Jon G. McCarty

Hydrogenation of surface carbon on alumina-supported nickel

Four types of carbon were observed to form on an alumina-supported nickel methanation catalyst on exposure to carbon monoxide at 550 ± 50 K. In order of their reactivity toward hydrogen the carbon species may be classified as: chemisorbed carbon atoms (α), bulk nickel carbide, amorphous carbon (β), and crystalline elemental carbon. The α-phase and the initial monolayers of Ni3C are much more reactive than the elemental forms as measured by temperature-programmed surface reaction in 100 kPa H2. At 550 K the α- and β-carbon species formed by CO exposure populate the surface at a ratio of about 2:1. Both phases are relatively stable on heating to 600 K in a He atmosphere. At higher temperatures, slow conversion of α- and β-carbon to graphite was observed. Hydrogenation of the α state at 550 K leads to methane at a sufficiently fast rate to make it a likely intermediate in nickel-catalyzed methane synthesis from hydrogen and carbon monoxide.

Kinetics of PdO combustion catalysis

Abstract The kinetics of the catalytic combustion of methane by supported palladium oxide catalysts (2 wt.-% Pd/La2O3·11A12O3 and 5 wt.-%Pd/ γ-A1203 were examined for several oxygen partial pressure levels over the temperature range from 40–900°C using temperature-programmed reaction and slow ramp and hold temperature-time transient techniques. Combustion rates were measured by differential reaction in a fixed bed of powdered catalyst at lower temperatures (200–500°C). Also, by preparing the catalysts as thin (ca. 10 μm) coatings on an alumina tube and conducting the experiments with very high flows of dilute methane and oxygen in helium, the rate measurements were extended up to 900°C without significant contribution from gas phase reactions. The specific combustion activity of supported PdO shows a persistent hysteresis between 450 and 750°C, i.e., the rate of combustion between these temperature limits depends strongly on whether the catalyst is cooling from above 750°C or heating from below 450°C. This region is also notable for negative apparent activation energy in the rate of methane oxidation, i.e., the rate increases with decreasing temperature during reoxidation of the Pd metal and decreases with increasing temperature (especially with low oxygen partial pressure) prior to decomposition of the bulk oxide. Detailed time-temperature transient kinetic analyses were performed for supported PdO catalysts within the 450–750°C temperature range. The hysteresis in methane combustion rate is caused by a higher activation energy for reduction of oxygen chemisorbed on metallic Pd and by suppressed reoxidation of Pd metal relative to PdO decomposition.

Perovskite catalysts for methane combustion

We have investigated the methane oxidation activity and structure of a number of complex metal oxides with the objective of determining the relationship between catalytic and such solid-state parameters as the type of transition metal cation incorporated in the oxide crystal structure, the metal cation valence state, oxygen stoichiometry, and defect structure

Stability of supported metal and supported metal oxide combustion catalysts

Abstract Catalysts used for high-temperature combustion of light hydrocarbons must maintain high activity over long time intervals by avoiding excessive sintering and deactivation in the hot and corrosive combustion environment. The sintering resistance and chemical stability of catalytically active phases is a key technical problem that must be solved for the development of commercially viable combustion catalysts. All noble metals and transition metal oxides that are catalytically active rapidly sinter at temperatures required for high combustion rates. Advanced materials used in the development of stable catalysts include highly sintering-resistant hexaaluminate supports for dispersion of noble metals, chemically and thermally stable supporting oxides for active transition metal oxides, and single-phase, substitutionally activated, sintering-resistant complex metal oxides. This paper will review deactivating phenomena, such as sintering and vapor transport and assess recent progress in the development of durable combustion catalysts.

Thermodynamics of sulfur chemisorption on metals. I. Alumina‐supported nickel

Sulfur chemisorption isosteres have been measured for nickel in powdered form and for nickel supported on two different alumina powders. The experiments were conducted in a closed‐loop gas recirculation system containing one atmosphere hydrogen. Isotherms were determined by stepwise injection of H2S aliquots into recirculating hydrogen gas and analyzing for the H2S concentration as equilibrium was approached. Isosteres were measured by varying the sample temperature and monitoring the H2S/H2 ratio in the gaseous environment. A gas chromatograph and a photoionization detector was used to measure the H2S concentration to levels below 1 ppb. As monolayer coverage is approached the H2S/H2 ratio attains the equilibrium values reported for the bulk sulfide, Ni3S2. Adsorbed sulfur is very strongly bound to the surface of nickel. The heat of formation of chemisorbed sulfur with respect to 1/2 S2(g) at 800 K is 247 kJ mol−1 more negative than the heat of formation of Ni3S2. The heat of segregation exceeds 190 kJ m...

Novel oxygen storage components for advanced catalysts for emission control in natural gas fueled vehicles

Abstract Advanced catalysts based on a novel oxygen storage component (OSC) were developed for emission control in natural gas fueled vehicles. The catalysts contain a manganese oxide (MnOx) as the OSC supported on an inert LaA1O3 perovskite and a noble metal component (Pd) supported on a separate high surface area refractory material, for example lanthana stabilized A12O3. The MnOx has higher oxygen storage capacity, and faster oxygen absorption and oxide reduction rates than the present commercial ceria-stabilized alumina support materials. Temperature programmed techniques and dynamic cycled experiments were used to measure oxygen storage capacity, activity for NO reduction and CO and CH4 oxidation rates. Durability tests on a physical mixture of Pd-Mn/LaAlO3 and Pt/A12O3(La) demonstrated that a temperature excursion to 950°C for 30 min did not cause significant loss in catalyst activity for either NO reduction or CO or CH4 oxidation. We have also shown that the MnOX can be added to conventional three-way catalysts (TWC) to enhance their performance for NOx reduction and CO or hydrocarbon oxidation.

Catalytic decomposition of nitrous oxide over Ru-exchanged zeolites

We report that ultrastable faujasite-based ruthenium zeolites are highly active catalysts for N2O decomposition at low temperature (120–200°C). The faujasite-based ruthenium catalysts showed activity for the decomposition of N2O per Ru3+ cation equivalent to the ZSM-5 based ruthenium catalysts at much lower temperatures (TOF at 0.05 vol.-% N2O: 5.132 × 10−4 s−1 Ru−1 of Ru-HNaUSY at 200°C versus 5.609 × 10−4 s−1 Ru−1 of Ru-NaZSM-5 at 300°C). The kinetics of decomposition of N2O over a Ru-NaZSM-5 (Ru: 0.99 wt.-%), a Ru-HNaUSY (Ru: 1.45 wt.-%) and a Ru-free, Na-ZSM-5 catalyst were studied over the temperature range from 40 to 700°C using a temperature-programmed micro-reactor system. With partial pressures of N2O and O2 up to 0.5 vol.-% and 5 vol.-%, respectively, the decomposition rate data are represented by: −dN2O/dt=itk(PN2O) (PO2)−0.5 for Ru-HNaUSY, −dN2O/dt=k(PN2O) (PO2)−0.1 for Ru-NaZSM-5, and −dN2O/dt=k(PN2O)−0.2 (PO2)−0.1 for Na-ZSM-5. Oxygen had a stronger inhibition effect on the Ru-HNaUSY catalyst than on Ru-NaZSM-5. The oxygen inhibition effect was more pronounced at low temperature than at high temperature. We propose that the negative effect of oxygen on the rate of N2O decomposition over Ru-HNaUSY is stronger than Ru-NaZSM-5 because at the lower temperatures (<200°C) the desorption of oxygen is a rate-limiting step over the faujasite-based catalyst. The apparent activation energy for N2O decomposition in the absence of oxygen is much lower on Ru-HNaUSY (Ea: 46 kJ mol−1) than on Ru-NaZSM-5 (Ea: 220 kJ mol−1).

Deactivation of alumina-supported nickel and ruthenium catalysts by sulfur compounds☆

Deactivation of alumina-supported nickel and ruthenium catalysts by sulfur compounds was studied by temperature-programmed desorption, temperature-programmed surface reaction, and in flow experiments of carbon monoxide and hydrogen on a commercial catalyst (G-65) containing 25% nickel on alumina (Ni/Al) and on this catalyst with 1% iridium added (Ni/Ir/Al) in the absence and presence of hydrogen sulfide. The Ni/Ir/Al catalyst adsorbed 30% more CO than Ni/Al, and it desorbed in three temperature peaks, compared with two desorption peaks for Ni/Al. Carbon dioxide and surface carbon were formed during the desorption. On Ni/Al, two types of surface carbon were formed with different reactivities towards hydrogen, and on Ni/Ir/Al only one highly reactive carbon species was detected. In the presence of hydrogen sulfide, adsorption decreased, less carbon was formed on Ni/Ir/Al and none on Ni/Al, and the methanation reaction decayed at a rate which was determined for the two catalysts. The iridium increased the sulfur resistance of the catalyst by a factor of two. An alumina-supported 5% ruthenium catalyst deactivated more rapidly than Ni/Al.

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

The N2O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N2O:1.47 × 10−4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10−4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N2O decomposition.

Effect of ruthenium-loading on the catalytic activity of Ru-NaZSM-5 zeolites for nitrous oxide decomposition

Abstract The effect of Ru-loading on the performance of Ru-NaZSM-5 catalysts prepared by ion exchange (extent of ruthenium exchange: 20–100%) in the decomposition of nitrous oxide was studied in the temperature range of 40–700°C. The turnover frequency (TOF) for nitrous oxide decomposition (number of nitrous oxide molecules converted per ruthenium site per unit time) increased significantly with decreasing Ru-loading. Although lower Ru-loading resulted in higher TOF and lower apparent activation energy for nitrous oxide decomposition, this occurred at the expense of stronger inhibition by oxygen. Therefore, to obtain catalysts that remain active in the presence of excess oxygen, high Ru-loaded catalysts (close to 100% of exchange) are needed; whereas in the absence of oxygen, low Ru-loadings (close to 20% of exchange) are preferred. The high TOF observed in Ru-NaZSM5-20 was attributed to the interaction between ruthenium and sodium cations.