Ceria Catalysts at Nanoscale: How Do Crystal Shapes Shape Catalysis?

Engineering the shape and size of catalyst particles and the interface between different components of heterogeneous catalysts at the nanometer level can radically alter their performances. This is particularly true with CeO2-based catalysts, where the precise control of surface atomic arrangements can modify the reactivity of Ce4+/Ce3+ ions, changing the oxygen release/uptake characteristics of ceria, which, in turn, strongly affects catalytic performance in several reactions like CO, soot, and VOC oxidation, WGS, hydrogenation, acid base reactions, and so on. Despite the fact that many of these catalysts are polycrystalline with rather ill-defined morphologies, experimental and theoretical studies on well-defined nanocrystals have clearly established that the exposure of specific facets can increase/decrease surface Oxygen reactivity and metal-support interaction (for supported metal nanoparticles), consequently affecting catalytic reactions. Here, we want to address the most recent developments in this area, showing that shape (and size) modification, surface/face reconstruction, and faceting of ceria at the nanoscale level can offer an important tool to govern activity and stability in several reactions and imagine how this could contribute to future developments

Potential of Ceria-Zirconia-Based Materials in Carbon Soot Oxidation for Gasoline Particulate Filters

ZrO(2)and Ce(0.8)Zr(0.2)O(2)mixed oxides were prepared and tested in the oxidation of carbon soot at different oxygen partial pressures and degrees of catalyst/soot contact to investigate their activity under typical gasoline direct injection (GDI) operating conditions. Under reductive atmospheres, generation of oxygen vacancies occurs in Ce0.8Zr0.2O2, while no reduction is observed on ZrO2. Both materials can oxidize carbon under high oxygen partial pressures; however, at low oxygen partial pressures, the presence of carbon can contribute to the reduction of the catalyst and formation of oxygen vacancies, which can then be used for soot oxidation, increasing the overall performance. This mechanism is more efficient in Ce(0.8)Zr(0.2)O(2)than ZrO2, and depends heavily on the interaction and the degree of contact between soot and catalyst. Thus, the ability to form oxygen vacancies at lower temperatures is particularly helpful to oxidize soot at low oxygen partial pressures, and with higher CO(2)selectivity under conditions typically found in GDI engine exhaust gases

The dynamics of PdO-Pd phase transformation in the presence of water over Si-doped Pd/CeO 2 methane oxidation catalysts

One of the main issues for the catalytic abatement of methane from natural gas fueled vehicles over Pd-based materials is due to the large amount of water vapor in the exhausts, which can severely deactivate the catalyst. In this work, we investigated the effect of water added during methane oxidation on a series of silica doped Pd/ceria catalysts prepared by solution combustion synthesis, using different characterization techniques. The results obtained by coupling Temperature Programmed Oxidation (TPO) experiments and High Resolution Transmission Electron Microscopy (HRTEM) indicate that the mechanism of PdO-Pd-PdO phase transformation over Si-doped catalysts is different in dry and wet conditions. The presence of water not only shifts the onset of PdO decomposition to higher temperature, but also PdO-Pd transition is found to proceed via the formation of multi-domain PdO/Pd particles. This effect is tentatively attributed to the suppression of oxygen exchange induced by the presence of stable hydroxyl groups on silica.Postprint (author's final draft

Ceria-Based Materials in Hydrogenation and Reforming Reactions for CO2 Valorization

Reducing greenhouse emissions is of vital importance to tackle the climate changes and to decrease the carbon footprint of modern societies. Today there are several technologies that can be applied for this goal and especially there is a growing interest in all the processes dedicated to manage CO2 emissions. CO2 can be captured, stored or reused as carbon source to produce chemicals and fuels through catalytic technologies. This study reviews the use of ceria based catalysts in some important CO2 valorization processes such as the methanation reaction and methane dry-reforming. We analyzed the state of the art with the aim of highlighting the distinctive role of ceria in these reactions. The presence of cerium based oxides generally allows to obtain a strong metal-support interaction with beneficial effects on the dispersion of active metal phases, on the selectivity and durability of the catalysts. Moreover, it introduces different functionalities such as redox and acid-base centers offering versatility of approaches in designing and engineering more powerful formulations for the catalytic valorization of CO2 to fuels

Sustainable solvent-free selective oxidation of benzyl alcohol using Ru(0) supported on alumina

The selective oxidation of primary alcohols into their corresponding carbonyl compounds is challenging because of the easy over oxidization to acids and esters. The traditional reaction requires large amounts of solvent and oxidant, causing serious environmental issues. Recently, several efforts have been made to transform the reaction into a more sustainable process. Here, we investigated the solvent-free oxidation of benzyl alcohol using air as a green oxidant in the presence of ruthenium supported on alumina and zirconia, thereby meeting atom economy and environmental requirements. The materials were extensively characterized and, in addition to their activity, selectivity, and reusability, the environmental sustainability of the process was assessed according to green chemistry metrics. XRD, TEM, and XPS analyses suggest that the formation of metallic Ru on the support plays a key role in the catalytic activity. Ru supported on alumina, after a reduction treatment, achieves good activity (62% conversion) and a complete selectivity in a very sustainable process (without a solvent and with air as oxidant), as indicated by the very low E-factor value. The formulation is very stable and maintains high activity after recycling.Peer ReviewedPostprint (published version

Abatement of the ecotoxicological risk of landfill leachate by heterogeneous Fenton-like oxidation

Landfill leachates are highly contaminated liquid waste, and their treatment and detoxification are a challenging task. The current system of ecotoxicological risk assessment is complex and time-consuming. It is of fundamental importance to develop simpler and faster tools for the evaluation of the treated liquid waste and for an easier preliminary screening of the most active catalytic formulation/reaction conditions of the Fenton-like process. Here, several analytical techniques have been used for the assessment of the reduction of toxicity of the landfill leachate after Fenton process over copper-zirconia catalyst (ZrCu). Ultraviolet-visible (UV-vis) spectroscopy and absorbable organic halogens (AOX) analysis have been coupled to achieve further insight into the degradation of contaminants. In addition, for the first time, the qualitative abatement of organic compounds is monitored through proton nuclear magnetic resonance (H-1 NMR) analysis, providing a new method for evaluating the effectiveness of the treatment. Spectroscopic techniques reveal that the Fenton process induces a significant abatement of the aromatic and halogen compounds (51%) in the landfill leachate with a reduction of the toxicity that has been confirmed by ecotoxicological test with algae. These results validate the investigated tool for a simple rapid preliminary evaluation of the detoxification efficacy

Insights on the Interfacial Processes Involved in the Mechanical and Redox Stability of the BaCe0.65Zr0.20Y0.15O3−δ–Ce0.85Gd0.15O2−δ Composite

Ceramic fuel cells and H2 permeation membranes are key technologies to accelerate the transition from a carbon economy based on fossil fuels to a H2 economy based on the use of renewable resources. The competitiveness of these technologies in the market depends on the identification and optimization of stable and effective low-cost materials. Perovskite-fluorite ceria-based composites show suitable properties, and studies on the mechanism that rules their mechanical, thermal, and redox stability are crucial for further technological advances. This study focuses on the redox behavior of BaCe0.65Zr0.20Y0.15O3-?-Ce0.85Gd0.15O2-? (BCZY-GDC) dual-phase ceramic. Temperature-programmed reduction, thermogravimetry, temperature-dependent X-ray diffraction, and Raman analyses are used to understand the dynamics of the interaction between the ceramic oxide components. It is shown how the simultaneous occurrence of structural changes in BCZY and GDC reduction helps in decreasing the mechanical stresses induced by temperature and by the reducing atmosphere. The interfacial processes between the single GDC and BCZY oxides contribute to limit reduction of GDC in the composite, which allows complete reversibility of the redox process investigated in this study. Thus, it is suggested that the redox behavior of this class of materials may be a descriptor of their mechanical and thermal stability

Silver-based catalytic materials for the simultaneous removal of soot and NOx

The potential of silver-based catalysts in the simultaneous removal of particulate matter (soot) and NOxis investigated in this work and compared with that of a model Pt-Ba/Al2O3catalyst. The Ag (5 wt%) - Ba (10 wt%)/MO (MO = CeO2, ZrO2, Al2O3) and Ag (5 wt%) - Sr (10 wt%)/CeO2catalysts have been prepared by incipient wetness impregnation and characterized by BET, XRD, HRTEM, XPS and temperature-programmed reduction (TPR) experiments. The behavior of the catalyst in the soot combustion (under loose conditions) and NOxremoval has been separately analyzed by means of temperature programmed oxidation (TPO) and isothermal concentration step change (ICSC) experiments, respectively. The results show that all the catalysts are active in soot combustion with a significant decrease of oxidation onset temperature compared to uncatalyzed soot oxidation. The removal of NOxin the absence and in the presence of soot was investigated under cycling conditions, i.e. alternating lean-rich phases according to the LNT strategy. It has been found that the Ag-based samples are able to simultaneously remove soot and NOx. In particular, comparing the behavior of the prepared catalysts, the Ba-containing systems showed higher NOxstorage capacity than Sr-catalyst; also, the nitrogen selectivity increased even if resulted lower than the traditional LNT Pt-based catalyst. A detrimental effect of soot on the NOxstorage activity has been also observed.Postprint (author's final draft

Chemoselective hydrogenation of cinnamaldehyde at atmospheric pressure over combustion synthesized Pd catalysts

A series of Pd-supported metal oxides (Al2O3, Fe2O3 and CeO2) have been prepared by a single step solution combustion synthesis (SCS) method. Their catalytic performance was evaluated for the selective hydrogenation of cinnamaldehyde (CAL) to hydrocinnamaldehyde (HCAL) under atmospheric pressure of hydrogen at 100 °C. Among these materials, combustion synthesized Pd (2 at.%)/Al2O3 catalyst exhibits the highest CAL conversion (69%) with complete HCAL selectivity. The analogous catalyst prepared by the incipient wetness impregnation (IWI) method shows an initially similar activity. X-ray diffraction and high resolution transmission electron microscopy analyses of the as prepared SCS sample show fine dispersion of PdO over the ¿-Al2O3 support. On ageing, a major portion of PdO is reduced to metallic Pd (Pd2+:Pd0 = 36:64 for the SCS catalyst and Pd2+:Pd0 = 26:74 for the IWI catalyst from X-ray photoelectron spectroscopy studies) suggesting comparatively more ionic character of palladium in the SCS catalyst. In the hydrogen atmosphere, without distinguishing the reductive pretreatment of catalyst and the beginning of hydrogenation subsequent to CAL addition, the Pd-species undergoes rearrangement to form a core–shell like structure of Pd (core)–PdO (periphery) covered with alumina layer, bringing in additional stability to the Pd-species in the SCS catalyst and making it highly recyclable. The analogous IWI catalyst, on the contrary, contains a mixed Pd–PdO ensemble that does not increase the stability causing continuous loss of activity in the consecutive cycles of hydrogenation.Peer ReviewedPostprint (author's final draft

The influence of nano-architectured CeOx supports in RhPd/CeO2 for the catalytic ethanol steam reforming reaction

The ethanol steam reforming (ESR) reaction has been tested over RhPd supported on polycrystalline ceria in comparison to structured supports composed of nanoshaped CeO2 cubes and CeO2 rods tailored toward the production of hydrogen. At 650-700 K the hydrogen yield follows the trend RhPd/CeO(2)cubes >RhPd/CeO2-rods >RhPd/CeO2-polycrystalline, whereas at temperatures higher than 800K the catalytic performance of all samples is similar and close to the thermodynamic equilibrium. The improved performance of RhPd/CeO2-cubes and RhPd/CeO2-rods for ESR at low temperature is mainly ascribed to higher water-gas shift activity and a strong interaction between the bimetallic-oxide support interaction. STEM analysis shows the existence of RhPd alloyed nanoparticles in all samples, with no apparent relationship between ESR performance and RhPd particle size. X-ray diffraction under operating conditions shows metal reorganization on {1 0 0} and {1 1 0} ceria crystallographic planes during catalyst activation and ESR, but not on {1 1 1} ceria crystallographic planes. The RhPd reconstructing and tuned activation over ceria nanocubes and nanorods is considered the main reason for better catalytic activity with respect to conventional catalysts based on polycrystalline ceria. (C) 2015 Elsevier B.V. All rights reserved.Postprint (author's final draft