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Date of Award
Stephanie L. Brock
Transition metal phosphides are emerging as efficient catalysts for different processes. Although binary phases have been extensively studied recently researchers have explored the synergism afforded by bimetallic ternary transition metal phosphides. The conventional catalyst preparation methods (temperature programmed reduction or solvothermal synthesis) yield inhomogeneous samples, preventing a detailed understanding of how active site density impacts catalytic activity and mechanism. In contrast, solution-phase arrested-precipitation reactions produce uniform nanoparticles with an excellent control on size, morphology and composition.
This dissertation describes the synthesis of ternary transition metal phosphide nanoparticles (Ni2-xCoxP and Ni2-xRuxP) by solution-phase arrested-precipitation reactions and evaluation of their composition-dependent catalytic activity (hydrodesulfurization (HDS) and oxygen evolution reaction (OER)).
Motivated by the enhanced HDS activity of Co-incorporated Ni2P catalysts produced by TPR methods, a synthetic protocol was developed to produce phase-pure Ni2-xCoxP (x≤1.7) nanoparticles with sizes ranging from 9-14 nm. From TEM analysis, nearly monodisperse particles were obtained (S.D <20%) and metal ratios from EDS closely follow the ratios employed in the synthesis. Attempts to synthesize more Co-rich compositions yielded CoP as an impurity phase. The size distributions broaden with increasing Co content and hollow particles were observed for Co-rich compositions, due to the Kirkendall effect. In order to identify synthetic levers and the mechanism of ternary phase formation, a systematic study was conducted for Ni:Co = 1:1 as a representative composition. It was revealed that heating temperature, heating time, and the P:M ratio, have a huge impact on the nature of both the intermediate and the final crystalline particles. By tuning these conditions, NiCoP nanoparticles could be produced with different morphology (hollow vs dense) in different sizes (from ca 7-25).
Three nickel rich compositions of Ni2-xCoxP (x= 0.08, 0.25, 0.5) prepared by solution-phase reactions were encapsulated in mesoporous silica and evaluated for dibenzothiophene (DBT) HDS activity. On a mass catalyst basis, the highest activity was observed for the x=0.08 composition at 623 K. A slight decrease in activity was observed with increasing the cobalt content. Nevertheless, the highest turnover frequency was observed for the most Co-rich composition tested (x=0.5). Detailed IR studies on CO adsorbed on the catalyst surface suggest an electron density increase in the Ni sites present in Co-rich materials, which could be attributed to the transfer of electron density from Co to Ni. All studied compositions prefer to undergo the direct desulfurization pathway (DDS) regardless of the Co amount.
Aiming to create an efficient, less-expensive catalyst for the oxygen evolution reaction (OER), a synthetic protocol was developed to prepare ternary metal phosphide nanoparticles, Ni2-xRuxP, incorporating Ru, the state-of-art-catalyst for OER, and Ni, a highly active but inexpensive metal. Using solution-phase arrested precipitation reactions, crystalline Ni2-xRuxP particles could be realized for compositions up to x≤1, whereas more Ru-rich compositions, including Ru2P, were amorphous. For x ≤ 1 particles are spherical, of sizes that vary between 5-10 nm in diameter (with a clear decreasing trend as the Ru amount is increased), and samples exhibit narrow size distributions (polydispersity <15%); whereas amorphous Ru-rich phases exhibit worm-like morphologies. ICP-MS data indicate the actual metal ratio closely follows the target ratio employed in the synthesis. OER electrocatalytic activity was evaluated for selected compositions over the entire synthesis range (0≤x≤2). Intriguingly, Ru2P proved to be the least active phase (overpotential of 0.56 V at 10 mA∙cm-2 in 1.0 M KOH) with the best performance observed for the bimetallic Ni1.25Ru0.75P phase (overpotential of 0.34 V). The augmented activity at x = 0.75 is attributed, at least in part, to electronic activation of Ni by Ru, facilitating Ni oxidation and thus decreasing the kinetic barrier for OER.
Some preliminary HDS catalytic data were obtained for Ru-rich Ni2-xRuxP materials (x=0.1, 0.25, 0.5) encapsulated in mesoporous silica. It was observed 4.6-dimethyldibenzothiophene (4,6-DMDBT) HDS catalytic activity decreased with increasing the Ru content and the highest activity in all temperatures was shown by Ni2P. Turnover frequencies calculated based on activity and chemisorption capacity mirrors the HDS activity trend, showing a decrease in activity with increasing Ru. Regardless of the Ru content all compositions undergo hydrogenation pathway (HYD).
Liyanage, Don Malinda Ruchira, "Synthesis And Characterization Of Transition Metal Phosphide Nanoparticles For Catalytic Applications: Model Catalysts For Hydrodesulfurization And Electrocatalysts For The Oxygen Evolution Reaction" (2016). Wayne State University Dissertations. 1652.