Silica-copper catalyst interfaces enable carbon-carbon coupling towards ethylene electrosynthesis

Membrane electrode assembly (MEA) electrolyzers offer a means to scale up CO2-to-ethylene electroconversion using renewable electricity and close the anthropogenic carbon cycle. To date, excessive CO2 coverage at the catalyst surface with limited active sites in MEA systems interferes with the carbon-carbon coupling reaction, diminishing ethylene production. With the aid of density functional theory calculations and spectroscopic analysis, here we report an oxide modulation strategy in which we introduce silica on Cu to create active Cu-SiOx interface sites, decreasing the formation energies of OCOH* and OCCOH*-key intermediates along the pathway to ethylene formation. We then synthesize the Cu-SiOx catalysts using one-pot coprecipitation and integrate the catalyst in a MEA electrolyzer. By tuning the CO2 concentration, the Cu-SiOx catalyst based MEA electrolyzer shows high ethylene Faradaic efficiencies of up to 65% at high ethylene current densities of up to 215mAcm(-2); and features sustained operation over 50h. CO2-to-ethylene conversion using renewable electricity provides a sustainable route to produce valuable chemicals. Here, the authors report high ethylene activity of 215mAcm(-2) and selectivity of 65% made possible by silica clusters in copper.

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

Nitrous Oxide Abatement over Fe-Zeolite

Bryan Bromley

The use of zeolites for catalytic reactions is a field in continuous development. Since it was demonstrated that metal-zeolites are efficient catalysts for NOx abatement, decomposition of nitrous oxide (N2O) over Fe-containing zeolites has attracted great interest by the scientific community mainly due: N2O is the third most important gas contributing to global warming, its concentration in atmosphere is still on the rise and direct N2O decomposition to N2 and O2 is a sustainable alternative. N2O interaction with Fe-zeolite leads to the formation of "special" adsorbed oxygen (often called α-oxygen) with great potential for selective oxidation. Although the focus of a number of studies, the nature of the actives sites for the formation of this adsorbed oxygen is still unclear. The main objectives of this work are: 1) study the decomposition mechanism of N2O over isomorpously substituted Fe-ZSM-5 zeolite aiming on catalyst optimization and 2) optimization and further use of structured Fe-zeolite in novel micro-structured reactor. A number of refined transient kinetics methods were intensively used such as transient response, Temperature Programmed Desorption (TPD) and Temporal Analysis of Product (TAP). Density Functional Theory (DFT) and in situ IR spectroscopy studies were conducted in collaboration and our results support the following: In chapter 3, quantitative measurements have been carrier out to analyze the effect of temperature on the activation (under helium flow) and the irreversible deactivations that can happen during the pretreatment. The results obtained indicate that high temperature treatment (1273 K) results in the formation of a high amount of α-sites. The addition of a very low fraction of oxygen (2%) in the feed demonstrates that the catalyst is sensitive to its presence. Certain sites can be irreversibly destroyed, a result ascribed to the oxidation of active Fe(II) to Fe2O3 clusters which contain inactive Fe(III). Catalyst treatment at high temperature (1273 K) in He for up to 3 hours, also reduced irreversibly the concentration of α-sites. TPD measurements have shown desorption of oxygen at 3 different temperatures. Desorption at the lowest temperature (650 K) was assign to the α-oxygen recombination. The oxygen which desorbs at 700 K was associated to the oxygen from the deactivated sites. Oxygen which desorbs at the highest temperature (730 K) is originated from the surface NOx that desorbed as NO and O2. After a comparison of the concentration of the molecules accumulated, we propose that NO2 is the adsorbed specie. The mechanism of N2O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [FeII(μ-O)(μ-OH)FeII]+ inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches (see chapter 4). Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N2O decomposition are strongly dependent on the temperature. This theoretical result was contrasted with the experimentally observed steady state kinetics of the N2O decomposition and TPD experiments. As predicted by the theoretical study, the switch of the reaction order with respect to N2O pressure (from zero to one) occurs at around 800 K which confirms a change of the rate determining step from the α-oxygen recombination to α-oxygen formation. The proposed mechanism was also confirmed by O2-TD which confirmed the viability of the binuclear complex. The α-oxygen recombination at low temperature (

EPFL2010

Widely available active sites on Ni2P for electrochemical hydrogen evolution - insights from first principles calculations

Ligang Feng, Martin Hangaard Hansen, Xile Hu, Lucas-Alexandre Stern

We present insights into the mechanism and the active site for hydrogen evolution on nickel phosphide (Ni2P). Ni2P was recently discovered to be a very active non-precious hydrogen evolution catalyst. Current literature attributes the activity of Ni2P to a particular site on the (0001) facet. In the present study, using Density Functional Theory (DFT) calculations, we show that several widely available low index crystal facets on Ni2P have better properties for a high catalytic activity. DFT calculations were used to identify moderately bonding nickel bridge sites and nickel hollow sites for hydrogen adsorption and to calculate barriers for the Tafel pathway. The investigated surfaces in this study were the (10 (1) over bar0), ((1) over bar(1) over bar 20), (11 (2) over bar0), (11 (2) over bar1) and (0001) facets of the hexagonal Ni2P crystal. In addition to the DFT results, we present experiments on Ni2P nanowires growing along the < 0001 > direction, which are shown as efficient hydrogen evolution catalysts. The experimental results add these nanowires to a variety of different morphologies of Ni2P, which are all active for HER.