derived rate expressions for overall kinetics with fitted parameters. Here a more detailed micro-kinetic model of steam reforming of methane has been developed by adding 3 steps pertinent to carbon formation on the catalyst to Xu and Froment\u27s mechanism. The complete set as well as the dominant reaction routes has been identified. This was accomplished first by enumerating the list of reaction routes and drawing this network. A program was written in Maple and was used to assist in creating the list of full routes, empty routes and intermediate nodes. This program reduces the amount of repetitive work that was needed in an earlier Matlab program when computing the list. After drawing the complete reaction network it was than converted into an equivalent electrical circuit and Multisim analysis was performed. Further, the resistances of various reaction steps were compared. From the reduced graph, it was determined that reaction steps pertaining to desorption of carbon dioxide, i.e., step s4, and intermediate methylene forming intermediate methylidyne, s11, are the rate limiting steps. Further, through simulation with Multisim, it was determined that in fact only 2 overall reactions are needed. Adding a third overall reaction results in a nodal balance error. A rate expression was developed based on assuming the above two rate determining steps, with remaining steps at pseudo equilibrium along with the quasi-steady state approximation. The rate expression however produced a substantial error in conversion when compared to the overall microkinetic model. In addition to computing the micro-kinetic model, experimental work for methane steam reforming was conducted. A steam to carbon ratio of 2:1 was fed to the packed bed reactor, where experimental conversion data were obtained. These data points for Ni and Rh catalyst were plotted against the model to see how well the simulation predicted the experimental results. Reasonable agreement was obtained

Ruthenium based Fischer-Tropsch synthesis on crystallites and clusters of different sizes : from 'Nano" to "Ångstrøm"

Includes bibliographical references (p. 137-151)

Methanol carbonylation with metal/zeolite catalysts

The industrial production of acetic acid currently uses a homogeneous methanol carbonylation process that requires the presence of a halide promoter. The commercially viable formation of further products, e.g. ethanol, via this route is restricted by the halide impurities in the intermediate acetic acid. Considering the growing importance of methanol, as a feedstock from natural gas, over that of the established petrochemical resources, there is an incentive for the development of a halide free acetic acid process. Heterogeneous catalysts, especially protonated zeolites incorporating copper, have previously been reported to produce acetic acid by methanol carbonylation, in the absence of a halide promoter, and these form the basis of this work. The carbonylation of methanol, has been carried out here in a fixed bed reactor under conditions typically of 350°C, 8 bar gauge and a CO : methanol ratio of 8.8 : 1. The two main aims of the work have been to determine (i) the role of copper and (ii) the effect of the zeolite’s pore structure. Mordenite zeolites of different silica/alumina ratios were studied both in their proton and copper forms. Copper was introduced using several techniques, including ionexchange, impregnation and solid state exchange. It was found that copper significantly promoted the proton form only when it was introduced by ion-exchange. Examples o f zeolites with different framework structures, e.g. ZSM-5, theta, and beta, were similarly studied in both their proton and copper forms. It was confirmed that the unique pore structure of mordenite, large pores containing narrow side pockets, was the most productive for methanol carbonylation. On comparing the reactivity of the different catalysts tested, this work concludes that the carbonylation of methanol occurs at Bronsted acid sites and is therefore in direct competition with hydrocarbon formation. Furthermore, if the Bronsted acid sites are isolated from each other, e.g. in the narrow channels of H/theta, then the carbonylation reaction is promoted. Conversely, high concentrations of methanol activated in the channel intersections of H/ZSM-5 or H/beta cause the preferential formation of hydrocarbons