Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties

The review is a summary and analysis of the data characterizing CO adsorption on surface cationic sites of oxides including supported materials and microporous and mesoporous materials. The contributions of various types of CO bonding to the IR frequency shifts of carbon-bonded molecules are analyzed, namely, the increase of the CO stretching frequency in cases of electrostatic and σ bonding and the decrease of the frequency with π bonding. Polycarbonyls, bridging CO, oxygen-bonded CO, and tilted CO are also considered. The main part of the review is a collection of the experimental results characterizing carbonyls of individual metal ions. The spectral behavior of CO bonded to metal atoms is also assessed in the cases when the metal ions are easily reduced to metal (Cu, Ag, Au, Pd, or Pt) or cationic carbonyls are produced after CO adsorption on supported metals (Ru, Rh, Ir, and Os). The interaction of CO with surface OH groups is also considered. It is demonstrated that IR spectroscopy of adsorbed CO is an efficient methodology to characterize cationic surface sites in terms of their nature, oxidation states, coordination environment and coordinative unsaturation, and location at faces, edges or corners of microcrystallites. When applied to materials with surface hydroxyl groups CO undergoes hydrogen bonding and information can be collected on the proton acid strength.

Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule

Palladium-based catalysts are widely applied in exhaust catalytic converter and catalytic combustion systems. The mechanism for methane oxidation on a Pd-based catalyst is complex. Catalyst activity is influenced by variations in the process pressure and temperature, by the gas mixture composition, by the type of support and various additives, and by pretreatment under reducing or oxidizing atmospheres. In this paper, we review the literature on supported Pd catalysts for combustion of methane. The mechanisms involved are discussed taking into consideration the oxidation/reduction mechanisms for supported palladium, poisoning, restructuring, the form of oxygen on the surface, methane activation over Pd and PdO phases, and transient behavior. Our review helps explain the array of experimental results reported in the literature.

Catalytic combustion of methane over palladium-based catalysts

Diesel particulate emission control

A large number of candidate soot oxidation catalysts were screened on their catalytic activity with a model soot. It was found that the intensity of contact between soot and catalyst is one of the major parameters that determine the soot oxidation rate. Two types of contact were studied; many catalysts increase the rate of soot oxidation considerably when the contact is intimate (‘tight’), whereas under conditions of poor (‘loose’) contact only some catalysts accelerate this oxidation reaction. It is tentatively suggested that (i) mobility of the catalyst is a major parameter determining the loose contact activity of catalysts and (ii) this mobility correlates with the melting point or the partial pressure of the catalyst.

Catalysts for the oxidation of soot from diesel exhaust gases. I. An exploratory study

Role of surface salt complexes in alkali-catalysed carbon gasification☆

Isotope tracer experiments prove the role of methyl in the oxidative coupling of methane and disprove a recently proposed mechanism involving methylene. The ethane product from oxidative coupling of CH4/CD4 mixtures over a Li/MgO catalyst consists of C2H6, C2D6 and CH3CD3 thus proving that ethane is formed by combination of methyl intermediates. The co-reaction of labelled CH4 (D and13C) with C2H4 produces propylene labelled predominantly at the methyl (3) position, thus proving C3 formation by terminal addition of methyl to ethylene rather than via a cyclic intermediate as has been proposed.

Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Processes for the conversion of hydrocarbons, and most preferably methane, to higher hydrocarbons and olefins are disclosed, including contacting the hydrocarbons with a mixed metal oxide catalyst having the pyrochlore structure Preferably, these catalysts have the general formula A n B 2 O 7-x , where A can be various mono-, di-, and trivalent metal cations, B can be various tri-, tetra-, penta-, and hexavalent metal cations, 0≦n≦20, and 0≦x≦10 Novel, high surface area, mixed metal oxide catalysts having the pyrochlore structure are also disclosed, including those having the formula A 2 B 2 O 7 , where A is divalent and B is niobium and/or tantalum, and those having the formula A 2 (Sn 2-y A y )O 7-z , where A is bismuth and/or lead, 00≦y≦10, and 00≦z≦10

High surface area oxide compositions with a pyrochlore structure, methods for their preparation, and conversion processes utilizing same

Fischer-Tropsch synthesis-A C2 initiator for hydrocarbon chain growth on ruthenium

The formation and reactivity of multilayer NiO films, both with and without added potassium, have been investigated using high‐resolution electron‐energy‐loss spectroscopy, low‐energy electron diffraction, and x‐ray photoelectron spectroscopy. We have found a strong interaction between K and NiO, as evidenced by the formation of K–O bonds, by a high‐temperature potassium thermal‐desorption peak and by an activated rate of NiO formation on K/Ni(100). We have also investigated the reactivities of the K‐doped NiO surface toward CH4 and H2 by monitoring the removal rate of oxygen from the surface. Our results demonstrate that the presence of K significantly enhances the reactivity of NiO with CH4, while the presence of K does not increase the reactivity of NiO toward H2.

Charles A. Mims

Papers

Role of surface salt complexes in alkali-catalysed carbon gasification☆

Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

High surface area oxide compositions with a pyrochlore structure, methods for their preparation, and conversion processes utilizing same

Fischer-Tropsch synthesis-A C2 initiator for hydrocarbon chain growth on ruthenium

Evidence for the potassium‐promoted activation of methane on a K‐doped NiO/Ni(100) surface