Activation Energy Of Rate-determining Step Research Articles

Mesoporous silicon-carbon composites: Novel supports of platinum nanoparticles for highly efficient selective oxidation of glycerol

Selective oxidation of glycerol is an extremely challenging reaction because of that three hydroxyl groups in glycerol are prone to randomly oxidize, resulting in unsatisfactory selectivity. In this work, silicon-carbon composites (Si-C-n) were designed and prepared as novel supports for nano Pt catalyst to boost the activity and selectivity in oxidation of glycerol to glyceric acid (GLYA). By the tannic acid/silicic acid self-assembly and carbonization process, mesoporous Si-C-n microspheres with high surface area. After the Pt loading, the as-prepared catalysts Pt/Si-C-3 exhibited significantly enhanced glycerol conversion (74.4%) and exceptional GLYA selectivity (86.7%). It also showed excellent stability, experiencing no obvious performance loss after six cycles. Characterizations and theoretical calculations verify that the introduction of Si species in the carbon framework provides preferred weak acid sites in Pt/Si-C-n, which could promote the desorption of product GLYA, enhance the electronic interaction between Pt and the support, and reduce the activation energy of rate-determining step, thus contributing to a high activity and selectivity to GLYA.

Effects of doping a boron atom in g-C3N4 on the catalytic activity of deposited Pd atom for the dehydrogenation of formic acid: A DFT study

The catalytic performance of palladium atom deposited on the cavity of the g-C3N4 sheet for the formic acid (FA) dehydrogenation was investigated using M06-L calculations. For the Pd1/g-C3N4 system, the preferable pathway for H2 production from FA decomposition occurs through the hydrocarboxyl intermediate with Ea of 21.2 kcal mol−1 for the rate-determining step (RDS). For B-doped g-C3N4, two different forms, which are the substitution of a boron atom at the nitrogen or the carbon atom, were used to examine the impact of the support on the catalytic activity of the deposited Pd atom. The boron dopant can enhance the basicity of the g-C3N4. As a result. the deposited Pd atom becomes more positively charged as compared to that on the pristine support. The greater the positive charge of the Pd atom, the stronger interaction with FA is found. Furthermore, the enhancement of basicity of the B-doped g-C3N4 sheet plays a key role in activating the O-H bond of FA as compared to that on the bare support. The reaction mechanism on Pd1/B-doped g-C3N4 favorably proceeds through the formate pathway. The activation energy of RDS is predicted to be 11.4 and 16.4 kcal mol−1 for BN-, BC-doped g-C3N4 supports, respectively.

Design of bimetallic Rh-M catalysts for N2O decomposition: From DFT calculation to experimental study

To understand the second doped metal influence on catalytic activity for N2O decomposition, the mechanisms of N2O decomposition over Rh-M(M = Co, Ni and Cu) bimetallic catalysts were investigated by density functional theory (DFT). Co doped into Rh catalyst can increase the activity for N2O decomposition due to the decrease of the activation energy of rate-determining step while Ni and Cu present contrary performance. A series of corresponding RhM catalysts supported on SBA-15 was prepared and applied in N2O catalytic decomposition. The catalytic activity trend of Rh-M catalysts for N2O decomposition is determined as Rh7Co1/SBA-15 > Rh/SBA-15 > Rh7Ni1/SBA-15 > Rh7Cu1/SBA-15, which is in consistency with the corresponding DFT calculation results.

CuI/BtH催化苯硫酚与对甲氧基溴苯C-S偶联合成(4-甲氧基)(苯基)硫醚反应机理理论研究

The mechanism of CuI/BtH catalyzed C-S coupling reaction of benzenethiol and 1-bromo-4- methoxybenzene to synthesize (4-methoxyphenyl)(phenyl)sulfane has been investigated by density functional theory (DFT). The geometries of reactants, intermediates, transition states, and products were optimized in both gaseous phase and solvent N , N -dimethyl formamide (DMF) at the B3LYP/6-31+G(d) level. The single point energies and zero point energy correction were calculated for the optimized configuration of each compound at B3LYP/6-311+ +G(d,p) level. Vibration analysis, energy calculation and IRC calculation proved the authenticities of the intermediates and the transition states. Atoms in molecules (AIM) and nature bond orbital (NBO) theories were used to discuss the bond nature and orbital interactions at the same of optimization calculation level. In CuI catalytic mechanism, there is one pathway, the rate-determining step activation energy of which is 180.49 kJ/mol in solution. In CuI/BtH catalytic mechanism, there are two possible pathways: I A is the optimal pathway and the activation energy of rate-determining step is 101.77 kJ/mol; the activation energy of rate-determining step in I B is 143.78 kJ/mol (sol). Since the ligand 1H-benzo[ d ][1,2,3]triazole (BTH) can reduce the activation energy of rate-determining step and the dissociation energy of catalyst, meanwhile, it is favorable for the separation of products and catalyst, which is in accordance with experimental results.

Theory studies on pyrolysis mechanism of phenethyl phenyl ether

In order to understand the mechanism of lignin pyrolysis, the pyrolysis of phenethyl phenyl ether (PPE) as dimeric lignin model compound was investigated by density functional theory methods at B3LYP/6-31G(d) level. Ten possible pyrolytic pathways were proposed including two concerted reactions and eight free-radical reactions. The standard thermodynamic and kinetic parameters in each reaction pathway were calculated at different temperatures. The calculation results show that all reactions are endothermic. Both thermodynamic and kinetic analyses support reaction pathways 9 and 10 in concerted mechanism as major reaction channels. The activation energies of rate-determining steps in pathways 9 and 10 are 60.1 kcal/mol and 52.8 kcal/mol respectively. We get four prior pathways from all the reaction pathways of PPE pyrolysis in four different bond rupture ways. Among these four reaction pathways, kinetic analysis prior supports pathway 2 whose rate-determining step is step 1 with an activation energy of 60.6 kcal/mol. It secondly supports pathway 4 whose rate-determining step is step 4 with an activation energy of 68.6 kcal/mol. Then it supports pathway 7 whose rate-determining step is step 12 with an activation energy of 93.6 kcal/mol. At last, kinetic analysis supports pathway 6 whose rate-determining step is step 10 with an activation energy of 96.7 kcal/mol. It can be inferred that major products of lignin pyrolysis are styrene, phenol and the compounds that can be formed by combinations of phenoxy, benzyl, phenyl and hydrogen radical.

Density functional theory studies on pyrolysis mechanism of β- d-glucopyranose

In order to understand cellulose pyrolysis mechanism, the pyrolysis of β- d-glucopyranose was investigated using density functional theory methods at B3LYP/6-31++G(d,p) level. Four possible pyrolytic pathways were proposed and geometries of reactants, transition states, intermediates and products were fully optimized. In pathway 1, the products are glycolicaldehyde, acetol, CO and H 2O; in pathway 2, the products are 5-hydroxymethylfurfural and H 2O; in pathway 3, the products are levoglucosan and H 2O; in pathway 4, the products are 3,4-anhydroaltrose and H 2O. The standard thermodynamic and kinetic parameters in each reaction pathway were calculated at different temperatures. The calculation results show that all reactions are endothermic and can take place spontaneously when reaction temperature exceeds 550 K. The changes of Gibbs free energies and the activation energies of rate-determining steps in reaction pathways 1 and 2 are less than that in reaction pathways 3 and 4. The activation energy of rate-determining step in pathway 1 is 297.0 kJ/mol and the activation energy of rate-determining step in pathway 2 is 284.5 kJ/mol. Based on thermodynamics and kinetic analysis, reaction pathways 1 and 2 are major pyrolysis reaction channels and the major products of β- d-glucopyranose pyrolysis are low molecular weight compounds such as glycolicaldehyde, 5-hydroxymethylfurfural, acetol and CO. The above results are in accordance with the related experimental results.

EXPLORING THE POTENTIAL ENERGY SURFACE OF THE CATALYZED ISOMERIZATION OF NITROSOMETHANE TO FORMALDOXIME

The mechanism of the isomerization of nitrosomethane to formaldoxime catalyzed by neutral molecule ( H 2 O and HCOOH ) has been investigated at the level of B3LYP/6-311+G**. Calculated results indicate that the rearrangement from nitrosomethane to more stable trans-formaldoxime can proceed via two different reaction channels, but the favorable reaction pathway catalyzed by water and formic acid is different from the one in the catalyst-free reaction. It is more favorable that the tautomeric reaction involves the formation of cis-formaldoxime and a subsequent rotation about the N – O bond to form trans-formaldoxime in the catalyzed reaction. The activation energy of rate-determining step was reduced from 197.9 kJ/mol to 138.7 kJ/mol in the water-catalyzed reaction and 79.6 kJ/mol in the formic acid-catalyzed reaction, respectively, due to the catalysis of hydroxylic groups, but the catalysis of more acidic hydroxyl group in the latter system has been shown to be more efficient.

Pyrolysis mechanism of carbon matrix precursor cyclohexane(I)

Possible pyrolysis reaction paths of cyclohexane were studied by UMP2 (FULL)/3-21G ∗, UB3LYP/3-21G ∗, UB3LYP/6-31G ∗ and RB3LYP/6-31G ∗ calculation. Pyrolysis mechanism of cyclohexane at high temperature ranges were studied by UB3LYP/6-31G ∗, data Δ E 0 θ , Δ E θ , Δ H θ , Δ G θ and Δ E 0 θ≠ , Δ E θ≠ , Δ H θ≠ , Δ G θ≠ of five reaction paths (13 reaction steps) and reaction rates at 298–1473 K were obtained. The calculations show: (1) the pyrolysis temperature of cyclohexane is about 873 K, and the products are 1-hexene, butadiene and butene, (2) as far as the reaction paths producing 1-hexene and producing butene are concerned, when the temperature is higher than 873 K, The reaction producing butene are more feasible thermodynamically and dynamically, and the activation energy of rate-determining step is ΔE 0 θ≠=374.46 kJ/ mol. Furthermore, at 1473 K, kinetic calculation suggests that the both reactions have almost equal reaction rates. (3) In the further pyrolysis reaction, reaction path D that produces butadiene from 2-butene is supported by kinetics, which means 1,3-butadiene is the main product. (4) At 298–1473 K, for the reaction paths producing 1-hexene and producing butadiene, the former is supported by kinetics, and the activation energy of rate-determining step is ΔE 0 θ≠=374.46 kJ/ mol. When reaching 1473 K, Δ G θ≠ of the rate-determining step of reaction path producing 1-hexene ( ΔG θ≠=284.19 kJ/ mol) is still smaller than Δ G θ≠ of the rate-determining step of reaction path producing 1,3-butadiene ( ΔG θ≠=313.10 kJ/ mol). The above results are basically in accord with mass spectroscopy analysis and GPC experiments.