Chemisorption isobar

The metal surface areas of Pt and Pt-Sn catalysts after carbon deposition can be detemiiRed by in sitn H2 chemisorptioR in our combined apparatus. According to tbe H2 chemisorption isobars shown in Hg.S, two kinds of adsorption sites corresponding to adsorbed H2 at different temperatures are tecogiuzed. Hydrogen uptakes determined at low temperature (0 O and hi temperature ( 200 C and 330°C) repiesem low temperature... 

An additional difficulty in studies of slow chemisorption is evidenced by the Taylor-Liang technique of measuring adsorption isobars at successively higher temperatures without pumping between runs, a technique that has been reviewed in an earlier volume of this series (8). The ad-sorption-desorption phenomena observed between room temperature and about 150°C. clearly show in this temperature range the existence on the surface of zinc oxide of two types of hydrogen chemisorption (9). 

We show in Fig. 9 the adsorption isobar of CO on our ZnO in the lower temperature range chemisorption rapidly decreases, the minimum being found at about 150° above this temperature the quantity adsorbed increases somewhat. The second type of chemisorption is associated with hysteresis and occurs over the temperature range in which oxygen exchange is readily measurable. 

Figure 5.3 Schematic adsorption isobar showing the transition between physical adsorption and chemisorption...

The in sim characterization of catalysts was earned out in an apparatus which included a quadiupole mass-spectrometer and a gas chromatograph for TPO and H2 chemisorption measurements. In situ coking was performed by injecting a mixture of He and n-hexane vapor over the reduced catalysts at 500 C, In TPO experiments, ihe coked sample was heated at a rate of 8 C/min in a stream of 2 voL% O2 + 98% He. The amount of CO2 produced was recorded. The chemisorption of H2 was carried out in the same appanitus by a flow method after reduction or caking. The flow rate of carrier gas (Ar) was maintained at 25 ml/min and the volume of H2 injected was 0.062 ml/pulse. Since the partial piessiire of H2 was very low in this system, the hydrogenation of coke was never observed. Isobaric H2 chemisorption measurements with fresh catalysts were carried out in a static adsorption apparatus. Dehydrogenation of n-butane was carried out in a flow micro-rcactor in H2 atmosphere at LHSV = 3 h-l and H2/HC=1. Reaction products were... 

Fig. 2. Linear isobars for chemisorption of hydrogen sulfide on nickel catalysts. 7-46 ppm, data from Alstrup et al, 0.001-0.1 ppm data from McCarthy et a 1. (I. Alstrup et al14, reproduced with the permission from Elsevier Scientific Publishing Company).

A schematic representation of these several conditions can be seen from Fig. 3. If the continuous lines AB and CD represent true equilibrium isobars for van der Waals and chemisorption respectively, it is obvious that, on raising and lowering the temperature in the CD region the same experimental values will be obtained. If the dotted line BC represents values obtained while the temperature is being raised it is obvious that these are below the true equilibrium values because the latter cannot be reached in practicable times of observation since the velocity is too low. If the... 

Figure 10 shows the isobars obtained starting with a clean surface at the several temperatures of —126, —95, and —78° C. and separate isobars representing the data obtained 10, 60, 120, and 250 minutes after hydrogen at 1 atmosphere pressure was introduced to the evacuated sample. It will be noted from the 10-minute isobar that already at —126° C. the van der Waals adsorption of hydrogen is low. Actually it must be less than the value after 10 minutes if any chemisorption has occurred. The van der Waals adsorption of hydrogen must be still lower than the —126° C. value at the higher temperatures of —95 and —78° C. Nevertheless the 10-... 

Figure 3.12. Isobar and isotherms for chemisorption of hydrogen on supported platinum cataly-jt5,122,126,135,136 The left-hand part shows the variation in the amount adsorbed (expressed as H/Pts) at a fixed pressure, and the right-hand part the corresponding variations with pressure at fixed temperature (see text for full description).

Chemisorption of hydrogen is a widely used method for determination of the surface area of nickel in supported catalysts. The H2/Ni is complicated because of different states of chemisorbed hydrogen. At low temperatures (-196 C), a fast uptake results in a saturation layer with H/Ni less than one [46] [381]. At higher temperatures, the chemisorption becomes activated with hysteresis phenomena in isobars — and with some of the hydrogen present in subsurface sites [389],... 

Figure 5.40 Isobars for chemisorption of H2S onNi catalysts [15]. The linear curves represent Equation (5.15). Reproduced with the permission of Elsevier.

Figure 10.4 Upper panel potential energy curves for physisorption and chemisorption. Lower panel adsorption isobars illustrating the presence of the two types of adsorption, one involving stronger binding forces than the other.

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