Applied Catalysis A: General 462–463 (2013) 64–74 Contents lists available at SciVerse ScienceDirect Applied Catalysis A: General j ourna l h omepa ge: www.elsevier.com/locate/apcata Influence of silver on the catalytic properties of the cryptomelane and Ag-hollandite types manganese oxides OMS-2 in the low-temperature CO oxidation Mahmut Özacar a,b,∗ , Altu˘ g S. Poyraz b , Homer C. Genuino b , Chung-Hao Kuo b , Yongtao Meng b , Steven L. Suib b,c a Department of Chemistry, Science & Arts Faculty, Sakarya University, 54187 Sakarya, Turkey b Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269-3060, USA c Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA a r t i c l e i n f o Article history: Received 7 December 2012 Received in revised form 15 March 2013 Accepted 19 April 2013 Available online 30 April 2013 Keywords: Cryptomelane Ag-hollandite OMS-2 Silver doped CO oxidation a b s t r a c t Manganese oxide octahedral molecular sieves (OMS) are important materials in environmental chem- istry, electrochemistry, and heterogeneous catalysis. Cryptomelane and Ag-hollandite type manganese oxides (OMS-2) were synthesized by microwave-reflux and hydrothermal methods, respectively. In this current study, silver doping of cryptomelane and Ag-hollandite was performed using both UV irradia- tion and KBH 4 reduction methods. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials were characterized by powder X-ray diffraction, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and nitrogen sorption. Studies by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed that the produced OMS-2 have a nanofiber structure. The produced catalysts showed high activity, as high as 100% in the low-temperature CO oxidation at 140 ◦ C. The cata- lysts doped with silver by KBH 4 reduction method showed higher activities than other doping methods and template catalysts. In longtime stability tests, 80% CO conversion can be maintained for 26 h at 120 ◦ C. The high activities, and stabilities of the Ag/K-OMS-2-KBH and Ag/Ag-OMS-2-KBH were attributed to the stable presence of Ag 0 and Ag + species and the unique morphologies of the cryptomelane and Ag- hollandite nanofibers. CO oxidation is believed to follow the Mars–van Krevelen mechanism via the Ag + - O 2- - Mn 4+ ↔ Ag 0 - Mn 3+ + O 2 redox reaction. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Carbon monoxide is produced in various human activities and is a strongly toxic gas for people and the environment. These human activity sources of CO emissions are due to inadequate combustion from cooking and heating in homes, industrial waste gases, and exhausts gases from automobiles. CO is also formed in the regen- eration of hydrocarbon cracking catalysts. CO concentrations have reached much higher levels in urban areas due to the increasing number of automobiles on roads [1–5]. CO is formed as an interme- diate reaction product during combustion. CO emissions are caused by incomplete combustion of fuels such as hydrocarbons, gas, char- coal, and wood. Incomplete combustion most often occurs in the ∗ Corresponding author at: Department of Chemistry, Science & Arts Faculty, Sakarya University, 54187 Sakarya, Turkey. Tel.: +90 2642956041. E-mail address: mozacar@hotmail.com (M. Özacar). cases of too low air-fuel ratios, low combustion temperature or too short reaction times. CO oxidation is of practical importance for controlling the CO poisons that come from incomplete combustion processes [3,6–8]. CO emissions cause potential harmful effects on human health, vegetation, and the environment. CO is a tasteless, colorless, and odorless poisonous gas. Despite CO being a very weak direct green- house gas, there are important indirect effects on global warming. CO takes part directly in the formation of ground-level ozone and influences the atmospheric chemistry and the climate [1,3–5]. Due to the adverse effect of CO emissions on living beings and the environment, the complete elimination or decrease below the per- missible levels of environmental regulations is a major concern. Although alternative energy sources such as hydrogen and elec- tricity have been developed to eliminate CO emissions, decreasing CO emissions from conventional combustion engines is critical. In recent years many methods have been used to reduce the emis- sions of CO. The catalytic oxidation of CO is an imperative process 0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcata.2013.04.027