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Dynamic observation: The rapid oxidative redispersion of large Pt nanoparticles supported on ceria‐based oxide in autoexhaust catalysts is demonstrated in the absence of Cl by in situ XANES analysis. An atomic migration model accounts for the observed redispersion through the trapping of Pt species at sites on the Ce support that exhibit strong interactions between the Pt oxide and the support.

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Noncatalytic reaction pathways and their rates of acetaldehyde were determined in the neat system and in supercritical water at 400 °C and the density of 0.1−0.6 g/cm3. In supercritical water, acetaldehyde undergoes five types of reactions:  (i) decarbonylation into methane and carbon monoxide, (ii) self-disproportionation producing ethanol and acetic acid, (iii) cross-disproportionation generating ethanol and carbonic acid, (iv) condensation forming crotonaldehyde, and (v) the subsequent polymerization of crotonaldehyde or its decarbonylated monomer. Reactions i and iv proceed irrespective of the presence of water, while water suppresses reaction i. Reactions ii and iii are characteristic of aldehyde under hydrothermal conditions. Although reaction ii produces the same products as the classical Cannizzaro reaction, it does not require any added catalysts. Reaction iii manifests the role of formic acid as a reducing aldehyde. Actually, it is shown that reaction iii involves a larger weight than reaction ii and leads to the excess production of ethanol. The rates of these reactions are sensitive to the water density, and path weight control is thus possible through variation of the thermodynamic conditions. New reaction mechanisms are proposed for the present set of high-temperature processes.