Surface analysis chamber

Campbell R A and Goodman D W 1992 A new design for a multitechnique ultrahigh vacuum surface analysis chamber with high-pressure capabilities Rev. Sc/. Instrum. 63 172... 

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. 

Potential Problems. Most, but not all, of the surface sensitive techniques require measurements to be made in a vacuum, frequently near room temperature. Because these conditions are usually different from the corrosion conditions, the possibility that the desired information will be lost in the transfer from the corrosion chamber to surface analysis chamber is a major concern. There is also a possiblity that the measurement itself will alter the composition or chemistry of interest. Various aspects of those problems may apply to any method for which analysis occurs under conditions different from those in which the sample is generated, but they are of particular concern for surface methods that examine the very outer layers of the material. 

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). 

Little can be gleaned about the nature of the alloy interface from only the cyclic current-potential curves. An important question that needs to be addressed is whether or not the cychc vol-tammograms are accompanied by changes in the surface composition of the alloy while a qualitative solution to this problem can easily be obtained from multiple voltammetric scans, a quantitative answer is fundamentally necessary. In fact, a more critical matter involves the stability the PtsCo alloy under fuel-cell operating conditions that is, after prolonged use at the OCP in an 02-saturated solution. All of these issues can be simultaneously tackled if the surface composition of the PtsCo alloy is monitored as a function of time at a given applied potential. For such measurements, the alloy electrode is withdrawn from the 02-saturated electrolyte at the test potential and, prior to transfer into the surface analysis chamber, rinsed in deaerated ultrapure (Millipore) water to remove emersed sulfuric acid. The results are shown in Fig. 11. 

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

Figure 6. Plan of the target preparation facilities consisting of UHV preparation chamber (a), (reactive) ion etching chamber (b), ion etching gun (c), laser (d), photon detector (e), transfer arms (f), Auger system for surface analysis (g), sample manipulator and annealing facility (h), load lock and optical microscope for viewing sample (i), evaporator (j), transmission diffractometer (k), and vacuum tank for main spectrometer (1).

Pump-down of the antechamber, following solution experiments, involved sorption pumping and cryo-pumping resulting in the pressure decreasing from ambient to 10 Torr in 5 minutes. The resulting sample surfaces were subsequently examined by both LEFT) and AES after transfer back to the analysis-chamber. 

Surface characterization studies by X-ray photoelectron spectroscopy (XPS) were conducted using DuPont 650 and Perkin Elmer 5300 instruments. Samples were prepared by placing solid material on double stick adhesive tape, or by allowing solvent to evaporate from an acetone dispersion of a suspension placed on a stainless steel probe. A magnesium anode was used as the X-ray source (hv 1253.6 eV). The temperature of samples during the analysis was approximately 30-40°C and the vacuum in the analysis chamber was about 10 torr. Potential... 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

UHV surface preparation and analysis chamber, a variable pressure STM chamber, and a load lock and sample transfer system. Fig. 11 shows a schematic of the system. 

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