High Surface Area SiO2 Research Articles

Synergetic effect on catalytic activity and charge transfer in Pt-Pd bimetallic model catalysts prepared by atomic layer deposition.

Pt-Pd bimetallic nanoparticles were synthesized on TiO2 support on the planar substrate as well as on high surface area SiO2 gel by atomic layer deposition to identify the catalytic performance improvement after the formation of Pt-Pd bimetallic nanoparticles by surface analysis techniques. From X-ray absorption near edge spectra of Pt-Pd bimetallic nanoparticles, d-orbital hybridization between Pt 5d and Pd 4d was observed, which is responsible for charge transfer from Pt to Pd. Moreover, it was found from the in situ grazing incidence X-ray absorption spectroscopy study that Pt-Pd nanoparticles have a Pd shell/Pt core structure with CO adsorption. Resonant photoemission spectroscopy on Pt-Pd bimetallic nanoparticles showed that Pd resonant intensity is enhanced compared to that of Pd monometallic nanoparticles because of d-orbital hybridization and electronic states broadening of Pt and Pd compared monometallic catalysts, which results in catalytic performance improvement.

Synthesis of β-SiC powders by the carbothermal reduction of porous SiO2–C hybrid precursors with controlled surface area

SiO2–C precursors with various surface areas were derived from tetraethyl orthosilicate and phenolic resin as Si and C sources, respectively, by a modified sol–gel process using the in situ precipitation of phenol resin in a prepared wet gel. The surface area of the SiO2–C precursors was varied from 20 to 175 m2/g by changing the C/Si molar ratio in the preform. β-SiC powders were synthesized using carbothermal reduction in vacuum at the temperature range of 1200–1600 °C. The effects of the temperature and heat treatment time as well as that of the surface area of the preform on the formation of β-SiC powders were studied. It was determined that the formation of β-SiC started at 1200 °C and was considerably promoted as the heat treatment temperature and time further increased during the carbothermal reduction of SiO2–C preforms with high surface area. When high surface area SiO2–C preforms were used, highly crystalline SiC powders were synthesized at 1600 °C in vacuum with a high yield of 85%.

Development of bio-inspired supports based on Ca–SiO2 and their use in hydrodeoxygenation of palmitic acid

Novel supports were developed for biomass conversion. A series of Ca–SiO2 materials were synthesized with a bio-inspired methodology. We used CaCO3 and CaSO4 for the modification of high surface area SiO2 to introduce basic and acid sites into the support. These two calcium sources were impregnated into the SiO2 in the presence of l-Alanine or l-Glycine. Both amino acids are able to bind calcium and those were used as a bio-inspiration procedure for changing the surface properties of SiO2. A series of MoO3 catalysts were prepared on the different supports and those were tested in the hydrodeoxygenation (HDO) of palmitic acid (PA). We found the formation of crystalline phases at the support surface due to the Ca addition. CaCO3 crystalline structure disappeared after the preparation of the catalysts because a new compound is formed, CaMoO4, while CaSO4 is not able to form any new compound with MoO3. Surface basicity and acidity were characterized by FT-IR using CO2 or pyridine adsorbed on the materials, respectively. We found that acidity plays a major role in the HDO of PA. Highly active MoO3-based catalysts were obtained by using SiO2–CaSO4 or SiO2–CaCO3 supports combined with amino acids.

A Molecular/Heterogeneous Nickel Catalyst for Suzuki–Miyaura Coupling

A molecular/heterogeneous catalyst motif based on an earth abundant nickel catalyst and SiO2 support has been designed and synthesized. Characterization and catalytic testing indicate that the molecular nickel catalyst [(2,2′:6′,2′′-terpyridine-4′-benzoic acid)Ni(II)]Cl2 attached to a high surface area SiO2 support is the active cross-coupling catalyst and increased product yields are a result of increased molecular stability of the catalyst while attached to the SiO2 support. This molecular/heterogeneous motif is easily separated from reaction mixtures and can be recycled for multiple catalytic reactions. The catalyst is active for Suzuki–Miyaura catalysis at catalyst loadings as low as 0.1 mol %, and turnover numbers nearing 2000 have been achieved.

Sustainable, inexpensive, naturally multi-functionalized biomass carbon for both Li metal anode and sulfur cathode

Lithium sulfur (Li-S) battery has been regarded as the promising energy storage device. However, this technology faces great challenges from both anode and cathode, which are mainly caused by the nature of materials. Here, we report a kind of multifunctional carbon derived from biomass like rice husk for optimizing both lithium (Li) metal anode and sulfur (S) cathode for Li-S batteries. It has been proved that the surface functionalized rice husk derived carbon could effectively achieve the controllable deposition of Li. Notably, the nucleation overpotential is reduced and the Coulombic efficiency is also improved. As for the cathode, the biomass carbon with high specific surface area and natural SiO2 nanoparticles is benificial for confining sulfur and sulfides. As a result, when the modified Li metal anode paired with carbon/sulfur composite, the battery delivers improved discharge capacity, rate capability and cycling stability. Moreover, the overpotential between charge and discharge process in Li-S batteries is obviously lowered compared with bare Li foil. This novel and straightforward design via employing sustainable biomass for constructing both anode and cathode materials may do great help to achieve the commercialization of Li-S batteries.

Silica‐Protected Ultrathin Ni3FeN Nanocatalyst for the Efficient Hydrolytic Dehydrogenation of NH3BH3

AbstractTransition metal nitrides (TMNs) inevitably aggregate in the high‐temperature thermal ammonolysis process, resulting in blocking of the active catalytic sites and serious decreases in their corresponding catalytic performance. Therefore, how to synthesize ultrasmall TMNs with higher exposed unsaturated coordination sites is still a challenge. Herein, ultrathin Ni3FeN (Ni3FeN@SiO2) with the size of ≈20 nm and thickness of ≈3.0 nm is fabricated by SiO2 protection strategy. The obtained Ni3FeN@SiO2 displays outstanding performance in NH3BH3 hydrolytic dehydrogenation due to the high specific surface area, intrinsic metal‐vacancies, and hydrophilic SiO2 layer.

Oxidative reforming of ethanol over CeO2-SiO2 based catalysts in a fluidized bed reactor

The Pt-Ni/CeO2-SiO2 catalyst, obtained by impregnation of high-surface area SiO2 (400m2g−1) with cerium nitrate hexahydrate and double activated by Nickel and Platinum, was used in the oxidative steam reforming (H2O/C2H5OH=4, O2/C2H5OH=0.5, T=500°C) of ethanol and the performances of packed (PBR) and fluidized bed reactors (FBR) were compared. In the fixed bed reactor, a faster deactivation and lower hydrogen yields with TOS were observed: fluidized bed operation reduced carbon formation rate (CFR) of almost 4 times. The stability of the Pt-Ni/CeO2-SiO2 catalyst was investigated at different operating conditions (WHSV=4.1–20.5h−1) and, independently from contact time, the catalyst, after a partial deactivation, reached a plateau performance with no more variation in ethanol conversion and hydrogen yield. Coke selectivity, in fact, decreases with TOS and the carbon formation rate remained unchanged at steady conditions. The dependence of plateau ethanol conversion and CFR on contact time was also investigated.

Influence of preparation conditions on the catalytic activity of high surface area silica in partial methanol oxidation

High surface area SiO2 samples were synthesized by one-step hydrolysis and co-condensation at different pH (between 3.0 and 6.0) using tetraethyl orthosilicate as the Si source and formamide as the drying additive. The effects of the presence of H2O, formamide and NH3 solution on the hydrophobic nature, textural properties and catalytic functionality of SiO2 were investigated. The physico-chemical properties of the samples were measured by XRD, FTIR, SEM, TEM, N2-physisorption and NH3-TPD techniques. It was observed that the use of formamide led to the formation of strong Si-O-Si bonds and the samples possessed a high surface area (325–853m2g−1), large pore size (42.5Å) and large pore volume (0.875cm3g−1). The characterization results also indicated that SiO2 sample prepared in the absence of water at pH=3 possessed fewer surface hydroxyl groups, leading the sample to have predominantly hydrophobic nature. An increase in water content in the SiO2 precursor solutions resulted in an increase in the average particle size with a more hydrophilic nature. All synthesized SiO2 samples were tested for partial oxidation of methanol at a temperature range of 200–400°C. The sample synthesized at pH=3 without water offer 90% conversion at 400°C; however the SiO2 sample synthesized with low water content (100mmol) at pH=4 offered the best selectivity (83%) to methyl formate and formaldehyde with moderate methanol conversion (72%). The SiO2 sample prepared at pH=6 with more water content offered less methanol conversion (60%) and low selectivity toward oxygenated products. It is evident that SiO2 samples that possessed better textural properties and moderate hydrophobicity performed better in partial methanol oxidation.

Thermal Stability Limits of Imidazolium Ionic Liquids Immobilized on Metal-Oxides.

Thermal stability limits of 33 imidazolium ionic liquids (ILs) immobilized on three of the most commonly used high surface area metal-oxides, SiO2, γ-Al2O3, and MgO, were investigated. ILs were chosen from a family of 13 cations and 18 anions. Results show that the acidity of C2H of an imidazolium ring is one of the key factors controlling the thermal stability. An increase in C2H bonding strength of ILs leads to an increase in their stability limits accompanied by a decrease in interionic energy. Systematic changes in IL structure, such as changes in electronic structure and size of anion/cation, methylation on C2 site, and substitution of alkyl groups on the imidazolium ring with functional groups have significant effects on thermal stability limits. Furthermore, thermal stability limits of ILs are influenced strongly by acidic character of the metal-oxide surface. Generally, as the point of zero charge (PZC) of the metal-oxide increases from SiO2 to MgO, the interactions of IL and metal-oxide dominate over interionic interactions, and metal-oxide becomes the significant factor controlling the stability limits. However, thermal stability limits of some ILs show the opposite trend, as the chemical activities of the cation functional group or the electron donating properties of the anion alter IL/metal-oxide interactions. Results presented here can help in choosing the most suitable ILs for materials involving ILs supported on metal-oxides, such as for supported ionic liquid membranes (SILM) in separation applications or for solid catalyst with ionic liquid layer (SCILL) and supported ionic liquid phase (SILP) catalysts in catalysis.

Catalytic and catalytic free process for cellulose conversion: fast pyrolysis and microwave induced pyrolysis studies

The catalytic and non-catalytic pyrolysis of microcrystalline cellulose and phosphoric acid pre-treated cellulose was investigated. The thermal processes were carried out applying two different methodologies: conventional fast pyrolysis and microwave-induced pyrolysis. For the catalytic experiments different catalysts were evaluated: CeO2, Nb2O5, SiO2, high surface area SiO2, Si-MCM-48 and Al-Fe-MCM-48. In all cases the liquid fraction was evaluated by quantifying the yields of anhydrosugars (mainly levoglucosan, levoglucosenone and 1,4:3,6-dianhydro-α-d-glucopyranose) and aromatic hydrocarbons. In the reaction of microcrystalline cellulose levoglucosan was the main product, while levoglucosenone was predominant in the pyrolysis of phosphoric acid pre-treated cellulose. Catalysts improved the fraction of bio-oil and the product distribution depended on the nature of catalytic materials as well as the starting cellulose. On the other hand, the microwave induced pyrolysis favored the formation of char at expenses of liquid fraction. In this case levoglucosenone and other anhydrosugars in conjunction with furan compounds were the main products.

Interfacial Bonding Stabilizes Rhodium and Rhodium Oxide Nanoparticles on Layered Nb Oxide and Ta Oxide Supports

Metal nanoparticles are commonly supported on metal oxides, but their utility as catalysts is limited by coarsening at high temperatures. Rhodium oxide and rhodium metal nanoparticles on niobate and tantalate supports are anomalously stable. To understand this, the nanoparticle-support interaction was studied by isothermal titration calorimetry (ITC), environmental transmission electron microscopy (ETEM), and synchrotron X-ray absorption and scattering techniques. Nanosheets derived from the layered oxides KCa2Nb3O10, K4Nb6O17, and RbTaO3 were compared as supports to nanosheets of Na-TSM, a synthetic fluoromica (Na0.66Mg2.68(Si3.98Al0.02)O10.02F1.96), and α-Zr(HPO4)2·H2O. High surface area SiO2 and γ-Al2O3 supports were also used for comparison in the ITC experiments. A Born-Haber cycle analysis of ITC data revealed an exothermic interaction between Rh(OH)3 nanoparticles and the layered niobate and tantalate supports, with ΔH values in the range -32 kJ·mol(-1) Rh to -37 kJ·mol(-1) Rh. In contrast, the interaction enthalpy was positive with SiO2 and γ-Al2O3 supports. The strong interfacial bonding in the former case led to "reverse" ripening of micrometer-size Rh(OH)3, which dispersed as 0.5 to 2 nm particles on the niobate and tantalate supports. In contrast, particles grown on Na-TSM and α-Zr(HPO4)2·H2O nanosheets were larger and had a broad size distribution. ETEM, X-ray absorption spectroscopy, and pair distribution function analyses were used to study the growth of supported nanoparticles under oxidizing and reducing conditions, as well as the transformation from Rh(OH)3 to Rh nanoparticles. Interfacial covalent bonding, possibly strengthened by d-electron acid/base interactions, appear to stabilize Rh(OH)3, Rh2O3, and Rh nanoparticles on niobate and tantalate supports.

Synthesis of mesoporous polymeric carbon nitride exhibiting enhanced and durable visible light photocatalytic performance

A thiourea precursor was employed to synthesize mesoporous carbon nitride (C3N4) by a thermal polycondensation process with high surface area SiO2 and nanosphere SiO2 as two types of hard templates. The resultant mesoporous C3N4 samples have high surface areas (105–112 m2/g) and mesopores with narrow sizes distribution (9.3 nm). Photocatalytic performance was evaluated by removal of NO in air under visible light irradiation. The results showed that mesoporous C3N4 samples exhibited significantly improved photocatalytic activity in comparison with bulk C3N4, which also exceeded that of N-doped TiO2 and C-doped TiO2. The activity enhancement can be ascribed to the synergistic effects of large surface area and pore volume, enhanced light-harvesting ability, increased redox potential, and reduced recombination of charge carriers. In addition to the high activity, the mesoporous C3N4 samples also showed high photochemical stability. The mesoporous C3N4 photocatalysts with enhanced and durable activity could provide a new efficient material for environmental pollution control.

A TGA–FTIR perspective of fatty acid adsorbed on magnetite nanoparticles–Decomposition steps and magnetite reduction

The fatty acid stabilization of magnetite nanoparticles is important for a broad field of studies and applications. In numerous previous studies TGA analyses are applied to investigate these compounds and draw conclusions such as magnetite concentration and surface grafting densities of the chemisorbed molecules. There are however deviations in interpretation of the analysis results. In the presented work we contribute to the discussion on the inert gas decomposition of the fatty acid ricinoleic acid adsorbed on the surface of magnetite nanoparticles with a priori knowledge of magnetite concentration. We report on impacts of autoxidation of the fatty acid as well as significant reduction of magnetite from carbonaceous residues. The findings are based on subsequent gas analysis with FTIR coupled to the TG device. We show how stoichiometric calculations on the reduction in the temperature range of 600–900°C let conclude that the residues are most probably from the chemisorbed fatty acid molecules. Only the physically adsorbed fatty acid molecules have decomposed or detached before 600°C. In context to the investigations on chemically adsorbed fatty acid on magnetite we compare the decomposition of pristine fatty acid and fatty acid physically adsorbed on a high surface area SiO2 nanopowder. Three distinct steps of decomposition which have often been reported before are found and accounted.

Thermal degradation of PEO on SiO 2 nanoparticles as a function of SiO 2 silanol density, hydrophobicity and size

The degradation behavior of semicrystalline PEO on silica (SiO 2) nanoparticles as a function of silanol density, hydrophobicity and nanoparticle size was investigated under N 2 purge by derivative thermogravimetric analysis (dTGA) for adsorption amounts at or below plateau adsorption. The PEO was adsorbed onto colloidal (Stöber) onto 100 nm colloidal that had been heat-treated to vary the silanol density or hydrophobically modified, (CH 3) 3–SiO 2, and onto 15, 40 and 100 nm SiO 2 made by a water-glass process. Either one or two degradation peaks, T d , were observed for the adsorbed PEO, one at ∼180–210 °C and one at ∼250–300 °C. The degradation peak at 180–210 °C appeared either at high adsorption amounts for the 100 nm SiO 2, on the 15 nm SiO 2, and on hydrophobically modified SiO 2, where PEO not directly hydrogen-bonded with SiOH was expected. The degradation peak at 250–300 °C correlated with SiOH density of the SiO 2, and thus SiOH/C O contacts, and the peak position increased with increase in SiOH density. With decreasing adsorption amount for the 100 nm SiO 2, the 180–210 °C peak decreased with respect to the 250–300 °C peak. Thus, the freer PEO segments appeared to degrade at lower temperatures than the H-bonded segments. However, both T d s were well below those observed for neat PEO, which occurs at ca 388 °C. This may arise since PEO is oxidatively unstable, and the volatile degradation products of the relatively small number of PEO chains on the high surface area SiO 2 are removed quickly, resulting in more rapid degradation.

Oxidation and capture of elemental mercury over SiO 2–TiO 2–V 2O 5 catalysts in simulated low-rank coal combustion flue gas

High surface area SiO 2–TiO 2–V 2O 5 (STV) catalysts of various titania loadings were synthesized by a sol–gel method. The STV catalysts were tested for oxidation of elemental mercury (Hg 0) and its capture in simulated coal combustion flue gas representing those from combustion of low-rank coals (sub-bituminous and lignite). Experiments were conducted in a fixed-bed reactor at temperatures ranging from 26 to 400 °C. In simulated flue gas, Hg 0 oxidation efficiency over the STV catalysts was found to decrease dramatically from 135 to 300 °C. At typical selective catalytic reduction (SCR) operating temperatures, the catalyst's oxidation activity increased as titania loading of the STV catalysts increased up to 18 wt%. The reaction mechanisms over the STV catalysts at SCR operating temperatures were investigated using individual flue gas components (HCl, NO, SO 2 and H 2O) with O 2 balanced in N 2. Hg 0 oxidation over STV catalysts follows the Eley–Rideal mechanism where active surface species generated from adsorbed flue gas components react with gas-phase or weakly adsorbed Hg 0. Fresh STV catalysts had some capability for adsorbing oxidized mercury (Hg 2+) at 350 °C, and no obvious effect of the adsorbed Hg 2+ on subsequent Hg 0 oxidation was observed. The presence of HCl with O 2 had excellent oxidation and capture efficiency; however, without O 2 it remarkably inhibited Hg 0 adsorption on the STV catalysts. NO and SO 2 promoted Hg 0 oxidation and capture in the presence of O 2, but their promotional effects were insignificant in the absence of O 2. Water vapor showed prohibitive effects on Hg 0 oxidation due to its competition with reactive species such as HCl and NO for active adsorption sites. This study demonstrates the feasibility of using STV catalysts for Hg 0 removal at typical SCR operating temperatures. The identification of the reaction mechanism provides critical information for developing effective SCR catalysts for Hg 0 oxidation in coal combustion flue gas.

ChemInform Abstract: Elementary Surface Reactions in the Preparation of Vanadium Oxide Overlayers on Silica by Chemical Vapor Deposition.

Elementary processes of the formation of vanadium oxide overlayers by the adsorption-decomposition of VO(OC 2 H 5 ) 3 vapor on high-surface area SiO 2 were studied by IR (infrared) spectroscopy, TPD (temperature-programmed decomposition), and the stoichiometry of the surface reactions. In this study the adsorption-decomposition process will be called a CVD (chemical vapor deposition) cycle. The structure of the vanadium oxide prepared by repeated CVD cycles was characterized by XRD (X-ray diffraction) and XPS (X-ray photoelectron spectroscopy)

Propylene Epoxidation: High-Throughput Screening of Supported Metal Catalysts Combinatorially Prepared by Rapid Sol–Gel Method

The gas phase oxidation of propylene using molecular oxygen was studied on a variety of supported metal catalysts. The most promising PO activity was obtained for Cu supported on high surface area SiO2 and the multimetallic systems exhibit synergistic effects that increased the desired PO yield by several folds for Ag promoted with Cu on SiO2 after screening a large number of catalysts by a high throughput testing technique.

Influence of K-doping on a Pd/SiO 2–Al 2O 3 catalyst

This work is devoted to understand the effect of K-doping (from K 2CO 3) on the reducibility, sintering, and surface properties of Pd metal nanoparticles deposited on a peculiar high surface area SiO 2–Al 2O 3 (SA) support. The effect of K-doping on the Pd/SA catalyst is systematically investigated by means of several complementary techniques (TPR, TEM, CO chemisorption and FTIR spectroscopy), trying to separate the effects of the doping loading (spanning the large 0.25–20.3 K/Pd atomic ratio interval), of the activation temperature (up to 823 K) and of the activation atmosphere (air or H 2). It is shown that the presence of dopant, already at the lowest loading, causes an important increase of the PdO reduction temperature. The sulphur poisoning released from the support in the undoped sample during the thermal treatments in H 2 atmosphere is inhibited in the presence of K 2CO 3. The dramatic loss of metal area measured by CO chemisorption for samples H 2-treated at high temperature is not justified by the small particle sintering observed by TEM, and is assigned to the progressive particle encapsulation due to the mobilization of the support by reaction between carbonate and support itself. FTIR spectroscopy of adsorbed CO, allowing to probe the nature of the Pd surface available for adsorption, confirms the hypothesis.

Determination of the Particle Size, Available Surface Area, and Nature of Exposed Sites for Silica−Alumina-Supported Pd Nanoparticles: A Multitechnical Approach

In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H2) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO2−Al2O3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6−8 Å) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/VChemi). The S/VChemi value drops significantly upon increasing the H2-reduction temperature. According to TEM, the sintering process can account only for a very small fraction of the S/VChemi decrease, suggesting an important poisoning of the potentially available Pd surface. This hypothesis is supported by a parallel experiment of thermal decomposition at the same temperature (in absence of H2), showing a S/VChemi value almost unchanged. FTIR spectroscopy of adsorbed CO, probing the nature of the Pd surface available for adsorption, confirms the hypothesis.

Solar thermochemical reactions III: A convenient one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles catalyzed by high surface area SiO 2 and induced by solar thermal energy

A simple, convenient and efficient method for the synthesis of 1,2,4,5-tetrasubstituted imidazole derivatives using benzoin, an aromatic aldehyde, an aromatic amine in the presence of ammonium acetate catalyzed by high surface area SiO 2 and induced by free solar thermal energy was reported.