Transfer to UHV

The first atomic level study of an EC-ALE system was of Te [235] and Cd atomic layers on Au single crystal surfaces [156, 157], using UHV-EC techniques [131] and some ex-situ STM. Those experiments involved emersion (removal) of the crystals from solution and transfer to UHV for analysis (Figure 42). 

It is clear that the best experimental designs for addressing structure-function relationships in catalysis are those that minimize exposure of the catalyst surface to undesired vapors between high-pressure kinetic measurements and surface analysis. Thus one aims for a system where the transfer to UHV is as rapid and as clean as possible, and where the sample can be cooled as rapidly as possible once inside UHV. 

It is difficult to observe tliese surface processes directly in CVD and MOCVD apparatus because tliey operate at pressures incompatible witli most teclmiques for surface analysis. Consequently, most fundamental studies have selected one or more of tliese steps for examination by molecular beam scattering, or in simplified model reactors from which samples can be transferred into UHV surface spectrometers witliout air exposure. Reference [4] describes many such studies. Additional tliemes and examples, illustrating botli progress achieved and remaining questions, are presented in section C2.18.4. 

A modified immersion method has been used by Hamm et al.140 to obtain electrochemical cell by a closed-transfer system, and immersed in 0.1 M HCIO4 solution at various . was derived from the charge flowing during the contact with the electrolyte under potential control. For the reconstructed Au(l 11M22 X Vayo.l M HCIO4interface, =0.31 0.04V (SCE) (Table 9). Using the impedance method, = 0.34 V (SCE) for recon-... 

Quantitatively, however, it is evident that directly measured A0 values are on average 0.2 to 0.3 eV higher than AX values. This shift in the potential scale has been discussed by Trasatti,31 34 who has attributed such a systematic difference to the different conditions of measurement (different temperatures, nonequivalence between thin water layers and bulk water, uncompensated partial charge transfer in UHV). For a more detailed discussion, the reader is referred to the original papers. 

XPS Analysis. The ultrahigh vacuum (OHV) catalyst treatment-surface analysis system employed to characterize and treat the cobalt catalysts has been described previously ( 1, 2 The catalyst treatment and data analysis procedures have also been described (JJ. Briefly, the samples were treated in quartz reactors and then transferred under UHV into a modified Hewlett-Packard 5950A BSCA spectrometer for emalysis. Peak areas were normalized with theoretical cross-sections (Z) to obtain relative atomic compositions. 

Especially in conjunction with the detection of water or OH species, ex situ XPS measurements have been critizised because of possible changes occurring during transfer and exposure of the sample to UHV. Kuroda et al. have demonstrated that structural changes of the passive film indeed occur when electron diffraction studies are performed in a hydrated and subsequently in a dehydrated environment. Structural changes, however, do not necessarily cause changes in elemental composition as determined by XPS. 

After adsorption of CO and solution exchange with pure base electrolyte, the oxidation of adsorbed CO during a triangular potential scan is observed (see Fig. 1.4a). In a second run after adsorption of CO the electrode is emersed and transferred to the UHV chamber in the same way as in the normal experimental procedure. The electrode is then transferred back to the cell and re-immersed in the base electrolyte. A potential scan is applied to oxidize the adsorbate. Fig. 1,4b shows... 

Fig. 2.4. (a) Thermal desorption blank experiment. The Pt electrode was held at 0.45 V vs. RHE in the base electrolyte (5 x 10 2 M H2S04) during 120 s and then transferred to the UHV. (b) Thermal desorption spectra of adsorbed CO on Pt after adsorption from an aqueous solution. Temperature scan 5 K/s. 

In order to check the survival of methanol adsorbate to the transfer conditions, the following experiment was performed. After adsorption of methanol and solution exchange with base electrolyte, the Pt electrode was transferred to the UHV chamber over a period of ca. 10 min, then back to the cell where it was reimmersed into the pure supporting electrolyte. A voltammogram was run and compared with that of an usual flow cell experiment. The results, (see Fig. 2.5a,b), show that the transfer procedure is valid. The areas under the oxidation curve are the same. As in the case of adsorbed CO on Pt (see Fig. 1.4), the change in the double peak structure indicates that some surface re-distribution may occur. 

The electrochemical experiments were conducted in an apparatus consisting of an electrochemical cell attached directly to a UHV system and has been described in detail elsewhere (16). The transfer between UHV and the EC was accomplished via a stainless steel air lock vented with ultra-pure Ar. Differentially pumped sliding teflon seals provided the isolation between UHV and atmospheric pressure. The sample was mounted on a polished stainless steel rod around which the teflon seals were compressed. All valves in the air lock were stainless steel gate valves with viton seals. Details of the electrochemical cell and conditions are contained in reference 16. Electrochemical potentials are referred to a saturated calomel electrode (SCE). 

Iwasita et used the similar techniques and confirmed that the adsorbate contains a proton atom but concluded that the adsorbate is C-OHad ie same group executed electrochemical thermal desorption mass spectroscopy, in which the methanol absorbing electrode was washed by the supporting electrolyte, transferred to the UHV environment, heated to desorb the adsorbates to analyze them by mass spectroscopy. They found hydrogen molecules in the desorbed gas as well as CO and the ratio of hydrogen to CO decreased as the concentration of methanol increased. 

The metal surfaces are always covered with a monolayer of CO upon evacuation of the reactor and transfer to the UHV system. On both Pd and Ir the CO, which desorbs as CO2 when reacted with the oxide species, desorbs at a much higher temperature than CO from the clean surface. This result implies that the oxide species forms an inactive complex with CO upon adsorption of CO under reaction conditions. While the presence of the oxide species reduces the overall rate of reaction, the activation energy is unchanged, suggesting that oxygen serves as a simple site blocker on the surface. 

The X-ray photoelectron spectroscopy (XPS) experiments were performed in an ultra-high vacuum (UHV) chamber coupled to an atmospheric pressure reaction cell. All XPS results were obtained from samples treated in situ in the reaction cell and transferred into UHV without exposure to air. Detailed sample mounting procedures and instrument details are described elsewhere.16 Ar+ bombardment was done with 3 KeV Ar+ ions at a current density of 0.8 pA/cm2 for 1 h in an attempt to remove the carbon overlayer and expose the underlying carbide phase. 

As with all surface analytical methods, surface preparation is critical to obtaining reproducible SHG from metallic surfaces and single crystals in particular. For surfaces prepared in UHV and then transferred to an electrochemical cell, sputtering and heating or annealing followed by Auger analysis of impurities should proceed inert transfer. Low energy electron diffraction (LEED) can also be used to check surface order. Metal electrode surfaces, particularly for the rotational anisotropy ex-... 

To verify the anisotropy observed on the silver surface and to attempt to understand the effect of the electrochemical solution on the surface electronic and structural properties, Bradley et al. [124] have examined the SH response from a Ag(111) surface in UHV. The experiments on this crystal were then repeated after an inert transfer to the electrochemical cell. The SH experiments performed in the electrochemical cell were again conducted at the PZC to minimize the effect of the dc electric field on the surface properties. Fig. 5.3 a and b show the results for the crystal examined in UHV for p- and s-polarized output at 532 nm. The solution data is consistent with the previous in-situ results of Koos et al. [122] shown in Fig. 5.1. More importantly, when the fits to the UHV data are compared to the subsequent results performed in solution, nearly identical values for the relative magnitudes of the a and c(3) coefficients are found (see Fig. 5.5 for comparison). Bradley et al. [124]... 

Underpotential deposition (UPD) is the electrochemical adsorption and (partial) reduction of a submonolayer or monolayer of cations on a foreign metal substrate at potentials more positive than the reversible potential of the deposited metal [141]. The UPD phenomenon is used in many fundamental and applied studies because it offers a means of controlling coverages during electrodeposition in a very concise manner. Until recently, most of the information obtained about the structure of the overlayers deposited on single crystal surfaces has come from indirect means such as current-voltage analysis or by analysis of the deposited films after transfer to a UHV chamber [141]. 

Therefore it is very important to complete the data obtained by (photo) electrochemical techniques with surface sensitive spectroscopic measurements. One promising possibility of gaining microscopic information on interfacial processes is the use of UHV surface science techniques. However due to the analysis requirements emersion of the samples from the electrolyte and transfer into UHV is necessary. During this procedure the semiconductor interface may change drastically. Alternatively the basic chemical and physical interactions of electrolyte components may be studied by adsorbing redox components on defined semiconductor surfaces thus simulating semiconductor/electrolyte junctions. 

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