The influence of the preparation method and the corresponding particle size distribution on hydrothermal deactivation behavior at 600-800°C and its performance during CO/NO oxidation was systematically investigated for a series of Pt/Al2O3 catalysts. Representative conventional (incipient wetness impregnation) and advanced preparation methods (flame spray pyrolysis, supercritical fluid reactive deposition and laser ablation in liquid) were selected, which generated samples containing narrow and homogeneous but also heterogeneous particle size distributions. Basic characterization was conducted by inductively coupled plasma-optical emission spectrometry, N2 physisorption and X-ray diffraction. The particle size distribution and the corresponding oxidation state was analyzed using transmission electron microscopy and X-ray absorption spectroscopy. The systematic study shows that oxidized Pt nanoparticles smaller than 2 nm sinter very fast, already at 600°C, but potential chlorine traces from the catalyst precursor seem to stabilize Pt nanoparticles against further sintering and consequently maintain the catalytic performance. Samples prepared by flame spray pyrolysis and laser ablation showed a superior hydrothermal resistance of the alumina support, although, due to small inter-particle distance in case of laser synthesized particles, the particle size distribution increases considerably at high temperatures. Significant deceleration of the noble metal sintering process was obtained for the catalysts containing homogeneously distributed but slightly larger Pt nanoparticles (supercritical fluid reactive deposition) or for particles deposited on a thermally stable alumina support (flame spray pyrolysis). The correlations obtained between Pt particle size distribution, oxidation state and catalytic performance indicate different trends for CO and NO oxidation reactions, in line with structure sensitivity.

The catalytic activity of Pd‐SnO2 core@shell nanocomposites in the oxidation of CO and their CO‐sensing behavior were compared. For this purpose, Pd particles were placed on the inside and the outside of SnO2 hollow spheres, as demonstrated by electron tomography, X‐ray photoelectron spectroscopy, and X‐ray absorption spectroscopy. Both the sensing and catalytic effect were studied in a systematic manner on such nanocomposites, and striking differences in the catalytic performance of the nanocomposites in CO oxidation and CO and H2 sensing were found. At low temperatures, SnO2@Pd was found to be a good sensor, and the light‐off temperature was significantly lower than that of Pd@SnO2. Above the ignition temperature, CO was probably rapidly removed from the gas so that the sensing effect disappeared. This demonstrated that understanding of the sensing and catalytic behavior can help in unraveling the functional properties of core@shell and Pd‐SnO2 nanocomposites in more detail.

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Catalyst aging effects were analyzed using X-ray absorption micro-computed tomography in combination with conventional characterization methods on various length scales ranging from nm to mm to gain insight into deactivation mechanisms. For this purpose, a 4 wt% Pt/Al 2 O 3 model exhaust gas catalyst was coated on a cordierite honeycomb and subjected to sequential thermal aging in static air at 950 C for 4, 8, 12 and 24 hours. The aging was followed on the one hand by traditional methods, i.e. CO-oxidation activity, scanning and transmission electron microscopy (SEM, TEM), and X-ray diffraction (XRD). On the other hand, all intermediate aging steps were captured by X-ray absorption micro-computed tomography (m-CT) with 1.27 mm voxel size using a quasi in situ approach as complementary tool. The m-CT data allowed comparing exactly the same position after each treatment using a special alignment procedure during data analysis which took into account that the sample was remounted on the sample holder. A growth of the initially nanometer-sized Pt particles into larger crystals as well as its agglomeration was found, preferentially in voids between support grains. Sintering occurred especially around the larger particles, which is in line with the Ostwald ripening mechanism reported for this system on a nanometer scale.The distribution of chemical elements in an embedded and mechanically cross-sectioned honeycomb was additionally mapped by an electron probe micro analyzer (EPMA), which in agreement to the m-CT results shows no diffusion of Pt into the cordierite. Together with studies on the nanometer scale, these results allow a more thorough multi-scale modeling of exhaust gas catalysts, especially also during aging.

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