Successive Dehydrogenation Research Articles

Mechanisms of Transforming CHx to CO on Ni(111) Surface by Density Functional Theory

To elucidate feasible routes of producing CO from CH3 and unravel the effect of adsorbed O on CHx transformation, the reactivity of CHx (x = 1–3) with and without the assistance of adsorbed atomic O on Ni(111) was explored using density functional theory calculations. The adsorption energies of CHx (x = 0–3) were found to be significantly reduced on an O-preadsorbed Ni(111) surface compared to a pure surface. Furthermore, O-assisted one-step dehydrogenation of CHx (x = 1–3) features energy barriers and thus is difficult to proceed. In terms of energy, the direct dissociation of CH3 is favorable, except for the last CH dehydrogenation, which is energy intensive. Interestingly, in O-assisted two-step CH transformation to CO via CHO intermediate, the barrier is dramatically lowered. The successive dehydrogenations of CHxO (x = 1–3) were also found to be a route for CO formation. Finally, two possible pathways from CH3 to CO are proposed: (a) CH3 → CH2 → CH → CHO → CO; (b) CH3 → CH3O → CH2O → CHO → CO.

A skeletal reaction scheme for selective catalytic reduction of NOx with NH3 over CeO2/TiO2 catalyst

CeO2/TiO2 has been regarded as the promising catalyst used for the selective catalytic reduction (SCR) of NOx with NH3. The complete NH3-SCR reaction mechanism of NO over CeO2/TiO2 catalyst was systemically investigated using density functional theory (DFT) calculations. Based on the DFT-derived energetics, a skeletal reaction scheme including 20 elementary reaction steps was proposed for NH3-SCR reaction of NO over CeO2/TiO2 catalyst. The results indicate that NH3, NO, O2, and NO2 adsorption on CeO2/TiO2(110) surface is dominated by the chemisorption mechanism. The successive dehydrogenation process of NH3 is cleaved by NH2 dehydrogenation step. NH2 derived from NH3 dehydrogenation step can stably exist on CeO2/TiO2 catalyst surface and serve as a key intermediate in NH3-SCR reaction. The activation energy barrier of N2O formation is higher than that of N2 formation, which is responsible for the excellent N2 selectivity of CeO2/TiO2 catalyst. NO2 formation under the typical low-temperature SCR conditions can make a contribution to NO reduction over CeO2/TiO2 catalyst. Reaction pathway analysis illustrates that the dominant reaction pathway of NO reduction by NH3 over CeO2/TiO2 catalyst is a three-step process: (1) NH2 formation from NH3 dehydrogenation, (2) N-N coupling reaction, and (3) NH2NO decomposition. During NO reduction, NH2 formation from NH3 dehydrogenation is the rate-determining step of N2 formation.

Ammonia oxidation kinetics on bimetallic clusters of platinum and iridium: A theoretical approach

Density functional theory was performed on Pt3-xIrx (x=0–3) clusters to investigate the ammonia electro-oxidation reaction to nitrogen. The adsorption of N2H4-x (x=0–3) and the effect of cluster composition on the adsorption were investigated in Gaussian 09. N and NH were found to be the most stable intermediates on these clusters, with a pronounced stability caused by the presence of iridium. On a Pt cluster, the ammonia oxidation mechanism involves hydrazine formation followed by hydrazine dehydrogenation to molecular nitrogen; however, on an Ir cluster, the ammonia undergoes successive dehydrogenation to form atomic nitrogen, followed by NN bond formation to N2. Moreover, rate of reaction constants, activation, and free energy calculations showed further evidence that production of N2 from its nitrogen atoms is sluggish and that the electro-catalyst may be considered as poisoned. Nonetheless, these calculations confirm that the onset potential for ammonia oxidation on iridium is lower than on platinum and this reaction starts at lower potential on Ir. In the presence of bimetallic catalysts, the iridium sites are more attractive for poisonous intermediates like NH and N, leaving platinum sites vacant for ammonia oxidation through hydrazine formation.

Dehydrogenative desaturation-relay via formation of multicenter-stabilized radical intermediates

In organic molecules, the reactivity at the carbon atom next to the functional group is dramatically different from that at other carbon atoms. Herein, we report that a versatile copper-catalyzed method enables successive dehydrogenation or dehydrogenation of ketones, aldehydes, alcohols, α,β-unsaturated diesters, and N-heterocycles to furnish stereodefined conjugated dienecarbonyls, polyenecarbonyls, and nitrogen-containing heteroarenes. On the basis of mechanistic studies, the copper-catalyzed successive dehydrogenation process proceeds via the initial α,β-desaturation followed by further dehydrogenative desaturation of the resultant enone intermediate, demonstrating that the reactivity at α-carbon is transferred through carbon–carbon double bond or longer π-system to the carbon atoms at the positions γ, ε, and η to carbonyl groups. The dehydrogenative desaturation–relay is ascribed to the formation of an unusual radical intermediate stabilized by 5- or 7,- or 9-center π-systems. The discovery of successive dehydrogenation may open the door to functionalizations of the positions distant from functional groups in organic molecules.

A new free radical scavenging cascade involving melatonin and three of its metabolites (3OHM, AFMK and AMK)

Melatonin and three of its metabolites [cyclic 3-hydroxymelatonin (3OHM), N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK)] are investigated to analyze their antioxidant capacity. The sequential scavenging action of melatonin and these metabolites has been named an antioxidant cascade, which greatly increases capacity of melatonin to scavenge free radicals and, therefore, decrease oxidative damage. In this theoretical study, we identified a new antioxidant cascade for melatonin and its metabolites by studying different antioxidant mechanisms: hydrogen atom transfer (HAT), radical adducts formation (RAF) and single electron transfer (SET). We demonstrate that melatonin and its metabolites 3OHM, AFMK and AMK are capable of scavenging free radicals through successive dehydrogenations, formation of stable radical adducts and by increasing electron donor-acceptor properties. The main conclusion in this report is that melatonin and its metabolites participate in a new antioxidant cascade and therefore scavenge many free radicals. These are new findings and help to explain the impressive ability of melatonin to reduce oxidative stress.

Synthesis of Pyrrole‐Fused Corannulenes: 1,3‐Dipolar Cycloaddition of Azomethine Ylides to Corannulene

AbstractIn the long history of corannulene chemistry, the 1,3‐dipolar cycloaddition to corannulene is unprecedented. Reported herein is the 1,3‐dipolar cycloaddition of a polycyclic aromatic azomethine ylide to corannulene, a reaction which occurs exclusively at the rim bond of corannulene, from the convex side in an exo fashion. The cycloadducts were successfully converted, by successive oxidative dehydrogenation, into pyrrole‐fused corannulenes, which exhibited pronounced solvatofluorochromism.

Synthesis of Pyrrole-Fused Corannulenes: 1,3-Dipolar Cycloaddition of Azomethine Ylides to Corannulene.

In the long history of corannulene chemistry, the 1,3-dipolar cycloaddition to corannulene is unprecedented. Reported herein is the 1,3-dipolar cycloaddition of a polycyclic aromatic azomethine ylide to corannulene, a reaction which occurs exclusively at the rim bond of corannulene, from the convex side in an exo fashion. The cycloadducts were successfully converted, by successive oxidative dehydrogenation, into pyrrole-fused corannulenes, which exhibited pronounced solvatofluorochromism.

Methanol oxidation on the PtPd(111) alloy surface: A density functional theory study

AbstractThe mechanisms of methanol (CH3OH) oxidation on the PtPd(111) alloy surface were systematically investigated by using density functional theory calculations. The energies of all the involved species were analyzed. The results indicated that with the removal of H atoms from adsorbates on PtPd(111) surface, the adsorption energies of (i) CH3OH, CH2OH, CHOH, and COH increased linearly, while those of (ii) CH3OH, CH3O, CH2O, CHO, and CO exhibited odd‐even oscillation. On PtPd(111) surface, CH3OH underwent the preferred initial CH bond scission followed by successive dehydrogenation and then CHO oxidation, that is, CH3OH → CH2OH → CHOH → CHO → CHOOH → COOH → CO2. Importantly, the rate‐determining step of CH3OH oxidation was found to switch from CO → CO2 on Pt(111) to COOH → CO2 + H on PtPd(111) with a lower energy barrier of 0.96 eV. Moreover, water also decomposed into OH more easily on PtPd(111) than on Pt(111). The calculated results indicate that alloying Pt with Pd could efficiently improve its catalytic performance for CH3OH oxidation through altering the primary pathways from the CO path on pure Pt to the non‐CO path on PtPd(111).

Alane adsorption and dissociation on the Si(0 0 1) surface

We used DFT to study the energetics of the decomposition of alane, AlH3, on the Si(0 0 1) surface, as the acceptor complement to PH3. Alane forms a dative bond with the raised atoms of silicon surface dimers, via the Si atom lone pair. We calculated the energies of various structures along the pathway of successive dehydrogenation events following adsorption: AlH2, AlH and Al, finding a gradual, significant decrease in energy. For each stage, we analyse the structure and bonding, and present simulated STM images of the lowest energy structures. Finally, we find that the energy of Al atoms incorporated into the surface, ejecting a Si atom, is comparable to Al adatoms. These findings show that Al incorporation is likely to be as precisely controlled as P incorporation, if slightly less easy to achieve.

Catalytic oxidative conversion of methane and ethane over polyoxometalate-derived catalysts in electric field at low temperature

We examined oxidative coupling of methane (OCM) and oxidative dehydrogenation of ethane (ODE) over various polyoxometalate supported CeO2 catalysts at 423K in an electric field. Tetrabutylammonium (TBA) salt of Keggin-type phosphotungstate (PW12O40) supported CeO2 catalysts ((TBA)3PW12O40/CeO2) showed high activities for OCM and ODE in the electric field (3mA) at 423K, although the catalyst without the electric field showed extremely low activities at such low temperature. FT-IR spectra and XRD patterns confirmed that the structure of (TBA)3PW12O40/CeO2 was changed to Ce2(WO4)3/CeO2 after reactions with the electric field, and it acted as an active site structure for OCM and ODE with the electric field. Results of activity tests revealed that the C2H6 production was a main reaction in OCM with the electric field, and C2H4 was formed through a successive oxidative dehydrogenation of the formed C2H6. Periodic operation tests demonstrated that reactive oxygen species suitable for ODE were formed on the catalyst surface of Ce2(WO4)3 in the electric field.

Theoretical study of NH3 decomposition on Pd-Cu (1 1 1) and Cu-Pd (1 1 1) surfaces: A comparison with clean Pd (1 1 1) and Cu (1 1 1)

The adsorption and successive dehydrogenation mechanisms of NH3 on Pd-Cu (111) and Cu-Pd (111) surfaces (the Pd atoms substitution of the first and second layers of Cu (111) surfaces) have been systematically investigated by density functional theory (DFT) method with a periodic slab model. All possible adsorption configurations of relevant intermediates on Pd-Cu (111) and Cu-Pd (111) surfaces are identified. It is revealed that the adsorption configurations and corresponding adsorption energies of adsorbates are slightly changed on Pd-Cu (111) and Cu-Pd (111) surfaces. The adsorption energies of NHx(x=0–3) species exhibit the following trend: NH3<NH2<NH<N. Then, the minimum energy path for the complete dehydrogenation of NH3 into adsorbed N and H is identified to explore the dehydrogenation mechanisms on different surfaces. The highest energy barrier and reaction energy on Pd-Cu (111) surface are greatly reduced to 1.56 and 0.99eV, implying that the complete dehydrogenation of NH3 on Pd-Cu (111) surface is favorable both kinetically and thermodynamically, namely, the doped-Pd atoms in the first layer are the reaction active center. Compared to that on clean Pd (111) and Cu (111) surfaces, it is found that the synergistic effect exits in different layers of catalyst surfaces. The calculated results show that the layer-substituted Pd atoms on the surface of Cu catalysts exhibit a better catalytic activity for NH3 dehydrogenation compared to the clean Cu (111) surface.

A density functional theory study of methanol dehydrogenation on the PtPd3(111) surface

Platinum (Pt)-based catalyst has been widely used as the catalyst in direct methanol fuel cells. This study aimed at determining the favorable pathway for methanol (CH3OH) dehydrogenation on the PtPd3(111) surface through the periodic density functional theory (DFT) calculation. The calculation evaluated the stable adsorption configurations of the possible intermediates produced from the successive dehydrogenation of methanol through C–H and O–H bond scission. The results showed that CH3OH, CH2OH, CH3O and CHO preferred to locate at the top site, COH, CO and H preferred to adsorb on the face centered cubic (fcc) site, while CH2O and CHOH would choose the bridge site. Both CH3OH and CH2O were bonded weakly on the PtPd3(111) surface due to their closed-shell electronic configurations. The other intermediate species were interacted strongly with the surface. The energy barriers in four possible pathways via initial breakage of O–H or C–H bond were calculated. By comparing the energy changes in the potential energy surface, the initial O–H scission of CH3OH as the most favorable reaction pathway was determined. The activation barrier by this pathway was only 0.955 eV. The dehydrogenation mechanism from CH3OH to CO via breaking O–H bond was derived as CH3OH → CH3O → CH2O → CHO → CO. The first dissociation step, CH3OH into CH3O and H was identified as the rate-limiting step. The theoretical results indicated that the proposed pathway for methanol dehydrogenation on PtPd3(111) surface was energetically favorable, and verified that the PtPd3 catalyst with Pt monolayer is a good candidate for methanol dissociation.

Molecular modelling of the decomposition of NH3 over CoO(100)

Spin-polarised density functional theory using the PBE + U approach are used to determine reaction pathways of successive NH3 dehydrogenation on the CoO(100) surface. NH3 dehydrogenation promotes noticeable displacements of the surface CoO sites, in particular due to the binding of NH2 and H species. Surface lattice O has low activity towards dehydrogenation, reflected in energy barriers that are in the range of 292 kJ mol−1 to 328 kJ mol−1. There is a preference of surface NH3 dehydrogenation to N2 rather than towards NO, due to a high-energy penalty of surface O vacancy formation. The presence of CoO in cobalt oxide catalysts not only may decline the ammonia conversion but also alter the selectivity towards N2 rather than NO.

A cuboctahedral platinum (Pt79) nanocluster enclosed by well defined facets favours di-sigma adsorption and improves the reaction kinetics for methanol fuel cells.

The methanol dehydrogenation steps are studied very systematically on the (111) facet of a cuboctahedral platinum (Pt79) nanocluster enclosed by well-defined facets. The various intermediates formed during the methanol decompositions are adsorbed at the edge and bridge site of the facet either vertically (through C- and O-centres) or in parallel. The di-sigma adsorption (in parallel) on the (111) facet of the nanocluster is the most stable structure for most of the intermediates and such binding improves the interaction between the substrate and the nanocluster and thus the catalytic activity. The reaction thermodynamics, activation barrier, and temperature dependent reaction rates are calculated for all the successive methanol dehydrogenation steps to understand the methanol decomposition mechanism, and these values are compared with previous studies to understand the catalytic activity of the nanocluster. We find the catalytic activity of the nanocluster is excellent while comparing with any previous reports and the methanol dehydrogenation thermodynamics and kinetics are best when the intermediates are adsorbed in a di-sigma manner.

DFT study of reaction processes of methane combustion on PdO(1 0 0)

The complex reaction mechanism of methane combustion on the PdO(100) surface is investigated within the framework of density functional theory. Driving forces and activation energies for the dissociative adsorption of methane and for the successive dehydrogenation of adsorbed hydrocarbons are calculated. Energy barriers of some of the dehydrogenation reactions are comparable to the barrier for the dissociative adsorption of methane, contrary to what is often assumed. Moreover, we find that reaction barriers for the early formation of C–O bonds are much lower than those for the complete dehydrogenation of CH4. In particular, reaction of oxygen molecules from the gas phase with suitable configurations of adsorbed H and CH3 can efficiently produce water and CH2O as oxidation products. Along this reaction path, the highest barrier is indeed given by the first dehydrogenation reaction.

Genetic engineering combined with random mutagenesis to enhance G418 production in Micromonospora echinospora.

G418, produced by fermentation of Micromonospora echinospora, is an aminoglycoside antibiotic commonly used in genetic selection and maintenance of eukaryotic cells. Besides G418, M. echinospora produces many G418 analogs. As a result, the G418 product always contains impurities such as gentamicin C1, C1a, C2, C2a, gentamicin A and gentamicin X2. These impurities are less potent but more toxic than G418, but the purification of G418 is difficult because it has similar properties to its impurities. G418 is an intermediate in the gentamicin biosynthesis pathway. From G418 the pathway proceeds via successive dehydrogenation and aminotransferation at the C-6' position to generate the gentamicin C complex, but genes responsible for these steps are still obscure. Through disruption of gacJ, which is deduced to encode a C-6' dehydrogenase, the biosynthetic impurities gentamicin C1, C1a, C2 and C2a were all removed, and G418 became the main product of the gacJ disruption strain. These results demonstrated that gacJ is in charge of conversion of the 6'-OH of G418 into 6'-NH2. Disruption of gacJ not only eliminates the impurities seen in the original strain but also improves G418 titers by 15-fold. G418 production was further improved by 26.6% through traditional random mutagenesis. Through the use of combined traditional and recombinant genetic techniques, we produced a strain from which most impurities were removed and G418 production was improved by 19 fold.

Effect of Ni(111) surface alloying by Pt on partial oxidation of methane to syngas: A DFT study

Density functional theory calculations were performed to study the adsorption and dissociation of methane as well as syngas formation over Ni(111) and NiPt(111) aiming to gain insight into the effect of Ni(111) surface alloying on partial oxidation of methane (POM). Results show that Pt-doped Ni(111) surface is beneficial to the adsorption of CHx and H species while unfavorable to the desorption of H2. The process of CH→C+H is found to be the rate-limiting step of successive dehydrogenation of methane on NiPt(111). Especially, the processes of CH4→CH3+H and CH3→CH2+H proceed more easily on NiPt(111) than on Ni(111). Three potential pathways for CO formation in the POM process were proposed and calculations identified that CH+O→CHO→CO+H are likely to be the main routes for CO formation on Ni(111) with CH+O→CHO as the rate-limiting step. Further calculations reveal that the formation of CO and H2 may be more difficult on NiPt(111) than on Ni(111).

Methane dissociation on Pt(1 1 1), Ir(1 1 1) and PtIr(1 1 1) surface: A density functional theory study

A periodic density functional theory (DFT) was utilized to calculate the dissociation process of methane (CH4) on Pt(111), Ir(111) and PtIr(111) surfaces. As compared to the adsorption energy, the most stable configurations of methane, methane dissociation species and co-adsorption of CHx (x=0–3) with H were obtained. The kinetic results of the CH4 dissociation indicated that the dissociating of CH4 into CH3 and H is the rate-limiting step on the PtIr(111) and Ir(111) surfaces. CH was the most abundant species that was difficult to dehydrogenate into C and H. Particularly, the activation barrier for CH3→CH2+H and CH2→CH+H on the Pt(111) surface is 3.5 and 1.4 times, respectively, higher than that on the PtIr(111) surface. According to the thermodynamics principles, the successive dehydrogenation of CH4 preferred to take place on the PtIr surface.

The carbon-tolerance mechanism of Ni-based alloy with coinage metals

Abstract Using the first-principles calculations, we investigate the successive dehydrogenation of CH4, as well as the diffusion of CH (the most important carbon-containing intermediate), on the Ni(111) surfaces doped with coinage metals. It is found that, although alloying of Ni with coinage metals can to some extent affect the CH4 dehydrogenation, the coking inhibition on the alloy surface mainly roots in the large diffusion barrier of CH, as well as the reduction of the number of active adsorption sites for CHx. These results give a clue for designing new catalyst with higher coking resistance.

Theoretical Study of Methanol Decomposition Mediated by Au3+, Au3 and Au3−: Mechanism and Effect of Charge State of Gold on its Catalytic Activity

By performing density functional theory calculations, we studied methanol decomposition promoted by neutral, anionic and cationic Au trimers, which represent the three simplest prototypes of Au-cluster-based catalysts with different charge states. The results show that the Au3– and Au3+-mediated reactions proceed via four successive single dehydrogenation steps, CH3OH → CH3O → CH2O → CHO → CO, while the Au3–-mediated reaction occurs through two double dehydrogenation steps, CH3OH → CH 2O → CO. The additional negative charge reduces remarkably the binding capability of CO (the completely dehydrogenated product of methanol) on the cluster, and is thus beneficial in reducing poisoning of the catalyst by CO. In contrast, the neutral and positively charged clusters present strong interactions with CO, making the catalyst readily poisoned by CO. Furthermore, the reaction promoted by the cationic cluster shows a much higher energy barrier than those by the neutral and anionic clusters. So selecting suitable substrates that make Au nanoparticles negatively charged may be a promising strategy for promoting methanol oxidation.