The properties of many functional materials depend critically on the spatial distribution of an active phase within a support. In the case of solid catalysts, controlling the spatial distribution of ...metal (oxide) nanoparticles at the mesoscopic scale offers new strategies to tune their performance and enhance their lifetimes. However, such advanced control requires suitable characterization methods, which are currently scarce. Here, we show how the background in small‐angle X‐ray scattering patterns can be analyzed to quantitatively access the mesoscale distribution of nanoparticles within supports displaying hierarchical porosity. This is illustrated for copper catalysts supported on meso‐ and microporous silica displaying distinctly different metal distributions. Results derived from X‐ray scattering are in excellent agreement with electron tomography. Our strategy opens unprecedented prospects for understanding the properties and to guide the synthesis of a wide array of functional nanomaterials.
The background is the data: The properties of many materials depend on the spatial distribution of nanoparticles at the mesoscopic scale. The latter distribution can be characterized quantitatively from the background intensity in X‐ray scattering patterns (see picture). Compared to electron tomography, this procedure enhances the sampling by twelve orders of magnitude, and it offers new prospects for in situ studies.
Quantitative insight into the three‐dimensional morphology of complex zeolite Y mesopore networks was achieved by combining electron tomography and image processing. Properties could be studied that ...are not measurable by other techniques, such as the size distribution of the intact microporous domains. This has great relevance in descriptions of the molecular diffusion through zeolite crystals and, hence, catalytic activity and selectivity.
Catalysts promoted with low amounts of sodium and sulfur exhibited higher selectivity to lower olefins and lower methane production. Promoted bulk and α-Al2O3-supported catalysts showed similar ...selectivities; however, bulk catalysts displayed lower catalytic activity and extensive coke formation.
•Supported iron catalysts promoted with Na and S exhibited high selectivity to lower olefins and low methane production.•Na- plus S-promoted bulk catalysts displayed high lower olefins selectivity but also a low mechanical stability.•Na increased alpha and coke formation while S presumably blocked chain termination through hydrogenation in a selective way.
The Fischer–Tropsch synthesis of lower olefins (FTO) is an alternative process for the production of major chemical building blocks from natural gas, coal, or biomass-derived synthesis gas. The addition of low concentrations of sulfur plus sodium to Fe/α-Al2O3 resulted in catalysts with high C2–C4 olefins selectivity (∼50%C), enhanced catalytic activity, and decreased methane production (<20%C) when the reaction was carried out at 340°C, 20bar and H2/CO=1. Sodium reduced methane selectivity by increasing the chain growth probability while sulfur probably reduced the hydrogen coverage of the catalyst resulting in even lower methane selectivities and higher olefin content of the products. The addition of extra sodium resulted in a detrimental effect on catalytic activity while favoring the formation of carbon deposits. Our results show that the nature and concentration of the promoters play a key role in the design of FTO catalysts with optimum catalytic performance.
The fundamentals of structure sensitivity and promoter effects in the Fischer–Tropsch synthesis of lower olefins have been studied. Steady state isotopic transient kinetic analysis, switching 12CO to ...13CO and H2 to D2, was used to provide coverages and residence times for reactive species on supported iron carbide particles of 2–7 nm with and without promoters (Na + S). CO coverages appeared to be too low to be measured, suggesting dissociative adsorption of CO. Fitting of CH4 response curves revealed the presence of parallel side-pools of reacting carbon. CH x coverages decreased with increasing particle size, and this is rationalized by smaller particles having a higher number of highly active low coordination sites. It was also established that the turnover frequency increased with CH x coverage. To calculate H coverages, new equations were derived to fit HD response curves, again leading to a parallel side-pool model. The H coverages appeared to be lower for bigger particles. The H coverage was suppressed upon addition of promoters in line with lower methane selectivity and higher lower olefin selectivity. Density functional theory (DFT) was applied on H adsorption for a fundamental understanding of this promoter effect on the selectivities, with a special focus on counterion effects. Na2S is a better promoter than Na2O due to both a larger negative charge donation and a more effective binding configuration. On the unpromoted Fe5C2 (111) surface, H atoms bind preferably on C after dissociation on Fe. On Na2S-promoted Fe5C2 surfaces, adsorption on carbon sites weakens, and adsorption on iron sites strengthens, which fits with lower H coverage, less CH4 formation, and more olefin formation.
The Fischer–Tropsch Synthesis converts synthesis gas from alternative carbon resources, including natural gas, coal, and biomass, to hydrocarbons used as fuels or chemicals. In particular, iron-based ...catalysts at elevated temperatures favor the selective production of C2–C4 olefins, which are important building blocks for the chemical industry. Bulk iron catalysts (with promoters) were conventionally used, but these deactivate due to either phase transformation or carbon deposition resulting in disintegration of the catalyst particles. For supported iron catalysts, iron particle growth may result in loss of catalytic activity over time. In this work, the effects of promoters and particle size on the stability of supported iron nanoparticles (initial sizes of 3–9 nm) were investigated at industrially relevant conditions (340 °C, 20 bar, H2/CO = 1). Upon addition of sodium and sulfur promoters to iron nanoparticles supported on carbon nanofibers, initial catalytic activities were high, but substantial deactivation was observed over a period of 100 h. In situ Mössbauer spectroscopy revealed that after 20 h time-on-stream, promoted catalysts attained 100% carbidization, whereas for unpromoted catalysts, this was around 25%. In situ carbon deposition studies were carried out using a tapered element oscillating microbalance (TEOM). No carbon laydown was detected for the unpromoted catalysts, whereas for promoted catalysts, carbon deposition occurred mainly over the first 4 h and thus did not play a pivotal role in deactivation over 100 h. Instead, the loss of catalytic activity coincided with the increase in Fe particle size to 20–50 nm, thereby supporting the proposal that the loss of active Fe surface area was the main cause of deactivation.
Metal nanoparticle growth represents a major deactivation mechanism of supported catalysts and other functional nanomaterials, particularly those based on low melting-point metals. Here we ...investigate the impact of the support porous structure on the stability of CuZnO/SiO2 model methanol synthesis catalysts. A series of silica materials with ordered cagelike (SBA-16 mesostructure) and disordered (SiO2-gel) porosities and varying pore sizes were employed as catalyst supports. Nitric oxide moderated nitrate decomposition enabled the synthesis of catalytically active Cu nanoparticles (3–5 nm) exclusively inside the silica pores with short interparticle spacings. Under relevant reactive conditions, confinement of the Cu particles in cagelike silica pores notably enhances catalyst stability by limiting Cu particle growth as compared to catalysts deposited in SiO2-gel host materials with also 3D and highly interconnected though unconstrained porosity. For both pore morphologies, we find a direct relationship between catalyst stability and support porosity, provided the narrowest characteristic pore dimension is employed as a porosity descriptor. For cagelike porosities this corresponds to the size of the entrances to the nanocages. Our results point to nanoparticle diffusion and coalescence as a relevant growth mechanism under reactive conditions and underscore the significance of the narrowest pore constrictions to mitigate growth and improve catalyst stability. This finding contributes to the establishment of general and quantitative structure–stability relationships which are essential for the design of catalysts and related functional nanostructures with long lifetimes under operation conditions.
The use of electron tomography in nanotechnology, particularly with regard to catalysts, is discussed. The importance of three-dimensional imaging to discerning the molecular structure of ...nanostructured materials is emphasized.
Nanoparticle growth has long been a significant challenge in nanotechnology and catalysis, but the lack of knowledge on the fundamental nanoscale aspects of this process has made its understanding ...and prediction difficult, especially in a liquid phase. In this work, we successfully used liquid-phase transmission electron microscopy (LP-TEM) to image this process in real time at the nanometer scale, using an Au/TiO2 catalyst in the presence of NaCl(aq) as a case study. In situ LP-TEM clearly showed that the growth of Au nanoparticles occurred through a form of Ostwald ripening, whereby particles grew or disappeared, probably via monomer transfer, without clear correlation to particle size in contrast to predictions of classical Ostwald ripening models. In addition, the existence of a significant fraction of inert particles that neither grew nor shrank was observed. Furthermore, in situ transmission electron microscopy (TEM) showed that particle shrinkage was sudden and seemed a stochastic process, while particle growth by monomer attachment was slow and likely the rate-determining step for sintering in this system. Identification and understanding of these individual nanoparticle events are critical for extending the accuracy and predictive power of Ostwald ripening models for nanomaterials.
We explored melt infiltration of mesoporous silica supports to prepare supported metal catalysts with high loadings and controllable particle sizes. Melting of Co(NO3)2·6H2O in the presence of silica ...supports was studied in situ with differential scanning calorimetry. The melting point depression of the intraporous phase was used to quantify the degree of pore loading after infiltration. Maximum pore-fillings corresponded to 70−80% of filled pore volume, if the intraporous phase was considered to be crystalline Co(NO3)2·6H2O. However, diffraction was absent in XRD both from the ordered mesopores at low scattering angles and from crystalline cobalt nitrate phases at high angles. Hence, an amorphous, lower density, intraporous Co(NO3)2·6H2O phase was proposed to fill the pores completely. Equilibration at 60 °C in a closed vessel was essential for successful melt infiltration. In an open crucible, dehydration of the precursor prior to infiltration inhibited homogeneous filling of support particles. The dispersion and distribution of Co3O4 after calcination could be controlled using the same toolbox as for preparation via solution impregnation: confinement and the calcination gas atmosphere. Using ordered mesoporous silica supports as well as an industrial silica gel support, catalysts with Co metal loadings in the range of 10−22 wt % were prepared. The Co3O4 crystallite sizes ranged from 4 to 10 nm and scaled with the support pore diameters. By calcination in N2, pluglike nanoparticles were obtained that formed aggregates over several pore widths, while calcination in 1% NO/N2 led to the formation of smaller individual nanoparticles. After reduction, the Co/SiO2 catalysts showed high activity for the Fischer−Tropsch synthesis, illustrating the applicability of melt infiltration for supported catalyst preparation.
The selective hydrogenation of cinnamaldehyde was studied over carbon nanofibers (CNF) supported platinum and ruthenium catalysts. The catalysts differed independently in their metal particle sizes ...and amount of acidic oxygen groups on the CNF surface. For the catalysts with oxygen on the CNF surface, the larger metal particles (∼3.5
nm) displayed the highest selectivity towards cinnamyl alcohol. Surprisingly, when the oxygen groups were removed from the catalyst surface, the smaller particles (∼2.0
nm) exhibited the highest selectivity to cinnamyl alcohol. Also the hydrogenation activity increased for all catalysts after oxygen removal. A model is proposed to account for the role of the metal particle size and oxygen surface groups in the hydrogenation of cinnamaldehyde.
Cinnamaldehyde hydrogenation was studied over carbon nanofibers (CNF) supported platinum and ruthenium catalysts. Catalysts with oxygen surface groups and larger metal particles resulted in the highest selectivity towards cinnamyl alcohol. Surprisingly, when the oxygen surface groups were removed, smaller metal particles exhibited the highest selectivity to cinnamyl alcohol. A model is proposed to explain this catalytic behavior.
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