Increasing Pt selectivity to vinylaniline by alloying with Zn via reactive metal-support interaction

The promoting effect of Zn on Pt-based catalyst was studied for the liquid-phase hydrogenation of 3-nitrostyrene (3-NS). The monodispersed Pt nanoparticles (ca. 2.3 nm) supported on ZnO catalyzed the target 3-vinylaniline (3-VA) formation (S3-VA = 97% at X3-NS approximate to 100%). Such catalytic performance was attributed to the intermetallic Pt/Zn active phase formed via reactive metal (Pt)-support (ZnO) interaction during reductive calcinations. The reference catalysts (Pt/Al2O3, Pt/C, Pt/MgO and Pt/TiO2) have not provided such high selectivity under the same reaction conditions. The solvent effect was studied showing the highest activity (TOF= 64 mm 1) and 3-VA selectivity (S3-VA = 97% at X3-Ns approximate to 100%) in ethanol. The obtained activation energy (E-a = 40 kJ mol(-1)) is in agreement with the literature values for nitroarenes hydrogenation. The variation of H-2 partial pressure had no significant effect on the products distribution while increasing reaction rate in accordance to the first-order kinetics. (C) 2015 Elsevier B.V. All rights reserved.

Pt-Zn nanoparticles supported on porous polymeric matrix for selective 3-nitrostyrene hydrogenation

Charline Berguerand, Fernando Cardenas Lizana, Lioubov Kiwi, Irina Prokopyeva, Artur Yarulin

We report the promoting effect of Zn on performance of Pt-based catalyst in liquid-phase hydrogenation of 3-nitrostyrene (3-NS) to 3-vinylaniline (3-VA). Bimetallic Pt-Zn nanoparticles (NPs) were prepared within the hypercross-linked polystyrene (HPS) support. The nanoporous structure of HPS allows a size control of Pt-Zn NPs by confining them in the cavities (ca. 4-5 nm) of the polymeric matrix. The TEM analysis showed that the mean size of the resulted metal particles (4.7 nm) corresponds to the HPS pore size. The properties of the bimetallic catalyst were assessed by IR spectroscopy of chemisorbed CO that suggested the modification of Pt surface and electronic structure invoked by Zn incorporation. The catalytic results demonstrated an increased yield of 3-VA over Pt-Zn/HPS catalyst (97%) relative to monometallic Pt/HPS (16%). This is the highest result reported over Pt catalysts for NS hydrogenation without any additional reaction modifiers. Furthermore, stability of Pt-Zn/HPS under reaction conditions was confirmed over repeated reaction runs. Our results demonstrate the Pt modification with Zn as efficient means to control 3-VA selectivity, whereas HPS serves as a suitable support to control NP size and avoid metal leaching. (c) 2014 Elsevier Inc. All rights reserved.

Elsevier2015

Investigation of the heterogeneously catalysed gas phase CO2 hydrogenation reaction: Development of analysis methods and reaction analysis on pristine metal catalysts

Robin Tobias Andreas Mutschler

The aim of this thesis is to contribute to the development of analysis methods that enable an improved investigation of the CO2 hydrogenation reaction such as quantitative mass spectrometry and infrared thermography. Furthermore, the contribution is in the characterization of the activity of basic catalyst building elements such as Fe, Co, Ni, Cu and Ru. To achieve these aims, we have developed and build a gas-controlling and analysis system to carry out the experiments in a wide range of conditions. In the frame of setting up the instruments and working on the analysis scripts, a rapid and quantitative gas analysis method by means of mass spectrometry was developed. In a second step, the CO2 hydrogenation reaction over Fe, Co, Ni and Cu in their pristine form was investigated with a particular focus on the catalytic activity and the activation energies. An Al2O3 supported Ru catalyst was used as reference catalyst. We showed that the reaction goes through a similar rate limiting step on Co, Ni and Ru/Al2O3, while the reaction rates are significantly different. Pristine Co showed minor activity in the formation of C2+ hydrocarbons. Fe was mostly active in the Carbon Monoxide formation. Cu exhibited no catalytic activity without a supporting metal-oxide phase. Based on these findings, we carried out an in-depth kinetical analysis of the catalytic CO2 hydrogenation over pristine Ni, Co and Ru/Al2O3 was carried out. A reaction model describing the CO2 hydrogenation over a broad temperature range, taking kinetically, diffusional and thermodynamic limitations into account, was developed. Diffusional limitation occurs at temperatures where the reaction kinetics is already rapid and diffusion of educts -and products to -and from the catalyst and its pores becomes rate limiting before the thermodynamic limitations are approached. The turnover rate of the CO2 conversion depends critically on the partial pressure of CO2 in the educts gas stream: With a high CO2 partial pressure, the catalyst surface saturates and the reaction order approaches zero. Out of the unsupported pristine metals Fe shows the lowest activation energy, but also the lowest activity while Co showed the highest activity and the highest activation energy for the overall CO2 conversion. Therefore, we have synthesized bimetallic nanosized pristine Fe-Co catalysts to investigate the influence of alloying Fe to Co and how this influences the activation energies of the formation of methane and C2-C5 hydrocarbons. We found clear trends towards higher activation energies with increasing Fe content- attributed to the stronger binding energy of CO2 to Fe. Furthermore, a new experiment for thermography was built with the aim to visualization the exothermic reaction dynamics. The dedicated reaction cell with an IR transparent window is coupled with the mass spectrometer. With the rapid gas sampling, the gas composition can directly be linked to the evolution of the thermal signal, contributing to the deeper understanding of the reaction start-up. We found and could prove that a CO2 covered surface has a strong inhibiting effect on the reaction startup while an H2 covered surface leads to a linear acceleration of the reaction front in the form of a thermal runaway.

EPFL2019

Fast temperature cycling of the catalytic CO oxidation using microstructure reactors

Martin Luther

The possibility to increase the performance (productivity or selectivity) of a chemical reactor by using periodic variations of reaction parameters (e.g. reactant concentration or temperature) has been theoretically envisaged since the beginning of the 70th. The experimental validation of the predicted positive effects was successful in the case of concentration variation but failed for temperature variation. This was mainly due to the high thermal inertia of the conventional chemical reactors used for the measurements which prevented to create variations having a sufficiently high frequency. Microstructure reactors own, at the contrary to conventional reactors, a very low thermal inertia and allow to generate temperature oscillations with an amplitude of about ten to hundred Kelvin at a frequency in the order of magnitude of 10-1 to 4 Hz. These properties, coupled with the possibility to introduce a catalytic active material within the devices, seem to make them well suited for the study of the effects of fast periodic temperature variations of a catalytic reaction. The objective of this work was to demonstrate that non-stationary temperature conditions may increase the reaction rate of a heterogeneously catalyzed reaction up to values not predicted by the classical Arrhenius dependency towards the temperature. Two different types of microstructure reactors have been used. The reaction was taking place in the first one (FTC-type 2) in microstructured channels on whose walls a catalytic layer was deposed. In the second one (FTC-type 3), the reaction was taking place on a piece of sintered metal fibres (SMF) plate placed in a reaction chamber. The catalytic active material was deposed on the SMF plate filaments. Both devices where permanently heated by electrical resistors and periodically cooled down with water flowing through cooling channels incorporated in the devices. The test reaction chosen for the experimental measurements was the CO oxidation reaction, heterogeneously catalyzed by platinum supported on alumina (Pt/Al2O3). The reaction behaviour under stationary thermal conditions was consistent with the one predicted by the Arrhenius law. The dependency of the reaction rate with the temperature was exponential with an apparent activation energy of 104 kJ·mol-1, a negative partial reaction order for CO and a positive one for O2. The measurements effectuated under quasi-stationary thermal conditions (slow temperature ramps) have shown that a temperature change rate between 7 and 14 K·min-1 was not sufficient to observe any non-trivial effect of the temperature. The reaction behaviour was always predicted by the Arrhenius law. The surface coverage of the reactive species is, in this case, always able to follow the slow temperature changes and the reaction behaves as being always under steady-state conditions. The experiments realized under non-stationary thermal conditions using the FTC-type 2 reactor have also failed to demonstrate any non-trivial effects of the temperature oscillations. This device allows indeed to generate temperature oscillations with an amplitude of up to 120 K with a frequency of 0.1 Hz but unfortunately correlated with a very high thermal inhomogeneity. A temperature difference of up to 80 K was measured between a cold spot and a hot spot inside the device. Due to their very low local reaction rate the colder areas of the reactor have attenuated the product concentration (CO2) oscillations which should have been created by the temperature variations. This attenuation prevented the temperature oscillations to have any positive effect compared to the stationary thermal conditions. The FTC-type 3 reactor allowed temperature changes of lower amplitude but the thermal homogeneity was much better. The maximal temperature difference measured between two points within the reactor was only 15 K. Under temperature oscillation conditions, the measured instantaneous CO2 concentration was higher for any temperature within the oscillation range compared to the one recorded under stationary thermal conditions. The increase obtained in the mean CO2 concentration was ranging from 34% for a frequency of the oscillations of 0.035 Hz to 85% for a frequency of 0.052 Hz. The amplitude of the oscillations was kept constant at approximately 40 K and the mean temperature value at 437 K. The simulations effectuated using a theoretical model for the catalytic CO oxidation including a feed-back step in the form of the oxidation-reduction of the catalyst have shown that the experimentally observed increase may be qualitatively explained. Above a certain frequency, the temperature oscillations are fast enough compared to the characteristic time of the feedback step and are able to perturb the stationary established reactive species surface coverage. The formation of a transient surface coverage more favourable for the reaction than the surface coverage at high temperature allows during the transient period to reach an instantaneous surface reaction rate values higher than the one at high temperature. This reaction rate peak is then responsible for the increase of the mean reaction rate under non-stationary thermal conditions and, thus, for an increased yield.