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. MayerMichael 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

220th ACS National Meeting August 21-24, 2000, Washington, D.C.

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

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Motivation

Lu Gang et al., J. Catal. 186 (1999) 100

Experimental Setup

Reactor:

Gas-mixturevia MFC

PumpMS

pressure-control

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Radiation source - Beamlines:

• U49/1 and UE56/2 (Undulator)• PM1 (bending magnet)

BESSY II

Detector (schematical):measure mode: Total Electron Yield

Sample current IS

Gasphase Signal IG

Cylinder Signal IZ

Collector Plate Signal IC

Sample heater Separation window

Synchrotronradiation

A.Knop-Gericke et al., Nucl. Instr. and Meth. In Phys. Res. A 406 (1998) 311-322

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

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

N-H* (NH3)

Int e

ns i

t y [

a. u

. ]

Photon energy [eV]

Temperature dependence @ p=0.4mbar

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

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

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

Temperature dependence @ p=1.2mbar

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

also no / low activity below

570K

at T570K 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

No deactivation within 2 hours @ 670K

Main product changes from NO to N2 with reduction of CuO to Cu2O

PM 1

Correlation of NO and N2 with CuO

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

CuO as catalyst mainly produces NO; Cu2O is catalysing the N2 production

@ 0.8mbar, 670K und NH3:O2=1:12

R.W. Mayer, Dept. Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany

Conclusions

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

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: