Our group's philosophy is to study catalytic surface reactions on heterogeneous catalysts via a two-fold approach, employing both surface science based planar model catalysts as well as industrial-grade catalysts. For both, the focus is on examining active functioning catalysts under operando conditions, at (near) atmospheric pressure and at elevated temperature. In particular for ultrahigh vacuum (UHV) based model catalysts that has been a challenge, requiring application of in situ surface spectroscopy, such as sum frequency generation (SFG) laser spectroscopy, polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS) and near atmospheric pressure x-ray photoelectron spectroscopy (NAP-XPS). To image (“see”) ongoing surface reactions by in situ surface microscopy, photoemission electron microscopy (PEEM) was applied to polycrystalline samples. For technological catalysts, analogous operando studies were performed by Fourier transform infrared spectroscopy (FTIR), X-ray absorption spectroscopy (XAS), NAP-XPS, and X-ray diffraction (XRD). Most of the operando studies were performed at synchrotron sources (BESSY (DE), MaxLab (SE), SLS (CH)), and in lock-step with theory. This two-fold approach yields a more “holistic” view of the catalytically relevant atomic and electronic surface structure of catalysts, as well as of molecular details that steer reaction activity and, even more important, reaction selectivity. Comparing surface science based single crystals with nanoparticle model catalysts elucidates the materials gap, and comparing UHV to ambient pressure studies reveals the pressure gap. In particular, synergisms between studies on model and technological catalysts often provide access to atomistic details. Among the systems studied are i) CO oxidation and H2/CO/O2 reaction (PROX) on supported and unsupported noble metals, Co3O4 and CoO thin films UHV-grown on Ir(100), and commercial cobalt oxides, ii) methane reforming on Ni-ZrO2 and CuNi-ZrO2, employing an ultrathin ZrO2 trilayer (O-Zr-O) film on Pt3Zr(0001) as well as commercial ZrO2 as support. For all, the operando studies identify the relevant surface processes, providing further atomistic insight on dynamics, such as changes of the surface and bulk oxidation state (oxygen vacancy formation), metal-support interface effects, reaction-induced surface segregation, surface hydroxylation, adsorbed species, and others.

Surface spectroscopy and surface microscopy of catalytic processes: From model to technological materials, from UHV to operando conditions

253rd National Meeting (2017)

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