A. Drochner

Investigation of NO adsorption and NO/O2 co-adsorption on NOx-storage-components by DRIFT-spectroscopy

NO adsorption and NO/O2 co-adsorption on CeO2 at different temperatures was studied by DRIFT-Spectroscopy. The results indicate that this oxide plays an important role in storing NOx. FTIR studies show that NO adsorption is dominated by the formation of nitrite species. Furthermore, cis- and trans hyponitrite species are detected. Co-adsorption of NO/O2 leads to the formation of nitrates. The experimental data show that the formation of nitrates is a consecutive reaction: adsorption of NO to form nitrite species (NO2−), followed by an oxidation to form nitrate species (NO3−).

A new DRIFTS cell for the in-situ investigation of heterogeneously catalyzed reactions

IR spectroscopy is a well-established method in the characterization of heterogeneous catalysts under reaction conditions. Working in transmission modes is the most common of currently applied technologies. Yet, there are problems inherent in both methods (KBr wafers and self-supporting wafers) for the in-situ investigation of heterogeneously catalyzed reactions. These problems can be avoided by using the DRIFT (diffuse reflection infrared Fourier transformation) spectroscopy. As is shown in this study, DRIFT spectroscopy proves to be a suitable method producing not only qualitative but also quantitative results of in-situ examinations of heterogeneous catalysts.

Oxygen exchange at Mo/V mixed oxides: A transient and 18O isotope study under technical conditions

Abstract Mo/V mixed oxides have proven themselves to be reliable for the partial oxidation of acrolein to acrylic acid. Methacrolein which is the homologue of acrolein, reacts with clearly worse selectivity and activity on this catalyst system. By means of transient investigations (TP reduction, CP reduction, CP reaction and 18 O -SSITKA) under reaction conditions, it was possible to provide a contribution towards clarifying the oxygen exchange of the Mo/V mixed oxides during the reaction of both aldehydes to the corresponding carboxylic acids. In the case of both aldehydes, an exchange of carbonyl oxygen and solid oxygen occurs on the Mo/V mixed oxide.

Characterisation of the structure of adsorbates : An in situ DRIFTS-study of the partial oxidation of unsaturated aldehydes on Mo/V oxide catalysts

Abstract Mo/V oxides are interesting catalyst materials for the selective partial oxidation of unsaturated aldehydes (acrolein and methacrolein) to the corresponding carboxylic acids. Two catalyst systems — a mixed oxide and a heteropolyacid salt — were investigated by in situ DRIFT-spectroscopy. Using a novel DRIFTS cell, in which both the catalyst powder and the reference (KBr) were in the same reaction chamber, we are able to detect different adsorbates of (meth)acrolein and (meth)acrylic acid under reaction gases. The investigations showed that in the case of the adsorption of acrolein and methacrolein a significant difference in the structure of corresponding adsorbates could not be proved. This indicates that the reaction mechanism of the selective partial oxidation should be the same for both aldehydes. With these kind of investigations we gain access to a deeper insight into the reaction mechanism.

Concentration-Programmed-Reaction, a Method to Evaluate the Potential of Application of Transient Process Control in Partial Oxidations

This work evaluates the potential of transient methods with an example of partial oxidation of acrolein with oxygen in gas-phase using a Mo-V-mixed-oxide. To reach this aim concentration programmed methods are introduced.

The Effect of Water on the Heterogeneously Catalyzed Selective Oxidation of Acrolein: An Isotope Study

The effect of water on the selective gas phase oxidation of acrolein to acrylic acid on a Mo/V/W mixed oxide catalyst was studied by performing steady‐state isotopic transient kinetic analysis experiments with H218O. Experiments were performed in the temperature range of 90–345 °C at ambient pressure. It could be shown that acrolein exchanges its carbonylic oxygen with oxygen from water even at low temperatures (<200 °C), at which no acrolein oxidation occurs. At higher temperatures (>200 °C), the oxygen atoms of the water molecules incorporate into all oxidation products such as acrylic acid, carbon monoxide, and carbon dioxide.

Interplay between Defect Structure and Catalytic Activity in the Mo10−xVxOy Mixed‐Oxide System

The Mo(10-x)V(x)O(y) solid-solution systems (0≤x≤10) were studied by electron paramagnetic resonance spectroscopy. The results show the existence of paramagnetic vanadyl VO(2+) species, whose concentration becomes maximal for Mo(5)V(5)O(y·). A quantitative analysis of the [VO(2+)] concentration as a function of the Mo/V ratio allows it to characterize the prevailing defect chemistry in the Mo(10-x)V(x)O(y) system. In this respect, the semi-conducting properties of Mo(10-x)V(x)O(y) are p-type in an interval of Mo(9)V(1)O(y)-Mo(5)V(5)O(y) and switch into n-type because of the conduction electrons in a composition range of Mo(5)V(5)O(y)-Mo(1)V(9)O(y). Highest catalytic activity is obtained when vanadium acts as an acceptor center and oxygen vacancies ν(··)(O) are formed for reasons of charge compensation. In addition to the surface, ν(··)(O) and VO(2+) centers in the bulk have to be considered too for heterogeneous catalysis.

Dynamics of Bulk Oxygen in the Selective Oxidation of Acrolein

The gas‐phase oxidation of acrolein to acrylic acid on a hydrothermally prepared mixed oxide catalyst was investigated by steady‐state isotopic transient kinetic analysis (SSIKTA) as well as different types of concentration‐programmed techniques (CPR‐pulse, CPO) under in situ or process‐relevant conditions. Balancing the amounts of active oxygen gives an overview of the quantities of participating bulk oxygen species. The dynamics of bulk oxygen lead to re‐oxidation processes on the catalyst surface and thus influence the selectivity pattern of the network of acrolein oxidation. Furthermore, the bulk dynamics are activated by temperature.

Infrared Spectroscopic Study of the NH3-SCR on Fe-Zeolites under Transient Conditions

The standard NH3-SCR (selective catalytic reduction) of NO/O2-mixtures on Fe-zeolites shows significant maxima in the NO-conversion as transient response, when NH3 is turned on and off. Introducing in situ DRIFT-spectroscopy of the catalyst two remarkable effects were observed. On the one hand a kink in the kinetics of the bands assigned to the 2H- and 3H-structure of adsorbed NH3 was detected. Furthermore the kinetics of several bands show a maximum, which gives hint to the existence of an intermediate, in the following called SCR-SI (SCR-Surface Intermediate). Due to the observed correlation between both, the maximum in the NO-conversion and the maxima in the band kinetics on the catalyst, the SCR-SI plays an important role in the mechanism and seems to be responsible for the increased NO-conversion. In order to gain more information about the structure and role of SCR-SI the influence of temperature, iron content, O2 concentration and NO/NH3–ratio was studied.

Interaction of surface- and bulk-oxygen in Mo/V-mixed oxides during the partial oxidation of an unsaturated Aldehyde: A concentration-programmed-reduction study

Mixed oxides based on molybdenum and vanadium are used as catalysts in partial oxidation reactions. Transient examinations, in particular the concentration programmed reduction method with acrolein, proofs the presence of at least two different oxygen species participating in the reaction. A mechanistic view involving surface- and bulk-oxygen as well as a simulation are given.