A series of thermally stable alpha-alumina supported and unsupported ceria catalysts with varying composition is systematically investigated under realistic coke - catalyst contact conditions for the oxidation of in-situ generated coke during ethylene pyrolysis. These catalysts have been designed for use in high temperature applications of steam cracking. The textural and structural characterization of alpha-alumina supported ceria catalysts show the absence of solid oxide solutions, with the supported catalysts having distinct ceria domains on the alumina support. For comparison, two types of coke - catalyst contacting are explored: (i) tight contact by grinding industrial coke with catalyst and (ii) realistic in-situ contact, where coke is deposited on the catalytic support. During in-situ coke deposition, ceria-containing catalysts demonstrate resistance to coking as compared to the bare alpha-alumina support. Ceria and ceria - alumina composites also enabled coke oxidation at lower temperatures than the un-catalyzed coke oxidation. The presence of both Ce3+ and Ce4+ is confirmed by X-ray absorption near edge structure (XANES), consistent with the well-known redox capability of ceria catalysts. Kinetic studies revealed 50-80 mol% Ce as the best compositions for oxidation activity towards both industrial and in-situ coke. The catalytic activity correlates with the presence of reactive lattice oxygen atoms on ceria for both types of contact, indicating a similar mechanism of carbon oxidation under in-situ contact and tight contact conditions. A mechanism of reaction involving lattice oxygen of ceria is proposed for the oxidation of coke as well as for retardation of coke deposition on ceria domains.