Processes at Solid Surfaces

Question 1

How are crystal lattices normally analysed? Can this be applied only to the surface structure?

Answer 1

X-ray diffraction is traditionally used to study surfaces, however the x-rays penetrate into the surface of the crystal.

Using a shallow angle of incidence (a glancing or grazing angle) enhances the sensitivity to surface atoms.

Question 2

How can low energy electrons be used to study solid surfaces?

Answer 2

Low energy electrons (K.E approximately 100 eV) will only travel approximately 0.7 nm into a surface before colliding and losing energy. This is true for most solids.

Question 3

Describe the experiment and results of low energy electron diffraction (LEED).

Answer 3

Monochromatic, low energy electorons are fired at a conducting sample and the back-scattered electrons are detected. This is similar to x-ray diffraction, however the electron wavelength is similar to the atomic spacing.

A 1-D LEED pattern can detect the interatomic distance between atoms using the bragg equation. The 2-D pattern can show the interatomic spacing in two directions.

Question 4

What factor of a crystal determines the surface energy of the solid? Use an FCC crystal as an example.

Answer 4

The number of nearest neighbours governs energy. A bulk FCC crystal has 12 nearest neighbours. The (111) plane has 9, the (100) has 8 and the (110) has 6.

This means that the (110) plane has the highest surface energy.

Question 5

In a LEED experiment, what is the key source of contamination and how can it be controlled?

Answer 5

Collisions with air are the main contamination. Collision frequency, Z_w, is proportional to pressure so forming a vacuum will decrease the collisions with air.

Most experiments are done at ultra-high vacuum.

Question 6

Define surface reconstruction and describe why it may occur for an FCC crystal.

How can this be detected by LEED experiments?

Answer 6

Surface reconstruction is where surface atoms rearrange spontaneously to form a lower energy arrangement. This can be catalysed by an adsorbate.

For an FCC surface, this is very likely for a (110) face as it is the highest energy, then (100), then (111).

LEED experiments can detect when atomic seperations have changed, meaning it can tell when a surface has reconstructed.

Question 7

Define surface relaxation and how LEED experiments can detect it.

Answer 7

Surface relaxation is where the atoms at a surface will shrink closer together, much closer than in the bulk of the material. This decreases the energy for the surface and will be the greatest for high energy surfaces (FCC 110).

3-D LEED can give information up to around the 4th layers and can show that the surface atoms are closer to the neighbour layers.

Question 8

Define and describe the basic thermodynamics of adsorption to a solid surface by a gas.

Answer 8

Gases (the adsorbate) bind to the surface (adsorbent), normally forming a monolayer on the solid surface.

As the entropy of adsorption is always positive, for adsoption to occur it must be an exothermic process with enough enthalpy to overcome the entropy term.

Question 9

How does adsorption occur during crystal growth? What factors decide the rate of adsorption/growth?

Answer 9

Here the adsorbate and adsorbent are the same atoms in different phases. The solution/gas atoms will adsorb to the surface and grow the crystal on a specifc face.

The highest surface energy (FCC 110) will grow the fastest as it is the least stable, therefore the slowest growing face dominates the crystal appearence.

Question 10

Describe the thermodynamics of desorption and how this can be measured experimentally.

Answer 10

As there is an attractive force between the surface and gas, the desorption will have an activation energy. The kinetics will be arrhenius-like (exp^(-Ea/RT)).

This can be tested by adsorbing molecules to a surface, then increasing the temperature while monitering desorption (normally MS). The desorption peaks will be the activation energies. There is often multiple as different modes of binding are common. The amount of each type can be worked out from the number of molecules desorbed (peak area).

Question 11

Summerise the main differences between chemisorption (C) and physisorption (P).

Answer 11

• C: large variations between materials, P: only slight variations.
• C: clear differences between adsorption on different planes, P: almost no plane variations
• C: Wide temperature range generally, P: only near/below gas condensation point.
• C: sometimes dissociative and irreversible, P: always non-dissociative and reversive.
• C: only monolayers, P: can form multilayers
• C: wide range of speeds, often requires activation, P: generally fast, non-activated.

Question 12

Give a term for surface coverage and the variables it depends upon.

Answer 12

Surface coverage, θ, is the number of occupied sites/overall number of sites.

It depends on the characteristics of the surface and gas, the concentration of the gas, and the temperature.

Question 13

What is an isotherm plot? Give the names of the two main isotherms.

Answer 13

An isotherm plot is a graph of how surface coverage depends on the gas pressure

The main isotherms are the Langmuir and the BET.

Question 14

Give the three main assumptions of the Langmuir isotherm and give the basic derivation of the isotherm.

Answer 14

1. There is a fixed number of identical sites (only a monolayer).
2. ΔH_ads is independent of coverage.
3. Adsorbates do not interact.

Question 15

Describe how to use the Languir isotherm to find the maximum monolayer coverage of a surface.

Answer 15

1. Do a quick plot to see if the data follows Languir behavour (increasing coverage with increasing pressure to a plateau).
2. Replace θ with A/A_max (where A is the amount of gas adsorbed).
3. Use the linearised form of the isotherm pictured.
4. Check the rough plot to make sure the A_max makes sense.

Question 16

How can we experimentally determine surface coverage?

Answer 16

By surface analysis methods, such as mass and radioactivity, or microscopy and spectroscopy.

By gas phase changes in volume, pressure, radioactivity, MS and spectroscopy.

Question 17

In which 2 ways does the BET isotherm improve over the Languir isotherm?

Answer 17

1. Assume random site distribution, increasing in layers.
2. For multilayers, θ > 1.

Question 18

Describe the principles, results and two different modes of an scanning tunnelling microscope.

Answer 18

A STM allows direct observation of surface atoms and features by allowing electrons to tunnel between an atomically sharp tip and the metal surface. The distance between the tip and the surface is related to the tunnelling current by I ∝ e^-d.

• Constant distance (measure current) which is not suitable for rough surfaces.
• Constant current (measure height) which is more common.

Question 19

What key features and processes can be visualised by STM and what are the limitations of the technique?

Answer 19

Surface terraces can clearly be seen, as well as the defects on the surface, such as steps, kinks and vacancies. Surface density, reconstruction and adsorbates/contamination can all be visualised as well.

The process can be done in a vacuum, air or liquid, however, it can only be used on conducting surfaces and requires expensive equipment.

Question 20

Briefly describe atomic force microscopy (AFM) and why we would use it.

Answer 20

AFM is similar to STM but can be used on insulators.

The set-up is a cantiliever with a sharp tip which is scanned across a surface. A laser reflects off the cantiliever to a photodiode which measures the force of deflection from the surface, giving information on the surface height.

Question 21

Briefly describe the set-up and results of x-ray photoelectron spectometry (XPS).

Answer 21

X-rays are fired at a sample at a known energy. The ionised electrons have their kinetic energy analysed to find their bonding energy. These are characteristic of each element and the amount of each element can be quantified.

The experiment must be done at ultra-high vacuum. By only considering low energy electrons the technique can be selective for the surface.

Ok for conductors and semi-conductors (and insulators if charge is dissapated).

Question 22

Briefly describe the set-up and results of Auger electron spectroscopy.

Answer 22

X-rays or an electron beam is fired at a surface at a shallow grazing angle (surface selective). This ionises a core electron. An electron in a higher orbital then relaxes which transfers energy to another electron which is ionised.

The second ionisation is the Auger emission. The AES spectrum is characteristic of the elemental composition.

Only works with conductors or semi-conductors.

Question 23

How do you measure the enthalpy of adsorption at different temperatures? What do the results show?

Answer 23

Use the integrated Clausius-Clapeyron equation pictured.

You need the adsorption as a measure of temperature using the same size/density of sites.

The results show what type of adsorption is occuring. Typical chemisorption values are from -50 to -600 kJ mol^-1.

Question 24

In what case will the temperature programmed desorption (TPD) signal depend on coverage of the solid?

Answer 24

When the absorbate is dissociating on the surface (N₂ → 2N). This is known as dissociative adsorption.

Question 25

Describe dissociative adsorption and how it can be shown to be occuring.

How can the Languir equation be adapted for this?

Answer 25

Dissociative adsorption is where the activation energy to break a bond can be overcome by a surface, to transition from weak physisorption to strong, dissociative chemisorption.

The enthalpy of dissociative chemisorption is typically higher but the values of physisorption and chemisorption strongly depend on the gas. Other ways of knowing is the desorption kinetics dependant to the second order to coverage and by comparing the Languir behaviour.

An adaptation can be made to the Languir isotherm pictured below to account for the additonal molecule produced.

Question 26

Briefly describe the set-up and results of a reflection absorption IR spectroscopy (RAIRS) experiment.

Answer 26

IR radiation is reflected by a surface to study the adsorbates on the surface. The surface must be highly reflective and strong IR absorbers will only be seen. Additionally, the oscillating dipole will only be seen if it is perpendicular to the metal surface (some modes are inactive). However it can be done in air.

Question 27

How is the rate of adsorption measured?

Answer 27

R_ads which is made up of two components. Z x s where Z is the collision frequency and s is the ‘sticking probability’.

s = rate of adsorption/rate of collision frequency

Question 28

Describe the three ways in which sticking probability can be modelled.

Answer 28

1. Using the simple Languir adsorption. s = (1 - θ)
2. Using the dissociative Languir. s = (1 - θ)²
3. Using a complex function to describe physisorption and multilayer formation. s = f(θ)

Question 29

Describe the two kinetic models for surface reactions and how to find out which one is occuring.

Answer 29

• Langmuir-Hinshelwood: Both gases adsorb seperately, react on the surface and desorb as the new compound
• Eley-Rideal: One gas adsorbs which then reacts with the other gaseous reactant. Then the product desorbs.

The LH mechanism rate depends on the coverage of both species and is maximised when the coverage of both is equal. This mechanism is more common.

The ER mechanism depends on the coverage of one species and the pressure of the other. Rate increases with coverage at first but then plateaus as the coverage maximises.

Question 30

Describe the set-up and results of a Second Harmonic Generation laser experiment (SHG).

Answer 30

A pulsed laser is fired at very shallow glancing angle. Some of the laser light is ‘doubled’. This can only be done by an assymetric medium (not the bulk).

SHG can detect for coverage, composition, binding site and adsorbate orientation, and since the photons can be pulsed very quickly, it is the only technique that allows for real-time adsorption studies.

It is only good for detecting changes, so use in conjuction with other techniques to find out physical information about the surface.

Question 31

Describe what makes a good heterogeneous catalyst. Which are the best catalysts and how can they be improved?

Answer 31

Good heterogeneous catalysts must adsorb the reactants well, allow the surface reaction to occur readily and desorb the products quickly. This requires a high sticking probability and normally high energy surface planes however the surface cannot bind too strongly to adsorbates.

The middle transition metals are the best catalysts from these requirements. Defects are often the key to catalytic mechanisms and the activity can be adjusted with poisons to remove some catalytic sites.