Invited Paper IS+SY+SS-WeM5
Nano Scale X-ray Absorption Spectroscopy of In Situ Modified Samples using Scanning Transmission X-ray Microscope

Wednesday, October 22, 2008, 9:20 am, Room 310

Observation and spectroscopy of in situ chemical reactions on nanoscale is important but not easy task. It is important in many fields, especially in research of heterogeneous catalysts. Typically, transmission electron microscopy has been used under in situ conditions, but the strict low pressure requirements for this technique prevent it from being carried out under catalytically relevant conditions. Recently however, by using a specially designed microreactor, promising TEM experiments have been performed under 1 bar conditions but with a limitation of gases which could be used.¹ Other techniques include STM, fluorescence and optical microscopy. Most of these techniques allow imaging of intermediate steps of reactions with limited information about reaction chemistry. Scanning Transmission X-ray Microscopy (STXM) with relatively good spatial resolution of 20-30 nm, high chemical sensitivity and much lower restriction on the environment of a sample is promising to provide new insides into the catalytic reactions. By using soft X-rays (130 -2000 eV) as a microprobe, it is possible to study both the active phase of the catalyst, through absorption edges of the inorganic species, and the organic, reactant phase of the catalyst through the absorption edges of carbon, oxygen and nitrogen. While the maximum spatial resolution of STXM is still significantly worse than for example STEM-EELS, there are important advantages in application STXM, most notably the superior spectral resolution and the reduced radiation damage in the study of softer core hole edges (C, N, O K-edges) and hydrated samples. Bell et al.² first demonstrated the use of STXM for the study of a catalytic material under in situ conditions. Their cell design allowed sample treatments in gases up to 260 °C. E. de Smit et al.³ used modified ETEM cell to in situ studies of a complex iron oxide based Fischer-Tropsch catalyst. They were able to heat the sample up to 500oC at gas pressure up to 1.2 bar opening a way to study real catalyst normal conditions. The cell, manufactured as a microelectromechanical system (MEMS), basically consists of a 500 * 500 mm reactor chamber connected by micrometer-sized gas-flow channels. Two amorphous Si3N4 optical windows, etched down to a thickness of 10 nm in certain places, separate the reactor from the outside environment. The thickness of the reactor, thus gas path length, is only about 50 mm, and therefore ensures minimum attenuation of the X-ray light by the gas phase molecules. Spacers, placed between the windows prevent the windows from sticking together. The sample is loaded by flowing a powder suspended in a solvent (e.g. ethanol) through the reactor and subsequent drying. After drying, the sample is supported on the SiNx windows. E. de Smit et al were able to track reaction steps measuring changes in chemical state of iron and oxigen. They also investigated a role of carbon in the reaction. Progress in technical development of reaction cell and increasing resolution of the x-ray microscope provide opportunity to investigate gas - solids or liquid - solids in situ, in relevant conditions on nanometer scale.

¹J. F. Creemer, S. Helveg, G. H. Hoveling, S. Ullmann, A. M. Molenbroek, P. M. Sarro and H. W. Zandbergen, Ultramicroscopy, In Press, Accepted Manuscript.
²I. J. Drake, T. C. N. Liu, M. Gilles, T. Tyliszczak, A. L. D. Kilcoyne, D. K. Shuh, R. A. Mathies and A. T. Bell, Review of Scientific Instruments, 2004, 75, 3242-3247.
³E. de Smit , I. Swart, J. F. Creemer , G. H. Hoveling, M. K. Gilles , T. Tyliszczak, C. Morina, P. J. Kooyman, H. W. Zandbergen, B. M. Weckhuysen and Frank M.F. de Groot, Nature, submitted manuscript.