Sol Immobilization Research Articles

Carbon Monoxide Oxidation by Polyoxometalate‐Supported Gold Nanoparticulate Catalysts: Activity, Stability, and Temperature‐ Dependent Activation Properties

AbstractNanoparticulate gold supported on a Keggin‐type polyoxometalate (POM), Cs4[α‐SiW12O40]⋅n H2O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was −67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U‐shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas‐phase reactions.

Carbon Monoxide Oxidation by Polyoxometalate-Supported Gold Nanoparticulate Catalysts: Activity, Stability, and Temperature- Dependent Activation Properties.

Nanoparticulate gold supported on a Keggin-type polyoxometalate (POM), Cs4 [α-SiW12 O40 ]⋅n H2 O, was prepared by the sol immobilization method. The size of the gold nanoparticles (NPs) was approximately 2 nm, which was almost the same as the size of the gold colloid precursor. Deposition of gold NPs smaller than 2 nm onto POM (Au/POM) was essential for a high catalytic activity for CO oxidation. The temperature for 50 % CO conversion was -67 °C. The catalyst showed extremely high stability for at least one month at 0 °C with full conversion. The catalytic activity and the reaction mechanism drastically changed at temperatures higher than 40 °C, showing a unique behavior called a U-shaped curve. It was revealed by IR measurement that Auδ+ was a CO adsorption site and that adsorbed water promoted CO oxidation for the Au/POM catalyst. This is the first report on CO oxidation utilizing Au/POMs catalysts, and there is a potential for expansion to various gas-phase reactions.

Synthesis and characterisation of supported Re nanoparticles for the synthesis of biofuels

Supported rhenium nanoparticles have been synthesised via wet impregnation and sol immobilisation. Deionised water was used as the solvent for both methods and the supports used were zirconia (ZrO2) and sulphated-zirconia (SO4-ZrO2). Characterisation involved BET surface area measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD showed that samples were crystalline while BET showed that nanoparticles synthesised via sol immobilisation had a higher surface area while those synthesised via impregnation had a higher pore volume and diameter. Adsorption-desorption isotherms revealed that the particles were mesoporous (2-50 nm), which is in agreement with the data obtained from TEM.

Gold as a modifier of metal nanoparticles: effect on structure and catalysis.

Bimetallic gold based catalysts have been prepared using a sol immobilisation technique. Despite a very similar metal dispersion, different structures are revealed depending on the second metal, with alloyed systems being preferred in the case of Pd, Pt and Cu, and core-shell in the case of Ru. A positive synergistic effect between the metals has been revealed only in the cases of Pd and Cu in the oxidation of benzyl alcohol. AuPd/C has been also studied in the hydrogenation of benzaldehyde where the bimetallic catalyst revealed a different selectivity compared to the monometallic counterpart.

Cinnamaldehyde hydrogenation using Au–Pd catalysts prepared by sol immobilisation

We report the catalytic performance of Au–Pd nanoparticles prepared via a sol immobilisation technique for the catalytic hydrogenation of cinnamaldehyde under mild reaction conditions.

Enhanced Activity of Au/NiO Nanohybrids for the Reductive Amination of Benzyl Alcohol.

Gold nanoparticles were prepared by sol immobilization (AuSI) or deposition precipitation (AuDP), then deposited on NiO and commercial TiO2 (P25). The Au/NiO catalysts showed higher activity and yield to the secondary amine, compared to Au/TiO2 catalysts, when tested for the reductive amination of benzyl alcohol with isopropylamine. We attribute this result to a synergistic effect between Au and NiO. Moreover, as a result of the protective effect of the polyvinyl alcohol used in the sol immobilization synthesis, the gold nanoparticles on NiO demonstrate an increased resistance to structural changes during the reaction. This effect results in enhanced catalytic efficiency in terms of activity, and better stability against deactivation.

Diols Production From Glycerol Over Pt-Based Catalysts: On the Role Played by the Acid Sites of the Support

A series of 1 wt% AuPt (6:4) catalysts were prepared by sol immobilization using acidic [TiO2, H-Mordenite, SiO2, MCM-41, sulphated ZrO2 (S-ZrO2)] and one basic (MgO) oxide supports. A careful characterization of the catalyst was performed by HRTEM and FTIR spectroscopy. EDX analysis showed that on all catalysts only alloyed AuPt nanoparticles are present, but the final size of AuPt particles is significantly affected by the support. Indeed, on TiO2 the mean AuPt nanoparticles diameter is 3.7 nm, whereas for all the remaining supports, larger AuPt nanoparticles with diameter of 6–7.5 nm were obtained. AuPt catalysts result very active in catalyzing the liquid phase hydrogenolysis of glycerol to 1,2-propandiol (68% conversion after 16 h) and 90% selectivity with ethylene glycol, 1-propanol and 2-propanol as main by-products. The role of the support has been highlighted in terms of acidic properties. The medium strength of Lewis acid sites of TiO2 leads to the best performance in terms of activity, selectivity and stability of the catalytic system.

An investigation into bimetallic catalysts for base free oxidation of cellobiose and glucose

AbstractBACKGROUNDThe selective conversion of cellulose to gluconic acid under mild conditions is challenging as it has abundant intra‐and inter‐molecular hydrogen bonds that protect the β‐1,4‐glycosidic bonds and make it intrinsically recalcitrant to depolymerize.RESULTSAu‐Pd/TiO2 and Au‐Pt/TiO2 catalysts prepared by sol immobilization method without pre‐treatment are active and selective for the oxidation of glucose and cellobiose to gluconic acid under base‐free conditions. Important preparation parameters are the pre‐treatment of the catalyst and the metal ratio.CONCLUSIONThe optimized catalyst provided a good yield of gluconic acid from cellobiose and has opened up a new catalyst system for cellobiose conversion in terms of a heterogeneous catalyst. © 2017 Society of Chemical Industry

Photocatalytic behavior of metal-decorated TiO2 and their catalytic activity for transformation of glycerol to value added compounds

In this work, TiO2 was decorated with one of four different metals (Bi, Pd, Pt and Au) by sol immobilization at ambient condition. The individual addition of each metals on the TiO2 did not affect the crystallite size or band gap energy of TiO2, but increased its visible light absorption ability due to the localized surface plasmon resonance effect of the doped metals, except for Bi, as well as the mechanism of the charge transfer. Among all the prepared metal-decorated TiO2 photocatalysts, Au/TiO2 exhibited the highest visible light absorption ability and an appropriate mechanism of charge transfer, which consequently alleviated the e−–h+ recombination. It also provided the highest photocatalytic activity for glycerol conversion (92.8% at 14h) and yield of dihydroxyacetone, glyceraldehyde, glycolic acid, hydroxypyruvic acid and formaldehyde of 12.0, 23.1, 10.7, 13.6 and 15.3%, respectively.

Monitoring in situ the colloidal synthesis of AuRh/TiO2 selective-hydrogenation nanocatalysts

AuRh/TiO2 nanocatalysts have been prepared by colloidal co-reduction followed by sol immobilization. The nanoparticle synthesis is monitored in situ by liquid TEM, DLS and UV-vis, and the catalyst performance in selective hydrogenation of cinnamaldehyde is correlated to structural information obtained from TEM and CO-FTIR.

The effects of impregnation of precious metals on the catalytic activity of titanium silicate (TS-1) in epoxidation of propene using hydrogen peroxide

Propene oxide is an important chemical intermediate and titanium silicalite (TS-1) has been widely investigated as a promising catalyst for the direct epoxidation of propene with ex-situ or in-situ produced hydrogen peroxide as an oxidant. In order to clarify the effects of the kind of precious metal and treatment process in the catalyst preparation on the propene epoxidation and the hydrogen peroxide decomposition, TS-1 was impregnated with gold and palladium via drying, calcination and reduction and the experiments to check its catalytic performance were conducted in a gas aspirating autoclave reactor in the absence of mass transfer limitations. The presence of precious metals vigorously catalyzed the side reactions and hydrogen peroxide decomposition. Some of the precious metal containing TS-1 catalysts showed high initial rates but there was no catalyst with a propene oxide yield after 5h reaction time comparable to TS-1 alone because of the enhancement of side reactions by precious metals. The significant decline in the selectivity to propene oxide over the dried precious metal containing TS-1 catalysts was attributed to the leaching of precious metals into the reaction medium. Palladium containing TS-1 showed exceptionally high decomposition of hydrogen peroxide. Reduction and calcination increased the decomposition by forming metallic gold and palladium. Homogeneous dispersion of gold nanoparticles was achieved by a sol immobilization method which led to a decrease of propene oxide selectivity and an increase of hydrogen peroxide decomposition.

Deposition of Gold Nanoparticles on Niobium Pentoxide with Different Crystal Structures for Room‐Temperature Carbon Monoxide Oxidation

AbstractNanoparticulate gold catalysts (NPGCs) supported on niobium oxides (Nb2O5) were prepared by different deposition methods. Au nanoparticles (NPs) with a mean diameter of approximately 5 nm were deposited by conventional deposition methods, which include the deposition‐precipitation (DP) and deposition‐reduction (DR) methods, on layered‐type Nb2O5 that possesses a deformed orthorhombic structure. Moreover, Au NPs with a mean diameter of approximately 2.7 nm could be deposited by the sol immobilization method on several crystalline forms of Nb2O5 (deformed orthorhombic, pseudohexagonal, and orthorhombic). NPGCs deposited on deformed orthorhombic Nb2O5 had a higher catalytic activity for CO oxidation. The temperature for 50 % CO conversion was 11 °C for 1 wt % Au/Nb2O5 (deformed orthorhombic) prepared by the sol immobilization method, and the CO conversion was 91 % at 28 °C. As far as we know, this is the first report of highly active Au NPGCs (smaller than 3 nm in diameter) supported on Nb2O5. NPGCs supported on acidic metal oxides will expand the utilization of gold catalysts for new applications.

Layered double hydroxides decorated with Au-Pd nanoparticles to photodegradate Orange II from water

MgZnAl-Cl layered double hydroxides were prepared by co-precipitation method and further used as support to immobilize gold-palladium (Au-Pd) bimetallic nanoparticles by colloidal sol immobilization method. These materials were characterized by XRD, FT-IR, N2 adsorption desorption, TGA to verify the structure and stability of the heterostructure. The pristine LDH and Au-Pd@LDH were investigated for an environmental purpose, where photocatalytic degradation of orange II (OII) dye was studied as model reaction. The performance of support and catalyst were determined by evaluating both degradation and adsorption phenomenon. Influence of parameters such as catalyst loading and Orange II concentration were evaluated. The current study holds importance in water treatment and similar environmental protection applications.

A micropacked-bed multi-reactor system with in situ raman analysis for catalyst evaluation

A micropacked-bed multi-reactor platform with integrated portable Raman spectrometer is presented for fast evaluation of catalyst activity and stability for gas/liquid/solid reactions. The silicon-glass microreactor was designed and fabricated so that pockets containing the liquid reaction mixture were created after each packed bed, into which the laser could be directed for Raman spectral acquisition. Using the oxidation of benzyl alcohol as a model reaction, the Raman spectrum was found to be affected both by temperature and by the composition of the multiphase reaction mixture which was related to the reaction conversion. These effects were accounted for by calibrating the Raman spectra at the reaction temperature using mixtures produced by the reactors that were analysed independently by gas chromatography. Fourteen catalysts containing different combinations of Au, Pd and Pt supported on TiO2 prepared by sol-immobilisation (SI) and standard impregnation (SImp) techniques were tested. The results showed that the activity of the catalysts prepared by SI was overall higher than those prepared by SImp, while the activity sequence followed the same pattern: Pd>AuPd>AuPdPt>PdPt>(Au, Pt, AuPt). The Pd and AuPd catalysts from both SI and SImp were stable in 5-h testing, however, for the PdPt and AuPdPt catalysts prepared by SI deactivation was observed.

Metal nanoparticles on carbon based supports: The effect of the protective agent removal

The sol immobilization technique is widely employed for the synthesis of size- and morphology-controlled supported nanoparticles. The protective agent adsorbed on the surface of nanoparticles can substantially affect the catalytic performances, by limiting the access to the active sites as well as by influencing metal–support and reactant–metal interactions, thus altering not only the activity but also the selectivity of the reaction. Herein, we studied the effect of the capping agent (specifically polyvinyl alcohol, PVA) removal on the catalytic activity and selectivity of gold nanoparticles supported on different carbon based materials using the glycerol oxidation as test reaction.

Pd/ZnO catalysts for direct CO2 hydrogenation to methanol

The direct hydrogenation of CO2 into methanol is crucial for providing a means of CO2 fixation and a way to store cleanly produced hydrogen in a more energy-dense and transportable form. Here we have prepared two series of Pd/ZnO catalysts, both by immobilisation of PVA-protected Pd colloids and by Pd impregnation of PdCl2 to investigate structure activity relationships for direct CO2 hydrogenation. Very different performances were found for the different preparation methods, and the Pd loading and pre-reduction of the catalysts were shown to be important factors for optimising methanol yield. The crucial factor for high methanol yield is the formation of a Pd–Zn alloy, either during the reaction itself, or better by high temperature pre-reduction. The formation of the alloy greatly reduces CO production by the reverse water gas shift reaction. The catalysts prepared by sol-immobilisation were relatively stable to thermal treatment. In contrast, the impregnated catalysts were much less thermally stable, due to the presence of remnant chloride on the surface of the catalyst, which was absent for the case of sol immobilisation preparation. The results illustrate the importance of controlling the PdZn particle size and its surface structure for the catalysts to achieve high methanol selectivity (60%, the rest being CO) and conversion (11%) at 250°C and 20bar. Selectivity for sol-immobilised catalysts decreases from 60% at 3nm average diameter, to 20% at 7nm.

Synthesis of Carboxylic Acid by 2-Hexenal Oxidation Using Gold Catalysts Supported on MnO2

Synthesis of carboxylic acid can be achieved by the oxidation of aldehyde using air as an oxidant in the presence of a potential catalyst. We demonstrated that 2-hexenal can be oxidized to carboxylic acid by Au, Pd, and Au-Pd catalysts and investigated the effects of catalyst support (graphite, TiO2, MgO, SiC, MnO2, CeO2, and Al2O3), preparation method for supported catalyst (sol immobilization, impregnation, and deposition precipitation), and choice of catalyst components. Analysis of conversion% and selectivity% for 2-hexenoic acid showed that MnO2-supported gold nanoparticles are the best catalysts for 2-hexenal oxidation. Moreover, catalysts prepared by sol immobilization are the most active possibly due to the much smaller gold nanoparticle size. Selectivity for 2-hexenoic acid is a major pathway of oxidation of 2-hexenal.

AuPt Alloy on TiO2: A Selective and Durable Catalyst for L-Sorbose Oxidation to 2-Keto-Gulonic Acid.

Pt nanoparticles were prepared by a sol immobilization route, deposited on supports with different acid/base properties (MgO, activated carbon, TiO2 , Al2O3, H-Mordenite), and tested in the selective oxidation of sorbose to 2-keto-gulonic acid (2-KGUA), an important precursor for vitamin C. In general, as the basicity of the support increased, a higher catalytic activity occurred. However, in most cases, a strong deactivation was observed. The best selectivity to 2-KGUA was observed with acidic supports (TiO2 and H-Mordenite) that were able to minimize the formation of C1/C2 products. We also demonstrated that, by alloying Pt to Au, it is possible to enhance significantly the selectivity of Pt-based catalysts. Moreover, the AuPt catalyst, unlike monometallic Pt, showed good stability in recycling because of the prevention of metal leaching during the reaction.

Oxidation of Aliphatic Alcohols by Using Precious Metals Supported on Hydrotalcite under Solvent- and Base-Free Conditions.

Precious metal nanoparticles supported on magnesium-aluminum hydrotalcite (HT), TiO2 , and MgO were prepared by sol immobilization and assessed for the catalytic oxidation of octanol, which is a relatively unreactive aliphatic alcohol, with molecular oxygen as the oxidant under solvent- and base-free conditions. Compared with the TiO2 - and MgO-supported catalysts, platinum HT gave the highest activity and selectivity towards the aldehyde. The turnover number achieved for the platinum HT catalyst was >3700 after 180 min under mild reaction conditions. Moreover, the results for the oxidation of different substrates indicate that a specific interaction of octanal with the platinum HT catalyst could lead to deactivation of the catalyst.

Operando Attenuated Total Reflectance FTIR Spectroscopy: Studies on the Different Selectivity Observed in Benzyl Alcohol Oxidation

AbstractAu, Pd, and AuPd supported on TiO2 were prepared by a sol immobilization route. Operando attenuated total reflectance (ATR) IR spectroscopy and catalytic batch reactor experiments were performed in parallel to elucidate the different catalytic performance of the catalysts in the liquid‐phase oxidation of benzyl alcohol. Pd/TiO2 exhibited a higher activity than AuPd/TiO2 and Au/TiO2, but the modification of Pd with Au demonstrated a significant stability enhancement. ATR‐IR spectroscopy evidenced that the presence of Au facilitates the desorption of byproducts, which thus reduces the extent of deactivation of the active sites caused by the irreversible adsorption of benzoate species. Although benzaldehyde was the main product in both catalysts, the nature of the byproducts differs. Pd/TiO2 favored the deoxygenation of benzyl alcohol to produce toluene as the main byproduct. Conversely, AuPd/TiO2 promoted the transformation of benzaldehyde to benzoic acid.