Catalytic performance of gold catalysts in the total oxidation of VOCs

Catalysts for the oxidation of volatile organic compounds (VOCs) were prepared by supporting 1% gold on cerium and zirconium oxides (CeO2, Ce_0.5Zr_0.5O₂, ZrO₂) using a simple impregnation method followed by reduction of gold in the presence or absence of ammonia (N). The catalysts were tested in model reactions, namely the total oxidation of benzene, hexane and chlorobenzene, using a micro flow reactor at atmospheric pressure in the temperature range 100–500°C and their activity was compared to that of Au/TiO₂ and Au/Fe₂O₃ reference catalysts supplied by the World Gold Council (WGC). Benefits on the light-off temperatures were observed by adding gold to cerium-containing oxides: 100% conversion of hexane was obtained with Au/Ce_0.5Zr_0.5O₂ at the lowest temperature (300°C). Full conversion of benzene was reached at only 250°C with Au/CeO₂ (N), at 290°C with Au/Fe₂O₃ (WGC) and at 300°C with Au/Ce_0.5Zr_0.5O₂. In the case of chlorobenzene oxidation, the addition of gold was of even greater relevance because of a drop in the light-off temperature of over 100°C for Au/Ce_0.5Zr_0.5O₂ and Au/CeO₂, with respect to the gold-free oxide supports; but in this case rapid deactivation took place.     

Catalytic performance and kinetics of Au/γ-Al_2O_3 catalysts for low-temperature combustion of light alcohols

Au/γ-Al2O3 catalysts were prepared by deposition-precipitation method for the catalytic combustion of low concentration alcohol streams(methanol,ethanol,iso-propanol and n-propanol).The catalysts were characterized by X-ray photoelectron spectroscopy(XPS),X-ray diffractometry(XRD) and energy dispersive X-ray micro analysis(EDS) techniques.The XPS results showed that there was only Au0 on the surface of catalysts.The XRD patterns showed that Au was presumably highly dispersed over γ-Al2O3.The temperatures for complete conversion of methanol,ethanol,iso-propanol and n-propanol with concentration of 2.0 g/m3 were 60,155,170 and 137 ℃,respectively,but they were completely mineralized into CO2 and H2O at 60,220,260 and 217 ℃ respectively over the optimized catalyst.The activity of the catalyst was stable in 130 h.The kinetics for the catalytic methanol elimination followed quasi-first order reaction expressed as r=0.652 8c0+0.084 2.The value of apparent activation energy is 54.7 kJ/mol in the range of reaction temperature.     

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.     

Oxide coatings modified with transition and rare-earth metals on aluminum and their activity in CO oxidation

Oxide catalysts, which, in addition to Al₂O₃ + SiO₂, contain from one (Ni) to four oxides or compounds of transition metals (Ni, Cu, Mn, and Co) and oxides or compounds of rare-earth elements (Ce, La), are produced on D16 aluminum alloy by plasma electrolytic oxidation combined with impregnation and subsequent annealing. The composites formed begin to catalyze the CO oxidation in a temperature range of from 100 to 300°C. The catalysts used can be arranged in the following series of decreasing catalytic activity: Ni-Cu-Mn-Co-Ce > Ni-Cu-Mn-Co-Ce-La ≈ Ni-Cu-Mn-Co > Ni-Cu-Mn > Ni-Cu > Ni. Oxygen compounds of Cu⁺, Cu²⁺, Mn⁴⁺, Co³⁺, Ce³⁺, and Ce⁴⁺, which seem to determine the catalytic properties of the oxide systems studied, are found on the surface and in the subsurface layer with a total thickness of ～6 nm of the most active Ni-Cu-Mn-Co-Ce catalyst.     

Selective catalytic reduction of NO with propene over Au/CeO2/Al2O3 catalysts

Selective catalytic reduction of NO by propene under an oxygen-rich atmosphere has been investigated over Au/ CeO₂, Au/CeO₂/Al₂O₃ and Au/Al₂O₃ catalysts prepared by deposition-precipitation. The results demonstrated that Au/16%CeO₂/Al₂O₃ had good low-temperature activity, selectivity towards N₂ and stability, which is superior to that of Pt/Al₂O₃. It was also found that adding 2% water vapour to the feed stream enhanced the NO conversions at low temperatures while the presence of 20 ppm SO₂ increased NO conversions at higher temperatures. It is particularly interesting that under the simultaneous presence of 2% water vapour and 20 ppm SO₂, the NO conversions to N₂ were significantly increased and the temperature window was widened significantly. The catalysts were characterized by Xray diffraction (XRD), high resolution transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (HRTEM-EDX) and temperatureprogrammed reduction (H2-TPR) techniques. Both XRD and HRTEM revealed that CeO₂ was highly dispersed on the alumina support, and HRTEM combined with EDX showed that gold particles were preferentially deposited on those highly dispersed CeO₂ particles. The gold deposition made CeO₂ more reducible and interaction between gold and those highly dispersed CeO₂ particles became stronger than that with the bulk CeO₂, and this interaction is probably responsible for the superior catalytic performance of the Au/CeO₂/Al₂O₃.     

Effects of Au nanoparticles on TiO2 in the photocatalytic degradation of an azo dye

The photocatalytic activity of Au modified titanium dioxide was evaluated in the photodegradation of the azo dye Acid Red 1 (AR1) under 254 nm irradiation. Noble metal nanoparticles were deposited on TiO₂ either through depositionprecipitation (DP), or by immobilisation of preformed metallic sols (polyvinylalcohol (PVA)/NaBH₄ or tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH systems). Gold nanoparticles on the photocatalyst surface had dimensions of around 3–4 nm in diameter, as determined by HRTEM analysis, and exhibited visible light plasmon absorption. THPC Au/TiO₂ appears to be the most photoactive amongst the photocatalysts with a 1 wt.% Au loading, while among THPC samples with different Au loadings (0.5–20.0 wt.%) the maximum photoactivity was attained with 5 wt.% Au/TiO₂. The higher AR1 photodegradation rate observed on Au/TiO₂ at basic pH can be related to the higher concentration of hydroxyl anions at the interface: these are able to effectively scavenge photoproduced valence band holes, possibly in competition with Au⁰ oxidation to Au⁺.     

Gold complexes with

The reactions of methane with the gold complexes [Au(OH)]⁻, [Au(OCH₃)₄]⁻, [Au(O(CO)₂O₂]⁻ and [Au(O₂CH)₂]⁺, [Au^I(acac)], [Au^III(acac)₂]⁺ (acac-acetylacetonato) were studied using the DFT/PBE method with the SBK basis set. High activation barriers were obtained for the electrophilic substitution in [Au(OH)]⁻, [Au(OCH₃)⁴]⁻, [Au(O(CO)₂O)₂]-and [Au^III(acac)₂]⁺ complexes, which excludes the possibility that these reactions might proceed under mild conditions. The reactions of the [Au(HCO₂)₂]⁺ and [AuI(acac)] complexes with methane have rather low energy barriers and proceed through the formation of an intermediate complex. The alternative mechanism of methane oxidation with a gold complex in the presence of oxygen is simulated.     

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.     

On the mechanism of selective CO oxidation on nanosized Auγ-Al2O3 catalysts

New findings give further information on the mechanism of carbon monoxide selective oxidation over γ-alumina supported nanoparticle sized gold catalysts. a) CO₂ formation, increasing with rising temperature, is observed in the absence of hydrogen and oxygen pointing to a model of active sites consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group, b) At high temperatures (>200 °C) in excess of H₂, reversed water gas shift (RWGS) reaction results in CO₂ consumption towards CO and H₂O formation. c) Hydrogen strongly influences the interaction of CO on Auγ-Al₂O₃, by weakening the CO adsorption. The presence of hydrogen plays an important role both decreasing the strength of CO bonding and in the prevention of deactivation and regeneration.     

Synthesis of high performance hydroxyapatite-gold catalysts for CO oxidation

Catalysts for low temperature CO oxidation were prepared by decorating hydroxyapatite (HAp) ceramic foam scaffolds with highly dispersed gold nanocrystals using a deposition-precipitation (DP) process. Catalytic activity, microstructure and crystallinity were studied as a function of reagent pH (4–12) and aging time (10, 30, 60 min) for powders and porous supports. Superior products with small (≤ 5 nm) gold crystals distributed homogeneously over HAp foam were obtained at pH 8–9. Larger crystal sizes and colloidal gold agglomeration appeared at longer aging times. The optimized catalyst prepared by reaction at pH 9 for 30 min showed 100% CO conversion to CO₂ at 150°C. The Au-HAp composite demonstrated excellent durability by retaining structural and crystallographic integrity with no loss of activity when tested at 65°C out to 166 h.     

制备方法对低温NH3-SCR催化剂Mn-La结构与性能的影响

采用固相反应法、共沉淀法制备了Mn-La复合氧化物催化剂,考察了制备方法对催化剂脱硝性能及抗SO₂/H₂O性能的影响。并通过XRD、BET、H₂-TPR、NH₃-TPD、XPS等手段对催化剂的结构和物理化学特性进行了表征。结果表明,La掺杂降低了MnO_x的结晶度,增大了催化剂的比表面积和孔体积。Mn-O-La键合作用促进了锰在催化剂表面的分散,而高分散的锰更容易被还原,催化剂的还原温度向低温方向迁移,氧化还原能力得到提高。掺杂La之后,催化剂表面Br?nsted酸和总酸量增加,同时Mn⁴⁺和表面化学吸附氧的浓度也得到提高。因此,La掺杂有利于促进催化剂的脱硝活性。反应评价结果表明,共沉淀法制备的MnLa-CPM催化剂表现出最佳的脱硝效率,在80 ℃时脱硝效率接近100%。在H₂O和SO₂存在的条件下,MnLa-CPM催化剂的脱硝效率仍能达到80%,表现出了较好的抗硫水性能。     

The combination effects of trivalent gold ions and gold nanoparticles with different antibiotics against resistant Pseudomonas aeruginosa

Despite much success in drug design and development, Pseudomonas aeruginosa is still considered as one of the most problematic bacteria due to its ability to develop mutational resistance against a variety of antibiotics. In search for new strategies to enhance antibacterial activity of antibiotics, in this work, the combination effect of gold materials including trivalent gold ions (Au³⁺) and gold nanoparticles (Au NPs) with 14 different antibiotics was investigated against the clinical isolates of P. aeruginosa, Staphylococcus aureus and Escherichia coli. Disk diffusion assay was carried out, and test strains were treated with the sub-inhibitory contents of gold nanomaterial. Results showed that Au NPs did not increase the antibacterial effect of antibiotics at tested concentration (40 μg/disc). However, the susceptibility of resistant P. aeruginosa increased in the presence of Au³⁺ and methicillin, erythromycin, vancomycin, penicillin G, clindamycin and nalidixic acid, up to 147 %. As an individual experiment, the same group of antibiotics was tested for their activity against clinical isolates of S. aureus, E. coli and a different resistant strain of P. aeruginosa in the presence of sub-inhibitory contents of Au³⁺, where Au³⁺ increased the susceptibility of test strains to methicillin, erythromycin, vancomycin, penicillin G, clindamycin and nalidixic acid. Our finding suggested that using the combination of sub-inhibitory concentrations of Au³⁺ and methicillin, erythromycin, nalidixic acid or vancomycin may be a promising new strategy for the treatment of highly resistant P. aeruginosa infections.     

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.     

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.     

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.     

Low temperature methane oxidation on differently supported 2 nm Au nanoparticles

Low temperature CH₄ oxidation was studied on 2 nm gold nanoparticles supported on various metaloxides. The differences in reaction rates for the different systems suggest that the support material has an effect on the activity. From TEM analysis, we found that the gold particles were stable in size during the reaction. In addition to full oxidation to CO₂, traces of C₂H₆ were detected when Au/TiO₂ was used, indicating limited partial CH4 oxidation. TiO₂ was found to be the best support for gold nanoparticles both in terms of activity and gold particle stability.     

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.     

Acetic acid production by selective oxidation of ethanol using Au catalysts supported on various metal oxide

The liquid-phase oxidation of ethanol to acetic acid using Au catalysts supported on various metal oxides was studied at 150°C using molecular oxygen as stoichiometric oxidant. Catalysts containing 1 wt% Au supported on TiO₂, Al₂O₃, and ZnO were examined for ethanol oxidation. The results showed that ZnO and TiO₂ gave higher initial activities as supports for gold in ethanol oxidation, followed by Al₂O₃. Ethanol conversions of >90% and selectivities to acetic acid of >95% were achieved when using ZnO and TiO₂ as supports under conditions where a slight oxygen deficiency was used. With a slight excess of oxygen present initially, ethanol conversions of 99.4%, and a selectivity to acetic acid selectivity of 99.8% could be achieved. Gold leaching seemed to be very apparent with alumina as support and also, after continued use with titania-based catalysts. The use of higher initial concentrations of ethanol (range studied 5–40 mass% ethanol in water) led to higher ethyl acetate selectivities. High acetic acid selectivities were seen for relatively low (5–10 mass%) initial ethanol concentrations.     

Au/CuMn催化剂对于C3H8还原NO催化性能的研究

以沉积-沉淀法制备了系列Au/CuMn催化剂,并对催化剂进行了XRD和XPS等表征,研究了Au负载量和催化剂焙烧温度对Au/CuMn催化剂性能的影响。实验结果显示,60℃烘干、Au理论负载量为3%的催化剂性能最佳,在50℃的反应条件下,NO的转化率达到72.4%,但催化剂失活较快。XPS分析显示,催化剂表面的Au3+与吸附氧物种Oads.是反应的活性中心,反应过程中催化剂表面氧化态Au3+与Oads.物种部分被还原,可能是催化剂失活的重要原因。另外,反应后在催化剂表面生成的亚硝酸盐、硝酸盐和碳酸盐等物种,覆盖了活性中心,也是导致催化剂失活的原因之一。     

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。