1.3.6 Solar Fuel Synthesis Through Photocatalysis
To reduce the dependency on fossil fuels and environmental pollution caused due to consumption of these fuels, renewable fuel production from biomass is in great demand and gained the focus of attention of academic and industrial researchers since the last decade.
In this process, water is used on a semiconducting material for photocatalyzing to generate hydrogen and oxygen [5]. In electrochemical cell, an electrode made of TiO2 is connected to black platinum electrode. Due to visible incident light, the flow of electrons starts in the direction from TiO2 electrode to black platinum electrode and at this point, the generation of hydrogen and oxygen can be done at cathode and anode, respectively. According to heterogeneous photocatalysis mechanism, oxygen is produced by oxidizing water through photo generated holes which acts as electron scavengers and rest electrons are responsible to generate hydrogen in reduction reaction. Research on photocatalytic conversion of biomass derived materials is available on use of pure organic molecule as model compound or biomass organic wastes. As we all know the model compounds are not directly found by organic biomass wastes so isolation or transformation reactions are required for this purpose. For example, methanol is procured from biomass wastes by using various valorization technologies such as gasification and some catalytic transformation of syngas. Undesired reactions are possible due to mixture in biomass wastes and this mixture passes through some transformation processes to reduce impurity or separation so that the good performance of desired reactions can be enhanced. Some other organic compounds as ethanol, acetic acid, formic acid are not suitable for raw biomass for conversion in high purity chemical products. But on the other side, glycerol, carbohydrate, and lignin derived from biomass provide high purity of product from various industries such as biodiesel refinery, agriculture, and wood or paper industry.
1.3.7 Photocatalytic Reforming
There are many technologies available for biomass utilization to produce valuable products such as seam reforming, dry reforming, pyrolysis, gasification, supercritical water reforming, partial oxidation, autothermal reforming, aqueous phase reforming, photocatalytic reforming, etc. Among all of them, photocatalytic reforming is a different technique because it is totally supported by solar energy as solar radiation energy is available free of cost on this globe. Photocatalytic reforming is recognized as a sustainable and promising process to convert solar energy into hydrogen as chemical energy for various necessary applications. Likewise, utilizing biomass via photocatalytic reforming is an effective route to convert to biofuel to chemical fuels such as clean hydrogen production which is a feasible alternate of fossil fuels for future development. Photocatalytic reforming minimizes the problems occurring in thermo-catalytic conversion of biomass reaction conditions and also combines solar energy with biofuels which is quite beneficial for our environment because it reduces the possibility of air and water pollution. A similarity is found in the mechanism of photocatalytic reforming of biomass and other materials. The substrates used as photocatalytic substrates are mainly semiconductors in which electron-hole pairs are formed to interact with incident solar heat waves and utilized in oxidative and reduction steps of photocatalytic reaction which is mostly based on titania catalysts. But, the addition of unwanted electron-hole pairs in the reactions is a drawback in the case of titania catalysts because they are the reason to achieve lower efficiency in photocatalytic reactions. Therefore, to improve the efficiency of the some specific photocatalysts, the addition of some metal particles in nano-size over the surface of titania is required so that the unwanted electrons can be trapped and the paring of electron-holes can be minimized. It has been seen that the gold oriented nanoparticles with the combination of platinum/palladium provides high conversion of some organic compounds like oxidation of alkanes, polylols, CO, and alcohols. But, these gold based photocatalysts are very costly and not feasible from the economic point of view. However, silver based nanoparticles are getting much attention by the researchers who are working in this field of photocatalysis due to wide range of applications such as sensors, catalysts, and microelectronics. The effects of gold/silver loadings with proper specified heat treatment over TiO2 based photocatalysts showed better catalytic activity for the production of hydrogen and other valuable by-products.
Table 1.5 Important studies on photocatalytic valorization of biomass substrates [5, 14, 15].
| S. no. | Biomass substrate with conditions | Conversion | Products (Y = yield, S = selectivity) | Light radiation | Photocatalyst |
|---|---|---|---|---|---|
| 1. | Glucose | 89% | H2 = 220 μmol (Y) | Ultraviolet | NiO/NaTaO3 |
| 2. | Glucose | 29% | H2 = 100 μmol (Y) | Visible | Pt/ZnS-ZnIn2S4 |
| 3. | Glucose | 83% | H2 = 4.8 mmol (Y) | Ultraviolet | Pt/TiO2 |
| 4. | Glucose | 11% | Glucaric acid + Gluconic acid + Arabitol = 71% (S) | Ultraviolet | TiO2 |
| 5. | HMF | 20% | FDC = 22% (S) | Ultraviolet | TiO2 |
| 6. | Glucose | 7% | Glucaric acid + Gluconic acid = 87% (S) | Ultraviolet | Cr/TiO2/zeolite |
| 7. | Glucose | 100% | H2 = 5,460 μmol (Y) | Visible | Ru-LaFeO3/Fe2O3 |
| 8. | Glucose | 65% | H2 = 850 μmol (Y) | Xenon Lamp | Rh/TiO2 |
| 9. | Arabinose + Glucose | 13.28% |
H2 = 60.1 μmol (Y)
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