Adsorption stoichiometry

In some specific cases one would like to convert the chemisorption data into an averaged particle size. In that case, the number of surface atoms per unit surface area (density of surface atoms) is an essential parameter. Since this number depends on the type of the crystallographic plane, (see Table 3.7), one also needs information on the types of crystallographic planes exposed to the gas phase. This is also important for another reason the adsorption stoichiometry may depend on the crystallographic plane. 

Some data on the adsorption stoichiometry of various gases on relevant transition metals have been collected in Table 3.7, which illustrates the usefulness of certain molecules for catalyst characterization by chemisorption. Note that Cu as active phase can be measured well with N2O and CO, but not with H2. It is not wise to determine Ni dispersion with CO, due to the possibility of carbonyl formation Ni carbonyls are volatile and poisonous. Note that in Table 3.7, for Rh the H/Me ratio is size dependent. This phenomenon is not restricted to Rh it is common in the chemisorption of metals. 

Chemisorption measurements (Quantachrome Instruments, ChemBET 3000) were conducted in order to determine the metal (Co) dispersion. Therefore, the nanomaterial catalysts were reduced under a hydrogen flow (10% H2 in Ar) at 633 K for 3 h. The samples were then flushed with helium for another hour at the same temperature in order to remove the weakly adsorbed hydrogen. Chemisorption was carried out by applying a pulse-titration method with carbon monoxide as adsorbing agent at 77 K. The calculation of the dispersion is based on a molar adsorption stoichiometry of CO to Co of 1. 

Figure 6.16 Reconformation of adsorbed cationic polyacrylamide (MW 4 x 106) on cellulose fibres as shown by the kinetics of adsorption and adsorption stoichiometry (measured by counter ion release).

The chemisorption of sulfur from mixtures of H,S and H2 has been widely studied we have discussed some of the results. Nevertheless, introduction of irreversible and reversible adsorbed sulfur, which is in line with adsorption stoichiometries varying from more than 1 to 0.4 sulfur atom by accessible platinum atom, shows that different adsorbed species are involved in sulfur chemisorption. In fact, electrooxidation of adsorbed sulfur on platinum catalysts occurs at two different electrochemical potentials (42) in the same way, two different species of adsorbed sulfur were identified on gold by electrochemical techniques and XPS measurements (43,44). By use of 35S (45) it was pointed out that, according to the experimental conditions, reducible PtS2 or nonreducible PtS mono-layers can be created. 

The characterization of pure platinum catalysts and of Pt catalysts modified by lead was achieved in situ by linear potential sweep cyclic voltammetry. This technique allowed to measure the active platinum surface area in the absence and in the presence of deposited lead and to determine the surface fraction covered by lead adatoms (9-12). The adsorption stoichiometry of lead on platinum was also evaluated by electrochemical techniques and found to be equal to two (one lead atom covers two platinum atoms on the surface) (II). 

Sulfur adsorption stoichiometries at saturation coverage for single-crystal surfaces of Pt (84, 85), Fe (72, 142), Mo (75,143, 144), Ag (53-56), and Cu (58,65), and for polycrystalline metal surfaces of Pt (145), Fe (101), Co (101), and Ru (101) have been reported. The general features observed for these metals are similar to those observed for Ni accordingly, only the more interesting observations will be discussed. 

This plot of fractional H2 adsorption (H2 uptake at 300 K of catalysts presulfided in 5, 10, or 25 ppm H2S at 725 K divided by initial H2 uptake) versus mean sulfur coverage (in molecules H2S adsorbed per molecule of surface nickel), suggests a linear relationship between H2 uptake and sulfur coverage. Interestingly, the intercept at zero H2 coverage (saturation sulfur coverage) is H2S/Nis = 0.75, in excellent agreement with the adsorption stoichiometry reported by Oliphant et al. 112) for adsorption of H2S at 725 K. 

Chemical Measurements. All the catalysts in Table I showed a suppression in hydrogen chemisorption to varying extent. Such a suppression manifested itself either as an overestimation of crystallite size from chemisorption data or as a lower adsorption stoichiometry of H/Ni,, than what would be expected of a comparable Ni/Si02 catalyst. In addition, the suppression was more severe with increasing reduction temperature (8-10). As mentioned... 

The metal accesibiMty of the catalysts was determined by gas chemisorption (hydrogen, oxygen) at room temperature in a conventional volumetric equipment. The widely admitted adsorption stoichiometries are H/Pt = 0/Pt = 1. No chemisorption of hydrogen and oxygen on gold was observed at this conditions (14). The carvone hydrogenation reaction, in... 

Table 1 compares our results obtained for the EuroPt-1 catalyst using the CO methanation technique with results reported by other groups [13-17] using x-ray diffraction, conventional chemisorption, and TEM. A dispersion of 45% was calculated using the spherical particle approximation and an assumed adsorption stoichiometry of 1 CO molecule per exposed Pt atom. If the adsorption stoichiometry is instead assiuned to be 0.7, in keeping with previous studies of CO chemisorption out on Pt catalysts [17,18], then the dispersion becomes 65% in close agreement with the studies shown in Table 1 employing other techniques. 

The results obtained for the fresh and aged commercial Pt/Rh and Pd/Rh TWCs are shown in Table 2. The first column contains the dispersions and calculated spherical particle sizes (in parentheses) derived from the CO methanation technique based on an assumed adsorption stoichiometry of 1 CO per exposed noble metal atom. The arbitrary choice of a stoichiometric factor of 1, rather than the value of 0.7 suggested by the EmoPt-l catalyst, was made on the basis of several factors. The main reason is that the presence of Rh in these catalysts (16% and 10% of the noble metal weight in the Pt/Rh and Pd/Rh catalysts, respectively) is likely to increase the average stoichiometric factor above 0.7 due to the presence of gem-dicarbonyl species on Rh. Bimetallic Pt/Rh particles have been found in automotive catalysts, sometimes with surface enrichment by Rh [20,21] or even bulk enrichment of selected particles as... 

The volumetric method has very often been used with platinum catalysts for which quite satisfactory results are generally obtained it is usual to assume that the monolayer volume or amount, obtained as just described or by extrapolation corresponds to an H Ms (hydrogen atom to metal surface atom) ratio of 1 1. Some justification for this assumption is to be found, at least for particles of moderate size, in the adsorption stoichiometry shown by films and single crystals, but for very small particles and at high pressures the H/Mj ratio can exceed unity quite substantially this is especially so with rhodium" and iridium (see below). Care is however needed with palladium " " because of the risk of forming the hydride however, monolayer coverage is obtained at pressures below which dissolution starts. The base metals iron, cobalt and nickel have been... 

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