Part 1—Structure-Sensitivity of Nanoparticle Catalysts: Relating Current Theories to Experimental Data (original) (raw)
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Nanowires and their morphology induced catalytic properties
Overall, this thesis aims to investigate the mechanism behind one dimensional nanostructure synthesis and catalytic performance, which hopes to contribute towards optimizing of catalytic process. More specifically, we planned to exploit the recent advances in one dimensional nanostructures synthesis, which we limits the scope to nanowires synthesis, and investigate the growth mechanism of the nanowires (NWs) synthesis if it was still not well understood. Next, with this understanding in synthesis mechanism, we hoped to create novel NWs structures, especially hybrid NWs structures, and apply them for photocatalysis and electrocatalysis. Besides that, we hoped to observe and identified specific advantages in catalysis arises from specific nanostructure, or in other words, morphology induced properties. TiO 2 nanowires had outstanding performance as light absorbing materials, and its large scale synthesis had been reported. However, there were not enough study done on the large scale synthesis mechanism, resulting in poor length and diameter control. We had demonstrated that large scale TiO 2 NWs synthesized via molten salt method adopts seed-assisted growth mode, with ripening as the main growth mechanism in chapter 2. Rutile NPs not only able act as seed to control the diameter of the resulting NWs, but also able to act as feedstock, where the selectivity comes from its own size. We had also identified the key to grow hybrid TiO 2 NWs was to control the rate of ligand adsorption and TiO 2 deposition in Chapter 3. We had tried various methods to increase ligand adsorption speed and decrease TiO 2 deposition rate. However, all the attempts had failed to control the lateral growth or the NWs obtained was not TiO 2. As we had exhausted all possible means but was unsuccessful to obtain the desired nanostructure, the project was halted. Ultrathin metal nanowires array was also an interesting system with great potential in catalysis. Although its synthesis mechanism is well studied, there had not been enough research conducted on the catalysis part. We have Abstract ii demonstrated that by simply changing the morphology of electrocatalyst from nanoparticles to aligned nanowire arrays, the catalytic activity can be improved dramatically by providing more electrochemical active surface for electrocatalysis, one-dimensional channels for improved mass transport and better conductivity in Chapter 4. Our approach provides a new and simple means to enhance the electrocatalytic activity, reduce the size of electrode for miniaturization of portable devices and improve the effectiveness of existing and emerging electrochemical technologies. We had proposed an EOR mechanism on Pd surface in Chapter 5, where OHions act as the main inhibitor that poison the Pd surface. We had discussed in details of the processes that might be happening in different part of the EOR CV. This understanding had helped us to identified the limiting factors of EOR mechanism and thus possible to suggest methods on improving the EOR activity and stability in DEFCs devices. A part from that, we had discovered that reservoir plays important role in determining the performance of Pd in EOR. The reservoir effect on our long and dense Au@Pd NWs array structure, proven to be a morphology induced property, might also be a contributing factor for its high performance in EOR. In addition, we had also shown that the present of two Pd facets on our Au@Pd NWs might be responsible for different ethanol oxidation product formation as proposed in our hypothesis through the studies conducted in Chapter 6. Pd (200) and (220) facets each have different affinity towards OHspecies, which results in different ethanol oxidation pathway on each facet, where Pd (200) facets responsible for the formation of acetic acid, and Pd (220) forms acetaldehyde. It was also discovered that some ligands bind specifically to one of the facets, which might served as a means to control ethanol oxidation product selectivity in the future. I would like to express my gratitude towards my supervisor, Professor Chen Hongyu, co-supervisor, Professor Liu Bin and my mentor, Professor Zhao Yanli. I would like to thanks Prof. Chen for inspired my interest in research and for all your teachings, guidance and advice for the past 7 years. I would like to show my gratefulness towards Prof. Liu for his patient teaching and guidance throughout my PhD studies. I also like to express my thankfulness to Prof. Zhao for all the care and resources he had provided for the past four years. I gratefully acknowledge the funding from the National Research Foundation (NRF), Prime Minister's Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program for supporting my 4 years course of PhD studies. I would also like to thanks my school, Interdisciplinary Graduate School for providing such a wonderful opportunity for me to experience multidisciplinary research.
The Journal of Physical Chemistry B, 2005
The particle size effect on the formation of OH adlayer, the CO bulk oxidation, and the oxygen reduction reaction (ORR) have been studied on Pt nanoparticles in perchloric acid electrolyte. From measurements of the CO displacement charge at controlled potential, the corresponding surface charge density versus potential curves yielded the potentials of total zero charge (pztc), which shifts approximately 35 mV negative by decreasing the particle size from 30 nm down to 1 nm. As a consequence, the energy of adsorption of OH is more enhanced, that is, at the same potential the surface coverage with OH increases by decreasing the particle size, which in turn affects the catalytic reactions thereon. The impact of the electronically induced potential shift in the OH adsorption is demonstrated at the CO bulk oxidation, in which adsorbed OH is an educt species and promotes the reaction, and the ORR, where it can act as a surface site blocking species and inhibits the reaction.
Faraday Discussions, 2013
Solid surfaces generally respond sensitively to their environment. Gas phase or liquid phase species may adsorb and react with individual surface atoms altering the solid-gas and solid-liquid electronic and chemical properties of the interface. A comprehensive understanding of chemical and electrochemical interfaces with respect to their responses to external stimuli is still missing. The evolution of the structure and composition of shape-selected octahedral PtNi nanoparticles (NPs) in response to chemical (gas-phase) and electrochemical (liquid-phase) environments was studied, and contrasted to that of pure Pt and spherical PtNi NPs. The NPs were exposed to thermal annealing in hydrogen, oxygen, and vacuum, and the resulting NP surface composition was analyzed using X-ray photoelectron spectroscopy (XPS). In gaseous environments, the presence of O 2 during annealing (300 C) lead to a strong segregation of Ni species to the NP surface, the formation of NiO, and a Pt-rich NP core, while a similar treatment in H 2 lead to a more homogenous Pt-Ni alloy core, and a thinner NiO shell. Further, the initial presence of NiO species on the as-prepared samples was found to influence the atomic segregation trends upon low temperature annealing (300 C). This is due to the fact that at this temperature nickel is only partially reduced, and NiO favors surface segregation. The effect of electrochemical cycling in acid and alkaline electrolytes on the structure and composition of the octahedral PtNi NPs was monitored using image-corrected high resolution transmission electron microscopy (TEM) and high-angle annular dark field scanning TEM (HAADF-STEM). Sample pretreatments in surface active oxygenates, such as oxygen and hydroxide anions, resulted in oxygen-enriched Ni surfaces (Ni oxides and/or hydroxides). Acid treatments were found to strongly reduce the content of Ni species on the NP surface, via its dissolution in the electrolyte, leading to a Pt-skeleton structure, with a thick Pt shell and a Pt-Ni core. The presence of Ni hydroxides on the NP surface was shown to improve the kinetics of the electrooxidation of CO and the electrocatalytic hydrogen
Size, Shape, Composition and Chemical state effects in nanocatalysis
2016
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.
Catalysis from First Principles: Towards Accounting for the Effects of Nanostructuring
Topics in Catalysis, 2013
The article deals with an issue of density-functional description of heterogeneous catalysts by nanoparticle models instead of still commonly employed slab models. Typically, active (metal) components are present in catalysts as nano-aggregates formed of many thousands atoms, remaining due to their size inaccessible even for modern first-principles computations. However, such species could be rather realistically represented by notably smaller computationally tractable model nanoparticles, whose surface sites only marginally change the reactivity with increasing particle size. Herein presented results are mainly related to methane dehydrogenation on Pt catalysts, methanol decomposition on Pd catalysts and the composition of active sites on Pt/ceria catalysts. They document feasibility of taking nanostructuring effects into account in density-functional modeling (at least for transition metals) and, more importantly, demonstrate that ignoring the nanoeffects in these systems leads to misrepresentation of their catalytic properties.
Size effects in the catalytic activity of unsupported metallic nanoparticles
Journal of Nanoparticle Research, 2003
The influence of the size of nanoparticles on their catalytic activity was investigated for two systems on unsupported, i.e. gasborne nanoparticles. For the oxidation of hydrogen on Pt nanoparticle agglomerates, transport processes had to be taken into account to extract the real nanoparticle size effects. The results indicate an optimum particle size for the catalytic activity below 5 nm which points clearly toward a real volume effect. In the case of the methanation reaction on gasborne Ni nanoparticles, no transport limitations were observed and the product concentration was directly proportional to the activity of the primary particles. We found an activity maximum for particles of about 19 nm in diameter. This size is too large to be attributed to a real nanoparticle size effect induced by the electronic band structure. Therefore, we concluded that the particle size influences the adsorption behavior of the carbon monoxide molecules. In fact, it is known that intermediate adsorption enthalpies may favor dissociation processes, which is an essential step for the reaction, as manifested in the so called volcano-shaped curve. Then, in addition to the material dependence of the adsorption, we would also encounter a direct size dependence in the case of methanation on gasborne Ni nanoparticles.
Geometric and electronic approaches to size effects in heterogeneous catalysis
Kinetics and Catalysis, 2011
The influence of electronic and geometric factors is considered in the context of the manifestation of size effects in heterogeneous catalytic oxidation and hydrogenation reactions. Both of the factors are inter dependent; however, the electronic factor predominates with regard to small metal and metal oxide particles (smaller than 10 nm), for which the energies of electron transitions in an activated complex are size depen dent. Only the geometry of active component nanoparticles exerts the main effect on the catalytic properties of coarser particles. In this case, the geometric factor depends on the accessibility of the active surface to reac tants. The probability of the occurrence of complex active centers including several surface atoms increases as the active component particles of a catalyst become larger. The efficiency of the approach proposed to study the activating effect of nanophase catalysts is demonstrated using the oxidation and hydrogenation reactions of carbon oxides and the hydrogenation of acetonitrile and acetone as examples.
The Journal of Physical Chemistry B, 2005
Colloidal metal nanoparticles have a high surface-to-volume ratio which makes them potentially attractive catalysts. Furthermore, atoms located at different facets, edges, or corners could show different catalytic activity. For this reason, different shapes could have different catalytic activities. In addition, surface atoms are so active that there could be significant changes in their shape and size during the course of nanocatalysis. As a result, a thorough examination on the effect of the catalytic process on the shape and size of colloidal metal nanoparticles after catalysis is necessary to fully evaluate their use in catalytic processes. In this paper, we briefly review our recent work on examining the shape dependence of nanocatalysis as well as the stability of platinum and palladium nanoparticles during the course of two reactions: the electron transfer reaction and the Suzuki reaction. It is found that nanocatalysis is indeed shape-dependent during the early stages of the electron transfer reaction. During the full course of the reaction, there are changes in the nanoparticle shape as well as changes in the activation energy that takes place. In the case of a relatively harsh reaction such as the Suzuki reaction, spherical palladium nanoparticles grow in size due to Ostwald ripening processes. Tetrahedral platinum nanoparticles catalyzing the Suzuki reaction transform into spherical shape and grow larger in size. We also conducted studies on the effect of individual reactants on the nanoparticle size and shape. In addition, the surface catalytic mechanisms of the reactions have been confirmed using spectroscopic tools such as FTIR and Raman spectroscopy. These kinds of studies will be very useful in the process of designing better nanocatalysts in the future.