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D4641-17 – Standard Test Method Technical Guide
This standard practice, designated D4641-17, provides a computational method for determining pore size distributions in catalysts and catalyst carriers using nitrogen desorption isotherms. The method is based on the Kelvin equation and assumes a cylindrical pore model. It is applicable for pore radii ranging from 1.5 to 100 nm (15 to 1000 Å).
📐 Scope and Application
The practice covers the calculation of pore size distributions for catalysts and catalyst carriers from nitrogen desorption isotherms. It is particularly useful for pores with sizes between approximately 1.5 and 100 nm (15 to 1000 Å) in radius. However, it should be used with caution when applied to samples containing pores both within this range and larger than 100 nm, as the isotherms may rise steeply near P/Po = 1, making total pore volume difficult to define. Calculations should begin at a point near saturation, preferably at P/Po = 0.99, to establish the upper limit of the pore size distribution. Definitions of terms can be found in Terminology D3766, and isotherm measurements follow Test Method D4222.
⚠️ Caution: For samples with pores larger than 100 nm, the total pore volume may not be well defined due to steeply rising isotherms near P/Po = 1.
⚙️ Calculation Methodology
The methodology employs the Kelvin equation to determine the radius of the inner core (rk) from the desorption pressure. The thickness of the adsorbed nitrogen film (t) is calculated and added to rk to obtain the pore radius (rp). A volume correction factor Q, defined as (r̄p/r̄k)², is applied. The calculation proceeds by determining the decrease in adsorbed amount (ΔVT), the volume desorbed from the film (ΔVf), and the volume from capillary condensation (ΔVk). The process is iterative for each data point.
Note that this practice is designed primarily for manual computation with simplified expressions. For computer computation, these can be replaced by exact expressions.
💡 Tip: Begin calculations at a point near P/Po = 0.99 to ensure an accurate upper limit for the pore size distribution.
📊 Key Measured Properties
The following table summarizes the key symbols used in the calculations:
| 🟦 Symbol | 📐 Definition | 🎯 Typical Value/Unit |
|---|---|---|
| rk(i) | Radius of inner core from Kelvin equation | Å |
| t(i) | Average thickness of nitrogen film on pore walls | Å |
| rp(i) | Pore radius, rk(i) + t(i) | Å |
| Q | Volume correction factor, (r̄p/r̄k)² | dimensionless |
| ΔVT(i) | Decrease in amount adsorbed | mm³/g |
| ΔVf(i) | Volume desorbed from film | mm³/g |
| ΔVk(i) | Volume from capillary condensation | mm³/g |
Additional parameters from the standard include:
| ⚡ Parameter | 📏 Value |
|---|---|
| Pore radius range | 1.5 to 100 nm (15 to 1000 Å) |
| Relative pressure at start | P/Po = 0.99 |
| Pore model | Cylindrical, non-intersecting |
❓ Frequently Asked Questions
🔍 What is the applicable pore size range for this practice?
The practice is applicable for pore sizes ranging from approximately 1.5 to 100 nm (15 to 1000 Å) in radius.
💡 Why is a cylindrical pore model assumed?
The cylindrical pore model simplifies the calculations by treating pores as non-intersecting, open-ended capillaries that function independently during adsorption or desorption.
⚡ What caution should be taken with pores larger than 100 nm?
For samples containing pores larger than 100 nm, the isotherms rise steeply near P/Po = 1, and the total pore volume may not be well defined. The practice should be used with caution in such cases.
📌 Is this practice suitable for computer computation?
The practice is designed primarily for manual computation with simplified expressions. For computer computation, the simplified expressions can be replaced by exact expressions for more accurate results.