Thermal activation and aging of a V2O3/WO3-TiO2 catalyst for the selective catalytic reduction of NO with NH3

Real-world vanadium-based catalysts for the selective catalytic reduction (SCR) of NO with NH3 are occasionally exposed to high temperatures, which can induce catalyst aging. In this work, a 2.0 wt% V2O5/WO3-TiO2 catalyst based on commercial WO3-TiO2 was lab aged in dry and wet feed at different time lengths and temperatures. Aging carried out in static atmosphere or in flow only marginally influenced its performance, while e.g. temperature and water in the feed heavily affected the SCR activity. The low temperature NO,, conversion (< 300 degrees C) increased after aging up to 600 C for 16 h and was more pronounced after hydrothermal aging compared to thermal aging, which was associated with the increased surface coverage of the SCR-active vanadyl groups. Measurements of vanadium volatility in the temperature range selected for the aging temperatures revealed the high mobility of V species induced by the (hydro-)thermal treatments. The onset of catalyst deactivation, observed at lower aging temperature for the hydrothermally aged catalyst compared to the thermally treated one, is possibly due to a larger amount of mobile V and W species and the concurrent loss of specific surface area. The set of catalytic and characterization data showed that water is an essential component in aging protocols because it heavily affects the structure and the resulting catalytic activity of the SCR catalyst. Removal of residual sulfate groups, which are present on the commercial support, also contributed to catalyst activation at 550 degrees C aging temperature as a result of structural changes evidenced by surface area measurements and by IR and Raman spectroscopy, including rearrangement of V species and apparent increase of Lewis acidity.

Transient kinetics and in situ spectroscopic techniques for the characterisation of V/Ti-oxide catalysts in toluene partial oxidation

Fabio Rainone

Development of selective oxidation catalysts is a major concern of modern chemical industry. Among several systems, VTi-oxide based catalysts showed the most promising and versatile, able to catalyse key reactions like oxidation of o-xylene to phtalic anhydride and the abatement of NOx for pollution control. Extensive investigation in the past decades concluded that supported vanadia is present as a layer of molecularly-defined surface compounds VxOy, ("monolayer species") on top of the carrier oxide which are the catalytically active sites. Despite all the efforts to correlate the activity of VTi-oxide with its structure, the catalytic role of the different active species in the selective oxidation of hydrocarbons and how to tune their properties are still open questions. The goal of the present thesis is the in situ characterisation of V/Ti-oxide by transient-response chemical (feed step-response, temperature-programmed analyses) and spectroscopic (infrared and Raman) techniques to answer these important questions by using toluene partial oxidation as test reaction. First, the identification and elucidation of the role of the vanadia species was carried out, using Raman spectroscopy and activity tests. The strongly-bound monolayer species which cannot be dissolved by acid treatment were found to have isolated vanadate structure and be catalytically active in toluene partial oxidation. They have redox properties close to more condensed vanadate ("polymeric") species, which are also active. Vanadium present as crystalline V2O5 is not directly active, but plays a role in the surface reduction mechanism. By in situ DRIFTS, it has been shown that V/Ti-oxide active sites work with a coupled mechanism of product desorption and site reoxidation. Re-oxidation can occur either by reaction with gas-phase O2 or by oxygen spillover from bulk V2O5. Acidity associated with V-O-V bridges in monolayer species is likely responsible for the reversible deactivation of the catalysts caused by strongly held surface coke. Second, the modification of the acid/base catalyst properties by K addition and the formation of surface V oxide species were studied. The presence of K modifies the molecular structure of the vanadia surface species and forms new K-containing ones. These species are difficult to reduce in hydrogen and they are not catalytically active. Their presence causes a diminished catalytic activity. At the same time, Lewis and Brønsted acidity is depressed. This eliminates the phenomenon of deactivation. The excessive basicity caused by potassium addition has a detrimental effect on selectivity in benzoic acid, which is strongly held on the surface because of their intrinsic acidity and undergoes overoxidation to CO2. Finally, the VTi-oxide was supported on structured supports based on woven fibreglass. The use of structured beds is interesting in chemical engineering due to their advantages in terms of increased mass and heat trasfer, reduced pressure drop and optimal flow distribution. In order to reach optimum performance, the rather inert silica surface must be modified by adding an alumina layer on top of the fibres. This modification is essential for an optimal support because of insufficient dispersion of the titania layer on silica due to phase repulsion between the two oxides. The structured catalysts possess comparable activity and identical selectivity to the studied non-structured VTi-oxide catalyst with the same content of vanadia layers.

EPFL2004

Development of heterogeneous catalytic systems for the transformation of carbon dioxide to methanol under mild conditions.

Aswin Gopakumar

Rising level of carbon dioxide (CO2) is a significant contributor to global warming and poses a major concern in modern society and environment. Anthropogenic activities such as fossil fuel combustion, cement production and deforestation are the primary causes of CO2 emissions. Creating sustainable pathways by converting abundantly available CO2 into fuel could help in mitigating the ever-growing global energy demand. Owing to the high intrinsic thermodynamic stability of CO2 molecule, the chemical reduction mechanism requires an appropriate catalyst system to reduce the high activation energy barrier. Additionally, the prospect of utilising CO2 as C1 synthon to convert the amines to afford value-added formamides is even more appealing. In the present dissertation, the development of efficient industrial friendly heterogeneous catalytic routes for CO2 fixation on amines by chemical reduction method to formamides, as well as their subsequent conversion to fuel, methanol under mild conditions are investigated. The first chapter describes an overview about the current trends in global climate change, about the challenges in CO2 capture and storage, the employment of CO2 as a source of carbon to synthesise industrially essential formamide products by catalytic reduction. Subsequently, the reduction pathway of formamides to afford methanol is also elucidated. Additionally, the prospect of using formic acid (a known CO2 and H2 carrier) to formylate amines and the consequent reduction to methanol is also given. The combination of these reactions presents a viable CO2 reduction system using amines. In the second chapter, N-formylation and N-methylation of amines using CO2 as the carbon source catalysed by palladium nanoparticles (Pd NPs) under mild conditions are discussed. The concluding results point out the high efficiency and high selectivity of Pd NPs in catalysing the amines to fix CO2 to afford formamides at room temperature. The Pd NPs exhibited excellent catalytic activity and selectivity on par with the reported catalysts. The overall selectivity and conversion of the products could be altered by adjusting the CO2 pressure, catalyst loading, temperature and solvent. The catalyst could be recycled for multiple reactions without significant loss of activity. The third chapter describes a âGeo-inspiredâ catalyst system with the use of an iron-rich natural mineral-Gibeon meteorite, as the catalyst for N-formylation and N-methylation of amines using CO2 as the carbon source at room temperature. Similar to the second chapter, the Gibeon meteorite exhibited excellent catalytic activity and selectivity on par with the reported catalysts. The overall selectivity and conversion could be altered by adjusting the CO2 pressure, catalyst loading, temperature and solvent. Comparative studies with the similar iron-containing alloys were also carried out. A plausible mechanism is proposed with the Gibeon meteoriteâs surface bound hydroxide ions. Moreover, the catalyst could be easily recovered and reused for multiple reactions as well without significant loss of activity. The fourth chapter encompasses the investigation of a metal-free catalytic system using polymerisable ionic liquid- (4-Vinylbenzyl)trimethylammonium chloride catalysed N-formylation and N-methylation of amines using CO2 as the carbon source under mild conditions. Similar to second and third chapters, the monomer and the polymer exhibited excellent catalytic activity (comparable

EPFL2018

Transient kinetics and in situ spectroscopic techniques for the characterisation of V/Ti-oxide catalysts in toluene partial oxidation

Fabio Rainone

EPFL2004

Development of heterogeneous catalytic systems for the transformation of carbon dioxide to methanol under mild conditions.

Aswin Gopakumar

EPFL2018