(571c) In-Situ Ftir and Xas Study of the Evolution of Surface Species during Transient Co Oxidation on Supported Au/Tio2 Conference: AIChE Annual MeetingYear: 2005Proceeding: 2005 AIChE Annual MeetingGroup: Catalysis and Reaction Engineering DivisionSession: Fundamentals of Supported Catalysis I Time: Friday, November 4, 2005 - 8:45am-9:05am Authors: Henao, J. D., Northwestern University Caputo, T., Università di Napoli Federico II Yang, J. H., Northwestern University Kung, M. C., Northwestern University Au/TiO2 catalyst prepared by deposition-precipitation of HAuCl4, is remarkably active for CO oxidation at low temperature. The as-prepared catalyst is inactive; it is activated by contact with flowing H2 at room temperature. Transient experiments involving the saturation of the catalyst either with CO or O2, followed by admission of O2 or CO, are performed. In-situ FTIR and XANES spectroscopy are used to monitor the development of surface species; the steady state CO oxidation activity of the catalyst is measured by gas phase FTIR. Exposure of the activated catalysts to 10 mbar of CO at -60 oC, leads to the formation of CO adsorbed species over only 13% of the gold. The process is verified by the development of a composite band at 2091 cm-1, which is mainly due to CO adsorbed on Au0 step sites close to the contact perimeter with the support. These species are very reactive toward CO2 in the presence of oxygen; they can also be slowly removed by He, undergoing transformation to less labile adsorbed species as the CO coverage decreases. Purging of a CO-saturated catalyst with He, reduces the coverage of gold to 5% after 30 min. These less labile species appear at 2101 cm-1; they correspond to CO adsorbed on step sites on top of the Au0 particles. Admission of 25 mbar of O2 to the CO-saturated catalyst induces drastic changes on the nature and population of surface species. CO2 is readily formed at the expense of the highly active CO-Au species (2091 cm-1), at a TOF of 1.4 mol/mol Au-min. The 2091 cm-1 species are also simultaneously transformed into CO co-adsorbed with oxygen on the gold step sites, as evidenced by the development of a band centered at 2114 cm-1. The contribution of the last species to the production of CO2 is minor; they appear to act as a CO sink as the CO surface coverage decreases. XANES data show that the activation of CO by Au0, which gives place to the CO-Au species characterized by the 2091 cm-1 band, is accompanied by a transfer of charge from Au0 to CO; thereby, an increment in the white line in the x-ray absorption edge of Au is observed by exposure to CO. After admittance of O2, gold returns to its zerovalent state at the time that CO is oxidized. When CO flows through a catalyst previously saturated with O2, a lag in the appearance of CO2 is observed. CO2 production is spread over a longer time scale, with a maximum TOF of 1 mol/mol Au-min. Whether the catalyst is initially saturated with CO or O2 before admitting the other reactant, does not affect the total amount of CO2 generated. Transformation of CO into CO2 entails a hydroxocarbonyl intermediate characterized by an absorption band at 1240 cm-1. Its rate of disappearing correlates with the rate of formation of CO2. At steady state, CO2 is generated at a TOF of 1.1 mol/mol Au-min, at a CO surface coverage of 10%, corresponding to 0.75 mol CO/mol surface Au. Oxygen co-adsorbs with CO on 3% of the available gold. All the results point to the main role of metallic Au as part of the active centers.