Equipment surface analysis

The importance of low pressures has already been stressed as a criterion for surface science studies. However, it is also a limitation because real-world phenomena do not occur in a controlled vacuum. Instead, they occur at atmospheric pressures or higher, often at elevated temperatures, and in conditions of humidity or even contamination. Hence, a major tlmist in surface science has been to modify existmg techniques and equipment to pemiit detailed surface analysis under conditions that are less than ideal. The scamiing tunnelling microscope (STM) is a recent addition to the surface science arsenal and has the capability of providing atomic-scale infomiation at ambient pressures and elevated temperatures. Incredible insight into the nature of surface reactions has been achieved by means of the STM and other in situ teclmiques. 

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

Whenever composite materials are used, the surface composition becomes an essential parameter to assess the actual electrocatalytic activity. The dominating role of surface composition in electrocatalysis was stressed by Frumkin et al. long ago [100]. This is especially the case with not well-defined compounds such as sulphides, carbides, etc. This task is undoubtedly tougher since the equipment for surface analysis is not an ordinary tool in electrochemical laboratories. As a matter of fact, the surface of electrodes remains insufficiently characterized in most instances, so that no more than a phenomenological observation can be made. In the cases where surface analysis has been carried out, it has usually opened new horizons to the understanding of the electrocatalytic action of materials [101,102], In some instances, the surface analysis has been essential to show that synergetic effects were only apparent [103]. 

Fig. 11. (a) Experimental apparatus combining a UHV surface analysis chamber with a UHV-high-pressure reaction cell optimized for PM-IRAS spectroscopy. Pre- and post-reaction surface analysis under UHV can be performed by XPS, LEED, AES, and TDS. The optical equipment and the high-pressure cell used for the PM-IRAS experiments are shown in (b) 84,113,114,171). 

It is important to consider the connection between the two types of studies. One often refers to the "pressure gap" that separates vacuum studies of chemisorption and catalysis from commercial catalytic reactions, which generally run above —often well above — atmospheric pressure. There is simply no way to properly simulate high pressure conditions in a surface analysis system. Reactions can be run in an attached reaction chamber, which is then pumped out and the sample transferred, under vacuum, into an analysis system equipped for electron, ion and photon spectroscopies. However, except for some optical and x-ray methods that can be performed in situ, the surface analytical tools are not measuring the system under reaction conditions. This gap is well recognized, and both the low- and high-pressure communities keep it in mind when comparing their results. 

The transmission electron microscopy was done with a 100-kV accelerating potential (Hitachi 600). Powder samples were dispersed onto a carbon film on a Cu grid for TEM examination. The surface analysis techniques used, XPS and SIMS, were described earlier (7). X-ray photoelectron spectroscopy was done with a Du Pont 650 instrument and Mg K radiation (10 kV and 30 mA). The samples were held in a cup for XPS analysis. Secondary ion mass spectrometry and depth profiling was done with a modified 3M instrument that was equipped with an Extranuclear quadrupole mass spectrometer and used 2-kV Ne ions at a current density of 0.5 /zA/cm2. A low-energy electron flood gun was employed for charge compensation on these insulating samples. The secondary ions were detected at 90° from the primary ion direction. The powder was pressed into In foil for the SIMS work. 

The operation and maintenance of ESCA equipment and interpretation of its data are quite complex. Samples intended for ESCA and other surface analysis methods should be handled carefully because minute contamination can mask the surface structure... 

Because many surface probes require high vacuum during their application, most surface science instruments are also equipped with high-pressure or environmental cells. The sample to be analyzed is first subjected to the usual high-pressure and/or high-temperature conditions encountered during reactions in the environmental cell. Then it is transferred into the evacuated chamber where the surface probe is located for surface analysis. One such apparatus is shown in Figure 1.13. 

Surface analysis investigations (XPS) were performed on a Leybold equipment already described in [16]. The fresh catalyst samples were stored imder argon prior to the catalytic tests and the surface analysis. The aged catalyst samples were also handled under argon. The precious metal dispersion was determined for some of the catalysts by a pulsed CO chemisorption technique [16]. 

Figure 6. Schematic of UHV surface analysis system equipped with sample isolation cell for high pressure (1-20 atm)...

As we will see, surface analysis equipment is very similar for several techniques, so most surface analysis instruments are configured for multiple surface analysis techniques. A schematic diagram of a commercial instrument showing the placement of the ion gun, energy analyzer, source, and so on is shown in Fig. 14.8. 

In tins section we wiU encounter some methods for characterizing catalysts and discuss their capabihties and limitations. Most chemical engineers working in catalyst development are not experts in complex industrial analytical chemistry, and only a few major companies and research institutes can afford surface analysis equipment. For these reasons we shall dispense with the details of methods and apparatus and concentrate on practical applications. 

After RGA data are collected and analyzed, the package is delidded to examine the inside components, interconnections, materials, and surfaces. Because removing the lid is a destructive process, it should be done in a lid-down position with special equipment to vacuum off any debris. After delidding, the components, surfaces, and connections are optically examined followed by analytical testing such surface analysis for contaminants, especially chloride and sodium ions. Scanning acoustical microscopy (SAM) is very useful in detecting voids, cracks or delamination within the adhesive or at the adhesive bond line. An excellent overview of SAM and its variations may be found in Ardebili and Pecht. ... 

Other examples of cleaning validation have been reported, but identities of the compounds were withheld due to proprietary considerations. " Nevertheless, quantitative determination of residual APIs and intermediates on equipment surfaces were described in detail. The linear dynamic ranges for the compounds investigated were 0.1-1.0 pg/mL for one compound and 1-10 pg/mL for the others. With IMS technology, they were able to evaluate 30 samples in less than 2 h. In another study, analysis required only 0.5 min/sample compared with 15-30 min/sample using the HPLC method. ... 

---