A Molecular Mechanism for the Chemoselective Hydrogenation of Substituted Nitroaromatics with Nanoparticles of Gold on TiO₂ Catalysts:  A Cooperative Effect between Gold and the Support

Abstract

Nanoparticles of gold on TiO2 are highly chemoselective for the reduction of substituted nitroaromatics, such as nitrostyrene. By combining kinetics and in situ IR spectroscopy, it has been found that there is a preferential adsorption of the reactant on the catalyst through the nitro group. IR studies of nitrobenzene, styrene, and nitrostyrene adsorption, together with quantum chemical calculations, show that the nitro and the olefinic groups adsorb weakly on the Au(111) and Au(001) surfaces, and that although a stronger adsorption occurs on low-coordinated atoms in gold nanoparticles, this adsorption is not selective. On the other hand, an energetically and geometrically favored adsorption through the nitro group occurs on the TiO2 support and in the interface between the gold nanoparticle and the TiO2 support. Such preferential adsorption is not observed with nanoparticles of gold on silica which, contrary to the Au/TiO2 catalyst, is not chemoselective for the reduction of substituted nitroaromatic compounds. Therefore, the high chemoselectiviy of the Au/TiO2 catalyst can be attributed to a cooperation between the gold nanoparticle and the support that preferentially activates the nitro group.

Sites for the selective hydrogenation of ethyne to ethene on supported NiO/Au catalysts

“…During transition to uncharged Au 12 0 (3D) structures that contain flat elements (planes) and structural defects (corners and edges), the Q(H 2 ) increases up to 78 kJ/mol. The obtained results confirmed the existing hypotheses on the positive impact of cluster defects to the activation of H 2 [13,30,[32][33][34][35] and are in agreement with explanation of NiO/ Au synergy (II).…”

“…The better hydrogenation activity of 2.5 nm gold particles can be explained if the chemisorption of H 2 is dependent on gold particle size. Our recent research on the TOF of Au particles of different sizes (2-30 nm) in hydrogenation of ethynylbenzene [13] and studies by Bus [30,31], Serna [32], Boronat [33], and Jia [28] have proved the positive influence of small gold clusters on hydrogenation rate. It was concluded that H 2 is dissociatively adsorbed only at the corners and edges of the supported gold particles [34,35].…”

Hydrogenation over gold catalysts: The interaction of gold with hydrogen

“…With decreasing particle size, increasing amounts of hydrogen are chemisorbed and an increasing fraction is adsorbed strongly. Thus it can be concluded, that the chemisorption is limited to low coordinated corner and edge atoms, which was recently confirmed by theory (50).…”

“…Gold catalyzes various hydrogenation reactions such as the hydrogenation of alkenes (3)(4)(5)(6)(7)(8)(9)(10)(11)(12), alkadiens (13)(14)(15)(16)(17)(18) alkynes (13,(19)(20)(21)(22)(23)(24)(25), , -unsaturated carbonyl compounds and nitro compounds (47)(48)(49)(50)(51)(52)(53). In contrast to traditional hydrogenation catalysts such as platinum, palladium or ruthenium, which require modification, it often shows remarkable selectivities when multiple functional groups are present.…”

Reduction of quinolines to 1,2,3,4-tetrahydroquinolines with hydrosilanes/ethanol catalyzed by TiO2-supported gold nanoparticles under solvent free conditions

“…Owing to the relative instability and the difficulty for the formation of Au-hydrides, Au NPs-catalyzed reduction protocols are more controllable and selective relative to the analogous Pd or Pt-catalyzed processes; the facile accumulation of Pd or Pt-hydrides on the catalyst's surface often leads to side reactions (e.g., dehalogenation or overeduction pathways). 7 While much effort has been devoted to Au-catalyzed reduction of carbonyl and nitro compounds, there are only a few examples regarding the reduction of π systems. Cao and co-workers have shown that Au NPs supported on high surface area titania (HAS-TiO 2 ) catalyze the chemoselective reduction of quinolines into 1,2,3,4-tetrahydroquinolines using H 2 .…”