The replacement of superheated steam with CO2 for ethylbenzene (EB) dehydrogenation to produce styrene (ST) is of great importance from both scientific and practical perspectives, while it is still a big challenge to design and synthesize catalysts for dehydrogenation in the presence of CO2. Research into shape-controlled nanocrystals has proved that well-defined facets are of great importance for understanding the catalytic phenomena due to their definite surface structure. In the present work, alpha-Fe2O3 nanocrystals enclosed by various crystal facets provide us the clue to understand the role of CO2 in dehydrogenation. O(III) (the triply coordinated surface oxygen atom) is found to play an important role in activating CO2 under reaction conditions, which is supported by both experimental and theoretical results. These results have deepened knowledge about the role of CO2 in dehydrogenation over pure alpha-Fe2O3 catalysts, which is significant in designing relative catalysts rationally. (C) 2016 Elsevier Inc. All rights reserved.