Introduction
In industry, the emphasis in catalyst characterization is mainly on developing an active, selective, stable, and a mechanically robust catalyst. In order for this to be done, tools are needed to identify the structural properties that discriminate efficient from less efficient catalysts and aid in catalyst design methods. X-ray photoelectron spectroscopy (XPS) is among one of the most common analytical methods used in determining catalyst characterization. XPS analyzes samples—determining properties such as chemical states and structures, catalyst surface compositions, and catalyst impurities.
Theory
XPS is based upon the principles of the photoelectric effect. The photoelectric effect was first proposed by Albert Einstein (1905), and later used in the development of XPS analytical methods by Swedish physicist Kai Siegbahn (1950’s). The photoelectric effect occurs when an x-ray (or any form of electromagnetic) beam of a given energy hn displaces a valence or inner shell electron from an atom.
(1)
Where A can be an atom, molecule, or ion, and A+ is an ion in an electrically excited state with a positive charge one greater than A. The ejected electron then possesses a kinetic energy Ek that is equal to the difference between the energy supplied by the photon, and the energy required to escape from the atom Eb (the binding energy). The following figure is a schematic representation of the physical processes involved in XPS.
Figure 1
The kinetic energy of the ejected emitted electron is measured in an electron spectrometer, and the binding energy is calculated using a corrected form of the equation for the photoelectric effect:
(2)
Where w is the work function of the spectrometer. The work function corrects for electrostatic effects of the environment in which the electron was measured. For example, differences in potential between the sample and the spectrometer could cause undesired acceleration or deceleration of the ejected electron, the work function accounts for such affects. The significance of this result is that the binding energy of an electron is characteristic of the atom and orbital from which the electron was ejected. And thus can be used to identify the atom.
XPS in Catalysis
For catalysts, XPS is most commonly used for surface analysis. Traditionally, the surface of a sample is irradiated by monochromatic x-rays, and the kinetic energy of each ejected electron is measured in an electron spectrometer. The binding energy is calculated, and a spectrum is produced giving the intensity of photoelectrons as a function of the their binding (or kinetic) energies. The binding energy of an electron may be regarded as an ionization energy of the atom for the particular electron shell involved. There are, for a given element, a variety of kinetic energies detected for an ejected electron because of the different energies associated with each shell. A set of binding energies is reasonably element specific, and differing elements show striking differences in their respective sets of binding energies. As a result of this, a table of standard electron binding energies can be used to determine the presence and composition of atoms in a sample. Below is the XPS spectrum of
Figure 2
The binding energies determined from an XPS analysis are not only element specific but give chemical information as well. This is because the energy levels of the core electrons also depend upon the chemical state of the atom, and chemical environment in which the atom resides. For example, the binding energy of an atom goes up with oxidation state. This is because the electrons of the nucleus of a cation feel a stronger attractive force than those of a neutral atom of the same element. In general, binding energy increases with increasing oxidation state, and with increasing electronegativity of ligands bonded to an atom (fixed oxidation state). A table showing the trend of bonding energy of an iron atom for the addition of ligands of increasing electronegativity is given below.
Table 1
Compound Eb (eV)
Iron Metal 706.7
Fe(CO)5 709.4
FeO 710.0
Fe2O3 710.7
FeBr3 710.0
FeCl3 711.1
FeF3 714.0
As a result, by comparing the bonding energies of XPS spectrum peaks to the tabulated bonding energies of elements contained within various molecular species, the molecular structure of sample can be arrived at.
XPS spectrums can also be used to determine the level of dispersion of a catalyst on a support phase surface. For a given intensity of x-ray emissions, the area of the spectral peak of a given analyzed substance is proportional to the concentration of the analyzed substance. For example, if a catalyst support surface has very low catalyst coverage, then a spectral analysis of the surface should show smaller relative peak area for the catalyst particles than for the support particles. Meijers et al used a spectral analysis of the aforementioned type to determine the best method of ZrO2/SiO2 preparation (see Fig. 3). Three ZrO2/SiO2 catalysts (labeled “nitrate”) were produced by wet deposition with an aqueous solution of zirconium nitrate. The fourth (labeled “ethoxide”) was prepared by contacting the support with a solution of zirconium ethoxide and acetic acid in ethanol.
Figure 3
The aforementioned characterization of catalyst surfaces is also used in studying the affects of different substances on the aging of catalysts, determining the composition of unstable intermediates, relating the effects of a catalyst’s structure on its efficiency, and detecting catalyst impurities.
Apparatus
Electron spectrometers used for XPS are made up of: (1) a source, (2) a sample holder, (3) a monochromater, (4) a detector, (5)a signal processor, and (6) a readout (see figure below).
Figure 4
Limitations
· Samples must be studied under high vacuum (as low as 10-10 torr), which is often very different from most catalyst’s operating environments.
· Only areas on the order of square millimeters can be studied at a time
· It is relatively surface specific—has a penetration depth of about 1.5 to 5nm.
References
Delglass et. al. Spectroscopy in Heterogeneous Catalysis. Academic Press: New York, 1979.
Nefedov, V. I. X-ray Photoelectron Sprctroscopy of Solid Surfaces. VSP: Utrecht, The Netherlands, 1988.
Niemantsverdriet, J. W. Spectroscopy in Catalysis. VCH: New York, 1993.
Skoog, Holler, and Nieman. Principals of Instrumental Analysis. Harcourt Brace & Company: Fort Worth, 1998.
Interesting Links
www.ornl.gov --Oak Ridge National Laboratatory
http://prins00.ethz.ch/index.htm --interesting links to various research topics in catalysis, surface science, zeolites, hydrotreating.