Effect of carbon deposits on the properties of co/y-zeolite catalysts for co hydrogenation

Co/Y-zeolite catalysts were prepared by three different preparation methods of excess-water (EW), ion-exchange(IE) and carbonyl complex-impregnation(CI), and they were deposited by carbon through the disproportionation of carbon monoxide at different temperatures. CO hydrogenation was performed on the fresh and carbon deposited Co/Y-zeolite catalysts respectively, and the results were compared with each other. CO hydrogenation was carried out in a differential reactor operating at atmospheric pressure, temperature of 270–400‡C and H₂/CO ratio of 2. Temperature programmed surface reaction (TPSR) of carbon deposits was performed in a thermogravimetric flow system to obtain a better understanding of the carbon deposits. The large cobalt metals existing mainly at the exterior surface of zeolite crystals in the EW catalyst had a great affinity to carbon deposits, but the small cobalt metals in the IE and CI ones were highly resistant to carbon deposits. The hydrocarbon product distributions were hardly affected by carbon deposits, while the production of olefinic hydrocarbons was enhanced significantly.     

Steam resistant CoLa-mordenite catalysts for the SCR of NOx with CH4

The stabilization of Co-mordenite catalysts through lanthanum exchange is reported here. The effect of exchange order and calcination conditions upon the reduction of NO_x to N₂ at 500 °C was tracked during 400 h on a stream containing NO_x, CH₄, O₂ and 10% H₂O. Both the fresh and used catalysts were characterized through TPR, Raman spectroscopy, FTIR spectroscopy using CO as probe molecule, and XPS. These techniques revealed that the CoLa-mordenite catalysts which were not affected by the severe hydrothermal treatment showed no sign of Co or La migration out of the exchange positions. Instead, those that rapidly deactivated showed the formation of cobalt oxides and, in some cases, the migration of the cations to other exchange positions. The presence of exchanged lanthanum seems to preserve the integrity of the zeolite structure preventing the migration of cobalt ions with the subsequent formation of cobalt oxides which favors the reaction of methane with O₂, thus decreasing N₂ production.     

n-Octane aromatization on Pt-containing non-acidic large pore zeolite catalysts

Platinum catalysts supported on the potassium-form of different large-pore zeolites (i.e. K-LTL, K-BEA, K-MAZ, and K-FAU) have been tested for n-octane aromatization at 500 °C. All catalysts were prepared by the vapor phase impregnation (VPI) method. It was found that the Pt/K-LTL catalyst exhibit a better aromatization performance than the other zeolite catalysts. However, due to secondary hydrogenolysis, the C8 aromatics produced inside the zeolite are converted to benzene and toluene. By contrast, a non-microporous Pt/SiO₂ catalyst did not present the secondary hydrogenolysis. Therefore, despite a lower initial aromatization activity, Pt/SiO₂ results in higher selectivity to C8 aromatics than any of the other zeolite catalysts. All fresh catalysts were characterized by hydrogen chemisorption and FT-IR of adsorbed CO. In addition, the residual acidity of the supports was analyzed by temperature programmed desorption (TPD) of ammonia. In agreement with previous studies, it was found that after reduction at either 350 or 500 °C, the Pt/K-LTL showed much higher Pt dispersion than other catalysts. It is known that the structure of L zeolite can stabilize the small Pt clusters inside the zeolite channel. By contrast, FT-IR indicated that a large fraction of platinum clusters were located outside the zeolite channels in the case of Pt/K-BEA and Pt/K-MAZ catalysts.     

Ultra-rapid synthesis of syngas by the catalytic reforming of methane enhanced byin-situ heat supply through combustion

Development in highly active catalysts for the reforming of methane with CO₂ and partial oxidation of methane was conducted to produce hydrogen and carbon monoxide with high reaction rates. An Ni-based four-components catalyst, Ni-Ce₂O₃-Pt-Rh, supported on an alumina wash-coated ceramic fiber in a plate shape was suitable for the objective reaction. By combining the catalytic combustion of ethane or propane, methane conversion was markedly enhanced, and a high space-time yield of syngas, 25,000 mol/l·h was obtained at a catalyst temperature of 700 ‡C or furnace temperature of 500 ‡C. The extraordinary high space-time yield of syngas was also confirmed even under the very rapid flow rate conditions as a contact time of 3 m-sec by using a monolithic shape of catalyst bed without back pressure.     

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

A series of Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C₂H₄ over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C₂H₄ were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al₂O₃ for CO or C₂H₄ oxidation. For NO decomposition, the H₂-reduced Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C₂H₄, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al₂O₃ has been reduced to zero valence during reduction by H₂ at 450°C, but in Co/γ-Al₂O₃ only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl₂O₄-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl₂O₄ which cannot be reduced by H₂ at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al₂O₃ has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al₂O₃. This difference mainly results from the distorting tetrahedrally- coordinated Co²⁺ ions which have lower coordination symmetry than Co⁰ in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al₂O₃ catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al₂O₃ makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co⁰ is the main active phase. For NO decomposition and selective reduction, Co⁰ is also catalytically active, but it can be oxidized into Co₃O₄ by oxygen at high reaction temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of nonreacting gases on the desorption of reaction-created CO from graphite

Temperature-programmed desorption (TPD) was used to determine whether inert gases influence the desorption step in the oxidation of carbon by CO₂. Surface oxides were formed on a graphite by reacting it at 1200 K with CO₂, and then quenching the reaction. The oxides then were removed by TPD to 1373 K, using He, Ar and Kr as carrier gases in separate desorptions. The oxides appeared as CO₂ and CO in the desorption carrier gases; some CO₂ was observed between 800 and 1050 K, but above 1050 K the product was almost entirely CO. The CO₂ desorption showed no effect from changing the carrier gas, but temperature of CO desorption peak and amount of CO desorbed by 1373 K both depended on which gas was used as a carrier. This shows that nonreactive gases affect the desorption step of the C-CO₂ reaction. Involved in the desorption step is movement of some surface oxide species to desorption sites; the nonreactive-gas influence occurs because the nonreactive gases affect this surface transport. In the presence of Ar and Kr, transport rates of surface oxides to desorption sites were higher than they were in the presence of He. Under reaction conditions this can result in greater CO desorption rates and faster overall reaction rates in the presence of Ar and Kr than occur in the presence of He.     

Synthesis,Characterization and Application of Co–MgO Mixed Oxides in Oxidation of Carbon Monoxide

The present work deals with the synthesis of nanostructured Co–MgO mixed oxides with different weight ratios of cobalt by a facile co-precipitation method as a catalyst for low-temperature CO oxidation. The prepared samples were characterized by X-ray diffraction (XRD), N₂ adsorption/desorption (BET), Fourier transform infrared spectroscopy (FTIR), and transmission and scanning electron microscopies (TEM and SEM) techniques. The results revealed that inexpensive cobalt–magnesium mixed metal oxide nanoparticles have a high potential as catalyst in low-temperature CO oxidation. The Co–MgO mixed oxide with 30 wt.% cobalt had the highest activity. The results showed that the catalysts pretreated under O₂-containing atmosphere possessed higher activity compared to the catalyst pretreated under H₂ atmosphere. Co–MgO catalyst showed a good repeatability in reaction condition. The stability test exhibited that the Co–MgO mixed oxides were highly stable for CO oxidation over a 30 h time on stream in the feed gas containing a high amount of moisture and CO₂.     

Temperature programmed desorption technique to predict the carbon oxygen reaction

The kinetics of the reaction o₂ oxygen with a sucrose char particle size: (88 μ<d_p<105 μrn) has been studied using a thermogravimetric analyzer (TGA) and a mass spectrometer (MS) to measure weight change and CO and CO₂ formation rates during reaction. Experiments were performed to determine the surface oxide formation rate and to determine the mechanism of CO desorption in the temperature range of 762 K to 851 K and for oxygen pressures of 0.04 to 0.3 atm, respectively. When the reaction rate at 30% conversion was used in the Arrhenius plot, an activation energy of 34±3 kcal/mol was obtained and the CO/CO₂ ratio was found to increase with increasing reaction temperature. Analysis of the rate of formation of CO and CO₂ shows the activation energy for CO formation is greater than for CO₂ formation. Temperature programmed desorption (TPD) studies of the surface oxides were made to provide a better understanding of the carbon oxidation process. The activation energy distribution function for desorption was approximately Gaussian and the average activation energy is 55 Kcal/mol for a preexponential factor of 10¹³ 1/sec. The peak of the energy distribution function shifts to higher activation energies for surface complexes formed at higher reaction temperature.     

Carbon Monoxide Hydrogenation on Cobalt/Zeolite Catalysts

The synthesis of hydrocarbons from catalytic hydrogenation of CO/H₂ was investigated over Co/zeolite catalysts at 1 atm, 493–553 K, H₂/CO = 2, and GHSV = 1200. Various zeolites, such as NaA, NaX, NaY, KL and NaMordenite, were used as the supports. The catalysts were prepared by impregnation and were characterized by H₂/CO chemisorption and temperature-programmed reduction (TPR). Based on TPD measurements, the CO/H₂ adsorption ratio can be used as an index for the extent of metal-zeolite interaction. The stronger the metal-zeolite interaction is, the higher the Co/H₂ adsorption ratio on metal is. The activity and selectivity of cobalt supported in zeolites were affected by complex factors such as framework structure, Si/Al ratio, and the complementary cations. The activity of the catalyst is in the order: Co/KL > Co/NaX > Co/NaY > Co/NaMordenite > Co/NaA. All of the Co/zeolite catalysts had a very high selectivity to C₂–C₄ olefins, which would decrease with increasing reaction temperature. Cobalt oxide supported in zeolite was difficult to reduce. Increasing the reduction temperature could increase the reducibility of cobalt and resulted in the increase of activity.     

NO oxidation over supported cobalt oxide catalysts

NO oxidation was conducted over cobalt oxides supported on various supports such as SiO₂, ZrO₂, TiO₂, and CeO₂. The N₂ physisorption, an inductively coupled plasma-atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), NO chemisorptions, the temperature-programmed desorption (TPD) with a mass spectroscopy after NO or CO chemisorptions were conducted to characterize catalysts. Among tested catalysts, Co₃O₄ supported on ceria with a high surface area showed the highest catalytic activity. This catalyst showed superior catalytic activity to unsupported Co₃O₄ with a high surface area and 1 wt% Pt/γ-Al₂O₃. For ceria-supported Co₃O₄, the catalytic activity, the NO uptake at 298 K and the dispersion of Co₃O₄ increased with increasing the surface area of CeO₂. The active participation of the lattice oxygen in NO oxidation could not be observed. On the other hand, the lattice oxygen participated in the CO oxidation over the same catalyst. The deactivation was observed over Co₃O₄/CeO₂ and 1 wt% Pt/γ-Al₂O₃ in the presence of SO₂ in a feed. 1 wt% Pt/γ-Al₂O₃ was deactivated by SO₂ more rapidly compared with Co₃O₄/CeO₂.     

CH4-SCR of NO over Co and Pd ferrierite catalysts: Effect of preparation on catalytic performance

Selective catalytic reduction of NO with methane (CH₄-SCR) in the presence of oxygen excess and water vapour was studied over two bimetallic cobalt/palladium-based FER catalysts, which differ on the order of introduction of metal ions. H₂-TPR and UV–vis analysis showed that the simple change in the order of addition of metals to catalyst, gives rise to totally diverse species (Co²⁺ ions, Co oxides, Co-oxo cations and Pd species) both in type and quantity but also in location within zeolite framework. Experiments of TPD and TPSR of NO and NO₂ provided important information to establish a relation between the various active sites formed on both catalysts and their function in the reaction mechanism. The importance of NO₂ in the mechanism of NO reaction with CH₄ and O₂ was explored and the catalyst with a higher capacity to retain adsorbed NO₂ is the less active for deNO_x. The preparation of a bimetallic catalyst active for NO reduction must provide the proximity between Co and Pd species, and the presence of Co-oxo cations together with palladium species seem to be essential. Furthermore, a suitable amount of cobalt oxides must exist in order to originate NO₂ that is the main reaction intermediate. Nevertheless, an excessive amount of these Co species can lead to an increase of adsorbed NO₂, which reduces the rate of the reaction of some of the mechanism steps.     

Higher alcohol and paraffin synthesis on cobalt based catalysts: Comparison of mechanistic aspects

Syngas reaction mechanism studies on cobalt based catalysts suggested that the CO dissociation step and the (alcohol and/or hydrocarbon) chain growth can be well mimicked by CO disproportionation and temperature programmed desorption (TPD) of acetaldehyde. Correlation between C₂₊ hydrocarbon selectivity and CO disproportionation on the one hand, C₂₊ alcohol selectivity and TPD after acetaldehyde adsorption on the other hand could be evidenced.     

Temperature-programmed desorption of CO from Ni/SiO2 Application of analytic techniques for strongly interacting adsorbates

The interactions of CO with a Ni/SiO₂ catalyst were studied using the temperature-programmed desorption (TPD) technique. Programmed heating following adsorption led to desorption of both CO and CO₂. C and O were found to remain on the catalyst surface after TPD of CO; both were removed by subsequent H₂ treatment. Four CO desorption peaks, designated β^*, β₁, β₂, and β′, were assigned to a surface-type carbonyl, linear-bonded CO, bridge-bonded CO, and the recombination of C and O adatoms, respectively. CO₂ desorption spectra showed a single desorption peak, P₂, when the catalyst weight was low. A new chemical pathway, revealed by the appearance of another peak, P₁, was opened for a higher weight of sample. The formation of P₁ and P₂ followed first-order and second-order kinetics, respectively.The strong readsorption properties of CO within the catalyst bed have made direct determination of kinetic parameters difficult. In a limited range of temperatures, however, a similarity of the CO desorption spectra to results from unsupported Ni was found. Numerical simulation of the TPD process for a flow system has shown that the differential bed assumption is reasonable, i.e. a uniform distribution of the adsorbate within the bed during desorption is approximated. With these observations serving as a basis, the catalyst weight, the amount adsorbed, and the heating rate were varied to obtain desorption energies by analysis of the TPD data for both CO and CO₂.     

Transesterification of tributyrin with methanol over calcium oxide catalysts prepared from various precursors

Calcium oxide catalysts were prepared by calcining various precursors such as calcium acetate, carbonate, hydroxide, nitrate and oxalate and their catalytic activities were examined in the transesterification of tributyrin with methanol. The prepared calcium oxide catalysts were characterized using thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption and temperature programmed desorption (TPD) of CO₂. The calcium oxide catalyst obtained by calcining calcium hydroxide at 600–800 °C showed the highest tributyrin conversion and methyl butyrate yield. The large desorption peak of CO₂ TPD confirmed that its numerous basic sites were responsible for its high activity. The low-temperature decomposition of calcium hydroxide provided many nano-sized pores with strong basic sites. Although the activity of the calcium oxide catalyst prepared from calcium hydroxide was high, its activity was one order of magnitude less than that of sodium hydroxide catalyst. The dissolution of calcium oxide catalysts in products and their repeated uses were also investigated to discuss their advantages as heterogeneous catalysts in the production of biodiesel.     

One step synthesis of mibk (methyl isobutyl ketone) from acetone over CaO-supported Ni catalyst

One step synthesis of MIBK from acetone over Ni/CaO catalysts was studied. 10 wt% Ni/CaO catalysts were prepared by conventional impregnation method (catalyst I), and liquid phase oxidation method using NaOCl as an oxidant (catalyst L). Catalyst L showed much higher activity than catalyst I because of recovered CaO pore structure and high BET surface area. Catalyst C, prepared by coprecipitation method, showed 60% of MIBK selectivity with a fairly high overall acetone conversion. Catalysts L and C had two CO₂ desorption states (α, Β). Incorporated Ni enabled support precursor [Ca(CO)₃] to decompose easily into CaO andCO₂even at low temperature and generated weakCO₂desorption state (α) which was from active state.     

Preparation of Co3O4/ZrO2(Y2O3) Catalyst and its Enhanced Catalytic Oxidation of CO

Yttria-stabilized zirconia powders were prepared by the sol–gel method coupled with supercritical CO₂ fluid-drying technology, using ZrOCl₂·8H₂O as the precursor, urea as the precipitant, and yttria as the stabilizer. The particles were characterized by X-ray diffraction, TEM and BET. The Co₃O₄/ZrO₂(Y₂O₃) catalysts were prepared by the impregnation method. The content of cobalt was varied from 5 to 12 wt%. The prepared catalysts were calcined at 200–500 °C and the pretreating temperature was varied from 200–400 °C. The performance of CO catalytic oxidation was tested and the catalyst with 8% Co loading, calcined at 200 °C, and with a pretreating temperature of 300 °C, showed the highest catalytic activity. The temperature for 95% CO conversion was as low as 113 °C; and, the catalyst showed both good cycling stability and excellent long-term stability.     

Adsorption and reaction of CO on CuO–CeO2 catalysts prepared by the combustion method

Temperature-programmed techniques were employed to investigate the interaction of CO with CuO–CeO₂ prepared by the urea-nitrates combustion method. These catalysts exhibited high and stable CO oxidation activity at relatively low reaction temperatures (< 150 °C). The CO adsorption capacity and catalytic activity of the catalysts was analogous to the concentration of easily-reduced copper oxide surface species. TPD and TPSR results can be explained by a dual scheme of CO adsorption: (i) on oxidized sites, which get reduced with simultaneous formation of surface CO₂ and (ii) on reduced sites created by the former interaction. 10–20% of adsorbed CO desorbs molecularly in the absence of gas-phase O₂, but reacts totally towards CO₂ in the presence of gas-phase O₂. Inhibition by CO₂ observed under steady-state CO oxidation conditions is due to CO₂ adsorption as found by CO₂-TPD.     

Study on the Mechanism of CO Formation in Reverse Water Gas Shift Reaction Over Cu/SiO2 Catalyst by Pulse Reaction, TPD and TPR

The mechanism of reverse water gas shift reaction over Cu catalyst was studied by pulse reaction with QMS monitoring, temperature programmed desorption (TPD) and temperature programmed reduction (TPR) of Cu/SiO₂ catalyst. The reduced and/or oxidized copper offered low catalytic activity for the dissociation of CO₂ to CO in the pulse reaction study with 1 ml volume of He/CO₂, but the rate of CO formation was significantly enhanced with H₂ participating in the reaction. The TPD spectra of CO₂ obtained by feeding H₂/CO₂ over copper at 773 K provided strong evidence of the formation of formate at high temperature. The formate derived from the association of H₂ and CO₂ is proposed to be the key intermediate for CO production. The formate dissociation mechanism is the major reaction route for CO production.     

Hydrogenation of carbon dioxide by hybrid catalysts,direct synthesis of aromatics from carbon dioxide and hydrogen

Direct synthesis of aromatics from carbon dioxide hydrogenation was investigated in a single stage reactor using hybrid catalysts composed of iron catalysts and HZSM-5 zeolite. Carbon dioxide was first converted to CO by the reverse water gas shift reaction, followed by the hydrogenation of CO to hydrocarbons on iron catalyst, and finally the hydrocarbons were converted to aromatics in HZSM-5. Under the operating conditions of 350°C, 2100 kPa, and CO₂/H₅ = 1/2, the maximum aromatic selectivity obtained was 22% with a CO₂ conversion of 38% using fused iron catalyst combined with the zeolite. Together with the kinetic studies, thermodynamic analysis of the CO₂ hydrogenation was also conducted. It was found that unlike Fischer Tropsch synthesis, the formation of hydrocarbons from CO₂ may not be thermodynamically favored at higher temperatures.     

Activity,selectivity, and adsorbed reaction intermediates/reaction side products in the selective methanation of CO in reformate gases on supported Ru catalysts

The reaction behavior and mechanistic aspects of the selective methanation of CO over two supported Ru catalysts, a Ru/zeolite catalyst and a Ru/Al₂O₃ catalyst, in CO₂ containing reaction gas mixtures were investigated by temperature-screening measurements, kinetic measurements and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. The influence of other components present in realistic reformate gases, such as H₂O and high amounts of CO₂, on the reaction behavior was evaluated via measurements in increasingly realistic gas mixtures. Temperature screening and kinetic measurements revealed a high activity of both catalysts, with the Ru mass-normalized activity of the Ru/zeolite catalyst exceeding that of the Ru/Al₂O₃ catalyst by about one order of magnitude. Approaching more realistic conditions, the conversion–temperature curve was shifted slightly upwards for the Ru/Al₂O₃ catalyst, whereas for the Ru/zeolite catalyst it remained unaffected. The selectivity was highest for the Ru/zeolite catalyst, where in parallel to full conversion of CO the conversion of CO₂ remained below 10% over a 40 °C temperature window. During selective methanation on the Ru/Al₂O₃ catalyst, CO₂ was converted even though CO was not completely removed from the feed. Transient DRIFTS measurements, following the build-up and decomposition of adsorbed surface species in different reaction atmospheres and in the corresponding CO-free gas mixtures, respectively, provide information on the formation and removal/stability of the respective adsorbed species and, by comparison with the kinetic data, on their role in the reaction mechanism. Consequences on the mechanism and physical reasons underlying the observed selectivity are discussed.