Adsorption metal surface area

In this article, we will discuss the use of physical adsorption to determine the total surface areas of finely divided powders or solids, e.g., clay, carbon black, silica, inorganic pigments, polymers, alumina, and so forth. The use of chemisorption is confined to the measurements of metal surface areas of finely divided metals, such as powders, evaporated metal films, and those found in supported metal catalysts. 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

It is often found that the ratio R (measured, for instance, by gas adsorption methods) of actual metal surface area accessible to the gas phase, to the geometric film area, exceeds unity. This arises from nonplanarity of the outermost film surface both on an atomic and a more macroscopic scale, and from porosity of the film due to gaps between the crystals. These gags are typically up to about 20 A wide. However, for film thicknesses >500 A, this gap structure is never such as completely to isolate metal crystals one from the other, and almost all of the substrate is, in fact, covered by metal. In practice, catalytic work mostly uses thick films in the thickness range 500-2000 A, and it is easily shown (7) that intercrystal gaps in these films will not influence catalytic reaction kinetics provided the half-life of the reaction exceeds about 10-20 sec, which will usually be the case. 

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 specific surface areas were determined by means of nitrogen adsorption and the metallic surface areas by using adsorption of 3-methylthiophen in liquid phase (ref. 9). The bulk composition of each sample was determined by chemical analysis and expressed by the atomic ratios Al/Ni and M/Ni. The catalysts were observed by transmission electron microscopy (JE0L 200 CX-TEM) and analysed either globally or at point level with a lateral resolution of 1.5 nm by means of a STEM (VG - HB 501) connected to an energy - dispersive X-ray analyser (EDAX). 

Table 1 gives the average sizes of nickel crystallites measured by X-ray line broadening analysis on (111) reflections, before and after the five hydrogenation runs. They increase moderately and even decrease for RNiFe. This confirms that the BET area loss could be due in part to a poisoning which reduces the capacity of nitrogen adsorption. However, measurements of the metallic surface area should also be done to confirm possible surface poisoning. 

The transitory poisoning by scavengers is explained by competitive adsorption of a halogen-containing species on catalyst sites that are needed for the oxidation of CO and hydrocarbons. In the case of EDB it is thermodynamically probable that HBr 33), or Br2 is the actual adsorbed species (66). The possible interactions of EDB and EDC with TEL and the resulting loss in noble metal surface area on the one hand, and catalyst activity on the other, are very complex (66). 

In order to obtain quantitative measurements of hydrogenation activity and acidity, various schemes are employed. For example, metal surface area has been related to hydrogenation activity and the adsorption of bases such as pyridine and ammonia have been correlated with acidity ((3). Some authors have used certain key reactions involving pure compounds as an indication of catalytic properties (16). Each of these methods is useful however, because of the complex interdependence of the catalytic functions of the hydrocracking catalysts and changes in these functions with catalyst aging, results from each method must be interpreted with caution. 

On metallic catalysts, sulfur is strongly adsorbed, and even if only minute amounts are found in the feedstock, accumulation can occur on a significant part of the metallic surface area. In the adsorbed state, the poison molecule will deactivate the surface on which it is adsorbed then the toxicity will depend on the number of geometrically blocked metal atoms. On the other hand, the chemisorption bond between the poison and the metal can modify the properties of the neighboring metallic atoms responsible for the adsorption of reactants. If the interaction between the poison and the metal is weak, the structure of the metal will remain unchanged, but it can induce a perturbation all around the adsorption site, which will be able to modify the catalytic properties of this surface. Yet if the interaction between the metal and the adsorbate is strong, it can go as far as to modify the metal-metal bond. The mobility of the surface atoms can be increased and a new superficial structure can appear. 

Competitive reactions and essentially competitive hydrogenations were often used to discuss the extent of the electronic transfers induced by poison adsorption. For instance, two model molecules with different electronic densities are chosen, e.g., benzene and toluene. In this case the electronic donor properties of the methyl group increase the electronic density on the insaturated bonds. During competitive hydrogenation of benzene and toluene, sulfur adsorption poisons the two reactions but is less toxic for toluene than for benzene hydrogenation (94-96). Sulfur, by its adsorption as an electron acceptor, is able to decrease the electronic density of the unpoisoned metallic surface area and could favor the adsorption of the reactant with the highest donor properties enhancing the hydro-... 

In hydrogenation conditions the effect of sulfur adsorption is the result of interactions between the metal, the hydrocarbon, and the sulfur-containing compound. As a consequence, for a given metal, the sulfur coverage, and its effect on the activity and selectivity of the unpoisoned metallic surface area, will be defined by the nature of the hydrocarbon. 

This organization has a long-established reputation for devising standard procedures for measurements of all kinds. A committee has recently drafted a specification for the determination of metal surface areas using gas adsorption techniques, and this has been published, as Part 4 of BS 4359, in 1995. Emphasis is placed on supported Pt catalysts, but guidance is given on other systems. 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

The metal surface area for one of the catalysts, catalyst A, was determined by hydrogen chemisorption. Prior to the adsorption experiment, the sample was reduced in hydrogen at 370°C for 16 hours and then evacuated for one and a half hours. The adsorption experiment was conducted at room temperature using one gram of catalyst. 

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... 

Hydrogen adsorpiion measurements were made on the catalysts in conventional static volumetric equipment Hydrogen uptake was obtained from the isotherm plateau. Metal surface area was calculated assuming dissociative adsorption of hydrogen with one hydrogen atom on each surface nickel atom and assuming the cross-sectional area of nickel to be 0.065 nm. ... 

In the first group belong the techniques which are also used in heterogeneous catalysis for determining the surface area of catalysts. Two such techniques are widely used The Brunauer-Emett-Teller (BET) method, based on the physical adsorption of N2 or Ar at very low temperatures [8, 44] and the H2 or CO chemisorption method [8, 44], The first method leads to the total catalyst surface area, whereas the second leads to the specific (active metal) surface area. In the case of supported electrocatalysts (e.g., Pt/C electrocatalysts used as anodes in PEM fuel cells) the two techniques are complementary, as the former can lead to the total electrocatalyst surface... 

Disproportionation. The hydrogenation of MNoP into TBHA is so fast that the disproportionation of TBHA may be approximated by 2 TBHA -> TBHA + TBA instead of 2 TBHA -> MNoP + TBA. This reaction decreases the TBHA concentration and increase the TBA formation. Depending on the adsorption coefficient value for each organic compound, this process may free a part of the metallic surface area, and therefore favor the MNP conversion. However, this hypothesis can not explain a slowing down of the reaction due to the addition of another active carbon. 

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