William H. Manogue

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

Abstract 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.

SO2 deactivation in NO reduction by NH3: III. Auger studies of deactivated catalysts

Deactivation of the supported metal catalysts by SO2 in the reduction of NO by NH3 was studied in a differential, packed-bed flow reactor. Foils of the same metals as the catalyst were placed in the reactor with the catalysts. Auger electron spectroscopy was used to study the metal foils subsequent to reaction. At 200 °C, 50 ppm SO2 reduced activity of RuAl2O3 by only fourfold as compared with three to five orders of magnitude activity reduction for PtAl2O3, PdAl2O3, andNiAl2O3. For PtAl2O3, PdAl2O3, andNiAl2O3, the surface of the metal foils was covered with approximately a monolayer of sulfur, and there was considerable incorporation of sulfur into the bulk of the metal. The Ru foil had a very small concentration of sulfur on the surface, and no sulfur was present in the subsurface layers. All of the sulfur present was in the form of sulfide; no sulfate was observed.

Batch reaction of methanol and oxygen over TiO2 at 400 °C: Observing surface species by infrared spectroscopy, and gas species by mass spectroscopy

Batch reactions of methanol-oxygen mixtures have been studied over TiO/sub 2/ at 400/sup 0/C. Analysis of the combined infrared and mass spectroscopic results suggests the following reaction mechanism: chemisorption of methanol to form surface methoxy; hydrogen abstraction from surface methoxy to give surface hydroxyl and gas-phase formaldehyde; formaldehyde chemisorption to give surface formate; and surface formate decomposition by dehydrogenation giving H/sub 2/ and CO/sub 2/, and by dehydration giving H/sub 2/O and CO. Kinetic isotope effects present in reactant consumption, product formation, and surface species evolution support assignments of the rate-limiting step in the reaction sequence to hydrogen abstraction from the surface methoxy. 16 references, 3 figures.

CO hydrogenation catalyzed by alumina-supported osmium: Particle size effects

Alumina-supported catalysts were prepared by conventional aqueous impregnation with [H2OsCl6] and by reaction of organoosmium clusters {[Os3(CO)12], [H4Os4(CO)12], and [Os6(CO)18]} with the support. The catalysts were tested for CO hydrogenation at 250–325 °C and 10 atm, the products being Schulz-Flory distributions of hydrocarbons with small yields of dimethyl ether. The fresh and used catalysts were characterized by infrared spectroscopy and high-resolution transmission electron microscopy. The catalyst prepared from [H2OsCl6] had larger particles of Os (~70 A). The cluster-derived catalysts initially consisted of molecular clusters on the support; the used catalysts contained small Os aggregates (typically 10–20 A in diameter). The catalytic activity for hydrocarbon formation increased with increasing Os aggregate size, but the activity for dimethyl ether formation was almost independent of aggregate size. The hydrocarbon synthesis was evidently catalyzed by the Os aggregates, and the ether synthesis was perhaps catalyzed by mononuclear Os Complexes.

SO2 deactivation in NO reduction by NH3: IV. Auger studies of deactivated catalysts in selective NO reduction

Deactivation of supported metal catalysts by SO2 in the selective reduction of NO by NH3 was studied in a differential, packed-bed flow reactor as a function of the gas-phase O2 concentration, and foils of the same metal as the catalyst were placed in the reactor with the catalyst. Auger electron spectroscopy (AES) was used to analyze the metal foils subsequent to reaction. With no SO2 in the feed gas, the rate of NO reduction over PtAl2O3, RuAl2O3, andNiA12O3 was enhanced by the presence of 0.5 mole% O2, whereas that for PdAl2O3 was reduced. AES studies confirm that the deactivation of Pd in the sulfur-free system is due to oxidation of the metal. Although SO2 severely poisons the catalytic activity in the NONH3SO2 system, addition of 0.5 mole% O2 to the feed gas largely restores the activity of all catalysts. This elimination of severe SO2 deactivation correlates well with the incorporation of oxygen to high concentrations into the subsurface layers of the metal; oxygen appears to counteract the presence of sulfur incorporation. Only metal oxides and metal sulfides were formed; there was no evidence of sulfate formation.

CO hydrogenation with alumina-supported catalysts prepared from [Os(CO)5]

Abstract γ-Al 2 O 3 -supported osmium was prepared from [Os(CO) 5 ] and characterized by infrared spectros-copy and transmission electron microscopy before and after use in catalytic hydrogenation of CO at 1 or 10 bar and 523–598 K. Mononuclear osmium carbonyl complexes were formed initially on the Al 2 O 3 surface, and when the samples were brought in contact with H 2 , the osmium was reduced, forming small metal aggregates. Samples were reduced either severely (in 10 bar of H 2 for 10 h), giving large aggregates, or mildly (in 1 bar of H 2 for 1 h), giving smaller aggregates. The properties of the catalysts for CO hydrogenation were compared with those of samples prepared from [H 2 OsCl 6 ] and [Os 3 (CO) 12 ], which consisted of mononuclear osmium complexes and ensembles consisting of three or fewer osmium ions. The activities of the latter catalysts for hydrocarbon formation and the chain growth probabilities were significantly less than those of the catalysts containing Os aggregates. The chain growth probability and the olefin: paraffin ratios were greater for the larger aggregates. Surface intermediates are suggested involving bifunctional interactions of CO with Os metal (or Os ion) sites and neighboring Al 3+ sites.