Active coke: Carbonaceous materials as catalysts for alkane dehydrogenation

The catalytic dehydrogenation (DH) and oxidative dehydrogenation (ODH) of light alkanes are of significant industrial importance. In this work both carbonaceous material deposited on VO_x/Al₂O₃ catalysts during reaction and unsupported carbon nanofibres (CNFs) are shown to be active for the dehydrogenation of butane in the absence of gas-phase oxygen. Their activity in these reactions is shown to be dependent upon their structure, with different reaction temperatures yielding structurally different coke deposits. Terahertz time-domain spectroscopy (THz-TDS), among other techniques, has been applied to the characterisation of these deposits – the first time this technique has been employed in coke studies. TEM and other techniques show that coke encapsulates the catalyst, preventing access to VO_x sites, without a loss of activity. Studies on CNFs confirm that carbonaceous materials act as catalysts in this reaction. Carbon-based catalysts represent an important new class of potential catalysts for DH and ODH reactions.     

The Chemical Evolution of the La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>CoO<Subscript>3−δ</Subscript> Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity

Owing to its extraordinary high activity for catalysing the oxygen exchange reaction, strontium doped LaCoO₃ (LSC) is one of the most promising materials for solid oxide fuel cell (SOFC) cathodes. However, under SOFC operating conditions this material suffers from performance degradation. This loss of electrochemical activity has been extensively studied in the past and an accumulation of strontium at the LSC surface has been shown to be responsible for most of the degradation effects. The present study sheds further light onto LSC surface changes also occurring under SOFC operating conditions. In-situ near ambient pressure X-ray photoelectron spectroscopy measurements were conducted at temperatures between 400 and 790 °C. Simultaneously, electrochemical impedance measurements were performed to characterise the catalytic activity of the LSC electrode surface for O₂ reduction. This combination allowed a correlation of the loss in electro-catalytic activity with the appearance of an additional La-containing Sr-oxide species at the LSC surface. This additional Sr-oxide species preferentially covers electrochemically active Co sites at the surface, and thus very effectively decreases the oxygen exchange performance of LSC. Formation of precipitates, in contrast, was found to play a less important role for the electrochemical degradation of LSC.     

In situ analysis of metal-oxide systems used for selective oxidation catalysis: how essential is chemical complexity?

The mode of operation of selective oxidation reactions is described by a series of chemical rules defining the catalyst and some reaction intermediates. In contrast to catalytic processes over metallic elements, little is known, however, about the atomistic details of selective oxidation. In particular, the participation of the subsurface region of the catalyst in the kinetically relevant elementary steps (Mars–van Krevelen mechanism) is not positively verified. Using in situ X-ray absorption techniques to study binary and ternary molybdenum oxides the present contribution shows that it is possible to tackle some of the problems in selective oxidation by direct experimental observation. The modification of the Mo–O local bonding interaction upon thermal reduction of MoO₃to MoO_3-x is illustrated. This was also found for mixed Mo–V oxides in which the chemical state of the vanadium seemed unaffected by the reaction but the surface Mo:V ratio varied substantially with the gas phase composition. It is further shown that the solid-state phase transformation between reduced and oxidised forms of molybdenum oxides occur so rapidly, that possibly relevant suboxide cannot be identified by ex situ phase analysis. Observation of the time-law of redox transformations showed that lattice oxygen is only available for selective oxidation if the associated solid-state transformation occurs in the kinetic regime of reaction control and not in that of diffusion control.     

Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

X-ray photoelectron and absorption spectroscopies (XPS and XAS) combined with on-line mass spectrometry were applied under working catalytic conditions to investigate methanol oxidation on cobalt. Two cobalt oxidation states (Co₃O₄ and CoO) were prepared and investigated as regards their influence on the catalytic activity and selectivity. In addition adsorbed species were monitored in the transition of the catalyst from a non-active state, to an active one. It is shown that the surface oxidation state of cobalt is readily adapted to the oxygen chemical potential in the CH₃OH/O₂ reaction mixture. In particular, even in oxygen-rich mixtures the Co₃O₄ surface is partially reduced, with the extent of surface reduction following the methanol concentration. The reaction selectivity depends on the cobalt oxidation state, with the more reduced samples favouring the partial oxidation of methanol to formaldehyde. In the absence of oxygen, methanol effectively reduces cobalt to the metallic state, also promoting H₂ and CO production. Direct evidence of methoxy and formate species adsorbed on the surface upon reaction was found by analysing the O 1s and C 1s photoelectron spectra. However, the surface coverage of those species was not proportional to the catalytic activity, indicating that they might also act as reaction inhibitors.     

X-Ray Absorption and Photoemission Studies of the Active Oxygen for Ethylene Epoxidation over Silver

Activation of clean polycrystalline silver by a C₂H₄ + O₂ reaction mixture has been studied by XANES, XPS, and UPS. In situ monitoring of the O K-edge XAS spectrum of the pre-treated silver surface revealed a broad signal at 10–20 eV above the threshold. The comparison of the X-ray absorption spectra with O 1s and valence band photoemission data allowed us to attribute this XAS signal to electrophilic oxygen (E _b(O 1s) = 530.4 eV) which is known to be active in ethylene epoxidation. The complete absence of XAS features in the photon energy range typical for ^* and ^* transitions of molecular oxygen (530–535 eV) indicates both the atomic origin of the electrophilic oxygen and the absence of molecular species on the catalyst surface under the present reaction conditions.     

Comparison of Oxidized Polycrystalline Copper Foil with Small Deposited Copper Clusters in Their Behavior in Ammonia Oxidation: An Investigation by Means of In Situ NEXAFS Spectroscopy in the Soft X-Ray Range

The oxidation of ammonia to nitrogen or nitric oxide was investigated using on the one hand a polycrystalline copper foil and on the other hand deposited copper clusters prepared with the inert gas aggregation technique. The behavior in the oxidation of ammonia of both model catalysts was studied using in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the soft X-ray range and mass spectrometry. It is shown that the copper foil reacts in a similar way to the copper clusters. Differences appear only with respect to the reaction temperature required, which is lower for the cluster sample. It can be concluded that the results obtained in experiments with polycrystalline copper foil are exemplary for and can be transferred to a supported copper catalyst consisting of small copper particles.     

Oxygen-induced activation of silica supported silver in acrolein hydrogenation

The effect of oxygen pre-treatment of Ag/SiO₂ catalysts used in the acrolein hydrogenation has been investigated from sub-atmospheric (266 mbar) to 10 bar reaction pressure. The pre-treatment has a profound influence on the overall hydrogenation activity, inasmuch the activity increased two- to three-fold. Oxygen pre-treatment also enhanced the selectivity towards the desired allyl alcohol. The beneficial effect of oxygen pre-treatment is ascribed to the formation of electronically modified Ag and/or morphology changes of the nanoparticles as no modification in particle size or dispersion was found.     

Investigation of Ammonia Oxidation over Copper with In Situ NEXAFS in the Soft X-Ray Range: Influence of Pressure on the Catalyst Performance

The oxidation of ammonia over polycrystalline copper was investigated by means of in situ NEXAFS (near-edge X-ray absorption fine structure) spectroscopy in the soft X-ray range. The reaction, carried out in a 1:12 excess of oxygen, was observed by mass spectrometry. The simultaneous detection of the surface electronic structure and its catalytic performance allows correlation of different reaction products to the current surface structure of the catalyst. It is shown that a change in total pressure from 0.4 to 1.2 mbar severely affects the reaction path. Copper(I) nitride was identified as poison and a copper oxide was found to be the active phase for the selective oxidation of ammonia to nitrogen.