A comparative study of alcohol oxidation over nanoporous gold in gas and liquid phase

Lackmann, Anastasia; Mahr, Christoph; Schowalter, Marco; Fitzek, Lisa; Weißmüller, J.; Rosenauer, Andreas; Wittstock, Arne

doi:10.1016/j.jcat.2017.07.008

Cited by 43 publications

(76 citation statements)

References 49 publications

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“…The results of this study are compiled in Figure 1. Interestingly and in line with a previous study [13], the surface roughness of npAu is not changed during catalysis staying within error bars nearly constant [5,13]. Based on HRTEM measurements and the evaluation method described in Section 2 and in Ref.…”

Section: Starting Alloysupporting

confidence: 90%

“…Based on HRTEM measurements and the evaluation method described in Section 2 and in Ref. [5], the surface roughness was determined to be ~22 ± 2 % after catalysis under basic conditions ( Figure S5), which is comparable to the roughness of ~25 ± 3 % reported previously for as-prepared samples and samples used as a catalyst under different catalytic conditions [36]. A detailed description of the procedure applied to extract these numbers from the HRTEM images is presented in Ref.…”

Section: Starting Alloysupporting

confidence: 72%

“…Under neutral conditions the catalyst is apparently inactive for the first 10 h and only subsequently the formation of methyl formate sets in. Such a behavior, i.e., the occurrence of an activation period, was already observed in previous studies in liquid [5] as well as in the gas phase [6]. The origin for this delayed activity is most likely twofold.…”

Section: Aerobic Catalytic Investigationssupporting

confidence: 69%

“…A detailed description of the procedure applied to extract these numbers from the HRTEM images is presented in Ref. [5].…”

Section: Starting Alloymentioning

confidence: 99%

“…In particular, aerobic oxidation, using molecular oxygen (O 2 ) from ambient air as oxidant, is an important step towards a more sustainable and "greener" chemistry. Accordingly, the growing interest in and research on npAu as a catalyst for highly selective oxidative self and cross-coupling of alcohols fits to this demand [5][6][7].…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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...

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Section: Starting Alloysupporting

confidence: 90%

Section: Starting Alloysupporting

confidence: 72%

Section: Aerobic Catalytic Investigationssupporting

confidence: 69%

“…A detailed description of the procedure applied to extract these numbers from the HRTEM images is presented in Ref. [5].…”

Section: Starting Alloymentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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Nanoporous Au/Ag Catalyzed Benzylic sp³C−H Oxidation of 9H‐Fluorene Derivatives and Similar Molecules with TBHP

Thayssen,

Riedel,

Risse

et al. 2024

ChemCatChem

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Nanoporous gold (npAu) was tested as a catalyst for oxidations of various benzylic sp3‐carbon positions in 9H‐fluorene derivatives and similar hydrocarbons using tertbutyl hydroperoxide (TBHP) as an oxidant. The silver content in the npAu catalyst had a major influence on the overall conversion: Lower silver amounts (0.53 mol%) led to higher conversions, whereas higher amounts of silver increasingly resulted in the disproportionation of TBHP as a competing reaction. Because residual silver from the bulk alloy was oxidized by TBHP and leached into the solution during the reaction, the Ag content diminished over time. Therefore, the recycling of the catalyst had a positive effect on the conversion after each use. Using electron paramagnetic resonance (EPR) spectroscopy, the formation of radicals from TBHP by npAu was evidenced indicating a radical‐based reaction mechanism which was further elucidated by NMR spectroscopic and GC‐MS (trace) product analysis. Subsequent oxidations of benzylic sp3‐carbon positions gave good to excellent conversions and chemoselectivities featuring npAu as a suitable catalyst for such reactions.

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Broadening the Realm of Nanoporous Gold Catalysts: Preparation and Properties When Emanating from AuCu as Parent Alloy

Tapia Burgos,

Mahr,

Silva Olaya

et al. 2024

ChemCatChem

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Nanoporous gold (npAu) attracted increasing attention over the last 20 years as a highly active and selective oxidation catalyst in particular at low temperatures. Previous research mainly focused on npAu that was fabricated by corrosive dealloying of AuAg parent alloys. Yet, the use of other binary alloys, such as AuCu, promises interesting variations of the catalytic properties, when considering that residual amounts of the less noble metal were shown to be co‐catalytically involved. Aiming at providing a platform for systematic studies in this direction for Cu, we not only dealt with strategies for a reliable and reproducible preparation of npAu(Cu) catalysts from AuCu, but also with their potential for CO oxidation in comparison to npAu(Ag). We were able to develop an approach based on thermally quenched Au0.3Cu0.7 alloys, providing distinct synthetic advantages as a starting material for the catalyst fabrication versus the thermodynamically more stable AuCu3 intermetallic compound. Using PCD (potentiostatically controlled dealloying), well‐defined pore structures with ligament diameters of ∼40 nm and variable residual Cu concentrations in the range between ∼0.6 at % and ∼1.2 at % could be straightforwardly obtained. After activating such catalysts at 150 °C, they reproducibly showed catalytic activity for aerobic CO oxidation in a broad temperature window between 40 °C and 250 °C. As opposed to npAu(Ag), the activity increased with decreasing residual Cu content, outperforming the former at temperatures above ∼60 °C not only with respect to CO2 formation rates but also with respect to thermal stability. Based on X‐ray photoelectron spectroscopic and transmission electron microscopic results, it was possible to conclude that Cu segregates to the surface and, with rising Cu bulk content, increasingly occurs in form of Cu2+ species at the surface. While the latter are expected to be catalytically inactive, Cu and Cu+ species are likely candidates for the activation of oxygen being not possible on pure Au.

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A comparative study of alcohol oxidation over nanoporous gold in gas and liquid phase

Cited by 43 publications

References 49 publications

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

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

Nanoporous Au/Ag Catalyzed Benzylic sp³C−H Oxidation of 9H‐Fluorene Derivatives and Similar Molecules with TBHP

Broadening the Realm of Nanoporous Gold Catalysts: Preparation and Properties When Emanating from AuCu as Parent Alloy

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A comparative study of alcohol oxidation over nanoporous gold in gas and liquid phase

Cited by 43 publications

References 49 publications

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

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

Nanoporous Au/Ag Catalyzed Benzylic sp3C−H Oxidation of 9H‐Fluorene Derivatives and Similar Molecules with TBHP

Broadening the Realm of Nanoporous Gold Catalysts: Preparation and Properties When Emanating from AuCu as Parent Alloy

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Product

Resources

About

Nanoporous Au/Ag Catalyzed Benzylic sp³C−H Oxidation of 9H‐Fluorene Derivatives and Similar Molecules with TBHP