•Simple and highly selective method to deposit Pt nanoparticles inside multi-walled carbon nanotubes (MWCNTs).•Simple image analysis method to obtain particle location selectivity inside/outside MWCNTs from normal 2D TEM images.•The selectivity of particles inside MWCNT reached 80%.A simple and efficient method was developed for the introduction of metal nanoparticles inside multi-walled carbon nanotubes (MWCNTs). The method is based on the incipient wetness impregnation using the aqueous solution of a metal salt precursor, followed by the reduction in hydrogen. A hydrophobic MWCNT outer surface is created, which minimizes the wetting of the impregnation solution on MWCNT outer surface, leading to the highly selective deposition of metal particles inside MWCNTs. The selectivity toward nanoparticles inside MWCNTs is about 80% based on a particle number average. Also, a new method was developed for the analysis of microscopic images from transmission electron microscope (TEM) so that the fraction of particles inside MWCNTs can be accurately characterized without 3D TEM. This method is based on the measurement of r/R , the ratio of the inside to outside diameters of the MWCNTs.

Pt and Pt–Co bimetallic catalysts supported on single-walled carbon nanotubes (SWNTs) were synthesized by a wet reduction–decoration method and tested for catalytic activity of aqueous phase reforming of ethylene glycol. The Pt decorated on SWNT achieves a catalyst mass time hydrogen yield of 890 micromole gcat–1 min–1, which is higher than the reported results for Pt–alumina catalyst. Experiments also show that this catalyst has better activity than Pt supported on activated carbon with a similar surface area, showing the advantage of SWNTs as a catalyst support. Factors affecting the aqueous phase reforming activity, such as temperature, pressure, WHSV, catalyst particle size, etc., were investigated. We have also explored Pt–Co bimetallic catalysts by combining the structural characterization results with the reactivity results and revealed that bimetallic catalysts may promote the catalyst performance in two different ways: either via the formation of Pt–Co alloy phase or via the synergistic catalytic activities of individual Pt and Co particles. The Pt–Co–SWNT catalyst achieved a hydrogen production activity as high as 4.6 mmol gcat–1 min–1.Keywords: aqueous phase reforming; bimetallic catalyst; biofuel; cobalt; EXAFS; hydrogen energy; platinum; single-walled carbon nanotubes;

In this paper, we present a direct one-step synthesis of Pt–Co–SWCNT from the catalytic CO disproportionation over Pt–Co–MCM-41 catalysts with different Pt loadings. The SWCNT produced by Pt–Co–MCM-41 shows higher yield than the SWCNT produced by Co–MCM-41 under the same reaction conditions, and also SWCNTs with diameter larger than 2 nm have been identified in the products. X-ray absorption and temperature programmed reduction studies indicate that Pt exists as metallic nanoparticles in the Pt–Co–MCM-41 catalysts, while Co is atomically dispersed and tetrahedrally coordinated in the MCM-41 framework as a Co2+ cation. Detailed structural study also suggested that in the Pt–Co–SWCNT hybrids produced by this method, the Pt–Co bimetallic nanoparticles consist of a Pt core and Co shell with no detectable alloy phase. This study provides a method for the direct synthesis of a Pt–M–CNT catalyst, where M is a non-precious metal.

In this study, silica-based mesoporous materials (the M41S family mesoporous molecular sieves) are synthesized using alkyltrimethylammonium bromide with different chain lengths (CnH2n+1N(CH3)3Br, n = 10, 12, 14, 16) as templates. The resulting silica structures are characterized by X-ray diffraction and are found to exhibit the phase transformation from the hexagonal mesophase MCM-41 to the cubic mesophase MCM-48 (with the space group of Ia3d). The structural phase transition in our study is controlled by the alkyl chain length of the surfactant: with an increase in the surfactant chain length (from C10 to C16), the structure goes from MCM-41 (synthesized by C10), through an intermediate structure (synthesized by C12), to MCM-48 (synthesized by C14 and C16). The amount of ethanol, which is used as a cosolvent, affects the pore size of the structured mesoporous silica, but only to a small extent. In the mean time, the autoclaving time has some effect, though not distinctively, on the structure integrity as well. With increased surfactant to silica ratio, the phase transformation can be shifted to longer chain template.Graphical abstractHighlights► Surfactant chain-length affects hexagonal to cubic transformation in M41S materials. ► A mechanism for chain-length phase transformation is hypothesized. ► Chain-length phase transformation can be shifted with surfactant/silica ratio.

The composite of multiwalled carbon nanotubes (MWCNTs) decorated with ZrO2 nanoparticles, synthesized by a grafting method followed by high-temperature annealing, was studied. The oxygen functionalized MWCNT surface uniformly disperses and stabilizes the oxide nanoparticles to an extent that is controlled by the metal oxide loading and thermal annealing temperature. This ZrO2/MWCNT also withstands decomposition in a hydrothermal environment providing potential applications in the catalysis of biomass conversion (e.g., aqueous phase reforming). The ZrO2/MWCNT have been characterized by (scanning) transmission electron microscopy ((S)TEM), X-ray diffraction (XRD), in situ small-angle X-ray scattering (SAXS), in situ wide-angle X-ray scattering (WAXS), and near edge X-ray fine structure (NEXAFS) for the purpose of a comprehensive analysis of the ZrO2 particle size and particle size stability.

We discuss the synthesis of a composite of ZrO2 nanoparticles supported on multiwalled carbon nanotubes (MWCNT). Using X-ray diffraction and high-resolution transmission electron microscopy (HR-TEM), the ZrO2 were found to be 2–3 nm tetragonal crystalline nanoparticles. Strong interfacial interaction between the ZrO2 nanoparticles and the MWCNT surface was observed by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge and the oxygen K-edge, and this strong metal oxide/support interaction leads to small ZrO2 particle size and thermal stability. The ZrO2/MWCNT was converted into a solid acid catalyst by sulfation, and the properties of S-ZrO2/MWCNT were studied. The nature of the acid sites was probed by S K-edge and Zr L-edges (L3, L2, L1) XANES as well as catalytic probe reaction of cyclohexane dehydrogenation/cracking. Such composites would be good candidates for potential catalysis applications in fuel cell electrodes and biomass processing.

Pt monometallic and Pt−Co bimetallic catalysts have been prepared on single-walled carbon nanotubes (SWNT) with and without HNO3 treatment. The HNO3 treatment introduced oxygen containing groups (OCGs), which affect both the structure and activity of the catalyst. The introduction of OCGs does not affect the structure of Pt monometallic catalysts but increased the dispersion in the bimetallic catalysts. The aqueous phase re-forming (APR) activity of the bimetallic catalysts is also affected by the OCGs, because the local concentration of the reactant around the SWNT support with OCGs is less than the case without OCGs. The two effects act on activity in opposing directions so the bimetallic catalysts on the two supports give similar APR yields, but this discovery gives us direction and a basis for the future design and improvement of SWNT supported catalysts.

Highly dispersed cobalt on SBA-15 was successfully prepared by a post synthesis grafting of cobalt. Of the cobalt precursors tested, Co(II) acetylacetonate was found to be the best source for high dispersion of cobalt. The Co-SBA-15 catalysts were characterized with different techniques: N2 physisorption, XRD, TPR, TEM, and X-ray adsorption analysis. The mesoporous structure of SBA-15 was retained after cobalt grafting with up to 10 wt % Co loading. There were no large cobalt oxide particles formed, which indicates all the cobalt ions are highly dispersed on the surface and the direct bonding to the silica surfaces results in a high reduction temperature (1123 K) relative to Co oxides. X-ray absorption analysis demonstrates a local structure of Co ions with all Co ions isolated and bonded with oxygen. XANES analysis requires that the local environment for Co ions be that of either a distorted tetrahedral or an octahedral structure and the fitting of EXAFS data further shows a Co−O bond coordination number of 3.58 ± 0.48, confirming that the Co is in a distorted tetrahedral environment. The catalytic activity of Co-SBA-15 catalyst was studied for the synthesis of carbon single walled nanotubes (SWNT). The high reduction stability of Co-SBA-15 is presumed to make a favorable catalyst for this high temperature reaction. Raman spectroscopy and TEM photographs show that good quality carbon SWNT was synthesized by Co-SBA-15. Moreover, Co-SBA-15 has a higher yield of carbon SWNT compared with Co-MCM-41 (C16 alkyl template) under the same reaction conditions.

High-quality single-walled carbon nanotubes (SWNT) with high yield were produced by using small-pore Co-MCM-41 catalyst, templated by a C10 surfactant and containing 3 wt % Co. A complete incorporation of Co ions in the silica matrix without formation of surface Co oxides and the contact time of the reaction, in the catalyst synthesis and the SWNT production, respectively, were the most critical factors to be considered. By controlling the reduction temperature and contact time in the reaction, the carbon yield could reach 34 wt % or higher with a selectivity of 96 wt % to SWNT. The metal content after purification of SWNT by base−acid treatments was 0.7 wt %, and the surface area was as high as 1800 m2/g. The metal surface occlusion effect by amorphous silica might play a key role in the stabilization of the completely reduced Co metallic clusters in the SWNT synthesis procedure, using small-pore C10 Co-MCM-41.

Transition metal incorporated MCM-41 materials were synthesized by careful control of multiple factors affecting the final product quality. Among them, the pH adjustment of the initial synthesis solution of Co-MCM-41 was crucial for the distribution of metallic ions in the silica framework. Higher pH significantly improved the physical structure of Co-MCM-41 and is accompanied by higher porosity. Each pore of Co-MCM-41, at the optimum pH, exhibited a hexagonal pore shaped entrance, not just the usual hexagonal array of pores of approximate circular shape. A mild acid treatment of Co-MCM-41 was a useful technique to remove surface metal oxide, which causes lowering of reduction stability by hydrogen spillover on the surface.

The influence of different ratios of colloidal (Cab-O-Sil, HiSil-915 and sodium silicate) and soluble (tetramethylammonium silicate) silica on the physicochemical properties (surface area, porosity, degree of structural order, thermal stability, density of free silanol group) of MCM-41 has been investigated. MCM-41 materials synthesized hydrothermally have been characterized by XRD, N2 physisorption, and IR spectroscopy. The results show that colloidal silica (HiSil-915 or Cab-O-Sil) to soluble silica ratio corresponding to 80:20 favors the formation of MCM-41 material with higher degree of structural order and uniformity of mesopores than those synthesized from the conventional ratios (∼70:30). Also, a new approach for tailoring the pore diameter within a modest range (∼20–30 Å) and controlling the other properties (surface area, porosity, and pore wall thickness) of MCM-41 has been demonstrated in this study. MCM-41 materials synthesized from Cab-O-Sil have thicker pore walls (15–20 Å) and show better thermal stability than when HiSil-915 is used.

The need for a multivariable designed experimental approach to the synthesis of Co-MCM-41 is described. The designed experiment was implemented with a total of 28 experiments that produced a quantitative model. This model allows the synthesis of Co-MCM-41 with varying pore size but constant composition and a good degree of structural order.

Vanadium substituted mesoporous molecular sieves (V-MCM-41) were prepared following a synthesis procedure developed in our laboratory [J. Phys. Chem. B 106 (2002) 8437]. N2 adsorption and X-ray diffraction experimental results show that our samples have a highly ordered hexagonal structure and fairly good stability. X-ray absorption and UV–vis spectra provide strong evidence that most vanadium ions are incorporated into the framework of siliceous MCM-41. The most important variable properties are the structural order, pore diameter, and vanadium loading which can be controlled easily by changing synthesis variables in such a way that collapse of the V-MCM-41 structure does not occur. A more quantitative model was built to correlate the structure, pore size, and vanadium content with the main synthesis variables and their cross terms. Modeling results show that our models are reasonably good at predicting the structure and pore diameter, it also can predict vanadium loading content well with longer alkyl chain length.

Chromium-substituted MCM-41 and MCM-48 mesoporous molecular sieves have been prepared via a direct hydrothermal synthesis. A change of structure of mesoporous molecular sieves and coordination of chromium was investigated by X-ray diffraction (XRD), nitrogen adsorption and X-ray absorption measurements after various redox gas treatments at high temperature. It was determined from the XRD patterns that the Cr containing material has a hexagonal MCM-41 and a cubic MCM-48 structure, respectively, and the structure is not affected significantly by redox gas treatments. The pore structure and surface area, analyzed from nitrogen isotherms, of Cr-substituted MCM-41 and MCM-48 are maintained after reduction and re-oxidation, which were performed sequentially after calcination. It is suggested from X-ray absorption near edge structure that the local environment of Cr in the MCM-41 and MCM-48 structures was transformed from tetrahedral to octahedral coordination by reduction, and vice versa by re-oxidation.

Several mesoporous aluminosilicate molecular sieves with the MCM-41 structure (Si/Al=24) have been synthesized using different aluminum and silica sources and varying several synthesis parameters during the preparation process such as the use of an anti-foaming agent, prior aging of the gel mixture or pH adjustment of the gel mixture under basic conditions followed by autoclaving three times. All samples were characterized by X-ray diffraction (XRD), N2 physisorption, TGA, MAS NMR and elemental analysis.There is a significant improvement in the structure by using an anti-foaming agent before adding the surfactant, by aging the mixture of silica and aluminum source prior to synthesis, and by using δ-alumina instead of PHF alumina. Also, when the sample is synthesized, alternating the pH adjustment and autoclaving three times in sequence, well-resolved XRD peaks are obtained. These Al-MCM-41 materials have BET areas around 1000 m2/g but the amount of tetrahedral coordinated Al, on the whole, is low.

Point of zero charge is a useful measurement to assess the surface acidity of multi-walled carbon nanotubes and to characterize functional groups on the multi-walled carbon nanotube surface. Knowledge of point of zero charge also assists the choice of an appropriate metal precursor to be used for electrostatic adsorption to prepare metal particles supported on multi-walled carbon nanotubes. Multi-walled carbon nanotubes with points of zero charge that range from 2.2 to 11.8 have been prepared and characterized. An efficient reduction method, utilizing ethanol at 20 atm and 180 °C, is described and can be used to reduce nitric acid functionalized multi-walled carbon nanotubes to convert the functional groups to mostly hydroxyls.

The methanation of carbon dioxide was carried out over Ni-incorporated MCM-41 catalysts. The Ni-MCM-41 catalysts containing 1–3 wt% Ni were prepared by incorporating the Ni ions into the framework of siliceous MCM-41 with the procedure developed in our previous work [S. Lim, G.L. Haller, J. Phys. Chem. B 106 (2002) 8437]. Pretreatment using pure H2 at different temperatures affected the reactivity. Pretreatment at 973 K for 0.5 h was the best among the conditions tested. At this temperature, the Ni was mostly reduced but remained highly dispersed. Significant selectivity to methane (85.1%) was obtained with 1 wt% Ni-MCM-41 at a reaction temperature as low as 573 K and gas hourly space velocity (GHSV) of 5760 l kg−1 h−1 under atmospheric pressure. Higher selectivity (96.0%) and space-time yield (91.4 g kg−1 h−1) were achieved with higher Ni loading catalysts (3 wt% Ni) at the same space velocity. This selectivity was maintained at higher temperature (673 K) with increased space-time yield (633 g kg−1 h−1). The X-ray absorption result demonstrates that the Ni particle size does not significantly change during reaction.

We have developed a simple method to create a catalyst with atomically dispersed Pt on top of Co nanoparticles on single walled carbon nanotubes (SWNT) supports by sequential impregnation of Pt(II) and Co(II) solutions following by hydrogen reduction. The aqueous phase reforming activity is much higher than for Pt monometallic catalysts on SWNT supports prepared by several methods, either pre-reduced in hydrogen or in the liquid phase. The high selectivity of the monometallic catalysts is maintained for the bimetallic systems. The Extended X-ray Absorption Fine Structure (EXAFS) results at the Pt LIII edge show no observable Pt–Pt bond. Only Pt–Co bonds were observed, indicating high dispersion of Pt. The enhanced activity comes from two sources: the high dispersion of Pt and the effect of the Co as co-catalyst or modifier. This contribution demonstrates the possibility to further engineer bimetallic catalysts to improve the aqueous phase reforming activity, especially to retain good selectivity at high conversion.

Vanadium oxide grafted on mesoporous silica SBA-15 has been synthesized using a controlled grafting process. Its structure has been thoroughly investigated using different characterization techniques, including N2-physisorption, X-ray diffraction, transmission electron microscopy (TEM), Raman spectroscopy, H2 temperature-programmed reduction, X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). The spectroscopic results revealed that under dehydrated conditions, the grafted vanadium domains are highly dispersed on the SBA-15 surface, composed predominately of isolated VO4 units with distorted tetrahedral coordination. The suggested (SiO)3VO sites on the silica surface include one short bond (∼1.54 Å) and three long bonds (1.74 Å). Methanol oxidation was used as a chemical probe reaction to examine the catalytic properties of these catalysts. At low vanadium loading, the vanadium species grafted on the surface show structural properties similar to those of vanadium-incorporated MCM-41 catalyst. However, the present mesoporous V-SBA-15 catalysts in the oxidation of methanol to formaldehyde show remarkable catalytic performance compared with that of VOx/SBA-15 catalysts synthesized through a conventional wet impregnation method, which has been attributed to the homogeneous dispersion and uniformity of the catalytic vanadium species achieved on the SBA-15 support with large pore diameter and surface area. The acidic properties of V-SBA-15 was investigated by pyridine temperature-programmed desorption, which indicated the existence of both Lewis and Brönsted acid sites of the surface.

In this paper, we present a direct one-step synthesis of Pt–Co–SWCNT from the catalytic CO disproportionation over Pt–Co–MCM-41 catalysts with different Pt loadings. The SWCNT produced by Pt–Co–MCM-41 shows higher yield than the SWCNT produced by Co–MCM-41 under the same reaction conditions, and also SWCNTs with diameter larger than 2 nm have been identified in the products. X-ray absorption and temperature programmed reduction studies indicate that Pt exists as metallic nanoparticles in the Pt–Co–MCM-41 catalysts, while Co is atomically dispersed and tetrahedrally coordinated in the MCM-41 framework as a Co2+ cation. Detailed structural study also suggested that in the Pt–Co–SWCNT hybrids produced by this method, the Pt–Co bimetallic nanoparticles consist of a Pt core and Co shell with no detectable alloy phase. This study provides a method for the direct synthesis of a Pt–M–CNT catalyst, where M is a non-precious metal.