Case Study 602 Acid-Base Parameters for Catalysts
The study by Majid Naderi and Dan Burnett (Surface Measurement Systems Ltd.) characterises the energetic and acid‑base surface properties of a calcined RuO/MCM‑41 catalyst and its pure MCM‑41 support using two complementary techniques: Dynamic Gravimetric Vapour Sorption (DVS) and Inverse Gas Chromatography Surface Energy Analysis (IGC SEA). DVS measurements with hexane and dichloromethane at 25 °C provide adsorption/desorption isotherms, spreading pressure curves, and enable calculation of work of adhesion, dispersive surface energy (γSd), and acid‑base numbers (γS⁺, γS⁻) via Young’s equation and Fowkes theory. IGC SEA experiments at 100 °C with a suite of alkanes and polar probes (e.g., toluene, ethyl acetate) yield retention volumes, from which dispersive surface energies, van Oss acid‑base numbers, and Gutmann Ka/Kb values are derived. Results show that the RuO/MCM‑41 catalyst has higher thermodynamic parameters (γSd, γS⁺, γS⁻, Ka, Kb) than the support despite a lower BET surface area (792 m² g⁻¹ vs 1250 m² g⁻¹). Both techniques indicate that the support is more acidic, while the catalyst exhibits relatively higher basicity (lower acid‑to‑base ratio) though still predominantly acidic. Discrepancies in absolute values between DVS and IGC SEA are attributed to differing pre‑treatment temperatures and measurement temperatures, affecting the activation of high‑energy sites. The paper concludes that the combined DVS/IGC SEA approach reliably reveals surface energetics relevant to catalytic activity, with the catalyst’s higher energy parameters suggesting altered active‑site characteristics.
Topics
Dual Characterisation Approach: Combining DVS and IGC SEA for Catalyst Surface Energetics
Explanation of why both Dynamic Gravimetric Vapour Sorption and Inverse Gas Chromatography Surface Energy Analysis were employed together to obtain complementary surface property data.
Thermodynamic Parameter Extraction from DVS: Work of Adhesion and Surface Energy Calculations
Details of the DVS experimental setup, probe molecules (hexane, dichloromethane), temperature, adsorption steps, and the equations (Young’s, Fowkes) used to derive WAd, γSd, γS⁺, γS⁻ from isotherms and spreading pressure curves.
IGC SEA Methodology: Retention Volume Analysis and Acid‑Base Number Determination
Description of IGC SEA procedures—column preparation, probe selection (alkanes, polar molecules), retention time measurement, James‑Martin correction, and application of van Oss and Gutmann models to obtain γSd, γS⁺, γS⁻, Ka, Kb.
Comparative Surface Area and Uptake Findings: Catalyst vs Pure MCM‑41
Presentation of BET surface areas derived from DVS (792 m² g⁻¹ catalyst, 1250 m² g⁻¹ support) and discussion of reduced hexane uptake for the catalyst, indicating blockage of surface sites by RuO.
Acid‑Base Profiling Results: Higher Dispersive Energies and Basicity in RuO/MCM‑41 Catalyst
Summarisation of the quantitative acid‑base parameters from Tables 1 and 2, highlighting that the catalyst exhibits larger γSd, γS⁻, γS⁺, Ka, and Kb values, and a lower acid‑to‑base ratio compared with the support.
Methodological Discrepancies Impacting Absolute Values: Temperature and Pre‑treatment Effects
Analysis of why IGC SEA yields higher absolute energy numbers for the catalyst than DVS, attributing differences to higher pre‑conditioning (350 °C) and measurement temperatures (100 °C vs 25 °C) that keep more high‑energy sites active.
Implications for Catalytic Performance: Relating Surface Energetics to Active Site Availability
Discussion of how the observed surface energetics and acid‑base balance may influence catalytic activity, with higher basicity potentially affecting hydrogenation behavior as noted in prior literature.