Gold Supported on Oxide Surfaces: Environmental Effects as Studied by STM

This paper begins by providing a short review of STM studies of gold particles supported on oxide surfaces. Following this, the morphology of Au particles deposited on thin FeO(111) films at elevated pressures of CO, O₂, CO+O₂, and H₂ has been examined using in situ STM at room temperature. The Au particles are found to be quite stable in oxygen and hydrogen environments at pressures up to 2 mbar. However, in CO and CO+O₂ atmospheres, the destabilization of Au particles located at the step edges occurs leading to the formation of mobile Au species, which migrate across the oxide surface. General problems encountered with high-pressure STM studies are discussed, and data clearly showing the effects of ambient gas impurities is provided. These effects may lead to erroneous conclusions, particularly about morphological changes of and CO dissociation on the gold surfaces at elevated pressures.     

Size and Support Effects for CO Adsorption on Gold Model Catalysts

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (3 nm), CO desorption temperature (at 170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.     

Initial Stages of Growth of Alumina on NiAl(001) at 1025 K

The initial stages of growth of ordered layers of Al₂O₃ on NiAl(001) single-crystal surfaces at 1025 K and 10⁻⁷ mbar (10⁻⁵ Pa) in O₂ have been studied using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM results evidence the formation of elongated strips (26 Å wide and 11 Å high) of Al₂O₃ oriented along the [100] and [010] directions of the substrate. With longer oxidation, the substrate is increasingly covered by rectangular and striped islands resulting from the vicinal and parallel growth of the strips. On the ultrathin oxide film formed after 500 L (1 L = 10⁻⁶ torr·s (∼1.33 × 10⁻⁴ Pa·s)) of exposure, STM atomic resolution images have been obtained for the first time. They evidence the [001] orientation of the oxygen sublattice in Bain epitaxy on the substrate. The observation of one-dimensional atomic trenches together with the strips observed on the nanometer scale is consistent with the growth of θ-Al₂O₃. The STS local measurements evidence the insulating behavior of the oxide layer formed with a gap value ranging from 7 and 8 eV for amorphous and ordered domains, respectively.     

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.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

Synthesis of doped ceria with mesoporous flowerlike morphology and its catalytic performance for CO oxidation

Mesporous flowerlike ceria Ce_0.9M_0.1O_2−δ (M = Y, La, Zr, Pr and Sn) have been synthesized successfully by a hydrothermal method. The impacts of doping on their physical properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, Raman and X-ray photoelectron spectroscopy (XPS). The doped materials show relatively high stability against the grain growth at 800 °C under reducing and oxidizing atmosphere. The catalytic activities of all flowerlike ceria materials for CO conversion are quite high (200 < T₅₀ < 320 °C) due to their high specific surface area (>100 m²/g), open mesoporous structure (pore size  3.9 nm) and nano-crystalline nature (grain size < 10 nm). Among flowerlike materials, Pr and Sn doped ceria show enhanced activity while La, Y, Zr doped ceria show decreased activity compared to undoped ceria.     

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.     

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 nanoparticles on ceria: importance of O vacancies in the activation of gold

Synchrotron-based techniques (high-resolution photoemission, in-situ X-ray absorption spectroscopy, and time-resolved X-ray diffraction) have been used to study the destruction of SO₂ and the water-gas shift (WGS, CO + H₂O → H₂ + CO₂) reaction on a series of gold/ceria systems. The adsorption and chemistry of SO₂ was investigated on Au/CeO₂(111) and AuO _x /CeO₂ surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO₂(111) was 4–7 kcal/mol larger than on Au(111). However, there was negligible dissociation of SO₂ on the Au/CeO₂(111) surfaces. The full decomposition of SO₂ was observed only after introducing O vacancies in the ceria support. AuO _x /CeO₂ surfaces were found to be much less chemically active than Au/CeO₂(111) or Au/CeO_2−x (111) surfaces. In a separate set of experiments, in-situ time-resolved X-ray diffraction and X-ray absorption spectroscopy were used to monitor the behavior of nanostructured {Au + AuO _x }–CeO₂ catalysts under the WGS reaction. At temperatures above 250 °C, a complete AuO _x → Au transformation was observed with high catalytic activity. Photoemission results for the oxidation and reduction of Au nanoparticles supported on rough ceria films or a CeO₂(111) single crystal corroborate that cationic Au^δ+ species cannot be the key sites responsible for the WGS activity at high temperatures. The active sites in {Au + AuO _x }/ceria catalysts should involve pure gold nanoparticles in contact with O vacancies of the oxide.     

Copper oxide catalysts supported on ceria for low-temperature CO oxidation

Copper oxide catalysts supported on ceria were prepared by wet impregnation method using finely CeO₂ nanocrystals, which was derived from alcohothermal synthesis, and copper nitrate dissolved in the distilled water. The catalytic activity of the prepared CeO₂ and CuO/CeO₂ catalysts for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The samples were characterized using BET, XRD, SEM, HRTEM and TPR.     

Gold/hydroxyapatite catalysts: Synthesis, characterization and catalytic activity to CO oxidation

This work reports the synthesis, characterization and catalytic activity for CO oxidation of gold catalysts supported on calcium hydroxyapatite. On both, the hydroxyapatite support and the gold-supported hydroxyapatite catalyst, the CO conversion shows a peak near 100% of conversion at room temperature. The generation of structural vacancies by interaction of CO with the solid provokes the formation of peroxide species in the presence of gaseous oxygen, which seems to be responsible of this high conversion of CO at room temperature. Moreover, the influence of the pre-treatment temperature on the activity has been observed and related with the elimination of carbonate species and the generation of structural defects in the apatite structure, which are able to modify the gold oxidation state.     

Negative activation energies in CO oxidation over an icosahedral Au/Mg(OH)2 catalyst

In the course of our studies on CO oxidation over Au/Mg(OH)₂ we have discovered a catalyst which exhibits an apparent negative activation energy when studied under ultra‐dry conditions (80 ppb moisture content). A review of current literature suggests that the oxidation of CO may occur by a reaction between CO and OH radicals and not by oxygen as previously thought. Substantial differences in catalytic behaviour between low and high temperature suggest that the reaction is complex and that more than one reaction pathway is present. This revised version was published online in July 2006 with corrections to the Cover Date.     

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。     

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。     

Application of alumina supported gold-based catalysts in total oxidation of CO and light hydrocarbons mixture

Gold nanoparticles supported on alumina have been produced using the anionic exchange method and ammonia washing procedure. The catalysts are tested in the reaction of total oxidation of a mixture of light hydrocarbons and carbon monoxide in order to study the possibility of application in the reduction of cold start emissions. The obtained results are promising according to the temperature range observed for the oxidation of unsaturated hydrocarbons. The results obtained for acetylene confirms the difference of oxidation of this hydrocarbon over gold catalysts. An ageing procedure has been employed. This procedure does not affect the comportment of the catalysts versus hydrocarbon oxidation.     

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.     

Evidence of active species in CO oxidation catalyzed by highly dispersed supported gold

This paper is a brief review of suggested structures of catalytic sites and reaction mechanisms in CO oxidation catalyzed by highly dispersed supported gold. The review is focused on evidence of the sites, which has been determined primarily by infrared, X-ray absorption, and other spectroscopies, often combined with transmission electron microscopy; theory has also been applied to characterize supported gold catalysts. The literature gives strong evidence of the reaction of oxygen with CO adsorbed on zerovalent gold at low temperature, consistent with the role of such adsorbed CO in the catalytic reaction. There is also strong evidence of the involvement of cationic gold, even isolated single sites, in the catalysis at higher temperatures (e.g., room temperature). Because CO is only weakly adsorbed on zerovalent gold and more strongly adsorbed on cationic gold, it is suggested that the cationic gold sites may be kinetically more significant than the zerovalent gold sites under potentially practical reaction conditions.     

Zirconium oxide supported on Pd(100): Characterization by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) were used to characterize the structure of a model metal-supported dispersed metal oxide, ZrO₂ on Pd(100). STM images illustrate changes in the surface morphology of the ZrO₂ resulting from various chemical treatments. When the sample was treated in O₂, the ZrO₂ appeared as a smooth, featureless overlayer of varying thickness wetting the Pd. After treatment in H₂, the ZrO₂ formed non-wetting particles on the Pd, with a sharp Pd-ZrO₂ interface. TS provided a fingerprint that verified the presence of a semiconducting overlayer on a metallic support. These results appear to be consistent with X-ray absorption spectra of ZrO₂ supported on Pd black, reported elsewhere.