Surface characterization and IR spectroscopy

Generally, the features of a surface determine the properties of a material and therefore must be characterized:

• Topology, morphology

• Elemental composition

• Chemical bonding of elements

• Structure (geometric and electronic)

Since high information content, high spatial resolution and high absolute and relative detection power are required, that can only be met by physical techniques based on the interaction of photons, electrons, ions and electrical fields with the material investigated [2].

Complete understanding of catalytic reactions mechanisms, including the nature of adsorbed intermediates is highly desirable. However, as such should reasonably be expected to provide major assistance in reaching the goals of better catalysts and improved catalytic processes from a better fundamental understanding of catalyst surface chemistry. This is an area in which infrared (IR) spectroscopy undoubtedly makes further major contributions. A variety of IR techniques can be used to obtain information on the surface chemistry of different solids.

Special meaning have investigations carried out under the reaction conditions. This includes spectral characteristics of reaction components, surface changes due to temperature treatment and many others. In principle, all forms of IR spectroscopy, including

transmission-absorption, diffuse reflectance (DRIFT), ATR (attenuates total reflection) and photo acoustic spectroscopy (PAS), are suitable for in situ measurements. The principal information obtained with all these techniques is equivalent and local availability and experimental necessities such as the sample particle size and the molecular extinction coefficient of the sample may dominate personal choices. For most practical experimental reasons the vast majority of experiments are currently performed in the transmission-absorption and the diffuse reflectance mode. This is more related to the design of cells to be used as reactor than to the principal problems of the other techniques. The IR cell in which the catalyst sample is pre-treated and subsequently studied is extremely important in surface studies [175, 176].

The cell is normally chosen to suit the purposes of a particular study. Some features are usually of overriding importance in a given application. Various complex schemes have been designed to seal the reactor cell with IR-transparent windows so that the IR cell can be operated at elevated temperatures and pressures. The development of high temperature and pressure IR cells has permitted the observation of adsorbates under reaction conditions. These cells may serve as a differential reactor for steady-state reaction, temperature-programmed reaction or desorption, and unsteady-state reaction studies. Therefore, the IR cell suitable for investigations of catalyzed reactions must fulfil two requirements: (a) it must allow the recording of IR spectra under in situ reaction conditions, and (b) its volume and construction must assure good mixing the gases inside, and the feasible space velocities must allow flexible variations of the conversion exposing the catalyst to the reactants and products that can be analysed precisely at the exit of the reactor cell [176].

However, the use of the infrared spectroscopy in heterogeneous catalysis can be classified: (1) the determination of catalyst bulk structures, (2) identification of adsorbed species and surface active sites, (3) characterization of surface hydroxyl groups (including Brönsted sites) and (4) examining the catalyst surface structures (Fig. 5).

Fig. 5. Applications of IR spectroscopy in catalysis and surface science

From the discussions above and studies of others, IR can provide information concerning the metal-oxygen vibrations in the region below 1000 cm^-1 [177]. Therefore, the metal-oxygen vibrational frequencies can be roughly divided into five characteristic ranges as shown in the table below:

Table 3. IR characteristics of metal-oxygen bonds

Vibrational mode Wavenumber (cm^-1)

Symmetric and asymmetric stretches of M=O 900-1100

Asymmetric stretches of M-O-M 700-900

Symmetric stretches of M-O-M 500-700

Bending vibrations of M=O 310-400

Deformation vibrations of M-O-M ~ 200

Diffuse reflectance spectroscopy (DRIFT)

The optical phenomenon known as diffuse reflectance is commonly used in the UV–Vis, NIR, and MIR regions to obtain molecular spectroscopic information [175-177]. When it is applied in MIR area with a Fourier transform it is known as diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). It is usually used to obtain spectra of powders with minimum sample preparation. The collection and analysis of surface-reflected electromagnetic radiation as a function of frequency or wavelength obtain a reflectance spectrum. Two different types of reflection can occur: regular or specular reflection usually associated with reflection from smooth, polished surfaces like mirrors, and diffuse reflection associated with reflection from so-called mat or dull surfaces textured-like powders. In diffuse reflectance spectroscopy, electromagnetic radiation reflected from dull surfaces is collected and analysed. If a sample to be analysed is not shiny, and whatever reason is not amenable to conventional transmission spectroscopy, diffuse reflectance spectroscopy is a logical alternative. The advent of FTIR spectrometers has led to the widespread application of DRIFT becoming a valuable technique since hardly any sample preparation is necessary. This implies that the DRIFT technique is potentially of great value for in situ studies of catalytic systems.

One of the interesting advantages of DRIFT is the prevention of typical transmission problems at high wavenumbers (due to scattering) and at low wavenumbers (due to strong absorption of catalyst carriers). Another advantage is the high sensitivity of DRIFT.

Furthermore, the catalyst powder need not be compressed to obtain high quality spectra. This improves the reproducibility and the activation of the catalyst. Nevertheless, there is a limited number of commercially available, heatable and evacuable DRIFT cells in combination with a specially designed optical system that is suitable for the in situ activation and pre-treatment of catalyst samples at high temperatures and pressures [176-178].