NANOSTRUCTURED MATERIALS FOR ENVIRONMENTAL AND ENERGY-RELATED APPLICATIONS

The world is facing an era of global environmental pollution, as a result of the tremendous population growth and the consequent massive fossil fuel-based energy consumption. A significant exploitation of renewable energies is needed to guarantee quality of human life and allow further sustainable growth, but this may take decades to happen. In order to mitigate the negative effect of human activities on the environment in the short- and mid-term, the development of more efficient technologies for emissions abatement and for renewable fuels production is imperative. Heterogeneous catalysis and photocatalysis are two key pillars of a multi-approach strategy to solve these issues. During the last century, catalysts were explored by changing the formulation of multi-component systems in order to find the best performing material for a certain reaction. Since the late 90's, a new approach to catalytic systems improvement emerged: nano-catalysis. Exploiting the tools of nanotechnology, tailored nanostructured materials can now be produced, which show different properties in comparison to their bulky counterparts, often resulting in better catalytic performances. Furthermore, combining the elements of the periodic table in nano-alloys allows to expand the possibility of catalyst generation. Consistently with these approaches, the main focus of this thesis is the synthesis and characterization of well-defined nanostructured and hierarchical materials for environmental and energy-related applications, such as emissions control, biofuels synthesis and photocatalytic H2 production. We show that structural control at the nanoscale is a great instrument for understanding reaction pathways, for studying the nature of catalytic active sites, and for synthesizing more selective, active and stable catalysts. Two synthetic strategies were followed to acquire nanostructural control: a self-assembly method was employed to prepare hierarchical materials starting from functional nanoparticles, and advanced solvothermal methods were used to prepare monodisperse nanocrystals having controlled size and composition. State-of-the-art hierarchical Pd-based catalysts embedded by metal oxide promoters were tested for methane catalytic oxidation in the presence of poisoning compounds typically found in real applications. Detailed surface studies allowed to propose deactivation mechanisms and strategies to improve catalysts resistance to deactivation. Well-controlled nanostructured Pt-based alloys and Ni-Cu alloys showed improved activity, stability and selectivity for hydrodeoxygenation reactions of biomass-derived feedstocks to produce biofuels. The control of nanostructure was pivotal to understand the reason for such enhanced performances. Finally, dye-sensitized photocatalysts were investigated in H2 photocatalytic production under visible light, and state-of-the-art stability and activities were demonstrated. All these findings greatly contributed to the development of catalytic materials for energy-related applications

Nanostructured Pd\u2013Pt nanoparticles: evidences of structure/performance relations in catalytic H2production reactions

A widespread approach to modulate the performances of heterogeneous catalysts is the use of bimetallic nanoparticles (NPs) as the active phase. However, studying the relationship between the NPs structure and catalytic properties requires well-defined systems, having uniform composition, size and nanostructure, which cannot be achieved by traditional methods (e.g. impregnation). Here, we developed wet-chemistry synthetic routes to prepare PdPt NPs or Pt-core@Pd-shell NPs of small size and well-controlled composition and structure, protected by mercaptoundecanoic acid (MUA) moieties. The pristine NPs were tested for H2 production by NH3BH3 hydrolysis, in order to systematically investigate the effect of composition and of synthetic route on the activity of the systems. Depending on the preparation method, two distinct trends of activity were observed, rationalized in terms of the extent of surface functionalization by MUA. The MUA protective layer was found to effectively stabilize the NPs dispersion while maintaining high activity in certain cases (Pt-rich NPs), and was demonstrated to be essential for catalyst recycling. In order to further study structure-activity relationships of PdPt NPs after ligand removal, nanostructured PdPt@CeO2-based catalysts were prepared by self-assembly route. Regardless of the starting NPs structure (alloy or core-shell), similar water gas shift reaction performances were observed, due to the structural rearrangements occurring upon oxidation and reduction thermal treatments, which led to the formation of Pt-rich core@PdPt-shell under reducing conditions

Structure Sensitivity of CO2Conversion over Nickel Metal Nanoparticles Explained by Micro-Kinetics Simulations

Nickel metal nanoparticles are intensively researched for the catalytic conversion of carbon dioxide. They are commercially explored in the so-called power-to-methane application in which renewably resourced H2 reacts with CO2 to produce CH4, which is better known as the Sabatier reaction. Previous work has shown that this reaction is structure-sensitive. For instance, Ni/SiO2 catalysts reveal a maximum performance when nickel metal nanoparticles of ∼2-3 nm are used. Particularly important to a better understanding of the structure sensitivity of the Sabatier reaction over nickel-based catalysts is to understand all relevant elementary reaction steps over various nickel metal facets because this will tell as to which type of nickel facets and which elementary reaction steps are crucial for designing an efficient nickel-based methanation catalyst. In this work, we have determined by density functional theory (DFT) calculations and micro-kinetics modeling (MKM) simulations that the two terrace facets Ni(111) and Ni(100) and the stepped facet Ni(211) barely show any activity in CO2 methanation. The stepped facet Ni(110) turned out to be the most effective in CO2 methanation. Herein, it was found that the dominant kinetic route corresponds to a combination of the carbide and formate reaction pathways. It was found that the dissociation of H2CO∗ toward CH2∗ and O∗ is the most critical elementary reaction step on this Ni(110) facet. The calculated activity of a range of Wulff-constructed nickel metal nanoparticles, accounting for varying ratios of the different facets and undercoordinated atoms exposed, reveals the same trend of activity-versus-nanoparticle size, as was observed in previous experimental work from our research group, thereby providing an explanation for the structure-sensitive nature of the Sabatier reaction

In Situ X-ray Raman Scattering Spectroscopy of the Formation of Cobalt Carbides in a Co/TiO2 Fischer–Tropsch Synthesis Catalyst

We present in situ experiments to study the possible formation of cobalt carbides during Fischer–Tropsch synthesis (FTS) in a Co/TiO2 catalyst at relevant conditions of pressure and temperature. The experiments were performed by a combination of X-ray Raman scattering (XRS) spectroscopy and X-ray diffraction (XRD). Two different experiments were performed: (1) a Fischer–Tropsch Synthesis (FTS) reaction of an ∼14 wt % Co/TiO2 catalyst at 523 K and 5 bar under H2 lean conditions (i.e., a H2:CO ratio of 0.5) and (2) carburization of pure cobalt (as reference experiment). In both experiments, the Co L3-edge XRS spectra reveal a change in the oxidation state of the cobalt nanoparticles, which we assign to the formation of cobalt carbide (Co2C). The C K edge XRS spectra were used to quantify the formation of different carbon species in both experiments.Peer reviewe

Phosphorus poisoning during wet oxidation of methane over Pd@CeO2/graphite model catalysts

10siThe influence of phosphorus and water on methane catalytic combustion was studied over Pd@CeO2 model catalysts supported on graphite, designed to be suitable for X-ray Photoelectron Spectroscopy/Synchrotron Radiation Photoelectron Spectroscopy (XPS/SRPES) analysis. In the absence of P, the catalyst was active for the methane oxidation reaction, although introduction of 15% H2O to the reaction mixture did cause reversible deactivation. In the presence of P, both thermal and chemical aging treatments resulted in partial loss of activity due to morphological transformation of the catalyst, as revealed by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) analysis. At 600 °C the combined presence of PO43− and water vapor caused a rapid, irreversible deactivation of the catalyst. XPS/SRPES analysis, combined with operando X-ray Absorption Near Edge Structure (XANES) and AFM measurements, indicated that water induces severe aggregation of CeO2 nanoparticles, exposure of CePO4 on the outer layer of the aggregates and incorporation of the catalytic-active Pd nanoparticles into the bulk. This demonstrates a temperature-activated process for P-poisoning of oxidation catalysts in which water vapor plays a crucial role.partially_openembargoed_20171009Monai, Matteo; Montini, Tiziano; Melchionna, Michele; Duchoň, Tomáš; Kúš, Peter; Tsud, Nataliya; Prince, Kevin C.; Matolin, Vladimir; Gorte, Raymond J.; Fornasiero, PaoloMonai, Matteo; Montini, Tiziano; Melchionna, Michele; Duchoň, Tomáš; Kúš, Peter; Tsud, Nataliya; Prince, Kevin C.; Matolin, Vladimir; Gorte, Raymond J.; Fornasiero, Paol

High-surface-area, iron-oxide films prepared by atomic layer deposition on \u3b3-Al2O3

High-surface-area iron oxides were prepared by Atomic Layer Deposition (ALD) on 130-m(2)/g gamma-Al2O3 for use as a catalyst support. Measurements of the sample mass, surface area, and pore-size distribution as a function of the number of ferrocene-O-2 ALD cycles at 623 K suggested that the iron oxide grew as a dense, conformal film with a growth rate similar to 0.016-nm per cycle. While films with 20 ALD cycles (20Fe(2)O(3)Al(2)O(3), 0.25 g Fe2O3/g Al2O3) were difficult to distinguish by HAADF STEM, EDS mapping indicated the Al2O3 was uniformly coated. Raman Spectroscopy showed the films were in the form of Fe2O3; but XRD measurements on samples with as many as 100 ALD cycles (100Fe(2)O(3)-Al2O3, 0.84g Fe2O3/g Al2O3) showed no evidence for crystalline iron-oxide phases, even after calcination at 1073 K. Specific rates for the water-gas-shift (WGS) reaction on the ALD-coated samples were significantly lower than those on bulk Fe2O3. However, addition of 1 wt.% Pd to Fe2O3/Al2O3 supports prepared by ALD exhibited specific rates that were much higher than that observed when I wt.% Pd was added to Fe2O3/Al2O3 prepared by conventional impregnation of Fe salts, suggesting more uniform contact between the Pd and FeOx phases on samples prepared by ALD

Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography

The conversion of methanol to valuable hydrocarbon molecules is of great commercial interest, as the process serves as a sustainable alternative for the production of, for instance, the base chemicals for plastics. The reaction is catalyzed by zeolite materials. By the introduction of magnesium as a cationic metal, the properties of the zeolite, and thereby the catalytic performance, are changed. With atom probe tomography (APT), nanoscale relations within zeolite materials can be revealed: i.e., crucial information for a fundamental mechanistic understanding. We show that magnesium forms clusters within the cages of zeolite SSZ-13, while the framework elements are homogeneously distributed. These clusters of just a few nanometers were analyzed and visualized in 3-D. Magnesium atoms seem to initially be directed to the aluminum sites, after which they aggregate and fill one or two cages in the zeolite SSZ-13 structure. The presence of magnesium in zeolite SSZ-13 increases the lifetime as well as the propylene selectivity. By using operando UV-vis spectroscopy and X-ray diffraction techniques, we are able to show that these findings are related to the suppression of aromatic intermediate products, while maintaining the formation of polyaromatic compounds. Further nanoscale analysis of the spent catalysts showed indications of magnesium redistribution after catalysis. Unlike zeolite H-SSZ-13, for which only a homogeneous distribution of carbon was found, carbon can be either homogeneously or heterogeneously distributed within zeolite Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon clusters were isolated, visualized, and analyzed and were assumed to be polyaromatic compounds. Small one-cage-filling polyaromatic compounds were identified; furthermore, large-cage-crossing aromatic molecules were found by isolating large coke clusters, demonstrating the unique coking mechanism in zeolite SSZ-13. Short-length-scale evidence for the formation of polyaromatic compounds at acid sites is discovered, as clear nanoscale relations between aluminum and carbon atoms exist

Elucidating the Structure and Composition of Individual Bimetallic Nanoparticles in Supported Catalysts by Atom Probe Tomography

Understanding and controlling the structure and composition of nanoparticles in supported metal catalysts are crucial to improve chemical processes. For this, atom probe tomography (APT) is a unique tool, as it allows for spatially resolved three-dimensional chemical imaging of materials with sub-nanometer resolution. However, thus far APT has not been applied for mesoporous oxide-supported metal catalyst materials, due to the size and number of pores resulting in sample fracture during experiments. To overcome these issues, we developed a high-pressure resin impregnation strategy and showcased the applicability to high-porous supported Pd-Ni-based catalyst materials, which are active in CO2 hydrogenation. Within the reconstructed volume of 3 × 105 nm3, we identified over 400 Pd-Ni clusters, with compositions ranging from 0 to 16 atom % Pd and a size distribution of 2.6 ± 1.6 nm. These results illustrate that APT is capable of quantitatively assessing the size, composition, and metal distribution for a large number of nanoparticles at the sub-nm scale in industrial catalysts. Furthermore, we showcase that metal segregation occurred predominately between nanoparticles, shedding light on the mechanism of metal segregation. We envision that the presented methodology expands the capabilities of APT to investigate porous functional nanomaterials, including but not limited to solid catalysts

The effect of sulfur dioxide on the activity of hierarchical Pd-based catalysts in methane combustion

SO2 poisoning of methane oxidation over alumina-supported, Pd@CexZr1-xO2 nanoparticle catalysts was systematically studied by means of advanced PhotoElectron Spectroscopy (PES) methods. The Pd@CexZr1-xO2 units were synthesized and deposited on two modified-alumina supports, i.e. high surface area modified alumina and a model alumina prepared by Atomic Layer Deposition (ALD) of alumina on Indium Tin Oxide (ITO)/quartz slides. The model support was designed to be suitable for PES analysis and was stable to high temperature treatments (850 degrees C). Characterization of the high-surface-area (HSA) catalysts by X-Ray Diffraction (XRD), N-2 physisorption, CO chemisorption and Transmission Electron Microscopy (TEM) indicated formation of CeO2-ZrO2 (CZ) mixed-oxide crystallites that stabilize the Pd active phase against sintering. Correlation of methane-oxidation rates with PES results demonstrated two distinct mechanisms for deactivation by SO2. Below 450 degrees C, the presence of SO2 in the feed led to partial reduction of the active PdO phase and to the formation of sulfates on the Pd. Above 500 degrees C, poisoning by SO2 was less severe due to spillover of the sulfates onto the oxide promoter. Pd@ZrO2 catalysts showed the best resistance to SO2 poisoning, outperforming analogous Pd@CZ mixed-oxide catalysts, because there was less sulfate formation and the sulfates that did form could be removed during regeneration