The NH3 + O-2 reaction over Pt(1 0 0) single-crystal surface was studied under UHV conditions using photoemission electron microscopy (PEEM) as spatially resolving method and work function measurements via a Kelvin probe and temperature programmed reaction (TPR) experiments as integral methods. A broad hysteresis in the reaction rates occurs and one finds, reaction fronts as well as spatially homogeneous transitions in the adsorbate layer. (C) 2007 Elsevier B.V. All rights reserved.