H-ZSM-5 zeolite-supported Ga (Ga/H-ZSM-5) has been considered as a selective catalyst for nonoxidative propane dehydrogenation (PDH) for decades; however, the reaction mechanism remains a topic of considerable discussion. In particular, the correlation between various Ga species present on the catalyst at the reaction conditions and the PDH activity has yet to be established. In this work, intrinsic PDH rates and activation energies were determined on Ga + −H + pair sites and isolated Ga + sites on Ga/H-ZSM-5 samples with a wide range of Si/Al and Ga/Al ratios. The turnover frequency on Ga + −H + pair sites in the PDH is higher than that of isolated Ga + sites by a factor of ∼15. Experimental measurements combined with a dual-site model show the activation energy in the PDH on the Ga + −H + pair sites and isolated Ga + sites to be 90.8 ± 1.5 and 117 ± 4.7 kJ•mol −1 , respectively. These results demonstrate that Ga + −H + pair sites are much more active in the PDH than isolated Ga + sites. The activation energy of GaH x decomposition to form H 2 was determined to be 40−60 kJ•mol −1 higher than that of the PDH on Ga species, suggesting that the GaH x decomposition is unlikely to be part of the PDH mechanism. Although both Brønsted acid and Ga sites interact with propane, Fourier transform infrared spectroscopy results provide strong evidence suggesting that the alkyl mechanism is more likely in the PDH on Ga/H-ZSM-5 catalysts.
[MoS] clusters were bridged between CoFe layered double hydroxide (LDH) layers using the ion-exchange method. [MoS]/CoFe-LDH showed excellent Hg removal performance under low and high concentrations of SO, highlighting the potential for such material in S-Hg mixed flue gas purification. The maximum mercury capacity was as high as 16.39 mg/g. The structure and physical-chemical properties of [MoS]/CoFe-LDH composites were characterized with FT-IR, XRD, TEM&SEM, XPS, and H-TPR. [MoS] clusters intercalated into the CoFe-LDH layered sheets; then, we enlarged the layer-to-layer spacing (from 0.622 to 0.880 nm) and enlarged the surface area (from 41.4 m/g to 112.1 m/g) of the composite. During the adsorption process, the interlayer [MoS] cluster was the primary active site for mercury uptake. The adsorbed mercury existed as HgS on the material surface. The absence of active oxygen results in a composite with high sulfur resistance. Due to its high efficiency and SO resistance, [MoS]/CoFe-LDH is a promising adsorbent for mercury uptake from S-Hg mixed flue gas.
Determination of the structure of Ga-containing catalytic sites in the propane dehydrogenation (PDH) reaction on Ga/H-ZSM-5 has been a long-standing challenge for understanding Ga-based catalysts but has been hampered by the complexity of the system. We employed quantitative pulse titration reactions to differentiate two types of Ga species based on their distinct redox properties: divalent Ga species balancing paired framework Al atoms and isolated Ga+ sites. While isolated Ga+ is redox-active, the divalent Ga species cannot be reduced or oxidized at reaction temperature (550 °C) by H2 or O2, respectively. Together with in situ infrared spectroscopic evidence, the highly active divalent Ga species is identified to be Ga2O2 2+. The PDH activity of Ga2O2 2+ is shown to be at least a factor of 18 higher than that of isolated Ga+, suggesting that increasing the density of the former could be an effective strategy in enhancing the catalytic performance. Interestingly, only 30% of Al pair sites observed through the Co2+ exchange protocol are able to stabilize Ga2O2 2+, that is, the latter has a more stringent requirement for the proximity between the two adjacent framework Al atoms, thus providing an additional method to probe the Al distribution in zeolites.
Ferromagnetic metal/alloy nanoparticles have attracted extensive interest for electromagnetic wave-absorbing applications. However, ferromagnetic nanoparticles are prone to oxidization and producing eddy currents, leading to the deterioration of electromagnetic properties. In this work, a simple and scalable liquid-phase reduction method was employed to synthesize uniform CoFe nanospheres with diameters ranging from 350 to 650 nm for high-performance microwave absorption application. CoFe@SiO core-shell nanospheres with SiO shell thicknesses of 30 nm were then fabricated via a modified Stöber method. When tested as microwave absorbers, bare CoFe nanospheres with a diameter of 350 nm have a maximum reflection loss (RL) of 78.4 dB and an effective absorption with RL > 10 dB from 10 to 16.7 GHz at a small thickness of 1.59 mm. CoFe@SiO nanospheres showed a significantly enhanced microwave absorption capability for an effective absorption bandwidth and a shift toward a lower frequency, which is ascribed to the protection of the SiO shell from direct contact among CoFe nanospheres, as well as improved crystallinity and decreased defects upon annealing. This work illustrates a simple and effective method to fabricate CoFe and CoFe@SiO nanospheres as promising microwave absorbers, and the design concept can also be extended to other ferromagnetic alloy particles.
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