J. Phys. Chem. B 2000, 104, 4649-4652
4649
Equilibrium Surface Composition of Sulfuric Acid Films in Contact with Various Atmospheric Gases (HNO3, CO2, CH2O, Cl2, NO, NO2) D. Howard Fairbrother* and Gabor A. Somorjai Department of Chemistry, UniVersity of California, Berkeley, CA 94720 ReceiVed: August 16, 1999; In Final Form: February 25, 2000
Differentially pumped X-ray photoelectron spectroscopy has been used to study the surface composition of sulfuric acid (H2SO4) and sulfuric acid/water mixtures in equilibrium with various atmospheric species. Nitric acid uptake in sulfuric acid solutions was observed below ≈ 220K. Compared to the theoretically predicted bulk composition, a significant enrichment of nitrogen was observed at the interface. The uptake of background water in sulfuric acid below 200 K was responsible for the reversible uptake of CO2, moderated through the formation of carbonic acid (H2CO3). For CH2O, Cl2, NO, and NO2, no observable surface species were observed over the temperature range studied (≈180-298 K).
I. Introduction
II. Experimental Section
In light of its significance in atmospheric chemistry, the uptake characteristics of molecules in contact with sulfuric acid solutions have been the focus of recent scientific interest.1 Studies by Nathenson et al.2 have also explored the detailed collision and solvation dynamics associated with molecular encounters at the sulfuric acid surface.3 The uptake of a molecule A from the gas phase into solution in its simplest form can be represented by the equilibrium:
The apparatus employed in this study has been described previously.6 Briefly, differentially pumped Auger electron and X-ray photoelectron spectroscopies (AES and XPS) were used to provide surface sensitive composition information on sulfuric acid surfaces. Initial experiments using AES were hampered by the ease with which the electron gun filament burned out under a partial pressure of H2SO4. Furthermore, AES measurements were often accompanied by significant surface charging, preventing routine spectral acquisition. These problems were largely overcome by the use of XPS, a photon-based spectroscopic probe. A. Preparation of Sulfuric Acid Surfaces. Sulfuric acid (Aldrich, 99.75%) surfaces were prepared under ambient conditions by applying the liquid directly to a hollowed gold cup.6 Following evacuation with a diffusion pump (5300 L/s), steadystate base pressures of 0.95.7 This assertion of an almost pure sulfuric acid surface has been supported by separate AES and XPS measurements made on the liquid surface at 290 K which revealed an O:S ratio of ≈ 4:1.6,8 B. Surface Composition Measurements. Surface uptake experiments were carried out in a constant partial pressure of the atmospheric species using QMS measurements to verify gas purity. Atmospheric gases were introduced into the chamber through a stainless steel leak valve using a stainless steel dosing line. Although we are dealing with a multicomponent system the situation is simplified by the low vapor pressure of sulfuric acid (1.4 × 10-5 Torr at STP).9 Consequently, the vapor phase is composed only of the atmospheric species of interest along with background water. The composition of the sulfuric acid surface therefore reflects the bulk uptake characteristics of the gas-phase atmospheric molecules, moderated by any surface specific effects. XPS measurements were made continuously as the surface was systematically cooled over the range indicated in Table 1. At each data point the system was allowed to equilibrate for
A(gas) h A(liquid),
H ) [A(liquid)]/[A(gas)], (H: Henry’s law constant)
However, this equilibrium is often modified by solvation and chemical reactions within the solution. Under these circumstances it is customary to replace H by an effective Henry’s law equilibrium constant H* to encompass all of the different forms of A in solution. At the surface, additional effects, such as surface tension, can also moderate the relative concentration of species.4 These considerations serve to highlight the possibility that situations could exist where the surface composition differs substantially from that of the bulk. Since heterogeneous atmospheric catalysis is mediated by reactions at the surface of acidified water droplets,5 an understanding of the surface composition in these systems represents an important step in understanding the mechanisms of atmospheric reactions. We report on results obtained on the composition of sulfuric acid surfaces between 150 and 240 K held in equilibrium with a variety of atmospheric species. Nitric acid uptake was observed throughout the 150-220 K range. Results revealed a significant excess of nitrogen at the surface compared to the predicted bulk composition. The uptake of carbon dioxide into sulfuric acid was also observed below 200 K. This has been attributed to the reversible formation of carbonic acid in the presence of dilute sulfuric acid. No detectable (
