H₂ Prodution Via Photocatalytic Reforming Of Methanol And Biomass-derived Compounds

Concerns about the depletion of fossil fuel reserves and the pollution caused bycontinuously increasing energy demands make H₂ an attractive alternative energy source. H₂production via photocatalytic reforming of biomass is a sustainable-energy technology due tothe potential application for the conversion of solar energy. Proton exchange membrane fuelcells (PEMFCs), using H2 as the fuel source, have received much attention as a potentialelectric power source. However, the Pt-anode catalyst of PEMFC is extremely sensitive to COcontaminant in H₂ feed gas. Small amount of CO can poison the catalyst and decrease thecatalytic performance. It is an urgent and challenging objective to reduce the COconcentration in H₂ for the applications in PEMFCs. Therefore, the effects of metals andmetal particle size on the CO formation, as well as the mechanism of CO formation areinvestigated using methanol, ethanol and glucose as the typical representatives ofbiomass-derived compounds.Among the gaseous products, the H₂, CO, CO₂ and CH₄ were detected as the majorproducts in the photocatalytic reforming of methanol, ethanol and glucose on M(Pt, Pd, Au,Rh, Ni etc.)/TiO₂ catalysts. It is shown that the loaded metals generally enhances the rate ofH₂ production while depresses the CO formation. Both H₂ production and CO formation arestrongly dependent on the kind of deposited metals. In the reforming of methanol and ethanol,the highest photoactivity was achieved on Pt/TiO₂ catalyst, while the CO formation was thelowest on Pd/TiO₂ catalyst. Rh/TiO₂ catalyst was found to be most active for H₂ productionwhile with the lowest CO concentration in the photocatalytic reforming of glucose. The factthat the molar ratio of CO₂/H₂ is much lower than the stiochiometric value indicates that a lotof oxidized fragments of biomass are produced and diffuse into the bulk solution. Theseintermediates are successively adsorbed and oxidized on the M/TiO₂ catalyst for several timesfinally forming CO₂ through the photo-Kolbe reaction.H₂ with low CO concentration was produced via photocatalytic reforming of methanol onAu/TiO₂ catalyst. The rate of H₂ production is greatly increased when the gold particle size isreduced from 10 nm to smaller than 3 nm. The concentration of CO in H₂ decreases withreducing the gold particle size of the catalyst. It is suggested that the by-product CO is mostlyproduced via decomposition of the intermediate formic acid species derived from methanol.The smaller gold particles possibly switch the HCOOH decomposition reaction mainly to H₂ and CO₂ products while suppress the CO and H₂O formation. In addition, some CO may beoxidized to CO₂ by photo-generated oxidizing species at the perimeter interface between thesmall gold particles and TiO₂ under photocatalytic condition.The loading of platinum on TiO₂ influences both the H₂ production and the CO formation.When the loading of Pt is higher than a threshold value, the rate of H₂ production is greatlyenhanced while the CO formation is largely depressed. The CO formation was significantlydepressed with a slight increase in H₂ production in the photocatalytic reforming of methanolon Pt/TiO₂ catalyst with addition of a small amount of inorganic anions, such as SO₄^2- andH₂PO₄^- in the reaction system. The ability of the anions for depressing CO formation is in theorder: Cl^-＜NO₃^-＜HCO₃^-＜SO₄^2-＜H₂PO₄^-. It is suggested that the byproduct CO is formedat oxygen vacancy sites on TiO₂ where formic acid species derived from methanol decomposeto CO and water. The deexcitation of the trapped photoexcited electrons at oxygen vacancysites may accelerate the dehydration reaction of formic acid. Sulfate and phosphate ions caneffectively suppress the CO formation via competing adsorption on oxygen vacancies with theformic acid species derived from methanol.The inhibition effect of the SO₄^2- anions on CO formation was also observed in thephotocatalytic reforming of glucose on Pt/TiO₂, Ni/TiO₂, and Pd/TiO₂ catalysts, indicatingthat the addition of some inorganic anions to the reaction solution is an efficient method todeppress CO formation in the photocatalytic reforming of biomass-derived compounds for H₂production.