On the nature of different Fe sites on Fe-containing micro and mesoporous materials and their catalytic role in the abatement of nitrogen oxides from exhaust gases

Gegenstand der Untersuchungen war die Reduktion von Stickoxiden (NOx und N2O) an verschiedenartig präparierten Eisenkatalysatoren (Fe-MF, Fe-beta, Fe-SBA-15). Die Katalysatoren wurden nach Synthese, Kalzinierung und Katalyse mittels EPR und UV/VIS-DRS charakterisiert, und darüber hinaus auch in-situ während des Kalzinierens. Isolierte Eisenspezies aggregieren im Verlauf der Kalzinierung bei 873 K. Sowohl höhere Heizraten beim Kalziniervorgang, als auch ein höheres Si/Al-Verhältnis des Trägermaterials verstärken die Neigung zur Aggregatbildung leicht. Die Verwendung des Katalysators für die SCR von NO führt zu weiterem Wachstum und zur Restrukturierung der FexOy-Cluster. Die Eisenkatalysatoren wurden weiterhin mittels in-situ Methoden (EPR, UV/VIS-DRS, FTIR) untersucht während der SCR von NO durch NH3 und Isobutan, der SCR von N2O mit CO, und im Strom der entsprechenden reinen Eduktgase. Die Ergebnisse korrelieren mit dem katalytischen Verhalten der Materialien. Verschiedene Fe3+-Spezies, welche sich durch ein unterschiedliches Redoxverhalten auszeichnen, wurden identifiziert. UV/VIS-Messungen erlauben die Schlußfolgerung, daß isolierte, oktaedrisch koordinierte Fe3+?Spezies leichter zu reduzieren sind als tetradrisch koordinierte. Im Gegensatz zu isoliertem Fe3+ lassen sich FexOx-Cluster leichter oxidieren als reduzieren, und verbleiben daher unter Reaktionsbedingungen trivalent. Durch ihr hohes Oxidationspotential kommt es, vor allem für die Reaktion mit Isobutan, zur unerwünschten Totaloxidation des Reduktanden. Der Anteil isolierter Fe3+ Spezies korreliert mit der Aktivität der Katalysatoren für die SCR von NO und N2O. Weiterhin hängt zumindest für die Reaktion zwischen N2O und CO der Reaktionsmechanismus von der Art der vorliegenden Eisenspezies ab: an isolierten Plätzen erfolgt die Reduktion des N2O an dem an Fe3+ gebundenen CO. An FexOy-Clustern hingegen läuft die Reaktion als Redoxprozeß (Fe3+ / Fe2+) unter Bildung eines radikalischen Intermediates O-. Der Einfluß der Porengeometrie des Trägermaterials auf die katalytische Aktivität wurde an Katalysatoren mit ähnlichem Eisengehalt und ähnlicher Art und Verteilung der Eisenspezies studiert (Fe-MFI, Fe-SBA-15). Die drastisch höhere Aktivität von Fe-MFI belegt, daß die Lokalisierung der aktiven Komponente in einer Pore mit passender Geometrie (Größe und Struktur) essentiell für die katalytischen Eigenschaften ist. Als weitere, die Aktivität stark beeinflussen Größe, wurde für die Reaktion von NO mit Ammoniak und auch mit Isobutan die Azidität identifiziert, die jedoch für die katalytische Zersetzung oder Reduktion mit N2O keine Rolle spielt.The reduction of nitrogen oxides (NOx and N2O) was investigated over Fe-catalysts (Fe-MFI, Fe-beta and Fe-SBA-15) which were prepared by different methods have been analyzed by EPR and UV/VIS-DRS ex situ after synthesis, calcination and use in catalysis as well as in situ during calcination. It was found that aggregated species are formed at the expense of isolated Fe species upon calcination at 873 K, and that aggregate formation is slightly favored by calcination with higher heating rates as well as by high Si/Al ratio of the support. Use in SCR of NO leads to further growth and restructuring of FexOy clusters. These Fe-catalysts were studied by in situ EPR, in situ UV/VIS-DRS and in situ FT-IR spectroscopy during SCR of NO with NH3 or isobutane and during SCR of N2O with CO as well as during interaction with single feed components. The results were related to the catalytic behaviour. Different types of isolated Fe3+ sites with different reducibility were identified. Based on FT-IR results which revealed that NO reacts preferably with trivalent Fe, it is concluded that Fe3+ ions reduced under reaction conditions to Fe2+ do probably not contribute to catalytic activity. In general, the degree of steady-state Fe site reduction during NH3-SCR is markedly lower than during isobutane-SCR. This might be the reason for the lower activity of Fe-catalysts in the latter reaction. UV/VIS-DRS results suggest that isolated Fe3+ in octahedral coordination is easier reduced than tetrahedral Fe3+. In contrast to isolated Fe3+ species, FexOy clusters are much faster reoxidized than reduced and, thus, remain essentially trivalent under reaction conditions. Due to their higher oxidation potential, they cause undesired total oxidation of the reductant being much more severe in the case of isobutane. A correlation was found between the fraction of isolated Fe+3 sites in the catalysts and their activity for SCR of NO and N2O. The reaction mechanism of SCR of N2O with CO is Fe site dependent. Over isolated Fe sites, the reduction of N2O occurs via coordinated CO species on Fe3+ sites. The reaction over FexOy sites proceeds via a redox Fe3+/Fe2+ process with intermediate formation of O- radicals. The effect of pore geometry of the support on the catalytic activity was studied by comparing catalytic performance of Fe-MFI and Fe-SBA-15 which contain similar iron content and show similar nature and distribution of Fe species as evidenced by UV/VIS-DRS and EPR. Fe-MFI revealed to be much more active than Fe-SBA-15 in all reactions studied. This clearly illustrates that the confinement of the iron species in pores of suitable geometry (structure and size) is essential to originate their remarkable catalytic properties. Acidity is essential for SCR of NO with NH3 or isobutane but it is not mandatory for direct decomposition or SCR of N2O

Observations on the Aging Environment Dependent NO Oxidation Activity of Model Pt/Al2O3 Diesel Oxidation Catalyst

The influence of aging environment of model diesel oxidation catalyst Pt/Al2O3 on the NO oxidation activity is studied. The fresh catalyst Pt/Al/F (calcined in air at 500°C) is aged with or without phosphorus (P) poisoning (7.5wt%) at 800°C either in air (P/Pt/Al/O or Pt/Al/O) or in simulated diesel exhaust (P/Pt/Al/R or Pt/Al/R). Catalyst aged under diesel exhaust environment (Pt/Al/R) surprisingly presents the best NO oxidation activity under excess of O2 followed by the fresh (Pt/Al/F) and thermally aged (Pt/Al/O) catalysts. The activity difference between the catalysts is quite large, especially between Pt/Al/R and Pt/Al/O that are aged at the same temperatures but under different environments suggesting the importance of the aging environment for the catalytic activity. The NO oxidation activity of P poisoned catalysts P/Pt/Al/R and P/Pt/Al/O is minute as compared to their P free counter parts indicating that chemical aging is more detrimental for catalytic efficiency than thermal agin

PdO x /Pd at Work in a Model Three-Way Catalyst for Methane Abatement Monitored by Operando XANES

The oxidation state of palladium in a model Pd/ACZ three-way catalyst was monitored by synchronous XANES and mass spectrometry during two consecutive heating (to 850°C) and cooling (to 100°C) cycles under stoichiometric conditions simulating exhaust after treatment of a natural gas engine. During heating in the first cycle, PdO reduction occurred around 500°C and the initial fully oxidized state of Pd was never recovered upon heating and cooling cycles. A mixed Pd2+/Pd oxidation state was at work in the second cycle. Hence, the operando XANES study reveals that the PdO x /Pd pair exists in a working catalyst but is less active than the catalyst in its initial state of fully oxidized palladium. It is also evident from XANES spectra that ceria-zirconia promotes re-oxidation of metallic Pd, thus reasonably sustaining catalytic activity after exposure to high temperature

Room temperature methoxylation in zeolite H-ZSM-5 : an operando DRIFTS/mass spectrometric study

The UK Catalysis Hub is thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC (grants EP/I038748/1, EP/I019693/1, EP/ K014706/1, EP/K014668/1, EP/K014854/1, EP/K014714/1 and EP/ M013219/1). Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service (www.archer.ac.uk). Johnson Matthey plc is thanked for the provision of the ZSM5. Dr A. J. O’Malley and Dr S. F. Parker are thanked for fruitful discussion.Peer reviewedPublisher PD

Lab Scale Fixed-Bed Reactor for Operando X-Ray Absorption Spectroscopy for Structure Activity Studies of Supported Metal Oxide Catalysts

Lab scale fixed-bed reactor is applied for operando transmission X-ray absorption spectroscopy (XAS) for structure-activity studies of supported metal oxide catalysts under real reaction conditions. This setup includes many properties of an optimal fixed-bed reactor for operando transmission XAS studies. For instance, it is usable in a wide range of temperature (up to 1,000°C), pressure and space velocity. Besides, this operando setup can be used for transmission XAS measurements in a wide edge energy range. The potential of this reactor for operando transmission XAS is demonstrated by, as examples, the three-way catalytic performance of Pd/Al2O3/CeZrO2 and Rh/Al2O

Methanol loading dependent methoxylation in zeolite H-ZSM-5

We evaluate the effect of the number of methanol molecules per acidic site of H-ZSM-5 on the methoxylation reaction at room temperature by applying operando diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS), which capture the methoxylation reaction by simultaneously probing surface adsorbed species and reaction products, respectively. To this end, the methanol loading in H-ZSM-5 (Si/Al ≈ 25) pores is systematically varied between 32, 16, 8 and 4 molecules per unit cell, which corresponds to 8, 4, 2 and 1 molecules per Brønsted acidic site, respectively. The operando DRIFTS/MS data show that the room temperature methoxylation depends on the methanol loading: the higher the methanol loading, the faster the methoxylation. Accordingly, the reaction is more than an order of magnitude faster with 8 methanol molecules per Brønsted acidic site than that with 2 molecules, as evident from the evolution of the methyl rock band of the methoxy species and of water as a function of time. Significantly, no methoxylation is observed with ≤1 molecule per Brønsted acidic site. However, hydrogen bonded methanol occurs across all loadings studied, but the structure of hydrogen bonded methanol also depends on the loading. Methanol loading of ≤1 molecule per acidic site leads to the formation of hydrogen bonded methanol with no proton transfer (i.e. neutral geometry), while loading ≥2 molecules per acidic site results in a hydrogen bonded methanol with a net positive charge on the adduct (protonated geometry). The infrared vibrational frequencies of methoxy and hydrogen bonded methanol are corroborated by Density Functional Theory (DFT) calculations. Both the experiments and calculations reflect the methoxy bands at around 940, 1180, 2868–2876 and 2980–2973 cm−1 which correspond to ν(C–O), ρ(CH3), νs(C–H) and νas(C–H), respectively

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

Methanol diffusion and dynamics in zeolite H-ZSM-5 probed by quasi-elastic neutron scattering and classical molecular dynamics simulations

Zeolite ZSM-5 is a key catalyst in commercially relevant processes including the widely studied methanol to hydrocarbon reaction, and molecular diffusion in zeolite pores is known to be a crucial factor in controlling catalytic reactions. Here, we present critical analyses of recent quasi-elastic neutron scattering (QENS) data and complementary molecular dynamics (MD) simulations. The QENS experiments show that the nature of methanol diffusion dynamics in ZSM-5 pores is dependent both on the Si/Al ratio (11, 25, 36, 40 and 140), which determines the Brønsted acid site density of the zeolite, and that the nature of the type of motion observed may vary qualitatively over a relatively small temperature range. At 373 K, on increasing the ratio from 11 to 140, the observed mobile methanol fraction increases and the nature of methanol dynamics changes from rotational (in ZSM-5 with Si/Al of 11) to translational diffusion. The latter is either confined localized diffusion within a pore in zeolites with ratios up to 40 or non-localized, longer-range diffusion in zeolite samples with the ratio of 140. The complementary MD simulations conducted over long time scales (1 ns), which are longer than those measured in the present study by QENS (≈1–440 ps), at 373 K predict the occurrence of long-range translational diffusion of methanol in ZSM-5, independent of the Si/Al ratios (15, 47, 95, 191 and siliceous MFI). The rate of diffusion increases slightly by increasing the ratio from 15 to 95 and thereafter does not depend on zeolite composition. Discrepancies in the observed mobile methanol fraction between the MD simulations (100% methanol mobility in ZSM-5 pores across all Si/Al ratios) and QENS experiments (for example, ≈80% immobile methanol in ZSM-5 with Si/Al of 11) are attributed to the differences in time resolutions of the techniques. This perspective provides comprehensive information on the effect of acid site density on methanol dynamics in ZSM-5 pores and highlights the complementarity of QENS and MD, and their advantages and limitations. This article is part of the theme issue ‘Exploring the length scales, timescales and chemistry of challenging materials (Part 2)’

Effects of crystal size on methanol to hydrocarbon conversion over single crystals of ZSM-5 studied by synchrotron infrared microspectroscopy

IBM and PAW would like to thank the EPSRC and CRITICAT Centre for Doctoral Training for Financial Support [PhD studentship to IBM, and supplementary equipment grant EP/L016419/1]. The UK Catalysis Hub is thanked for resources and support provided via membership of the UK Catalysis Hub Consortium and funded by EPSRC (grants EP/I038748/1, EP/I019693/1, EP/K014706/1, EP/K014668/1, EP/K014854/1, EP/K014714/1 and EP/M013219/1). We thank the Diamond Light Source for provision of beam time and support facilities at the MIRIAM beamline B22 (Experiments SM13725-1, SM16257-1, SM18680-1, SM20906-1). IBM and PAW thank EPSRC and CRTICAT Centre for Doctoral Training for a PhD Studentship (grant EP/IO17008/1) and Supplementary Equipment Grant (EP/L016419/1). We thank Pit Losch and Hans J. Bongard, Max-Planck-Institut fur Kohlenforschung for cross-sectional SEM-EDX analysis, Daniel M. Dawson, University of St Andrews, for solid state NMR, and Juan M.Gonzalez-Carballo, University of St Andrews, for assistance with ammonia TPD. The research data supporting this publication can be accessed at https://doi.org/10.17630/306bd3c3-014b-466f-9538-b107628c847d.Peer reviewedPostprin