This is an author-produced version of a paper published in Angewandte Chemie International Edition (ISSN 1433-7851). This version has been peerreviewed, but does not include the final publisher proof corrections, published layout, or pagination. The final published version is available to subscribers from http://www3.interscience.wiley.com/cgi-bin/jhome/26737All articles available through Birkbeck ePrints are protected by intellectual property law, including copyright law. Any use made of the contents should comply with the relevant law. The term "Fe-Mo-O" is used to denote a mixed phase catalyst system as used, for example, in the production of formaldehyde which is an important building block molecule for complex chemicals in many consumer products. Almost all formaldehyde is manufactured by either: (i) dehydrogenation of a methanol-rich air mixture over a silver catalyst, or (ii) direct oxidation of methanol over a Fe-Mo-O based catalyst. [1,2a] The latter process is advantageous in that the exothermic reaction can be carried out at comparatively lower temperatures, typically 350°C, and the catalyst offers comparatively higher activity and selectivity. [2b] The mechanism is assumed to follow a Mars-van Krevelen like process of hydrocarbon oxidative dehydrogenation and this has been demonstrated for an industrial Fe-Cr-Mo-O preparation [3] . Methanol oxidation proceeds, as shown in equation (1), with a partial reduction of the active iron molybdate (Fe 2 (MoO 4 ) 3 ) phase, followed by catalyst regeneration [4,5] [3,[6][7][8][9][10][11][12][13][14] covering aspects of catalytic activity, deactivation and mechanism. In particular the Mo/Fe ratio is considered to be important; industrial preparations use a Mo/Fe ratio >1.5 comprising a mixture of two phases, Fe 2 (MoO 4 ) 3 and MoO 3 . It is thought that the primary function of MoO 3 is to provide a reservoir of molybdenum to balance losses through sublimation at reactor hot-spots which would otherwise prevent regeneration of the active phase and lead to the formation of Fe 2 O 3 which favours total oxidation [1,7,15] . However, there have not been any previous diffraction studies which focus on the interplay between these phases and their effect on catalytic reaction and regeneration. Here we show the importance of observing these reactions using rapid time-resolved in situ powder diffraction; in particular we capture the responses of the Fe-Mo-O catalyst under accelerated aging conditions and show the importance of the multiplephase form in maintaining catalyst longevity.A special environmental cell system has been designed to apply redox cycling conditions to catalyst material while collecting (in situ) powder diffraction data on the synchrotron; this cell is illustrated in Figure 1 and described, with operational details, in the Experimental Section. In order to confirm starting materials and basic methodology, diffraction patterns of the catalyst material, both as prepared and after reduction, were first analysed. The three component phases expected, Fe 2...
