EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Operando characterization of working catalysts, requiring the simultaneous measurement of catalytic performance, is crucial to identify the relevant catalyst structure/composition and how molecules interact with surfaces/interfaces. The two examples, covering nanoparticles, thin films and meso-scale aggregates, bridge the "materials and pressure gaps".
i) Area-averaging operando spectroscopy: CO oxidation on Pt/ZrO2 prepared by atomic layer deposition (ALD) was examined by sum frequency generation (SFG) spectroscopy and near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), combined with mass spectrometry (MS) [1]. Combining experimental data with ab initio Density Functional Theory (DFT), we show that the reaction onset is determined by a delicate balance between CO disproportionation (Boudouard reaction) and oxidation. Disproportionation occurs on low-coordinated Pt sites at high CO coverages, when the remaining C-atom is stabilized by a favorable coordination. At variance with the general expectation, rough Pt nanoparticles are seemingly less active than smoother Pt films.
ii) Spatially-resolved operando microscopy: H2 oxidation on polycrystalline Rh was studied by scanning photoelectron microscopy (SPEM) and photoemission electron microscopy (PEEM), which allow local surface analysis and visualising the heterogeneity of ongoing reactions on a µm-scale [2]. This revealed an anisotropy of surface oxidation (depending on the local step/edge density, yielding an oxidation map), as well as its effect on catalytic activity. In situ PEEM imaging of ongoing H2 oxidation directly compares the local reactivity of metallic and oxidised Rh, demonstrating the effect of surface oxides. Employing the velocity of propagating reaction fronts as indicator of reactivity, a high transient activity of Rh surface oxide was observed. The corresponding velocity map reveals the structure-dependence of such activity, representing a direct imaging of a structure-activity relation for plenty well-defined surface structures within one sample.
[1] V. Pramhaas et. al, ACS Catalysis, 11 (2021) 208.
[2] P. Winkler et al, Nature Communications, 12 (2021) 69.