The concept of ensembles is based on the idea that adsorbed species and intermediates in catalytic reactions require more than one surface atom. This model can probably be traced to the mul-tiplet theory of Balandin [1], who, as early as 1929, made the assumption that a reacting molecule can be simultaneously adsorbed on several atoms.

A Quantitative Approach to the Ensemble Model of Catalysis by Metals

This paper reviews recent developments in the application of magnetic methods for investigation of Fischer-Tropsch catalysts involving cobalt, iron and nickel. Magnetic characterization provides valuable information about catalyst reduction, sizes of ferromagnetic nanoparticles, chemisorption on ferromagnetics and topochemical reactions which occur with the catalysts during the genesis of the active phase and in the conditions of Fischer-Tropsch synthesis. The capabilities and challenges of the magnetic methods are discussed.

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Magnetic Characterization of Fischer-Tropsch Catalysts

Carbon monoxide hydrogenation at 5 MPa was investigated on silica-supported Ni, Ni-Cu, Ni-Li and Ni-Cu-Li catalysts, with special focus on C 2+ OH formation Copper addition does not result in formation of higher alcohols while lithium does Over Ni-Cu-Li/SiO 2 , selectivities towards alcohols as high as 85% are observed and higher alcohols such as ethanol and propanol were effectively produced Addition of copper to nickel results in a remarkable stabilization of the catalytic activity while lithium promotion increases aging of the catalyst significantly Infrared spectroscopy of adsorbed carbon monoxide shows that copper addition inhibits subcarbonyl formation (Ni(CO) x , x = 2 or 3) and increases the relative abundance of linear species When lithium promoter is added to Ni/SiO 2 , some is deposited on the silica support whilst the remaining decorates the metal particles Subcarbonyl species are stabilized by lithium addition The spectra obtained over Ni-Cu-Li/SiO 2 are practically composed of only two bands, subcarbonyl and linear By comparing catalytic pattern and infrared data it is concluded that the presence of subcarbonyl species is related to catalyst aging and that active sites for C 2+ OH formation are, either isolated nickel atoms associated with Li + , or linked to subcarbonyl formation

Alcohol synthesis from syngas over nickel catalysts: Effect of copper and lithium addition

Theoretical model of chemisorption on metals: I. Formalism

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co, Ni, Cu, Rh, Pd and Ag

H2 adsorption studies have been carried out on a series of supported Ni–Cu alloys ranging from pure Ni to 60 Cu at.%. Reduction and homogeneity of the metallic phase are controlled by Curie point and spontaneous magnetization determinations. When H2 is admitted, the 4 K saturation magnetization decreases linearly as the quantity of adsorbed gas increases. Desorption and adsorption curves suggest only one state of chemisorption. The decrease of magnetization per adsorbed molecule (α) is twice as large as the magnetic moment of Ni in the alloys. This result rules out the collective model to describe H2 chemisorption on Ni, and suggests that Ni sites are magnetically decoupled from subjacent moments upon chemisorption. A complete demetallization of Ni atoms would lead to another expression for α. Thus, it may be concluded that Ni atoms remain in their metallic state. This result is compared to magnetic effect observed on other ferromagnetic catalysts (Fe, Co): in all cases, H2 chemisorption results in a change of the superficial magnetic structure.

https://iopscience.iop.org/article/10.7567/JJAPS.2S2.261/pdf

B. Imelik

Papers

Adsorption of H2on Ni-Cu Alloys Studied by Magnetic Measurements