Revealing Hydrogenation Reaction Pathways on Naked Gold Nanoparticles

Abstract

Gold nanoparticles (AuNPs) display distinct characteristics as hydrogenation catalysts, with higher selectivity and lower catalytic activity than group 8−10 metals. The ability of AuNPs to chemisorb/activate simple molecules is limited by the low coordination number of the surface sites. Understanding the distinct pathways involved in the hydrogenation reactions promoted by supported AuNPs is crucial for broadening their potential catalytic applications. In this study, we demonstrate that the mechanism of the hydrogenation reactions catalyzed by AuNPs with "clean" surfaces may proceed via homolytic or heterolytic hydrogen activation depending on the nature of the support. The synthesis of naked AuNPs employing γ-Al 2 O 3 and ionic liquid (IL)-hybrid γ-Al 2 O 3 supports was accomplished by sputtering deposition using ultrapure gold foils. This highly reproducible and straightforward procedure furnishes small (∼6.6 nm) and well-distributed metallic gold nanoparticles (Au(0)NPs) that are found to be active catalysts for the partial and selective hydrogenation of substituted conjugated dienes, alkynes, and α,β-unsaturated carbonyl compounds (aldehydes and ketones). Kinetic and deuterium labeling studies indicate that heterolytic hydrogen activation is the primary pathway occurring on the AuNPs imprinted directly on γ-Al 2 O 3 . In contrast, AuNPs supported on IL-hybrid γ-Al 2 O 3 materials cause the reaction to proceed via a homolytic hydrogen activation pathway. The IL layer surrounds the AuNPs and acts as a cage, influencing the frequency of the interaction of the catalytically active species and the metal surface and, consequently, the catalytic performance of the AuNPs. The IL layer is shown to improve the product selectivity by the enhancement of the substrate/product discrimination, and to decrease the catalytic activity by shifting the rate-determining step to the H 2 and substrate competitive adsorption/activation on the same active sites. A series of kinetic experiments suggest that AuNPs imprinted on an IL-hybrid γ-Al 2 O 3 support are more efficient (lower activation energy, E a ) than group 8−10 metal based catalysts for hydrogenation reactions at moderate to high temperatures (75−150°C).

Catalytic Semi‐Water–Gas Shift Reaction: A Simple Green Path to Formic Acid Fuel

“…The alloy is mainly composed of oxidized metals (Figure ) and has the tendency to be accommodated close to more ionic regions of the material (surface of the hybrid silica) rather than to the IL contact‐ion‐pair regions . This behavior is opposite, as expected, to that observed for Pd 0 and Au 0 NPs imprinted in similar hybrid silica supports . It is important to note that XPS analysis was used to study the location of the metal only in the surface region of the supports (Table ), because XPS is a surface‐sensitive technique.…”

“…The geometric and electronic properties of NPs supported in ILs can be tuned by the proper choice of the IL cations and anions, along with NPs, which strongly influence the residence time/diffusion of the reactants, intermediates, and products in the nano‐environment . Interestingly, the hydrophobic and hydrophilic nature of the SILP shows a remarkable influence on the conversion of CO and H 2 O to FA.…”

“…[41] This behavior is opposite, as expected, to that observed for Pd 0 and Au 0 NPsi mprinted in similar hybrid silica supports. [42,43] It is important to note that XPS analysisw as used to study the location of the metal only in the surface region of the supports (Table 1), because XPS is as urface-sensitive technique. This difference also corroborates the RBS results, which indicated that the RuFe/SILP-OAc sample presents am uch higher NP concentration in the surface region compared with the RuFe@SILP-NTf 2 .T he chemical environments of the Ru and Fe surface atoms were investigated by high-resolution XPS data of the Ru 3p (Figure 3d)a nd Fe 2p (Figure 3c)r egions.…”

Arene Hydrogenation by Metal Nanoparticles in Ionic Liquids

“…In reactions performed with these IL/metal catalysts, there is an appearance of flowing liquid(s) (reagents and products) through a chemically active membrane composed of NPs in a confined IL space. Hence, the catalysts in the IL nanocontainer under the multiphase regime (“dynamic asymmetric mixture”) operate akin to catalytically active membranes: i. e., away from the thermodynamic equilibrium ,. The IL effect has been explored in both soluble metal nanoparticles (NPs in ILs) and classically supported heterogeneous catalysts (supported catalyst with an IL layer, SCILL) for the selective partial hydrogenation of benzene and other arenes (Figure ).…”

“…Hence, the catalysts in the IL nanocontainer under the multiphase regime ("dynamic asymmetric mixture") operate akin to catalytically active membranes: i. e., away from the thermodynamic equilibrium. [19,20] The IL effect has been explored in both soluble metal nanoparticles (NPs in ILs) and classically supported heterogeneous catalysts (supported catalyst with an IL layer, SCILL) for the selective partial hydrogenation of benzene and other arenes ( Figure 2). Herein, we will present and discuss in detail the IL effect on the structural, electronic and geometric properties of metal catalysts for the hydrogenation of arenes, in particular benzene.…”