Gold‐Nanotube Membranes for the Oxidation of CO at Gas–Water Interfaces

Abstract

An exciting discovery in the area of heterogeneous catalysis is the observation that nanoparticles of gold on high-surfacearea supports exhibit high activity for oxidation of CO with O 2 (CO + 1/2 O 2 !CO 2 ) at room temperature. [1,2] This discovery is of particular relevance for the production of fuel-cell-grade hydrogen, since carbon monoxide must be removed from hydrogen streams generated by catalytic reforming of hydrocarbons.[3] The origins for the unique catalytic properties of supported gold catalysts are still unresolved. These properties have been related to changes in the electronic properties, the presence of defect sites, and the existence of strain for metallic gold nanoparticles. [4][5][6][7][8] Unique catalytic properties have also been related to the presence of sites associated with the catalyst support, such as cationic gold species, and sites at gold-support interfaces. [9,10] Here we show that it is possible to study the catalytic properties of metallic gold, without interference from a catalyst support, by using nanotubes of gold in polycarbonate membranes. These nanotubes exhibit catalytic activity for the oxidation of carbon monoxide by O 2 at room temperature, and this activity is enhanced by liquid water, promoted by increasing the pH of the solution, and increased using H 2 O 2 as the oxidizing agent. The rate can also be increased by depositing KOH within these nanotubes. These rates are comparable with those found in heterogeneous catalysis studies with gold nanoparticles on oxide supports, which suggests that the high activity of these latter catalysts may be related to the promotional effect of hydroxyl groups.Gold nanotubes of uniform size were prepared via a template-synthesis method by electroless deposition of gold [11] within the pores of a 10-mm-thick, track-etched polycarbonate membrane containing 220-nm-diameter pores.[12] All surfaces of the template membrane were first sensitized with a Sn II salt, activated by formation of a metallic Ag layer, followed by electroless deposition of gold for a period of 2 h. The gold nanotubes were cleaned with a 25 % HNO 3 solution for 15 h.[13] Hydrophobic or hydrophilic selfassembled monolayers were formed on gold nanotubes by rinsing the samples in ethanol for 20 min, followed by immersion for 17 h in solutions of ethanol containing HS(CH 2 ) 15 CH 3 or HS(CH 2 ) 15 COOH, respectively. [14] Gold nanotubes embedded within the pores of the polycarbonate template membranes were exposed by reactive ion etching (RIE) using an oxygen plasma to selectively etch approximately 2.3 mm of polycarbonate, leaving the gold nanotubes intact.[15] Figure 1 shows images from field-emission scanning electron microscopy (SEM) of the top surface of a template membrane after electroless deposition of gold followed by RIE. The images in Figure 1 reveal a high level of surface roughness; the length scale is % 53 nm. The membrane reactor used to study CO oxidation over gold nanotubes is shown schematically in Figure 2.[12] Table 1 shows results for the rate of...

Water-assisted oxygen activation during selective oxidation reactions

“…CO 2 ) is of substantial interest [27][28][29][30]. CO oxidation is routinely used as the prototypical model reaction in heterogeneous catalysis research because it is seemingly simple, yet exhibits a variety of complex phenomena [27,28,31].…”

On the Impact of Solvation on a Au/TiO2Nanocatalyst in Contact with Water

“…The so-called carboxyl and redox mechanisms have been proposed for the WGS reaction on metal/oxide surfaces; in the former mechanism the CO species reacts with terminal hydroxyl groups, whereas in the latter CO reacts with an O atom from OH dissociation or from the oxide support. In both mechanism proposed the starting point is a H 2 O molecule that is initially adsorbed on the metallic cluster with its oxygen atom being attached to a metal atom.Moreover, even large (as opposed to nanoscale) gold particles, which are usually catalytically inert, show considerable oxidation activity at aqueous conditions [11,12]. Because of its high dielectric constant, water may actively participate in chemical reactions for instance by stabilizing ionic species, thus speeding up reactions at liquid-solid interfaces, which opens up novel avenues to improve the performance of traditional heterogeneous catalysis at the gas-solid interface.…”

Nanoporous Microtubes via Oxidation and Reduction of Cu–Ni Commercial Wires

“…In particular, metallic nanotubes attract attention because of magnetic properties [4], surface enhanced Raman scattering (SERS) [5] and catalytic performance [6]. The synthesis of microtubes and nanotubes is typically run by self-organization of constituent atoms or molecules, template-assisted method (assisted by electroless deposition, electrodeposition or atomic layer deposition), Kirkendall-based route or hydrothermal method [1,[7][8][9].…”