Application Of Nanoporous Metals In Gas-Phase Catalysis

Recently, nanoporous metal prepared by dealloying is sought for promising applications in catalysis, electronics, optic and drug delivery, due to its unique porous structure and property. Extending the facile method to prepare novel nanoporous material, further investigating its structure, and exploring its applications are very important to nanomaterial and industry field. Here we fabricate several nanoporous materials based on the dealloying process. The structure and possible formation mechanism are also investigated. Moreover, we explore their applications for the CO oxidation, preferential CO oxidation, gas-phase selective oxidation of alcohols, and electrooxidation of formic acid. The main informations are as follows:1. Insights into the reactivity of dealloyed nanoporous gold:combining the intrinsic activity of gold and residual impurityDealloyed nanoporous gold (NPG) has been found to possess remarkable catalytic activity for some of the typical gold-catalyzed reactions. It is suggested that there may be several effects contributing to the special catalytic properties. To date, no consistent picture regarding effects related to the intrinsic catalytic activity of gold and residual elements has arisen, so the debate still continues. In this work, we choose three typical NPG catalysts fabricated by dealloying of AuAg, AuCu, and AuAl alloy to investigate the catalytic activity of gold by using CO oxidation as the model reaction. Moreover, we first introduce AuAl-based ternary alloy to prepare NPG with different amounts of Ag content by dealloying of AuAgAl alloys. Theoretical analysis indicates that such unique nanostructure of NPG films, a smaller nanopore size with a closer interligament distance and large local curvature, possess a high concentration of low-coordination gold atoms (such as corner and step sites). Our data suggest that the most important effect is related to the intrinsic catalytic activity of gold. Residual impurity may also contribute to the special catalytic properties, but is not mandatory for catalytic reactions. 2. Nanoporous gold as an active low temperature catalyst toward CO oxidation in hydrogen-rich stream.Preferential CO oxidation (PROX) was investigated by using dealloyed nanoporous gold (NPG) catalyst under ambient conditions. Systematic investigations were carried out to characterize its catalytic performance by varying reaction parameters such as temperature and co-existence of CO2and H2O, which revealed that NPG was a highly active and selective catalyst for PROX, especially at low temperature. At20℃, the exit CO concentration could be reduced to less than2ppm with a turnover frequency of4.1×10-2s-1at a space velocity of120,000mL h-1g-1cat. and its high activity could retain for more than24hours. The presence of residual Ag species in the structure did not seem to improve the intrinsic activity of NPG for PROX; however, they contributed to the stabilization of the NPG structure and apparent catalytic activity. These results indicated that NPG might be readily applicable for hydrogen purification in fuel cell applications.3. Preparation of nanoporous platinum-based bimetallic nanocomposites by dealloying ternary alloys and research on their catalytic and electrocatalytic activity.Nanoporous PtCo (NP-PtCo) alloy was synthesized by dealloying a ternary PtCoAl alloy and applied in CO oxidation (COOX) and preferential CO oxidation (PROX). This interesting structure consists of layered nano-array around hundreds of nanometers, in which the array was composed of a three-dimensional bicontinuous network structure with a ligament/pore size down to3nm. NP-PtCo exhibited superior activity for COOX with good structure durability. The formed nanoporous structure facilitated the diffusion of the reactants and products, making contributions for the improved activity. This improvement of stability was related to the formation of a surface oxidized Co species layer and small surface diffusivity of Pt. Kinetic studies indicated that this reaction proceeded through Langmuir-Hinshelwood mechanism with low apparent activation energy (31.6kJ mol-1). Dealloying method is employed to fabricate three-dimensional nanoporous AuPt (NP-AuPt) alloys with low content of Pt, which favor the high dispersion of Pt atoms. The as-prepared alloys were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The NP-AuPt alloys composed of low Pt content show superior electrocatalytic activity and long stability toward the electrooxidation of formic acid in the acidic solution in comparison to the commercial Pt/C catalyst, due to the porous structure, the single-atom effect, the electronic property, and the assembly effect. This material can find potential applications in direct formic acid fuel cells.4. Gas-phase selective oxidation of methanol and ethanol over nanoporous AuAg alloys.Both nanoporous gold and nanoporous silver are interesting catalytic materials, because of their remarkable catalytic activities towards some reactions. Recent reports point to the crucial role of residual silver in very small concentrations in CO oxidation. Here we prepared a series of nanoporous AuAg alloys (NP-AuAg) prepared by dealloying of AuAgAl alloys, including NP-AuAg alloys with low content of Ag (Au99.5Ago.5, Au99Agi, Au98Ag2, Au95Ag5), with low content of Au (Ag99.5Auo.5, Ag99Au1, Ag98Au2, Ag95Au5), Au20Ag80, and Au50Ag50, which were applied in gas-phase selective oxidation of methanol and ethanol. In details, as for the catalytic oxidation of methanol on samples with low content of Ag, the conversion, selectivity and TOF decrease with the increase of Ag contents, however the catalytic activity of ethanol on the same samples exhibit contrary tendency. The activity varies with the Au/Ag molar ratios and attains the best activity when Au/Ag is1:4.