Page 380 - Adsorbents fundamentals and applications

NO X REMOVAL 365

NO 2 can be removed as nitric acid. The catalytic oxidation activity for NO was
also observed with activated carbon by Rubel and Stencel (1997). The mechanism
of the catalytic oxidation reaction is not understood, but it is possibly related to
the surface oxides on carbon.
Iwamoto and co-workers (Zhang et al., 1993) studied NO adsorption on vari-
ous ion-exchanged zeolites. The best sorbent was Cu-MFI, Co-MFI, and Ag-MFI.
They measured both “reversibly” (that could be desorbed at ambient tempera-
ture) and “irreversibly” adsorbed NO. The amounts given in Table 10.21 are for
the irreversibly adsorbed. IR spectroscopy indicated that most of the reversibly
2+
+
adsorbed NO was NO adsorbed on Cu , and that the irreversibly adsorbed NO
−
−
+
+
was in the forms of NO , nitrate (NO 3 ), nitrite (NO 2 ), and NO 2 . Unfortu-
nately, exposure to water vapor severely deactivated the zeolites.
H 3 PW 12 O 40 belongs to a large class of heteropoly acids and salts, which have
been the subject of long-standing investigation (Pope, 1983). The crystal structure
of the PW 12 O 40 anion belongs to the Keggin structure of XM 12 O 40 , shown in
Figure 10.63. In this structure, 12 MO 6 octahedra surround a central XO 4 tetrahe-
dron, where M is usually W or Mo and X can be P, As, Si, Ge, B, etc. Although
the structure of the heteropoly anion (e.g., the Keggin structure) is well-defined
and stable, the structure by which the Keggin structures are linked together is
less understood. However, a distinct X-ray diffraction pattern is obtained for
H 3 PW 12 O 40 · 6H 2 O. As shown in Figure 10.63, the Keggin polyanions are linked
+
in a 3-dimensional network through H (H 2 O) 2 bridges, and these linkages can
be easily (i.e., at ambient temperature) replaced by polar molecules such as alco-
hols and amines (Misono, 1987). Generally, 2–6 alcohol or amine molecules
per Keggin anion irreversibly replace the water linkages (of 6H 2 O). The Keggin
structure is called the primary structure and the linked structure is called the
secondary structure. Due to the flexible form of the secondary structure, Misono
referred to the solid structure as a “pseudo-liquid phase.” The importance of the
“pseudo-liquid phase” in many reactions catalyzed by the heteropoly compounds
has been demonstrated (Misono, 1987).
Yang and Chen observed that a large amount of NO could be absorbed by the
heteropoly compounds (Yang and Chen, 1994; Yang and Chen, 1995; Chen and
Yang, 1995). The water linkages in the secondary structure can be substituted
◦
readily by NO linkages at 50–230 C at low NO concentrations (i.e., under flue
gas conditions). The absorption product for H 3 PW 12 O 40 · 6H 2 OisH 3 PW 12 O 40 ·
3NO. Moreover, a substantial fraction of the absorbed NO is decomposed into N 2
upon rapid heating of the NO-linked compound (Yang and Chen, 1994; 1995).
The bond energy of the NO linkages is 25 kcal/mol. CO 2 does not adsorb/absorb
in the heteropoly compounds. SO 2 has no effect on the absorption of NO. Because
of this unusual property of the heteropoly compounds, a number of studies were
undertaken after the work of Yang and Chen. Herring and McCormick (1998)
determined the mechanism of the NO absorption. In the presence of O 2 ,the
surface of H 3 PW 12 O 40 is involved in the catalytic oxidation of NO to NO 2 via an
y−
adsorbed NO x intermediate. This species can be absorbed into the H 3 PW 12 O 40
as a form of NOH , displacing water, or can leave as NO 2 gas molecules.
+