James R. Katzer

Sulfur Poisoning of Metals

Publisher Summary This chapter provides an overview of the sulfur poisoning of metals. Sulfur apparently bonds so strongly to metal surfaces that marked activity reduction occurs at extremely low gas-phase concentrations of sulfur-containing compounds. In commercial practice, the life of supported metal catalysts may be reduced to only a few months or weeks in the presence of only ppm quantities of sulfur contaminants in the feed. Because of the essentially irreversible adsorption of sulfur compounds on metals, regeneration is usually impossible or impractical. This chapter integrates available information on the interaction of sulfur with metal surfaces with that of poisoning studies to provide a more complete picture of sulfur poisoning and of the mechanism. The chapter stresses the importance of experimental techniques that provide definitive, fundamental information regarding sulfur adsorption and poisoning.

Involvement of free radicals in the aqueous-phase catalytic oxidation of phenol over copper oxide

Abstract Aqueous-phase oxidation of phenol over copper oxide was studied between 369–393 °K and 1–17 atm oxygen pressure in batchwise mode. The reaction involves an induction period and a steady-state activity regime. The initial rate is first order in phenol and oxygen, but the rate is one-half order in oxygen in the steady-state activity regime. Heterogeneously catalyzed aqueous-phase phenol oxidation occurs by a free-radical mechanism which involves initiation on the catalyst surface, homogeneous propagation and either predominantly homogeneous or heterogeneous termination depending on catalyst concentration. That a free-radical mechanism is involved is indicated by the induction period, by the observed reaction kinetics, by free-radical inhibitor studies which show the same inhibitor efficiencies as observed in free-radical oxidation and by pH studies. The involvement of a heterogeneous-homogeneous reaction mechanism is indicated by dependence of the reaction rate on catalyst concentration.

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.

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.

X-Ray absorption edge and extended x-ray absorption fine structure studies of PtTiO2 catalysts

X-Ray absorption edge and extended x-ray absorption fine structure (EXAFS) spectroscopy was used to study PtTiO2 prepared by two different techniques. Edge studies show that the observable electronic structure of very small particles of reduced PtTiO2 are independent of preparation technique. After H2 reduction at 473 °K, PtTiO2 has about 10% fewer unfilled (vacant) d states (0.03 fewer d state vacancies) per Pt atom than PtSiO2, prepared and reduced the same way, and has about 15% more unfilled d states (0.04 more d state vacancies) per Pt atom than bulk Pt. Reduction of PtTiO2 in H2 at 698 °K results in less than a 4% reduction in the number of unfilled d states per Pt atom (less than 0.015 hole reduction per Pt atom) as compared to the 473 °K reduction, indicating a very small amount of electron transfer to the Pt induced by the high-temperature H2 reduction of PtTiO2. It was concluded that the reported effect of high-temperature reduction of PtTiO2 on CO and H2 chemisorption is not due to a substantial change in the extent of electron transfer from the support but is due to more subtle and specific electronic changes.

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.

X-ray absorption spectroscopy of Pt-ReAl2O3 catalysts

The state of Re in reduced Pt-ReAl2O3 catalysts has been a subject of controversy for some years. X-ray absorption spectroscopy at the Stanford Synchrotron Radiation Laboratory was used to examine two laboratory-prepared, commercial-type catalysts after reduction. Data from the Re LIII absorption edge show that Re is neither zero valent nor significantly associated with Pt. The data also suggest that the Re is in the 4+ valence state.

Determination of support and reduction effects for Pt/Al2O3 and Pt/SiO2 by X-ray absorption spectroscopy

By use of a recently developed technique to quantitatively determine the number of unfilled d states in a material from measurements of the LIIand LIII X-ray absorption edge spectra, the electronic properties were examined in terms of the change in the number of unfilled d states of PtAl2O3 and PtSiO2 as a function of reduction conditions and of the support relative to that of the bulk metal. PtSiO2 is fully reduced after 473 K H2 reduction and undergoes no further measurable changes in electronic properties with higher-temperature reduction. PtAl2O3 still exhibits a significant extent of electron deficiency after 473 K H2 reduction; this is largely eliminated by H2 reduction at 723 K.

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.