The porosity in powder and the porosity in porous solids are analyzed in the same way as porosity measurement in porous materials. Physical gas adsorption or physisorption is important in the determination of porosity in general and more specific the specific surface area (BET surface area, according to the Brunauer, Emmet and Teller theory), pore volume and pore size distribution of porous materials and non-porous materials. The analysis of porosity by the adsorption isotherm and desorption isotherm of nitrogen, adsorption isotherm of argon or adsorption isotherm of krypton is performed at different low pressures and low temperature, often the boiling point of liquid argon or nitrogen. Prior to the analysis of the adsorption isotherm the sample is pretreated or degassed (outgassed) in vacuum at elevated temperatures. The measurement of the adsorption isotherm by the volumetric technique to calculate the porosity and BET surface area SBET, the total pore volume and pore size distribution is performed on the Quantachrome Autosorb-6B analyzer, NOVA or Autosorb-1C and Micromeritics ASAP 2010 and ASAP 2400. Screening of the surface area of a catalyst is done by dynamic adsorption where very rapidly information is obtained on the surface area and total pore volume. The dynamic adsorption measurements are performed on the Qsurf M3 surface area analyzer and measurement of the multi point BET surface area is done within minutes. Analysis of CO2 adsorption is performed at 273 K on microporous carbons or activated carbons or charcoals for determination of the micropore size and micropore surface area and microporosity. The Dubinin theory is used to calculate the surface area and micropore volume. The CO2 adsorption isotherm is analyzed on the Autosorb-6B. Mercury intrusion porosimetry is measured for surface characterization of mesoporous and macroporous materials and mesoporous solids and macroporous solids and results in information on overall porosity as pore size distribution, pore surface area and pore volume and apparent density. The Thermofinigan PASCAL 140 and PASCAL 440 are used to measure the mercury intrusion curve and extrusion curve at pressures between vacuum and high pressure. Mercury porosimetry is a useful technique that gives rapidly information on the texture and porosity of your porous samples. Chemisorption or chemical gas adsorption is used to analyze mostly catalysts for determination of the active metal surface area and metal dispersion and metal crystallite size by chemical adsorption of reactive gases as hydrogen (H2) and carbonmonoxide (CO) on metals as platinum (Pt), palladium (Pd), nickel (Ni), ruthenium (Ru) and rhodium (Rh). The chemical adsorption isotherm is measured after in-situ reduction and evacuation of the catalyst. This technique does not give information on porosity or pore size distribution but only on the active metal surface area and metal dispersion of the catalyst. The measurement of the chemical adsorption isotherm is performed on the Quantachrome Autosorb-1C in the pressure range vacuum to atmospheric pressure. Helium pycnometry gives information on the skeletal density of solid materials and combined with the apparent density derived form mercury porosimetry can give information on the overall porosity. Helium does not adsorb at ambient conditions and can therefore be used to determine the density of solid samples. The analysis is performed on the Quantrachrome penta pycnometer. Porosity and surface area are important characteristics of solid materials that highly determine the properties and performance of catalysts, sorbents, medicines, rocks, and so on. "How active is your catalyst" and "how efficient will medicines work" are questions related to these characteristics. Physical gas adsorption and chemical gas adsorption as well as mercury intrusion porosimetry are the most widely used techniques to characterize the above-mentioned parameters. Our laboratory can measure specific surface area (SBET), pore volume, pore size distributions in the micro-, meso- and macropore range (0.5 nm - 200 µm), density and active metal surface areas of various materials using many dedicated instruments for each application. In physical gas adsorption an inert gas as nitrogen (or argon, krypton, carbon dioxide) is adsorbed on a solid material. Prior to the measurement the sample is pretreated at elevated temperature in vacuum or flowing gas to remove any contaminants. Two different techniques can be distinguished: The flow technique uses a TCD detector to obtain information on the amount of physisorbed gas only resulting in a BET surface area and/or total pore volume. The volumetric technique provides more information, since many adsorption and/or desorption points are measured to give a full isotherm. The flow measurements are performed on a Qsurf M3 analyzer and the report consists of a single value for the BET surface area or total pore volume. The volumetric adsorption measurements are performed on a Quantachrome Autosorb-6B (N2 or CO2), or Micromeritics ASAP 2010 (Ar) and the report can be a single value for surface area up to a full report on isotherm, surface area, total pore volume and pore size distribution. In mercury intrusion porosimetry non-wetting mercury is used to gain information on the porous characteristics of solid materials. As a consequence of this non-wetting behaviour, higher pressures are required as the pores size decreases. In this way a broad range of pores can be measured starting from 4 nm (pressure = 430 MPa) up to 200 µm (vacuum), and therefore this technique is extremely suitable for materials showing broad distributions of pores or mainly larger pores. Prior to the analysis, vacuum degassing is used to remove moisture from the porous structure. The low and high-pressure intrusion measurements are performed on the CE Instruments Pascal 140 and 440, respectively. The report consists of graphs on the intrusion and extrusion curve and the corresponding pore size distribution and data on pore volume and porosity. In chemical gas adsorption a reactive gas as hydrogen or carbon monoxide is used to obtain information on the active properties of the metal phase of a (supported) metal catalyst. The sample is first reduced in hydrogen and then evacuated to retrieve the active metal phase. In the volumetric method, known amounts of hydrogen (Pt, Ni, Rh, Ru) or CO (Pd, Pt) are dosed and subsequently adsorbed at different partial pressures, resulting in a chemisorption isotherm. This isotherm measurement is repeated after applying an evacuation step at the analysis temperature, to remove weakly adsorbed species (back-sorption method). The difference between the two isotherms represents the chemically bonded reactive gas and is used in the calculations. The measurements are performed on a Quantachrome Autosorb-1C and the report consists of a graphic showing the two measured isotherms and its corresponding difference and a table giving information on specific metal surface area, metal dispersion, and average crystallite size. Helium-pycnometry is a technique to obtain information on the true density of solids. Since helium, which can enter even the smallest voids or pores, is used to measure the volume per unit weight, the final result is often referred to as skeletal density. The density measurements are performed on a Quantachrome pentapycnometer and by application of many different sample cell sizes, a broad range of sample volumes and weights can be measured. Especially physical gas adsorption techniques are widely used in the characterization of these hierarchically ordered porous materials and provide information on the total surface area, pore volume, and pore size distribution. Pore size distributions (PSDs) derived from these adsorption measurements can be used as a fingerprint of the resulting pore structure. With the development of the M41S family and related materials, new models were launched and adapted models were tested for their accuracy in the calculation of PSDs from nitrogen and argon adsorption measurements. However, the conventional models as BJH for mesoporosity and Horvath-Kawazoe (HK model) or Saito-Foley (SF model) for microporosity are still used to large extent and especially for comparative purpose. Problems have arisen in the interpretation of the porous properties obtained from adsorption data. The term sorption is a general expression encompassing both processes. Physical adsorption is caused mainly by van der Waals forces and electrostatic forces between adsorbate molecules and the atoms, which compose the adsorbent surface. Thus adsorbents are characterized first by surface properties such as surface area and polarity. A large specific surface area is preferable for providing large adsorption capacity, but the creation of a large internal surface area in a limited volume inevitably gives rise to large numbers of small sized pores between adsorption surfaces. The size of the micropores determines the accessibility of adsorbate molecules to the internal adsorption surface, so the pore size distribution of micropores is another important property for characterizing adsorptivity of adsorbents. Especially materials such as zeolite and carbon molecular sieves can be specifically engineered with precise pore size distributions and hence tuned for a particular separation. Surface polarity corresponds to affinity with polar substances such as water or alcohols. Polar adsorbents are thus called "hydrophillic" and aluminosilicates such as zeolites, porous alumina, silica gel or silica-alumina are examples of adsorbents of this type. On the other hand, nonpolar adsorbents are generally "hydrophobic". Carbonaceous adsorbents, polymer adsorbents and silicalite are typical nonpolar adsorbents. These adsorbents have more affinity with oil or hydrocarbons than water. Adsorption, the binding of molecules or particles to a surface, must be distinguished from absorption, the filling of pores in a solid. The binding to the surface is usually weak and reversible. Just about anything including the fluid that dissolves or suspends the material of interest is bound, but compounds with color and those that have taste or odor tend to bind strongly. Compounds that contain chromogenic groups (atomic arrangements that vibrate at frequencies in the visible spectrum) very often are strongly adsorbed on activated carbon. Decolorization can be wonderfully efficient by adsorption and with negligible loss of other materials. The most common industrial adsorbents are activated carbon, silica gel, and alumina, because they present enormous surface areas per unit weight. Activated carbon is produced by roasting organic material to decompose it to granules of carbon - coconut shell, wood, and bone are common sources. Silica gel is a matrix of hydrated silicon dioxide. Alumina is mined or precipitated aluminum oxide and hydroxide. A surface already heavily contaminated by adsorbates is not likely to have much capacity for additional binding. Freshly prepared activated carbon has a clean surface. Charcoal made from roasting wood differs from activated carbon in that its surface is contaminated by other products, but further heating will drive off these compounds to produce a surface with high adsorptive capacity. Although the carbon atoms and linked carbons are most important for adsorption, the mineral structure contributes to shape and to mechanical strength. Spent activated carbon is regenerated by roasting, but the thermal expansion and contraction eventually disintegrate the structure so some carbon is lost or oxidized. Temperature effects on adsorption are profound, and measurements are usually at a constant temperature. Graphs of the data are called isotherms. Most steps using adsorbents have little variation in temperature. When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that dissappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption. When the phenomena occur simultaneously, the process is termed sorption. The phenomenon of adsorption was disovered over two centuries ago. The uptake of gases by charcoal was studied by C. W. Scheele in 1773 and by the F. Fontana in 1777. In 1785, charcoal was found to decolorize' solutions by a surface adsorption mechanism. Since these processes have such a long history, we will make little attempt to be fair to the early pathfinders in this field nor to the historical development of the science. The solid that takes up the gas is called the adsorbent, and the gas or vapor taken up on the surface is called the adsorbate. It is not always easy to tell whether the gas is inside the solid or merely at the surface because most practical' absorbents are very porous bodies with large internal' surfaces. It is not possible to determine the surface areas of such materials by optical or electron microscopy because of the size and complexity of the pores and channels of the material. The gas adsorption itself, however, can be used to determine the accesible surface area of most absorbents. Gas adsorption is of practical consequence to engineers and chemists in many ways. It can provide a convenient, cheap and reusable method for fluid purification and purification. Gas masks used in WWI (and even in present day) are an example of the utility of charcoal as an absorbent. More significantly, perhaps, the phenomenon of surface adsorption has been used to modify the rates of product yields of chemical reactions through heterogeneous catalysis. For a catalyst to be useful, it must have a large surface area, bind the reactants quickly and effectively, stabilize the activated complex, and release the products of the reaction. Thus the attraction of various molecules on the surface, as well as the total surface area of the catalyst, are extremely important properties of potential catalytic materials. Gas adsorption has been studied theoretically for most of this century and the simplist of the resulting theories provide the insight needed for most applications. We will investigate two such treatments, one attributed to Langmuir and one to Brunauer, Emmett and Teller (BET) and apply their equations to our experimental data. We will investigate the adsorption of N2 at cryogenic temperatures on common high area supports such as alumina. We will use this information to test simple adsorption theory, determine the specific area of the absorbent, and estimate the heat of adsorption of N2. Molecules and atoms can attach themselves onto surfaces in two ways. In physisorption (physical adsorption), there is a weak van der Waals attraction of the adsorbate to the surface. The attraction to the surface is weak but long ranged and the energy released upon accommodation to the surface is of the same order of magnitude as an enthalpy of condensation (on the order of 20 kJ/mol). During the process of physisorption, the chemical identity of the adsorbate remains intact, i.e. no breakage of the covalent structure of the adsorbate takes place. Physisorption, to be a spontaneous thermodynamic process, must have a negative . Because translational degrees of freedom of the gas phase adsorbate are lost upon deposition onto the substrate is negative for the process, physisorption must be exothermic. In chemisorption (chemical adsorption), the adsorbate sticks to the solid by the formation of a chemical bond with the surface. This interaction is much stronger than physisorption, and, in general, chemisorption has more stringent requirements for the compatibility of adsorbate and surface site than physisorption. The chemisorption may be stronger than the bonds internal to the free adsorbate, which can result in the dissociation of the adsorbate upon adsorption (dissociative adsorption). In some cases for dissociative adsorption can be greater than zero, which means endothermic chemisorption, although uncommon, is possible. The energetics of adsorption depend on the extent to which the available surface is covered with adsorbate molecules. This is because the adsorbates can interact with each other when they lie upon the surface (in general they would be expected to repel each other). The fractional coverage of a surface is defined by the quantity theta: At any temperature, the adsorbate and the surface come to a dynamic equilibrium, that is, the chemical potentials of the free adsorbate and the surface bound adsorbate are equal. The chemical potential of the free adsorbate depends on the pressure of the gas, and the chemical potential of the bound adsorbate depends on the coverage theta. Thus the coverage at a given temperature is a function of the applied adsorbate pressure. The variation of with p at a given T is called an adsorption isotherm. Several adsorption isotherms have proven useful in understanding the process of adsorption. The simplest isotherm is attributed to a pioneer in the study of surface processes, Langmuir, and is called the Langmuir isotherm. If one assumes: Adsorption cannot proceed beyond the point at which the adsorbates are one layer thick' on the surface (monolayer) All adsorption sites are equivalent. The adsorption and desorption rate is independent of the population of neighboring sites. The Langmuir isotherm gives us a wonderfully simple picture of adsorption at low coverage and is applicable in some situations. At high adsorbate pressures and thus high coverage, this simple isotherm fails to predict experimental results and thus cannot provide a correct explanation of adsorption in these conditions. What is missing in the Langmuir treatment is the possibility of the initial overlayer of adsorbate acting as a substrate surface itself, allowing for more adsorption beyond a saturated (monolayer) coverage. This possibility has been treated by Brunauer, Emmett, and Teller [J. Amer. Chem. Soc., 60, 309 (1938)] and the result is named the BET isotherm. This isotherm is useful in cases where multilayer adsorption must be considered. Adsorption is defined as the concentration of gas molecules near the surface of a solid material. The adsorbed gas is called adsorbate and the solid where adsorption takes place is known as the adsorbent. Adsorption is a physical phenomenon (usually called physisorption) that occurs at any environmental condition (pressure and temperature) but only at very low temperature it becomes measurable. Thus physisorption experiments are performed at very low temperature, usually at liquid nitrogen or liquid argon boiling temperature at atmospheric pressure. Adsorption takes place because of the presence of an intrinsic surface energy. When a porous material is exposed to a gas, an attractive force acts between the exposed surface of the solid and the gas molecules. The result of these forces is characterized as physical (or Van der Waals) adsorption, in contrast to the stronger chemical attractions associated with chemisorption. The surface area of a solid includes both the external surface and the internal surface of the pores. Due to the weak bonds involved between gas molecules and the surface (less than 10 Kcal/mole), adsorption is a reversible phenomenon. Gas physisorption is considered non-selective, thus filling the surface step by step (or layer by layer) depending on the available solid surface and the relative pressure. Filling the first layer enables the measurement of the surface area of the material because the amount of gas adsorbed when the mono-layer is saturated is proportional to the entire surface, that includes the internal and external surface. The complete adsorption/desorption analysis is called adsorption isotherm. Six adsorption isotherms are well known and defined representing different gas/solid interactions on the majority of solid materials. The gas adsorption technique may be used to measure the specific surface area and pore size distribution of powdered or solid materials. The dry sample is usually evacuated of all gas and cooled to a temperature of 77K, the temperature of liquid nitrogen. At this temperature inert gases such as nitrogen, argon and krypton will physically adsorb on the surface of the sample. This adsorption process can be considered to be a reversible condensation or layering of molecules on the sample surface during which heat is evolved. Nitrogen gas is ideal for measuring surface area and pore size distribution. Adsorption Isotherm An adsorption isotherm (one temperature) is usually recorded as volume of gas adsorbed (cc/g @ STP) versus relative pressure (i.e., sample pressure / saturation vapor pressure). Using relative pressure to construct the isotherm eliminates changes in pressure from small changes in temperature. A small change in temperature changes the saturation vapor pressure considerably. For example, 0.1K increase in temperature changes the saturation pressure of nitrogen from approx. 760 mm Hg to 800 mm Hg. The use of relative pressure is convenient and is scaled from 0 to 1. A relative pressure of 1 represents a completely saturated sample, i.e., all of the available surface structure is filled with liquid-like gas. Different Methods of Measurement There are three instrument methods in common use today for measuring adsorption isotherm data. These volumetric methods use the "GAS LAWS" to calculate the volume of gas adsorbed at measured relative pressures and are known as 1) Static, (classic) fully equilibrated, 2) Continuous Flow, or quasi-equilibrated, and Dynamic or Chromatographic. Each have their own advantages and disadvantages. Beckman Coulter manufactures instruments which use the static and continuous flow methods. BET Surface Area Determination One or more data points of the adsorption isotherm must be measured and the BET (after Brunauer, Emmett and Teller) equation is used to give specific surface area from this data. The BET equation is used to give the volume of gas needed to form a monolayer on the surface of the sample. The actual surface area can be calculated from knowledge of the size and number of the adsorbed gas molecules. Nitrogen is used most often to measure BET surface, but if the surface area is very low, argon or krypton may be used as both give a more sensitive measurement, because of their lower saturation vapor pressures at liquid nitrogen temperature. We will perform the adsorption measurements in a commercial vacuum manifold called the Omnisorb 360, manufactured by Omnicron. A schematic diagram of the relevant portion of the vacuum system is shown in Figure 4. Although this system is designed for semi-automated use, we will use the equipment manually. The numbers on the above schematic represent some of the numbered valves on the system and the diagram above is similar to the layout of the valve controls on the machine itself. Each valve is pneumatically controlled by a numbered push button switch. Simply push the button to open any valve; an open valve is indicated by the switch lamp on. To close, hit the button again. Several portions of the system require some discussion. The adsorbtive gases enter the system through valves 10(N2), 13(CO2, not used in this experiment), and 12(He). The inlet is the portion of the system between these valves, valve 11, the flow controller, and valve 8. The manifold is the portion of the system between valves 7,8, and the sample port valves 3,4,5 and 6. The manifold is evacuated by a vacuum pump when valves 9 and 7 are open. The vacuum pump pressure is monitored by a Pirani gauge on the upper left of the console. The manifold pressure is monitored by two (0-1000 torr, 0-10 torr) capacitance manometers (Baratrons). (Please note where the pressure measurement is made on the manifold in Figure 3. Since there is a slight change in the manifold volume depending on the state of valves 3 - 8, all pressure readings should be made with these valves closed whenever possible). Sample preparation Weigh (tare) a sample bulb with valve. Introduce less than one gram of sample into the bulb. Degas the sample (Degassing is accomplished in the furnace at the left bottom of the instrument). Determine as accurately as possible the degassed weight of the sample plus bulb and thus the sample mass. Attach the bulb to sample port #3 or #6 and fill the Dewar with LN2 to a level about 2" above the bulb. Record the atmospheric pressure (barometer) and manifold temperature (displayed on the upper center of the control panel in oC). System Setup You will find the system with the mechanical and diffusion pumps already on. Evacuate the inlet and manifold by opening valves 9,7,8. Close the Flow controller by setting the flow rate to 0.00. Evacuate the sample and known volume by opening the appropriate sample port valves. Note the base pressure indications on all three pressure gauges (One Pirani, and the two Baratrons) and record these in your notebook. As the system will base at a lower pressure than the manometers can indicate, the Baratron readings you take now will be used as a zero offset and will be used to correct all subsequent readings. Close the sample port valves and flush the inlet and manifold with the gas to be used (initially He). Gas is introduced into the manifold by the following procedure: Close valve 8 and momentarily open (open, then close) the inlet valve for the desired gas. The inlet is now charged. Now open and close valve 8. A slug of gas has been introduced into the manifold and you should see the pressure rise (to about 300 torr at this point). Now pump out the manifold and inlet by opening the appropriate valves. Repeat this procedure a couple of times each time you change gases. Volume Determinations Only one volume in the system is known at the onset, Vk, the volume of the bulb on port #5. We will use it to determine the manifold volume, Vm, as well as the free volume in our sample bulb, Vs. Manifold Volume Fill the manifold and known volume bulb to about 700 torr with He and record the pressure. This may take two 'charges' from the inlet. Evacuate the inlet and manifold but leave the known volume pressurized, and record the value. Now fill the manifold with gas from the known volume bulb. Record the pressure again. The reduction in pressure will allow you to calculate the ratio of the manifold volume to the known volume through the ideal gas relation. Close valve #5, evacuate the manifold, and refill from the known volume again. This will give you another determination of the manifold volume. Repeat this procedure several times to obtain an average value of Vm. Sample Void Volume Now that the manifold volume is known, determine the sample void volume by the same procedure as above but fill the manifold from the sample bulb instead of the bulb on port #5. Helium is used for volume determinations because it does not adsorb to anything at 77K. Adsorption Studies Evacuate the sample and close the valve connecting it to the manifold. Close valve #5 and isolate the known volume from the manifold for the remainder of the experiment. Flush the inlet and manifold with N2. Fill the manifold with N2 to about 40 torr and record the pressure. Since one 'charge' from the inlet will produce more pressure than this, you will have to reduce the pressure by closing #8, evacuating the manifold, momentarily opening #8, and repeating until the manifold pressure is about the desired value. With all valves closed open the sample port valve and expose the sample to the sealed off manifold. The pressure will drop when the sample bulb is opened to the manifold filled with nitrogen but it will drop significantly more more than would occur if the gas in the manifold were He. This is because some of the nitrogen gas is adsorbing to the sample surface. Allow the sample and adsorbate to equilibrate at 77K. NOTE: The approach to equilibrium is perceptably slow, especially at high coverage. It would in principle take an infinite time for equilibrium to be exactly established. The nature of the isotherm and the required precision of the measurement suggests that equilibrium pressures need be known only to a few percent. Take a few point allowing a long time for equilibrium to be acheived and plot the approach to equilibrium as a function of time. Is this a first order process in adsorbate? Estimate the time it takes to get to within 5% of the equilibrium pressure and us this as a cutoff time for data taken at similar coverage. Correct for the incomplete equilibration in working up the data. In summary, use your judgement as to how long to wait at each pressure. Analyse and exloit kinetic information about the apporach to equilibrium. Record the equilibrium pressure. This procedure generates the data you need to allow you to determine the number of moles adsorbed on the solid as a function of (equilibrium) pressure. Close the valve to the sample tube and refill the manifold. 'Dose' the sample again and record the pressures. You now have the next point on the isotherm. Repeat the above with ca. 50 torr in the manifold until the equilibrium pressure reaches ca. 20 torr. Then increase the initial manifold pressure to 300 torr and repeat until the equilibrium pressure reaches about 200 torr. Then increase the initial manifold pressure to ca. 700 torr until the equilibrium pressure reaches ca. 500 torr. This completes the data acquisition needed to generate the adsorption isotherm. Desorption Studies The desorption measurement is performed to see if there is any hysterisis or non-equilibrium effects in the adsorption/desorption cycle.
This is basically performed in reverse of the procedure above. You should already have the sample and manifold in equilibrium at some pressure ca. 450 torr from the adsorption run. Close the sample off from the manifold and evacuate the manifold. Open the sample to the manifold and allow equilibrium to be achieved. Record this pressure. You now know how many moles of gas have been removed from the sample and the new equilibrium pressure --- so this is the first point on the desorption isotherm. Repeat this procedure until the equilibrium pressure reaches ca. 5 torr. If time permits, perform adsorption and desorption studies for all three solids: alumina, silica, and charcoal. Data Analysis · Calculate the manifold and sample void volumes. Use this and the manifold temperature to determine the number of moles of N2 adsorbed as a function of equilibrium pressure for the adsorption run and plot this isotherm. · Apply the BET theory to the isotherm by replotting the data as (z/n(1-z)) vs z. This should give a straight line in the range of x=0.05-0.2. · Determine nmono, and c. Repeat for the desorption data. · Determine the specific area and estimate the heat of adsorption for each of the samples. The tendency of all solid surfaces to attract surrounding gas molecules gives rise to a process called gas sorption. Monitoring the gas sorption process provides a wealth of useful information about the characteristics of solids. Before performing gas sorption experiments, solid surfaces must be freed from contaminants such as water and oils. Surface cleaning (degassing) is most often carried out by placing a sample of the solid in a glass cell and heating it under a vacuum. Once clean, the sample is brought to a constant temperature by means of an external bath. Then, small amounts of a gas (the absorbate) are admitted in steps into the evacuated sample chamber. Absorbate molecules quickly find their way to the surface of every pore in the solid (the adsorbent). These molecules can either bounce off or stick to the surface. Gas molecules that stick to the surface are said to be adsorbed. The strength with which adsorbed molecules interact with the surface determines if the adsorption process is to be considered physical (weak) or chemical (strong) in nature. The theory of all mercury Porosimeters is based on the physical principle that a non-reactive, non-wetting liquid will not penetrate pores until sufficient pressure is applied to force its entrance. The relationship between the applied pressure and the pore size into which mercury will intrude is given by the Washburn equation, where P is the applied pressure, D is the diameter, is the surface tension of mercury (480 dyne cm-1 and is the contact angle between mercury and the pore wall, usually near 140°. As pressure increases, the instrument senses the intrusion volume of mercury by the change in capacitance between the mercury column and a metal sheath surrounding the stem of the sample cell. As the mercury column shortens, the pressure and volume data are continuously acquired and displayed by an attached personal computer. Many gases hi react with surfaces by chemically bonding. In contrast to physisorption, chemical adsorption (chemisorption) involves the formation of strong chemical bonds between adsorbate molecules and specific surface locations known as chemically active sites. Chemisorption is thus used primarily to count the number of surface active sites which are likely to promote chemical and catalytic reactions. One of the most common density measurements involves the determination of the geometric space occupied within the envelope of a solid material... including any interior voids, cracks or pores. This is called geometric, envelope or bulk density and only equals true density when there are no internal openings in the material being measured. Tap Density Each particle of a solid material has the same true density after grinding, milling or processing, but more geometric space is occupied by the material. In other words, the geometric density is less... approaching 50% less than true density if the particles are spherical. Handling or vibration of powdered material causes the smaller particles to work their way into the spaces between the larger particles. The geometric space occupied by the powder decreases and its density increases. Ultimately no further natural particle packing can be measured without the addition of pressure. Maximum particle packing is achieved. Under controlled conditions of tap rate, tap force (fall) and cylinder diameter, the condition of maximum packing efficiency is highly reproducible. This tap density measurement is formalized in the British Pharmacopoeia method for Apparent Volume, ISO 787/11 and ASTM standard test methods B527, D1464 and D4781 for tap density. Automated tap density determinations are performed either by the Quantachrome Autotap or the two sample Dual Autotap. True Density The true density of powders often differs from that of the bulk material because the process of comminution, or grinding will change the crystal structure near the surface of each particle and therefore the density of each particle in a powder. In addition, voids at the surface of a particle, into which liquids will not penetrate, can generate apparent volume which will cause serious errors when density is measured by liquid displacement. The pycnometers from Quantachrome are specifically designed to measure the true volume of solid materials by employing Archimedes' principle of fluid (gas) displacement and the technique of gas expansion. True densities are measured using helium gas since it will penetrate every surface flaw down to about one Angstrom, thereby enabling the measurement of powder volumes with great accuracy. The measurement of density by helium displacement often can reveal the presence of impurities and occluded pores which cannot be determined by any other method. Surface area helps determine such things as how solids dissolve, burn, and react with other materials. To determine the surface area, solid samples are pretreated by applying some combination of heat, vacuum and/or flowing gas to remove adsorbed contaminants acquired from atmospheric exposure. The solid is then cooled, under vacuum, usually to cryogenic temperature. An adsorptive (typically nitrogen) is admitted to the solid in controlled increments. After each dose of adsorptive, the pressure is allowed to equilibrate and the quantity of gas adsorbed is calculated. The gas volume adsorbed at each pressure (at one constant temperature) defines an adsorption isotherm, from which the quantity of gas required to form a monolayer over the external surface of the solid and its pores is determined. With the area covered by each adsorbed gas molecule known, the surface area can also be calculated. Mercury porosimetry can determine a broader pore size distribution more quickly and accurately than other methods. Besides offering speed, accuracy, and a wide measurement range, mercury porosimetry permits you to calculate numerous sample properties such as pore size distributions, total pore volume, total pore surface area, median pore diameter, and sample densities (bulk and skeletal). Mercury Porosimetry can also analyze a wide variety of materials including reservoir rocks, refractory materials, resins, pigments, carbons, catalysts, textiles, leather, adsorbents, pharmaceuticals, membranes, filters, ceramics, papers, fuel cell components, and other porous materials. A chemisorption analysis evaluates the catalytic activity by measuring the amounts and types of reactive gas chemisorbed. This volume of gas, along with reaction stoichiometry, is used to calculate metal dispersion, active surface area, size of crystallites, and surface acidity. Our instrumentation is specifically designed for chemisorption. The internal stainless steel components will not react with most commonly used chemisorptives. Heated manifolds, gas lines, and detectors allow analysis with condensable vapors. Sample preparation occurs in situ to prevent contamination prior to analysis. Samples can be prepared at high temperatures (up to 1100 deg C) and at low pressures (10-5 mmHg). The TriStar 3000 Analyzer uses physical adsorption and capillary condensation principles to obtain information about the surface area and porosity of a solid material. The analytical technique is simple: a sample contained in an evacuated sample tube is cooled (typically) to cryogenic temperature, then is exposed to analysis gas at a series of precisely controlled pressures. With each incremental pressure increase, the number of gas molecules adsorbed on the surface increases. The equilibrated pressure (P) is compared to the saturation pressure (Po) and their relative pressure ratio (P/Po) is recorded along with the quantity of gas adsorbed by the sample at each equilibrated pressure. As adsorption proceeds, the thickness of the adsorbed film increases. Any micropores in the surface are filled first, then the free surface becomes completely covered, and finally the larger pores are filled by capillary condensation. The process may continue to the point of bulk condensation of the analysis gas. Then, the desorption process may begin in which pressure systematically is reduced resulting in liberation of the adsorbed molecules. As with the adsorption process, the changing quantity of gas on the solid surface at each decreasing equilibrium pressure is quantified. These two sets of data describe the adsorption and desorption isotherms. Analysis of the shape of the isotherms yields information about the surface and internal pore characteristics of the material. Instruments:
The following instruments are at our disposal: Surface Area and Porosity of Solids Quantachrome Autosorb-6B analyzers (2) / N2 adsorption Quantachrome Autosorb-1C analyzers (2) / CO or H2 chemisorption Quantachrome PentaPycnometer / He density Quantachrome NOVA 1200 analyzers (2) / CO2 adsorption Quantachrome Autosorb degassers (3) / vacuum pre-treatment Quantachrome NOVA flow degasser / Flow pre-treatment Micromeritics ASAP 2010 analyzers (2) / Low-pressure Ar adsorption or Kr Qsurf M3 / N2 adsorption flow CE Instruments Pascal 140 porosimeter / Low-pressure mercury intrusion CE Instruments Pascal 440 porosimeter / High-pressure mercury intrusion Particle Size of Solids Coulter LS230 / Forward light scattering + PIDS Malvern Mastersizer S / Forward light scattering Cilas 1064 / Forward light scattering Malvern 2600 / Forward light scattering Coulter N4 / Dynamic light scattering (PCS) ALV 5000/ Static and dynamic light scattering (PCS) Coulter Counter / Electrical sensing zone Micromeritics Sedigraph / Sedimentation PCO ccd imaging / Image Analysis Philips XL 20 / Scanning electron microscopy Particle Size of Sprays and Aerosols TSI PDPA / Phase Doppler velocimetry …..and more coming soon Chemical composition of gases VG Prima 600 / Mass Spectrometry Siemens SESAM 1 / FTIR Siemens MIPAN / Microwave spectrometry Chemical composition of solids Philips PW 1400 spectrometer / X-Ray Fluorescence HOR / Instrumental Neutron Activation Analysis PerkinElmer DSC7 / Differential Scanning Calorimetry PerkinElmer TGA7 / Thermogravimetry Porosity N2 Physical Gas Adsorption at 77 K (Flow Technique) Single point BET (0.30 p/p0) Multi-point BET (0.05-0.10-0.15-0.20-0.25 p/p0) Total pore volume (0.99 p/p0) 100 150 125 N2 Physical Gas Adsorption at 77 K (Volumetric Technique) Single point BET (0.30 p/p0) Multi-point BET (0.05-0.10-0.15-0.20-0.25 p/p0) Multi-point BET + pore volume (0.99 p/p0) 150 200 250 Adsorption isotherm, multi-point BET, total pore volume, BJH adsorption pore size distribution (2-200 nm) 375 Adsorption and desorption isotherm, multi-point BET, total pore volume, BJH desorption pore size distribution (2-200 nm) 400 Ar Low Pressure Physical Gas Adsorption at 87 K (Volumetric Technique) Adsorption isotherm (10-6 0.3 p/p0), SF micropore size distribution 0.5 2 nm 550 CO2 Physical Gas Adsorption (Volumetric Technique) Adsorption and desorption isotherm, micropore surface area and micropore volume (Dubinin-Radushkevic) 400 Kr Physical Gas Adsorption (Volumetric Technique) Multi-point BET (0.05-0.10-0.15-0.20-0.25 p/p0) 300 H2 or CO Chemical Gas Adsorption (Volumetric Technique) Dual adsorption isotherm (1-100 kPa), metal surface area, metal dispersion and average crystallite size 425 Mercury Intrusion Porosimetry Intrusion curve 0.1 400 MPa, porosity, apparent density, pore size distribution 4 nm 17 µm 275 Intrusion curve vacuum 0.1 MPa, porosity, apparent density, pore size distribution 17µm 120 µm 200 Intrusion curve vacuum 400 MPa, porosity, apparent density, pore size distribution 4 nm 120 µm 375 He Pycnometry Skeletal density 125 Sizing Laserdiffraction (dry, aqueous or organic fluid dispersion) Forward light scattering, size distribution 0.5 µm 2000 µm 275 Forward light scattering incl. backscattering information, size distribution 0.04 µm 2000 µm 300 Photon Correlation Spectroscopy (PCS) Particle size distribution 3 nm 3 µm 225 Phase Doppler Velocimetry Spray droplet velocity and size distribution 0.5 µm 90 µm p.o.a. Electrical Sensing Zone Coulter Counter narrow size distribution 0.6 µm 1200 µm 325 Scanning Electron Microscopy Surface and size scan incl. photographs 200 Image Analysis Distribution and size scan incl. photographs 200 Gravitational Sedimentation Sedigraph size distribution 1 µm 300 µm 200 Sieve Analysis Dry sieving, range 37 µm 1700 µm 150 Composition Microwave Spectrometry Gas sample analysis of 0.2 up to 2000 ppmv NH3 150 FTIR Spectroscopy Gas sample analysis of NO, NO2, H2O, NH3, SO2, SO3, p.o.a. Mass Spectrometry Mass spectrum from m/e 4 300 for identification Gas sample analysis on C3H6 150 150 Instrumental Neutron Activation Analysis (INAA) Package A: F, Se 140 Package B: Al, Ba, Ca, Cl, Cu, Dy, Er, Ga, K, I, In, Mg, Mn, Na, Rh, Si, Ti, V 140 Package C: As, Au, Ba, Br, Ca, Cd, Ce, Cr, Fe, Ga, K, La, Mo, Na, Pd, Pt, Re, Sb, Sc, Sm, U, W, Yb, Zn 140 Package D: Ag, Ba, Ca, Ce, Cr, Co, Cs, Eu, Fe, Hf, Hg, Ir, Lu, Nd, Ni, Os, Rb, Sb, Sc, Se, Sn, Sr, Ta, Tb, Te, Th, Yb, Zn, Zr 140 Package E: As, Ba, Br, Cd, Co, Cr, Hg, Mo, Ni, Sb, Se, Sn, Zn 140 X-Ray Fluorescence (XRF) 72 components in solids 130 Thermogravimetric analysis (TGA) TGA analysis 300 Differential Scanning Calorimetry (DSC) DSC analysis 300 p.o.a.="price" on application Priority Service: 40% surcharge Consultancy:€ 90,00 per hour Publications J.C. Groen and L.A.A. Peffer, "Influence of dead space measurement on adsorption characteristics of microporous zeolites", The MicroReport, 3rd quarter 1997, Vol. 8, No. 3, p.8. J.C. Groen, M.C. Doorn, L.A.A. Peffer, in D.D. Do (Eds.), "MCM-41 and the BdB corrected Kelvin equation for accurate mesopore size distributions from gas adsorption data", Adsorpti. Sci. Technol., Proc. 2nd Pacific Basin Conference, Brisbane, Australia (2000) p. 229. J.C. Groen, J. Pérez-Ramírez, L.A.A. Peffer, "Different chemisorption methods applied to zeolite supported Pt-catalysts", in: A. Galarneau, F. Di Renzo, F. Fajula, J. Vedrine (Eds.), Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Studies in Surface Science and Catalysis, Vol. 135, Elsevier, Amsterdam, 2001, 2862. J.C. Groen, J. Pérez-Ramírez, and L.A.A. Peffer, "Formation of uniform mesopores in zeolite ZSM-5 upon alkaline post-treatment?" Chem. Lett. 2002 94. J.C. Groen, J. Pérez-Ramírez, and L.A.A. Peffer, Comments on "Vanadium- and chromium-containing mesoporous MCM-41 molecular sieves with hierarchical structure", Micropor. Mesopor. Mater. 51 (2002) 75
Sizing Particle and droplet size are basic features that influence the properties and performance of solids, emulsions and sprays. Conductivity, flexibility, gloss, hardness, stability, strength and taste are some of the properties that can be influenced by particle size. Size and size distributions are important in various areas such as: atmospheric aerosol dispersion, ceramic and alloy properties, catalyst performance, crystal growth, contamination of soil and liquids, wastewater management, medicine and spray effectiveness, paint performance, emulsions stability and so on. Several light scattering and electrical techniques are available in our laboratory, as well as more conventional methods as sedimentation and microscopy. Disclaimer: Instrumental particle and droplet sizing techniques do not measure the geometrical size but give an equivalent spherical diameter depending on the technique
Composition Knowledge of the chemical composition of raw materials, semi products, process streams and final products is essential for behavior, quality or for example health risk of each and every product or medium. Therefore man puts huge effort in trying to determine even the most untraceable elements. In particular for analysis of liquids and gases a wide variety of analyzers is available. Measuring the composition of solids without destruction appeared to be difficult. Apart from the traditional semi quantitative method of X-ray fluorescence, currently the more accurate and more 'multi-component' Instrumental Neutron Activation Analysis is available
Analysis Sample analysis can be requested on this site. Please fill in the appropriate analysis request form below and submit it to us. A copy of the form should be signed and enclosed with the samples for dispatch. Make sure the samples are well packed to prevent damage during transport. The postal address is given below: Delft Solids Solutions B.V. Kluyverweg 2A Innovation Centre 2629 HT Delft The Netherlands Delft Solids Solutions B.V. (not for delivery services) PO Box 1038 2600 BA Delft The Netherlands A customs declaration is required for samples from outside the European Union. For countries inside the European Union, a declaration is recommended. Templates of the customs declaration are available on this site. Should there be any questions concerning the sample dispatch or analysis, please do not hesitate to contact us.
How to contact us Delft Solids Solutions cooperates with partners such as the Delft University of Technology and employs academically educated and scientific experienced specialists who earned their well respected place in the Dutch scientific world (see our publications - item references). visiting address postal address Delft Solids Solutions B.V. Kluyverweg 2A Innovation Centre 2629 HT Delft The Netherlands (not for delivery services) Delft Solids Solutions B.V. PO Box 1038 2600 BA Delft The Netherlands Phone: +31 (0) 15 26 825 16 Fax: +31 (0) 15 26 825 30 If you have any questions about our company, analyses we provide or any other observations, feel free to contact us. E-mail is merely meant for business to business information exchange. For information about instruments and techniques, please consult the manufacturers and relevant books. Appropriate contact addresses and literature are available on our website.
light scanning techniques
Photon Correlation Spectroscopy In photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS) the Brownian motion (movement in random direction) of sub-micron particles is measured as a function of time. A laser beam is diffracted by particles in suspension. The diffusion of particles causes rapid fluctuations in scattering intensity around a mean value at a certain angle (varying from 10 to 90°). These intensity fluctuations depend on particle size. The calculated correlation function results in a diffusion coefficient for a given temperature and viscosity which can be converted to particle size. The technique is used for determination of average particle size in a range between 3 and 3000 nm. The measurements are performed on a Coulter N4 or an ALV 5000. The report consists of a table with average and mode effective hydrodynamic diameter and polydispersity index, which is a measure for the width of the distribution.
Electrical sensing zone In electrical sensing zone an electrolyte solution is used to disperse particles. A tube with a narrow aperture is submerged in the solution and two electrodes are placed on both sides of the aperture. Electrolyte and particles travel through the aperture and the resistance proportional to the particle volume is measured. Each individual particle is counted and categorized in the appropriate size class. The technique is used for characterizing and counting narrow distributions within the range of 0.6 to 1200 µm. Particles in low concentration solutions, powders and biological material can be analyzed. The measurements are performed on a Coulter Multisizer II. The report consists of a graph showing the cumulative and differential volume or number distributions and statistics as the mode, mean and median diameter, skew ness and kurtosis, standard deviation etc.
Scanning electron microscopy In scanning electron microscopy (SEM) a source of electrons is focused into a fine probe that is rastered over the surface of the specimen. The sample is coated with a thin gold layer and bombarded with electrons to visualize the surface, which is constantly scanned and reconstructed. A detector collects a part of the emitted electrons and an image is built by signal modulation and amplification which looks just like the object. Magnifications up to 20.000 times can be used. The technique is often used when visualization of a sample is required in order to detect size and shape effects or to create a better understanding of the material behavior. The measurements are performed on a Philips SEM XL20; the report consists of pictures and a brief evaluation of the analysis.
Gravitational sedimentation In gravitational sedimentation (originally the pipette method) the settling rate of particles in liquid is measured and related to the mass by use of the Stokes law. Nowadays the settling rate is determined by measuring x-ray transmission in the liquid at specific heights and time intervals and a size distribution based on difference in mass is calculated. The technique is suitable for any material containing elements with Z > 12 and is popular for determination of clay fractions in soil samples. The sizing range, determined by laminar flow, is from 1 to 300 µm. The measurements are performed on a Micromeritics Sedigraph 5100 and the report consists of a graph showing the cumulative and differential mass distribution and statistics as the mode, mean and median diameter, standard deviation etc.
Phase Doppler Velocimetry In laser Doppler velocimetry particles are radiated by two laser beams and the phase shift of the scattered light signals is measured. Two laser beams are split into four beams of equal intensity. The beams are focused and made to intersect. The scattered light from particles passing through the beams at their intersection is mixed at the photo detector surface and gives a ‘difference’ signal. Since the rays enter the particle at different angles, the optical paths to a common arbitrary point on the detector differ; the light waves are shifted relatively to each other. Two detectors allow determination of particle size. The technique is used for determination of droplet size and velocity distribution in sprays and nozzles from 0.5 to 90 µm. The measurements are performed on a TSI Phase Doppler Particle Analyzer and the report consists of size and velocity distribution results.
Laser diffraction In laser diffraction (static light scattering) the scattering pattern, obtained from illumination of dispersed particles with a laser beam, contains information about particle size. The interaction between particles and light is mainly dependent on particle size, shape, surface roughness and refractive indices of material and dispersing medium. For a specific material, the scattering pattern of a particle is unique for its size. Deconvolution of the sample scattering pattern with an optical model such as Mie or Fraunhofer results in the particle size distribution. The technique is especially applicable to samples with a broad or bimodal distribution and for information on size trends in series of samples. Materials can be characterized in the range of 0.04 to 2000 µm and dispersion can be made in water, organic liquid as well as air. The measurements are performed on a Malvern Mastersizer, Coulter LS 230 or a Cilas 1064. The report consists of a graph showing the cumulative and differential volume distributions and statistics as the mode, mean and median diameter, skewness and kurtosis, standard deviation etc.
Sieve analysis In sieve analysis a powder is separated into specified size fractions. Both mechanical sieving and sonic sieving are available. The separation range is from 37 µm to 10 mm. The measurements are performed on ATM, Retsch or Stork Veco sieves. The results are given in sieve fractions and cumulative mass distribution.