By preparing at 1200°C solid solutions of formula NixMg1–xO, where ions of stable valency (Mg2+) are interposed between nickel ions without change in structure, it has been possible to study how the chemisorption properties and the catalytic activity of the transition metal ions are influenced by isolation. Solid solutions of the type NixLiyMg1–x–yO have also been investigated.
Chemisorption of oxygen has been studied from –78 to 550°C and catalysis of N2O decomposition from 300 to 500°C. The activity per nickel ion is found to increase as the ions are diluted by magnesium ions, reaching a maximum at about 1 % nickel. Chemisorption occurs preferentially on nickel ions, and activity in the catalysis is paralleled by the occurrence in chemisorption studies of a reversible type of oxygen adsorption which develops as nickel ions are diluted.
The drift of charge across an array of Ni2+/Ni3+ ions, which gives rise to p-type semiconductivity in NiO, is considered to be much less relevant for the catalysis of N2O decomposition than the nature of the octahedral complex formed at the surface by chemisorbed oxygen. Several forms of oxygen chemisorption are discussed, including a strong form which occurs on lithium-containing specimens and inhibits the catalysis.
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