Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption

Abstract

The process of chemisorption of nitric oxide (NO) on a carbon derived from phenol-formaldehyde resin has been studied at temperatures between 323 and 473 K and at N O partial pressures between 2.02 and 10.1 kPa. The process of chemisorption can be split into two separate pathways-one a reversible chemisorptive pathway forming nitric oxide surface complexes C ( N 0 ) and the other an irreversible chemisorptive pathway yielding N2 and carbon surface oxides C ( 0 ) and C(02). The enthalpy of formation of C ( N 0 ) is between 42 and 46 W/mol exothermic. The reversible pathway is governed by isotherms that are linear in NO partial pressure, within the range of pressures studied here. This same process, however, appears to be kinetically second order with respect to N O partial pressure, suggesting a possible role of NO dimer as an intermediate in the process. IatradUcti0nThe reactions of nitric oxide with carbons have been considered as offering interesting possibilities for reduction of NO emissions from combustion systems. Some of the relevant literature on these reactions has been reviewed in a paper on the global kinetics of the gasification reaction.'s2 The first step of the process, involving the chemisorption of NO on the carbon surface, has been separately con~idered.~ The literature on chemisorption and/or physisorption of NO on carbons is l i m i t~d .~-~~ Most of these studies acknowledge that if temperatures are kept low (Le., well below ambient) mainly physisorption occurs.*J0 It is generally agreed that chemisorption occurs to a significant extent at temperatures above ambient. The chemisorption is generally accompanied by formation of surface oxides and release of N2.The actual mechanism of the chemisorption processes is not yet established in any detail.One study has reported that the chemisorption of nitric oxide affects the spin-resonance absorption of charcoal in a manner similar to oxygen; there is an increase of ESR absorption linewidth with increasing extent of absorption on a cleaned carbon surface.I2 The difference is that oxygen adsorbed at room temperature can be desorbed by evacuation, whereas nitric oxide cannot. The initial absorption appears, on the basis of magnetic susceptibility, infrared, and thermal studies, to involve the addition of nitric oxide in an "N-down" config~ration.~,~ Another study casts doubt on the notion that the sites reactive toward nitric oxide addition could be spin center^.^ It also appeared that more highly heattreated carbons gave nitric oxide surface complexes of lower thermal stability.' This suggests that addition to aromatic ring structures is involved and that the number of resonance structures affects the stability of the NO adduct. In summary, it appears that the literature implies that radical addition processes occur on the surface of carbons, involving the paramagnetic nitric oxide (which is essentially free radical in nature). These addition processes appear to affect the ESR spectra but do not destroy the measurable free radicals in carbons (which ar...

Production of activated carbons from waste tyres for low temperature NOx control

“…The addition of O2 was essential, since it increases the adsorption of nitric oxide via the catalytic oxidation of NO to NO2, producing in an increase of the NO adsorption because of the higher adsorption potential of NO2 (Yang et al, 2000;Klose and Rincon, 2007). In the absence of oxygen the NO removal efficiency was only about 5%, similar findings have been reported in the literature (DeGroot et al, 1991;Teng and Suuberg, 1993). Prior to any test, the samples were dried under an inert N2 atmosphere.…”

Waste-derived activated carbons for control of nitrogen oxides

“…The presence of oxygen in the gas mixture is essential, since it increases the adsorption of nitric oxide through the catalytic oxidation of NO to NO 2 , which results in an increase of NO adsorption due to the higher adsorption potential of NO 2 (Klose and Rincón, 2007;Yang et al, 2000). In this work, experiments in the absence of oxygen showed that NO removal efficiency was only about 5%; similar findings have been reported in the literature (DeGroot et al, 1991;Teng and Suuberg, 1993). The outlet gas was directed to a gas analyser system.…”

“…The kinetics of NO x removal by carbon of commercial origin has been studied by many researchers (Claudino et al, 2004;Klose and Rincón, 2007;Mochida et al, 1994;Shirahama et al, 2002;Teng and Suuberg, 1993;Zhang et al, 2008) and it has been concluded that the origin, the natural impurities in the feedstock, surface chemistry and the conditions used to prepare activated carbon are the main factors that can control its efficiency for NO x reduction (Xu et al, 2003). Although there has been much research into NO x control using activated carbons, the factors controlling the NO x adsorption process are still not clear.…”

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

“…Moreover a deeper knowledge of the chemistry involved in the formation and the reduction of NO X is required. Many authors have performed NO heterogeneous reduction studies on carbonaceous materials using, mainly, chars from phenol-formaldehyde resins [4][5][6][7][8][9], graphites [10] and activated carbons [11][12][13]. The use of model compounds, with controlled heteroatom content and well defined functionalities, is a useful approach that has been used by other authors [14,15] to study the release of nitrogen compounds from char combustion.…”