Comparing ammonia diffusion in NH3-SCR zeolite catalysts: a quasielastic neutron scattering and molecular dynamics simulation study

Temperature-Programmed Desorption of Single Zeolite Nanoparticles

Zeolites are essential solid acid catalysts in various chemical processes. Temperature-programmed desorption (TPD) is one of the most established techniques used to characterize the acidity of zeolites by measuring the desorption kinetics of probes from bulk samples. However, conventional TPD can hardly deliver the intrinsic acid properties of zeolites because the apparent desorption kinetics are inevitably mixed with mass transfer and thermal conduction due to the large sample amount (∼0.1 g). Herein, we developed an optical microscopy approach to measure the TPD spectra of single zeolite nanoparticles, termed oTPD, by in situ monitoring of the reduced scattering intensity of individuals as a result of the desorption of probe molecules during heating. A significantly reduced sample amount contributed to the oTPD spectrum, revealing an intrinsic desorption temperature of ∼300 °C lower than the apparent value and also a greatly narrowed peak width from ∼150 to ∼15 °C. Correlating oTPD and micro-Raman spectra of the very same individuals further uncovered a linear dependence between the acidity and the content of silicon islands. This study provided unprecedented capabilities for measuring the intrinsic acid properties and the desorption kinetics of single zeolite nanoparticles, with implications for better understanding the structure-acidity relationship and for designing better zeolite catalysts.

Neutron Scattering Studies of Heterogeneous Catalysis

Understanding the structural dynamics/evolution of catalysts and the related surface chemistry is essential for establishing structure-catalysis relationships, where spectroscopic and scattering tools play a crucial role. Among many such tools, neutron scattering, though less-known, has a unique power for investigating catalytic phenomena. Since neutrons interact with the nuclei of matter, the neutron-nucleon interaction provides unique information on light elements (mainly hydrogen), neighboring elements, and isotopes, which are complementary to X-ray and photon-based techniques. Neutron vibrational spectroscopy has been the most utilized neutron scattering approach for heterogeneous catalysis research by providing chemical information on surface/bulk species (mostly H-containing) and reaction chemistry. Neutron diffraction and quasielastic neutron scattering can also supply important information on catalyst structures and dynamics of surface species. Other neutron approaches, such as small angle neutron scattering and neutron imaging, have been much less used but still give distinctive catalytic information. This review provides a comprehensive overview of recent advances in neutron scattering investigations of heterogeneous catalysis, focusing on surface adsorbates, reaction mechanisms, and catalyst structural changes revealed by neutron spectroscopy, diffraction, quasielastic neutron scattering, and other neutron techniques. Perspectives are also provided on the challenges and future opportunities in neutron scattering studies of heterogeneous catalysis.

SHERPA: A Spectrometer with High Energy Resolution and Polarisation Analysis

SHERPA is a proposed quasielastic neutron spectrometer with polarisation analysis, intended to replace the ageing Iris instrument at the ISIS neutron and muon source. In this paper we present a concept of the instrument along with Monte-Carlo simulations and analysis of possible instrument location. We expect greatly increased count rate compared to Iris (expected from 49 to 660 × Iris) in unpolarised mode and dedicated polarisation analysis capabilities at a more modest count rate increase (~5-70 × Iris). This huge gain in the count rate would be achieved from the combination of three factors: modern neutron guide with high-m coating, and prismatic effect and larger solid angle coverage at the energy analyser. Such an instrument would be the first of its kind and has incredible potential to revolutionise quasielastic neutron scattering technique through the separation of the coherent and incoherent scattering contributions.

Local and Nanoscale Methanol Mobility in Different H-FER Catalysts

The dynamical behaviour of methanol confined in zeolite H-FER has been studied using quasielastic neutron scattering (QENS) and classical molecular dynamics (MD) simulations to investigate the effects of the Si/Al...

Methanol dynamics in H-ZSM-5 with Si/Al ratio of 25: a quasi-elastic neutron scattering (QENS) study

Methanol dynamics in zeolite H-ZSM-5 (Si/Al of 25) with a methanol loading of ~ 30 molecules per unit cell has been studied at 298, 323, 348 and 373 K by incoherent quasi-elastic neutron scattering (QENS). The elastic incoherent structure factor (EISF) reveals that the majority of methanol is immobile, in the range between 70 and 80%, depending on the measurement temperature. At 298 K, ≈ 20% methanol is mobile on the instrumental timescale, exhibiting isotropic rotational dynamics with a rotational diffusion coefficient (DR) of 4.75 × 1010 s−1. Upon increasing the measurement temperature from 298 to 323 K, the nature of the methanol dynamics changes from rotational to translational diffusion dynamics. Similar translational diffusion rates are measured at 348 and 373 K, though with a larger mobile fraction as temperature increases. The translational diffusion is characterised as jump diffusion confined to a sphere with a radius close to that of a ZSM-5 channel. The diffusion coefficients may be calculated using either the Volino–Dianoux (VD) model of diffusion confined to a sphere, or the Chudley–Elliot (CE) jump diffusion model. The VD model gives rise to a self-diffusion co-efficient (Ds) of methanol in the range of 7.8–8.4 × 10–10 m2 s−1. The CE model gives a Ds of around 1.2 (± 0.1) × 10–9 m2 s−1 with a jump distance of 2.8 (either + 0.15 or − 0.1) Å and a residence time (τ) of ~ 10.8 (either + 0.1 or − 0.2) ps. A correlation between the present and earlier studies that report methanol dynamics in H-ZSM-5 with Si/Al of 36 is made, suggesting that with increasing Si/Al ratio, the mobile fraction of methanol increases while DR decreases.

On the Redox Mechanism of Low-Temperature NH3 -SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

Cu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications. Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathways in SCR working conditions. Herein we address the redox mechanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques, transient-response methods, operando UV/Vis-NIR spectroscopy with modelling tools based on transient kinetic analysis and density functional theory calculations. We show that the rates of the Reduction Half-Cycle (RHC) of LT-SCR display a quadratic dependence on Cu^II , thus questioning mechanisms based on isolated Cu^II -ions. We propose, instead, a Cu^II -pair mediated LT-RHC pathway, in which NO oxidative activation to mobile nitrite-precursor intermediates accounts for Cu^II reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade.

Recent Understanding of Low-Temperature Copper Dynamics in Cu-Chabazite NH3-SCR Catalysts

Dynamic motion of NH3-solvated Cu sites in Cu-chabazite (Cu-CHA) zeolites, which are the most promising and state-of-the-art catalysts for ammonia-assisted selective reduction of NOx (NH3-SCR) in the aftertreatment of diesel exhausts, represents a unique phenomenon linking heterogeneous and homogeneous catalysis. This review first summarizes recent advances in the theoretical understanding of such low-temperature Cu dynamics. Specifically, evidence of both intra-cage and inter-cage Cu motions, given by ab initio molecular dynamics (AIMD) or metadynamics simulations, will be highlighted. Then, we will show how, among others, synchrotron-based X-ray spectroscopy, vibrational and optical spectroscopy (diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and diffuse reflection ultraviolet-visible spectroscopy (DRUVS)), electron paramagnetic spectroscopy (EPR), and impedance spectroscopy (IS) can be combined and complement each other to follow the evolution of coordinative environment and the local structure of Cu centers during low-temperature NH3-SCR reactions. Furthermore, the essential role of Cu dynamics in the tuning of low-temperature Cu redox, in the preparation of highly dispersed Cu-CHA catalysts by solid-state ion exchange method, and in the direct monitoring of NH3 storage and conversion will be presented. Based on the achieved mechanistic insights, we will discuss briefly the new perspectives in manipulating Cu dynamics to improve low-temperature NH3-SCR efficiency as well as in the understanding of other important reactions, such as selective methane-to-methanol oxidation and ethene dimerization, catalyzed by metal ion-exchanged zeolites.

Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study on the Molecular Behaviour of Catechol in Zeolite Beta

The dynamics of catechol in zeolite Beta was studied using quesielastic neutron scattering (QENS) experiments and molecular dynamics simulations at 393 K, to understand the behaviour of phenolic monomers relevant in the catalytic conversion of lignin via metal nanoparticles supported on zeolites. Compared to previous work studying phenol, both methods observe that the presence of the second OH group in catechol can hinder mobility significantly, as explained by stronger hydrogen-bonding interactions between catechol and the Brønsted sites of the zeolite. The instrumental timescale of the QENS experiment allows us to probe rotational motion, and the catechol motions are best fit to an isotropic rotation model with a $$D^{rot}$$ D rot of 2.9 × 10$$^{10}$$ 10 s$$^{-1}$$ - 1 . While this $$D^{rot}$$ D rot is within error of that measured for phenol, the fraction of molecules immobile on the instrumental timescale is found to be significantly higher for catechol. The MD simulations also exhibit this increased in ‘immobility’, showing that the long-range translational diffusion coefficients of catechol are lower than phenol by a factor of 7 in acidic zeolite Beta, and a factor of $$\sim$$ ∼ 3 in the siliceous material, further illustrating the significance of Brønsted site H-bonding. Upon reproducing QENS observables from our simulations to probe rotational motions, a combination of two isotropic rotations was found to fit the MD-calculated EISF; one corresponds to the free rotation of catechol in the pore system of the zeolite, while the second rotation is used to approximate a restricted and rapid “rattling”, consistent with molecules anchored to the acid sites through their OH groups, the motion of which is too rapid to be observed by experiment.

Octane isomer dynamics in H-ZSM-5 as a function of Si/Al ratio: a quasi-elastic neutron scattering study

Dynamical behaviour of n-octane and 2,5-dimethylhexane in H-ZSM-5 zeolite catalysts of differing Si/Al ratios (15 and 140) was probed using quasi-elastic neutron scattering, to understand molecular shape and Brønsted acid site density effects on the behaviour of common species in the fluid catalytic cracking (FCC) process, where H-ZSM-5 is an additive catalyst. Between 300 and 400 K, n-octane displayed uniaxial rotation around its long axis. However, the population of mobile molecules was larger in H-ZSM-5(140), suggesting that the lower acid site concentration allows for more molecules to undergo rotation. The rotational diffusion coefficients were higher in H-ZSM-5(140), reflecting this increase in freedom. 2,5-dimethylhexane showed qualitative differences in behaviour to n-octane, with no full molecule rotation, probably due to steric hindrance in the constrictive channels. However, methyl group rotation in the static 2,5-dimethylhexane molecules was observed, with lower mobile fractions in H-ZSM-5(15), suggesting that this rotation is less hindered when fewer Brønsted sites are present. This was further illustrated by the lower activation barrier calculated for methyl rotation in H-ZSM-5(140). We highlight the significant immobilizing effect of isomeric branching in this important industrial catalyst and show how compositional changes of the zeolite can affect a range of dynamical behaviours of common FCC species upon adsorption. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

SAPO-34 Framework Contraction on Adsorption of Ammonia: A Neutron Scattering Study

Neutron scattering data was recorded from SAPO-34 using the OSIRIS instrument before and after repeated ammonia adsorption at pressures up to 8 bar. Coherent scattering from the zeolite framework provides the neutron powder diffraction pattern and gave evidence for anisotropic contraction on ammonia dosing. Incoherent quasielastic scattering from the hydrogen of the ammonia showed that mobile ammonia was present in the framework. The quasielastic data was fitted to a model where the ammonia was confined within the chabazite cage in the c direction of the crystal lattice, with diffusion solely occurring through the perpendicular 8-membered rings. The calculated diffusion constant reached a maximum of 6.3×10^-8  m²  s^-1 at 5 bar.

Molecular behaviour of phenol in zeolite Beta catalysts as a function of acid site presence: a quasielastic neutron scattering and molecular dynamics simulation study

The dynamic behaviour of phenol in zeolite Beta is strongly influenced by the presence of Brønsted acid sites.