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

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

Rebound Tonometry in Cataract Surgery: Comparison with Goldmann Applanation Tonometry

Purpose: to compare the rebound tonometer ICare\uae (RT) with the Goldmann applanation tonometer (GAT) in cataract surgery and to assess the influence of central corneal thickness (CCT) on intraocular pressure (IOP) measurements. Design: retrospective, comparative study. Methods: a total of 472 patients underwent IOP measurement using RT (time 0 = RTa). GAT IOP measurement was performed 5 minutes later, followed by a second RT IOP measurement after other 5 minutes (RTa + 10 min = RTb). CCT was obtained by ultrasound pachymetry. In 106 patients IOP was measured by means of RT and GAT before clear corneal cataract surgery (RT1 and GAT1) and at one day postoperatively (RT2 and GAT2). Results: RT IOP values > 5 mmHg were overestimated, while RT IOP values < 15 mmHg were underestimated. Every 100 \ub5m of CCT an increase of 0.97 mmHg and 0.33 mmHg was found for RT and GAT respectively. The difference between RT1 and RT2, GAT1 and GAT2, RT1 and GAT1 was not statistically significant; while the difference between RT2 and GAT2 was statistically significant (p < 0.04). The difference between RTa and RTb was not statistically significant whereas the difference between RTa and GAT and RTb and GAT was statistically significant (p < 0.001). Conclusion: our results showed a good agreement between measurements obtained with RT and GAT in pre and postoperative cataract surgery, although a significant statistically difference was found between RT and GAT measurements performed postoperatively. Moreover, CCT is a parameter to be considered for the IOP measurement, especially for values obtained with RT

Methanol steam reforming behavior of copper impregnated over CeO2-ZrO2 derived from a surfactant assisted coprecipitation route

A series of ceria-zirconia solid solutions has been prepared by a surfactant assisted coprecipitation method. After impregnation of copper, their activities have been assessed for methanol steam reforming. The results indicate that the compositions with 10 and 15 at.% loading of copper on Ce0.6Zr0.4O2 exhibit maximum catalytic efficiency. Detailed structural analyses reveal high degree of copper dispersion on the ceria-zirconia matrix. In situ XPS studies confirm reduction of surface CuO species with concomitant lowering of Cu-surface atomic composition and increase of carbon. These evidences point to the formation of large aggregates of copper covered with coke that is suggested to be responsible for on stream activity loss. On regeneration, these aggregates break into a mixture of oxidized (Cu2+) and reduced (Cu-o and Cu+) copper species showing similar activity to the as prepared catalysts. In general, we have attributed catalytic activity to different proportions of copper components in the various forms of these catalysts. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.Postprint (author's final draft

In situ investigation of the mechanochemically promoted Pd–Ce interaction under stoichiometric methane oxidation conditions

The optimization of the supported Pd phase for CH4 activation on Pd/CeO2 catalysts has been a matter of great interest in the recent literature, aiming at the design of efficient methane abatement catalysts for Natural Gas fueled Vehicles (NGVs). Under lean conditions, a mixed Pd0 /PdO combination has been indicated as exhibiting the best performance, while controversial results have been reported under stoichiometric conditions depending on the support oxide, where either Al2O3 or zeolite-based supports are usually considered. Here, by means of synchrotron-based in situ NAP-XPS and XRD measurements, we follow the evolution of Pd species on Pd/CeO2 samples prepared by dry mechanochemical synthesis (M) under stoichiometric CH4 oxidation feed, unravelling a stable Pd0 /Pd2+ arrangement in a close to 1 : 1 ratio as the most active palladium state for CH4 activation when excess oxygen is not available, in contrast to what was reported for Pd/alumina materials, where metallic Pd0 nanoparticles showed the highest activity. The combination of NAP-XPS analysis and activity test results highlights the promotional effect of the Pd–Ce interaction, resulting in enhanced oxygen transfer and improved activity and stability of the Pd/CeO2 catalyst prepared by a novel mechanochemical approach even under low O2 content, large excess of water vapor (10 vol%) and high temperature exposure (4700 1C)

Identification of highly selective surface pathways for methane dry reforming using mechanochemical synthesis of Pd-CeO2

The methane dry reforming (DRM) reaction mechanism was explored via mechanochemically prepared Pd/CeO2 catalysts (PdAcCeO2M), which yield unique Pd–Ce interfaces, where PdAcCeO2M has a distinct reaction mechanism and higher reactivity for DRM relative to traditionally synthesized impregnated Pd/CeO2 (PdCeO2IW). In situ characterization and density functional theory calculations revealed that the enhanced chemistry of PdAcCeO2M can be attributed to the presence of a carbon-modified Pd0 and Ce4+/3+ surface arrangement, where distinct Pd–CO intermediate species and strong Pd–CeO2 interactions are activated and sustained exclusively under reaction conditions. This unique arrangement leads to highly selective and distinct surface reaction pathways that prefer the direct oxidation of CHx to CO, identified on PdAcCeO2M using isotope labeled diffuse reflectance infrared Fourier transform spectroscopy and highlighting linear Pd–CO species bound on metallic and C-modified Pd, leading to adsorbed HCOO [1595 cm–1] species as key DRM intermediates, stemming from associative CO2 reduction. The milled materials contrast strikingly with surface processes observed on IW samples (PdCeO2IW) where the competing reverse water gas shift reaction predominates.Peer ReviewedPostprint (published version

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.Peer ReviewedPostprint (published version

Investigation of Iron Vanadates for Simultaneous Carbon Soot Abatement and NH3-SCR

FeVO4 and Fe0.5Er0.5VO4 were prepared and loaded over standard Selective Catalytic Reduction (SCR) supports based on TiO2-WO3-SiO2 (TWS) and redox active supports like CeO2 and CeZrO2 with the aim of finding a suitable formulation for simultaneous soot abatement and NH3-SCR and to understand the level of interaction between the two reactions. A suitable bi-functional material was identified in the composition FeVO4/CeZrO2 where an SCR active component is added over a redox active support, to increase carbon oxidation properties. The influence of the presence of ammonia in soot oxidation and the effect of the presence of soot on SCR reaction have been addressed. It is found that the addition of NO and NO/NH3 mixtures decreases at different levels the oxidation temperature of carbon soot, while the presence of carbon adversely affects the NH3-SCR reaction by increasing the oxidation of NH3 to NO, thus lowering the NO removal efficiency

Pd/REOs catalysts applied to the Suzuki-Miyaura coupling. A comparison of their catalytic performance and reusability

Two new palladium catalysts based on rare earth oxide supports, namely, Pd/Dy2O3 and Pd/Yb2O3 were prepared by the incipient wetness impregnation method, followed by calcination at 600 \ub0C. Both compounds, which were fully characterized, were tested as catalysts in the Suzuki-Miyaura reaction and their catalytic activity was compared with that of already known Pd/La2O3, Pd/CeO2, Pd/Pr6O11, Pd/Sm2O3, and Pd/Gd2O3. It has been found that the nature of the support strongly affects the catalytic activity. The scope of the catalytic protocol was investigated and quantitative yields of the coupling products (21 examples are reported) were always observed using a low metal amount (0.05 mol%). A systematic investigation on the reusability of the entire set of catalytic systems has been carried out. All catalysts showed to be successfully recyclable, and also in this case the nature of the rare earth element is crucial. The highest reusability was observed for catalysts Pd/CeO2 and Pd/Sm2O3. \ua9 2017 Elsevier B.V