Direct Formation Of CO Research Articles

Isomer-specific influences on ignition and intermediates of two C5 ketones in an RCM

Ketones have been considered as potential biofuels and main components of blend stock for internal engines. To better understand the chemical kinetics of ketones, ignition delay times of 2-pentanone (propyl methyl ketone, PMK) and 3-pentanone (diethyl ketone, DEK) were measured at temperatures of 895–1128 K under 10 and 20 bar, at equivalence ratios (ϕ) of 0.5 and 1.0 in a rapid compression machine (RCM). To explore the impact of carbonyl functionality and resonance stabilized structures of fuel radicals on their combustion kinetics, high-temperature pyrolysis at 1130 K and relatively low-temperature oxidation at 950 K studies were performed in an RCM, and the time-resolved species concentration profiles under these two conditions were quantified using a fast sampling system and gas chromatography (GC). A new kinetic model containing low-temperature reactions was built aiming at predicting the pyrolysis and oxidation behaviors of both ketones. The consumption pathways of the resonance stabilization fuel radicals through oxygen addition and following reactions are promoted since the decomposition rates of these radicals are about 4 orders magnitudes lower than regular fuel radicals. The occurrences of the so-called “addition-dissociation reactions”, i.e., ketones reacting with a hydrogen yielding aldehyde or reacting with a methyl radical yielding shorter-chain-length ketones, are verified in pyrolysis experiments. Based on experiments and model analysis, the carbonyl functionality in both ketones is preserved during the process of β-scissions of fuel radicals and α-scissions of fuel-related acyl radicals, resulting in the direct formation of CO and ketene. However, the position of carbonyl functionality has a significant impact on the species pools.

The CO Formation Reaction Pathway in Steam Methane Reforming by Rhodium

Three different pathways toward CO formation from adsorbed CH and O are compared by quantum-chemical density functional theory (DFT) calculations for planar and stepped Rh surfaces. The conventional pathway competes with the pathway involving a formyl (CHO) species. This holds for both types of surfaces. The barrier for carbon-oxygen bond formation for the planar surface (180 kJ/mol) is substantially higher than that for the stepped surface (90 kJ/mol). The reaction path through intermediate formyl formation competes with direct formation of CO from recombination via adsorbed C and O atoms. Calculations are used as a basis for the analysis of the overall kinetics of the methane steam reforming reaction as a function of the particle size and the metal.

Investigation of the selective sites on graphitic carbons for oxidative dehydrogenation of isobutane

Reaction network analysis of the oxidative dehydrogenation (ODH) reaction of isobutane over model carbon catalysts with tailored open-edge graphitic structure and quinone-type oxygenated functionalities was used to identify the selective pathways for the formation of isobutene. Carbon-based materials have been widely used in catalysis, but the active sites are not well-understood due to the complexity of the carbon structure. Correct identification of these sites is essential for learning how to manipulate material structure to achieve high catalytic yields of the desired products. In this study, we created model catalysts with controllable surface concentration of oxygen based on graphitized mesoporous carbon (GMC). Our studies reveal that the ODH reaction of isobutane on carbon catalysts is a parallel-consecutive pathway with partial oxidative dehydrogenation for the formation of isobutene and deep oxidation pathway for the direct formation of CO and CO 2 from isobutane. These two pathways show different dependence on the quinone-type oxygen sites: the rate constant leading to the desired partial oxidation product does not show a strong correlation to the density of the oxygen sites, whereas the rate constant leading to the unselective CO x products increases continuously with the density of oxygen sites.

Vacancy formation and CO adsorption on gold-doped ceria surfaces

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO 2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.

The selective epoxidation of conjugated olefins containing allylic substituents and epoxidation of propylene in the presence of butadiene

The epoxidation of isoprene (2-methyl-1,3-butadiene) and piperylene (1,3-pentadiene), both conjugated olefins containing allylic methyl groups, has been conducted using conventional, CsCl-promoted, Ag/ α-Al 2O 3 catalysts. Selectivities to the allylic olefin epoxide isomers are over 20% and are much higher than expected due to the presence of the conjugated olefin structure. The epoxidation of propylene over the same catalyst under the same conditions is only 2.5%. The epoxidation of propylene in the presence of butadiene also yields PO in much higher selectivities. The presence of as little as 1% C 4H 6 in the reaction feedstream increases the selectivity to PO from 2.5% to over 40% at the expense of overall activity for C 3H 6 conversion. The upper limit of selectivity to PO in the presence of C 4H 6 appears to be approximately 50%, suggesting an upper limit for the effectiveness of this methodology. Epoxidation of C 4H 6 alone on similar Ag catalysts indicates that the consecutive reaction of EpB to CO 2/H 2O is strongly limited by the presence of excess C 4H 6 in the feedstream. In addition, the selectivity to EpB is directly proportional to the amount of C 4H 6 in the reaction feed stream. Selectivities >90% are obtained only when there is sufficient C 4H 6 in the reaction feedstream to control the concentration of the reactive Ag–O surface. For C 4H 6 epoxidation, all CO 2/H 2O is formed by a consecutive reaction pathway from EpB; there is no parallel pathway for the direct formation of CO 2/H 2O from C 4H 6. Using the selective epoxidation of C 4H 6 as the model for understanding the enhancement in selectivity for allylic olefin epoxide formation, the most likely reason for improved selectivities is that strongly adsorbed C 4H 6 (or other conjugated olefins) limits the ensemble size of contiguous Ag–O surface sites. These ensembles are too small for PO combustion, but not too small for PO formation.

The selective epoxidation of conjugated olefins containing allylic substituents and epoxidation of propylene in the presence of butadiene

The epoxidation of isoprene (2-methyl-1,3-butadiene) and piperylene (1,3-pentadiene), both conjugated olefins containing allylic methyl groups, has been conducted using conventional, CsCl-promoted, Ag/ α -Al 2 O 3 catalysts. Selectivities to the allylic olefin epoxide isomers are over 20% and are much higher than expected due to the presence of the conjugated olefin structure. The epoxidation of propylene over the same catalyst under the same conditions is only 2.5%. The epoxidation of propylene in the presence of butadiene also yields PO in much higher selectivities. The presence of as little as 1% C 4 H 6 in the reaction feedstream increases the selectivity to PO from 2.5% to over 40% at the expense of overall activity for C 3 H 6 conversion. The upper limit of selectivity to PO in the presence of C 4 H 6 appears to be approximately 50%, suggesting an upper limit for the effectiveness of this methodology. Epoxidation of C 4 H 6 alone on similar Ag catalysts indicates that the consecutive reaction of EpB to CO 2 /H 2 O is strongly limited by the presence of excess C 4 H 6 in the feedstream. In addition, the selectivity to EpB is directly proportional to the amount of C 4 H 6 in the reaction feed stream. Selectivities >90% are obtained only when there is sufficient C 4 H 6 in the reaction feedstream to control the concentration of the reactive Ag–O surface. For C 4 H 6 epoxidation, all CO 2 /H 2 O is formed by a consecutive reaction pathway from EpB; there is no parallel pathway for the direct formation of CO 2 /H 2 O from C 4 H 6 . Using the selective epoxidation of C 4 H 6 as the model for understanding the enhancement in selectivity for allylic olefin epoxide formation, the most likely reason for improved selectivities is that strongly adsorbed C 4 H 6 (or other conjugated olefins) limits the ensemble size of contiguous Ag–O surface sites. These ensembles are too small for PO combustion, but not too small for PO formation.

Kinetic model of the partial oxidation of methane to synthesis gas over Ru/TiO2 catalyst

The kinetic behavior of the Ru/TiO2 catalyst in the partial oxidation of methane to synthesis gas was investigated as a function of temperature and partial pressures of CH4 and O2. It was found that the catalyst promotes, to a large extent the direct formation of CO and H2 from the reaction of methane and oxygen, while the reforming and water–gas shift reactions are negligible under these conditions. A kinetic model, based on formation of CO and H2 as primary products and of CO2 and H2O as secondary oxidation products, describes satisfactorily the observed kinetic behavior. Values of model parameters satisfy thermodynamic constraints and agree well with data derived from surface science techniques and kinetic studies of elementary surface processes that are in the literature.

Mechanistic investigations on the partial oxidation of methane to synthesis gas over a nickel-on-alumina catalyst

The mechanism of the partial oxidation of methane to synthesis gas (CO+H 2) was investigated over a nickel-on-alumina catalyst. The pulse reaction experiments combining XPS and TPR investigations were used to verify the active sites and selective O species present on the working catalyst. It was found that both, reduced Ni and NiO x species are present on the catalyst surface during the reaction. Unlike the highly dispersed surface NiAl 2O 4 species on the fresh catalyst which are reducible at ca. 800°C, these NiO x species present on the working catalyst can be reduced at 520–560°C, they are thus assigned to adsorbed O atoms. The comparisons of the pulse reactions of CH 4, CH 4/O 2 (2 : 1) and CH 4/CO 2(1 : 1) indicate that the reduced Ni sites play a role in dissociating methane into surface C atoms and H 2, while the direct formation of CO occurs via reacting surface C atoms with adsorbed O atoms present as NiO x . We propose that the primary surface reaction of the partial oxidation of methane over Ni/Al 2O 3 proceeds via a CH 4 pyrolysis followed by the reduction of NiO x by the surface C atoms (arising from CH 4 dissociation), which is reoxidized by incorporation of O 2 into the catalyst.

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H2 followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.