Insight into the Synergic Effect of Fe-SSZ-13 Zeolite and FeMnTiZrO_<i>x</i> Catalyst with Enhanced Reactivity in NH₃–SCR of NO_<i>x</i>

The relatively low activity at low temperatures and poor performance at high temperatures with low N2 selectivity are the major problems of Fe-SSZ-13 zeolite and Mn-based metal oxides for NH3 selective catalytic reduction (SCR) of NOx, respectively. Hybrid catalysts have been prepared by mechanically mixing Fe(1.6)-SSZ-13 zeolite and Fe0.2Mn0.4TiZr0.03Ox to remedy the defects of both catalysts. The optimized mass ratio of zeolite and metal oxide components in hybrid catalysts is 7:1, which shows superior NH3 SCR performances with NO conversion above 80% and N2O yield below 12 ppm at wide-temperature window ranging from 200 to 550 °C. NO-TPD and in situ DRIFTS studies indicate that the metal oxide component in hybrid catalyst can provide more active nitrite/nitrate species as well as more active sites for NH3 activation to promote low-temperature activity. The migration of active intermediate from NO on the metal oxide component to react with NH3 adsorbed on zeolite component may be responsible for the enhancement of low-temperature activity. Strong adsorption of NH3 on zeolite component during SCR reaction can hinder NH3 oxidation, so high NO conversions and N2 selectivity at high temperatures are maintained for hybrid catalyst. These insightful understandings on the synergic effect may be beneficial to rational design of high-performance hybrid catalysts in NH3-SCR reaction

<i>In Situ</i> High Temperature High Pressure MAS NMR Study on the Crystallization of AlPO₄‑5

A damped oscillating crystallization process of AlPO₄-5 at the presence of small amount of water is demonstrated by <i>in situ</i> high temperature high pressure multinuclear MAS NMR. Crystalline AlPO₄-5 is formed from an intermediate semicrystalline phase via continuous rearrangement of the local structure of amorphous precursor gel. Activated water catalyzes the rearrangement via repeatedly hydrolysis and condensation reaction. Strong interactions between organic template and inorganic species facilitate the ordered rearrangement. During the crystallization process, excess water, phosphate, and aluminums are expelled from the precursor. The oscillating crystallization reflects mass transportation between the solid and liquid phase during the crystallization process. This crystallization process is also applicable to AlPO₄-5 crystallized in the presence of a relatively large amount of water

Impact of Adsorption Configurations on Alcohol Dehydration over Alumina Catalysts

The dehydration of alcohols to value-added chemicals using alumina catalysts has been industrialized decades ago; however, despite years of fundamental research, the molecular-level understanding of the reaction process is still lacking. Herein, the dehydration reactions of ethanol on two representative aluminas, i.e., γ-Al2O3 and five-coordinated Al-rich Al2O3-nanosheet (Al2O3-NS), are comparatively investigated by a combination of solid-state NMR spectroscopy and reaction evaluations. NMR results reveal that the presence of hydroxyl groups nearby the Lewis acid sites (LASs) leads to different ethanol adsorption modes. The surface hydroxyls nearby LASs interfere with ethanol adsorption, which causes the methyl group to move away from the alumina surface and hinders the elimination of β-H, resulting in low reaction activity. In addition, the desorption property of surface hydroxyls will affect the dehydration reaction as they participate in the catalytic reaction cycles. Such effects may also explain the lower reaction activities for diethyl ether to ethylene compared with ethanol. Although both four- and five-coordinated Als are the possible active sites for ethanol dehydration, the reactivity of alcohol dehydration may also be substantially affected by the local environment of undercoordinated Als besides coordination numbers. These new insights demonstrate that the host–guest interaction regulated by the local environment of active sites plays an important role in the catalytic reaction

Untangling Framework Confinements: A Dynamical Study on Bulky Aromatic Molecules in MFI Zeolites

In MFI zeolites, differentiating the molecular dynamics as a function of pore structures, that is, straight-channel versus zigzag-channel versus channel-intersection, has always been challenging, mainly due to small differences in pore size but is of great interest because these subtle structural differences can remarkably influence shape selectivity. Herein, 1,2,4-trimethyl benzene (1,2,4-TMB), a characteristic molecule larger than the 10-MR channel diameter, while smaller than the channel intersection, is chosen to probe the pore-confined dynamical behaviors in MFI via 2H NMR spectroscopy and density functional theory calculations. Our results show that in the absence of acid sites, that is, in the siliceous MFI silicalite-1, 1,2,4-TMBs can only diffuse along the straight channels; while at equilibrium, they incline to occupy the channel intersections with structure-defined orientations. Furthermore, a series of dynamic motions of 1,2,4-TMBs under different types of pore confinements are revealed and evaluated at a molecular level over a wide range of timescales, concluded, in short, as methyl C3-rotation > 112°-flip > 90°-flip > translational diffusion. With the presence of acid sites, that is, in H-ZSM-5; however, 1,2,4-TMBs are strongly adsorbed on Brønsted acid sites and the confined motions are further impeded. The findings in this work may provide insights to the catalytic roles of polymethyl-benzene intermediates, including 1,2,4-TMB, which usually serve as active centers or deactivation precursors in zeolite-based hydrocarbon conversion processes

Dynamic Structural Changes of SiO₂ Supported Pt–Ni Bimetallic Catalysts over Redox Treatments Revealed by NMR and EPR

SiO₂ supported Pt–Ni bimetallic catalysts with different nickel loadings were prepared and their structural changes after redox treatments were studied by XRD, NMR, and EPR. It is found that the paramagnetic Ni species are mainly located on the surface of silica lattice. The relaxation of detected ²⁹Si nuclei in our samples is mainly governed by a spin-diffusion mechanism. The paramagnetic effects are reflected in the spin–lattice relaxation of Q⁴ species, with the oxidized samples presenting faster relaxation rates than the corresponding reduced ones. Meanwhile the Q³ species, which are in close contact with the paramagnetic nickel centers, are “spectrally invisible”. In reducing atmosphere Ni gradually diffuses into Pt NPs to form PtNi alloys. While under oxidization treatment, the alloyed Ni atoms migrate outward from the core of Pt NPs and are oxidized. The main EPR spectrum results from reduced nickel species, and the reduced samples show stronger EPR signal than the corresponding oxidized ones. However, in the reduced samples, the superparamagnetic or ferromagnetic metallic Ni particles were inside the PtNi NPs, making their influence on the ²⁹Si relaxation in the SiO₂ support weaker than the oxidized samples

Nature of Five-Coordinated Al in γ‑Al₂O₃ Revealed by Ultra-High-Field Solid-State NMR

Five-coordinated Als (Al­(V)) on the surface of aluminas play important roles when they are used as catalysts or catalyst supports. However, the comprehensive characterization and understanding of the intrinsic structural properties of the Al­(V) remain a challenge, due to the very small amount in commonly used aluminas. Herein, the surface structures of γ-Al2O3 and Al­(V)-rich Al2O3 nanosheets (Al2O3–NS) have been investigated and compared in detail by multinuclear high-field solid-state NMR. Thanks to the high resolution and sensitivity of ultra-high-field (up to 35.2 T) NMR, the arrangements of surface Als were clearly demonstrated, which are substantially different from the bulk phase in γ-Al2O3 due to the structure reconstruction. It reveals for the first time that most of the commonly observed Al­(V)­s tend to exist as aggregated states on the surface of γ-Al2O3, like those in amorphous Al2O3–NS liable to structure reconstruction. Our new insights into surface Al­(V) species may help in understanding the structure–function relationship of alumina

Pt/Fe-TiO₂‑Catalyzed Selective Carbonyl Hydrogenation: Fe-Promoted Hydrogen Spillover

PtFe catalysts have demonstrated their unique properties in the hydrogenation of carbonyl compounds, but the promotion effect of Fe is still under debate. Herein, a series of Pt/Fe-TiO2 catalysts with controllable Fe dispersion and similar Pt particle size were prepared as model catalysts to elucidate the role of Fe in the selective hydrogenation of furfural to furfuryl alcohol. The catalytic data suggest that the states of Fe significantly affect the catalytic performance of Pt/Fe-TiO2 and Pt/Fe-TiO2 with dispersed Fe oligomers is ca. 20-fold more active than Pt/TiO2. The combined results of reaction kinetic analysis and characterizations suggest that the high activity of Pt/Fe-TiO2 in furfural hydrogenation is mainly attributed to the promotion effect of Fe in hydrogen spillover, a phenomenon seldom reported for PtFe catalysts. The remote hydrogen spillover test by a physical mixture of Pt/TiO2 and Fe-TiO2 implies that the dispersed Fe oligomers on TiO2 are more favorable to enhance the hydrogen spillover than large Fe aggregates, which is possibly related with the high electron/charge conductivity of the former one. This explains the higher activity of Pt/Fe-TiO2 with dispersed Fe oligomers than with large Fe aggregates. The use of model catalysts is beneficial to elucidate the key factors affecting the catalytic performance of multicomponent catalysts and sheds light on the catalyst design

The Role of Organic and Inorganic Structure-Directing Agents in Selective Al Substitution of Zeolite

Organic and inorganic structure-directing agents (SDAs) impact Al distributions in zeolite, but the insights into how SDAs manipulate Al distribution have not been elucidated yet. Herein, the roles of different SDAs such as cyclohexylamine (CHA), hexamethylenimine (HMI), and Na+ in selective Al substitution of MCM-49 zeolite are investigated comprehensively by multinuclear solid-state NMR. The results demonstrate that MCM-49 synthesized with HMI shows relatively more T6 and T7 Al, while more T2 Al is observed using CHA. The formation of T2 Al in both MCM-49­(HMI) and MCM-49­(CHA) is derived from Na+, while protonated HMIs show bias in incorporation of T6 and T7 Al. Most HMIs are occluded in protonated status, and about half of CHAs are occluded in nonprotonated status. The close spatial proximity between nonprotonated CHAs and Na+ synergistically promotes the formation of zeolite structure, leading to more Na+ ions occluded in the zeolite channel with preferential T2 Al substitution

Mapping Al Distributions in SSZ-13 Zeolites from ²³Na Solid-State NMR Spectroscopy and DFT Calculations

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na⁺ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining ²³Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na⁺ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na⁺ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na⁺ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a portion of Na⁺ ions with high mobility among different SII sites may result in the SIII’b site. In Al-rich SSZ-13 zeolite (Si/Al = 4), the presence of Si­(2Al) and Si­(3Al) may lead to Na⁺ being located at 6-MR SIIa2, with ²³Na NMR signals exhibiting relatively large quadrupolar interactions, and at 8-MR SIII’a2, with NMR signals overlapped with that of the SIII’a1 site. This approach may expand its application in discrimination of Al distributions in other zeolites to gain deeper understanding of the relationship between the structure and the property for their applications in catalysis and adsorption

Direct Detection of Reactive Gallium-Hydride Species on the Ga₂O₃ Surface <i>via</i> Solid-State NMR Spectroscopy

Surface metal hydrides (M–H) are ubiquitous in heterogeneous catalytic reactions, while the detailed characterizations are frequently hindered by their high reactivity/low concentration, and the complicated surface structures of the host solids, especially in terms of practical solid catalysts. Herein, combining instant quenching capture and advanced solid-state NMR methodology, we report the first direct and unambiguous NMR evidence on the highly reactive surface gallium hydrides (Ga–H) over a practical Ga2O3 catalyst during direct H2 activation. The spectroscopic effects of 69Ga and 71Ga isotopes on the 1H NMR signal are clearly differentiated and clarified, allowing a concrete discrimination of the Ga–H signal from the hydroxyl crowd. Accompanied with quantitative and two-dimensional NMR spectroscopical methods, as well as density functional theory calculations, information on the site specification, structural configuration, and formation mechanism of the Ga–H species has been revealed, along with the H2 dissociation mechanism. More importantly, the successful spectroscopic identification and isolation of the surface Ga–H allow us to clearly reveal the critical but ubiquitous intermediate role of this species in catalytic reactions, such as propane dehydrogenation and CO2 hydrogenation reactions. The analytic approach presented in this work can be extended to other M–H analysis, and the insights will benefit the design of more efficient Ga-based catalysts