Kinetics falsification by symmetry breaking. 2. Olefin oxidation on a platinum wire

Abstract

The multiplicity patterns of olefin oxidation, catalyzed by a Pt wire controlled to maintain a preset average temperature, are traced by varying the reactant concentrations and are mapped in the concentration plane. The similar bifurcation maps of ethylene and propylene oxidations exhibit regions of tristability and isolated branches in the directions of olefin and oxygen concentrations.Isobutylene oxidation exhibits mushroom-shaped bistability. Several features indicate the occurrence of symmetry breaking to form a partially ignited wire. Comparison of these results with the characteristic bifurcation maps of inhomogeneous solutions, drawn in part 1 of this work, shows that these complex patterns are induced by coupling of reactant inhibition with strong exothermicity. This work also develops an efficient methodology for fast tracing of all the multiplicity features of reacting systems. Bifurcation diagrams are traced automatically with a microcomputer-governed experimental system, and the continuity of features in the second and third dimensions is studied off-line to decide on further experimentation.In the first part of this work (Sheintuch, 1989), we demonstrated that surprisingly complex multiplicity patterns can be induced by relatively simple kinetics in systems that attain stable inhomogeneous solutions. We reviewed previous observations of bistability in reactions catalyzed by an "isothermal" wire, i.e., controlled to maintain a preset (average) temperature, and showed that the patterns with counterclockwise hysteresis or with isolated branches can be accounted for by stationary thermal fronts in systems with simple kinetics or with reactant inhibition. The analysis was also extended to systems that show tristability in order to account for the observations reported in this part.This work presents a comparative study of multiplicity patterns and bifurcation maps in olefin oxidation on a Pt wire, using a thermochemic method. Regions of uniqueness, bistability, and tristability are mapped in the oxygen versus olefin concentration plane at several temperatures.

Kinetics falsification by symmetry breaking. 1. Steady-state analysis

“…Figure 3) and is discontinuous at Ts < Ta, showing either two hysteresis loops (i.e., mushroom shaped, line 4) or an isolated branch (isola, line 5). It is single valued at sufficiently low temperatures: These patterns were observed during isobutylene oxidation and are presented in part 2 of this work (Sheintuch et al, 1989).…”

“…Analysis of such diagrams that assumes homogeneity of all states results in significant falsification of kinetics. Our review of previous work and the work presented in the second part of this work (Sheintuch et al, 1989) show that clockwise hysteresis is probably the only truly isothermal pattern that has been observed with varying reactant concentrations.…”

“…Studies on olefin oxidation over a Pt wire showed three solutions organized in a pattern, which suggests the existence of three homogeneous steady states. These results are presented in part 2 of this work (Sheintuch et al, 1989). With these modifications, all observations of isothermal multiplicity can be accounted for by thermal effects in reactions that are inhibited by reactants.…”

Micro-Kinetics of Ethylene and Methane Oxidation on Platinum

“…Previous experimental studies have analysed the C 2 H 4 catalytic oxidation on Pt at ambient pressure under lean-fuel conditions [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. These studies were carried out with a C 2 H 4 /O 2 molar ratio between 10 − 7 and 10 − 1 , and employed different forms of Pt, including foil [9], wires [22,24], deposited film [10][11][12][13][14][15][16][17][18], and nanoparticles [8,[20][21][22]. The ignition temperature of the C 2 H 4 on the Pt surface is lower than 650 K, and the concentration of C 2 H 4 positively affects its consumption rate and its oxidation to CO 2 .…”

Modelling concentration oscillations in the oxidation of carbon-monoxide/propene mixtures over supported platinum catalysts

“…Isotherm al reaction rate oscillations have been described in the oxidation of propene over a P t wire by Scheintuch & Luss (1981) in the tem perature range 175-228 °C. Multiple steady states were also reported, and have been analysed using singularity theory w ith particu lar atten tio n being paid to th e interaction between m ultiple steady states, sym m etry breaking and the in terp retatio n of kinetic m easurem ents (Scheintuch & Luss 1983;Scheintuch et 1989). R ate oscillations have also been observed during th e sim ultaneous oxidation of CO and one or more alkenes, when th e alkene is 1-butene, ethene, 2-butene and a m ixture of propene and ethene (Capsaskis 1985).…”