ADSORPTION ON SURFACES Kinetics of small molecule binding to solid surfaces
When the reactants arrive at the catalyst surface, reactions are accelerated Physisorption and Chemisorption 1- diffusion to the surface 2- adsorption on the surface 3- chemical reaction 4- desorption of the products from the surface 5- diffusion into the bulk Substrate: The substance that is adsorbing on the surface (Hydrogen) adsorbate: the material which adsorbs the substrate (char coal) adsorption: the process in which the molecule is attached to the surface coverage: the fraction of the surface occupied by the adsorbate exposure : the product of gas pressure and the time of exposure residence time: average time molecule resides on the adsorbent surface
Physical Adsorption The bonding with the surface is quite weak and mostly involves van der Waals forces. There will be minor changes in the free energy of the system and there will be no significant changes in the electron distributions of the adsorbate or the adsorbent H is similar to that of liquifaction of the adsorbent G small activation barrier may exist! Rxn Coordinate
Chemical Adsorption Often, one or more chemical bonds are formed/broken when the adsorbate binds on the surface of the adsorbent. (Physisorption precedes chemisorption) There will be significant changes in the free energy of the system and there will be changes in the electron distribution in both the adosrbate as well as the adsorbent. - monolayer binding is typical, - strong enthalpy changes may occur G Rxn ->
Surface Tension/Surface Energy Air Molecules at the surface experience pull to the bulk Liquid Surface molecules have a higher energy than in the bulk dx L work = f. dx = 2L dx γ γ = f/ 2L Film This work is stored as energy in the surface. Liquids minimize thier surface areas to lower their energy and form droplets.
Surface Texture of Solids
Actual Images of Solid Surfaces
Adsorption Kinetics Rate of adsorption = k (p) x k = A e (E*/RT) where k = rate constant for adsorption p = gas pressure x = order of the reaction where E* is the activation energy for adsorption Rate is controlled by diffusion rate of the adsorbate to the surface from the bulk (flux) fraction of molecules that adsorb and not leave (probability) Rate = (flux)(sticking probability) = [(p/(2πmkt) 1/2 ](1-fraction surface covered)(e (E*/RT) ) proprtional to pressure, surface coverage, activation energy, temperature
PE Curves/Surfaces single molecule approaching a clean surface 1 dimensional case Physisorption neglect angular orientation of the molecule neglect surface roughness or crystallinity neglect molecular shape and size neglect the intervening medium Chemisorption R(equilibrium) G ads
Dissociation - Chemisorption Step 1: Physisorption Step 2: Chemisorption Step 3: Chemical bond breaking Energy of desorption could be far greater than energy of adsorption!
Desorption Kinetics Adsorbed molecule may be desorbed by a number of means temperature, pressure, ph, equilibrium redistribution etc., (Thermal energy of the adsorbed species outweighs the adsorption energy) Rate of desorption = k (N) x = A e (-E*/RT) k = rate constant N = surface concentration in moles/m 2 x = order of the reaction Simple desorption Most desorption reactions are unimolecular, elementary processes, x= 1 A(surface) à A(gas) x=1 Recombinative desorption A(surface) + A(surface) à A 2 (gas) x=2 rate of desorption = A e (-E*/RT) = (frequency of desorption) N x e (-E*/RT)
Residence Time Average time the adsorbate spends on the surface before desorption tau = 1/(rate constant for desorption) = 1/((frequency of desorption) N x e (-E*/RT) ) = tau(zero) (e (E*/RT) ) = tau(zero)(e (- Hadsorption/RT) ) tau(o) is the frequency of vibration between the adsorbate and adsorbent it is taken as about 10-13 s or one hundred fempto seconds