Talk about catalysts

[sebosfato]

i discovered something about catalysts

"
A catalyst is a substance that increases the speed of a chemical reaction without being consumed by the reaction."

i found that NO is a catalyst for the dissociation of ozone and that it work like this...


"For example, nitric oxide (NO) is a catalyst in the breakdown of ozone (O3) in the upper atmosphere. An oxygen atom (O) will react with an ozone molecule form two oxygen molecules (O2), but at a very slow rate. However, in the presence of nitric oxide, ozone is quickly broken down with the following series of steps:

Step 1 - An oxygen atom combines with a molecule of nitric oxide, forming a molecule of nitrogen dioxide (NO2):

O + NO --> NO2

Step 2 - A molecule of nitrogen dioxide combines with a molecule of ozone, forming two molecules of oxygen and a molecule of nitric oxide:

NO2 + O3 --> 2O2 + NO

Although the molecule of nitric oxide participates in the reaction, it is not consumed by it and is available for additional reactions. "

Glossary Item - Catalyst


Thats one of the reasons why we need to control emissions of NO and NO2.


Another Catalyst is platinum

Platinum is able to transform hydrogen peroxide H202 into H2O. I'm studying Atomic and Molecular and material physics and I learned today that this happens because platinum have something special in its D orbit.

than i found that:

"

The Science of Catalysts And Catalytic Converters

A catalyst is a substance that makes a chemical reaction happen more easily. You can get some really stroppy reactions in which you want to rearrange the atoms in them to make something useful, but it's just not playing. The starting material just isn't interested into becoming the product that you want. IF you put the right catalyst into the reaction, you can make this reaction happen either more quickly or using a lot less energy. Often you have to go into high temperatures and pressures to get the reaction to work. -How do they do that? -

Imagine that you've got yourself a beach cottage and the beach is about a mile away from your cottage. Between you and the beach there is this massive great mountain. You have several options for reaching the beach. One of them is that you put lots of energy into it, so you put lots of energy to walk up the mountain and down the other side. Not great. The second option is to walk all the way around the mountain but it takes a heck of a long time. But if someone's gone and dug a tunnel from one side to the other then you can get to the beach pretty quickly and with a lot less energy. This is exactly what a catalyst does. So the catalyst does for a chemical reaction what the tunnel does for you: it takes an alternative pathway that allows the reaction to happen a lot more easily. -

And just like a tunnel, it's not used up in the reaction. It's available forever if you like. -

Yeah and that's why you only need a very small amount of catalyst when you have a chemical reaction. Because although the catalyst is changed during the reaction, it's regenerated at the end of it. So each little atom or each little molecule of catalyst that's in there can go on a react with hundreds and thousands of millions of reactant molecules. -

Sounds fantastic, but how do we find these things? Why isn't there a catalyst for everything? Why isn't there a catalyst for my homework? How do we discover the chemicals that do these clever jobs? -

There are quite a lot of metals that are used as catalysts because there are two different types of catalysts: homogeneous catalysts and heterogeneous catalysts. In homogeneous catalysts, the reactants and the products are in the same phase. So if there are gases reacting, the catalyst will also be a gas. In heterogeneous reaction, the reagents are in a different phase from the catalyst. An example of homogeneous would be making plastic bags - high density polyethylene. The ethene and the catalyst are all going on in the same phase, in solution. That would be a homogeneous catalyst. In a heterogeneous catalyst, that would be carbon monoxide turning into carbon dioxide. That would happen on platinum metal. -

They've got expensive tastes these things! -

Platinum is very expensive but you often find that some of the most expensive metals turn out to be the best catalysts. -

Why is platinum so good? What's special about the metal? How does it do what it does? -

When people ask questions like this, a scientist's usual answer is: oh well, it's quantum. Actually it's to do with how well these gas molecules can stick to the surface. Platinum is very good at sticking molecules onto the surface of it. That's very important because that's where the reaction actually happens. The other thing that platinum is good for is, well, when you have a molecule, the atoms in the molecule are stuck together with chemical bonds, which are electrons. Platinum is very good at rearranging these electrons and allowing the molecules to turn into something else. Again, forming this alternative pathway by which a chemical reaction can happen. -

So if you could zoom in to the surface of the platinum, what would it look like to make it so sticky and that things like it? -

We always imagine it as lots of little balls stuck next to each other. One of the aims of being a catalyst chemist, which is what I am, is to try and make as much surface as possible. So we have our tiny little pieces of platinum which are stuck onto a ceramic support. We want as much platinum on the surface and as little platinum in the middle of these balls as possible. The platinum is part of the periodic table which has lots of d-orbitals, and it's these magic d-orbitals that makes it so good at catalysis and making things stick to it. So it very easily forms bonds with lots of different types of molecule. -

And it brings them together in just the right way that they want to get married or do whatever you want them to do. -

And provides them with a route which requires so little energy that it can happen essentially spontaneously or with very little energy on the metal's surface. -

Ok so turning now to what comes out of your exhaust pipe, how does a catalytic converter on a car actually work? What are they doing? -

The catalyst on a catalytic converter is essentially a can which is next to the engine. What it does is it purifies the exhaust gases. If it was ideal, we'd just get carbon dioxide and water out when fuel was burnt. -

I'm sure people would argue that it would be ideal if we just had water and burning hydrogen, which is what Fraser is going to be talking to us about in a minute. -

The problem being that you put fuel in at the beginning and you can destroy atoms as you go along. But what we also get out is carbon monoxide, which is a poison and binds so strongly to the blood that you can't bind oxygen anymore. There are also what's called NOx gases, which are oxides of nitrogen responsible for acid rain; and hydrocarbons which come together with NO-x to form smog. This is why in the 1970s Los Angeles got buried under this cloud of photochemical smog and what triggered all of the legislation about car pollutants. Also what comes out are particulates, which are essentially soot. This is linked with respiratory illnesses as well as cancer. So we obviously don't want these coming out of the backs of our cars. We need to put the catalytic converter between the engine and the exhaust pipe to catch these things as they go out. Inside the catalytic converter we have the monolith and the metal. The monolith is a ceramic and it's a honeycomb with a very large surface area and it's coated to give it an even greater surface area. If you spread it out it would cover about three football pitches. On the channels of this monolith you have little globules of the metal platinum, palladium, rhodium in various mixtures depending on whether you're a petrol or a diesel car, and these are so small that we call them nanoparticles. This is what we were talking about before. As these gases go past, which is a very quick reaction, it goes from the engine through the catalytic converter in less than a tenth of a second. -

So it must be very fast. -

Yes and it wouldn't happen normally unless there was a catalyst there. -

So how much of the gases does the catalytic converter scavenge or convert? Does it do the lot? -

It causes about a 90% decrease in the amount of pollution coming out, and what you mostly get out of the other end is nitrogen, water and carbon dioxide. -

So it does a good job but there was a motivation for people to stop using leaded fuel because it makes your brain rot and causes dementia but also lead's quite toxic to catalysts. -

Exactly. If you think about these little metal particles, the lead will stick onto the surface of them. The more you reduce the amount of surface there is, the less chance these pollutant gases have of sticking to the surface and making the catalyst catalyse. -

So it's better to do without lead if we can for more reason than one. -

It's better to do without lead and it's better to do without sulphur in petrol too, because sulphur is responsible, or used to be responsible when we had high sulphur petrol, for this eggy smell some people associate with catalytic converters. Now there's less sulphur in fuel, this is much less of a problem. -

Now Emma, it's impossible to miss your t-shirt and on the subject of noisy engines, I was wondering if this was what we're talking about! It says NOISE. What is NOISE and why are you here today? -

NOISE is the New Outlooks In Science and Engineering campaign. It's a group of young scientists who are there to give an alternative image for what scientists are like. Chris, when you think about the stereotype of a scientist, what is it that springs to mind? -

Glasses more powerful than the Hubble space telescope, shocking teeth, 1960s get-up and muttering unintelligibly in a way that no-one can understand. -

And the words fun and dynamic don't really feature in those descriptions. -

But that's why people listen to the Naked Scientists! -

And that's why NOISE is there. We need to change this. We're the new generation of young scientists and we have a website NOISE - New Outlooks In Science & Engineering. where there's this whole group of scientists that do lots of fun science that we want to tell people about. We have a snowboarding physicist and we have somebody who does robotics who is a scuba diver. The idea is to point out to people, especially kids who are thinking of going to university, that there's more to being a scientist than a white coat. -


September 2006

Dr Emma Schofield, Johnson Matthey Technology Centre"

The Science of Catalysts And Catalytic Converters - The Naked Scientists September 2006

[sucahyo]

electrolysis catalyst

In electrolysis the catalyst is NaOH and KOH. When you put salt in water, it eventually turn into NaOH and Cl2. Baking soda and other salt or addition will end up as KOH or NaOH.

Carbon will increase electrolysis efficiency up to twice, but will be consumed. Which is why alcohol accelerate electrolysis but will only last few minutes.

[sebosfato]

In electrolysis the catalyst is NaOH and KOH. When you put salt in water, it eventually turn into NaOH and Cl2. Baking soda and other salt or addition will end up as KOH or NaOH.

Carbon will increase electrolysis efficiency up to twice, but will be consumed. Which is why alcohol accelerate electrolysis but will only last few minutes.

Hi sucahyo

Whenever the catalyzing agent is consumed it is not actually a catalyst because of absorption but chemisorption because chemical bounds are formed. (the case of alcohol and salt (ionic bounds)
I guess NaOh is not actually a Catalyst it only ionize the water allowing for the current to pass. I'm not sure but i think it create an ionic bound.

"Heterogeneous Catalysts

A Study Guide

Discussion Questions

What are chemical absorptions and how do they promote chemical reactions?
What types of chemisorption lead to the poisoning of a catalyst?
How transition metals are chosen as catalysts?
What are syngases and how are they prepared?
Why metal clusters will be excellent potential catalysts?
Are non-stoichiometric oxides potential catalysts for redox reactions?
What type of catalysts can be made of stoichiometric oxides?
What are photocatalytic reactions?
Heterogeneous Catalysts

A catalyst is another substance than reactants products added to a reaction system to alter the speed of a chemical reaction approaching a chemical equilibrium. It interacts with the reactants in a cyclic manner promoting perhaps many reactions at the atomic or molecular level, but it is not consumed. Another reason for using a catalyst is that it promote the production of a selected product.
A catalyst changes the activation energy, Ea, of a reaction by providing an alternate pathway for the reaction. The rate and rate constant k of a reaction are related to Ea in the following ways:

rate = k * function of concentration
k = A exp (- Ea / R T)
where A is a constant related to collision rates. Thus, a change in Ea changes the rate of a reaction.
A catalyst in the same phase (usually liquid or gas solution) as the reactants and products is called homogeneous catalyst.

A catalyst that is in a separate phase from the reactants is said to be a heterogeneous, or contact, catalyst. Contact catalysts are materials with the capability of adsorbing molecules of gases or liquids onto their surfaces. An example of heterogeneous catalysis is the use of finely divided platinum to catalyze the reaction of carbon monoxide with oxygen to form carbon dioxide. This reaction is used in catalytic converters mounted in automobiles to eliminate carbon monoxide from the exhaust gases.

Promoters are not catalysts by themselves but increase the effectiveness of a catalyst. For example, alumina Al2O3, is added to finely divided iron to increase the ability of the iron to catalyze the formation of ammonia from a mixture of nitrogen and hydrogen. A poison reduces the effectiveness of a catalyst. For example, lead compounds poison the ability of platinum as a catalyst. Thus, leaded gasoline shall not be used for automobils equiped with catalytic converters.

Because heterogeneous catalysts often are used in high temperatures reactions, they are usually high melting (refractory) materials, or else they can be supported by refractory materials such as alumina.

Today, catalysts design is a challenge for chemists and engineers for effective productions, pollution prevention, and waste treatments.

What are chemical absorptions and how do they promote chemical reactions?

As mentioned in solid defects, solid surfaces are two-dimensional defects. They offer a potential for attraction to molecules of gases and liquid. Adsorption takes place as molecules are attracted to the surface, and when molecules penetrate through the bulk material, the term absorption is used. Absorption with no chemical bonds formed or broken is called physical absorption or physisorption, whereas chemisorptions refer to processes when new bonds are formed or broken.
Inorganic Chemistry by Swaddle (page 117) gives an excellent example to illustrate the chemisorption of hydrogen by a nickel catalyst. The bond energy of H2 is 435 kJ/mol. Thus, in a hydrogenation reaction, energy has to be made available for the reactions:

H2 -> 2 H, H = 435 kJ
| |
>C=C< + 2 H -> H-C--C-H
| |
In the above reaction, the activation energy, Ea is close to 435 kJ. However, when hydrogen is absorbed by nickel, the breakage of the H-H bond is facilitated by a series of steps.
2 Ni + H2 --->2 Ni...H2----> 2 Ni-H
solid gas physisorption chemisorption
The activation energy is thus lowered due to the formation of Ni-H bonds. A change in activation energy changes the rate of reaction.
In the activation of O2 by a metal M, the O=O bond is weakend or broken via these steps:

O=O O--O O O O- O-
| | || || | |
-M--M- ==> -M---M- ==> -M M- ==> -M M-
In these steps, the oxygen is activated at verious stages.
With sophiscated experimental techniques, we can study the chemisorbed species in details. For example, the chemisorbed ethylene is believed to be an ethylidyne radical


H H H
\ | /
C
|
C
/|\
PtPtPtPtPt
Metal Metal Metal
The chemisorbed ethylidune radical.

What types of chemisorption lead to the poisoning of a catalyst?

If the absorbed species are very stable, and much energies are release in the chemisorption process, the absorbed species are not reactive. Their absorbtions prevent further absorption of other species, making the catalyst inactive. These phenomena are known as catalyst poisoning.
A poison reduces the effectiveness of a catalyst. Tetraethyl lead has allways been additive to the gasoline. For environmental protection, catalytic converters have been installed in automobiles to oxidize carbon monoxide and hydrocarbons. However, lead compounds poison the ability of platinum as a catalyst. Thus, leaded gasoline should not be used for automobiles equipped with catalytic converters.

There are many types of catalyst in the market place, for example MIRATECH oxidation catalyst can also reduce carbon monoxide and hydrocarbon emissions. The most common catalytic converter uses Pt metal.

Recently, there is a concern over the reduction of sulfur in gasoline and other engine fuels for the purpose of reducing sulfur oxides emission. Technically, sulphur compounds are not catalyst poisons (i.e. they do not cause an irreversible reduction in catalyst efficiency). However, they will occupy part of the precious metal surface, thereby reducing the active conversion of exhaust emissions until the sulphur gets de-sorbed from the precious metal sites again (short-term effect).

[sebosfato]

How transition metals are chosen as catalysts?

The first period of transition metals are represented by these metals.
Sc Ti V Cr Mn Fe Co Ni Cu and Zn
Typical common features among them are the presences of d electrons, and in many of them, and their unfilled d orbitals. As a result, transition metals form compounds of variable oxidation states. Thus, these metals are electron banks that lend out electrons at appropriate time, and store them for chemical species at other times.
Tranisition metals are used in hydrogenation reactions mentioned earlier. These reaction are represented by

| |
>C=C< + 2 H -> H-C--C-H
| |
For example, the hydrogenation of unsaturated oil in the manufacture of margarine is such an application. Special catalysts such as ICT-3-25-P is made of palladium supported on the special wide-porous carbon carrier Sibunit.
Other processes catalyzed by transition metals are oxidation-reduction reactions:

NH3 + 5/4 O2 = NO + H2O
2 CO + O2 = 2 CO2
The oxidation of CO takes place in catalytic converters, platinum is often, but not always used as a catalyst in them. The picture shown here is a dual catalytic converter showing its gas flow path.
For most transition metals except gold, the chemisorption strength follows a general sequence for gaseous reagents:

O2 > alkynes > alkenes > CO > H2 > CO2 > N2
The chemisorption strength also varies with the metals. In general, the chemisorption is the strongest for metals on the left, and it decreases for transition metals in a period as the atomic number increases:
Sc Ti V Cr Mn Fe Co Ni Cu Zn
Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
La Hf Ta W Re Os Ir Pt Au Hg
The chemisorptions are too strong for Sc, Ti, V, Cr, and Mn groups and these metals are not effective catalysts.
Fe, Ru, and Os chemisorb most gases firmly, and barely chemisorb N2.
Chemisorb strengths for Co and Ni are weaker than those of the Fe group. Their absorptions for CO2 and H2 are very weak.
Rh, Pd, Ir, and Pt barely chemisorb H2, but not CO2.
Cu, Ag, barely chemisorb CO and ethylene.
These relative chemisorption strengths enable us to make some simple predictions regarding their sutability as catalysts for specific reactions. For example, a catalyst for the Haber process to produce ammonia must chemisorb nitrogen. Iron, ruthenium, or osmium may be considered.
For hydrogenation reactions, the catalyst must chemisorb H2. Metals Co, Rh, Ir, Ni, Pd, and Pt are suitable. Availability and costs are additional factors for the consideration. Nickel is actually a good choice, all considered.

These guidelines are very crude, and each case must be carefully studied. Fortunately, many catalysts are commercially available. The research and development of catalyst are left for many companies.

What are syngases and how are they prepared?

Syngas is a general term used to mean synthetic gases suitable as fuel or for the production of liquid fuel. Often, it is a mixtuure of H2 and CO, and this mixture can be converted into methanol, CH3OH. The well known catalysts are Pt and Rh, but other technology such as memberanes are also used for syngas productions.
Selection of a catalyst is important in industrial productions. For example, using rhodium or platinum as catalysts have shown to give very different distribution of products when methane or ethane were used.

CH4 (65%) + O2 (35%) ---Rh--> H2 (60%) + CO (30%) + CO2 (2%) + H2) (5%)
When platinum is used, more of the undsirable products H2O and CO2 were obtained. Swaddle has described the difference between using these two metals as catalysts (Inorganic Chemistry, page 120), but much more details is required when it comes to application. The data provided evidence to show that a slight difference in chemisorption led to very different results.
Why metal clusters will be excellent potential catalysts?

The surface area per unit weight is an important consideration when solids are used as catalysts. There are many studies related to the study of surface area of particulate metals. Various methods are developed to measure the surface areas of solid materials. One such method is the surfact area determination from gas adsorption.
Clusters are the limiting sizes of metal particles, each of which are made up only a few atoms. There is no need to rigorously define the number of atoms in a particulate to be called clusters, but a general view is that when the number of atoms at the surface of the particle is more than the nuber of atoms in the interior, the particle is a cluster. Thus, a cluster can have as few as 3 atoms, and as large as a few tens of atoms.

By the way, the term cluster have been used in other areas of study. For example, in organometallic chemistry, compounds with a few metals bonded together by metal-metal bonds are also called metal clusters. Many carbonyl compounds belong to this category. For example,

Co2(u-CO)2(CO)6, (u-CO meaning CO bridged between two metal atoms)
Mn2(CO)10
Fe3(CO)12
Co4(CO)12
Rh4(CO)12
CFe5(CO)15
Rh6(CO)16
Os6(CO)18
Metal carbonyls have been studies as homogeneous catalysts. They are mentioned here so that you will be able to appreciate their usage in other literatures.
All catalytic activities occur at the surface, because the surface atoms have tendencies for chemisorption of gas molecules. Thus, clusters will naturally be excellent potential catalysts. Thus, the study of heterogeneous catalysts may involve the study of metal cluster ion chemistry, and encapsulated silver clusters as oxidation catalysts. Clusters can be made from vapour deposition. The title of this link sounds very interesting: Metal Atom Vapor Chemistry: A Field Awaits Its Breakthrough.
Are non-stoichiometric oxides potential catalysts for redox reactions?

Due to their ability to have various oxidation state, transition metals form non-stoichiometric oxides, and they have excellent potentials for oxidation and reduction (redox) reactions, because they can both give and accept electrons.
Mn+ => M(n+1)+ + e-
M(n+1)+ + e- => Mn+
Furthermore, they resemble metals, and they catalyze hydrogenation and isomerization reactions.
A p-type metal oxide have excess positive charges in the solid, and they can adsorb oxygen to form anions such as O-, O2-, O2-, and O22- on their surfaces. Nickel oxide is such an oxide. It turns out that the adsorbed O- species is the most active,

O2 (g) + 2 Ni2+ => 2 O- (ads) + 2 Ni3+
2 O- (ads) + 2 CO (ads) => 2 CO2 + 2 e-
2 Ni3+ + 2 e- => 2 Ni2+
When an oxide gives up oxygen, electrons were left behind and the negative charge in it makes it a n-type oxide. Zinc oxide is such an n-type oxide, and the reaction mechanism may be represented as follows:
CO (g) + 2 O2- (lattice) => CO32- (lattice) + 2 e-
0.5 O2 + 2 e- => O2-
CO32- (lattice) = CO2 + O2- (lattice)
The overall reaction is ---
CO + 0.5 O2 => CO2
In these primary steps, the oxygen is consumed via adsorption on the solid.
A sulfide, such as MoS2, can loose sulfur atoms to become a n-type solid, Mo1+xS2 or gain a sulfur atom to become a p-type solid Mo1-xS2, depending on the vapour pressure of S2 gas surrounding the solid. By doping MoS2 with oxide can also make it a p-type solid for a catalyst.

One of the useful applications of MoS2 as a catalyst is to reduce sulfur in gasoline. For example, the cyclic thiophene C4H4S can be converted to a hydrocarbon by using a p-type MoS2,

C4H4S + 4 H2 == p-type MoS2 == > C4H10 + H2S
This is accomplished by a typical commercial hydrodesulfurization catalyst, which may contain 14% MoO3, and 3% CoO on alumina support.
What type of catalysts can be made of stoichiometric oxides?

It has been well known that metal oxides dissolve in water to form basic solutions whereas non-metalic oxides dissolve in water to give acidic solutions. Some metal oxides such as Al2O3, Fe2O3, Cr2O3 etc dissolve in strong acid and bases. Thus, we can divide oxides into acidic and basic oxides for catalytic activities.
Acidic oxides

Acidic oxides such as Al2O3 and SiO2 catalyze dehydration reactions such as
R-CH2CH2OH (g) == (Al2O3, 600 K) ==> R-CH=CH2
If we consider the oxide a Lewis acid, it adsorbs the OH group, facilitating the reaction in the following steps.
R-CH2CH2OH (g) => R-CH2-CH2+ + OH- (adsorbed)
R-CH2-CH2+ => R-CH+-CH3
R-CH+-CH3 + OH- (adsorbed) => R-CH=CH2 + H2O
Zeolites, which are alumniosilicates, function as acidic catalysts. They also catalyze isomerization, cracking, alkylation and other organic reactions.

Basic oxides

Basic oxides such as MgO and ZrO favor reactions involving anionic species. When a proton, H+, is adsorbed onto the surface close to an O2- ion in the metal oxide, an OH- group is formed, leaving the organic molecule a negative charge.
CH3-CH2-CN + MO (solid) => -CH2-CH2CN + M-OH+ (solid)
=> CH2=CH-CN + MOH2 (solid)
= + oxygen => CH2=CH-CN + MO (solid) + H2O (product)
The over all reaction is a selective oxidation
CH3-CH2-CN + MO (solid) + 0.5 O2 => CH2=CH-CN + MO (solid) + 0.5 H2O
The oxidation eliminated two hydrogen atoms per molecule in the process, and the proposed mechanism suggests a two step elimination process.
Mixtures of basic oxides have been used as catalysts in the oxidative coupling of methane. In some cases, special reactors and catalysts are designed for this type of application. TAP Reactor is one such an application. In this case, zeolites were used.


What are photocatalytic reactions?

Reactions caused by photons, bundles of radiation energy, are called photolysis. Photocatalyic reactions imply photolysis in the presence of a catalyst. In most cases, however, the catalysts are semiconductors and the reactions are semiconductor assisted photolysis reactions. In this aspect, the photocatalyst has a slightly different function than those in thermal chemical process.
The simulation below shows that when the yellow beam strikes the semiconductor TiO2, an electrons are excited from the valance band into the conduction band. This band gap is 3.2 V. The excited electron then promote the production of H2. The holes take electrons from OH- groups converting them to active OH radicals. The radicals break up forming O2 or react with CHCl3 converting it into the harmless CO2, H+, and Cl-. This simulation of photocatalysts is prepared by a Japanese group, and it illustrates the concept rather well. In reality, the process is rather complicated.



In the photodecomposition of water, the excited electrons react with hydrogen ions (protons)

2 H+ + 2 e- = H2
2 OH- + 2 e+ (hole) = H2O + 0.5 O2
Thus, the products, H2 and O2, are potential fuels for the supply of energy, especially for fuel cells.
As another example, fluoroboric acid is used in electroplating and metal finishing. To treat wastewater from these industries requires the removal of fluoroboric acid. Existing methods of adsorption, coagulation, precipitation methods do not work. Thus, photocatalytic decomposition of fluoroboric acid has been studied, and it showed that TiO2 being rather effective when it doped with Cr and Fe oxides. The above link showed that doping of Cr or Fe drastically enhanced the activity. Moreover, 0.5 wt% Cr/TiO2 and 1.0 wt% Fe/TiO2 showed maximum activities of 61 % and 41 %, respectively.

Recently, a news article has an attractive claim on Indoor Air Cleaner. Judge it yourself to see if it is something worth investigating.

[sebosfato]

Example 1

The energy gap of TiO2 is 3.2 V. What is the frequency of the photons that has just eneough energy to excite the electrons from the covalent band of TiO2 into the conduction band?
Solution
The energy to excite an electron up 3.2 V is 3.2 eV.

1.6022e-19 J 1
3.2 eV ------------- ------------- = 7.74e14 Hz
1 eV 6.626e-34 J s
Discussion
The wavelength of these photons are

3e8 m/s
-------- = 387e9 m (or 387 nm)
7.74 /s
These photons are at the just within visible limit of 350 - 700 nm.
Example 2

A mole of photons is called an einstein. How much energy in J is an einstein of the photons described in Example 1.
Solution
The energy is

1.6022e-19 J 6.022e23
3.2 eV -------------- ------------- = 308000 J = 308 kJ
1 eV 1 photon
Discussion
When we discuss Gibbs energy, we have learned that the enthalpy of formation for H2O is - 285.83 kJ. This means that we need a minimum of 286 kJ to decompose water. Thus, 1 einstein of photons has more energy to decompose a mole of water than the minimum. However, an overpotential is required to decompose water. The titanium oxide is mixed with platinum metal and ruthenium oxide to facilitate the formation of bubbles in these process. (See Inorganic Chemistry by Swaddle(page 125).

"
Heterogeneous Catalysts

[sebosfato]

More about metals and platinum and hydrogen

Hydrogen is a very small atom.

Metals otherwise specially heavy ones like platinum are very big.

How can platinum be used as a permeable electrode in fuel cells ???

To visualize it:

Think about big balls all inside a container, (this are the platinum atoms) and now look at the holes witch are created because one ball don't touch the other completely.

This holes is where the hydrogen can get into. And pass thru it.

Atoms are not as this balls but this way is easier to understand.

However because of the D orbital vacancies, the hydrogen atoms in the hydrogen molecule tends to link with this platinum atoms lets say one atom to one platinum atom and the other atom to other platinum atom. This action makes that the covalent link of the H2 is weaken because both atoms in the molecule are kind of bounded with the platinum atoms. Thus you basically ionize the hydrogen, splitting it into 2 atoms...

Another interesting thing about hydrogen molecule is that its binding energy or energy to split the molecule into two atoms is close to 2,7 ev.

This action is what makes the catalysts to help reactions, for example:

Think of a hydrocarbon molecule... when it get into contact with the platinum the hydrogen will stick to it allowing to the oxygen to bound with it easily because it is no longer covered with carbon atoms...

Gases are never stopped, even if they seem to. They are aways colliding against their selves and walls of the container where they are inside, and this agitation or speed raise whenever temperature or pressure raise. ( this clear up why a catalyst can act on a traveling gas for those who didn't knew this)

A catalyst to accomplish a reaction it can cause what is called transition reaction, or transition state, where it creates molecules witch can't exist normally in the process of creating the final molecule. (i don't remember exactly the example that was gave to me =( )

This is known even if it occur very fast because tests have being made with mass spectrometers during the reaction.

So thats why plastic container is a better way of storing hydrogen, plastics structure are very closed (remember the big balls).

[sebosfato]

I 'm not sure, but i guess that not much. Because it would be more reactive with NOX's and Hydrocarbons and i think that maybe it need a starting energy (min temperature) for work. I think that the car one is designed especially for the gas exhaust. Thus i think that if you want to create an air cleaner it should be designed for that.

Example: a car have 2 stage catalyst the first is for hydrocarbon and the second for NO's

Hope it's cleaner ..
Thanks for reading
Best Regards