Active sites for methanol partial oxidation on nanoporous gold catalysts

Wang, Lu‐Cun; Personick, Michelle L.; Karakalos, Stavros; Fushimi, Rebecca; Friend, Cynthia M.; Madix, Robert J.

doi:10.1016/j.jcat.2016.08.012

Cited by 48 publications

(79 citation statements)

References 41 publications

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“…It is well known that the first step in the catalytic cycle for methanol self-coupling is activation of methanol to formaldehyde 17,18 ; the transient experiments probe the initial step in the reaction pathway that governs the steady-state reaction. The difference in product selectivity in the titration with methanol pulses and in steady flow is related to the large difference in contact times (10 −4 s versus 10 −1 s, respectively) with the catalyst and will be addressed in a separate paper 19 .…”

Section: Resultsmentioning

confidence: 99%

“…Furthermore, the catalyst facilitates the dissociation of O 2 after complete reaction of the 'selective' oxygen via reaction with methanol. Exposure of this material to dioxygen produces adsorbed O, but only from reaction with the minority sites 19 ; this adsorbed oxygen is unreactive with CO but reacts readily with methanol to give selective oxidation products. This is in agreement with previous studies of ozone-activated npAu 13 .…”

Section: Resultsmentioning

confidence: 99%

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Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

et al. 2016

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Bimetallic, nanostructured materials hold promise for improving catalyst activity and selectivity, yet little is known about the dynamic compositional and structural changes that these systems undergo during pretreatment that leads to efficient catalyst function. Here we use ozone-activated silver-gold alloys in the form of nanoporous gold as a case study to demonstrate the dynamic behaviour of bimetallic systems during activation to produce a functioning catalyst. We show that it is these dynamic changes that give rise to the observed catalytic activity. Advanced in situ electron microscopy and X-ray photoelectron spectroscopy are used to demonstrate that major restructuring and compositional changes occur along the path to catalytic function for selective alcohol oxidation. Transient kinetic measurements correlate the restructuring to three types of oxygen on the surface. The direct influence of changes in surface silver concentration and restructuring at the nanoscale on oxidation activity is demonstrated. Our results demonstrate that characterization of these dynamic changes is necessary to unlock the full potential of bimetallic catalytic materials.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

et al. 2016

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“…As far as the catalytic mechanism of methanol oxidation on Au is concerned there is general agreement that it proceeds as follows: methanol reacts with adsorbed oxygen atoms originating from the activation and dissociation of molecular oxygen on Ag sites in the case of npAu to form methoxy species (OCH 3 ), which in turn undergo further hydrogen elimination resulting in surface-bonded formaldehyde (O=CH 2 ). The latter species reacts further with surrounding methoxy species to yield the self-coupling product MeFm, which finally desorbs from the surface [2,[5][6][7]43,44]. The loss of the hydrogen atom from the methyl group (H-elimination step) is believed to exhibit the highest activation barrier (based on UHV-studies on Au(111)) and thus to represent the rate-limiting step [45].…”

Section: Aerobic Catalytic Investigationsmentioning

confidence: 99%

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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“…The current research in gold catalysis consists of the study of catalytic reactions on nanoporous catalysts [261][262][263][264][265]. The nanoporous Au, npAu, consists of a 3D structure of interconnected pores and ligaments with a ligament size in the 20-100 nm range.…”

Section: Processes On Self-supported Aumentioning

confidence: 99%

“…In this work, the authors focused on influence of ozone activation on the catalyst to improve the selectivity towards alcohol oxidation reactions. One of the key steps towards this goal is the dissociation of oxygen species at the surface of the catalyst, and Ag provides active sites for oxygen dissociation [261][262][263][264][265].…”

Section: Nanoporous Au As Catalyst For Selective Alcohol Oxidationmentioning

confidence: 99%

Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

et al. 2017

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Abstract:Since the early discovery of the catalytic activity of gold at low temperature, there has been a growing interest in Au and Au-based catalysis for a new class of applications. The complexity of the catalysts currently used ranges from single crystal to 3D structured materials. To improve the efficiency of such catalysts, a better understanding of the catalytic process is required, from both the kinetic and material viewpoints. The understanding of such processes can be achieved using environmental imaging techniques allowing the observation of catalytic processes under reaction conditions, so as to study the systems in conditions as close as possible to industrial conditions. This review focuses on the description of catalytic processes occurring on Au-based catalysts with selected in situ imaging techniques, i.e., PEEM/LEEM, FIM/FEM and E-TEM, allowing a wide range of pressure and material complexity to be covered. These techniques, among others, are applied to unravel the presence of spatiotemporal behaviours, study mass transport and phase separation, determine activation energies of elementary steps, observe the morphological changes of supported nanoparticles, and finally correlate the surface composition with the catalytic reactivity.

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Active sites for methanol partial oxidation on nanoporous gold catalysts

Cited by 48 publications

References 41 publications

Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

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

Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

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