Hydrogen fuel formation using visible light

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2015

A two-part process involving a specific type of titanium dioxide and a unique substrate produced a high photohydrogen yield.

KEY CONCEPTS
• Use of titanium dioxide in the manufacture of hydrogen from water in a process similar to photosynthesis has been hindered in the past because visible light cannot be used.
• New research enables visible light to be used to produce hydrogen through the chemical modification of titanium dioxide and the use of a light-diffuse- reflected surface.
• Initial experimental work leads to a photohydrogen yield that is one to two orders of magnitude higher than most previously reported results.

FUEL CELLS ARE EMERGING AS A POTENTIAL PROPULSION SYSTEM for automobiles, with Toyota being one of the first manufacturers offering a vehicle based on this technology. Fuel cells use hydrogen as a power source, which represents a technical challenge because there is no efficient process available for its production. Currently hydrogen is mainly produced through steam-reforming of natural gas that is energy intensive.

Efficient production of hydrogen as a fuel continues to be a goal of researchers. In a previous TLT article, a pathway for producing hydrogen from biomass, which is more abundant and more evenly distributed globally than hydrocarbon fuels, is described (1). Invitro metabolic engineering was used to convert the cellulose and xylose present in biomass into hydrogen. Mild process conditions enabled hydrogen to be produced in an efficient manner.

The ideal approach to manufacturing hydrogen from water in a manner similar to photosynthesis is using light in a photocatalytic process. Titanium dioxide is a very strong oxidizing agent when exposed to light. Upon exposure to a photon, an electron and a hole (positive charge) are generated with the electron able to move from the valence band to the conduction band of titanium dioxide if exposed to light with the correct wavelength.

Once an electron reaches the conduction band, titanium dioxide can split water to produce oxygen and hydrogen. Currently the problem is that operating conditions have not been found to enable titanium dioxide to effectively convert water into hydrogen.

Yun Hang Hu, the Charles and Carroll McArthur professor of materials science and engineering at Michigan Technological University in Houghton, Mich., says, “Titanium dioxide has a relatively large band gap energy between the valence band and the conduction band of 3.0 and 3.2 electron volts. Moving an electron through this gap using light is very difficult because only ultraviolet (UV) light contains sufficient energy. UV light represents about 4% of total energy, making it very inefficient to use sunlight.”

The more viable option is to use visible light that accounts for 45% of the total solar energy but does not have the same energy level as UV. If a process can be found to use visible light to facilitate the use of titanium dioxide as a photocatalyst, then researchers may be a step closer to using hydrogen as a fuel. Such a process has now been developed.

TWO-PART PROCESS
Hu, in collaboration with his doctoral student Bing Han, has bridged the energy gap and found a way to use visible light to enable titanium dioxide to oxidize water and produce hydrogen. He says, “We used a two-part process that involved working with a specific type of titanium dioxide on a particular substrate. Typically titanium dioxide is a white-colored substance that is widely employed as a pigment. Hydrogenation of titanium dioxide leads to the formation of black titanium dioxide, which becomes the photocatalyst. Hydrogenation enables titanium dioxide to be reduced leading the oxidation state of titanium to change from Ti⁺⁴ to Ti⁺³.”

The use of Ti⁺³ is very important because it closes the band gap energy between the valence band and the conduction band to only 1.3 electron volts. At this energy state, visible light can now be used to excite an electron from black titanium dioxide to the conduction band. Figure 2 shows the difference in the gap between valence band of titanium dioxide and the valence band of reduced titanium dioxide with respect to the conduction band.


Figure 2. Hydrogenation of titanium dioxide produces Ti⁺³, which as shown in this figure closes the gap between the valence band of titanium dioxide and the conduction band, enabling visible light to be used in producing hydrogen from water. (Figure courtesy of Michigan Technological University.)

To further improve the process, approximately 1% platinum is used in the black titanium dioxide photocatalyst.

The second element in the catalytic system is the type of substrate used. Hu says, “Application of incident light on the titanium dioxide photocatalyst works only if the light is absorbed. Otherwise the light is lost. But the use of a light-diffuse-reflected surface increases the ability of light to be absorbed by 100 times because the bumpy surface traps light waves and bounces them around the surface so the chance they will be absorbed by the catalyst increases dramatically.”

A final concern in putting together the photocatalytic surface is to make sure that the photo-excited electron moves to the conduction band and does not combine back with a hole in the valence band. Hu says, “We utilize methanol as a sacrificial oxidation agent to donate electrons to the oxidized Ti⁺³ at elevated temperature to prevent the photo-excited electron from going back to the hole in the valence band.”

The researchers conducted the water splitting reaction in a quartz tube reactor containing the titanium dioxide photocatalyst placed on a light-diffuse-reflected silicon dioxide surface. After heating to an elevated temperature, water containing 30% methanol was introduced and the reactor was illuminated with light. Hu says, “We conducted the reaction at temperatures between 150 C and 300 C. The reaction started at 200 C and reached an optimum point at 280 C.”

At that temperature the researchers achieved a photohydrogen yield of 497 millimoles per hour per grams of catalyst, which is one to two orders of magnitude higher than most previously reported results.

The success of the catalyst system in using visible light is leading the researchers to optimize the process. Hu says, “We intend to scale-up the process and are looking for industry partners to assist with commercialization.

Additional information can be found in a recent article (2) or by contacting Hu at yunhangh@mtu.edu.

REFERENCES
1. Canter, N. (2015), “Hydrogen production from low-cost biofuel,” TLT, 71 (7), pp. 14-15.
2. Han, B. and Hu, Y. (2015), “Highly efficient temperature-induced visible light photocatalytic hydrogen production from water” The Journal of Physical Chemistry C, 119 (33), pp. 18927-18934.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be
submitted to him at neilcanter@comcast.net.