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Physical y3 term 1 > Phys Need To Know > Flashcards

Phys Need To Know Flashcards

1
Q

TF

the Langmuir isotherm assumes that adsorption is non dissociative

A

True

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2
Q

TF
THE Langmuir isotherm assumes coverage stops after one mono layer

A

True

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3
Q

TF
the Langmuir isotherm assumes absorption sites can be different

A

False
They are all identical in this assumption

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4
Q

TF
the langmiur isotherm assumes there is no interaction between sites

A

True

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5
Q

In the Langmuir isotherm, what relationship do pressure and adsorption have ?

A

Absorbance increases as pressure increases until a point B where absorbance plateaus as we have reached full mono layer coverage

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6
Q

For a single atom molecule rate of adsorption is …

A

Rads = ka Ns (1-theta) S

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7
Q

If an isotherm has a hysteresis loop it indicates what

A

Pores filling them emptying (mesoporous material)

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8
Q

If an isotherm takes a shape where it increases plateaus and then increases again what is this indicative of?

A

Multilayer adsorption, it plateaus after one mono layer coverage and then increases as a multilayer is formed

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9
Q

At EQBM surface coverage in terms of rate =

A

Theta = Kp/ (1+Kp)

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10
Q

For isotherms that don’t take Langmuir shape we use the …… isotherm instead

A

Braunder emmett teller (BET) isotherm

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11
Q

In a rate eqn (surface coverage) :
What are powers to the half indicative of ?

A

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.

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12
Q

In a rate eqn :
Squared implies …

A

Competitive adsorption in a langmiur hi shell wood mechanism

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13
Q

Signifies what type of adsorption?

A

Dissociative chemisorption

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14
Q

What can we tell about the adsorption from this eqn ?

A

Competing species for the same site

We have an extra denominator for the competing species

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15
Q

What type of mechanism does this rate eqn demonstrate?

A

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.

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16
Q

What happens in the elay rideal mechanism

A

A ads reacts with B2 gas to produce product (gas)

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17
Q

RDS of elay rideal mechanism

A

The reaction
A ads + B2gas - - - - products (gas)

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18
Q

Langmiur hinshelwwod mechanism RDS?

A

The reaction fully in the adsorbed layer
Aads + B ads ==== products ads

(Reversible)

19
Q

What happens in the langmiur hinshelwood mechanism

A

A and B are both adsorbed and they react whilst adsorbed to create an adsorbed product that is then described

20
Q

If there is a product term in the rate eqn for a catalytic process, which mechanism has it gone by?

A

Langmiur hinshelwood because the product is formed in the adsorbed layer so can compete with a and b

21
Q

What imechansim is this process indicative of?

A

Langmiur hinshelwwod

22
Q

If B is a poison is present how does it effect the eqn for surface coverage A (theta A)?

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

23
Q

TF

in a volcano plot, the RDS is different at either side

A

True

24
Q

TF
promoters are electronegative elements

A

FLASE
they are electro positive elements

25
Q

TF
promotors have low surface energies compared to catalyst

A

True
They will segregate to the surface so stabilise the surface and keep the particles small
Increase surface area

26
Q

TF
Promotors can increase surface area of catalyst

A

True

27
Q

TF

Binding strength of adsorbed molecules is dependent on energy of fermi level EF

A

True

28
Q

TF

For a single atom on m catalyst, across a period binding strength between an adsorbed molecule and a catalyst increases

A

False
Binding strength decreases across a period as delta h decreases

29
Q

WHat type of bonding is dominant between catalyst surface and adsorbent species

A

Covalent

30
Q

TF
BINDING STRENGTH OF ABSORBED SOECIES AND CATALYST IS DEPENDENT ON BAND OCCUPATION

A

True - this is dependent on fermi level which effects binding strength

31
Q

TF
FOR DIATOMIC ATOMS ON M CATALYST, EG H2
ELECTRON DENSITY IN SIGMA * OF H-H BOND MAKES IT MORE LIEKLY TO DISSOCIATE

A

True

32
Q

What effect will this have on M-H bond and H-H bond

A

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

33
Q

TF

FOR diatomic molecules the position of Ef determines if dissociative adsorption occurs

A

True

34
Q

For CO
What effect would scenarios 1 and 2 have on M-C and C-O bonds

A

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

35
Q

TF

binding strength and reactivity of adsorbates on a catalytic surface are independent of fermi level

A

FALSE, they are highly influence pd by fermi level

36
Q

TF
Activity / catalytic performance depends on surface type

A

True (but not much)

37
Q

TF
Activity / catalytic performance depends on dektaHads

A

True

38
Q

Relationship between activity and strength of chemisorption

A

Activity (alpha) = 1/ strength of chemisorption

39
Q

Lifetime of absorbance on a surface (Tao)

A

Tao = 1/v

V = vibrational frequency

40
Q

How far does a molecule travel on a surface before diffusing?

A
41
Q

TF
Catalytic performance depends on deltaH ads

A

TRUE
DEPENDS CRITICALLY ON IT
refer to volcano plots

42
Q

TF
catalytic supports maximise sintering effects

A

False they minimise sintering effects (agglomeration)

43
Q

TF
Catalyst supports allow high diesperion of small particles

A

True
Small particles on surface to maximise surface area

44
Q

TF bulk solids are always preferred as supports

A

False
When unsupported metal catalysts are sufficiently active, porous sponges or gauze or wire like materials are preferred over bulk solids as supports.

Physical y3 term 1 (3 decks)

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