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Abstract: The mechanism of N2O decomposition was studied on a series of Fe-ZSM-5 catalysts prepared by ion exchange (IE) and chemical vapor deposition (CVD). N2O decomposition activity depends strongly on the extent of autoreduction of the catalyst during the pretreatment in He at high temperatures. The extent of autoreduction is significantly lower for catalysts prepared by CVD compared with that for catalysts prepared by aqueous ion exchange. As a result, the global turnover frequencies of the ion-exchanged catalysts are significantly higher. If the activity is normalized by the concentration of Fe2+ sites, however, then the CVD catalyst is the most active sample. This is attributed to the fact that high iron loading favors the surface migration and recombination of surface oxygen atoms to O2, which is the rate-limiting step of the reaction. O2 formation by slow recombination of two surface oxygen atoms is the dominating mechanism even for IE samples with very low iron loadings. [Copyright &y& Elsevier]

Abstract: Fe-FER is an active catalyst for the abatement of N2O in the tail gas of nitric acid plants. The activity of Fe-FER can be increased if Ru is added as a second active component. This is a surprising finding, because noble metals are usually strongly inhibited by NO, which is always present in tail gas. Yet the bimetallic FeRu-FER catalyst is more active than the sum of the components, Fe-FER and Ru-FER. A synergy between Fe and Ru can explain this phenomenon. This work discusses the role of Fe and Ru in the reaction mechanism as well as the interplay of these two components. In situ IR measurements show that the preferential adsorption of NO and its reaction products on Fe in the bimetallic catalyst reduces the inhibiting effect of NO on the Ru component; this effect largely contributes to the synergy between Fe and Ru. Moreover, in situ X-ray absorption data are presented, which allow for tracing the average oxidation state of the two active components Fe and Ru under reaction conditions. [Copyright &y& Elsevier]

Abstract: NO-assisted N2O decomposition over four different Fe-ZSM-5 samples prepared by wet ion exchange (WIE) or chemical vapor deposition (CVD) of FeCl3 was investigated by steady-state kinetics, in situ infrared (IR) spectroscopy, and transient response methods. Despite their lower iron loading, the samples prepared by WIE had the highest activity for N2O decomposition in the presence of NO. The in situ IR experiments showed that the most active sample was characterized by a high concentration of adsorbed NO, as well as adsorbed NO2 under reaction conditions. Step response experiments proved that NO2 is an intermediate of the catalytic cycle and functions as intermediate oxygen storage. IR and transient kinetic experiments showed that WIE catalysts behave qualitatively different in NO-assisted N2O decomposition than CVD catalysts. These differences are discussed in terms of the different structure of the iron species in the two types of samples. [Copyright &y& Elsevier]

The effect of pretreatment conditions on the N2O-decomposition activity of Fe-ZSM-5 prepared by chemical vapor deposition (CVD) was studied by steady-state kinetics and step-response experiments. Pretreatment at 873 K in H2 or He significantly increased the N2O-decomposition activity of Fe-ZSM-5 CVD. In addition to the increased steady-state activity, the catalyst exhibited a very high initial activity in the step-response experiments, which slowly decayed to steady state. This “transient” activity was particularly high after pretreatment of the samples in He, whereas it vanished completely after pretreatment in O2 or exposure to water vapor. This led to the conclusion that the transient activity is related to strongly dehydroxylated Fe2+ sites created by autoreduction at high temperatures. On the basis of isotope-labeling data a mechanism for N2O decomposition is proposed. The first step is the reaction of Fe2+ with N2O to give a Fe3+O- species, which quickly transforms into a Fe2+–peroxo complex. Desorption of O2 occurs upon reaction of the complex with another N2O molecule (mainly during the transient) or via the recombination of two peroxo species after their migration over the iron oxide cluster (mainly during steady state). [Copyright &y& Elsevier]

Iron-containing zeolites were prepared by aqueous ion exchange, sublimation of FeCl3, or hydrothermal synthesis and were modified by steaming at 873 K. The samples contained different kinds of iron species ranging from isolated iron atoms to small clusters that contain a few atoms and somewhat larger oligomers. Yet, the dynamic behavior of all samples in N2O decomposition was qualitatively similar. Pulse experiments showed that a slow and fast pathway of O2 formation, which is the rate-limiting step of the reaction, exists. Both contributed to the overall activity. The fast pathway dominated on isolated iron sites, whereas the contribution of the slow pathway increased on catalysts with high iron loadings. Steaming led to an agglomeration of the iron clusters and increased the activity (with one exception). The steamed samples had a peculiar time-on-stream behavior. They exhibited a very high initial activity and then slowly decayed to steady state. This indicates that steaming did not merely agglomerate the iron clusters, but qualitatively changed the nature of the active sites. [Copyright &y& Elsevier]

The nitration of toluene and 2-nitrotoluene was investigated using different batches of zeolite BEA. In the nitration of toluene, notable differences in the product composition were obtained depending on the batch of BEA used. In the nitration of 2-nitrotoluene, all the catalysts displayed a good selectivity, but different conversions. Steaming improved the performance of the catalysts, but strong dealumination with oxalic acid had a negative effect. The positive effect of steaming was attributed to the enhanced transport of the reaction products out of the pores. Lewis acidity did not seem to play a role in the nitration reaction. A high concentration of Brønsted sites was detrimental, because it slowed down the diffusion of the products, but a small concentration of Brønsted sites was required to catalyze the heterogeneous reaction. [Copyright &y& Elsevier]

The reaction of Fe-zeolites prepared by sublimation of FeCl3 with N2O was studied by step- and pulse-response experiments. Steps from 0 to 5000 ppm N2O were performed at 673 K, after pretreatment of the catalyst in H2, He, or O2. When the catalysts were prereduced in H2, N2O was first consumed for the reoxidation of the catalyst, before steady-state N2O decomposition started. Also pretreatment in He reduced the catalyst. This autoreduction was not spontaneous, but caused by a small amount of hydrocarbons adsorbed on the catalyst. Prereduced catalysts had a higher steady-state activity, indicating that the incorporation of oxygen from N2O changed the structure of the active sites. Pulse responses proved that O2 desorption is the rate-limiting step of N2O decomposition. O2 desorption was relatively fast from a small fraction of sites, while a second oxygen pool desorbed only slowly from the catalyst. Only the former is responsible for catalytic activity. [Copyright &y& Elsevier]

The nitration of toluene and 2-nitrotoluene with acetyl nitrate was investigated using several solid acid catalysts. In the nitration of toluene, enhanced para-selectivity was observed only with zeolite BEA. For the nitration of 2-nitrotoluene, all the catalysts displayed good para-selectivity, but only BEA led to high yields. The superior performance of BEA was due to the fact that only with this zeolite did the heterogeneously catalyzed reaction compete successfully with the homogeneous nitration in the liquid phase. Geometrical considerations showed that the high para-selectivity of BEA was not caused by classical transition-state selectivity but was induced by the reaction of the arene with a surface-bound acetyl nitrate complex. The existence of this complex was supported by NMR measurements, which showed that its formation is a specific property of zeolite BEA. [Copyright &y& Elsevier]

[Display omitted] • Co promotion do not alter metal-support interactions compared to non-promoted systems. • Co promotion is not surface-dependent but contingent to the formation of MoS 2. • Highest catalytic activity for metal-support interactions of intermediate strength. The current environmental and industrial contexts require constant revision and improvement of hydrotreating processes for obtaining cleaner (bio)fuels through the development of supported transition metal sulfide (TMS) catalysts. The present work proposes a surface-science approach to describe the surface-dependent genesis of the cobalt-promoted active phase (CoMoS) supported on α-Al 2 O 3 single crystals used as surrogates of the industrial γ-Al 2 O 3 support. Co-Mo precursor solutions are deposited by aqueous-phase impregnation onto four α-Al 2 O 3 single crystals of C(0001), A (11 2 ¯ 0) , M (10 1 ¯ 0) , and R (1 1 ¯ 02) orientations, which display specific OH sorption sites. The model catalysts are characterized in the oxide and sulfide states by XPS, AFM, and TEM revealing a surface dependent sulfidation and dispersion as a result of metal-support interactions of diverse nature. The surfaces can be ranked following metal-support interactions of increasing strength C(0001) < A (11 2 ¯ 0) , M (10 1 ¯ 0) < R (1 1 ¯ 02) in good agreement with model non-promoted catalysts. However, AFM and TEM imaging also reveal that cobalt incorporation leads to an increase in the stacking of CoMoS particles with respect to non-promoted catalysts. The activity of the model catalysts in thiophene hydrodesulfurization cannot be simply related to the common features of the active phase (sulfidation, dispersion, promotion extent) but a volcano-shape curve is obtained when plotting the activity as a function of metal-support interactions. These results suggest that high activity is obtained on surfaces with intermediate metal-support interactions that lead to an optimal metal-sulfur bond energy in agreement with the Sabatier principle. Transposing these results to the traditional γ-Al 2 O 3 support suggests an enhanced activity of the (1 0 0) facet, which can lead to the development of improved supports for hydrotreating catalysts. [ABSTRACT FROM AUTHOR]

The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the support-dependent genesis of MoS 2 on four planar single-crystal α-Al 2 O 3 surfaces with different crystal orientations (C (0 0 0 1), R ( 1 1 ¯ 0 2 ) , M ( 1 0 1 ¯ 0 ) , and A ( 1 1 2 ¯ 0 ) ). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, Mo is introduced by impregnation from an aqueous solution in order to mimic standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. Atomic force microscopy, X-ray absorption spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the α-Al 2 O 3 orientation has a clear impact on the strength of metal–support interactions in the oxide state with consequences for the sulfidation, size, stacking, and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0 0 0 1) plane, suggesting weak metal–support interactions leading to a high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R ( 1 1 ¯ 0 2 ) plane implies stronger metal–support interactions, leading to a low sulfidation degree and shorter MoS 2 slabs. Both A ( 1 1 2 ¯ 0 ) and M ( 1 0 1 ¯ 0 ) facets, of similar crystallographic structure, display intermediate behavior in terms of sulfidation rate and MoS 2 size, in line with intermediate metal–support interactions. Polarization-dependent grazing-incidence-EXAFS experiments, as well as high-angle annular dark-field (HAADF) scanning transmission electron microscopy analysis, allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. Predominant basal bonding is suggested on the C (0 0 0 1) plane, in agreement with the existence of weak metal–support interactions. Conversely, a random orientation (edge and basal bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed, based on comparison of the surface structure of the various model α-Al 2 O 3 orientations used in this work and the predominantly exposed γ-Al 2 O 3 surfaces ((1 1 0), (1 0 0), and (1 1 1)). [ABSTRACT FROM AUTHOR]