Surface X-Ray Diffraction on model catalysts in action

Click here to check out our results on catalytic reaction oscillations and the role of steps in catalysis (including SXRD movies; Nature Chemistry)

It has been known for many years that there is a strong interplay between the surface structure of a catalyst and the actual catalytic reaction process. Figure 1 shows the dramatic effect of the catalytic reaction conditions on the surface structure of a catalyst particle. The Pt particle has served as a CO oxidation catalyst under realistic pressure and temperature conditions. One can clearly see that the catalyst surface has been altered strongly by the presence of the reactants and the product. The process that has caused this alteration is not revealed by such post-reaction observations. The only way to actually ‘see’ and understand surface rearrangements and correlate them with the catalytic mechanism at work, is to ‘look’ at the surface of the catalyst during the reaction, and while all reactant and product gasses are present. It is expected that similar structure modifications take place during many catalytic reactions on a wide variety of catalyst surfaces, i.e. that the alteration of the catalyst during and due to the reaction is a general effect in catalysis.

Figure 1: Pt catalyst particle before (left panel) and after exposure to elevated pressures of CO and O2 (middle and right panel). The dramatic effect of the catalytic conditions on the surface morphology is obvious. Reproduced from: Flytzani-Stephanopoulos et al., J. of Catalysis, 49 (1), (1977).

Traditionally, in surface science, the characterization of surface structure or morphology is carried out under ultrahigh vacuum (UHV, pressure < 10^-9 mbar) conditions, allowing researchers to work with clean and ordered surfaces and to control the type and number of molecules interacting with the surface accurately. This is advantageous when trying to understand the fundamental interaction of molecules with singlecrystalline surfaces. In UHV conditions electrons and ions also have a very long mean free path compared to ambient pressure conditions, allowing one to use techniques that require these long mean free paths (e.g. LEED, LEIS, SEM and TEM). However, in ‘real life’ catalysis the vast majority of processes takes place at elevated temperatures and at high pressures (> 0.1 bar). This constitutes a formidable pressure gap of easily nine or more orders of magnitude between realistic and laboratory conditions. In addition, there is also an important materials gap, since a catalyst is usually not a single-crystal surface but very often it consists of oxide supported, nanometer size particles. This means that particle size effects and particle-support interactions possibly contribute to and influence the catalytic mechanisms and performance.

The interface physics group uses multiple techniques to study model catalysts at elevated pressures: two scanning probe microscopy techniques, namely Reactor-STM and Reactor-AFM, and two x-ray based techniques, surface x-ray diffraction (SXRD) and grazing incidence small angle x-ray scattering (GISAXS). Since the x-ray based techniques require very intense x-ray radiation they are performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France.  

Instrument development
In collaboration with the beamline staff of the ID3 surface diffraction beamline at the ESRF we have developped a high pressure/UHV flow reactor, shown in fig. 2. This setup allows us to prepare model catalyst surfaces under UHV conditions after which they can be studied under realistic conditions (pressure up to 1200 mbar and temperatures up to 950 K). The UHV system is also equipped with a Quadrupole Mass Spectrometer (QMS), with which we can analyze the composition of the gas in the reactor. In this way we can correlate the atomic-scale surface structure with the chemical activity of a model catalyst surface. A full technical description of the setup can be found here. If you are interested in buying the instrument, go to the Leiden Probe Microscopy website.

Figure 2: Flow reactor mounted on the diffractometer at the ID3 beamline.

Research
The central question of our research is: what is, under realistic conditions, the structure of the catalyst surface and how does this, in turn influence the reaction mechanism and, thus, the reactivity and selectivity?

Most of the experiments up to now have focussed on the study of CO oxidation and NO reduction on single crystals of Pt-group metals and role surface oxides play in these reactions. Several (co-)publications on the subjects can be found below or on the groups publication list.

Recent Publications

	"Generation of Pd model catalyst nanoparticles by spark discharge" M.E. Messing, R. Westerström, B.O. Meuller, S. Blomberg, J. Gustafson, J.N. Andersen, E. Lundgren, R. van Rijn, O. Balmes, H. Bluhm, K. Deppert
	J.Phys. Chem. C 114 (2010) 9257-9263	Full Text [PDF 4 MB]
	"Comment on "CO oxidation on Pt-group metals from ultrahigh vacuum to near atmospheric pressures. 2. Palladium and Platinum"" R. van Rijn, O. Balmes, R. Felici, J. Gustafson, D. Wermeille, R. Westerström, E. Lundgren, J.W.M. Frenken
	J. Phys. Chem. C 114 (2010) 6875-6876	Full Text [PDF 707 kB]
	"Ultrahigh vacuum/high-pressure flow reactor for surface x-ray diffraction and grazing incidence small angle x-ray scattering studies close to conditions for industrial catalysis" R. van Rijn, M.D. Ackermann, O. Balmes, T. Dufrane, A. Geluk, H. Gonzalez, H. Isem, E. de Kuyper, L. Petit, V.A. Sole, D. Wermeille, R. Felici, J.W.M. Frenken
	Rev. Sci. Instrum. 81 (2010) 014101	Full Text [PDF 821 kB]
	"Catalytic activity of the Rh Surface Oxide: CO oxidation over rh(111) under realistic conditions" J. Gustafson, R. Westerström, O. Balmes, A. Resta, R. van Rijn, X. Torrelles, C.T. Herbschleb, J.W.M. Frenken, E. Lundgren
	J. Phys. Chem. C 114 (2010) 4580-4583	Full Text [PDF 609 kB]
	"The ID03 surface diffraction beamline for in-situ and real-time X-ray investigations of catalytic reactions at surfaces" O. Balmes, R. van Rijn, D. Wermeille, A. Resta, L.Petit, H. Isern, T. Dufrane, R. Felici
	Catalysis Today 145 (2009) 220-226	Full Text [PDF 518 K]