Measuring Transient Reaction Rates from Nonstationary Catalysts

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 CO 2 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.

Chemical dynamics from the gas‐phase to surfaces

“…Recently, velocity‐resolved kinetics using high‐repetition‐rate pulsed laser ionization and high‐speed ion imaging detection has been achieved 472 . This overcomes the time‐consuming scanning of the delay between the reaction initiating molecular beam pulse and the laser ionization pulse—Figure 30(a).…”

“…Using high rep-rate velocity-resolved kinetics, the diffusional influence could be observed and quantitatively modeled. 472 Exploiting our dynamical understanding of surface reactions to obtain a predictive theory of reaction rates remains yet an unfulfilled goal, as current limitations of the tools needed to make accurate measurements of reaction rates at well-characterized active sites have prevented meaningful tests of rate-theory. However, with experiments and theory both improving, we are increasingly benefitting from the fruitful interplay and we stand at the threshold of bringing this effort to fruition.…”

“…Recently, velocity-resolved kinetics using high-repetition-rate pulsed laser ionization and high-speed ion imaging detection has been achieved. 472 This overcomes the time-consuming scanning of the delay F I G U R E 3 0 Comparison of delay scanning (a) versus high-rep-rate detection (b). In delay scanning, we average many ion images at each delay between the initiating molecular beam pulse and the ionizing laser pulse, generating the kinetic trace.…”

“…For high rep-rate detection, we record many points in the kinetic trace for each molecular beam pulse, increasing the duty cycle by orders of magnitude, enabling measurements while the catalyst is changing. 472 Reprinted from Borodin D, Golibrzuch K, Schwarzer M, et al Measuring transient reaction rates from nonstationary catalysts. ACS Catal.…”

“…As time passes, most CO 2 formed from CO that adsorbed near the center of the beam where O* is depleted, and diffused to the edges of the beam where O* is still present. Using high rep‐rate velocity‐resolved kinetics, the diffusional influence could be observed and quantitatively modeled 472 …”