DOI: 10.1002/cctc.201000061 Dynamic Structure Changes of a Heterogeneous Catalyst within a Reactor: Oscillations in CO Oxidation over a Supported Platinum Catalyst Jagdeep Singh, [a] Maarten Nachtegaal, [b] Evalyn M. C. Alayon, [a, b] Jan Stçtzel, [c] and Jeroen A. van Bokhoven* [a, b] The rational design and synthesis of tailored catalysts and cata- lytic processes require understanding of the structure–perfor- mance relationship under reaction conditions involving high temperature and pressure. The catalytic oxidation of CO is one of the best studied reactions in catalysis. [1] This reaction occurs in automotive catalysts and is important in removal of CO from streams of hydrogen in fuel cell applications. [2, 3] Surface sensitive techniques, such as low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelec- tron spectroscopy (XPS), and Auger electron spectroscopy (AES), have greatly contributed to the understanding of the ox- idation of CO taking place on well defined single crystal surfa- ces. [1, 4] During CO oxidation, two or more reaction regimes with distinct CO oxidation rates have been identified on model single crystal catalysts and technical catalysts. [5, 6] There is con- sensus that a CO-covered metallic surface shows low activity because of poisoning by the CO. [7] The surface structure of a highly active state can be either a surface oxide [8–12] or an oxygen-covered surface. [13] Surface science studies showed that under specific conditions the oxidation of CO will oscillate on well-defined single crystal surfaces such as the Pt(110) sur- face. [14, 15] Oscillations are not limited to vacuum conditions and also occur on technical catalysts under actual catalytic condi- tions. [16, 17] Most of the above stated techniques are limited to low pressure environments, and are thus far away from indus- trially relevant conditions. Furthermore, the structure of single crystal model catalyst surfaces deviates from that of technical catalysts, especially when the particles are nanometer in size and supported on an (active) support to increase their reactivi- ty. Deriving a fundamental understanding of the correlation between structure and performance of a technical catalyst inside a reactor under operating conditions is thus at the fron- tier of contemporary catalytic research. [18–21] Thanks to the penetration depth of hard X-rays, synchro- tron-based X-ray techniques allow for the study of the catalyst structure in a reactor under operating conditions. X-ray absorp- tion spectroscopy (XAS) is particularly powerful for in situ stud- ies of nano particles, because it probes the average local geo- metric (up to about 5 ) and electronic bonding environment of the element of interest. As a result, XAS has been increas- ingly applied over the last three decades to uncover the struc- ture of catalysts under operating conditions. Recent develop- ments in beamline optics allow for the study of the kinetics of structural changes of the catalyst in the reactor with a subsec- ond time resolution [22–26] and for investigating the catalyst structure with a (sub)micrometer resolution. [14] We combined in situ time- and space- resolved X-ray absorption spectrosco- py with mass spectrometry and infrared (IR) spectroscopy to derive at a spatially resolved understanding of dynamic struc- ture–performance relationships in a reactor [28] during a famous oscillating reaction: the oxidation of carbon monoxide (CO). In our experiments, XAS at the Pt L 3 edge enables determin- ing the oxidation state and local structure of the platinum under actual reaction conditions. The white line in the L 3 edge XAS spectra is sensitive to the structure of the metal and to the number of holes in the d band. Complimentary in situ in- frared spectroscopy detects adsorbates on the surface of the catalyst. Figure S1 (see the Supporting Information) shows the pellet and packed-bed reactors used to study the oscillations in CO conversion over supported Pt nanoparticles with infrared respectively quick XAS spectroscopy. Both reactors were cou- pled to a mass spectrometer to follow the integrated activity of the catalyst at the end of the reactor. CO conversion was achieved by heating reduced Pt particles in a flow of O 2 and CO at a molar ratio of 19:1 to 382 K. At this temperature, the catalyst showed high activity and full conversion of CO. Then, cooling of the reactor was started. Figures 1 a and b show the traces of CO 2 , CO, and O 2 as detected in the mass spectrome- ter upon cooling in the infrared and XAS experiments, respec- tively. In both reactors, oscillations in the signal of CO 2 were detected with increasing intensity over time. One oscillation (Figure 1 a) was characterized by a slow de- crease in the amount of CO 2 followed by a sharp increase of the signal to a level higher than characteristic of a full CO con- version. This can be explained by storage of CO on the catalyst during the decrease of CO 2 production, which is suddenly re- leased during the sharp rise in CO 2 . With decreasing CO 2 , an enhanced CO signal was detected. The CO signal showed a maximum, and thus the catalyst showed its lowest activity, a few seconds past the minimum in the CO 2 signal. The CO signal returned to its base level at the same time as the CO 2 signal came back to its base level. Within an oscillation, the de- crease in CO 2 production was larger than its excess during the spike. This indicates that the integral activity of the catalyst de- creased in the first part of the oscillation, which was also indi- cated by the enhanced CO signal. The decrease of the CO 2 and increase of CO signal at the exhaust were paralleled by an in- [a] J. Singh, E. M. C. Alayon, Prof. Dr. J. A. van Bokhoven Institute of Chemical and Bioengineering ETH Zurich, 8093 Zurich (Switzerland) Fax: (+ 41) 43-362-1162 E-mail : j.a.vanbokhoven@chem.ethz.ch Homepage: http://www.vanbokhoven.ethz.ch [b] Dr. M. Nachtegaal, E. M. C. Alayon, Prof. Dr. J. A. van Bokhoven Paul Scherrer Institute PSI, CH-5232 Villigen (Switzerland) [c] J. Stçtzel Dept. of Physics, Faculty C, University of Wuppertal Gaubstrasse 20, 42119 Wuppertal (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201000061. ChemCatChem 2010, 2, 653 – 657  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 653