Further reading: G. Rupprechter, Annual Reports on the Progress of Chemistry, Section C (Physical Chemistry), 100 (2004) 237 or Physical Chemistry Chemical Physics, 3 (2001) 4621
High surface area ("real") technical catalysts are often too complex to allow molecular level investigations of surface processes. Planar model catalysts, prepared by evaporation methods under well controlled ultrahigh vacuum conditions (UHV; 10-10 mbar), are better suited for fundamental studies because
- they can be characterized on an atomic level by surface sensitive techniques and
- their structure and composition are well-defined.
We utilize metal nanoparticles supported on thin oxide films and single crystal surfaces as model catalysts.
The model catalysts are characterized with respect to their structure (low energy electron diffraction LEED; scanning tunnelling microscopy STM) and composition (Auger electron spectroscopy AES; X-ray photoelectron spectroscopy XPS), and also the interaction with gas molecules is examined (thermal desorption spectroscopy TDS; sum frequency generation (SFG); polarization-modulation infrared reflection absorption spectroscopy PM-IRAS).
SFG and PM-IRAS can not only be applied under UHV but also at pressures up to 1 bar. This makes them excellent in situ techniques able to bridge the pressure gap between traditional surface science and applied catalysis.
Experiments are performed as follows: After preparation and characterization (LEED, TDS, XPS, etc.) under UHV, the model catalysts are transferred under UHV to a UHV-high pressure reaction cell which is equipped with IR transparent CaF2 windows for SFG and PM-IRAS spectroscopy at mbar pressure. Kinetic measurements are performed simultaneously by gas chromatography, infrared spectroscopy or mass spectrometry. After the reaction the model catalyst are re-inspected to detect structural or compositional changes induced by the catalytic reaction.
Correlations between the concentration of the observed reaction intermediates, the catalytic activity and the surface structure of the catalyst can help to elucidate reaction mechanisms. Systems previously studied include CO adsorption, CO/H2 coadsorption, CO hydrogenation, ethylene and 1,3-butadiene hydrogenation and methanol decomposition and oxidation on Al2O3 supported Pd nanoparticles and Pd(111).
Combining in-situ methods (SFG/PM-IRAS) with a well-defined nanoparticle model catalyst is certainly a promising route to better connect surface science and applied catalysis.
In the following a few cases studies are discussed to illustrate the techniques and our current research interests.
Further reading: M. Morkel et al. Surface Science Letters, 588 (2005) L209
1. Infrared-visible sum frequency generation (SFG) laser spectroscopy
IR-vis sum frequency generation (SFG) is a surface-specific vibrational spectroscopy that can operate in a pressure range from ultrahigh vacuum (UHV) to atmospheric pressure. Due to its intrinsic surface sensitivity, SFG is particularly suited to monitor adsorbates during a catalytic reaction at 1 bar.
SFG is a second-order nonlinear optical process which involves the mixing of infrared (ωir) and visible light (ωvis) to produce light at the sum of these two frequencies (ωsf = ωir + ωvis). To carry out an SFG experiment, the visible laser beam is held at fixed frequency while the infrared beam is tuned through the vibrational range of interest. An adsorbate spectrum is obtained by plotting the SFG intensity vs. the IR wavenumber. SFG is uniquely surface monolayer specific because such a non-linear process can occur only in media without inversion symmetry. The laser setup consists of a 50 Hz-Nd:YAG laser (1064 nm, 30 mJ/pulse, 20 ps), followed by an SHG (second harmonic generation)-unit to produce 532 nm visible light and an OPG/DFG(optical parametric generation/difference frequency generation)-unit, which provides the tunable IR-light.
Due to the importance of CO as reactant or probe molecule, its adsorption on noble metal surfaces has been extensively studied under UHV. However, studies at mbar pressures are more relevant for heterogeneous catalysis. SFG is among the few techniques that can provide this information. Figure 2 shows the SFG spectrum of 600 mbar CO on Pd nanoparticles (~5 nm in size) supported by a thin Al2O3 film grown on NiAl(110). The "high pressure" spectrum indicates that CO adsorbs on threefold-hollow sites and on-top sites of (111) nanoparticle facets (peaks at 1895 und 2100 cm-1), and at the particle edges and steps (peak at 1990 cm-1). The site occupation strongly depends on particle size and surface roughness. The spectrum on the right shows the CO site occupation during methanol synthesis pointing to adsorbate disordering and/or surface roughening at high temperature/high pressure.
2. Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PM-IRAS)
Further reading: M. Borasio et al. Journal of Physical Chemistry B Letters, 109 (2005) 17791
PM-IRAS allows to obtain vibrational spectra of molecules adsorbed on model catalysts at mbar pressure. Compared to SFG it offers advantages with respect to resolution, frequency range and acquisition time. PM-IRAS utilizes the polarization modulation (PM) of the incident infrared light and is based on the predominance of p- over s-polarized light at a metal surface. Accordingly, the differential reflectance ΔR/R that is measured with PM-IRAS provides the surface vibrational spectrum while no bulk (gas phase) species are detected.
PM-IRAS was used to examine methanol oxidation on Pd(111) and Pd nanoparticles (Figure 3). Various surface species (methoxy CH3O, formaldehyde CH2O, formyl CHO, CO) and their adsorption configurations were identified.
3. X-ray Photoelectron Spectroscopy (XPS)
Further reading:M. Borasio et al. Journal of Physical Chemistry B Letters, 109 (2005) 17791
XPS Spectroscopy is typically applied at low pressure or before and after the catalytic reaction. For instance, XPS was utilized to determine the catalyst surface composition during methanol oxidation. Methanol oxidation takes place on metallic Pd(111) while Al2O3 supported Pd nanoparticles appear to be partially oxidized during the reaction. Carbonaceous overlayers resulting from C-O bond scission are present under realistic conditions and strongly affect the reaction selectivity towards CH2O.