CH5713 Flashcards
2 problems Gerhard Ertl addressed regarding ‘The Surface Science Approach’
Pressure gap: measuring from 12 or more orders of magnitude away from catalyst, how can this be relevant?
Structure gap: Single crystal ordered, support is nowhere near as ordered
Give two examples of materials with each of these structures Simple Cubic FCC BCC HCP Diamond
Na, K Pt, Pd Mo, W Co, Lanthanides Diamond, Si
How do the presence of steps on a surface effect its properties?
The ‘long’ steps and ‘short’ steps have different energies and hence chiral molecules on a catalyst cab direct a reaction a certin way
How can you tell if an FCC surface is chiral?
They are chiral i the length of step wither side of the kink is different
What is the order of priority for kinked surfaces?
From looking above
Highest –> lowest
Long step, short step, terrace between
In catalysis how is surface area maximised?
Metals are present as nano particles exposing thermodynamically most favourable atomic arrangements
What is the typical particle shape for an fcc metal?
A cube-octahedron exposing the (111) and (100) faces
Why is heterogeneous catalysis not always uniform?
Because edges and faces all have different reactivities depending on tom density on the surfaces
Why do some catalysts need to work at very low pressures?
because even at low pressures, the rate of bombardment is such that it would only take a few seconds for the surface to be covered in contaminants.
What does the sticking probability depend on?
composition
crystal face (coordination number)
adsorbate
Draw schematic for 001 fcc
1
draw schematic for 101 fcc
2
draw schematic for 111 fcc
3
draw schematic for all possible processes when a diatomic molecule hits a surface
4
Example of physical adsorption
Xe (g) –> Xe (ads)
Example of chemical adsorption
Co(g) –>CO (ads) on Pd
Example of dissociative adsorption
CO(g) —> C(ads) + O(ads) on W
Example of dissociative adsorption and desorption
CH3OH (g) —> CH3O (ads) + 0.5H2 (g) on Cu at 300 K
describe physisorption in more detail
Weak atom (molecule) surface bonds e.g. rare gas adsorption or non-dissociative alkane adsorption Van der Waals' forces, so no electron rearrangement. deltaH ~ 25 kJ.mol
draw and annotate the Lennard Jones potential for physisorption
5
Describe chemisorption in more detail
Strong atom (molecule) to surface bonds Electron rearrangement leading to covalent or ionic or metallic bonds eg C2H4/Ni delta H = 210 kJ/mol
metal to metal is especially high
Draw potential energy curve for chemisorption
6
draw lennard jones for A2 + metal and 2A +metal
7
draw potential energy profiles for not activated and activated adsorption, with examples
8
Compare Langmuir vs precursor adsorption
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state the Langmuir isotherm equation and related equations
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Discuss CO incident on Fe
Molecule comes in from gas phase. Very briefly physisorbed and then chemisorbed then flies over the the cross-over peak and into the v. deep well due to the strength of the Fe-O and Fe-C bonds. Thermodynamics dictates that the depth of he well will induce breaking, so the molecule dissosiates. Non-activated
Discuss CO incident on Pd
Atoms are less strongly bound to Pd than Fe. The physisorbed well ill be the same but the chemisorbed well will be shallower so less likely to overcome barrier into deep well. Molecule stays associated. Activated
What do you plot to graphically show Langmuir Isotherm?
11
What are the assumptions in the LI theory
Non-dissosiative adsorption
No lateral ineractions
draw adsoprtion energy per molecule vs coverage for CO/Pd(111) and evaluate enery gain
13
Why is there a drop off at 0.3?
14
Draw the mo diagram for a molecular adsorbate (M) forming a bond with a d-band metal surface (d) to form bonding (Md) and antibonding (Md*) energy levels and also draw density of states
15
Will a molecule dissociate?
18
Why do only some molecule dissociate CO?
19
draw energy density driagrams for Fe, Ni and Cu and draw a conclusion
20
How far below the fermi level can electrons realistically form bonds?
21
Draw diagram to show how stm can be misleading
25
2 cool things you can do with stm
Atoms and molecule manipulation (IBM)
Quantum corrals to see electron wave function for single electron in a circle.
How can one achieve good resolution even when not done at UHV?
26
Draw set up of STM
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Draw 2 energy diagrams for STM
23
Show how current changes with distance
24
Why is definition often lost in imaging?
Because movement across the surface is faster than the image is taken
Name 6 crystal defects
Adatom Terrace Kink Vacancy Step Incorporated impurity
What is relaxation?
On the surface the atoms have less interaction so they club together a little close to make up for this. This means that the top few layers of atoms are spaced closer than the bulk atoms. Drawing on page 27
Describe reconstruction metals
28
give the domains and angles for: square diamond hexagonal rectangular (centred) rectangular (primitive) Oblique
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2 types of diffraction structure determination
Photoelectron diffraction (XPD) Low energy electron diffraction (LEED)
2 types of indirect structure determination
Low energy ion scattering (LEIS)
Medium energy ion scattering (MEIS)
1 Microscopy structure determination
Scanning tunneling microscopy (STM)
2 Spectroscopic methods structure determination
Surface extended x-ray absorption fine structure (SEXAFS)
Normal Incidence Standing X-ray Wavefield (NISXW)
When is electron diffraction strongest?
when the wavelength is similar to the gap width of the splitter
Sketch inelastic mean free path vs Energy
30
draw a schematic of LEED
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state all reciprocal lattice equations
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draw LEED set-up
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Why might some diffraction peaks not be visible even when they are expected?
They may be outside of the wavevector radius, k=2pi/1.37
Give an example of Diastereomeric recognition
the nucleic acid/base adenine forms mirror equivalent chains of dimers on Cu(110). The subsequent adsorption of (S)-phenylglycine is readily nucleated at one type of chain but rarely at the other type. H-bonding, dipole-dipole interactions and preferred adsorption sites all play a role
Describe and give an example of asymmetric amplification
The use of proline to produce an aldol product. When proline is in solution the entantiomeric excess of the aldol product increases linearly with proline ee. When proline is in a saturated solution (co-existing with the solid proline) the product ee tracks with the solution ee of proline, not the overall ee of proline.
Which amino acid has the highest product ee: amino acid ee and which has the worst
best is serine and worst is threonine
Derive the TPD equation
34
derive the Redhead approximation
35
Why does increased temp increase rate of desorption?
Faster vibrations, so if a bond break 1/10000 vibrations, it will break faster
How can Ea be foudn experimentally
36
draw graph of energy vs distance for desorption
37
What does the area under the TPD peak show?
Is proportional to the amount originally adsorbed ie. proportional to the surface coverage
What does the peak profile (and the coverage dependence of the desorption characteristics) show?
Gives info on the state of aggregation of the adsorbed species e.g. molecular vs dissociative
What does the position of the peak tell you?
The peak temp is related to the enthalpy of adsorption i.e. to the strength of binding to the surface
In a first order reaction how does the spectrum change withcoverage?
The position of max temp doesnt change but the peak does change in intensity
How can you identify a zero-order TPD?
Very sharp and at very low temp. Probably physisorbed.
How does second order cange with coverage? How does second-order desoprtion work?
O(a) + O(a) —-> O(g)
T max decreases with coverage but the peak is always symmetric
What are the physical causes of zero, first and second order peaks?
zero - large molecules coming off eg benzene
first - smaller molecules coming off
second - atom recombination then desorption
What does the coincident desorption of two species well above their normal desorption temp suggest?
Suggests decomposition of a more complex surface species
How does chirality effect peak desoprtion temp?
R/R or S/S molecules have higher temp than R/S or S/R
what is rate proportional to?
theta(1-theta)
What effect does changing chirality have?
It switches the desorption temperature of the chiral adsorpate temperatures. eg if SS-TA has a higher desorb temp on Cu(s) then RR-TA will have a higher desorb temp on Cu(R)
Name four types of surface vibrational spectroscopies
High res electron energy loss spectroscopy
electron energy loss spectroscopy
reflection absorption infrared spectroscopy
Sum frequency generation
What does specular refer to?
When the angle of incidence is the same as the angle of reflection. Only picks up dipole active modes (only totally symmetric stetches appear) but off-spec picks up all vibrational modes
specifically describe off-spec
Impact scattering where electrons collide with an atom and is scattered in all directions and every vibration can be seen.
schematically draw EELS
38
draw detector current vs resolution
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Describe EELS generally
In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will undergo inelastic scattering, which means that they lose energy and have their paths slightly and randomly deflected. The amount of energy loss can be measured via an electron spectrometer and interpreted in terms of what caused the energy loss.
Describe Reflection absorption infrared spectroscopy?
Standard IR spec but where the IR beam is specularly reflected from the front face of a highly-reflective sample, such as a metal single crystal surface.
draw schematic of RAIRS
40
Pros and cons of using grazing angle for RAIRS
Good as you get a larger spot area but you do also get more noise
Describe surface dipole selection rule
When a molecule is adsorbed on a substrate, the molecule induces opposite image charges in the substrate. The dipole moment of the molecule and the image charges perpendicular to the surface reinforce each other. In contrast, the dipole moments of the molecule and the image charges parallel to the surface cancel out. Therefore, only molecular vibrational peaks giving rise to a dynamic dipole moment perpendicular to the surface will be observed in the vibrational spectrum.
If a spectrum shows that the molecules are orientated in a ‘standing’ position, what does this day about surface coverage?
Suggests that there is high coverage as they are probably standing due to no room for lying
why is EELS better than IR?
Gives much more info especially better in low frequency range because it utilises inelastic scattering of low energy electrons. Although, Since the technique employs low energy electrons, it is usually necessary to use UHV
Compare IR and EELS in a a table
41
Describe sum frequency generation
When is SFG not allowed?
When is SFG finite?
Sum-frequency generation (SFG) is a nonlinear optical process. This phenomenon is based on the annihilation of two input photons at angular frequencies A and B while, simultaneously, one photon at frequency C is generated.
- One input is visible (fixed) and one is IR (variable).
- SFG is not allowed in media with inversion symmetry (e.g. gas, liquid, fcc metal).
- SFG is finite when inversion symmetry is broken eg at an interface
In what ways must a vibrational mode be ‘active’ to be SFG active? Define these
Raman = change in polarizability IR = Change in dipole moment
draw a schematic energy diagram for SGF for CO
43
Application of RAIRS
44
What is polarisation modulation in RARS?
P polarised light will detect both surface adsorbed species and any molecules in solvent or gas above the
surface.
S polarised light will not detect surface adsorbed species, but will detect molecules in solvent / gas.
So measure: page 45
