Size, Shape, Composition and Chemical state effects in nanocatalysis (original) (raw)

The field of nanocatalysis has gained significant attention in the last decades due to the numerous industrial applications of nanosized catalysts. Size, shape, structure, and composition of the nanoparticles (NPs) are the parameters that can affect the reactivity, selectivity and stability of nanocatalysts. Therefore, understanding how these parameters affect the catalytic properties of these systems is required in order to engineer them with a given desired performance. It is also important to gain insight into the structural evolution of the NP catalysts under different reaction conditions to design catalysts with long durability under reaction condition. In this dissertation a synergistic combination of in situ, ex situ and operando state-of-the art techniques have allowed me to explore a variety of parameters and phenomena relevant to nanocatalysts by systematically tuning the NP size, chemical state, composition and chemical environment. environments as long as oxygen species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270°C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion towards the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established. In Chapter 9 the chemical and morphological stability of size-and shape-selected octahedral PtNi NPs was investigated after different annealing treatments up to a maximum temperature of 700°C in vacuum and under 1 bar of CO. AFM was used to examine the mobility of the NPs and their stability against coarsening, and XPS to investigate the surface composition, chemical state of Pt and Ni in the NPs and thermally and CO-induced atomic segregation trends. Exposing the samples to 1 bar of CO at room temperature before annealing in vacuum was found to be effective at enhancing the stability of the NPs against coarsening. In contrast, significant coarsening was vii observed when the sample was annealed in 1 bar of CO, most likely as a result of Ni(CO)4 formation. Sample exposure to CO at room temperature prior to annealing lead to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective of the initial sample pretreatment, Ni surface segregation was observed upon annealing in vacuum at moderate temperature (T<300°C). Interestingly, a distinct atomic segregation trend was detected between 300°C-500°C for the sample pre-exposed to CO, namely, Ni surface segregation was partially hindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1-bar CO results in the occupation of the NP surface by Ni atoms at 400°C as a result of Ni(CO)x formation. Above 500°C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni-Pt alloy is observed. Chapter 10 contains the summary and outlook of the thesis. viii This dissertation is dedicated to my parents and my wife who were extremely patient and supportive with me during all these years. ix ACKNOWLEDGMENT My deepest thanks is to my advisor Prof. Beatriz Roldan Cuenya for her support and guidance during my academic and personal Phd life. Prof. Roldan gave me the freedom to approach different project in my own way, which gave me the opportunity of tackling problems independently, while at the same time her guidance in critical moments wouldn't allow me to get lost in the passage. Her broad mindedness toward diverse ideas taught me the team working and tolerance in a research group. Her hard working, honesty and commitment to scientific values would be my example of a successful researcher. I am also thankful to her for carefully reading, revising and commenting my manuscripts. I want to also thank my group members Dr.