Page 385 - Adsorbents fundamentals and applications

370 SORBENTS FOR APPLICATIONS

before the air is fed to the cryogenic separator. Essentially all commercial sor-
bents, including various zeolites (Golden et al., 2000; Miller et al., 2000; Centi
et al., 2000) have been studied for N 2 O removal from air, as reviewed by Ackley
et al. (2002). From the comparison made by Ackley et al. (2002), clinoptilolite
and chabazite appeared to be the best sorbents. For air prepurification, the sor-
bent for N 2 O cannot be used alone, because other sorbents must be used to first
remove CO 2 ,H 2 O, C 2 H 2 , and other hydrocarbons. A transition metal sorbent
would not have such limitation, that is it could selective adsorb N 2 O.

Higher Temperatures. NO removal/reduction in power plants has been per-
formed worldwide by the selective catalytic reduction (SCR) process, with ammo-
nia injection and by using vanadia/TiO 2 (with W or Mo additive) as the catalyst.
◦
Over 80% NO is reduced to N 2 at an operating temperature near 350 C. NO
in automotive emissions is reduced by three-way catalytic converters. For the
new generation of lean-burn engines (i.e., fuel-lean, for fuel economy), however,
these three-way catalytic converters are not adequate. The most promising can-
didate (over the past 7 years) for lean-burn NO control has been the “NO trap.”
In this scheme, the engine is operated in lean-rich cycles (i.e., air rich–air lean
cycles). The NO trap sorbent is added to the three-way catalyst. During the lean
phase (or air-rich phase), the sorbent adsorbs/absorbs NO x , and forms nitrates
and nitrites. During the rich phase, these nitrates/nitrites decompose into N 2 (over
◦
the noble metal catalysts). The system is operated around 300 C. BaO and SrO
were the trap sorbents used in Toyota vehicles. These sorbents are deactivated by
SO 2 (forming sulfate), as well as CO 2 (forming carbonate). The automotive NO
trap has been an incentive for sorbent development for hot gases. Such sorbents
would also be useful in many other possible applications.
Tabata et al. (1988) reported that NO and CO could be adsorbed rapidly
◦
on superconducting YBa 2 Cu 3 O 7 . After pre-evacuation at 300 C, the sample
adsorbed approximately 2 mol/mol oxide for NO at the same temperature. The
adsorbed NO molecules were almost completely desorbed when the temperature
◦
was increased to 400 C. For these Y-containing oxides, Kishida et al. (1991)
reported that the NO adsorption amount decreased according to the order:
YSr 2 CO 3 O x >YBa 4 Co 8 O x >YSr 2 Mn 3 O x >YSr 2 V 3 O x . TPD and IR results showed
−
that the adsorbed NO molecules were oxidized to NO 3 by lattice oxygen. The
adsorbed NO was desorbed as a mixture of NO/O 2 . Arai et al. (1994) reported
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that Ba-Cu-O mixed oxides had a high adsorption capacity for NO/NO 2 at 200 C.
This uptake was accelerated by the presence of oxygen. XRD results indicated
the formation of Ba(NO 3 ) 2 /CuO. In the presence of O 2 , a large amount of NO x
◦
was liberated from the sample at temperatures above 500 C. However, the NO
adsorption capacity of this sorbent vanished completely after exposure to 8%
CO 2 because of the formation of surface BaCO 3 . Since the sorbents containing
Ba are easily deactivated by CO 2 , Eguchi et al. (1996) developed sorbents that did
not contain alkaline earth metals. A series of mixed-oxide sorbents containing
Mn and/or Zr were investigated. The mixed Mn-Zr oxide (at 1 : 1 mole ratio)
was the best.