Importance of high pressure, surface sensitive in situ methods: 220th ACS National Meeting August 21-24, 2000, Washington, D.C. Importance of high pressure, surface sensitive in situ methods: A study of ammonia oxidation over copper with in situ NEXAFS in the soft X-ray range Ralf W. Mayer Michael Hävecker, Axel Knop-Gericke, Robert Schlögl Fritz-Haber-Institut der Max-Planck-Gesellschaft Dept. Inorganic Chemistry, Berlin, Germany Lu Gang, Bruce G. Anderson, Rutger A. van Santen Eindhoven University of Technology Laboratory of Inorganic Chemistry and Catalysis, Eindhoven, The Netherlands

Motivation NEXAFS in the soft X-ray energy range provides information of local electronic structure of all involved species (gasphase and solid state)  Determination of chemical structure/environment Measurement when reaction takes place = in situ in pressure range up to 10 mbar  „bridging the pressure gap“ High surface sensitivity (80-100Å) Aim: Determination of catalytically active surface species or intermediates Ammonia oxidation: 4NH3 + 3O2 2N2 + 6H2O partial oxidation* 4NH3 + 5O2 4NO + 6H2O total oxidation * industrially used reaction, e.g. for ammonia slipstream treatment Lu Gang et al., J. Catal. 186 (1999) 100 R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Experimental Setup Detector (schematical): Reactor: measure mode: Total Electron Yield Sample current IS Gasphase Signal IG Cylinder Signal IZ Collector Plate Signal IC Sample heater Separation window Synchrotron radiation  A.Knop-Gericke et al., Nucl. Instr. and Meth. In Phys. Res. A 406 (1998) 311-322 Reactor: Gas- mixture via MFC Pump MS pressure- control Radiation source - Beamlines: U49/1 and UE56/2 (Undulator) PM1 (bending magnet) BESSY II R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Data Treatment (N K-edge) 1) Normalization to beam intensity and window absorption 2) Adjust the gasphase and collector signal until white line of gaseous component (e.g. N-H*) disappears in difference signal 3) Substracting gasphase signal from collector signal gives difference signal with resonances from sample only N-H* (NH3) Intensity [a.u.] Photon energy [eV] R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Temperature dependence @ p=0.4mbar no / low activity below 570K initial higher activity with increasing temperature but: deactivation of the catalyst R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

@ p=0.4mbar, NH3:O2=1:12, var. temperature Deactivation NEXAFS-spectra @ p=0.4mbar, NH3:O2=1:12, var. temperature Deactivation due to formation of some N-species on the surface Cp. with reference spectra shows: Formation of Cu3N R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Temperature dependence @ p=1.2mbar also no / low activity below 570K at T570K significant higher activity than at p=0.4mbar no deactivation of the catalyst detectable R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Deactivation at p=1.2 mbar with CuO ? NEXAFS-spectra @ p=1.2mbar, NH3:O2=1:12, var. Temperature, init. state: CuO No deactivation within 2 hours @ 670K No formation of Cu3N or other nitrogen species R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Deactivation at p=1.2 mbar with Cu2O ? MS and NEXAFS-spectra @ p=1.2mbar, NH3:O2=1:12, 670K, init. state: Cu2O Deactivation within 2 hours @ 670K due to Cu3N-formation Only small amounts of NO in product gas, mainly nitrogen R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Temperature dependence @ p=0.8 mbar MS and NEXAFS-spectra @ p=0.8mbar, NH3:O2=1:12, var. Temperature, init. state: CuO PM 1 No deactivation within 2 hours @ 670K Main product changes from NO to N2 with reduction of CuO to Cu2O R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Correlation of NO and N2 with CuO @ 0.8mbar, 670K und NH3:O2=1:12 CuO as catalyst mainly produces NO; Cu2O is catalysing the N2 production R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Conclusions Big differences in temperature and time dependence between p=0.4, 0.8 and 1.2 mbar At p=0.4 mbar fast deactivation within 1 hour due to copper nitride formation At 1.2 mbar also copper nitride formation and therefore deactivation but only after copper (I) oxide is used as initial catalyst At 0.8 mbar reduction of copper (II) oxide to copper (I) oxide visible while main reaction product changes from NO to N2 Correlation of nitrogen and nitric oxide with copper (II) oxide possible:  CuO catalyzes the total oxidation of ammonia to nitric oxide  CuO inhibits the partial oxidation to nitrogen  Partial oxidation catalyzed by Cu2O, but fast formation of copper nitride R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany