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Aiming at a mechanistic understanding of the methanol (MeOH) synthesis from CO 2 /H 2 over Au/CeO 2 catalysts and the activation/deactivation of these catalysts, we have investigated these processes by a combination of kinetic measurements, time-resolved in situ diffuse reflectance Fourier transform infrared (FTIR) spectroscopy (DRIFTS) measurements, and structural characterization by X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). Kinetic measurements indicated a rapid activation phase, followed by a continuous slow deactivation. A faster deactivation of CO formation (reverse water−gas shift reaction) compared to that of methanol formation results in an increasing selectivity toward MeOH formation with time on stream. The activation of the catalyst is attributed to a rapid initial reduction of the support (formation of O vacancies). Since based on STEM imaging and XRD measurements sintering of Au nanoparticles is negligible, the subsequent deactivation is attributed to the slow buildup of site-blocking adsorbates, specifically surface carbonates, and/or over-reduction of the catalyst. This is supported also by the reversible nature of the deactivation upon recalcination in O 2 /N 2 . Steady-state isotopic transient kinetic analysis (SSITKA) measurements, following the buildup/decay of adsorbed formate and methoxy species by DRIFTS upon changing from a CO 2 /H 2 to a CO 2 /D 2 mixture and back under steadystate conditions, indicate that surface formate species are reaction intermediates in the dominant reaction pathway for CO 2 hydrogenation to methanol, with the calculated rates of formation/decay comparable to the rate of methanol formation. The consequences of these results for the mechanistic understanding of this reaction are discussed.

Atomic-scale characteristics of individual nanocrystals (NCs), such as the crystallographic orientation of their facets and the kind and density of crystal structure defects, play a tremendous role for the functionality and performance of the whole NC population. However, these features are usually quantified only for a small number of individual particles, and thus with limited statistical relevance. In the present work, we developed the multiscale approach available in transmission electron microscopy (TEM) further, and applied it to describe features of different types of Au NCs in a statistical and scale-bridging manner. This approach combines high-resolution TEM, which is capable of describing the characteristics of NCs on atomic scale, with a semi-automatic analysis of low-magnification high-angle annular dark-field scanning TEM images, which reveals the nanoscopic morphological attributes of NCs with good statistics. The results of these complementary techniques are combined and correlated. The potential of this multiscale approach is illustrated on two examples. In the first one, the habitus of Au NCs was classified and assigned to multiply twinned nanoparticles and nanoplates. These classes were quantified and related to different stacking fault densities. The second example demonstrates the statistical determination of crystallographic orientations and configurations of facets in Au nanorods.

Die elektronischen und strukturellen Eigenschaften von Au/ZnO-Katalysatoren und deren Zusammenhang mit der Aktivitätf ürd ie Methanolbildung unter realistischen und idealisierten Reaktionsbedingungen wurden mittels kinetischer und zeitaufgelçster operando bzw.i n-situ spektroskopischer Messungen untersucht. Während der Reaktion unter realisti-schenBedingungen durchgeführte operando IR-Spektroskopie (DRIFTS) Messungen an adsorbiertem CO belegen, dass zu Beginn der Reaktion negativ geladene Au-Nanopartikel/Au-Adsorptionsplätzeg ebildet werden. In-situ Messungen mittels Rçntgenphotoelektronen-Spektroskopie (NAP-XPS) und Nahkanten-Rçntgenabsorptions-Spektroskopie (XANES) zeigen, dass dies mit der Bildung von O-Leerstellen und einer erheblichen Zunahme der Methanolbildungsrate einhergeht. Konsequenzen dieser Ergebnisse fürd en Reaktionsablauf werden diskutiert und mit früheren Befunden an Cu/ZnO-und Au/ZnO-Katalysatoren verglichen.

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