Au Particle Size Research Articles

CO Adsorption on Supported Gold Nanoparticle Catalysts: Application of the Temkin Model

The adsorption of CO on the supported gold nanoparticle catalysts Au/TiO2, Au/Fe2O3, and Au/ZrO2 was examined using infrared transmission spectroscopy to quantify the isobaric CO coverage as a function of temperature. The Temkin adsorbate interaction model was then applied to account for the adsorption behavior. To test the general applicability of the Temkin model, this treatment was also applied to three data sets from the literature. This included another real-world catalyst and two model catalysts. All data sets were accurately represented by the Temkin adsorbate interaction model. The resulting thermodynamic metrics are consistent with previous determinations and reflect a particle size-dependence. In particular, the intrinsic adsorption enthalpy at zero CO coverage varies almost linearly with Au particle size, and this trend appears to be correlated with the abundance of low-coordinate Au sites (cf., CN = 6 and 7 for corners and edges, respectively). For very small particles with mostly CN = 6 corne...

Isokinetic behavior in the gas phase hydrogenation of nitroarenes over Au/TiO2: application of the selective energy transfer model

The gas phase selective hydrogenation of a series of nitroarenes (nitrobenzene, p-chloronitrobenzene, p-bromonitrobenzene, p-nitroaniline, p-nitrotoluene, p-nitrophenol and p-nitroanisole) has been examined over Au/TiO2 (0.3 % w/w Au, mean Au particle size = 3.9 nm). Compensation behavior is demonstrated with an associated isokinetic temperature (Tiso) of 558 ± 32 K. We account for this response in terms of the selective energy transfer (SET) model where the occurrence of resonance between catalyst and reactant vibrations generates the activated complex. An analysis of the stepwise variation of the activation energies has identified a critical vibrational frequency of 853 cm−1, which is close (±2 cm−1) to the reference value for nitro-group (in-plane symmetric O–N–O bending and stretching) vibrations. Application of SET suggests activation of weakly adsorbed nitroarene (at the support or metal/support interface) by excitation of the nitro-group via IR radiation from a strongly adsorbed surface nitroarene component. The excited nitroarene is then attacked by reactive hydrogen supplied by the Au sites to generate the respective aromatic amine with 100 % selectivity. Agreement of the SET predicted Tiso with the experimental value requires the incorporation of a term due to C–N torsional entropy resulting from distortion of the O–N–O plane.

Comparative Study of Au Modified Beta, MCM-22, Mordenite, ZSM-5, MCM-41, Ce-MCM-41 Catalysts in Piperonyl Alcohol Oxidation

Au and Au-Ce promoted catalysts supported on zeolites (Beta, H-MCM-22, H-Mordenite, ZSM-5), mesoporous material MCM-41 and metal oxides (TiO2, SiO2, Al2O3) were prepared. Catalysts were characterized by a range of methods: nitrogen physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS), transmission electron microscopy (TEM) and activity was evaluated in selective aerobic piperonyl alcohol oxidation to piperonal. The effect of such catalyst ’ s properties as Au particle size and Ce addition on the catalytic activity was highlighted.

Determination of the Au active site and surface active species via operando transmission FTIR and isotopic transient experiments on 2.3 wt.% Au/TiO2 for the WGS reaction

The kinetics and the active site for the water gas shift (WGS) reaction were investigated by operando FTIR during steady state and isotopic transient experiments on a 2.3wt.% Au/TiO2 catalyst. The WGS rate at 120°C and activation energy measured in the operando IR reactor for the WGS reaction (7% CO, 11% H2O, 9% CO2, and 37% H2) on the Au/TiO2 catalyst were 1.1±0.1×10−2 (molH2) (mol−1Au) s−1 and 56±3kJmol−1, respectively. We have shown that the kinetics measured over a pressed wafer in a transmission FTIR were similar to those measured in a regular plug flow reactor (PFR). Operando IR data were used to follow the catalyst deactivation as a function of time, and the amount of CO adsorbed on metallic Au species (Au0, 2100cm−1) correlated with the WGS rates, while CO adsorbed on partially negatively charged Au species (Auδ−, 2040cm−1) did not. The rate loss did not correlate with the amounts of formate- and carbonate-like species measured by IR but was consistent with a loss of active sites by sintering.Measurements by isotopic transient experiments (13CO–12CO) on the Au/TiO2 catalyst with an average Au particle size of 3nm showed that less than 3% of the total surface Au atoms on the Au nanoparticles were responsible for the majority of the catalytic activity. Time-resolved IR spectra collected during the isotopic transient experiments showed that CO adsorbed on Au0 species were involved in the reaction pathway, while the surface formate and carbonate species were spectators under these WGS conditions. Based on these results, we conclude that metallic corner Au atoms are the dominant active sites for WGS on Au/TiO2 catalysts and that CO adsorbed on Au0 is an active intermediate. In addition, we present a detailed description of the design, construction, and operation of a transmission FTIR cell capable of performing operando kinetic measurements and isotope switching experiments with a signal decay time of about 95% in 1.5s.

Au/FeOx catalysts of different degree of iron oxide reduction

Au/FeOx catalysts were prepared by deposition of Au on Fe3O4 (commercial Cerac, a mixture of magnetite and maghemite) with the deposition-precipitation method, and heating of the precursors in air or in Ar. The catalysts were characterized by XPS, Mössbauer spectroscopies, H2TPR, TEM, decomposition of isopropanol (probe reaction for acid–base properties), and tested in CO oxidation.The heating atmosphere influences composition, physicochemical and catalytic properties. The catalysts prepared in Ar contain magnetite, MT and maghemite, MG as the main iron oxide phases, for those calcined in air MG is the only phase observed. Higher amounts of weakly bound oxygen are observed in H2TPR measurements for the catalysts prepared in air. They exhibit smaller acidity and higher basicity compared to the samples prepared in Ar. The mean particle size of nano Au is not affected markedly by the preparation conditions and amounts to ∼4.5nm. Calcination in Ar appears, however, to stabilize smaller particles (<4nm) against the sintering. Catalytic activity of the Au/FeOx catalysts in CO oxidation is higher for catalysts prepared in air, containing MG, as compared to those containing MT. This can be related to the higher amounts of weakly bound oxygen for the former catalysts. In the reaction mixture CO+O2, MT phase is oxidized to MG. For all the catalysts the activity is considerably enhanced by pretreatment in hydrogen. This can be associated with the increase in the content of the surface OH groups.

Size and Support Effects for the Water–Gas Shift Catalysis over Gold Nanoparticles Supported on Model Al2O3 and TiO2

The water-gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO(2), 22% H(2)O, and 37% H(2) at 1 atm for Au/Al(2)O(3) catalysts at 180 °C and Au/TiO(2) catalysts at 120 °C varies with the number average Au particle size (d) as d(-2.2±0.2) and d(-2.7±0.1), respectively. The use of nonporous and crystalline, model Al(2)O(3) and TiO(2) supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au(0) (near 2100 cm(-1)) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al(2)O(3) and Au/TiO(2) catalysts, respectively, and 10 times more active for Au on TiO(2) versus Al(2)O(3). From operando Fourier transform infrared spectroscopy (FTIR) experiments, it is determined that the active Au sites are metallic in nature. In addition, Au/Al(2)O(3) catalysts have a higher apparent H(2)O order (0.63) and lower apparent activation energy (9 kJ mol(-1)) than Au/TiO(2) catalysts with apparent H(2)O order of -0.42 to -0.21 and activation energy of 45-60 kJ mol(-1) at near 120 °C. From these data, we conclude that the support directly participates by activating H(2)O molecules.

Activation and Deactivation of Au–Cu/SBA-15 Catalyst for Preferential Oxidation of CO in H2-Rich Gas

This work aims to develop an efficient catalyst for preferential oxidation (PROX) of CO in a H2-rich gas and obtain a clear structure–property relationship of the catalyst. A Au/CuO/SBA-15 catalyst was designed and prepared via a nanoengineering approach in which the metallic particles of average size 3 nm located in the vicinity of CuO particles were highly dispersed on the SBA-15 support. It was found that the CuO particles could reduce the gold (Au) particle size, stabilize the Au particles, and facilitate the activation of molecular oxygen. Although this catalyst is extremely active for the PROX reaction at room temperature and superior to the Au/SBA-15 and CuO/SBA-15 catalysts, it deactivates easily. Clear experimental evidence showed that CuO was reduced to Cu2O and Cu in the reductive reaction gas mixture, and the Cu further combined/dissolved into the Au particles during the reaction. The alloying of Au and Cu not only decreased the catalytic activity of the Au particles but also reduced the ability of CuO to activate molecular oxygen. Fortunately, this alloying process could be reversed via a simple calcination in air to activate the catalyst. The identification of the catalyst structural evolution and this new contributing factor toward the deactivation of the Au catalysts has provided insights into the field of Au catalysis.

Synthesis of Au Nano-Particles Using Plasma Sputtering into Oil

It has been examined to disperse the Au nano-particles directly into liquid by replacing the solid substrate with low-vapor oil in a plasma sputtering apparatus. The dispersing particles in oil are much smaller than the conventional methods. Under low density of particles on oil, the intense absorbance peak is detected around 325nm and the shape and peak of absorption spectra don’t almost change following time, and the size of Au particles is steady. With higher density, the absorbance peak is stronger and shifts to visible light about 20nm. With prolonging measuring time, absorption peaks fall and a new absorption locating on around 550nm gradually appears, large numbers of particles gradually convert and produce the new particles sizes around 3-5nm. under high density, the other intense absorbance peak exists around 540nm besides the peak of 350 nm. High particles density produces the particles sizes around 5-7nm, and the contents and sizes of particles are basic invariablenes against measured period.

Synthesis of Au Nano-Particles Using Plasma Sputtering into Oil

Abstract It has been examined to disperse the Au nano-particles directly into liquid by replacing the solid substrate with low-vapor oil in a plasma sputtering apparatus. The dispersing particles in oil is much smaller than the conventional methods. Under low density of particles on oil, the intense absorbance peak is detected around 325 nm and the shape and peak of absorption spectra don’t almost change following time, and the size of Au particles is steady. With higher density, the absorbance peak is stronger and shifts to visible light about 20 nm. With prolonging measuring time, absorption peaks fall and a new absorption locating on around 550 nm gradually appears, large numbers of particles gradually convert and produce the new particles sizes around 3-5 nm. under high density, the other intense absorbance peak exists around 540 nm besides the peak of 350 nm. High particles density produces the particles sizes around 5-7 nm, and the contents and sizes of particles are basic invariablenes against measured period.

Nano-Scale Au Supported on Carbon Materials for the Low Temperature Water Gas Shift (WGS) Reaction

Au-based catalysts supported on carbon materials with different structures such as graphite (G) and fishbone type carbon nanofibers (CNF-F) were prepared using two different methods (impregnation and gold-sol) to be tested in the water gas shift (WGS) reaction. Atomic absorption spectrometry, transmission electron microscopy (TEM), temperature-programmed oxidation (TPO), X-ray diffraction (XRD), Raman spectroscopy, elemental analyses (CNH), N2 adsorption-desorption analysis, temperature-programmed reduction (TPR) and temperature-programmed decomposition were employed to characterize both the supports and catalysts. Both the crystalline nature of the carbon supports and the method of gold incorporation had a strong influence on the way in which Au particles were deposited on the carbon surface. The higher crystallinity and the smaller and well dispersed Au particle size were, the higher activity of the catalysts in the WGS reaction was noted. Finally, catalytic activity showed an important dependence on the reaction temperature and steam-to-CO molar ratio.

Nanoscale Laser Processing of Hollow Silica Microbeads Assisted by Surface Plasmon Resonance of Gold Particles

Abstract Irradiation with a nanosecond pulse laser (λ = 532 nm) was performed on spherical hollow silica particles modified with gold nanoparticles to photoexcite the localized surface plasmon resonance, resulting in the formation of pores on the shell wall accompanied with the fragmentation of the original Au particles to much smaller ones. The size of pores was tunable from 17 to 56 nm by varying the size of original Au particles on SiO2 shell from 6 to 39 nm.

Gas-phase propene epoxidation over coinage metal catalysts

Propene oxide (PO) is a very important bulk chemical and is produced on a scale of about 7.5 million tons per year. In industry, PO is produced via multiple reaction steps in the liquid phase, using hazardous chlorine or costly organic hydroperoxides as oxidants. Accordingly, development of a simple and green process to produce PO has been desired. This paper presents an overview of one-step propene epoxidation in the gas phase over coinage metal catalysts with a mixture of O2 and H2 or with molecular O2 alone as oxidant. Silver (Ag) and gold (Au) catalysts can catalyze propene epoxidation with a mixture of O2 and H2, with high selectivity, whereas copper (Cu) catalysts cannot. In this reaction Au catalysts are much more active than Ag catalysts. All the coinage metals can catalyze propene epoxidation by molecular O2, but with selectivity usually below 60%. The valence states of Cu species and the sizes of Ag particles and Au particles are of crucial importance in PO synthesis.

CuxCryOz mixed oxide as a promising support for gold – The effect of Au loading method on the effectiveness in oxidation reactions

CuxCryOz containing copper–chromium oxide spinel and CuO was used as a support for gold loaded by different methods: deposition–precipitation using KOH, deposition–precipitation using urea and gold–sol method with tetrakis(hydroxymethyl)phosphonium chloride. The technique of modification with gold determined not only the Au particle size (the smallest if gold–sol method was used) but also the number of oxygen vacancies and copper concentration on the surface. All these parameters were considered in the study of CO, glycerol and methanol oxidation. The size of gold particles is a key feature determining the activity in the oxidation of CO. Glycerol oxidation is also sensitive to this parameter but less than CO oxidation. The oxidation of methanol is not influenced by the presence of gold and it occurs on the CuxCryOz support.

Analysis of Measuring Time for Au Nanoparticle Size Measurement by Photon Cross Correlation Spectroscopy

It introduced the basic principle of measuring the size of nanoparticles by the method of Photon Cross-Correlation spectroscopy (PCCS). The measuring time for Au particle size measurement at different concentration using PCCS was studied. We measured gold particles with an average diameter of 24.5 nm at different concentration and different measuring times by PCCS. The measuring results demonstrate that for the same measuring time, the higher the concentration is, the smaller the standard deviation of repeated measurements is; for suspension of the same concentration, the longer the measuring time is, the smaller the standard deviation of repeated measurements is. It fitted the relationship between the standard deviation of measuring results and the measuring time at difference concentration. For the needed measuring accuracy, we can get the measuring time for some concentration according to the curve fitting.

Selective hydrogenation of benzaldehyde to benzyl alcohol over Au/Al2O3

We provide the first evidence of exclusive benzyl alcohol production in the continuous gas phase hydrogenation of benzaldehyde (T = 393 K; P = 1 atm) over Au/Al2O3, where activity was largely maintained for up to 15 h on-stream. XRD and DRS UV–vis have established the presence of metallic Au post temperature programmed reduction to 473 K, with a surface area weighted mean Au particle size (from TEM analysis) of 7.9 nm. Under the same reaction conditions, Ni/Al2O3 and Pd/Al2O3 were non-selective, generating toluene as the principal product via hydrogenolysis of benzaldehyde and exhibiting an appreciable temporal loss of activity. The results establish Au/Al2O3 as a promising catalyst for the production of benzyl alcohol.

Au/MCr2O4 (M = Co, Mn, Fe) catalysts in the oxidations of CO, C2, and C3 hydrocarbons

Abstract Au/MIICr2O4 (MII = Co, Mn, Fe) catalysts have been tested in the total oxidation of ethane, propane, propene at 300 °C and of CO at 35 °C, and characterized by TEM, XPS and hydrogen thermo-programmed reduction (H2TPR). The catalytic activity has been found to depend on the nature of the oxidized compound. For the CO and ethane oxidation, the activity decreases in the sequence Au/CoCr2O4 > Au/MnCr2O4 ≫ Au/FeCr2O4 and follows the sequence of the decreasing reducibility of the supports and the catalysts, estimated from H2TPR measurements. The activity decreases with the increasing Au particle size in the same order, the reactions then seem structure sensitive. No correlations between the activity and reducibility or the particle size are observed for oxidation of the C3 hydrocarbons: the sequence of the activity for propane is: Au/FeCr2O4 > Au/CoCr2O4 > Au/MnCr2O4, whereas activity in propene oxidation does not change markedly within the studied catalysts. The reaction mechanism in the oxidation of ...

Influence of gold particle size on the photocatalytic activity for acetone oxidation of Au/TiO2 catalysts prepared by dc-magnetron sputtering

Two series of Au/TiO2 materials with different gold content have been prepared by dc-magnetron sputtering on ceramic shaped pure anatase or Degussa P25 TiO2. The time of deposition was varied between 1 and 20min in order to obtain different thickness and nanoparticle size of the gold films. For comparison samples with Au loadings in the range 0.3–0.9wt% were prepared using the deposition–precipitation methodology. The obtained materials were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), DR-UV-Vis and atomic force and scanning electron microscopy techniques. The photocatalytic activity was checked in the photo-oxidation of acetone under both UV and visible irradiation conditions. Several factors were found to influence the photoactivity. An optimal Au content corresponding to a maximum activity is observed and attributed to the occurrence of adequate titania surface coverage and Au particle size. The support plays an important role and it was found that pristine anatase on which gold (average particle size 7.7nm) was deposed by dc-reactive sputtering exhibits the maximum of the activity. Samples prepared by dc-sputtering were more active than samples prepared by deposition–precipitation method. Also, the photocatalysts prepared using dc-reactive sputtering showed activity under both UV and visible light irradiation, while those prepared using the deposition–precipitation technique are active only under UV light. The most likely mechanism of gold activation of titania is that upon light absorption, gold nanoparticles inject electrons into the titania conduction band.

Selective hydrogenation of nitrocyclohexane to cyclohexanone oxime by alumina-supported gold cluster catalysts

Metal oxides (Al 2O 3, SiO 2, MgO)-supported Au cluster catalysts prepared by colloid deposition method and well established Au/TiO 2 prepared by deposition–precipitation method were tested for the selective reduction of nitrocyclohexane into cyclohexanone oxime. The activity and selectivity depended strongly on the size of Au and support material. Au/Al 2O 3 with smaller Au particle size (2.5 nm) was the most effective catalyst, exhibiting a high cyclohexanone oxime yield (86%) under mild condition (0.6 MPa H 2, 100 °C). To study the origin of support and size effects, in situ FTIR experiments for OH/D 2 exchange reaction and nitrobenzene adsorption combined with poisoning experiments using pyridine and acetic acid were carried out. It is suggested that cooperation of coordinatively unsaturated Au atoms and the acid–base pair site (Al δ+–O δ− site) plays an important role in H 2 dissociation step. Al δ+–O δ− site also acts as the adsorption site of the nitro group.

Study on morphology and photoluminescence of Au/SiO 2 nanocomposite films

Au/SiO 2 nanocomposite films were fabricated on Si (111) substrates by radio frequency (RF) magnetron sputtering technique and annealing at different temperature for 20 min (mode A) and at 1000 °C for different annealing time (mode B). The nanocomposite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). SEM results demonstrate that the size of Au crystallites in mode A first increases and then decreases, on increasing annealing temperature, according to the results of XRD spectra. Analysis of PL spectra in mode B shows that the intensity of the emission peak at 440 nm and 523 nm early increases and late decreases, with increasing annealing time at 1000 °C. The origin of the emission peak at around 440 nm was related to the size and quantity of Au particles and one of the emission peak at around 523 nm was related to the nanostructure of films in agreement with SEM imagines. Experimental results indicated that morphology, microstructure and luminescence of Au/SiO 2 nanocomposite films showed close affinity with annealing temperature and annealing time.

The studies of performance of the Au electrode modified by Zn as the anode electrocatalyst of direct borohydride fuel cell

The carbon supported Au-base electrocatalysts (Au(1−x)Znx/C, 0 ≤ x < 1) modified by Zn were synthesized by reverse microemulsion method and employed as anode electrocatalysts of direct borohydride fuel cell (DBFC). The physical and electrochemical properties were investigated by energy dispersive X-ray (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM), cyclic voltammetry (CV), chronopotentiometry and fuel cell test. The results showed that the morphologies of Au(1−x)Znx nanoparticles all were uniformly spherical no matter what Zn content changed, and the average particle size of Au(1−x)Znx bimetallics varied from 3 to 6 nm. The electrochemical measurements revealed that the Au(1−x)Znx/C electrocatalysts showed no activity toward the NaBH4 hydrolysis reaction and obviously improved the catalytic activity of borohydride oxidation. Compared with Au/C anode electrocatalyst, the stability of DBFC using the Au0.65Zn0.35/C as anode electrocatalyst was apparently improved, and the maximum power density of 39.5 mW cm−2 was obtained at 20 °C.