Quiz3 Flashcards
What did Kirchoff do?
Kirchoff in 1814 noted that acids aid hydrolysis of starch to glucose.
What did Faraday do?
Faraday (and Davy) studied oxidation catalysts in the 1820’s.
What did Berzelius do?
Catalyst defined by Berzelius in 1836.
“A catalysts increases the rate of a chemical reaction without itself being consumed”
Reaction rate (formulas):
Chemical reaction:
A+B → C
Reaction rate:
r=kc_Ac_B
Arrhenius equation:
k = A*e^Ea/RT
A catalyst affects the activation energy Ea
Main processes catalysis:
1. Adsorption (reactants entering) 2. Rearrangement (of chemical bonds) 3. Desorption (product leaving)
Catalytic transport processes:
- charge transport
- energy transport
- heat transport
- mass transport
Catalytic surface reactions:
• Transport of reactants (gaseous/liquid phase) • Adsorption (dissociation) • Surface diffusion • Surface reaction • Surface diffusion • Desorption • Transport of products (gaseous/liquid phase)
Temp diff from catalytic oxidation of CO:
Requires about 700deg without 100deg with catalysis.
Ficks law:
The flux of the surface diffusion is given by Ficks law:
F = -D*dC/dx
Diffusivity D:
D = D0*e^-dE/kT
Langmuir-Hinshelwood mechanism:
Aad+Bad → ABad → ADg
COad+Oad → CO2ad → CO2g
Two different reactants enter the surface, reacts, leave as a new compound.
Rate passes a maximum and ends up at zero, when surface (S) covered by A.
Eley-Rideal Mechanism:
Ag + Bad → ABg
One reactant enters the surface, the second reacts with the first without entering the surface, leave as a new compound.
Rate increases until surface is covered by A
Mars - van Krevelen Mechanism:
A,ad + O,surf → AO,ad → AO,g
O2,g → 2O,ad → 2O,surf
C3H6 + 2O,surf → C3H4O + H2O + 2s(?)
O2,g + 2s (locations) → 2O,surf
The surface absorbs oxygen and binds it. An reactant enter the surface and oxygen is released to the reactant. They react and are released together.
The reaction to absorb a reactant must be “lagom”, why?
Sabatier principle
Too strong:
Takes over instead of the other reaction, between the reactants.
Too week:
Not enough happens.
Selectivity:
Some reactions have “multiple choices” and a catalyst can decide what product to get, what reaction to boost.
Ammonia synthesis:
When nitrogen and hydrogen is absorbed by an iron catalyst they can form ammonia 2NH3
Wilhelm Osswald
Fritz Harber
Carl Bosch
The Haber–Bosch process:
- N2 (g) → N2 (ad)
- N2 (ad) → 2 N (ad)
- H2 (g) → H2 (ad)
- H2 (ad) → 2 H (ad)
- N (ad) + 3 H(ad)→ NH3 (ad)
- NH3 (ad) → NH3 (g)
T = 400deg P = 300bar Catalyst = Iron F (K Al Ca O)
Most of the H comes from fossil fuels today
Could be gathered from water.
1.9 metric tons fossil CO2 are released per
metric ton ammonia produced!
2% of the global energy consumption!
Emission cleaning
Major pollutants are:
- Carbon monoxide, CO
- Nitrogen oxides, NOx
- Hydrocarbons, HC
- Particulate matter, PM
Reduction of NOx
Adding CH4 over a honnycomb platinum pipe
2NO2 + CH4 → N2 + H2O + CO2
Catalyst in cars (3):
- CO + O2 → CO2
- HC + O2 → CO2 + H2O
- NO + CO → N2 + CO2
Lambda sensor:
Controls the air/fuel sensor
Monolith catalyst
Ceramics
Chanel 1mm
washcoat partcle 20um
Noble metal particle 1-10 nm
Photocatalysis definition:
Photocatalysis:
“acceleration of a photoreaction in the presence of a catalyst”
Photocatalysis:
“Change in the rate of a chemical reaction or its initiation under the action of radiation (UV-VIS-IR) in the presence of a substance—the photocatalyst—that absorbs light and is involved in the chemical transformation of the reaction partners”.
Catalyst (thermal) vs photocatalyst:
Thermal catalyst
kT (0.03 - 0.1 eV)
Enhancement of reaction rate or the change of reaction path through interaction with catalyst surface.
Photocatalyst
hν (1 - 4 eV)
Generation of electrons and holes by excitation of photocatalyst and their electron transfer reactions.
(The photon is consumed and therefore adds energy to the reaction)
Photocatalysis energy transfer:
on TiO2 Surfaces
Energy transfer takes place through charge transfer
In regular catalyst:
Heat transfer, vibration, collision
Lifetime of the exciton (interacting e-h) depends on:
- Energy
- Effective mass (smaller overlap for lighter electrons)
- Interaction with the phonon bath (long!)
- Internal conversions (fs)
- Spin-orbit interactions (ps)
- …
This is how you can create EXCELLENT, semiconductors
PHOTOCATALYSIS, the elementary steps (3):
1. Light absorption and charge carriers generation 2. e-h pair separation, transport and recombination 3. Chemical work
STARK-EINSTEIN law:
“For each absorbed photon only one molecule is activated for reaction.”
1 Einstein=1 mol of photons=NA photons (6.022×1023 )
LAMBERT’s law:
Ix = I0 * exp(-alpha*x)
GROTTHUSS-DRAPER law:
“Light must be absorbed in order for a chemical reaction to take place”
Direct and Indirect band gaps:
Examples direct BG:
CdTe, CIGS, CZTS
Examples indirect BG:
Si, Ge, as well III-V materials
(direct better that indirect)
Generation and Recombination:
• In thermal equilibrium the net generation rate is zero.
• Each recombination process is associated with a lifetime, τ
• The presence of defects, level of doping and the band
gap- type (is it direct or indirect) determines what types of recombination are present and which one is dominant
• Reducing the rate of recombination processes is what
photocatalysis is ultimately about!
E-field:
direction or no direction
- Electrons are in constant random motion. When subjected to an electric field, the motion along field vector is superimposed on the random motion.
- Nett effect is that the electrons (and holes) drift in the direction expected from classical electromagnetism.
- In external field electrons and holes go in opposite directions!
Typical absorption depths in c:Si:
Crystalline Silicon has an indirect bandgap – this means it is not an efficient light absorber.
EG = 1.1 eV
→ / = 1100 nm
→ Thickness required ~ 100 µm
Charge carrier trapping:
• Depending on energy – shallow (just below the CBM or above the VBM) or – deep (in the band gap) • Depending on location – bulk (defects, dislocations) – surface (dangling bonds, adsorbates)
Where the fields come from?
- Light - sun
- Interface - material prop
- Defects - introduces gradients
- External - ex. electric field
particle plasmon 2 ways:
radiative decay:
photon
equilibrium
nonradiative decay: e-h pair Hot e- phonons equilibrium
TiO2 properties:
TiO2 is desirable for photocatalysis due to its:
• inertness, stability, and low cost.
• It is self regenerating and recyclable.
• Its redox potential of the H2O/*OH couple (-2.8 eV)
• lies within the band gap.
However, its large band gap (Eg=3.2 eV)
only allows absorption of the UV part of solar spectrum;
→ an absorber in the visible range is desired.
• Absorption in the visible range can be
improved by dye sensitization, doping, particle size
modification, and surface modification by noble metals.
Hematite:
\+Reasonably low band gap, 2.4 eV \+favourable valence band edge, \+high stability \+low cost \+Mars
- poor conversion efficiencies due to short minority carrier collection length (2-4-6? nm)
- to be overcome with nanostructured electrodes or ??? (M. Grätzel)
- GOOD for detailed mechanistic investigations (?)
H2O photodissociation:
“The absorption spectrum of the
system must overlap the emission
spectrum of the sun”.
CO2 reduction:
CO2 + H + 2e → HCOOCO2 \+ 2H + 2e → CO + 2H2O CO2 + 8H + 8e → CH4+ 2 H2O 2CO2 + 12H + 12e → C2H4+ 4 H2O 2CO2 + 10H + 10e → CH3OH+ 3 H2O
Why do we need nano for sunlight?
Electron with this energy (visible light) move in the nm length path.
UV cleaning:
UV cleaning does not add chemicals to the water. Kills and neutralizes everything. No bacteria resistance.
Electrical conductivity (sigma):
sigma = IL/A1/dU
I: current
Thermal conductivity (k)
k = PL/A1/dT
P: heat
Thermoelectric power (Seebeck coefficient, S):
S = (Uh-Uc)/(Th-Tc)
[V/K]
n-type & p-type:
- n-type conductors have negative electrons as majority carriers. Negative Seebeck. Donor states.
- p-type conductors have positive holes as majority carriers. Positive Seebeck. Acceptor states.
Positive aspects of thermoelectrics:
- Solid-state devices
- Simple set-up
- No moving parts
- No greenhouse gases
- High reliability
- Low maintenance
- Long lifetime
Combined TEG Systems Applications:
- Internal combustion/Thermoelectric
- Thermoelectric/Photovoltaic
- Fuel cell/Thermoelectric
- Thermoelectric/Electrocatalytic
- Self-powering devices (e.g. sensors, displays, etc.)
Thermoelectric figure of merit - ZT:
and basic physics for it
ZT is dimensionless and should be high
ZT = (S2sigmaT)/k
T: Absolute temperature (K)
S (alpha): Seebeck coefficient (V/K)
sigma: Electrical conductivity (Wm)-1
k: Thermal conductivity (W/mK)
k = ke + kL
Try to lower kL
lattice conduction
Definition of thermoelectric efficiency, TEG:
energy supplied to the load
/
heat energy absorbed at hot junction
Best material or compound for thermoelectric devices:
Bi2Te3 material
- Binary semiconductors are found among the group III, IV, V and VI elements
- Heavy elements reduce speed of sound
- Mass fluctuation increases phonon scattering
Summary of part 1 (thermoelectricity):
- Size of band gap to guides us to temperature of operation (10 kBT rule)
- Carrier tuning to achieve the best electronic performance.
- Lattice thermal conductivity reduction through lowering the acoustic phonon velocity (heavy elements)
concepts in thermoelectricity:
- Nanosizing
- Phonon-Glass Electron-Crystal (PGEC) concept
- Nanoinclusions
Lowering the thermal to electrical
conductivity ratio:
In Semiconductors, due to low charge carrier concentration, most of the heat is transported by phonons (lattice vibrations). Which provides an opportunity to improve the TE efficiency by reducing the lattice thermal conductivity using phonon scattering mechanisms.
”New” strategies for improving bulk:
a) A polycrystalline microstructure.
b) Preferential alignment of grains along
favorable transport directions.
c) Reduced grain size to take advantage
of favorable interfacial scattering
processes.
d)-f) Nanocomposites:
d) Nanocoated grains
e) Embedded nanoinclusions
f) Lamellar/multilayer structures.
Enhanced Seebeck by energy filtering:
• Metallic nanoinclusion in semiconductor matrix results in bandbending at interface • Generates potential that preferentially scatters low energy charge carriers and enhances S.
Improved Seebeck by energy filtering
The Seebeck coefficient is related to the
scattering time t through the Mott relation,
where n is the electronic group velocity.
“Summary” of ways to reduce k:
• Heavy elements reduce velocity of phonons vp ~ m-1/2
, kL = 1/3·C·lp·vp
• Complex structures have relatively fewer acoustic phonon modes Þ low kL
• Mass fluctuation scattering of phonons
• ”Atomic rattling” scattering of phonons
• Grain boundary scattering of phonons
• Interface scattering of phonons
• Energy filtering by nanoinclusions
• (Nano)structuring over wide length scale makes efficient phonon scattering
over wide energy range
Summary of part 2 (thermoelectricity):
- Lattice thermal conductivity reduction through combination of structuring over wide range of length scales
- Extrinsic effects of grain boundaries, heterostructured interfaces and energy levels need to be controlled to improve zT
- Elaborate synthesis methods, detailed structural, electrical and thermal characterization, and theoretical modelling need to be combined to improve understanding
Summary (thermoelectricity):
• Large progress has been made in thermoelectric
research over the past 20 years.
• Offers a stimulating interdisciplinary research area with many scientific and engineering challenges.
• Thermoelectric heat recovery is becoming a viable route for automotive applications.
• Sufficiently efficient AND sustainable compounds are still lacking, but promising candidates exist.
What is a Fuel Cell?
A Fuel Cell is a “factory” that converts chemical energy (fuel) to electricity (and heat). Delivers electricity as long as fuel is supplied (continuous operation).
Chemical Energy → Electrical Energy
What is a Battery?
A Battery is a “factory” that converts chemical energy to electricity (and heat). Delivers electricity as long as there are reactants in the battery (batch operation).
Chemical Energy → Electrical Energy
Sustainable Energy and Fuel Cells
Dream scenario:
• Renewable energy sources (Solar, Wind etc.) • Clean fuel production • Easy distribution and storage • Clean, high efficiency energy conversion
Fuel Cells, batteries, and IC engines:
Combustion engine: \+ Mature technology \+ High energy and power density \+ Low price \+ Lifetime – Low efficiency
Battery: \+ Very high efficiency – Low energy density – Lifetime? – Price?
Fuel Cell \+ High efficiency \+ High energy and power density – High price – Lifetime?
Energy density:
Volumetric energy density is often more important than Gravemetric energy density. Especially for transport applications.
How does a Fuel Cell/Battery work?
Combustion: 1. Molecules collide (H2 and O2) 2. Bonds are broken/formed • H-H and O-O bonds are broken • H-O bonds are formed • Redistribution of electronic charge 3. Product is formed
- Takes place on the order of ps (10-12)
- Energy difference is released as heat
H2 → Electricity:
By spatially separating reactants it is possible to harness the electrons from one reactant and let them do work before they are recombined with the other reactant and forms the product.
H2-O2 FC:
Lead-acid battery:
𝐻2 + 1 /2 𝑂2 → 𝐻2𝑂
𝑃b + 𝑃b𝑂2 + 2𝐻(2)𝑆𝑂4 → 2𝑃b𝑆𝑂4 + 2𝐻2𝑂
Li-ion batteries:
Li-ion technology represents a family of batteries with different materials and electrochemical reactions. There is no metallic Li here! Li-ions move between the electrodes where it is intercalated or chemically bounded.
Positive electrode (cathode): • LiCoxO2 • LiNixMnyCozO2 (NMC) • LiMn2O4 (LMO) • LiFePO4 (LFP)
Negative electrode (anode): • Graphite • Li4Ti5O12 (LTO) • Hard Carbon • Ti/Sn-alloy • Si/C
Types of Fuel Cells:
- PEMFC – Polymer Electrolyte Fuel Cell
- PAFC – Phosphoric Acid Fuel Cell
- AFC– Alkaline Fuel Cell
- MCFC – Molten Carbonate Fuel Cell
- SOFC – Solid Oxide Fuel Cell
- BFC – Biological Fuel Cell
Possibilities of BFC (bio):
• Self Sustaining Robots:
flies
• Implants:
electricity from compounds in the blood
• Wastewater treatment:
Cleaning and producing electricity
Running a FC in “reverse” – Electrolysis:
At times in some areas, there is surplus of electricity. With electrolysis, this excess can be stored in the chemical bonds of the produced chemicals (fuels)
Fuel Cells vs. Batteries
FC: • Includes only the “reaction zone” • Requires fuel • Simple chemicals • Complex reactions • Safety!
Battery: • Includes reactants and products • Stand alone unit (closed system) • Needs to be replaced/recharged • Complex chemicals • Simple reactions • Safety!
More complementary than competing
Nanotechnology for Fuel Cells and Batteries:
• Reducing component size → Increases capacity → Reduces amount of materials needed • Detailed analysis → Accurate descriptions → Increases understanding
Cheaper and more efficient devices
Optimization of Fuel Cells
The fuel cells has a high theoretical efficiency, but it has not been realized yet.
- Activation losses
- Ohmic losses
- Mass transport losses
The Fuel Cell Stack:
- Current depends on the total area of all cells connected in parallel
- Voltage depends on the number of cells connected in series
- Power = Current ∙ Voltage
N&N challenge for electrolyte (?):
High catalyst surface area:
• Good mass transport properties (gas and water)
• High electrical conductivity
• High proton conductivity
N&N for battery development – examples
• Silicon nanowires: capacity • CuO nanorods: capacity • Si/C nanoparticles: stability
Summary (Fuel cells):
• Fuel Cells are promising candidates for energy conversion in a sustainable energy system
– High power density
– No harmful emissions
• With N&N Fuel Cells and Batteries (has been and) will be improved
– More efficient catalyst structures
– More efficient catalyst materials
– Smaller scale → Higher power density and cheaper
systems
– N&N tools are valuable in characterization
