Evidence from in Situ X-ray Absorption Spectroscopy for the Involvement of Terminal Disulfide in the Reduction of Protons by an Amorphous Molybdenum Sulfide Electrocatalyst

The reduction of protons into dihydrogen is important because of its potential use in a wide range of energy applications. The preparation of efficient and cheap catalysts for this reaction is one of the issues that need to be tackled to allow the widespread use of hydrogen as an energy carrier. In this paper, we report the study of an amorphous molybdenum sulfide (MoSx) proton reducing electrocatalyst under functional conditions, using in situ X-ray absorption spectroscopy. We probed the local and electronic structures of both the molybdenum and sulfur elements for the as prepared material as well as the precatalytic and catalytic states. The as prepared material is very similar to MoS3 and remains unmodified under functional conditions (pH = 2 aqueous HNO3) in the precatalytic state (+0.3 V vs RHE). In its catalytic state (-0.3 V vs RHE), the film is reduced to an amorphous form of MoS2 and shows spectroscopic features that indicate the presence of terminal disulfide units. These units are formed concomitantly with the release of hydrogen, and we suggest that the rate-limiting step of the HER is the reduction and protonation of these disulfide units. These results show the implication of terminal disulfide chemical motifs into HER driven by transition-metal sulfides and provide insight into their reaction mechanism.

Study of N2O decomposition over Fe-ZSM-5 with transient methods

Petra Prechtl

Nitrous oxide is the third most important gas contributing to global warming and its concentration in the atmosphere is still on the rise. Nitric acid production represents the largest source of N2O in the chemical industry, with a total annual emission of about 400 kt N2O. An efficient way to reduce the N2O is the catalytic decomposition into N2 and O2. ZSM-5 zeolites containing iron are known to be effective in this reaction involving Fe(II)-extraframework species as active sites. But the nature of these sites and the reaction network is still under debate. Often the N2O is in the exhaust together with other gases like NOx, COx, oxygen, and water vapor, which may influence the catalyst efficiency towards N2O decomposition. The effect of these gases on the reaction mechanism is still not understood yet. The aim of this work is to study the decomposition mechanism of N2O over isomorphously substituted Fe-ZSM-5 (0.02-0.55% Fe) using transient response methods. Kinetic studies under steady-state conditions give overall information about the reaction, whereas transient experiments lead to information on the individual steps and the possibility to suggest a reaction model. Transient response methods are applied in vacuum using a Temporal Analysis of Products (TAP) setup and at ambient pressure using a Micromeritics AutoChem 2910 analyzer. The influence of different gases such as NO, H2O and NO2 on the decomposition of N2O over Fe-ZSM-5 is investigated to gain insight into their effect on this reaction. The TAP multifunctional reactor system is used to perform transient pulse experiments for micro-kinetic analysis of complex heterogeneous catalytic reactions. This setup allows to dose precisely reactants and to monitor product formation with submillisecond time resolution implying the absence of external mass or heat transfer. A reproducible low hydroxylation level of the catalyst can be attained in vacuum. The results from the transient responses with the Micromeritics analyzer are obtained under conditions more relevant to "operando". The mechanism of N2O decomposition over Fe containing ZSM-5 was studied by transient response methods at ambient pressure with the Micromeritics analyzer and under vacuum with the TAP setup. Both approaches showed that the mechanism strongly depends on the reaction temperature, but in all cases involves atomic oxygen loading from N2O with N2 evolution as the first reaction step. The surface oxygen, (O)Fe, formed from N2O possesses very high reactivity and oxidizes CO to CO2 already at 373 K. A complete saturation of the active sites can be reached by the catalyst exposure to N2O at low temperatures (T < 600 K in vacuum, T < 523 K at ambient pressure). Under these conditions oxygen is stored and does not desorb. On that basis, the concentration of sites active for the surface atomic oxygen loading from N2O is determined by pulse experiments using the TAP setup. The concentration of active sites is found to be the same as in a flow experiment with the Micromeritics analyzer. The mechanism of surface atomic oxygen recombination and desorption as O2 depends on the temperature. At temperatures higher than 773 K, the mechanism probably involves direct (O)Fe recombination. The investigation of the response profiles upon N2O pulses at 803 K evidences the O2 formation is the rate-determining step since the oxygen desorption is found to be slow compared to N2O and N2. Furthermore, a fit of the oxygen desorption curve corresponded to a first order dependence on the oxygen surface coverage. In the range of 523 to 773 K, the O2 formation is facilitated by NO, which is formed on the catalyst surface from N2O. The addition of NO to the gas phase confirmed the accelerating effect of NO on the oxygen desorption. NO and (O)Fe are adsorbed on adjacent, but different sites and are not competing for the same ones. Adsorbed NO accommodates oxygen from N2O forming higher oxidized nitrogen oxide species NO2/NO3 which decompose back to NOads and molecular oxygen. Thus, the recombination of surface oxygen loaded from N2O is accelerated by a NO/NOx redox cycle whereas adsorbed NOx acts as co-catalyst. Adsorbed NO is oxidized by N2O easily as shown also by temperature-programmed desorption (TPD) experiments. Therefore, the interaction of gaseous NO2 with Fe-ZSM-5 at 523 K in vacuum and at ambient pressure is investigated and demonstrates the formation of two different NOx surface species accompanied by the evolution of gaseous NO. These surface species block the sites for oxygen loading from N2O whereas the amount of reversible adsorbed N2O stays constant compared to the standard pre-treated catalyst. The formation of reactive surface oxygen from NO2 could neither be evidenced in TPD nor with CO oxidation experiments. Adsorbed water strongly influences the N2O decomposition inhibiting it completely, at least at temperatures ≤ 673 K. Water pulses at 523 and 593 K result in partial desorption of the loaded atomic surface oxygen as O2, indicating the competition of water for the same adsorption sites. The residual adsorbed oxygen desorbs during TPD experiments at 849 K instead of 715 K for the dry catalyst and it can not oxidize CO to CO2. It means that the active sites are transformed into an inactive form in the presence of water. The oxygen which stays on the catalyst seems to participate in the Fe(II) oxidation to the hydroxylated Fe(III) species. These Fe(III) species are inactive in N2O decomposition and could be activated again by high temperature treatment in He or in vacuum. During TAP experiments only small amounts of water are dosed and the water desorption is facilitated due to vacuum. Therefore, the effect of water could be shown to be reversible at temperatures > 700 K. At 803 K oxygen desorbs significantly faster with increasing water amount suggesting a water assisted O2 desorption. Simultaneously, the catalyst activity decreases to about a half of the initial activity of the dry zeolite. To conclude, from the TAP reactor data obtained under vacuum conditions and the results from the transient flow experiments at ambient pressure it follows that the mechanism of the N2O decomposition over Fe-containing ZSM-5 depends strongly on the reaction temperature and on the presence of NO, H2O and NO2. In general, below 523 K only surface atomic oxygen loading from N2O takes place with simultaneous evolution of N2 in the gas phase. The O2 formation proceeds via direct (O)Fe recombination at temperatures above 773 K. In the range of 523 to 773 K, the recombination of the atomic oxygen involves NOx adsorbed on the catalyst surface.

EPFL2007

Chemical and electrochemical promotion of supported rhodium catalyst

Olena Baranova

The chemical and electrochemical promotion of highly dispersed nanofilm Rh catalysts (dispersion: about 10 %, film thickness: 40 nm) has been investigated for the first time. To this end Rh metal was sputter-deposited, either on a purely ionic conductor (8 % Y2O3-stabilized ZrO2) or on a mixed ionic-electronic conductor (TiO2), the latter being a highly dispersed layer of TiO2 (4 µm) deposited on YSZ. These catalysts are designated as Rh/YSZ and Rh/TiO2/YSZ, respectively. It was established analytically that both in the Rh/YSZ and in the Rh/TiO2/YSZ system, the catalyst films have a nanoparticle-size grain structure. The Rh supported on titania is rather porous, exhibiting a higher dispersion and surface area than Rh on YSZ. Both after reduction (H2, T=400 °C) and after oxidation (O2, T=400 °C), Rh supported on TiO2 was found to be in a more highly reduced state than Rh on YSZ. After reducing treatment, the Rh/TiO2/YSZ samples contain a larger amount of weakly bonded oxygen, which can be attributed to oxygen "backspillover" from the TiO2. A major feature of this research was the electrochemical characterization of the oxygen/Rh/solid electrolyte three-phase boundary by steady-state polarization measurements and by impedance spectroscopy. These are powerful techniques for extracting experimental trends and details that are useful for an understanding of the electrochemical promotion principles. It was shown that the exchange current densities at the Rh/solid electrolyte interface are lower on account of the TiO2 layer. The exchange current densities are more than twice lower at Rh/TiO2(4 µm)/YSZ than at Rh/YSZ, demonstrating that the former interface is much more polarizable. The mechanism of oxygen exchange occurring close to equilibrium (O2/O2- couple) was investigated for the first time at Rh catalyst electrodes interfaced with solid electrolyte. The processes of cathodic oxygen reduction and anodic oxygen evolution are not symmetric, but they are similar in the two systems, Rh/YSZ and Rh/TiO2(4 µm)/YSZ. The cathodic process consists of three steps: dissociative adsorption of oxygen at the gas-exposed Rh surface, atomic oxygen diffusion to the electrochemical reaction sites (ERS), and a two-electron transfer to this oxygen on the ERS. The cathodic process is limited by interfacial diffusion of oxygen atoms from the gas-exposed metal surface to the ERS. The anodic process includes two steps: two-electron transfer reaction, which is the rate-determining step, and oxygen desorption to the gas phase. With data obtained from impedance spectroscopy at the equilibrium potential, it was possible to confirm the reaction scheme proposed. It was demonstrated that Rh nanofilm catalysts interfaced with YSZ or TiO2/YSZ can be electrochemically promoted for the reaction of ethylene oxidation. Small anodic currents cause periodic oscillations in catalytic rate and potential of the Rh/YSZ catalyst, while at Rh/TiO2/YSZ they give rise to a stable and reversible rate enhancement by up to a factor 80. The increase in ethylene oxidation rate is up to 2000 times larger than the electrochemical rate, I/2F, of O2- oxidation. The pronounced electrochemical promotion behavior that has been observed is due to the anodically controlled migration of O2- species from the electrolyte to the Rh/gas interface. At the Rh surface, these species destabilize the formation of rhodium surface oxide (Rh2O3). The existence of backspillover oxygen species has been confirmed by impedance measurements under positive applied potential. Another significant result for heterogeneous catalysis is the finding that thick as well as thin films of Rh/TiO2/YSZ catalyst are open to chemical promotion of ethylene oxidation and partial methane oxidation, ie, they offer a higher catalytic activity and stability than the Rh/YSZ catalysts. The modification of catalytic activity observed for Rh/TiO2/YSZ was attributed to either a "long-range" electronic-type SMSI mechanism or to a self-driven electrochemical promotion mechanism. In both cases, the ultimate cause of promotion are different work functions of catalyst and support. Equilibration of the work functions of two solids in contact induces surface charging, a migration of O2- ions to the catalyst/gas interface, and a weakening of the Rh-O chemisorptive bonds. It facilitates reduction of oxidized surface sites. Another important achievement of the present work was that of exploring and confirming the possibilities of current-assisted activation of Rh/TiO2/YSZ catalysts. In partial methane oxidation using close to stoichiometric gas compositions (CH4 : O2 = 2 : 1) at 550 °C, the inactive (oxidized) Rh/TiO2/YSZ catalyst was successfully activated by applied currents, either positive or negative. This phenomenon is an example of "permanent" electrochemical promotion furnishing a permanent rate enhancement ratio of γ = 11. The activation by negative currents is explained in terms of an electrochemical reduction of rhodium surface oxide, while the activation by positive currents can be explained by the mechanism of electrochemical promotion.

EPFL2005

Nanofibres de carbone sur filtre métallique comme support catalytique structuré

Pascal Tribolet

The main objectives of this work are 2-fold : The development of a novel type of composite material based on carbon nanofibers supported on sintered metal fibers filters (CNF/SMF) and to explore the use of this material as a catalytic support for a model reaction: selective hydrogenation of acetylene. The advantages of this novel support are: Due to carbon nanofibers, it combines a graphitic structure with a high specific surface area without microporosity. Supported carbon nanofibers can be used in a fixed bed reactor without very high pressure drop. Combination of two materials with high thermal conductivity gives a composite which keeps this property. It is suitable for highly exo/endothermic reactions, diminishing temperature gradients in the catalytic bed. The synthesis of carbon nanofibers on SMF filters was carried out by ethane catalytic decomposition at 655°C directly on metallic filters. An oxidation of SMF followed by a reduction in H2 create surface roughness, leading to a larger area for CNF nucleation. The CNF formation mechanism includes several steps: it starts with carbon deposition on metallic surface followed by its diffusion into the metal leading to carbides formation. When the limit of carbon solubility is attained, a layer of carbon is formed on the surface of metallic fibers. High tension within the bulk metal leads to carbide dissociation and by phase separation to graphite formation, which pushes the metallic particles through the carbon layer. Nickel, stainless steel an Inconel (an alloy of mainly nickel, iron and chromium) filters were used. The latter gave the highest uniformity of the carbon nanofibers layer showing an excellent mechanical stability. In order to control carbon deposition on the filters, a formal kinetic analysis of the decomposition reaction was undertaken. Partial orders of 1 for ethane, -2 for hydrogen and an apparent activation energy of 235 kJ/mol were obtained. The observed kinetics suggests strongly that the C – C bond scission is the rate determining step and not carbon diffusion into the metal which is often reported in the literature. The synthesis procedures like time on stream and reaction temperature were optimized: 1 hour of a mixture C2H6:H2:Ar (3:17:80) decomposition over SMFInconel at 655°C lead to ∼6% of carbon, leading to a CNF layer of 1 µm thickness covering the metallic fibers entirely. This material presents a high specific surface area (22.2 m2/g) and a low pressure drop for the gas flowing through. The carbon nanofibers have a diameter between 20 and 50 nm and a crystalline structure with platelets morphology where the graphene layers are stacked one above the other. Their surface is composed almost exclusively of carbon and hydrogen. Amongst the multiple possible applications of this novel composite material, its use as a catalyst support was assessed. Selective hydrogenation of acetylene was carried out over palladium supported on CNF/SMF and compared with commercial supports (ACF, EGF and AGF). The activated carbon fibers cloth (ACF) led to a Pd activity at least twice as E-type glass fibers tissue (EGF) or EGF covered by an alumina layer (AGF). Reaction kinetics was studied for Pd/ACF, leading to acetylene and hydrogen partial orders of -0.5 and 2.4, respectively and apparent activation energy of 50 kJ/mol. The negative partial order for C2H2 suggests high adsorption strength of this compound compared to other reagents on palladium. This particularity allows Pd to be selective. Hydrogenations are known to be structure sensitive. Therefore the catalyst activity for the model reaction depends on Pd particle size. It increases for large particle sizes. The control of this parameter was a priority. Variations of the palladium precursor, of the support and catalyst treatment were carried out. Micro-wave plasma was used since it is fast and has low energy consumption allowing chemically modifying the carbon support surface and activating catalyst after impregnation. This controls not only palladium sintering, but also decomposes tetraaminopalladate more efficiently, resulting in a catalyst activity of one order of magnitude higher. In order to control the palladium deposition on CNF/SMFInconel composite, it is important to have surface functional groups. The activation of carbon nanofibers was carried out with a 4 hours treatment in a boiling aqueous solution of hydrogen peroxide. This led to an amount on surface oxygen containing groups, per mass of carbon, higher than in ACF. Palladium particle size on the composite was controlled by support activation, the use of several metal precursors or different loadings or by a thermal treatment in argon at 500°C. Optimisation of the procedure allowed to obtain in a controlled way an average palladium particle sizes between 3.6 and 25 nm. The activity and selectivity towards ethylene of Pd/CNF/SMFInconel were observed to be higher compared to palladium supported on activated carbon, alumina or barite. Graphitic nature of carbon nanofibers, leading to strong metal-support interaction, was accounted as responsible for these improvements. The high thermal conductivity of the composite CNF/SMF ensured good heat transfer released due to the exothermicity of the reaction and allowed to work close to isothermal conditions in the catalytic bed. Finally it can be concluded, that for the first time carbon nanofibers synthesis was carried out in one step on sintered metal fibers filters, resulting in a novel structured composite material combining different specific properties and having promising potential applications.

EPFL2006