Measuring Transient Reaction Rates from Nonstationary Catalysts.

Dmitriy Borodin,Georgios Skoulatakis,Dirk Schwarzer,Alec M Wodtke,Michael Schwarzer,Kai Golibrzuch,Theofanis Kitsopoulos,Jan Fingerhut,Thomas Seelemann

doi:10.1021/acscatal.0c03773

Abstract

Up to now, methods for measuring rates of reactions on catalysts required long measurement times involving signal averaging over many experiments. This imposed a requirement that the catalyst return to its original state at the end of each experiment—a complete reversibility requirement. For real catalysts, fulfilling the reversibility requirement is often impossible—catalysts under reaction conditions may change their chemical composition and structure as they become activated or while they are being poisoned through use. It is therefore desirable to develop high-speed methods where transient rates can be quickly measured while catalysts are changing. In this work, we present velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection. The reaction is initiated by a single molecular beam pulse incident at the surface, and the product formation rate is observed by a sequence of pulses produced by a high-repetition-rate laser. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the principle of this approach by rate measurements on two simple reactions: CO desorption from and CO oxidation on the 332 facet of Pd. This approach overcomes the time-consuming scanning of the delay between CO and laser pulses needed in past experiments and delivers a data acquisition rate that is 10–1000 times higher. We are able to record kinetic traces of CO2 formation while a CO beam titrates oxygen atoms from an O-saturated surface. This approach also allows measurements of reaction rates under diffusion-controlled conditions.

Highlights

Methods to measure the kinetics of surface reactions are fundamental to improving our understanding of heterogeneous catalysis
The surface temperature is controlled so that a 1 kHz detection rate is sufficiently rapid to follow the desorption kinetics, while ensuring that all CO molecules desorb between molecular beam pulses
Looking in more detail (Figure 9C), we find that the single-pulse transient rate is decreasing with increasing titration time; the transient rates are well reproduced by the kinetic model as is the continuous production of CO2 seen in the later stages of the titration

Summary

INTRODUCTION

Methods to measure the kinetics of surface reactions are fundamental to improving our understanding of heterogeneous catalysis. Temperature-programmed reaction, molecular beam relaxation spectrometry, and phase-lag detection have been available to experimentalists.[1−4] Recently, the kinetic trace was obtained using velocity-resolved methods[5] based on ion imaging.[6−8] This is essentially a pump−probe technique where a molecular beam pump−pulse initiates the reaction and pulsed laser ionization probes the desorbing products. The ionized products are recorded with ion imaging providing product velocity information with every detection pulse. This allows measured product densities to be converted to product flux, which is by definition the reaction rate for a surface reaction. The reaction starts when a pulse of molecules arrives at the surface, and ion images are recorded for each pulse of a high-repetition-rate laser that ionizes desorbing products.

EXPERIMENT

Proof of Principle

CONCLUSIONS

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