Catalytic activity of nanostructured Au: Scale effects versus bimetallic/bifunctional effects in low-temperature CO oxidation on nanoporous Au

Abstract

SummaryThe catalytic properties of nanostructured Au and their physical origin were investigated by using the low-temperature CO oxidation as a test reaction. In order to distinguish between structural effects (structure–activity correlations) and bimetallic/bifunctional effects, unsupported nanoporous gold (NPG) samples prepared from different Au alloys (AuAg, AuCu) by selective leaching of a less noble metal (Ag, Cu) were employed, whose structure (surface area, ligament size) as well as their residual amount of the second metal were systematically varied by applying different potentials for dealloying. The structural and chemical properties before and after 1000 min reaction were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic behavior was evaluated by kinetic measurements in a conventional microreactor and by dynamic measurements in a temporal analysis of products (TAP) reactor. The data reveal a clear influence of the surface contents of residual Ag and Cu species on both O2 activation and catalytic activity, while correlations between activity and structural parameters such as surface area or ligament/crystallite size are less evident. Consequences for the mechanistic understanding and the role of the nanostructure in these NPG catalysts are discussed.

Active sites for methanol partial oxidation on nanoporous gold catalysts

“…at.%) [10,11,[14][15][16]21].The np(Ag)Au catalyst used in these studies were treated in ozone to activate them for selective oxidation of methanol [7].The studies herein indicate that Ag is an integral part of the active site for O 2 activation and subsequent reaction with methanol and that the oxygen atoms do not migrate significantly away from the dissociation site under reaction conditions.…”

“…Dioxygenwas deposited on this activated catalyst using a series of microsecond pulses of O 2 to thenp(Ag)Au at 423 K [10,11,21,23]. The number of O 2 pulses was varied between 1 and 200 in order to determine both the sticking coefficient and the saturation site density.The methanol pulses were generated from a methanol/Ar mixture (approximately 1:4 ratio), which was prepared in a heated blend tank and maintained at a constant total pressure (~2.4 atm).…”

“…Key to the design of Au-based catalysts, including np(Ag)Au, is understanding how atomic O is produced from molecular O 2 .Adsorbed atomic oxygen is the active site for reaction ofalcohols, initiating the catalytic cycles for alcohol coupling [4][5][6][7][8].The nature and density of reactive centers for O 2 dissociation on np(Ag)Auis still a controversial issue. On one hand, the residual Ag is thought to play a critical role [9][10][11][12][13]. Specifically, silver multi-atom (pairs) sites have been proposed as the active sites for O 2 activation over Au-Ag bimetallic systems including both supported AuAg alloy particles and high Ag-loaded np(Ag)Au catalysts [14][15][16].The essential role of Ag in O 2 activation is also supported by the qualitative correlation between the total number of surface Ag atoms and the oxygen storage capacity (OSC) for a series of np(Ag)Au catalysts with different surface Ag contents (16.6 at.% -30 at.%) [10,11].On the other hand, others have proposed that low-coordinated Au sites, such as edge, step or kink sites, are the sites for O 2 activation and catalytic activity [3,17,18].…”

“…On one hand, the residual Ag is thought to play a critical role [9][10][11][12][13]. Specifically, silver multi-atom (pairs) sites have been proposed as the active sites for O 2 activation over Au-Ag bimetallic systems including both supported AuAg alloy particles and high Ag-loaded np(Ag)Au catalysts [14][15][16].The essential role of Ag in O 2 activation is also supported by the qualitative correlation between the total number of surface Ag atoms and the oxygen storage capacity (OSC) for a series of np(Ag)Au catalysts with different surface Ag contents (16.6 at.% -30 at.%) [10,11].On the other hand, others have proposed that low-coordinated Au sites, such as edge, step or kink sites, are the sites for O 2 activation and catalytic activity [3,17,18]. This viewpoint is consistent withtransmission 4 electron microscopy (TEM) studies that reveal a high incidence of step and kink sites on the material, which are stabilized bya high content of residual Ag by s uppressing (111) faceting kinetics [19].However, recent studies with extremely pure np(Ag)Au synthesized from an intermetallic AuZr alloy (bulk residual Zr content ~0.03%) was inactive for O 2 activation and exhibited very low activity toward CO oxidation(several orders of magnitude lower than np(Ag)Au), which leads to the conclusion that Ag impurities play a key role in the dissociation of O 2 on np(Ag)Au [20].…”

Nanoporous Gold by Alloy Corrosion: Method-Structure-Property Relationships

“…for CO oxidation) is under discussion. [22][23][24][25] As another instance, reports on the mechanical strength and stiffness of nominally comparable structures disagree by up to one order of magnitude. 26,27 Therefore it is significant to note that a number of different dealloying protocols are applied within these studies.…”