Vacuum apparatus, ultrahigh

Figure 1. An ultrahigh vacuum apparatus used for the study of single crystal catalysis before and after operation at atmospheric pressure in a catalytic reactor.

Fig. 1. An ultrahigh vacuum apparatus for studying single crystal catalysts before and after reaction at elevated pressures.

The residual gas analyzer sensing head is generally supplied on an ultrahigh-vacuum flange which can be readily accommodated on most metal vacuum apparatus. 

Fig. 3. Ultrahigh vacuum apparatus for studying single-crystal catalysts before and after operation at high pressure in catalytic reactor. Position 1 crystal is in position for Auger electron spectroscopy study of surface composition or of ultraviolet photoemission spectrum of surface species. Position 2 crystal is in position for deposition of a known coverage of poisons or promoters for a study of their influence on the rate of a catalytic reaction. Position 3 crystal is in position for a study of catalytic reaction rate at elevated pressures (s2 atm). Gas at high pressure may be circulated by using a pump mass spectrometric-gas chromatographic analysis of the reactants and products is carried out by sampling the catalytic chamber. From Ref. 5.

Figure 4 Ultrahigh-vacuum apparatus for studying single-crystal catalysts before and after poisoning and/or reactions at higher pressure (c 2 atm). Position 1 is for AES or UPS, position 2 is for deposition of poisons, position 3 is for reaction in the catalytic reactor. Product analysis is by GC or GC/MS.

The present results clearly suggest that well-characterized singlecrystal samples can serve as models of practical, working catalysts. The ultrahigh vacuum apparatus described herein will be used further to study the pressure dependence of reaction kinetics, and in particular, the systematics of catalyst poisoning in a quantitative fashion (using the molecular beam doser in conjunction with AES). 

UHV surface analysis, apparatus designs, 36 4-14 see also Ultrahigh vacuum surface analysis mechanisms, 32 313, 319-320 Modified Raney nickel catalyst defined, 32 215-217 hydrogenation, 32 224-229 Modifying technique of catalysts, 32 262-264 Modulated-beam mass spectrometry, in detection of surface-generated gas-phase radicals, 35 148-149 MojFe S CpjfCOlj, 38 352 Molar integral entropy of adsorption, 38 158, 160-161... 

One approach is to carry out reactions of interest in attached chambers under conditions approaching atmospheric and then do the actual surface analyses under ultrahigh-vacuum conditions. For example, XPS has been used to follow the formation of nitrate on the surface of NaCl exposed to HNCL (Laux et al., 1994, 1996 Vogt et al., 1996 Hemminger, 1999). Figure 5.30 shows the apparatus used to dose known quantities of HN03 onto the NaCl surface (Laux et al., 1994). After each dose, the loss of Cl and uptake of N and O... 

For Dewar flasks, metal evaporation apparatus, and most research apparatus, a vacuum of 10 ° to 10 ° Torr is sufficient this is in the high vacuum range, while 10 ° Torr would be termed ultrahigh vacuum. However, for many routine purposes a utility vacuum or forepump vacuum of about 10 ° Torr will suffice, and for vacuum distillations only a partial vacuum of the order of 1 to 50 Torr is needed. 

Figure 4 shows a schematic diagram of an ultrahigh vacuum (5 x 10 ° Torr) apparatus that integrates LEED, XPS, TPD, LEISS, and electrochemistiy (EC). The base pressure of the chamber is 5 x 10" Torr. The sample is mounted on a probe, a tube fabricated out of stainless steel, at the top of the chamber. The probe allows experiments to be performed at very low temperatures for example, the probe is filled with hquid nitrogen for experiments at 77 K. The sample can also be heated resistively (up to 1500 K) via copper wires attached to the sample for still higher temperatures, an electron beam from a tungsten wire located behind the sample is employed. Temperature is monitored via a ReAV-Re thermocouple. 

Experimental studies of the formation of molecules within astrochemical ices are now being conducted in several international laboratories. A typical apparatus is shown in Figure 5. An ultrahigh vacuum chamber is required (typically less than 10 torr) to mimic the ultralow pressures found in space within which is mounted a cryostat, cooled by liquid helium to temperatures typical ofthe ISM (10 K). Gases are then introduced into the vacuum... 

Figure 5. A typical experimental apparatus used to explore astrochemistry, comprising an ultrahigh vacuum chamber to mimic low pressures in space, a cryostat to prepare surfaces at very low temperatures, an irradiation source to provide energy to induce chemistry and a detection system (e.g. Fourier transform infrared spectroscopy or FTIR) to determine what molecules are formed in the astrochemicalprocesses.

Much recent research with metal catalysts has been done with very clean single crystals under ultrahigh vacuum. The structures of the crystal surfaces can be determined precisely by low-energy electron diffraction (LEED), and catalytic reactions in the same apparatus have allowed comparisons of catalytic activity for various surface structures. 

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