Catalytic C2H2 synthesis via low temperature CO hydrogenation on defect-rich 2D-MoS2 and 2D-MoS2 decorated with Mo clusters (original) (raw)
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Generation and Reactions of CH2 and C2H5 Species on Mo2C/Mo(111) Surface
Journal of Catalysis, 1999
The adsorption and dissociation of CH 2 I 2 and C 2 H 5 I on Mo 2 C/Mo(111) surface have been investigated with the purpose of producing adsorbed CH 2 and C 2 H 5 species. Methods used include high resolution electron energy loss, X-ray photoelectron, Auger electron, and temperature programmed desorption spectroscopies. Independently of the coverage, CH 2 I 2 adsorbs molecularly at 90-100 K. The dissociation of an adsorbed layer starts around 180-190 K. The primary products of thermal dissociation are adsorbed CH 2 and I. The species CH 2 undergoes self-hydrogenation to CH 4 at T p = 300 K and dimerization into C 2 H 4 at and above 222-280 K. Ethylene formed desorbs above 400 K. C 2 H 5 I also adsorbs molecularly on Mo 2 C at 90-100 K and dissociates to C 2 H 5 and I above 150 K. The reaction of C 2 H 5 on Mo 2 C/Mo(111) surface yielding C 2 H 6 and C 2 H 4 proceeds at a much lower temperature, above 180 K, than that of CH 2. Neither the cleavage of the CC bond nor the coupling of C 2 compounds occurred to detectable extent under the reaction conditions. The ethylene formed in the reactions of both CH x species exhibited the same features as observed following C 2 H 4 adsorption on Mo 2 C: the stable di-σ-bonded ethylene is transformed into ethylidyne at higher temperature. The results are discussed in relevance to the conversion of methane into benzene on Mo 2 C deposited on ZSM-5.
Journal of Catalysis, 2008
MoS 2 prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at temperatures up to 773 K was activated by treatments involving thermoevacuation at 723 K and/or reduction at 573 K. The effect of different activations on the activity in ethylene hydrogenation, cistrans isomerization of 2-butene, and H 2 /D 2 isotope exchange (all measured at temperatures around 473 K) was compared, taking into account the extent of Mo exposition as measured by oxygen chemisorption. It was found that activation had a widely varying impact on the test reactions, indicating that these proceed on sites with different numbers of vacancies. Hydrogenation activity was boosted by two orders of magnitude when reduction of the catalyst at 573 K was followed by thermoevacuation at 723 K, whereas this change was moderate with H 2 /D 2 exchange and negative with cis-trans isomerization. For the latter reaction, mere thermoevacuation was a suitable activation, and the impact of subsequent catalyst reduction was negative, whereas appreciable activity in the former reactions was obtained only when thermoevacuation was combined with a subsequent reduction. The data suggest that all three reactions proceeded on different sites, probably 3 vacancies per Mo for olefin hydrogenation, 2 vacancies per Mo for H 2 /D 2 exchange, and 1 vacancy per Mo for cis-trans isomerization, with the latter two reactions involving adjacent -SH groups. Sites with greater Mo exposure than stated may be suitable for H 2 /D 2 exchange but not for cis-trans isomerization. With increasing severity of the activation treatments, sites with a small number of vacancies appeared to combine and migrate toward the rims or to escape into the bulk. Therefore, very high hydrogenation activity may be seen on surfaces with an oxygen chemisorption capacity far from the maximum value occurring after less drastic treatments. Our findings imply that a combination of chemisorption studies with test reactions may be a valuable tool for surface characterization of sulfide catalysts.
Journal of Catalysis, 2005
The breaking of the C-S bond is a crucial step in hydrodesulfurization, the removal of the sulfur atom from sulfur-containing molecules in crude oil. Thus the hydrogenolysis reaction of CH 3 SH to CH 4 was studied by means of density functional theory on the catalytically active (100) edge of 2H-MoS 2 , with and without Co and Ni promoter atoms. Thiol adsorption, C-S bond breaking, and the formation and desorption of CH 4 were investigated with different sulfur and hydrogen surface coverages. CH 3 SH first adsorbs molecularly with its S atom in a bridging mode between two surface Mo atoms, followed by S-H bond cleavage with moderate activation energy. The subsequent concerted C-S bond breaking and CH 4 formation occurs through a reaction of the adsorbed CH 3 S group with the H atom of a neighbouring SH group at the molybdenum sulfide surface. Sulfur atoms, hydrogen atoms adsorbed on sulfur atoms, and promoter atoms (Co and Ni) at the catalyst surface weaken the bonding of adsorbed CH 3 S and lower the energy barrier for CH 4 formation. Although the reactions of thiols on the metal sulfide surface are similar to reactions on metal surfaces, the chemistry is different. The reactions occur between intermediate alkyl and hydrogen fragments bonded to sulfur atoms, not to metal atoms. 2005 Elsevier Inc. All rights reserved.
Journal of Catalysis, 2008
MoS 2 was prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at 723-773 K and activated by procedures involving evacuation at 723 K and/or reduction at different temperatures, predominately 573 K. Bulk and surface properties of these samples were studied by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, oxygen chemisorption (273 K, pulse mode), and isotope exchange with D 2 to determine the quantity of exchangeable hydrogen. In addition, the quantity of hydrogenation-active surface hydrogen was determined by hydrogenation of ethylene in absence of gas-phase hydrogen. No obvious relationship was found between the S/Mo stoichiometry and the degree of Mo exposure; depending on the pretreatment, exposed Mo ions can be detected by oxygen chemisorption at widely varying quantities while the S/Mo ratio remains 2. At more severe reduction treatments to remove additional sulfur, the oxygen chemisorption capacity decreases, likely due to migration of defects into the bulk. The reactivity of MoS 2 , as expressed by the temperatures required to expose Mo ions and the quantity of exchangeable hydrogen, varies significantly between batches prepared by similar routes. Therefore, thorough surface characterization by a set of techniques including reactivity studies is an indispensable prerequisite for any meaningful study investigating the relationship between catalytic properties with preparation procedures. After activation, MoS 2 holds hydrogen, which is able to hydrogenate ethylene in absence of gas-phase H 2 ; the quantity of this hydrogenation-active surface hydrogen is not correlated with the hydrogenation activity, however.
Physical Review Letters, 2001
By means of scanning tunneling microscopy measurements and density functional theory calculations, we identify the reaction mechanism for the oxidation of carbon monoxide to carbon dioxide on the Rh(110) surface at 160 K, which appears to be completely different than the one active at room temperature. The reasons for these different behaviors are determined. Our results demonstrate that even for a very simple catalytic reaction, the microscopic mechanism can dramatically change with temperature, following pathways that differ for nucleation sites and surface propagation and involve different surface moieties.
Theoretical studies on the C2H+O2 reaction: mechanism for HCO+CO, HCCO+O and CH+CO2 formation
Chemical Physics Letters, 1998
The reaction of the ethynyl radical with molecular oxygen has been examined using density functional theory. Two major reaction routes are open to the chemically activated HCCOO adduct 1: dissociation to HCCOq O 16 and formation of the thermodynamically most stable products HCO q CO 12: HCCOO 1 ™ dioxirenyl 2 ™ oxyrenyloxy 3 ™ oxo-ketene 5 Ž. Ž. Ž. ™ HCO q CO 12 ™ H q 2CO 15. The CCSD T r6-311qqG d,p rr B3LYPr6-311qqG d,p energies of the respective rate controlling transition states, 1 r r r r r16 and 1 r r r r r2, indicate that the route leading to H q CO q CO should w x dominate. Several other C ,H,O isomers and other, minor pathways have also been characterised. The present study 2 2 reveals this reaction to be a capture-limited association-elimination reaction with a high and pressure-independent rate coefficient.
Adsorption and decomposition of C6H5I on the Mo2C/Mo(1 0 0) surface
Surface Science, 2003
The adsorption and surface reactions of phenyl iodide on Mo 2 C/Mo(1 0 0) surface have been investigated by thermal desorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy in the 100-1200 K temperature range. Phenyl iodide adsorbed molecularly on the Mo 2 C/Mo(1 0 0) surface at 100 K. At submonolayer coverages the molecules adsorbed in a flat-lying, and in the condensed layer in a random position. The desorption of the weakly bonded C 6 H 5 I occurred in a peak with T p ¼ 200 K. Phenyl iodide bonded in the chemisorbed state underwent dissociation at 160-300 K, as evidenced by XPS data, while photolysis of the monolayer by UV light resulted in a complete dissociation even at 100 K. Iodine atoms formed in the decomposition process were released into the gas phase with T p ¼ 980 and 1080 K. The phenyl groups formed as a result of C-I cleavage reacted in three different ways. A very limited part is coupled into biphenyl (T p ¼ 510 K). Other part was hydrogenated to benzene which desorbed with a T p ¼ 290-278 K. The third part of C 6 H 5ðaÞ decomposed to hydrogen and benzyne groups. This species could be also hydrogenated into benzene, but it mostly decomposed at higher temperature, as shown by H 2 desorption peaks at 600 and 700 K. HREEL spectra suggested that the aromatic ring was preserved on the surface up to 430K.Elevatingtheadsorptiontemperatureto400KenhancedtheamountofstronglybondedC6speciesbyafactorof430 K. Elevating the adsorption temperature to 400 K enhanced the amount of strongly bonded C 6 species by a factor of 430K.Elevatingtheadsorptiontemperatureto400KenhancedtheamountofstronglybondedC6speciesbyafactorof3 as evidenced by the increased hydrogen desorption.
The C 4 H 6 •+ Potential Energy Surface. 2. The Reaction of Ethylene Radical Cation with Acetylene
The Journal of Physical Chemistry A, 1998
The reaction of the ethylene radical cation (Et •+ ) with acetylene (Ac) to form stable C 4 H 6 •+ intermediates and the subsequent fragmentation of these to C 3 H 3 + + CH 3 • or to C 4 H 5 + + H • have been studied by the UMP2, RMP2, and B3LYP methods with the 6-31G* basis set, as well as by single-point calculations at the RCCSD-(T)/cc-pVTZ level of theory. The aim of this study was to identify all stationary points that might be relevant to explain the course of the observed reactions. According to their stability to dissociation, we distinguish three classes of C 4 H 6 •+ structures: weakly bonded complexes, structures of medium stability, and tightly bonded complexes. Methylcyclopropene radical cation seems to be the most likely ultimate precursor for the formation of the observed fragmentation products.
Thermodynamic Study: C-H Bond Activation of Methane with OsO⁺
Acta Physica Polonica A, 2015
Catalysis plays a critical role in the accomplishment of industrially significant chemical transformations, by requiring less energy investment in underlying processes. Computational chemistry has had a pronounced impact on the understanding of the role of catalysts at the atomic and molecular level, contributing to design of more efficient catalysts. In this study, we compute thermochemical properties attending C-H bond activation of methane by OsO + and enabling subsequent dehydrogenation and dehydration reactions. It is found that the dehydrogenation channel is thermodynamically more favorable. This study should contribute to the understanding of C-H bond activation using homogeneous catalysis of partial oxidation of natural gas (methane) leading to formation of the easily transported liquid fuel methanol.
Potential Energy Surfaces for the Reactions of HO2 Radical with CH2O and HO2 in CO2 Environment
The journal of physical chemistry. A, 2016
We report on potential energies for the transition state, reactant, and product complexes along the reaction pathways for hydrogen transfer reactions to hydroperoxyl radical from formaldehyde H2CO + HO2 → HCO + H2O2 and another hydroperoxyl radical 2HO2 → H2O2 + O2 in the presence of one carbon dioxide molecule. Both covalently bonded intermediates and weak intermolecular complexes are identified and characterized. We found that reactions that involve covalent intermediates have substantially higher activation barriers and are not likely to play a role in hydrogen transfer kinetics. The van der Waals complexation with carbon dioxide does not affect hydrogen transfer from formaldehyde, but it lowers the barrier for hydroperoxyl self-reaction by nearly 3 kcal/mol. This indicates that CO2 environment is likely to have catalytic effect on HO2 self-reaction, which needs to be included in kinetic combustion mechanisms in supercritical CO2.