Adhesion of small catalyst particles to gas bubbles: determination of small effective solid—liquid—gas contact angles

The adhesion of small particles to a gas bubble in water is studied with the aid of a modified bubble pick-up (BPU) method. The particle-to-bubble adhesion is revealed in the angle αmax by which the gas-bubble surface is covered by adhering particles under static conditions. The value of αmax depends on the particle and bubble sizes and on the physical properties of the three phases involved, especially on the effective contact angle θE. A “particles-to-bubble adhesion” (PBA) model, based on a balance of forces under static conditions, is developed to calculate the value of θE from the measured values of αmax. The value of θE measured according to the BPU method reflects the particle-to-bubble adhesion encountered during flotation processes and in slurry reactors. Experiments carried out under static conditions with air bubbles in water and commercially available catalyst particles showed that θE is smaller than the contact angle θ which occurs at “flat” solid and liquid surfaces. In accordance with the extended Young—Dupré equation, the value of θ can be obtained by extrapolation of measured θE values to a value where the curvatures of the surfaces are zero. The particle-to-bubble adhesion for the hydrogen—water—Pd/C system is much larger than that for the air—water—Pd/C system. For the hydrogen—water—Pd/AI2O3 and the hydrogen—water—Pd/BaSO4 systems, the particle-to-particle cohesion dominates the particle-to-bubble adhesion resulting in hardly any attachment of the catalyst particles to the hydrogen bubbles.