Oxidation of mono- and polyalcohols with gold: Comparison of carbon and ceria supported catalysts

Aim of this work is the investigation of the gold catalysed liquid-phase oxidation of polyalcohols with both, primary and secondary alcohol groups, in order to evaluate the potential of gold catalysts to oxidise a secondary alcohol group in presence of a primary group. For that purpose we investigated the heterogeneously catalysed liquid-phase oxidations of n-propanol, as reference for the oxidation of the primary alcohol group only, of propylene glycol, where the competitive reaction can be examined and finally of glycerol, which is the target reaction due to known economic aspects. We report on the performance of ceria supported gold catalysts in these reactions and discuss the results in dependency of the specific surface area of the support, the catalyst and support preparation method and the catalyst activation conditions. Finally, in order to estimate the relative activity and selectivity of the ceria supported catalysts we compared the catalytic results with the performance of carbon and titania supported gold catalysts.     

In situ measurements under flow condition of the CO oxidation over supported gold nanoparticles

In situ FT-IR measurements for Au/TiO₂ and Au/Al₂O₃ have been carried out under the flow condition of CO oxidation at atmospheric pressure. It has been found that the Au particles remain neutral (Au⁰) in the presence of oxygen, while negatively charged particles (Au^δ−) is formed in the absence of oxygen, as a result of the charge transfer from the oxygen vacancies. Moisture did not significantly affect the adsorption states of CO over Au/TiO₂ and Au/Al₂O₃. Enhancement of the CO₂ production by moisture was observed over Au/Al₂O₃, which is accompanied by the decomposition of carbonate-like species by moisture.     

Research on unsupported nanoporous gold catalyst for CO oxidation

Nanoporous gold (NPG), a novel unsupported gold catalyst prepared by dealloying, exhibits exceptional catalytic activity for CO oxidation. Systematic studies were carried out on this new catalytic system, including the active sites of catalysts, the reaction kinetics, and activity dependence as functions of space velocity and temperature. Our results show strong evidence that metallic gold atoms on NPG are the intrinsic active sites at which the reaction of CO with O₂ occurs. The kinetic study found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O₂ concentration. We suggest that CO adsorption plays a decisive role in CO oxidation on NPG as the rate-limiting step. By completely ruling out the support influence, our findings provide considerable insight into the role of gold catalysts.     

d-Lactose oxidation over gold catalysts

Various supported gold catalysts were utilized in aerobic lactose oxidation. Comparison between these catalysts revealed, that gold catalysts are sensitive to the type of support. Kinetic regularities, e.g. dependence on pH, temperature and oxygen feed rate were established. Gold catalysts were selective for the production of the first reaction product, sodium salt of lactobionic acid. The electrochemical catalyst potential response was measured “in situ” and then utilized in the reaction process characterization.     

Comparison of the activity of Ru and Pt catalysts for the oxidation of carbon by NO2

The role of two catalysts Pt/Al₂O₃ and Ru/NaY on the oxidation of carbon by NO₂ was investigated in the temperature range 300–400 °C. In the case of Pt/Al₂O₃ no significant catalytic effect on the carbon oxidation rate is observed although decomposition of NO₂ takes place on the noble metal and leads to the formation of NO. This result suggests that the amount of the oxygen atoms transferred from the metallic surface sites to the carbon surface to form C(O) complex is negligible. In contrast, in presence of Ru/NaY the oxidation rate of carbon by NO₂ is markedly increased. Hence, a significant part of the formed O through catalytic decomposition of NO₂ on Ru surface sites is transferred to the carbon surface leading to a larger amount of C(O) complexes on the carbon surface. Thus, the ruthenium surface is a generator of active oxygen species that are spilled over on the carbon surface at 350 °C.     

From Renewable to Fine Chemicals Through Selective Oxidation: The Case of Glycerol

Catalytic selective oxidation of glycerol is presented in terms of catalytic systems and experimental conditions. Unsupported gold nanoparticles (AuNPs), AuNPs on carbon and on TiO₂ were employed and compared in terms of reaction selectivity and activity. The role of the base and the formed hydrogen peroxide has been considered. Gold based catalysts showed selectivity that is strongly dependent of the reaction conditions. In particular C–C scission products increases by increasing the reaction temperature but correlated only partially with the rate of degradation of the H₂O₂ formed under the operative conditions. Moreover, under neutral/acidic conditions glycerol can be oxidised also by increasing the temperature slightly, but it leads to a detrimental effect on selectivity and catalyst life.     

Synthesis of carbon nanotubes over gold nanoparticle supported catalysts

The synthesis of carbon nanotubes (CNTs) through the catalytic decomposition of acetylene was carried out over gold nanoparticles supported on SiO₂-Al₂O₃. Monodispersed gold nanoparticles with 1.3-1.8 nm in diameter were prepared by the liquid-phase reduction method with dodecanethiol as protective agent. The carbon products formed after acetylene decomposition consist of multi-walled carbon nanotubes with layered graphene sheets, carbon nanofilaments (CNFs), and carbon nanoparticles encapsulating gold particles. The observed CNTs have outer diameters of 13-25 nm under 850 °C. The influence of several reaction parameters, such as kind of carriers, reaction temperature, gas flow rate, was investigated to search for optimum reaction conditions. The CNTs were observed at a relatively low temperature (550 °C). The silica-alumina carrier showed higher activity for the formation of CNTs than others used in the screening test. With increasing temperature, the CNTs showed cured structures having thick diameters and inside compartments. When Au content on the support was over 5 wt.%, the gold nanoparticles coagulated to form large ones >20 nm in diameter and became encapsulated with graphene layers after decomposition of acetylene.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

Synergism in methanol synthesis from carbon dioxide over gold catalysts supported on metal oxides

Gold deposited on various oxides with high dispersion was found to be active for the hydrogenation of CO₂ at temperatures between 150 and 400°C. Product selectivities greatly depended on the nature of support oxide. Acidic oxides like TiO₂ gave higher CO₂ conversions but low methanol yields. Zinc oxide component was indispensable for selective methanol synthesis. Significantly, large particle size effect of gold was observed and smaller gold particles gave higher methanol productivity per exposed surface area of gold. This can be explained by an increase in the perimeter area of gold particles with a decrease in particle size. Methanol yield was greatly enhanced in a Au/ZnO---TiO₂ catalyst probably due to an increase in gold-zinc oxide interface.     

Low-temperature prepared highly effective ferric hydroxide supported gold catalysts for carbon monoxide selective oxidation in the presence of hydrogen

Ferric hydroxide supported Au catalysts prepared with co-precipitation method at room temperature without any heat treatment hereafter exhibited high catalytic activity and selectivity for CO oxidation in air and CO selective oxidation in the presence of H₂. With calcination temperature rising, both activity and selectivity decreased. X-ray Photoelectron Spectra (XPS) indicated that Au existed as Au⁰ and Au⁺ in the catalyst without heat treatment and even after being calcined at 200 °C, while after being calcined at 400 °C, Au existed as Au⁰ completely. X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopic (HRTEM) investigations indicated that both the supports and Au species were highly dispersed as nano or sub-nano particles even after being calcined at 200 °C, but after being calcined at 400 °C the supports transformed to crystal Fe₂O₃ with typical diameter of 30 nm and Au species aggregated to nano-particles with typical diameter of 2–4 nm. HRTEM investigations also suggested that the supports calcined at 200 °C were composed of amorphous ferric hydroxide and crystal ferric oxide. Results of computer simulation (CS) showed that O₂ was adsorbed on Au crystal cell and then were activated, which should be the key factor for the subsequent reaction. It also suggested that O₂ species were more easily adsorbed on Au⁺ than on Au⁰, indicating that higher positive charge of the Au species possessed the higher activity for CO oxidation.     

Preparation of supported gold catalysts from gold complexes and their catalytic activities for CO oxidation

A phosphine-stabilized mononuclear gold complex Au(PPh₃)(NO₃) (1) and a phosphine-stabilized gold cluster [Aug(PPh₃)₈](NO₃)₃ (2) were used as precursors for preparation of supported gold catalysts. Both complexes 1 and 2 supported on inorganic oxides such as -Fe₂O₃, TiO₂, and SiO₂ were inactive for CO oxidation, whereas the 1 or 2/ oxides treated under air or CO or 5% h₂/Ar atmosphere were found to be active for CO oxidation. The catalytic activity depended on not only the treatment conditions but also the kinds of the precursor and the supports used. The catalysts derived from 1 showed higher activity than those derived from 2. -Fe₂O₃ and TiO₂ were much more efficient supports than SiO₂ for the gold particles which were characterized by XRD and EXAFS.     

Palladium and platinum catalysts supported on carbon nanofiber coated monoliths for low-temperature combustion of BTX

In this work carbon nanofiber (CNF)-coated monoliths with a very thin, homogeneous, consistent and good adhered CNF layer were obtained by means of catalytic decomposition of ethylene on Ni particles.The catalytic behaviour of Pt and Pd supported on the CNF-coated monoliths was studied in the low-temperature catalytic combustion of benzene, toluene and m-xylene (BTX) and compared with the performance of Pt and Pd supported on γ-Al₂O₃ coated monoliths.The catalysts supported on CNF-coated monoliths were the most active, independent of the metal catalyst or the type of the tested aromatic compound. TPD experiments showed that the γ-Al₂O₃ phase retained important amounts of the water molecules produced during the reaction. When water vapour was supplied to the reactant flow, the activity of Pd catalysts decreased much stronger than the Pt ones, and the activity of the Pt catalysts supported on the γ-Al₂O₃ was more affected than that of the catalysts supported on CNF.BTX combustion reactions seem to be catalyzed by Pt and Pd through different kinetic mechanisms, explaining why Pt catalysts always were more active than the Pd ones deposited on the same type of support. Pd catalyzed combustion of benzene is strongly inhibited by oxygen and by water.Catalysts supported on CNF-coated monoliths showed a selectivity to burn benzene better than toluene or m-xylene, attributed to a better aromatic-CNF surface interaction.     

Influence of the surface area of the support on the activity of gold catalysts for CO oxidation

In the preparation of 1% Au/TiO₂ catalysts supported on either Degussa P-25 or anatase (90 m² g⁻¹) by deposition–precipitation, the gold content passes through a maximum at about the isoelectric point (pH 6), but maximum specific rates occur at pH 8–9 because the Au particle size becomes smaller as the pH is further increased. The gold uptake increases with the surface area of the support (anatase, rutile, P-25) and is complete above 200 m² g⁻¹; adsorption of the gold precursor at pH 9 is shown to be equilibrium-limited. Highest activities are found with supports of 50 m² g⁻¹. Catalysts made with high-area anatase (240 or 305 m² g⁻¹) are least active but show least deactivation.With Au/SnO₂ catalysts, gold uptake does not depend on the area of the support, and is highest at pH 7–8; very active catalysts (T₅₀ = 230–238 K) are obtained using SnO₂ of 47 m² g⁻¹. Storing a catalyst at 258 K for 1 week dramatically improves its stability. Results for Au/CeO₂ and Au/ZrO₂ catalysts confirm that moderate support areas give the most active catalysts, and suggest that surface area is often more important than chemical composition.     

Gold and iron supported on Y-type zeolite for carbon monoxide oxidation

Gold and iron supported on Y-type zeolite for carbon monoxide oxidation have been studied. They were prepared by ion exchange of gold(III) compounds with Y-type zeolite, iron-impregnated Y-type zeolite, or iron-exchanged Y-type zeolite in a chloroauric acid solution. The as-prepared gold/Y sample possessed high activity for carbon monoxide oxidation, but it gradually and irreversibly deactivated during the reaction. The deactivation was attributed to the transformation of some gold ions to gold metal. The activities of as-prepared gold/iron/Y samples were poor when compared with that of gold/Y, but their activities were markedly promoted by a high-temperature reduction treatment at 570°C. This is ascribed to the fact that the reduced gold/iron/Y catalyst could catch and release oxygen at low temperature.     

Oxidation of carbon monoxide over barium cuprate catalysts

Catalytic activities of BaCuO₂, Ba₂Cu₃O₅ and CuO for CO oxidation were investigated. At 250 ° C, BaCuO₂ was found to be about 7 times more active than CuO, while Ba₂Cu₃O₅ was found to be only slightly more active than CuO. This result also demonstrates that expensive rare earth elements such as La and Y are not necessary for a cuprate to have good activity for CO oxidation. After sintering at 940 ° C in air, the conversion substantially decreased for CuO. At steady state, both barium cuprates exhibited higher activity than in the fresh state. Based on the absence of significant changes in the XRD spectra, the change in catalytic activity is attributed to changes at the surface and possibly slight reduction of Cu²⁺. Reaction orders of CO were found to be 1.2 and 0.3, and reaction orders of O₂ were found to be 0 and 0.3 for BaCuO₂ and Ba₂Cu₃O₅, respectively.     

Partial oxidation of methane towards hydrogen production over a promising class of catalysts: Rh supported on Ti-modified MgO

The partial oxidation of methane over the supported Rh (0.8 wt.%) catalysts was investigated. Two kinds of supports were used, MgO and Ti-modified MgO (prepared by grafting technique). Among the Ti-modified MgO supports, two different compounds were used as source of Ti: inorganic (chloride) and organic (alkoxide). The catalytic performance of Rh-supported catalysts depends on the support and varies in the sequence: Ti-MgO/I > Ti-MgO/O > MgO. Ti-containing catalysts exhibited higher activity and selectivity compared to MgO, which is especially noticeable at low temperature. Possible explanations for the phenomena observed were proposed on the basis of characterization results.     

Enhanced methanol electro-oxidation activity of PtRu catalysts supported on heteroatom-doped carbon

A typical heteroatom (nitrogen)-doped carbon materials were successfully synthesized through the carbonization of a hybrid containing traditional carbon black covered by in situ polymerized polyaniline. The nitrogen content onto carbon can be adjusted up to 5.1 at.% by changing the coverage of polyaniline. The effects of nitrogen doping on the surface physical and electrochemical properties of carbon were studied using XPS, XRD and HRTEM, as well as CV and EIS techniques. With increasing nitrogen doping, the carbon structure became more compact, showing curvatures and dislocations in the graphene stacking. The nitrogen-doped carbon also exhibited a higher accessible surface area in electrochemical reactions, and a lower charge transfer resistance at the carbon/electrolyte interface. Moreover, to investigate the influence of nitrogen doping on the electrocatalytic activity of the PtRu/C catalyst, comparisons in CO stripping and methanol oxidation were carried out on PtRu catalysts supported by non-doped and nitrogen-doped carbon. Since the promotional roles of nitrogen doping, including the high electrochemically accessible surface area, the richness of the disordered nanostructures and defects, and the high electron density on N-doped carbon supports, contribute to the synthesis of well-dispersed PtRu particles with high Pt utilization and stronger metal-support interactions, an enhanced catalytic activity for methanol oxidation was obtained in the case of the PtRu/N-C catalyst in comparison with the traditional PtRu/C catalyst.     

High yield preparation of tubular carbon nanofibers over supported Co-Mo catalysts

In the search for high yield synthesis of carbon nanotubes (CNTs) at lower temperatures, Co-Mo catalysts on carbon black were investigated with ethylene and CO as carbon sources in catalytic gas-phase pyrolysis in comparison to that on TiO₂. The carbon black support was expected to be advantageous because of the feasibility of a CNT/carbon black composite possibly fabricated for several applications without removal of the support. Depending on the catalyst support, the catalytic activity toward CO and ethylene showed great differences. Co-Mo (9:1) catalysts on titania or carbon black provided a high carbon yield from CO and ethylene at the rather low temperatures of 450-530 °C.     

Partial oxidation of methane over silica supported molybdenum oxide catalysts

Two kinds of MoO₃/SiO₂ catalysts, MoO₃-I and MoO₃-S, were prepared by impregnation and sol-gel method, respectively. When MoO₃ loading was increased, formation of MoO₃ crystals was observed to begin at a MoO₃ loading of 8 and 16 wt% with MoO₃-I and MoO₃-S, respectively. The highest yield of formaldehyde from methane oxidation was attained also at those critical values of MoO₃ loading of 8 and 16 wt% over MoO₃-I and MoO₃-S, respectively. It is suggested that the active species for formaldehyde formation is well dispersed molybdenum oxide clusters on SiO₂ support: the optimum dispersion of the clusters affords the highest activity for formaldehyde formation.     

Influence of the preparation conditions on the properties of gold catalysts for the oxidation of glucose

In this work, two deposition–precipitation methods for the preparation of gold catalysts for glucose oxidation were investigated. Thus far, gold colloids immobilized on carbon have been used for catalytic glucose oxidation, but the long-term stability of these systems was not sufficient. To improve the long-term stability we used the deposition–precipitation methods using NaOH (DP NaOH) or urea (DP urea) as precipitation agents as they were described by Haruta and Dekkers, respectively, using alumina as a support material. With these methods, it was possible to prepare highly active and selective catalysts which showed an excellent long-term stability. DP urea was found to be the preferred method, because in contrast to DP NaOH, no losses of gold occurred during the preparation, and it was possible to adjust various gold contents up to 10 wt% Au.