Engineering topics

Li, Junhua

Publications and source records attributed to Li, Junhua.

3 records

DOE OSTI2022

Atomically dispersed Ag on δ-MnO 2 via cation vacancy trapping for toluene catalytic oxidation

The regulation of metal–support interaction (MSI) and the improvement of noble metal utilization are the frontiers of volatile organic compound (VOC) catalytic oxidation. Here, we report a H 2 O 2 -assisted synthesis process for trapping Ag + onto the engineered Mn vacancies of δ-MnO 2 for the catalytic oxidation of toluene. Due to the strong MSI between Ag and δ-MnO 2 , the monoatomic Ag + anchored on cation vacancies forms longer and weakened Ag–O bonds compared with Ag 2 O, which can extra promote the formation of oxygen vacancies and the activation of adjacent lattice oxygen in Ag–O–Mn units compared with regular nanoparticle counterparts. It is beneficial to the steps of methyl dehydrogenation, demethylation, and benzene ring cleavage. The catalyst yields excellent toluene oxidation activity, enhanced turnover frequency in terms of Ag, and good stability. Finally, we propose in this study a facile design strategy for fabricating stably atomically dispersed metal sites, as well as clarify the structure–activity relationships during toluene catalytic oxidation, especially the important effect of single-atom Ag on the adjacent local structure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Quantitative Cu Counting Methodologies for Cu/SSZ-13 Selective Catalytic Reduction Catalysts by Electron Paramagnetic Resonance Spectroscopy

Here, two Cu/SSZ-13 selective catalytic reduction (SCR) catalysts with distinct Si/Al ratios and isolated Z2Cu and ZCuOH distributions are prepared for in situ electron paramagnetic resonance (EPR) spectroscopic studies. These in situ studies include dehydration, titration of dehydrated samples with NO+O 2 and NH 3 , titration of NH 3 saturated samples with NO+O 2 , and finally steady-state standard NH 3 -SCR reaction. During dehydration, EPR active hydrated ZCuOH loses H 2 O ligands and becomes EPR silent due to a pseudo Jahn-Teller effect; a portion of ZCuOH also undergoes autoreduction to ZCu(I) species, a process that also induces EPR invisibility. During NO+O 2 treatment of dehydrated samples, ZCu(I) species are oxidized to Cu(II)-NO 3 - species, regaining EPR visibility. During NH3 titration, EPR silent dehydrated ZCuOH can also regain EPR visibility by coordinating with NH 3 ligands. During NO+O 2 titration of NH 3 -saturated samples, EPR active Cu contents first decrease due to Cu(II) reduction to Cu(I), and then increase due to Cu(II)-NO 3 - species formation. However, the Cu(II)-NO 3 - formation chemistry is substantially slower for the Si/Al = 36 catalyst. In steady-state SCR studies, the EPR active content decreases with increasing temperature in the kinetically controlled low-temperature regime and becomes largely invariant in mass-transfer limited regime. Importantly, Cu sites in the SCR more active Si/Al = 6 catalyst display substantially higher EPR visibility than the SCR less active Si/Al = 36 catalyst at any reaction temperatures tested. The higher Cu loading for the former catalyst is believed to be key for this difference.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2020

Probing Active-Site Relocation in Cu/SSZ-13 SCR Catalysts during Hydrothermal Aging by in situ EPR Spectroscopy, Kinetic Studies, and DFT Calculations

Cu/SSZ-13 selective catalytic reduction catalysts display activity loss after mild hydrothermal aging without support degradation or loss of isolated Cu ion content. By applying electron paramagnetic resonance spectroscopy to compare hydrated, dehydrated and NH3-saturated catalysts before and after hydrothermal aging, as well as density functional theory calculations, it is found that hydrothermal aging induces Cu relocation, leading to stronger Cu-support interactions. This is particularly evidenced by ZCuOH conversion to Z2Cu, which leads to the generation of a unique structure with two Cu(II) ions positioned in a double 6-membered ring prism. The Cu(II) ions that do not undergo such dramatic changes also appear to stay closer to framework windows as evidenced by the overall increased anisotropy during EPR measurements. In situ EPR measurements under SCR conditions demonstrate that, in comparison to the fresh catalyst, higher percentages of Cu species stay as Cu(II) in the aged catalyst. This suggests that the reduction half-cycle of the SCR redox, i.e., Cu(II) ? Cu(I), becomes slower for the hydrothermally aged catalyst.

42 ENGINEERING↗