Dramatic promotion of gold/titania for CO oxidation by sulfate ions P. Mohapatra, a John Moma, b K. M. Parida, a W. A. Jordaan c and Mike S. Scurrell* b Received (in Cambridge, UK) 3rd October 2006, Accepted 4th December 2006 First published as an Advance Article on the web 8th January 2007 DOI: 10.1039/b614267b The activity of gold/titania catalysts for the room-temperature oxidation of CO can be dramatically enhanced by the addition of sulfate ions to the support; it appears that anion promotion of gold may be a general phenomenon and may be related to the direct modification of active gold sites in the case of sulfate ions, as evidenced by secondary ion mass spectrometry. Gold has been known for a long time as an inert metal that does not possess much catalytic activity. However the pioneering work of Haruta et al. showed that nanoparticles of gold are catalytically highly active for CO oxidation. 1 Gold also exhibits high catalytic activity for many reactions when it is highly dispersed on the metal oxides. 2,3 In particular, CO oxidation on gold-supported oxides has been investigated in detail for its simplicity as well as on its technological importance. 4–6 The catalysts that were found to be most selective towards CO 2 contained transition-metal oxides that are also good oxidation catalysts. Gold nanoparticles have been deposited on various supports such as TiO 2 , Al 2 O 3 , ZnO, Co 3 O 4 , MnO 2 , MgO, Fe 2 O 3 , ZrO 2 etc. However, gold highly dispersed on TiO 2 was found to be more active for CO oxidation than on other supports. 2,7,8 The low-temperature oxidation of CO over sup- ported gold catalyst is thought to proceed through the reaction of CO adsorbed on the gold surface with molecular oxygen activated at perimeter sites around the gold particles. 9,10 Recent studies showed that sulfate ion could be anchored on surface of TiO 2 developing strong acidity. The sulfate ion forms SLO and O–S–O bonds in bulk and surface of TiO 2 , creating unbalanced charge on Ti and vacancies and defects in the TiO 2 network. 11,12 For this, sulfated metal oxides can be used as solid acid catalysts in heterogeneous catalytic reactions for a wide variety of applications. The crystallinity is more or less dependent on the presence of sulfate ion, but not on the percentage of sulfate loading. The crystallite size decreases in the presence of sulfate ion as SO 4 22 species could possibly interact with the TiO 2 network and thus hinder the growth of the particle. The strength as well as the number of Lewis acid sites is higher in sulfated TiO 2 . Our earlier studies 13–15 revealed that the method of preparation, source and concentration of sulfate ions affected the surface, textural as well as catalytic activities of TiO 2 . The method of preparation plays an important role towards the physicochemical properties and catalytic activity of the catalyst. Several approaches have been considered for the synthesis of gold nanoparticles on metal oxide supports such as co-precipitation, deposition precipitation, gas phase grafting, liquid-phase grafting 16 co-sputtering 17 colloidal mixing 7 etc. Similarly the support TiO 2 can be prepared by methods such as hydrolysis of titanium salt, sol–gel, microemulsion etc. There is an advantage of synthesizing TiO 2 by gelling titanium alkoxides as in such preparations specific surface area, titania crystallite size and crystalline titanium phase can be controlled. Final properties of sol–gel catalysis can be controlled by varying the hydrolysis pH. 18 The objective of the present research is to study the effect of method of preparation of the support and the effect of sulfate ion on the catalytic activity of gold-promoted TiO 2 towards CO oxidation. During the course of our study we noted that an unexpected promotional effect of nitrate ions on Au/TiO 2 for CO oxidation was reported by Hutchings and co-workers. 19 The titania was a high purity form, prepared from Ti(IV) isopropoxide.{ Sulfate was introduced by impregnation with dilute sulfuric acid and gold subsequently introduced using the single- step borohydride method. 20 The dramatic effect of sulfate treat- ment on CO oxidation activity, recorded at room temperature is shown in Fig. 1, where a .5-fold higher activity is found for relatively low sulfate loadings. We note that the absolute rate of CO oxidation on the unsulfated catalyst compares favour- ably with the highest rates found for other Au/TiO 2 catalysts, including those based on the use of Degussa P25 titania (specific rates of CO oxidation at 303 K are 0.05 (this work) and 0.10 mol CO (mol Au) 21 s 21 for P25 based catalysts (see also ref. 25). The promotional effect of sulfate is clearly not due to the base catalyst being of unusually low activity. We further note that over several hours on stream none of the catalysts exhibited any marked tendency to increase or decrease their activity (less than 5% change relative in the absolute CO conversion over ca. 10 h). In an attempt to understand something of the origins of the sulfate promotional effect we have established that there are no dramatic a Colloids & Materials Chemistry Cell, Regional Research Laboratory (CSIR) Bhubaneswar, Orissa, 751 013, India b Molecular Sciences Institute, School of Chemistry, University of the Witatersrand Johannesburg, PO WITS, South Africa 2050 c National Metrology Laboratory, CSIR, Pretoria, South Africa 0001 Fig. 1 Activity at 303 K for the various catalysts for CO oxidation as a function of the sulfate content. COMMUNICATION www.rsc.org/chemcomm | ChemComm 1044 | Chem. Commun., 2007, 1044–1046 This journal is ß The Royal Society of Chemistry 2007 Published on 08 January 2007. Downloaded by University of the Witwatersrand on 02/03/2017 07:49:46. View Article Online / Journal Homepage / Table of Contents for this issue