Journal of Catalysis 200, 298–308 (2001) doi:10.1006/jcat.2001.3209, available online at http://www.idealibrary.com on Effect of Drying Conditions of Au–Mn Co-Precipitates for Low-Temperature CO Oxidation Seung-Jae Lee, ∗ Asterios Gavriilidis, ∗,1 Quentin A. Pankhurst,† Andreas Kyek,‡ Friederich E. Wagner,‡ Philip C. L. Wong,§ and King Lun Yeung¶ ∗ Department of Chemical Engineering, University College L ondon, Torrington Place, L ondon WC1E 7JE, United Kingdom; †Department of Physics and A stronomy, University College L ondon, Gower Street, L ondon WC1E 6BT, United Kingdom; ‡Physik Department, Technische Universit¨ at M¨ unchen, D-85747 Garching, Germany; and §Material Characterization and Preparation Facility, and ¶Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China Received October 13, 2000; revised February 8, 2001; accepted February 8, 2001; published online May 15, 2001 Au–Mn co-precipitates were dried at different temperatures and in different atmospheres. The co-precipitates were tested for low- temperature CO oxidation without any additional thermal treat- ment. While samples dried at 120 ◦ C in air exhibited the high- est activity for CO oxidation, samples dried at room temperature under vacuum were less active. The catalytic properties of most fresh samples improved after the first light-off test, during which a temperature of 400 ◦ C was reached. To investigate the physical and chemical state of gold and manganese support, dried samples were examined by 197 Au M¨ ossbauer spectroscopy, X-ray diffrac- tion (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The 197 Au M¨ ossbauer spectra showed that gold existed only in metallic form and was produced during the co-precipitation procedure. XRD and XPS analyses demonstrated that fresh samples contained manganese carbonate and oxides such as MnO 2 . The carbonate was converted to man- ganese oxides after the light-off test. TEM analysis indicated that the manganese support was present in plate-like shapes and the shape of gold particles was rectangularwith rounded corners. Au particle sizes were in the range 4.5–6.5 nm before the light-off test. After heat treatment at 400 ◦ C, catalytic activity improved and a strong interaction between gold particles and the MnO x support was induced. c  2001 Academic Press Key Words: gold catalyst; carbon monoxide oxidation; co- precipitation; drying; manganese carbonate. INTRODUCTION Gold supported on reducible oxides is known to catalyse CO oxidation at low temperatures (cf. 1, 2). This reaction could be important for CO safety gas masks (2), purifica- tion of air in CO 2 lasers (3), and CO sensors (4, 5). For example, Gardner’s group reported that Au/MnO x cata- lystsshowbetter catalyticperformance than Pt/SnO 2 (6–8). TorresSanchez et al. (9) observed 100% CO conversion on 1 To whom correspondence should be addressed. E-mail: a.gavriilidis @ucl.ac.uk. Fax: +44-20-7383-2348. Au/MnO x catalysts in H 2 stream, which can be applicable for selective CO oxidation in H 2 streams of polymer elec- trolyte fuel cells. Various preparation methods have been utilised to ob- tain active supported gold catalysts. Bamwenda et al. (10) reported that Au/TiO 2 prepared by conventional impreg- nation is considerably less active than Pt/TiO 2 prepared in the same way, but Au/TiO 2 prepared by deposition– precipitation exhibits better catalytic performance than platinum catalysts. Hutchings et al. (11) conducted durabil- itytestsofAu/ZnO prepared byco-precipitation and found that CO conversion wasgreater than 90% over 10h ofreac- tion time.Haruta’sgroup prepared highlyactive supported gold catalysts by co-precipitation (12–15), deposition– precipitation (16–19), and chemical vapour deposition (20, 21). These preparation methods can produce small Au par- ticles below 10 nm in size, which are strongly attached to metal oxide supports. Iwasawa’s group (22–25) attempted to prepare supported gold catalystsusinga gold–phosphine complex. Baiker’s group (26–28) has prepared Au/TiO 2 and Au/ZrO 2 catalysts using gold sol formed by reduc- tion ofHAuCl 4 withtetrakis(hydroxymethyl)phosphonium chloride. Co-precipitation of supported gold catalysts is usually performed by adding an aqueous solution of HAuCl 4 and a metal nitrate into an aqueous solution of Na 2 CO 3 . The co- precipitates formed are washed, dried, and calcined in air at a temperature above 250 ◦ C. However, it has been found that the preparation conditions such as precursor concen- trations, pH, calcination temperature, and atmosphere can significantly influence catalyst activity for CO oxidation (7, 12, 29, 30). For Au/MnO x catalysts, Hoflund et al. (7) suggested that the ratio of Au to Mn in the precursor solution must be properly chosen to obtain high CO conversion. In their study, they reported that the optimum Au to Mn molar ratio in the precursor solution is 1:10. This observation is 0021-9517/01 $35.00 Copyright c  2001 by Academic Press All rights of reproduction in any form reserved. 298