Stuart Taylor

Question 1

What is a VOC?

Answer 1

The term VOC refers to a wide-ranging class of compounds

Definition by the US Environmental Protection Agency (EPA):

“.…..Any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metal carbides or carbonates and ammonium carbonates which participates in atmospheric photochemical reactions…..”

Basically, any organic compound with vapour pressure (volatility) exceeding 0.1 mmHg under standard conditions (STP = 25^oC and 1 atmosphere) may be regarded as a VOC.

Equates to organic compound with boiling point < 250 °C at STP.

EPA classify over 400 compounds as VOCs. E.g. Hydrocarbons; Alcohols; Aromatics; Carboxylic acids; Esters; Ethers; halogenated compounds; organo-sulphur compounds

Question 2

What are the sources of VOCs?

Answer 2

1. Natural: Swamps, Forests, Volcanic activity, animals and insects. VOC emissions are natural
2. Anthropogenic (man-made): chemical and processing industries, manufacturing industries, vehicles, solvent (paints, adhesives).

Cannot easily control natural sources but it is possible to influence man made emissions.

Question 3

What is the scale of VOC emissions? (LARGE SCALE)

Answer 3

In 2004 estimates of VOC emission from US were 20,000,000,000 kg pa. Worldwide estimates are at least twice US figure. Emissions are on a vast scale. VOCs emitted from a wide range of areas.

• 28% solvent use
• 38% transport
• 3% forests
• 10% fossil fuels

Question 4

What are the environmental problems caused by VOCs?

Answer 4

1. ozone depletion
2. Greenhouse effect
3. Photochemical smog/formation of low-level ozone
4. Toxicity

Question 5

Discuss Ozone depletion:

Answer 5

Consider the photochemical reactions of O2 in the atmosphere; all models predict the concentration profile of O3. Predicts higher concentration of O3 in the stratosphere rather than the actual concentration.

Why is this so?

Reactive species present in the atmosphere can catalytically remove ozone:

Question 6

Which catalysts can cause ozone depletion?

Answer 6

Chloro Fluoro Carbons: CFCs

Now banned but were used as refrigerants and aerosol propellants. Homolytic fission of C-Cl bond by UV process gives active chlorine radicals.

hv

CF₃Cl –> CF₃• + Cl•

Takes place more readily: In higher atmosphere (stratosphere), and over the earth’s poles. Lifetime of CFCs in atmosphere is 100s of years.

Question 7

How do VOCs contribute to the greenhouse effect?

Answer 7

Radiation from the sun is absorbed by the earth. Earth re-radiates the energy as infrared radiation. Molecules in the atmosphere absorb in the IR and heat the atmosphere. Natural process with molecules such as H₂O, CO₂ and CH₄ contributing. Steady state temperature attained, the process is one of the main factors leading to development of life on Earth. Manmade emission of greenhouse gasses disturbs the equilibrium. Emissions have dramatically increased in recent years.

• Predicted temp rise if nothing changes?*
• 50 years: 0.6-2.5°C*
• 100 years: 1.6-5.6°C*

Question 8

What is GWP and how is it related to VOCs?

Answer 8

VOCs are very potent greenhouse gases. Molecules in the atmosphere have been allocated a Global Warming Potential (GWP). Based on their ability to:

• absorb IR and
• their lifetime in the atmosphere
• VOCs have much greater GWP than CO₂. Some VOCs have GWP 1000s times greater than CO₂. CO₂ emissions receive a lot of attention, but VOC emissions also significantly contribute to global warming. CO₂ readily absorbs IR, however it has a poor lifetime in the atmosphere. CH₄ has the effect of a lot less carbon dioxide.
• Compound*
• GWP*
• CO₂*

1

CH₄

88

CCL₃F

7020

Question 9

Discuss how VOCs contribute to Photochemical smog/formation of low-level ozone

Answer 9

Formed from VOCs and NOX (Discussed later with NO_X control)

Question 10

Why are VOCs toxic?

Answer 10

Many VOCs are simply toxic when released into the atmosphere. Examples:

1. Benzene is carcinogenic, even low levels can have serious health effects.
2. Formaldehyde very common, new car and new carpet smell. Toxic and can create indoor health problems.
3. Polyaromatic hydrocarbons (PAHs), released from fuel combustion, toxic and involved in air borne formation of particulates (e.g. naphthalene).

Question 11

How can we control VOC emissions?

Answer 11

VOC emissions subject to ever stricter legislation. 1990 EPA clean air act in the US called for a 90% reduction by 2010. In Europe similar targets agreed in the Gothenburg Protocol, current emissions will be reduced by 40% by 2010. Worldwide agreements, e.g. Kyoto, are also in place. How are we doing? Difficult to find figures!

Question 12

How can we reduce VOC emissions?

(2 points?)

Answer 12

Several technologies are available:

1. Thermal oxidation,
2. catalytic oxidation

Question 13

What is thermal inscineration of VOCs? How does it work?

Answer 13

Air stream containing VOCs is mixed with supplementary fuel and incinerated in a flame. Temperatures in excess of 1000°C are achieved. VOCs are mainly oxidized to CO₂ and H₂O. Oxidation process takes place by a complex network of gas phase radical reactions. Incineration process has to be carefully controlled and monitored.

Question 14

What are some potential problems of thermal inscineration of VOCs?

Answer 14

Conversion to CO₂ may not be 100%. Many possible organic species produced. Toxic by-products (partially oxidized products), such as phosgene (COCl₂), dibenzofurans and dioxins are often produced. High temperatures used lead to N oxidation to NO_X. The use of excess supplementary fuel increases the economic operating costs. Process is popular because it is simple and low tech to operate.

Question 15

What is catalytic oxidation? Discuss any pros and cons?

Answer 15

The introduction of a suitable catalyst: Decreases the temperature required (lowering the activation energy of the reaction!) to oxidise VOCs to CO₂ and H₂ O. Increases the selectivity to CO₂; reduces/ eliminates the harmful organic by-products.

Further benefits from the lower temperature: Less fuel required, minimal formation of NO_X.

Compared to incineration:

• Cleaner
• Cheaper to operate

Question 16

What types of catalyts are used for catalytic oxidation?

Answer 16

Catalysts must: Have high activity for VOC oxidation to CO₂. Show stable activity with no appreciable deactivation over a prolonged period.

Two classes of catalysts used:

1. Metal oxide based or
2. precious metal based

Question 17

Discuss properties and examples of metal oxides as catalyts for catalytic oxidation of VOCs

Answer 17

Most active catalysts are based on:

• Transition metal oxides
• Mixed metal oxides

Example: Oxidation of naphthalene – don’t need to remember

Constructing a ‘light off’ curve – increase temperature and measure activity

The most active catalyst will get to a high yield at lowest temperature

♦ CeO₂ (urea), ● MnO_x, ■ CoO_x, ▲ CuO_x, ◊ Fe₂O₃, x CeO­₂ (carbonate), ○ ZrO₂, ∆ TiO₂, □ Al₂O₃: GHSV = 60000 h^-1, naphthalene (C₁₀H₈) = 100 ppm in air

Question 18

How is the performance of a catalyt for catalytic oxidation expressed?

Answer 18

a. Conversion: how much of the VOC is converted, the higher the conversion more active the catalyst.
b. Rate: many different units, e.g. mol g_cat^-1 s^-1, higher the rate the more active the catalyst.
c. Yield of CO₂: higher the yield the greater VOC oxidised to CO2, higher the yield the more active the catalyst.

Question 19

Discuss general supports of metal oxide catalyts and the advantages and disadvantages of these:

Answer 19

Catalysts may be formed from oxide alone or more usually on a support to increase active surface area. Metal oxides are cheap so there is a large driving force for making use of these.

Typical supports:

• Alumina, Al₂O₃, γ, Δ, θ phases, surface area 100-300 m² g^-1
• Silica, SiO₂, e.g. fumed silica, surface area 200-400 m² g^-1

Advantages/disadvantages

• Generally, metal oxides are more tolerant to deactivation by poisoning.
• Generally, have lower activity than precious metal catalysts.

Question 20

CASE STUDY: Example of understanding metal oxide VOC oxidation catalyst

Nanocrystalline cobalt oxide. Catalyst of small crystallites of Co₃O₄ is very active for total oxidation of hydrocarbons. Propane conversion to CO₂. Propane is one of the most difficult to convert and is very in vogue. 5% platinum is a very high amount so very expensive.

QUESTION: Why is nano Co₃O₄ so active?

1. How is it prepared?

Answer 20

Need to understand structure and features of the catalyst–> structure-activity relationship.

Preparation

• Cobalt nitrate and ammonium bicarbonate (NH₄HCO₃) ground together. We want to make metal oxide cat.
• Material dried at 100 °C for 16 hours (precursor).
• Precursor calcined (heated in air) at 400 °C for 4 hours–> catalyst.

Question 21

CASE STUDY: Example of understanding metal oxide VOC oxidation catalyst

Nanocrystalline cobalt oxide. Catalyst of small crystallites of Co3O4 is very active for total oxidation of hydrocarbons. Propane conversion to CO2. Propane is one of the most difficult to convert and is very in vogue. 5% platinum is a very high amount so very expensive.

QUESTION: Why is nano Co3O4 so active?

2. How is X-ray diffraction used to deduce the crystal structure?

Answer 21

Powder X-ray diffraction (XRD)

Sample must have crystalline order. Because of the crystal structure, we get a powder diffraction pattern.

Bragg law: nl= 2 d sinq

n = order of diffraction (1, 2, 3, etc.)

l = X-ray wavelength

d = inter planar spacing

q = diffraction angle

From d-spacing and relative intensity can identify phases present.

Question 22

CASE STUDY: Example of understanding metal oxide VOC oxidation catalyst

Nanocrystalline cobalt oxide. Catalyst of small crystallites of Co3O4 is very active for total oxidation of hydrocarbons. Propane conversion to CO2. Propane is one of the most difficult to convert and is very in vogue. 5% platinum is a very high amount so very expensive.

QUESTION: Why is nano Co3O4 so active?

2. How is X-ray line broadening used?

Answer 22

X-ray line broadening:

Diffraction peaks broadened as a function of crystallite size. Smaller crystals = broader peak. Described by the Scherrer equation. Don’t need to know these calculations

average crystallite size/Å = 0.9l/B cos q

l = radiation wavelength/Å

B = line broadening constant

q = diffraction angle

B is derived from the peak full width at half maximum (FWHM) by:

B² = B_u² - B_s²

B_u = FWHM for unknown in radians

B_s = FWHM for standard in radians

Standard is a highly crystalline sample (>1000Å crystallite size).

Question 23

CASE STUDY: Example of understanding metal oxide VOC oxidation catalyst

Nanocrystalline cobalt oxide. Catalyst of small crystallites of Co3O4 is very active for total oxidation of hydrocarbons. Propane conversion to CO2. Propane is one of the most difficult to convert and is very in vogue. 5% platinum is a very high amount so very expensive.

QUESTION: Why is nano Co3O4 so active?

3. How is XRD used?

Answer 23

Phases: highly active catalyst and commercial catalyst are both Co₃O₄ with the same phase.

• Difference of activity is not due to different cobalt oxide phases. Mixed oxidation states. Difference in their activity not solely due to their composition.

Line broadening: highly active catalyst has broad peaks, commercial has narrow peaks. High activity Co₃O₄ has very small crystallites (named nanocrystalline, low activity commercial Co₃O₄ has very large crystallites.

• Small crystallites seem important.

Heterogeneous catalysis is surface process, so catalyst surface area has a role. Geometric surface areas are very different. Surface areas are very important for heterogenous catalysis. This is where all the chemical reactions take place, we are talking about surface phenomena.

• Catalyst*
• Crystallite size/nm*
• High activity Co₃O₄*

12

Commercial Co₃O₄

>1000

Question 24

CASE STUDY: Example of understanding metal oxide VOC oxidation catalyst

Nanocrystalline cobalt oxide. Catalyst of small crystallites of Co3O4 is very active for total oxidation of hydrocarbons. Propane conversion to CO2. Propane is one of the most difficult to convert and is very in vogue. 5% platinum is a very high amount so very expensive.

QUESTION: Why is nano Co3O4 so active?

4. What is BET surface area and how is it used? - What is the equation?

Answer 24

Determined by measuring the nitrogen adsorption isotherm (amount adsorbed verses relative pressure) at 77 K. IUPAC classification of isotherms. This is how we can measure surface areas. Isotherms take place at constant temperature. We are putting gas on a sample and measuring the amount absorbed. We add more pressure and look at the difference again. Type II is a BET isotherm, type I is the Langmuir isotherm.

BET (Brunauer Emmett & Teller) isotherm is type II. Isotherm described by BET equation.

P = equilibrium pressure

Po = saturation pressure of adsorbate

Va = volume adsorbed

VM = volume of monolayer

C = BET constant

Form of a straight line (y = mx +c), plot P/Va(Po-P) against P/Po (over the range 0.05-0.35).

Question 25

What does the graph look like when the following BET equation is plotted?

P = equilibrium pressure

Po = saturation pressure of adsorbate

Va = volume adsorbed

VM = volume of monolayer

C = BET constant

How is surface area calculated from this and what are the approximate values of Co₃O₄?

Answer 25

Form of a straight line (y = mx +c), plot P/Va(Po-P) against P/Po (over the range 0.05-0.35).

Question 26

Is the increased activity due to surface are only? How can we find out?

Catalyst

Surf. area/m² g^-1

Nano Co₃O₄

160

Commercial Co₃O₄

4

Answer 26

1. Calculate the reaction rate per unit area.

If the increased activity was due to higher surface area only, the surface area normalised rates would be the same.

Nano Co₃O₄ rate per m^-2 is much higher than commercial Co₃O₄.

• Some other effect of nanocrystalline CO₃O₄ is controlling catalyst activity.

Need to consider catalyst mechanism

Mars-van Krevelen mechanism is common for oxidation reactions.

• VOC oxidised by O^2- (oxide anion) of the metal oxide
• As O^2- is removed from the metal oxide it becomes reduced
• Molecular oxygen (O₂) from the gas phase reoxidises metal oxide catalyst

Mechanism is a redox cycle. Key feature: oxygen from catalyst lattice ends up in oxidised products. The ease that O^2- can be removed from metal oxide lattice is important for activity. Temperature Programmed Reduction probes ease of O^2- removal. Pulling oxygen out of the catalyst. Oxygen then is incorporated from the atmosphere into the catalyst. Redox behaviour is critical.

Mars-van Krevelen mechanism

Question 27

What is Temperature Programmed Reduction?

How does it work?

Give an example

Answer 27

Common technique for heterogeneous catalysis research. Relatively quick and easy, actually measures properties of the catalyst. Measuring catalytic efficiency. Used widely in heterogenous catalyst research. We are measuring a property of a catalyst instead of observing a catalyst via spectroscopic techniques. We are probing how easy it is to remove oxygen from the lattice, and this relates to the first step of the mechanism above.

How does it work?

• Powdered catalyst packed into a tube
• Catalyst sample in a flow of H₂/Ar; Ar is diluent (reducing atmosphere)
• Tube with catalyst placed in a furnace and temperature increased in a linear manner (start around RT, change to around 5-10 degrees/min up to several hundred degrees)
• Measure amount of H₂ consumed (detector signal) as a function of temperature

A metal oxide is reduced

MO(s) + H2(g) –> M(s) + H2O(g)

1. Reduction temperature: tells you how easily sample is reduced (low temp easy, higher temp more difficult). We get the temperature at which the reduction takes place. We are using the oxygen within the lattice to oxidise the hydrogen instead of using an organic compound.
2. Signal size: extent of reduction, peak area relates to amount of H₂ reacted (large peak a lot of reduction and vice versa).

_Example_ – copper(II) oxide CuO. Reduction temperature around 275 °C. Reduction take place in a single step (Cu²⁺ à Cu⁰) Process taking place:

• CuO_(s) + H_2(g)* à Cu_(s) + H₂O_(g)
• Cu²⁺ + 2e^-* à Cu⁰

Question 28

Why can TPR be more complicated?

Answer 28

Reduction process can be more complex.

• There can be many peaks, e.g. Iron(III) oxide Fe2O3:*
• 3Fe₂O_3(s) + H_2(g) –>* 2Fe₃O_4(s) + H₂O_(g)
• Fe₃O_4(s) + H_{2(g) –>}* 3FeO_(s) + H₂O_(g)
• FeO_(s) + H_2(g) –>**Fe_(s) + H₂O_(g)*
• Process: Fe³⁺ –>* Fe^3+/2+ –> Fe²⁺ à Fe⁰

Question 29

What does the TPR look like for nanocrystaline Co₃O₄

Answer 29

Nano Co3O4 (a) shows low temperature peak (ca. 100 °C), other Co3O4 inactive samples (b) and (c) do not show peak. Nano Co3O4 has highly reactive catalyst lattice oxygen species – likely to be O2-. Often these reactive species are related to defects in the metal oxide lattice. This is very low temperature. Very active species has very reactive oxygens species present on the catalyst, however fewer active species have less active oxygen species that can’t do the reaction at this temperature

Question 30

What types of lattice defects can occur?

Answer 30

1. Point defects: MIssing cation or anion
2. 2D defects

Question 31

Discuss what point defects are:

Answer 31

1. Point defects: Missing cation or anion

• In metal oxides oxide anion (O^2-) is often lost to create an oxygen vacancy.
• To preserve charge neutrality one or more metal cations are reduced.
• e.g. For TiO₂: Loss of O^2- leads to 2 Ti⁴⁺ being reduced to Ti^{3+ (often called} TiO_2-X).*
• Can create oxygen vacancies in an oxide by doping (adding small amount of another metal ion).
• e.g. Introducing Cu²⁺ into CeO₂ lattice.*
• Substitution of Ce⁴⁺ by Cu²⁺ means that oxygen vacancy is created to preserve charge neutrality (one O^2- lost for each Cu²⁺ added).
• Point defects on surface, so as surface area increases more defects of this type.

Question 32

What are two dimentional defects?

Answer 32

• Atoms/ions on surface have a lower coordination number than they ideally like, they are coordinatively unsaturated. Surface has higher energy than the bulk and is therefore reactive.
• Surfaces are higher energy than the bulk
• This example has a rock salt structure (ex. FeO)

Ions at edges and corners are even more coordinatively unsaturated and are more reactive. These edge and corner ions are called 2 dimensional defects. Number of these defects increase as particle size decreases. Think about the surface to volume ratio (S:V). The more of the lower coordination sites the easier. More and more reactive species as surface:volume increases.

Particle size decreases

• Number of surface sites increases rapidly
• Smaller particles give more corner and edge sites
• Increased number of high energy surface defect sites.

Question 33

How do more defect sites promote catalysis?

Answer 33

• Promote catalyst redox properties: Lattice oxygen is more easily removed and can react (remember the Mars-van Krevelen mechanism).
• Defects provide surface sites to adsorb hydrocarbon VOC.
• Defects help promote reoxidation of the catalyst – in some cases this can be rate determining step.

e.g. 2M⁰ + O_2(s) –> 2M²⁺O^2-

Reduced oxidised

Question 34

Discuss prescious metal catalysts and usual supports, characteristics:

Answer 34

• Mostly: Platinum, Palladium
• Tend to be more expensive than metal oxides, but they are more active catalysts.
• Can be susceptible to deactivation by poisoning by S, Cl and/or other metals.
• Precious metal catalysts consist of the metal dispersed on a support.
• Maximise the amount of metal exposed

Usual supports:

g-Al₂O₃ surface area – ca. 200 m² g^-1

SiO₂ surface area - ca. 350 m² g^-1

Produces highly dispersed small metal particles.

Transmission electron micrograph of supported metal catalyst

• High metal surface area and many active sites.
• Interaction of metal particles with support modifies properties of the metal.

Benzene combustion over Pd/VO_X/TiO₂. Light off curve. Most activity due to presence of Pd.

Pd/VO_X/TiO₂ solid lines; VO_X/TiO₂ dashed line

Catalyst mechanism. Hydrocarbon oxidation over a palladium catalyst: Complete oxidation occurs via a surface redox cycle known as the Mars-van Krevelen mechanism. We need to assume this is all Pd does.

Question 35

What is the Mars Van Krevelen mechanism?

Answer 35

Question 36

What features make a good supported Pd Catalyst?

Answer 36

Pd good catalyst because it adsorbs hydrocarbons and oxygen well and can undergo redox cycle relatively easily.

1. Metal dispersion
   - Want high metal surface area
   - Metal should be accessible
2. Metal electronic properties (don’t worry about this)
   - Ability to donate/accept electron density
   * Both can be influenced by:*

• Preparation methods (heat treatment, metal precursor).
• Choice of support (Prof Golunski metal-support interactions, SMSI).
• Addition of other components.

Example of making a better supported Pd catalyst. Performance of supported Pd can be enhanced by addition of metal oxides. E.g. Pd/TiO2 modified by tungsten oxide

Question 37

Discuss Propane conversion by 0.5Pd/xWO_X/TiO₂

Answer 37

• Very low loading of Pd (0.5 wt%)
• Adding WO_X improves conversion
• Conversion increases with WO_X content

Catalyst structure complex, determined:

Powder X-ray diffraction, Temperature programmed reduction, Laser Raman Spectroscopy, X-ray photoelectron spectroscopy, Transmission electron microscopy

TPR and XPS:

• With W present only Pd²⁺ on surface, without W mix of Pd⁰ and Pd²⁺.
• With W a lot more easily reducible species are present (more than can be accounted for based on Pd alone).

Laser Raman

• Highly dispersed monotungstate and polytungstate species

High resolution TEM

Very useful, state-of-the-art

Question 38

How do WO_X species decorate the periphery of Pd particles, synergy between W and Pd. Develop a model of the catalyst:

Answer 38

Synergistic interaction at WO_x/Pd interface gives rise to increased activity.

Question 39

How are VOC oxidation catalysts used?

Answer 39

Catalysts located in a reactor towards the end of polluting Process. Used in a fixed bed reactor.

• catalyst secured in place
• Heating supplied from external furnace

To avoid excessive pressure, drop the catalyst is often supported on a honeycomb ceramic support called a monolith

Question 40

What are some other technologies for VOC control?

Answer 40

1. Adsorption
2. Absorbtion
3. Condensation

Question 41

Discuss Adsorption for VOC control:

Answer 41

Remember adsorption is the interaction of molecules at a surface. VOCs are removed by passing the effluent through a bed of solid adsorbent.

• VOCs interact strongly and are adsorbed onto the surface.

Interactions are strong and VOCs retained for long period. Different types of adsorbents are used:

• Silica, SiO₂
• Alumina, g-Al₂O₃
• Activated carbon
• Zeolites

Adsorbents must have large adsorption capacity. Activated carbon is used most widely, surface area >1000 m² g^-1

Compared to oxidation processes:

Advantages

• No fuel costs
• Removes VOCs without producing greenhouse gas
• Can recover VOCs, may be economical for higher VOC concentrations

Disadvantages

• More difficult to remove VOCs to low levels
• Large volume of waste produced, have to dispose adsorbent in landfill site

Question 42

Discuss Absorption for control of VOCs

Answer 42

This is not a surface process. Absorption of VOCs from a gas stream can be absorbed into:

• Solid
• Liquid

Absorption into a liquid is most common.

Process takes place in a reactor vessel shown schematically

• Reactor packed with bed of inert plastic material.
• Absorbing liquid is flowed over inert packing to form thin liquid film.
• Gas stream containing VOCs is flowed through the reactor.

Rate of VOC absorption into the liquid solvent is diffusion controlled. As VOC content in solvent increased eventually solvent becomes saturated. To continue absorbing need fresh solvent.

Disadvantages

• Cost of large solvent volume
• Careful choice of solvent so it is not too volatile adding to emissions
• Large volume of liquid waste

Question 43

Discuss condensation as a method to control VOCs:

Answer 43

Cooling effluent stream to temperatures below the dew point of the VOCs results in their condensation (liquefaction). Cryogenic condensation is now most common and uses liquid N₂ at -196°C. Heat exchanger is used:

1. Coolant flows through thin pipes covered with fins.
2. VOC stream flows in opposite direction to coolant around the outside of pipes.
3. Liquid VOCs collected.

If VOC freezing point too high, problems with freezing on heat exchangers à reduces efficiency. Use two condensers in parallel one operating while the other defrosts.

Advantages

• Complete recovery of VOCs
• Universal process for all VOCs

Disadvantages

• Only works with low flow rates of gas
• May still have to destroy VOCs
• Very expensive to operate

Question 44

What are nitrogen oxides?

Answer 44

What is NO_x? Mix of nitrogen oxides, oxides of nitrogen are numerous: N₄O, N₂O, NO, NO₂, N₂O₄.

At high temperature (>1000 degrees):

N₂ + O₂ –> 2NO

On cooling, some of the NO is oxidized further:

2NO + O₂ –> 2NO₂

The mixture of nitrogen oxides is often called NO_x and contains mainly NO and NO₂.

Question 45

What are the sources of nitrogen oxides?

Answer 45

Natural:

1. Volcanic activity
2. Electrical storms
3. Bacteria

Anthropogenic sources

1. High temperature combustion processes:

• Stationary sources: power generation, gas turbine incineration
• Mobile sources: motor vehicles

1. Processes using nitric acid and nitrates

• Nuclear reprocessing industry
• Metal processing industry

Question 46

What are the environemental problems of NOx?

Answer 46

1. Low level ozone/smog formation
2. Acid rain
3. Toxicity

Question 47

Discuss how NOx forms low level ozone/smog formation:

Answer 47

1. NOx pollution results in formation of ground level ozone and consequently smog. Ozone is formed by the reaction of atomic oxygen with molecular oxygen:

O2 + O –>O₃

Natural processes occur in the stratosphere. NO₂ is active in photochemical reactions.

NO2 –> NO + O

This monoatomic oxygen can then form with O₃. NO also reacts with O₃.

NO + O₃ –> NO₂ + O₂

Combining these reactions, there is no net formation of ozone as it is formed and then destroyed.

• How is NO_X involved in the process of O₃ formation?*
• O₃ is formed by the reaction of NO with VOCs.

RCH₃ + 2NO + O₂ –> RCHO + 2NO₂ + H₂O

• Reaction cycle leads to O₃ formation.

Increased formation of NO₂, which leads to O₃ formation. Decreased concentration of NO, which is normally destroying ozone and regulating the levels. Also leads to partially oxidised hydrocarbons which are generally more toxic.

Process takes place at/close to ground level to produce toxic atmosphere (smog).

Question 48

Discuss how NOx leads to acid rain:

Answer 48

Rain water is slightly acidic (pH 5.5) due to the presence of CO₂ in the atmosphere.

CO₂ + H₂O à H₂CO₃ Carbonic acid

NO_X in the atmosphere reacts with atmospheric water and oxygen.

4NO + O₂ + 2H₂O à 4HNO₂

Nitrous acid

4NO₂ + O₂ + 2H₂O –> 4HNO₃ Nitric acid

pH of rainwater is decreased and this is called acid rain. Acid rain has caused particular problems in northern Europe (Scandinavia) and Eastern provinces of Canada.

Acid rain has had a devastating effect on the biosphere:

• Acidification of lakes killing fish
• Acidification of soils killing vegetation
• Increased rates of weathering

Question 49

How does NOx lead to toxicity?

Answer 49

• NO_X can cause respiratory irritation

Question 50

How can we reduce NOx emissions?

Answer 50

1. Selective catalytic reduction (SCR):
2. Selective non-catalytic reduction (SNCR)

Question 51

Outline the process of reducing NOx emissions with Selective catalytic reduction (SCR):

We want to pass dirty air over a reducing species (ammonia)

Answer 51

The process of SCR is now the most widely adopted technique for NO_X removal. The polluted stream containing NO_X is mixed with NH₃ and reacted over a heterogeneous catalyst. Oxygen is required for the reaction and if not present in the effluent stream it must be added.

Reactions: (LEARN) – very clean reactions, we need an efficient catalyst.

2NO + 2NH₃ + 0.5O₂ –> 2N₂ + 3H₂O

NO₂ + 2NH₃ +0.5O₂ –> 1.5N₂ + 3H₂O

The NO_X is reduced and the NH₃ is oxidized (so it is acting as a reducing agent!) to form the benign products N₂ and H₂O. The process is described schematically:

Heater is where the NOx is being formed. Don’t worry too much about the detail, just known rough order, the flow and how if it isn’t reduced it gets recycled. The ammonia is the key step, either as gaseous ammonia or as liquid urea. If we use urea then we get carbon dioxide which isn’t as ideal.

NH₃ is either fed as a gas or sprayed in as a liquid. Sometimes urea, CO(NH₂)₂ is used instead of NH₃

To achieve maximum performance it is important to:

• Carefully controlled the ratio of NO_X: NH3, otherwise NO_X will be produced from NH₃ oxidation.
• Make sure that the NH₃ is well mixed with the NO_X stream, or the process is less efficient.

The catalytic reactor is a fixed bed type, shown schematically:

The catalyst is supported on brick shaped monoliths. This design allows:

• Bricks to be stacked together and the bed size can be increased or reduced easily.
• Low pressure drop through catalyst bed.

The monolith is formed from an Mg/Si/Al ceramic called Cordierite, and channels are usually rectangular or hexagonal. In a well-mixed system with controlled NH₃ delivery NO_X can be converted to N₂ with > 95% efficiency.

Question 52

What are appropriate catalysts for SCR (dealing with NOx)?

How much vanadium is required?

Answer 52

Two different types of catalysts are used depending on the temperature of gas to be treated.

a. Low temperature SCR catalysts; Operate in the temperature range 250-450°C.

The best catalyst is:

• V₂O₅ supported on TiO₂
• TiO₂ is often on top of g-Al₂O₃ to increase surface area
• All supported on a monolith

The structure is very complex:

Simplistically it is made up of five co-ordinate vanadium in a square pyramidal coordination. The square pyramidal units link together by edge and corner sharing. Gives 2 general types of vanadium oxygen bonds.

1. V-O-V bridging species, found along the square base.
2. V=O terminal species, found at the apex of pyramid.

Question 53

Are these different vanadium-oxygen species important? (SCR for NOx)

Answer 53

Consider the catalytic mechanism:

• Complex requiring bi-functionality

Acid sites; cycle using V⁵⁺ Bronsted acid sites from surface OH. (need these because ammonia is basic)

• Required for activation of NH₃

Redox sites; V⁵⁺ –> V⁴⁺ cycle of V=O species. (Rate determining step)

• Required for NOX reduction

Acid and redox sites in close proximity. The redox cycle is the critical cycle. Hydroxyl groups on the left and NOx on the right. Don’t need to focus on this in too much detail.

• V=O are the critical active sites for the reduction of NO_X.

Question 54

Hypothesis**: Catalysts on which the surface concentration of V=O species is maximised will give the maximum performance. Consider volcano plot of activity:

What happens with low and high loading

Answer 54

Question 55

What is the importance of TiO2 for NOx catalysts?

Answer 55

• TiO₂ is critical as there is interaction between the V₂O₅ and TiO₂ to give the V=O species. Titanium is critical because there is a strong interaction of Vanadium and Titanium. Titanium presents the vanadium in the correct way.
• The footprint of a V₂O₅ unit can be calculated from Crystallographic data.

Footprint of V₂O₅ = 20.2 Ų

• If the surface area of the TiO₂ is measured then the V₂O₅ loading for maximum performance can be calculated.

Question 56

How can the vanadium species be characterised?

Answer 56

a. Vibrational spectroscopy

Laser Raman spectroscopy particularly useful.

920 cm^-1 V=O species

997 cm^-1 crystalline V₂O₅

1028 cm^-1 isolated VO_X units

b. Powder X-ray diffraction

Identify crystalline V₂O₅ formed when the vanadium loading is above monolayer coverage

As the vanadium loading is increased start to see diffraction peaks from V₂O₅ as well as crystalline TiO₂.

Question 57

What are high temperature SCR catalysts?

Answer 57

Sometimes flue gases are at temperatures above 450 °C and it is not possible to use V₂O₅/TiO₂ catalysts as they deactivate. Sintering is the combination of crystals to make ‘big lumps. High temperature SCR catalysts operate in the temperature range 450 - 650 °C. These catalysts are less common. Types used:

1. Metal exchanged zeolites/zeotypes (copper pours material with Silica and Phosphorus)
2. WO₃/TiO₂ embedded in a ceramic matrix

Are there any other processes for NOX abatement from stationary sources?

Question 58

What is selective non-catalytic reduction (SNCR)?

Answer 58

The process of SNCR is now not widely used as it has been largely replaced by SCR. The process and chemistry is the same as SCR but no catalyst is used. Remember:

2NO + 2NH₃ + 0.5O₂ –> 2N₂ + 3H₂O

NO₂ + 2NH₃ +0.5O₂ –> 1.5N₂ + 3H₂O

The NO_X is reduced and the NH₃ is oxidized to form the benign products N₂ and H₂O. The SNCR process:

• Takes place at high temperature, ca. 900°C

Question 59

Compare SCR and SNCR and give advantages and disadvantages

Answer 59

• SCR operates at ca 350°C whilst SNCR is ca 900°C, more economic
• SCR removes NOx to lower levels than SNCR
• SCR catalyst has limited lifetime and must be replaced, not a problem with SNCR
• SCR more susceptible to operating conditions, requires higher degree of control

Question 60

Exam question: Briefly explain why the release of Volatile Organic Compounds to the atmosphere is detrimental to the environment. [4]

Answer 60

4 main effects:

Stratospheric ozone depletion [1]

Greenhouse gasses [1]

Production of ground level ozone [1]

Toxic e.g. benzene

Question 61

Exam question: What are THREE problems associated with VOC removal by thermal incineration? [3]

Answer 61

• Toxic by products formed by partial oxidation
• formation of NOx
• Expensive fuel use (>1000 degrees)

Question 62

Exam question

c) What are the general requirements of a catalyst for VOC oxidation? Using a supported metal catalyst as an example describe important features of the catalyst necessary for high activity. [9]

Answer 62

High conversion of VOC to CO₂

Stable activity without deactivation

Specific features:

Large number of metal sites

Achieved by small metal particles

High dispersion from supporting on high surface area support

Metal needs to be accessible

Should also be some identification of metal support interaction, influencing electronic properties of metal (redox mechanism)

Question 63

Exam question: d) What are the important characteristics of a metal oxide catalyst that contribute to high VOC oxidation activity?

Answer 63

High surface area Reactive lattice oxygen, facile redox cycle for effect MVK mechanism Defects help redox etc

Question 64

Exam question: b) Using balanced chemical equations describe how NO and NO2 are removed in selective catalytic reduction.

Answer 64

2NO + 2NH₃ + 0.5O₂ –> 2N₂ + 3H₂O

NO₂ + 2NH₃ +0.5O₂ –> 1.5N₂ + 3H₂O

Question 65

Exam question: c) Using a diagram show the equipment that is used to carry out the process of SCR.

Answer 65

Question 66

Exam question: Explain briefly how NOX is involved in the formation of acid rain. [4]

Answer 66

Question 67

Exam question: What are the advantages and disadvantages of the process of SCR compared to SNCR? [4]

Answer 67

Question 68

Exam question: a) Explain briefly why it is important to control the release of Volatile Organic Compounds into the atmosphere. [5]

Answer 68

Question 69

Exam question: Two metal oxides, Mn3O4 and Si2, were tested for the catalytic total oxidation of a hydrocarbon. Mn3O4 was very active, whilst SiO2 was inactive, Given that both metal oxides have similar surface areas, discuss the reasons why Mn3O4 is more active. In your answer refer to the catalytic mechanism operating for oxidation of the hydrocarbon. [9]

Answer 69

Question 70

Exam question: c) Two Mn3O4 catxalysts show very different activity for the total oxidation of a hydrocarbon. One with a high surface area is very active, whilst the one with a low surface area is much less active. Once the influence of surface area has been taken into account, the rate per unit area is much greater for the high surface area catalyst. Explain why this is the case and provide brief details of an experiment that could be used to provide supporting evidence.

Answer 70

Question 71

2013-2014 Exam Paper Question 1: (a) How is a VOC defined? [3]

Answer 71

VOCs encompass a large number of compounds containing carbon (excluding CO, CO2, carbonic acid, carbides and carbonates) that participate in atmospheric photochemical reactions. VOCs have a vapour pressure > 0.1 mmHg under standard conditions equating to a boiling point < 250 degrees at STP.

Question 72

2013-2014 Exam Paper Question 1: (b) Compare and contrast the processes of incineration and catalytic oxidation for the abatement of VOCs from aerial effluent [9]

Answer 72

Thermal inscineration involves streaming air containing VOCs with supplementary fuel in a flame. Temperature is in the region of 1000 degrees allowing NOx to form which isn’t favourable and safe. Also this process is expensive due to the fuel used. In comparison, catlaytic oxidation uses a catalyst meaning a lower temperature is required to oxidise VOCs and less fuel is required. This is more economic and more environmentally friendly.

For thermal inscineration and catalytic oxidation VOCs are oxidised to CO2 and H2O., For thermal incineration, conversion to CO2 is not always 100% . Many possible side products arrise some are toxic.

Catalytic oxidation employs a variety of catalyts for example metal oxides or transition metal oxides. thermal inscineration process occurs via a network of gas phase chemical reactions.

Thermal inscineration has to be closely monitored. Popular because it is simple and requires low tehcnological experise to operate. Catalytic oxidation is cheaper and cleaner to operate.

Question 73

2013-14 Paper: Question 1 (c): Explain in detal the catalytic mechanism that leads to total oxidation of a hydrocarbon VOC catalyt over supported palladium catalyst. Discuss ket features which are required for high activity. [8]

Answer 73

For catalytic oxidation, the process follows the Mars Van Krevelen Mechansim: VOC is oxidised by oxide of the metal oxide. As the ozide is remove, the metal oxide becomes reduced. O2 from the gas phase re-oxidises metal oxide catalyst. *DRAW and DRAW EQUATIONS*

Mechansim is a redox cycle. Oxygen from the catalyst ends up in the oxidised products.

The ease that oxide can be removed from the metal oxide lattice is important for activity. TPR can proble the removal of ozide pulling ozygen out of the catalyst. Oxygen is the incorporated from the atmosphere into the catalsyt. Redx behaviour is critical.

Need a high surgace area with many active sites. Interaction of metal particles with the supposrt is key.

Other key features include: metal dispersion, metal electronic properties

Question 74

Exam question 2013-14: Q2

Answer 74

(i) . 0.586 x 0.136 = 7.97 %
(ii) 0.532 x 0.015 = 0.798 %
7. 97/0.798 = 10, therefore 10x activity
(iii) CO2 + H2O
(iv) More point defects and 2D defects if greater surface area (co-precipitated)/Pt encapsulated. Greater conversion with catalyst because of lower temperature, elss by-product
(v) Calcinationa and drying? Grinding reagents to give small molecules