Structure-insensitive Reaction Research Articles

Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis

Some fundamental concepts of catalysis are not fully explained but are of paramount importance for the development of improved catalysts. An example is the concept of structure insensitive reactions, where surface-normalized activity does not change with catalyst metal particle size. Here we explore this concept and its relation to surface reconstruction on a set of silica-supported Ni metal nanoparticles (mean particle sizes 1–6 nm) by spectroscopically discerning a structure sensitive (CO2 hydrogenation) from a structure insensitive (ethene hydrogenation) reaction. Using state-of-the-art techniques, inter alia in-situ STEM, and quick-X-ray absorption spectroscopy with sub-second time resolution, we have observed particle-size-dependent effects like restructuring which increases with increasing particle size, and faster restructuring for larger particle sizes during ethene hydrogenation while for CO2 no such restructuring effects were observed. Furthermore, a degree of restructuring is irreversible, and we also show that the rate of carbon diffusion on, and into nanoparticles increases with particle size. We finally show that these particle size-dependent effects induced by ethene hydrogenation, can make a structure sensitive reaction (CO2 hydrogenation), structure insensitive. We thus postulate that structure insensitive reactions are actually apparently structure insensitive, which changes our fundamental understanding of the empirical observation of structure insensitivity.

Facet-dependent catalytic activities of Pd/rGO: Exploring dehydrogenation mechanism of dodecahydro-N-ethylcarbazole

Facet structure plays an important role in many reactions. Generally, dehydrogenation reactions with only CH bond cleavage are often considered as structure insensitive reactions and therefore, there is no report on facet effect in reactions of LOHC for now. Herein, we report a facile synthesis of Pd nanoparticles enclosed by {100}, {110} and {111} facets supported on reduced graphene oxide. Combined with catalyst characterization, dehydrogenation performance study and DFT calculations, we found that the conversion rate and selectivity were strongly correlated with the facet structure of Pd, and {100} facet exhibited superior activity. We also identified the rate-determining step in the dehydrogenation of tetrahydro-N-ethylcarbazole to N-ethylcarbazole on Pd {100} facet. This work provides a basis for research requiring template Pd catalysts on rGO and broaden design methods for LOHC catalysts.

Is the CO2 methanation on highly loaded Ni-Al2O3 catalysts really structure-sensitive?

Highly loaded Ni-Al2O3 catalysts (14–88 wt.-% Ni) were prepared by dry impregnation, wet impregnation, deposition precipitation and coprecipitation and tested in the CO2 methanation. By means of ICP-OES, XRD, TPR, H2 chemisorption, N2 physisorption and thermogravimetry, different Ni species were identified, and their dependence on the synthesis method and the Ni loading was investigated. The catalysts prepared had Ni particle sizes of 5–91 nm and Ni surface areas of 5–51 m2 g−1. It could be shown that the turnover frequency (TOF) does not depend on metal-support interactions, the metal-support interface or the particle size. Hence, the CO2 methanation on Ni-Al2O3 catalysts is clearly a structure-insensitive reaction. There is a linear correlation between the conversion and the Ni surface area at constant modified residence time. Hence, catalysts with the highest Ni surfaces, i.e., the samples prepared by deposition precipitation and coprecipitation, reached the highest weight time yields. The prerequisites for high Ni surface areas at high Ni loadings could be elucidated: During the synthesis, Ni has to be able to completely penetrate the mesopores of the support. Furthermore, a linear correlation between the selectivity to CO and the weight percentage of the impurity Na could be revealed. The stability of the supported catalysts was highly dependent on the spatial distribution of Ni and the stability of the support: A high dispersion or the presence of unsupported Ni reduced the stability. Furthermore, the stability of the supported catalysts was smaller than the one of the coprecipitated catalysts, where the stability decreased with increasing nNi/nAl.

Structure-Sensitive and Insensitive Reactions in Alcohol Amination over Nonsupported Ru Nanoparticles

The reaction rates and selectivity of many metal-catalyzed reactions depend on the size of the metal particles in the nanoscale range. Primary amines are important platform molecules in the chemical industry. In this work, the catalytic performance of nonsupported Ru nanoparticles with sizes from 2 to 9 nm was investigated in direct amination of octanol and other alcohols into primary amines in the presence of ammonia. The 90% selectivity to octylamine was obtained over small Ru nanoparticles (d = 2 nm) even at 92% conversion, whereas for larger Ru nonsupported and supported nanoparticles, the octylamine selectivity dropped as the octanol conversion approached 70–80%. The primary reaction of alcohol amination into octylamine was found to be nearly a structure-insensitive reaction. The selectivity to primary amine drops over large Ru particles at higher conversions, because of the secondary highly structure-sensitive reaction of amine self-coupling. Over small metal nanoparticles, amine self-coupling is hi...

Effect of MoO3 on catalytic performance and stability of the SBA-16 supported Ni-catalyst for CO methanation

A series of Ni/SBA-16 and NiMo/SBA-16 catalysts were prepared by the impregnation methods and their performance on synthetic natural gas (SNG) production were systematically investigated in a continuous flow fixed-bed reactor. Catalysts were characterized by nitrogen physisorption, powder X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 pulse chemisorption, temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS). Catalysts with the addition of MoO3 exhibited larger metal surface area and better heat-resistant performance, because the strong interaction between Ni and SBA-16 was formed. Mo–Ni/SBA-16(1wt%) achieves the best activity with 100% CO conversion and 97% CH4 selectivity at 350°C, 0.1MPa, and 15,000mL/g/h. The activity of Mo–Ni/SBA-16(1wt%) remain stable after calcination in reactant gases at 700°C and during the 100h reaction period. It was noting that for catalysts with large metal particle size (more than 20nm) CO methanation turned to be a structure-insensitive reaction, which the activity is related to the amount of active site rather than the particle size.

Tuning Pt dispersion and oxygen mobility of Pt/γ-Al2O3 by Si addition for CO oxidation

γ-Al2O3 was doped with TEOS solution to obtain SiO2 content in the range of 0.6–12.4 wt%. Pt dispersion was effectively enhanced by 82 and 45 % by doping 0.6 and 1.2 wt% SiO2, and was slightly declined afterward. However, oxygen mobility as described by the reduction of basicity was monotonically decreased with increasing SiO2 content. The results from CO oxidation tests reveal a linear relationship between TOFs and desorbed-CO2. It is suggested that oxygen mobility plays a distinct role in controlling the surface reaction between CO and O surface species, which were activated by different adjacent Pt clusters. The highly dispersed Pt catalysts obtained by this technique can also be applied to other structure-insensitive reactions such as propane dehydrogenation.

Single-step synthesis of dimethyl ether from syngas on Al2O3-modified CuO–ZnO–Al2O3/ferrierite catalysts: Effects of Al2O3 content

Single step synthesis of dimethyl ether (DME) from syngas on the bifunctional catalysts of CuO–ZnO–Al2O3 deposited on Al2O3-modified H-ferrierite was investigated to elucidate effects of Al2O3 modification of H-ferrierite in terms of CO conversion and DME yield. The enhanced catalytic activity was observed on the bifunctional catalysts at an optimum content of Al2O3 on H-ferrierite around 2.5–5wt%. It is attributed to a higher dispersion of Al2O3 on H-ferrierite with an enhanced interaction of copper particles with Al2O3 modified H-ferrierite with a lower aggregation property by adjusting acidic sites and by increasing the dispersion of copper particles. The main role of Al2O3 on H-ferrierite is found to reduce strong acidic sites by depositing on the sites selectively, which resulted in showing a lower formation of byproducts. Both of copper surface area and amount of acidic sites are well correlated with an intrinsic activity (turn-over frequency; TOF), which revealed a structure-insensitive reaction character of a single-step synthesis of DME. The measurements of the dispersion of Al2O3 and copper particles with the number of acidic sites concomitantly were employed to verify the effects of Al2O3 modification of H-ferrierite though X-ray photoelectron spectroscopy, temperature-programmed methods and surface morphology characterizations.

Structure sensitivity via decoration of low-coordination exposed metal atoms: CO oxidation catalysis on Pt clusters

The effects of CO and O2 concentrations on turnover rates and 18O2–16O2 exchange rates during catalysis are used to assess the relevant elementary steps and the consequences of Pt coordination for CO oxidation catalysis at moderate temperatures (700–800K) on supported Pt clusters 1.8–25nm in diameter. Turnover rates, measured under conditions of strict kinetic control, are proportional to O2 pressure and inhibited by CO; these data are consistent with kinetically-relevant O2 dissociation steps on cluster surfaces covered partially by chemisorbed CO (CO*). O2 dissociation also limits CO oxidation rates at higher temperatures, which lead to bare Pt surfaces, and at lower temperatures, where saturation CO* coverages require O2 dissociation to be assisted by CO* because of a dearth of vacant sites. At the intermediate temperatures used here, kinetic coupling between irreversible O2 activation and CO* reactions with O* causes edge and corner atoms to become decorated by unreactive O* species; consequently, turnovers occur predominantly on exposed low-index planes, which account for a decreasing fraction of exposed atoms with increasing metal dispersion. These decoration effects confer the appearance of structure sensitivity to the prototypical structure insensitive reaction by rendering only a fraction of exposed metal atoms able to turnover. These active sites, residing at exposed low-index planes, show similar CO* binding energies on large and small Pt clusters, but their relative abundance decreases as clusters become smaller, leading to a sharp decrease in turnover rates with increasing Pt dispersion. These trends stand in marked contrast with the absence of cluster size effects on CO oxidation rates at low temperatures, where high CO* coverages dampen the intrinsic site non-uniformity of metal clusters, and at high temperatures, where all Pt atoms remain accessible irrespective of coordination and active for catalytic turnovers.

Preparation of platinum-on-carbon catalysts via hydrolytic deposition: Factors influencing the deposition and catalytic properties

The deposition of Pt oxide from aqueous chloroplatinate solutions onto carbon supports (activated carbons, carbon blacks, carbon nanofibers) with the aid of Na2CO3 and without adding the reductants or wetting agents was studied. It has been revealed that carbons accelerate greatly the hydrolysis of chloroplatinate, and the hydrolytic deposition can therefore interfere with the other preparation methods. Neutral pH was found optimum for heterogeneous nucleation and formation of smaller PtOx particles. High temperature (80°C) allowed depositions of 5–30wt.% of Pt for a short time (1–3h) but did not affect Pt dispersions (15–90% at surface concentration of Pt 0.5–30μmol/m2). The supported particles could be further reduced in the liquid phase with minimal losses of the dispersion (Na-formate as reductant). The heterogeneous nucleation ensured a uniform distribution of Pt particles but provoked particle confinement in narrow pores. The possibility of influencing catalytic properties with the aid of the support, Pt loading and deposition conditions was demonstrated (hydrogenation of cycloalkenes as a model structure-insensitive reaction and oxidation of isopropyl alcohol as structure-sensitive one).

Highly dispersed Pt–Ir nanoparticles on titania nanotubes

Bimetal ∼1.4nm Pt–Ir nanoparticles were homogeneously dispersed on titania nanotubes by a conventional successive wet impregnation method, by first adding Ir and then, the Pt chloride precursors. Ir precursor interacts strongly with the nanotubular surface yielding highly dispersed ∼1.4nm nanoparticles, and later acts as a nucleation site for the assembly of Pt atoms. The mean sizes of Ir, Pt–Ir were 1.4nm and that of Pt nanoparticles was 3.5nm. Bimetal Pt–Ir nanoparticles were more active in cyclohexene (CHE) disproportionation reaction than the corresponding monometal catalysts. At reaction temperatures <423K, CHE is hydrogenated to cyclohexane (CHA) as the main product, whereas at >473K, CHE is dehydrogenated to produce mainly benzene (BZ). The activity of Pt–Ir nanoparticles increased as the reduction temperature rose from 573 to 673K, suggesting that electronic transfer between reducible NT support and metal nanoparticles favors CHE hydrogenation/dehydrogenation, which is a structure insensitive reaction.

ChemInform Abstract: Olefin Hydrogenation on Pd Model Supported Catalysts: New Mechanistic Insights

AbstractReview: 39 refs.

Olefin hydrogenation on Pd model supported catalysts: New mechanistic insights

Hydrogenation of unsaturated hydrocarbon compounds, catalyzed by transition metals, is traditionally believed to be a structure-insensitive reaction. Recent progress in understanding the microscopic details of hydrogenation processes now challenges the universality of this common belief. In this perspective article, we present results on the hydrogenation of cis-2-butene: over Pd model nanoparticles supported on an Fe 3O 4/Pt(1 1 1) oxide thin film and over a Pd(1 1 1) single crystal. We provide direct experimental evidence that the hydrogenation activity of Pd is strongly dependent on the presence of hydrogen species absorbed in the subsurface region of the metal catalyst and is governed by the permeability of the surface for hydrogen. Since the formation of subsurface hydrogen species in Pd is a strongly structure-sensitive process, the low-coordinated surface sites, such as edges and corners on Pd nanoclusters, play a crucial role in replenishment of the subsurface hydrogen reservoir and maintaining the hydrogenation activity under steady-state conditions. We show that selective modification of these low-coordinated surface sites on Pd nanoclusters with carbon allows for faster subsurface hydrogen diffusion and thereby results in sustained hydrogenation activity, a result that cannot be achieved on C-free Pd nanoparticles, nor on C-free Pd(1 1 1), nor on C-containing Pd(1 1 1). Theoretical calculations support the proposed facilitation of subsurface hydrogen diffusion through C-modified low-coordinated surface sites on Pd nanoclusters and show the conceptual importance of atomic flexibility of nanoparticles in hydrogenation catalysis.

Structure sensitivity of cyclopropane hydrogenolysis on carbon-supported platinum

There has been debate in the past as to whether or not cyclopropane hydrogenolysis is a structure sensitive reaction. This paper addresses the structure sensitivity of cyclopropane hydrogenolysis on Pt using K + addition to Pt/C and compares the results to those for CO hydrogenation, a classic structure insensitive reaction. Kinetic parameters determined for both reactions show the effect of K + on Pt to be limited to simple site blockage at the reaction conditions used. Determination of the site ensemble size (number of contiguous surface metal atoms) required for reaction using Martin’s model suggests that cyclopropane hydrogenolysis requires a site ensemble size of ca. 7, whereas the structure insensitive CO hydrogenation reaction requires only an ensemble size of ca. 1. In addition, evidence suggests that K + decorates Pt non-uniformly.

ChemInform Abstract: Recently Published Work on EUROPT-1, a 6% Pt/SiO2 Reference Catalyst

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Vapour Phase Hydrogenation of Cinnamaldehyde over Ni/γ-Al2O3 Catalysts: Interesting Reaction Network

The vapour phase hydrogenation of cinnamaldehyde over Ni loading γ-Al2O3 catalysts was performed at 1 atm and 300 °C in a fixed-bed reactor. The major product was hydrocinnamaldehyde. Unexpected products of styrene and ethylbenzene due to new reaction pathway of hydrodeformylation were observed. The results indicated that Ni metal was the active centers and the catalytic activity was parallel to the Ni surface area. The stability of TOF values implied that the cinnamaldehyde hydrogenation over Ni/γ-Al2O3 catalysts was a structure insensitive reaction.

Properties of Pt/C catalysts prepared by adsorption of anionic precursor and reduction with hydrogen. Influence of acidity of solution

Carbon-supported Pt/C catalysts prepared with H2PtCl6 as metal precursor have been studied by temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and CO chemisorption in order to investigate the effect of acidity of the solution on the catalyst properties. The catalytic activity of the samples has been tested in structure insensitive reaction of cyclohexene hydrogenation. The catalysts were prepared by adsorption of platinum precursor on activated carbons with different pore structure followed by drying and reduction in flowing hydrogen at 523 K. The use of acidic and highly basic solution during the impregnation of microporous carbons led to highly dispersed catalysts with low activity. It has been suggested that a part of the supported metal became inaccessible to the organic substrate due to localization of metal particles in narrow pores of the support. However, in the range of intermediate solution pH, the activity per surface metal atom remained at a high level. Such phenomenon is attributed to the formation of bulky species of platinum precursor, which hardly penetrates into the small pores of the support, thus preventing a blocking effect. Effect of acidity of the solution proved significant for Pt/C catalysts on the activated carbons with the smaller micropores and higher concentration of oxygen-containing groups.

Study of benzene hydrogenation catalyzed by nickel supported on alumina in a fixed bed reactor

The Ni/Al2O3 catalysts were prepared for studying benzene hydrogenation. It was observed that the activity for Ni loading under 5% is very low due to the formation of nickel aluminate (spinel) in low nickel loading. It has been found that Ni/Al2O3 systems exhibit SMSI (strong-metal support interaction) to a much lesser extent. It could be concluded that the hydrogenation of benzene on supported Ni catalysts is a structure insensitive reaction.

Transesterification of Dimethylcarbonate and Phenol Over Silica Supported TiO2 and Ti-MCM 41 Catalysts: Structure Insensitivity

Silica-supported titanium dioxide (TiO2/SiO2) and Ti-MCM 41 catalysts have been used for transesterification of dimethylcarbonate (DMC) and phenol to methylphenylcarbonate (MPC). The structure and the chemical state of titanium species in TiO2/SiO2 and Ti-MCM 41 have been investigated by means of X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) for Ti K-edge and X-ray photoelectron spectroscopy (XPS). To understand the role of pore size on the activity of catalysts, different pore size silica supports (Q-series) were utilized in TiO2/SiO2 catalysts. Similarly, to understand the effect of Ti symmetry on the activity of catalysts, Ti-MCM 41 was used with different Ti-loadings. It was observed that the Ti surface area was an only important factor to achieve highest activity. In case of Ti-MCM 41 catalysts, as the Ti-loading increased octahedral symmetry increased and tetrahedral symmetry decreased. But, turnover rates based on the surface Ti atoms were independent of the Ti symmetry. They are also similar to those obtained for TiO2/SiO2 catalysts. Showing that transesterification of DMC and phenol over Ti-based catalysts is a structure insensitive reaction.

Effect of Electrode Pt Loading on the Oxygen Reduction Reaction Evaluated by Rotating Disk Electrode and Its Implication on the Reaction Kinetics

The oxygen reduction reaction (ORR) activity was evaluated over film electrodes prepared from different Pt catalysts using rotating-disk electrode (RDE) and rotating-ring-disk electrode (RRDE) systems. The ORR activity as a function of Pt loading using the same catalyst was examined first, and it is concluded to be strongly influenced by mass transfer limitation. Two types of mass transfer limitation were identified. One is the external diffusion as described by the rotating-speed-dependent Levich equation. The other one is the internal diffusion within the electrode in which the electrode thickness is the governing parameter. The electrode thickness is closely related to the catalyst loading of the electrodes. After corrections of these mass transfer limitations, all the tested Pt catalysts under the conditions of this study show similar specific ORR activity with a Tafel slope near −120 mV/dec This result suggests that the ORR over Pt catalysts is a structure-insensitive reaction and the overall electro...

Surface-structure sensitivity of CeO 2 for several catalytic reactions

Vapor-phase catalytic reactions of several reactants were investigated over CeO 2 with different particle sizes, and the surface-structure sensitivity, i.e. crystal plane dependency, was discussed. The particle size of CeO 2 was regulated by the calcination temperature. In dehydration of 1,4-butanediol and 1,3-butanediol into unsaturated alcohols, and ketonization of propanoic acid, the activities increased with increasing the CeO 2 particle size. CeO 2 {1 1 1} facets became predominant on the large particles. This indicates that CeO 2 {1 1 1} facets have active sites for the reactions of butanediols and of propanoic acid. Therefore, dehydration of 1,4-butanediol and 1,3-butanediol, and ketonization of propanoic acid are categorized into surface-structure-sensitive reaction. The structure-sensitive reactions have feature that the main product is formed with high selectivity at temperatures lower than 400 °C. In contrast, activities of dehydration of 2-heptanol, methylation of phenol with methanol, and ketonization of propanal and 1-propanol were independent of the CeO 2 particle size. These reactions are categorized into structure-insensitive reaction.