Stroboscopic operando spectroscopy of the dynamics in heterogeneous catalysis by event-averaging

Abstract

Heterogeneous catalyst surfaces are dynamic entities that respond rapidly to changes in their local gas environment, and the dynamics of the response is a decisive factor for the catalysts’ action and activity. Few probes are able to map catalyst structure and local gas environment simultaneously under reaction conditions at the timescales of the dynamic changes. Here we use the CO oxidation reaction and a Pd(100) model catalyst to demonstrate how such studies can be performed by time-resolved ambient pressure photoelectron spectroscopy. Central elements of the method are cyclic gas pulsing and software-based event-averaging by image recognition of spectral features. A key finding is that at 3.2 mbar total pressure a metallic, predominantly CO-covered metallic surface turns highly active for a few seconds once the O2:CO ratio becomes high enough to lift the CO poisoning effect before mass transport limitations triggers formation of a √5 oxide.

Aligning time-resolved kinetics (TAP) and surface spectroscopy (AP-XPS) for a more comprehensive understanding of ALD-derived 2D and 3D model catalysts

“…For example, the actual partial pressure of reactants in front of the sample area probed by AP-XPS is usually lower than the nominal pressure measured in the chamber due to the differential pumping through the analyzer. 84 Considerable progress has been made in improving the understanding and control of the gas transport in AP-XPS devices, 14,85–87 and further corrections can be devised for the exposure models in both devices. We envision that these advanced capabilities will enable more robust and flexible transient experiments to be performed in kinetic and spectroscopic devices with eventual data reconciliation in mind from the start of experimental planning and data acquisition.…”

Spiers Memorial Lecture: prospects for photoelectron spectroscopy

“…65 In the last decade, the potential of NAP-XPS to contribute to the understanding of commercially important thin film growth processes has been realised, and although there remain rather few published studies, investments are being made to support these challenging experiments at a number of synchrotron sources, including at the SPECIES and HIPPIE beamlines at MAX-IV. 66,67 The pressure used in atomic layer deposition (ALD) (and the useful range in some chemical vapour deposition (CVD) processes) is typically in the 10 −3 to 20 mbar range, which is well-matched to NAP-XPS. 23 However, there are also formidable challenges.…”

“…This capability was realised for a test reaction (CO oxidation on Pd(100)) in 2021. 67 Because the mass flow controllers that switch the gas pulses in the cell are generally distant from the surface, gas switching may not be a suitable signal for triggering the averaging. In this demonstration, the work-function-induced binding energy shift of the gas-phase O 1s peak as the gas environment around the sample changes from CO 2 (at the start of a CO pulse) to O 2 (at the end of the pulse) was used, and recognised using image recognition software.…”

“…In this demonstration, the work-function-induced binding energy shift of the gas-phase O 1s peak as the gas environment around the sample changes from CO 2 (at the start of a CO pulse) to O 2 (at the end of the pulse) was used, and recognised using image recognition software. 67 It was possible to record data at frame rates of 6–17 Hz, giving a time resolution of the order of 150–50 ms, limited by the software and frame rate of the detector camera. 67 Clearly, with a delay-line or similar detector, the time resolution can be increased substantially if there is a sufficiently large trigger signal.…”

“…67 It was possible to record data at frame rates of 6–17 Hz, giving a time resolution of the order of 150–50 ms, limited by the software and frame rate of the detector camera. 67 Clearly, with a delay-line or similar detector, the time resolution can be increased substantially if there is a sufficiently large trigger signal. Indeed, a demonstration of sub-ms time resolution in gas-pulsing experiments was published in 2021.…”