Adsorbed nitrogen

Fig. XVn-24. Site energy distribution for nitrogen adsorbed on Silica SB. (From Ref. 160.) (Reprinted with permission from J. Phys. Chem. Copyright by the American Chemical Society.)...

Deitz and Carpenter [176] found that argon and nitrogen adsorbed only... 

In Section XVII-16C there is mention of S-shaped isotherms being obtained. That is, as pressure increased, the amount adsorbed increased, then decreased, then increased again. If this is equilibrium behavior, explain whether a violation of the second law of thermodynamics is implied. A sketch of such an isotherm is shown for nitrogen adsorbed on a microporous carbon (see Ref. 226). 

Kjems J K, Passell L, Taub H, Dash J G and Novaco A D 1976 Neutron scattering study of nitrogen adsorbed on basal plane-oriented graphite Rhys. Rev. B 13 1446-62... 

Fig. 2.7 The BET plot for nitrogen adsorbed at 78 K on sodium chloride. (p/p°)/e(l — pfp°) is plotted against p/p° v = amount adsorbed in cm (stp). (Courtesy Maciver and Emmett.)...

Fig. 2.16 The molar entropy for nitrogen adsorbed ongraphitizedcarbon (Graphon) at — 189-3°C, as a function of the amount adsorbed s, = molar entropy of adsorbed nitrogen s, = molar entropy of liquid nitrogen. (Courtesy Hill, Emmett and Joyner.)...

The point is illustrated by the figures in Table 3.11 for hexane and nitrogen (adsorbed on carbon black). Though the value of yVJT is almost twice as high for hexane as for nitrogen, the corresponding values of t for hexane are... 

Fig. 4.7 Isosteric heat of adsorption q of nitrogen adsorbed at 77 K on non-porous and porous silica.(I) 4. Fransil", non-porous (2) gel A, mesoporous (. ) A gel B, mesoporous and microporous (4) gel D,...

The reported surface area is the combined surface area of zeolite and matrix. In zeolite manufacturing, the measurement of the zeolite surface area is one of the procedures used by catalyst suppliers to control quality. The surface area is commonly determined by the amount of nitrogen adsorbed by the catalyst. 

The analytical data of the catalysts used are given in Table II. The number of B5 sites was determined from infrared measurements and nitrogen adsorption isotherms in the way outlined by van Hardeveld and Montfoort 10). The values found are higher than those mentioned in an earlier paper 24), owing partly to an improvement of the method for determining the extinction coefficient per molecule of nitrogen adsorbed,... 

Data taken from the adsorption leg of the isotherm of Figure 17.11 are listed in the first two columns of the following table. Test the applicability of the following equilibrium theories (a) Langmuir (b) infinite BET and (c) Harkins and Jura. From (a) and (b) obtain estimates of the surface area of the adsorbent and compare the values with that obtained by the point B method. One molecule of nitrogen adsorbed on alumina occupies 0.162 nm2. 

We have so far discussed two possibilities alternative to the assumption of the existence of multimolecular films and enquiry is necessary to examine how far all existing data can be reconciled to the assumptions either of a capillary surface or a surface variable in accessibility. It must be admitted that these views do not seem adequate to explain all cases of adsorption. Thus in the data presented by Evans and George it is rather singular that the amount of nitrogen adsorbed on glass should be equal to the computed unimolecular film whilst the other easily liquefiable gases exceed this thickness. Langmuir s data on the adsorption of carbon... 

The kinetics of the electrochemical oxidation of ammonia on platinum to dinitrogen in basic electrolytes has been extensively studied. In the widely supported mechanism originally suggested by Gerischer and Mauerer[ll], the active intermediate in the selective oxidation to N2 is a partly dehydrogenated ammonia adsorbate, NH2 ads or NHaatomic nitrogen adsorbate N ag, which is apparently formed at more positive potentials, is inactive toward N2 production at room temperature. Generally, only platinum and iridium electrodes exhibit steady-state N2 production at potentials at which no sur-... 

The usual method of measuring the statistical thickness is to divide the liquid volume of nitrogen adsorbed, by the BET surface area... 

The results obtained by Loebenstein and Deitz agreed with vacuum volumetric measurements on a large variety of samples with a wide range of surface areas. They were also able to establish that the quantities of nitrogen adsorbed were independent of the presence of helium. 

Fig. 2. STM image (78 A x 76 A) of nitrogen atom adsorbates on an Fe(l 00) surface. Because the nitrogen adsorbates deplete the LDOS at the Fermi level, the nitrogen atoms are imaged as depressions in accord with the Tersoff-Hamann model. From the STM image it is concluded that nitrogen atoms adsorb in fourfold hollow sites on Fe(l 0 0). This is just one of many examples illustrating how the STM contrast may depend on the details of the LDOS around an adsorbate and produce a somewhat counterintuitive picture. Adapted from Reference (dd).

Fig. 15. The variation of the sticking probability of Nj on W with the amount of nitrogen adsorbed.

From 500°C to 600°C, the samples maintain pratically their high surface area (more than 600 m2/g), even if from a general point of view the value decreases slowly. For both MCM-41 and MCM-48 materials, the value of specific surface area decreases dramatically after calcination at 600°C. The maximum volume of nitrogen adsorbed by samples decreases from 1200 cm3/g for calcination at 500°C to 180 cm3/g for calcination at 700°C for the MCM-48 materials and from 1130 cm3/g to 200 cm3/g for the MCM-41 one. These observations indicate that a calcination temperature superior to 600°C can destroy almost completely the structure of MCM-41 and MCM-48 materials. 

The sharp increase in the adsorbed volume of nitrogen due to capillary condensation for the sample obtained with decane is relatively vertical, reflecting the homogeneity of the sample which has a high specific surface area of 750 m2/g. For other compounds, the capillary condensation is less pronounced, meaning that only a part of material is well crystallized. This is confirmed by the low value of nitrogen adsorbed and low specific surface areas (500 m2/g for heptane, 405 m2/g for undecane). 

From the quantity of nitrogen adsorbed at the three nitrogen flowrates which, in turn, correspond to three relative pressures, a plot of the BET equation is obtained. The surface area is then calculated according to Equation 11.1. 

Iron has a body-centred cubic lattice (see Figure 5.16) with a unit cell side of 286 pm. Calculate the number of iron atoms per cm2 of surface for each of the Fe(100), Fe(110) and Fe(lll) crystal faces. Nitrogen adsorbs dissociatively on the Fe(100) surface and the LEED pattern is that of a C(2 x 2) adsorbed layer. Assuming saturation of this layer, calculate the number of adsorbed nitrogen atoms per cm2 of surface. 

Vapors in equilibrium with liquid in fine capillaries or pores will have depressed vapor pressure as a result of the Kelvin effect. In fact, if the pores are adequately small, the vapor will condense at pressures far below normal. By measuring the volume of nitrogen adsorbed at a relative pressure, i.e., p/po, of 0.99 and with prior knowledge of the surface area, the average pore diameter can be calculated. 

There are two values of surface area and volume of nitrogen adsorbed (BJH method), obtained with the parent H-Y zeolite and the H-Y/TFA sample (Table 1) the first corresponds to the zeolite-type micropores and the other, to the mesopores. Figure 1 shows the pore size distribution of the H-Y/TFA catalyst there is a sharp peak (not shown here) in the micropore region and another peak at 4nm in the mesopore region. Such a bimodal pore size distribution was also observed with the parent zeolite. 

Figure 1 Pore size distribution (mesopore region) of the H-Y/TFA catalyst. V (Volume of nitrogen adsorbed) and D (pore diameter) are expressed in cc/g and nm (10 9 m), respectively.

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