Measuring transient reaction rates from non-stationary catalysts

TL;DR

This paper introduces a high-speed laser ionization and imaging method to measure transient reaction rates on non-stationary catalysts, enabling rapid data collection and analysis of catalyst activation, poisoning, and reaction kinetics.

Contribution

The work presents a novel velocity resolved kinetics technique using high repetition rate pulsed laser ionization and ion imaging for real-time catalyst reaction rate measurements.

Findings

Achieved 10-1000 times higher data acquisition rates than traditional methods.

Successfully measured CO desorption and oxidation kinetics on Pd(332) in single experiments.

Demonstrated the ability to analyze reaction rates under diffusion-controlled conditions.

Abstract

Up to now, the methods available for measuring the rate constants of reactions taking place on heterogeneous catalysts require that the catalyst be stable over long measurement times. But catalyst are often non-stationary, they may become activated under reaction conditions or become poisoned through use. It is therefore desirable to develop methods with high data acquisition rates for kinetics, so that transient rates can be measured on non-stationary catalysts. 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 molecular beam pulses incident at the surface and the product formation rate is observed by a sequence of laser pulses at a high repetition rate. Ion imaging provides the desorbing product flux (reaction rate) as a function of reaction time for each laser pulse. We demonstrate the method using a 10 Hz pulsed CO molecular beam pulse train to initiate CO desorption from Pd(332) - desorbing CO is detected every millisecond by non-resonant multiphoton ionization using a 1-kHz Ti:Sapphire laser. 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 also apply this method to CO oxidation on Pd(332) - we record kinetic traces of CO$_2$ formation while a CO beam titrates oxygen atoms from an O-saturated surface. This provides the reaction rate as a function of O-coverage in a single experiment. We exploit this to produce controlled yet inhomogeneously mixed reactant samples for measurements of reaction rates under diffusion-controlled conditions.