Carbon monoxide adsorption isotherms

The TPR experiments were performed in an Altamira AMIl instrument with a flow of 10 vol.-% H2 in Ar. A heating rate of 3 K / min and approx. 100 mg of sample were chosen to enhance resolution and avoid "hot spots" Prior to the reduction experiments, the samples were activated in a helium flow containing 10 vol.-% O2 up to a maximum temperature of 400 °C at a rate of 5 K / min. The carbon monoxide adsorption isotherms were measured volumetrically at 25 °C in a home-build all-steel apparatus. The samples were dehydrated at 250 °C for 18 h prior to the adsorption measurements, ESR spectra were recorded on a Bruker ESP 300 spectrometer at 77 K. 

The BET surface areas and the hydrogen and carbon monoxide adsorption isotherms were determined by volumetric adsorption performed with a Texas Instrument quartz spiral BOURDON gauge in a system already described elsewhere (ref. 3). 

Fig. 4 Energy distribution function, (p(s t) (cmol kJ mol ), against the dimensionless product of the lateral interaction energy (P) and the local isotherm (0)P0, for carbon monoxide adsorption over a bimetallic Pto.25-Rho.75 silica-supported catalyst, at 698 K. (From Ref. [12].)...

Figure 2 Carbon monoxide adsorption in silicalite-I (a) net enthalpies and isotherm at 77K, (b) Clapeyron phase diagram.

Fia. 6. Comparison of the isotherms for total carbon monoxide adsorption at —183° and for the physical adsorption on about 45 g. of pure iron synthetic ammonia catalyst (No. 973) and on a similar quantity of a doubly promoted iron catalyst (No. 931) (39). 

Carbon monoxide chemisorption was used to estimate the surface area of metallic iron after reduction. The quantity of CO chemisorbed was determined [6J by taking the difference between the volumes adsorbed in two isotherms at 195 K where there had been an intervening evacuation for at least 30 min to remove the physical adsorption. Whilst aware of its arbitrariness, we have followed earlier workers [6,10,11] in assuming a stoichiometry of Fe CO = 2.1 to estimate and compare the surface areas of metallic iron in our catalysts. As a second index for this comparison we used reactive N2O adsorption, N20(g) N2(g) + O(ads), the method widely applied for supported copper [12]. However, in view of the greater reactivity of iron, measurements were made at ambient temperature and p = 20 Torr, using a static system. 

Titrations of carbon monoxide and hydrogen sulfide up to 800 torr were performed at 30°C each volumetric titration was composed of two adsorption isotherms the first isotherm was a combination of chemisorption and physisorption. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

Volumetric measurements at room temperature showed that by far the greater part of the adsorption of carbon monoxide on Au/TiC>2 occurred on the support it followed the Langmuir equation and most of it was removable by pumping.23,83 About one-third of the titania surface was able to retain it, but there was little uptake on Au/SiC>2. Use of the double-isotherm method with Au/MgO showed that adsorption onto the metal was complete at about 1 atm, but on various samples the coverage never rose above 18%. On model Au/MgO(100) the maximum coverage attained using a pulsed molecular beam at room temperature was < 10%.54... 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

The same sample of AlP04-5 was used to obtain the results shown in Figure 12.13. Here, the isotherms of methane, argon, nitrogen and carbon monoxide and the corresponding net adsorption energies were determined at 77 K after the adsorbent had teen outgassed by CRTA to 353 K. 

This equation thus gives Ceo s as a function of the partial pressure of carbon monoxide, and is an equation for the adsorption isotherm. This particular type of isotherm equation is called a Langmuir isotherm. Figure 10-8... 

Langmuir isotherm for adsorption of molecular carbon monoxide... 

Figure 3 Adsorption isotherms of carbon monoxide at 25 °C on mesoporous molecular sieves ( CuAlMCM-41-(15), CuZnAlMCM-41-(15), CuAlMCM-41-(30), O CuZnAlMCM-41-(30), AZnAlMCM-41-(15), AZnMCM-41)...

Figure 6 Net enthalpies and isotherms at 77K for the adsorption of (a) carbon monoxide on 5 A and (b) methane on 13X.

Figure 8 shows the adsorption isotherms of carbon monoxide at 25° for two temperatures of activation. If one takes the linear portions of the isotherms at pressures above 60 torr as the Henry s law portion of physical adsorption and extrapolates to zero pressure, one obtains values of chemisorption nearly identical with those measured at —78° as described above. 

We attempted to measure an adsorption isotherm for the adsorption of ethylene at 25° on a chromia activated at 337°. At 2, 8, 18, 28, 36, and 48 torr, the weight rapidly reached a steady state at 60 torr, the weight very slowly increased at 90 torr it increased rather rapidly. In 12 minutes at 90 torr, 0.5 rnolecules/100 of extra ethylene adsorbed. Adsorption from about 25 to 50 torr is linear with pressure. If one extrapolates the linear region to zero pressure as we did with carbon monoxide, one computes a chemisorption of 0.6 molecules/100 A. Most of the ethylene adsorbed at 90 torr is not removable by helium flushing at either 25 or 100°. 

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