Reaction Mechanism Of Hydrogenation Research Articles

Research Progress of Vegetable Oil Hydrogenation Catalyst

The hydrogenation of vegetable oil can increase the oxidation stability and shelf life of hydrogenated oils, which is a common industrial method to change the chemical and physical properties of oil. Meanwhile, it is an important catalytic process in oils and fats industry. In this article, from the aspects of catalyst types and hydrogenation reaction mechanism, the research progress of vegetable oil hydrogenation catalysts are reviewed, and the new hydrogenation catalysts and processes are summarized. At the end, some insights of the hydrogenation catalyst improving and the development of the process are suggested.

Study on the reaction mechanism of acetylene selective hydrogenation catalysts Pd-Ag/Al2O3

Taking active component Pd-Ag alloy and catalyst carrier Al2O3 as the research object, the catalytic mechanism of acetylene hydrogenation reaction was explored by quantum mechanics theory and Monte Carlo method. In this paper, the activation energy on the catalyst surface, H2 dissociation process, acetylene and ethylene hydrogenation reaction were calculated. From the thermodynamic point of view, the increase of the co-catalyst Ag content enhances the desorption of ethylene on the catalyst surface, reduces the probability of ethane formation, and thus enhances the selectivity of acetylene hydrogenation. Through further process simulation studies, the quantum chemistry method and Monte Carlo method can be used to obtain the mechanism and optimal conditions for the selective catalytic hydrogenation of acetylene, with saving a lot of manpower and material resources.

A brief review on the reaction mechanisms of CO2 hydrogenation into methanol

The catalytic reduction of CO2 to methanol is an appealing option to reduce greenhouse gas concentration as well as renewable energy production. In addition, the exhaustion of fossil fuel, increase in earth temperature and sharp increases in fuel prices are the main driving factor for exploring the synthesis of methanol by hydrogenating CO2. Many studies on the catalytic hydrogenation of CO2 to methanol were published in the literature over the last few decades. Many of the studies have presented different catalysts having high stability, higher performance, low cost, and are immediately required to promote conversion. Understanding the mechanisms involved in the conversion of CO2 is essential as the first step towards creating these catalysts. This review briefly summarizes recent theoretical developments in mechanistic studies focused on using density functional theory, kinetic Monte Carlo simulations, and microkinetics modeling. Based on these simulation techniques on different transition metals, metal/metal oxide, and other heterogeneous catalysts surfaces, mainly, three important mechanisms that have been recommended are the formate (HCOO), reverse water–gas shift (RWGS), and trans-COOH mechanisms. Recent experimental and theoretical efforts appear to demonstrate that the formate route in which the main intermediate species is H2CO* in the reaction route, is more favorable in catalytic hydrogenation of CO2 to chemical fuels in various temperature and pressure conditions.

CO2 hydrogenation to formic acid over platinum cluster doped defective graphene: A DFT study

The hydrogenation of CO2 to formic acid is an important reaction in environmental catalysis, which can both alleviate the greenhouse effect and produce useful chemicals. Platinum element catalysts need to be investigated to realize large-scale development due to the expensive and scarce features. In this work, CO2 hydrogenation to formic acid over Pt4 cluster doped single-vacancy graphene was investigated using density functional theory. Catalyst configuration was optimized to perform corresponding gas adsorption and reduction reaction. Four reaction pathways were explored according to the reaction mechanism of Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R) and termolecular Eley-Rideal (TER). Kinetic analysis was used to evaluate the reaction rate of different processes under the given temperature range, and the potential effect of CO molecule was also considered to better understand the feasibility. Results showed that Pt4/SV was a stable and high activity catalyst. The minimum activation energy among different pathways was 0.56 eV and TER could be the dominant reaction mechanism of CO2 hydrogenation. This work not only provides a promising catalyst but also gives a more deep understanding of CO2 reduction technology and the future applicability of platinum metal catalysts.

Study of mechanism and kinetic modeling of CO hydrogenation reaction over the impregnated Co‐Ni/Al2O3 catalyst

AbstractAn investigation of the kinetic and mechanism of CO hydrogenation reaction was performed on impregnated Co‐Ni/Al2O3. Determination of kinetic parameters from the experiments was carried out in a micro fixed‐bed reactor. Kinetic evaluations were performed under various operational conditions of T = 473–673 K, p = 1–14 bar, H2/CO = 1–3, and GHSV = 4,500 hr−1. Kinetic models and rate equations for CO consumption were obtained by using two main‐type rate equations of Langmuir‐Hinshelwood‐Hougen‐Watson (LHHW) and Eley‐Rideal (ER). Estimation of various kinetic parameters was performed using a nonlinear regression method. According to the obtained experimental results and using statistical criteria, one kinetic expression based on the LHHW mechanism (‐rCO = kp.bCO.PCO. bH2. PH2/[1+ bCO.PCO + bH2.PH2]2) was chosen as the best‐fitted model. For this fitted model, the activation energy was found to be 109.2 kJ/mol. Characterization of the catalyst was also performed using X‐ray diffraction (XRD), BET, scanning electron microscopy (SEM), and energy‐dispersive x‐ray spectrometer (EDS) techniques.

Homogeneous catalytic transfer semihydrogenation of alkynes – an overview of hydrogen sources, catalysts and reaction mechanisms

Chemoselective semihydrogenation of alkynes to alkenes with E- or Z-stereoselectivity is among the most important transformations in the synthesis of highly functional organic building blocks.

Generation of molybdenum hydride species via addition of molecular hydrogen across metal-oxygen bond at monolayer oxide/metal composite interface

Generation of molybdenum hydride species on monolayer oxide/metal composite via addition of molecular hydrogen across metal-oxygen bond is investigated for the first time utilizing periodic Van der Waals density-functional calculations. Lewis acid-base pair constructed by the interfacially defected oxide film and the metal support provides novel active sites for activating H2. The produced heterolytic dissociative state exhibits negative dissociative adsorption energy of −0.315 eV which thermodynamically facilitate the dissociation process of H2 on insulating oxide films. The penitential energy pathways are calculated to reveal the dynamics and reaction processes for H2 splitting at the oxide-metal interface. The differential charge density contour, electronic density plots, particular occupied orbitals, work function and electron localization function of H2 dissociation are interpreted to better understand the electronic properties of the unique dissociation behavior of H2 at interfacially defected magnesia. It is anticipated that the results here could help understand the mechanism of hydrogenation reactions on nanostructured oxide film and provide useful clue for enhancing the reactivity of insulating oxide toward activating H2.

A first-principles microkinetic study on the hydrogenation of carbon dioxide over Cu(211) in the presence of water

The hydrogenation of carbon dioxide (CO2) is one of important processes to effectively convert and utilize CO2, which is also regarded as the key step at the industrial methanol synthesis. Water is likely to play an important role in this process, but it still remains elusive. To systematically understand its influence, here we computationally compare the reaction mechanisms of CO2 hydrogenation over the stepped Cu(211) surface between in the absence and presence of water based on microkinetic simulations upon density functional theory (DFT) calculations. The effects of water on each hydrogenation step and the whole activity and selectivity are checked and its physical origin is discussed. It is found that the water could kinetically accelerate the hydrogenation on CO2 to COOH, promoting the reverse water gas shift reaction to produce carbon monoxide (CO). It hardly influences the CO2 hydrogenation to methanol kinetically. In addition, the too high initial partial pressure of water will thermodynamically inhibit the CO2 conversion.

Investigation of hydrogen oxidation in supercritical H2O/CO2 mixtures using ReaxFF molecular dynamics simulation

Hydrogen oxidation kinetics in supercritical H2O/CO2 mixtures is a fundamental topic in supercritical water oxidation (SCWO) technology. A series of reactive molecular dynamics (ReaxFF-MD) simulations were performed to investigate hydrogen oxidation process in supercritical mixtures. The results showed that HO2 and H2O2 radicals played key roles in the reaction kinetics. High concentration H2O suppressed the production of OH radical and increased steric hindrance for effective collisions, exerting negative influence on hydrogen oxidation. The presence of CO2 advanced the oxidation rate of hydrogen mainly through the elementary reaction CO2 + H → CO + OH. It was found that high O2 concentration promoted the oxidation of hydrogen and also affected the reaction induction time. The hydrogen reaction mechanisms under supercritical conditions were illustrated, showing different characteristics from those at atmospheric condition. The results will facilitate the development of continuum-scale kinetic models for accurate prediction of hydrogen oxidation behavior in supercritical systems.

Electrochemical Impedance Analysis on Hydrogen Penetration Mechanisms on Steel

Environmental degradation of steels associated with the hydrogen embrittlement has been a serious problem worldwide. The hydrogen evolution reaction (HER) on steel has been studied by some electrochemical methods [1-3] in order to explore the hydrogen penetration behavior on steel. In the present study, the electrochemical impedance spectroscopy was applied to the investigation of the hydrogen reaction mechanisms on steel because the time constants observed in impedance spectrum can be discriminated. The reaction scheme of HER on a metal were proposed to be a multistep as described below [4]. H+ + M + e- → MHads (1) MHads + H3O+ + e- → H2 + H2O + M (2) The reaction related to the hydrogen adsorption and absorption on steel was expressed by the following reaction [4]. MHads ↔ MHabs (3) In these reactions, the M is metal, the MHad is the hydrogen adsorption, the MHab is the hydrogen absorption. The first step (Eq. (1)) and the second step (Eq. (2)) are Volmer reaction and Heyrovsky reaction, respectively. The third step is the hydrogen penetration reaction on steel. The reactions (2) and (3) are competition reactions when the MHad generated by the reaction (1). In order to investigate these competition reactions, the Faraday impedance simulations were carried out based on these reactions. When the reaction rate constant of hydrogen penetration reaction is small compared to the HER, the capacitive loop related to the electrode/solution interface was observed in the high frequency range and the capacitive loop appeared in the low frequency range. In the case of the large reaction rate constant of hydrogen penetration compared to the HER, the impedance spectrum described the capacitive loop related to the electrode/solution interface in the high frequency range and the inductive loop in the low frequency range, respectively. These results indicated that the low frequency impedance is related to the rate-determining step of the reactions (2) or (3). The electrochemical impedance measurement of steel was performed under potentiotatic polarization in order to discuss the HER and the hydrogen penetration mechanisms on steel based on the results of simulations.

Simultaneous absorption and hydrogenation of CO2 from flue gas by KBH4 catalyzed by nickel nanoparticles supported on TiO2

Considering that the carbon dioxide (CO2) emissions resulting from the combustion of fossil fuels significantly aggravate the global greenhouse effect and frequently lead to extreme environmental problems, it was proposed in this work to carry out the simultaneous capture and resource utilization of CO2 from flue gas by using a low-energy method. Importantly, nickel nanoparticles supported on titanium dioxide (Ni NPs@TiO2) was prepared to catalyze potassium borohydride (KBH4) to be transformed into BH4-n(OH)n− (n = 0–3) species with strong hydride donation capacity, which hydrogenated CO2 to formate under atmospheric pressure and mild temperature. The optimum experimental conditions for the catalytic hydrogenation of CO2 were determined by investigating the effects of various factors on the CO2 conversion: the Ni NPs@TiO2 dosage was 1 g·L−1, KBH4 concentration was 0.2 mol·L−1, solution pH was 9.5, reaction temperature was 60 °C and flow velocity of flue gas was 0.5 L·min−1. The average CO2 conversion efficiency was achieved as 51.2% under the optimal conditions. Based on a series of catalyst characterizations, product analyses and related literatures, the catalytic hydrogenation reaction mechanism for the conversion of CO2 to formate was analyzed. This work can provide reference for developing the processes for CO2 emission reduction combined with resource utilization.

Metal-free carbocatalyst for catalytic hydrogenation of N-containing unsaturated compounds

Eco-friendly carbocatalyst is extremely desirable in organic catalysis and wastewater treatment. Herein, we reported a one-pot synchronous hydrothermal process to synthesize a highly efficient N doped holey graphene (NHG) carbocatalyst, in which graphene oxide nanosheets are self-assembled, reduced, chemically etched, and simultaneously doped with nitrogen to form a graphene hydrogel in the presence of H2O2 and NH4OH. Benefiting from the good hydrophilicity, unique hierarchical porous structure, large specific surface area (322.1 m2 g−1), and high N content (9.77 at. %), the resultant NHG carbocatalyst exhibits an excellent catalytic performance for the hydrogenation of N-containing unsaturated compounds. In 4-nitrophenol reduction reaction, the NHG carbocatalyst delivers a turnover frequency of 3.32 × 10−2 min−1. This catalytic activity is superior to that of the commercial Pd/C (5.0 wt%), other previously reported carbocatalysts, and many noble-metal-based catalysts. Furthermore, the molecular structure analysis combined with density functional theory calculations elucidate metal-free catalytic mechanism of hydrogenation reaction for unsaturated chromophore groups in N-containing organic dyes.

Intensive investigation on hydrogen storage properties and reaction mechanism of the NaBH4-Li3AlH6 destabilized system

Notable effects of Li3AlH6 on the hydrogen storage properties of the NaBH4 are studied intensively. Li3AlH6 is synthesized by milling 2LiH-LiAlH4 mixture for 12 h. The best molar ratio of the NaBH4- Li3AlH6 destabilized system is 1:1 which has decomposed at two stages; Li3AlH6 decomposition stage at 170 °C and NaBH4decomposition stage at 400 °C. As no significant effect on the decomposition temperature between 1 h and 24 h of milling time can be observed, the 1-hour milling preparation method is selected for the characterization. Isothermal absorption has shown that the system is able to absorb 4.2 wt% and 6.1 wt% of hydrogen in 60 min at 330 °C and 420 °C under 30 atm of hydrogen pressure. In contrast, only about 3.4 wt% and 3.7 wt% of hydrogen can be absorbed by the milled NaBH4 under a similar condition. Meanwhile, the system is able to desorb 2.0 wt% and 4.1 wt% of hydrogen in 60 min at 330 °C and 420 °C in isothermal desorption while only 0.3 wt% and 2.1 wt% can be released by the milled NaBH4 under the similar condition. The decomposition activation energy and enthalpy of the NaBH4 stage are calculated to be 162.1 kJ/mol and 68.1 kJ/mol H2. Based on the X-ray diffraction analysis, Na, Al and AlB2 are formed during the dehydrogenation process. The formation of Al and AlB2 are the keys to the improvement of hydrogenation properties. It is concluded that Li3AlH6 is a good destabilizing agent for the NaBH4 system.

Tuning CO2 hydrogenation selectivity via metal-oxide interfacial sites

CO2 hydrogenation over ZrO2-supported NiFe catalysts is investigated to illustrate the role of Fe in controlling the activity and selectivity, and to reveal the structure-function relationship between metal-oxide interfaces and catalytic selectivities. The Ni-ZrO2 interfaces and Ni-FeOx interfaces are identified as the most likely active sites for the methanation reaction and the reverse water-gas shift reaction, respectively, using combined in-situ and ex-situ characterization techniques. The reaction mechanisms of CO2 hydrogenation to CH4 on the Ni-ZrO2 interfacial sites and to CO on the Ni-FeOx interfacial sites are further revealed by combined in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density functional theory (DFT) calculations. Both experimental and theoretical results demonstrate that the binding energy of absorbed CO (*CO) is a key descriptor to predict CO2 hydrogenation selectivity: weak interaction (e.g., Ni-FeOx interfaces) promotes *CO desorption to increase CO selectivity, while moderate interaction (e.g., Ni-ZrO2 interfaces) facilitates further hydrogenation of *CO to produce CH4.

Hydrogen Evolution and Oxidation: Mechanistic Studies and Material Advances.

Electrochemical energy storage and conversion through hydrogen is essential for a clean and sustainable energy system. Highly efficient hydrogen electrocatalysts play a key role in the electrochemical transformation reactions. A comprehensive understanding of the hydrogen reaction kinetics and mechanisms is critical for the catalyst design and development. Especially pH-dependent hydrogen evolution and oxidation reaction (HER/HOR) kinetics receives increasing interest, and understanding its origin adds new knowledge to fundamental hydrogen electrocatalysis. Here, a detailed description of kinetic analysis and reaction mechanisms for HER/HOR, and a brief summary about recent development of highly efficient and cost-effective hydrogen electrocatalysts are presented. Lastly, recent advances in the fundamental understanding of pH-dependent hydrogen electrocatalysis are discussed.

Mechanistic study of methanol synthesis from CO2 hydrogenation on Rh-doped Cu(111) surfaces

Self-consistent periodic density functional theory (DFT) calculations were employed to explore the adsorption and reaction mechanisms of CO2 hydrogenation to methanol via the reverse water-gas-shift pathway on Cu(111) and three RhCu(111) surfaces with different doped-Rh atoms on Cu(111) surfaces, denoted as Rh3Cu6(111), Rh6Cu3(111) and Rh ML surfaces. All the possible favored adsorption sites, structures and adsorption energies of the relative species on Cu(111) and RhCu(111) were determined. H2CO*, HCO*, CO*, cis-COOH* and trans-COOH* adsorbing at Rh atoms through C atom are strengthened by the doped Rh, and the adsorption energies of species on Cu sites through O atom are not altered obviously, which can be explained by d-PDOS analysis. Through the analyses of Brønsted-Evans-Polanyi (BEP) relationships, it is found that the adsorption of trans-COOH* and co-adsorbed CO*/H* is stronger on RhCu(111) surfaces, while the energy of CO* in the TS geometry is dramatically decreased by doping with Rh atoms, leading to the alternation of rate-limiting step from CO2 hydrogenation forming trans-COOH* on Cu(111) and Rh3Cu6(111) surfaces to CO hydrogenation forming HCO* on Rh3Cu6(111) and Rh ML (monolayer) surfaces. The order of the highest activation barriers for the overall reaction is Cu(111) > Rh6Cu3(111) > Rh ML > Rh3Cu6(111). Moreover, due to the dissociative adsorption of H2, the yield of CO byproduct is suppressed. The calculated results show that the overall reaction process of CH3OH synthesis is facilitated kinetically and thermodynamically, especially on Rh3Cu6(111) surface. The present insights are helpful for the rational design and optimization of Rh-Cu bimetallic catalysts used in the process of CO2 hydrogenation to CH3OH synthesis.

Hydrogenation and hydration of carbon dioxide: a detailed characterization of the reaction mechanisms based on the reaction force and reaction electronic flux analyses.

A computational DFT study of the reaction mechanism of hydrogenation and hydration of carbon dioxide is presented. It has been found that hydrogenation and hydration are endoenergetic reactions that are carried out in two steps, passing by a stable intermediate that is surrounded by energy barriers of 70 kcal/mol and 10 kcal/mol for hydrogenation and 50 kcal/mol and 10 kcal/mol for hydration. Using the reaction force analysis, we were able to characterize the physical nature of the activation barriers and found that activation energies are mostly due to structural rearrangements. An interesting difference in the reaction mechanisms disclosed by the reaction force and electronic flux analyses is that while in the hydrogenation reaction the mechanisms is conditioned by the H2 cleavage with a high energy barrier, in the hydration reaction the formation of a transient four member ring structure favoring an attractive local hydrogen bond interaction pushes the reaction toward the product with a considerably lower energy barrier.

H2 dissociation by Au1-doped closed-shell titanium oxide cluster anions

Dissociation of molecular hydrogen (H2) is extensively studied to understand the mechanism of hydrogenation reactions. In this study, H2 dissociation by Au1-doped closed-shell titanium oxide cluster anions AuTi3O7− and AuTi3O8− has been identified by mass spectrometry and quantum chemistry calculations. The clusters were generated by laser ablation and mass-selected to react with H2 in an ion trap reactor. In the reaction of AuTi3O8− with H2, the ion pair Au+−O22− rather than Au+−O2− is the active site to promote H2 dissociation. This finding is in contrast with the previous result that the lattice oxygen is usually the reactive oxygen species in H2 dissociation. The higher reactivity of the peroxide species is further supported by frontier molecular orbital analysis. This study provides new insights into gold catalysis involving H2 activation and dissociation.

Hydrogen storage performances and reaction mechanism of non-stoichiometric compound Li1.3Na1.7AlH6 doped with Ti3C2

Our previous work found that Li1.3Na1.7AlH6 possesses the lowest dehydrogenation onset temperature and the most favorable dehydrogenation thermal stability among Li-Na-Al-H compounds (Fan et al., 2017). In this work, additional Ti3C2 was introduced into Li1.3Na1.7AlH6 by ball milling to further enhance its dehydrogenation performances. According to the volumetric release curves, the initial dehydrogenation temperature of the sample Li1.3Na1.7AlH6 + 5 wt.%Ti3C2 was lowered by 35 K from that of the pristine Li1.3Na1.7AlH6. The dehydrogenation enthalpy values of Li1.3Na1.7AlH6 + 5 wt.%Ti3C2 (46.9 KJ mol H2−1) was also slightly lower than that of pristine Li1.3Na1.7AlH6 (49.7 KJ mol H2−1). It means that the doped Ti3C2 was actively involved in the reaction with Li1.3Na1.7AlH6 during the dehydrogenation process. The investigation by X-ray photoelectron spectroscopy (XPS) demonstrated that the reaction between Ti element and Li1.3Na1.7AlH6 was definitely carried out in which the zero valence of Ti was transformed into Ti3+.

Front Cover: Phenol Catalytic Hydrogenation over Palladium Nanoparticles Supported on Metal‐Organic Frameworks in the Aqueous Phase (ChemCatChem 12/2018)

The Front Cover illustrates the different reaction barriers that must be overcome in the phenol hydrogenation to either cyclohexanol or cyclohexanone. In their Full Paper, H. Chen et al. investigated the reaction mechanism of phenol hydrogenation catalyzed by Pd nanoparticles supported on a hollow box-shaped Ni-MOF in aqueous phase. Experimental and theoretical results demonstrate that phenol is first hydrogenated to cyclohexanone, which is then hydrogenated to cyclohexanol, and finally, cyclohexanol is hydrogenated to cyclohexane. More information can be found in the Full Paper by H. Chen et al. on page 2558 in Issue 12, 2018 (DOI: 10.1002/cctc.201800211).