Frequency Response Method for Investigation of Kinetic Details of a Heterogeneous Catalyzed Reaction of Gases

Abstract

A novel frequency response (FR) method is proposed: (i) Partial pressure(s) of reactant(s) in a steady-flow reactor is perturbed sinusoidally by varying gas space of the reactor with a definite angular frequency ω; (ii) amplitude and phase shift of every partial-pressure variation of both reactant(s) and product(s) is measured over a wide range of ω; (iii) on the other hand, characteristic functions of ω to explain "reaction-rate spectra" obtained from the data in (ii) are deduced analytically on the basis of a reaction mechanism; and (iv) complex rate coefficients, (k + iωl), involved in the functions are determined by numerical simulation to the spectra. The FR method is applied to CO oxidation over Ru/Al 2 O 3 at ca. 10 2 Pa and 623 K in order to confirm its efficiency: (i) As many as thirteen rate coefficients at five elementary steps were determined; (ii) chemical potentials of surface-intermediates, CO(a), O 2 (a), and O(a), were derived from l's; and (iii) free-energy dissipations via the three intermediates were deduced from chemical potential changes at five elementary steps. It is concluded that the reaction sequence of carbon monoxide, CO(g) f CO(a) f CO 2 (g), could occur spontaneously but the other sequence of oxygen, 1 / 2 O 2 (g) f 1 / 2 O 2 (a) f O(a) f CO 2 (g), would be against the reaction, although they are coupled.

Identification of mass transfer resistances in microporous materials using frequency response methods

“…Schrieffer and Sinfelt [104] carried out an FR analysis for the surface processes occurring in flow systems, and illustrated the utility of FR data in yielding more basic kinetic information than the conventional steady-state mode of operation. Other application could also involve the measurement of reaction rates of a heterogeneous catalytic reaction by periodically varying gas space of a steady-flow reactor [35,36,105]: in this instance, a mass spectrometer would be used to monitor the pressure variation of each component in the system. Yasuda et al [36] applied this approach to study CO oxidation over a Ru/Al 2 O 3 catalyst in order to confirm its efficacy in determining thirteen rate coefficients at five elementary steps.…”

“…Other application could also involve the measurement of reaction rates of a heterogeneous catalytic reaction by periodically varying gas space of a steady-flow reactor [35,36,105]: in this instance, a mass spectrometer would be used to monitor the pressure variation of each component in the system. Yasuda et al [36] applied this approach to study CO oxidation over a Ru/Al 2 O 3 catalyst in order to confirm its efficacy in determining thirteen rate coefficients at five elementary steps. Since all elementary reactions are more or less coupled to each other, the range of frequency played an essential role in measuring "reaction rate spectroscopy".…”

“…Naphtali and Polinski [31] first applied the volume-swing frequency response (VSFR) technique to study chemisorption kinetics. Since then, it has been developed by Yasuda et al [13,[32][33][34][35][36][37][38] to measure diffusion coefficients in gas-zeolite systems. Other research groups (e.g.…”

Periodic perturbation of the kinetics of heterogeneous catalytic reactions

“…Yasuda et al [31] have recently studied CO oxidation on Ru/Al 2 O 3 at 623 K in a flow reactor with periodic perturbation (3 v 100 min À1 ) of reactant pressures due to varying gas space of the reactor. A gas mixture contained CO, O 2 and Ar with partial pressures of ' 10 À4 bar.…”

Master curves for mass transfer in bidisperse adsorbents for pressure‐swing and volume‐swing frequency response methods

“…Most theoretical and experimental aspects of FR have been studied for a batch system in which gas pressure is changed by a forced periodic volume fluctuation 1–9. Yasuda applied this volume‐swing technique with an isothermal model to study physical and chemical phenomena occurring on zeolites, including adsorption,2, 10 diffusion–controlled processes,11–13 combinations of adsorption–diffusion processes,14 and reactions 15–18. Rees and coworkers8, 9, 19 used a square‐wave volume perturbation instead of the traditional sinusoidal wave to investigate hydrocarbons adsorbed on silicate adsorbents.…”