surfaces

Question 1

definition of a surface

Answer 1

the outermost layer of a material

Question 2

how are surface atoms different from atoms buried in the bulk

Answer 2

a surface is coordinatively unsaturated

Question 3

what is a interface

Answer 3

the layer where the two solids are in contact with each other

Question 4

what is a catalyst

Answer 4

something that increases the rate at which a chemical system approaches equilibrium

Question 5

name some applications of surfaces

Answer 5

1. painting
2. growth of crystal
3. coatings
4. metal plating
   surface tension on liquid

Question 6

what are the 5 steps in heterogeneous catalysis

Answer 6

1. diffusion of reactant to catalyst surface
2. reactant adsorbed over catalyst surface
3. reaction at the surface
4. desorption of product
5. diffusion of product to the fluid phase

Question 7

what is physisorption and chemisorption

Answer 7

physi - adsorption of molecules onto the surface through weak intermolecular forces
chemi - adsorption of surfaces through the formation of chemical bonds

Question 8

what is the fractional coverage equation (θ)

Answer 8

θ = no. of adsorption sites occupied/no. of adsorption site available

Question 9

what is the Langmuir isotherm model and the three assumptions

Answer 9

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

Question 10

what is the rate of adsorption (r_a)

Answer 10

r_a = k_a * p * N(1-θ)
ka is rate constant
θ - fraction of site occupied
N = total sites
p = pressure

Question 11

what is the rate of desorption (r_d) equation

Answer 11

r_d = -kd * N * θ
kd = rate constant
N = total sites
θ = fraction of sites occupied

Question 12

what is the rate at equilibrium

Answer 12

θ = Kp/1+Kp

Question 13

what is the trend in the Langmuir isotherm model when temperature increases and why

Answer 13

θ decreases with increasing T (ie decreasing K) because high temperature promotes desorption off the surface so more unoccupied sites

Question 14

on a real flat surface, how many adsorption modes are there

Answer 14

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

Question 15

for adsorbed molecule to react what is needed

Answer 15

1. change in electron density
2. bond cleavage (bond formation)

Question 16

according to the Sabatier principle what does strong interaction possibly lead to

Answer 16

1. preclude a desorption step as such interrupts the 5-step catalyst step
2. Carry on covering the surface (poisoning the surface)

Question 17

what is Sabatier principle

Answer 17

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.”

Question 18

pros and cons of the Sabatier principle

Answer 18

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

Question 19

what is the equation of θ involving volume

Answer 19

V/Vm
V = volume of adsorbed gas
Vm = volume needed to cover surface

Question 20

what is the linearization equation for the Langmuir model

Answer 20

p/V = p * 1/Vm + 1/KVm
y=mx+c

Question 21

what are the expansions of the Bruauer isotherm model

Answer 21

1. account for multilayer formation
2. account for enthalpy of adsorption ΔH
3. go up to infinite adsorption
4. Heats of adsorption of successive layers equal the enthalpy of liquefaction, ∆𝐻𝐿

Question 22

whats uses do we get from X-ray photoelectron spectroscopy

Answer 22

1. quantify the chemical composition of the surface
2. identify the chemical elements that are present on the surface
3. identify oxidation state

Question 23

the equation for Binding energy for X-ray photoelectron spectroscopy

Answer 23

BE = hv - Ek - Φ
hv - light absorbed
Ek - energy
Φ - work function

Question 24

what is the Frenkel equation and what is it

Answer 24

𝜏 = 𝑎0 ∙ 𝑒^𝐸𝑑/𝑅𝑇
t = sticking time
Ed = energy of desorption
number of collisions of a gas over a surface