Alessandro Longo

Bio: Alessandro Longo is an academic researcher from European Synchrotron Radiation Facility. The author has contributed to research in topics: Catalysis & Extended X-ray absorption fine structure. The author has an hindex of 35, co-authored 165 publications receiving 4215 citations. Previous affiliations of Alessandro Longo include National University of Ireland & Netherlands Organisation for Scientific Research.

XPS study of supported gold catalysts: the role of Au0 and Au+δ species as active sites

Abstract: Gold nanoparticles supported on different oxides (SiO2, CeO2 and TiO2) were prepared by the SMAD (solvated metal atom dispersion) and deposition–precipitation (DP) techniques. The physical and chemical characterization of the catalysts was performed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the catalytic activity was tested during the reaction of low temperature CO oxidation. The structural and surface analyses evidenced the presence of small gold crystallites (cluster size ∼2–5 nm) in all the SMAD-prepared samples and oxidized gold species in the case of the DP catalysts. A different surface distribution of ionic gold species was found on the different supports. By comparing the catalytic activities of the samples, the presence of Au+1 species seems to be the main requisite for the achievement of the highest CO conversion at the lowest temperature. The higher activity of Au/CeO2(DP) catalysts at T ≈ 250 K can be ascribed to a better stabilization of the AuO− species by the cerium oxide. Nanosized metallic gold particles exhibit a worse catalytic performance, both on ‘reducible’ and ‘inert’ supports, being significantly active only in the temperature range: 400–600 K. Copyright © 2006 John Wiley & Sons, Ltd.

In Situ Observation of Active Oxygen Species in Fe-Containing Ni-Based Oxygen Evolution Catalysts: The Effect of pH on Electrochemical Activity

Abstract: Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni(Fe)−Bi) and nickel oxyhydroxide (Ni(Fe)OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO–), which can be described as adsorbed “active oxygen”. Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negati...

Relationship between structure and CO oxidation activity of ceria-supported gold catalysts.

Abstract: Gold catalysts supported on cerium oxide were prepared by solvated metal atom dispersion (SMAD), by deposition-precipitation (DP), and by coprecipitation (CP) methods and were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity was tested in the CO oxidation reaction. The structural and surface analyses evidenced the presence of a modified ceria phase in the case of the DP sample and the presence of pure ceria and gold metal crystallites in the case of the SMAD and CP samples. The DP sample, after a mild treatment in air at 393 K, exhibited only ionic gold, and it was very active below 273 K. By comparing the activities of the different catalysts, it is suggested that the presence of small gold particles, as obtained by the SMAD technique, is not the main requisite for the achievement of the highest CO conversion. The strong interaction between ionic gold and ceria, by enhancing the ceria surface oxygen reducibility, may determine the particularly high activity.

Interface dynamics of Pd–CeO2 single-atom catalysts during CO oxidation

Abstract: In recent years, noble metals atomically dispersed on solid oxide supports have become a frontier of heterogeneous catalysis. In pursuit of an ultimate atom efficiency, the stability of single-atom catalysts is pivotal. Here we compare two Pd/CeO2 single-atom catalysts that are active in low-temperature CO oxidation and display drastically different structural dynamics under the reaction conditions. These catalysts were obtained by conventional impregnation on hydrothermally synthesized CeO2 and one-step flame spray pyrolysis. The oxidized Pd atoms in the impregnated catalyst were prone to reduction and sintering during CO oxidation, whereas they remained intact on the surface of the Pd-doped CeO2 derived by flame spray pyrolysis. A detailed in situ characterization linked the stability of the Pd single atoms to the reducibility of the Pd–CeO2 interface and the extent of reverse oxygen spillover. To understand the chemical phenomena that underlie the metal–support interactions is crucial to the rational design of stable single-atom catalysts. Single-atom catalysts have become a frontier of heterogeneous catalysis, but to achieve a high stability under turnover is often a challenge. Now, a Pd/CeO2 single-atom catalyst prepared using flame spray pyrolysis is able to stabilize the isolated Pd species during CO oxidation due to a high mobility of surface lattice oxygen.

Near-complete suppression of surface losses and total internal quantum efficiency in BiVO4 photoanodes

Abstract: Bismuth vanadate (BiVO4) is one of the most efficient light absorbing metal oxides for solar water splitting. BiVO4 photoanodes immersed in an electrolyte in an open circuit configuration and exposed to simulated solar illumination for prolonged time achieve superior photoelectrochemical (PEC) activity. This photocharging (PC) effect is capable of almost completely overcoming the surface and bulk limitations of BiVO4. Herein we show that alkaline conditions favor the PC effect; specifically BiVO4 photoanodes subjected to PC treatment at pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO4 of 4.3 mA cm−2 @ 1.23 VRHE, an outstandingly low onset potential of 0.25 VRHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95% respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in situ XAS study suggest that electronic, structural and chemical properties of the bulk of these films remain unchanged during the PC treatment. However, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO4 is enhanced by surface reaction pathways of the semiconductor–liquid junction, which is directly correlated with the electrochemical environment in which it is modified.

Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives

Abstract: There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.

Ultrathin metal–organic framework nanosheets for electrocatalytic oxygen evolution

Abstract: The design and synthesis of efficient electrocatalysts are important for electrochemical conversion technologies. The oxygen evolution reaction (OER) is a key process in such conversions, having applications in water splitting and metal–air batteries. Here, we report ultrathin metal–organic frameworks (MOFs) as promising electrocatalysts for the OER in alkaline conditions. Our as-prepared ultrathin NiCo bimetal–organic framework nanosheets on glassy-carbon electrodes require an overpotential of 250 mV to achieve a current density of 10 mA cm−2. When the MOF nanosheets are loaded on copper foam, this decreases to 189 mV. We propose that the surface atoms in the ultrathin MOF sheets are coordinatively unsaturated—that is, they have open sites for adsorption—as evidenced by a suite of measurements, including X-ray spectroscopy and density-functional theory calculations. The findings suggest that the coordinatively unsaturated metal atoms are the dominating active centres and the coupling effect between Ni and Co metals is crucial for tuning the electrocatalytic activity. Efficient electrocatalysts for the oxygen–evolution reaction are desired due to their importance in applications such as water splitting and metal–air batteries. Here, the authors engineer ultrathin metal–organic frameworks that require low overpotential to generate oxygen from alkaline media.

Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts

Abstract: ion from a primary OH group of glyc-erol. 223,231 A similar mechanism was proposed manyyears ago for alcohol oxidation on Pt/C, involving asecond step, the transfer of a hydride ion to the Ptsurface (Scheme 11). 8,87,237 We consider it more feasible that the rate-deter-mining step is the cleavage of the C-H bond at theR-carbon atom. A similar mechanism is now generallyaccepted for Au electrodes (Scheme 12). 238 Despite thestructural differences between Au nanoparticles andan extended Au electrode surface, there are alsosimilarities, such as the critical role of aqueousalkaline medium and the absence of deactivation dueto decomposition products (CO and C x H y frag-ments). 239,240 An important question is the nature of active siteson Au nanoparticles. Electrooxidation of ethanol onAu nanoparticles supported on glassy carbon re-quired the partial coverage of Au surface by oxides. 241 Another analogy might be the model proposed for COoxidation. 219,242,243 According to this suggestion, theactive site consists of an ensemble of metallic Auatoms and a cationic Au