Au-Promoted Pt nanoparticles supported on MgO/SBA-15 as an efficient catalyst for selective oxidation of glycerol

A systematic study of Au-promoted and unpromoted Pt/MgO/SBA-15 catalyst is developed to separate the promoter effect from electron transfer effect between Au and Pt. Multi-characterizations revealed that Au and Pt metals in these bimetallic catalysts mainly exist in the form of alloy, and the main role of Au is to reduce the size of AuPt alloy nanoparticles, thus enhancing the adsorption and activation of intermediate products. Through the optimization of various factors (including MgO content, Au/Pt molar ratio, reaction temperature and time), the Au₁Pt₂/MgO/SBA-15 (0.05) catalyst exhibits excellent catalytic activity and glyceric acid selectivity for the selective oxidation of glycerol. Density functional theory calculation confirmed that the synergistic effect between Pt and Au active sites could facilitate the oxidation of primary hydroxyl group by promoting the activation of C H bond and the oxidation of aldehyde group. The results may give insights on designing effective Pt based bimetallic catalyst for selective oxidation of glycerol.     

Exploratory Catalyst Screening Studies on the Base Free Conversion of Glycerol to Lactic Acid and Glyceric Acid in Water Using Bimetallic Au–Pt Nanoparticles on Acidic Zeolites

The base free oxidation of glycerol with molecular oxygen in water using bimetallic Au–Pt catalysts on three different acidic zeolite supports (H-mordenite, H-β and H-USY) was explored in a batch setup. At temperatures between 140 and 180 °C, lactic acid formation was significant and highest selectivity (60 % lactic acid at 80 % glycerol conversion) was obtained using Au–Pt/USY-600 (180 °C). A selectivity switch to glyceric acid (GLYA) was observed when the reactions were performed at 100 °C. Highest conversion and selectivity towards GLYA were obtained with Au–Pt/H-β as the catalyst (68 % selectivity at 68 % conversion).     

Exploratory Catalyst Screening Studies on the Base Free Conversion of Glycerol to Lactic Acid and Glyceric Acid in Water Using Bimetallic Au–Pt Nanoparticles on Acidic Zeolites

The base free oxidation of glycerol with molecular oxygen in water using bimetallic Au–Pt catalysts on three different acidic zeolite supports (H-mordenite, H-β and H-USY) was explored in a batch setup. At temperatures between 140 and 180 °C, lactic acid formation was significant and highest selectivity (60 % lactic acid at 80 % glycerol conversion) was obtained using Au–Pt/USY-600 (180 °C). A selectivity switch to glyceric acid (GLYA) was observed when the reactions were performed at 100 °C. Highest conversion and selectivity towards GLYA were obtained with Au–Pt/H-β as the catalyst (68 % selectivity at 68 % conversion).

     

胶体负载法制备Au/C催化剂用于乳酸合成的研究

将预先制备好的十二烷基硫醇保护的纳米金胶体负载到用硝酸处理过的活性炭上,制备得到纳米Au/C催化剂,并用1,2-丙二醇直接氧化合成乳酸的反应考评催化剂。重点讨论载体的预处理方法和负载温度对金胶体负载的影响以及载体上金的含量、粒径、热处理方式等对纳米金催化剂活性的影响。结果表明:胶体负载法制备的纳米Au/C催化剂在1,2-丙二醇直接氧化合成乳酸反应的转化率达到50%以上,选择性达到80%以上。     

Solvent free liquid phase oxidation of benzyl alcohol using Au supported catalysts prepared using a sol immobilization technique

Solvent free oxidation of benzyl alcohol was investigated in the absence of a base using Au catalysts prepared by sol immobilization on titania and carbon supports. Comparison between the Au supported catalysts revealed that activity and distribution of products was dependent on the nature of support and heat treatment. Specifically, heat pre-treatment of the Au catalysts has a beneficial effect in terms of activity, but is detrimental in terms of selectivity to the benzaldehyde. We conclude that sol immobilization is a suitable technique for preparing gold catalysts with small particle size and narrow particle size distributions and very high activity and selectivity for benzyl alcohol oxidation.     

New approaches to designing selective oxidation catalysts: Au/C a versatile catalyst

Selective oxidation is of key importance in the synthesis of chemical intermediates. For many years a number of oxides and supported metal catalysts have been used. The key questions involved in the design of selective oxidation catalysts are discussed in the initial part of this paper. One of the most exciting recent developments in the field of selective oxidation has been the discovery that supported gold catalysts are active. The second part of the paper discusses Au/C catalysts, which are shown to be particularly versatile for oxidation reactions. Four examples of selective oxidation are described using molecular oxygen as oxidant: (a) selective oxidation of glycerol to glycerate in the presence of base; (b) the oxidation of cyclohexane to cyclohexanol and cyclohexanane in the presence of a radical initiator; (c) the oxidation of hydrogen to hydrogen peroxide, and (d) the oxidation of benzyl alcohol to benzaldehyde under solvent free conditions. In contrast, the Au/C catalysts are not active for oxidation of carbon monoxide at ambient temperature. These examples demonstrate that there exists a rich potential for Au/C as a selective oxidation catalyst and that research efforts should now be focussed on selective oxidation using supported gold catalysts.     

Magnetron sputtering of gold nanoparticles onto WO3 and activated carbon

In this paper we describe the production and investigation of two supported gold catalyst systems prepared by magnetron sputtering: Au on WO₃ and Au on activated carbon. The magnetron sputtering technique entails using an argon plasma to sputter a high purity gold target producing a flux of gold atoms which are deposited onto a constantly tumbling support material. This technique offers a number of advantages over conventional chemical preparation methods. One advantage is the ability to create gold nanoparticles (diameters <3 nm) on unusual support materials, such as WO₃ and carbon, which are generally not accessible using the ubiquitous deposition-precipitation technique. We present data demonstrating the formation of catalytic gold nanoparticles with average diameters of 1.7 nm (Au/C) and 2.1 nm (Au/WO₃), as well as a substantial number of single atom species on the Au/C sample. Prototypical carbon monoxide oxidation (Au/WO₃) and glycerol oxidation (Au/C) reactions were performed in order to gauge the activity of these catalysts. The WO₃ supported catalyst exhibits substantial catalytic activity from room temperature to 135 °C (0.0018–0.082 mol CO/mol Au s) with an activation energy near 23 kJ/mol. The activity of the Au/C catalyst was compared to a Au/C catalyst prepared from a poly(vinyl alcohol) (PVA) sol. The smaller catalysts prepared by sputtering are more active than the large gold particles prepared using the PVA sol, however the larger gold nanoparticles are substantially more selective towards the production of intermediate products from the oxidation of glycerol.     

Platinum modification of gold and electrocatalytic oxidation of ethylene glycol on Pt-modified Au electrodes

The surface modification of gold electrodes with platinum and the electrocatalytic oxidation of ethylene glycol on Pt-modified Au electrodes are investigated by cyclic voltammetry. Platinum modification is performed by the electrochemical deposition of platinum on polycrystalline gold electrodes, and the Pt-modified Au electrodes with different amount of the deposited platinum are used for the ethylene glycol oxidation in alkaline and acidic solutions. It is shown that oxidation potential for the ethylene glycol oxidation on the Pt-modified Au electrodes shifts significantly negative compared with that on Au electrodes, nearly same oxidation potentials as that on Pt electrodes are observed, and peak current density of the ethylene glycol oxidation is dependent on the amounts of the deposited platinum on gold surface, much higher peak current densities than that on Au and Pt electrodes can be obtained. The low oxidation potential and high peak current density indicate the enhanced reactivity of Au electrodes by the platinum modification. The characteristics of the Pt-modified Au electrodes are found to be similar to that of Pt electrodes, and the reactivity of the Pt-modified Au electrodes is mainly attributed to the deposited platinum.     

Bimetallic Pt―Cu catalysts for glycerol oxidation with oxygen in a base-free aqueous solution

A series of carbon supported bimetallic Pt―Cu catalysts were prepared and used for glycerol oxidation with oxygen in a base-free aqueous solution. It was found that bimetallic Pt―Cu/C was more active than monometallic Pt/C towards selective oxidation of glycerol to glyceric acid. The selectivity of free glyceric acid reached 70.8% at an 86.2% conversion of glycerol over 5Pt―Cu/C. Highly dispersed bimetallic Pt―Cu nanoparticles with small particle size in dominant alloyed phase of PtCu₃ were confirmed by XRD and TEM in the bimetallic Pt―Cu/C catalyst, which is proposed to contribute to the improved performance.     

Oxidation of Glycerol Using Supported Gold Catalysts

A series of supported gold catalysts (0.25, 0.5, 1.0 wt% Au/graphite) have been investigated for the oxidation of glycerol and propan-1,2-diol. The 1 wt% Au/graphite catalyst is found to give 100% selectivity to the mono acid product, isolated as the sodium salt, as long as NaOH is present. The catalysts are characterized by TEM and cyclic voltammetry. By TEM, active catalysts all comprise fairly broad-size distributions (5–50 nm diameter) for the gold nanoparticles, although most are ca. 25 nm in diameter. An inactive 1 wt% Au/graphite is shown to have considerably larger particle sizes (>50 nm) and this indicates that there may be an optimum particle size for the desired catalysis. Characterization using cyclic voltammetry of active Au/graphite catalysts carried out in NaOH reveals the presence of an oxide species that may be responsible for the observed catalysis. In contrast, the inactive 1 wt% Au/graphite catalyst shows no oxidation in the cyclic voltammetry experiments.     

Selective oxidation with air on metal catalysts

Oxidation of organic molecules with air on metal catalysts has been known for a long time but there has been a renewed interest in recent years because these catalytic reactions are environmentally safe and could replace stoichiometric oxidations. This paper describes several oxidation reactions conducted either at high temperatures in the gas phase or at moderate temperatures in the liquid phase; in both cases they proceed via a mechanism of oxidative dehydrogenation on the metal surface. Ethylene glycol was converted to glyoxal at 550°C on Ag/SiC catalyst with a 70% yield provided promoters were added to the reaction feed (diethylphosphite or iodine) or deposited on the catalyst (LiPO₄ or H₃PO₄). The promoters improve the conversion and selectivity by modifying the structure and the oxygen concentration on the surface of silver. Oxidation of glyoxal to glyoxylic acid, glucose to gluconic acid and glycerol to various oxygenated derivatives were conducted in water at 60°C in the presence of carbon-supported palladium or platinum catalysts. Bismuth promoter, deposited on the platinum metals by redox reaction, improves the catalyst activity by preventing over-oxidation of the metal surface and favors the oxidation of secondary alcohol functions into keto-derivatives. At higher reaction temperatures, platinum catalysts produce C-C bond rupture with the formation of carboxylic acids with smaller chains. Thus, cyclohexanol was converted into C₆, C₅, and C₄ diacids with a 45% selectivity to adipic acid on Pt/C catalysts at 150°C.     

炭载体对Pt-C复合物非碱性条件下催化甘油氧化性能的影响

采用等量浸渍法制备了具有相似平均粒径的活性炭（AC）和碳纳米管（CNTs）负载的Pt催化剂,并比较研究了非碱性条件下两种催化剂催化甘油氧化反应的性能。结果表明,炭载体对Pt-C复合物催化甘油氧化反应的活性、选择性和稳定性有重要影响。相对于Pt/CNTs催化剂,Pt/AC催化剂中Pt 4f结合能较低,导致其表面氧的覆盖度相对较高,因而抑制了甘油的吸附,降低了甘油氧化反应的初始活性;Pt/AC催化剂会促进甘油醛进一步氧化成甘油酸以及C₃产物的氧化断键;Pt/AC催化剂失活的主要原因是氧中毒和中间产物的吸附,而Pt/CNTs催化剂的失活主要是由于甘油酸的吸附堵塞Pt表面的活性位造成的。     

Synergistic effect of CeO2 modified Pt/C catalysts on the alcohols oxidation

The effect of the addition of CeO₂ to Pt/C catalysts on electrochemical oxidation of alcohols (methanol, ethanol, glycerol, ethylene glycol) was studied in alkaline solution. The ratios of Pt to CeO₂ in the catalysts were optimised to give the better performance. The electrochemical measurements revealed that the addition of CeO₂ into Pt-CeO₂/C catalysts could significantly improve the electrode performance for alcohols oxidation, in terms of the reaction activity and the poisoning resistance, due to the synergistic effect. The electrode with the weight ratio of Pt to CeO₂ equals 1.3:1 with platinum loading of 0.30 mg/cm² showed the highest catalytic activity for oxidation of ethanol, glycerol and ethylene glycol.     

Influence of Reaction Conditions on Diacid Formation During Au-Catalyzed Oxidation of Glycerol and Hydroxymethylfurfural

The selective oxidation of glycerol and 5-hydroxymethylfurfural (HMF) to diacids over supported gold catalysts (Au/C and Au/TiO₂) in liquid water at mild temperatures was a strong function of the added base such as NaOH. Use of hydrotalcite as a solid base in place of NaOH in the HMF reaction medium facilitated the production of diacid over Au/TiO₂, but extensive leaching of magnesium suggested that hydrotalcite was consumed stoichiometrically in the reaction. Production of diacids from glycerol oxidation over supported Au catalysts was promoted by operating in a continuous flow reactor and by increasing the catalyst loading in a semi-batch reactor. Trace inhibitors formed by conversion of the product monoacid are proposed to account for the generally low selectivity to diacids over gold catalysts.     

A new environmental friendly method for the preparation of sugar acids via catalytic oxidation on gold catalysts

A novel and efficient way of producing of aldonic acids is reported. The selective catalytic oxidation of monosaccharides (arabinose, ribose, xylose, lyxose, mannose, rhamnose, glucose, galactose, N-acetyl-glucosamine) and disaccharides (lactose, maltose, cellobiose, melibiose) on Au, Pd and Pt catalysts was investigated. By using the gold catalyst, a total selectivity with respect to aldonic acids and a high catalytic activity for all investigated sugars was found. The reaction conditions for the production of maltobionic and lactobionic acids via oxidation on gold catalysts were optimised. A high long-term stability of two different gold catalysts for the oxidation of maltose and lactose, respectively, was found.     

A Comparative Study of Partial Oxidation of Methanol over Zinc Oxide Supported Metallic Catalysts

Au/ZnO, Pd/ZnO and Au–Pd/ZnO catalysts were prepared by PVP-stabilized reduction method by C₂H₅OH. The catalysts have been used successfully for hydrogen production by partial oxidation of methanol (POM). The influence of Au, Pd and Au–Pd on the performance of supported catalysts for POM has been investigated. The prepared samples were characterized by ICP, XRD, BET, TPR and TPD. The results show that the Au–Pd/ZnO catalyst are more active and exhibit higher hydrogen selectively compared to the Pd/ZnO and Au/ZnO catalyst, the methanol conversion of 99.5% and hydrogen selectivity of 65.6% were obtained at 573 K. The enhanced activity and stability of the bimetallic Au–Pd/ZnO catalyst has been explained in terms of the higher dispersion and basic density, smaller particles of gold and synergetic effect between gold and palladium.     

Electro-Oxidation of Glycerol on Platinum Modified by Adatoms: Activity and Selectivity Effects

Antimony irreversibly adsorbed on a carbon supported platinum electrode oxidizes glycerol selectively to dihydroxyacetone with a lower onset potential (ca. 150 mV) and a higher peak current density (ca. 170 %) compared to clean Pt/C. Pb, In, and Sn also promote the catalytic activity of glycerol oxidation, however the reaction pathway towards the primary alcohol oxidation remains unchanged. Higher surface coverage by adatoms on Pt/C generally increases the activity of glycerol oxidation.     

Effect of gold addition on Pt and Pd catalysts in liquid phase oxidations

Single phase Au–Pd and Au–Pt on carbon catalysts have been compared in the liquid phase oxidation of glycerol (representative for polyols) and n-octanol (representative for long chain aliphatic alcohol). The observed overall enhancement of catalytic activity appeared to be function of support, substrate and reaction conditions. Effect of substrate structure has been disentangled: synergistic effect between Au and Pt was maximized when polyol-like substrates were oxidized whereas Au–Pd based catalyst showed a more general match.     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.