Catalytic Cracking of Cycloparaffins Admixed with Olefins:1. Single-Event Microkinetic(SEMK) Modeling

Single-event microkinetic(SEMK) model of the catalytic cracking of methylcyclohexane admixed with 1-octene over REUSY zeolites at 693 K—753 K in the absence of coke formation is enhanced. To keep consistency with the wellknown carbenium ion chemistry, hydride transfer forming and consuming allylic carbenium ions in the aromatization of cycloparaffins are further investigated and differentiated. The reversibility of endocyclic β-scission and cyclization reactions is refined by accounting explicitly for the reacting olefins and resulting cycloparaffins in the corresponding thermodynamics. 24 activation energies for the reactions involved in the cracking of cycloparaffins are obtained by the regression of 15 sets of experimental data upon taking the resulting 37 main cracking products, i. e., responses into account. The enhanced SEMK model can adequately describe the catalytic behavior of 37 main products with conversion and temperature.

Standard-state entropies and their impact on the potential-dependent apparent activation energy in electrocatalysis

The apparent activation energy,Eapp,is a common measure in thermal catalysis to discuss the activity and limiting steps of catalytic processes on solid-state materials.Recently,the electrocatalysis community adopted the concept of Eappand combined it with the Butler-Volmer theory.Certain observations though,such as potential-dependent fluctuations of Eapp,are yet surprising because they conflict with the proposed linear decrease in Eappwith increasing overpotential.The most common explanation for this finding refers to coverage changes upon alterations in the temperature or the applied electrode potential.In the present contribution,it is demonstrated that the modulation of surface coverages cannot entirely explain potential-dependent oscillations of Eapp,and rather the impact of entropic contributions of the transition states has been overlooked so far.In the case of a nearly constant surface coverage,these entropic contributions can be extracted by a dedicated combination of Tafel plots and temperature-dependent experiments.

La/Ni(111)表面CO甲烷化:助剂La对活性和选择性的影响

针对Ni(111)表面上因副产物CH_3OH形成而导致CH_4选择性低的问题,采用量子化学密度泛函理论(density functional theory,DFT)计算的方法,通过添加富电子的助剂La调节表面Ni原子的电子状态,增大Ni的d电子平均能以增加La/Ni(111)表面的反应性,实现La与Ni的协同催化,从而提高CO甲烷化活性和CH_4生成选择性。同时,基于DFT结果,以Microkinetic modeling模拟实验条件下CH_4和CH_3OH的生成速率,结果表明,反应速率r随着温度升高而增大;在同一温度下,CH_4生成速率r(CH_4)远大于CH_3OH的生成速率r(CH_3OH),且S_(CH4)在反应温度550~750 K内高达100%,表明在La/Ni(111)表面上的CO甲烷化过程中没有副产物CH_3OH的生成。究其原因,在电子水平上通过对CH_4形成路径Path1、Path4、Path9和Path10所对应的关键中间体CO、HCO、CH_2O和COH进行Bader电荷和pDOS分析,发现C—O键明显弱化,C—O断键能垒显著降低,因而无CH_3OH生成的微观机理是La→Ni电子离域和La与O强相互作用而产生的"给电子诱导"效应。进一步通过La与邻近Ni原子的三维差分电荷密度分析得知,La原子的电荷损耗是沿着"La→Ni"方向,这就给出了助剂La与Ni协同催化CO甲烷化并高活性高选择性地生成CH_4的微观解释。

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.

Mechanism investigation and catalyst screening of high-temperature reverse water gas shift reaction

Reverse water gas shift(RWGS)catalysis,a prominent technology for converting CO2 to CO,is emerging to meet the growing demand of global environment.However,the fundamental understanding of the reaction mechanism is hindered by the complex nature of the reaction.Herein,microkinetic modeling of RWGS on different metals(i.e.,Co,Ru,Fe,Ni,Cu,Rh,Pd,and Pt)was performed based on the DFT results to provide the mechanistic insights and achieve the catalyst screening.Adsorption energies of the carbon-based species and the oxygen-based species can be correlated to the adsorption energy of carbon and oxygen,respectively.Moreover,oxygen adsorption energy is an excellent descriptor for the barrier of CO2 and CO direct dissociation and the difference in reaction barrier between CO2(or CO)dissociation and hydrogenation.The reaction mechanism varies on various metals.Direct CO2 dissociation is the dominating route on Co,Fe,Ru,Rh,Cu,and Ni,while it competes with the COOH-mediated path on Pt and Pd surface.The eights metals can be divided into two groups based on the degree of rate control analysis for CO production,where CO–O bond cleavage is rate relevant on Pt,Pd,and Cu,and OH–H binding is rate-controlling on Co,Fe,Ru,Ni,and Rh.Both CO-direct dissociation and hydrogen-assisted route to CH4 contribute to the methane formation on Co,Fe,Pt,Pd,Ru,and Rh,despite the significant barrier difference between the two routes.Besides,the specific rate-relevant transition states and intermediates are suggested for methane formation,and thus,the selectivity can be tuned by adjusting the energy.The descriptor(C-and O-formation energy)based microkinetic modeling proposed that the activity trend is Rh~Ni>Pt~Pd>Cu>Co>Ru>Fe,where Fe,Co,Ru,and Ni tends to be oxidized.The predicted activity trend is well consistent with those obtained experimentally.The interpolation concept of adsorption energy was used to identify bimetallic materials for highly active catalysts for RWGS.

An efficient single atom catalysts Os/P_(3)C sheet for ammonia borane dehydrogenation

Ammonia borane(NH_(3)BH_(3),AB)has been considered to be a promising chemical hydrogen storage material.Based on density functional theory,a series of transition metal atoms supported P_(3)C(P_(3)C_O)sheet is systematically investigated to screen out the most promising catalyst for dehydrogenation of AB.The results indicate that the Os/P_(3)C and Os/P_(3)C_O could be an efficient single atom catalyst(SACs)and the stepwise reaction pathway with free energy barrier of 2.07 and 1.54 e V respectively.Remarkably,the rate constant further quantitatively confirmed the real situation of the first step of dehydrogenation of AB on the Os/P_(3)C and Os/P_(3)C_O substrates.We found that k_(f1)at 400 K is equivalent to k_(f2)at 800 K,which greatly improves the temperature of the first step of AB dehydrogenation on P_(3)C_O.We hope this work can provide a promising method for the design of catalysts for AB dehydrogenation reactions on the surface of two-dimensional materials(2D).