Clean surfaces measurement

Measure the surface profile of the freshly blast-cleaned surface and ensure that it is within specification. 

Davies et al. (D9) have recently measured the rates of absorption of various gases into turbulently stirred water both with carefully cleaned surfaces and with surfaces covered with varying amounts of surfactants. That hydrodynamic resistances, rather than monolayer resistances, are predominant in their work is consistent with the high sensitivity of kL to very small amounts of surface contamination and also with the observation that a limit to the reduction in kL is found (D7, D9). This is in agreement with the results of Lindland and Terjesen (L9), who found that after a small concentration of surfactant had been used further additions caused but little change in terminal velocity (L9). 

More than with continuous improvement in the preparation of clean surfaces in electrochemistry, the measurement of 0 is rather casual in surface science at present. In particular, work functions are mostly measured for d-metals rather than for sp-metals, which are more common in electrochemical double-layer studies. As a consequence, compilations of work function values report data for sp-metals that are 20 to 30 years old.63,856,857 This does not imply that the data are unreliable, but imparts to the situation a sense of frustration related to the immobility in one of the variables. 

Figure 1 shows two reactor configurations we have used to measure reaction rates on clean surfaces. In Figure 1(a) is shown a high pressure cell inside the UHV system ( ) with the sample mounted on a bellows so it can be moved between the reaction cell and the position used for AES analysis. In Figure 1(h) is shown a reaction cell outside the analysis system with the sample translated between heating leads in the reactor and in the UHV analysis system ( ). 

Transient kinetic experiments have also been carried out to complement the information deduced from the steady-state measurements [33], Systematic variations were observed during the transition from the clean surface to the steady-state catalytic regime that correlate well with the overall reaction rates in the latter. Specifically, there is a time delay in the production of molecular nitrogen because of the need to buildup a threshold of atomic nitrogen coverage on the surface. This atomic nitrogen coverage, which could... 

Thermal desorption measurements were carried out by first exposing the clean surface to a 15 L dose of ethylene at 323 K. 

The sensitivity of y using these procedures has been found to be very high ( 0.001 dyn/cm [mN/m]) (Birdi, 2008). The change in surface tension (surface pressure = II) is then determined by measuring the change in F for a stationary plate between a clean surface and the same surface with a monolayer present. If the plate is completely wetted by the liquid (i.e., cos(0) = cos(0) = 1), the surface pressure is then obtained from the following equation ... 

It is apparent from the foregoing that the B.E.T. method will give values for the monolayer which are too large when applied to clean metal surfaces using a gas whose heat of adsorption is large relative to that in ordinary van der Waal s adsorption. With a heat of adsorption of 10,000 calories on iron, nitrogen is unsuitable for surface measurements of such metals by the B.E.T. method when absolute best values are of interest. 

Using this apparatus, Binnig et al. demonstrated the exponential distance dependence of tunneling current over four orders of magnitude. Their first results are summarized in Fig. 1.6. The value of the work function was found to depend sharply on the condition of the surfaces. Initially, the measured values were around 0.6 - 0.7 eV. After repeated cleaning, the slope became much steeper. A value of 3.2 eV was obtained, which can last for several minutes. Being still lower than the value for clean surfaces, 4 — 5 eV, it was... 

Spurious effects due to incompletely removed oxides layers are very likely to be recorded and misinterpreted in photoemission experiments from the very oxidizable U-metal surface. However, considering only high resolution XPS and UPS data for clean surfaces as well as the measurements using synchrotron radiation , it can be... 

At elevated temperatures physical adsorption does not occur or is sufficiently slight that relatively clean surfaces can be prepared on which to make accurate surface area measurements. 

Sachtler and Dorgelo (74) measured the change in photoelectric work function when Na and Ha were adsorbed on evaporated films of Ni and Ta. These films were deposited under a vacuum of 10 mm. Hg, and the maximum surface potentials observed were 0.1 v. for the system Ni + Ha, —0.44 V. for Ta + Ha, and —0.38 v. for Ta + Na. On the other hand, the adsorption of Ha on a Ni surface prepared under less satisfactory experimental conditions decreased the work function (7S). As a result of this work, some doubt arises as to whether the positive S.P. values reported for the adsorption of Ha on various metals, particularly Pt (68), refer to clean surfaces. 

When adsorption occurs on the clean surface, heat is liberated during the formation of the surface bond. The heat of adsorption, AH ds, associated with the layer of adsorbates reveals the strength of interaction between atoms and molecules in the monolayer and the surface on which they are adsorbed. These two macroscopic, experimentally measurable parameters, d and AH ds, usually well characterize the adsorbed monolayer and the form of their interdependence often reveals the nature of bonding in the adsorbed layer. 

Figure 4.17. Measured sticking probability (relative to the clean surface) of a methane molecular beam on Ni(l 11) surfaces with varying amounts of Au alloying into the surface. The result of a model (prediction) based on DFT calculations of the change in the activation energy due to the addition of Au atoms is also shown. The beam data primarily measures methane sticking on the facets. Adapted from Ref. [59].

---