在双促进铁催化剂上氨合成反应机理与动力学方程

本文从已知实验事实的理论分析,包括使用EHMO量子化近似计算及反应速率方程的推导等理论分析方法,提出了氮的活化、氢的活化、~(28)N_2—~(30)N_2同位素交换的机理.并提出氮的分子吸附以及H~(+δ)对吸附N_2~(-δ)作用生成NH_x是氨合成反应的二个决定性步骤.N_2~(-δ)与NH_x的相对含量决定于催化剂组成和反应条件(温度、压力、原料组成).文中根据微观分子催化作用机理,推导出相应的动力学方程式,该方程形式上与Temkin推广式一致,二者仅差一氢吸附常数项,但微观作用机理与某些动力学参数的物理意义则有重要的不同

CATALYSIS MECHANISM AND KINETIC EQUATION OF AMMONIA SYNTHESIS ON DOUBLY-PROMOTED IRON CATALYSTS

On the basis of the theoretical analysis (including the EHMO approximation calculationand the derivations of differential rates) of many known experimental facts, the modes ofactivation of N_2 and H_2 and the mechanism of ~(28)N_2-~(30)N_2 isotopic exchange have been put for-ward, together with the proposal of the kinetic mechanism of two rate controlling steps ofammonia synthesis on doubly-promoted iron catalysts, i.e. to form adsorbed molecular N_2~(-δ)and to form NH_x (x=0, 1, 2) from reaction of N_2~(-δ) with 2H~(+δ).The relative concentrationsbetween N_2~(-δ) and NH_x vary with catalyst composition and reaction conditions (pressure, temper-ature,gas composition). From the proposed molecular catalysis mechanism,a kineticequation of ammonia synthesis on doubly-promoted iron catalyst can be derived.This equa-tion, except the hydrogen adsorption constant term, is in agreement in mathematical formwith the extended kinetic equation given by Temkin in 1963, but there are significant dif-ferences in k

在双促进铁催化剂上氨合成反应动力学及作用机理

在前文的基础上,根据红外吸收光谱,场离子质谱,俄歇电子能谱,低能电子衍射,吸附后金属输出功增量,~(15)N_2-~(14)N_2同位素交换,D_2对氨合成反应速率的反同位素效应,氢对氮吸附量、吸附速度、氨生成速度的影响,同位素方法测出的化学计量比等方面的实验事实,以及量子化学计算结果,提出在铁催化剂上氨合成的催化作用机理,并推导出相应的动力学方程式。这方程形式上与1963年推广式一致,但动力学机理与微观参数物理化学意义不同。本文不主张气相H_2分子与吸附氮作用而主张吸附的N_2δ~-与诱生吸附的H~δ~+(或H_2δ~+)相作用,为速率控制步骤之一,从而较好地解释氮解离的化学推动力。同时,说明了N_2与H_2不互相竞争吸附位的微观原理。综合评论了国际上主要的氨合成动力学方程式及其互相间的联系。 根据本文推得的动力学方程,将0zaki-Taylor实验数据重新处理,结果表明本文方程与实验值符合程度更好。例如,由该方程处理实验数据求得的动力学参数a与压力的关系,αH/αD随温度变化的关系,都与理论估计符合。本文还求得某些新的动力学参数,并讨论其物化涵义。结果表明,D_2对氨合成反应速率的反同位素效应是热力学因素(K_(ND_3)>K_(NH_3),K_(D_2)>K_(H2))以及动力学因素[(k_2)_D>(k_2)_H]的加和,而不是Ozaki所提出

氨合成铁催化剂活性中心模型及分子氮的络合活化

根据α-Fe晶格参数,表面原子的配位不饱和性,反应的动力学几何因素,络合催化原理,以及由表面原子剩余杂化轨函所组成的基函的量子化学近似计算,本文提出N_2的端基吸附不是目前一般认为的垂直吸附在(111)晶面的底端原子上,而是N_2端基吸附在(111)晶面配位较底端原子不饱和的一个表面原子上,端基吸附与(111)面斜交约20°角,同时有三个与N_2分子中心相距约2.48A的最邻近铁原子侧基络合,生成端基加三侧基ω1,μ_3(η~2)型络合物。氨合成铁催化剂活性中心,除了这样的4-Fe原子簇吸附中心外,还需要一个和吸附中心毗邻并合用二原子的4-Fe原子簇氮分子离解中心,共构成6-Fe原子簇活性中心。本文提出的活性中心模型及吸附态,解释了H_2在铁屑上及钼屑上的吸附场电子发射显微镜图象(Brill et al.,＆ Ishizaka et al.);N_2或NH_3对催化剂预处理所引起的Mossbauer谱图象改变及氨合成活性的增加(Boudart et al.);N_2在Fe上的X光光电子能谱(Kishi et al.);说明了由场离子质谱证实的N_3~+,N_4~+的生成机理(Schmidt);~(14)N_2-~(15)N_2在Fe,Fe-Al_2O_3-K_2O上同位素交换机理,提出N或NH的表面迁移是同位素交换决定性步骤,从而解释了在Fe,Fe-Al_2O_3-K_2O上及H_2存在下同位素的显著差异及交换动力学(Ozaki et al.)。 ω_1,μ

表征催化剂性能的动力学色谱微机系统 1.乙烯环氧化银催化剂的表征

一、前言评价催化剂质量的优劣,需要测定催化剂的动力学性能(活性、选择性等),且常常是用气相色谱仪来检测反应物和产物各组份的峰面积,然后选用适当的定量方法计算出各组份的定量数据,再根据反应的计量关系计算出催化剂的活性、选择性等数值.为了简化这种处理手续,我们将动力学色谱系统与微机联机,实现数据采集、处理过程的全自动化,大大提高了工作效率

沙溪水污染对微生物群落变化的影响

沙溪是闽江上游3大支流之一.近年来,随着三明市经济的发展,局部污染事件仍时有发生.对沙溪三明河段的细菌、真菌、大肠菌群以及降解污染物的主要微生物进行了数量跟踪,结合COD、总氮和总磷等污染物对沙溪水环境与微生物群落变化的关系进行了初步的研究.结果表明:各种微生物的数量变化与主要的有机污染物值之间存在显著的相关性,所有的相关系数均大于0.7,表明微生物在降解与净化中起着重要的作用,同时对水环境的污染状况具有一定的指示作用;降解有机物的主要微生物的数量于COD显著相关,可作为该河段有机污染物水平的指标;研究结果还显示,三化总口和三农电化口是主要的有机污染点,应加强对这两个位点的监测和排污管理

流动反应体系的若干宏观动力学问题

本文简单综述了 Denbigh 有关流动反应体系的生产效率,最优反应条件的选择,以及动态规划(dynamic programming)的一些新发展;同时对简单的均相反应及多相催化反应在畿种反应器中的时空产率作了公式推导,从而说明连续流动管式反应器的若干优黠

TRANSITION-METAL BONDING FUNCTIONS AND THEIR APPLICATION IN ADSORPTIONS AND CATALYTIC REACTIONS 1 THEORETICAL-MODEL

A new method has been proposed for rapidly and chemically-intuitively giving correct information on the relative abilities or relative data (binding energy, bond structure, bond strength, vibration frequencies of surface species, etc.) of chemisorptions, dissociation and reactions on various transition metals, and the effects caused by the Metal-Promoter (or -Support) Interactions. In order to achieve this purpose, the LCAO method is first used to derive the wave function of the mono-transition-metal atom, represented by the combination of molecular orbital (M.O.) bands, where each M.O. is described as the linear combination of s and d orbitals. Second, based on various electronic spectra before and after adsorption, we assume that the adsorption and reaction occur chiefly on the valence band around the Fermi level. The valence band consists of three M.O. Groups (MOGs): the vacant MOG at the bottom of the s band, the vacant or fractional half-occupied MOG of the d band and the occupied MOG at the top of the occupied band near the Fermi level, denoted as PSI-(Mi, Vs), PSI-(Mi, Vd) and PSI-(Mi, d(occ)), respectively. Assuming that the M.O.'s energy distribution is even, the concept of mean energy and mean chemisorption binding energy is employed, and the three MOGs can be simplified as three representative M.O.s. According to the principle of Bloch energy band formation, the concept of d-s band overlap and the probability theory, some simple formulas with the parameters of metal band structure such as the width of the d band, the atomic orbital effective exponents and the total electron number of the s and d orbitals have been derived to calculate coefficients of the s and d orbitals of the three representative M.O.s. The interaction between metal and absorbate is characterized by bonding functions which depend on three factors: the overlap integral between the wave function of three representative M.O.s and the adsorbate, the thermodynamic potential and the ability of electron transfer. The bonding functions D,A,B and AB have been proposed to characterize the bonds involved in metal electron donation, metal electron acceptance, d electron back-donation and sigma-pi coordination, respectively. Our model involves intuitive chemical localized bonds and represents the delocalized effects of energy bands. Its advantages (relatively realistic, intuition, rapidity, convenience and practice) and drawbacks (the inability to obtain the absolute amounts of surface bond strength, electron charge distribution and detailed energy levels) were discussed in comparison with the EHMO, CNDO, X-alpha-SW, LDF, GVB and EMT methods

TRANSITION-METAL BONDING FUNCTIONS AND THEIR APPLICATIONS IN CATALYTIC ADSORPTIONS AND REACTIONS 3 EFFECT OF METAL VALENCE BAND AND PERIODIC TREND OF CO SIGMA-PI BONDING FUNCTIONS ON TRANSITION-METALS

Our model of metal valence band and our new concept of sigma-pi coordination are further discussed and confirmed in this paper. The infrared stretching frequencies of C-O decrease in the order 2056, 1886 and 1786 cm-1 in Ni(CO)4, Co(CO)4(-1) and Fe(CO)4(-2), which parallels the increase in d electron back-donation functions (B metal bonding functions) from 1.539, 2.121 to 2.895 on Ni, Co and Fe metals, respectively. On the other hand, the M-C bond orders increase from 1.33, 1.89 to 2.16 for Ni(CO)4, Co(CO)4(-1) and Fe(CO)4(-2), which parallel the increase in A(CO5-sigma-M-sigma)-B(CO2-pi-M-pi) metal bonding functions from 24.61, 30.01 to 33.19, respectively. They are in agreement with our new concept of sigma-pi coordination proposed in the previous paper. This new concept has also been used to analyze the mechanism of the formation of Ni(CO)4, Co(CO)4(-1) and Fe(CO)4(-2), and to explain why they can automotively hybridize each other despite the energy differences between 3d and 4s, 4p, which are very large. The effects of metal valence bands have been accounted for on all transition metals (d1 to d8), and it is demonstrated that d orbitals increase from the Vd band upward to the Vs band, and s orbitals from the Vs band downward to the Vd band, which is equivalent to a change in orbital potential, and would modify their orbital overlap integrals with the adsorbate M.O.s and the A, B metal bonding functions significantly. The effective potentials and the percentage s, d functions of Vs, Vd and d(occ) bands are the most important factors for determining the effect of the metal valence band. The effects of promoter and support are also altered by changes in the above factors. For Group VIII metals, the valence band provides various s and d orbitals at various potentials, in which a certain number of s and d orbitals can match better with CO adsorbate M.O.s, which explains why CO adsorbed species on Group VIII metals are all stable and adsorption rates are all relatively rapid. The periodic trends of metal A, B, AB and D(c) bonding functions depend on the structures of the metal valence band, i.e. the potential levels and s, d percentage functions of Vs, Vd and d(occ) bands. For 4d and 5d metals, the potential levels of the Vs band are high, which cannot form a strong CO 5-sigma-M sigma bond, but the potential levels of Vd band are higher and the width of the d band is wider than those of 3d metal, so their B bonding functions are larger, and they can be used to activate saturated and unsaturated hydrocarbons. In contrast, for 3d metals, the potentials of the Vs band are lower, which favour formation of strong CO 5-sigma-M sigma and M-C bonds, i.e. their A and D(c) bonding functions are larger, which can promote coke formation. While ABD(c)D(o) can be used to characterize CO dissociation, B/A can be used to characterize C-C formation. The characteristics of various metal bonding functions on each transition metal are useful for designing catalyst composition. A typical example has been illustrated, using the possibility to select non-noble metals instead of noble metals in hydrocarbon reactions

TRANSITION-METAL BONDING FUNCTIONS AND THEIR APPLICATIONS IN CATALYTIC ADSORPTIONS AND REACTIONS 4 CHARACTERIZATION OF CO ADSORPTION BONDING, IR-SPECTRA, SURFACE SPECIES POPULATION AND ADSORPTION HEATS

Calculations of metal bonding functions show that the controlling factors for forming end-on monocarbonyl, bridge-on carbonyl and three-fold sites on carbonyl are their corresponding functions AB, A2B2 and A3B3, respectively. A2B2 = 2A(s)B(s) cos-alpha. A3B3 = A2B2 + AB when the terminal CO adsorbs on the sublayer metal atom. A and A(s) represent the various vacant s and d orbitals of metal M.O. bands which accept electrons from the CO 5-sigma and CO 1 pi M.O.s to form end-on and side-on sigma bonds, respectively. B and B(s) represent the occupied d orbitals which back-donate d electrons to that part of the CO 2-pi M.O. located near the terminal carbon and that part of the CO 2-pi M.O. located on the side of the CO axis, to form end-on and side-on pi bonds, respectively. alpha is the angle between the CO bridging bond and the CO side-on bond. Using the above metal bonding functions, it is possible to demonstrate the following experimental facts: (1) Among the Group VIII metals, excepting Pd, A(s) is large than A, and the value of A3B3 or A2B2 is significantly larger than AB; thus at room temperature the multi-site carbonyls prevail on Pd, whereas the end-on monocarbonyls are predominant on most Group VIII metals except Pd. (2) On Ni, the sequence of bonding functions is: A3B3 > AB > A2B2. Thus at low CO exposures, the three-fold site carbonyl begins to form on Ni(111), whereas the end-on monocarbonyl begins to form on Ni(100), since Ni(100) has no appropriate sublayer atom for forming the three-fold site bonding. (3) The end-on monocarbonyl prevails on Rh(111) and Pt(111) at low CO exposures, because in the neutral state AB > A2B2. But if the values of B/A and B(s)/A(s) on Rh and Pt are large, the metal is induced to a positive valence after CO adsorption, resulting in A2B2 > AB, which is why the bridge-on species begin to form on Rh and Pt at intermediate coverages. (4) On Ru, AB is significantly larger than A2B2 at a valence state between 0 and +0.5; thus Ru is totally different from Pd, Ni, Rh and Pt, and only a single peak of end-on carbonyl is present at all coverages. (5) The metal bonding functions semiquantitatively characterize the populations of various carbonyl structures on the surface and can be used to estimate adsorption heats on Group VIII metals