A Study of Nanostructuring Effects on Model Heterogeneous Catalysts

Abstract

The role of heterogeneous catalysts in modern society is immense. The majority of catalysts are nanostructured solely in order to increase the fraction of atoms on their surface. The nanostructuring, however, introduces important changes in properties of the catalysts. Since commercial heterogeneous catalysts have a very complex hierarchical structure many academic studies are performed on so-called model catalysts. The latter are simplified systems that have a realistic degree of nanostructuring, but lack complexity on micrometer and millimeter scales.

This thesis summarizes results of computational investigations of different ways how nanostructuring may affect properties and activity of model heterogeneous catalysts. Among various forms of model catalysts this thesis considers supported and unsupported transition metal nanoparticles, steps on surfaces and nanometer thick films. In all cases computational models were designed to be as similar as possible to respective experimental systems. In particular, the simulated nanoparticles were sufficiently big to be scalable with size. That is, their properties can be safely extrapolated to respective properties of much bigger nanoparticles present in the majority of experiments and applications. Since these models often contain hundreds of atoms they were investigated with density functional theory methods that yield a good compromise between accuracy and computational cost for systems of this size.

In particular, the following studies were performed:

•Adsorption and infrared spectroscopy properties of CHxOy (x=1-3, y=0-1) species on

Pd nanoparticles were characterized.

•The activity of edges Pd nanoparticles in methane dissociation was critically analyzed compared to that of Pd(111).

•The activity of {111} terraces of Pd, Pt, Ni, and Rh nanoparticles in ethyl hydrogenation was compared to the activity of respective (111) single crystal surfaces.

•A new method to optimize chemical ordering in bimetallic nanoparticles was proposed and applied to catalytically active PdAu, PdAg, PdCu and PdZn alloy particles.

•The effect of MgO(100) support on physical and adsorptive properties of Pd and Pt nanoparticles was quantified.

•H absorption into Pd and Pt nanoparticles supported on MgO(100) was simulated.

•Atomic and electronic structure of steps on CeO2(111) was determined.

•Two novel methods to calculate specific energy of steps on a surface were proposed and applied to steps on CeO2(111)

•The ability of steps on CeO2(111) to form O vacancies was investigated using a newly proposed prescreening procedure.

•Structure of ~1 nm thick Ce2O3 films was investigated by means of the simulated mechanical annealing method.

•Reconstructrion of PdZn films on Pd(111) for monolayer thick films or under CO atmosphere was characterized.

This diverse list of studies fulfills the goal of exploring various effects of different forms of nanostructuring on heterogeneous catalysts. Many of these studies are one-of- a-kind investigations describing certain effects induced by the nanostructuring on catalytically active materials for the first time. Another part of these studies describe novel simulation and analysis techniques that advance the methodology of computational investigations of nanostructured solid state systems.

La catálisis heterogénea juega un gran papel en el mundo. El material activo de un catalizador normalmente se dispersa en forma de nanopartículas. En ciertos casos se ha demostrado que la nanoestructura altera completamente las propiedades catalíticas del material.

El objetivo principal de esta tesis es investigar cómo varias formas de nanoestructuración pueden alterar los materiales usados en catálisis heterogénea, a través de cálculos DFT.

Los resultados de esta tesis incluyen:

1. Los filos de nanopartículas de Pd adsorben ciertas especies CHxOy significativamente más fuertamente que las caras {111} de las nanopartículas.

2. Los cálculos muestran que nanopartículas de Pd son catalíticamente más activas en la descomposición de metano que las superficies de Pd(111).

3. Las caras {111} de nanopartículas de Pd presentan una actividad catalítica mucho mayor en procesos de hidrogenación comparando con monocristales de Pd(111), siempre y cuando el sistema presenta H absorbido.

4. La aplicación de los Hamiltonianos topológicos propuestos en este trabajo a nanoparticulas de aleaciones bimetálicas permite optimizar su estructura en base a los resultados de cálculos de estructura electrónica.

5. El efecto de la superficie de MgO(100) en las propiedades físicas y adsorbentes de nanopartículas de Pd127 y Pt127 de 1.6 nm de tamaño depositadas puede ser insignificante.

6. La saturación de la superficie de las nanopartículas Pd127 y Pt127 con átomos de H afecta a su estructura geométrica y electrónica y a sus propiedades adsorbentes mucho más que la presencia del soporte de MgO(100).

7. Nanoislas formadas sobre la superficie de CeO2(111) exponen escalones con una estructura electrónica modificada y una mayor reducibilidad.

8. Los métodos (aquí propuestos) para calcular las energías absolutas de los escalones proporcionan una exactitud estadística superior al método preexistente a pesar del menor número de cálculos necesarios.

9. La estructura de las capas de Ce2O3 depende del substrato en el que crecen.

10. La estructura distorsionada (“zigzag”) de capas de PdZn sobre Pd(111) es más estable que la estructura convencional para monocapas y para capas cubiertas por moléculas de CO.