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Core 9 > Processes and Solid Surfaces > Flashcards

Processes and Solid Surfaces Flashcards

1
Q

What is the problem with using diffraction patterns to give information on crystal lattice structure?

A

The x-rays penetrate the bulk of the crystal and therefore the technique isn’t sensitive to the few surface atom layers - we’re only interested in the surface atoms.

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2
Q

How can you combat the problem with diffraction for looking at surface atoms?

A

Use a shallow angle of incidence which will enhance sensitivity. This is because the shallower angle will cause many more surface atoms to be sampled before probing the bulk atoms.

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3
Q

What is the electron universal escape depth curve?

A

It’s a curve showing how far electrons can travel through various solid materials with varying kinetic energy.

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4
Q

Describe what the electron universal escape depth curve shows.

A
  • at both high and low energies, an electron is able to travel a long way through a material
  • electrons with intermediate energies are unable to travel very far through a material and therefore must have come from the surface of the sample
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5
Q

What are low energy electrons (LEEs)?

A

Electrons with around 100 eV of kinetic energy with an escape depth of around 0.7 nm before they collide and lose energy = low energy electrons. If we observe these electrons then we know they’re from the surface layers.

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6
Q

Describe the features of good surface experiments.

A
  1. surface selective - must give information of surface atoms, often use LEE
  2. sensitive - must be sensitive to the surface which consists of relatively few atoms compared to the bulk, use a glancing angle
  3. avoid contamination
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7
Q

What is LEED?

A

It is low energy electron diffraction. It uses the idea of wave-particle duality where electrons have wave properties that lend themselves to diffraction.

The wavelength is from the de Broglie equation in quantum mechanics:

λ = h/p

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8
Q

What are the key features of LEED?

A
  • monochromatic electron beam
  • detects elastic back-scattered electrons
  • only works for conducting surfaces
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9
Q

What does a 1D LEED pattern consist of?

A

It consists of parallel lines from constructive interference, which denotes the position of atoms. The pattern spacing decreases with increases interatomic separation (a). This can be related back to the Bragg equation:

2asinθ = nλ

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10
Q

What happens to the LEED pattern when moving on to include the second dimension?

A

A second interatomic distance, b, is introduced. Another pattern of parallel lines, perpendicular to the first, is yielded from constructive interference.

2bsinθ = nλ

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11
Q

What is the resulting 2D LEED pattern?

A

The 2D pattern is yielded from the sum of both dimensions. Constructive interference requires both

2asinθ = nλ

2bsinθ = nλ

It generates a 2D spot pattern of a 2D surface with the spots representing the atoms. Pattern spacing still decreases with increasing interatomic seperation, either a or b.

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12
Q

Describe the results of LEED.

A

One experiment shows an array of spots. The resulting pattern is a regular 2D pattern where the spacing gives an atom density of around 10-19 m2, both of these telling us that (at a glance) the surface atoms look like extensions of the bulk atoms.

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13
Q

Describe Miller indices in terms of the example plane below.

A
  • this plane cuts the x-axis at a and doesn’t cut the y- or z- axes = (a ∞ ∞)
  • divide by the unit cell dimensions = (1 ∞ ∞)
  • take the reciprocal = (100) plane
  • other planes can be found in a similar way
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14
Q

Describe the bulk of face-centred cubic structures and what happens when the structure is cleaved.

A
  • in the bulk of FCC, each atom is surrounded by 12 nearest neighbours which makes the atoms stable
  • cleaving to give a (111) plane decreases the number of nearest neighbours to 9
  • cleaving to expose the (100) plane decreases it further to 8
  • cleaving to expose the (110) plane decreases it to 6
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15
Q

What is surface energy and what is it governed by?

A

Surface energy is governed by the number of nearest neighbours. Fewer near neighbours means more exposed/reactive atoms and therefore a higher surface energy.

All surface atoms have fewer near neighbours than bulk atoms, so they have a higher surface energy and are more reactive.

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16
Q

What is the order of surface energy for different FCC planes?

A

(110) > (100) (111)

high to low

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17
Q

What else can LEED be used to observe?

A

It can also be used to observe surface reconstruction, which is the rearrangement of atoms to a lower surface energy. It causes there to be fewer surface atoms which reduces the overall surface energy.

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18
Q

When is surface reconstruction more likely and what does it cause in the LEED pattern?

A

Reconstruction is more likely to occur for high surface energy arrangements of atom: (110) > (100) > (111).

Evidence from LEED shows that pattern spacing decreases with interatomic separation (a or b increases) so here the 2D pattern of spots gets closer together.

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19
Q

How do you avoid contamination?

A

When in air, every atom is constantly colliding with N2 or reactive O2. Because of this, the time for a monolayer to form is 3 x 10-9 s. This means it’s impossible to do any experiment before a monolayer forms e.g. build up of O2 on the surface.

In order to avoid contamination, ulta-high vacuum (UHV) needs to be used to increase the time taken for a monolayer to form. This also means that the probe beam isn’t affected by gases.

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20
Q

How can LEED give 3D information?

A

Spots converge as eV increases, giving a full 2D geometry - bond angles, unit cell, distinguish e.g. FCC from BCC, different planes, etc. The intensities of the spots change as they fade and new spots appear. This gives information on 3D structure - 1st, 2nd, 3rd, etc. layer. There’s also evidence of surface ‘relaxation’.

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21
Q

What is surface relaxation?

A

3D LEED shows a surface layer that has relaxed and therefore sits closer to its neighbouring layers. This relaxation causes a decrease in surface energy and the effect is greatest for higher energy surfaces.

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22
Q

How can surface relaxation be described quantitatively?

A

There is a maximum relaxation of around 10% for the first surface layer for ‘open’ surfaces e.g. FCC (110). The relaxation effect also affects deeper layers, but the effect is decreased for each subsequent layer.

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23
Q

Describe the process of adsorption.

A

A gas-phase molecule (adsorbate) binds to the surface (adsorbent).

  • ΔS is negative, so -TΔS is positive
  • for ΔG to be < 0, ΔH has to be negative (to be spontnaeous)
  • this means that adsorption is always an exothermic process
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24
Q

Describe adsorption for crystal growth.

A
  • special case of adsorption where adsorbate and adsorbent are chemically identical
  • speed of growth depends on surface energy of crystal plane
  • high surface energy faces grow fastest, e.g. (110)>(100)>(111)
  • slowest growing faces dominate the crystal appearance
  • low surface energy crystal
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25
Q

Describe the process of desorption.

A
  • reverse of adsorption
  • thermodynamics are therefore reversed - ΔS is positive
  • to desorb, molecules must overcome attractive forces - ΔH = positive
  • arrhenius-like kinetics
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26
Q

Describe the 3-step process of Temperature Programmed Desorption (TPD).

A
  1. absorb molecules of interest on the surface
  2. increase temperature (controlled, linear ramp)
  3. monitor the gas evolved = desorption
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27
Q

What information can be gathered from the peaks in TPD?

A
  • peak position = activation energy (energy needed to overcome the desorption barrier)
  • peak area ∝ molecules of gas desorbed
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28
Q

Describe the process of physisorption.

A
  • Physical adsorption
  • Van der Waals interactions form between the adsorbate and the surface (induced dipole-induced diple interactions)
  • no barrier to physisorption - shallow attractive well, ΔHads
  • ΔHads always small (≤ 40)
  • all gases physisorb below their condensation temperature
  • always reversible
  • can be multi-layer
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29
Q

Describe the process of chemisorption.

A
  • chemical adsorption
  • a true chemical bond forms between the adsorbate and the surface involving electron transfer
  • bonds within the adsorbate molecules are weakened (fundamental for catalysis)
  • ΔH varies a lot but always larger than for physisorption
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30
Q

What is surface coverage?

A
  • 0 = nothing
  • 1 = monolayer
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31
Q

What are adsorption isotherms?

A
  • recording surface data at one temperature
  • characteristic of the chemical system A + M
  • plot of surface coverage vs gas pressure
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32
Q

What is the simplest adsorption isotherm model?

A

The simplest model is from Langmuir. The key assumption are:

  • surface has fixed number of identical sites (monolayer only)
  • ΔHads independent of coverage
  • adsorbates do not interact.
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33
Q

What is the coverage equation for the Langmuir isotherm?

A
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34
Q

Describe the different kinds of surface coverage experiments.

A

In a real experiment we rarely observe moles molecules. However, there are many ways of probaing surface coverage:

  1. by surface analysis
    • traditional methods e.g. mass/weight, radioactivity
    • modern techniques e.g. microscopy, spectroscopy
  2. by looking at change in gas phase
    • volume, pressure, radioactivity, mass-spec, spectroscopy
  3. with desorption methods
35
Q

What TPD evidence contradicts the Langmuir isotherm?

A
  • some TPD graphs, e.g. for H2, have 3 peaks which suggests 3 distinct binding sites
  • contradicts assumption 1 - surface has a fixed number of identical sites
  • suggests the Langmuir isotherm is not always appropriate
36
Q

Describe the BET isotherm.

A
  • extension of Langmuir model
  • deals with multiple layers
  • assumes random sit distribution
  • for multilayers: N > Nsites, coverage > 1
37
Q

Describe the electron microscope.

A
  • based on de Broglie equation
  • compared with visble light: increased magnification and resoltuion
  • can observe living cells and even chemical structure
38
Q

Describe the STM.

A
  • scanning tunnelling microscope
  • allows magnification x 108
  • direct observations of surface atoms and features
39
Q

How is a STM used?

A
  • tip placed close (0.5 nm) to surface with a small 1/2V potential applied
  • tunnelling = quantum mechanical effects
  • electrons ‘tunnel’ from surface to probe tip
  • tunnelling current (nA) is related to separation, d

I ∝ e-d

  • atomic resolution, very surface selective
40
Q

What are the two modes of operation for STM?

A
  1. constant distance and voltage
  2. constant current
41
Q

Describe the constant distance and voltage mode of STM.

A
  • only moving the tip in horizontal plane, not up and down
  • map current, I / nA, as the surface is scanned
  • not good for rough surfaces as the tip will hit the atoms
42
Q

Describe the constant current mode of STM.

A
  • more common and safe for rough surfaces
  • scan tip over surface
  • adjust probe height to mantain constant current (I/ nA)
  • map V applied to piezotube as surface is scanned - map displacement up and down
43
Q

Give an overview of STM.

A
  • good surface detail
  • can use in vacuum, air or liquid
  • technically demanding
  • only for conducting surfaces
44
Q

Describe AFM.

A
  • atomic force microscopy
  • tip is placed right up to the surface and is repelled by the surface atoms
  • atoms deflect the sharp tip which changes the laser reflected on the back
  • used for insulators
45
Q

What is the relationship between defects and crystal growth?

A
  • speed of growth depends on surface energy of crystal plane
  • high surface energy = fast growth

defects >> terraces

vacancies > kinks > steps

  • defects rapidly filled in
  • terraces dominate crystal appearance
  • low surface energy crystal
  • ‘screw dislocation’ defect is one exception
46
Q

What techniques are used for elemental idenification?

A
  • STM gives only limited chemical contrast info
  • use spectroscopy to identify surface composition: XPS, AES
  • these techniques probe core electron configurations = elemental identification
47
Q

What is XPS?

A
  • a core electron is ionised by x-ray and travels to the electron energy analyser
  • gives the kinetic energy of the ionised electron
  • we want binding energy:

Ex-ray (tot) = EKE + EBE

  • core electrons are independent of bonding/environment therefore characterisitic of each elements and can be tabulated
48
Q

Give an overview of the XPS technique.

A
  • conducted under UHV
  • sensitive
  • surface selective - LEE
  • can get some chemical environment info
  • quantitative - integrate peak area = % composition
  • some 3D info with ‘depth profiling’ - change angle
  • poor spatial resolution
49
Q

What is the Auger process in AES?

A
  • x-ray or electron beam induces ionisation
  • initial excited state is unstable so an electron drops down the fill the hole
  • relaxation energy transferred to higher energy electron
  • this second ionisation is the Auger emission
50
Q

Describe the energy of AES emission.

A
  • energy of AES emission depends on 3 electron energy levels:

EAUGER = EW - EX - EY

  • characteristic of elemental composition
  • independent of excitation energy
  • only for conductors
51
Q

How do we learn more about adsorption thermodynamics?

A

From experiments at different temoeratures.

Recall phase changes and phase diagrams - slope of phase change boundaries given by Clapeyron and Clausius-Clapeyron equations.

52
Q

What is the Clausius-Clapeyron equation?

A
  • ΔHads = isosteric enthalpy of adsorption (same size/density, here = same coverage)
53
Q

Describe chemisorption ΔHads values.

A
  • they can be very large = values for covalent bonds
  • exothermicity can also impact on surface reconstruction
54
Q

Describe surface reconstruction PES with + without chemisorption.

A

Without:

Large Ea barrier for spontaneous reconstruction (no adsorption) - only for highest energy surfaces.

With:

The surface is reconstructed during the exothermic adsorption process with a much snaller barrier. The reconstruction is essentially catalysed by the adsorbate molecule. Still faster for higher energy surfaces.

Both lead to a lower surface energy structure.

55
Q

Give a summary of reconstruction.

A
  • change in surface layer geometry
  • can be spontaneous or adsorbate-catalysed
  • reconstruction rate ∝ surface energy
  • can result in surface composition change for alloys
56
Q

Describe dissociation adsorption PES.

A
  • the adsorbate molecule dissociates (breaks down) and the individual fragments adsorb to the surface
    e. g. N2 -> N + N
57
Q

How can thermochemistry give evidence of dissociative adsorption?

A
  • non-dissociative chemisorption has lower ΔHads due to weaker interactions
  • stronger interactions (larger ΔHads) tend to be dissociative chemisorption
  • not conclusive as values can overlap - thermochemistry alone does not prove dissociation
58
Q

How can coverage data give evidence for dissociative adsorption?

A
  • dissociative adsorption is a special case of chemisorption
    • Langmuir-like assumptions
  • as X2 dissociates, fractional surface coverage is of X
    • Langmuir-like derivation
  • gives dissociative Langmuir isotherm
59
Q

What is the coverage equation for the dissociative Langmuir isotherm?

A
60
Q

Compare the two different Langmuir isotherms.

A
  • the non-dissociative isotherm is able to near coverage = 1
  • the dissociative isotherm is less likely, as it’s harder to get to full coverage as it’s harder to find 2 free sites for a molecule compared to 1 free site for an atom
  • however, coverage data can be noisy so it’s rarely enough to prove dissociation
61
Q

How can desorption kinetics give evidence for dissociative adsorption?

A
  • if adsorbate X2 dissociates:
    • coverage is of X
    • X must meet another X prior to desorption as X2
    • desorption is second order wrt coverage
  • perform TPD experiments at different surface coverages
  • good evidence for dissociation
62
Q

What is UPS?

A
  • ultraviolet photoemission spectrometry
  • related to XPS
  • under UHV
  • sensitive
  • surface selective - LEE
63
Q

What is IR spectroscopy? How can it give evidence for dissociative adsorption?

A
  • absorptions are derived from intramolecular vibrations = chemical bonds
  • if adsorbate XY dissociates on the surface = change in X-Y bonding so check IR spectrum
64
Q

What is RAIRS?

A
  • reflection absorption IR spectroscopy
  • reflect IR off of the surface
  • doesn’t need a vacuum
  • sensitive
  • surface selective - shallow glancing angle
65
Q

How does RAIRS work?

A
  • highly reflective surface needed
  • strong IR absorbers only
  • generate a difference spectrum by subtracting: surface only, adsorbent only, other gas-phase
  • selection rule: oscillating dipole must be perpendicular to surface
  • some modes are inactive - RAIRS absence is not proof of dissociation
66
Q

A surface catalysed reaction can be broken down into steps. What are they?

A
  1. diffusion of reactants to active surface
  2. adsorption of one or more reactants onto the surface
  3. surface reaction
  4. desorption of products from the surface
  5. diffusion of products away from the surface
67
Q

Describe the rate of adsorption.

A

There are two components to Rads:

Rads = Z x s

68
Q

What are the three different descriptions of sticking probability, s?

A
69
Q

What is s in reality?

A

In reality, s = f(coverage).

  • complex
  • larger for high surface energy planes
  • often larger than Langmuir s attained by precursor states (physisorption)
70
Q

What are the two mechanisms for surface reactions?

A

There are two basic reaction mechanisms - two distinct kinetic regimes observable.

71
Q

What is the Eley-Rideal mechanism for surface reactions?

A
72
Q

What is the Langmuir-Hinshelwood mechanism for surface reactions?

A
73
Q

Which surface reaction is the reality?

A
  • the L-H mechanism more commonly fits with observations of surfce reactions
  • however, adsorption is a complex process
  • many real reactions proceed by a mixture of L-H + E-R mechanisms
74
Q

Why are lasers ideal for realistic surface kinetic studies?

A
  1. fast - short pulse duration, high repetition rate
    • can work in air (no UHV)
    • use light (not LEE)
  2. sensitivity - highly collimated light source
    • intense beam at grazing angle
  3. surface selective
75
Q

What is SHG?

A
  • second harmonic generation aka frequency doubling
  • selection rule: assymetric medium required for ‘doubling’ = surface can double (surface selective)
  • pulses of doubled-light readily distinguished from probe beam = enhanced sensitivity
  • in general, λout = 1/2λin
76
Q

What is SHG sensitive to changes in?

A
  • coberage
  • composition
  • binding site
  • adsorbate orientation

SHG is the only technique to allow realistic, real-tie adsorption studies.

77
Q

What is heterogeneous catalysis?

A
  • catalyst is a different phase to reactant(s)
    • catalyst is easily removed from products
  • most large-scale industrial processes are het. catalysed
  • catalyst promotes desired reaction rate by lowering activation barrier
  • catalyst itself is unaltered following the recycled
78
Q

What makes an efficient catalyst?

A
  1. separable from reactants
  2. large surface area
  3. enhances surface area
    • complex mechanism where steps 1-5 are all fast
79
Q

How do you make step 2 (adsorption) faster?

A
  • create a high sticking probability due to a large ΔHads for chemisorption
    • high surface energy plane / defects
    • transtion metal
80
Q

How do you make step 3 (surface reaction) faster?

A
  • weaken adsorbate bonds for chemisorption
    • high surface energy plane / defects
    • transition metal
81
Q

What about step 4 (desorption)?

A
  • desorption must overcome attractive ‘binding’ forces
  • factors that promote adsorption (step 2) also inhibit product desorption (step 4)
82
Q

What is a consequence of the step 2/step 4 relationship?

A
  • surfaces can bind too strongly
    • slow desorption
    • inefficient catalysis
  • ‘volcano curve’ of catalysis rates showing that the middle of the transition metals are catalysts
83
Q

How do defects affect catalytic activity?

A
  • kinetic measurements show surface composition and detaile structure are crucial for catalytic activity:
    • step defects for H-H bond cleavage
    • kinks for C-H/C-C bond cleavage
    • terraces much less active
  • explains ‘poisoning of catalysts by small amounts of impurities
  • catalytic control - can supress some reactions (dehydrogenation) by deliberate poisoning

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