W.H. Manogue

Crystallite size effects in the low-temperature oxidation of ammonia over supported platinum

Abstract The effect of crystallite size on the specific catalytic activity of supported platinum catalysts in ammonia oxidation with molecular oxygen was determined. Rates of ammonia oxidation were measured in a differential, fixed bed, flow reactor between 393 and 473 °K, and catalysts containing average crystallite sizes of 2.0, 2.7 and 15.5 nm (1 nm = 10 A) were used. Nitrogen and nitrous oxide were the only nitrogen-containing products with about four times as much nitrogen as nitrous oxide. The initial specific catalytic activity of the large crystallite catalyst was higher than the 2.0 and 2.7 nm catalyst by factors of 5.7 and 3.7, respectively. All catalysts showed a marked decline in activity in the first 6 hr of operation with steady-state activity being reached in about 12 hr. Smaller crystallites were more severely deactivated so that the resultant steady-state specific activity of the large crystallite catalyst was greater by factors of 14 and 8. Both the specific catalytic activity and the selectivity, also a function of crystallite size, are explained in terms of the changes in average surface concentration of active oxygen with changes in crystallite size. The reaction rate data are best represented by a Langmuir-Hinshelwood model involving dissociative adsorption of both reactants.

Sulfur deactivation of nickel methanation catalysts

Sulfur deactivation of supported Ni in CO hydrogenation was studied in an all-quartz internal-recycle reactor with a feed containing 4% CO in H/sub 2/. Thirteen ppB H/sub 2/S reduced the steady-state methanation activity of Ni/..gamma..-Al/sub 2/O/sub 3/ about 200-fold at 661 K; 100 ppB H/sub 2/S reduced the activity 5000-fold. A dual site Langmuir-Hinshelwood rate expression predicts both the CO partial pressure dependence and the S poisoning. Poisoning and chemisorption data indicate formation of a stable two-dimensional surface sulfide with a S:Ni surface atom ratio of 1:2 for 13 ppB H/sub 2/S in H/sub 2/ at 661 K. The surface sulfide has a free energy of formation of at least -26 kcal/mole which is 15 kcal/mole more stable than bulk Ni/sub 2/S/sub 3/. Sulfur poisoning is due to geometric effects, i.e., site blockage, rather than electronic effects since the activation energy for methanation over S-poisoned Ni was the same as that over unpoisoned Ni, 24 kcal/mole.

Methanation over transition metal catalysts: II. carbon deactivation of CoAl2O3 in sulfur-free studies

Abstract Hydrogenation of CO catalyzed by Co Al 2 O 3 was studied in an all-quartz internal-recycle reactor between 200 and 400 °C and from 0.1 to 20% CO in H 2 at atmospheric pressure. The surface and subsurface regions of the aged Co Al 2 O 3 catalysts were investigated using Auger electron spectroscopy (AES). Two pseudo steady states were observed. The upper pseudo steady state corresponds to a cobalt surface mainly containing active reaction intermediates; the lower pseudo steady state corresponds to deactivated cobalt which has large deposits of graphitic carbon on its surface, and for which the bulk CO is carburized to great depths as shown by AES. Methane is the primary reaction product; other hydrocarbons constitute less than 10% of the total products formed. The activation energy for methanation over Co Al 2 O 3 in the upper pseudo steady state is 28 ± 2 kcal/mole, whereas that in the lower pseudo steady state is 16 ± 2 kcal/mole. The change in the activation energy appears to be caused by changes in the electronic structure of CO due to bulk carburization.

Methanation over transition metal catalysts: III. CoAl2O3 in sulfur-poisoning studies

Poisoning of CoAl2O3 by H2S in CO hydrogenation was studied in an all-quartz recycle reactor; the studies were carried out at atmospheric pressure and at 390 °C with feed gas containing 1 to 4% CO in H2 and 13 to 100 ppb H2S. Sulfur-poisoned catalysts were analyzed using Auger electron spectroscopy (AES). Addition of 13 ppb H2S reduced the steady-state methanation activity of CoAl2O3 by more than 103-fold at 390 °C; 90 ppb H2S reduced the activity 104-fold. AES studies showed that the loss in methanation activity resulted from two-dimensional surface sulfide formation; no sulfur was present in the subsurface regions. In the sulfur poisoning of CoAl2O3, carbon plays only a secondary role. Poisoning by sulfur appears to be due primarily to geometric blockage of sites, with one sulfur atom adsorbed per two surface Co atoms. Electronic effects due to sulfur adsorption are also important as evidenced by a 12 kcal/mole reduction in the activation energy for methanation upon poisoning.

Methanation over transition-metal catalysts: V. RuAl2O3—Kinetic behavior and poisoning by H2S

Abstract Hydrogenation of CO over Ru films supported on Al 2 O 3 was investigated in a gradientless, allquartz, internal-recycle reactor at temperatures between 523 and 673 °K, at CO concentrations from 0.1 to 4%, and at H 2 S concentrations between 0 and 100 ppb in H 2 at atmospheric pressure. Methane is the major reaction product. The measured activation energy of 27 ± 1 kcal/mole is in good agreement with the values reported for highly dispersed Ru catalysts. Methanation is strongly inhibited by CO in the gas phase at temperatures between 523 and 673 °K. AES studies of aged catalysts show that there is no carbon present either on the surface or in the subsurface regions. In the presence of 13 ppb H 2 S in the gas phase the methanation activity is reduced by more than 10 3 -fold at 663 °K. The activation energy remains 27 kcal/mole indicating that poisoning of Ru by H 2 S is primarily a geometric effect. AES studies show the formation of a saturated two-dimensional surface sulfide, corresponding to each sulfur atom blocking two surface Ru atoms. There is no subsurface sulfur. This poisoning by sulfur is in marked contrast with the tolerance of Ru to poisoning by SO 2 during NO reduction by NH 3 , and suggests that the nature of the reaction environment (strongly reducing vs mildly reducing) may have a significant effect on the tolerance of a catalyst to poisoning by sulfur.