Negative Reaction Order Research Articles

Theory of competitive adsorption and its application to the anodic dissolution of nickel and other iron-group metals—II. The steady state in the prepassive, passive and transpassive potential ranges

The theory developed in Part I has been extended to include the action of chemisorbed hydroxide. Equilibrium charge transfer adsorption is able to predict the negative reaction orders with respect to hydroxide ions which have been reported for solutions high in chloride and hydrogen ions, as well as the occurrence of a maximum and its dependence on the composition of the solution. It has been concluded that it is likely that passivation is in the first instance due to the chemisorption of hydroxide. If, on the other hand, hydroxide ions are assumed to have a catalytic action in metal dissolution, albeit with a very low anodic desorption rate, then the relevant equations are in addition able to predict passive and transpassive behaviour. Thus the steady state anodic current—potential relationship in the potential range between hydrogen evolution and oxygen evolution can in principle be expressed in one equation. The remarkable agreement between predicted and observed anodic behaviour makes it likely that competitive adsorption plays an important role in the passive range, even in the presence of an oxide film. Finally it has been investigated to what degree the theory can contribute to a better understanding of the behaviour of passive metals.

Effects of experimental variables on the determination of kinetic parameters with differential scanning calorimetry. II. Calculation procedure of Freeman and Carroll

The effects of sample mass, heating rate and particle size on the determination of reaction order, activation energy and pre-exponential factor with the calculation procedure of Freeman and Carroll in differential scanning calorimetry was studied with factorial designs. Sodium bicarbonate, three organic hydrates (potassium oxalate hydrate, mercaptopurine hydrate, and sodium citrate dihydrate) and two substances giving solid-solid transformations (potassium nitrate and hexamethylbenzene) were studied. It was found that the extent of these effects depended on the nature of the decomposition and on the evaluated section of the transition peak. It was not justified to use data from the complete peak in the calculation, as different processes contribute to the total solid-state reactions. Arrhenius plots with such data were curved. In the initial section of the peak heat transport is the main process; high values of the kinetic parameters are then obtained. Lower values, with smaller error, were found for the middle section where mass transport is the predominant process. Narrow peaks gave rise to very high activation energies and large error. Even negative reaction orders were then found which are indicative of the explosive character of the transition. As the kinetic parameters differ considerably during the peak, it is not justified to attribute any physical meaning to them.

Isothermal nth order reaction in catalytic pellets : Effect of external mass transfer resistance

The effectiveness factor for the isothermal power-law decomposition of a reactant within symmetrical catalytic pellets is obtained from the solution of a single first order differential equation. The effects of external mass transfer are taken into account, as well as the effect of simple types of non-uniform distribution of the catalyst within the particle. For fractional and negative reaction orders there are solutions with zero concentration in a central region of the particle. For negative reaction orders intervals of multiplicity of solutions are found.

Catalytic dehydrogenation of propane to propene over platinum and platinum-gold alloys

The rates of dehydrogenation of propane to propene over platinum and very dilute platinumin-gold alloys have been measured. In the composition range of 0.5–14.0 atom % platinum, the rates per unit surface area of the alloy powders vary linearly with the bulk platinum concentration in the alloys. From this it is concluded that only one platinum atom is involved in the rate-determining step. For both platinum and the alloys, activation energies of some 29 ± 2 kcal/mole were measured. The reaction rate order in hydrogen, however, is different. It is proposed that propane dehydrogenation over platinum and over the alloys occurs via the same reaction mechanism, namely, dissociative chemisorption of propane on a single platinum atom to which two adsorption sites are associated, one of which carries a hydrogen atom. The subsequent conversion of the propyl radical into π-bonded propene via β-hydrogen elimination appears to be rate determining. The last step, desorption of π-bonded propene, has a comparatively low activation energy. The difference in negative reaction order, with respect to hydrogen, between platinum and its diluted alloys reflects a lower steady-state θ H on Pt atoms surrounded by Au atoms vis-à-vis Pt atoms on a Pt surface.

Über einige besonderheiten der polymerisation von 2,3‐dimethylbutadien in durch lithiumorganische verbindungen initiierten prozessen

AbstractThe polymerization of 2,3‐dimethylbutadiene (DMB) induced by butyllithium (BuLi), oligo‐2,3‐dimethylbutadienyllithium (ODMBL), and by the system ODMBL/N,N,N′,N′‐tetramethylethylenediamine (TMD) was investigated in heptane at 60°C. The reaction order with respect to the organolithium compound was found to be for these cases −0,32, 0,28, and 0,56 respectively. The negative reaction order for BuLi is caused by the existence of the mixed associates (BuLi)x. (MnLi)y which follows from the negative influence of BuLi on the propagation reaction rate in the system DMB/ODMBL. Contrary to butadiene and isoprene, catalytic amounts of TMD decrease the propagation rate of DMB. This phenomenon is ascribed to a weak tendency of the complexes of the DMB‐living chains to dissociate into the monomeric form MnLi. TMD. The UV‐ and IR‐spectra of the living DMB‐chains and their complexes with TMD are studied.

Behaviour of surface oxide in ethylene oxidation at platinum in the oxide region

The development of surface oxide species at platinum anodes in the course of electro-oxidation of ethylene at intermediate potentials (0·7–1·2 V(nhe)) has been investigated in relation to the kinetics and mechanism of ethylene oxidation in this potential range. Effects of electrode rotation on the oxidation rate, and on the steady- and non-steady-state surface-oxide coverages are reported, and enable the true activation-controlled, inhibited current/potential curve to be delineated. It is shown that in the potential region where surface oxide coverage becomes appreciable, negative reaction orders can arise for a reaction step involving oxidative removal of chemisorbed “ethylene” species, and the results allow an examination of the extent to which a pathway involving blocked surface oxidation of the metal can be rate-determining over the range of intermediate potentials studied in the present work.

Parahydrogen conversion on the first transition series

The effect of pressure on the reaction velocity has revealed a new phenomenon, in which zero, or negative reaction order changes over to a fractional or first order reaction at higher pressures. This was found for Mn, Fe, Co, Ni at 90 °K, and Co at 293 °K. The mechanisms suggested are hydrogen atom recombination in a saturated layer, probably accompanied by hydrogen poisoning at low pressures and hydrogen molecule-atom exchange in a partly filled “second layer” at higher pressures. The reactions on Cr at 90 °K, and Cr, Fe, and Ni at 293 ° K are simple zero order, and Mn, 293 °K, Zn, 368 °K, fractional and first order, respectively. Hydrogen-deuterium equilibration goes at comparable rates with the conversion, for Ti, V, Cr, and Ni; there is evidence for Ni that this holds down to 77 °K. Attempts to estimate the rate of the paramagnetic conversion at 90 °K using the Wigner equation confirm the dominance of the chemical mechanisms for all metals except Zn, where the mechanism is probably paramagnetic. A correlation between reaction velocity and sublimation energy for the range of metals at 293 °K is associated with a mechanism of recombination of H atoms held on sites of minimum bond energy. The low frequency of these sites for body-centered cubic metals gives a frequency factor of 10 16 molecules cm −2 sec −1 compared with 10 19 for close-packed metals. A lower activation energy is found for bcc metals and is associated with a greater interaction energy between adsorbed atoms, in its turn associated with the broad character of the d band in this group of metals.