Davide Ferri

Sorption enhanced CO2 methanation

The transformation from the fatuous consumption of fossil energy towards a sustainable energy circle is most easily marketable by not changing the underlying energy carrier but generating it from renewable energy. Hydrocarbons can be principally produced from renewable hydrogen and carbon dioxide collected by biomass. However, research is needed to increase the energetic and economic efficiency of the process. We demonstrate the enhancement of CO2 methanation by sorption enhanced catalysis. The preparation and catalytic activity of sorption catalysts based on Ni particles in zeolites is reported. The functioning of the sorption catalysis is discussed together with the determination of the reaction mechanism, providing implications for new ways in catalysis.

Au on Nanosized NiO: A Cooperative Effect between Au and Nanosized NiO in the Base‐Free Alcohol Oxidation

Nanosized NiO has been synthesized and used as a support for polyvinyl alcohol‐protected Au nanoparticles. This catalytic system exhibits an extraordinary performance in the base‐free liquid phase oxidation of alcohols compared to the same Au supported on a commercial, micrometersized NiO. This enhancement in activity cannot be solely attributed to the improved basic properties of the support. A cooperative effect between Au nanoparticles and nanosized NiO is envisaged.

Probing boundary sites on a Pt/Al2O3 model catalyst by CO2 hydrogenation and in situ ATR-IR spectroscopy of catalytic solid–liquid interfaces

In situ ATR infrared spectroscopy was used to follow the reaction of CO2 with hydrogen at the solid–liquid interface of a model Pt/Al2O3 catalyst in cyclohexane at 313 K. CO2 adsorbs on Al2O3 to form carbonate-like species, which react with hydrogen to form CO. The observed spectrum exhibits unusually low vibrational frequencies and ratio of linearly to bridged bonded CO. In contrast, CO adsorbed from the solution onto the Pt/Al2O3 model catalyst results in a normal spectrum for CO on Pt. The results suggest that the reaction of CO2 and hydrogen takes place at the boundary sites of the interface between the Pt and the Al2O3 and that the resulting CO selectively probes these sites. The unusual CO spectrum indicates that these sites are characterised by a low ionisation potential of the d-valence orbitals of Pt. It is likely that the unusual property of these Pt sites are caused by a nearby oxygen vacancy of the Al2O3, onto which the CO2 initially adsorbs.

Subsecond and in Situ Chemical Speciation of Pt/Al2O3 during Oxidation Reduction Cycles Monitored by High-Energy Resolution Off-Resonant X-ray Spectroscopy

We report an in situ time-resolved high-energy resolution off-resonant spectroscopy study with subsecond resolution providing insight into the oxidation and reduction steps of a Pt catalyst during CO oxidation. The study shows that the slow oxidation step is composed of two characteristic stages, namely, dissociative adsorption of oxygen followed by partial oxidation of Pt subsurface. By comparing the experimental spectra with theoretical calculations, we found that the intermediate chemisorbed O on Pt is adsorbed on atop position, which suggests surface poisoning by CO or surface reconstruction.

NiO as a peculiar support for metal nanoparticles in polyols oxidation

The peculiar influence of a NiO support was studied by preparing gold catalysts supported on NiO1−x–TiO2 x mixed oxides. PVA protected Au nanoparticles showed high activity when supported on NiO for the selective oxidation of glycerol and ethane-1,2-diol. A detailed characterization of the resulting Au catalysts revealed a preferential deposition of the metal nanoparticles on the NiO phase. However, the activity of Au on NiO1−x–TiO2 x decreased with respect to pure NiO and the selectivity evolved with changes in the support.

Revealing the dynamic structure of complex solid catalysts using modulated excitation X-ray diffraction

X-ray diffraction (XRD) is typically silent towards information on low loadings of precious metals on solid catalysts because of their finely dispersed nature. When combined with a concentration modulation approach, time-resolved high-energy XRD is able to provide the detailed redox dynamics of palladium nanoparticles with a diameter of 2 nm in 2 wt % Pd/CZ (CZ = ceria-zirconia), which is a difficult sample for extended X-ray absorption fine structure (EXAFS) measurements because of the cerium component. The temporal evolution of the Pd(111) and Ce(111) reflections together with surface information from synchronous diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements reveals that Ce maintains Pd oxidized in the CO pulse, whereas reduction is detected at the beginning of the O2 pulse. Oxygen is likely transferred from Pd to Ce(3+) before the onset of Pd re-oxidation. In this context, adsorbed carbonates appear to be the rate-limiting species for re-oxidation.

The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium-Based Catalysts

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

Adding diffuse reflectance infrared Fourier transform spectroscopy capability to extended x-ray-absorption fine structure in a new cell to study solid catalysts in combination with a modulation approach

We describe a novel cell used to combine in situ transmission X-ray absorption spectroscopy (XAS) with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in a single experiment. The novelty of the cell design compared to current examples is that both radiations are passed through an X-ray and IR transparent window in direct contact with the sample. This innovative geometry also offers a wide surface for IR collection. In order to avoid interference from the crystalline IR transparent materials (e.g., CaF2, MgF2, diamond) a 500 μm carbon filled hole is laser drilled in the center of a CaF2 window. The cell is designed to represent a plug flow reactor, has reduced dead volume in order to allow for fast exchange of gases and is therefore suitable for experiments under fast transients, e.g., according to the concentration modulation approach. High quality time-resolved XAS and DRIFTS data of a 2 wt.% Pt/Al2O3 catalyst are obtained in concentration modulation experiments where CO (or H2) pulses are alternated to O2 pulses at 150 °C. We show that additional information can be obtained on the Pt redox dynamic under working conditions thanks to the improved sensitivity given by the modulation approach followed by Phase Sensitive Detection (PSD) analysis. It is anticipated that the design of the novel cell is likely suitable for a number of other in situ spectroscopic and diffraction methods.

Conformational isomerism of α-ketoesters. A FTIR and ab initio study

The conformational behaviour of several α-ketoesters was investigated using solution FTIR in combination with ab initio calculations. The α-ketoesters show marked differences in the OC–CO torsional potential energy surface depending on the substituent at the α-keto group. In general the torsional potential is characterised by broad minima corresponding to s-cis and s-trans conformations and low interconversion barriers. The s-trans conformation is more stable but the fraction of s-cis is considerable at room temperature and increases with solvent polarity due to the higher dipole moment of the latter. Hydrogen bonding with alcoholic solvents also leads to a stabilisation of the s-cis conformer. The interaction of ethyl pyruvate with R3N+–H is much stronger when ethyl pyruvate adopts an s-cis conformation due to strong ion–dipole interaction. This type of interaction between ethyl pyruvate and protonated cinchonidine is considered to be crucial for the enantio-differentiation in the heterogeneous enantioselective hydrogenation of ethyl pyruvate over cinchonidine modified platinum in acidic media.

Ageing induced improvement of methane oxidation activity of Pd/YFeO3

The ageing characteristics of flame-made 2 wt% Pd supported on YFeO3 were analysed in comparison with a Pd/Al2O3–CeO2–ZrO2 three-way catalyst (TWC) with respect to structural changes and catalytic performance for methane oxidation under stoichiometric reaction conditions. Thermal treatment under lean conditions (air, 900 °C) resulted in slight decrease in the methane oxidation activity of the TWC. In marked contrast, YFeO3-supported Pd catalysts exhibit an increase in activity after such treatment. Activity enhancement is even higher when the treatment was performed under stoichiometric conditions (air–fuel equivalence ratio, λ = 1, 900 °C). To explain this observation, in-depth characterization (BET, STEM, OSCC, XAS, and CO chemisorption) of fresh and aged catalysts was performed. Both thermal and stoichiometric ageing cause a severe sintering of the support particles and the phase transformation from hexagonal to orthorhombic YFeO3. Despite the absence of a mixed Pd–YFeO3 phase, the growth of Pd particles appears to be limited under the λ = 1 atmosphere. In contrast to thermally aged catalysts where large PdO particles are formed, well-defined metallic Pd nanoparticles of 10–20 nm are present after stoichiometric ageing along with higher methane oxidation activity. Although it is tempting to conclude that metallic Pd is active for methane oxidation under the given conditions, reversible and periodic partial oxidation of the large metallic particles is observed in modulation excitation high energy X-ray diffraction (HXRD) experiments designed to simulate the oscillating redox conditions experienced during operation. These results indicate that large Pd particles exhibit improved methane oxidation activity but equally confirm that activity under stoichiometric conditions is the result of a delicate equilibrium dictated by the bulk-Pd/surface-PdO pair.