nm0693: i guess in the last three lectures what we've been looking at has been 
has been the generality of photochemical processes and today i want to continue 
with the notion of specific problems and looking looking at them in some depth 
as illustrating er the sorts of things that er you can do with photochemistry 
now last week we were looking at the splitting of water with sunlight and we 
recognized that water doesn't draw sunlight at all but if you put it in with 
the right catalyst and the catalyst we talked about very much was this 
ruthenium catalyst then provided you had a cocatalyst there as well then you 
had the possibility of the ruthenium complex splitting the water into hydrogen 
and oxygen and this could be done but there were drawbacks and that the 
drawbacks you remember were that er particularly er the catalyst didn't go on 
for ever the yields were rather low and you had to have a sacrificial donor in 
the system now if you were going to do that well you might 
as well say we'll go to biomass but it looks as though photoelectrochemistry 
where you actually generated an electric current from such a system was a much 
better bet and work is still going on with that now what i'd like to talk about 
today is something er which is related but different and that is the use of 
irradiated er catalyst that absorbs the U-V components of sunlight to destroy 
organic pollutants and er that will be the subject of today and the f-, the 
first question is er why should one er choose titanium dioxide as a a a thing 
to do this er there are various other things that one could have maybe thought 
about but er i'll put down er almost a kind of summary of the lecture er before 
i begi-, er before i've given it and that is that er it's very cheap and it's 
readily available T-I-O-two as i said before is used as a er the whitener in 
emulsion paints er for getting a white finish on any gloss paints as well it's 
also environmentally harmless there's er there's 
sacks of titanium all around the world and nobody suffers from it we know f-, 
er that it's got a very good turnover number in other words you can irradiate 
it again and again and again and again and again and it will still function er 
it turns out to be efficient for a wide range of pollutants and i'll illustrate 
that in the lecture er another advantage is you can attach it to supports 
rather readily in other words er if you take T-I-O-two which is a er a white 
powder a bit like talcum powder and you stood it up in water or even sonicate 
it with s-, with a s-, with a sonicator you know sound wave emitter er you get 
a suspension if you lay the suspension onto glass and allow it to evaporate the 
T-I-O-two powder sticks to the glass really very fiercely and it's really quite 
hard to get off particularly if after you've layered it you you heat it maybe 
to er a hundred degrees or so for a twenty-four hours it becomes even more 
firmly attached to glass and it means that er you've got a stable thin 
layer er which will be c-, really quite a useful advantage you might have p-, 
predicted at the outset from what we said last week from the er the other 
lecture where we talked about T-I-O-two it's got rather a large band gap er 
under solar irradiation there isn't a lot of U-V but there is some so that 
there's a reasonable chance that you'll get something out of it but clearly if 
you could activate it by er putting in maybe a t-, an inorganic ion strip in 
the lattice to move the action spectrum towards the visible then you could 
probably increase its er its efficiency er as i've as i've indicated there so 
those are the er those are the main features and i shall spend quite a bit of 
time er underlining those notes er throughout the lecture okay have you have 
you got have you got all that 
sm0694: not yet 
nm0693: not yet okay i'll hol-, i'll hold for on a sec while you er try and get 
it down 
nm0693: right what i'd like to do next is t-, for us to sort of think about 
what a T-I-O-two particle titanium dioxide particle is like and we have 
here a very er schematic view of what it's like so it's irregular it's roughly 
roughly kind of spherical and essentially the two points to note are these 
you've got within the crystal because it's a semiconductor you've got a valence 
band and a conduction band got these two energy levels but they're really bands 
where i've just drawn them as lines for the to simplify things and when you 
optically pump the thing by shining light on it you push an electron out of the 
valence band into the conduction band just as just as i talked about last week 
and the a l-, hole here this H-plus or the absent electron this will migrate 
from within the crystal to the surface so you've got to imagine this in three 
dimensions of course er so you can imagine to get to the surface and you've got 
holes dotted around the surface as i've indicated there the electrons also 
migrate to the surface they also of course stand a good chance of er 
recombining and if you didn't have anything else in the system they would 
recombine and 
nothi-, not very much would happen but er what happens is that you're able to 
fix the charge separation because essentially it's in water and the water is 
being purged with oxygen or even less extreme if you've got some air in in the 
water and we would normally have some air there and certainly if you were 
talking about er river water lake water something like that er then there will 
be there would be oxygen present from the air at about you know twenty twenty 
per cent of the er dissolved gases would would be oxygen so the oxygen in the 
solution will migrate around of course it will collide with the particles and 
it will trap the electron and make it O-two-minus the so-called superoxide ion 
and if it's trapped as this then the electron is no longer mobile because it's 
become O-two-minus the other thing that happens is that the H-plus is a very 
powerful oxidizing agent and it will oxidize water to O-H radicals so quite 
quickly all of these H-pluses will react with the water on the 
surface of the particle and they will be converted to O-H so you've got O-H 
which is an oxidizing agent upon the surface and you've got O-two-minus which 
is also an oxidizing agent it's not as powerful as O-H but it's still certainly 
not a reducing agent er the other thing that can happen is that the O-H is 
absorbed going to move that into a solution so you've got the absorbed O-H 
becoming a solution O-H the O-two-minus is absorbed and with that become an O-
two-minus in solution so all of the species produced by further excitation end 
up either by being oxidizing agents absorbed on the surface or they become 
oxidizing agents out in solution and this is the this is the basis of the the 
photoreactivity o-, of T-I-O-two you're generating all these oxidizing species 
now before going any further into T-I-O-two and er er its various merits i will 
er briefly er cast an eye over one or two competitor type systems that you 
might have thought about and these have all been tried er as 
you can imagine er this is quite a complicated er transparency you may not be 
able to draw all of this but i'll just highlight what's important so what have 
i got down here well this is quite a a complex picture er the point here is 
that er you can look at a whole series of other semiconductors and all of the 
all of these that are quite well known because they've all been used er in 
photoelectrochemical cells so the technology has already been explored for 
these things and what i've got here are the band gaps the band gaps er the 
distance between the valence band and the conduction band the T-I-O-two is 
about three-point-two electron volts as you go to some of these ones then it's 
getting less one-point-seven one-point-four er two-point-four these are 
coloured of course because the band gap is now in the visible and these things 
are either orange or yellow er or whatever er zinc sulphide white 'cause you've 
got really really quite a big band gap there and strontium titanate is very 
similar to this W-O-three is probably just about on the 
edge of the o-, of the on the edge of the visible so it will have a faint maybe 
a probably a a a a a slight colour you know slightly coloured yellow er the 
point is that er the the band gaps for these are very attractive but 
unfortunately er the flip side of having a narrow band gap which would be very 
sensitive to visible light which is a good thing is that you don't actually get 
enough in the way of potential to er oxidize the water through to O-H you've 
got if you want to get O-H radicals you've got to get past this well this 
potential here as we run across the bottom so in a sense er there's a whole all 
all of these ones are going to fail this might just just about creep in er but 
the ones that are going to win on energy are those those three there er these 
which might have seemed attractive simply don't win on potential you've got to 
get the oxidizing potential to get the oxidation to O-H but you've also got to 
try and match the band gap as best you can to what's in sunlight if you're 
going to use 
sunlight now if you decide you're not going to use sunlight at all but you're 
going to use fluorescent tubes you stand a much better chance because here you 
can make tubes emitting U-V with a a a a high level of efficiency so the way 
thinking has gone is to move away from er using sunlight er not completely away 
by the way but more towards using U-, U-, U-V tubes so the idea would be if you 
had er a a factory emitting some polluted water say polluted with low levels of 
really poisonous materials but down at say a micromolar if you passed that 
effluent past a bank of U-V tubes all switched on and if you had the T-I-O-two 
present in the system in some form it wouldn't be a suspension obviously 'cause 
it would be washed away but if you can immobilize the T-I-O-two on sheets or 
rods or fibres then you would destroy the polluting organic and you stand a 
reasonable chance of cleaning up because i can s-, i can tell you sort of in 
anticipation this is a remarkably 
efficient system it works really quite well and much better than you'd ever 
believe from the outset okay so the the messa-, the message from that slide 
really is that er you got to get past this point here to get O-H radicals and 
those three work quite well this is another possibility and those don't really 
s-, you know stand much of a chance okay right so i'll take that off now hoping 
that you've got the the the basic message without copying everything down 
furiously er what i'd like to do next is er perhaps show you how some of these 
er semiconductors compare with each other so you can just see whi-, you know 
which really are the good ones and and the not so good ones and what i've got 
here is a slide and this is where we're looking at er pentachlorophenol now 
this is all the chlorophenols are quite good things to look at because many of 
the pollutants that er er eman-, emanate from er waste sites and dumps and the 
like they've been partly converted by bacteria from 
something like er polychlorobiphenyls into chlorophenols and so the 
chlorophenols tend to be the water soluble form which gets leached into the 
aquifers or the water table and get into rivers so the chlorophenols have been 
looked at very closely because they are really ideal model compounds to work 
with and here we've got pentachlorophenol er and it's a concentration of about 
i suppose four times ten-to-the-minus-five mole per litre this has been 
irradiated various semiconductors are present and you can see T-I-O-two is here 
Z-N-O is there this is cadmium sulphide and then up here you've got W-O-three 
and tin oxide and you can see that by far the best two are the T-I-O-two and 
the Z-N-O which the previous slide would have would have led you to believe so 
the previous slide was a kind of theoretical prediction if you like the 
rationalization this is the experimental result er i suppose the thing is that 
er er C-D-S is rather better than we might have thought it was but it's still 
nothing like as good as T-I-O-two 
or zinc oxide so er that's er a a a n-, a nice sort of comparator experiment on 
pentachlorophenol the next question is i suppose what actually happens when the 
er the light falls on the particle what are the subsequent reactions after what 
i've described so i'll er i'll show you er that as on on the next slide this is 
quite a complicated one and i'll follow it up with one or two rather easier 
ones so here we go er and i'll take you through it fairly slowly okay can you 
actually read read those on there yeah i'll i'll call them out anyway so we 
start off on with T-I-O-two with light we get the whole we get the electron so 
i said before the electron picks up O-two to give O-two-minus er it's not very 
well done as a as a minus but it ought to be a minus dot really 'cause it's a 
free radical this is a O-two-minus-dot an O-two-minus-dot can go on er it can 
react two of these react together to give you these it gives you H-O-two you've 
got it can react with protons and it's 
slightly acidic to give you H-O-two radicals and H-O-two itself is is er a weak 
oxidant it can attack organics in the absence of the organics er the thing will 
go on it will form oxygen and hydrogen peroxide er it can pick up electrons to 
give this that's E-minus of the proton is equivalent of H atom and H atom will 
react with H-O-two to give you H-two-O-two and then further electrons will give 
you O-H radicals and then you've got further reactions here so you end up by 
making some more O-H radicals if the thing is not intercepted by an organic so 
that's the er that's what's happening to the electron it's being converted 
ultimately in a number of steps through to hydroxyl radicals er unless the 
thing gets picked up beforehand this side you've got H-plus and the H-plus 
surface gets on to the surface it ma-, it the contribution system gets to the 
surface oxidizes the water to O-H radicals the O-H radical sticks er on the 
surface near to a titanium er what's going to happen then well basically if 
you've got 
er in the system er an organic which we can denote R or a f-, organic radical 
or just set it on carbon or another organic radical which is basically a a semi-
oxidized alcohol er all of these things react with er all of those things which 
have come from here you see all most of these will spill down over into there 
now and you get oxidized species so ultimately after many many steps and if you 
think about it if you've got something like pentachloro phenol or even a thing 
like say phenol itself to get phenol through to carbon dioxide you've got many 
many electrons needed you need to take many electrons out of phenol to get it 
to a point where the phenol molecule is often being converted right through to 
C-O-two the process of converting these organics right through to either C-O-
two or if you've got a chloryl organic to chloride anion which is harmless er 
both of these are p-, are pretty harmless er compared with the start material 
then it's called mineralization because carbonic 
acid and H-shell are mineral acids so the whole process is called 
mineralization or if you like photomineralization so this this is a f-, a a f-, 
a fairly fairly complete mechanism er it's told you what's happening to all 
these oxygen radicals it hasn't said very much of what what's happening er 
department there all you can er er all this says at the moment is that these c-,
these carbon centres are either organic er or an organic radical or a 
hydroxylated organic radical all of these three will all of these will get 
oxidized in in the process so that's a k-, a kind of total scheme so it's an 
elaboration of my particle picture i've now got a lot more reactions there okay 
have you have you got that well now just in slightly more detail er i want to 
try and explain the kind of reactions that go on when any organic system is 
being oxidized so thi-, thi-, what i've got next is not just peculiar to this 
system this is a generality for the oxidization of all organics including 
polymers as it happens so 
here we go well the one i've got i've put it down er it's actually for the 
oxidation using hydrogen peroxide er you can use that instead of T-I-O-two er 
but it's not as good because you use it up but the chemistry is exactly the 
same if you irradiate hydrogen peroxide with pentachlorophenol you destroy it 
er but of course you destroy the H-two-O-two as well the T-I-O-two you 
constantly regenerate the T-I-O-two it goes on and on and on er going through 
all these steps but the the key point is this right so we start with off with 
an organic and we'll call the organic molecule whatever it you know it can be 
anything you like phenol benzene er ethanol a dye stuff we'll call it H-R-H 
that's two hydrogens bonded to quite a complex network of carbons but we'll 
just call it H-R-H you've got the O-H radical formed i've got it formed from H-
two-O-two there but of course it's formed as we've already said from T-I-O-two 
under photolysis so the O-H radical is formed here and it attacks the H-R-H it 
pulls a 
hydrogen off to give us water so the O-H becomes water the H-R-H comes through 
here and becomes R-H-dot so there's our organic radical the organic radical if 
you didn't have any oxygen in the system it would almost certainly dimerize to 
give you a polymeric product and you do actually get some if you have a look at 
these phenols you do get some er polyphenols in the system you can get dimers 
from trimers of phenol as a side product but they tend to be fairly minor 
normally of course you make a point of having oxygen there you make sure the 
system is has got air absorbed in it or if you're a bit worried about that you 
can blow air through it and if you want to be really definite you can blow some 
oxygen through it because what we tend to do in the laboratory is not really a 
viable proposition for an industrial plant mind you could do it but it would 
make it more expensive you don't need to really R-H will will react with oxygen 
to give a peroxide radical that's an organic 
peroxide radical and it's got er normally it's written as R-O-two-dot but 
because we've written this as R-H-dot we'll just put the oxygen on and call it 
R-H-O-two-dot so this is this is a peroxyl radical R-O-two-dot now the R-O-two-
dot can do all sorts of things er and there's a there's a variety of things 
that it can do there er it can reverse that is not particularly important i 
will say er but what it can also do and this is the important one it can attack 
another R-H-R-H which i've written here by pulling off a hy-, a hydrogen and we 
end up by getting R-H-O-two-H there and we go back to R-H here so basically 
you've got little a little chain reaction going along here where you're 
constantly diverting H-R-H through to the this hydroperoxide so you convert it 
through to the hydroperoxide and then you've got some other steps up here er 
you've got various si-, scissions this is to actually cleave to give an R-O 
radical er you can go up to R-H-plus you can go round to O-two-minus 
and get back to H-two-O-two but i would say the important the really important 
steps here what the first one is the abstraction by O-H to take you through to 
here the next important step is the picking up of oxygen to go through to there 
the next step is to go round this way to give you a hydroperoxide so you've now 
taken your organic which started out like this through to there but of course 
it doesn't stop there because what then happens is the the whole thing starts 
all over again this time you would write R-H-O-two-H in here and again you 
would pull off another hydrogen atom and you'd take it to one higher state of 
oxidation so it goes round and round and round and round and every time it goes 
round you strip one hydrogen out and you put an O-H on so you know you in the 
end you end up with if you like if you imagine a carbon with four hydroxy 
groups around it that's all that really is C-O-two and some protons so er you 
are really you're really taking the through through the 
thing through from a fully reduced form with carbons with lots of hydrogens 
through to a very highly oxygenated form that finally becomes C-O-two so that 
is the er that's the sequence of reactions in in the H-two-O-two U-V process 
it's exactly the same if you have the T-I-O-two there as well if i wrote T-I-O-
two for H-nu it's ju-, it's exactly it's just the same sort of thing in fact 
people working on these systems there are three favourite things to work on 
one's T-I-O-two one's H-two-O-two and the third one's ozone 'cause if you 
irradiate ozone you get O-two molecule and an oxygen atom and the oxygen atom 
inserts into water it's extremely reactive and gives you two O-H radicals so 
the whole thing is centred on making O-H radicals O-H radicals are the great 
purifying radicals in this life now this system has been looked at very very 
extensively er with all manner of pollutants and i'll give you a t-, a a 
another slide now with about a million compounds on and you're not to try and 
write 
all these down but you could just maybe write down well well one or two 
examples so er here we go so these are photomineralization of organic 
pollutants sensitized by T-I-O-two examples of compound studied and the very 
simplest compound methane pentane dodecane they go hologenated hologenated 
alkanes tetrachloroethane dibromoethane so for all these rather dangerous 
solvents you know which at one time were very beloved of the dry cleaning 
industry er can be degraded using the T-I-O-two system alcohols absolutely no 
problem acids the next stage up from alcohols no problem alkenes straight away 
chloroalkenes again dry cleaning solvents they can be destroyed and the dreaded 
aromatics like benzene they can be destroyed halogens well here we've got 
dichlorophenol dichlorophenol these are all the de-, degraded products from 
things like polychloro biphenyls they're also degraded products from dioxins 
the famous er Seveso disaster in Italy where er a whole area er was heavily 
contaminated 
a lot of people er died from ingesting er the these toxic materials the 
straightforward phenols are vulnerable carboxylic acids polymers are not 
they're not quite so easy to do with polymers but they you can degrade polymers 
with T-I-O-two er 'cause these aren't water soluble so it's it's not it's not 
straightforward you have to grind them into a powder or do something to them to 
er expose them properly to the T-I-O-two but T-I-O-two will will will remove 
them whole series of er surfactants er all manner of surfactants of course get 
out into the aqueous systems into rivers streams and lakes er they're used very 
extensively in industry and also domestically and they're really quite di-, 
difficult things to get rid of er but T-I-O-two will er attack them all then 
you get down to herbicides er simazine and things like that the pesticides like 
D-D-T parathion and lindane and then also dye stuffs they're quite important 
because a lot of factories that use dye stuffs they're only allowed 
to discharge at very very low levels so they have to process their effluent 
themselves otherwise they're subject to heavy fines and they're always looking 
for they normally do this by some sort of chemical means but if you get down to 
very low levels then you can use T-I-O-two to finish off the to finish off the 
job so a vast array of compounds and they all go through this organic radical 
route er by going to giving you er O-H attack it to give a carbon radical which 
picks up oxygen to give a peroxy radical and you get a hydroperoxide and then 
that in turn is destroyed further so that's the kind of thing that er that goes 
on er this is er er not as not as important a slide but i'll i'll i'll just er 
show you the the type of thing that people study here er there are two ways of 
going at this really one is to try and analyse the pollutant disappearance er 
which you can do by monitoring the pollutant level by G-C-M-S or something like 
that the other is to look at C-O-two evolution and 
er here are we well beyond looking at C-O-two er evolution and we are measuring 
pollutants at various concentrations in milligrams per per cubic decimetre and 
er as you can imagine as you begin to increase the concentration of course 
you'd expect the thing to plateau out because as you raise the concentration 
you're beginning to saturate the surface of the T-I-O-two particle with the 
thing you're trying to destroy and so if you go to er er to to levels more than 
er around here then you're not going to have such an efficient process simply 
because you've reached a kind of a saturation of the level it's very much a 
kind of Langmuir type kinetics er whether you can remember i think i talked 
about Langmuir kinetics even in first year to you but er Langmuir's idea was 
that every every surface of a catalyst has a certain number of sites and when 
the sites were filled then you've got no more catalytic action and you could 
double treble multiply this concentration by a ten or a 
hundred you'd get no more joy in the way of them producing C-O-two or in this 
case or whatever it was because you've s-, saturated the catalyst you can see 
the saturation coming in here so this illustrates that the the Langmuir type of 
idea is valid for T-I-O-two a lot of people have done done very detailed 
surface kinetic study of these systems for a number of things a number of 
different pollutants and they find all the time the sort of behaviour i've 
illustrated here you you plateau out now this doesn't matter too much because 
industrially the sorts of things you're trying to remove like the chlorophenyls 
and dioxins and things surfactants are actually at very very low levels they're 
very harmful but they are at very very le-, very low levels and so the problem 
is one that you can reasonably attack because you do not normally have for the 
pollutant levels sort of you know at this end you tend to be more towards that 
end so the t-, the the thing is in your sights you can er stand a 
reasonable chance o-, of getting a a conversion to er free up the system the 
question of how good a catalyst i-, the the T-I-O-two is of course er also 
revolves around does it wear out do you come up against having to replace it 
quite frequently how often does it turn over and this is a a a n-, a nice 
little slide here i think er it's only er ten cycles but what what have we got 
here well we've actually got four-chlorophenol which is again a very typical 
type of er thing to be looking at and we got one portion of T-I-O-two only one 
portion we start off er at this concentration here of about er three times ten-
to-minus-er - er - six micromolar you shine the light on and you see the level 
falls away rapidly and as the f-, four-chlorophenol is is being destroyed then 
what you do is er inject into the system er a fresh quantity syringe the four-
chlorophenol away it goes you inject f-, a fresh amount away it goes so you're 
you're acid we use in a catalyst to completely destroy or very nearly destroy 
your four-chlorophenol 
then you if you use some fresh four-chlorophenol and you see it goes on and on 
and on although i think they did they had ten goes at it and then they probably 
got bored er after that 'cause they got up to twelve-hundred minutes er which 
is i suppose er quite a long time and probably the graduate student who er was 
doing this just got tired and wanted to go home so that er it w-, it wasn't 
taken any further but i think you get the you get the picture the thing is very 
er adaptable it's very recyclable it takes a long time to exhaust it i i i can 
add a sort of footnote to that even even when it begins after many many many 
cycles maybe hundreds to become exhausted you can actually reactivate it you 
can you can er take it wash it er and heat it to quite a high temperature let 
it cool down it starts off all over again so you i-, it is really renewable so 
it it's really kind of er er almost hypnotically successful you know people er 
really become very enthusiastic about it a-, as a way of er er trying to 
degrade noxious organics er 
how do you sort of set up things like this how do you get it to work these 
these are some of the practical points now and then i n-, i g-, moving away 
from theory there are there are there are s-, various ways of doing it er what 
you can do where we've got we've got er two two set ups and these are two flow 
reactors flow reactors are probably much better in a way because they model 
what you want to do in treating industrial effluent much better than batch 
reactors now you you don't just sort of have to take er a batch of water purify 
it and then take another batch and purify it you want to be able to purify a 
constant stream as it comes from some site or other so what have we actually 
got here well er you've got er in this one you you're flowing your material er 
well you you've got you start o-, to start off with you've got your fluorescent 
tube okay just like an ordinary ordinary fluorescent tube and then coaxially 
positioned to this you've got an outer glass jacket and you flow your solution 
through 
here up there and out the top and all the way through here these sort of things 
are a glass mesh or they could be glass helices they're all coated with T-I-O-
two it's very easy to coat the glass with that er you know we sort of do it all 
the time er and as the stuff flows up the fluorescent light is on the U-V is 
coming this way it hits the helices where the T-I-O-two is layered with a thin 
layer the T-I-O-two becomes activated by the light you get the degradation 
occurring so you pass in the chlorophenol or whatever it is at the bottom and 
at the top you get C-O-two and chloride and that's one way of doing it er 
another way of doing it is in fact to er it would simply spiral the ethene 
round the fluorescent tube and the inside of the glass tube is coated with the 
T-I-O-two if you have a very thin layer that's okay the light the light will 
get through a very very thick layer of T-I-O-two so er you can you can do it 
this way so these these are two kind of flow systems that you can you can 
operate so 
that's a that's a kind of a just just a technical point but i mean you know you 
might wonder how you're going to set about doing these things er if you want to 
do a er a batch reaction well you can do it er i guess the first time you ever 
study these things you tend to do them in batch so but you this would be n-, of 
not much help in er trying to er establish a kind of industrial process but 
essentially here you er have got er er yeah you've got all your tubes in here 
you've got a bank of tubes maybe six or eight in in each of those you you sort 
of bring them together er in a core here B is where you've actually got your er 
reaction where there's your sample er and you're passing through some oxygen 
continuously er through through the top through through this er this cylinder 
here and er it bubbles round and uses the pumping system to pump it around so 
and you've also got the stirrer wheel okay so that that would be a typical 
batch thing er i've got a system here that er i run from time to time which is 
slightly different 
from that i blow the oxygen up the bottom of the tube and at the sort of bottom 
of the vessel through a glass sinter and as oxygen goes through the oxygen 
pressure er keeps the aqueous solution above the sinter and the sinter the 
oxygen into thousands and thousands of tiny streams and so you get a very very 
good sparging of the oxygen as it goes up through the solution very good er er 
er use uses of the oxygen because you you've broken it down into tiny bubbles 
you get maximum bubble exposure to to the solution er another way of er er i've 
got another another slide here er if you're looking at er i mentioned before 
when you were trying to analyse what was going on you can with er 
pentachlorophenol you can analyse it by G-C-M-S but much easier is to look at C-
O-two evolution and also quite easy is to have a sensor electrode for chloride 
and again in the studies we do here we use a chloride unselective electrode 
which simply d-, measures chloride as it's 
developed as the organic chloride breaks down to chloride you can follow the 
chloride electrochemically you might remember in the first year i think there 
was a experiment where you hydrolyse T-butyl chloride and you measure the 
chloride evolution fro-, from the hydrolysis by looking at the conductivity do 
you remember this experiment from your your misspent youths well anyway er that 
used to be in the first year even if it's isn't isn't now it's quite easy to 
have a a little sensor electrode which is sensitive to chloride you have this 
dipping in and you simply measure the chloride that's produced if you're 
looking at dye stuff degradation it's much easier so this is if you've got a a 
a ni-, a nice big chromoform on your molecule and here this is metheylene blue 
which is a good model for quite a few dyes as quite a few dyes have got a 
structure similar to metheylene blue and metheylene blue absorbs er well it's 
blue of course it absorbs in the red a bit of a bit like a sort of 
solution and you start off by having an absorption band A there and as you 
irradiate at various times the thing falls away like so so you end up with that 
as your baseline and you you s-, you your baseline's rather high 'cause you've 
got T-I-O-two buzzing around giving you a quite a bit of a background and if 
you measure the peak maximum there the plot of the absorbence versus time 
normalize it to the baseline you get that sort of thing occurring and you can 
see that er with this metheylene blue that's ten micromolar roughly er you only 
need to expose er for five minutes and it's totally destroyed er i've seen this 
done as a demonstration it's quite effective starts off with deep blue ends up 
water white so it's really quite a good er quite a good demonstration type 
experiment and here we're actually we've got a a a project running at the 
moment with one of the M-chem students who's going to be looking at other dyes 
er being destroyed by by this type of system er one or two 
other small er kinetic points er that er are p-, are probably worth making i 
mentioned before that if you vary the concentration of the molecule you're 
attacking your substrate then it follows Langmuir type kinetics in other words 
to begin with if you double the amount of organic you double the rate of 
degradation but quite soon it turns over to a plateau because you're saturating 
the surface of the particles with the organic you're wanting to destroy and 
that any more isn't isn't mu-, really much much use the other thing you can do 
is to look at the dependence on the light intensity and you might say well if i 
go from a hundred watt bulb to a two-hundred watt bulb to a four-hundred watt 
bulb to a kilowatt bulb am i gaining or is it worth the extra energy input 
there's been quite a bit of work on that and again we've done some work here on 
it as well this is not our results but our results are very similar this is 
actually degrading isopropanol er okay and you're destroying 
isopropanol using T-I er well it says rutile that's one of the forms of T-I-O-
two and you can see you what you what what's been plotted here is the log of 
the rate of the acetone formation you can measure acetone quite easily by again 
by G-C er and here is the log of the light intensity and if you've got if you 
remember that's really if you remember from your again your again from your 
first year kinetics lectures that er if you've er got a dependence which is 
like this and you've got R is equal to K times some sort of thing like 
concentration if i call it I for light intensity and you've got that power A 
okay then what you can do is take logs say log- R is equal to K log-K sorry 
plus A- log- I okay that's taking logs that's actually f-, you know first year 
type work and to find out what A is what is the value is it depending t-, on 
the light intensity of the first power the second power no power at all it does 
it not depend on the amount of light you put in then you can 
find that out by plotting the log of the rate of the reaction 
photomineralization in this case versus the log of the light intensity okay so 
that that that is er first year revisited and you ought to keep you you rev-, 
revisiting first year chemistry for as long as you do chemistry because all the 
fundamentals come out there you can see here that this this line going up here 
is is is a good nice forty-five degrees it's a forty-five degree line it means 
the thing is strictly first order the slope is one-point-nought and so to begin 
with the and if you double the light intensity you double the rate but when you 
get past a certain point you find that the slope falls exactly to half it falls 
from one-point-nought to nought-point-five so it seems that we can go to very 
high light intensities that at the rate of evolution of your product is no 
longer first order in the light it becomes half order in the light so if you 
double the light you only increase the intensity by one-point-four er 
that's an interesting observation er and you might think well i can understand 
easily why it is if you increase the organic concentration you saturate the 
surface but surely the more light you put on the system surely all the time the 
more efficient the process would be 'cause you're getting more excited states 
which would give you more O-H radicals et cetera et cetera has anyone got any 
idea why it is that it falls away as you get to very high light intensities any 
thoughts on that imagine you've got a particle tiny particle it's being 
irradiated with light and you go on increasing the light more and more and more 
these are logs here you know this is a er enormous dis-, you're going through 
between thirteen and eighteen that's five orders of magnitude on the light 
intensity a hundred-thousand times this is a very detailed study when you go to 
extremely high light intensities what do you think happens to the whole 
electron pairs formed in the particle any ideas 
well you've got a plus and a minus they're going to move to the surface to s-, 
to localize this plus and minus but what happens when the concentration of the 
pluses and the minuses on the surface becomes extremely large what will they do 
to each other 
sf0695: they destroy each other 
nm0693: they yeah they they mutually destroy each other if you get to growing 
high light intensities you get to the point where the positive centres and the 
negative centres although they want to react with the water they want to react 
with the oxygen they're in such a high concentration now there's quite a good 
chance they'll actually kill each other off and so you actually begin to lose 
out once you've gone beyond a certain light intensity the benefits of going to 
even higher light intensities and i mean enormously powerful lamps tend to be 
lost so that that again that's an a a a an environmental point well worth kind 
of taking a note of okay er i can er maybe er finish off with a a few more 
advanced experiments 
that have been done and er i'll just look at the kinetic-, i've ju-, talked 
about the kinetics several times i'll maybe underline that now by putting up 
the normal rate of degradation okay so this is for any system now er using a 
semi-, any semiconductor under illumination and what i've got there is the rate 
of degradation we've got you see got a Langmuir term here for oxygen and what 
this what it is what this implies is that if you keep increasing the oxygen you 
gain benefit but eventually you're going to saturate the surface with oxygen 
this is the organic chlorophenol you can go on adding chlorophenol but 
eventually when it when it becomes very high er that term there one plus you 
know on the right here if C-P is very very high the one is almost negligible 
and that will then cancel with the thing above and so then it becomes zero 
order in the chlorophenol in other words it's reached a plateau same with the 
oxygen so you can you can oxygen is a good thing and 
chlorophenol's a good thing but you can have too much of both okay that's the 
that's the message there and then er you've also well if you've er yeah i think 
i'll i'll probably skip that i mean er er i think i mi-, i might skip the next 
bit i think i'll leave it at that with just this warning er i've talked about 
the light intensity before er it's gamma-I-A-to-the-power-M and M is one-point-
nought low light intensities and it's nought-point-five at high light high 
light intensities okay so i think that's m-, kind of summarizes what i what i 
said before in trying to go on and look at er other detailed aspects of the 
process one of the questions is this does all the reaction take place at the 
surface of the particle or do some of the O-H radicals escape into solution and 
some of the superoxide ions escape into solution as well and do we have a 
solution process as well as a surface process and the question er is is here 
again quite an in-, quite an instru-, important one mechanistically is it the 
whole thing surface limited or is we've got a surface process and a degradation 
solution so we began to think was there a way of probing this in some detail 
and what i've got here is an experiment er which has only been i've only done 
about two or three years ago and this is er one where you've got er this glass 
slide here with a very thick coating of T-I-O-two okay inside this little bath 
you've got a dish you've got solution of chlorophenol so the chlorophenol is in 
there here you've got a microelectrode and if i say the word microelectrode you 
ought to think of the w-, the two words namex 'cause he's the chap who makes 
all the microelectrodes here he's one of probably the U-K's leading expert on 
microelectrodes and this this electrode here is as low as five microns in 
diameter five microns i think the one we used was about twenty-five microns but 
he's improved them quite a bit since then this is simply the reference 
electrode saturated calomel electrode in the 
usual way you always have to have a reference electrode the light comes from a 
lamp here going through a bank of lenses and bounced off a mirror to arrive 
there so collect at a highly focused spot of light there on the T-I-O-two film 
and the experiment consisted of looping this microelectrode nearer and further 
away from the T-I-O-two and the question was would you get a bigger chloride 
ion development very close or would you get any further away and could you 
model it with a computer so this is er a er a probe this is a probe type 
experiment these are becoming really very important er all manner of probe 
apparatuses have been er designed and perfected in in recent years and people 
are now able to look at almost atomic resolution with some kind of probe with 
this electrochemical probe rather than optical probe and this is the this is 
just the result of the experiment er when i-, if you turn the light on here you 
watch the voltage develop here and the voltage 
records the amount of chloride that's developed okay and you could see at ten 
microns you turn the light on very quickly you get chloride developing and then 
gradually you e-, you're exhausting the chlorophenol it tapers off you turn the 
light off and the chloride ion dissipates and moves through the solution so 
clearly mass transport's quite important the chloride ion er moves away from 
the electrode tip very soon after you turn the light off that's at ten microns 
between the tip and the T-I-O-two so imagine there's the tip of the electrode 
here is the T-I-O-two here and you're measuring chloride being produced in that 
region if you move the tip eighty microns away then the development is much 
much much smaller and what this tells us is that virtually all of the action is 
taking place very close to the surface and we can model these curves very 
precisely on the basis of purely a surface process you don't need to invoke a 
reaction occurring in solution and 
always in chemistry and in science in general if you've got two possible 
explanations one is simple and one is more complicated and the simple one is 
actually works slightly better than the complicated one then you always say the 
simple one is right and that's called the principle of Occam's Razor er after 
William of Occam who was from Northumberland okay er i think the i've got about 
two minutes to go so i will er er try and er summarize things just er i haven't 
got a slide to summarize this up but essentially there's a lot of interest in 
this area because of the environmental possibilities of converting very toxic 
organics at low concentrations in the aquatic environment to harmless 
substances by shining U-V light on them in the presence of various catalysts 
and what i've talked about today is T-I-O-two er it's the one that probably on 
which most work has been done but you can also l-, use hydrogen peroxide as a 
as a that's not a catalyst but it gives you the O-H 
radicals it will degrade the organics very well you can also use ozone all 
three of these systems are called in the water industry advanced oxidation 
processes or A-O-Ps so er if you get a if you get go to a job interview at 
Severn Trent and they say what do you know about purifying water you can say i 
know about advanced oxidation processes or A- O- Ps and all of these fall er 
fall into that group a lot of work going on and the work is interesting not 
merely from the point of view of the end product that is that er one is trying 
to perfect systems people are using light pipes to transmit the light they are 
using er glass wool er there are hospital tiles that are being tried out in 
America where you take the ho-, you take the tile and you coat it with a very 
thin coating of T-I-O-two and this tile will absorb enough moisture from the 
air to give a kind of thin monolayer or a few layers of water on the tile and 
if there is a bacterium floating around in 
the hospital and it alights on the tile then believe it or not the action of 
the fluorescent tubes in the room on the tile surface is to degrade the 
bacterium the bacteria are actually killed by their location on the surface of 
the semiconductor because it's able to produce O-H radicals in the thin 
monolayer which then attack the bacteria so these tiles are are are are have 
been patented they're now being produced and they're being tried out in 
hospitals in the States you know the so they're self it's a kind of self-
cleaning tile or self-sterilizing tile obviously if somebody puts a muddy muddy 
hand on the wall that's not going to influence things very much but it's the 
bacteria that you're interested in you want to sterili-, keep the hospital 
sterile these tiles are s-, autosterilizing and also in the States there are 
patents taken out on light pipes made of er very thin glass fibres coated with 
T-I-O-two and you can have these in a in the form of i think if you 
can imagine a mop er with with a handle and a whole series of er er fibres at 
the end of it that you normally use for mopping well if you imagine those are 
glass fibres you could immerse that into a tank of er of toxic water and just 
rely on sunlight actually and you actually autoclean the water using a system 
like that so there are a lot there's a there's a lot of technology being 
developed it hasn't really hit the market place in a big way yet but i think 
watch this space five years down the line and you'll be very surprised okay has 
anyone got any questions anything that er that they didn't understand or would 
like me to go through again any points no okay well we'll we'll wrap it up 
there and i think er we'll i think tomorrow we got to start looking at the 
effects of er alpha rays beta particles on substances okay