Properties of TiO2 support and the performance of Au/TiO2 Catalyst for CO oxidation reaction

Gold catalysts were prepared on TiO₂ supports of different phase structures (i.e., anatase, rutile and biphasic), TiO₂ crystal size (i.e., 9–23 nm), surface and textural properties (i.e., hydration and surface area). The CO oxidation on the gold catalysts was carried out in an operando-DRIFTS set-up equipped with DRIFTS reactor cell connected on-line to CO gas analyser and gas chromatograph enabling real time monitoring of surface reaction and simultaneous reaction rate measurements. Gold catalysts supported on pure anatase TiO₂ were more resistant to sintering compared to catalysts supported on rutile and bi-phasic TiO₂. Besides catalyst sintering, deposition of surface carbonates is an important cause of catalyst deactivation. The best gold catalyst was prepared on 13 nm anatase TiO₂. It displays both increased activity and stability for CO oxidation reaction at room temperature. Surface and textural properties of TiO₂ also play a role on the performance of the Au/TiO₂ catalyst.     

Role of promoting oxide morphology dictating the activity of Au/SiO2 catalyst in CO oxidation

The interfacial interaction of gold nanoparticles deposited on either model SiO₂/Si(100) or high surface area amorphous or mesoporous silica with minute amounts of promoter oxide like “active” FeO_x, TiO₂ and CeO₂ has been discussed. The role of the active oxide, its contribution to the perimeter along the gold nanoparticles has been interpreted. The oxide may invoke electronic interaction and simultaneously the defect structure of oxides likely has a key issue in the formation and stabilization of very small Au particles. The activity of the Au/oxide perimeter depends not only on the size of the Au particles, but also on the size and morphology of the oxide component (likely amorphous structure) regardless of whether it is supporting Au nanoparticles or decorating them. The activity in CO oxidation over Au catalysts is strongly affected by the length of the Au/“active” oxide perimeter which is regarded as the “active interface”. The longer length of the perimeter is evidenced by the enhanced CO oxidation activity.     

The role of nanosized gold particles in adsorption and oxidation of carbon monoxide over Au/Fe2O3 Catalyst

The presence of gold is found to promote the development of weakly bonded (CO)_ad species over the surface of Au/Fe₂O₃ catalyst during interaction with carbon monoxide (CO) or a mixture of carbon monoxide and oxygen. The concentration of these species and the nature of the bonding depend on the gold particle size. No such species are formed for gold particles larger than ∼11 nm or over gold-free iron oxide. The bulk carbonate-like species, formed in the process with the involvement of the hydroxy groups of the support, are merely side products not responsible for the low temperature activity of this catalyst.     

The activation of supported Au catalysts prepared by impregnation

In this study, Au/TiO₂, Au/γ-Al₂O₃, and Au/C catalysts were prepared by an incipient-wetness impregnation method. The CO oxidation activity after different pretreatment was analyzed. Two pretreatments are found possible to activate these catalysts for CO oxidation: (i) high-temperature hydrogen reduction and (ii) aqueous base treatment using NH3(aq). The high-temperature hydrogen reduction is effective for Au/TiO₂ and Au/C, but not Au/Al₂O₃. The base treatment is effective for Au/Al₂O₃ and Au/TiO₂, but not Au/C. Small Au particles of ca. 2 nm size were observed in activated Au catalysts from both pretreatments; however, the high-temperature H₂ reduction procedure also resulted in large Au particles of ca. 25 nm size which makes it a less efficient pretreatment than the base treatment. The activated Au/TiO₂ catalysts show comparable turnover frequency as the Au/TiO₂ catalyst prepared by a deposition precipitation method. The effective pretreatments were found to be accompanied by a reduced acidity, which is measured by the pH of aqueous catalyst suspension. Reasons for such activation are discussed.     

Preparation and catalytic behavior of reduced graphene oxide supported cobalt oxide hybrid nanocatalysts for CO oxidation

The reduced graphene oxide (rGO) supported cobalt oxide nanocatalysts were prepared by the conventional precipitation and hydrothermal method. The as-prepared rGO-Co₃O₄ was characterized by the XRD, Raman spectrum, SEM, TEM, N₂-sorption, UV-Vis, XPS and H₂-TPR measurements. The results show that the spinel cobalt oxide nanoparticles are highly fragmented on the rGO support and possess uniform particle size, and the as-prepared catalysts possess high specific surface area and narrow pore size distribution. The catalytic properties of the as-prepared rGO-Co₃O₄ catalysts for CO oxidation were evaluated through a continuous-flow fixed-bed microreactor-gas chromatograph system. The catalyst with 30% (mass fraction) reduced graphene oxide exhibits the highest activity for CO complete oxidation at 100 °C.     

Enhanced catalytic performance for selective oxidation of propene with O_2 over bimetallic Au-Cu/SiO_2 catalysts

Au-Cu bimetallic nanoparticles with uniform size,shape,and compositions were synthesized by wet chemistry method,and then the Au-Cu/SiO_2 catalyst supported on SiO_2 was prepared.Meanwhile,their catalytic activity for the selective oxidation of propene to acrolein using O_2 as an oxidant was evaluated.The bimetallic catalyst shows a significantly enhanced catalytic performance comparing with Au and Cu monometallic catalysts.Characterization of the materials and kinetic study was conducted to explore the cooperating mechanism of Au and Cu for improving the catalytic activity of the bimetallic catalyst.Cu component can segregate to the alloy surface and the Au-Cu alloy transferred to Au-CuO core/shell structure after annealing during the preparation process.Based on the Mars-van Krevelen mechanism for the selective oxidation of propene over the prepared catalysts,the coexistence of CuO can promote the adsorption and activation of O_2.Meanwhile,the electrons transfer from Au to Cu in the catalyst can facilitate the adsorptions of both oxygen on CuO sites and propene on Au sites.The combined effects of the above two aspects result in the high catalytic activity of the Au-Cu/SiO_2 catalyst for selective oxidation of propene to acrolein,compared to the Au/SiO_2 and CuO/SiO_2 catalysts.     

Au/ZrO2 catalysts for low-temperature water gas shift reaction: Influence of particle sizes

The size effects of Au and ZrO₂ particles on the structural property and the catalytic performance of Au/ZrO₂ catalysts for the water gas shift reaction were extensively investigated. It was found that the Au-ZrO₂ contact boundaries played essential roles in determining the catalytic reactivity. By keeping the size of Au particle to be ∼3 nm, the increase in the particle size of ZrO₂ from ∼7 nm to ∼55 nm caused significant decrease in the reaction rate. When the particle size of ZrO₂ was fixed at ∼20 nm, the conversion of CO decreased greatly with increasing the size of gold particle from 2.9 to 6.2 nm. IR spectroscopy and kinetic study revealed that the water gas shift reaction occurred at the Au-ZrO₂ contact boundaries, where CO is adsorbed on the Au species and H₂O is activated on the surface of ZrO₂ through the formation of formate species, acting as key reaction intermediates.     

New gold catalysts for liquid phase oxidation

A pre-requisite for good catalytic activity with supported gold is that the metal is present as nanoparticles, but no suitable method for depositing gold on carbon as nanoparticles has previously been reported. The deposition precipitation method which has been used successfully for catalyst preparation with other supports, produces large aggregates with carbon. It is now shown, however, that pre-reduced gold sol is a satisfactory method for preparing gold on carbon catalysts as long as an appropriate choice of sol is made. In this paper the activity of gold on carbon catalysts prepared in a number of different ways is compared in a standardized liquid phase organic oxidation reaction. The activity for Au/C catalysts prepared by the sol method can show a two-fold increase compared with a similar catalyst prepared by deposition precipitation.     

Application of heterogeneous gold catalysis with increased durability: Oxidation of CO and hydrocarbons at low temperature

2% Au/Al₂O₃ catalysts were prepared by a novel method involving Direct Anionic Exchange (DAE). The method produces strong bonding of the gold complex (HAuCl₄) to the alumina support with no loss of gold during the subsequent steps of preparation. The complete removal of chloride from the catalyst was achieved by washing with concentrated ammonia. This procedure ensures a better activity and prevents sintering during calcination as shown by TEM. The catalysts were tested for the oxidation of CO and of saturated and unsaturated hydrocarbons (C₁ to C₃). The catalysts showed high activities over a range of concentrations and temperatures relevant to applications in automotive exhaust cleaning. Furthermore, a remarkable resistance to thermal ageing at 600°C in the absence or presence of water was observed, due to the presence of the strongly anchored nanosized gold particles obtained during the preparation step.     

Effect of the oxidation state of gold on the complete oxidation of isobutane on Au/CeO2 catalysts

Catalysts of highly dispersed gold supported on ceria were prepared by deposition precipitation method. Au is dispersed as Au⁰, Au⁺ and Au³⁺ species on ceria. The content of Au⁺ and Au³⁺ was highest on catalyst prepared on uncalcined ceria, which possess least ordered surface. It is inferred that oxygen vacancy on disordered ceria surface is essential for the preparation of highly dispersed gold catalysts and in stabilizing monolayer surface Au⁺ clusters while cationic vacancies are sites for substitutional Au³⁺. Au/CeO₂ catalysts showed low-temperature isobutane oxidation activity with maximum conversion at 150–180°C. Ex-situ XPS results demonstrated that the low temperature isobutane oxidation activity was closely related to the content of Au⁺ which we interpreted as surface gold oxide formed under reaction conditions. Isobutane oxidation activity associated with ceria at temperature above 300°C was enhanced by substitutional Au³⁺.     

喷雾干燥法制备碳载铂钴镍合金催化剂及其甲醇催化氧化性能研究

本文以氯铂酸、氯化镍和硝酸钴为原料,XC-72炭黑为载体,通过雾化干燥法结合煅烧还原制备碳载铂基（PtCo Ni）分散性好的多元合金纳米粒子催化剂。重点研究表面经过改性的炭黑对合金纳米粒子形成和分散的影响规律,研究碳载PtCoNi（原子比为1:1:1）合金纳米粒子的甲醇催化氧化活性、抗CO中毒能力和耐久性,以及不同原子比对催化氧化甲醇活性和抗CO中毒能力的影响规律。研究结果表明,采用表面改性后的炭黑作为载体,制备的碳载铂基（PtCoNi）催化剂为合金纳米粒子,且纳米粒子在炭黑表面分散均匀,粒径分布在1-4nm,平均粒径为2.3nm;与商用的Pt/C催化剂相比,PtCoNi/C（原子比为1:1:1）催化剂具有更高的甲醇催化氧化活性、耐久性和抗CO中毒性;不同原子比铂基多元催化剂在催化氧化甲醇活性上的顺序为：PtCoNi/C>Pt3CoNi/C>Pt5CoNi/C,抗CO中毒性顺序为：PtCoNi/C>Pt3CoNi/C>Pt5CoNi/C。     

Analysis of the Active Au Species on Au/α-Fe2O3 Catalyst

采用沉积沉淀法制备了CO低温氧化 Au/α-Fe2O3催化剂,利用 X 射线衍射(XRD)、X 射线光电子能谱(XPS)、BET 比表面测定、程序升温还原(H2-TPR)等表征技术,对比了制备过程 pH 值的微小变化、焙烧及光线照射对催化剂结构及催化性能的影响,探明了 Au/α-Fe2O3催化剂的活性物种。结果表明,110 ℃处理的 Au/α-Fe2O3催化剂表面同时存在 Au3+、Au0以及过渡态 Auδ+(0<δ<1),它们对 CO 氧化的活性顺序为 Au3+>Auδ+>Au0;pH 值为 8 条件下制备的催化剂 Au3+含量高、比表面积大,催化性能最好;高温焙烧使氧化态金还原的同时也使载体比表面积严重缩小,催化活性显著下降;紫外线照射可以引起 Au3+的逐渐还原以及 Au0 颗粒的生长,其催化失活作用弱于高温焙烧。     

Electrochemical oxidation of dissolved carbon monoxide on gold electrode in alkaline medium

The study of the oxidation of dissolved CO is not much seen in the literature probably due to the strong adsorption of CO on platinum metals in acidic media. This work was thus undertaken to study the electrochemical oxidation of dissolved CO on Au in alkaline medium without the influence of adsorbed CO. Our results show that the electrochemical oxidation of dissolved CO on gold electrode is mainly diffusion-controlled process at the potential greater than 0.3V (vs. RHE). At the potential range between 0.0V and 0.3V, the oxidation process is a mixed-control process. Our data also confirm that the electrochemical oxidation of dissolved CO is affected by the presence of surface oxide on gold. The electrochemical oxidation of the dissolved CO at liquid-solid interface may provide certain practical uses in CO removal or low temperature fuel cell.     

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

CO is a useful probe in the characterization of surface properties of both metal and metal oxide via adsorption. Adsorption of CO was used to monitor the possible active site of an Au/TiO₂ catalyst for the CO oxidation reaction. CO adsorption on the reduced catalyst results in the band at 2104 cm⁻¹ indicative of Au⁰. During the reaction (in the presence of both CO and O2 present) the band is shifted to higher wave numbers indicating non-competitive adsorption on the surface of Au species. This study also reveals the relationship between the presence of CO (in the absence of oxygen) and the build-up of surface species such as bicarbonates, formates and carbonate species which decreases the activity of the catalyst. The presence of both the reduced and the cationic species of Au seem to be requirement for the activity of the catalyst.     

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al₂O₃, TiO₂, Fe₂O₃ and CeO₂) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO₂ bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO₂ catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.     

Investigation of the active site and the mode of Au/TiO2 catalyst deactivation using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS)

CO is a useful probe in the characterization of surface properties of both metal and metal oxide via adsorption. Adsorption of CO was used to monitor the possible active site of an Au/TiO₂ catalyst for the CO oxidation reaction. CO adsorption on the reduced catalyst results in the band at 2104 cm⁻¹ indicative of Au⁰. During the reaction (in the presence of both CO and O₂ present) the band is shifted to higher wave numbers indicating non-competitive adsorption on the surface of Au species. This study also reveals the relationship between the presence of CO (in the absence of oxygen) and the build-up of surface species such as bicarbonates, formates and carbonate species which decreases the activity of the catalyst. The presence of both the reduced and the cationic species of Au seem to be requirement for the activity of the catalyst.     

金在低温燃料电池技术中的应用前景

金因其良好的物理化学性质和催化活性在低温燃料电池技术中具有很好的应用前景.本文就金在此方向的研究进展进行综述,包括:金催化剂选择氧化燃料电池氢燃料中的CO以及Pt-Au系电催化剂、Pd-Au系电催化剂、以金为主体的电催化剂的研究状况.同时简要介绍了低温燃料电池电催化剂的发展现状和金应用于电催化剂的基础.     

New gold and silver-gold catalysts in the shape of sponges and sieves

Gold with a nanoporous sponge-like morphology, generated by leaching of AuAg alloys is presented as a new unsupported material system for catalytic applications. The role of residual silver for catalytic activity towards CO oxidation in the temperature range from −20 to 50°C has been investigated by comparison with Au and Au/Ag zeolite catalysts. As revealed by a systematic variation of the silver content in the zeolite catalysts, bimetallic systems exhibit a significantly higher activity than pure gold, probably due to activation/dissociation of molecular oxygen by silver. By STEM tomography we can unambiguously prove that at least some of the particles form inside the zeolite lattice.     

制备条件对Au/AC催化分解臭氧性能的影响

研究了活性炭负载的金催化剂(Au/AC)对低浓度臭氧的催化分解性能,考察了前驱体溶液pH值、干燥方式、焙烧温度等制备条件对催化剂活性的影响,以及空速对臭氧分解率的影响。结果表明,在金前驱体溶液pH值为10、经微波辐射干燥、200℃氢气还原制备的催化剂,金颗粒在活性炭表面的分布更加均匀,尺寸更小,具有更高的催化活性。在室温、相对湿度45%和空速72000h-1时,对浓度为50mg/m3臭氧的分解率保持在90%以上(在2400min内)。N2吸附-脱附和XPS表征结果表明,活性炭载金催化剂在催化分解臭氧后,比表面积和孔体积略有下降;Au4f的XPS峰虽稍向高能方向移动,但仍保持催化活性;活性炭表面碳含量显著下降而氧含量大幅增加,说明活性炭上负载的Au在自身催化分解臭氧的同时,还起到了促进臭氧与单质碳反应的作用。     

金核/铂壳纳米粒子的光化学合成及电催化活性

在含不同摩尔比的Au(Ⅲ)和Pt(Ⅳ)离子的PEG(聚乙二醇)-丙酮溶液中,采用光化学共还原法合成了一组Au@Pt复合纳米粒子,并以炭黑分别对其负载制成Au@Pt/C催化剂。借助于UV-Vis、TEM和HR-TEM的表征,证实复合纳米粒子为球形的核/壳结构;分别以XPS、EDS和电化学方法分析了复合粒子的化学状态、结构特点和Au@Pt/C催化剂的催化性质。结果表明,不同Au:Pt摩尔比的Au@Pt/C催化剂对甲醇氧化反应具有良好的催化活性和稳定性,其中Au:Pt=1:1时形成的Au@Pt/C催化剂电催化活性最高,约为商品Pt/C催化剂的4倍。简要讨论了核/壳结构产生高催化活性的主要原因。