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XPS on heterogeneous catalysts – beware of BE scale calibration!

September 13, 2013

X-ray photoelectron microscopy (XPS) is very commonly used by researchers in the field of heterogeneous catalysis. What is more important than the surface of solid catalyst, where all reactions take place? XPS allows us to analyze the composition of the catalyst surface. XPS allows us to grab some info on the dispersion of active species of the supports. XPS allows us to determine the oxidation state or describe the chemical environment of some elements. XPS allows us to track impurities responsible for catalyst deactivation. XPS is indeed quite useful.

An XPS spectrum gives the count of emitted electrons as a function of their binding energy (BE). One crucial step of XPS data precessing is when we calibrate the BE scale. Indeed, surface charges shift the energy of the emitted electrons. So we need an element for which we know a priori the BE to shift back the whole spectrum in the appropriate position.

In the field of heterogeneous catalysis, the classical practice is to use the carbon contamination as an internal standard. This strategy is satisfactory in many cases, especially when people analyze a set of samples with similar composition (same catalyst treated under different conditions, same formulations with different loading, etc.).

We show in our recent paper (Jacquemin et al. ChemPhysChem 2013) that this strategy is however not appropriate when analyzing a set of samples which differ significantly in terms of bulk composition. The chemical nature of the inorganic matrix onto which the carbon contamination is deposited actually affects the energy of the electrons emitted by the latter. And the calibration based on the C peak is thus biased. We study this phenomenon, tracking down the possible technical explanations, discarding them systematically and proposing two possible “chemical” explanations. Our work should be seen as a warning at the address of the community: be careful when you use the C 1s peak as an internal standard for the BE scale calibration!

Interestingly, some researchers seem to be well aware of that problem. In a minority of catalysis papers, another reference element is chosen. For example, one element of the inorganic support for which we have a BE value with a good consensus. This choice – though more appropriate in some cases – appears as arbitrary and is never explained.

Our paper provides a systematic study and hopefully helps giving more rationality behind calibration practices.

DD

ref: “Calibration of the X-Ray Photoelectron Spectroscopy Binding Energy Scale for the Characterization of Heterogeneous Catalysts: Is Everything Really under Control?” Marc Jacquemin, Michel J. Genet, Eric M. Gaigneaux, Damien P. Debecker, ChemPhysChem 2013 DOI: 10.1002/cphc.201300411

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2 Comments
  1. Our paper is spotlighted in the current issue of Angewandte Chemie! http://onlinelibrary.wiley.com/doi/10.1002/anie.201384413/abstract

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