Supercritical Fluid Reactive Deposition Research Articles

Deposition of Pd, Pt, and PdPt Nanoparticles on TiO2 Powder Using Supercritical Fluid Reactive Deposition: Application in the Direct Synthesis of H2O2.

In this study, we investigated the catalytic properties of mono- and bimetallic palladium (Pd) and platinum (Pt) nanoparticles deposited via supercritical fluid reactive deposition (SFRD) on titanium dioxide (TiO2) powder. Transmission electron microscopy analyses verified that SFRD experiments performed at 353 K and 15.6 MPa enabled the deposition of uniform mono- and bimetallic nanoparticles smaller than 3 nm on TiO2. Electron-dispersive X-ray spectroscopy demonstrated the formation of alloy-type structures for the bimetallic PdPt nanoparticles. H2O2 is an excellent oxidizing reagent for the production of fine and bulk chemicals. However, until today, the design and preparation of catalysts with high H2O2 selectivity and productivity remain a great challenge. The focus of this study was on answering the questions of (a) whether the catalysts produced are suitable for the direct synthesis of hydrogen peroxide (H2O2) in the liquid phase and (b) how the metal type affects the catalytic properties. It was found that the metal type (Pd or Pt) influenced the catalytic performance strongly; the mean productivity of the mono- and bimetallic catalysts decreased in the following order: Pd > PdPt > Pt. Furthermore, all catalysts prepared by SFRD showed a significantly higher mean productivity compared to the catalyst prepared by incipient wetness impregnation.

TiO2−CeOx−Pt Hollow Nanosphere Catalyst for Low‐Temperature CO Oxidation

AbstractTiO2−CeO2−Pt hollow nanospheres (1 wt‐% Pt) are realized using a liquid‐phase strategy using NaCl as a template. The NaCl template is first coated with TiO2 and thereafter with CeO2 via the hydrolyzation of TiCl(OiPr)3 and Ce(OiPr)4 as suitable alkoxides. Finally, the NaCl template is removed by washing with water. The resulting @TiO2−CeO2 hollow nanospheres (□: inner cavity) exhibit an outer diameter of 140–180 nm, a wall thickness of 30–40 nm, an inner cavity of 80–100 nm, a specific surface area of 210 m2/g, a pore volume and area of 0.08 cm3/g and 191 m2/g, mainly with micropores ≥5 Å and ≤14 Å. The hollow nanosphere support is impregnated with Pt nanoparticles, using two different methods – a wet‐chemical deposition (Pt(ac)2, acetone, 25 °C) and a supercritical fluid reactive deposition (SFRD) process ([Pt(COD)Me2], supercritical CO2, 80 °C, 15.6 MPa) resulting in an uniform size distribution with Pt nanoparticles 2.5±0.1 nm (TiO2−CeO2−PtWCD) and 2.3±0.1 nm (TiO2−CeO2−PtSFRD) in size. The catalytic properties of the TiO2−CeO2−Pt hollow nanospheres are evaluated for CO oxidation between 50 and 500 °C. A promising catalytic activity and stable light‐out/light‐off temperatures are observed especially for the TiO2−CeO2−PtSFRD sample, indicating the suitability of hollow nanospheres as high‐porosity catalyst material.

Supercritical deposition of mono- and bimetallic Pd and Pt on TiO[formula omitted] coated additively manufactured substrates for the application in the direct synthesis of hydrogen peroxide

In this study we investigated the properties of mono- and bimetallic Pd and Pt deposited via supercritical fluid reactive deposition (SFRD) on TiO2 coated additively manufactured substrates. The focus of this work lies on the suitability of these catalysts for the direct synthesis of H2O2 in the liquid phase. All catalysts produced showed a high activity towards the desired reaction, derived from high productivities towards the desired product. Thereby, the productivity of the different catalysts decreased in the following order: Pd = PdPt > Pt. In agreement with literature, an increase of the Pd loading from 1 to 2 wt.-% led to a decrease in the productivity. Resulting from comparison with productivities from literature, this method indicates a high suitability of the SFRD method for the suggested application. Due to the easy and eco-friendly nature of this deposition method the catalyst production process can be intensified.

Adsorption of Precursors on Substrates in the Presence of scCO2 for the Synthesis of Supported Metallic Nanoparticles: Experiments and Modeling

Supercritical fluid reactive deposition is an environmentally friendly technique for the synthesis of supported mono- or bimetallic nanoparticles. Experimental results show that the adsorption of a precursor on a substrate is the crucial process step that controls the loading and the size of the deposited metal nanoparticles. In this review, an overview of experimental and modeling work is given and selected experimental data were correlated with the following adsorption isotherm models: Henry, Freundlich, Langmuir, Toth, and Langmuir–Freundlich equations. As a result, in the case of precursors with a low CO2 solubility and therewith low uptake, the adsorption behavior can be described with sufficient accuracy by the Henry approach. Furthermore, the Freundlich and Langmuir equations enable sufficiently accurate descriptions of the experimental data. In the end, strategies for overcoming the knowledge gaps for essential future research directions are suggested.

Supercritical fluid reactive deposition: A process intensification technique for synthesis of nanostructured materials

Supercritical fluid reactive deposition (SFRD) is a promising process intensification technique for synthesis of a wide variety of nanostructured materials. The enhanced mass transfer characteristics of supercritical fluids (SCFs) coupled with high solubilities of reducing gases in SCFs provide many advantages related to equipment size and time minimization over conventional techniques. Among SCFs, the emphasis has been placed on supercritical CO2 (scCO2) which is non-toxic, cheap and leaves no residue on the treated medium. Moreover, in SFRD, multiple processes such as dissolution, adsorption, reaction, and purification are combined in a single piece of equipment which is an excellent example of process integration for process intensification. In this review, the fundamental thermodynamic and kinetic aspects of the technology are described in detail. The studies in the literature on synthesis of a wide variety of nanostructured materials including supported nanoparticles, films, and ion-exchanged zeolites by SFRD are reviewed and summarized. The applications of these materials as catalysts and sensors are described. The review hopes to lead to further studies on further development of this technology for a wide variety of applications.

Synthesis of Metal Nanostructures Using Supercritical Carbon Dioxide: A Green and Upscalable Process.

Metallic nanostructures have numerous applications as industrial catalysts and sensing platforms. Supercritical carbon dioxide (scCO2 ) is a green medium for the scalable preparation of nanomaterials. Supercritical fluid reactive deposition (SFRD) and other allied techniques can be employed for the mass production of metal nanostructures for various applications. The present article reviews the recent reports on the scCO2 -assisted preparation of zero-valent metal nanomaterials and their applications. A brief description of the science of pure supercritical fluids, especially CO2 , and the basics of binary mixtures composed of scCO2 and a low volatile substance, e.g., an organometallic precursor are presented. The benefits of using scCO2 for preparing metal nanomaterials, especially as a green solvent, are also being highlighted. The experimental conditions that are useful for the tuning of particle properties are reviewed thoroughly. The range of modifications to the classical SFRD methods and the variety of metallic nanomaterials that can be synthesized are reviewed and presented. Finally, the broad ranges of applications that are reported for the metallic nanomaterials that are synthesized using scCO2 are reviewed. A brief summary along with perspectives about future research directions is also presented.

Synthesis of supported nanoparticles in supercritical fluids by supercritical fluid reactive deposition: Current state, further perspectives and needs

Supercritical fluid reactive deposition is an environmentally friendly technique for the synthesis of supported mono- or bimetallic nanoparticles. Firstly, the present article gives a short introduction into the fundamental aspects of the particle formation process. A critical survey of published knowledge about synthesis of nanostructured materials and their deposition on different kind of supports is given and the underlying process basics i.e. dissolution of the precursor in the supercritical phase, adsorption or sorption of the precursor on the substrate and reduction of the precursor to its metal form are discussed. Examples for successful applications of these supported nanoparticles are highlighted. Based on a critical analysis and evaluation of the current status, strategies for overcoming the knowledge gaps, for essential future research directions are suggested. Throughout the manuscript, the challenges that need to be overcome for an improved understanding of the relationship between process conditions and obtained product properties are stated.

Preparation of SBA-15 supported Pt/Pd bimetallic catalysts using supercritical fluid reactive deposition: how do solvent effects during material synthesis affect catalytic properties?

Supercritical carbon dioxide is the preferred reaction medium for catalyst synthesis as exemplified for the hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL).

Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al2O3

The hysteresis in the CO oxidation profile over Pt/Al2O3 catalysts has been intensively studied, but its origin is still controversial. In this work, the influence of the Pt particle size and size distribution was systematically investigated. By the application of conventional and advanced preparation methods, such as flame spray pyrolysis and supercritical fluid reactive deposition, a series of catalysts containing homogeneous distributions of Pt particles were obtained. An optimal Pt particle size of 2–3 nm was identified for the CO oxidation light-off on Pt/Al2O3 catalysts. The CO oxidation results show a clear correlation between the Pt nanoparticle size and the ignition/extinction profile, including the switch of the hysteresis loop. On the basis of high-angle annular dark-field scanning transmission electron microscopy characterization combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and operando X-ray absorption spectroscopy measurements, the appearance of inverse h...

Optimization and modelling of the supercritical CO2 deposition of CoxOy nanoparticles in MCM41

Nanocomposites of cobalt oxide on mesoporous silica support (Co/MCM41) have been synthesized by supercritical fluid reactive deposition (SCFRD) using cobaltocene (CoCp2) as precursor. The study of adsorption isotherm for CoCp2 on MCM41 at 70°C and 11MPa has revealed an unfavourable adsorbate–adsorbent system with a maximum load of 43mgCo/gMCM41. Thus, an in situ thermal decomposition of precursor is necessary to obtain higher loadings of active phase. The modelling of the SCFRD process reveals that dissolution is very fast compared to the adsorption of cobaltocene on MCM41, and that Co content is maximum after 1h of decomposition at 200°C and 14MPa. The composite material with 4.3wt.% Co has a BET surface area of 1034m2g−1, a pore size of 3.9nm and metal nanoparticles of 20–30nm. This load can be increased by successive batches of 1h.

Preparation of supported Pt nanoparticles by supercritical fluid reactive deposition: Influence of precursor, substrate and pressure on product properties

Nanoparticles with high surface area deposited on different substrates are currently used extensively as catalysts for chemical transformations. Numerous experiments demonstrate that metallic nanoparticles can directly be deposited on various solid substrates by supercritical fluid reactive deposition. This technique involves the dissolution of the organometallic precursor in a supercritical fluid and the exposure of a substrate to this solution. In general, different methods such as chemical or thermal reduction under different pressures can be used to convert the precursor to its metal form. Results of these experiments show that uniform metallic nanoparticles in the range of 2nm can be produced. Average particle size and size distribution can be affected by the precursor reduction method, type and amount of the precursor, the surface properties of the solid substrate such as surface area and chemical nature of the surface. Metal loading depends on the metal content of the precursor and its concentration in CO2.

Production of supported gold and gold–silver nanoparticles by supercritical fluid reactive deposition: Effect of substrate properties

Supported monometallic gold and bimetallic gold–silver nanoparticles on different substrates are prepared by reactive deposition of Au(acac)Me2 and Ag(cod)(hfac) from supercritical carbon dioxide. Among others such supported metallic nanoparticles are suitable as catalysts for chemical reactions, e.g. CO oxidation. The influence of the substrate properties on the resulting mean particle size, distribution, particle structure and metal loading with focus on the provided surface area was investigated. The obtained mono- and bimetallic nanoparticles were characterized by transmission electron microscopy, electron dispersive X-ray spectroscopy, optical emission spectrometry with inductively coupled plasma and UV–Vis spectroscopy. The mean particle size varied between 2 and 26nm for monometallic gold and between 4 and 28nm for bimetallic gold–silver nanoparticles depending on the used substrate (mean particle size on BP2000<γ-Al2O3<TiO2<β-CD). EDXS line scans of single particles show both monometallic and bimetallic gold–silver core/shell particles consisting of a gold core which is surrounded by an intermetallic shell with an average composition of Ag1Au3.

Influence of Perfluorinated End Groups on the SFRD of [Pt(cod)Me(CnF2n+1)] onto Porous Al2O3in CO2under Reductive Conditions

The optimized synthesis of a range of cyclooctadiene-stabilized Pt complexes that contained different perfluoro-alkane chains, [Pt(cod)Me(Cn F2n+1 )], is presented. These metal-organic compounds were employed in the so-called supercritical fluid reactive deposition (SFRD) in CO2 under reductive conditions to generate metallic nanoparticles on aluminum oxide as a porous support. Thus, Al2 O3 -supported Pt nanoparticles with a narrow particle-size distribution were obtained. At a reduction pressure of 15.5 MPa and a temperature of 353 K, particle diameters of d50 =2.3-2.8 nm were generated. Decreasing the pressure during the reduction reaction led to slightly larger particles whilst decreasing the amount of organometallic precursor in CO2 yielded a decrease in the particle size from x50 =3.2 nm to 2.6 nm and a particle-size distribution of 2.2 nm. Furthermore, substitution of the CH3 end group by the Cn F2n+1 end groups led to a significant drop in Pt loading of about 50 %. Within the series of perfluorinated end groups that were considered, the Pt complex that contained a branched perfluoro-isopropyl group showed the most-interesting results when compared to the control precursor, [Pt(cod)Me2 ] (1).

Novel PtCuO/CeO2/α-Al2O3 sponge catalysts for the preferential oxidation of CO (PROX) prepared by means of supercritical fluid reactive deposition (SFRD)

Ceramic sponges (α-alumina) supporting ceria wash-coats and PtCuO layers of varying compositions were tested in the preferential oxidation (PROX) of CO in the presence of excess hydrogen. Copper and platinum were loaded simultaneously on the ceria-coated sponge by means of supercritical fluid reactive deposition (SFRD) which includes adsorption and reduction of metal-organic complexes dissolved in supercritical CO2. Products were characterized by means of SEM, STEM HAADF, EDX, H2-TPR, CO-TPD, CO2-TPD and N2-adsorption. Catalysts prepared after the SFRD-method exhibit high selectivity. The CO oxidation activity roughly correlates with the copper content and is superior as compared to the activity of PROX-catalysts reported in the literature so far.

Platinum nanoparticles and their cellular uptake and DNA platination at non-cytotoxic concentrations

Three differently sized, highly dispersed platinum nanoparticle (Pt-NP) preparations were generated by supercritical fluid reactive deposition (SFRD) and deposited on a β-cyclodextrin matrix. The average particle size and size distribution were steered by the precursor reduction conditions, resulting in particle preparations of <20, <100 and >100 nm as characterised by TEM and SEM. As reported previously, these Pt-NPs were found to cause DNA strand breaks in human colon carcinoma cells (HT29) in a concentration- and time-dependent manner and a distinct size dependency. Here, we addressed the question whether Pt-NPs might affect directly DNA integrity in these cells and thus behave analogous to platinum-based chemotherapeutics such as cisplatin. Therefore, DNA-associated Pt as well as the translocation of Pt-NPs through a Caco-2 monolayer was quantified by ICP-MS. STEM imaging demonstrated that Pt-NPs were taken up into HT29 cells in their particulate and aggregated form, but appear not to translocate into the nucleus or interact with mitochondria. The platinum content of the DNA of HT29 cells was found to increase in a time- and concentration-dependent manner with a maximal effect at 1,000 ng/cm(2). ICP-MS analysis of the cell culture medium indicated the formation of soluble Pt species, although to a limited extent. The observations suggest that DNA strand breaks mediated by metallic Pt-NPs are caused by Pt ions forming during the incubation of cells with these nanoparticles.

Cellular Uptake of Platinum Nanoparticles in Human Colon Carcinoma Cells and Their Impact on Cellular Redox Systems and DNA Integrity

Supercritical fluid reactive deposition was used for the deposition of highly dispersed platinum nanoparticles with controllable metal content and particle size distribution on beta-cyclodextrin. The average particle size and size distribution were steered by the precursor reduction conditions, resulting in particle preparations <20, <100, and >100 nm as characterized by transmission electron microscopy and scanning electron microscopy (SEM). These particle preparations of different size distributions were used to address the question as to whether metallic platinum particles are able to invade cells of the gastrointestinal tract as exemplified for the human colon carcinoma cell line HT29 and thus affect the cellular redox status and DNA integrity. Combined focused ion beam and SEM demonstrated that platinum nanoparticles were taken up into HT29 cells in their particulate form. The chemical composition of the particles within the cells was confirmed by energy-dispersive X-ray spectroscopy. The potential influence of platinum nanoparticles on cellular redoxsystems was determined in the DCF assay, on the translocation of Nrf-2 and by monitoring the intracellular glutathione (GSH) levels. The impact on DNA integrity was investigated by single cell gel electrophoresis (comet assay) including the formation of sites sensitive to formamidopyrimidine-DNA-glycosylase. Platinum nanoparticles were found to decrease the cellular GSH level and to impair DNA integrity with a maximal effect at 1 ng/cm(2). These effects were correlated with the particle size in an inverse manner and were enhanced with increasing incubation time but appeared not to be based on the formation of reactive oxygen species.