Opportunities and prospects in the chemical recycling of carbon dioxide to fuels

Oxidative dehydrogenation of ethane and propane : How far from commercial implementation?

Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided. Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species. Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H2, C or CO. The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H2 and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced. The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Small clusters are known to possess reactivity not observed in their bulk analogues, which can make them attractive for catalysis. Their distinct catalytic properties are often hypothesized to result from the large fraction of under-coordinated surface atoms. Here, we show that size-preselected Pt(8-10) clusters stabilized on high-surface-area supports are 40-100 times more active for the oxidative dehydrogenation of propane than previously studied platinum and vanadia catalysts, while at the same time maintaining high selectivity towards formation of propylene over by-products. Quantum chemical calculations indicate that under-coordination of the Pt atoms in the clusters is responsible for the surprisingly high reactivity compared with extended surfaces. We anticipate that these results will form the basis for development of a new class of catalysts by providing a route to bond-specific chemistry, ranging from energy-efficient and environmentally friendly synthesis strategies to the replacement of petrochemical feedstocks by abundant small alkanes.

http://www.dstuns.iitm.ac.in/grouppresentations/2009/Subnanometre%20platinum%20clusters%20as%20highly%20active%20and%20selective%20catalysts%20for%20the%20oxidative%20dehydrogenation%20of%20propane.pdf

Subnanometre platinum clusters as highly active and selective catalysts for the oxidative dehydrogenation of propane.

Bimetallic catalysts have attracted extensive attention for a wide range of applications in energy production and environmental remediation due to their tunable chemical/physical properties. These properties are mainly governed by a number of parameters such as compositions of the bimetallic systems, their preparation method, and their morphostructure. In this regard, numerous efforts have been made to develop “designer” bimetallic catalysts with specific nanostructures and surface properties as a result of recent advances in the area of materials chemistry. The present review highlights a detailed overview of the development of nickel-based bimetallic catalysts for energy and environmental applications. Starting from a materials science perspective in order to obtain controlled morphologies and surface properties, with a focus on the fundamental understanding of these bimetallic systems to make a correlation with their catalytic behaviors, a detailed account is provided on the utilization of these systems in the catalytic reactions related to energy production and environmental remediation. We include the entire library of nickel-based bimetallic catalysts for both chemical and electrochemical processes such as catalytic reforming, dehydrogenation, hydrogenation, electrocatalysis and many other reactions.

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Ni-based bimetallic heterogeneous catalysts for energy and environmental applications

Steam methane reforming was studied at 823 K, a total pressure of 20 bar and steam to carbon ratios from 0.08 to 2.4 over conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 catalysts as well as catalysts supported on hydrotalcite derived materials. Catalyst activity, coke formation and deactivation at steam reforming conditions were studied using the tapered element oscillating microbalance (TEOM). Nickel supported on hydrotalcite derived materials had a smaller crystal size and a higher resistance to coke formation than the conventional NiO/α-Al 2 O 3 and NiO/CaO-Al 2 O 3 . The higher resistance to carbon formation could be due to a higher saturation concentration of carbon in the smaller nickel crystals. Sintering experiments were performed at 903 K and 20 bar on the hydrotalcite derived catalysts and compared with an industrial NiO/CaAl 2 O 4 catalyst. The particle growth for the hydrotalcite derived catalysts was larger than for the industrial catalyst, but the hydrotalcite derived catalysts had the smallest size of stabilized Ni crystals.

Effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming

Catalytic hydrodeoxygenation (HDO) of phenol over supported molybdenum carbide, nitride, phosphide and oxide catalysts

Deactivation during carbon dioxide reforming of methane over Ni catalyst: microkinetic analysis

Pre-reforming of natural gas has been studied on a nickel catalyst at 480–550 °C and 20 bar using a TEOM (Tapered Element Oscillating Microbalance) reactor. The focus has been on carbon formation and the main objective was to study the influence of C 2 –C 3 hydrocarbons in the methane feed on carbon deposition on the catalyst. Coking thresholds for different mixtures of hydrocarbons were determined by varying the steam to carbon (S/C) ratio at various temperatures. The steady-state coking rate decreases with increasing S/C ratio, and increases with increasing carbon number of the hydrocarbon. Unsaturated hydrocarbons show a strong effect on coking rates and on carbon thresholds. For mixtures of methane and propane/propene, the steady-state coking rate as well as the coking threshold decreased with a decrease in the mole fraction of propane/propene and with an increase in the hydrogen mole fraction. The coking rate revealed a complicated temperature dependency, and a minimum in the coking rate and in the coking threshold was detected at 500 °C. A relationship between the critical steam to carbon ratio and the thermodynamic carbon activity is also developed based on a suggested reaction mechanism, which can properly predict the role of hydrocarbon, hydrogen and water in carbon formation.

Rune Lødeng

Papers

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Effect of supports and Ni crystal size on carbon formation and sintering during steam methane reforming

Catalytic hydrodeoxygenation (HDO) of phenol over supported molybdenum carbide, nitride, phosphide and oxide catalysts

Deactivation during carbon dioxide reforming of methane over Ni catalyst: microkinetic analysis

Pre-reforming of natural gas on a Ni catalyst Criteria for carbon free operation