The Synthesis of Metal Phosphides: Reduction of Oxide Precursors in a Hydrogen Plasma

Abstract

Phosphorus can react with most elements of the periodic table to form different classes of phosphides, ranging from ionic for the alkali and alkaline-earth elements to metallic or covalent for the transition elements and covalent for the main-group elements. Among them, InP and GaP (III-V semiconductors) have various applications in telecommunications, optoelectronic devices and solar cells, [1,2] while transition-metal phosphides are attractive candidates for high-performance catalytic, electronic, and magnetic applications. [3] A variety of methods for synthesizing bulk metal phosphides, including direct reaction of the appropriate elements for prolonged periods at high temperature, reaction of phosphine with metals or metal oxides, reduction of metal phosphates by hydrogen, electrolysis of molten metal phosphate salts, solid-state metathesis, thermal decomposition of single-source precursors, and self-propagation high-temperature synthesis, are known. [4][5][6] These methods typically require extremely high reaction temperatures (sometimes above 1000 8C) and/or long reaction times. Metal phosphides can be obtained under milder conditions in a solvothermal approach, but this method is not feasible for the deposition of metal phosphides on supports, and in cases where yellow phosphorus and sodium are employed, care must be taken to ensure the rigorous absence of oxygen and water. [7] Only two methods, namely the reduction of metal phosphates by H 2[4] and the phosphidation of metals or metal oxides with PH 3 /H 2 , [5] are generally feasible for the preparation of supported transition-metal phosphides for use as hydrotreating or hydrogenation catalysts. Supported metal phosphides are usually prepared by the reduction method due to the high toxicity of PH 3 . Nevertheless, the conversion of oxide precursors to phosphides is neither thermodynamically nor kinetically favorable. Since the formation of Ni 2 P from the oxides by means of the temperature-programmed reduction (TPR) method (Scheme 1) is thermodynamically unfavorable, the forward reaction has to be aided by high temperature and low water vapor pressure.[8] Thus, Ni 2 P can only be obtained at a low heating rate (e.g. 1 8C min À1 ) and a high H 2 flow velocity to purge the water (by-product) off the solid surface. The forward reaction is slow because the H 2 molecules must be split into hydrogen atoms, therefore the metal oxide must first be reduced and then spilt-over hydrogen atoms can reduce the phosphorus oxide, followed by a solid-state reaction to form the metal phosphide. As a consequence, Ni 2 P can only be prepared by means of the TPR method above 550 8C.Herein we describe a new strategy for synthesizing metal phosphides that uses nonthermal H 2 plasma as the reduction medium instead of the H 2 used in the TPR method. Highenergy electrons collide inelastically with hydrogen molecules in the plasma and transfer their energy to the latter, which leads to the production of excited hydrogen species and ions with a significantly higher reduction abilit...

Ni2P clusters on zeolite nanosheet assemblies with high activity and good stability in the hydrodesulfurization of 4,6-dimethyldibenzothiophene

“…The characteristic Ni 2 P (PDF: 03-0953) peaks at 40.6°, 44.5°, and 47.1°are observed for the Ni 2 P/ SiO 2 and Ni 2 P/NS-NaMOR catalysts (Fig. 4a, b, and d) [31][32][33]. However, these peaks are not detected for the Ni 2 P/NS-HMOR catalyst (inset in Fig.…”

“…It has been reported that the hydrogenation activity of Ni 2 P catalysts can be improved by decreasing the crystallite size, increasing the particle dispersion, and modifying the Ni 2 P active phase morphology [32][33][34]. On the other hand, the metal catalyst stability can be improved by strengthening the metal-support interactions [35].…”

Metal Phosphides as High-performance Hydrotreating Catalysts

“…Investigation of the reaction bed revealed that the reaction temperature was lower than 150 , which implies that the rapid reduction is attributable to the excited H species. Such low synthesis temperature also suppressed sintering of metal phosphide particles in supported catalysts, leading to increased dispersion of the active phase and resultant enhanced HDS performance 13) . Another advantage of the H2 plasma method is that metal phosphides are stoichiometrically synthesized, probably because of the high reactivity of excited H species and the low reaction temperature.…”

A Novel Route to the Preparation of Carbon Supported Nickel Phosphide Catalysts by a Microwave Heating Process

“…In this respect, the temperature programmed reduction (TPR) of phosphates which was first developed by Oyama et al [11], seems to be the most effective method. However, the formation process of Ni 2 P from the phosphates by TPR method is neither thermodynamically nor kinetically favorable, and thereby requires a high temperature and a slow heating rate [20]. Typically, the TPR process for preparing a Ni 2 P catalyst takes 8 h or even longer.…”