An integrated surface science approach towards metal oxide catalysis

The function of a metal oxide catalyst was investigated by an integrated approach, combining a variety of surface science techniques in ultrahigh vacuum with batch reactor conversion measurements at high gas pressures. Epitaxial FeO(111), Fe₃O₄(111) and α‐Fe₂O₃(0001) films with defined atomic surface structures were used as model catalysts for the dehydrogenation of ethylbenzene to styrene, a practized selective oxidation reaction performed over iron‐oxide‐based catalysts in the presence of steam. Ethylbenzene and styrene adsorb onto regular terrace sites with their phenyl rings oriented parallel to the surface, where the π‐electron systems interact with Lewis acidic iron sites exposed on Fe₃O₄(111) and α‐Fe₂O₃(0001). The reactant adsorption energies observed on these films correlate with their catalytic activities at high pressures, which indicates that the surface chemical properties do not change significantly across the pressure gap. Atomic defects were identified as catalytically active sites. Based on the surface spectroscopy results a new mechanism was proposed for the ethylbenzene dehydrogenation, where the upward tilted ethyl group of flat adsorbed ethylbenzene is dehydrogenated at Brønsted basic oxygen sites located at defects and the coupling of the phenyl ring to Fe³⁺ terrace sites determines the reactant adsorption–desorption kinetics. The findings are compared to kinetic measurements over polycrystalline catalyst samples, and an extrapolation of the reaction mechanism found on the model systems to technical catalysts operating under real conditions is discussed. The work demonstrates the applicability of the surface science approach also to complex oxide catalysts with implications for real catalysts, provided suitable model systems are available.     

In-situ pyrolyzed polymethylsilsesquioxane multi-walled carbon nanotubes derived ceramic nanocomposites for electromagnetic wave absorption

Iron acetylacetonate (Fe(acac)₃) modified polymethylsilsesquioxane (PMS), simplified as PMS(Fe), was firstly obtained from PMS and Fe(acac)₃ via the condensation reaction. Multi-walled carbon nanotubes (MWCNTs) were then introduced to fabricate the corresponding MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composites via pyrolyzed process. Owing to the catalytic effect of iron and heterogeneous nucleation promoted by MWCNTs, SiC nanocrystals were separated from SiOC amorphous ceramic matrix under 1400?°C. When the mass fraction of MWCNTs was 9?wt%, the obtained MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composite (C9) demonstrated high microwave-absorbing properties. The minimum reflection loss (RL_min) and effective absorption bandwidth (EBA) of the obtained C9 at X-band (8.2–12.4) reached ?61.8?dB and 2.6?GHz (a thickness of 2.19?mm), respectively. Compared with other polymer-derived ceramics (PDCs), the RL_min was higher and the required thickness was thinner. This excellent microwave-absorbing property was due to the interfacial polarization relaxation generated between nanocrystals (MWCNTs & SiC) and amorphous SiOC, and the formed complete conductive networks inside the ceramic composites.     

Dehydrogenation of ethylbenzene in the presence of CO2 using a catalyst synthesized by polymeric precursor method

Catalysts of iron oxide dispersed on Al or Si oxides were prepared via a polymeric precursor derived from the Pechini method and tested in the dehydrogenation of ethylbenzene in the presence of CO₂, in order to contribute with the studies of this reaction. The catalysts were characterized by thermogravimetric analysis (TG), temperature-programmed reduction (TPR), X-ray diffraction (DRX) and temperature-programmed desorption of CO₂ (TPD-CO₂). Analysis of the spent catalysts by TG and Fourier transformed infrared spectroscopy (FT-IR) pointed to the contribution of CO₂ to the coke deposition. The catalytic results suggest that the high initial ethylbenzene conversion is due to the contribution of basic sites, and the CO₂ adsorption in the basic site (lattice oxygen) may compete with the oxidative dehydrogenation of ethylbenzene. Although CO₂ provides the appropriate conditions to lower the consumption of the basic site, it is not able to promote the Fe²⁺ oxidation or to regenerate the basic site (lattice oxygen) in the iron oxide dispersed on Al or Si oxide catalysts.     

Iron oxide supported sulfated TiO2 nanotube catalysts for NO reduction with propane

Sulfated TiO₂ nanotubes and a series of iron oxide loaded sulfated TiO₂ nanotubes catalysts with different iron oxide loadings (1 wt%, 3 wt%, 5 wt% and 7 wt%) were prepared and calcined at 400 °C. The physico-chemical properties of the catalysts were studied by using XRD, N₂-physisorption, Raman spectroscopy, SEM-EDX, TEM, XPS, and pyridine adsorption using FTIR and H₂-TPR techniques. It was observed that iron oxide was highly dispersed on the sulfated TiO₂ nanotube support due to its strong interaction. The activity of these catalysts in the catalytic removal of NO with propane was also studied in the temperature range of 300–500 °C. Highest activity (90% NO conversion) was observed with 5 wt% iron oxide supported on sulfated TiO₂ catalyst at 450 °C. Selective catalytic reduction of NO activity of the catalysts was correlated with iron oxide loading, reducibility, and the Brönsted and Lewis acid sites of the catalysts. The catalyst also showed good stability under studied reaction conditions that no deactivation was observed during the 50 h of reaction.     

IR study of reaction of 2‐butene adsorbed on deuterated ZSM‐5 and mordenite

The dehydrogenation of ethylbenzene to styrene was studied over single-crystalline iron oxide model catalyst films grown epitaxially onto Pt(111) substrates. The role of the iron oxide stoichiometry and of atomic surface defects for the catalytic activity was investigated by preparing single-phased Fe₃O₄(111) and α-Fe₂O₃(0001) films with defined surface structures and varying concentrations of atomic surface defects. The structure and composition of the iron oxide films were controlled by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES), the surface defect concentrations were determined from the diffuse background intensities in the LEED patterns. These ultrahigh vacuum experiments were combined with batch reactor experiments performed in water–ethylbenzene mixtures with a total gas pressure of 0.6 mbar. No styrene formation is observed on the Fe₃O₄ films. The α-Fe₂O₃ films are catalytically active, and the styrene formation rate increases with increasing surface defect concentration on these films. This reveals atomic surface defects as active sites for the ethylbenzene dehydrogenation over unpromoted α-Fe₂O₃. After 30 min reaction time, the films were deactivated by hydrocarbon surface deposits. The deactivation process was monitored by imaging the surface deposits with a photoelectron emission microscope (PEEM). It starts at extended defects and exhibits a pattern formation after further growth. This indicates that the deactivation is a site-selective process. Post-reaction LEED and AES analysis reveals partly reduced Fe₂O₃ films, which shows that a reduction process takes place during the reaction which also deactivates the Fe₂O₃ films.     

Anisotropic conductivity of magnetic carbon nanotubes embedded in epoxy matrices

Maghemite (γ-Fe₂O₃)/multi-walled carbon nanotubes (MWCNTs) hybrid-materials were synthesized and their anisotropic electrical conductivities as a result of their alignment in a polymer matrix under an external magnetic field were investigated. The tethering of γ-Fe₂O₃ nanoparticles on the surface of MWCNT was achieved by a modified sol–gel reaction, where sodium dodecylbenzene sulfonate (NaDDBS) was used in order to inhibit the formation of a 3D iron oxide gel. These hybrid-materials, specifically, magnetized multi-walled carbon nanotubes (m-MWCNTs) were readily aligned parallel to the direction of a magnetic field even when using a relatively weak magnetic field. The conductivity of the epoxy composites formed in this manner increased with increasing m-MWCNT mass fraction in the polymer matrix. Furthermore, the conductivities parallel to the direction of magnetic field were higher than those in the perpendicular direction, indicating that the alignment of the m-MWCNT contributed to the enhancement of the anisotropic electrical properties of the composites in the direction of alignment.     

Effect of silane structure on the properties of silanized multiwalled carbon nanotube-epoxy nanocomposites

Multiwalled carbon nanotubes (MWCNTs) were functionalized with aminosilanes via an aqueous deposition route. The size and morphology of siloxane oligomers grafted to the MWCNTs was tuned by varying the silane functionality and concentration and their effect on the properties of a filled epoxy system was investigated. The siloxane structure was found to profoundly affect the thermo-mechanical behavior of composites reinforced with the silanized MWCNTs. Well-defined siloxane brushes increased the epoxy T_g by up to 19 °C and significantly altered the network relaxation dynamics, while irregular, siloxane networks grafted to the MWCNTs had little effect. The addition of both types of silanized MWCNTs elicited improvements in the strength of the nanocomposites, but only the well-defined siloxane brushes engendered dramatic improvements in toughness. Because the silanization reaction is simple, rapid, and performed under aqueous conditions, it is also an industrially attractive functionalization route.     

Multi-walled carbon nanotubes modified by sulfated TiO2 - A promising support for Pt catalyst in a direct ethanol fuel cell

We report the synthesis of multi-walled carbon nanotubes coated with sulfated TiO₂ (S-TiO₂/MWCNTs) as a promising support for Pt catalyst in a direct ethanol fuel cell. Highly dispersed Pt nanoparticles were supported on the S-TiO₂/MWCNT composites by NaBH₄ reduction procedure (Pt-S-TiO₂/MWCNTs). The presence and nature of the catalyst were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy. The size of the sulfated TiO₂ product was about 8 nm, and that of the Pt nanoparticle on the S-TiO₂/MWCNT composites was about 5 nm. The Pt-S-TiO₂/MWCNTs were used to study the electrochemical ethanol oxidation reaction using cyclic voltammetry, chronoamperometry and impedance spectroscopy. The results show that Pt-S-TiO₂/MWCNT catalysts show higher catalytic activity for ethanol oxidation compared with Pt supported on non-sulfated TiO₂/MWCNT composites and commercial Pt/C catalysts.     

Kinetics of the dehydrogenation of ethylbenzene to styrene over unpromoted and K-promoted model iron oxide catalysts

The dehydrogenation of ethylbenzene to styrene over unpromoted and potassium-promoted model iron oxide catalysts has been studied using ultrahigh vacuum techniques in conjunction with elevated pressure reaction kinetics. Model iron oxide catalysts were prepared by oxidizing a polycrystalline Fe sample that was subsequently dosed with metallic potassium. At 875 K the unpromoted catalyst exhibited a turnover frequency of 5×10^–4 molecules/ site s and an activation energy of 39 kcal/mol, both in excellent agreement with the results found for an analogous iron oxide powder catalyst. Potassium promotion increased the turnover frequency to 1.0×10^–3 molecules/site s and lowered the activation energy to 36 kcal/mol for the dehydrogenation reaction. Similarities between the activation energies on the unpromoted and promoted catalysts indicate that the active site is the same on both catalysts. Creation of the active site was dependent upon the formation of an Fe³⁺ metastable species, consistent with the formation of a KFeO₂ phase, upon the addition of potassium.     

Catalytic Performance and Sulfur Dioxide Resistance of One-Pot Synthesized Fe-MCM-22 in Selective Catalytic Reduction of Nitrogen Oxides with Ammonia (NH3-SCR)—The Effect of Iron Content

The catalytic performance of Fe-catalysts in selective catalytic reduction of nitrogen oxides with ammonia (NH₃-SCR) strongly depends on the nature of iron sites. Therefore, we aimed to prepare and investigate the catalytic potential of Fe-MCM-22 with various Si/Fe molar ratios in NH₃-SCR. The samples were prepared by the one-pot synthesis method to provide high dispersion of iron and reduce the number of synthesis steps. We have found that the sample with the lowest concentration of Fe exhibited the highest catalytic activity of ca. 100% at 175 °C, due to the abundance of well-dispersed isolated iron species. The decrease of Si/Fe limited the formation of microporous structure and resulted in partial amorphization, formation of iron oxide clusters, and emission of N₂O during the catalytic reaction. However, an optimal concentration of Fe_xO_y oligomers contributed to the decomposition of nitrous oxide within 250–400 °C. Moreover, the acidic character of the catalysts was not a key factor determining the high conversion of NO. Additionally, we conducted NH₃-SCR catalytic tests over the samples after poisoning with sulfur dioxide (SO₂). We observed that SO₂ affected the catalytic performance mainly in the low-temperature region, due to the deposition of thermally unstable ammonium sulfates.     

Distribution of alkaline promoters within the structure of iron oxide catalyst for dehydrogenation

Along with potassium, cesium is an efficient promoter of catalytic activity of iron oxide catalysts for dehydrogenation of olefins and alkylaromatic hydrocarbons. In the reaction medium, a catalyst is a ferrite system consisting of potassium β″-polyferrite, potassium and cesium monoferrites, and magnetite. The character of the distribution of alkaline promoters within the catalyst structure is studied to provide the theoretically substantiated calculation of the optimal composition of this type of catalysts. The preferred location of cesium ions is shown to be the structure of β″-polyferrite of K_{2 ? z} Cs _z Fe²⁺Fe ₁₀ ³⁺ O₁₇ composition. The catalytic activity of this system with different contents of cesium in the dehydrogenation of ethylbenzene to styrene (flow-type reactor; 0.1 MPa; 600°C; hourly space velocity of ethylbenzene, 1 h^?1; ethylbenzene : steam weight ratio, 1 : 3) was tested. The maximum specific rate of styrene formation is attained at a Cs : Fe ratio in the interval 0.023–0.027, corresponding to the coefficient z = 0.26–0.30. It is impractical to introduce more cesium. Theoretical propositions for targeted transporting of a promoting agent to a given phase of a catalytically active ferrite system are developed. The content of expensive cesium compounds in iron oxide catalyst is optimized.     

Multi-walled carbon nanotubes modified Bi2S3 microspheres for enhanced photocatalytic decomposition efficiency

A series of multi-walled carbon nanotubes (MWCNTs)-loaded Bi₂S₃ nanomaterials composites (MWCNTs/Bi₂S₃) were prepared by hydrothermal method. The crystallization, morphology and other properties of the obtained MWCNTs/Bi₂S₃ composites were completely characterized by powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FT-IR), Thermal gravimetric analysis and differential thermal analysis (DTA-TG), N₂ adsorption-desorption isotherm, and UV–vis diffuse reflectance spectroscopy (DRS). In this work, the photocatalytic activity of the as-prepared materials was evaluated for methylene blue (MB) degradation under visible light irradiation. Comparing with others work, the composites showed excellent photocatalytic performance and maintained a good stability during the constant cycling experiment. Finally, a possible photocatalytic reaction mechanism on the MWCNTs/Bi₂S₃ composites was proposed.     

Catalytic effect of iron oxide on carbon/carbon composites during graphitization

Coal tar pitch matrix was modified by addition of iron oxide in different proportions i.,e. 0, 1, 3 and 5% by weight. The matrix was used to develop carbon fibre reinforced composites heat treated to 1000 and 2500 °C, respectively. The catalytic effect of iron oxide was ascertained by measuring the physical properties viz. the inter layer spacing, thermal conductivity and flexural strength of the composites. Low concentrations of the catalyst resulted in improvement in the thermal conductivity of composites from 68 × 10⁻² W/m K for 0% to 127 × 10⁻² W/m K for 1% iron oxide concentration. The flexural strength of graphitized composites, however, showed a remarkable increase from 325 MPa for 0% to 450 MPa for 5% iron oxide concentrations. The increase in flexural strength was probably due to the development of large numbers of grain boundaries whereas the increase in the thermal conductivity was most likely due to larger crystallite size i.e. decreases in the interlayer spacing (d₀₀₂) of the graphitized composites.     

Catalytic decomposition of biomass tars with iron oxide catalysts

Catalytic gasification of wood (Cedar) biomass was carried out using a specially designed flow-type double beds micro reactor in a two step process: temperature programmed non-catalytic steam gasification of biomass was performed in the first (top) bed at 200–850 °C followed by catalytic decomposition gasification of volatile matters (including tars) in the second (bottom) bed at a constant temperature, mainly 600 °C. Iron oxide catalysts, which transformed to Fe₃O₄ after use possessed catalytic activity in biomass tar decomposition. Above 90% of the volatile matters was gasified by the use of iron oxide catalyst (prepared from FeCl₃ and NH_3aq) at SV of 4.5 × 10³ h^?1. Tar was decomposed over the iron oxide catalysts followed by water gas shift reaction. Surface area of the iron oxide seemed to be an important factor for the catalytic tar decomposition. The activity of the iron oxide catalysts for tar decomposition seemed stable with cyclic use but the activity of the catalysts for the water gas shift reaction decreased with repeated use.     

Polypropylene as a carbon source for the synthesis of multi-walled carbon nanotubes via catalytic combustion

Multi-walled carbon nanotubes (MWCNTs) were efficiently synthesized by catalytic combustion of polypropylene (PP) using nickel compounds (such as Ni₂O₃, NiO, Ni(OH)₂ and NiCO₃ · 2Ni(OH)₂) as catalysts in the presence of organic-modified montmorillonite (OMMT) at 630-830 °C. Morphologies of the sample undergoing different combustion times were observed to investigate actual process producing MWCNTs by this method. The obtained MWCNTs were characterized by X-ray diffraction (XRD), transmission electron microscope and Raman spectroscopy. The yield of MWCNTs was affected by the composition of PP mixtures with OMMT and nickel compounds and the combustion temperature. The proton acidic sites from the degraded OMMT layers due to the Hoffman reaction of the modifiers at high temperature played an important role in the catalytic degradation of PP to supply carbon sources that are easy to be catalyzed by nickel catalyst for the growth of MWCNTs. The XRD measurements demonstrated that the nickel compounds were in situ reduced into the Ni(0) state with the aid of hydrogen gas and/or hydrocarbons in the degradation products of PP, and the Ni(0) was really the active site for the growth of MWCNTs. The combination of nickel compounds with OMMT was a key factor to efficiently synthesize MWCNTs via catalytic combustion of PP.     

Microwave-assisted epoxidation of styrene with molecular O2 over sulfated Co–Y–ZrO2 solid catalyst

The microwave-assisted styrene epoxidation reaction with molecular O₂ as an oxidant was studied over a sulfated Co–Y-doped ZrO₂ solid catalyst. The microwave irradiation (400 W) resulted in similar styrene conversion and styrene oxide selectivity, in reduced time, as compared to conventional thermal heating. Higher power (800 W) of microwave irradiation decreased the styrene oxide selectivity as well as leading to the formation of styrene glycol. DMF was found to be the most suitable solvent for epoxidation of styrene with molecular O₂ under microwave irradiation and yielded maximum oxide selectivity (91%) at 120 °C. The microwave-assisted oxidation reaction resulted in time saving and is energy conserving method.     

Synthesis of multi-walled carbon nanotube-tungsten carbide composites by the reduction and carbonization process

Multi-walled carbon nanotube-tungsten carbide composites were prepared by the reduction and carbonization process using multi-walled carbon nanotubes (MWCNTs) and WO₃ precursor by molecular level mixing and calcination. The pre-prepared MWCNT-tungsten carbide composites were characterized by scanning electron microscope and transmission electron microscope. Furthermore, the crystal phase was identified by X-ray diffraction. The results showed that the one-dimensional (1D) nanostructure of the MWCNTs was destroyed by the direct carbonization reaction between the MWCNTs and the WO₃ precursor without an additional carbon source. Moreover, pure MWCNT-tungsten carbide composites were difficult to obtain. With the additional carbon source CH₄, pure MWCNT-tungsten carbide composites were prepared, and the 1D nanostructure of the MWCNTs was retained.     

Catalytic Redox Transformations in Rock Matrices

The properties of catalytic systems based on iron oxide and inorganic matrices of oil-bearing rocks (basalt, clay, sandstone) in the decomposition of ammonium nitrate, oxidation of methane, and hydrocracking of asphaltenes were studied. The catalytic systems were iron oxide (hematite with a particle size of D = 11.0–20 nm, preparation temperature 453–473 K) fixed on matrices during co-hydrolysis of carbamide and iron chloride under hydrothermal conditions at temperatures of T = 433–473 K and pressures of 0.6–1.6 MPa. The iron oxide catalysts based on basalt and clay were most active in deep oxidation of methane (at 773 K, \({X_{C{H_4}}}\) = 83% and 72.9%, respectively); the Fe₂O₃/basalt and Fe₂O₃/sandstone systems were more active in the decomposition of ammonium nitrate. In hydrocracking of asphaltenes to maltenes, the catalyst activity decreased in the series Fe₂O₃/basalt > Fe₂O₃/clay > Fe₂O₃/sandstone, the iron oxide catalysts on clay being most selective. The obtained experimental data confirm that natural materials (oil-bearing rocks: basalt, clay, and sandstone) may be used for the development of catalytic systems for reactions in oil beds and of advanced technologies for increasing the oil recovery.     

Preparation of titania/carbon nanotube composites using supercritical ethanol and their photocatalytic activity for phenol degradation under visible light irradiation

Titanium dioxide (anatase, TiO₂) nanoparticles have been successfully deposited onto multi-walled carbon nanotubes (MWCNTs) via hydrolysis of titanium isopropoxide in supercritical ethanol. The as-prepared composites were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. It was demonstrated that the MWCNTs were decorated with well-dispersed anatase nanoparticles less than 7 nm in diameter. The size and loading content of the nanoparticles on MWCNTs could be tuned by manipulating the ratio of precursor to MWCNTs, and the formation mechanism of the composites was also discussed. The absorbance spectrum of the resultant TiO₂/MWCNT composites extended to the whole UV-visible region due to the decoration of TiO₂ on MWCNTs. The TiO₂/MWCNT composites were used as photocatalyst for phenol degradation under irradiation of visible light, which showed higher efficiency compared to a mixture of TiO₂ and MWCNTs.     

Influence of MWCTs and nanometal oxide/MWCTs hybrids on the thermal conduction of their acrylic-based nanocomposites

In this study, acrylic-based nanocomposites containing different contents of multi-walled carbon nanotubes (MWCNTs) and metal oxide nanoparticles (i.e., TiO₂, CuO and Fe₂O₃) were fabricated by solvent mixing method. The thermal conductivity of these samples was evaluated. The results indicated that the thermal conductivity of all fabricated samples was significantly improved even at small loading of MWCNTs. It was found that the thermal conductivity was enhanced by increase in MWCNTs content up to 5 wt%. Similarly, the metal oxide nanoparticles caused up to 75 % increment in thermal conductivity at 1.5 wt% of their loading in acrylic film. Contrary to expectations, the thermal conductivity of acrylic film was more increased by nanometal oxides (i.e., TiO₂, CuO and Fe₂O₃) than MWCNTs. The effect of hybridizing of nanometal oxide particles (1.5 wt%) and MWCNTs (1.5 wt%) on thermal conduction was investigated as well. It was found that hybridizing improved thermal conductivities by about 85, 94 and 97 % for Fe₂O₃, TiO₂ and CuO, respectively. Finally, the effects of TiO₂ pigment and CaCO₃ extender on the thermal conductivity of acrylic polymer and nano-TiO₂ acrylic composites were studied. It was found that TiO₂ could increase considerably thermal conduction of its acrylic films and acrylic nanocomposites.