Hydrogen adsorption isobars

Before leaving the nickel experiments, it may be well to refer to the experiments on hydrogen adsorption variously reported in the literature. As an example, the work of Maxted and Hassid (13) had as its main objective the measurement of the slow activated adsorption of hydrogen on reduced nickel oxide catalysts. It has been proved by the foregoing that the slow adsorption is actually absorption. When plotting their data as isobars, as was done in Fig. 9, the similarity between these isobars and those obtained with sintered nickel films is evident. 

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). 

As a matter of fact there is a striking parallelism between both systems, H2-ZnO and O2-NiO. Thus the adsorption isobar (60 mm. Hg) corresponding to the amount of oxygen taken up after 100 min. by NiO between room temperature and 700°C. has the characteristic shape of hydrogen adsorption lobars a curve with a flat minimum between 200° and 300°C. (37). 

Fig. 9. Adsorption isobar of hydrogen on zinc oxide (after Taylor and Strother).

Taylor (37) describes the shape of adsorption isobars for hydrogen on an iron catalyst and curves showing desorption, readsorption when the temperature is increased stepwise. These observations are best explained by the existence of an a priori heterogeneity. This heterogeneity may well be connected with the existence of abnormal lattice distances on the surface. 

Fig. 20. Isobaric representation of spiltover hydrogen adsorption on activated aluminas, amorphous or d (and silica) (A) <5-alumina (and silica), with Pt/Al203 (B) amorphous alumina with Pt/Al203 (C) i5-alumina without Pt/Al203 (D) amorphous alumina without Pt/Al203.

Fia. 2. Adsorption isobar (hydrogen on ZnO III). Roman numeral indicates order of run. 

Flo. 7. Isobaric hydrogen adsorption on iron CF 413-9 at 1 atmosphere. O Raising temperature A Lowering temperature... 

Also in 1950 a paper was published describing adsorption isobars for hydrogen on nickel films from 20 K to room temperature ( ). These results were achieved with the aid of the newly developed Collins helium cryostat. 

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... 

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... 

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 3.10. Dependence of surface coverage by hydrogen atoms on (a) temperature at various fixed pressures (isobars), and (b) on pressure at various fixed temperatures (isotherms). Isosteric heats of adsorption at (say) half-coverage are derived from the pressures giving this coverage at each temperature.

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