Synthesis and properties of PdO/CeO2-Al2O3 catalysts for methane combustion

This study focuses on the loading of catalytic materials, e.g., palladium on the surface of supporting materials, with the aim to obtain catalysts with high activity for methane combustion. The catalyst PdO/CeO₂-Al₂O₃ was prepared by impregnation under ultrasonic condition. The effect of different activation methods on the activity of catalysts for methane catalytic combustion was tested. The properties of reaction and adsorption of oxygen species on catalyst surface were characterized by H₂-temperature programmed reduction (H₂-TPR), and O₂-temperature programmed desorption (O₂-TPD). Furthermore, the sulfur tolerance and sulfur poisoning mode were investigated. The results indicate that the catalyst PdO/CeO₂-Al₂O₃ activated with rapid activation shows higher activity for methane combustion and better sulfur tolerance. The result of sulfur content analysis shows that there is a large number of sulfur species on the catalyst’s surface after reactivation at high temperature. It proves that the activity of catalysts cannot be fully restored by high-temperature reactivation.     

过渡金属Ni对Pd/Al_2O_3催化剂甲烷催化燃烧性能的影响

考察过渡金属Ni对Pd/Al_2O_3催化剂甲烷催化燃烧活性的影响以及过渡金属负载量及循环条件对甲烷降解性能的影响,采用扫描电镜、N_2吸附-脱附以及H_2程序升温还原技术对催化剂进行表征。结果表明,过渡金属Ni的添加对催化剂在(375~475)℃下的甲烷催化燃烧活性产生影响。催化剂经多次重复使用后,催化燃烧活性提高。分析原因为经过渡金属Ni改性后,可与载体形成NiAl_2O_4尖晶石,促进活性组分形成较小晶粒,并改善活性组分分散度,提高催化剂催化活性。     

The effect of ceria content on the performance of Pt/CeO2/Al2O3 catalysts in the partial oxidation of methane

The effect of CeO₂ loading (1–20 wt.%) on the properties and catalytic behaviors of CeO₂–Al₂O₃-supported Pt catalysts on the partial oxidation of methane was studied. The catalysts were characterized by S_BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR) and oxygen storage capacity (OSC). XRD and TPR results showed that the pretreatment temperature of the support influences on the amount of CeO₂ with fluorite structure. The pretreatment temperature of the support and CeO₂ loading influenced the morphology of Pt. OSC analysis showed a significant increase in the oxygen storage capacity per weight of CeO₂ for samples with high CeO₂ loading (12 and 20 wt.%). TPR analyses showed that the addition of Pt promotes the reduction of CeO₂. This effect was more significant for the catalysts with high CeO₂ loading (≥12 wt.%). The dispersion of Pt, measured by the rate of cyclohexane dehydrogenation, increases with increasing of the pretreatment temperature of the support. It was shown that the kind of the support is very important for obtaining of catalysts resistant to carbon formation. The catalysts with high CeO₂ loading (≥12 wt.%) showed the highest catalytic activity and stability in the reaction of partial oxidation of methane due to a higher Pt–CeO₂ interface.     

Coke study on MgO-promoted Ni/Al2O3 catalyst in combined H2O and CO2 reforming of methane for gas to liquid (GTL) process

MgO-promoted Ni/Al₂O₃ catalysts have been investigated with respect to catalytic activity and coke formation in combined steam and carbon dioxide reforming of methane (CSCRM) to develop a highly active and stable catalyst for gas to liquid (GTL) processes. Ni/Al₂O₃ catalysts were promoted through varying the MgO content by the incipient wetness method. X-ray diffraction (XRD), BET surface area, H₂-temperature programmed reduction (TPR), H₂-chemisorption and CO₂-temperature programmed desorption (TPD) were used to observe the characteristics of the prepared catalysts. The coke formation and amount in used catalysts were examined by SEM and TGA, respectively. H₂/CO ratio of 2 was achieved in CSCRM by controlling the feed H₂O/CO₂ ratio. The catalysts prepared with 20 wt.% MgO exhibit the highest catalytic performance and have high coke resistance in CSCRM. MgO promotion forms MgAl₂O₄ spinel phase, which is stable at high temperatures and effectively prevents coke formation by increasing the CO₂ adsorption due to the increase in base strength on the surface of catalyst.     

Catalytic combustion of natural gas as the role of on-site heat supply in rapid catalytic CO2---H2O reforming of methane

Development in highly active catalysts for the reforming of methane with H₂O, CO₂, and H₂O+CO₂, and partial oxidation of methane was conducted to produce hydrogen with high reaction rates. A Ni-based three-component catalyst such as Ni---La₂O₃---Ru or Ni---Ce₂O₃---Pt supported on alumina wash-coated ceramic fiber in a plate shape was very suitable for both reactions. The catalyst composition was set at 10 wt.-% Ni, 5.6 wt.-% La₂0₃, and 0.57 wt.-% Ru for example, or molar ratios of these components were 1:0.2:0.03. Even with such a low concentration, the precious metal enhanced the reaction rate markedly, and this synergistic effect was ascribed to the hydrogen spillover effect through the part of precious metal and it resulted in a more reduced surface of the main catalyst component. In particular, a marked enhancement in the reaction rate of CO₂-reforming of methane was observed by the modification of a low concentration Rh to the Ni---Ce₂0₃---Pt catalyst. Very high space-time yields of H₂ (i.e., 8300 mol/1 h in partial oxidation of methane at 600°C with a methane conversion of 37.5%, and 3585 mol/1 h in CO₂reforming of methane at 600°C with a methane conversion of 58%) were realized in those reactions. By combining the catalytic combustion reaction, methane conversion to syngas was markedly enhanced, and even with a very short contact time (10 ms) the conversion of methane increased more than that at 50 ms. The space-time yield of hydrogen amounted to 2,780 mol/1 h with a methane conversion of 90% at 700°C. Furthermore, in a reaction of CH₄---CO₂---H₂O---O₂ on the four components catalyst, an extraordinarily high space-time yield of hydrogen, 12 190 mol/1 h, could be realized under the conditions of very high space velocity (5 ms).     

Role of CeO2 in Ni/CeO2–Al2O3 catalysts for carbon dioxide reforming of methane

Ni catalysts supported on γ-Al₂O₃, CeO₂ and CeO₂–Al₂O₃ systems were tested for catalytic CO₂ reforming of methane into synthesis gas. Ni/CeO₂–Al₂O₃ catalysts showed much better catalytic performance than either CeO₂- or γ-Al₂O₃-supported Ni catalysts. CeO₂ as a support for Ni catalysts produced a strong metal–support interaction (SMSI), which reduced the catalytic activity and carbon deposition. However, CeO₂ had positive effect on catalytic activity, stability, and carbon suppression when used as a promoter in Ni/γ-Al₂O₃ catalysts for this reaction. A weight loading of 1–5 wt% CeO₂ was found to be the optimum. Ni catalysts with CeO₂ promoters reduced the chemical interaction between nickel and support, resulting in an increase in reducibility and stronger dispersion of nickel. The stability and less coking on CeO₂-promoted catalysts are attributed to the oxidative properties of CeO₂.     

Performance of solvothermally prepared Ga2O3-Al2O3 catalysts for SCR of NO with methane

The solvothermal reaction of mixtures of aluminum isopropoxide (AIP) and gallium acetylacetonate (Ga(acac)₃) directly yielded the mixed oxides of γ-Ga₂O₃-Al₂O₃. In the solvothermal synthesis, the crystal structure of mixed oxides was controlled by the initial formation of γ-Ga₂O₃ nuclei. The mixed oxides prepared in diethylenetriamine have extremely high activities for selective catalytic reduction (SCR) of NO with methane as a reducing agent. With increasing crystallite size of the spinel structure, the catalytic activity increased. The ratio of the amount of methane consumed by combustion to total methane conversion was proportional to the density of acid sites on the surface of the mixed oxides. The mixed oxide catalysts prepared in diethylenetriamine had lower densities of acid sites and showed a higher methane-efficiency for CH₄-SCR than those prepared in other solvents. These catalysts maintained their high activity even when the reaction was carried out under the severe conditions (i.e., high space velocity and low NO concentration).     

Superior catalytic behaviour of Pt-doped Pd catalysts in the complete oxidation of methane at low temperature

The catalytic combustion of methane at low temperature under lean conditions was investigated over bimetallic palladium-platinum catalysts supported on alumina. Pd-Pt catalysts with constant 2 wt.% metal loading and varying compositions in Pt and Pd were prepared by successive impregnations of the metal salts. The catalysts were characterised by powder X-ray diffraction, transmission electron microscopy/electron dispersion X-ray spectroscopy (TEM/EDX), volumetry of H₂ chemisorption, FTIR study of CO adsorption and temperature-programmed oxidation (TPO). In the absence of water added to the feed, the methane conversion over Pd-rich bimetallic catalysts (Pt/Pt + Pd molar ratios less than 0.3) was found to be the same as that of the reference Pd/Al₂O₃ catalyst. Interestingly, under wet conditions, these bimetallic catalysts exhibited an improved performance with respect to Pd/Al₂O₃. This effect was found to be maintained upon mild steam ageing. An interaction between both metals was suggested to explain the enhanced activity of bimetallic catalysts. This was confirmed by TPO experiments indicating that formation and decomposition of PdO is affected upon Pt addition even for very low amounts of Pt. The adsorption of CO on reduced catalysts studied by FTIR revealed new types of adsorbed CO species, suggesting again an interaction between two metals.     

Tri-reforming of methane: a novel concept for catalytic production of industrially useful synthesis gas with desired H2/CO ratios

A novel process concept called tri-reforming of methane has been proposed in our laboratory using CO₂ in the flue gases from fossil fuel-based power plants without CO₂ separation [C. Song, Chemical Innovation 31 (2001) 21–26]. The proposed tri-reforming process is a synergetic combination of CO₂ reforming, steam reforming, and partial oxidation of methane in a single reactor for effective production of industrially useful synthesis gas (syngas). Both experimental testing and computational analysis show that tri-reforming can not only produce synthesis gas (CO + H₂) with desired H₂/CO ratios (1.5–2.0), but also could eliminate carbon formation which is usually a serious problem in the CO₂ reforming of methane. These two advantages have been demonstrated by tri-reforming of CH₄ in a fixed-bed flow reactor at 850 °C with supported nickel catalysts. Over 95% CH₄ conversion and about 80% CO₂ conversion can be achieved in tri-reforming over Ni catalysts supported on an oxide substrate. The type and nature of catalysts have a significant impact on CO₂ conversion in the presence of H₂O and O₂ in tri-reforming in the temperature range of 700–850 °C. Among all the catalysts tested for tri-reforming, their ability to enhance the conversion of CO₂ follows the order of Ni/MgO > Ni/MgO/CeZrO > Ni/CeO₂ ≈ Ni/ZrO₂ ≈ Ni/Al₂O₃ > Ni/CeZrO. The higher CO₂ conversion over Ni/MgO and Ni/MgO/CeZrO in tri-reforming may be related to the interaction of CO₂ with MgO and more interface between Ni and MgO resulting from the formation of NiO/MgO solid solution. Results of catalytic performance tests over Ni/MgO/CeZrO catalysts at 850 °C and 1 atm with different feed compositions confirm the predicted equilibrium conversions based on the thermodynamic analysis for tri-reforming of methane. Kinetics of tri-reforming were also examined. The reaction orders with respect to partial pressures of CO₂ and H₂O are different over Ni/MgO, Ni/MgO/CeZrO, and Ni/Al₂O₃ catalysts for tri-reforming.     

Oxidation behaviour and catalytic properties of Pd/Al2O3 catalysts in the total oxidation of methane

A series of Pd/Al₂O₃ catalysts with a wide range of mean Pd particle sizes (ca. 2–30 nm in diameter) was prepared by using various precursors (H₂PdCl₄, Pd(NO₃)₂ and Pd(AcAc)₂) and pre-treatments. The mean particle size of reduced samples was determined by H₂ chemisorption. The catalytic activity in methane oxidation under lean burn conditions was measured. The oxidation of reduced samples was studied at 300 °C. The extent of oxidation was found to decrease with increasing mean particle size. While small particles (<5 nm) oxidised very rapidly, the oxidation of large particles (ca. >15 nm) proceeded via a two-step process, being first fast and then slow. The decomposition of oxide species was studied by temperature-programmed experiments under vacuum. Two distinct oxidised species with different stability were evidenced depending on the particle size. Oxidised species in larger particles were found of lower stability than in smaller ones. A correlation between the existence of distinct types of oxide species and catalytic properties in methane oxidation was discussed.     

Effect of niobium addition to Co/γ-Al2O3 catalyst on methane combustion

In this work, the effect of niobium addition on textural, structural, acidic, and catalytic properties of Co/γ-Al₂O₃ catalysts for use in the total combustion of methane was studied. The catalysts were prepared by using the sol–gel technique and characterized by X-ray diffraction (XRD), infrared spectroscopy of adsorbed pyridine (IR), nitrogen adsorption (BET surface area), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), transmission electron microscopy in conjunction with energy-dispersive X-ray analysis (STEM–EDX), and activity in the total oxidation of methane. Results show that all cobalt-containing catalysts, regardless of the type of support, decrease the light off temperature of methane compared to pure γ-Al₂O₃. Therefore, the addition of 1.0% (by weight) of niobium to cobalt-containing alumina catalysts promoted a negative effect on the catalytic activity. Both cobalt-containing catalysts (4.5 and 9.3%, by weight, of Co) show a higher catalytic activity when compared to cobalt-containing niobia–alumina catalyst (6.3%, by weight, of Co). The lower activity of the niobia-containing catalysts may be associated to their acidic and textural properties.     

Transformation of methane formation sites into methanol formation ones during CO---H2 reaction over Pd/CeO2 in its SMSI state

The phenomena of induction period of methanol formation, observed in CO---H₂ reaction over Pd/CeO₂ catalysts under SMSI state, was investigated in depth. The magnitude of the induction period was dependent on the extent of SMSI, and higher temperature H₂ reduction lengthened it accompanied with the increase of the number of active sites for methane formation. On the contrary, by the pretreatment of SMSI surface with water vapor, this induction period almost disappeared with the drastic decrease of methane formation rate. These results indicate that methane formation sites would be transformed into methanol formation sites by the oxidation of water vapor formed during CO---H₂ reaction. Infrared spectroscopic investigation of adsorbed CO after various pretreatments indicated that during the induction period thin layers of reduced ceria, which preferentially covered Pd(1 1 1) plane under SMSI state, were removed from the Pd(1 1 1) plane by formed water vapor during CO---H₂ reaction. It was concluded that Pd(1 1 1) plane adjacent to ceria would be the efficient active sites for methanol formation.     

Autothermal reforming of methane over Ni catalysts supported over CaO–CeO2–ZrO2 solid solution

Ni catalysts supported on various solid solutions of ZrO₂ with alkaline earth oxide and/or rare earth oxide were synthesized. The catalytic activities were compared for partial oxidation of methane and autothermal reforming of methane. For partial oxidation of methane, the Ni catalyst supported on a CaO–ZrO₂ solid solution showed a high activity. Incorporation of CaO in the ZrO₂ matrix was effective for increasing the reduction rate of the NiO particles and for decreasing the coke formation. On the other hand, the Ni particles supported on the CaO–CeO₂–ZrO₂ solid solution had a strong interaction with the support, and the Ni particles showed high activity and stability for autothermal reforming of methane.     

Transient studies of carbon dioxide reforming of methane over Pt/ZrO2 and Pt/Al2O3

The mechanism of the CO₂ reforming of methane reaction over the Pt/ZrO₂ catalyst was investigated using a temporal analysis of products (TAP) reactor system. For comparative purposes, the reaction pathway using a Pt/Al₂O₃ catalyst was also examined. A reaction sequence is suggested for both catalysts. Over both catalysts, methane decomposition takes place over platinum. The main difference between the two catalysts concerns the carbon dioxide dissociation. Over Pt/Al₂O₃ this step is assisted by hydrogen. Over Pt/ZrO₂ this step takes place over the zirconia support and involves surface vacancies. Moreover, large pools of formate and carbonate species are present on the zirconia. Transient studies conducted to determine the origin of carbon species accumulated during CO₂ reforming revealed that more than 99% of the carbon was derived from the methane molecule over both catalysts. Over the Pt/ZrO₂ catalyst, only a single very reactive carbon species was detected, while over the Pt/Al₂O₃ a second less active species was also formed.     

Carbon formation and its influence on ethanol steam reforming over Ni/Al2O3 catalysts

Ethanol steam reforming was studied over Ni/Al₂O₃ catalysts. The effect of support (- and γ-Al₂O₃), metal loading and a comparison between conventional H₂ reduction with an activation method employing a CH₄/O₂ mixture was investigated. The properties of catalysts were studied by N₂ physisorption, X-ray diffraction (XRD) and temperature programmed reduction (TPR). After activity tests, the catalysts were analyzed by scanning electron microscopy (SEM) and thermogravimetric analysis (TG/DTA). Ni supported on γ-Al₂O₃ was more active for H₂ production than the catalyst supported on -Al₂O₃. Metal loading did not affect the catalytic performance. The alternative activation method with CH₄/O₂ mixture affected differently the activity and stability of the Ni/γ-Al₂O₃ and the Ni/-Al₂O₃ catalyst. This activation method increased significantly the stability of Ni/-Al₂O₃ compared to H₂ reduction. SEM and TG/DTA analysis indicate the formation of filamentous carbon during the CH₄/O₂ activation step, which is associated with the increasing catalyst activity and stability. The effect of temperature on the type of carbon formed was investigated; indicating that filamentous coke increased activity while encapsulating coke promoted deactivation. A discussion about carbon formation and the influence on the activity is presented.     

Preparation and characterisation of supported La0.8Sr0.2MnO3+x

Three supported La_0.8Sr_0.2MnO_3+x catalysts were prepared, one supported on lanthanum-stabilised alumina and two supported on a NiAl₂O₄ spinel. The catalysts were characterised using X-ray diffraction, transmission electron microscopy and surface area measurements following heat-treatments at temperatures up to 1200°C in air. In the alumina-supported catalyst, a reaction occurred between the active phase and the support at high temperatures, indicating that these materials would be unsuitable for high temperature catalytic combustion. Only in the NiAl₂O₄-supported catalysts were the supported perovskite phases found to be stable at high temperature. These catalysts showed good methane combustion activity.     

Partial oxidation of methane to synthesis gas over Co/Ca/Al2O3 catalysts

A series of calcium-modified alumina-supported cobalt catalysts were prepared with a two-step impregnation method, and the effect of calcium on the catalytic performances of the catalysts for the partial oxidation of methane to syngas (CO and H₂) was investigated at 750 °C. Also, the catalysts were characterized by XRD, TEM, TPR and (in situ) Raman. At 6 wt.% of cobalt loading, the unmodified alumina-supported cobalt catalyst showed a very low activity and a rapid deactivation, while the calcium-modified catalyst presented a good performance for this process with the CH₄ conversion of 88%, CO selectivity of 94% and undetectable carbon deposition during a long-time running. Characterization results showed that the calcium modification can effectively increase the dispersion and reducibility of Co₃O₄, decrease the Co metal particle size, and suppress the reoxidation of cobalt as well as the phase transformation to form CoAl₂O₄ spinel phases under the reaction conditions. These could be related to the excellent catalytic performances of Co/Ca/Al₂O₃ catalysts.     

Light alkanes CO2 reforming to synthesis gas over Ni based catalysts

The CO₂ reforming of methane and propane has been compared over two different Ni catalysts: one reference Ni/SiO₂ system and a Ni/Mg(Al)O hydrotalcite-derived catalyst, shown previously to display high catalytic stability for long term reforming. By combining the Tapered Element Oscillating Microbalance (TEOM), Temperature Programmed Hydrogenation (TPH), Transmission Electron Microscopy (TEM) and magnetic measurements, the formation of coke and its role on the catalyst activity has been investigated and compared for both hydrocarbons. It was found that Ni/SiO₂ and Ni/Mg(Al)O are both more active for methane reforming than for propane reforming. Coke formation is much more pronounced for propane than for methane over both catalysts. However, for both hydrocarbons a much faster carbon formation is observed over the Ni/SiO₂ catalyst than over the Ni/Mg(Al)O catalyst. The difference in the rates of coke formation for methane and propane is ascribed in the case of propane to partially dehydrogenated C₃ adspecies, which are good coke precursors. The superior stability of the hydrotalcite-derived catalyst is due to the strong interaction of the nickel phase with the support and the capacity of the support to activate CO₂ and channel oxygen to the nickel phase.     

Low temperature oxidation of methane over Pd/SnO2 catalyst

Catalytic activities of supported Pd were investigated for low temperature oxidation of methane. Pd/SnO₂ catalysts demonstrated excellent activity for methane oxidation in spite of their low surface area. The catalytic activity of Pd/SnO₂ was strongly affected by the preparation procedure. Impregnation of Pd on SnO₂ using aqueous solution of Pd(CH₃COO)₂ was most effective in enhancing the catalytic activity. The catalytic activity was also improved when well-crystallized SnO₂ was employed as a support material. TEM observations revealed that catalytic activity is strongly influenced by the dispersion state of Pd. For the active catalysts, strong interaction between Pd and SnO₂ support was observed in the adsorption of oxygen.     

NiBa catalysts for CO2-reforming of methane

Ni–Ba catalysts supported on γ-Al₂O₃ for the dry reforming of methane were prepared, characterized and studied under reaction conditions. Ba incorporation inhibits the formation of Ni spinel. All the Ni–Ba catalysts studied are highly active for the CO₂-reforming of methane. However, the Ni–Ba catalyst with high Ba and Ni content was the most active and stable catalyst, due to the presence of accessible Ni particles stabilized by the formation of BaAl₂O₄.