Platinum catalyst deactivation in low-temperature ammonia oxidation reactions: I. Oxidation of ammonia by molecular oxygen

• Published in: Journal of catalysis 1976, Vol.41 (2), p.277-292
• Main Authors: Ostermaier, J.J., Katzer, J.R., Manogue, W.H.
• Format: Article
• Language: English
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(76)90343-2

Deactivation of 2.0–15 nm Pt crystallites supported on alumina and Pt black was studied in a fixed-bed differential reactor during NH 3 oxidation by molecular oxygen between 368 and 473 °K. Marked deactivation caused by surface oxidation took place during the first 12 hr on stream. Deactivation occurred only in the presence of both reactants. It was more severe with smaller crystallites and at lower temperatures. The rate decay was best described by a second-order deactivation process. At 433 °K oxygen present in the deactivated 2.7 nm crystallites was about 5 atoms per surface Pt atom, giving an estimated bulk composition of PtO 1.7. For the 15.5 nm crystallites the oxygen present in the deactivated crystallites was about 7 atoms per surface Pt atom but the resultant bulk composition was PtO 0.5. Deactivation was reversible and activity could be restored by raising the temperature above 473 °K or by reacting with H 2 or NH 3. Deactivated catalyst slowly released oxygen in reaction with NH 3 chemisorbed on the support after the feed O 2 and NH 3 were turned off. This is a form of reverse spillover. Platinum black studies confirmed the results with the supported catalysts.