Search

Heterogeneous supported catalysts are often synthesized by impregnation or precipitation methods. Recently, melt infiltration has emerged as an alternative method that allows high metal loadings and eliminates the need for a solvent, but challenges arise regarding control over the particle size and distribution. In this work, melt infiltration for the synthesis of supported silver catalysts is explored. The narrow pore size distribution of the chosen ordered mesoporous silica support, SBA-15, allowed in depth in-situ and ex-situ characterization of the infiltration of the precursor, molten silver nitrate, into the support and its subsequent decomposition to form metallic silver nanowires or nanoparticles. The heat treatment parameters during decomposition played a key role in determining whether nanowires or nanoparticles were formed. The supported silver catalysts containing high silver weight loadings were investigated in the selective hydrogenation of cinnamaldehyde, where the silver nanowires showed superior activity and selectivity over the nanoparticles. Hence, melt infiltration shows great promise for the synthesis of supported silver catalysts containing high silver weight loadings, which are applicable in, e.g., selective oxidation or hydrogenation reactions.

Colloidal synthesis of metal nanocrystals (NC) offers control over size, crystal structure and shape of nanoparticles, making it a promising method to synthesize model catalysts to investigate structure-performance relationships. Here, we investigated the synthesis of disk-shaped Co-NC, their deposition on a support and performance in the Fischer–Tropsch (FT) synthesis under industrially relevant conditions. From the NC synthesis, either spheres only or a mixture of disk-shaped and spherical Co-NC was obtained. The disks had an average diameter of 15 nm, a thickness of 4 nm and consisted of hcp Co exposing (0001) on the base planes. The spheres were 11 nm on average and consisted of ε-Co. After mild oxidation, the CoO-NC were deposited on SiO2 with numerically 66% of the NC being disk-shaped. After reduction, the catalyst with spherical plus disk-shaped Co-NC had 50% lower intrinsic activity for FT synthesis (20 bar, 220 °C, H2/CO = 2 v/v) than the catalyst with spherical NC only, while C5+-selectivity was similar. Surprisingly, the Co-NC morphology was unchanged after catalysis. Using XPS it was established that nitrogen-containing ligands were largely removed and in situ XRD revealed that both catalysts consisted of 65% hcp Co and 21 or 32% fcc Co during FT. Furthermore, 3–5 nm polycrystalline domains were observed. Through exclusion of several phenomena, we tentatively conclude that the high fraction of (0001) facets in disk-shaped Co-NC decrease FT activity and, although very challenging to pursue, that metal nanoparticle shape effects can be studied at industrially relevant conditions.

The effect of oxidation of a carbon nanotube (CNT) support on the structure and catalytic performance in the Fischer-Tropsch (FT) synthesis of pre-synthesized cobalt nanocrystals (NC) has been investigated. 6 nm CoO-NC were prepared using colloidal techniques and impregnated on pristine and surface-oxidized CNT. The CoO-NC on pristine CNT were reduced completely to Co, while ∼50 % CoO reduction was observed on oxidized CNT (oxCNT), indicating a stronger Co-support interaction on oxCNT. In FT, the turnover frequency (TOF) after 100 h was exceptionally high on pristine CNT (150·10−3 s-1) against 63·10−3 s-1 on oxCNT. In the initial stage of FT, the TOF was 35 % lower on oxCNT than on pristine CNT, which matched with 32 % lower crystallinity of cobalt determined using in situ XRD. Furthermore, deactivation was observed mainly on oxCNT. These results demonstrate the large impact of the support on the structure and performance of nanocrystal-based catalysts.

Stability of metal nanoparticles under reaction conditions is crucial in many catalytic processes. Nickel-based catalysts often encounter severe particle growth in the presence of carbon monoxide due to the formation and migration of nickel carbonyl. In this research, we showed that the reduction temperature of nickel oxide supported on niobia (Nb2O5) influenced the stability of the resulting nickel catalyst during subsequent carbon monoxide hydrogenation. Low reduction temperatures resulted in high initial nickel-normalized activity towards long-chain hydrocarbons (C5+), but fast deactivation throughout the experiment. High reduction temperatures led to a shift in product distribution towards shorter hydrocarbons and a decreased initial nickel-normalized activity, while during the first hours of the experiment an increase in turnover frequency and nickel-normalized activity was observed, resulting eventually in a stable catalytic performance. Electron microscopy analysis revealed extensive particle growth after catalysis when the catalyst had been reduced at low temperatures and no significant changes in particle size when reduced at high temperatures. By use of in-situ FT-IR spectroscopy, nickel subcarbonyl species which are precursors of volatile nickel tetracarbonyl were detected on Ni/Nb2O5 after low temperature reduction and exposure to CO, but not after high temperature reduction. Hence, particle growth is explained by the formation and diffusion of nickel carbonyl and subsequent Ostwald ripening, that leads to larger nickel particles with concomitant decrease in nickel-normalized activity. The stability of the catalyst reduced at high temperature was linked to the formation of niobium suboxides and their partial coverage of the nickel particles limiting the formation of nickel carbonyl and slowing down particle growth.

Using colloidal iron oxide nanoparticles with organic ligands, anchored in a separate step from the supports, has been shown to be beneficial to obtain homogeneously distributed metal particles with a narrow size distribution. Literature indicates that promoting these particles with sodium and sulfur creates an active Fischer-Tropsch catalyst to produce olefins, while further adding an H-ZSM-5 zeolite is an effective way to obtain aromatics. This research focused on the promotion of iron oxide colloids with sodium and sulfur using an inorganic ligand exchange followed by the attachment to H-ZSM-5 zeolite crystals. The catalyst referred to as FeP/Z, which consists of iron particles with inorganic ligands attached to a H-ZSM-5 catalyst, was compared to an unpromoted Fe/Z catalyst and an Fe/Z-P catalyst, containing the colloidal nanoparticles with organic ligands, promoted after attachment. A low CO conversion was observed on both FeP/Z and Fe/Z-P, originating from an overpromotion effect for both catalysts. However, when both promoted catalysts were washed (FeP/Z-W and Fe/Z-P-W) to remove the excess of promoters, the activity was much higher. Fe/Z-P-W simultaneously achieved low selectivity toward methane as part of the promoters were still present after washing, whereas for FeP/Z-W the majority of promoters was removed upon washing, which increased the methane selectivity. Moreover, due to the addition of Na+S promoters, the iron nanoparticles in the FeP/Z(-W) catalysts had grown considerably during catalysis, while those in Fe/Z-P(-W) and Fe/Z(-W) remained relatively stable. Lastly, as a large broadening of particle sizes for the used FeP/Z-W was found, where particle sizes had both increased and decreased, Ostwald ripening is suggested for particle growth accelerated by the presence of the promoters.

The effect of oxidative treatments on the depostion of cobalt nanocrystals (Co-NC) onto a support and subsequent ligand removal was investigated. Deposition of ϵ-cobalt NC led to extensive clustering of NC and low Fischer-Tropsch synthesis activity. Low-temperature oxidation of ϵ-cobalt NC resulted in a very uniform CoO-NC distribution and high activity whereas high-temperature oxidation to Co3O4 led to less uniform NC distributions and lower activity.

The production of dimethyl ether (DME) as compared to methanol from synthesis gas allows for increased conversions in a single pass. A challenge in this process is the combination of methanol synthesis and dehydration functionalities in a single catalyst without mutual negative interference of their performances. Here, we studied the use of hydrated niobium pentoxide (Nb2O5·nH2O) and niobium phosphate (NbOPO4) in combination with a copper-based methanol synthesis catalyst in the direct synthesis of DME, while gamma-alumina (γ-Al2O3) was used as a reference material. The three solid acids combined with the copper-based catalyst proved active and selective in the production of DME, however all of them showed some degree of deactivation throughout the reaction. Characterization of the used catalysts pointed out that while the γ-Al2O3-based mixture deactivated most likely due to coke deposition on the alumina and structural changes in the methanol synthesis catalysts, the Nb2O5·nH2O and NbOPO4-based catalysts lost activity probably as a result of copper migration from the methanol synthesis catalyst to the acid component. In view of the high volume-based activity of the Nb-based solid acids it is concluded that these are promising components for the direct catalytic conversion of synthesis gas to DME.

Structure and catalytic performance of niobia-supported cobalt catalysts were studied based on crystal phase, porosity and cobalt loading. Crystalline niobia as support proved to be a prerequisite to obtain highly active and selective Co/niobia Fischer–Tropsch catalysts, whereas amorphous niobia showed minimal activity. Crystallization changed the porous morphology of Nb 2 O 5 · n H 2 O resulting in a dense material with low specific pore volume and specific surface area. Multiple impregnations on crystalline Nb 2 O 5 were necessary to achieve cobalt loadings higher than 6 wt.%; this led to larger cobalt particles, diminished interaction of cobalt with niobia and therefore decreased activity per unit weight of cobalt and C 5+ selectivity. Carbon deposition via sucrose pyrolysis was employed in order to partly maintain the porosity during crystallization. The obtained porous crystalline niobia was used as support for cobalt catalysts with higher metal loadings. STEM-EDX mapping characterization of the catalysts provided unique information for this kind of materials, e.g. cobalt distribution and particle size. The catalysts showed high cobalt-normalized catalytic activity and C 5+ selectivity for the Fischer–Tropsch synthesis under industrially relevant conditions. Moreover, higher cobalt loadings led to an increased catalyst-weight normalized catalytic activity.

In the epoxidation of ethylene, catalyzed by supported Ag particles, the support plays not only a beneficial role, but can also negatively impact the selectivity to ethylene oxide. Especially high surface area supports, and supports containing acid groups, seem detrimental for the selectivity, which is attributed to secondary reactions on the support surface. We prepared Ag nanoparticles on supports with a wide range of specific surface areas and different chemical nature, but with similar Ag metal loading and particle size. Indeed, a strong dependence of selectivity on both the specific surface area, and the chemical nature of the oxide support was found, with α-Al 2 O 3 giving much more selective catalysts than SiO 2 . Furthermore, the conversion was an important factor in determining the selectivity, while on the other hand the support had no influence at all on the ethylene conversion. We described our experimental selectivities with a reaction model that used the rate constant for the secondary reaction of oxidation of ethylene oxide to carbon dioxide and water as the only variable parameter. The model was able to adequately describe the experimental results. It gave insight into the dependence of selectivity on conversion, how the secondary reaction depended on the chemical nature of the support, and how its rate scaled linearly with the support's specific surface area. Building on this insight we modified a SiO 2 support to passivate its OH-groups, which indeed yielded a 94% decrease in the rate of the secondary reaction, performing even better than α-Al 2 O 3 with similar specific surface area would. We have shown that the selectivity towards ethylene oxide is dependent on a metal specific, intrinsic selectivity, and the decrease in selectivity with conversion to be support dependent. This decrease is caused by the isomerization of ethylene oxide, which is found to be first order in ethylene oxide and the active sites on the catalyst support.

In this study Co/Nb2O5 catalysts and the effect of Pt-promotion thereon are investigated in comparison with γ-Al2O3- and α-Al2O3-supported catalysts for the Fischer Tropsch (FT) synthesis. Upon Pt-promotion of Co/Nb2O5 the cobalt-weight normalized FT activity was found to increase by a factor of 2.4, while the high C5+ selectivity of 85 wt% was maintained. Based on environmental TEM results no indications were found that Pt affected the cobalt particle size in Co/Nb2O5 catalysts. A kinetic study indicates an increased number of active sites upon Pt-promotion whereas Steady-State Isotopic Transient Kinetic Analysis experiments show that a combination of an increased number of active sites and an increased turnover frequency is at the origin of the enhanced activity in Co/Nb2O5 catalysts upon Pt-promotion. Pt was tentatively proposed to bring about more efficient promotion of Co by NbOx being present as smaller clusters.

Synthesis gas conversion to short olefins and aromatics using bifunctional catalysts has gained much attention in recent years. Here, we study the interaction between the components of bifunctional catalysts to design a more stable catalyst system. Mixing α-alumina supported iron (-carbide) promoted with sodium and sulfur with an H-ZSM-5 zeolite to convert synthesis gas to aromatics and short olefins we observed selectivity loss of the iron (-carbide) catalyst as well as the acid function. This was displayed by increasing methane and decreasing aromatics selectivity when the two individual catalysts were mixed in close proximity. We introduced different approaches to understand this selectivity related deactivation. Larger spatial separation of the iron and zeolite allowed a more stable system with constant methane and aromatics selectivity. Alternatively, iron supported on carbon nano tubes mixed with the zeolite in close proximity did not display selectivity related deactivation. We conclude that the selectivity loss was caused by migration of sodium ions that were used next to sulfur as promoters on the iron catalyst over the α-alumina support to the zeolite, which was supported by XPS model experiments. This migration seems hindered on carbon supported iron catalysts.

© The Royal Society of Chemistry 2016. Lower C 2 -C 4 olefins are important commodity chemicals usually produced by steam cracking of naphtha or fluid catalytic cracking of vacuum gas oil. The Fischer-Tropsch synthesis of lower olefins (FTO) with iron-based catalysts uses synthesis gas as an alternative feedstock. Nanostructured carbon materials are widely applied as supports for the iron nanoparticles due to their weak interaction with the metal species, facilitating the formation of catalytically active iron carbide. Numerous synthetic approaches towards carbon-supported FTO catalysts with various structures and properties have been published in recent years but structure-performance relationships remain poorly understood. We apply ordered mesoporous carbon (CMK-3) as a support material with well-defined pore structure to investigate the relationships between calcination/activation conditions and catalytic properties. After loading of iron and sodium/sulfur as the promoters, the structures and properties of the FTO catalysts are varied by using different calcination (300-1000°C) and activation (350 or 450°C) temperatures followed by FTO testing at 1 bar, 350°C, H 2 /CO = 1. Carbothermal reduction of iron oxides by the support material occurs at calcination temperatures of 800 or 1000°C, leading to a higher ratio of catalytically active iron(carbide) species but the catalytic activity remains low due to particle growth and blocking of the catalytically active sites with dense graphite layers. For the samples calcined at 300 and 500°C, the formation of non-blocked iron carbide can be enhanced by activation at higher temperatures, leading to higher catalytic activity. Olefin selectivities of ∼60% C in the formed hydrocarbons with methane of ∼10% C are achieved for all catalysts under FTO conditions at low CO conversion. The influence of the calcination temperature is further investigated under industrially relevant FTO conditions. Promoted CMK-3-supported catalysts obtained at low calcination temperatures of 300-500°C show stable operation for 140 h of time on stream at 10 bar, 340°C, H 2 /CO = 2. crosscheck: This document is CrossCheck deposited related_data: Supplementary Information identifier: W. S. Lamme (ORCID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: This article is freely available. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence (CC BY 3.0) history: Received 8 June 2016; Accepted 13 November 2016; Accepted Manuscript published 14 November 2016; Advance Article published 24 November 2016; Version of Record published 5 December 2016 ispartof: Catalysis Science & Technology vol:6 issue:24 pages:8464-8473 status: published

Currently, for the industrial ethylene epoxidation α-alumina supported silver catalysts are the only catalyst of choice. We demonstrate a novel method to produce these catalysts with different silver particle sizes, but without changing other key parameters that may affect the catalytic performance such as support specific surface area or metal precursor. α-Alumina was impregnated with a silver oxalate solution, and was subsequently dried and treated in different gas atmospheres and at different temperatures to tune the silver particle sizes in the range of 20–500 nm. Particles of 20 nm exhibited a lower turnover frequency than particles of 70 nm and larger, which exhibit a constant turnover frequency, in accordance with results in literature. However, the selectivity, when measured at constant conversion, was particle size independent. This is the first time that the effect of the particle size on the selectivity of ethylene epoxidation is reported at constant conversion. This was made possible by a new method of producing supported silver catalysts, which we expect that is also applicable for silver catalysts with other supports and for the preparation of other supported metal catalysts.