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's no direct measurement of the surface, people use gas molecules with known molecular cross-sections as probes. These gas molecules are made to cover the entire sample surface through adsorption. The number of adsorbed molecules multiplied by the molecular cross-sectional area gives the specific surface area. This includes all surfaces, internal or external. Physical adsorption is typically a weak and reversible process, so the solid must be cooled to the gas's boiling point. Theoretical models then calculate the surface area from single molecule coverage. This instrument creates the conditions needed for complex calculations. 2. Is the specific surface area value measured? No, it's calculated. We measure the adsorption isotherm and use theoretical models to compute the specific surface area based on the sample's characteristics. Different people might interpret the same data differently, leading to varying results. Therefore, when "measuring" surface area, it's actually an analytical 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. Langmuir introduced monolayer adsorption theory, suitable for microporous samples. The BET theory, developed in 1938, allows multi-layer adsorption and is widely used. However, it's not applicable to all samples. ISO9277-2010 and IUPAC specify BET for microporous materials. Different models like Langmuir, BET, BJH, DR, and NLDFT exist. NLDFT is considered the most accurate for microporous and mesoporous materials. 4. What is the principle of measuring specific surface by physical adsorption? Nitrogen is commonly used due to its availability, liquid nitrogen cooling, and well-known cross-sectional area (0.162 nm²). Relative pressures are measured using pressure sensors. Data points between 0.025 and 0.30 are collected, forming adsorption isotherms. 5. Which important terms should you know at least in physisorption analysis? Key terms include Avogadro constant, BET, cross-sectional area, molar volume, monolayer, relative pressure, saturated vapor pressure, 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's inert, easily cooled, and follows the ideal gas law. Other gases like argon or carbon dioxide may be used depending on the material and pore size. However, IUPAC recommends argon for microporous samples due to nitrogen's reactivity. 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 77.35K, ensuring accurate measurements. Impure nitrogen can lead to errors, so it must be pure. Mechanical refrigeration systems are also available for precise temperature control. 8. How to judge the liquid nitrogen is not pure? Impure nitrogen may show higher saturated vapor pressure than atmospheric pressure, or contain impurities like oxygen. A failed sensor or inconsistent data may indicate contamination. 9. Why should the sample be degassed before performing the sorption analysis? Degassing removes contaminants like water and oil, ensuring accurate adsorption measurements. Proper pretreatment ensures reliable results. 10. How to choose the degassing temperature of the sample? The temperature should be high enough to remove contaminants without damaging the sample. Safe temperatures vary by material, with oxides up to 350°C and organic compounds lower. 11. How to determine the degassing time of the sample? Longer times improve degassing efficiency, but too long may damage the sample. The optimal time depends on the sample's complexity and pore structure. 12. When degassing the sample, should you choose vacuum degassing or flow degassing? What are the characteristics of each of the two methods? Vacuum degassing is more effective for removing deep-seated moisture, while flow degassing is faster but less efficient. For ultra-microporous samples, molecular pumps are recommended. 13. What are the requirements for degassing a hydrophilic ultra-microporous sample? Use a molecular pump for oil-free degassing. High temperatures and long times are required to remove adsorbed water. 14. What gas should be backfilled after degassing and unloaded? Nitrogen is best to minimize buoyancy errors. Helium can cause significant weighing inaccuracies, especially for small samples. 15. What are the experimental techniques for physical adsorption measurements? Gravimetric and volumetric methods are used. Gravimetric measures mass changes, while volumetric calculates adsorption from pressure changes. Continuous flow methods are also used for rapid analysis. 16. What is free space? What is the dead volume? How does it affect measurement sensitivity? Free space is where gas molecules diffuse, and dead volume is the unused space in the system. Smaller dead volumes increase measurement accuracy. 17. What are the methods for determining the dead volume of free space? Helium calibration is the most accurate method, while blank experiments can also be used. Calibration curves help improve measurement precision. 18. What is the relationship between micropore diameter and gas pressure? Micropores fill at low pressures due to strong adsorption forces. Gas fills the smallest pores first, followed by larger ones. 19. What are the components of the static capacity method physical adsorption analyzer? A vacuum pump, gas sources, manifold, coolant Dewar, sample tube, and pressure sensors are essential components. Dead volume calibration is crucial for accuracy. 20. What are the requirements for physical purity of a physical adsorption analyzer? Gases must be at least 99.99% pure, with 99.999% recommended for high-precision measurements. 21. Why do you want to weigh the sample (weighing)? How large is the sample size? Sample weight affects measurement accuracy. Larger samples reduce weighing errors, while smaller samples require careful selection to avoid inaccuracies. 22. What are the specifications for the sample tube? What are the selection principles for sample tubes and filler rods? Sample tubes vary in size and design. Smaller tubes reduce dead volume but make loading harder. Filler rods help minimize free space in some cases. 23. What is a manifold? How does it affect the accuracy of the instrument? The manifold connects various components and its volume affects measurement accuracy. Smaller manifolds improve precision. 24. Why record the manifold temperature? What is the effect of manifold temperature control on measurement accuracy? Temperature affects gas behavior, so accurate control is necessary for reliable results. High-resolution sensors require stable temperatures. 25. Why do you want to get rid of helium before the analysis process begins? Helium can be adsorbed by microporous materials, affecting the isotherm shape. Removing it ensures accurate measurements. 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 in the liquid nitrogen bath, while warm free space is at room temperature. Cold free space has a greater impact on 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? Controlling the liquid level maintains consistent sample immersion, ensuring accurate pressure measurements. Servo feedback and jacket methods are common. 28. What is saturated vapor pressure? Why measure saturated vapor pressure? Saturated vapor pressure determines the maximum adsorption pressure. Accurate measurement is critical for pore size and surface area analysis. 29. How to measure saturated vapor pressure? Real-time monitoring using a separate P0 tube ensures accurate results. Temperature and purity affect vapor pressure, requiring careful control. 30. What are the different gas injection modes in a physical adsorption system? What are their characteristics? Two main modes are constant gas injection and constant pressure. Each has advantages and limitations, particularly for microporous materials. Adsorption equilibrium conditions are set by allowing sufficient time for pressure stabilization. Insufficient time leads to inaccurate results. For microporous materials, longer equilibrium times are necessary to capture true adsorption behavior.

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