A new reactor for the in-situ study of model catalyst surfaceS by Surface XRD and GISAXS from UHV up to reaction conditions

A new reactor for the in-situ study of model catalyst surfaceS by Surface XRD and GISAXS from UHV up to reaction conditions

M.-C. Saint-lager, P. Taunier, A. Bailly, R. Baudoing Savois, S. Cuccaro, P. Dolle, S. Garaudee, J.-P. Lévy, G. Perroux, O. Tissot, O. Ulrich, J.S. Micha

It is difficult to characterize industrial catalysts (usually nanoparticles supported on oxide powders) at the atomic scale. Thus the fundamental research in heterogeneous catalysis, aiming at understanding the catalytic processes at the molecular scale, is performed under UHV using model catalysts such as single-crystal surfaces. The recent development of surface science tools, such as surface X-ray diffraction and GISAXS, allows to overcome the two limits of this approach which are the so-called « pressure gap » (studying catalyst surfaces in pressure and temperature conditions close to the ones of the reaction) and the « material gap ».

This reactor project is inserted in this very active research field. The aims are:

(i) to follow the surface structure, composition and morphology of the model catalysts from the UHV environment where they are elaborated up to the reaction conditions (atmospheric pressure and temperature above the room temperature)

(ii) to extend the study from single-crystal surfaces to thin epitaxial films and to nanoparticles supported on oxide.

Fig.1: (a) Section of the reactor designed for GMT: the sample holder (green at the centre of the picture) receives the laser beam on the back for annealing. The x-ray beam enters the characterisation-reactor chamber (pink) through a Be window. The chamber is fixed and separated from the rotating mount (yellow) using a differentially pumped circle (DPC). Clean surfaces under reactive gas pressure or UHV can be then aligned by a goniometer head working in air. Total weight is about 50 kilograms. (b) Laser annealing setup used for surface cleaning and ordering.

The experimental set-up is composed of two main parts: (1) the reactor itself with beryllium windows for X-ray experiments. It is equipped with pumping systems and suitable pressure measurements for UHV up to atmospheric pressure, a reactive gas input and a mass spectrometer. The connection to the GMT diffractometer is ensured via a goniometer head for high-accuracy sample alignment. It is also equipped for UHV preparation of single-crystal surfaces (Ar+ sputtering and sample annealing). (2) the choice of a vertical surface allows a simple transfer from a UHV preparation chamber, equipped with the usual tools of surface science (LEED, AES, evaporation sources, …). It takes also benefit from the direction with the smallest natural divergence of the x-ray beam delivered by a bending magnet: Bragg's peaks of the (vertical) surface plane are studied with the best angular resolution and the maximum photon flux (slits opened).

Fig.2: Reactor mounted in vertical geometry on the GMT goniometer. Primary and secondary pumps are necessary to achieve UHV in the reactor (10-10 mbar). From the introduction of the reactive gases (for instance butadiene or H2) catalysis gas products and their partial pressure are determined by mean of a mass spectrometer.

The first experiment to check the sole reactor part was performed in January 2005. It has confirmed and completed the results previously obtained on ID03 dealing with the selective butadiene hydrogenation on Pd8Ni92(110). The whole system (reactor + preparation chambers) should be checked in march 2006 in GMT.