Tuning stability of titania-supported Fischer-Tropsch catalysts:Impact of surface area and noble metal promotion

Cobalt oxidation is a relevant deactivation pathway of titania-supported cobalt catalysts used in Fischer-Tropsch synthesis (FTS). To work towards more stable catalysts, we studied the effect of the surface area of the titania support and noble metal promotion on cobalt oxidation under simulated high conversion conditions. Mössbauer spectroscopy was used to follow the evolution of cobalt during reduction and FTS operation as a function of the steam pressure. The reduction of the oxidic cobalt precursor becomes more difficult due to stronger metal-support interactions when the titania surface area is increased. The reducibility was so low for cobalt on GP350 titania (surface area 283 m2/g) that the catalytical activity was negligible. Although cobalt was more difficult to reduce on P90 titania (94 m2/g) than on commonly used P25 titania (50 m2/g), the Co/P90 catalyst showed increased resistance against cobalt sintering and higher FTS performance than Co/P25. The addition of platinum to Co/P90 led to a higher reduction degree of cobalt and a higher cobalt dispersion, representing a catalyst with promising performance at relatively low steam pressure. Nevertheless, the stronger cobalt-titania interactions result in more extensive deactivation at high steam pressure due to oxidation.</p

In situ Mössbauer Spectroscopy Study of the Activation and Reducibility of Chromium- and Aluminium-doped Iron Oxide based Water-Gas Shift Catalysts under Industrially Relevant Conditions

The influence of chromium and aluminium doping on the over-reduction during activation of iron-oxide-based water-gas shift catalysts was investigated using Mössbauer spectroscopy for the first time. In situ Mössbauer spectra of catalysts exposed to industrially relevant gas compositions were recorded with increasingly reducing R factors R = [CO]*[H2]/[CO2]*[H2O]. Whereas α-Fe and cementite formed during exposure of a non-doped iron-oxide catalyst to process conditions with an R factor of 2.09, such phases were only observed at R = 4.60 for a chromium-doped catalyst, showing that chromium stabilizes the catalyst. Over-reduction was enhanced to R = 2.88 in a chromium-copper co-doped catalyst. α-Fe was already observed at R = 1.64 in an aluminium-doped catalyst, while cementite formation occurred at R = 2.09, showing that over-reduction was enhanced, the presence of aluminium delaying carburization. Co-doping copper in the aluminium-doped catalyst showed cementite formation at R = 2.09, the same as a non-doped catalyst

Influence of temperature during pyrolysis of Fe-alginate: Unraveling the pathway towards highly active Fe/C catalysts

Transition metals supported on carbons play an important role in catalysis and energy storage. By pyrolysis of metal alginate, highly active catalysts for the Fischer-Tropsch synthesis (FTS) can be produced. However, the evolution of the carbon (alginate) and transition metal (Fe3+) during pyrolysis remains largely unknown and was herein corroborated with several advanced in situ techniques. Initially, Fe3+ was reduced to Fe2+, while bound to alginate. FeO nucleated above 300 °C, destabilizing the alginate functional groups. Increasing temperatures improved carbonization of the carbon support, which facilitated reduction of FeO to α-Fe at 630 °C. Catalysts were produced by pyrolysis between 400 and 700 °C, where the highest FTS activity (612 µmolCO gFe−1 s−1) was achieved for the sample pyrolyzed at low temperature. Lower metal loading, due to less decomposition of alginate, moderated sintering and yielded larger catalytic surface areas. The results provide valuable knowledge for rational design of metal-alginate-based materials.publishedVersio

Synthesis and activation for catalysis of Fe-SAPO-34 prepared using iron polyamine complexes as structure directing agents

This work was supported by Johnson Matthey PLC, UK. Solid-state NMR spectra were obtained at the EPSRC UK National Solid-state NMR Service at Durham.The use of transition metal cations complexed by polyamines as structure directing agents (SDAs) for silicoaluminophosphate (SAPO) zeotypes provides a route, via removal of the organic by calcination, to microporous solids with well-distributed, catalytically-active extra-framework cations and avoids the need for post-synthesis aqueous cation exchange. Iron(II) complexed with tetraethylenepentamine (TEPA) is found to be an effective SDA for SAPO- 34, giving as-prepared solids where Fe2+-TEPA complexes reside within the cha cages, as indicated by Mössbauer, optical and X-ray absorption near edge spectroscopies. By contrast, when non-coordinating tetraethylammonium ions are used as the SDAs in Fe-SAPO-34 preparations, iron is included as octahedral Fe3+ within the framework. The complex- containing Fe-SAPO-34(TEPA) materials give a characteristic visible absorption band at 550 nm (and purple colouration) when dried in air that is attributed to oxygen chemisorption. Some other Fe2+ polyamine complexes (diethylenetriamine, triethylenetetramine and pentaethylenehexamine) show similar behaviour. After calcination in flowing oxygen at 550 °C, ‘one-pot’ Fe(TEPA) materials possess Fe3+ cations and a characteristic UV-visible spectrum: they also show appreciable activity in the selective catalytic reduction of NO with NH3.PostprintPostprintPeer reviewe

Site-specific iron substitution in STA-28, a large pore aluminophosphate zeotype prepared using 1,10-phenanthrolines as framework-bound templates

Funding: UK Engineering and Physical Sciences Research Council (Grant Number(s): EP/N50936X/1, EP/S016201/1, EP/S016147/1); The Royal Society (Grant Number(s): INF\R2\192052).An AlPO4 zeotype has been prepared using the aromatic diamine 1,10‐phenanthroline and some of its methylated analogues as templates. In each case the two template N atoms bind to a specific framework Al site to expand its coordination to the unusual octahedral AlO4N2 environment. Furthermore, using this framework‐bound template, Fe atoms can be included selectively at this site in the framework by direct synthesis, as confirmed by annular dark field scanning transmission electron microscopy and Rietveld refinement. Calcination removes the organic molecules to give large pore framework solids, with BET surface areas up to 540 m2 g‐1 and two perpendicular sets of channels that intersect to give pore space connected by 12‐ring openings along all crystallographic directions.Publisher PDFPeer reviewe

A Guideline to Mitigate Interfacial Degradation Processes in Solid-State Batteries Caused by Cross Diffusion

Diffusion of transition metals across the cathode–electrolyte interface is identified as a key challenge for the practical realization of solid-state batteries. This is related to the formation of highly resistive interphases impeding the charge transport across the materials. Herein, the hypothesis that formation of interphases is associated with the incorporation of Co into the Li7La3Zr2O12 lattice representing the starting point of a cascade of degradation processes is investigated. It is shown that Co incorporates into the garnet structure preferably four-fold coordinated as Co2+ or Co3+ depending on oxygen fugacity. The solubility limit of Co is determined to be around 0.16 per formula unit, whereby concentrations beyond this limit causes a cubic-to-tetragonal phase transition. Moreover, the temperature-dependent Co diffusion coefficient is determined, for example, D700 °C = 9.46 × 10−14 cm2 s−1 and an activation energy Ea = 1.65 eV, suggesting that detrimental cross diffusion will take place at any relevant process condition. Additionally, the optimal protective Al2O3 coating thickness for relevant temperatures is studied, which allows to create a process diagram to mitigate any degradation with a minimum compromise on electrochemical performance. This study provides a tool to optimize processing conditions toward developing high energy density solid-state batteriesD.R. acknowledges financial support by the Austrian Federal Ministry for Digital and Economic Affairs, the National Foundation for Research, Technology, and Development, and the Christian Doppler Research Association (Christian Doppler Laboratory for Solid-State Batteries). D.R. and J.F. acknowledges financial support by the Austrian Science Fund (FWF) in the frame of the project InterBatt (P 31437). D.K. acknowledges funding by the European Union’s Horizon 2020 Research and Innovation Programme (Grant No. 823717, project “ESTEEM3”) and by the Zukunftsfond Steiermark. J.G.S. and D.J.S. acknowledge financial support from the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy , Office of Science, Basic Energy Sciences. Technical assistance of M. Stypa in crystal growth experiments is greatly acknowledge

Benzyl alcohol valorization via the in situ production of reactive oxygen species

In this contribution, we outline the efficacy of Pd-based bimetallic catalysts toward the oxidative upgrading of benzyl alcohol via the in situ synthesis of H2O2 (and related reaction intermediates) from the elements. In particular, the formation of PdAu and PdFe nanoalloys is observed to be highly effective, offering high yields of benzaldehyde and near total selectivity to the desired product, with these catalysts outperforming alternative materials reported in the literature. Notably, the PdFe formulation also achieves high selective utilization of H2, a key requirement if the in situ approach to chemical synthesis is to become economically viable. Correlative studies, focusing on the direct synthesis of H2O2 and further experiments utilizing preformed H2O2, coupled with Electron Paramagnetic Resonance (EPR) spectroscopy indicate that H2O2 itself is not primarily responsible for the observed catalysis, but rather, the performance of the PdAu and PdFe formulations can be related to the generation of reactive oxygen species (ROS). While the origin of these ROS is not fully understood, it is hypothesized that they are generated through a combination of (i) the desorption of reaction intermediates formed during H2O2 synthesis and (ii) through Fenton-mediated chemistry involving the synthesized H2O2, in the case of the PdFe-based materials. Importantly, our EPR studies also identify the noninnocent nature of the reaction solvent