Reactions of Nitrogen and Oxygen Surface Groups in Nanoporous Carbons under Inert and Reducing Atmospheres

Abstract

International audienceThe reactions of surface functional groups have an important role in controlling conversion of char nitrogen to NOx during coal combustion. This study involved an investigation of the thermal stability and reactions of nitrogen surface functional groups in nanoporous carbons. Four suites of carbons, which were used as models for coal chars, were prepared with a wide range of nitrogen and oxygen contents and types of functional groups. The porous structures of the carbons were characterized by gas adsorption methods while chemical analysis, X-ray photoelectron spectroscopy, and X-ray near edge structure spectroscopy were used to characterize the surface functional groups. Temperature programmed desorption and temperature programmed reduction methods were used to study the reactivity of the surface functional groups during heat treatment under inert and reducing conditions. Heat treatment studies show that the order of stability of the functional groups is quaternary nitrogen>pyridinic>pyrrolic>pyridineN-oxide. PyridineN-oxide surface groups desorb NO and formN2 via surface reactions at low temperature. Pyrrolic and pyridinic functional groups decompose and react with surface species to give NH3, HCN, and N2 as desorption products, but most pyrrolic groups are preferentially converted to pyridinic and quaternary nitrogen. The main desorption product is N2. Approximately 15-40 wt % of the original nitrogen was retained in the carbons mainly as quaternary nitrogen after heat treatment to 1673 K. The results are discussed in terms of decomposition ranges for surface functional groups and reaction mechanisms of surface species

Organic nitrogen chemistry during low-grade metamorphism

“…Following extensive high-temperature pyrolysis of nitrogencontaining amorphous carbon precursors in the laboratory, essentially all edge-located nitrogen atoms (Fig. 2) are eliminated and remaining nitrogen atoms are preferentially located in the interior of the carbon lattice as thermally stable quaternary nitrogen (Isaacs, 1970;Pels et al, 1995;Stanczyk et al, 1995;Xiao et al, 2005). These trends have not been observed in nature where fluids, temperature, pressure, time and catalysts play important roles in the transformation of carbonaceous matter during metamorphism (e.g., Price and Dewitt, 2001;Schwab et al, 2005;Guedes et al, 2005).…”

“…The N-X sub-peak in Fig. 5b may relate to organic R=N-O-R where nitrogen is linked to carbon (R) and to one oxygen atom (e.g., Gong et al, 1999;Xiao al., 2005).…”

“…Experimental dry, open-system programmed pyrolysis of model substances shows that pyridinic, pyrrolic and N-C 3 -type nitrogen species are remarkably stable at high temperature (e.g., Stanczyk and Boudou, 1994;Schmiers et al, 1999;Xiao et al, 2005;Boudou et al, 2006). Modelling with Optkin or 2D Petromod software, for example, justifies the early pyrolytic elimination of only a minor amount of anthracitic N org , even under high heat flow of up to 80 W/cm 2 from the Carboniferous to present time (Littke et al, 1995), in reality most N org is not eliminated throughout low grade metamorphism (Boudou and Espitalié, 1995).…”

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

“…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. However, this picture leaves out important aspects such as coal structure, interactions between different surface groups and cross-linking reactions.…”

N-containing carbons from styrene–divinylbenzene copolymer by urea treatment

“…X-ray photoelectron spectroscopy (XPS) is one of the few methods available to get information on the local bonding environment of nitrogen in carbon materials [19] and, consequently, XPS alone [19][20][21] or in combination with other techniques [2,12,[22][23][24] has been extensively used with this purpose.…”