Recent Advances in Mechanistic Understanding of Metal-Free Carbon Thermocatalysis and Electrocatalysis with Model Molecules

Metal-free carbon,as the most representative heterogeneous metal-free catalysts,have received considerable interests in electro-and thermo-catalytic reac-tions due to their impressive performance and sustainability.Over the past decade,well-designed carbon catalysts with tunable structures and heteroatom groups coupled with various characterization techniques have proposed numerous reaction mechanisms.However,active sites,key intermediate species,precise structure-activity relationships and dynamic evolution processes of carbon catalysts are still rife with controversies due to the monotony and limitation of used experimental methods.In this Review,we sum-marize the extensive efforts on model catalysts since the 2000s,particularly in the past decade,to overcome the influences of material and structure limitations in metal-free carbon catalysis.Using both nanomolecule model and bulk model,the real contribution of each alien species,defect and edge configuration to a series of fundamentally important reactions,such as thermocatalytic reactions,electrocatalytic reactions,were systematically studied.Combined with in situ techniques,isotope labeling and size control,the detailed reaction mechanisms,the precise 2D structure-activity relationships and the rate-determining steps were revealed at a molecular level.Furthermore,the outlook of model carbon catalysis has also been proposed in this work.

Microstructure and mechanical properties of in-situ TiB_2/A356 composites and a prediction model of yield strength

The in-situ TiB2/A356 composites were successfully synthesized through the mixed salt reaction method. The advantage of this technique was that the particle sizes and morphology can be controlled by the melt reaction. Therefore, the technique can be designed to obtain expected properties, such as high strength at high and room temperatures, high damping capacity, high modulus and good fatigue life. Results showed that in the as-cast state of A356 alloy and TiB2/A356 composites, the eutectic Si phase is normally in the needle shape, and TiB2 particles are mostly in the cubic or near spherical shape, with the size ranging from 30 to 500 nm uniformly distributed in the grains. Also, TiB2 particle clusters are observed in composites. With an increase in TiB2 particles, the average grain size of composites decreases both in as-cast and T6 state. It is found that both the yield stength and ultimate tensile strength increase with an increase in the TiB2 volume fraction. On the contrary, the elongation reduces with the addition of TiB2 particles. Based on the experimental results and Clyne＇s report, a revised model related to particle strengthening mechanism was proposed to fairly predict yield strengths of TiBJA356 composites. The satisfactory agreement between the calculated values and experimental data reported in the literature was obtained.

Study on Lumped Kinetic Model for FDFCC I. Establishment of Model

According to the process features and the reaction mechanism of FDFCC technology, its two reaction subsystems, one for heavy oil riser reactor, the other for gasoline riser reactor, were respec-tively studied. Correspondingly, a 12-lump kinetic model for heavy oil FCC and a 9-lump kinetic model for gasoline catalytic upgrading were presented. Based on this work, mathematical correlation of the lumps in the feeds and products involved in the reaction subsystems and those of the overall reaction system were analyzed in detail. Then, a combined kinetic model for FDFCC, which was based on the data recovered from a commercial unit, was put forward. The reaction performance embodied by the kinetic constants for the combined model of FDFCC was in accordance with catalytic cracking reaction mechanism. The model-calculated values were close to the data obtained in commercial scale. The model was easy to be applied in practice and could also provide some theoretical groundwork for further re-search on kinetic model for FDFCC.

A Review of In‑Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction(ORR)is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal–O2/air batteries,etc.However,the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process,and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction.This makes them difficult to be accurately captured,making the identification of ORR active sites and the elucidation of ORR mechanisms difficult.Thus,it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR.This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts.Specifically,the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized,such as phase,valence,electronic transfer,coordination,and spin states varies.In-situ revelation of intermediate adsorption/desorption behavior,and the real-time monitoring of the product nucleation,growth,and reconstruction evolution are equally emphasized in the discussion.Other interference factors,as well as in-situ signal assignment with the aid of theoretical calculations,are also covered.Finally,some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Unraveling the advantages of Pd/CeO_(2)single-atom catalysts in the NO+CO reaction by model catalysts

Selective catalytic reduction of NO by CO is challenging in environmental catalysis but attractive owing to the advantage of simultaneous elimination of NO and CO.Here,model catalysts consisting of Pd nanoparticles(NPs)and single-atom Pd supported on a CeO_(2)(111)film grown on Cu(111)(denoted as Pd NPs/CeO_(2)and Pd_(1)/CeO_(2),respectively)were successfully prepared and characterized by synchrotron radiation photoemission spectroscopy(SRPES)and infrared reflection absorption spectroscopy(IRAS).The NO+CO adsorption/reaction on the Pd_(1)/CeO_(2)and Pd NPs/CeO_(2)catalysts were carefully investigated using SRPES,temperature-programmed desorption(TPD),and IRAS.It is found that the reaction products on both model catalysts are in good agreement with those on real catalysts,demonstrating the good reliability of using these model catalysts to study the reaction mechanism of the NO+CO reaction.On the Pd NPs/CeO_(2)surface,N_(2)is formed by the combination of atomic N coming from the dissociation of NO on Pd NPs at higher temperatures.N_(2)O formation occurs probably via chemisorbed NO combined with atomic N on the surface.While on the single-atom Pd_(1)/CeO_(2)surface,no N_(2)O is detected.The 100%N_(2)selectivity may stem from the formation of O-N-N-O^(*)intermediate on the surface.Through this study,direct experimental evidence for the reaction mechanisms of the NO+CO reaction is provided,which supports the previous density functional theory(DFT)calculations.

Study of the reaction mechanism for preparing powdered activated coke with SO_(2)adsorption capability via one-step rapid activation method under flue gas atmosphere

In this study,the impact of different reaction times on the preparation of powdered activated carbon(PAC)using a one-step rapid activation method under flue gas atmosphere is investigated,and the underlying reaction mechanism is summarized.Results indicate that the reaction process of this method can be divided into three stages:stage I is the rapid release of volatiles and the rapid consumption of O_(2),primarily occurring within a reaction time range of 0-0.5 s;stage II is mainly the continuous release and diffusion of volatiles,which is the carbonization and activation coupling reaction stage,and the carbonization process is the main in this stage.This stage mainly occurs at the reaction time range of 0.5 -2.0 s when SL-coal is used as material,and that is 0.5-3.0 s when JJ-coal is used as material;stage III is mainly the activation stage,during which activated components diffuse to both the surface and interior of particles.This stage mainly involves the reaction stage of CO_(2)and H2O(g)activation,and it mainly occurs at the reaction time range of 2.0-4.0 s when SL-coal is used as material,and that is 3.0-4.0 s when JJ-coal is used as material.Besides,the main function of the first two stages is to provide more diffusion channels and contact surfaces/activation sites for the diffusion and activation of the activated components in the third stage.Mastering the reaction mechanism would serve as a crucial reference and foundation for designing the structure,size of the reactor,and optimal positioning of the activator nozzle in PAC preparation.

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity,in general,can promote a given reaction by stabilizing a transition state or a product via a mobility ofπelectrons in a cyclic structure.Similarly,such a promotion could be also achieved by destabilizing an antiaromatic reactant.However,both aromaticity and transition states cannot be directly measured in experiment.Thus,computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms.In this review,we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity.Specifically,we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation,Csingle bondF bond activation,rearrangement,as well as metathesis reactions.In addition,antiaromaticity-promoted dihydrogen activation,CO_(2)capture,and oxygen reduction reactions will be also briefly discussed.Finally,caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases.Thus,a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Study on Lumped Kinetic Model for FDFCC Ⅱ.Validation and Prediction of Model

On the basis of formulating the 9-lump kinetic model for gasoline catalytic upgrading and the 12- lump kinetic model for heavy oil FCC, this paper is aimed at development of a combined kinetic model for a typical FDFCC process after analyzing the coupled relationship and combination of these two models. The model is also verified by using commercial data, the results of which showed that the model can better predict the product yields and their quality, with the relative errors between the main products of the unit and commercial data being less than five percent. Furthermore, the combined model is used to predict and optimize the operating conditions for gasoline riser and heavy oil riser in FDFCC. So this paper can offer some guidance for the processing of FDFCC and is instructive to model research and development of such multi-reactor process and combined process.

Suggestion of global model for carob batch fermentation to produce bioethanol

Modelling of carob batch fermentation is established basing on mass transfer balances. The modelling treats the reaction kinetics of substrate （S）, the micro-organisms （X） and the ethanol （E）. Nine models are taken from the literature to describe specific organism growth rate and specific ethanol development rate. These models treat all types of fermentation. The Phisalapbong et al. model and the Ghose and Tyagi model show the best fit of the experimental data. This affirms that the batch fermentation of carob is conducted with substrate and/or ethanol inhibition. Some simulations and relationships （X = f（S）, E = f（S）） are obtained from the Phisalaphong et al. model. Those simulations show a lot of important and useful results of carob batch fermentation process.

Understanding reaction mechanisms of metal-free dinitrogen activation by methyleneboranes

Dinitrogen activation under mild conditions is important but extremely challenging due to the inert nature of the N≡N triple bond evidenced by high bond dissociation energy(945 k J/mol) and large HOMOLUMO gap(10.8 e V). In comparison with largely developed transition metal systems, the reported main group species on dinitrogen activation are rare. Here, we carry out density functional theory calculations on methyleneboranes to understand the reaction mechanisms of their dinitrogen activation. It is found that the methyleneboranes without any substituent at the boron atom performs best on dinitrogen activation, which could be contributed to its small singlet-triplet gap. In addition, strong correlations are achieved on dinitrogen activation between the singlet-triplet energy gap and the reaction energies for the formation of the end-on products as well as the side-on ones. The principal interacting orbital analysis suggests that methyleneboranes can mimic transition metals to cleave the N≡N triple bond. Our findings could be helpful for experimental chemists aiming at dinitrogen activation by main group species.

Mechanism and active sites of CO oxidation over single-crystal Au surfaces and a Au/TiO_2(110) model surface

We describe the reaction mechanism and active sites for CO oxidation over a Au/TiO2（110） model surface and Au single‐crystal surfaces, along with the role of H2O, on a molecular scale. At low tem‐perature （＆lt;320 K）, H2O played an essential role in promoting CO oxidation, and the active site for CO oxidation was the perimeter of the interface between the gold nanoparticles and the TiO2 sup‐port （Auδ＋–Oδ––Ti）. We believe that the O–O bond was activated by the formation of OOH, which was produced directly from O2 and H2O at the perimeter of the interface between the gold nanoparticles and the TiO2 support, and consequently OOH reacted with CO to form CO2. This reaction mechanism explains the dependence of the CO2 formation rate on O2 pressure at 300 K. In contrast, at high temperature （＆gt;320 K）, low‐coordinated gold atoms built up on the surface as a result of surface reconstruction due to exposure to CO. The low‐coordinated gold atoms adsorbed O2, which then dissociated and oxidized CO on the metallic gold surface.

The Effect of Different Reaction Mechanisms on Combustion Simulation of a Reverse-Flow Combustor

Three different reaction mechanisms of kerosene and flamelet models were used to simulate combustion in a reverse-flow combustor.By comparing the effects of different mechanisms on the flow field characteristics,components and temperature distribution of the combustion chamber,the results showed that:Under different reaction mechanisms,there was a strong similarity between flow filed and temperature field,but the penetration depth and temperature distribution of local jets were affected by the mechanism.Due to the different reaction paths and reaction rates,the distribution of major components had a great degree of similarity,but the concentration of intermediate components varied greatly.Comprehensive analysis,the 16 species and 17 species reaction mechanisms can simulate the flow field and outlet temperature distribution of the combustor well.

New progress in theoretical studies on palladium-catalyzed C-C bond-forming reaction mechanisms

This review reports a series of mechanistic studies on Pd-catalyzed C-C cross-coupling reactions via density functional theory(DFT) calculations.A brief introduction of fundamental steps involved in these reactions is given,including oxidative addition,transmetallation and reductive elimination.We aim to provide an important review of recent progress on theoretical studies of palladium-catalyzed carbon-carbon cross-coupling reactions,including the C-C bond formation via C-H bond activation,decarboxylation,Pd(Ⅱ)/Pd(Ⅳ) catalytic cycle and double palladiums catalysis.

Water Gas Shift Reaction： A Monte Carlo Simulation

The water gas shift （WGS） reaction is reacts with water on a catalytic surface a process of industrial importance to form CO2 and H2. We study this In this reaction carbon monoxide reaction with thermal （Langmuir- Hinshelwood） and non-thermal （precursor and Eley-Rideal） reaction mechanisms using the techniques of Monte Carlo computer simulation. The details of surface coverages and production rates are given as a function of CO partial pressure. The diffusion of species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system have been drawn to observe the behaviour of reacting species on the surface. The study reveals that the production rates are higher for non-thermal precursor mechanism and are in agreement with the experimental finding.

Investigation of the kinetic mechanism of the demanganization reaction between carbon-saturated liquid iron and CaF2–CaO–SiO2-based slags

The demanganization reaction kinetics of carbon-saturated liquid iron with an eight-component slag consisting of CaO–SiO2–MgO–FeO–MnO–Al2O3–TiO2–CaF2 was investigated at 1553, 1623, and 1673 K in this study. The rate-controlling step（RCS） for the demanganization reaction with regard to the hot metal pretreatment conditions was studied via kinetics analysis based on the fundamental equation of heterogeneous reaction kinetics. From the temperature dependence of the mass transfer coefficient of a transition-metal oxide（MnO）, the apparent activation energy of the demanganization reaction was estimated to be 189.46 kJ·mol^–1 in the current study, which indicated that the mass transfer of MnO in the molten slag controlled the overall rate of the demanganization reaction. The calculated apparent activation energy was slightly lower than the values reported in the literature for mass transfer in a slag phase. This difference was attributed to an increase in the ＂specific reaction interface＂（SRI） value, either as a result of turbulence at the reaction interface or a decrease of the absolute amount of slag phase during sampling, and to the addition of calcium fluoride to the slag.

Combustion characteristics of unburned pulverized coal and its reaction kinetics with CO2

The combustion characteristics of two kinds of unburned pulverized coal (UPC) made from bituminous coal and anthracite were investigated by thermogravimetric analysis under air. The reaction kinetics mechanisms between UPC and CO2 in an isothermal experiment in the temperature range 1000–1100°C were investigated. The combustion performance of unburned pulverized coal made from bituminous coal (BUPC) was better than that of unburned pulverized coal made from anthracite (AUPC). The combustion characteristic indexes (S) of BUPC and AUPC are 0.47 × 10^-6 and 0.34 × 10^-6 %2·min^-2·°C^-3, respectively, and the combustion reaction apparent activation energies are 91.94 and 102.63 kJ·mol^-1, respectively. The reaction mechanism of BUPC with CO2 is random nucleation and growth, and the apparent activation energy is 96.24 kJ·mol^-1. By contrast, the reaction mechanism of AUPC with CO2 follows the shrinkage spherical function model and the apparent activation energy is 133.55 kJ·mol^-1.

A review on the structure-performance relationship of the catalysts during propane dehydrogenation reaction

Dehydrogenation of propane(PDH)technology is one of the most promising on-purpose technologies to solve supply-demand unbalance of propylene.The industrial catalysts for PDH,such as Pt-and Cr-based catalysts,still have their own limitation in expensive price and security issues.Thus,a deep understanding into the structure-performance relationship of the catalysts during PDH reaction is necessary to achieve innovation in advanced high-efficient catalysts.In this review,we focused on discussion of structure-performance relationship of catalysts in PDH.Based on analysis of reaction mechanism and nature of active sites,we detailed interaction mechanism between structure of active sites and catalytic performance in metal catalysts and oxide catalysts.The relationship between coke deposition,co-feeding gas,catalytic activity and nanostructure of the catalysts are also highlighted.With these discussions on the relationship between structure and performances,we try to provide the insights into microstructure of active sites in PDH and the rational guidance for future design and development of PDH catalysts.

Mechanism of non isothermal reaction between elemental powders Ti and Al

The mechanism of the reaction between elemental Ti and Al powders in continuous heating was studied through DSC and XRD phase analyses. Results show that, only one exothermic peak appeared on DSC curves for blended elemental Ti and Al powder compacts; and the onset temperature increased with increasing heating rate. After heating to 1 200 ℃, the main phases of the heating products were Ti 3Al and TiAl phases. By kinetic calculation, the apparent activation energy for the exothermic reaction was determined as 340±20 kJ/mol. Based on these results, it is suggested that the reaction between elemental Ti and Al powders be a complex one. During this reaction, TiAl 3 is formed first, finally Ti 3Al and TiAl. The rate and intensity of the reaction are inherently dependent on the composition and morphology of raw materials, as well as the heating rate.

Effect of reaction conditions and kinetic study on the Fischer-Tropsch synthesis over fused Co-Ni /Al_2O_3 catalyst

Co-Ni/Al2O3catalyst was prepared by the fusion method and used in Fischer-Tropsch synthesis(FTS).The catalysts were characterized by means of nitrogen sorption and scanning electron microscopy.The effect of some reaction conditions such as temperature,pressure and H2/CO feed ratio on the catalytic performance of Co-Ni/Al2O3in CO hydrogenation was investigated in a fixed-bed reactor.The results indicate that the optimum reaction conditions are 250℃,0.3 MPa,H2/CO feed ratio of 2.0,and GHSV of 3 000 h-1.Kinetically,the reaction rate was correlated with the Langmuir-Hinshelwood-Hougen-Watson type models.The activation energy for the best fitted model is 88.41 kJ/mol,suggesting that the intra-particle mass transport is not significant.

Theoretical Study on the Reaction Mechanism of CH2F Radical with HNCO

The reaction mechanism of CH2F radical with HNCO was investigated by density functional theory （DFT） at the B3LYP/6-311＋＋G（d,p） level. The geometries of the reactants, the intermediates, the transition states and the products were optimized. The transition states were verified through the vibration analysis. The relative energies were calculated at the QCISD（T）/6-311＋＋G^＊＊//B3LYP/6-311＋＋G（d,p） level. Seven feasible reaction pathways of the reaction were studied. The results indicate that the pathway （5） is the most favorable to occur, so it is the main pathway of the reaction.