Mechanistic Studies of Heterogeneously Catalyzed Reactions of Ammonia and Acetic Acid on Platinum Surfaces

Citation

Vajo, John J. (1987) Mechanistic Studies of Heterogeneously Catalyzed Reactions of Ammonia and Acetic Acid on Platinum Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/vayx-4k53. https://resolver.caltech.edu/CaltechTHESIS:04152019-173425189

Abstract

The design and operation of a versatile microreactor capable of studying the rates of both steady-state and batch heterogeneous reactions on a wire, a foil or a single crystalline surface at pressures between 10^-7 and 1000 Torr are described. The residence time distribution of the microreactor was characterized in order to evaluate the validity of using the continuous stirred tank reactor approximation to calculate reaction rates.

Absolute reaction rates (i.e. the rate-per-unit catalyst surface area) have been measured for both the catalytic decomposition of NH₃ and ND₃ and the NH₃ + D₂ exchange reaction over a polycrystalline platinum wire. The pressure was varied between 5 x 10^-7 and 0.5 Torr, and the temperature ranged from 400 to 1200 K. At relatively low pressures and/or high temperatures, the order of the decomposition reaction is unity with respect to ammonia, and the reaction rate is dictated by a competition between the surface reaction and the desorption of molecularly adsorbed ammonia. Under these conditions a primary isotope effect was observed for the decomposition of ND₃. At relatively high pressures and/or low temperatures, the reaction rate is independent of ammonia pressure, and the recombinative desorption of nitrogen controls the rate of ammonia decomposition. The measured kinetics of the NH₃ + D₂ exchange reaction were employed together with adsorption-desorption parameters of NH₃, N₂ and H₂ to develop a mechanistic model that describes the reaction rate over the entire (wide) range of conditions studied.

Steady-state absolute reaction rates are reported also for the catalytic decomposition of NH₃ on the Pt(110)-(1x2) single crystalline surface at pressures between 1 x 10^-6 and 2.6 x 10^-6 Torr and at temperatures between 400 and 1000 K. Qualitatively, the kinetics is similar to those observed for ammonia decomposition on the polycrystalline platinum surface. Thermal desorption measurements conducted during the steady-state decomposition reaction demonstrate directly that nitrogen adatoms are the predominant surface species, and that the recombinative desorption of nitrogen is the major elementary reaction that produces molecular nitrogen.

The decomposition of CH₃¹³COOH at 7 x 10^-4 Torr on a polycrystalline platinum wire at temperatures between 300 and 900 K was examined in the microreactor. The major reaction products on the initially clean surface are ¹³CO, CO, ¹³CO₂, H₂ and adsorbed carbon-12. The adsorbed carbon accumulates on the surface until the reactions that produce these products are poisoned by the graphitic overlayer that is formed. On the graphitized platinum surface, acetic acid dehydrates catalytically to ketene and water. The relative quantities of ¹³CO and ¹³CO₂ that are formed depend both on the surface temperature and on the surface carbon coverage.

The catalytic dehydration of acetic acid to ketene was investigated over a graphitized polycrystalline platinum surface at pressures between 8 x 10^-7 and 7 x 10^-4 Torr and temperatures between 500 and 800 K. Steady-state absolute reaction rates, thermal desorption measurements, and the reactivities of functionally related compounds suggest that the reaction proceeds via an irreversibly adsorbed intermediate, which is formed by dissociation of the oxygen-hydrogen bond of acetic acid. For temperatures below 540 K at pressures of 3.5 x 10^-4 Torr and above, the rate of decomposition of the surface intermediate controls the overall rate of the reaction. At 675 K or above for the entire range of pressures studied, the rate of dehydration is determined by a competition between the rates of desorption and surface reaction of molecularly adsorbed acetic acid.

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