Article pubs.acs.org/EF
Reactions of Hydrogen Chloride with Carbonaceous Materials and the Formation of Surface Chlorine Species Naoto Tsubouchi,*,† Noriaki Ohtaka,‡ and Yasuo Ohtsuka‡ †
Center for Advanced Research of Energy and Materials, Hokkaido University, Kita 13 Nishi 5, Kita-ku, Sapporo, Hokkaido 060-8628, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai, Miyagi 980-8577, Japan ABSTRACT: Secondary reactions of hydrogen chloride (HCl) during high-temperature combustion processes were elucidated using a model carbon prepared from a commercially available phenol resin. This resin was O2-activated, followed by doping with K+, Ca2+, Cu2+, or Zn2+, and subsequently exposed to a stream of 100 ppm of HCl in N2 at 500 °C, during which HCl reacts with the carbon samples to form surface chlorine species. In the absence of metal cations, the extent of reaction increases almost linearly up to 500 °C as the number of carbon active sites, determined by temperature-programmed desorption, increases. The addition of Ca, Cu, or Zn but not K significantly promotes the formation of carbon sites as well as the reaction with HCl, with the latter effect increasing in the order of Ca < Cu < Zn on a molar basis. Cl 2p X-ray photoelectron spectroscopy (XPS) data show that inorganic chlorides of Ca, Cu, and Zn are formed and that the organic Cl/C ratio is 2.0−2.3 times greater when using these additives. It is thus evident that the addition of Ca, Cu, or Zn can promote the formation of organic chlorine species. Possible mechanisms for the Ca-, Cu-, and Zn-enhanced formation of carbon active sites and organic chlorine forms are discussed on the basis of the results of XPS and temperature-programmed desorption analyses.
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INTRODUCTION A portion of hydrogen chloride (HCl) evolved in various industrial processes, such as waste incineration, iron ore sintering, electric arc furnace steelmaking, and pulverized coal combustion, is known to be transformed into hazardous chlorinated organic compounds through secondary reactions with unburned carbon downstream of the initial combustion site. It is thus important to determine the most important factors controlling the extent of such reactions. According to previous studies,1−5 metallic chlorides present in unburned carbon can work as not only the chlorine sources but also the catalysts for the formation of dioxins, a family of organochlorine compounds, with the catalytic activity increasing in the sequence of NaCl, KCl, CaCl2, MnCl2, FeCl2, NiCl2, CdCl2, SnCl2 < FeCl3, ZnCl2, PbCl2 < CuCl ≪ CuCl2. It has also been reported that, when the rate of O2 gasification of carbonaceous materials increases, yields of organic chlorides increase significantly.4,5 However, the role of unburned carbon, in particular carbon active sites, in the formation of organochlorines remains unclear. It is well-known that oxygencontaining functional groups on carbonaceous materials are decomposed into CO and/or CO2 upon heating to yield carbon active sites.6,7 Because it has been accepted that such carbon sites are about 100−1000 times more reactive than basal plane carbon,8 they may work efficiently as carbon sources for the formation of organic chlorides. It has been widely accepted that the carbon sites can work as reactive sites for gasification reactions.6,7 In the present work, the main objective was to ascertain the mechanisms of HCl−carbon interactions. For this purpose, phenol resin-derived carbon was used as a model of unburned carbon, because it does not contain significant amounts of © 2016 American Chemical Society
metal impurities that may affect the reactions that are the focus of this study. We first investigated the effects of O2 activation and metal addition on the reaction of HCl with this carbon, then ascertained the roles of carbon active sites and some metal catalysts in the formation of organic chlorine species, and finally elucidated possible mechanisms for HCl−carbon interactions by means of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) analyses.
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EXPERIMENTAL SECTION
Carbon Samples. A commercially available phenol formaldehyde resin (
