Samuel ShissoProject SEEDSummer 2010

Abstract

Background

Procedures and Results

With this research, I was assigned to work specifically with the TiO₂ catalyst, placed in a solution containing Acetonitrile and isopropanol, to study what would be reduced or oxidized. Below is the equation used as a building block for the reduction and oxidation reactions.

Optimization of TiO₂ Based SolutionsFor Solar Applications/ CO₂ Reduction

Solutions in which the titanium dioxide (TiO₂) catalyst was present were prepared so that the optimization of photocatalytic reduction of carbon dioxide (CO₂) by TiO₂ could be studied. The objective of this process was to discover a way to convert or even store CO₂ into a more efficient solar fuel such as Methanol (MeOH or CH₃OH). However, Gas Chromatogram (GC) results showed that there was no confirmed reduction of CO₂. However, oxidation of isopropanol, which was used as the main reduction agent, to Acetone ((CH₃)₂CO) was confirmed.

•CO₂ + 2H₂O + photons CH₃OH + 3/2O₂

This equation would guide us in creating our solutions. The CO₂ combines with the H₂O to form methanol and oxygen gas (O₂). The H₂O serves as the solvent or the universal reducing agent. However, in our reaction we substituted H₂O and instead we used isopropanol (IPA) as our sacrificial reducing agent. This was done because IPA is easily detectable on the GC and it is more thermodynamically favored than a system using water as the reducing agent. As a result of this change we expected acetone as an oxidation product instead of oxygen. Using this information we were able to create a few TiO₂ based solutions to begin our research. There have been a few proposed mechanisms on how the photocatalytic reduction process could be approached. Earlier research showed that the major reduction products were Formate (HCOO ) and carbon monoxide (CO). The picture to the right illustrates what is occurring at the semiconductor surface.⁻

In our reaction above, the TiO₂ photocatalyst is irradiated with a photon of light of the appropriate wavelength. On the valence band isopropanol reacts with two holes to form acetone and two protons. On the conduction band CO₂ reacts with an electron to form the CO₂ radical anion. Depending on the reaction conditions, the CO₂ radical anion combines with a proton and an electron to create the formate (HCOO ) ion or it creates CO and a hydroxide (OH ) ion. The ⁻ ⁻likeliness of this occurring is displayed in the following graphs. Here the determining factor is proposed to be the solvent used in the reaction.

From the graph, depending on the dialecture constant of the solvent, we can predict the formation of either HCOO or CO. Yoneyama and colleagues ⁻suggest that the more polar the solvent, then the presence of formic would be greater, but the more nonpolar the solvent, the more CO is present in the solution.

First a solution, that contained 80ml of Acetonitrile (CH₃CN), 0.5M of isopropanol, and 0.08g of TiO₂, was made. After the solution was made, it was then placed in a sonicator for about twenty minutes so that all of the contents would be mixed in completely. After letting the solution sonicate T (time) =0 filtered sample was taken so we could to run GC tests. The GC allows us to inject, usually 2µl, a sample of our solution in a column and then the sample is burned off of the column. The gas chromatograph then characterizes what was burned off at what time frame, and through peaks. For example we’ve been getting a lot of acetone in almost all of the runs, and it has peaked off at about the 2-2.5 minute mark. Also, our reducing agent, isopropanol, has peaked off consistently at about the 4.5-4.9 minute mark. This was because our heating profile changed variously. The graphs below are the gas chromatogram results of our TiO₂ solution. The left graph shows the injection of a sample that was irradiated with UV light for 75 minutes and the right displays an injection of a sample taken after being irradiated with light for 915 minutes.

After taking that sample of the solution, the rest of it was placed in our photochemical pressure reactor. Before using the reactor it had to be cleaned thoroughly so that nothing from the previous reaction would be added to our solution. Anything extra would foil the entire process and definitely give strange results in the GC. The reactor was just a vessel used to keep our solution in a closed space so that light from the UV lamp would shine on the solution, but it also enabled us to apply pressure to the solution. We usually kept the pressure at about 200-600 psi. After the solution would sit irradiated with light for x hours we would have to shut everything down before taking a sample out. Shutting everything down meant de-gassing the reactor by letting all the CO escape, and lowering the pressure. At times we let the solution irradiate with light for an hour and run a GC sample hourly or at other times it would irradiate with light overnight.

This process was repeated for about 2 ½ weeks but no promising results really came through. The gas chromatograph was the instrument that would best show us if we were creating any MeOH and how much of it were we creating. With the TiO₂ the GC showed no signs of any MeOH. This is known because earlier in the program we started our first tests with silicon carbide (SiC) and our MeOH peak, which appeared occasionally, came up at about the 4-4.5 minute mark just before the isopropanol peak at 4.9 minutes. However, we did consistently create acetone whose concentration grew steadily over time spent under light as shown in the graph.

It is presumed that the reduction of CO₂ with the TiO₂ catalyst in acetonitrile and 2-propanol formed reduction products of only HCOO and CO. Acetone was an ⁻oxidation product of the isopropanol and detectable with the gas chromatographer. Furthermore, we await utilization of the HPLC to confirm formic acid products. Overall it can be conclude that TiO₂ is not the best catalyst for this project because we never really saw any formation of CO. Even when we took gas samples of the solution there was no sign of CO present.

References1.Liu, B., Torimoto, T., Matsumoto, H., Yoneyama, H. Effect of solvents on photocatalytic reduction of carbon dioxide using TiO2 nanocrystal photocatalysts embedded in SiO2 matrices. J. Photochem.

Photobiol., A: Chem. 1997, 108, 187-192.2.Jiang, Z., Xioa, T., Kuznetsov, V., Edwards, P. Turning Carbon Dioxide into fuels. Phil. Trans. R. Soc. A. 2010, 368, 3343-4464.

2

This summer was a great experience. I enjoyed working in the labs and learning chemistry at the college level. I thank Dr. Asala and ACS for allowing me to be a part of this program. It was a pleasure working with Dr. Schmedake and the whole TAS group; Chika Chukwu, Matt Huff, Nick Lepore, and Devante Bledsoe. I would really recommend any one to take this opportunity because it really does teach a lot.