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At present the primary difficulty with employing carbon for the purposes of NOx removal is the lack of selectivity. Carbon generally reacts as readily with oxygen as with NO. Thus, carbons in lean-burn exhausts (O2/NO > 100) generally “burn” rather than selectively removing NO. Here we report on microcalorimetric studies which show that high temperature (950 °C) hydrogen-treated carbons will adsorb NO at room temperature, but will not adsorb significant quantities of oxygen. In contrast, the same activated carbon treated at high temperature in nitrogen will strongly adsorb both species. The selective nature of the hydrogen-treated material and the less-selective nature of the nitrogen-treated material is fully consistent with an earlier model of carbon surface chemistry which highlights the contrast in the character and concentration of active surface species created by each of these treatments.

The use of inexpensive potassium-containing coal chars for NOx reduction in a fuel-lean (oxygen-rich) environment has been investigated. Coals of different rank have been used as char precursors. Two types of experiments were performed to study the kinetics of the NOx/carbon reaction: (i) temperature-programmed reaction in a 15NO(0.5%)/O2(5%)/He mixture and (ii) isothermal reaction at 300 and 350 °C. NOx adsorption at room temperature was also carried out using an identical mixture. Two characteristic temperature regimes were identified: (i) At temperatures below 200 °C, NOx adsorption on the char surface is the more important phenomenon. (ii) At temperatures above 200 °C, the true NOx reduction by carbons starts with N2 and CO2 being the main reaction products. A very active potassium-containing coal char has been prepared by using the following char preparation conditions: pyrolysis at 900 °C and a KOH/coal ratio of 0.5/1.

In this last paper of a series devoted to uncatalyzed and catalyzed reduction of NO by carbon, we discuss some mechanistic aspects of this reaction, which is thought to be a viable alternative to selective catalytic reduction by ammonia or hydrocarbons. We present a summary of the results of isothermal and temperature-programmed reduction studies on a metal-free activated carbon and carbon loaded with K, Ca, Fe, Cu, Cr, Co, and Ni catalysts. All the results obtained on catalyzed NO reduction are consistent with a simple oxidation-reduction (redox) mechanism involving the following kinetically significant steps : (1) chemisorption of NO on the catalyst surface ; (2) transfer of oxygen from the catalytically active sites to the carbon reactive sites ; and (3) desorption of oxygen from the carbon surface. A comparison with the proposed mechanisms for selective catalytic reduction is offered, as well as a mechanistic interpretation of the NO reduction enhancement in the presence of O 2 .

The effect of iron as catalyst of the N0-carbon reaction has been investigated. A coal-derived carbon was loaded with iron using different methods and different precursors. A brief exploratory study was also conducted with pitch-derived carbon fibers. The iron-loaded and/or parent carbons were characterized by physical adsorption of CO 2 (at 0 °C) and N 2 (at -196 °C), X-ray absorption fine structure spectroscopy (XAFS), and chemisorption of CO at 25 °C. The N0-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure using two types of experiments: (i) temperature-programmed reaction (TPR) in a NO/He mixture, and (ii) isothermal reaction at 300-600 °C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Iron was found to catalyze NO reduction by carbon through an oxidation/reduction (redox) mechanism similar to that reported previously for potassium- and calcium-catalyzed reaction. Nevertheless, the iron species present on the carbon surface before NO reduction (Fe x O y or Fe0) are less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer less oxygen to the carbon active sites. The results show also that the nature of the catalyst precursor, the catalyst preparation conditions and the reducibility of the catalyst by the carbon determine the chemical state of the catalyst, its dispersion and catalyst/substrate contact, and hence control the catalytic activity of iron in NO reduction by carbon

The results of an investigation are reported concerning the NO reduction activity of a carbon loaded by impregnation from solution with varying amount of a potassium catalyst. The samples were characterized by CO 2 chemisorption at 250 o C and NO chemisorption at 60 o C. The catalytic effect of potassium in NO reduction was evaluated at atmospheric pressure in a fixed-bed flow reactor. Two types of experiments were performed: (i) temperature-programmed reaction (TPR) in a NO/He mixture; and (ii) isothermal reaction at 300-600 o C. Both TPR and temperature-programmed desorption experiments were also conducted subsequent to NO chemisorption. The reaction products were monitored in all cases, thus allowing detailed oxygen and nitrogen balances to be determined. Three characteristic reactivity regions were found in the TPR profiles: (a) high catalytic activity at low temperatures due to dissociative NO chemisorption, accompanied by N 2 and N 2 O evolution and oxygen accumulation on the catalyst surface; (b) severe catalyst deactivation at intermediate temperatures as the catalyst becomes fully oxidized; (c) recovery of high catalytic activity at high temperatures as the catalyst is reduced by the carbon, accompanied by the evolution of CO 2 and CO. An increase in potassium loading was found to affect only the first region, extending the period of high activity. A good correlation was found between steady-state isothermal catalytic activity and the amount of oxygen accumulated on the surface upon NO chemisorption at 60 o C, especially for NO reduction at low temperatures (e.g., 300 o C). Therefore, dissociative NO chemisorption seems to be a promising technique for titrating the active sites in potassium-catalyzed NO reduction by carbon

The reactions of brown coal chars with nitric oxide, oxygen, and their mixture were carried out at 300 °C. The copper-catalyzed char-NO reaction was remarkably enhanced by the presence of O 2 . The formation of reactive surface intermediates (C(O)) and stable carbon-oxygen complexes (C-O) during these reactions was investigated by the combination of transient kinetics and temperature-programmed desorption techniques. The majority of the carbon-oxygen complexes generated during the reactions were the stable C-O species. The concentration of reactive C(O) was increased by the presence of both O 2 and Cu catalyst

Transient (unsteady-state) kinetics, a relatively new technique for studying noncatalytic gas/solid reactions, has been used successfully to further our understanding of char (carbon) gasification. It provides the unique capability of separately determining the reaction rate constant (site reactivity or turnover frequency) and the number of active sites participating in the reaction (reactive surface area). Its application to the uncatalyzed gasification of coal-derived chars and polymer-derived carbons is illustrated. In particular, the heretofore elusive quantitative understantding of their reactivity variations with conversion has been achieved for gasification in both carbon dioxide and oxygen