Invited Paper SS-WeA7
Multiple Time and Length Scales in Nanocatalysts Probed in a Single Synchrotron Experiment: The Combined use of XAFS, XRD, DAFS and IR

Wednesday, October 20, 2010, 4:00 pm, Room Santa Ana

X-ray absorption fine structure (XAFS) and x-ray diffraction (XRD) techniques give complementary information about the structure of catalytic materials. XRD is effective in crystalline materials that possess medium to long range order (bulk catalysts, substrates and templates) while XAFS provides short range structural details in disordered, amorphous and/or low-dimensional materials. In addition, XAFS gives information about the electronic properties of the catalysts. These two methods have been developed and advanced independently from each other at synchrotron sources in the US and abroad. To analyze catalysts in situ, in particularly in operando (under their operating conditions), a new approach is needed, namely, the simultaneous collection of the XRD and XAFS data in real time as the reaction progresses, together with the online product analysis. Diffraction Anomalous Fine Structure (DAFS) is a structure-sensitive technique that allows to deconvolute multiple phases of the same element (e.g., metal nanoparticles and metal oxide) that can coexist in the sample. Application of quick scanning monochromator mode to XAFS measurement (called QEXAFS) allows to study kinetics of structural transformations within the reactants, catalysts and the reaction products. Such combinations allow to measure the time-dependent changes in the actual structure (in the short, medium and long range order), electronic properties and chemical activity of catalysts synchronously.

The first in US dedicated instrument for such combined measurements was built at the beamlines X18A and X18B of the National Synchrotron Light Source at Brookhaven National Laboratory. The current setup includes transmission and fluorescence XAFS detectors, QEXAFS monochromator enabling 10ms time EXAFS scan time, 2D area detector for XRD, residual gas analyzer and the automated gas mixing system. The upgrades currently under way include the addition of Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) instruments developed jointly by members of Synchrotron Catalysis Consortium at BNL and Harrick Scientific.

I will present several applications of the combined use of these instruments for catalysis research. including the studies of knietics of reduction and oxidation of Cu-ceria catalysts, the investigation of the mechanism of reduction of CuFe₂O₄ with CO, and others.