Oxidation state | EPFL Graph Search

In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Conceptually, the oxidation state may be positive, negative or zero. While fully ionic bonds are not found in nature, many bonds exhibit strong ionicity, making oxidation state a useful predictor of charge. The oxidation state of an atom does not represent the "real" charge on that atom, or any other actual atomic property. This is particularly true of high oxidation states, where the ionization energy required to produce a multiply positive ion is far greater than the energies available in chemical reactions. Additionally, the oxidation states of atoms in a given compound may vary depending on the choice of electronegativity scale used in their calculation. Thus, the oxidation state of an atom in a compound is purely a formalism. It is nevertheles

Liquid phase in situ spectroscopic investigations of heterogeneous catalysts

Thibault Xavier Florian Fovanna

In this work, we have studied various aspects of catalysts and catalytic processes at the solid-liquid interface, e.g. the reduction of supported PdO, the formation of Pd hydrides, their quantification, the their reactivity in a transient in time-resolved manner, Ru and P-modified Ru based catalysts and zeolites. In order to provide structural information mainly X-ray absorption spectroscopy, infrared spectroscopy and X-ray diffraction were used, while the measurements were made possible by the realization of a dedicated setup including reactor cells. The cells are designed to allow for monitoring structural changes of oxidation state and nature of adsorbates in a time-resolved manner. This is of particular interest to study the reactivity of species, such as the Pd hydrides, formed on the catalyst in the liquid environment exploiting tools that are already widely applied to study gas phase reactions.

EPFL2020

Operando XAS insight in the redox dynamics of VOx species and reducible supports involved in oxidative dehydrogenation of alcohols

Anna Zabilska

Supported vanadia (VOx) belongs to some of the most versatile selective oxidation and reduction catalysts applied for many reactions in the chemical industry and pollution control. Among them, the oxidative dehydrogenation (ODH) of alcohols is particularly attractive since it is one of the main industrial methods of formaldehyde production and a possible route for the transformation of renewable bioethanol to value-added products, such as acetaldehyde and acetic acid. Despite many spectroscopic methods used to investigate the mechanism of this reaction, the details about the redox involvement of VOx species and promoting supports (CeO2 and TiO2) remain debated. In this study, we used an element-specific operando X-ray absorption spectroscopy (XAS) to probe the redox transformations of active sites in vanadia-titania and vanadia-ceria catalysts during ethanol and methanol ODH. By carefully designing the steady-state and transient operando XAS experiments and by applying advanced chemometric methods, we could discriminate specifically and often quantitatively, which role plays each redox couple during the alcohol ODH cycle and uncover hidden structure activity-relationships.Chapter 1 introduces the relevant literature and summarizes the goals and the scope of the thesis.In Chapter 2, we investigate the redox dynamics of active sites in a bilayered VOx/TiOx/SiO2 catalyst (an analog of a widely used VOx/TiO2 catalyst with comparable activity) during ODH of ethanol. Operando time-resolved V and Ti K-edge XAS coupled with a transient experimental strategy quantitatively showed that the formation of acetaldehyde over VOx/TiOx/SiO2 catalyst is kinetically coupled to the formation of a V4+ intermediate, while the formation of V3+ is delayed and its rate is 10-70 times lower. The only very weak redox activity of the Ti4+/Ti3+ couple was detected, which suggests that TiOx species promote the redox activity of VOx indirectly, either via the fast electron transport between VOx species or by changing the electronic structure of VOx species.In Chapter 3, we report on possible difficulties, which can occur during in situ XAS studies of supported vanadia (VOx) catalysts at synchrotron beamlines related to X-ray-induced photochemical reduction of vanadium. We rigorously tested this phenomenon and suggest approaches, which can help to identify and mitigate vanadium photoreduction interfering chemical speciation.In Chapter 4, we focused on the structure-activity relationships in VOx/CeO2 system. We prepared a series of VOx/CeO2 catalysts using high-surface-area CeO2 Using time-resolved operando quick XAS at V K- and Ce L3-edges, we demonstrated that during oxygen cut-off experiments vanadium and cerium change their oxidation states in concert with each other and with the production of acetaldehyde. It suggests that both reducible vanadium and cerium are parts of the same active sites. The maximum VOx/CeO2 activity per mass of catalyst observed at ca. 3 V/nm2 coincides with the maximal amount of cerium atoms, which reversibly change their oxidation state during oxygen cut-off experiments.In Chapter 5, we prove that the mechanistic findings described in Chapter 4 for ethanol ODH over VOx/CeO2 catalysts are also valid for methanol ODH. Moreover, we confirm that the VOx surface density is the main descriptor of the activity for VOx/CeO2 catalysts in alcohol ODH.

EPFL2022

Understanding the anomalous behavior of Vegard's law in Ce1-xMxO2 (M = Sn and Ti; 0 < x

Olga Safonova

The dependence of the lattice parameter on dopant concentration in Ce1-xMxO2 (M = Sn and Ti) solid solutions is not linear. A change towards a steeper slope is observed around x similar to 0.35, though the fluorite structure (space group Fm3m) is preserved up to x = 0.5. This phenomenon has not been observed for Ce1-xZrxO2 solid solutions showing a perfectly linear decrease of the lattice parameter up to x = 0.5. In order to understand this behavior, the oxidation state of the metal ions, the disorder in the oxygen substructure and the nature of metal-oxygen bonds have been analyzed by XPS, Sn-119 Mossbauer spectroscopy and X-ray absorption spectroscopy. It is observed that the first Sn-O coordination shell in Ce1-xSnxO2 is more compact and less flexible than that of Ce-O. The Sn coordination remains symmetric with eight equivalent, shorter Sn-O bonds, while Ce-O coordination gradually splits into a range of eight non-equivalent bonds compensating for the difference in the ionic radii of Ce4+ and Sn4+. Thus, a long-range effect of Sn doping is hardly extended throughout the lattice in Ce1-xSnxO2. In contrast, for Ce1-xZrxO2 solid solutions, both Ce and Zr have similar local coordination creating similar rearrangement of the oxygen substructure and showing a linear lattice parameter decrease up to 50% Zr substitution. We suggest that the localized effect of Sn substitution due to its higher electronegativity may be responsible for the deviation from Vegard's law in Ce1-xSnxO2 solid solutions.

Royal Soc Chemistry2016

Operando XAS insight in the redox dynamics of VOx species and reducible supports involved in oxidative dehydrogenation of alcohols

Anna Zabilska

EPFL2022