Halogen-Induced Corrosion of Platinum

Abstract

The interaction of halogens with platinum surfaces is an extremely interesting problem in catalysis and semiconductor technology. Halogens serve as platinum mobilizers in both deposition and corrosion or etching processes. [PtCl 4 ] 2-adsorption from an electrolyte and subsequent reduction to metallic Pt clusters is an electrochemical pathway for obtaining highly dispersed, catalytically active Pt surfaces. 1 Halogen-induced etching of platinum surfaces is widely applied in device fabrication, where platinum functions as the electrode metal. 2,3 Cl 2 is routinely used in the etch gas, but its role is not clear. 4,5 Enhancement of etching by addition of CO in the Cl 2 etching plasma was attributed to the formation of volatile species. 6 Here we investigate the halogen-platinum surface interaction in an ultrahigh vacuum (UHV) environment by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. We show that the dry deposition of Cl 2 in vacuo on the initially clean Pt(110) surface yields an array of Cl and PtCl 4 almost identical to that from direct deposition of [PtCl 4 ] 2-onto Au(100) in an aqueous environment, as described by Kolb and co-workers. 1 Furthermore, coadsorption of other less reactive species is observed to promote corrosion by converting adsorbed chlorine into PtCl 4 . This indicates that the highly ordered Cl/PtCl 4 /metal system reported here is a universal transition state in both erosion and deposition. In addition, the results provide new insights into the role of promoters of the etching process.To study the platinum-halogen interaction, we chose the Pt(110) surface. Baking in oxygen, Ar ion sputtering, and finally cycles of low-temperature oxygen adsorption (T < 140 K) and flash desorption yielded an extremely clean (1 × 2) missing-row reconstructed surface ( Figure 1a). Halogen adsorption was carried out by means of solid-state electrolysis of AgBr or AgCl. The silver halide pellet was kept in a heated glass collimator tube, and the amount of halide molecules produced during electrolysis was calculated from the time-integrated electrolysis current. The sample was positioned in front of the collimator, thereby avoiding contamination and corrosion of the UHV chamber.First-principles DFT calculations served to determine the Br and Cl bond energies on different adsorption sites and the optimum bonding geometries. The calculations were performed using both the all-electron full-potential linearized augmented plane-wave method (as implemented in the FLAIR package) 7 and the Vienna ab initio simulation package (VASP) 8 within the local density approximation (LDA) and the generalized gradient approximation (GGA). The surface was modeled either by a single slab (FLAIR) or repeated slabs (VASP) up to 11 layers thick. In VASP, the repeated slabs were separated by vacuum layers at least 1 nm thick.The geometry was optimized until all of the forces were less than 0.01 eV/Å. More computational details have been given previously. 9...

Halogen Phases on Pd(110): Compression Structures, Domain Walls, and Corrosion

“…We interpret the appearance of these fragments as evidence of corrosion, that is, the formation of PdBr x clusters along the lines reported in ref 14 for Cl on Pt(110). The desorption spectra of Figure 4 then indicate that, at around 700 K, corrosion products start to desorb from the surface.…”

“…In general, the Pd(110) surface is much less stable against reconstruction 17 and corrosion by Br than the Pt(110) surface. 14 Next, we address half-monolayer coverage. Because local patches of c(2 × 2) structure appear already at around Θ = 0.25 ML (compare Figure 6c), one should expect a global c(2 × 2) structure to develop at Θ = 0.5 ML.…”

“…At room temperature and 0.5 ML coverage, the c(2 × 2) structure is well-ordered and stable for Br/Pt(110), 22 I/Pt(110), 23 Cl/ Pd(110), I/Pd(110), 12 Cl/Cu(110), 24,25 and I/Cu(110). 18 The exceptions are Cl/Pt(110) 14 and Br/Pd(110). For Cl/Pt(110), a (2 × 1) structure is formed that is also the low-temperature phase (ground state) of Br/Pt(110).…”

Nanoscale kinetics of asymmetrical corrosion in core-shell nanoparticles

“…The relevant galvanic etching reactions are as follows 67 : In our in situ experiment using liquid cell, O 2 can be generated by beam irradiation 68 , 69 . When Br − ions are introduced, Br − -induced oxidative etching can also occur along with galvanic etching 7 , 68 , 70 , in which Pd atoms react with Br − and form [PdBr 4 ] 2− with a standard reduction potential of 0.49 V. This halogen-induced oxidative etching is thus thermodynamically preferable than galvanic etching 71 .The relevant reaction equation is as follows 67 : This resulted in the exposure of internal Pd atoms to the electrolyte directly and enabled the reaction of Pd atoms with Br − ions. Two types of defects resulted in different etching phenomena (Figs.…”

Surface concentration dependent structures of iodine on Pd(110)

“…3 This effect on selectivity is related to the strength of the palladiumhalide interaction and the surface bond geometry. Moreover, halogens are used to etch metals 4 and semiconductors 5 and a fundamental understanding of halogen surface properties is crucial. Iodine/iodide is often used as redox couple in dye sensitized solar cells (the Grätzel cell).…”