Surface chemistry of Cu(100) in acidic sulfate solutions

Abstract

The surface chemistry of Cu(100), in H 2SO 4 and acidic K 2SO 4 solutions, has been studied by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and thermal desorption spectroscopy (TDS). Experiments were conducted in an electrochemical cell coupled directly to a UHV surface analysis chamber. Structures formed on the Cu(100) surface emersed (removed) from sulfate solutions were the principal focus of this study. Structures and coverages were primarily dependent on sulfate concentration and the presence of coadsorbates such as Cl and K +. Emersion of Cu(100) from 1 mM H 2SO 4 resulted in a Cu(100)(2 × 2)−SO 2− 4 adlattice with a 1 4 coverage of sulfate. A 1 5 coverage Cu(100)( 5 × 5 ) R26.6°− SO 2− 4 structure formed upon emersion mM H 2SO 4 containing trace Cl − contaminants, and a 1 3 coverage Cu(100)(2 × 100 )− SO 2− 4 structure formed following e from 10 mM H 2SO 4. Emersion of Cu(100) from 1 mM K 2SO 4 (pH = 3.6) resulted in a Cu(100)(4 × 2 5 )− SO 2− 4 , K + surface structure, with K + and SO 2− 4 coadsorbed in a 1:1 stoichiometry, each at 1 4 coverage. No significant potential-dependent variatio in surface structure or coverage was observed when the electrode was emersed from H 2SO 4 at potentials in the double-layer charging region. For emersion from 1 mM K 2SO 4 (pH = 3.6), a reduction in K + coverage along with a change in the surface structure to a (2 × 2) occurred at positive potentials. Comparisons were made of sulfate adsorption on the low-index planes of Cu using a Cu single crystal polished on three different faces to the (111), (110) and (100) planes. This electrode was emersed from 1 mM K 2SO 4 (pH = 3.6) at several potentials. A c(8 × 2) and a diffuse (1 × 1) LEED pattern were observed on Cu(110) and Cu(111), respectively. The K + and SO 2− 4 coverages differed appreciably between the three surfaces. Significant differences were observed in the thermal desorption spectra of Cu(100) emersed from H 2SO 4 and K 2SO 4. Water desorption occurred following emersion from H 2SO 4, but not after emersion from K 2SO 2. For Cu(100) emersed from 1 mM H 2SO 4 and l mM K 2SO 4 (pH = 3.6), sulfate species decomposed into SO 2(g) and adsorbed oxygen at 670 and 780 K, respectively for the two solutions. This behavior indicates sulfate stabilization by coadsorbed K + following emersion from K 2SO 4. Lastly, TDS of an Au(100) surface following emersion from 1 mM H 2SO 4 indicated that sulfate is considerably more strongly bound to Cu(100).