| 423 / | HD | CHEMISTRY OF INORGANIC COMPOUNDS |
| 351 | DF | NITROGEN OR COMPOUND THEREOF (EXCEPT AMMONIUM SALT OF NON-NITROGEN ACID) {5} |
| 385 | DF | .~ Oxygen containing {4} |
| 400 | DF | .~.~ Binary compound (oxide of nitrogen) {3} |
| 402 | | .~.~.~ Utilizing catalyst {1} |
| 403 | DF | .~.~.~.~> And ammonia as reactant {1} |