Page 378 - Adsorbents fundamentals and applications

NO X REMOVAL 363

10.9. NO X REMOVAL
Adsorption has been playing an increasingly important role in environmental
control. The sorbents being used in common industrial adsorption systems for
the removals of SO 2 and volatile organic compounds (VOCs) are quite well-
established. The VOC removal systems often use activated carbon, polymeric
resins, and hydrophobic zeolites, for both gas and aqueous systems. Activated
carbon (and alkalized forms) and hydrophobic zeolites are used for SO 2 removal.
Lime injection is used for SO 2 removal from hot gases. For NO x removal, on
the contrary, no suitable sorbents have been established. For this reason, selec-
tive sorbents for NO x remain an active research area, and will be discussed. The
search for CO 2 sorbents is also of interest. The subject of CO 2 sorbents has been
discussed recently in an excellent review by Yong et al. (2002) and will not be
covered here.
Selective NO x removal is an area where the boundary between sorbent and cat-
alyst tends to disappear. Many different types of sorbents have been investigated
◦
for NO x sorption for both cold (near ambient temperature) and hot (200–400 C)
applications. Transition metal oxides appear to be the best. In both temperature
ranges, a significant amount of adsorption is achieved usually when assisted by
catalytic oxidation of NO to NO 2 as an intermediate step. In air near ambient
temperature, about 5% of the NO is in the form of NO 2 .NO 2 adsorbs more eas-
ily than NO for both chemisorption and physical adsorption. The normal boiling
◦
◦
point of NO is −152 C and that of NO 2 is 21 C. Thus, NO 2 adsorbs read-
ily near ambient temperature in micropores and mesopores by pore filling. In
chemisorption, NO 2 readily forms nitrite and nitrate on transition metal oxides.
These chemisorbed species can be decomposed or desorbed only at elevated tem-
◦
peratures, e.g., 200–300 C. Other surface species are also formed, such as NO +
(nitrosyl). These species could be desorbed at near-ambient temperature. The
complex chemistry of NO is due to the fact that there is one electron occupying
the antibonding orbital of NO, which is empty in most other molecules.

Near-Ambient Temperature. There has been a long search for sorbents for
NO x at near-ambient temperature. Table 10.21 is a summary of the equilibrium
capacities for these sorbents. The equilibrium capacity can be deceiving, because
of the following problems. The NO x capacities are decreased by the presence of
water vapor. The most severe case is with the ion-exchanged zeolites, such as MFI
(or ZSM-5); exposure to water vapor destroys the sorbent quickly and completely.
SO 2 and CO 2 also have some effects on some sorbents, as to be discussed.
Chemisorption of NO on various metal oxides was studied before 1982, as
shown in Table 10.21. The partial pressure of NO is not important for the NO
capacity because the isotherms are usually very steep for chemisorption. The
sorbents will be discussed following the sequence listed in Table 10.21.
Kaneko and co-workers (summarized by Kaneko, 1998) studied the adsorption
of NO on activated carbon fibers (ACFs). The capacity of NO on ACF is not
high. However, large capacities were obtained after iron oxide or FeOOH was
dispersed on the ACF. The reason for the increase is not clear. An explanation