AVS 56th International Symposium & Exhibition
    Applied Surface Science Friday Sessions
       Session AS-FrM

Paper AS-FrM5
Comparison of Supported Catalyst Particle Size Determination by Low Energy Ion Scattering (LEIS) and Transmission Electron Microscopy (TEM)

Friday, November 13, 2009, 9:40 am, Room C2

Session: Practical Surface Analysis
Presenter: T. Grehl, ION-TOF GmbH, Germany
Authors: R.A.P. Smith, Johnson Matthey Technology Centre, UK
D. Ozkaya, Johnson Matthey Technology Centre, UK
H. Brongersma, ION-TOF GmbH, Germany
T. Grehl, ION-TOF GmbH, Germany
H.R.J. ter Veen, Tascon GmbH, Germany
Correspondent: Click to Email
Particle size determination for supported metal catalysts is vital for the technological improvement of such systems in most areas of catalysis; automotive emission control catalysts, fuel cell materials to name but two. Improved measurements of the dispersion of a supported metal can be fed into product research and development with benefits including improved control of metal deposition and thereby, ultimately, efficient material use, particularly when the supported metal is from the platinum group of metals and so very expensive. The particle sizes of interest for many supported metal catalysts are in a single-digit nanometre range and for many techniques this poses a problem.

TEM is one of a limited number of techniques that can be applied in this particle size range but there is an inherent statistical difficulty - the areas studied are limited so might not be truly representative of a well-dispersed material although any information obtained is usually very accurate.

As a laboratory technique rather than large-scale facility such as is often required for effective spectroscopic determination of particle size, LEIS is of great potential use in this area. LEIS signals are directly related to the surface area of a given element that is exposed for the study and a reduction in particle size leads to higher surface area to volume ratio and therefore inherently larger signal from scattered ions for the amount of material present. In addition, although the single outermost layer of a surface is the principal signal of interest, a signal due to ions scattered from deeper layers can be measured and is useful for determination of extremely thin (up to 10 nm) layers. This makes the technique useful for electronics applications but it can also be applied to characterise how "thick" a particle is, leading to a second method of particle size determination. For these two reasons, smaller particles yield good information when studied by LEIS. The technique is a large-area (1 mm scale) technique so provides statistically representative average information. LEIS also has advantages over TEM in that it works just as easily on supports with heavy elements (zirconia, ceria) as light elements (carbon, alumina).

In this presentation we show results of a comparative study of LEIS and TEM used for the characterisation of model fuel cell materials, specifically platinum supported on carbon with a range of particle sizes under 10 nanometres. The results show that LEIS is an effective laboratory technique that can provide excellent characterisation data on small particles that can be correlated with other techniques to improve understanding of commercial systems.