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.     

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.     

Gold supported on well-ordered ceria films: nucleation, growth and morphology in CO oxidation reaction

Structure of gold nanoparticles formed by physical vapor deposition onto thin ceria films was studied by scanning tunneling microscopy (STM). Gold preferentially nucleates on point defects present on the terraces of the well-ordered, fully oxidized films to a low density. The nucleation expands to the terrace step edges, providing a large variety of low-coordinated sites. Only at high coverage, the Au particles grow homogeneously on the oxygen-terminated CeO₂(111) terraces. The morphology of Au particles was further examined by STM in situ and ex situ at elevated (up to 20 mbar) pressures of O₂, CO, and CO + O₂ at 300 K. The particles are found to be stable in O₂ ambient up to 10 mbar, meanwhile gold sintering emerges at CO pressures above ∼1 mbar. Sintering of the Au particles, which mainly proceeds along the step edges of the CeO₂(111) support, is observed in CO + O₂ (1:1) mixture at much lower pressure (∼10⁻³ mbar), thus indicating that the structural stability of the Au/ceria catalysts is intimately connected with its reactivity in the CO oxidation reaction.     

纳米金催化剂在CO低温氧化和选择性氧化中的研究进展

介绍了纳米金催化剂在CO低温氧化和丙烯直接环氧化反应中的研究进展。在CO低温氧化反应中,催化剂的活性相和载体都具有明显的尺寸效应,纳米金颗粒和载体之间的相互作用主要表现载体不仅可以改变纳米金颗粒的大小和形状,而且也影响了氧的活化,从而提高反应活性;在丙烯直接环氧化反应中,由H2和O2在金颗粒表面反应生成的过氧化物种是反应中间体;在选择性氧化和选择性加氢反应中,金催化剂表现出优良的活性和稳定性。     

A comparative study of the effect of addition of CeO x and Li2O on γ-Al2O3 supported copper, silver and gold catalysts in the preferential oxidation of CO

In the study described in this paper we deposited gold, silver and copper on γ-Al₂O₃ as nanoparticles (<4 nm) and investigated the behavior of these nanoparticles in the preferential oxidation of CO in presence of H₂. In addition, the effect of addition of CeO _x and/or Li₂O was investigated. All the three metals show preferential oxidation of CO at low temperatures. The oxides added to Au/γ-Al₂O₃, Ag/γ-Al₂O₃ and Cu/γ-Al₂O₃ improve the catalytic performance of the gold, silver and copper. Interesting and synergistic effects were observed when both the CeO _x and Li₂O were added. Possible mechanisms are proposed.     

CO oxidation activity of gold catalysts supported on various oxides and their improvement by inclusion of an iron component

A number of oxide-supported gold catalysts have been prepared by deposition–precipitation, with variation of the pH over a wide range, the optimum pH for high activity being 9 for TiO₂, 7.5 for Fe₂O₃, and 7 for SnO₂ and CeO₂. Whereas the activity shown by Au/TiO₂ and Au/Fe₂O₃ decreased linearly with time, Au/CeO₂ and Au/SnO₂ underwent an initial major deactivation. Addition of iron in the preparation lowered the rate of deactivation when TiO₂, SnO₂ and CeO₂ were used as supports, and imparted activity when as with Bi₂O₃ it was previously lacking. XPS revealed the existence of a broad multi-state iron-containing region, and TEM and STEM/EDX indicated that small gold particles (1.5–4 nm) were partly in contact with it. Improved stability is therefore due to gold particles being in contact with an iron phase such as FeO(OH); calcination removed the stabilisation.     

负载银催化剂的氧性质和CO氧化活性

运用ＸＲＤ、ＴＰＤ、ＴＰＲ技术研究了催化剂Ａｇ／Ａｌ２Ｏ３、Ａｇ／ＣｅＯ２、Ａｇ／ＴｉＯ２的氧性质及ＣＯ氧化活性。Ａｇ／Ａｌ２Ｏ３催化剂的ＣＯ氧化活性最高。催化剂的ＣＯ氧化活性顺序与还原易难顺序相一致，但与催化剂氧脱出顺序没有对应关系。     

Zeolite NaY-supported gold complexes prepared from Au(CH3)2(C5H7O2): reactivity with carbon monoxide

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH₃)₂(C₅H₇O₂), and their reactions with CO in a flow system at 298 K and 760 Torr were investigated by infrared (IR) spectroscopy and mass spectral analysis of the effluent gases. CH₄ and CO₂ were formed as CO flowed through the sample either steadily or as successive pulses. The results are consistent with the inferences that (a) CO reacted with the supported gold to form gold carbonyls, (b) CH₄ formed by reaction of methyl groups on gold with traces of H₂O or hydroxyl groups on the zeolite and (c) CO on cationic gold reacted with traces of O₂ and/or H₂O to form CO₂. In samples treated in steadily flowing CO, cationic gold was reduced to zerovalent gold, but the cationic gold in samples exposed to CO pulses was not reduced to zerovalent gold, although CO₂ formed. Thus, CO adsorbed on cationic gold reacts to give CO₂ in the absence of zerovalent gold, consistent with the inference that gold catalysts for CO oxidation need not contain zerovalent gold.     

Structural and chemical promoter effects of alkali (earth) and cerium oxides in CO oxidation on supported gold

Nanostructured Au/Al₂O₃ catalysts prepared by deposition–precipitation with urea were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) and tested for low temperature CO oxidation. The paper is focused on the effect of two different kinds of additives on the structure and reactivity of Au/Al₂O₃ for CO oxidation: (1) alkali (earth) metal oxides and (2) ceria. The structural properties of nanosized Au particles are drastically affected by the presence of additives. The main role of the alkali (earth) metal oxides is to stabilize the small Au particles against sintering, such that when BaO is added to Au/Al₂O₃, full CO conversion is already achieved at room temperature. For this reaction ceria addition does not result in a higher activity. However, when both BaO and CeO_x are added to Au/Al₂O₃, the catalytic performance of the multicomponent catalyst resembles that of Au/BaO/Al₂O₃.     

CO dissociation and oxidation on small supported rhodium particles: SSIMS and TPR study

The dissociation and oxidation of carbon monoxide on small rhodium particles prepared by vapour deposition of Rh on either MgO or alumina substrate has been investigated by means of static secondary ion mass spectrometry (SSIMS), and temperature programmed reaction (TPR). The intensity ratios Rh _n C⁺/Rh⁺ _n measured by SSIMS, have been used to monitor the build-up of surface carbon concentration. It was shown that a part of the CO molecularly adsorbed on clean particles undergoes dissociation during heating. The dissociation is more important for smaller particles. This behaviour is explained in terms of increase of CO dissociation probability in the case of CO adsorption near structural irregularities of a surface (edges, corners, steps). During the reaction of CO oxidation the intermediate carbon formation, which is more pronounced for smaller particles, is observed. The temperature dependent carbon concentration exhibits a maximum resulting from a counterbalance between CO dissociation first and carbon reaction with adsorbed oxygen consequently.     

On the kinetics of CO oxidation by O2 over RhI(CO)2 catalytic species anchored to a zeolitic support

The reactivity of Rh^I(CO)₂ towards CO oxidation was studied on a model Rh(0.7 wt%)/HY material. The kinetic results show that Rh^I(CO)₂ exhibit a fairly low activity. It is therefore suggested that the catalytic species responsible for the enhanced activity of Rh/Ce_0.68Zr_0.32O₂ [Manuel et al., J. Catal. 224 (2004) 269] would rather be electron-deficient Rh clusters (Rh _n ^δ+ ).     

Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2

In order to clarify the effect of metal oxide support on the catalytic activity of gold for CO oxidation, gold has been deposited on SiO₂ with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex. Comparison of Au/SiO₂ with Au/Al₂O₃ and Au/TiO₂, which were prepared by both CVD and liquid phase methods, showed that there were no appreciable differences in their catalytic activities as far as gold is deposited as nanoparticles with strong interaction. The perimeter interface around gold particles in contact with the metal oxide supports appears to be essential for the genesis of high catalytic activities at low temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO adsorption and correlation between CO surface coverage and activity/selectivity of preferential CO oxidation over supported Ag catalyst: an in situ FTIR study

In situ IR measurements for CO adsorption and preferential CO oxidation in H₂-rich gases over Ag/SiO₂ catalysts are presented in this paper. CO adsorbed on the Ag/SiO₂ pretreated with oxygen shows a band centered around 2169 cm^–1, which is assigned to CO linearly bonded to Ag⁺ sites. The amount of adsorbed CO on the silver particles (manifested by an IR band at 2169 cm^–1) depends strongly on the CO partial pressure and the temperature. The steady-state coverage on the Ag surface is shown to be significantly below saturation, and the oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir–Hinshelwood mechanism on the silver catalyst which occurs in the high-rate branch on a surface covered with CO below saturation. A low reactant concentration on the Ag surface indicates that the reaction order with respect to Pco is positive, and the selectivity towards CO₂ decreases with the decrease of Pco. On the other hand, the decrease of the selectivity with the reaction temperature also reflects the higher apparent activation energy for H₂ oxidation than that for CO oxidation.     

Characterization and catalytic activity of unpromoted and alkali (earth)-promoted Au/Al2O3 catalysts for low-temperature CO oxidation

Various unpromoted and alkali (earth) promoted gold catalysts were characterized by means of XRD, HRTEM, DR/UV–Vis and TPR. Based on the results we conclude that metallic Au is the active species in CO oxidation and that the reduction of Au³⁺ to Au⁰ proceeds below 200 °C. Pretreatment at mild temperatures, viz. 200 °C, results in the highest catalytic performance of Au/Al₂O₃ in low-temperature CO oxidation. Alkali (earth) metal oxide additives are most probably structural promoters. The best promoting effect is found for BaO.     

Ceria‐based nanomaterials as catalysts for CO oxidation and soot combustion: Effect of Zr‐Pr doping and structural properties on the catalytic activity

In this work, we investigated a set of ceria‐based catalysts prepared by the hydrothermal and solution combustion synthesis. For the first time to our knowledge, we synthesized nanocubes of ceria doped with zirconium and praseodymium. The catalysts were tested for the CO and soot oxidation reactions. These materials exhibited different surface reducibility, as measured by H₂‐TPR, CO‐TPR and Soot‐TPR, despite their comparable chemical compositions. As a whole, Soot‐TPR appears a suitable characterization technique for the soot oxidation catalysts, whereas CO‐TPR technique allows to better discriminate among the CO oxidation activities. Praseodymium contributes positively toward the soot oxidation. On the other hand, it has an adverse effect on the CO oxidation over the same catalysts, as compared to pure ceria. The incorporation of zirconium into the ceria lattice does not have a direct beneficial effect on the soot oxidation activity, although it increases the catalyst performances for CO oxidation. © 2016 American Institute of Chemical Engineers AIChE J, 63: 216–225, 2017     

Catalytic oxidation of benzene,toluene and p-xylene over colloidal gold supported on zinc oxide catalyst

Au was loaded (1.5 wt.%) on the supports (ZnO, Al₂O₃ and MgO) by a colloidal deposition method. For a range of low temperatures (50–300 °C), the catalytic activity of Au/ZnO was much greater than that of Au/Al₂O₃ and Au/MgO. In particular, for the Au/ZnO, the benzene conversion exceeded 80% at 150 °C. The results of catalyst characterization suggested that the high catalytic activity of the Au/ZnO might be attributed to the effects of strong metal–oxide interaction which is possibly originated from the small lattice parameter difference between Au {111} and ZnO {101} lattice planes.     

Gold supported CeO2/Al2O3 catalysts for CO oxidation: influence of the ceria phase

A series of low loading gold supported ceria/alumina catalysts have been prepared by the deposition–precipitation method, varying the pH of the synthesis. The catalysts were characterised by means of XRD, TEM, S_BET, XRF and UV–Vis techniques, and their catalytic activity towards CO oxidation in the absence and in presence of water in the stream, were tested. It has been found that in this low loading gold catalysts, where the metallic particles are far away one from another and the oxygen transportation is not the limiting step of the reaction, the electronic properties of the ceria phase and the structure of the metal-support perimeter more than the diameter of the gold nanoparticles is the determinant factor in the catalytic performances of the solid.     

锰改性氧化硅负载金催化剂用于醇气相选择性氧化的研究

为实现醇的高效气相选择性氧化,以四氧化三锰改性的氧化硅为载体制备了负载型金催化剂。实验结果表明,氧化硅经过四氧化三锰改性后,苯甲醇转化率和选择性明显提高。通过进一步调节四氧化三锰和金的负载量,可以实现醇的高效气相选择性氧化。当反应温度为250℃时,苯甲醇转化率和选择性可分别达到96%和98%。采用X射线粉末衍射(XRD)和透射电镜(TEM）等手段对催化剂进行了表征,结果表明,低负载量的四氧化三锰在氧化硅表面实现了单层分散,金颗粒粒径为12-14 nm,但是由于金和单层分散的四氧化三锰之间的相互作用,使得该催化剂体现出良好的醇气相选择性氧化催化性能。     

Effect of the promoter and support on the catalytic activity of Pd?CeO2‐supported catalysts for CH4 combustion

A series of palladium–ceria‐supported catalysts prepared by the sol–gel method are characterized and their catalytic results are reported. It was found that the addition of ceria to alumina produces a highly dispersed catalyst when compared with their zirconia counterparts. However, the catalytic activity of the ceria zirconia series is higher, essentially due to the larger particle size of palladium. This result can be attributed to the structure‐sensitive reaction of the combustion of methane. Copyright © 2004 Society of Chemical Industry     

Fabrication and characterization of La-added MgFe2O4 as catalyst support for CO oxidation

Fe-containing oxides can serve as excellent supports for precious metal catalysts. Therefore, we investigated the catalytic properties of noble metals supported on Fe-containing mixed oxides. MgFe₂O₄ and La-added MgFe₂O₄ (La-MgFe₂O₄) were prepared via complexation with malic acid and characterized by X-ray diffraction, N₂ adsorption–desorption, and Fe K-edge X-ray fine structure analysis. MgFe₂O₄ calcined at 400–800 °C has a spinel structure and is porous. The addition of La to MgFe₂O₄ increased the local structural disorder around Fe, suppressed grain growth, affected the pore size, and increased the specific surface area. In addition, a Pd-loaded La-MgFe₂O₄ catalyst was prepared and found to exhibit higher activity for CO oxidation than a representative Pd/γ-Al₂O₃ catalyst. Further, temperature-programmed reduction studies revealed that the reactivity of the surface lattice oxygen of La-MgFe₂O₄ was enhanced by the Pd loading. Further, diffuse reflectance Fourier transform infrared spectroscopy studies showed that the surface lattice oxygen reacted with CO to form CO₂.