A common expectation in heterogeneous catalysis is that the optimal activity will occur for the particle size with the highest concentration of undercoordinated step, edge, or corner sites, expectedly in the <5 nm range. However, many metal-catalyzed reactions follow a different trend, where the turnover frequency (TOF, here rate per surface atom) is instead lower for these smaller particles and increases strongly with increasing size toward a stabilized level with a size-independent TOF. Here, we use one of these reactions, the Rh-catalyzed CO hydrogenation to hydrocarbons and C2-oxygenates, to illuminate the origin of this effect. Studying Rh/SiO2 catalysts, we show that smaller (<4 nm) Rh particles are richer in undercoordinated edge, corner, and step sites, but are nevertheless of lower activity because the entire surface, including the planar facets, is shifted to a prohibitively high adsorbate coveragein this case of CO. In transient experiments, where the inhibiting adsorbates are allowed to desorb, smaller 1.7 nm Rh particles and larger 3.7 nm Rh particles reach similar rates of CO activation despite the steady-state TOF being an order of magnitude higher on the larger particles. This shows that it is a prohibitive adsorbate coverage under reaction conditions rather than a lower number of active sites or a lower intrinsic activity of the sites that causes the lower activity of the smaller particles. In steady-state experiments at 20 bar, the TOF for CO hydrogenation increases by 55% from 3.7 nm Rh particles to 5.3 nm Rh particles even though the measured concentration of step sites decreases by 30% in this size range. This indicates that such undercoordinated sites are not necessarily the primary active centers and that the reaction is instead focused on the planar facets. The reaction kinetics show that the reaction becomes increasingly pressure-dependent with increasing particle size, implying that the surface becomes increasingly free of adsorbates on larger particles. Taken together with the indications that the reaction may be focused on the planar facets, this leads to the new insight that it is a prohibitively high adsorbate coverage on the entire surface (and not just on a minority of undercoordinated sites) that is the primary reason for the low activity of small nanoparticles. The identification of a detrimental high-coverage state for small particles is expected to be of general relevance to the many industrially important reactions sharing the same behavior. The high-coverage state is not exclusively negative, but can also facilitate different reaction pathways. It is the higher CO coverage on small particles that drives the C2-oxygenate formation and is the reason for the high selectivity of rhodium to such complex products, which is at its highest for the smallest (∼2 nm) investigated particles.

The structural changes of an iron molybdate/molybdenum oxide (Mo/Fe = 2.0) catalyst for the selective oxidation of methanol to formaldehyde were studied using combined operando X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) as well as operando Raman spectroscopy. Under operating conditions, the Mo K-edge XANES spectra showed a transition from a mixture of α-MoO 3 and Fe 2 (MoO 4 ) 3 towards only Fe 2 (MoO 4 ) 3 . XRD and Raman spectroscopy also showed disappearance of the α-MoO 3 phase with time on stream. The results evidenced that the α-MoO 3 component evaporated completely, while the Fe 2 (MoO 4 ) 3 component remained stable. This was linked to a decrease in catalytic activity. Further studies unraveled that the rate of α-MoO 3 evaporation increased with increasing MeOH concentration, decreasing O 2 concentration and increasing temperature. The simultaneous measurements of catalytic activity and spectroscopy allowed to derive a structure-activity relationship showing that α-MoO 3 evaporation needs to be prevented to optimize MoO 3 -based catalysts for selective oxidation of methanol.

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A combination of different in situ techniques were used to investigate the activation and deactivation processes of supported metal catalysts such as α-Fe2O3 for Fischer-Tropsch synthesis and Rh based catalysts for the synthesis of higher alcohols. The pressure was varied between a few millibars and atmospheric pressure.Iron-based catalysts are typically used commercially for Fischer-Tropsch synthesis, as they are cheaper than their cobalt and ruthenium-based counterparts [1]. Therefore, looking at their activation process is critical for gaining further insight on the nature of the active sites in order to further enhance their performance. α-Fe2O3, used as precursor for Fischer-Tropsch catalysts, is typically activated in either H2, CO or syngas before synthesis reaction. As CO or syngas activation lead to iron carbides and iron oxides we look at the unsupported α-Fe2O3 precursors exposed to hydrogen to gain information about their evolution in time under in situ conditions. The activation involve interaction between the precursor, α-Fe2O3, and the surrounding gas constituents, in this case hydrogen, which is not only altering the morphology and surface structure but eventually also the catalytic performance.To better understand the gas-induced shape evolution of the α-Fe2O3 precursor, we look at the activation process at different pressures. First, we look at low pressure (a few millibars) using a CS-corrected FEI Titan 80-300 environmental TEM and a DENSsolution Wildfire S3 MEMS based heating holder, see Figure 1. Under these conditions, the catalyst can be reduced from Fe2O3 to Fe3O4 and finally to metallic Fe. In order to further understand what happens at higher pressure, a DENSsolutions Climate system was used. Here the catalyst can be exposed to the same gas composition at pressures up to 1 bar. While reducing α-Fe2O3 to Fe3O4 and further to α-Fe, we aim to look into the shape evolution of iron and to demonstrate the difference that occur at different pressures, which is an important result of the influence of pressure in terms of obtaining the real structure-activity correlation for nanoparticles in reaction condition. Furthermore, since the high electron energies and current densities typically used in HRTEM measurements often causes beam induced dynamic processes, we also want to address some of the effects that occur at higher pressures.In order to further understand pressure effects under in situ investigations of supported metal catalysts, strong metal support interactions (SMSI) were investigated on Rh based catalysts for the synthesis of higher alcohols, see