Martin Makosch

Particle size and support effects in hydrogenation over supported gold catalysts

Particle size and support effects were determined for the hydrogenation of nitrobenzene over gold supported on alumina and titania by kinetic experiments, TEM and in situ high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS). Especially when supported on alumina, the catalytic activity correlated well with the fraction of particles smaller than 2 nm. These particles are able to split hydrogen on undercoordinated gold atoms, hence the strong particle size effect. In addition, on titania the increased length of the metal–support interface for smaller particles leads to enhanced activity because of the beneficial effect of the metal–support interface on the splitting of hydrogen. Reporting the average particle size is not always relevant in describing catalytic performance.

Hydrogenation of Nitrobenzene Over Au/MeOx Catalysts—A Matter of the Support

The heterogeneous hydrogenation of substituted nitrobenzenes is a reaction of great interest, because aniline and its derivates are valuable substances in the chemical industry for the production of polymers, pharmaceuticals, herbicides, and dyes. The state-of-the-art catalysts are mostly active metals, such as Pt, Pd, Ni, Cu, and Ir, which are supported on various materials, such as activated C, CaCO3, and SiO2, depending on their application. To achieve high selectivity to substituted anilines in the presence of other reducible groups and to prevent arylhydroxylamine accumulation in the reaction mixture, state-of-the-art catalysts are often modified with environmentally harmful additives, such as Pb and V promoters and Fe salts. Since the discovery that Au, when present as nanoparticles in the range of 1–3 nm, catalyzes CO oxidation, more and more reactions have been shown to be catalyzed by Au, among them the hydrogenation of nitrobenzene. Hydrogenation of nitroaromatics containing additional unsaturated groups over unmodified Au/TiO2 and Au/Fe2O3 shows a high selectivity to the nitro group. Thus, Au/MeOx (Me corresponds to a metal) catalysts have been presented as a “green” alternative in reactions where a high selectivity under moderate reaction conditions is required. Haber proposed a reaction scheme (Scheme 1) for the electrochemical hydrogenation of nitrobenzene and its derivates in 1898; however, there is an ongoing debate about the reaction mechanism over heterogeneous catalysts. Haber proposed two main reaction routes, namely the “direct” (left hand side) and the “condensation” route (right hand side). In the direct route, nitrobenzene is reduced to nitrosobenzene, then to phenylhydroxylamine, and finally to aniline (Steps I–III). A variation of the direct route is the “no-nitroso route” (Step IV), in which nitrobenzene directly reacts to phenylhydroxylamine and then to aniline. The condensation route occurs when the two intermediates nitrosobenzene and phenylhydroxylamine condensate to form azoxybenzene (Step VI). This species is then hydrogenated to aniline in consecutive steps via the intermediates azobenzene and hydrazobenzene (Steps VII–IX). Another possible step in the transformation of nitrobenzene to aniline is the decomposition of phenylhydroxylamine into nitrosobenzene and aniline (Step V). Aniline is produced by the disproportion of phenylhydroxylamine. The nitrosobenzene generated by the disproportion reenters the catalytic cycle and is subsequently transformed into phenylhydroxylamine. These findings are based on measurements of nitrobenzene hydrogenations over Ir/C poisoned by Hg. Azoxybenzene is the first intermediate that is formed in the condensation route, which is observed when the reactions are performed in the presence of a base. Azoxybenzene can also be detected at slow reaction rates, for example, over Pd/SiO2 in methanol at 25 8C. Recently, the selective catalytic hydrogenation of functionalized nitroarenes has been reviewed. The authors describe precisely the tailoring of selective catalysts by using organic and inorganic modifiers and their application for different catalytic problems. Also, the effect of solvent, particle size, and support are discussed. The discussion on the influence of the support focuses on selectivity, activity, and stabilization of the metal nanoparticles. Other reports detail the effect of the composition of the reaction mixture, the noble metal, and the Scheme 1. Possible reaction pathways for the hydrogenation of aromatic nitro compounds to the corresponding anilines. NB: nitrobenzene, NSB: nitrosobenzene, PHA: phenylhydroxylamine, AN: aniline, AOB: azoxybenzene, AB: azobenzene, HAB: hydrazobenzene. Adapted from Ref. [6] .

Evaluation of Pt and Re oxidation state in a pressurized reactor: Difference in reduction between gas and liquid phase

Determination of metal oxidation state under relevant working conditions is crucial to understand catalytic behaviour. The reduction behaviour of Pt and Re was evaluated simultaneously as a function of support and solvent in a pressurized reactor (autoclave). The bimetallic catalysts are used in selective hydrogenation of carboxylic acids and amides. Gas phase reduction reduced the metals more efficiently, in particular Pt.

The Dynamic Structure of Gold Supported on Ceria in the Liquid Phase Hydrogenation of Nitrobenzene

With regard to heterogeneous catalysis by gold and, in particular, hydrogenation reactions, there is no consensus on the oxidation state of the catalytically active species. By means of high‐resolution X‐ray absorption spectroscopy, we determined in situ the oxidation state of gold in the functioning catalyst Au/CeO2 in the slurry phase of the hydrogenation of nitrobenzene at high pressure. The conversion of nitrobenzene was monitored simultaneously by means of online attenuated total reflectance–Fourier transform infrared spectroscopy. We found that catalysts without measurable amounts of cationic gold were more active than catalysts with cationic gold. Any cationic gold that remained after pretreatment was reduced under reaction conditions, without any loss of activity. No evidence was found that the cationic gold contributed to the catalytic activity.