Quantum Mechanical Treatment of Gold Adsorption/Reduction on Galena: Calculations of the Thermodynamics, Surface Diffusion, XPS and STS Spectra

Udo Becker Virginia Polytechnic Institute and State University, Dept. of Geological Sciences,

4044 Derring Hall, Blacksburg VA 24061, USA,

present address: University of Manchester, Dept. of Earth Sciences,

Oxford Road, Williamson Bldg., Manchester M13 9PL, UK


Michael F. Hochella Jr. Virginia Polytechnic Institute and State University, Dept. of Geological Sciences,

4044 Derring Hall, Blacksburg VA 24061, USA

Thermodynamics of gold adsorption/reduction and surface diffusion

Reaction energies were calculated for the adsorption of AuCl4- from solution onto the galena surface and for the stepwise reduction of gold from Au(III) to Au(0) by using an ab-initio cluster approach and by including hydration energies for all dissolved species. The adsorption of gold chloride onto galena and the reduction from Au(III) to Au(I) were found to be exothermic, but only if hydration energies are considered. Both the galena-Au(III)Cl3 and the galena-Au(I)Cl structure occupy local energy minima, that is they are potentially metastable. In the galena-Au(III)Cl3 cluster, the electron density for the bond between sulfur and gold is provided by all sulfur atoms in the cluster (approximately 1/10 of a unit charge from each sulfur atom) explaining the unique affinity and bonding mechanism of gold to sulfide surfaces. The reduction of Au(I) to elemental gold was calculated to be only exothermic if stabilized by a gold dimer formation (a gold-gold metal bond in addition to the Au-S bond). Therefore, the energetics of gold diffusion had to be examined. A diffusion path of gold which proceeds from sulfur to sulfur was chosen because galena exhibits a diffuse valence band electronic density along this path as observed with using scanning tunneling microscopy (STM) or molecular orbital calculations. Along this diffusion path, an activation energy has to be overcome which is only about four times higher than the thermal energy at room temperature. From this, it can be estimated that on average, one out of about 200 gold atoms can hop from one sulfur atom to the next at any time. This finding explains the formation of gold islands during sorption experiments.

Calculation of XPS and STS spectra

X-ray photoelectron (XPS) spectra were calculated for the adsorbed structures galena-Au(III)Cl3, galena-Au(I)Cl, galena-Au and galena-Au2. We calculated the peak chemical shifts for sulfur atoms that are bonded to gold to be very similar to peak shifts that are conventionally interpreted as polysulfides, if only bulk structures are considered for calibration of the XPS peak shift. Therefore, it can be derived from the stoichiometry of the adsorption/reduction reaction that approximately 40% of the polysulfide peak is from sulfur bonded to gold at the initial stage of the adsorption process.

Scanning tunneling (STS) spectra were calculated for gold adsorption sites in order to develop a tool that can help to identify which adsorbate structure is present at the surface and to examine the local electronic structure of such sites. The most significant difference is the change of the local band gap which decreases from a little more than 1 eV in case of galena-Au(III)Cl3 to about 0.5 eV for galena-Au(I)Cl to a metal-like local electronic environment (no bandgap) for elemental gold in galena-Au and galena-Au2 (the bulk band gap of fresh galena is on the order of 0.4 eV). The calculated STS spectrum for galena-Au2 was found to be very similar to experimental STS spectra near the rim of a gold island.