Phys Need To Know Flashcards
TF
the Langmuir isotherm assumes that adsorption is non dissociative
True
TF
THE Langmuir isotherm assumes coverage stops after one mono layer
True
TF
the Langmuir isotherm assumes absorption sites can be different
False
They are all identical in this assumption
TF
the langmiur isotherm assumes there is no interaction between sites
True
In the Langmuir isotherm, what relationship do pressure and adsorption have ?
Absorbance increases as pressure increases until a point B where absorbance plateaus as we have reached full mono layer coverage
For a single atom molecule rate of adsorption is …
Rads = ka Ns (1-theta) S
If an isotherm has a hysteresis loop it indicates what
Pores filling them emptying (mesoporous material)
If an isotherm takes a shape where it increases plateaus and then increases again what is this indicative of?
Multilayer adsorption, it plateaus after one mono layer coverage and then increases as a multilayer is formed
At EQBM surface coverage in terms of rate =
Theta = Kp/ (1+Kp)
For isotherms that don’t take Langmuir shape we use the …… isotherm instead
Braunder emmett teller (BET) isotherm
In a rate eqn (surface coverage) :
What are powers to the half indicative of ?
Dissociative chemisorption
They come from the individual rate equations for dissociative chemisorption where they are squared as we need 2 vacant sites adjacent for this to occur.
In a rate eqn :
Squared implies …
Competitive adsorption in a langmiur hi shell wood mechanism
Signifies what type of adsorption?
Dissociative chemisorption
What can we tell about the adsorption from this eqn ?
Competing species for the same site
We have an extra denominator for the competing species
What type of mechanism does this rate eqn demonstrate?
ELAY RIDEAL
No k terms for B2 as there is no eqbm established between B2 gas and B2 ads on the surface
Mechanism in which A ads and it reacts with Bgas that’s not adsorbed to give a products in gas phase.
What happens in the elay rideal mechanism
A ads reacts with B2 gas to produce product (gas)
RDS of elay rideal mechanism
The reaction
A ads + B2gas - - - - products (gas)
Langmiur hinshelwwod mechanism RDS?
The reaction fully in the adsorbed layer
Aads + B ads ==== products ads
(Reversible)
What happens in the langmiur hinshelwood mechanism
A and B are both adsorbed and they react whilst adsorbed to create an adsorbed product that is then described
If there is a product term in the rate eqn for a catalytic process, which mechanism has it gone by?
Langmiur hinshelwood because the product is formed in the adsorbed layer so can compete with a and b
What imechansim is this process indicative of?
Langmiur hinshelwwod
If B is a poison is present how does it effect the eqn for surface coverage A (theta A)?
Theta A = kapa/ 1 +kapa +kbpb as b and a are competing
If b is a poison ! Kb» ka so kbpb term dominates
Therefore thetaA = kapa/ kbpb
Surface coverage of A is LIMITED BY POISON B
TF
in a volcano plot, the RDS is different at either side
True
TF
promoters are electronegative elements
FLASE
they are electro positive elements
TF
promotors have low surface energies compared to catalyst
True
They will segregate to the surface so stabilise the surface and keep the particles small
Increase surface area
TF
Promotors can increase surface area of catalyst
True
TF
Binding strength of adsorbed molecules is dependent on energy of fermi level EF
True
TF
For a single atom on m catalyst, across a period binding strength between an adsorbed molecule and a catalyst increases
False
Binding strength decreases across a period as delta h decreases
WHat type of bonding is dominant between catalyst surface and adsorbent species
Covalent
TF
BINDING STRENGTH OF ABSORBED SOECIES AND CATALYST IS DEPENDENT ON BAND OCCUPATION
True - this is dependent on fermi level which effects binding strength
TF
FOR DIATOMIC ATOMS ON M CATALYST, EG H2
ELECTRON DENSITY IN SIGMA * OF H-H BOND MAKES IT MORE LIEKLY TO DISSOCIATE
True
What effect will this have on M-H bond and H-H bond
m-h weakened because there is energy in anti bonding bands (sigma)
H-h weakened because there is E density on the sigma * bands which will weaken the h-h bond
TF
FOR diatomic molecules the position of Ef determines if dissociative adsorption occurs
True
For CO
What effect would scenarios 1 and 2 have on M-C and C-O bonds
1) increase M-C strength because bonding band for the sigma donation of c onto m (lowest band)
Increase m-c strength because bonding band of backdonation from m-c is filled (higher bonding band)
C=O strength . Whilst b2 is a bonding band, it arises from the interaction of an anti bonding orbital on c=o so will actually weaken c=o bonds.
2) anti binding band decreases m-c strength so it may react more readily as it is not held so tight to the catalyst
Binding band 2 has same effect at weakening CO as above
TF
binding strength and reactivity of adsorbates on a catalytic surface are independent of fermi level
FALSE, they are highly influence pd by fermi level
TF
Activity / catalytic performance depends on surface type
True (but not much)
TF
Activity / catalytic performance depends on dektaHads
True
Relationship between activity and strength of chemisorption
Activity (alpha) = 1/ strength of chemisorption
Lifetime of absorbance on a surface (Tao)
Tao = 1/v
V = vibrational frequency
How far does a molecule travel on a surface before diffusing?
TF
Catalytic performance depends on deltaH ads
TRUE
DEPENDS CRITICALLY ON IT
refer to volcano plots
TF
catalytic supports maximise sintering effects
False they minimise sintering effects (agglomeration)
TF
Catalyst supports allow high diesperion of small particles
True
Small particles on surface to maximise surface area
TF bulk solids are always preferred as supports
False
When unsupported metal catalysts are sufficiently active, porous sponges or gauze or wire like materials are preferred over bulk solids as supports.
