Si Catalysts Research Articles

Ir‐Au Bimetallic Nanoparticle Modified Silicon Nanowires with Ultralow Content of Ir for Hydrogen Evolution Reaction

AbstractReducing the size and increasing the dispersity of the noble metals are effective ways to decrease the dosage and simultaneously increase their electrocatalytic activities for hydrogen evolution reaction (HER). The Ir−Au bimetallic nanoparticle modified silicon nanowires (Ir−Au−Si) with ultralow content of Ir are prepared and served to catalyze HER in the acid condition. The small Ir−Au nanoparticles are in‐situ grown and uniformly dispersed on the surface of silicon nanowires. Ir−Au−Si exhibits a significantly increased activity in HER catalysis compared with Ir−Si and Pt−Au−Si. Ir provides the active sites for HER, Au increases the electron transfer rate, and the silicon nanowires improve the speed of the hydrogen gas evolution. The optimal Ir−Au−Si‐2 shows a low Tafel slope of 24 mV dec−1 and high mass activity, which are better than those of Ir−Si and Pt−Au−Si catalysts.

Oxidative desulfurization of model and real oil samples using Mo supported on hierarchical alumina–silica: Process optimization by Box–Behnken experimental design

Abstract The catalytic performance of Mo supported on hierarchical alumina–silica (Si/Al = 15) with Mo loadings of 3, 6 and 15 wt% was investigated for the oxidative desulfurization (ODS) of model and real oil samples. Hierarchical alumina–silica (hAl–Si) was synthesized by economical and ecofriendly silicate-1 seed-induced route using cetyltrimethylammonium bromide (CTAB) as mesoporogen. The effect of CTAB on the structure of catalyst was studied by characterization techniques. The results revealed that 6%Mo/hAl–Si had the highest sulfur removal compared to the other catalyst loadings. The effect of operating parameters was evaluated using Box–Behnken experimental design. The optimal desulfurization conditions with the 6%Mo/hAl–Si catalyst were determined at oxidation temperature of 67 °C, oxidation time of 42 min, H2O2/S molar ratio of 8 and catalyst dosage of 0.008 g·ml−1 for achieving a conversion of 95%. Under optimal conditions, different sulfur-containing compounds with initial concentration of 1000 ppm, Dibenzothiophene (DBT), Benzothiophene (BT) and Thiophen (Th), showed the catalytic oxidation reactivity in the order of DBT > BT > Th. According to the regeneration experiments, the 6%Mo/hAl–Si catalyst was reused 4 times with a little reduction in the performance. Also, the total sulfur content of gasoline and diesel after ODS process reached 156.6 and 4592.2 ppm, respectively.

Combined steam and CO2 reforming of methane over one-pot prepared Ni/La-Si catalysts

The highly dispersed mNi/xLa−Si catalysts with varied weight percentages of Ni and La were synthesized via one-pot sol-gel process and subsequently applied to combined carbon dioxide and steam reforming of methane (CSDRM) for syngas production. The addition of La improved the catalytic activity and stability as well as the coke resistance of the mNi/xLa−Si catalysts. The effects of preparation routes, Ni contents and CO2/steam (C/S) ratios on the performances of the Ni/LaSi catalysts were studied in detail for the CSDRM. The 17.5Ni/3.0LaSi catalyst synthesized with the assistance of poly (ethylene glycol) and ethylene glycol exhibited the most excellent catalytic activity, stability and coke resistance. In addition, the H2/CO ratios in the product gas could be tuned by changing the C/S ratios in the feed. When the C/S ratio was 0.5, the H2/CO ratio of about 2 was achieved for the 17.5Ni/3.0LaSi catalyst.

Suppression of N2O formation by H2O and SO2 in the selective catalytic reduction of NO with NH3 over a Mn/Ti–Si catalyst

Proposed mechanism of NO reduction and N2O formation as well as H2O/SO2 suppression effects with participation of (a) Lewis acid sites and (b) Brønsted acid sites over a Mn/Ti–Si catalyst.

Methane decomposition into COx-free hydrogen and carbon nanomaterials over ZrO2–M (M = MgO, Al2O3, SiO2, La2O3 or CeO2) binary oxides supported cobalt catalysts

Hydrogen production via methane decomposition has attracted great attention as a source of clean energy. In this work, the catalytic decomposition of undiluted methane into COx-free hydrogen and carbon nanomaterial over ZrO2–M (M = MgO, Al2O3, SiO2, La2O3 and CeO2) binary oxides supported Co catalysts was studied. The fresh and spent catalysts were characterized by XRD, H2-TPR, N2 adsorption–desorption, TEM and Raman spectroscopy. The results revealed that the incorporation of secondary oxide to ZrO2 support played a vital role in the activity and stability of cobalt metal. The activity results illustrated that Co/Zr–Mg exhibited better activity in terms of hydrogen yield compared with the other Zr–M supported catalysts. This is ascribed to the moderate cobalt oxide-support interaction and forming CoMgOx species which enhance the Co3O4 dispersion and prevent its aggregation on the catalyst surface. On the other hand, the Co/Zr–Si catalyst showed the lowest activity due to the agglomeration of Co3O4 on the surface of the SiO2 support. High-resolution TEM images illustrated that almost the deposited carbon on the surface of spent catalysts was in the form of multi-walled carbon nanotubes.

Synthesis of mesoporous Ni–La–Si mixed oxides for CO2 reforming of CH4 with a high H2 selectivity

Ni–La–Si mixed oxides with different La loadings were first synthesized through one-step polyethylene glycol-assisted hydrolysis of tetraethoxysilane and inorganic salts in a water–ethylene glycol solution of nitric acid. The Ni–La–Si materials exhibited mesoporous structures with narrow pore size distributions and large specific surface areas. Effects of La loadings on material structure, surface properties, interaction between Ni species and supports, and dispersion of Ni species were discussed in detail. Homogeneously-dispersed NiO species were reduced in situ with H2 to form ultrafine metallic Ni nanocrystallites of 5–7 nm throughout the mesoporous silica. The obtained Ni/La–Si catalysts were applied for CO2 reforming of CH4 and exhibited high catalytic activity and excellent anti-coking ability. Addition of La in the Ni–La–Si catalysts not only enhance dispersion of Ni crystallites, reforming rate of Ni sites and catalytic stability, and more importantly, but also strongly inhibited reverse water gas shift reaction, resulting in almost 100% selectivity to H2 or a syngas with H2/CO molar ratio at the La loading of 3.0 wt%.

Partial Hydrogenation of Benzene to Cyclohexene on Ru@XO2 (X = Ti, Zr, or Si)

The effect of coating Ru particles with a film of hydrophilic materials on the tetra-phase partial hydrogenation of benzene to cyclohexene was examined. By the sol–gel method, amorphous mesoporous TiO2, ZrO2, and SiO2 were coated on hexagonal Ru particles, by which Ru@XO2 (X = Ti, Zr or Si) catalysts were prepared. During the partial hydrogenation of benzene on Ru@XO2, the diffusion of benzene and cyclohexene dissolved in the water phase onto Ru particles is suppressed by the presence of mesoporous coating film. The diffusion of benzene is retarded further by the hydrogen bonding with the structural OH groups contained in hydrophilic materials. Because of the delicate diffusion behavior of benzene and cyclohexene through coating film, a decreasing rate of benzene conversion but a significantly increasing cyclohexene selectivity was observed in the partial hydrogenation on different Ru@XO2 catalysts. A maximum cyclohexene yield of 52.6 mol % occurs on Ru@TiO2 at the benzene conversion of 85.0%.

The deactivation of a ZnO doped ZrO2-SiO2 catalyst in the conversion of ethanol/acetaldehyde to 1,3-butadiene.

A deactivation study on the ethanol/acetaldehyde conversion to 1,3-butadiene over a ZnO promoted ZrO2–SiO2 catalyst prepared by a sol–gel method was performed. The samples were characterized by N2 adsorption–desorption isotherms, scanning electron microscopy (SEM), NH3 temperature programmed desorption (NH3-TPD), X-ray powder diffraction characterization (XRD), thermogravimetric analyses (TGA), Fourier transform infrared resonance (FT-IR), 13C magic-angle spinning nuclear magnetic resonance (13C NMR) and X-ray photoelectron spectroscopy (XPS). The pore structure characteristics and surface acidity of Zn0.5–Zr–Si catalysts were largely decreased with time-on-stream and no crystal structure was formed in the used catalyst, indicating that the deactivation was caused by carbon deposition. Two main types of carbon deposition were formed, namely low-temperature carbon deposition with the oxidation temperature of around 400 °C and high-temperature carbon deposition with the oxidation temperature of 526 °C. The carbon species were mainly composed of graphitized carbon, amorphous carbon, carbon in C–O bonds and carbonyls. The deactivated catalyst could be regenerated by a simple oxidation process in air. Adding a certain amount of water into the feed had a positive effect on reducing the carbon deposition.

Fermentation–pervaporation–catalysis integration process for bio-butadiene production using sweet sorghum juice as feedstock

Abstract Fermentation–pervaporation–catalysis (FPC) integration process was employed to produce bio-butadiene using sweet sorghum juice as feedstock. After in situ ethanol removal from fermentation broth, permeate of pervaporation membrane (478.6 g/L bio-ethanol) was directly converted into butadiene by an Ag/Mg–Si catalyst. A butadiene yield of 64.1% with a butadiene selectivity of 75.8% was achieved within 6 h of operation. The integration process showed a great potential on high final product yield in a long-term operation. Following this protocol, about 16 g of bio-butadiene was produced from 1 kg sweet sorghum stalk. The integration process showed a potential in further application for bio-butadiene and materials production.

Ethylene Polymerization over MgCl2/SiO2 Bi‐Supported Ziegler–Natta Hybrid Titanium/Vanadium Catalysts

MgCl2/SiO2 bi‐supported Ziegler–Natta hybrid titanium/vanadium catalysts are synthesized with magnesium acetate as Mg source under different reaction sequences with TiCl4 and VOCl3. The activities of Ti/V hybrid catalysts are almost between those of MgTi/Si and MgV/Si catalysts. The hybrid titanium/vanadium catalyst prepared by coreaction with TiCl4 and VOCl3 shows higher activity than those prepared via two‐step reaction method. Regarding kinetic curves of ethylene polymerization, comonomer effect, and polymer properties, hybrid catalysts prepared through two‐step reaction method are inclined to exhibit more traits of the second active species supported during catalyst preparation. The vanadium species can adjust the molecular weight of polymers more efficiently by adding hydrogen with the sacrifice of catalytic activity for MgTiV/Si and MgTi‐V/Si catalysts. The one‐step coreaction method is more advantageous in catalytic activity, hydrogen response, and 1‐hexene incorporation than the two‐step reaction method for the MgCl2/SiO2 bi‐supported Ziegler–Natta hybrid titanium/vanadium catalyst system. image

Bifunctional gold catalysts: Relationship between preparation method and catalytic performance in tandem cellobiose valorization

Au was supported on carbon xerogel (CX) using various techniques and tested in tandem oxidation of cellobiose to gluconic acid (combining hydrolysis and oxidation steps in one-pot) in order to establish the relationship between the physicochemical properties of these materials and their performance as bifunctional catalysts. Notably higher selectivity to gluconic acid was obtained by catalysts with larger Au particle size showing moderate TOF. The performance of Au/CX catalysts was also affected by changes in surface chemistry of CX, introduced during deposition of Au. A direct link between the catalyst modifications and the reaction pathway was established by applying a simple reaction model to compare the rate constants of the intermediate processes. The experimental and modelled results revealed that reduction with citric acid was the most suitable method of preparation of the bifunctional catalyst. This catalyst was almost inactive in conversion of glucose and gluconic acid to side products, resulting in a 4 times higher yield of the desired product as compared to its counterpart prepared by sol-immobilization method (SI). On the other hand, the presence of PVA stabilizer on the surface of the SI catalyst resulted in the preferential oxidation of alcohol over aldehyde group in glucose, leading to poor selectivity of the cascade process. The reaction kinetics was examined and the apparent activation energy of the one-pot oxidation of cellobiose to gluconic acid was determined.

SBA‐15‐Supported Metal Silicides Prepared by Chemical Vapor Deposition as Efficient Catalysts Towards the Semihydrogenation of Phenylacetylene

AbstractMetal silicide (Pd2Si, Ni2Si, CoSi, FexSi, and Cu6.69Si) catalysts supported by SBA‐15 have been prepared by a chemical vapor deposition method under a H2 atmosphere using (CH3)2SiCl2 as the Si source. Of the SBA‐15‐supported metal silicide catalysts, Ni2Si/SBA‐15 presented a modest activity, notable selectivity to styrene (>90 %), and high stability with respect to the selective hydrogenation of phenylacetylene, which is attributed to the active‐site isolation and electron transfer in the silicide. Furthermore, Ni2Si/SBA‐15 also has a high selectivity to diphenylacetylene and 1,4‐butynediol, which demonstrates that Ni2Si/SBA‐15 is a promising, low‐cost catalyst alternative to noble‐metal catalysts as a selective hydrogenation catalyst.

Nucleation, evolution, and growth dynamics of amorphous silica nanosprings

The initial phases of amorphous silica nanospring formation via a vapor–liquid–solid mechanism are reported. The low temperature eutectic of Au–Si results in the formation of an asymmetrical shaped catalyst at the early stages of nanospring formation. As solid silica is formed below the Au–Si catalyst the system lowers its surface free energy and forms multiple amorphous silica nanowires beneath a common catalyst, as opposed to a single nanowire. The diameter of one of the nanowires forming the nanospring ranges between 10–20 nm. The difference in growth rates of the individual nanowires creates an asymmetry in the interfacial surface tension on the boundaries of the Au–Si catalyst/nanowires interface. Using Stokes’ theorem it is shown that there is a variable work of adhesion on the outer boundary of the Au–Si catalyst/nanowire interface of a nanospring, which is defined as an effective contact angle anisotropy. The anisotropic growth on the catalyst/nanowire boundary results in the nanowires coherently coiling into to a single, larger, helical structure with an overall diameter of 70–500 nm.

Succinic Acid Esterification on Mixed Oxides with Titanium

Ti–M (M˭Si, Al, Zr) mixed oxides exhibited better performances than single oxides for catalyzing succinic acid (SA) and butyric acid (BA) esterifications, because mixed oxide had larger surface area and acidity. The optimum catalyst compositions occurred at Ti/M atomic ratios of 1/3(TS13), 1/1 (TA11), and 3/1 (TZ31) for M˭Si, Al, and Zr, respectively. TS13 had the best activity (reached a diester yield of 98.1%), but had much smaller acidity than TA11, which was ascribed to the stronger interaction between the latter and water by-product. For Ti–Si catalysts, the pseudo first-order esterification rate constant increased linearly with increasing acid site amount. The rate constant of SA was about two times that of BA, suggesting that COOH on SA and BA had similar reactivity. For Ti–Zr oxides, the activity order was different from the acidity order, which might be due to their acid-base bi-functional property.

Pd-on-Si catalysts prepared via galvanic displacement for the selective hydrogenation of para-chloronitrobenzene.

The direct redox reaction (galvanic displacement) between Pd(2+) and substrate Si was used to deposit Pd on Si, and the Pd-Si catalysts enabled a chemoselective hydrogenation of para-chloronitrobenzene with the selectivity for para-chloroaniline higher than 99.9% at complete conversion of para-chloronitrobenzene.

Ni–MnOx Catalysts Supported on Al2O3-Modified Si Waste with Outstanding CO Methanation Catalytic Performance

A series of MnOx-promoted Ni catalysts supported on Si waste contact mass (W) modified by Al2O3 were successfully prepared by the deposition–precipitation method for CO methanation. Compared with the Ni catalysts directly supported on W, Ni/Al2O3–Si and MnOx-promoted Ni–MnOx/Al2O3–Si catalysts exhibited much better catalytic performance for CO methanation, particularly the latter, which exhibited the least decrease in CO conversion (1%) and lowest carbon deposit (0.3 wt %) in a 110 h lifetime test. The structural characterizations showed the highest Ni dispersion and highest oxygen vacancy concentration in the Ni–MnOx/Al2O3–Si catalyst. The added Al2O3 improves the dispersion of Ni, and the MnOx promoter can restrain the sintering and aggregation of Ni particles during the process of reduction and reaction at high temperatures and provides more oxygen vacancies, which is conducive to the removal of carbonaceous species on the catalyst surface for anticoking.

Effect of silica and alumina promoters on co-precipitated Fe–Cu–K based catalysts for the enhancement of CO2 utilization during Fischer–Tropsch synthesis

Silica and alumina were used as structural promoters to increase the catalytic activity of a co-precipitated Fe–Cu–K catalyst for the CO2 and CO hydrogenation during Fischer–Tropsch (FT) synthesis. The doubly-promoted Fe–Cu–K–Si–Al catalyst achieved higher CO and CO2 conversions than the Fe–Cu–K catalyst and singly-promoted Fe–Cu–K–Al and Fe–Cu–K–Si catalysts. The CO and CO2 conversions of the syngas with 54% H2/10% CO/29% CO2/7% N2 over the doubly-promoted catalyst were 88.3% and 25.2%, respectively, compared to 81.8% and 18.5% for the Fe–Cu–K catalyst. In this case, the C5+ selectivity of the doubly-promoted catalyst was 71.9%, which was slightly lower than 75.5% for the Fe–Cu–K catalyst. The CO2 was converted to hydrocarbons using the doubly-promoted catalyst when the CO2/(CO+CO2) ratio was higher than 0.35 for H2-balanced syngas at H2/(2CO+3CO2)=1.0, and 0.5 for H2-deficient syngas at H2/(2CO+3CO2)=0.5. The increase of hydrogen content in the syngas increased the methane selectivity at the expense of decrease in the liquid hydrocarbon selectivity.

Synthesis and Electrochemical Properties of CNFs-Si Composites as an Anode Material for Li Secondary Batteries.

We have performed a study on the electrochemical and structural characteristics of CNFs-Si composites which are active anode material for lithium secondary batteries. Carbon nanofibers (CNFs) have been synthesized by Chemical Vapor Deposition (CVD) using Co and Cu catalysts. The CNFs on the surface of the Si particle can provide a flexible space to relieve the volumetric expansion during a charge. Therefore, the CNFs composites on Si particles were prepared on the basis of the following two processes: (1) CNFs were grown on the simple mechanical mixture of Si particles and catalysts (CNFs/Si); (2) CNFs were grown on the surface of a pyrolytic carbon that was coated with Si particles (CNFs/PC/Si). The morphology and composition of CNFs-Si composites were analyzed by SEM and EDS measurements. Crystallinity and amorphicity were investigated using XRD and Raman spectroscopy. The characteristics of the synthesized CNFs-Si composites were analyzed through XPS, TGA, and BET. The two different CNFs-Si composite materials were evaluated as the anodic material in three different electrode cells. We found that the initial capacity of the CNFs/PC/Si composite electrode was 1,361 mAh/g with retention rate of 28.4%, which was better than the retention rate of 4.9% with the CNFs/Si electrode.

Synthesis and Characterization of Vapor‐grown Si/CNF and Si/PC/CNF Composites Based on Co–Cu Catalysts

We report a study on the synthesis and structural characteristics of Si/carbon nanofiber (CNF) and Si/PC/CNF composites. CNFs were synthesized by the chemical vapor deposition using Co and Cu catalysts at a Co:Cu ratio of 6:4. This study focused on the fabrication processes of Si/PC/CNF composites to confirm the contact of Si particles with CNFs. The CNF composites on Si particles were prepared on the basis of the following two processes—Sample 1: CNFs were grown on a simple mechanical mixture of Si particles and catalysts; and Sample 2: CNFs were grown on the surface of pyrolytic carbon‐coated Si particles. The morphology and composition of the Si/CNF composites were analyzed by scanning electron microscope and energy dispersive spectroscopy measurements. Physical properties were investigated using X‐ray diffraction, Raman, X‐ray photoelectron spectroscopy, and thermogravimetric analysis, and the specific surface area was measured by Brunauer, Emmett and Teller. Consequently, pyrolytic carbon coating on the surface of Si particles showed the ability to increase the contact of CNFs to the silicon particles. The resulting pyrolytic carbon‐coated Si/CNF (Si/PC/CNF) composite also demonstrated better crystallizability compared with the Si/CNF composite prepared through the first method.

Pt—Si Bifunctional Surfaces for CO and Methanol Electro-Oxidation

Bimetallic surfaces offer activity benefits derived from synergistic effects among active sites with uniquely different functions, which is particularly important for the development of highly effective heterogeneous catalysts for specific technological applications, such as energy conversion and storage. Here we report on Pt—Si bulk samples prepared by arc-melting, for the first time, with high activities toward the electro-oxidation of CO and methanol. Increasing the Si concentration on the surface was correlated with the shifts of onset oxidation potentials to lower values and higher activities for CO and methanol electro-oxidation. It is proposed that the reaction on the Pt—Si catalyst could follow a Langmuir–Hinshelwood type of mechanism, where substantially enhanced catalytic activity is attributed to the fine-tuning of the surface Pt—Si atomic structure.