H2S exposure

Figure 2.35. Effect of varying chlorine, sulphur and phosphorus precoverages on the H2 TPD spectra from Ni(100). H2 exposure 10 L Ta = 100 K.86 Reprinted with permission from Elsevier Science.

Figure 1. HREELS spectra for Increasing H2S exposures on the Pt(lll) surface at 11 OK. A value of one In the exposure units corresponds to saturation of the first layer.

The hydrogen molecule does not chemisorb onto clean sintered gold surfaces at or above 78 K [147] but on unsintered films, a small amount of H2 is chemisorbed if gold surface atoms of low coordination number are present [148]. Stobinski [149] found that H2 can also chemisorb on thin sintered Au films if the surface is covered at low temperatures with a small amount of gold equivalent to 1-3 Au monolayers prior to H2 exposure. This suggests a fundamental role of surface Au atoms of low coordination number in the chemisorption process. Deuterium molecules also chemisorb in a similar fashion on gold films at 78 K and isotope effects were... 

Figure 5.9 Images of the Ni(110)-O surface for two oxygen coverages before and after exposure to H2 at 300 K. (a) 0 = 0.2 ML and (c) after H2 exposure (b) 0 =0.33 ML and (d) after H2 exposure. (Reproduced from Ref. 29).

Fig. 3.5.10 Percent conversion of Cd + ions in the film, assuming stoichiometry in Eq. (4), versus total H2S exposure lime for different thickness of CdAr films. Filled squares. 10-layer CdAr circles, 20-laycr CdAr films open squares, 40-layer CdAr film. QCMs with LB films were exposed to H2S in a sealed cell and then removed from the cell and left in air lor a period prior to frequency measurements. (From Ref. 46.)...

Moriguchi et al. (43) noted a correlation between the change in the basal plane spacing with H2S exposure of CdSt, and the ionic radius of sulfide, and used this as evidence for CdS monolayer formation. Measurements of high lateral conductivity in metal ion fatty acid films exposed to H2S (20,23) and of photoelectric properties (21) have also been used to invoke the concept of continuous sheets of metal sulfide forming. 

Real-time mass increases of MBe (M = Cd, Hg) films on a QCM have been measured as a function of H2S exposure time (58,64). The results from one of the studies, presented in Figure 3.5.13 (58), show the mass increase reached at equilibrium was proportional to film thickness and corresponded to 83-88% conversion of Cd2+ ions to CdS. The rate at which equilibrium was reached was strongly dependent on film thickness, in contradiction to the results presented in Figure 3.5.10 (46). 

XPS also provides evidence that, at its endpoint, the reaction of metal ions in LB films with H2S is not stoichiometric as depicted in Eq. (4). For example, XPS analysis for a number of MBe films (M = Cd or Hg) to H2S for 1 h gave an average S M ratio of 0.76 instead of 1 as predicted by Eq. (4). Both QCM and UV/visible absorbance measurements indicate that 1 h of H2S exposure is more than enough for the reaction to reach its endpoint. In another XPS investigation of films of calixerenes containing Cd2+ ions, S Cd ratios of 0.84 0.1, on average, were obtained (37). 

H2S exposure may be monitored by diffusion type colorimetric dosimeters (such as Vapor Gard) color changes from white to brown-black dosage exposure measured from the length of stain in the indicator tube. 

Ultraviolet photoemission spectroscopy( UPS) measurements of a Pd/ SiCVa-Si H structure indicate (Fortunato et al., 1984) that the mechanism responsible for the transport property variations is a change in the contact potential. In these experiments a few angstroms of a-Si have been deposited by an in situ evaporation onto a Pd/SiO substrate. Figure 5 shows the photoemission spectra obtained by synchrotron radiation at a photon energy of 30 eV and for three different conditions (1) after the a-Si deposition, (2) after the H2 exposure at 10-2 Torr for 2 min, and (3) after 02 exposure at 5 X 10-5 Torr and 110°C for 7 min. 

Fig. 5. Energy distribution curves at a photon energy of 30 eV for three different conditions (a) after a-Si deposition (b) after H2 exposure at 10 2 Torr for 2 min (c) after 02 exposure at 5 X 10 5 Torr for 7 min at 110°C. The tops of valence tend of SiOx and a-Si H are also extrapolated (dashed lines). The top of SiO, VB shifts from 2.7 e V under the Fermi level to 2.45 eV (AE = 0.25 eV), while the top of a-Si H shifts from 0.6 to 0.35 eV, going from condition (b) to condition (c).

From the slope of the lines in Fig. 10, the calculated space-charge density N is equal to 6 X 1015 cm-3 (in air) and seems to change only a little under H2 exposure (NH = 5.3 X 1015 cm-3). These results, in conjunction with the observation that no feature appears in the C-V curves when the device is exposed to H2, confirm the change in the contact potential as the main mechanism for the H2 sensitivity. 

Guerra (187) reported shifts in the M—CO bond to lower frequencies due to poisoning of Ir and Os by H2S. Exposure of Rh to H2S enhanced the bond attributed to two CO molecules per metal site and eliminated bridged Rh2-CO species. Thus sulfur appears to affect the adsorption states for CO on some group VIII metals in a manner similar to nickel. 

H2S exposure (mol x 107) Surface carbon (mol x 107) Fractional deactivation of surface carbon (%)... 

Fig. 31. Sequential dosing of H2 and CO on Pd/Al203 and Pd(l 11) After cooling in H2 at 2 X 10 mbar from 300 to 150 K (approximately 150 L), SFG (a, c), and TDS (b, d) measurements were made (lower traces). After a repeat of the H2 exposure and subsequent adsorption of 10-20 L of CO at 150 K, SFG and TDS spectra were again acquired (a-d upper traces). The models illustrate the locations of CO and hydrogen adapted from (dS) with permission from Elsevier.

This technique enables detennination of catalyst activity as a function of H2S exposure time. The normalized activity, a, is defined as the ratio of reaction rate at any time to initial reaction rate (prior to poisoning). In line with other authors (ref. 8), as a direct measure of thioresistance, we define the half life as the time it takes to achieve a nrsmalized activity of O.S. Thus, the greater the half life, the mote thiOTesistant the catalyst is. 

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