Bare alumina support transforms into an active catalyst for the dehydrogenation of ethylbenzene to styrene in CO2 or N-2. During the first 15 h time on stream in CO2, or the first 10 h time on stream in N-2, the alumina shows an increase in ethylbenzene conversion and styrene selectivity from 15% to 60% and from 60% to 92%, respectively, under industrially relevant conditions of 600 degrees C and 10 vol% ethylbenzene. Thereafter, the system slowly deactivates, but remains highly selective. TGA analysis shows an increase in coke content. The specific surface area and pore volume show a decrease with time on stream. TEM-imaging reveals that the spent catalyst surface is completely covered by several layers of coke. These results combined suggest that the carbon deposits on the alumina are responsible for the increase in activity and selectivity, and also are the cause of deactivation once a monolayer of carbon is deposited on the support surface. Similar trends are observed for zirconia support. Supported vanadium and chromium oxides on alumina all give similar results, but after a faster activity development. Also for these supported catalysts and even carbon samples, deposited coke is the active and selective phase. (C) 2012 Elsevier B.V. All rights reserved.