surfaces Flashcards
definition of a surface
the outermost layer of a material
how are surface atoms different from atoms buried in the bulk
a surface is coordinatively unsaturated
what is a interface
the layer where the two solids are in contact with each other
what is a catalyst
something that increases the rate at which a chemical system approaches equilibrium
name some applications of surfaces
- painting
- growth of crystal
- coatings
- metal plating
surface tension on liquid
what are the 5 steps in heterogeneous catalysis
- diffusion of reactant to catalyst surface
- reactant adsorbed over catalyst surface
- reaction at the surface
- desorption of product
- diffusion of product to the fluid phase
what is physisorption and chemisorption
physi - adsorption of molecules onto the surface through weak intermolecular forces
chemi - adsorption of surfaces through the formation of chemical bonds
what is the fractional coverage equation (θ)
θ = no. of adsorption sites occupied/no. of adsorption site available
what is the Langmuir isotherm model and the three assumptions
its a simple physical chemisorption model
- adsorption cannot proceed beyond monolayer coverage
- all adsorption sites are equivalent, surface is uniform
- molecules adsorb at a given site is independent of the occupation of the neighbouring sites
what is the rate of adsorption (r_a)
r_a = k_a * p * N(1-θ)
ka is rate constant
θ - fraction of site occupied
N = total sites
p = pressure
what is the rate of desorption (r_d) equation
r_d = -kd * N * θ
kd = rate constant
N = total sites
θ = fraction of sites occupied
what is the rate at equilibrium
θ = Kp/1+Kp
what is the trend in the Langmuir isotherm model when temperature increases and why
θ decreases with increasing T (ie decreasing K) because high temperature promotes desorption off the surface so more unoccupied sites
on a real flat surface, how many adsorption modes are there
there are 4
- substrate directly above a surface atom
- substrate between two surface atoms
- substrate between two but buried in centre
- substrate between four atoms
for adsorbed molecule to react what is needed
- change in electron density
- bond cleavage (bond formation)
according to the Sabatier principle what does strong interaction possibly lead to
- preclude a desorption step as such interrupts the 5-step catalyst step
- Carry on covering the surface (poisoning the surface)
what is Sabatier principle
the binding energy of the reactants to the surface must be strong enough that they produce reaction intermediates,
but has to be weak enough to allow the products to leave the surface thus allowing more reactions to occur at that site.”
pros and cons of the Sabatier principle
pros
- simple
- distinguish between the adsorption site and the active site
-
cons
- tries to correlate thermodynamic parameters with kinetic behaviour
- doesn’t work for charge transfer reactions
- assumes all active sites are identical
what is the equation of θ involving volume
V/Vm
V = volume of adsorbed gas
Vm = volume needed to cover surface
what is the linearization equation for the Langmuir model
p/V = p * 1/Vm + 1/KVm
y=mx+c
what are the expansions of the Bruauer isotherm model
- account for multilayer formation
- account for enthalpy of adsorption ΔH
- go up to infinite adsorption
- Heats of adsorption of successive layers equal the enthalpy of liquefaction, ∆𝐻𝐿
whats uses do we get from X-ray photoelectron spectroscopy
- quantify the chemical composition of the surface
- identify the chemical elements that are present on the surface
- identify oxidation state
the equation for Binding energy for X-ray photoelectron spectroscopy
BE = hv - Ek - Φ
hv - light absorbed
Ek - energy
Φ - work function
what is the Frenkel equation and what is it
𝜏 = 𝑎0 ∙ 𝑒^𝐸𝑑/𝑅𝑇
t = sticking time
Ed = energy of desorption
number of collisions of a gas over a surface
