Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D3908-20 – Standard Test Method Technical Guide
🔍 Test Overview and Applicability
ASTM D3908-20 specifies a static volumetric vacuum method for determining the chemisorption of hydrogen on supported platinum catalysts. The analysis is conducted at a controlled temperature of 298 K (25 °C) following a standardized reduction of the catalyst in flowing hydrogen at 723 K (450 °C).
This test method is technically validated for unused supported platinum on alumina catalysts with metal loadings exceeding 0.3 weight percent. Application to other supports or lower platinum loadings requires independent validation by the user, as defined in the scope of the standard.
| 🟦 Parameter | 📐 Specification |
|---|---|
| Chemisorption Temperature | 298 K (25 °C) |
| Reduction Temperature | 723 K (450 °C) |
| Primary Support | Alumina (Al₂O₃) |
| Platinum Loading | > 0.3 wt % |
| Analytical Technique | Static Volumetric Vacuum at Constant Volume |
⚙️ Test Method and Reduction Procedure
The analysis begins by loading a precisely weighed mass (Wcat) of the catalyst into a sample cell with a known dead space volume (Vd). The sample is reduced in a flowing stream of hydrogen at 723 K to ensure the platinum is in its metallic state. After reduction, the system is evacuated and cooled to the chemisorption temperature of 298 K.
Precise hydrogen doses are admitted from a calibrated bulb (Vc) into the manifold (Vm). The equilibrium pressure (Pex) is measured after each expansion. Accurate temperature compensation relies on recording the manifold temperature before (TmAx) and after (TmBx) expansion. The average temperature of the manifold is T = (TmAx + TmBx)/2. The dead space determination also uses specific temperature terms (Tm and TmD) to correct for thermal gradients between the manifold and the sample zone.
💡 Tip for Accurate Measurements: The precision of D3908-20 heavily relies on the accurate determination of the dead space volume (Vd) and the application of the correct temperature terms. Small errors in these parameters can lead to significant deviations in the cumulative adsorbed volume (Vads(STP)cx).
📊 Key Measured Properties and Symbols
The primary result of the test is the monolayer volume of gas adsorbed at STP (VS). This is determined from the adsorption isotherm constructed by plotting Vads(STP)cx against the equilibrium pressure (Pex). The standard defines a comprehensive set of symbols to ensure rigorous calculation of the adsorbed volumes throughout the analysis.
| 🔣 Symbol | 📖 Definition |
|---|---|
| Vc |
📥 Standard Documents Download |