Single Catalyst Particle Research Articles

Visualization of the Heterogeneity of Cerium Oxidation States in Single Pt/Ce2Zr2Ox Catalyst Particles by Nano‐XAFS

The cerium oxidation states in single catalyst particles of Pt/Ce2Zr2O(x) (x=7 to 8) were investigated by spatially resolved nano X-ray absorption fine structure (nano-XAFS) using an X-ray nanobeam. Differences in the distribution of the Ce oxidation states between Pt/Ce2Zr2O(x) single particles of different oxygen compositions x were visualized in the obtained two-dimensional X-ray fluorescent (XRF) mapping images and the Ce L(III)-edge nano X-ray absorption near-edge structure (nano-XANES) spectra.

Optical investigation of carbon nanotube agglomerate growth on single catalyst particles

A setup for optically monitoring the agglomerate growth of multiwalled carbon nanotubes (MWCNTs) by catalytic chemical vapor deposition on single Co–Mn–Al–Mg oxide catalyst particles with ethene as carbon precursor has been developed. Ethene concentrations and temperatures were varied between 5–75Vol.% and 550–770°C, respectively. It could be shown that the agglomerate growth is rapid and the final diameter is reached after a few ten seconds to about 3min depending on the reaction conditions. The average enlargement factor of the agglomerates over all experiments was found to be 6.5±1.2 compared to the original diameter of the catalyst particle. The growth rate is enhanced by both, reaction temperature and ethene concentration. Hence it is concluded that the agglomerate growth rate is associated with the reaction rate of MWCNT synthesis. Short time experiments and analysis of the resulting agglomerates have confirmed an earlier proposed growth mechanism.

Correlating metal poisoning with zeolite deactivation in an individual catalyst particle by chemical and phase-sensitive X-ray microscopy.

Fluid catalytic cracking (FCC) is the main conversion process used in oil refineries. An X-ray microscopy method is used to show that metal poisoning and related structural changes in the zeolite active material lead to a non-uniform core–shell deactivation of FCC catalyst particles. The study links the detrimental effect of V and Ni poisoning with zeolite destruction and dealumination in a spatial manner within a single FCC catalyst particle.

Development of Fast Scanning Microscopic XAFS Measurement System

We have developed a fast scanning microscopic X-ray absorption fine-structure (XAFS) measurement system using a 100–300 nm focused X-ray beam at the BL39XU nano-scale analysis station at SPring-8. This system provides two-dimensional XAFS images constructed from X-ray fluorescence (XRF) images measured by fast continuous 2-dimensional scanning at all XAFS measurement energies. High-quality XAFS spectra at each position were obtained by correcting shift in the XRF images. We applied our proposed method to measuring single catalyst particles, and succeeded in measuring the microscopic x-ray absorption near-edge structure (XANES) images that were 2–4 μm square with a spatial resolution of 300 nm.

Toward bifunctional catalysts for the direct conversion of syngas to gasoline range hydrocarbons: H-ZSM-5 coated Co versus H-ZSM-5 supported Co

One step production of gasoline range hydrocarbons from syngas is demonstrated by combination of Fischer–Tropsch synthesis (FTS) and acid functionalities in one single bifunctional catalyst particle. Two different catalyst configurations were studied in which the acid functionality of H-ZSM-5 zeolite conjoins the cobalt FTS active phase: (i) H-ZSM-5 as catalytic coating on Co and (ii) H-ZSM-5 as catalytic support for Co. Spherical shaped Co/SiO2 was chosen as a conventional FTS catalyst for comparison and used as precursor to synthesize the H-ZSM-5 coated Co catalyst. Various Silicalite-1 and H-ZSM-5 coated reference samples were prepared by subjecting Co/SiO2 to a direct hydrothermal procedure. Thorough characterization and catalytic performance tests reveal that direct hydrothermal synthesis results in transformation of SiO2 from the Co/SiO2 particles into an MFI coating of Co agglomerates. The silica support does not only act as precursor but also as nano-mold during the preparation of the zeolite coated catalysts as the original Co/SiO2 particle shape is preserved. The close vicinity of the acid sites and Co function in the zeolite coated catalysts promotes the acid catalyzed conversion of the produced FTS hydrocarbons and reduces the production of C12+. Alternatively, mesostructured H-ZSM-5 was used to support Co. Mesoporous hierarchy in the latter case improves both the Co dispersion and the proximity of FTS and acid sites. Thus, Co supported on mesoporous H-ZSM-5 is a much more effective catalyst for the direct production of gasoline range hydrocarbons than the H-ZSM-5 coated Co catalyst.

Breaking the Fischer–Tropsch synthesis selectivity: direct conversion of syngas to gasoline over hierarchical Co/H-ZSM-5 catalysts

We report the combination of Fischer–Tropsch catalyst with acid functionality in one single catalyst particle. The resulting bifunctional catalyst is capable of producing gasoline range hydrocarbons from synthesis gas in one catalytic step with outstanding activities and selectivities.

Cover Picture: Staining of Fluid-Catalytic-Cracking Catalysts: Localising Brønsted Acidity within a Single Catalyst Particle (Chem. Eur. J. 4/2012)

Cover Picture: Staining of Fluid-Catalytic-Cracking Catalysts: Localising Brønsted Acidity within a Single Catalyst Particle (Chem. Eur. J. 4/2012)

Fischer–Tropsch reaction–diffusion in a cobalt catalyst particle: aspects of activity and selectivity for a variable chain growth probability

The reaction–diffusion performance for the Fischer–Tropsch reaction in a single cobalt catalyst particle is analysed, comprising the Langmuir–Hinshelwood rate expression proposed by Yates and Satterfield and a variable chain growth parameter α, dependent on temperature and syngas composition (H2/CO ratio). The goal is to explore regions of favourable operating conditions for maximized C5+ productivity from the perspective of intra-particle diffusion limitations, which strongly affect the selectivity and activity. The results demonstrate the deteriorating effect of an increasing H2/CO ratio profile towards the centre of the catalyst particle on the local chain growth probability, arising from intrinsically unbalanced diffusivities and consumption ratios of H2 and CO. The C5+ space time yield, a combination of catalyst activity and selectivity, can be increased with a factor 3 (small catalyst particle, dcat = 50 μm) to 10 (large catalyst particle, dcat = 2.0 mm) by lowering the bulk H2/CO ratio from 2 to 1, and increasing temperature from 500 K to 530 K. For further maximization of the C5+ space time yield under these conditions (H2/CO = 1, T = 530 K) it seems more effective to focus catalyst development on improving the activity rather than selectivity. Furthermore, directions for optimal reactor operation conditions are indicated.

Tracing Catalytic Conversion on Single Zeolite Crystals in 3D with Nonlinear Spectromicroscopy

The cost- and material-efficient development of next-generation catalysts would benefit greatly from a molecular-level understanding of the interaction between reagents and catalysts in chemical conversion processes. Here, we trace the conversion of alkene and glycol in single zeolite catalyst particles with unprecedented chemical and spatial resolution. Combined nonlinear Raman and two-photon fluorescence spectromicroscopies reveal that alkene activation constitutes the first reaction step toward glycol etherification and allow us to determine the activation enthalpy of the resulting carbocation formation. Considerable inhomogeneities in local reactivity are observed for micrometer-sized catalyst particles. Product ether yields observed on the catalyst are ca. 5 times higher than those determined off-line. Our findings are relevant for other heterogeneous catalytic processes and demonstrate the immense potential of novel nonlinear spectromicroscopies for catalysis research.

Staining of Fluid‐Catalytic‐Cracking Catalysts: Localising Brønsted Acidity within a Single Catalyst Particle

A time-resolved in situ micro-spectroscopic approach has been used to investigate the Brønsted acidic properties of fluid-catalytic-cracking (FCC) catalysts at the single particle level by applying the acid-catalysed styrene oligomerisation probe reaction. The reactivity of individual FCC components (zeolite, clay, alumina and silica) was monitored by UV/Vis micro-spectroscopy and showed that only clay and zeolites (Y and ZSM-5) contain Brønsted acid sites that are strong enough to catalyse the conversion of 4-fluorostyrene into carbocationic species. By applying the same approach to complete FCC catalyst particles, it has been found that the fingerprint of the zeolitic UV/Vis spectra is clearly recognisable. This almost exclusive zeolitic activity is confirmed by the fact that hardly any reactivity is observed for FCC particles that contain no zeolite. Confocal fluorescence microscopy images of FCC catalyst particles reveal inhomogeneously distributed micron-sized zeolite domains with a highly fluorescent signal upon reaction. By examining laboratory deactivated FCC catalyst particles in a statistical approach, a clear trend of decreasing fluorescence intensity, and thus Brønsted acidity, of the zeolite domains is observed with increasing severity of the deactivation method. By comparing the average fluorescence intensities obtained with two styrenes that differ in reactivity, it has been found that the Brønsted acid site strength within FCC catalyst particles containing ZSM-5 is more uniform than within those containing zeolite Y, as confirmed with temperature-programmed desorption of ammonia.

Stain and shine

Catalyst particles for fluid catalytic cracking are vital for the oil-refinery industry, but their activity is hard to diagnose because of their inter- and intra-particle structural inhomogeneity. With fluorescence confocal microscopy and selective staining, one can now pinpoint the catalytic activity within single catalyst particles from an industrial reactor.

μ-XAFS of a single particle of a practical NiOx/Ce2Zr2Oy catalyst

μ-XAFS analysis using an X-ray μ-beam (1000 nm (h) × 800 nm (v)) was successfully carried out on a single particle of a practical catalyst NiO(x)/Ce(2)Zr(2)O(y) (0 ≤x≤ 1, 7 ≤y≤ 8). The oxidation state and local coordination structure of the NiO(x)/Ce(2)Zr(2)O(y) particle were characterized by Ni K-edge μ-XANES and μ-EXAFS, which showed the catalytically active and inactive phases of a single catalyst particle.

High platinum utilization in ultra-low Pt loaded PEM fuel cell cathodes prepared by electrospraying

Cathode electrodes for proton exchange membrane fuel cells (PEMFCs) with ultra-low platinum loadings as low as 0.012 mg Ptcm −2 have been prepared by the electrospray method. The electrosprayed layers have nanostructured fractal morphologies with dendrites formed by clusters (about 100 nm diameter) of a few single catalyst particles rendering a large exposure surface of the catalyst. Optimization of the control parameters affecting this morphology has allowed us to overcome the state of the art for efficient electrodes prepared by electrospraying. Thus, using these cathodes in membrane electrode assemblies (MEAs), a high platinum utilization in the range 8–10 kW g −1 was obtained for the fuel cell operating at 40 °C and atmospheric pressure. Moreover, a platinum utilization of 20 kW g −1 was attained under more suitable operating conditions (70 °C and 3.4 bar over-pressure). These results substantially improve the performances achieved previously with other low platinum loading electrodes prepared by electrospraying.

Hydrogenation of polystyrene in CO 2-expanded liquids: The effect of catalyst composition on deactivation

The effect of catalyst composition on the hydrogenation of the aromatic rings of polystyrene dissolved in carbon dioxide-expanded decahydronaphthalene (CO 2-DHN) was studied using a slurry reactor at 150 °C. Catalyst deactivation occurred within 15 min of the addition of CO 2 to the reactor when 5% Pd/SiO 2 or 5% Pd/Al 2O 3 was used as the catalyst. The deactivation is believed to be a consequence of CO formation via the reverse water-gas shift reaction, as CO was observed at the end of each hydrogenation reaction. In an effort to convert CO to CH 4 before the poisoning of Pd sites occurred, a methanation catalyst such as 5% Ru/Al 2O 3 or 65% Ni/SiO 2–Al 2O 3 was physically mixed with 5% Pd/SiO 2 or 5% Pd/Al 2O 3 and used to hydrogenate PS. Carbon monoxide concentrations decreased, but no significant improvements in aromatic ring hydrogenation were observed. When a catalyst containing both a hydrogenation and methanation function in a single catalyst particle (1.6% Ru/4.2% Pd/SiO 2) was used, the degrees of hydrogenation in CO 2-DHN matched those obtained when neat DHN was the solvent. Furthermore, no CO was detected when the reaction was catalyzed by the bimetallic catalyst. These results suggest that the methanation function must be proximate to the hydrogenation (reverse water-gas shift) function in order for aromatic ring hydrogenation to proceed as rapidly in a CO 2-expanded solvent as in the unexpanded solvent.

A general formula for reactant conversion over a single catalyst particle in TAP pulse experiments

Abstract We obtain a general formula for reactant conversion in diffusion–reaction TAP systems over single non-porous catalyst particles as a product of two terms: α = P H × K . The first term, P H , is a purely geometric factor dependent only on the shape of the reactor and the shape and position of the catalyst particle, interpreted as the probability that an individual molecule of reactant will hit the catalyst before leaving the reactor. The second term, K = k τ H / ( 1 + k τ H ) , is a geometrical/chemical term involving the kinetic constant k and a transport characteristic (residence time in the catalyst zone, τ H ). Both P H and τ H can be effectively calculated for any given reactor-particle configuration. Our formula greatly extends the validity of a formula given in Shekhtman et al. [1999. Thin-zone TAP-reactor—theory and application. Chem. Eng. Sci. 54, 4371–4378] for thin-zone TAP-systems. It is derived by a probabilistic analysis of the residence time of individual molecular trajectories in chemically active regions. Our results are based on the theory previously developed in our paper [Feres, R., Yablonsky, G.S., Mueller, A., Baernstein, A., Zheng, X., Gleaves, J., 2009. Probabilistic analysis of transport-reaction processes over catalytic particles: theory and experimental testing. Chem. Eng. Sci. 64, 568–581].

Nanoscale Chemical Imaging of the Reduction Behavior of a Single Catalyst Particle

A closer look: Investigation of the reduction properties of a single Fischer-Tropsch catalyst particle, using in situ scanning transmission X-ray microscopy with spatial resolution of 35 nm, reveals a heterogeneous distribution of Fe(0), Fe(2+), and Fe(3+) species. Regions of different reduction properties are defined and explained on the basis of local chemical interactions and catalyst morphology.

Nanoscale Chemical Imaging of the Reduction Behavior of a Single Catalyst Particle

AbstractKatalysator unter der Lupe: Die Untersuchung der Reduktion an einem Fischer‐Tropsch‐Katalysatorpartikel mithilfe von In‐situ‐Röntgentransmissionsmikroskopie im Rastermodus zeigte bei einer räumlichen Auflösung von 35 nm eine ungleichmäßige Verteilung von Fe0‐, Fe2+‐ und Fe3+‐Spezies. Regionen mit unterschiedlichen Reduktionseigenschaften können abgegrenzt werden; ihr Zustandekommen lässt sich auf der Grundlage lokaler chemischer Wechselwirkungen und der Katalysatormorphologie erklären.magnified image

Cover Picture: Nanoscale Chemical Imaging of the Reduction Behavior of a Single Catalyst Particle (Angew. Chem. Int. Ed. 20/2009)

AbstractIron‐based catalysts for the Fischer–Tropsch synthesis bring about the conversion of synthesis gas (CO/H2) derived from coal or biomass into liquid transportation fuels. In their Communication on page 3632 ff., B. M. Weckhuysen, F. M. F. de Groot, and co‐workers provide insights into the difference in behavior of the catalyst precursors during pretreatment in H2 on both the nanoscopic and the bulk scale. These findings enable further understanding of how the activated catalyst works.magnified image

Three‐dimensional shapes and spatial distributions of Pt and PtCr catalyst nanoparticles on carbon black

High-angle annular dark-field scanning transmission electron microscopy tomography is applied to the study of Pt and PtCr nanoparticles supported on carbon black, which are used as heterogeneous catalysts in the electrodes of proton exchange membrane fuel cells. By using electron tomography, the three-dimensional architecture of the heterogeneous catalyst system can be determined, providing high-spatial-resolution information about the shapes, faceting and crystallographies of 5-20 nm single and multiply twinned catalyst particles, as well as their positions with respect to the carbon support. Approaches that can be used to provide improved information about the distribution and orientation of the particles on their support are proposed and discussed. Our results show that electron tomography provides important information that is complementary to high-resolution lattice imaging. Both techniques are required to understand fully the nature and role of the surfaces of faceted catalyst particles.

Modeling of single catalyst particle in cathode of PEM fuel cells

A microscopic catalyst model is proposed for predicting the local mass diffusion of a single catalyst particle in the cathode catalyst layer. The numerical model is employed to obtain a geometric description of the active layer. The cathode catalyst particle model proposed here is based on the microstructure of the catalyst layer. The catalyst particle is treated as many small platinum particles (1–10.0 nm) embedded in the larger carbon particle support (30–100 nm). This assembly is surrounded by an ionomer film with thickness ranging from 0.5 nm to 10.0 nm. The modeling results confirm that the platinum particle size, platinum loading and ionomer thickness can each play an important role on local mass and charge transport in the PEM fuel cell catalyst particle agglomerate. The local spherical diffusion, reactant distribution and electrochemical kinetics are strongly influenced by particle size, platinum loading and ionomer thickness.