Physical adsorption application articles - Database & Sql Blog Articles

Physical Adsorption Application 1. What is the working principle of a physical adsorption analyzer (specific surface and porosity analyzer)? Since there is no direct measurement of the surface of the tool, people use gas molecules with known molecular cross-sections as probes to create conditions for the gas molecules to cover the entire surface of the sample being tested (adsorption). The number of adsorbed molecules multiplied by the molecular cross-sectional area is considered to be the specific surface area of the sample. Measuring specific surface area includes all gases that can reach the surface, whether external or internal. Physical adsorption is generally a weak reversible adsorption, so the solid must be cooled to the boiling temperature of the gas, and a theoretical method is chosen to calculate the surface area from the single molecule coverage. The specific surface and porosity analysis instrument creates the corresponding conditions for complex calculations. 2. Is the specific surface area value measured? The specific surface area value is not directly measured but calculated. We measure the adsorption isotherm of the sample and then select the appropriate theoretical model to calculate the specific surface area based on the characteristics of the sample. Therefore, the specific surface measurement process is actually an analysis process. Different people may have different knowledge about the sample, leading to variations in the results of the same set of adsorption isotherms. When "measuring" the surface, remember that it's actually an "analysis" process. 3. Is BET a better surface? How many methods are there to calculate the specific surface area? The BET method is one theory among several used in surface analysis. Langmuir introduced the concept of monolayer adsorption, suitable for samples with only micropores. The BET theory, published in 1938, is a multi-layer adsorption theory and is currently the most popular due to its wide applicability. However, it's not suitable for all samples. For microporous materials, the BET method might give incorrect results. ISO 9277:2010 and IUPAC specify the BET method for microporous materials. Different theoretical models yield different results, so the most suitable one should be selected based on the sample's nature. Common models include Langmuir, BET, BJH, DR, and NLDFT. NLDFT is particularly accurate for microporous and mesoporous materials. 4. What is the principle of measuring specific surface by physical adsorption? Nitrogen is commonly used as the adsorbent gas due to its availability, liquid nitrogen cooling, and well-known cross-sectional area (0.162 nm²) for BET calculations. In conventional volumetric techniques, relative pressures less than an integer are achieved through partial vacuum conditions. High-precision pressure sensors monitor pressure changes caused by adsorption. A series of adsorption measurements at different relative pressures are taken, typically collecting at least three data points between 0.025 and 0.30 relative pressure. The recorded data pairs, volume of gas at standard temperature and pressure (VSTP), correspond to the relative pressure (P/Po). These data form adsorption isotherms. 5. What important terms should you know in physisorption analysis? Key terms include Avogadro constant (6.022×10²³), BET (Brunauer, Emmet, Teller), cross-sectional area, molar volume (22.414 liters at STP), molar (Avogadro’s number), monolayer, relative pressure (P/Po), saturated vapor pressure (Po), and standard temperature and pressure volume. 6. Why is nitrogen commonly used for surface and pore size analysis? Can you use other gases? Nitrogen is preferred because it is inert, easily cooled to its boiling point, and fits the ideal gas equation. Other gases like argon and CO₂ are also used, depending on the sample and desired resolution. Nitrogen is cost-effective and widely available, but IUPAC recommends argon at 87 K for microporous samples. 7. Why do you use liquid nitrogen for surface and pore size analysis? Don't you have to? Liquid nitrogen is used to cool the sample to 77.35 K, which is necessary for physical adsorption. It's inexpensive and easy to obtain, but impure nitrogen can cause errors. Mechanical refrigeration systems can also achieve the required temperatures, offering flexibility. 8. How to judge the liquid nitrogen is not pure? Impure liquid nitrogen shows higher saturated vapor pressure, blue color (indicating oxygen contamination), or inconsistent pressure readings. The linearity of the analysis may also deviate significantly from expected values. 9. Why should the sample be degassed before performing the sorption analysis? Degassing removes contaminants like water and oil from the sample surface. This ensures accurate adsorption measurements. The image shows the cleaned surface of the sample after pretreatment. 10. How to choose the degassing temperature of the sample? Higher temperatures improve degassing efficiency, but must not damage the sample. Safe temperatures vary: 350°C for oxides, 300°C for carbon materials, and lower for hydrates. Organic compounds require careful consideration to avoid structural changes. 11. How to determine the degassing time of the sample? Longer degassing times improve pretreatment. The time depends on the sample's complexity and pore structure. IUPAC recommends at least 6 hours for general samples, with longer times for microporous materials. For example, magnesium stearate requires only 2 hours. 12. When degassing the sample, should you choose vacuum degassing or flow degassing? Flow degassing is faster for removing weakly bound water, while vacuum degassing is better for deep-seated moisture. Molecular pumps are essential for ultra-microporous samples to remove adsorbed water effectively. 13. What are the requirements for degassing a hydrophilic ultra-microporous sample? Use a molecular pump for oil-free degassing. Pressure-controlled heating is recommended for sensitive samples to avoid structural damage during degassing. 14. What gas should be backfilled after degassing and unloaded? Nitrogen is best to prevent buoyancy errors. Using helium can lead to significant weighing errors, especially for small samples. 15. What are the experimental techniques for physical adsorption measurements? Adsorption isotherms are measured using volumetric or gravimetric methods. Gravimetric involves a microbalance and pressure sensor, while volumetric uses calibrated volumes and pressure changes. Continuous flow methods are used for rapid analysis. 16. What is free space? What is the dead volume? How does it affect measurement sensitivity? Free space is where adsorbate molecules diffuse. Dead volume is the unoccupied space in the system. Smaller dead volumes increase measurement sensitivity and accuracy. 17. What are the methods for determining the dead volume of free space? ISO 15901 outlines two methods: helium calibration and blank experiments. Helium is non-adsorbed and provides high accuracy, while the calibration curve method is efficient for repeated measurements. 18. What is the relationship between micropore diameter and gas pressure? Micropores fill at low relative pressures. Gas molecules fill the smallest pores first, followed by larger ones. This relationship helps determine pore sizes based on adsorption behavior. 19. What are the components of the static capacity method physical adsorption analyzer? Key components include a vacuum pump, gas sources, manifold, coolant Dewar, sample tube, saturation pressure measuring tube, and pressure sensors. The manifold volume must be calibrated for accurate measurements. 20. What are the requirements for physical purity of a physical adsorption analyzer? Gases like helium and nitrogen must be at least 99.99% pure. IUPAC recommends 99.999% for adsorbate gases to ensure accurate measurements. 21. Why do you want to weigh the sample (weighing)? How large is the sample size? Specific surface area is per unit mass, so the sample must be weighed before and after analysis. Sample size varies depending on the material's surface area. Larger samples reduce weighing error, while smaller samples require precise control. 22. What are the specifications for the sample tube? What are the selection principles for sample tubes and filler rods? Sample tubes come in various diameters (6mm, 9mm, 12mm). Smaller tubes reduce dead volume but make loading more difficult. Filler rods help minimize free space, improving accuracy. 23. What is a manifold? How does it affect the accuracy of the instrument? A manifold connects various parts of the system. Smaller manifolds improve accuracy by reducing dead volume and increasing pressure sensitivity. 24. Why record the manifold temperature? What is the effect of manifold temperature control on measurement accuracy? Temperature affects gas behavior. Accurate temperature control is crucial for reliable adsorption measurements. Modern instruments use high-resolution sensors and stable heating systems to maintain precision. 25. Why do you want to get rid of helium before the analysis process begins? Helium can be adsorbed by microporous materials, affecting the adsorption isotherm. Removing helium ensures accurate measurements, especially for ultra-low pressure ranges. 26. What is cold free space? What is warm free space? What is the significance of the relative size of the free space? Cold free space is immersed in liquid nitrogen, while warm free space is at room temperature. Cold free space contributes more to dead volume, affecting measurement accuracy. 27. Why do you want to control the liquid level of liquid nitrogen or liquid argon? What are the methods for controlling the liquid level? Maintaining a constant liquid level ensures stable temperature and pressure. Methods include real-time feedback servo systems and jacket methods to keep the liquid level consistent. 28. What is saturated vapor pressure? Why measure saturated vapor pressure? Saturated vapor pressure is the equilibrium pressure of a substance at a given temperature. Measuring it is crucial for accurate pore size and surface area analysis. 29. How to measure saturated vapor pressure? Accurate measurement of saturated vapor pressure is done using a separate P0 tube. Real-time monitoring ensures reliability, especially for microporous materials. 30. What are the different gas injection modes in a physical adsorption analysis system? There are two main modes: fixed gas injection and fixed pressure. Fixed gas injection allows for dynamic studies but may introduce inaccuracies. Fixed pressure mode is more accurate but requires advanced software for precise control. 31. How are adsorption equilibrium conditions set? Adsorption equilibrium is reached when pressure changes remain within acceptable limits over time. Proper balance time and pressure tolerance are essential for accurate results, especially for microporous materials. Flexible settings are needed to adapt to new materials.

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