When TiO2 captures UV rays, it causes a photocatalytic reaction and oxidizes organic compounds present into water and carbon dioxide. Can someone help or point me in the right direction in tring to figure out how much CO2 and H2O is given off in this reaction? I am trying to figure how much CO2 & H2O is given off in a given time per square foot (or sample area) of TiO2.
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Titanium Dioxide (TiO2) photocatalytic properties 5
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HydroScope
Mechanical
The only thing I know about TiO2 is that it will give off H2 when emersed in water and a small potential difference is applied between the water (actually electrolyte, I think) and the Ti02. The university of New south Wales (Australia) has a project running now and for another 7 years to investigate this property.
You are talking about the decomposition of organic compounds? So what I have mentioned is different, since you have listed your question under the Hydrogen and FC section then I figure you might be referring to the above reaction. But a little confused? Also the reaction I speak of does not utilise ultra violet spectrum but the light waves in much the same manner that Photovoltalic cells do (solar cells)
This reaction is less than 1% efficient (actually less than .1% but they expect to reach 1% soon) thus if you take how much solar energy hits the ground, 1000watts/m2 multiply it by the efficiency rate. (Not sure where you would obtain accurate numbers) you can then use these numbers to obtain hydrogen quantity.
You are talking about the decomposition of organic compounds? So what I have mentioned is different, since you have listed your question under the Hydrogen and FC section then I figure you might be referring to the above reaction. But a little confused? Also the reaction I speak of does not utilise ultra violet spectrum but the light waves in much the same manner that Photovoltalic cells do (solar cells)
This reaction is less than 1% efficient (actually less than .1% but they expect to reach 1% soon) thus if you take how much solar energy hits the ground, 1000watts/m2 multiply it by the efficiency rate. (Not sure where you would obtain accurate numbers) you can then use these numbers to obtain hydrogen quantity.
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moltenmetal
Chemical
Anatase TiO2 does produce hydroxyl radicals, "holes", hydrated electrons etc. when irradiated with solar light (both short-wave visible up to ~ 430 nm max) and ultraviolet fractions). The quantum yield is quite low for useful oxidizing species: the literature would give you a more accurate numbers, but from memory it's around 6% at its peak wavelength (in the UV) in deionized water. The yield is low because radicals tend to recombine at the surface before contaminants have a chance to diffuse to the surface- the self-reaction rate constant for hydroxyl radicals is on the order of 10e-8 L/mol*s, which is roughly as fast as they react with most organic contaminants in the free aqueous phase. The effective quantum yield increases to ~ 20% max if you add the right amount of hydrogen peroxide (i.e. H2O2 made by processes using fossil fuels rather than solar energy).
For comparison purposes, the effective quantum yield for UV photolysis of hydrogen peroxide is unity (i.e. every photon generates 1 useful hydroxyl radical). But since peroxide only starts to absorb below about 280 nm, UV/peroxide is dependent on artificial light sources- unless you sensitize the process. There are patents out there on iron-based soluble photosensitizers which drive the useful absorption out well into the visible spectrum.
The real effective quantum yields for contaminant destruction will depend on many factors including contaminant type, geometry, light intensity, and the absorptivity of the water, the tendency for it to form absorptive or radical-quenching solid precipitates etc. The 6% figure was for slurry anatase in water: if you immobilize the anatase with a support matrix to make it easier to handle, the quantum yield decreases.
So: assuming you're using something (ideally suited) like methanol as the contaminant, you'll almost certainly need at least two hydroxyl radicals to get all the way to CO2 and water as products (in reality you'll probably stop at formaldehyde or formic acid and won't actually go all the way to carbon dioxide- but who's looking? Most people only care that the contaminant itself disappears and aren't concerned about low molecular weight, water-soluble by-products). The quantum yield for CO2 production would therefore be 6%*6%. Another way of putting this is that your TiO2 will need to absorb ~278 useful photons to produce one molecule of CO2. Note that the "useful" fraction of the solar spectrum as far as TiO2 is concerned is quite a small fraction of the 1000 W/m2 on average people use as a nominal figure for solar radiation to earth- again, the literature has good data on the solar spectrum and the photon counts by wavelength cut-off etc..
The problem with TiO2 is that it is a "doubly heterogeneous" catalyst: not only is it a solid in a liquid system, such that contaminants have to diffuse to the surface to react with the short-lived radical species before they recombine or are quenched unproductively, but the solid also must be irradiated with light to be active. Because the process is already diffusion limited at normal solar intensities, it responds quite poorly to increased intensity. Experiments by NREL in Colorado in the '90s showed that a doubling of intensity by using solar reflector/collectors increased the rate of contaminant destruction by only 5% in a slurry TiO2/added peroxide system. That means that the process's efficiency of photon use dropped almost in half with a doubling of intensity- bad news for real solar-driven processes.
So if your question is related to the solar energy capture efficiency of anatase TiO2, the answer is that it is quite poor. It's also extremely well studied, despite the fact that the key limitations were known in industry as early as the late '70s. There's something about the combination of "solar" and "environmental remediation" in grant applications which seems to attract government research grant money like nobody's business.
From what I've seen in the literature, nature's already given us an excellent solar-driven non-invasive groundwater treatment device: it's called a poplar tree...It digs its own wells, pumps its own water, and metabolizes or immobilizes a wide range of contaminant species, all the while producing oxygen and sequestering atmospheric CO2- and makes a nice sound in the breeze to boot. Better than TiO2 by a long shot.
For comparison purposes, the effective quantum yield for UV photolysis of hydrogen peroxide is unity (i.e. every photon generates 1 useful hydroxyl radical). But since peroxide only starts to absorb below about 280 nm, UV/peroxide is dependent on artificial light sources- unless you sensitize the process. There are patents out there on iron-based soluble photosensitizers which drive the useful absorption out well into the visible spectrum.
The real effective quantum yields for contaminant destruction will depend on many factors including contaminant type, geometry, light intensity, and the absorptivity of the water, the tendency for it to form absorptive or radical-quenching solid precipitates etc. The 6% figure was for slurry anatase in water: if you immobilize the anatase with a support matrix to make it easier to handle, the quantum yield decreases.
So: assuming you're using something (ideally suited) like methanol as the contaminant, you'll almost certainly need at least two hydroxyl radicals to get all the way to CO2 and water as products (in reality you'll probably stop at formaldehyde or formic acid and won't actually go all the way to carbon dioxide- but who's looking? Most people only care that the contaminant itself disappears and aren't concerned about low molecular weight, water-soluble by-products). The quantum yield for CO2 production would therefore be 6%*6%. Another way of putting this is that your TiO2 will need to absorb ~278 useful photons to produce one molecule of CO2. Note that the "useful" fraction of the solar spectrum as far as TiO2 is concerned is quite a small fraction of the 1000 W/m2 on average people use as a nominal figure for solar radiation to earth- again, the literature has good data on the solar spectrum and the photon counts by wavelength cut-off etc..
The problem with TiO2 is that it is a "doubly heterogeneous" catalyst: not only is it a solid in a liquid system, such that contaminants have to diffuse to the surface to react with the short-lived radical species before they recombine or are quenched unproductively, but the solid also must be irradiated with light to be active. Because the process is already diffusion limited at normal solar intensities, it responds quite poorly to increased intensity. Experiments by NREL in Colorado in the '90s showed that a doubling of intensity by using solar reflector/collectors increased the rate of contaminant destruction by only 5% in a slurry TiO2/added peroxide system. That means that the process's efficiency of photon use dropped almost in half with a doubling of intensity- bad news for real solar-driven processes.
So if your question is related to the solar energy capture efficiency of anatase TiO2, the answer is that it is quite poor. It's also extremely well studied, despite the fact that the key limitations were known in industry as early as the late '70s. There's something about the combination of "solar" and "environmental remediation" in grant applications which seems to attract government research grant money like nobody's business.
From what I've seen in the literature, nature's already given us an excellent solar-driven non-invasive groundwater treatment device: it's called a poplar tree...It digs its own wells, pumps its own water, and metabolizes or immobilizes a wide range of contaminant species, all the while producing oxygen and sequestering atmospheric CO2- and makes a nice sound in the breeze to boot. Better than TiO2 by a long shot.
Hi to everyone, pls consider myself as newbie in TiO2 technology.
I’ve been attended to seminar about TiO2 technology, which fascinated me for its easy application and low cost.
My field is environmental engineering, so I was looking for a technology which could replace (if it is possible) the biological treatment for small and high fluctuate flow rates (during summers) with physical-chemical treatment.
I would appreciate if anyone, who is already in this line of business, informs me about applications and problems of TiO2 technology.
I’ve been attended to seminar about TiO2 technology, which fascinated me for its easy application and low cost.
My field is environmental engineering, so I was looking for a technology which could replace (if it is possible) the biological treatment for small and high fluctuate flow rates (during summers) with physical-chemical treatment.
I would appreciate if anyone, who is already in this line of business, informs me about applications and problems of TiO2 technology.
moltenmetal
Chemical
Dimitri: I'd call TiO2 water treatment technology neither "easy to apply" nor "low cost". Relative to typical biological treatment options, cost per unit volume treated for TiO2 or UV/peroxide treatments is at least an order of magnitude higher. TiO2 is usually used slurry-phase because immobilizing the TiO2 tends to reduce its efficiency as a photocatalyst. Any water containing solids or producing solids as a by-product of treatment therefore is a problem for this process, because these solids will accumulate. There are other problems too. It's a great process for ultrapure water preparation because if you can tolerate the lower quantum efficiency you can actually treat the water without using additives. But in my experience, if you're paying to make the light, UV/peroxide is a better process for treating real waters.
Thank you for your quick response. The problems which you mentioned had crossed my mind, but during seminars (which I mentioned above) there have been presented some successful applications, such as treatment of leachates from landfills, wastewater form olive treatment and others heavy loaded wastewaters (high COD, high SS) which can be difficult treated by any other methods, that’s why I turn my self to look for this kind of technology.
Also during my small research I’ve found companies (mainly from Far East) which already merchandise this kind of equipment, but there is also a big research activity in that filed all over the world.
So my question is, are those companies in Far East so advanced so they already selling the technology which is still under research.
Thank you in advance
Also during my small research I’ve found companies (mainly from Far East) which already merchandise this kind of equipment, but there is also a big research activity in that filed all over the world.
So my question is, are those companies in Far East so advanced so they already selling the technology which is still under research.
Thank you in advance
moltenmetal
Chemical
Dimitri:
I am no longer in the advanced oxidation water treatment business, though I once was, so I can give you the straight goods on it. Water treatment is like any other business- there is no "magic bullet" which is the perfect solution for every problem, but some people want to sell you their technology as if it were the "magic bullet". If you go to the guy who sells hammers and tell him that you have screws to drive, don't expect him to tell you that there's a better tool for the job called a screwdriver- he'll try to sell you a very big hammer. It will work, sort of, but it will not be the best choice!
All treatment technologies have good points and bad points, benefits and problems. They work well for some groups of contaminants in some circumstances, and very poorly for the same contaminants in other circumstances. Some contaminants are almost totally refractory to treatment. TiO2 is not unusual in this regard- it's true of all treatment technologies.
Personally I think TiO2 is one of the worst choices for high COD, high TSS wastewater treatment. It wouldn't even be on my list, unless there were absolutely no alternatives. For low/no TSS, low COD groundwater treatment of typical organic micropollutants, it can certainly be a choice- but I would absolutely want to see an apples-to-apples comparison in terms of both operating and capital costs for treating the same water, based on actual TESTWORK using a split sample of the real water, before I will believe that it is more energy- and cost-efficient than properly designed UV/peroxide treatment. And unlike TiO2, UV/peroxide has no solid catalyst to deal with, which makes it operationally much simpler. And unlike TiO2, UV/peroxide has no intensity dependence, such that one can use more powerful and more compact UV lamps, making the treatment system physically smaller.
One of the problems for potential users of TiO2 photooxidation and all other "advanced oxidation" technologies is that although there are a few reputable firms who have well-developed and reliable treatment systems to offer, there are also a large number of disreputable companies out there who have nice-looking websites and virtually no successful installations in the field. Choosing an unproven firm can be a significant risk. Even if they have the process photochemistry working well, there's no guarantee that they have done the process and mechanical engineering properly and have a treatment system which works reliably without significant operator intervention and maintenance. Something as simple as the wrong choice of metering pump can result in an operational headache that may result in the discharge of improperly treated water, depending on the system design. Some of the firms mis-state or mis-calculate the costs of treatment, neglecting such costs as lamp or catalyst replacement, or understating the costs of reagent addition.
My strong suggestion when dealing with this or any other treatment technology: get it tested on a representative sample of actual water. NEVER trust a "spiked tapwater" test as representative of your real water- it virtually never will be. Believe me, that goes for treatment technologies as simple as activated carbon- if you don't do the testwork, chances are you'll be surprised by how high the real operating cost is on your real water. Most reputable advanced oxidation firms will do this test work either for free or for a fee which is credited against the purchase price of a system. Get the reagent addition, total electrical consumption and lamp replacement/lamp lifetime cost rates per unit volume of water treated, and then get your own, local costs for the reagents and lamps. Estimate the required operator intervention and the resulting labour cost. Use these values to calculate the actual operating cost of treatment, and make your decision on the basis of both capital and operating costs.
I hope this is of help to you.
I am no longer in the advanced oxidation water treatment business, though I once was, so I can give you the straight goods on it. Water treatment is like any other business- there is no "magic bullet" which is the perfect solution for every problem, but some people want to sell you their technology as if it were the "magic bullet". If you go to the guy who sells hammers and tell him that you have screws to drive, don't expect him to tell you that there's a better tool for the job called a screwdriver- he'll try to sell you a very big hammer. It will work, sort of, but it will not be the best choice!
All treatment technologies have good points and bad points, benefits and problems. They work well for some groups of contaminants in some circumstances, and very poorly for the same contaminants in other circumstances. Some contaminants are almost totally refractory to treatment. TiO2 is not unusual in this regard- it's true of all treatment technologies.
Personally I think TiO2 is one of the worst choices for high COD, high TSS wastewater treatment. It wouldn't even be on my list, unless there were absolutely no alternatives. For low/no TSS, low COD groundwater treatment of typical organic micropollutants, it can certainly be a choice- but I would absolutely want to see an apples-to-apples comparison in terms of both operating and capital costs for treating the same water, based on actual TESTWORK using a split sample of the real water, before I will believe that it is more energy- and cost-efficient than properly designed UV/peroxide treatment. And unlike TiO2, UV/peroxide has no solid catalyst to deal with, which makes it operationally much simpler. And unlike TiO2, UV/peroxide has no intensity dependence, such that one can use more powerful and more compact UV lamps, making the treatment system physically smaller.
One of the problems for potential users of TiO2 photooxidation and all other "advanced oxidation" technologies is that although there are a few reputable firms who have well-developed and reliable treatment systems to offer, there are also a large number of disreputable companies out there who have nice-looking websites and virtually no successful installations in the field. Choosing an unproven firm can be a significant risk. Even if they have the process photochemistry working well, there's no guarantee that they have done the process and mechanical engineering properly and have a treatment system which works reliably without significant operator intervention and maintenance. Something as simple as the wrong choice of metering pump can result in an operational headache that may result in the discharge of improperly treated water, depending on the system design. Some of the firms mis-state or mis-calculate the costs of treatment, neglecting such costs as lamp or catalyst replacement, or understating the costs of reagent addition.
My strong suggestion when dealing with this or any other treatment technology: get it tested on a representative sample of actual water. NEVER trust a "spiked tapwater" test as representative of your real water- it virtually never will be. Believe me, that goes for treatment technologies as simple as activated carbon- if you don't do the testwork, chances are you'll be surprised by how high the real operating cost is on your real water. Most reputable advanced oxidation firms will do this test work either for free or for a fee which is credited against the purchase price of a system. Get the reagent addition, total electrical consumption and lamp replacement/lamp lifetime cost rates per unit volume of water treated, and then get your own, local costs for the reagents and lamps. Estimate the required operator intervention and the resulting labour cost. Use these values to calculate the actual operating cost of treatment, and make your decision on the basis of both capital and operating costs.
I hope this is of help to you.
Hey I have really liked this discussion on Tio2 properties and have a question for moltenmetal. I agree with most of your points, but I am curious to know if n-type doping on Tio2 will help increase the quantum yield. I have read in a patent that it will and it kind of makes sense to me since a semi conductor application will reduce the band gap. I would want to know if you have any comments on this and would be glad if you could share information on the practicality of this approach.
Thanks
Inqiztiv
Thanks
Inqiztiv
moltenmetal
Chemical
I am by no means an expert in TiO2 photochemistry and my knowledge is limited to TiO2's use in photochemical water treatment. From what I do know, the practical quantum yield, expressed in terms of the number of lamp photons needed to destroy/transform a given mass of target material in a given volume of water, is not so much determined by the properties of the semiconductor photocatalyst as by the short-lived nature of the product radicals and the limitation of diffusion of contaminants to the surface and radicals from the surface, along with a number of other practical factors. I also know that most attempts to support the catalyst to make it easier to handle (i.e. not a slurry) result in considerable decreases in practical quantum yield.
What results from the diffusion limitation is an intensity dependence, a "double heterogeneity" if you will, which limits the amount of energy which a given area of exposed TiO2 slurry can absorb before a doubling of UV intensity (photons per unit area) results in only a modest increase in the destruction rate of contaminants. As that occurs, the effective quantum yield drops. What results is that to remain efficient in use of photons, an impractical number of low-intensity lamps required, and using solar concentrators as collectors to make the apparatus physically smaller and less expensive becomes impossible.
A number of other practical factors combine with this effect to render TiO2 photolysis less practical than other photochemical methods like UV-peroxide, particularly if enhanced by the use of soluble photocatalysts.
What results from the diffusion limitation is an intensity dependence, a "double heterogeneity" if you will, which limits the amount of energy which a given area of exposed TiO2 slurry can absorb before a doubling of UV intensity (photons per unit area) results in only a modest increase in the destruction rate of contaminants. As that occurs, the effective quantum yield drops. What results is that to remain efficient in use of photons, an impractical number of low-intensity lamps required, and using solar concentrators as collectors to make the apparatus physically smaller and less expensive becomes impossible.
A number of other practical factors combine with this effect to render TiO2 photolysis less practical than other photochemical methods like UV-peroxide, particularly if enhanced by the use of soluble photocatalysts.
moltenmetal
Chemical
I'm no expert in band-gap photolysis, but excited states capable of generating radicals don't hang around long- their mean lifetime before quenching is measured in nanoseconds. They're quenched by production of some other species- a hydrated electron, "hole", or radical species (i.e. hydroxyl radical etc.) rather quickly if they're formed at the surface. Considering that the bimolecular reaction rate coefficient for the reaction of hydroxyl radical with most species of interest is on the order of 10^-8 L/mol s, you can see that these radicals are very short lived also.
What exactly is your concern?
What exactly is your concern?
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