Deposition Precipitation Technique Research Articles

Effect of copper loading on texture, structure and catalytic performance of Cu/SiO2 catalyst for hydrogenation of dimethyl oxalate to ethylene glycol

Cu/SiO2 catalysts prepared by a convenient and efficient method using the urea hydrolysis deposition-precipitation (UHDP) technique have been proposed focusing on the effect of copper loading. The texture, structure and composition are systematically characterized by ICP, FT-IR, N2-physisorption, N2O chemisorption, TPR, XRD and XPS. The formation of copper phyllosilicate is observed in Cu/SiO2 catalyst by adopting UHDP method, and the amount of copper phyllosilicate is related to copper loading. It is found the structure properties and catalytic performance is profoundly affected by the amount of copper phyllosilicate. The excellent catalytic activity is attributed to the synergetic effect between Cu0 and Cu+. DMO conversion and EG selectivity are determined by the amount of Cu0 and Cu+, respectively. The proper copper loading (30 wt%) provides with the highest ratio of Cu+/Cu0, giving rise to the highest EG yield of 95% under the reaction conditions of p = 2.0 MPa, T = 473 K, H2/DMO = 80 and LHSV = 1.0 h−1.

Reductive dehydration of ethanol to hydrocarbons on Ni- and Au-containing nanocomposites

Monometallic and bimetallic nanocomposites M/Al2O3 (M = NiO, Au, NiO + Au) with metal contents of 0.1–0.65 wt % were prepared by the deposition-precipitation and/or impregnation technique. The surface of the composites was investigated by transmission electron microscopy (TEM), EDX, adsorption spectroscopy (AAS), and XAFS. The features of the M/Al2O3 catalysis of the reductive dehydration of ethanol at 350°C were investigated. The activity of NiO/Al2O3 containing 3.5 nm particles of NiO was 130 h−1. A strong size effect in activity was revealed for Au/Al2O3 nanocomposites with activity increasing from 341 to 1384 h−1 as the size of gold particles decreased from 13–20 to 5 nm. The selectivity of 5-nm particles of Au/Al2O3 to the fraction of C3-C8 hydrocarbons was 11.69 mass %. The growth of M particles due to the segregation of individual metals and/or the formation of mixed (Au + NiO) particles resulted in an increase in the selectivity of C3-C8 up to 34.19 mass %. The causes of the observed catalytic phenomena are discussed considering the reaction mechanisms and the specific structure features of the obtained catalysts.

Catalytic transformation of glycerol on several metal systems supported on ZnO

Different metal systems consisting in platinum supported on several reducible supports (5% by weight) were synthesized through the deposition–precipitation technique and tested for glycerol hydrogenolysis. Interestingly, supports exhibiting the highest conversions were those with the greatest strong metal–support interaction (SMSI) effect, ZnO and SnO2, eventually forming alloys (Pt–Zn and Pt–Sn, respectively). ZnO was subsequently selected for further studies as support for Rh, Pt, Pd and Au and the resulting solids were tested again for glycerol catalytic transformation under reductive or inert atmosphere at 453K. Under similar reaction conditions, glycerol conversion order followed the sequence Pt>Rh≫Pd≫Au. Moreover, the solids reduced at 473K were more active than those activated at 673K, which evidences the detrimental effect of the increase in metal particle size and/or alloy formation on the catalytic performance. Quite high yields to lactic acid were achieved in a basic medium (e.g. 68% for Rh/ZnO-473 under H2) whereas yield to 1,2-PDO was more modest (26% for Rh/ZnO-673 under similar reaction conditions).

Noble Metal‐Modified Porous Titania Networks and their Application as Photocatalysts

AbstractIn order to enhance the photocatalytic activity of titania materials, which suffer from high percentages of photon‐induced electron and hole pair recombination, noble metal (Pd, Au, Ag, and Pt) modified porous titania materials were prepared. A modified deposition precipitation technique was used to deposit the metal onto a porous TiO2 support preformed using an agarose gel templating technique coupled with sol‐gel chemistry. The final composites were characterized by use of SEM–EDX, TEM, XRD, diffuse reflectance UV/Vis spectroscopy, X‐ray photoelectron spectroscopy, FTIR, inductively coupled plasma mass spectroscopy, zeta potential, and secondary ion mass spectrometry. The photocatalytic activity of the samples was investigated by monitoring changes in methylene blue concentration under UV light irradiation. The photocatalytic activity was enhanced significantly, relative to the pure titania support, by Pd (63 %) and Au (31–37 %), and slightly by Ag (24 %) and Pt (12 %) at the respective optimum metal deposition amount. A uniform distribution of metal in the TiO2 network, the metallic form of the noble metal, and an increased organic pollutant adsorption contributed positively to the photocatalytic activity of the final materials.

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.

Solvent-free selective oxidation of primary alcohols-to-aldehydes and aldehydes-to-carboxylic acids by molecular oxygen over MgO-supported nano-gold catalyst

Magnesium oxide supported nano-gold catalyst (prepared by the homogeneous deposition precipitation technique) showed high activity/selectivity and excellent reusability in the oxidation of different primary alcohols and aldehydes to corresponding aldehydes and carboxylic acids, respectively, by molecular oxygen (under atmospheric pressure) in the absence of any solvent. Influence of the catalyst calcination temperature (400–900 °C), reaction temperature (50–120 °C) and use of different solvents (viz. toluene, p-xylene, DMF or DMSO) on the oxidation reaction has also been studied.

Water–gas shift activity of ceria supported Au–Re catalysts

Abstract Au–Re/ceria, Au/ceria and Re/ceria catalysts were prepared using deposition precipitation and impregnation techniques for Au and Re addition, respectively, except the sample prepared by sequential impregnation. Catalysts were characterized by HRTEM-EDS, SEM-EDS, XPS and XRD. WGS activity tests on the samples were performed in the temperature range 200–450 °C. The effects of Re incorporation, metal addition sequence, space velocity and H 2 O/CO ratio on the catalytic performance were investigated. The novel Au–Re/ceria catalysts showed high activity in WGS reaction, especially at high H 2 O/CO ratios, led by the presence of catalytically active and steam tolerant sites formed on the bimetallic catalysts.

Preparation of ultra low loaded Au catalysts for oxidation reactions

Cyanide leaching was used to obtain Au/SiO2 catalysts with very low gold loadings. A number of catalysts with a nominal 5 wt% target loading were prepared using impregnation and deposition precipitation techniques and these were found to be active catalysts for the solvent-free aerobic oxidation of benzyl alcohol. Exposure of these catalysts to a basic solution of NaCN for 10 min leached gold from the materials to give very pale pink catalysts which were found to contain just 0.06 wt% Au. The concentration of Au removed from the catalyst was constant regardless of the length of NaCN exposure. When the cyanide leached materials were employed for benzyl alcohol oxidation under the same conditions as the unleached parent catalysts, the conversions were identical. However, when the catalytic activity was normalised to the Au content (determined by ICP analysis) the TOFs were much higher for the NaCN treated catalysts (> 400,000 h(-1)). These results clearly demonstrate that NaCN leaching is an effective route to the development of catalysts containing very low gold content, whilst maintaining high activity. The leached materials were found to comprise metallic Au nanoparticles. The successful utilisation of ultra low loaded Au catalysts for selective oxidation, where the majority of the Au present is active and 98% of the spectator Au is removed, makes the industrial application of such materials more economically viable. Information obtained through the identification of these active structures using state of the art techniques may provide useful insights into how the reaction proceeds on the Au surface.

Impacts of MgO promoter and preparation procedure on meso-silica supported nano gold catalysts for carbon monoxide total oxidation at low temperature

The meso silica supported novel nano-size gold catalysts promoted by MgO have been synthesized using deposition–precipitation technique for the total oxidation of carbon monoxide. The effects of MgO promoter and preparation procedure on the structure and performance of gold catalysts have been investigated. Better gold dispersion, smaller average size of Au particles and more oxygen mobility were observed in the Au/MgO/SiO 2-SH catalyst using NaOH as precipitation agent, over which the turnover frequency (TOF) was much better than that of the sample without promotion. The temperature of the CO total conversion over the sample Au/MgO/SiO 2-SH is about 250 K lower than that over the silica which directly supported gold catalyst.

Sol–gel silver hexatitanates as photocatalysts for the 4-chlorophenol decomposition

Sodium hexatitanate (Na 2Ti 6O 13) and silver-supported sodium hexatitanates were prepared by the sol–gel and solid state reaction methods. Fibril structures and octagonal microbar hexatitanates were obtained. Silver nanoparticles were incorporated into the sodium hexatitanate supports by the deposition–precipitation technique. HRTEM (high resolution transmission electron microscopy) and high angle annular dark field (HAADF) digital images showed silver particles with nanometric sizes (5–7 nm). E g band gap values around 3.3–3.2 eV were obtained in the solids. UV–vis–DRS spectra showed in the visible region, the silver plasmon surface resonance which intensity was taken as an indirect way to determine the enhanced photoactivity of the semiconductors. In the 4-chlorophenol photodecomposition, it was found that the sol–gel silver fibril solid was more active ( t 1/2 = 57 min) than the silver solid state microbars ( t 1/2 = 92 min). It is proposed that the highest photoactivity showed by the sol–gel fibril preparation is an effect due to the formation of highly dispersed silver nanoparticles on the sol–gel hexatitanate support.

Acetylene hydrogenation on SiO2 supported gold nanoparticles

Acetylene hydrogenation has been investigated on 1.8 wt.% Au(I)/SiO2 and 1.9wt.% Au(II)/SiO2 catalysts prepared by fixation of Au sol to SiO2 (Aerosil 200). The mean particle size measured by TEM is 3.7 and 6.1 nm, respectively. For the sake of comparison a 2.1 wt.% Au/TiO2 sample was prepared by deposition-precipitation (DP) technique (mean particle size of Au is 3.3 nm). Transformation of acetylene was measured at 5 K/min ramp rate with gas mixtures containing the reactants at H2/C2H2=2 and 70 ratios. The C2H2 content of the gas mixture was 0.11% (0.11 kPa C2H2). The activity sequence at 423 K was: Au/TiO2>Au(I)/SiO2≫Au(II)/SiO2. Both the partial pressure of hydrogen and the temperature significantly affect the activity (acetylene conversion) and ethylene selectivity. Above 500–550 K over-hydrogenation (ethane formation) and hydrogenolysis (methane formation) decrease the ethylene selectivity. Faster deactivation and larger amount of deposit was observed on Au/TiO2 than on Au(I)/SiO2. A reaction scheme is proposed suggesting formation of sigma bonded intermediates as sp carbon hybridises to sp2 and sp3.

Nano-gold supported on Fe2O3: A highly active catalyst for low temperature oxidative destruction of methane green house gas from exhaust/waste gases

Abstract A number of nano-gold catalysts were prepared by depositing gold on different metal oxides (viz. Fe 2 O 3 , Al 2 O 3 , Co 3 O 4 , MnO 2 , CeO 2 , MgO, Ga 2 O 3 and TiO 2 ), using the homogeneous deposition precipitation (HDP) technique. The catalysts were evaluated for their performance in the combustion of methane (1 mol% in air) at different temperatures (300–600 °C) for a GHSV of 51,000 h −1 . The supported nano-gold catalysts have been characterized for their gold loading (by ICP) and gold particle size (by TEM/HRTEM or XRD peak broadening). Among these nano-gold catalysts, the Au/Fe 2 O 3 (Au loading = 6.1% and Au particle size = 8.5 nm) showed excellent performance. For this catalyst, temperature required for half the methane combustion was 387 °C, which is lower than that required for Pd(1%)/Al 2 O 3 (400 °C) and Pt(1%)/Al 2 O 3 (500 °C) under identical conditions. A detailed investigation on the influence of space velocity (GHSV = 10,000–100,000 cm 3 g −1 h −1 ) at different temperatures (200–600 °C) on the oxidative destruction of methane over the Au/Fe 2 O 3 catalyst has also been carried out. The Au/Fe 2 O 3 catalyst prepared by the HDP method showed much higher methane combustion activity than that prepared by the conventional deposition precipitation (DP) method. The XPS analysis showed the presence of Au in the different oxidation states (Au 0 , Au 1+ and Au 3+ ) in the catalyst.

Preparing Au/ZnO by precipitation–deposition technique

Abstract This report presents results related to the preparation of Au/ZnO using the deposition–precipitation technique. Many samples were synthesized taking into account different pH values, gold concentration, procedures and contact times between the gold precursor and support. The exposure of gold compounds to radiation in the visible range was also examined during the preparation. These syntheses show that precipitation–deposition is, in this case, an ionic exchange. It could be observed that exposure to light and competition among anions change the gold content of the catalyst. Therefore, these are important parameters in the preparation of gold catalyst by ionic exchange. The results also show that by using this technique it is possible to prepare active catalysts for the preferential oxidation of carbon monoxide (PROX) reaction.

Magnetron sputtering of gold nanoparticles onto WO 3 and activated carbon

In this paper we describe the production and investigation of two supported gold catalyst systems prepared by magnetron sputtering: Au on WO 3 and Au on activated carbon. The magnetron sputtering technique entails using an argon plasma to sputter a high purity gold target producing a flux of gold atoms which are deposited onto a constantly tumbling support material. This technique offers a number of advantages over conventional chemical preparation methods. One advantage is the ability to create gold nanoparticles (diameters <3 nm) on unusual support materials, such as WO 3 and carbon, which are generally not accessible using the ubiquitous deposition-precipitation technique. We present data demonstrating the formation of catalytic gold nanoparticles with average diameters of 1.7 nm (Au/C) and 2.1 nm (Au/WO 3), as well as a substantial number of single atom species on the Au/C sample. Prototypical carbon monoxide oxidation (Au/WO 3) and glycerol oxidation (Au/C) reactions were performed in order to gauge the activity of these catalysts. The WO 3 supported catalyst exhibits substantial catalytic activity from room temperature to 135 °C (0.0018–0.082 mol CO/mol Au s) with an activation energy near 23 kJ/mol. The activity of the Au/C catalyst was compared to a Au/C catalyst prepared from a poly(vinyl alcohol) (PVA) sol. The smaller catalysts prepared by sputtering are more active than the large gold particles prepared using the PVA sol, however the larger gold nanoparticles are substantially more selective towards the production of intermediate products from the oxidation of glycerol.

Nanocatalysis on Tailored Shape Supports: Au and Pd Nanoparticles Supported on MgO Nanocubes and ZnO Nanobelts

Active gold and palladium nanoparticles supported on MgO nanocubes and ZnO nanobelts and transition-metal-containing MgO nanobelts were synthesized by combining evaporation and deposition-precipitation techniques. The high activity and stability of the Au/CeO2 and Pd/CeO2 nanoparticle catalysts deposited on the MgO cubes are remarkable and imply that a variety of efficient catalysts can be designed and tested using this approach. The significant increase in the concentration of corner and edge sites in MgO nanocubes make them well-defined supports to study the detailed mechanism of the catalytic activity enhancement.

Preparation of Fischer–Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation

Homogeneous deposition-precipitation on either a silica or carbon nanofiber (CNF) support of cobalt from basic solution using ammonia evaporation was studied and compared with conventional deposition from an acidic solution using urea hydrolysis. In the low-pH experiment, the interaction between precipitate and silica was too high; cobalt hydrosilicates were formed requiring a reduction temperature of 600 °C, resulting in low cobalt dispersion. Lower interaction in experiments performed in a basic environment yielded a well-dispersed Co 3O 4 phase on silica, and after reduction at only 500 °C, a catalyst with 13-nm cobalt particles was obtained. On CNF from an acidic solution, cobalt hydroxy carbonate precipitated and displayed a low interaction with the support resulting after reduction at 350 °C in a catalyst with 25-nm particles. From basic solution we obtained high dispersion of cobalt on the CNF, probably related to the greater ion adsorption. After drying, Co 3O 4 crystallites were obtained that, after reduction at 350 °C, resulted in a catalyst with 8-nm Co particles. Samples prepared in the high-pH experiment had 2–4 times higher cobalt-specific activity in the Fischer–Tropsch reaction than their low-pH counterparts. CNF support materials combined with the high-pH deposition-precipitation technique hold considerable potential for cobalt-based Fischer–Tropsch catalysis.

Highly active gold-ceria catalyst for the room temperature oxidation of carbon monoxide

Ceria supported gold catalysts containing 0.1–5 wt.% gold are prepared by deposition–precipitation technique in which the precipitate is subjected to ultrasound treatment and aging in the parent solution at 65 °C for 2 h prior to filtration and drying. The catalysts are found to be very active for the room temperature oxidation of CO. Au/CeO 2 catalyst containing as low as 1 wt.% Au is found to be a highly active catalyst for the room temperature oxidation of CO to CO 2 with complete conversion of CO and specific activity up to 490 × 10 −4 mol CO s −1 g Au −1. The catalyst shows good stability and sustains its high activity for a prolonged period. Aging of the catalyst in the parent solution is found to be important in obtaining a highly active catalyst. The catalysts are characterized by XRD, SEM, TEM, XPS and TPR/TPD. TPR, TPD and XPS studies suggest that the active site for CO oxidation may be a combination of Au + OH − and metallic gold in interface with reduced Ce 3+ sites. In other words, the presence of highly dispersed gold particles in close contact with the ceria surface defects is vital for the superior performance of the catalyst.

Au/Fe 2O 3 nanocatalysts for CO oxidation: A comparative study of deposition–precipitation and coprecipitation techniques

Two synthetic approaches to the preparation of Au/Fe 2O 3 nanocatalysts were compared: a novel deposition–precipitation technique utilizing thermal decomposition of urea and the conventional coprecipitation method. The products were characterized by X-ray powder diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and temperature programmed reduction. The catalysts prepared by both methods demonstrated very high activity toward CO oxidation in the “as prepared” state. Starting with the same amount of gold precursor, the deposition–precipitation technique produces a catalyst with higher final gold content than the coprecipitation method because it achieves more complete precipitation of the gold from solution. Heat treatment of the samples prepared by both methods resulted in a decrease of the catalytic activity by about an order of magnitude. However, we conclude that heat-treated Au/Fe 2O 3 catalysts are more suitable for practical applications because of stability issues with the “as prepared” samples.

Synthesis of PdO/CeO 2 mixed oxides catalyst for automotive exhaust emissions control

PdO/CeO 2 mixed oxide catalysts were synthesized using a modified deposition-precipitation technique. The catalyst was quite effective in oxidizing hydrocarbons (HC) and carbon monoxide (CO), but was not efficient in reducing nitrogen oxides (NO x ). The light-off temperature for CO and HC under stoichiometric conditions was about 130 °C. The presence of CeO 2 enhanced the low temperature activity of the catalyst. About 100% CO conversion was achieved at 235 °C, whereas the conversion for HC reached 77.8% at 245 °C. The promoting effect of CeO 2 also improved the catalyst’s ability to operate under lean conditions. The catalyst was able to reduce up to 80% of CO at 250 °C and 66% of HC at 315 °C. The efficiency of the catalyst in reducing NO x was low and the light-off temperature was not reached over the duration of the experiment. The catalyst started to reduce NO x only above 240 °C. The highest efficiency of about 27% was achieved at 275 °C.

Au/Fe2O3 nanocatalysts for CO oxidation by a deposition–precipitation technique

Nanoscale Au/Fe2O3 catalysts for CO oxidation were prepared by a deposition–precipitation techniqueusing the thermal decomposition of urea. Freshly precipitated amorphousFe(OH)3 was used as a support. The resulting products were characterized by transmission electronmicroscopy and x-ray powder diffraction. The catalysts in the ‘as prepared’ statedemonstrated exceptionally high activity toward CO oxidation. Compared to conventionalcoprecipitation with the same amount of gold precursor, the deposition–precipitationmethod was shown to produce more active catalysts because it allows completeprecipitation of gold from solution.