PhD

dissertationThis dissertation describes size-dependent investigations of supported palladium and iridium clusters, used as model catalysts for carbon monoxide oxidation and hydrazine decomposition reactions. Chapter 1 provides an introduction into the role of size-dependent studies within the greater scope of catalysis research, with a primary focus on the changing nature of metal particles with size at the small scale (<25 atoms per particle), and the important role of support interactions in determining particle chemistry. An investigation of CO oxidation over TiO2 (110) supported Pdn particles (n = 1,2,4,7,10,16,20,25) via stepwise dosing and reacting protocols is given in Chapter 2, with experimental results showing a correlation between nonmonotonically varying cluster activities and their accompanying electronic properties as a function of size. In Chapter 3, the same catalyst-reaction system is studied as a function of surface temperature and total oxygen exposure during the oxygen dose, and is found to be limited by oxygen binding under the conditions investigated within the previous chapter

Strong Catalytic Activity Of Iron Nanoparticles On The Surfaces Of Reduced Olivine

It is demonstrated that olivine powders heated to subsolidus temperatures in reducing conditions can develop significant concentrations of 10–50 nm diameter Fe nanoparticles on grain surfaces and that these display strong catalytic activity not observed in powders without Fe nanoparticles. Reduced surfaces were exposed to NH3, CO, and H2, volatiles that may be present on the surfaces of comet and volatile-rich asteroids. In the case of NH3 exposure, rapid decomposition was observed. When exposed to a mixture of CO and H2, significant coking of the mineral surfaces occurred. Analysis of the mineral grains after reaction indicated primarily the presence of graphene or graphitic carbon. The results demonstrate that strong chemical activity can be expected at powders that contain nanophase Fe particles. This suggests space-weathered mineral surfaces may play an important role in the synthesis and processing of organic species. This processing may be part of the weathering processes of volatile-rich but atmosphereless solar-system bodies

A Bifunctional Catalyst For Efficient Dehydrogenation And Electro-Oxidation Of Hydrazine

The chemical energy stored in energetic materials may often be utilized in various ways, which motivates the development of multifunctional catalysts for flexible and efficient utilization of the chemical energy. Hydrazine is a promising energy carrier due to its high energy density and high hydrogen content, which can be utilized as a chemical hydrogen storage medium or a fuel for direct fuel cells. Herein, we propose a bifunctional catalyst for efficient dehydrogenation and electro-oxidation of hydrazine. As a proof-of-concept study, a carbon-black-supported Pt0.2Ni0.8 nanoparticle catalyst has been developed with high activity and durability for both complete dehydrogenation (with a turnover frequency of 673 h-1 and a H2 generation rate of 188 L h-1 gmetal-1) and electro-oxidation (with a mass activity of 132 mA mgmetal-1) of hydrazine under mild conditions, outperforming other catalysts including Pt, Ni, Pd0.2Ni0.8, and Au0.2Ni0.8 nanoparticles. Such a bifunctional catalyst can enable the utilization of hydrazine as a promising energy carrier for both on-demand hydrogen generation and electricity generation via direct hydrazine fuel cells, enhancing its flexibility for future onboard applications

Ambient Electrochemical Ammonia Synthesis With High Selectivity On Fe/Fe Oxide Catalyst

Electrochemical reduction of N2 to NH3 under ambient conditions can provide an alternative to the Haber-Bosch process for distributed NH3 production that can be powered by renewable electricity. The major challenge for realizing such a process is to develop efficient electrocatalysts for the N2 reduction reaction (N2RR), as typical catalysts show a low activity and selectivity due to the barrier for N2 activation and the competing hydrogen evolution reaction (HER). Here we report an Fe/Fe3O4 catalyst for ambient electrochemical NH3 synthesis, which was prepared by oxidizing an Fe foil at 300 °C followed by in situ electrochemical reduction. The Fe/Fe3O4 catalyst exhibits a Faradaic efficiency of 8.29% for NH3 production at -0.3 V vs the reversible hydrogen electrode in phosphate buffer solution, which is around 120 times higher than that of the original Fe foil. The high selectivity is enabled by an enhancement of the intrinsic (surface-area-normalized) N2RR activity by up to 9-fold as well as an effective suppression of the HER activity. The N2RR selectivity of the Fe/Fe3O4 catalyst is also higher than that of Fe, Fe3O4, and Fe2O3 nanoparticles, suggesting Fe/Fe oxide composite to be an efficient catalyst for ambient electrochemical NH3 synthesis

Hydrogen Evolution from Metal–Surface Hydroxyl Interaction

The redox interaction between hydroxyl groups on oxide surfaces and metal atoms and clusters deposited thereon, according to which metals get oxidized and hydrogen released, is an effective route to tune both the morphological (particle size and shape) and electronic (oxidation state) properties of oxide-supported metals. While the oxidation state of the metals can straightforwardly be probed by X-ray based methods (e.g., XPS), hydrogen is much more difficult to capture, in particular in highly reactive systems where the redox interaction takes place directly during the nucleation of the metals at room temperature. In the present study, the interaction of Pd with a hydroxylated MgO(001) surface was studied using a combination of vibrational spectroscopy, electronic structure studies including Auger parameter analysis, and thermal desorption experiments. The results provide clear experimental evidence for the redox nature of the interaction by showing a direct correlation between metal oxidation and hydrogen evolution at slightly elevated temperature (390 K). Moreover, a second hydrogen evolution pathway opens up at 500 K, which involves hydroxyl groups on the MgO support and carbon monoxide adsorbed on the Pd particles (water–gas shift reaction)