Part 2 - Nanocatalysts. Supported metal particles - Surface investigation Flashcards
Give examples of metals with a hcp-structure.
Co, Ru, Re, Os
Draw the different adsorption sites for fcc (100), (110) and (111) planes.
See slide 19, lecture 2, part 2.
Give a basic description of the Wood notation for superlattices.
2x2 would describe a superlattice where each lattice point has twice the length between them as the underlying lattice. c(2x2) would describe a lattice described above, but with another atom in the center of this lattice. This can then also be described as sqrt(2)xsqrt(2) (R 45 deg), as it also makes up a lattice that is turned 45 degrees wrt the underlying lattice, with lattice vector length sqrt(2).
In ammonia synthesis, using Fe as a catalyst, arrange the crystal faces from highest to lowest activity.
111»_space; 100 > 110
How does the cyclohexane hydrolysis/dehydrogenation reactions depend on particle size?
For increasing particle size, the dehydrogenation increases and the hydrolysis decreases. The reason being the former is favored by low index planes and the latter is favored by corner and edge sites, which decreases relative to planes.
What is the definition of dispersion in catalysis?
Fraction of surface atoms of a metal in a catalyst. D = N_s / N_t
What is the relation between the dispersion and mean particle size?
D = 6 * (v_m / a_m) / d_va
where v_m is the volume of one atom in the bulk phase, a_m is the area one atom occupies on a polycrystalline surface and d_va = ∑n_i (d_i)^3 / ∑ n_i (d_i)^2 = mean particle size.
What is the definition of chemisorption?
Electronic interaction (chemical bonding) between molecule and surface.
What is associative chemisorption?
Molecular adsorption. Normally non-activated.
What is dissociative chemisorption?
Breaking of bonds upon adsorption. Normally activated with activation energy Ea
What is physisorption?
Weak physical interaction (vdW) between molecule and surface, but no bonding.
Give a list of techniques we can use to determine particle sizes.
- Electron microscopy (direct observation)
- XRD (line broadening analysis)
- SAXS
- XPS (ratio between surface concentration of support component and active phase)
- Magnetic methods
- Chemisorption of probe molecules
What is selective chemisorption?
A technique we can use to measure the accessible catalytic component on the surface by probing the amount of gas adsorbed per unit weight of catalyst metal
What is the Scherrer equation?
For XRD-measurements, the Scherrer-equation can give information on the particle size.
L = K lambda / beta cos theta
L is the crystallite size, beta is the full width at half maximum of the peak, lambda is the incident x-ray wavelength, theta is the Bragg angle, and K is a constant ≈ 1.
What group of materials are commonly catalytic active?
Transition metals, due to their partially filled d-bands and possibility of having more than one oxidation state.
