Oxidation of isobutane over Mo–V–Sb mixed oxide catalyst

The catalytic performances of Mo–V–Sb mixed oxide catalysts have been studied in the selective oxidation of isobutane into methacrolein. V–Sb mixed oxide showed the activity for oxidative dehydrogenation of isobutane to isobutene. The selectivity to methacrolein increased by the addition of molybdenum species to the V–Sb mixed oxide catalyst. In a series of Mo–V–Sb oxide catalysts, Mo₁V₁Sb₁₀O_x exhibited the highest selectivity to methacrolein at 440°C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO₄ and -Sb₂O₄ phases in the Mo₁V₁Sb₁₀O_x. The high catalytic activity of Mo₁V₁Sb₁₀O_x can be explained by the bifunctional mechanism of highly dispersed molybdenum suboxide and VSbO₄ phases. It is likely that the oxidative dehydrogenation of isobutane proceeds on the VSbO₄ phase followed by the oxidation of isobutene into methacrolein on the molybdenum suboxide phase.     

Controlled partial oxidation of methane to methanol/formaldehyde over Mo–V–Cr–Bi–Si oxide catalysts

The non-catalytic and catalytic oxidations of CH₄ over Mo–V–Cr–Bi–Si oxide catalysts were investigated in a tubular reactor and the catalysts were characterized by XRD, XPS and TPR. Contents of Bi in the catalysts influenced the combination of Mo–V–Bi–O species and, consequently, influenced the TPR reduction temperature of the catalysts. The catalysts exhibited more selective production of methanol when the TPR reduction peaks shifted to lower temperature.     

Selective oxidation of styrene over Mg–Co–Al hydrotalcite like-catalysts using air as oxidant

A set of synthesized Mg/Co/Al hydrotalcites was synthesized and characterized by XRD, XPS, BET, SEM, TEM, and FT-IR physical techniques. The partial substitution of Mg^2 + by Co^2 + in brucite layers has not significantly affected the layered double hydroxide structure, but plays a crucial role in the oxidation of styrene in the presence of air. The prepared Mg/Co/Al hydrotalcite-like compounds express a good activity and stability in the oxidation of styrene in the free-solvent condition. Both styrene conversion and desired product selectivities are strongly dependent on the cobalt substitution content. The intra-hydrotalcite lattice Co^2 + ions are active sites for the epoxidation of styrene.     

Low temperature catalytic oxidation of nitric oxide over the Mn–CoOx catalyst modified by nonthermal plasma

Nonthermal plasma (NTP) treatment was investigated to modify the Mn–CoO_x catalyst for the low-temperature oxidation of nitric oxide. The catalysts were characterized by XRD, BET, TGA and XPS techniques. The results showed that the activity of NTP-treated catalysts improved significantly, and that NTP treatment has the advantage of changing the structural and morphological properties (higher surface areas and pore volume) and varying the relative surface concentration and oxidation states of surface species over catalysts. High surface areas and pore volume, high concentration of chemisorbed oxygen, Mn^4 + and Co^2 +, and the efficient synergetic catalytic effect between Co and Mn ions were thought to be the main reasons for the high activity of NTP-treated catalysts.     

Selective Oxidation and Oxidative Dehydrogenation of Isobutane over Hydrothermally Synthesized Mo–V–O Mixed Oxide Catalysts

A series of Mo–V–O catalysts were prepared by calcining the orthorhombic (M1) Mo–V–O phase containing precursors under different conditions (T = 500 or 600 °C in atmosphere of N₂ or air) and tested for the oxidation of isobutane and isobutene. Characterization results (BET, XRD, XPS, FTIR, and TPR) showed that their structure and properties depend on the composition of the calcined samples and the calcined conditions. Catalytic tests showed that relatively high isobutane conversion and desired product selectivity can be achieved over MoV_0.3-500-N and MoV_0.3-600-A catalysts. It is also found that both orthorhombic M1 phase and (V_0.07M_0.93)₅O₁₄ phase are active and selective for the selective oxidation of isobutane to methacrolein, whereas higher selectivity toward methacrolein (40.4%) can be achieved over the former phase at a moderate isobutane conversion (6.4%). Moreover, (Mo_0.3V_0.7)₂O₅ phase may be propitious to complete oxidation for the selective oxidation of isobutane. On the other hand, the presence of V affects the activity and selectivity, and a low surface V⁴⁺ concentration prefers selective oxidation products. In addition, specific surface areas of the catalysts appear to be little important in determining the catalytic activation.     

Selective aerobic oxidation of alcohols over Au–Pd/sodium titanate nanotubes

The catalytic application of Au–Pd nanoparticles supported on sodium titanate nanotubes (NaTNTs) for liquid-phase aerobic oxidation of alcohols is reported, for the first time. This reaction occurs at 80–120 °C, 1 atm and solvent-/alkali-free conditions yielding the corresponding carbonyls in high selectivity. This catalyst was reusable and found to be more active/selective than the corresponding monometallic Au and Pd catalysts and Au–Pd/TiO₂. Higher dispersion, smaller particle size and higher amount of electron density at gold are the causes for the superior activity of Au–Pd/NaTNT catalyst.     

Partial oxidation of ethane to syngas over nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide

The catalytic activity, thermal stability and carbon deposition of various modified NiO/γ‐Al₂O₃ and unmodified NiO/γ‐Al₂O₃ catalysts were investigated with a flow reactor, XRD, TG and UVRRS analysis. The activity and selectivity of the NiO/γ‐Al₂O₃ catalyst showed little difference from those of the modified nickel‐based catalysts. However, modification with alkali metal oxide (Li, Na, K) and rare earth metal oxide (La, Ce, Y, Sm) can improve the thermal stability of the NiO/γ‐Al₂O₃ and enhance its ability to suppress carbon deposition during the partial oxidation of ethane (POE). The carbon deposition contains graphite‐like species that were detected by UVRRS. The nickel‐based catalysts modified by alkali metal oxide and rare earth metal oxide have excellent catalytic activities (C₂H₆ conversion of ~100%, CO selectivity of ~94%, 7 × 10⁴ l/(kg h), 1123 K), good thermal stability and carbon‐deposition resistance. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

NO presence effects on the reduction of N2O by CO over Al–Pd–Co oxide catalyst

Nitrous oxide (N₂O) is known as one of the greenhouse gases of which the emission levels are to be controlled and also an ozone-depleting material in the stratosphere. For more than a last couple of decades, various catalysts have been investigated for the reduction of N₂O emission. However, most of those catalysts require reaction temperatures as high as 450 °C even with the use of effective reducing agents. Furthermore, NO, which is usually present with N₂O, significantly interferes with the removal efficiency of N₂O. Al–Pd–Co oxide catalyst which is a type of mixed metal oxides (MMO) has shown 100% conversion of N₂O at temperatures as low as 200 °C using CO. This paper examines the effect of NO on the reduction of N₂O with CO over Al–Pd–Co oxide catalyst. The experiments were carried out in the temperature range of 100–500 °C with space velocity of 10,000–50,000 h^?1. Though the efficiency of N₂O reduction is decreased significantly when NO is present, the efficiency increases when sufficient CO is supplied. In this study, the reduction mechanisms of N₂O and NO by CO have been confirmed, and it was shown that MMO catalyst can simultaneously remove N₂O and NO using CO with high efficiency.     

Anodic oxidation behaviour of Al–Ti alloys in acidic media

The anodic oxidation behaviour of Al–Ti alloys in several acidic solutions was investigated. The influence of conditions such as alloy composition, electrolytic solution, electrolyte temperature and formation current density on the formation rate of oxides on Al–Ti alloys and the dielectric properties of the anodic films were analysed. It was shown that the oxide formation rate for the Al–Ti alloy containing 54 at % aluminium was the highest and the dielectric property of its anodic oxide was also the best. In addition, by means of several surface analytical techniques, the chemical composition of the films were determined as (TiO₂) _n (Al₂O₃) _m . AES (Auger electron spectroscopy) profiling analysis data showed that Al–Ti alloys had preferential oxidation behaviour, that is, the aluminium was oxidized preferentially.     

Selective hydrogenation of styrene oxide to 2-phenyl ethanol over polyurea supported Pd–Cu catalyst in supercritical carbon dioxide

2-Phenyl ethanol (2-PEA) is an important chemical which finds several applications in perfumes, deodorants, soaps and detergents. It is prepared by different polluting and dangerous routes. The current work is concerned with production of 2-PEA by hydrogenation of styrene oxide using polyurea encapsulated catalysts (EnCat) in methanol and supercritical carbon dioxide (scCO₂). Bimetallic Pd–Cu catalyst encapsulated with polyurea, Pd–Cu EnCat, is the best catalyst. The epoxide ring in styrene oxide is selectively hydrogenated to give 2-PEA in scCO₂ without formation of any isomerization or deoxygenated products, which are formed in methanol. A complete conversion of styrene oxide with 100% selectivity to 2-PEA was obtained without addition of any promoter. Effects of various parameters were studied and a bifunctional site Langmuir–Hinshelwood–Hougen–Watson kinetic model was found to be in good agreement with the experimental data. The process is clean and green.     

Oxidative dehydrogenation of propane to propylene over Al2O3-supported Sr–V–Mo catalysts

This work presents the results of propane oxidative dehydrogenation on alumina-supported V–Mo oxides and Sr–V–Mo oxide catalysts. The reaction was conducted at atmospheric pressure and 500 °C. Catalysts composed of 2:4 vanadium to molybdenum ratio showed the best performance. The presence of strontium in the catalyst's matrix enhanced its performance (increase in conversion and selectivity) and decreased its reducibility by changing the nature of its surface as confirmed by BET, XRD and TGA techniques. Moreover, the presence of strontium improved the stability of the catalyst. Hence, Sr–V–Mo stands as a promising catalyst for oxidative dehydrogenation of propane.     

Selective Synthesis of Perillyl Alcohol from β-Pinene Epoxide over Ti and Mo Supported Catalysts

Catalysis Letters - Isomerization of β-pinene epoxide to perillyl alcohol using several materials based on titanium and molybdenum is reported. The metals were incorporated on SBA-15, MCM-41...     

Selective oxidative dehydrogenation of ethane over MoO3/V2O5–Al2O3 catalysts: Heteropolymolybdate as a precursor for MoO3

Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO₃ added V₂O₅–Al₂O₃ catalysts. The catalysts were characterized by BET, XRD, Laser-Raman and FT-IR spectroscopies and TPR technique. Catalytic tests were carried out in a fixed bed stainless steel reactor in the temperature range from 450 to 600 °C. Results revealed that the loading of molybdophosphoric acid (MPA) and the method of preparation had a clear influence on the catalytic performance. Among all, 10 wt.% MPA/V₂O₅–Al₂O₃ solid was found to possess superior activity and selectivity (X-C₂H₆ ~ 35% and S-C₂H₄ ~ 65%). Formation of Mo–V mixed oxide phases on Al₂O₃ appeared to be responsible for this improved performance. This best catalyst also exhibited good long-term stability over a period of ca. 36 h.     

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H₂O₂ of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ～ 2, 1 M H₂O₂, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H₂O₂. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm²) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.     

The performance of Ba in total oxidation of chlorinated hydrocarbons over La–Ba–Ni-mixed oxide catalysts

This study examined the catalytic behavior of La–Ba–Ni catalysts regarding the total oxidation of five CVOCs. The experimental results demonstrate that La–Ba–Ni catalysts are superior in suitability for the total oxidation of CVOCs. More than one Ba(3d_5/2), La(3d_5/2), and Ni(2p_1/2) peaks appeared in each XPS spectrum due to a chemical shift to a higher binding energy. Since the electropositivity of Ba is higher than that of Ni, chloride is transferred from the Ni-site to the Ba-site with ease, thereby enhancing activity.     

Evolution of Ti–Sn-rutile-supported V2O5–WO3 catalyst during its use in nitric oxide reduction by ammonia

This paper concerns the relation between surface structure of crystalline vanadia-like active species on vanadia–tungsta catalyst and their activity in the selective reduction of NO by ammonia to nitrogen. The investigations were performed for Ti–Sn-rutile-supported isopropoxy-derived catalyst. The SCR activity and surface species structure were determined for the freshly prepared catalyst, for the catalyst previously used in NO reduction by ammonia (320 ppm NO, 335 ppm NH₃ and 2.35 vol% O₂) at 573 K as well as for the catalyst previously annealed at 573 K in helium stream containing 2.35 vol% O₂. The crystalline islands, exposing main V₂O₅ surface, with some tungsten atoms substituted for V-ones, were found, with XPS and FT Raman spectroscopy, to be present at the surface of the freshly prepared catalyst. A profound evolution of the active species during the catalyst use at 573 K was observed. Dissociative water adsorption on V⁵⁺OW⁶⁺ sites is discussed as mainly responsible for the catalyst activity at 473 K and that on both V⁵⁺OW⁶⁺ and V⁴⁺OW⁶⁺ sites as determining the activity at 523 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

Selective growth of single-walled carbon nanotubes over Co–MgO catalyst by chemical vapor deposition of methane

The selective synthesis of SWCNTs with narrow chirality and diameter distribution by methane decomposition over a Co–MgO catalyst is reported. Raman spectroscopy, temperature programmed oxidation (TPO), UV–Vis–NIR absorption spectroscopy, and nitrogen physisorption were used to probe SWCNTs morphology, reaction selectivity, SWCNTs chirality and diameter distribution, and carbon yield. The catalyst was examined by nitrogen physisorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), and UV–Vis-diffuse reflectance spectroscopy to elucidate the structure and chemical state of the species responsible for SWCNT growth. The results established a clear link between the degree of dispersion of Co species inside the MgO lattice and the catalyst activity and selectivity for SWCNT growth. High dispersion and stabilization of Co species influenced catalytic activity for methane decomposition and the high SWCNT selectivity. The yield of carbon and SWCNT selectivity increased with an increase in temperature, however, SWCNTs diameter distribution shifts to larger diameter tubes as synthesis temperature was increased.     

Joining of Ti–Al–C ceramics by oxidation at low oxygen partial pressure

The joining of titanium aluminum carbides has been successfully performed at high temperature and low oxygen partial pressure. The mechanism of the bonding is attributed to the preferential oxidation of Al atoms in the titanium aluminum carbides at low oxygen partial pressure, which leads to the formation of an Al₂O₃ layer through the joint interface. The specimens joined at 1400 °C exhibit a high flexural strength of 315 ± 19.1 MPa for Ti₂AlC and 332 ± 2.83 MPa for Ti₃AlC₂, which is about 95% and 88% of the substrates, respectively, and the high flexural strength can be retained up to 1000 °C. The high mechanical performance of the joints is attributed to the similar density and thermal expansion coefficient values of Al₂O₃ to those of the Ti₂AlC and Ti₃AlC₂ substrates. It indicates that bonding via preferential oxidation at low oxygen partial pressure is a practical and efficient method for Ti₂AlC and Ti₃AlC₂.