Doped TiO2 Catalysts for "Artificial Photosynthesis" with High Energy and Visible Photons.

[NNadir]

As we all know, photosynthesis is driven by visible light, with blue light at around 430 nm representing the strongest absorption peak for chlorophyll. (Chlorophyll finds green light almost useless for chemical transformation which is why leaves are green, that color is transmitted without use. I also find many "Greens" to be useless, since one type of green consists of people who will do anything for the environment except open a science book, as P.J. O'Rouke put it.)

Plant based photosynthesis of course makes sugar, but there are other possible products besides sugar that are photochemically synthesized. For instance, in theory, the following endothermic reaction is conceivable. CO₂ + 2H₂O <-> C2H₄ + 2O₂. Endothermic reactions can be driven by energy, and one form of energy is electromagnetic radiation including, but not necessarily limited to visible light.

Of course the mixture formed would be explosive unless a method were provided to remove the oxygen from the mixture. Happily there are many very good technological approaches for accomplishing this separation.

A relatively recent paper reporting a methane product from this kind of reaction was reported in the literature, and I came across it in connection with my interest in the reduction of carbon dioxide to give products useful as fuels.

The paper is entitled, "Dye sensitized artificial photosynthesis in the gas phase over thin and
thick TiO₂ films under UV and visible light irradiation."

The abstract is here: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TF6-4M936HF-1&_user=10&_coverDate=02%2F15%2F2007&_alid=1043673275&_rdoc=1&_fmt=high&_orig=search&_cdi=5218&_sort=r&_docanchor=&view=c&_ct=2&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=e4fcfcb5507b8909b2e5c0ad6d969369">Applied Catalysis B: Environmental 71 (2007) 291–297

TiO₂, is a very common compound on earth, most commonly used for white paint. It is the ore of titanium metal, although producing the metal from the ore has historically been very difficult, making the metal, but not the oxide, expensive. (This was once the case with aluminum until the invention of the Hall Process, an electrolytic process.)

In the paper, the oxide is doped either on the surface or as a mixture in thin films.

Here is some text from the paper.

1. Introduction

The most important challenge in the design of artificial photosynthetic systems is in the field of material selection and preparation. Semiconductor materials under irradiation at the proper wavelength can eject an electron from the valence band to the conduction band, where it is mobile and ready to be used for catalytic reactions. If the band gap of the semiconductor is wide, then high-energy radiation is necessary to excite electrons from the valance band. The photon energy of visible light is between 1.8 and 3.1 eV range. Therefore in order to be able to design a system operating with visible light, the photocatalytic material has to possess a band gap which can be overcome by electrons excited with photons of energy in the range of 1.8–3.1 eV. In addition to the photocatalytic properties, the successful candidate for a global scale catalyst has to be a non-toxic, inexpensive, stable, and widely available material. Considering all those criteria, scientists have mainly focused on TiO₂ based systems since 1980s. TiO₂, with a band gap of 3.2 eV, can be activated by UV light with wavelengths of 400 nm or shorter <1>.
For semiconductor photocatalysis, the primary challenge is to prevent the electron–hole recombination phenomena, which decrease the efficiency of the process. In general, an excited state molecule can undergo relaxation via three mechanisms: fluorescence or back radiation, heat release (internal conversions) or electron transfer.

As we know, ideally UV radiation doesn't strike the earth but is filtered out by stratospheric ozone. Thus to the extent that this system works, it is not a solar energy scheme. (There have been many research efforts on making solar hydrogen, producing oodles of wishful thinking about solar hydrogen HYPErcars, but thus far none of these have proved industrially useful. This work may not prove industrially useful either. Does this particular work have much solar application, does TiO₂, photoreduce CO₂? It was examined.

First lets look at the UV case. TiO₂ modified with surface platinum did the best without dye sensitizers, producing about 33 micromoles (about 0.000053 grams) of methane in 180 minutes per gram of catalyst using UV light sources.

Dye sensitation is adding a third component to increase photochemical yield:

UV tests on Ru(Bpy)₃²⁺ containing thin films are presented in Fig. 2. Due to the limited amount of dyes investigated in this work, UV tests on thin films were only performed with Ru(Bpy)₃]²⁺ dye. At the earlier stages of the reaction, hydrogen evolution on 2+ containing catalysts was also observed but not quantified. It can be seen from Fig. 2 that the addition of 3²⁺ enhanced methane yields of the reaction both in the presence and in the absence of Pt in the catalyst. Repeated tests on the same films indicated a continuous deactivation, shown in Fig. 3, which we attributed to the degradation of the dye under UV light.

Bpy is a bipyridine complexation agent used with the element ruthenium. It is quite possible that a substitute complexation agent might prove more stable. Nevertheless, yields were almost a 0.62 millimoles, or about a tenth of a milligram of methane per gram of catalyst in 180 minutes. That I think is impressive, although obviously still very weak in terms of being of any practical use.

To get to the solar case, these catalysts were tested with visible light and slowed much lower activity.

Since UV does not (ideally) penetrate earth's atmosphere, sources of UV are not available from solar energy. They are available from nuclear energy, using scintillation devices that transform gamma and x-rays into UV/visible light. The most commonly used example of a scintillator is cesium iodide which is, in fact, a common constituent of so called "nuclear waste," used nuclear fuels. In fact radioactive cesium iodide will continuously without any intervention radiate both visible and UV light by stepping down the energy of x-rays and gamma rays that it autogenerates.

Although my interest in this paper was motivated by my interest in nuclear science, it was not my original idea when opening it to imagine schemes for the direct reduction of carbon dioxide with radiation. I can think of superior ways to use nuclear energy to reduce carbon dioxide to methane and other fuels, means that are not necessarily photochemical.

[AlecBGreen]

1. This is an important field of research

Not only must reduce our net CO2 emissions to zero but we must actively remove the surplus CO2 we have dumped into the atmosphere. Artificial photosynthesis is one such method but its hard to concieve the scale necessary to remove the megatons or gigatons of CO2 we have added.

[NNadir]

2. I agree wholeheartedly. There are, of course, intermediate steps.

I have some very good ideas and have been thinking about this for some time.

My own approach is somewhat limited in scalability owning to certain materials limitations, but I will pursue them for other reasons. They could so something to remove CO2 from the atmosphere, but no where near everything necessary.

Writing this post last night however, led me to consider a different kind of system, and I'm checking out the literature on that.

I think CO2 reduction strategies - and not the dumb sequestration garbage - should be a primary area of research in addressing climate change.

I oppose the use of all fossil fuels, but I have written elsewhere about a plan to phase them out using CO2 reduction as a means:

http://www.dailykos.com/storyonly/2007/6/17/19109/0486">The Utility of Light: Getting Real with the Existing Energy Infrastructure.

If you don't want to listen to all of my flowery bull, just scroll to the bottom.

[Tesha]

3. Here's a hint...

> (I also find many "Greens" to be useless, since one type of green consists of
> people who will do anything for the environment except open a science
> book, as P.J. O'Rouke put it.)

Here's a hint: When you're trying to convince people of the rightness
of your position, don't take cheap shots at them in the first paragraph
of your pitch.

Here's a second hint: P.J. O'Rourke is a big-time Libertarian asshole
and no supporter of Democratic causes. So he makes a bad source
for quotes if you're trying to convince liberals, progressives, greens,
and others who read here of the rightness of your positions.

Tesha

[Orrex]

4. +1

:thumbsup:

[NNadir]

5. I know who P.J. O'Rouke is. His statement however, in reference to a certain sect,

was, um, right on.

If Dick Cheney says that the sky is blue, that does not make it green or pink.

I am sick, frankly, of people who identify themselves as "progressives" and "greens," especially when they viciously maligned the 2000 Democratic nominee as not being "progressive" and "green" enough, and so worked to throw their country and their planet into the abyss to placate their stupid little egos and oversized view of their own rectitude.

Right then and there I decided to be dismissive and hostile toward their abysmal and loud ignorance.

Thanks for your advice. I couldn't, however, fucking care less about it. Why?

Let me quote from the discoverer of plane polarized light, David Brewster, to explain why I am not inclined to kiss up to people on the grounds that they don't know shit from shinola and insist I coddle them for it:

...the keenness of his temper, his clear perception of the truth and his indistinguishable love of it, combined to exasperate and prolong the hostility of his enemies. When argument failed to enlighten their judgment and reason to dispel their prejudices, he wielded against them his powerful weapons of ridicule and sarcasm; and in his unrelenting warfare, he seems to have forgotten that Providence had withheld from his enemies those very gifts which had so liberally received.

The bold is mine.

Brewster was critical of the person he described, but on ethical review, I have decided that the person described and not Brewster was correct. Had the person described not been correct, the history of Western Science would be completely different.

I have been living by the words for quite some time, and I will continue to do so until I no longer can.

Ignorance is not neutral. Ignorance does not deserve to be excused. Ignorance kills.