Atomic-level insights in catalytic nanomaterials by in situ surface spectroscopy and microscopy

ABSTRACT

Operando spectroscopy of catalytic reactions has been very successful in mechanistic studies.¹ However, as spectroscopy typically examines large areas/volumes, this averaging “smoothens out” local variations that may be critical to understand how a reaction proceeds. Dynamics in catalyst structure, composition and adsorbate coverage may also go unnoticed by averaged spectral data. A way overcoming these limitations is to use correlative surface microsopy to directly “watch” ongoing catalytic reactions, i.e. to apply several microscopic and spectro-microscopic techniques to the same catalysts locations under identical reaction conditions.² Most of the methods herein not only image catalyst structure or composition, but also the adsorbed reactants, so that active and inactive states can be discerned (kinetics by imaging), active regions identified and mechanisms elucidated.³

Examples of real-time in situ imaging of H₂ oxidation include meso-scale polycrystalline Rh surfaces and Rh nanotips (as small as 30 nm, enabling single particle catalysis). For planar catalysts, photoemission electron microscopy (PEEM), low energy electron microscopy (LEEM) and scanning photoelectron microscopy (SPEM) were used with resolution up to 3 nm. For nanotips, field emission microscopy (FEM) and field ion microscopy (FIM) were applied with up to atomic resolution.

The direct, real-time and locally-resolved observation of H₂ oxidation on Rh-based catalysts revealed:

(i) the transition from inactive to active states via catalytic ignition and spreading of chemical waves,³

(ii) the mechanism of oscillatory H₂ oxidation involving subsurface oxygen,³

(iii) whether different facets on a single Rh nanoparticle communicate via hydrogen diffusion or not (coupled monofrequential vs. (uncloupled) multifrequential oscillations),⁴ and

(iv) how La modifies the reaction dynamics on a Rh nanotip.⁵

Microkinetic modelling and density functional theory (DFT) rationalized the experimental observations. The novel nanoscale insights in the dynamics of reactants and surfaces, including the identification of active regions, may stimulate new ways of catalyst design and operation.

References:

[1] G. Rupprechter, Small 2021, 2004289.

[2] J. Zeininger et al., ACS Catalysis 12 (2022) 11974.

[3] P. Winkler et al., Nature Communications 12 (2021) 69 and 6517.

[4] Y. Suchorski et al., Science 372 (2021) 1314.

[5] M Raab et al., Nature Communications 14 (2023) 7186.

BIO

The main research interests of Günther Rupprechter are in heterogeneous catalysis and nanomaterials, particularly in situ (operando) spectroscopy/microscopy of model and technological catalysts, applied to studies of the mechanisms and kinetics of processes relevant for energy and environment: hydrogen as clean fuel, methane reforming, CO2 and olefin hydrogenation, efficient automotive catalysis, sensing and waste remediation. In 2005 he received the Jochen Block Award of the German Catalysis Society for “the application of surface science methods to heterogeneous catalysis”, became corresponding member of the Austrian Academy of Sciences (ÖAW) in 2012 and Fellow of the European Academy of Sciences (EurASc) in 2023. He is Editorial Board Member of “Catalysis Letters” and “Topics in Catalysis”, and Vice-Chair of the Austrian Catalysis Society. From 2011 to 2019 he was Speaker of the Collaborative Research Center “Functional Oxide Surfaces and Interfaces (FOXSI)” of the Austrian Science Fund (FWF). He is “Renowned Overseas Professor” of Shanghai University of Engineering Science and Guest Professor at Kasetsart University Bangkok. Rupprechter is the Director of Research (Speaker) of a new Austrian Cluster of Excellence “Materials for Energy Conversion and Storage (MECS)”, including 5 Austrian universities/institutions.