K.P. de Jong

On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis

The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

Base-catalyzed condensation of citral and acetone at low temperature using modified hydrotalcite catalysts

A study on the catalytic properties of properly activated hydrotalcite (HT) with special attention to the nature and amount of active sites present in this solid base catalyst has been undertaken. Only a small fraction (5%) of the available basic sites in the rehydrated calcined HT is active in liquid-phase aldol condensations. These sites exhibit high catalytic activity and are most likely localized at the edges of the HT-platelets. Besides a high activity, these modified HTs also show a high selectivity. No further condensation products other than diacetone alcohol (DAA) in the acetone self-condensation could be observed. Initial results with the citral‐acetone condensation show that even at 273 K this reaction is catalyzed by modified HTs with a conversion of 65% and a selectivity of 90%, when the citral concentration is not too high (1 wt.%). At higher citral concentrations, no reaction is observed indicating a negative order in citral concentration.

Plugged hexagonal templated silica: a unique micro- and mesoporous composite material with internal silica nanocapsulesElectronic supplementary information (ESI) available: Fig. S1: X-ray diffractogram of a PHTS material. Fig. S2: TEM images of SBA-15 and PHTS-2. Fig. S3: hydrothermal stabilities. See http://www.rsc.org/suppdata/cc/b2/b201424f/

We describe in this paper the development of plugged hexagonal templated silicas (PHTS) which are hexagonally ordered materials, with internal microporous silica nanocapsules; they have a combined micro- and mesoporosity and a tuneable amount of both open and encapsulated mesopores and are much more stable than other tested micellar templated structures.

Synthesis of highly loaded highly dispersed nickel on carbon nanofibers by homogeneous deposition-precipitation.

Highly loaded (45 wt%) Ni on graphitic carbon nanofiber (diameter 50nm) catalysts were prepared by means of homogeneous deposition–precipitation (HDP) from an aqueous solution. The obtained Ni metal particles were small (9nm). This shows clearly that HDP can be used for the preparation of catalysts based on inert supports, like carbon nanofibers, which cannot form surface co-precipitates between the catalyst support and the metal precursor. The latter is known to be a crucial step in the preparation of metal-on-oxidic support catalysts. The oxygen-containing groups on the surface of the carbon nanofiber act as nucleation and anchoring sites in the deposition of highly dispersed Ni.

Condensation of citral and ketones using activated hydrotalcite catalysts

Activated hydrotalcite catalysts were prepared via calcination and room-temperature rehydration. Replacement of water from the pores after rehydration resulted in an increased amount of accessible active sites. The performance of the catalyst was explored in liquid-phase aldol condensations at low temperatures. In the citral–acetone condensation high activities and selectivities were obtained. Usage in the citral–MEK condensation showed also a high citral conversion, with methyl pseudoionones as primary products.

Cobalt supported on carbon nanofibers- a promising novel Fischer-Tropsch catalyst

Abstract The potential of carbon nanofibers supported cobalt catalysts for the Fischer-Tropsch reaction is shown. Using the wet impregnation method cobalt on carbon nanofiber catalysts were prepared with cobalt loadings varying from 5 to 12 wt%. The cobalt particle size of the catalysts varied with increasing loading from 3 to 13 nm. Cobalt particles were localized both at the external and at the internal surface of the fibers. The activity at 1 bar syngas varied with increasing loading from 0.71 to 1.71 10−5 molco·gco−l·s−1. This might indicate that smaller particles are less active in the Fischer-Tropsch reaction, but may also be provoked by different fractions of cobalt present inside the fibers. Stable activity of 225 gCH2·1cat−1·h−1 for 400 h was obtained at pressures of 28–42 bar syngas. A C5+ selectivity of 86 wt% was found, which is remarkably high for an unpromoted catalyst.

An infrared spectroscopic study of the adsorption of carbon monoxide on silica-supported copper oxide

Adsorption of carbon monoxide at room temperature (0.1–50 Torr) on silica-supported copper oxide was studied by infrared spectroscopy. Catalysts were prepared by deposition-precipitation or impregnation. After calcination two types of adsorbed CO were identified showing absorption bands at 2136 ± 3 and 2204 ± 1 cm−1, which are ascribed to CO adsorbed on copper(II) oxide and on isolated copper(II) ions in the silica surface, respectively. Reduction and reoxidation removed the band at 2204 cm−1 with all samples and raised the intensity of the 2136-cm−1 band with the precipitated catalysts but not with the impregnation catalyst. Evidence is brought forward that the isolated copper ions are mobilized during reduction and generate new copper (oxide) surface. The change in background transmission of the samples could be used to obtain further information about the interaction of O2 and CO with copper oxide.

Heterogeneities of the Nanostructure of Platinum/Zeolite Y Catalysts Revealed by Electron Tomography

To develop structure-performance relationships for important catalysts, a detailed characterization of their morphology is essential. Using electron tomography, we determined in three dimensions the structure of Pt/zeolite Y bifunctional catalysts. Optimum experimental conditions enabled for the first time high-resolution 3D imaging of Pt particles as small as 1 nm located inside zeolite micropores. Semiautomated image analysis of 3D reconstructions provided an efficient study of numbers, size distributions, and interparticle distances of thousands of Pt particles within individual zeolite crystals. Upon extending this approach to a number of zeolite crystals of one batch of Pt/zeolite Y catalyst, heterogeneities were revealed. The Pt loading, an important parameter for catalyst performance, varied between zeolite crystals up to a factor of 35. This discovery calls for re-evaluation of catalyst preparation methods and suggests potential for lowering the nominal loading with noble metals.

Development and Application of 3-Dimensional Transmission ElectronMicroscopy (3D-TEM) for the Characterization of Metal-Zeolite CatalystSystems

With electron tomography (3D-TEM) a 3D-reconstruction is calculated from a series of TEM images taken at a tilt angle range (tilting range) of +70° to −70°. The reconstruction can be visualized with contour surfaces that give information about the surface of the sample as well as with slices through the reconstruction that give detailed information on the interior of the sample. Electron tomography gives much more information than Scanning Electron Microscopy (SEM), since SEM gives only information about the surface of a sample. As a case study, the imaging of silver clusters on zeolite NaY is given. The reconstruction shows silver particles at the external surface as well as a silver particle in a mesopore of the zeolite crystallite. It is concluded that 3D-TEM comprises a breakthrough in the characterization of nano-structured solid catalysts.

Stability and activity of carbon nanofiber-supported catalysts in the aqueous phase reforming of ethylene glycol

Abstract Nickel, cobalt, copper and platinum nanoparticles supported on carbon nano-fibers were evaluated with respect to their stability, catalytic activity and selectivity in the aqueous phase reforming of ethylene glycol (230 °C, autogenous pressure, batch reactor). The initial surface-specific activities for ethylene glycol reforming were in a similar range but decreased in the order of Pt (15.5 h−1) >Co(13.0 h−1) >Ni(5.2 h−1) while the Cu catalyst only showed low dehydrogenation activity. The hydrogen molar selectivity decreased in the order of Pt (53%)>Co(21%)>Ni (15%) as a result of the production of methane over the latter two catalysts. Over the Co catalyst acids were formed in the liquid phase while alcohols were formed over Ni and Pt. Due to the low pH of the reaction mixture, especially in the case of Co (as a result of the formed acids), significant cobalt leaching occurs which resulted in a rapid deactivation of this catalyst. Investigations of the spent catalysts with various techniques showed that metal particle growth is responsible for the deactivation of the Pt and Ni catalysts. In addition, coking might also contribute to the deactivation of the Ni catalyst.