Aerobic Methanol Oxidation over Unsupported Nanoporous Gold: The Influence of an Added Base

Abstract

We studied the aerobic oxidation of methanol over nanoporous gold catalysts under neutral and alkaline conditions. We find that under neutral conditions the catalyst has an activation period of about 10 h while upon addition of a base the catalyst becomes active right away. After this activation period, however, the activity of the catalyst is in both cases similar. Moreover, the selectivity was not affected by the base. We tested different bases and found the largest effect when adding OH − . The cation, however, does not play a role. We conclude that it is OH − , which is impacting the reaction and propose a mechanism for the suppression of the activation period. While the catalytic cycle, i.e., the reaction of methanol on the catalyst surface seems unaffected, the transient adsorption of OH − onto the surface can facilitate the activation of molecular oxygen by donating electrons to the surface. Due to the intermediate formation of oxidic Ag species, an effective segregation of surface-near Ag can be induced, which increases the abundance of Ag being essential for the activation of oxygen at the surface. In this way, a more efficient pathway for the generation of active oxygen is opened, allowing the reaction to set in faster.Catalysts 2019, 9, 416 2 of 14 intermetallic compound) and-as the atomic radii are also very similar-stress generation during the dealloying process is avoided. During removing the less-noble metal chemically or electrochmically, an interconnected three-dimensional network is formed with ligament and pore sizes typically in the range of 20-50 nm. Whereas Au nanoparticles in the range of a few nm supported on suitable supports are known to be catalytically active for a long time, catalytic activity of this unsupported material was initially unexpected. Extensive research after first reports in 2006 and 2007 [9,10] revealed that its catalytic activity can be attributed to two primary factors [11]. One is the surface roughness, i.e., the density of low-coordinated atoms [12,13] on the surface of the npAu ligaments, which, according to TEM studies, is similar to 5 nm Au nanoparticles so that the surfaces of the npAu ligaments expose a comparable density of edge and kink sites. Such low-coordinated sites are known to increase the adsorption strength of reactants and at the same time lower the activation barriers for breakage of, e.g., C-H bonds [14].The second factor, explaining the ability of npAu to activate molecular oxygen even in the absence of an oxide support, has been attributed to residual Ag [15][16][17], which remains in the material in the range of a few atom% after the dealloying process. It has been suggested [9] and supported by theoretical DFT studies [18][19][20][21][22] that Ag clusters or bimetallic AuAg sites are able to activate molecular oxygen (O 2 )-a crucial step that is not possible on pure Au. The reactive O atoms formed from O 2 dissociation in this way can subsequently spill over to surrounding Au sites and oxidize adsorbed reactants and intermediates. Theoretical...

What Changes on the Inverse Catalyst? Insight From CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

“…[1][2][3][4][5][6][7] In contrast, the mechanistic understanding of catalytic processes ocurring on nanoporous gold (np-Au), a three-dimensional nanoporous material, is still far less developed, even though high activities and selectivities have been observed in (partial) oxidation reactions. [8][9][10][11][12] Theoretical studies have been debating on the mechanism of such reactions with dioxygen on np-Au, especially on the role of under-coordinated surface atoms and impurities of a less noble metal (usually silver) left over from the preparation of npAu, which is obtained by selective leaching this less noble metal out of a corresponding Au alloy. [13][14][15][16][17][18] Whereas most studies suggest a significant participation of the second metal (Ag) in the activation of O2 on the surface of npAu, some studies also suggest a possible role of chemisorbed oxygen on Au sites in facilitating O2 dissociation, 17,19 or as a driving force for Ag segregation to the surface.…”

Exploiting the Liquid Phase to Enhance the Cross-Coupling of Alcohols over Nanoporous Gold Catalysts

“…Au catalysts have also been shown to promote oxygen-assisted alcohol coupling in the liquid phase at elevated pressures, though direct comparisons with gas-phase coupling are infrequently made. , There is a general consensus that the combination of O 2 and Au is also responsible for C–H activations in the liquid phase, and this mode of operation introduces the possibility that alcohol/aldehyde coupling occurs homogeneously, producing a liquid-phase hemiacetal, which is then oxidized by Au to yield the final ester product. − Hemiacetals are known to form noncatalytically in mixtures of alcohols and aldehydes, − though these reactions are quite slow in the absence of an acid or base. , Co-catalysts in the form of a homogeneous base (e.g., NaOH, NaOMe, K 2 CO 3 ) or a support oxide with modest acidobasicity (e.g., TiO 2 , CeO 2 , MgO, Ga 2 O 3 ) are therefore commonly employed to increase the catalytic activity of Au in alcohol coupling. − ,− Proof that a liquid-phase hemiacetal intermediate is involved is lacking, however, and the increased reaction rates observed upon base addition may be attributable to the donation of reactive OH – species to the catalyst surface, OH-induced restructuring of the catalyst, or neutralization of site-blocking acidic intermediates rather than promotion of homogeneous hemiacetal formation. It therefore remains possible that oxygen-assisted coupling in the liquid phase proceeds solely through Au-catalyzed steps as it does in the gas phase, especially in the absence of an acid or base co-catalyst.…”

Photocatalytic coatings based on a zinc(ii) phthalocyanine derivative immobilized on nanoporous gold leafs with various pore sizes

“…[12][13][14] While gold as bulk material is relatively inactive in chemical reactions, npAu was shown to be a very efficient catalyst for several oxidation reactions in both, gas phase and liquid phase even at comparably low temperatures. [15][16][17][18] Besides this, the formation of porosity in the nanometer scale is also the reason for arising special optical features such as surface plasmon resonance similar to gold nanoparticles (AuNPs). [19][20][21] Using surface gold chemistry, which is well established in literature, the nanoporous material can be easily functionalized in many ways, for example with different metal oxide nanoparticles as well as several organometallic complexes or even proteins using the principles of self-assembled monolayers.…”