Synthesis, characterization, and catalytic properties of some transition metal-doped nickel phosphide nanoparticles

Abstract

Energy crisis and global warming are two most critical issues emerged in the 21st century. To cope with these problems, development of robust catalysts which can facilitate the production of renewable and sustainable alternative energy source is needed. For instance, water splitting reaction is one of the important reactions for production of hydrogen as a clean renewable energy source. In addition, the efficiency improvements in industrial processes such as ammonia synthesis can reduce the amount of energy required, thus relieves the high global energy demand. Metal phosphide nanoparticles have recently attracted great attention owing to their potential catalytic activity in organic transformation and energy-related researches. Among them, nickel phosphide is of particular interest because of its versatile catalytic applications with considerable reactivity and high abundance, yet its catalytic performance still lags far behind the noble metal-based catalysts and demands further development. Metal doping is one of the efficient ways to improve the reactivity of catalyst, by altering the local structure and bonding environments of catalyst. However, due to the difficulties involved in synthetic procedures, studies on phosphide with two or more metal atoms are still rare. In the study, a series of transition metal-doped nickel phosphide nanoparticles were prepared and their catalytic activities in various energy conversion reactions were explored. In Chapter 2, a systematic synthetic procedure of incorporating various transition metal ions, including cobalt, iron, manganese, and molybdenum ions, into the crystal lattice of nickel phosphide nanoparticle was described. Using different characterization techniques, including X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and extended X-ray absorption fine structure, the morphology and crystal structure of prepared transition metal-doped nickel phosphide nanoparticles were studied. Water splitting is an important reaction to simultaneously generate hydrogen and oxygen gases as alternative energy sources in an environmental-friendly fashion. In Chapter 3, the catalytic performance of the as-synthesized metal-doped nickel phosphide nanoparticles in both electro- and photocatalytic hydrogen evolution reaction (HER) were studied. Among all the metal-doped nickel phosphide nanoparticles investigated, molybdenum-doped nickel phosphide nanoparticle showed the highest reaction rate for both electro- and photocatalytic HER. The electrocatalytic HER performance of molybdenum-doped nickel phosphide nanoparticle in alkaline medium was comparable to that of platinum, with an overpotential of 0.34 V (versus RHE at 10 mA cmˉ²) and Tafel slope of 163 mV decˉ¹. In the dye-sensitized photocatalytic HER, molybdenum-doped nickel phosphide nanoparticle achieved a hydrogen production rate of 268 mmol hˉ¹ gˉ¹. In Chapter 4, a series of as-prepared metal-doped nickel phosphide nanoparticles were investigated as potential electrocatalysts for the other half-reaction of water splitting reaction, oxygen evolution reaction (OER). Different from the HER, iron-doped nickel phosphide nanoparticle was found to be the most active catalyst with an overpotential of 0.33 V and Tafel slope of 39 mV decˉ¹, which demonstrates a comparable catalytic activity to the benchmark OER electrocatalysts (RuO₂ and IrO₂). In addition, brief kinetic and mechanistic studies on electrochemical OER were conducted. The reaction order of hydroxide ion in electrochemical OER was estimated to be close to unity. The rate determining step in electrochemical OER was determined to be the formation of Ni=O species, as implied by the Tafel slope and reaction order of hydroxide ion. In addition to the water splitting reaction, the study of using metal-doped nickel phosphide nanoparticles for catalytic ammonia synthesis is presented in Chapter 5. The synthesis of ammonia is an important industrial process, since ammonia is one of the essential ingredients for making useful chemicals in daily life including polymers and fertilizers. It is the first example of using phosphide-based nanomaterial as catalysts for the synthesis of ammonia. In our studies, iron-doping was shown to play a critical role in enhancing the catalytic activity of nickel phosphide nanoparticles toward ammonia synthesis. The catalytic reaction mechanism for iron-doped nickel phosphide nanoparticle is proposed where iron plays a role of the reaction center while nickel phosphide base assists iron to produce ammonia.