Toshio Uchijima

Analysis of thermal desorption curve for heterogeneous surfaces: II. Nonlinear variations of activation energy of desorption

We reported analyses of the desorption curve for the simple heterogeneous surface. This paper deals with the more general heterogeneous surface with nonlinear variation of energy of desorption as a function of surface coverage. Computer simulation shows that the characteristic changes of the curve shapes are caused by variations of the distribution pattern of the activation energy of desorption. Thus, the determination of the profile of Ed(θ) is possible by the analysis of the peak shape with a few assumptions; the rate parameters, Ed(θ) and A, of desorption can be deduced as a function of surface coverage, θ, by solving a number of simultaneous equations Ed(θ) − RT ln A = RT ln (θnβC) , where n is the order of desorption, β is the heating rate, C is the normalized concentration, A is the frequency factor, and Ed(θ) is the activation energy of desorption, assuming the relation Ed(θ)=∑k=0Nαk(1−θ)k. The above analysis was verified by its application to the various simulated desorption curves. The thermal desorption curves of oxygen on ZnO and silver catalyst were analyzed by this method and found to give the reliable values of rate parameters of desorption as a function of the surface coverage; Ed(θ) varying from 21.5 ± 1.5 kcal/mol at the initial stage of desorption to 32.0 ± 3.0 kcal/mol at the end for ZnO and from 28.5 ± 0.5 to 35.0 ± 0.5 kcal/mol for silver catalyst. The result was much more improved by the simultaneous analysis of the plural desorption curves obtained at different heating rates.

The measurement of the distribution of oxidation power on nickel oxide catalysts

Abstract Acid strength distribution has been demonstrated to be a useful variable for the explanation of the catalytic activities on some solid acids. A similar state of affairs would also be expected in oxidation catalysis on oxides. Therefore, the present investigation was intended to provide a method for determining the distribution of oxidation power of the surface excess oxygen. This was accomplished by the combination of several aqueous reduction methods, each of which uses a certain reducing reagent having a different oxidation-reduction potential. The hydrazine method and a series of the KI(pH x) method with pH between 7.5 to 11.3 were used and applied to nickel oxide catalysts. The total amount of surface excess oxygen is obtained by the hydrazine method; it decreases monotonously against the calcination temperature. The KI(pH 7.5) method provides a decreasing curve with a flat part at the range of 550–600 °C. In the cases of the other KI(pH x) methods with pH above 9.3, the curves turn out to have one or two sets of maxima and minima against the calcination temperature. These characteristic behaviors are more emphasized by the excess oxygen having the higher oxidation power. Taking account of the preliminary results on the catalytic decomposition of hydrogen peroxide, the distribution in the oxidation power seems to be a useful variable in explaining the catalytic activities on nickel oxide catalysts.

Linear free energy relationships in heterogeneous catalysis: IX. Kinetic study of catalytic dehydrogenation of cyclohexanes by the pulse technique

The dehydrogenation of methyl-substituted cyclohexanes catalyzed by chromiaalumina and molybdena-alumina was studied by means of the pulse technique. In the case, where the rate is expressed as v = αβp(1 + αp), the relation between the conversion, x, and the maximum partial pressure, pm, of a triangular pulse can be formulated as follows, under the ideal conditions: xF2W = βαpm − ln(αpm + 1)αpm2 By using this equation, α and β of all the reactants used were obtained at a few temperatures. From the fact that α increases with increasing temperature as well as from the results of some supplementary experiments, it is demonstrated that αβ stands for the rate constant of the first hydrogen abstraction and β for that of the second one. It was found that αβ varied remarkably with the reactants used and was correlated to the delocalizability, a quantum-chemical reactivity index, though β was almost invariant. Therefore, it is concluded that the slowest step is the dehydrogenation to cyclomonoolefin, where each of two hydrogens is abstracted stepwise, and that the first hydrogen abstraction from an original hydrocarbon determines the relative reactivity.

Linear free energy relationships in heterogeneous catalysis: VII. Reactivity of ring hydrogens in catalytic dehydrogenation of cyclohexanes

The rates of dehydrogenation of various methyl-substituted cyclohexanes catalyzed by chromia-alumina and molybdena-alumina were numerically analyzed in terms of the varying reactivities of the ring hydrogens classified into several groups, from the viewpoint of the LFER. It is demonstrated that the rate, v(R,T), can be represented as the sum of the reactivities of the ring hydrogens, vH(m,T), as v(R,T) = ∑ w(R,m) vH(m,T), where w(R,m) is the number of mth hydrogens in a reactant R. The logarithm of the reactivity of ring hydrogen is, in turn, linearly related to the delocalizability, DrR(H), a quantum-chemical reactivity index for hydrogen abstraction reactions, as log vH(m,T) = log vH(O,T) + γ(T)ΔDrR(Hm)2.303RT, where vH(O,T) is the reactivity of an imaginary hydrogen whose delocalizability is 1.0, and γ(T) is a proportional constant. Furthermore, from the temperature dependence, γ(T) has been correlated to the isokinetic temperature, T8, and as the result the rates are expressed as follows: v(R,T) = vH(O, ∞) exp (−EA(0)RT) ∑ w(R,m) exp γD (1 − TT8)ΔDrR(Hm)RT where γD is a constant independent of temperature. Thus, the rate of dehydrogenation of any reactant at any temperature can be calculated with the knowledge of delocalizabilities inherent to ring hydrogens and four parameters characteristic for a catalyst, i.e., vH(O, ∞), EA(O), γd, and T8. This equation is in accordance with the reaction scheme where the dehydrogenation to monoolefin is the slow step.

Linear free energy relationships in heterogeneous catalysis: XI. Deep oxidation of lower olefins on nickel oxide catalyst

Linear free energy relationship studies were made on the rates of deep oxidation of some lower olefins over a nickel oxide catalyst. Since the trend of reactivities of olefins seems to be determined by the type of allylic hydrogens and their number for each olefin, quite similar treatments as previously reported on the dehydrogenation of cyclohexanes may be applied to this case. That is, the overall rate of deep oxidation of olefin R at temperature T, υ(R,T), can be represented as the weighted sum of the characteristic rates for allylic hydrogen atoms as v(R,T)=∑mw(R,m)·v(m,T) where w(R,m) means the statistical factor, i.e., the number of mth allylic hydrogen (primary, secondary, or tertiary) for olefin R and υ(m,T), the rate of the mth hydrogen at temperature T. The logarithm of υ(m,T) is demonstrated to have a linear relationship with delocalizability, DrR(H), a quantum chemical reactivity index for hydrogen abstraction, as log υ(m,T) = log υ(0,T) + γ(T) · ΔDrR(Hm)2.3RT where υ(0,T) is the rate of hypothetical hydrogen with delocalizability of 1.00 and γ(T) is a proportional constant. Furthermore, γ(T) is proved to be practically independent of temperature. This means that the preexponential factor of the mth hydrogen, υ(m,∞), is independent of the type of allylic hydrogen but its activation energy, EA(m), decreases proportionately with DrR(Hm). Finally, the overall rates can be expressed as follows: v(R,T)=∑mw(R,m)·v(0,∞)·exp{−[EA(0)−λD·ΔDrR(Hm)]/RT} where υ(0,∞) and EA(0) are the preexponential factor and the activation energy, respectively, for hypothetical hydrogen defined above and γ(T) is rewritten as γD. Finally, the rate of deep oxidation of any olefin at any temperature can be predicted according to the above equation using the delocalizability inherent to allylic hydrogen and three parameters characteristic of the type of catalyst, i.e., υ(0,∞), EA(0) and γD. This treatment is compatible with the tentative mechanism that the deep oxidation of olefins over nickel oxide involves an abstraction of allylic hydrogen as a rate determining step.

nippon kagaku kaishi

担持白金触媒や種々の金属酸化物触媒および両者の物理的混合系,さらに金属酸化物に白金を直接担持した触媒系について,パルス法によるNO分解活性の比較検討を行なった。いずれの触媒でも触媒の還元によりNO分解活性はいちじるしく上昇し,酸化により低下する。このとき,NO分解による生成酸素が触媒上に不可逆的に吸着し,活性低下の原因となっている。物理的混合触媒系におけるNO分解活性については,定常的には金属酸化物の共存効果は認められなかった。しかし,[Pt15(CO)80]・2N(C2H5)4/アセトンや[Pt(NH3)2](NO3)2/H2Oから調製した種々の金属酸化物担持白金触媒系では,担体によって活性値にかなりの差異がみられ,活性の順にZnO,Cr2O3>γ-Al2O3,Co3O4,NiO>MgO,Fe2O3,SiO2,SiO2,Al203,Pr6O11,Mn2O3>CuO>V2O3の序列が得られた。また,N2/N2O生成比や見かけの活性化エネルギー(3～19kcal/mol)にも担体による差が認められた。担体の効果を比較するため,一部の触媒,すなわちγ-Al2O3,SiO2,Al2O3,MgO,SiO2,ZnOに担持した白金触媒について白金分散度を測定して,400℃の活性を白金分散度で規格化した結果,白金粒径が大きいほど比活性が高い傾向があり,顕著な白金の分散効果が認められた。

Determination of Heats of Formation of Coordination Compounds between a Lewis Acid and Bases with a Solution Microcalorimeter

Making use of a commercial conduction type calorimeter, we constructed a solution microcalorimeter which is applicable to entropy titration involving reactive reagents difficult to handle in the air, and tested its performance. The heats of coordination of various organic bases (amines, pyridines and ethers) to triethyl-aluminium were measured with the aid of this apparatus. As to the equilibrium constants of coordination, only approximate values could be estimated from the thermometric titration curves, because they were too large for exact analysis. The value of the enthalpy change of each system was obtained from the thermometric titration curve as a mean value of several titrations of a base solution fairly before the equivalent point. Comparing the enthalpy changes thus obtained with the pKa values of the bases used, it was found that linear relations hold individually for amines and ethers, but pyridines form an iso-lated group apart from the other series (cf. Fig. 5). The chemical shifts of methyl and methylene protons of the ethyl groups bonded to aluminium were calculated from the NMR spectra of the coordination compounds. Then, the internal chemical shift, defined as a difference between those of methyl and methylene protons, can be correlated with the electronegativity of aluminium in the coordination compounds according to the Dailey-Shoolery equation (cf. Eq. (7)). As a result of comparison of the enthalpy changes with the values of the electronegativity thus obtained, it was shown that a simple linear re-lation holds throughout, with the only exception of pyridines which exert a marked shielding effect due to the ring current to the methylene proton (cf. Fig. 7).

Fully Automatic Microcatalytic Reactor and Its Application to Oxidation of Lower Olefins

固体触媒による触媒反応の研究手段として,流通法およびパルス法が用いられているが,近年パルス法によってその特徴を生かした研究が数多く見られるようになった。著者らは,ガス状反応物を対象としてパルス反応装置の完全自動化に関する試作研究を行なった。自動化の目的の第一は,夜間の連続無人運転による実験の能率化であり,第二は人為的誤動作を避けることによる再現性の向上にある。装置の設計にあたっては次の3種の機能に重点をおいた。(1)反応ガスを順次とりかえながら自動的に速度を測定すること(反応物効果δR)。(2)反応ガス分圧を順次変えながら速度を自動測定すること(分圧依存性δP)。(3)反応温度を順次変えながら速度を自動測定すること(温度依存性δT)。また,より広範な化合物を測定の対象として可能にするため,少量の試料を効率良く使用し得るように装置上の工夫を行なった。本試作装置を低級オレフィンの完全酸化反応に適用した結果,装置の信頼性および再現性において充分満足すべき結果が得られた。