Effect of Gold Electronic State on the Catalytic Performance of Nano Gold Catalysts in n-Octanol Oxidation (original) (raw)
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Catalysts, 2021
Au and Ag were deposited on TiO2 modified with Ce, La, Fe or Mg in order to obtain bimetallic catalysts to be used for liquid-phase oxidation of 1-octanol. The effects of the deposition order of gold and silver, and the nature of the support modifying additives and redox pretreatments on the catalytic properties of the bimetallic Au-Ag catalysts were studied. Catalysts were characterized by low-temperature nitrogen adsorption–desorption, energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and ultraviolet-visible diffuse reflectance spectroscopy. It was found that pretreatments with hydrogen and oxygen at 300 °C significantly decreased the activity of AuAg catalysts (silver was deposited first) and had little effect on the catalytic properties of AgAu samples (gold was deposited first). The density functional theory method demonstrated that the adsorption energy of 1-octanol increased for all positively...
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The complete oxidation of isobutane has been studied using the oxidation catalysts-MnO 2 ,-Fe 2 O 3 , Co 3 O 4 and NiO, prior to and following addition of 5 wt% Au nanoparticles. The activity order is Co 3 O 4 >-MnO 2 >-Fe 2 O 3 , with the position of NiO dependent on the Ni 3+ content which changes with temperature. Preformed n-hexanethiolate-stabilized gold nanoparticles, following adsorption and thermolysis in air, introduce a small amount of sulfur as adsorbed sulfate. The sulfate appears to block the reoxidation step in the Mars-van Krevelen mechanism. This can have a significant effect on catalytic activity, as observed for-MnO 2. TEM/STEM studies indicate that gold nanoparticles of 2–4 nm in diameter form, which depends on the identity of the metal oxide and its specific surface area. Gold nanoparticle size effects have been studied on NiO, and show that the apparent activation energy and temperature of initial reaction depend on nanoparticle size. Comparisons of the multicomponent Au/MO x /-Al 2 O 3 (M:Al = 1:10) catalysts, where M = Mn, Fe, Co, Ni, have also been studied, and all are more active catalysts than Au/-Al 2 O 3 , but less active than the unsupported catalysts. Gold 4f 7/2 XPS studies on Au/MO x and Au/MO x /-Al 2 O 3 have shown that the only common species present is Au(0), suggesting that higher oxidation states of Au are not important in oxidation catalysis.
Gold nanoparticles supported on SBA-15 are prepared by homogenous deposition-precipitation method (HDP) using urea as the precipitating agent. The structural features of the synthesized catalysts were characterized by various techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption–desorption (BET), pore size distribution (PSD), CO chemisorption and X-ray photoelectron spectroscopy (XPS). The catalytic activity and stability of the Au/SBA-15 catalysts are investigated during the vapor phase aerobic oxidation of benzyl alcohol. The BJH pore size distribution results of SBA-15 support and Au/SBA-15 catalysts reveals that the formation of mesoporous structure in all the samples. TEM results suggest that Au nanoparticles are highly dispersed over SBA-15 and long range order of hexagonal mesopores of SBA-15 is well retained even after the deposition of Au metallic nanoparticles. XPS study reveals the formation of Au (0) after chemical reduction by NaBH4. The particle size measured from CO-chemisorption and TEM analysis are well correlated with the TOF values of the reaction. Au/SBA-15 catalysts are found to show higher activity compare to Au/TiO2 and Au/MgO catalysts during the vapor phase oxidation of benzyl alcohol. The catalytic functionality are well substantiated with particle size measured from TEM. The crystallite size of Au in both fresh and spent catalysts were measured from X-ray diffraction.
Catalysts
CO preferential oxidation (PROX) is an effective method to clean reformate H2 streams to feed low-temperature fuel cells. In this work, the PROX and CO oxidation reactions were studied on preformed Au nanoparticles (NPs) supported on TiO2 anatase. Preformed Au NPs were obtained from gold cores stabilized by dodecanethiols or trimethylsilane-dendrons. A well-controlled size of ca. 2.6 nm and narrow size distributions were achieved by this procedure. The catalysts were characterized by high-resolution transmission electron microscopy and ex situ and in situ X-ray photoelectron spectroscopy (XPS). The XPS results showed that the preformed Au NPs exhibited high thermal stability. The different ligand-derived Au catalysts, as well as a conventional gold catalyst for comparison purposes, were loaded onto cordierite supports with 400 cells per square inch. The activity and selectivity of the samples were evaluated for various operation conditions. The catalyst prepared using dodecanethiol-...
Catalysis Today, 2021
Gold nanocatalysts, active in several oxidation reactions, suffer of insufficient time-on-stream stability. The easiest way to solve this problem is modifying the support, due to metal-support interaction. This study compares modifying effects of MgO and La 2 O 3 on textural, electronic, and catalytic properties of Au nanoparticles (NPs) supported on inert nanostructured SiO 2 in CO oxidation and liquid phase 1-octanol oxidation. Modification of the silica support surface with La and Mg increased metal support interaction, leading to gold particles with primary size of 1 nm but with different stability: stable under different pretreatment conditions on Mg-modified samples but highly sensible to the pretreatments on La-modified samples. Both modifiers changed electronic properties of supported gold favoring formation and stabilization of Auδ + states, which are probable gold active sites in catalytic redox processes. Modification with La and Mg oxides changed catalytic properties in CO oxidation before and after pretreatment in H 2 at 300 • C for 1 h. Gold catalysts supported on La-and Mg-modified silica showed similar performance in 1-octanol oxidation with higher conversion than unmodified Au/SiO 2. La and Mg showed better promoting effects of catalytic properties in this reaction than redox modifiers (Fe and Ce) supported on small SiO 2 particles.
The vapor phase oxidation of benzyl alcohol was investigated over gold nanoparticles supported on mesoporous titanium dioxide (anatase) catalysts under aerobic conditions. The catalysts were prepared by homogeneous deposition–precipitation method using urea as the precipitating agent. The physico-chemical properties of the synthesized catalysts were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), pore size distribution (PSD), CO-chemisorption and X-ray photoelectron spectroscopy (XPS) techniques. The crystallite size of gold in Au/TiO2 catalysts was measured from XRD. The mesoporosity of TiO2 support and Au/TiO2 catalysts were confirmed by PSD analysis. TEM results suggest that gold nanoparticles are well dispersed over mesoporous TiO2. The catalytic functionality is well substantiated with particle size measured from TEM. XPS results reveal the formation of Au(0) after chemical reduction by NaBH4. The vapor phase benzyl alcohol oxidation was used as a test reaction to investigate the influence of the metal, nature of the support, and of metalsupport interactions in Au/TiO2 catalysts and also the catalytic activity and stability of the Au/TiO2 catalysts. The conversion of benzyl alcohol was found to increase with decrease in the size of gold particles. Smaller gold particles and a higher amount of small gold particles had a beneficial effect on the catalytic activity. The catalytic activity in the presence of oxygen is believed to be associated with the transport of electrons through the catalyst to the adsorbed oxygen on the surface.
The myth that gold cannot act as a catalyst has been discarded in view of recent studies, which have demonstrated the high catalytic efficiency of pure nano-gold and supported nano-gold catalysts. In recent years, numerous papers have described the use of supported nano-gold particles for catalysis in view of their action on CO and O 2 to form CO 2 , as well as a variety of other reactions. Special emphasis is placed on the oxidation studies undertaken on model nano-Au systems. In this work a solvent free oxidation of 1-phenyl ethanol was carried out using gold supported on ceria-silica, ceriatitania, ceria-zirconia and ceria-alumina at 160 0 C. Almost 88-97% conversion was obtained with >99% selectivity. Temperature screening was done from 70 to 160 0 C.Catalysts were prepared by deposition co-precipitation method and deposition was determined by EDEX analysis.
Gold Nanoparticle: Enhanced CO Oxidation at Low Temperatures by Using Fe-Doped TiO2 as Support
Catalysis Letters, 2017
Iron doped TiO 2 materials were prepared by the sol-gel method and used as supports of gold nanoparticles synthesized by the deposition-precipitation technique. The gold-iron-titania catalysts were characterized by X-ray diffraction, Raman spectroscopy, N 2 physisorption, UV-Vis spectroscopy as a function of temperature, H 2-temperature programmed reduction, transmission electronic microscopy and X-ray photoelectron spectroscopy. The gold-iron catalysts were catalytically active during the CO oxidation reaction at low temperatures, reaching CO conversion percentages of almost 80% at room temperature. The Au/TiO 2-Fe catalyst surface was characterized through infrared spectroscopy (DRIFTS) during the CO oxidation reaction to elucidate the active sites and the real carbon monoxide interaction during the reaction. A 24-h deactivation test corroborated a final deactivation of the catalysts of only 25% for both Au/TiO 2-Fe 1 and the bare Au/TiO 2. The results here obtained corroborate that the activity of the iron-doped TiO 2 catalyst was higher than that of the bare TiO 2 due to the iron incorporation into the TiO 2 lattice, which allows the formation of surface oxygen vacancies and new adsorption sites which favor the CO adsorption and its oxidation to CO 2 .