Ind. Eng. Chem. Res. 2002, 41, 27-31
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A Hydrophobic Pt/Fluorinated Carbon Catalyst for Reaction of NO with NH3 Weizhu An, Qinglin Zhang, Karl T. Chuang,* and Alan R. Sanger Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
A novel hydrophobic fluorinated carbon (FC) supported Pt catalyst has been developed that shows high activity for reaction of NOx with NH3 in the temperature range 170-275 °C. No water inhibition effect on the catalytic activity was observed with a vapor content of 4% (v/v) in the feed. A slight increase in N2O formation at 275 °C and an increase in N2 formation at 175 °C were observed when water vapor was present in the feed. No deactivation of the Pt/FC catalyst occurred during 80 h on stream under either wet feed or dry feed conditions. Substantially complete NO and NH3 conversion can be achieved at temperatures above 170 °C and gas hourly space velocities below 14 400 h-1, and N2 and N2O are the main products. Introduction Selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia is one of the most effective postcombustion DeNOx technologies to control NOx emissions from chemical plants and stationary sources.1,2 Most DeNOx installations are based on TiO2-supported V2O5 series catalysts, which have a typical working temperature range of 300-450 °C and are usually located at the upstream of the particulate collector (PC) and/or flue gas desulfurization (FGD) systems in order to utilize the heat energy from the boilers.2-4 Economic evaluation of the SCR process for power plant applications revealed that a low-temperature SCR DeNOx process could significantly decrease both initial capital and ongoing operating costs if the catalyst is sufficiently active at low temperatures and the catalyst bed is located downstream from the PC and FGD devices.5 Water vapor is one of the main components of NOxcontaining flue gases and frequently leads to catalyst deactivation.4,6 The water content in coal-fired NOx sources is typically 6%. Natural gas combustion leads to a water vapor content of 5-16% in the tail gas.7 Therefore, resistance of DeNOx catalysts to inhibition by water vapor is very important for industrial application of a low-temperature DeNOx process. Precious metal based catalysts, platinum in particular, are effective DeNOx catalysts at low temperatures. However, the performance of these catalysts is significantly inhibited by the presence of water vapor.8 Numerous studies have been devoted to improving the performance of supported Pt catalysts for SCR of NOx with hydrocarbons as reducing agents, particularly under humid feed conditions.9,10 Studies of supported Pt catalysts for SCR of NOx using ammonia as a reducing agent are scarce.11 In general, the reported catalysts are hydrophilic. The competitive adsorption of water and reactants on the catalyst surface is an important factor leading to low activity under humid conditions and/or hydrothermal instability of the catalyst. Thus, it is highly desirable to develop alternative, water-resistant catalysts. * To whom correspondence should be addressed. Tel: 780492-4676.Fax: 780-492-2881.E-mail:
[email protected].
In the present study the performance of a new hydrophobic fluorinated carbon (FC) supported Pt catalyst for reaction of NOx with NH3 will be examined. Catalysts comprising Pt supported on FC and nonfluorinated carbon supports will be described, and the activity and selectivity of these catalysts for the reaction of NO with ammonia will be compared under both dry and humid conditions. FC has been selected because it is a well-known hydrophobic material and has unique physicochemical properties, such as a strong hydrophobic nature and a variety of bond characters between fluorine and carbon,12,13 and the potential application of FC as catalyst supports has not been fully explored.14 Experimental Section 1. Catalyst Preparation and Characterization. (a) Preparation of a 10 wt % Pt/FC Catalyst. FC with a fluorine content of 28% [Brunauer-EmmettTeller (BET) surface area 130 m2/g; Allied Corp.] was used as the hydrophobic support material. The hydrophobicity of the FC support was characterized by contact angle measurements using a calorimeter. Details of measurement and calculation of the contact angle are described elsewhere.14 The calculated contact angle for water on FC used in this study is 125°, indicating the material is highly hydrophobic. A 4% Pt solution of tetraammineplatinum(II) nitrate (Colonial Metals Inc.) in water was used as the Pt catalyst precursor. To prepare a 10 wt % Pt/FC catalyst, the desired amount of platinum-containing solution was first diluted with a 50 wt % mixture of methanol and water, to which FC powder was then added. After thorough mixing under agitation for 30 min, the mixture was reduced to dryness in an inclined rotary evaporator under an infrared light. The Pt was reduced in a stream of H2 at 300 °C for 14 h. (b) Preparation of a Pt/FC/Ceramic Catalyst. To examine the hydrophobic catalyst in a form similar to that which would be used in industrial applications, the 10 wt % Pt/FC catalyst [BET surface area 157 m2/g; X-ray diffraction (XRD) Pt dispersion 7.5%] was anchored on ceramic rings. The ceramic rings (diameter 6 mm × 6 mm; BET surface area
