| 423 / | HD | CHEMISTRY OF INORGANIC COMPOUNDS |
| 210 | DF | MODIFYING OR REMOVING COMPONENT OF NORMALLY GASEOUS MIXTURE {11} |
| 212 | DF | .~ Mixture is exhaust from internal-combustion engine {1} |
| 213.2 | DF | .~.~ Utilizing as solid sorbent, catalyst, or reactant a material containing a transition element {2} |
| 213.5 | | .~.~.~ Group VIII element |