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Ind. Eng. Chem. Res. 1993, 32,416-423
Chemisorption of Nitric Oxide on Char. 2. Irreversible Carbon Oxide Formation Hsisheng Teng and Eric M. Suuberg’ Division of Engineering, Brown University, Providence, Rhode Island 02912
The process of chemisorption of nitric oxide (NO) on a carbon derived from phenol-formaldehyde resin has been studied a t temperatures between 323 and 473 K, and a t NO 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(N0) and the other an irreversible chemisorptive pathway yielding Nz and carbon surface oxides C(0) and C(02). The processes of formation of the surface oxides both appear to involve dual-site mechanisms, and all these processes are kinetically second order with respect to NO. The latter has been hypothesized to be a consequence of the tendency of NO to form a dimer species within microporous carbons. Within the temperature range studied, the irreversible chemisorptive processes can only be studied on a surface that has been first cleaned by desorption of oxides. There are particular sites created by surface cleaning that, once filled, are not regenerated. The irreversible processes that form the surface oxides have an extremely low activation energy barrier. Introduction The reactions of nitric oxide with carbons have been studied with a view toward their potential for reduction of NO emissions from combustion systems, and relevant literature on these reactions has been reviewed in papers dealingwith the global kinetics of the gasificationreaction (Suuberg et al., 1991; Teng et al., 1992). The first step of the process, involving the chemisorption of NO on the carbon surface, has been separatelyconsidered by us (Teng et al., 1990; Teng and Suuberg, 1993) and others (Zarifayanz, 1964; Zarifayanz et al., 1967; Smith et al., 1956, 1959; Shah, 1929; Harker et al., 1966; Brown and Hall, 1971, 1972; Pastor et al., 1957; Dianis and Lester, 1974; Richter, 1983; Richter et al., 1985; Cascarini de Torre and Arvia, 1968;Kaneko, 1987,1988,1989;Kanekoand Shindo, 1989; Kaneko et al., 1987a,b, 1988a-c, 1989a,b, 1991; Mataumoto et al., 1989; Uchiyama et al., 1990; DeGroot et al., 1991). Most of these studies have concluded that if temperatures are kept low (i.e., well below ambient) mainly physisorption occurs (Smith et al., 1956; Brown and Hall, 1971). 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 Nz. One study has reported that the chemisorption of nitric oxide affects the spin-resonanceabsorption of charcoal in a manner similar to oxygen; there is an increase of ESR absorption line width with increasing extent of absorption on a cleaned carbon surface (Pastor et al., 1957). 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”configuration (Zarifayanz, 1964; Zarifayanz et al., 19671, but apparently not at spin centers (Harker et al., 1966). It also appeared that more highly heat-treated carbons gave nitric oxide surface complexes of lower thermal stability (Harker et al., 1966). 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 impliesthat radical addition processes occur on the surface of carbons,
* To whom correspondence should be addressed. OSSS-5SS5/93/2632-0416~04.00/0
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 are probably of the ?r type). Thus the addition process does not appear to involve the “titration” of the measurable radicals in carbon by the nitric oxide. Hence the identity of the sites that are active toward chemisorption of NO remains unclear. This paper focuses on the irreversible processes in sorption of NO on a particular carbon, at temperatures slightly above ambient. An earlier paper dealt with the formation of reversibly held NO complexes, C(NO), on carbon (Teng and Suuberg, 1993). That earlier study concluded that the mechanism of reversible C(N0) complex formation most likely involved a sequence of reactions that could be represented by 2NO(g)
-
(NO),(ads)
(NO),(ads) + C
-
2C(NO)
(R1) (R2)
where R1 reflecta the formation of dimer intermediates in the porous structure of the carbon, and R2 represents the reversible formation of C(N0) complexes on the carbon surface. This mechanism is consistent both with earlier theories of dimer formation in the carbon and with the data on observed kinetics of C(N0) formation (second order with respect to NO) and the thermodynamics of the system (isotherm linear in NO partial pressure). The separation of a study on the chemisorption process into two parts, one part involving only reversible NO complex formation and the other involving irreversible NO decomposition (to give oxides and Nz),is possible because there appears to be a definite population of surface sites capable of decomposing NO which is separate from the population of sites capable of reversibly binding NO. The former type of site is irreversibly saturated, provided that the temperature of chemisorption is so low that actual steady gasification is not possible. For the material of interest in this study, temperatures below about 473 K seem to fulfill this criterion, and this defines the upper limit of temperatures of interest in this paper. At the temperatures of interest in this study, the role of external mass transfer and macropore diffusion will not be significantin determining observed rates of reaction. 0 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 3, 1993 417 Experiments on diffusional time scales, associated with determining the Surface areas of the chars, suggest that small gases (e.g., Nz, C02) can penetrate the micropore structure in times of less than minutes, much shorter than the observed reaction times. The role of micropore limitations however remains difficult to assess for this reaction, just as for all other gasificationreactions, because it is unclear how deeply the micropores are penetrated by different species. 0
Experimental Section
A standard thermogravimetric analyzer (TGA)was used for studying the kinetics of NO chemisorption on chars. Experiments were performed in a static gas environment, in He/NO mixtures at 101-kPatotal pressure. The volume of the vessel was large enough to ensure that, under any reaction conditions, the consumption of NO was not significant. Pulverized char samples (50-100 mg) were held in a quartz bucket suspended in the heated zone of a quartz tube. A thermocouple placed within a few millimeters of the bucket served to indicate its temperature. The vessel could be purged following experiments and the contents analyzed by gas chromatography. The cham used in present study were derived from phenol-formaldehyde resins. These resins were synthesized in house in order that they contain few catalytic impurities (
