Highlights from recent literature

have described the interesting results obtained by the addition of gold to nickel catalysts, arising from the large difference in the heats of chemisorption of NO and CO on these metals. The aim of the present research was to examine the course of NO reduction by CO and to follow the changes in the activity and selectivity of the process for alloys of varied Ni-Au composition. The results of adsorption measurements indicated that changes in the surface which binds hydrogen in Ni-Au catalysts, produced by the presence of gold, may be seen as a consequence of an overlapping of two mutually opposed factors, ie the increase in the rate of NiO reduction and surface segregation of gold. The introduction of even a small quantity of gold produces a change in the energy character of nickel centres. With these nickel-gold systems the reaction rate is limited by dissociative NO chemisorption. Title compounds are the first examples of mixed valent phases containing gold in the and 29-36, report that they have a combination of distinctive structural features of Au(I) oxides and aurides: for Au + a typical linear coordination by oxygen is found and the surroundings of Au – bear a close resemblance to the binary 1:1 aurides. In consequence, the overall structures of Rb 5 Au 3 O 2 and M 7 Au 5 O 2 can be described as intergrowths of M 3 AuO 2 and MAu (M = Rb, Cs), constituting members of a homologous series [MAu] n [M 3 AuO 2 ] with n = 2 and 4, respectively. The crystal chemistry evidence for the valence states assumed, also confirmed by Mössbauer spectroscopy, is supported by various band structure calculations (Hartree-Fock and density functional) clearly indicating the coexistence of two different oxidation states. The compounds 0.05% has and a to corrode. See Gold


Iridium-Gold Carbonylation Catalysts
A carbon-supported solid catalyst suitable for the vapour-phase carbonylation of lower aliphatic alcohols (especially methanol), ethers, esters, and ester-alcohol mixtures to produce carboxylic acids and/or their esters includes an effective amount of iridium and gold associated with a solid carbon support, and a halogen promoter, and has been patented by J.R. Zoeller, A.H. Singleton G.C. Tustin and D.L. Carver of the Eastman Chemical Company, US Patent 6177380 B1 (23 January 2001). The halogen promoter component of the catalyst is preferably vaporous, thus providing a two-phase catalyst system. Thus, activated carbon particles were impregnated with an aqueous solution of HAuCl 4 , dried at 300°C, impregnated with an aqueous solution of IrCl 3 , and dried at 300°C to give a catalyst. A 7:3 (wt) MeOH-MeI mixture at 12 mL/h was evaporated and passed over 0.5 g of the catalyst, which had been pre-equilibrated to 240°C and 250 psia, together with 25 cm 3 /min hydrogen and 100 cm 3 /min carbon monoxide. The reaction produced acetic acid and methyl acetate combined at a rate of 227 mol/kg catalyst per h, compared with 56 and 97 mol/kg catalyst per h, respectively, for catalysts containing only gold or only iridium in equivalent amounts.

Supported Gold Catalysts Prepared from a Gold Phosphine Precursor and As-Precipitated Metal-Hydroxide Precursors: Effect of Preparation Conditions on the Catalytic Performance
Supported gold catalysts which are highly active for lowtemperature carbon monoxide oxidation were prepared by using AuPPh 3 NO 3 complex as a gold-metal precursor and wet, asprecipitated iron hydroxide and titanium hydroxide as support precursors. The effects of catalyst preparation conditions on the performance for low-temperature carbon monoxide oxidation have been studied by A.I. Kozlov, A.P. Kozlova, K. Asakura, Y. Matsui, T. Kogure, T. Shido and Y. Iwasawa, Department of Chemistry, Graduate School of Science, The University of Tokyo, Japan, J. Catal., 2000, 196(1), 56-65. Metal-hydroxide precipitation conditions and heating rates of temperature-programmed calcination significantly altered the performance of the supported gold catalysts. Changes in the activities of both Au/Fe oxide and Au/Ti oxide catalysts originated mainly from changes of the gold particle size distribution. It was also found that states of the mesoporous support precursors upon attaching the gold precursor onto them and during the temperature-programmed calcination appeared to be of great importance in obtaining dispersed gold nanoparticles on oxide surfaces.

Catalytic Combustion of Volatile Organic Compounds on Gold/Iron Oxide Catalysts
Catalytic oxidation of 2-propanol, methanol, ethanol, acetone and toluene was studied by S. Minico, S. Scire, C. Crisafulli, R. Maggiore and S. Galvagno, Dipartimento di Scienze Chimiche, Universita di Catania, Italy, Appl. Catal., B, 2000, 28(3,4), 245-251, on coprecipitated Au/Fe 2 O 3 catalysts in the presence of an excess of oxygen. Catalysts were very active in the oxidation of volatile organic compounds (VOCs). The high activity of these systems has been related to the capacity of highly dispersed gold to weaken the Fe-O bond thus increasing the mobility of the lattice oxygen which is involved in oxidation of the VOCs, probably through a Mars-van Krevelen reaction mechanism.

Influence of Gold Content in Nickel Catalysts on the Course of the Carbon Monoxide + Nitric Oxide Reaction
W. Cwikla and D. Nazimek of the Faculty of Chemistry, University of Marie Curie-Sklodowska, Lublin, Poland, Technol. Chem. Przelomie Wiekow, 2000, 627-630, have described the interesting results obtained by the addition of gold to nickel catalysts, arising from the large difference in the heats of chemisorption of NO and CO on these metals. The aim of the present research was to examine the course of NO reduction by CO and to follow the changes in the activity and selectivity of the process for alloys of varied Ni-Au composition.
The results of adsorption measurements indicated that changes in the surface which binds hydrogen in Ni-Au catalysts, produced by the presence of gold, may be seen as a consequence of an overlapping of two mutually opposed factors, ie the increase in the rate of NiO reduction and surface segregation of gold. The introduction of even a small quantity of gold produces a change in the energy character of nickel centres. With these nickel-gold systems the reaction rate is limited by dissociative NO chemisorption.

Highly Dispersed Supported Gold Catalysts for Methanol and Formaldehyde Oxidation
In order to develop a catalyst system for the complete removal of the unburned methanol and formaldehyde in exhaust gas from methanol-fuelled vehicles, the activity of various supported metal oxides or gold catalysts has been studied using a conventional flow reactor. It is reported by H.  Kongop Hwahak, 2000, 11(5), 529-534, that the activity of the gold catalysts for methanol and formaldehyde oxidation was much higher than that for supported metal oxide catalysts, eg MoO 3 /Al 2 O 3 . The order of the activity of gold catalysts for those prepared by deposition precipitation was greater than for coprecipitation which was greater than for impregnation. Gold particles on Co 3 O 4 were more active than on Al 2 O 3 . In particular, methanol was completely oxidized to carbon dioxide and water. The reaction mechanism proposed is that the oxygen atom dissociated on the active site reacts with the methanol or the intermediates adsorbed on the support adjacent to active sites, and then gold particles promote the dissociative adsorption of oxygen.

Uptake of Au(III) Ions by Aluminum Hydroxide and their Spontaneous Reduction to Elemental Gold (Au 0 )
The behaviour of AuCl 4 ions during the formation of aluminium hydroxide at pH 6 has been studied by T. Yokoyama, Y. Matsukado, A. Uchida, Y. Motomura, K. Watanabe and E. Izawa of the Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka, Japan, J. Colloid Interface Sci., 2001, 233(1), 112-116. With an increase in sodium chloride concentration, the content of gold taken up by the aluminium hydroxide decreased, suggesting that chloro-hydroxy complexes of Au(III) ion were taken up due to the formation of Al-O-Au bonds. It was unexpectedly found that the Au(III) ions taken up were spontaneously reduced to Au 0 without addition of a specific reducing reagent and then colloidal gold particles were formed. The mechanisms for the uptake of Au(III) ions by aluminium hydroxide and for their spontaneous reduction are discussed. in a molar ratio of 2:1. Some of these derivatives were characterized by x-ray diffraction studies and show unusual geometries, associated with Au-Au interactions. The latter were studied theoretically using [Se(AuPH 3 ) n ] (n-2) + (n = 2-6) models at the HF and MP 2 levels with quasi-relativistic pseudopotentials. There is good agreement between experimental and theoretical geometries. AuO 2 ] with n = 2 and 4, respectively. The crystal chemistry evidence for the valence states assumed, also confirmed by Mössbauer spectroscopy, is supported by various band structure calculations (Hartree-Fock and density functional) clearly indicating the coexistence of two different oxidation states. The compounds were synthesized by reacting binary aurides MAu and alkali monoxides M 2 O (M = Rb, Cs) with elemental gold in the required stoichiometric amounts. A further astonishing parallel to the chemistry of halogens was thus revealed. Like these, gold disproportionates upon interaction with bases.    (2), 237-247, have described transmission electron microscope observations of the effects of stress in polycrystalline gold thin films. The films exhibit a very strong {111} fibre texture, and are loaded in biaxial tensile stress. Under these conditions, each grain exhibits the same Schmid factor, within a very small variance. Despite the identical loading conditions and resolved shear stresses from grain to grain, they observed that yield was isolated in only a few grains. Yield initiates only at specific triple junctions, and the conditions required for a triple junction to be active in this manner were defined. The required conditions for yield to occur at a triple junction include (1) an appropriate slip-plane inclination relative to the grain boundaries at the junction; and

The Early Stages of Plastic Yielding in Polycrystalline Gold Thin Films
(2) at least one grain boundary of appropriate character, adjacent to the slip plane specified in (1).

Assembling Gold Nanoparticles as Nanostructured Films using an Electrophoretic Approach
N. Chandrasekharan and P.V. Kamat, Notre Dame Radiation Laboratory, IN, USA, Nano Lett., 2001, 1(2), 67-70, describe how a three-dimensional array of gold nanoparticles was assembled on a nanostructured TiO 2 template by subjecting the colloidal gold suspension to a d.c. electric field (50-500 V). By controlling the concentration of gold colloids in toluene and the applied voltage, it is possible to control the thickness of the nanostructured gold film without inducing aggregation effects. As indicated by AFM, these gold films are highly porous and consist of a nanoparticle assembly of fairly uniform size. The surface plasmon characteristics observed in these nanostructured gold films suggest that the particles retain the identity of individual nanoparticles.

Focused Electron Beam Induced Deposition of Gold
Codeposition of hydrocarbons is a severe problem during focused electron beam writing of pure metal nanostructures. When using organometallic precursors, a low metal content carbonaceous matrix embedding and separating numerous nanosized metal clusters is formed. In this work, I. Utke, P. Hoffmann, B. Dwir, K. Leifer, E. Kapon and P. Doppelt, of the Microengineering Department, EPFL-DMT-IOA, Swiss Federal Institute of Technology, Lausanne, Switzerland, J. Vac. Sci. Technol., B, 2000, 18, 3168-3171, present a new and easy approach to obtain high purity gold lines: ie the use of inorganic PF 3 AuCl as a precursor.
Electrical resistivities as low as 22 μω-cm at 295 K (ten times the bulk gold value) were obtained. This is to the authors' knowledge the best value for focused electron beam deposition obtained from the vapour phase so far. No special care was taken to prevent hydrocarbon contamination. The deposited nanostructure consists of gold grains varying in size and percolation with beam parameters.

Size-controlled Colloidal Gold Nanoparticles Dispersed in Organic Solvents
The preparation of well-size-controlled colloidal gold nanoparticles in an organic solvent has been investigated by G. After the preparation of aqueous colloidal gold particles, they changed the solvent to an organic one. This technique is required to enable a chemical reaction between the gold particles and hydrophobic molecules, since a colloidal gold solution is typically prepared in water using a reduction process. The stability of the gold particle suspension was also investigated, and found to decrease in the sequence of water, ethanol, chloroform, and benzene. A simple analysis using the determined threshold energy and the nanorod concentration showed that it takes an average of ca 60 fJ to melt a single gold nanorod. Experiments using 820 nm as well as 410 nm femtosecond laser pulses yield similar values, indicating that the laser induced shape transformation of the nanorods is independent of the irradiation wavelength and that this process is therefore photothermal in origin.  , 167(1-2), 191-198, have studied the nucleation and growth of gold clusters supported on SiO 2 /Mo(110) using XPS, low energy ion scattering, and temperature programmed desorption. Stoichiometric ultrathin SiO 2 films were synthesized and characterized by Auger electron spectroscopy and XPS. At 300 K, the growth mode of gold at fractional monolayer coverages is quasi-2D; at higher coverages, 3D growth was found with no evidence of a significant chemical interaction between gold and silica. Annealing Au/SiO 2 /Mo(110) to 1000 K led to sintering of the gold clusters. The desorption activation energies for gold coverages of <0.2 monolayer equivalent were determined by TPD to be significantly lower than the sublimation energies found for higher coverages of gold clusters and for bulk gold.

Nonlinear Optical Properties of Molecularly Bridged Gold Nanoparticle Arrays
Hyper-Rayleigh scattering spectroscopy has been used by J.P.

Fabrication of Nanostructures by Hydroxylamine Seeding of Gold Nanoparticle Templates
Hydroxylamine-seeding of colloidal gold particles was used by S.  ) were randomly deposited on a silica surface that had been modified with aminopropyltrimethoxysilane (APTS). The nanoparticles were then manipulated using a scanning force microscope (SFM) tip to produce 1-dimensional templates for gold deposition. The authors demonstrate the utility of this approach by fabricating a gold nanowire by using 13 nanoparticles as a template. This approach was also used to fabricate nanostructures in a small gap between two gold electrodes. Particles were pushed into the gap, and then gold deposition was used to connect the particles and the electrodes.

Gold Plating
Gold has been applied to surfaces of electronic components by electroplating, thermal decomposition of a screen-printed paste, vacuum deposition or thermal decomposition of an organometallic compound, or by mechanical rolling of inlay material. The plating solution usually contains KAu(CN) 2 together with substances for the stabilization of the conductivity and buffers. Grain refining additives such as As 3+ , Ti 2+ , and Pb 2+ are added in the plating solutions at low concentrations. Post-plating treatments, environmental status, health impact, and trends are outlined in this review by A. Blair, Plat. Surf. Finish., 2000, 87(8), 46, 48.

Electrocatalytic Oxidation of Phenol at Potentiodynamically Preanodized Gold Electrodes in Alkaline Medium
The catalytic effect of potentiodynamically grown oxide films on gold during the electrooxidation of phenol in an alkaline medium has been studied by P.I. Iotov and S.V. Kalcheva, University of Chemical Technology and Metallurgy, Sofia, Bulgaria, Bull. Electrochem., 2000, 16(9), 407-414. The ␣and ␤-oxides grown during the anodization provide redox mediator systems. Other advantages of the preoxidized gold electrode are connected with hindering the polymerization and easier in-situ removal of the polymer film. A tentative mechanism based on the experimental results, reference data and in particular the incipient hydrous oxide/adatom mediator model of electrocatalysis was proposed.

Proximity Effect and Multiple Andreev Reflections in Gold Atomic Contacts
The electronic transport properties of gold point contacts with superconducting aluminum leads have been investigated by E. Scheer, W. Belzig, Y. Naveh, M.H. Devoret, D. Esteve and C. Urbina, Physikalisches Institut, Universität Karlsruhe, Germany, Phys. Rev. Lett., 2001, 86(2), 284-287. The modifications induced by the proximity effect in the quasiparticle density of states at the contact region are measured by tunnel spectroscopy. The theory of transport through multiple Andreev reflections is extended to incorporate these effects and used to determine the number and transmission coefficients of the conduction channels in the contact regime. .5/1-H12.5/6, have studied the deposition chemistry of hexanethiol-stabilized gold nanoclusters selfassembled on gold and silica surfaces by reaction with a series of α,ω-alkanedithiols using XPS and electrical measurements. Testing of micron-scale devices shows that high conductivity is achieved only for a narrow range of dithiols and with a sharp efficiency threshold. Nanoscale devices exhibit similar behaviour though with much evidence of 'granularity' in the deposition.

Room-Temperature Single Electron Charging in Gold Nanoparticle Networks Formed on Biopolymer Templates
The

Metal-Insulator Transition in Stable One-Dimensional Arrangements of Single Gold Atoms
The atomic arrangement and conductance during the separation process of gold point contacts were simultaneously observed in-situ by high-resolution TEM microscopy (T. Kizuka, S. Umehara and S. Fujisawa, Department of Applied Physics, School of Engineering, Nagoya University, Japan, Jpn. J. Appl. Phys., Part 2 (2001), 40(1A/B), L71-L74). Onedimensional arrangements of gold single atoms, ie, atomic wires, appeared between two tips at the point contacts. They were stable when their length was increased up to 2.6 nm. The interatomic distance of the wires was 0.25-0.31 nm. It was found that a metal-insulator transition occurs in the wires.

Production of a Purple Pigment Based on Colloidal Gold
Z. Gasiorowski and K. Kazimierz of Mennica Panstwowa S.A., Poland, Polish Patent 177180 B1 (29 January 1999) describe how the colloidal gold pigment is produced from AuCl 3 or HAuCl 4 by reduction by using an anhydrous or hydrated Sn 2+ salt and/or its solution in the presence of an anhydrous or hydrated Sn 4+ salt and/or its solution. The reaction is carried out in the presence of a neutral solid reaction support (eg, SiO 2 ) with a surface area of >1,000 cm 2 /g. The resulting pigment is suitable for decoration of ceramics.

Preparation of Gelatin Layer with Gold Clusters in Photographic Film
A photographic process involving gold has been described by K. Kuge, M. Arisawa, N. Aoki and A. Hasegawa of the Department of Information and Imaging Sciences, Faculty of Engineering, Chiba University, Japan, Jpn. J. Appl. Phys., Part 1, 2000, 39(12A), 6550-6554. A gelatin layer film with gold clusters is produced by taking advantage of the photosensitivity of silver halide photography. Through exposure, silver latentimage specks, which are composed of several reduced silver atoms, are formed on the surface of silver halide grains in the photographic film. As the latent-image specks act as a catalyst for redox reactions, reduced gold atoms are deposited on the latent-image specks when the exposed film is immersed in a gold(I) thiocyanate complex solution for 5-20 days. Subsequently, when the silver halide grains are dissolved and removed, the gelatin layer film with gold clusters remains. The film produced by this method is purple and showed an absorption spectrum maximum of ca 560 nm as a result of plasmon absorption. The clusters continued to grow with immersion time, and the growth rate increased as the concentration of the gold complex solution was increased. The cluster diameter increased from 20 to 100 nm. By this method, it is possible to produce a gelatin film of a large area with evenly dispersed gold clusters, and since it is produced only on the exposed area, pattern forming is also possible.

Magnetic-Resonance-Compatible Balloon-Expandable Endoprosthesis (MR Stent) made from a High-Gold-Content Alloy
A balloon-expandable endoprosthesis, consisting of the following high-gold-content alloy (data in mass per cent): Au 70.0, Pt 4.4, Pd 2.0, Ir 0.1, Ag 13.5, Cu 8.8 and Zn 1.2, with other metals altogether under 0.05% and a material density of 15.7 g/cm 3 ; and a wire structure, being cut from a circular cylinder with given diameter and wall thickness by means of laser technology has been patented by A. Rübben (German Patent DE 29914244 U1 (8 February 2001).

Gold and Palladium Burden from Dental Restoration Materials
From 81 volunteers (16 without dental restorations, 65 with gold crowns or inlays) samples of saliva before and after chewing gum, were analysed for gold and palladium. G. Drasch, C. Muss and G. Roider of the Institut für Rechtsmedizin der Ludwig-Maximilians-Universität Munchen, Germany, J. Trace Elem. Med. Biol., 2000, 14(2), 71-75, report that the gold concentration in all analysed biomonitors correlates significantly with the number of teeth with gold restorations. Persons with gold restorations show maximal gold and palladium concentrations, 102-103 times higher than the background burden. The calculated maximal daily gold load in saliva (1.38 mg Au per day) reaches the range of an oral gold therapy for rheumatoid arthritis with 6 mg Auranofin (= 1.74 mg Au per day).

Dental Material Containing at Least 98% gold
This patent (U.K. Rabe, Degussa-Huls, Austria, German Patent DE 10039168, 22 March 2001) covers dental material that contains at least 98wt% gold and at least one of the following metals: rhodium, iridium, ruthenium, platinum, palladium, silver, rhenium or their combination. The preferred alloy contains 99 wt% gold and 1 wt% rhodium. The alloy can be blended with low and high melting point dental ceramics and resins and used as dental crowns.

Kinetic Electron Emission from Clean Polycrystalline Gold Induced by Impact of Slow C + , N + , O + , Ne + , Xe + , and Au + ions
Ion-induced kinetic electron emission is commonly attributed to collisions of an energetic projectile with quasi-free electrons, and to the promotion of atomic levels in binary collisions of the projectile with atomic particles in the solid. The contribution of the promotion processes to the electron emission has been estimated theoretically for all studied systems from MO correlation diagrams (J. As quasi-free electron collisional excitations have a sharp threshold at relatively high velocities of the projectiles, their contribution to the electron emission at lower impact velocities should be negligible. The authors show, however, that the partial localization of the quasi-free electrons due to the presence of the solid surface 'washes out' this sharp threshold. This can lead to one-electron excitations at low impact velocities that may be more significant than excitations due to promotion. At the lowest impact velocities, the electron emission yields conspicuously level off in some of the cases studied. Such behaviour cannot be reconciled with any existing one-electron model (including the one proposed here), as they all predict a rapid decrease of the electron emission with decreasing impact velocity. In this paper, the authors interpret the levelling-off of the yield in terms of a many-electron excitation mechanism, based on the assumption of spatial and temporal localizations of electronic excitation in the impact zone.
The models discussed in this paper are compared with experimental data on kinetic electron emission from polycrystalline gold bombarded by C + , N + , O + , Ne + , Ne 0 , Xe + , and Au + , with kinetic energies below ca 15 keV, and perpendicular incidence on the surface.

Thiosulfate Leaching of Gold
This review, covering many references, was written by M.G. Aylmore and D.M. Muir of the CSIRO Division of Minerals, Bentley, Australia, Miner. Eng., 2001, 14(2), 135-174. The ammoniacal thiosulfate leaching process for gold and silver extraction has been reviewed in terms of leaching mechanism, thermodynamics, thiosulfate stability, and gold recovery options. The application to different ore types and process options are also discussed. The thiosulfate leaching process is catalysed by copper and has several advantages over the conventional cyanidation process. Thiosulfate leaching can be considered a non-toxic process, the gold dissolution rates can be faster than conventional cyanidation and, due to the decreased interference of foreign cations, high gold recoveries can be obtained from the thiosulfate leaching of complex and carbonaceous-type ores. While difficulties remain to be overcome, thiosulfate leaching has considerable potential as an effective and less hazardous procedure for gold and silver extraction from auriferous ores.  Min. Metall., 2000, 5/2000(MINPREX 2000, 139-146 have included many references in this review. As remaining gold deposits tend to become more refractory, some form of oxidative pretreatment involving hydro-or pyrometallurgy is necessary to maintain high metal recoveries. These pretreatments include pressure oxidation, bio-leaching, or roasting. In the past, the primary criterion for selection of oxidative pretreatment relied mainly on the extent of gold recovery and relative economics. In this study, a different approach has been adopted, ie the quantitative determination of environmental performance of the oxidative pre-treatment processes bio-leaching, roasting, and pressure oxidation, to assess which had the least impact on the environment. The decreasing order of contribution to the environmental index was found to be acidification potential, toxicity, resource depletion, solid waste generation and greenhouse enhancement for bio-leaching and pressure oxidation; whereas it was acidification potential, solid waste generation, toxicity, resource depletion and greenhouse enhancement for roasting.

Environmental Performance of Oxidative Pre-Treatments in the Extraction of Gold
This study demonstrated the potential for the development of a predictive tool which could be used in determining the environmental performance, and thus the selection, optimization, and improvement of pre-oxidation processes for least impact on the environment.