Transient experiments on the selective oxidation of methane to formaldehyde over V2O5/SiO2 studied in the temporal-analysis-of-products reactor

Methane oxidation to formaldehyde was studied over a vanadium oxide catalyst supported on silica at 630 °C using the technique known as temporal analysis of products with sequential pulsing of methane and oxygen. This work shows that methane interacts very weakly and oxygen very strongly with the catalyst surface and it is concluded that the initial activation of methane involves an adsorbed oxygen species. Methyl radicals formed in the first step subsequently extract lattice oxygen to yield formaldehyde.     

Selective oxidation of methane to formaldehyde and C2 hydrocarbons over double layered Sr/La2O3 and MoO3/SiO2 catalyst bed

A double layered catalyst bed of Sr/La₂O₃ followed by MoO₃/SiO₂ has been used to produce C₂ hydrocarbons and formaldehyde from a CH₄/air mixture with a formaldehyde space time yield of 187 g (kg cat)^–1 h^–1, which is significantly higher than those yields obtained with single bed catalysts or with mechanically mixed catalyst bed at ambient pressure and 630 ° C.     

Vanadium species in new catalysts for the selective oxidation of methane to formaldehyde: Activation of the catalytic sites

New vanadium oxide supported on mesoporous silica catalysts for the oxidation of methane to formaldehyde were investigated by infrared and Raman spectroscopies to identify and characterize the molecular structure of the most active and selective catalytic sites. In situ and operando experiments have been conducted in order to understand the redox and hydroxylation/dehydroxylation processes of the vanadium species. (SiO)₂VO(OH) species were identified in these catalysts in reaction conditions and shown to undergo a deprotonation at 580 °C under vacuum, leading to a site giving a photoluminescence band at 550 nm attributed to reverse radiative decay from the excited triplet state:

(V⁴⁺–O⁻)^*  (V⁵⁺O²⁻). An activation mechanism of vanadium monomeric species with electrophilic oxygen species is proposed.     

Partial oxidation of ethane over K2MoO4/SiO2 and potassium promoted MoO3/SiO2 catalyst

Silica supported K₂MoO₄ and potassium-promoted MoO₃ were used as catalysts for the partial oxidation of ethane in fix-bed continuous-flow reactor at 770–823 K using N₂O as oxidant. The main products of the oxidation reaction were ethylene, acetaldehyde, CO and CO₂. Addition of various compounds of potassium to the MoO₃/SiO₂ greatly enhanced the conversion of ethane and influenced the product distribution. The highest rate and selectivity for acetaldehyde formation was found on a K₂MoO₄/SiO₂ catalyst.     

Preparation and characterization of WO3/SiO2 catalysts

Two sets of WO₃/SiO₂ catalysts were prepared from (NH_{4 6}H₂W₁₂O₄₀ (aqueous method) and W(³-C₃H₅)₄ (non-aqueous method). The molecular structures and dispersions of the surface tungsten oxide species for the WO₃/SiO₂ catalysts under ambient and in situ dehydrated conditions were investigated by Raman spectroscopy. The samples prepared from (NH₄)₆H₂W₁₂O₄₀ (aqueous method) exhibit very strong Raman features due to the presence of crystalline WO₃ and the samples prepared from W(³-C₃H₅)₄ (non-aqueous method) do not possess crystalline WO₃. These results suggest that the preparation method exerts an influence on the dispersion of the surface tungsten oxide species on SiO₂. The surface tungsten oxide species under ambient conditions possess polytungstate clusters, W₁₂O ₄₂ ^12– , on the silica support. Upon dehydration at elevated temperatures, the hydrated polytungstate clusters decompose and interact with the silica support via the formation of isolated, octahedrally coordinated tungsten oxide species.     

FTIR spectroscopic investigation of the active sites on different types of silica catalysts for methane partial oxidation to formaldehyde

Several commercial silica samples showing different catalytic activities in the partial oxidation of methane (MPO) to formaldehyde have been investigated using FTIR technique. Two IR absorption bands at 893 and 909 cm^–1, observed upon dehydroxylation of the silica catalysts and assigned to reactive siloxane sites on the surface (strained siloxane bridges), were found to disappear upon heating in methane at high temperature. The catalytic activity increases together with the intensity of the bands due to such strained sites in the different SiO₂ samples.     

Selective oxidation of methane to formaldehyde on supported molybdate catalysts

A series of silica-supported molybdena catalysts with variable molybdenum content has been prepared and tested in the selective oxidation of methane to formaldehyde. The nature of the supported oxide phases has been studied by Fourier transform IR of chemisorbed NO molecule, X-ray photoelectron spectroscopy, UV-Visible reflectance and X-ray diffraction techniques. The results show that molybdenum oxide is highly dispersed on the silica at low molybdenum concentrations where two-dimensional polymolybdates are developed. Three-dimensional MoO₃ crystals grow in the region of high molybdenum concentrations. The analysis of the combined data suggests that there is a close relationship between methane conversion and formaldehyde selectivity and the presence of highly dispersed polymolybdate structures on the silica surface.     

The effect of Mo on the catalytic and surface properties of Rh-Mo/ZrO2 catalysts

A series of Rh-Mo/ZrO₂ catalysts with fixed Rh and different Mo loadings were prepared and characterized by H₂ chemisorption, XRD, TPR, TEM and XPS. The catalysts were studied in two reactions: the hydrogenation of carbon monoxide and the hydrogenation of toluene. The results suggest that the increase in the Mo content produces a partial coverage of the support and the Rh particles. Moreover, at high Mo coverage, an increase of the MoO₃ layer thickness is produced. After being treated in hydrogen, the molybdenum oxide remains as slightly reduced particles, while Rh is essentially as Rh⁰, with only a small contribution of Rh+ species. The Mo promotes the formation of oxygenates in the CO hydrogenation and it does not affect the activity in the hydrogenation of toluene.     

Characterization of surface species on V/SiO2 and V,Na/SiO2 and their role in the partial oxidation of methane to formaldehyde

Silica-supported vanadium (1–8 wt%) and vanadium (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy, temperature-programmed reduction, X-ray photoelectron spectroscopy, NO + NH₃ rectangular pulses and oxygen chemisorption. The presence of different vanadium species was correlated with activity and selectivity during the methane partial oxidation reaction. The pre-impregnation of the silica support with sodium favors vanadium dispersion, but strongly diminishes V=O concentration due to the formation of orthovanadate-like compounds. As a result of these modifications, methane conversion is strongly inhibited while formaldehyde decomposition is favored.     

Effects of NO and solids on the oxidation of methane to formaldehyde

The selective oxidation of methane has been studied both in the presence and absence of solids (inert or catalysts) with and without NO added, at 1 bar of total pressure. NO enhances the yield to formaldehyde, while the solids favor its decomposition. These results, together with abundant literature data, show a maximum for formaldehyde yield of about 4.0%.     

Effect of copper species and the presence of reaction products on the activity of methane oxidation on supported CuO catalysts

The activity of the various CuO species found in supported copper catalysts and the effect of the presence of reaction products, CO₂ and H₂O, was studied during the complete oxidation of methane. Series of copper catalysts supported on ZrO₂, Al₂O₃ and SiO₂ with different metal concentrations were analyzed under identical experimental conditions of reactant concentration and temperature. The catalysts were characterized by TPR, UV–vis spectroscopy and XRD. The results show that the activity of supported CuO is closely related to the kind of Cu species formed on the different supports. It was found that the Cu species formed on ZrO₂ and Al₂O₃ are dependent on the metal loading/support's surface area ratio, and that the activity of highly dispersed Cu is substantially higher than that of bulk CuO. In the case of silica, only the formation of bulk CuO was detected, accounting for the low activity of CuO/SiO₂ catalysts. The activity of highly dispersed Cu species formed on ZrO₂ is higher than those formed over Al₂O₃, and it is not significantly affected by the formation of bulk CuO on the surface. On the contrary, the activity of copper species formed on alumina decreases continuously as the Cu loading is increased. Thus, for the range of copper loading studied in this work, the activity of the catalysts, per gram of loaded Cu, follows the sequence: CuO/ZrO₂ > CuO/Al₂O₃  CuO/SiO₂. It was also found that CO₂ does not inhibits the activity of the CuO/ZrO₂ catalysts, while water inhibits the combustion reaction of methane, with an estimated reaction order of about −0.2 for temperatures between 360 °C and 420 °C.     

Partial oxidation of methane with N2O over Fe2(MoO4)3 catalyst

The product distributions for partial oxidation of methane on Fe₂(MoO₄)₃ catalyst were changed remarkably when the oxidant was switched from oxygen to nitrous oxide. When oxygen was used as the oxidant, the main products were HCHO and CO. However, when nitrous oxide was used, the formation of HCHO was greatly suppressed and C₂ hydrocarbons (C₂H₆ and C₂H₄) were newly produced. The difference in kinetic behaviors between the two reactions using nitrous oxide and oxygen as the oxidant can be explained in terms of the competitive conversions of methyl intermediate into HCHO and C₂H₆. In the case of nitrous oxide as the oxidant, the adsorbed methyl intermediate would be transformed predominantly into C₂H₆ due to a low steady-state concentration of the active oxygen species on Fe₂(MoO₄)₃.     

Selective oxidation of methane to CO and H2 over unreduced NiO-rare earth oxide catalysts

NiO-LnO _x (Ln = lanthanide) catalysts (with NiLn=11) without prereduction show high activity/selectivity and very high productivity in the oxidative conversion of methane to CO and H₂. The catalysts are first activated in the initial reaction, which is started at 535–560°C, by the reduction of NiO and creation of active sites. The carbon deposition on the catalysts in the reaction, particularly for the NiO-Gd₂O₃, NiO-Tb₄O₇ and NiO-Dy₂O₃ catalysts, is quite fast but it has caused a little or no influence on the catalytic activity/selectivity. Pulse reaction of pure methane on NiO-Nd₂O₃ (at 600°C) shows involvement of lattice oxygen in the initial reaction and also reveals formation of carbon from CO on the catalyst reduced in the reaction.     

Effect of cobalt oxide on surface structure of alumina supported molybdena catalysts studied by in situ Raman spectroscopy

A set of Co promoted 10% Mo/Al₂O₃ samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerization of surface molybdenum oxide species decreases with increasing Co loading. Under dehydrated conditions, the polymeric molybdenum oxide species is absent with the addition of only 0.2% Co. At low Co loadings (2%), before the formation of CoMoO₄ compound, the spectral features are very similar under ambient conditions. Dehydration causes the upward shift of the Mo=O symmetric stretching mode. A broad band around 920–930 cm^–1 was thus observed. This band has been suggested to be associated with the Co-Mo interaction species. In contrast to crystalline CoMoO₄, this species shows a reversibility on H₂ reduction-O₂ reoxidation treatments. From the results obtained, it is proposed that cobalt oxide interacts with the most polymerized molybdenum oxide species to form Co-Mo interaction species and/or crystalline CoMoO₄; therefore, the amount of the surface molybdenum oxide species decreases with a change in the molecular structure as a function of the Co concentration.     

Effect of the particle size on the activity of MoO x C y catalysts for the isomerization of heptane

Molybdenum trioxide samples having apparent particle sizes (APS) of 5 and 20 .m were partially reduced under flow of a mixture of H₂/n-heptane during 4 h. X-ray diffraction and Raman spectroscopy showed the typical structural transformation of MoO₃ into MoO_xC_y and MoO₂. These structural changes occur preferentially on the {0k0} planes. After the reduction treatment the resulting materials, having surface areas of 23 and 53 m²/g, were evaluated in the isomerization of n-heptane at 643 K and 18.5 bar. The catalyst with an APS of 20 .m showed a maximum conversion around 70%, while for the catalyst with an APS of 5 .m the maximum conversion was 34%. The lower activity of the 5 .m MoO_xC_y catalyst seems to be related to a faster rate of formation of oxygen vacancies and rearrangement of the lattice into a more stable and less active structure in the case of small-size particles, due to a higher concentration of terminal Mo=O bonds along the a- and b-axes, which facilitate the electrophilic attack by hydrogen on the (010) plane.     

Nickel-calcium phosphate/hydroxyapatite catalysts for partial oxidation of methane to syngas: Effect of composition

Nickel-calcium phosphate/hydroxyapatite catalysts have recently been reported to exhibit high activity and selectivity in partial oxidation of methane (POM). In this work the optimum composition was determined. The optimum mole ratio of Ca/PO₄ was around 10/6 and that of Ni/PO₄ was in a range from 1.0/6 to 3.0/6 with the optimum Ca/PO₄, and the activity could be related with the amount of metallic nickel. In a temperature range from ca. 400 to 700 K, an apparent autothermal reaction was observed to occur in some cases. This is due to the fact that the actual catalyst temperature is higher than the measured temperature, which comes from the exothermic nature of the reaction. The mixing sequence of the precursors during the catalyst preparation does little affect the catalyst activity and characteristics. Deactivation of the catalyst occurred slowly, but the catalyst could easily be regenerated. Moreover, the nickel-calcium phosphate/hydroxyapatite catalyst showed higher activity than the nickel-strontium phosphate catalyst. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Effect of the metal oxide loading on the activity of silica supported MoO3 and V2O5 catalysts in the selective partial oxidation of methane

Precipitated silica catalysts loaded with either MoO₃ (0.2–4.0 wt%) or V₂O₅ (0.2–5.3 wt%) have been studied in the selective partial oxidation of methane to formaldehyde with molecular oxygen at 520 °C. The functionality of the SiO₂ surface towards the formation of HCHO is significantly promoted by V₂O₅, while it is depressed by the MoO₃.     

Effect of Rh loading on the performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al₂O₃ catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH₄ and reaction products (CO, H₂, and CO₂) were observed. Comparative studies using H₂-TPR, O₂-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al₂O₃ with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO_x), that intimately interacting with the Al₂O₃ surface (RhⁱO_x) and the Rh(AlO₂)_y species formed by diffusion of rhodium oxides in to sublayers of Al₂O₃ [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H₂-TPR and O₂-TPD experiments. Among them, the first two species can be easily reduced by H₂ at temperature below 350 °C, while the last one can only be reduced by H₂ at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO_x and RhⁱO_x species can be reduced by CH₄ in the reaction mixture. Compared to the Rh/Al₂O₃ with high Rh loadings, the catalysts with low Rh loadings contain more RhⁱO_x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO_x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al₂O₃ with low Rh loadings can be related to the behaviour of Rh(AlO₂)_y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction.     

Re-investigating the CO oxidation mechanism over unsupported MnO, Mn2O3 and MnO2 catalysts

Kinetic study of CO oxidation in combination with experiments of temperature-programmed oxidation (TPO) and reduction (TPR) have been performed on various unsupported crystalline manganese oxides (MnO_x); while the reactivity shows an order of MnO ≤ MnO₂ < Mn₂O₃ in a mixture of unit ratio of O₂/CO at/below 523 K. We propose that under the current conditions the interaction of adsorbed CO and O is mainly responsible for CO₂ formation on Mn₂O₃ and MnO₂ catalysts, following either the Langmuir–Hinshelwood mechanism or Eley–Rideal mechanism. Meanwhile, direct evidence from transient CO oxidation suggests that the Mars-van-Krevelen mechanism may occur for all catalysts simultaneously, especially, it is predominant for the MnO catalyst. The evidence of structural modifications during reaction was confirmed by Raman spectra obtained from used MnO.     

Total oxidation catalysts based on manganese or copper oxides and platinum or palladium I: Characterisation

Temperature-programmed reduction (TPR), oxidation (TPO), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterise catalysts based on manganese oxides, copper oxides or one of them mixed with platinum or palladium-supported on γ-alumina. The catalysts were characterised before and after they had been exposed either to high temperature in the presence of steam or to sulphur dioxide. Raman spectroscopy, XRD, XPS and TPR performed on the fresh samples of MnO_x, mixed MnO_x–Pt and MnO_x–Pd revealed the presence of a mixture of manganese oxides, particularly Mn₂O₃. In the fresh mixed MnO_x–Pd and CuO_x–Pd samples, Pd catalysed the reduction of both MnO_x and CuO_x, whereas Pt only catalysed the reduction of MnO_x. After hydrothermal treatment at 900°C of the MnO_x, mixed MnO_x–Pt and MnO_x–Pd samples, there was a formation of new manganese oxide phase, Mn₃O₄ detected by Raman spectroscopy. TPR revealed increasing interaction between the metal oxides and the noble metals in the hydrothermally treated mixed MnO_x–Pd and CuO_x–Pd samples, and also the appearance of interaction in the treated mixed CuO_x–Pt sample. The sulphur adsorbed in all the MnO_x samples formed sulphate, which was more difficult to reduce than the oxides. Also, the reduction temperature of sulphates was lowered when noble metals are present.