The Research On Nanoporous Gold Catalyst For The Oxidation Of CO, D-glucose And Benzyl Alcohol

Gold has been generally considered catalytically inactive for a long time. However, Haruta et al. reported that nanosized gold particles supported on oxides show high catalytic activity for CO oxidation, and then supported gold catalysts attracted increasing interest because they exhibited exceptional catalytic activity. Nanogold was widely used for many important reactions, for gas phase reaction including the selective oxidation of CO, NO reduction, water gas shift reaction and epoxidation of propene; for liquid phase reaction including the oxidation of benzyl alcohol, glycol, glycerol, cyclohexene and styrene, as well as the synthesis of hydrogen peroxide, and so on. In recent years, studying on the properties of nanoporous gold (NPG), especially its catalytic performance, has become a new hotspot. NPG, made by dealloying of Ag from Ag/Au alloys, has a unique bi-continuous foam structure. As an unsupported gold catalyst, NPG has displayed exceptional catalytic performances for many reactions, such as low-temperature CO oxidation, electro-oxidation of methanol and reduction of hydrogen peroxide.In this paper, we carried out the studying on the catalytic performance of NPG for CO, D-glucose and benzyl alcohol oxidation.1. NPG catalysts with various residual silver contents were fabricated, and their micro-morphologies were characterized; then we investigated the effect of residual Ag on the catalytic activity of NPG for CO oxidation at room temperature and changing temperature, the results showed that the existence of Ag played a key role for the CO oxidation. The catalytic activity of these NPG catalysts for CO oxidation is closely correlation with surface silver contents and oxidation state, whereas the ligament size effect is not obvious. The original NPG samples exhibited very low catalytic activity for CO oxidation at room temperature, and the presence of residual Ag is found to be unfavorable under this condition. Upon Pretreated in oxygen at elevated temperature (above 150℃), NPG with appropriate Ag residues showed enhanced catalytic activity for CO oxidation at low temperature, though the ligament sizes dramatically increased to above 50 nm synchronously. Especially, for the temperature changing reaction, these samples exhibited a similar catalytic performance to supported Au/Ag alloy particle catalysts previous reported, which strongly implies NPG with certain Ag residues to be virtually a novel unsupported nanoporous Au/Ag alloy catalyst, and the best activity can be obtained with surface Au/Ag ratio around 2:1. We suppose that a thin surface silver oxide-like structure on NPG ligaments might play an important role for the catalytic activity of these catalysts toward CO oxidation.2. According to the corresponding references, we reported the promotional effect of hydroxyl on the CO oxidation over nanoporous gold (NPG) catalysts. The experimental results showed that the catalytic activity of NPG catalyst pretreated with alkaline solution was enhanced obviously; ammonia comparative experiment proved that OH^- was the promoter, not Na⁺; the high catalytic activity of alkaline pretreatment NPG catalysts for CO oxidation was closely related with pH value of alkaline solution and immersing time; moreover, an investigation of the catalytic activity of NPG-16 (24.9at%Ag) pretreated with alkaline solution for CO oxidation was carried out, and we got that after being pretreated with alkaline solution the activity of this sample was still very low at temperature, while increasing temperature a significant activity increasing was obtained.3. We prepared two different sizes NPG samples with electrochemical corrosion method and free corrosion method, respectively; subsequently, we used these samples as catalysts for D-glucose oxidation, obtaining that the catalytic activity of 6nm NPG sample was much higher than the 30nm one's. Meanwhile, the effect of residual Ag on catalytic activity of free corrosion NPG for D-glucose oxidation was investigated, and the more Ag residues, the less catalytic activity; moreover, nanoporous Au-Pt alloy was made, and the addition of Pt greatly improved the catalytic stability for D-glucose oxidation.4. We took NPG as the catalyst for solvent-free selective oxidizing benzyl alcohol to benzaldehyde. Experiment showed that increasing the temperature increased the conversion of benzyl alcohol and the selectivity of benzaldehyde; however, the increasing of pO₂ increased the conversion of benzyl alcohol, whereas reduced the selectivity of benzaldehyde. An induction phase was needed before the reaction carryied out, and the lower reaction temperature, the longer induction phase, but the addition of a little water could initiate this reaction under lower temperature. The result of KI-containing starch detection showed that hydroperoxy species were probably involved in the oxidation as intermediate. Moreover, the introducing of anhydrous K₂CO₃ significantly promoted the activity of NPG catalyst for benzyl alcohol oxidation.