Formation of sulfite-like species on Cr 2 O 3 after SO 2 chemisorption V.A. Ranea a , S.N. Hernandez b , S. Medina b , I.M. Irurzun a , I.D. Coria b , E.E. Mola a,b, ⁎ a CCT-La Plata-CONICET, Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calle 64 y Diagonal 113, 1900 La Plata, Argentina b Facultad de Química, e Ingeniería, Pontificia Universidad Católica Argentina, Mendoza 4197 CP 2000 Rosario, Argentina abstract article info Article history: Received 9 September 2010 Accepted 3 December 2010 Available online 13 December 2010 Keywords: Density functional calculations Chemisorption Thermal desorption Sulfur dioxide Polycrystalline surfaces Low index single crystal surfaces Solid-gas interfaces The adsorption of sulfur dioxide (SO 2 ) on polycrystalline Cr 2 O 3 was experimentally investigated using temperature-programmed desorption (TPD). The chemisorption of SO 2 on the (0001) surface was also studied using theoretical methods. Different adsorption geometries were explored for SO 2 adsorption on the α-Cr 2 O 3 (0001) surface. Two similar adsorption configurations were found to be the most stable with chemisorption energies of −3.09 and −2.79 eV/molecule. In both calculated stable adsorption configurations the appearance of sulfite-like species is predicted on the (0001) surface after adsorption. It is important to emphasize that these results are predicted only within the DFT + U framework. Under these conditions and despite great efforts, no stable sulfate-like geometry was found on this surface. The TPD spectrum exhibit a desorption peak at T p ≈870 °C with a heating rate of β ≈0.12 °C/s. The desorption energy calculated by the analysis given by Redhead and Adams, assuming the rate of desorption is given by a Polanyi–Wigner equation, is ≈−3.12 eV. This value is in good agreement with the predicted one using DFT + U calculations. To our knowledge, this is the first theoretical study of SO 2 adsorption on the Cr 2 O 3 (0001) surface. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The metal oxide surfaces are of great interest in topics such as catalysis and corrosion. Environmental science has received much attention and catalytic surfaces have been used in order to eliminate harmful molecules (such as SO 2 , NH 3 , CO and NO x ) from air due to pollution. Among these molecules, SO 2 is one of the products emitted to the atmosphere from natural and human sources that can be converted into acid rain. Most of the SO 2 that reach the atmosphere (≈3/4) is produced by human activities, mainly, the combustion of fossil fuels. More than half of the world production comes from a few developed countries. Looking for the best surface to capture SO 2 , its adsorption has been investigated on several selected polycrystalline transition metal oxides (Co, Ni, Fe, V, Mn, Cr and Mo) supported on alumina [1]. At high temperature, Cr 2 O 3 is the oxide with the maximum capacity to adsorb SO 2 . Compounds formation on the surface and the desorption process have also been investigated. The surface can be regenerated by an 800 °C air passing treatment, and can recover its original adsorption capacity. It is proposed in that article [1] that the SO 2 retention on the Cr 2 O 3 surface is a chemisorption process with sulfite formation on the surface. The α-Cr 2 O 3 (0001) has been experimentally investigated using different techniques such as low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) [2–5]. The surface is Chromium-terminated and undergoes profound vertical relaxations of the external layers without lateral distortions. Different theoretical approaches have also been used to study chromia as well as the α-Cr 2 O 3 (0001) surface [6–11]. There is a general agreement among the different methodologies that the surface undergoes strong vertical relaxations. In this article we take into account the strong electron correlation effects described by a Hubbard-type on-site Coulomb repulsion, not included in a density functional description [10]. The inclusion of the DFT + U approach has critical importance in the description of the SO 2 adsorption on the α-Cr 2 O 3 (0001) surface. To our knowledge, this is the first theoretical investigation of SO 2 chemisorption on the α-Cr 2 O 3 (0001) surface. The work methodology, which includes the experimental setup as well as the theoretical approach, is presented in the next section. In the following section the experimental and theoretical results are reported and discussed and finally the conclusions are exposed. 2. Methodology 2.1. Experimental setup 2.1.1. Gas supply Sulfur dioxide at a concentration of 10 ppm was circulated inside our equipment. Nitrogen was the carrier gas used; it was supplied by a gas container with a gas meter to control the gas flow according to Surface Science 605 (2011) 489–493 ⁎ Corresponding author. CCT-La Plata-CONICET, Instituto de Investigaciones Fisico- químicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Calle 64 y Diagonal 113, 1900 La Plata, Argentina. E-mail address: eemola@inifta.unlp.edu.ar (E.E. Mola). 0039-6028/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2010.12.004 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/ locate/susc