H2 Production Using Thermal Decomposition of Hcooh with Pd/ZnO Catalyst

Abstract

Introduction The worldwide problem of air pollution induced by the excessive use of fossil fuels has become increasingly serious issues for human beings in modern times. Hydrogen gas has been recognized as one of clean and renewable energy sources to replace fossil fuels for the sustainable future. Hydrogen has extremely low density and highly explosive character. As a consequence, hydrogen storage and distribution remains a challenging issue. The formic acid decomposition into H2 and CO2 (HCOOH → H2 + CO2) is thermodynamically favored (⊿G = −32.8 kJ mol−1). However, a competitive route of formic acid decomposition into CO and H2O (⊿G = −20.7 kJ mol−1) usually occurs, which is undesirable. Therefore, the major challenge of developing formic acid as a practical carrier of hydrogen is finding suitable catalysts that enable highly selective formic acid decomposition into H2/CO2 rather than CO/H2O to occur with high efficiency and large throughput in ambient condition. Experimental The catalytic reaction was performed for 5 hours in Pyrex reactor (volume: 123 mL) equipped with a magnetic stirrer with the heater. The reactor with sitter was placed on the stirrer to keep the suspension during the reaction and the constant temperature of reactor. An optimum amount of sodium hydroxide solution was added into the formic acid solution to adjust the pH. The base solution suppressed the dissolution of ZnO in the formic acid solution. In a typical experiment, 50 mg of ZnO was dispersed in a Pyrex glass reactor containing 40 mL of formic acid solution. An optimum Pd amount was added in the solution. Formic acid was adsorbed onto ZnO surface. The catalytic hydrogen production form the formic acid was analyzed by gas chromatography with thermal conductivity detector (TCD). Catalysts after the reaction were recovered by centrifugal separation. The prepared samples were characterized by X-ray diffraction (XRD, Rigaku Ultima IV), Fourier transform infrared spectroscopy (FT-IR, PerkinElmer Spectrum Spectrum 100), X-ray photoelectron spectroscopy (XPS, Ulvac PHI Quantera SXM), scanning electron microscopy (SEM, JEOL JEM-1011) and N2 adsorption–desorptionisotherms. Result and discussion We examined the effect of pH, co-catalysts, Pd concentration, formic acid concentration, and temperature to optimize the experimental conditions. The composition and crystalline structures of the ZnO and Pd/ZnO were characterized by XRD. As compared Pd/ZnO with ZnO, there are the same XRD patterns between ZnO and Pd/ZnO. SEM was used to characterize the morphologies and structures of the products. SEM images confirmed that the diameter of ZnO and Pd/ZnO were approximately a few hundred nanometers. As compared Pd/ZnO with ZnO, there are almost same SEM images between ZnO and Pd/ZnO. XPS were employed to investigate the surface chemical state of the Pd/ZnO. In the initial stage of the reaction, the existence of PdZn alloy was confirmed by XPS. ZnO may react with metallic Pd. Then, this reaction generated PdZn alloy. In the latter stage of the reaction, XPS results suggested that PdO was existence. The result indicated it that PdZn alloy changed into PdO with progress of time. The composition, structure and surface state of the Pd/ZnO were characterized by FT-IR. FT-IR spectra of Pd/ZnO in the latter stage of the reaction have a lot of peak. These peaks are a carbonyl group and a hydroxyl group. As a reaction time, surface state of Pd/ZnO was contaminated with formic acid. BET was employed to investigate the surface area of the ZnO and Pd/ZnO. As compared Pd/ZnO with ZnO, BET surface result showed it that increase in surface area of Pd/ZnO was confirmed by loading of Pd. By using 50 mg of the Pd/ZnO containing the 0.1wt.% Pd, we could achieve an optimal H2 generation rate of 220 mol h−1. This hydrogen evolution rate is about 760 times than that of the pure ZnO. However, the hydrogen production rate decreased with progress of time. The results may be owing to it that PdZn alloy changed into PdO. PdZn alloy catalyst produced carbon dioxide (CO2) selectively. The generation of CO could be not confirmed. Mechanism of this reaction was explained as follows. Because the isoelectric point of ZnO is pH 9.3～10.3, the surface of ZnO in pH 6.0 became a positive charge. In the formic acid thermolysis course, O species of formic acid was attracted ZnO, and that O was adsorbed on PdZn alloy surface. In this experiment, a reaction course generating H2 and CO2 progresses preferentially. Formic acid, which was adsorbed onto PdZn alloy surface, produced carboxyl (COOH) and formate (HCOO) intermediates. H2 and CO2 were generated from the intermediates.