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Access Type

WSU Access

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stephanie L. Brock

Abstract

Transition metal phosphides are promising catalysts for hydrodesulfurization (HDS). The size-dependent catalytic activity of these metal phosphides for hydrodesulfurization (HDS) remains unstudied because the traditional temperature programmed reduction (TPR) method used in catalyst preparation results in highly polydisperse particles.

The main goals of this dissertation research are (1) synthesize metal phosphide nanoparticles (Ni2P, Rh2P, and Pd5P2) with control of size and morphology using a solution based arrested precipitation method; (2) develop large scale synthesis of mesoporous silica encapsulated nanoparticles (Ni2P and Pd5P2) with controlled loading to prevent sintering at high temperatures; (3) establish the structure activity relationship of Ni2P nanoparticles in HDS and study the deep-HDS activity of noble metal phosphides.

The ability to control the Ni2P particle size on the nanoscale using solution-phase arrested precipitation is reported in this dissertation. Ni2P particles were introduced to a high surface area silica support (Cab-O-Sil, M-7D grade, 200 m2/g) via incipient wetness and HDS activity was probed against dibenzothiophene (DBT). All samples were less active than TPR prepared materials and the smallest particles were the least active, contrary to expectation. This is attributed in part to particle sintering under HDS conditions. Sintering occurs independently of wt% loading of catalyst, time, incipient wetness procedure and ionic additives, at all temperatures greater than 200 °C.

To minimize sintering, we developed a route for in-situ mesoporous silica encapsulation of Ni2P nanoparticles. We optimized the Ni2P@mSiO2 synthesis to yield samples on large scale with controlled loading. The generality of this approach is employed to other systems such as CdSe and Au. Sintering is minimized by encapsulation of Ni2P nanoparticles in a mesoporous silica shell, resulting in a doubling of HDS activity. This enables us to study size-dependent HDS of Ni2P nanoparticles.

The synthesis of monodisperse 5-10 nm Pd5P2 catalytic particles by encapsulation in a mesoporous silica network, along with preliminary data on hydrodesulfurization (HDS) activity, is also reported in this dissertation. Precursor Pd-P amorphous nanoparticles are prepared by solution-phase reaction of palladium (II) acetylacetonate with trioctylphosphine at temperatures up to 300 ºC. Direct crystallization of Pd5P2 in solution by increasing temperatures to 360 ºC leads to sintering, but particle size can be maintained during the transformation by encapsulation of the amorphous Pd-P particles in a mesoporous silica shell, followed by treatment of the solid at 500 ºC under a reducing atmosphere, yielding Pd5P2@mSiO2. The resultant materials exhibit high BET surface areas (> 1000 m2/g) and an average pore size of 3.7 nm. Access to the catalyst surface is demonstrated by dibenzodithiophene (DBT) HDS testing. Pd5P2@mSiO2 shows a consistent increase in HDS activity as a function of temperature, with DBT conversion approaching 60% at 675 K. The ability to control particle size, phase, and sintering is expected to enable the fundamental catalytic attributes that underscore activity in Pd5P2to be assessed