Hydrogen adsorption detection

The major role of TOF-SARS and SARIS is as surface structure analysis teclmiques which are capable of probing the positions of all elements with an accuracy of <0.1 A. They are sensitive to short-range order, i.e. individual interatomic spacings that are <10 A. They provide a direct measure of the interatomic distances in the first and subsurface layers and a measure of surface periodicity in real space. One of its most important applications is the direct determination of hydrogen adsorption sites by recoiling spectrometry [12, 4T ]. Most other surface structure teclmiques do not detect hydrogen, with the possible exception of He atom scattering and vibrational spectroscopy. 

For comparison we also show a cyclic voltammogram of a Au(lll) electrode (see Fig. 13.4). There is no detectable hydrogen adsorption region the hydrogen evolution reaction is kinetically hindered, and sets in with a measurable rate only at potentials well below the thermodynamic value. There is a much wider double-layer region in which other... 

Effects of Hydrogen Adsorption on Pd-Y Zeolite Samples. The oxidized samples in which Pd(III) ions have been detected, when exposed to hydrogen at 25 °C, turned black instantaneously, and ESR measurements showed that the Pd(III) ions had disappeared. Simultaneously, a strong ESR signal developed. 

Fig. 5. Hydrogen adsorption isotherms at 293 K with platinum-gold/Aerosil catalysts V, Pt 98, Au 2 mol %, 1.0 wt % metal 0.516 g catalyst A, Pt 90, Au 10 mol %, 0.9 wt % metal, 0.510gcatalyst , Pt 67, Au 33 mol %,0.9wt % metal, 0.500g catalyst O.Pt 15, Au 85 mol %, 1.0 wt % metal, 0.450 g catalyst standard pretreatment (cf. text). Filled symbols, amount of adsorbed hydrogen remaining after pumping at 293 K for 30 min, after equilibration at indicated pressure. Catalyst samples identified from corresponding symbols above. Within the limits of experimental accuracy, no adsorption could be detected on a Pt 0, Au 100 mol %, 1.0 wt % catalyst, using a 0.500 g sample (20).

Kaneko et al attempted to apply He adsorption at 4.2 K for evaluation of ultramicropores in activated carbon. [34] Although He adsorption at 4.2 K is efficient for detection of presence of ultramicropores, quantitative evaluation of ultramicroporosity is still difficult a kind of quantum effect is speculated. Johnson et al pointed that quantum effect is predominant in hydrogen adsorption in SWNT from their theoretical studies. [35] We need a small probe molecule for ultramicropore characterization. At the same time, contribution by quantum effect must be understood in order to establish nanopore characterization using the small probe molecule. 

For the polycrystal exposed first to near-saturation doses of CO and then to H2, hydrogen adsorption into the 62 state was completely blocked. In addition, no Z-states were formed. Consistent with published spectroscopic characterization which showed that no C-H or 0-H bonds are formed under UHV coadsorption conditions, no CO hydrogenation products could be detected desorbing from the surface during heating. 

At least five types of hydrogen adsorption states on platinum electrodes have been postulated. These are the so-called strongly bound hydrogen, the weakly bound hydrogen, the on-top hydrogen, the dihydride state, and the sub-surface state. Most of these can be detected by cyclic voltammetry and characterized at single crystal electrodes [37-42]. 

A big advantage of cyclic voltammetry is the detection of surface processes like adsorption, oxide layer formation, etc. In the anodic scan in Figure 4.14 the oxidation of weakly and strongly bound hydrogen (peaks a and b) is followed by hydroxide adsorption (peak c) and oxide layer formation (d). In the cathodic scan the reduction of the oxide (peak e) is followed by hydrogen adsorption strongly and weakly bound to the platinum atoms (peaks f and g). Further examples will be shown in Chapters 4 (Section 4.4) and 9. In these applications cyclic voltammetry is very similar to thermodesorption spectroscopy in surface science. Cyclic voltammetry can also be used to study diffusion and kineticaUy controlled processes. This will be discussed in more detail in Chapters 5 and 6. 

Glass and metal containers are not recommended for collection of HCN samples this is due to adsorption of the compound onto the walls of such containers. Good results are achieved when HCN is adsorbed onto porous materials, from which it can be desorbed with a solvent or by using thermal methods. For HCN detection in biological samples (e.g., in blood), a HS analysis method may be applied. Hydrogen cyanide detectability with a thermionic nitrogen detector may reach 1 pg in a sample. 

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