Physical adsorption measurements

This paper is concerned with a description cf the experimental findings including the influence of thermal pretreatment. A subsequent paper will be concerned with the influence of the amount of gas introduced. The range of surface coverage which leads to the delay in adsorption is far removed from the usual region in which physical adsorption measurements are made, for example, in surface area determinations. This may be one reason why the delay has not been reported before. 

Figures 1.10 and 1.11 show, respectively, the mercury intriision curve and resulting pore size distribution of a commercial alumina. This solid is thus characterised by the presence of mesopores with an average diameter of 12 nm and macropores of approximately 3 600 nm. The value of this technique in supplementing physical adsorption measurement is clearly shown here as nitrogen adsorption cannot be used to characterise pores greater than approximately 50 nm in size.

Surface properties of all the samples were determined from physical adsorption measurements by volumetric techniques, using Nj at -196 °C and COj at 25 °C. The complementary use of these adsorbates allows characterisation of complex networks of pores of different sizes (5). A Micromeritics Gemini 2360 surftce area analyser was employed for Nj adsorption experiments, whereas adsorption measurements performed with CO2 were carried out in a Micromeritics Accusorb 2100E soiption instrument. All the samples were outgassed overnight at 120°C at a final pressure of 1.33 x lO Pa (10 mm Hg) prior to adsorption measurements. 

Physical-adsorption studies are valuable in determining the physical properties of solid catalysts. Thus the questions of surface area and pore-size distribution in porous catalysts can be answered from physical-adsorption measurements. These aspects of physical adsorption are considered in Secs. 8-5 and 8-7. 

As with all scientific writing there are various levels that can be presented. For example, infrared spectroscopy could be used on simply the pattern recognition level or at the more sophisticated level of quantum mechanics. So it is with physical adsorption. One can use the data from physical adsorption measurements as a simple control device, i.e. Does this powder have the right adsorption isotherm to meet production requirements , or on a different level What is the meaning of the isotherm in terms of surface and pore structure and chemical attractions For most applications, the level of sophistication is somewhat intermediate. 

Give four specific experimental tests, measurements, or criteria that would be considered good evidence for characterizing adsorption in a given system as either physical adsorption or chemisorption. 

Surface areas are deterrnined routinely and exactiy from measurements of the amount of physically adsorbed, physisorbed, nitrogen. Physical adsorption is a process akin to condensation the adsorbed molecules interact weakly with the surface and multilayers form. The standard interpretation of nitrogen adsorption data is based on the BET model (45), which accounts for multilayer adsorption. From a measured adsorption isotherm and the known area of an adsorbed N2 molecule, taken to be 0.162 nm, the surface area of the soHd is calculated (see Adsorption). 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Physical adsorption—surface areas of any stable solids, e.g., oxides used as catalyst supports and carbon black Chemisorption—measurements of particle sizes of metal powders, and of supported metals in catalysts... 

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. 

In particular, emphasis will be placed on the use of chemisorption to measure the metal dispersion, metal area, or particle size of catalytically active metals supported on nonreducible oxides such as the refractory oxides, silica, alumina, silica-alumina, and zeolites. In contrast to physical adsorption, there are no complete books devoted to this aspect of catalyst characterization however, there is a chapter in Anderson that discusses the subject. 

As this field is very wide, we will discuss first the gases that can be used to study metal dispersion by selective chemisorption, and then some specific examples of their application. The choice of gases, is, of course, restricted to those that will strongly chemisorb on the metal, but will not physically adsorb on the support. Prior to determining the chemisorption isotherm, the metal must be reduced in flowing hydrogen details are given elsewhere. The isotherm measurement is identical to that used in physical adsorption. 

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. 

Barometric chemisorption. Chemisorption on catalysts is measured routinely by the barometric method. The equipment is very similar to that commonly used in texture determination by physical adsorption (see Section 3.6.2), except that for chemisorption measurements facilities for pretreatment of the samples should be present. In particular for metal catalysts often the catalyst is received in a partly or fully oxidized form and, as a consequence, reduction is required when one wants to measure the amount of active sites. Moreover, during storage adsorption of various molecules might occur and evacuation is... 

There is further emphasis on adsorption isotherms, the nature of the adsorption process, with measurements of heats of adsorption providing evidence for different adsorption processes - physical adsorption and activated adsorption -and surface mobility. We see the emergence of physics-based experimental methods for the study of adsorption, with Becker at Bell Telephone Laboratories applying thermionic emission methods and work function changes for alkali metal adsorption on tungsten. 

The conclusions from this work were (i) that the mechanism that operates is of wide applicability, (ii) that exchange proceeds by either the dissociative chemisorption of benzene or by the dissociation of benzene which has previously been associatively chemisorbed, and (iii) that M values of about 2 indicate that further dissociation of surface-area measurements. Surface areas of metal films determined by the chemisorption of hydrogen, oxygen, carbon monoxide, or by physical adsorption of krypton or of xenon concur... 

Alloy films are commonly sintered during preparation by deposition on substrates heated to, say, 400°C or by subsequent annealing at such temperatures, and, consequently, rather small surface areas have to be measured, perhaps in vessels of substantial volume. Krypton adsorption at liquid nitrogen temperature was used with induction-evaporated Cu-Ni, Fe-Ni, and Pd-Ni films, and BET surface areas of 1000-2000 cm2 were recorded (48), after correction for bare glass. The total area of Cu-Ni films was measured by the physical adsorption of xenon at — 196°C (70) in addition, the chemisorption of hydrogen on the same samples enabled the quantity a to be determined where... 

Physical adsorption of nitrogen was carried out on an ASAP 2400 Micromeritics apparatus. Before measurements, samples were evacuated overnight at 350 °C at vacuum of 2 Pa. For all samples the same adsorption data table was used. Collected adsorption data were treated by BET-isotherm in the range 0.05 < P/micropore volume and mesopore + external surface, t-plot method, with master isotherm of nonporous alumina (Harkins-Jura) was used, t-plot was linearized in the range of 0.35 < t < 0.6 nm. 

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

To avoid chemisorption problems, an alternative and more universal adsorption method was proposed [89,90], The method is based on the differences in potentials of physical adsorption on the different components. The background of this method is discussed in Problem 8. It advances a CM, and its efficiency was tested on the model mechanical mixtures and PS type of C/SiOz, C/A1203, C/MgO, Si02 + A1203, etc. by adsorption of COz or hydrocarbons. The main requirement of the adsorbate for such measurements is the existence of some specificity of adsorption on different components. 

The presence of pre-adsorbed polyacrylic acid significantly reduces the adsorption of sodium dodecylsulfonate on hematite from dilute acidic solutions. Nonionic polyacrylamide was found to have a much lesser effect on the adsorption of sulfonate. The isotherm for sulfonate adsorption in absence of polymer on positively charged hematite exhibits the typical three regions characteristic of physical adsorption in aqueous surfactant systems. Adsorption behavior of the sulfonate and polymer is related to electrokinetic potentials in this system. Contact angle measurements on a hematite disk in sulfonate solutions revealed that pre-adsorption of polymer resulted in reduced surface hydrophobicity. 

Electrochemical impedance measurements of the physical adsorption of ssDNA and dsDNA yields useful information about the kinetics and mobihty of the adsorption process. Physical adsorption of DNA is a simple and inexpensive method of immobilization. The ability to detect differences between ssDNA and dsDNA by impedance could be applicable to DNA biosensor technology. EIS measurements were made of the electrical double layer of a hanging drop mercury electrode for both ssDNA and dsDNA [34]. The impedance profiles were modeled by the Debye equivalent circuit for the adsorption and desorption of both ssDNA and dsDNA. Desorption of denatured ssDNA demonstrated greater dielectric loss than desorption of dsDNA. The greater flexibility of the ssDNA compared to dsDNA was proposed to account for this difference. 

Our data indicate that ammonia acts as a mild inhibitor for hydrogen absorption in Ni-containing AB5 alloys. The measured heat of adsorption of NH3 on Ni is about -11 kcal/mol NH3, suggesting weak, physical adsorption. Since Ni is viewed to be a catalyst for the hydriding reaction, weak physical adsorption of NH3 at these Ni sites would retard the reaction and promote a broadening of the reaction front as shown in Figures 12 and 13. 

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