Ultralow specific surface area vermiculite supporting Mn-Ce-Fe mixed oxides as “curling catalysts” for selective catalytic reduction of NO with NH3

https://doi.org/10.1016/j.gce.2021.03.002

“In this study, the ultralow specific surface area clay vermiculite (VMT) was selected to be a catalyst support for the NH₃-SCR process, and the active components MnCeFeO_x loaded on vermiculite was just like curling on ice from the TEM results. The de-NO_x performance of Mn–Ce–Fe/VMT exhibited almost complete NO conversion with a gas hourly space velocity (GHSV) of 15,300 h⁻¹ at 150 °C, which was 25% and 10% higher than that of Mn/VMT and Mn–Ce/VMT, respectively. Ce and Fe co-doping improved the BET surface area, the quantities of active Mn⁴⁺, the acid sites and NH₃ adsorption energy of Mn/VMT, all of which contributed to the increase in low-temperature SCR activity. In situ DRIFT measurements suggested that NO_x removal over Mn–Ce–Fe/VMT followed both Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms at 150 °C, but the E-R mechanism played a dominant role. Corresponding Mn–Ce–Fe/VMT monolithic catalysts reached 90% NO conversion with a GHSV of 4000 h^-1.”

“The adsorption/desorption isotherms were measured with a Micromeritics ASAP 2460 instrument using N₂ at −196 °C. Total surface area was calculated with Brunauer-Emmet-Taller (BET) method. ”

“The agglomeration phenomenon was also proved by the specific surface area value of between VMT and Mn/VMT (Fig. 2 and Table 2). For Mn–Ce/VMT, the MnO_x nanoparticles have a little small average diameter (about 8.75 nm) than Mn/VMT, and the reunion phenomenon of Mn/VMT was alleviated by the dopant of Ce, which was also in accordance with the BET results (Fig. 2 and Table 2). The Mn–Ce–Fe/VMT have the smallest nanoparticles stack with an average diameter of about 2.75 nm, and it also have the highest BET surface area than Mn/VMT and Mn–Ce/VMT (Fig. 2 and Table 2), it meant that Fe-modified Mn–Ce/VMT obviously reduced catalyst particles [22]. And the STEM-HAADF (Fig.1e-i) further confirmed that Mn, Ce, Fe and O were uniformly distributed on the VMT. In other words, Ce and Fe-modified Mn/VMT was beneficial to increase the active sites to promote the NO conversion. Based on the above morphological characteristics, we supposed that the mixed oxides supported VMT with irregular nanoparticles is just like curling on ice. To know the BET area of mixed oxides, the theoretical specific area of removing VMT support was calculated (Table 2).”

Fig. 2. Adsorption isotherms and pore size distribution (inset) of all samples.