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The selective hydrogenation of nitrobenzene (NB) over Ni/γ-Al 2 O 3 catalysts was investigated using different media of dense phase CO 2 , ethanol, and n-hexane. In dense phase CO 2 , the total rate of NB hydrogenation was larger than that in organic solvents under similar reaction conditions; the selectivity to the desired product, aniline, was almost 100% over the whole conversion range of 0-100%. The phase behavior of the reactant mixture in/under dense phase CO 2 was examined at reaction conditions. In situ high-pressure Fourier transform infrared measurements were made to study the molecular interactions of CO 2 with the following reactant and reaction intermediates: NB, nitrosobenzene (NSB), and N-phenylhydroxylamine (PHA). Dense phase CO 2 strongly interacts with NB, NSB, and PHA, modifying the reactivity of each species and contributing to positive effects on the reaction rate and the selectivity to aniline. A possible reaction pathway for the hydrogenation of NB in/under dense phase CO 2 over Ni/γ-Al 2 O 3 is also proposed.

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