6 Basic properties of nanoparticles – V
Dr. Anchal Srivastava
Contents of this Unit
1. Wettability
2. Surface area of the nanoparticles
3. Porous materials
4. Summary
Learning Outcomes
- After studying this module, you shall be able to understand
- Contact angles between the two surfaces
- The action of surface area in the action of catalysts Porous materials
1. Wettability
There exists a surface tension between a fluid and a solid, in the same way that a surface tension exists between an interface between two immiscible fluids (cf. the surface tension on water under air that is sufficient to support the weight of a needle.). When two fluids are in contact with a solid surface, the equilibrium configuration of the two fluid phases (say air and water) depends on the relative values of the surface tension between each pair of the three phases (Figure 5.1). Let us denote surface tension as γ, and solid, liquid and gas as s, l, and g respectively. Each surface tension acts upon its respective interface, and define the angle θ at which the liquid contacts the surface. This is known as the wetting (or dihedral) angle of the liquid to the solid in the presence of the gas. Equilibrium considerations allow us to calculate the wetting angle from the surface tensions:
Figure 5.1. Liquid/solid/gas wetting angle
This is known as Young’s equation (1805). Table 5.1 shows some contact angles and surface tensions for common fluids in the hydrocarbon industry.
Table 5.1 Contact angles and interfacial tension for common fluid–fluid interfaces
For a stable contact | Cos θ | or equivalently | γsg – γsl | ≤ γgl. This inequality is not satisfied when γlg + γsl = γsg, when liquid covers the whole solid surface. Alternatively, when γlg + γsg = γsl, the gas displaces the liquid away from the surface completely. Figure 5.2 shows a range of different wetting conditions.
Figure 5.2 Wetting angles for various wetting properties
When one fluid preferentially covers the surface, it is called the wetting fluid, and the other fluid is called the non-wetting fluid. The origin of these surface tensions arises in the different strengths of molecular level interactions taking place between the pairs of fluids. For example a quartz sandstone grain generally develops greater molecular forces between itself and water than between itself and oils. Clean sandstones are therefore commonly water wet. However, should that same grain have been baked at high temperatures in the presence of oil at depth, then (i) its surface chemical structure can be altered, or (ii) the surface itself can become coated, in such a way that results in the grain having greater molecular interaction with oils than water, and hence become oil wet.
IMPORTANT NOTE: properly measured wettability on well preserved core and core plugs should form the initial step in choosing relative permeability test methods.
2. SURFACE AREA OF NANOPARTICLES
Nanoparticles have an appreciable fraction of their atoms at the surface, as the data in Tables 5.3 demonstrate. A number of properties of materials composed of micrometre-sized grains, as well as those composed of nanometre-sized particles, depend strongly on the surface area. For example, the electrical resistivity of a granular material is expected to scale with the total area of the grain boundaries. The chemical activity of a conventional heterogeneous catalyst is proportional to the overall specific surface area per unit volume, so the high areas of nanoparticles provide them with the possibility of functioning as efficient catalysts. It does not follow, however, that catalytic activity will necessarily scale with the surface area in the nanoparticle range of sizes.
Figure 5.3 Reaction rate of hydrogen gas with iron nanoparticles versus the particle size.
Figure 5.3, which is a plot of the reaction rate of H2 with Fe particles as a function of the particle size, does not show any trend in this direction, and neither does the dissociation rate plotted in Fig. 5.4 for atomic carbon formed on rhodium aggregates deposited on an alumina film. Figure 5.5 shows that the activity or turnover frequency (TOF) of the cyclohexene hydrogenation reaction (frequency of converting cyclohexene C6H4 to cyclohexane C6H6) normalized to the concentration of surface Rh metal atoms decreases with increasing particle size from 1.5 to 3.5 nm, and then begins to level off.
Figure 5.4 Effect of catalytic particle size on the dissociation rate of carbon monoxide. Rhodium aggregates of
on alumina (Al2O3) films. The rhodium was given a saturation carbon monoxide (CO) coverage, then the material was heated from 90 to 550 K (circles), or from 300 to 500 K (squares), and the amount of atomic carbon formed on the rhodium provided a measure of the dissociation rate for each aggregate (island) size.
The Rh particle size had been established by the particular alcohol CnH2n+1OH (inset of Fig. 5.5) used in the catalyst preparation, where n = 1 for methanol, 2 for ethanol, 3 for 1-propanol, and 4 for 1-butanol. The specific surface area of a catalyst is customarily reported in the units of square meters per gram, denoted by the symbol S, with typical values for commercial catalysts in the range from 100 to 400 m2/g. The general expression for this specific surface area per gram S is
Where ρ is the density, which is expressed in the units g/cm3. A sphere of diameter d has the area A =
πr2 and the volume V = 6 3, to give A/V = 6/d. A cylinder of diameter d and length L has the volume V = 4 2. The limit L << d corresponds to the shape of a disk with the area A = 22, including both sides, to give A/V = 2/d. in like manner, a long cylinder or wire of diameter d and length L >> d has A = 2πrL, and A/V = 4/V. Figures 2.1 provide the sketches of these figures.
Figure 5.5 Activity of cyclohexane hydrogenation, measured by the turnover frequency (TOF) or rate of conversion of cyclohexane to cyclohexane, plotted as a function of the rhodium (Rh) metal particle size on the surface. The inset gives the alcohols (alkanols) used for the preparation of each particles size.
Using the units square meters per gram, m2/g, for these various geometries we obtain the expressions
Table 5.2 Densities in g/cm3 of group III-V and II-VI compounds
The densities of type III-V and II-VI semiconductors, from table 5.2 are in the range from 2.42 to 8.25 g/cm3, with GaAs having the typical value ρ = 5.32 g/cm3. Using this density we calculated the specific surface area of the nanostructures represented by equations (5.3a), (5.3c) and (5.3d), for various values of the size parameters d and L, and the results are represented in the table 5.3.
Table 5.3 Specific surface areas of GaAs spheres, long cylinders (wires) and thin disks as a function of their size
The specific surface areas for the smallest structures listed in the table correspond to quantum dots, quantum wires and quantum wells, as discussed in the module II. Their specific surface areas are within the range typical of the commercial catalysts.
The data tabulated 5.3 represent minimum specific surface areas in the sense that for a particular mass, or for a particular volume, a spherical shape has the lowest possible area, and for a particular linear mass density, or mass per unit length, a wire of circular cross section has the minimum possible area. It is of interest to examine how the specific surface area depends on the shape. Consider a cube of side a with the same volume as a sphere of radius r
With the aid of equations (5.3a) and (5.3b) we obtain for this case Scub = 1.24 Ssph, so a cube has 24% more specific surface area than a sphere with the same volume.
To obtain a more general expression for the shape dependence of the area: volume ratio, we consider a cylinder of diameter D and length L with the same volume as a sphere of radius r, specifically, 4πr3/3 =πD2L/4, which gives r = ½ [3D2L/2]1/3. It is easy to show that the specific surface area S(L/D) from equation 5.2 is given by
This expression S(L/D), which has a minimum Smin = 1.146Ssph for the ratio (LID) = I, is plotted in figure 5.6, normalized relative to Ssph. The normalization factor Ssph was chosen because a sphere has the smallest surface area of any object with a particular volume.
Figure 5.6 Dependence of the surface area S(L/D) of cylinder on its length: diameter ratio L/D. the surface area is normalized relative to that of a sphere Ssph = 3/ρr with the same volume.
Figure 5.6 shows how the surface area increases when a sphere is distorted into the shape of a disk with a particular L/D ratio, without changing in its volume. This figure demonstrates that nanostructures of a particular mass or of a particular volume have much higher surface area S when they are flat or elongated in shape, and further distortions rom s regular shape will increase the area even more.
3. POROUS MATERIALS
In the previous section we saw that an efficient way to increase the surface area of a material is to decrease its grain size or its particle size. Another way to increase the surface area is to fill the material with voids or empty spaces. Some substances such as zeolites crystallize in structures in which there are regularly spaced cavities where atoms or small molecules can lodge, or they can move in and out during changes in the environmental conditions. A molecular sieve, which is a material suitable for filtering out molecules of particular sizes, ordinarily has a controlled narrow range of pore diameters. There are also other materials such as silicas and aluminas which can be prepared so that they have a porous structure of a more or less random type; that is, they serve as sponges on a mesoscopic or micrometre scale. It is quite common for these materials to have pores with diameters in the nanometre range. Pore surface areas are sometimes determined by the Brunauer-Emmett-Teller (BET) adsorption isotherm method in which measurements are made of the uptake of a gas such as nitrogen (N2) by the pores.
Most commercial heterogeneous catalysts have a very porous structure, with surface areas of several hundred square meters per gram. Ordinarily a heterogeneous catalyst consists of a high-surface-area material that serves as a catalyst support or substrate, and the surface linings of its pores contain a dispersed active component, such as acid sites or platinum atoms, which bring about or accelerate the catalytic reaction. Examples of substrates are the oxides silica (SiO2), gamma alumina (γ-Al2O3), titania (TiO2 in its tetragonal anatase form), and zirconia (ZrO2). Mixed oxides are also in common use, such as high-surface-area silica-alumina. A porous material ordinarily has a range of pore sizes, and this is illustrated by the upper right spectrum in Fig. 5.7 for the organosilicate molecular sieve MCM-41, which has a mean pore diameter of 3.94 nm (39.4 A). The introduction of relatively large trimethylsilyl groups (CH3)3Si to replace protons of silanols SiH3OH in the pores occludes the pore volume, and shifts the distribution of pores to a smaller range of sizes, as shown in the lower left spectrum of the figure 5.8. The detection of the nuclear magnetic resonance (NMR) signal from the 29Si isotope of the trimethylsilyl groups in these molecular sieves, with its +12 ppm chemical shiR shown in Fig. 5.8, confirmed its presence in the pores after the trimethylsilation treatment.
Figure 5.7 Distribution of pore diameters in two molecular sieves with mean pore diameters of 3.04 and 3.94 nm, determined by the physisorption or argon gas.
Figure 5.8 Silicon (29Si) nuclear magnetic resonance (NMR) spectrum of an organosilicate molecular sieve before (lower spectrum) and after (upper spectrum) the introduction of the large trimethylsilyl groups (CH3)3Si to replace the protons of the silanols SiH3OH arises from 29Si of trimethylsilyl, and the strong signal on the right at -124 ppm is due to 29Si in silanol SiH3OH.
The active component of a heterogeneous catalyst can be a transition ion, and traditionally over the years the most important active component has been platinum dispersed on the surface. Examples of some metal oxides that serve as catalysts, either by themselves or distributed on a supporting material, are NiO, Cr2O3, Fe2O3, Fe3O4, Co3O4, and β-Bi2MO2O9. Preparing oxides and other catalytic materials for use ordinarily involves calcination, which is a heat treatment at several hundred degrees Celsius. This treatment can change the structure of the bulk and the surface, and Fig. 5.9 illustrates this for the catalytically active material β-Bi2Mo2O9. We deduce from the figure 5.9 that for calcinations in air the grain sizes grow rapidly between 300 and 350″C, reaching 20 nm, with very little additional change up to 500°C. Sometimes a heat treatment induces a phase change of catalytic importance, as in the case of hydrous zirconia (ZrO2), which transforms from a high-surface-area amorphous state to a low-area tetragonal phase at 450°C, as shown in Fig. 5.10. The change is exothermic, that is, one accompanied by the emission of heat, as shown by the exotherm peak at 450°C in the differential thermal analysis (DTA) curve of Figure 5.11. For some reactions the catalytic activity arises from the presence of acid sites on the surface. These sites can correspond to either Brcansted acids, which are proton donors, or Lewis acids, which are electron pair acceptors. Figure 5.12 sketches the structures of a Lewis acid site on the left and a Brransted acid site on the right located on the surface of a zirconia sulphate catalyst, perhaps containing platinum (Pt/ZrO2-SO4). The figure 5.9 shows a surface sulphate group SO4 bonded to adjacent Zr atoms in a way that creates the acid sites. In mixed-oxide catalysts the surface acidity depends on the mixing ratio. For example, the addition of up to 20 wt% ZrO2 to gamma-alumina (γ-A12O3) does not significantly alter the activity of the Brransted acid sites, but it has a pronounced effect in reducing the strength of the Lewis acid sites from very strong for pure γ-alumina to a rather moderate value after the addition of the 20wt% ZrO2. The infrared stretching frequency of the -CN vibration of adsorbed CD3CN is a measure of the Lewis acid site strength, and Figure 5.13 shows the progressive decrease in this frequency with increasing incorporation of ZrO2 in the γ-A12O3.
Figure 5.9 X-ray diffraction patterns of the catalysts β-Bi2Mo2O9 taken at 1000C. Interval during calcination in air over the range 100 – 5000C.
Figure 5.10 Temperature dependence of the surface area of hydrous zirconia, showing the phase transition at 4500C.
Figure 5.11 Differential thermal analysis curves of hydrous zirconia catalysts. The heat emission peak at 4500C arises from the exothermic phase transition shown in figure 5.10.
Figure 5.12 Configuration of sulphate group on the surface of a zirconia-sulphate catalyst, showing the Lewis acid site Zr– at the left, and the Bronsted acid site H+ at the right.
Figure 5.13 Cyanide group (-CN) infrared stretching frequency of CD3CN adsorbed on zirconia alumina catalysts as a function of zirconia content after evacuation at 250C (upper curve) and 1000C (lower curve). The stretching frequency is a measure of the Lewis acid site strength.
4. SUMMARY
- About the contact angle for the wettability of surfaces. Role of specific surface area for catalyst action
- About the properties of porous materials.
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Learn More:
1. H.S. Nalwa, ed., Handbook of nanostructured materials and nanotechnology, Vols. 1-5. Academic Press,
Boston, 2000.
2. W.R. Moser. Ed,. Advanced catalysis and nanostructured materials, Academic Press, San Diego, 1996.
3. J.L. Wilber and G.M. Whitesides, in Nanotechnology, G. Timp, Ed., Springer-Verlag, Berlin, 199, Chapter 8