ABSTRACT
Small pore zeolites have shown great potential in a number of catalytic reactions. While Mo-containing medium pore zeolites have been widely studied for methane dehydroaromatisation (MDA), the use of small pore supports has drawn limited attention due to the fast deactivation of the catalyst. This work investigates the structure of the small pore Mo/H-SSZ-13 during catalyst preparation and reaction by operando X-ray absorption spectroscopy (XAS), in situ synchrotron powder diffraction (SPD), and electron microscopy; then, the results are compared with the medium pore Mo/H-ZSM-5. While SPD suggests that during catalyst preparation, part of the MoOx anchors inside the pores, Mo dispersion and subsequent ion exchange was less effective in the small pore catalyst, resulting in the formation of mesopores and Al2(MOO4)3 particles. Unlike Mo/H-ZSM-5, part of the Mo species in Mo/H-SSZ-13 undergoes full reduction to Mo0 during MDA, whereas characterisation of the spent catalyst indicates that differences also exist in the nature of the formed carbon deposits. Hence, the different Mo speciation and the low performance on small pore zeolites can be attributed to mesopores formation during calcination and the ineffective ion exchange into well dispersed Mo-oxo sites. The results open the scope for the optimisation of synthetic routes to explore the potential of small pore topologies.
Subject(s)
Methane/chemistry , Molybdenum/chemistry , Zeolites/chemistry , Catalysis , PorosityABSTRACT
In this study we present the results from two in situ X-ray diffraction computed tomography experiments of catalytic membrane reactors (CMRs) using Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) hollow fibre membranes and Na-Mn-W/SiO2 catalyst during the oxidative coupling of methane (OCM) reaction. The negative impact of CO2, when added to the inlet gas stream, is seen to be mainly related to the C2+ yield, while no evidence of carbonate phase(s) formation is found during the OCM experiments. The main degradation mechanism of the CMR is suggested to be primarily associated with the solid-state evolution of the BSCF phase rather than the presence of CO2. Specifically, in situ XRD-CT and post-mortem SEM/EDX measurements revealed a collapse of the cubic BSCF phase and subsequent formation of secondary phases, which include needle-like structures and hexagonal Ba6Co4O12 and formation of a BaWO4 layer, the latter being a result of chemical interaction between the membrane and catalyst materials at high temperatures.
ABSTRACT
The small pore zeolite Cu-SSZ-13 is an efficient material for the standard selective catalytic reduction of nitrogen oxides (NO x ) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analysed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the experiments were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) experiments provide spectroscopic evidence for previously postulated intermediates Cu-N([double bond, length as m-dash]O)-NH2 and Cu-NO3 in the NH3-SCR deNO x mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.
ABSTRACT
The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic reduction by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3-SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3-SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3-SCR of NOx occurring at Cu six ring sites within the zeolite.
