Improved resistance against coke deposition of titania supported cobalt and nickel bimetallic catalysts for carbon dioxide reforming of methane

The catalytic behavior of bi-metallic Co–Ni/TiO₂ catalysts for CO₂ reforming of CH₄ to synthesis gas was investigated under atmospheric pressure with a particular attention to carbon deposition. The catalysts with optimized Co/Ni ratios showed high catalytic stability towards the reaction with very little amount of deposited carbon at a wide range of reaction temperature (773–1123 K). The results suggest that adjusting of composition of the active metals (Co and Ni) can kinetically control the elementary steps (formation of carbon species and its removal by oxygen species) of CH₄/CO₂ reaction.     

Sequential production of H2 and CO over supported Ni catalysts

Methane decomposition into H₂ and carbon nanofibers at 823 K and subsequent gasification of the carbon nanofibers with CO₂ into CO at 923 K were performed over supported Ni catalysts (Ni/SiO₂, Ni/TiO₂ and Ni/Al₂O₃). Supported Ni catalysts were deactivated for CH₄ decomposition with time on stream due to deposition of a large amount of carbon nanofibers. Subsequent contact of CO₂ with carbon nanofibers on the deactivated catalysts resulted in the formation of CO with a conversion of the carbons higher than 95%. In addition, gasification with CO₂ regenerated the activity of supported Ni catalysts for CH₄ decomposition, indicating that H₂ formation through CH₄ decomposition and CO formation through gasification with CO₂ could be carried out repeatedly. Conversions of carbon nanofibers into CO were kept higher than 95% in the repeated gasification over all the catalysts, while change in the catalytic activity for CH₄ decomposition with the repeated cycles depended on the kind of catalytic supports. Catalytic activity of Ni/SiO₂ for CH₄ decomposition was high at early cycles, however, the activity decreased gradually with the repeated cycles. On the other hand, Ni/TiO₂ and Ni/Al₂O₃ showed high activity for CH₄ decomposition and the activity was kept high during the repeated cycles. These changes of catalytic activities for CH₄ decomposition could be explained by changes in particle sizes of Ni metal, i.e. Ni metal particles in Ni/SiO₂ aggregated into ones larger than 150 nm with the repeated cycles, while the particle sizes of Ni metal in Ni/TiO₂ and Ni/Al₂O₃ remained at an effective range for CH₄ decomposition (60-100 nm).     

Noble metal promoted Ni0.03Mg0.97O solid solution catalysts for the reforming of CH4 with CO2

Reforming of CH₄ with CO₂ to produce syngas was studied over Ni_0.03Mg_0.97O solid solution catalyst and its bimetallic derivative catalysts which contained small amounts of Pt, Pd or Rh (the atomic ratio M/(Ni + Mg) was about 2 × 10^–4, M = Pt, Pd or Rh). It was found that although the Ni_0.03Mg{0.97}O catalyst showed an excellent stability and activity at the reaction temperature of 1123 K, it lost its activity completely within 51 h when the reaction temperature was as low as 773 K. However, both the activity and the stability at 773 K were improved significantly by adding Rh, Pt, or Pd. This synergistic effect is rationally explained by the promoted reducibility of Ni. On all these catalysts, the amount of deposited carbon during the reaction was very low, suggesting that carbon deposition was not the main cause of the deactivation. Also, the catalytic activity of bimetallic catalysts increased gradually with the noble metal loading, but after passing through a maximum, it decreased with superfluous addition. The maximum was found to be located at around the atomic ratio of M/(Ni + Mg) 0.02% (M = Pt, Pd and Rh). This phenomenon could most probably be attributed to the different composition of Pt-Ni alloy particles formed after the reduction.     

Hydrogen Production by Steam Reforming of Commercially Available LPG in UAE

Steam reforming of commercially available LPG using Ru/Al₂O₃ and Ni/Al₂O₃ catalysts has been studied at temperatures between 573 and 773 K. Ru/Al₂O₃ catalyst showed higher rates of reaction and lower activation energies of the three main components of LPG, compared with Ni/Al₂O₃. However, Ni/Al₂O₃ catalyst showed a better H₂:CH₄ selectivity. The activation energy of n-butane was the lowest over Ru/Al₂O₃, whereas over Ni/Al₂O₃, propane had the lowest activation energy. The activation energy of i-butane was always the highest over both catalysts, which suggests that both catalysts performed better with unbranched molecules. A slight increase in activation energy was observed, when each component of the LPG mixture was studied separately as a pure gas, compared with being mixed in LPG. At a constant temperature of 773 K, hydrogen production yield and H₂:CH₄ selectivity were determined using Ru/Al₂O₃ at different steam:carbon (S:C) ratios and LPG flow rates. It was found that the yield and selectivity increased with the increase in S:C ratio and the decrease in the flow rate. The highest yield of 0.64 was achieved using S:C ratio of 6.5 and a LPG flow rate of 50 mL min^?1. The work provides valuable information on steam reforming of pure components of LPG, compared with when they are in the mixture. The comparison is done using conventional steam reforming catalyst, Ni/Al₂O₃, and compared with Ru/Al₂O₃. The observed trends and variations in reaction rates, in pure and mixed gases, indicated that the mechanism of steam reforming of a hydrocarbon mixture depends on its composition.     

Investigation of CH4 Reforming with CO2 on Meso-Porous Al2O3-Supported Ni Catalyst

Meso-porous Al₂O₃-supported Ni catalysts exhibited the highest activity, stability and excellent coke-resistance ability for CH₄ reforming with CO₂ among several oxide-supported Ni catalysts (meso-porous Al₂O₃ (Yas1-2, Yas3-8), -Al₂O₃, -Al₂O₃, SiO₂, MgO, La₂O₃, CeO₂ and ZrO₂). The properties of deposited carbons depended on the properties of the supports, and on the meso-porous Al₂O₃-supported Ni catalyst, only the intermediate carbon of the reforming reaction formed. XRD and H₂-TPR analysis found that mainly spinel NiAl₂O₄ formed in meso-porous Al₂O₃ and -Al₂O₃-supported catalysts, while only NiO was detected in -Al₂O₃, SiO₂, CeO₂, La₂O₃ and ZrO₂ supports. The strong interaction between Ni and meso-porous Al₂O₃ improved the dispersion of Ni, retarded its sintering and improved the activated adsorption of CO₂. The coking reaction via CH₄ temperature-programed decomposition indicated that meso-porous Al₂O₃-supported Ni catalysts were less active for carbon formation by CH₄ decomposition than Ni/-Al₂O₃ and Ni/-Al₂O₃.     

Catalysis of NiO–Al2O3 aerogels for the CO2-reforming of CHF4

Uniform and monolithic NiO–Al₂O₃ aerogels were prepared from cyclic nickel glycoxide, (CH₂O)₂Ni, and boehmite sol, AlOOH, and the catalyst performance of the aerogels for the CO₂-reforming of methane was investigated. The NiO–Al₂O₃ aerogels showed higher activity than impregnation NiO/Al₂O₃ catalysts, while the aerogels exhibited much less activity for coking than the impregnation catalysts. Less deactivation was also observed on the aerogel catalysts than on the impregnation catalysts in the continuous-flow reaction. The Ni was uniformly incorporated throughout alumina where both the metal and the support exist in the aerogel form, i.e., Ni–O–Al bond was considered to be formed in the aerogels. As a result, fine Ni particles appeared after H₂ reduction throughout the alumina support with high dispersion, which brought about not only higher activity but also much less activity for coking on the aerogels. Retardation of catalyst deactivation was ascribed to the suppression of both coking and sintering of Ni particles on the aerogels. This revised version was published online in July 2006 with corrections to the Cover Date.     

Characterization of aerogel Ni/Al2O3 catalysts and investigation on their stability for CH4-CO2 reforming in a fluidized bed

The CH₄-CO₂ reforming was investigated in a fluidized bed reactor using nano-sized aerogel Ni/Al₂O₃ catalysts, which were prepared via a sol–gel method combined with a supercritical drying process. The catalysts were characterized with BET, XRD, H₂-TPR and H₂-TPD techniques. Compared with the impregnation catalyst, aerogel catalysts exhibited higher specific surface areas, lower bulk density, smaller Ni particle sizes, stronger metal-support interaction and higher Ni dispersion degrees. All tested aerogel catalysts showed better catalytic activities and stability than the impregnation catalyst. Their catalytic stability tested during 48 h reforming was dependent on their Ni loadings. Characterizations of spent catalysts indicated that only limited graphitic carbon formed on the aerogel catalyst, while massive graphitic carbon with filamentous morphology was observed for the impregnation catalyst, leading to significant catalytic activity degradation. An aerogel catalyst containing 10% Ni showed the best catalytic stability and the lowest rate of carbon deposition among the aerogel catalysts due to its small Ni particle size and strong metal-support interaction.     

Methane Oxidation Over M–8YSZ and M–CeO2/8YSZ (M = Ni, Cu, Co, Ag) Catalysts

The oxidation of methane has been studied by sequential flow reaction experiments over M–8YSZ and M–CeO₂/8YSZ (M=Ni, Cu, Co, Ag) catalysts as a function of CH₄/O₂ from 773 to 1073 K. Over Ni–8YSZ and Ni–CeO₂/8YSZ, methane pyrolysis is dominant leading to surface carbon formation at temperatures of 873 K and above. While the addition of ceria to Ni–8YSZ to produce Ni–CeO₂/8YSZ does not significantly affect the reaction kinetics, the activity of Cu–CeO₂/8YSZ, Co–CeO₂/8YSZ, and Ag–CeO₂/8YSZ are higher than their M–8YSZ counterparts. The activity of Co–CeO₂/8YSZ at high temperatures (973 K and above) is higher than Ni-8YSZ with selectivity towards partial methane oxidation and CO formation. Considering Ni-based catalysts are prone to deactivation due to surface carbon accumulation, Co–CeO₂/8YSZ, Cu–CeO₂/8YSZ, and Ag–CeO₂/8YSZ are possible alternative anode cermets for direct hydrocarbon oxidation solid-oxide fuel cells (SOFC).     

The electronic effects of Pr on La1−xPrxNiAl11O19 for CO2 reforming of methane

The CO₂ reforming of CH₄ to synthesis gas by using praseodymium modified hexaaluminate La_1−xPr_xNiAl₁₁O₁₉ (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) as catalysts was studied. The modifier Pr improved the reducibility and catalytic activity of Ni ions as active component in the hexaaluminate lattices, especially the conversion of CH₄ and CO₂ reached 89.62% and 92.94%, respectively over La_0.8Pr_0.2NiAl₁₁O₁₉. It was found that the addition of Pr can promote the electronic transformation between the Ni ions and the La ions to maintain Ni at a lower valence, which promotes the activation of CH₄.     

Studies on the Stability of a La0.8Pr0.2NiAl11O19 Catalyst for Syngas Production by CO2 Reforming of Methane

CO₂ reforming of CH₄ was studied over a magnetoplumbite-type hexaaluminate La_0.8Pr_0.2NiAl₁₁O₁₉ catalyst, which showed very high activity for over 300 h without deactivation at 1023 K. This catalyst showed good resistance to carbon deposition, which in this reaction and in CH₄ decomposition was investigated by means of XPS and TEM. It is suggested that nano-tube-like carbon is an intermediate in this reaction and a spillover of carbon from crystalline Ni onto the hexaaluminate oxide occurred during the reaction.     

Spectroscopic evidence for the formation of CHx species in the hydrogenation of carbidic carbon on Ni(100)

The CH _x species formed on Ni(100) by hydrogenation of carbidic carbon have been detected using high resolution electron energy-loss spectroscopy (HREELS). Exposures of carbidic carbon to 1×10^–7 Torr H₂ and D₂ at 313 K for 20 min produce CH _x and CD _x species, respectively. These species are identified by two energy-loss peaks for CH _x at 2970 and 1380 cm^–1 and only one peak for CD _x at 1980 cm^–1. Because of the existence of the intense peak at 1380 cm^–1, in the range of a scissors mode for CH₂ and a symmetric deformation mode for CH₃, the CH _x species are tentatively ascribed to CH₂ and/or CH₃. The CD _x species undergo decomposition at 330–370 K in UHV as well as in hydrogen below 10^–7 Torr. No stable CH _x species are observed above 400 K, which is lower than the normal reaction temperature of the methanation reaction (500 K).     

Integrated coal pyrolysis with CO2 reforming of methane over Ni/MgO catalyst for improving tar yield

A new process to integrate coal pyrolysis with CO₂ reforming of methane over Ni/MgO catalyst was put forward for improving tar yield. And several Chinese coals were used to confirm the validity of the process. The experiments were performed in an atmospheric fixed-bed reactor containing upper catalyst layer and lower coal layer to investigate the effect of pyrolysis temperature, coal properties, Ni loading and reduction temperature of Ni/MgO catalysts on tar, water and char yields and CH₄ conversion at fixed conditions of 400 ml/min CH₄ flow rate, 1:1 CH₄/CO₂ ratio, 30 min holding time. The results indicated that higher tar yield can be obtained in the pyrolysis of all four coals investigated when coal pyrolysis was integrated with CO₂ reforming of methane. For PS coal, the tar, water and char yield is 33.5, 25.8 and 69.5 wt.%, respectively and the CH₄ conversion is 16.8%, at the pyrolysis temperature of 750 °C over 10 wt.% Ni/MgO catalyst reduced at 850 °C. The tar yield is 1.6 and 1.8 times as that in coal pyrolysis under H₂ and N₂, respectively.     

Evaluation of different oxygen carriers for biomass tar reforming (II): Carbon deposition in experiments with methane and other gases

This work is a continuation of a previous paper by the authors [1] which analyzed the suitability of the Chemical Looping technology in biomass tar reforming. Four different oxygen carriers were tested with toluene as tar model compound: 60% NiO/MgAl₂O₄ (Ni60), 40% NiO/NiAl₂O₄ (Ni40), 40% Mn₃O₄/Mg-ZrO₂ (Mn40) and FeTiO₃ (Fe) and their tendency to carbon deposition was analyzed in the temperature range 873-1073 K. In the present paper, the reactivity of these carriers to other compounds in the gasification gas is studied, also with special emphasis on the tendency to carbon deposition. Experiments were carried out in a TGA apparatus and a fixed bed reactor. Ni-based carriers showed a tendency to form carbon in the reaction with CH₄, especially Ni60. The addition of water in H₂O/CH₄ molar ratios of 0.4-2.3 could decrease the carbon deposited, but not in the case of Ni60. Mn-based sample reacted with CH₄ almost completely and with low tendency to carbon deposition, while the Fe-based sample showed low reactivity. Ni40 showed more reactivity to CO than Mn40, although in both cases carbon was deposited, especially at 873 K. When H₂ was present, it reacted rapidly with both carriers, decreasing the amount of carbon deposited. The presence of CO₂ could also decrease the carbon deposited on Ni40 at 1073 K. According to both these and the previous results [1], it can be concluded that Mn40 is the most adequate for minimization of carbon deposition in Chemical Looping Reforming (CLR).     

Nature of Surface Deposits on Sulfated Zirconia Used as Catalyst in the Benzoylation of Anisole

The effects of CO₂, CO and H₂ co-reactants on CH₄ pyrolysis reactions catalyzed by Mo/H-ZSM-5 were investigated as a function of reaction temperatures and co-reactant and CH₄ concentrations. Total CH₄ conversion rates were not affected by CO₂ co-reactants, except at high CO₂ pressures, which led to the oxidation of the active MoC _x species, but CH _x intermediates formed in rate-determining C–H bond activation steps increasingly formed CO instead of hydrocarbons as CO₂ concentrations increased. CO formation rates increased with increasing CO₂ partial pressure; all entering CO₂ molecules reacted with CH₄ within the catalyst bed to form two CO molecules at 950-1033 K. In contrast, hydrocarbon formation rates decreased linearly with increasing CO₂ partial pressure and reached undetectable levels at CO₂/CH₄ ratios above 0.075 at 950 K. CO formation continued for a short period of time at these CO₂/CH₄ molar ratios, but then all catalytic activity ceased, apparently as a result of the conversion of active carbide structures to MoO _x . The removal of CO₂ from the CH₄ stream led to gradual catalyst reactivation via reduction-carburization processes similar to those observed during the initial activation of MoO _x /H-ZSM-5 precursors in CH₄. The CO₂/CH₄ molar ratios required to inhibit hydrocarbon synthesis were independent of CH₄ pressure because of the first-order kinetic dependencies of both CH₄ and CO₂ activation steps. These ratios increased from 0.075 to 0.143 as reaction temperatures increased from 950 to 1033 K. This temperature dependence reflects higher activation energies for reductant (CH₄) than for oxidant (CO₂) activation, leading to catalyst oxidation at higher relative oxidant concentrations as temperature increases. The scavenging of CH _x intermediates by CO₂-derived species leads also to lower chain growth probabilities and to a significant inhibition of catalyst deactivation via oligomerization pathways responsible for the formation of highly unsaturated unreactive deposits. CO co-reactants did not influence the rate or selectivity of CH₄ pyrolysis reactions on Mo/H-ZSM-5; therefore, CO formed during reactions of CO₂/CH₄ mixtures are not responsible for the observed effects of CO₂ on reaction rates and selectivities, or in catalyst deactivation rates during CH₄ reactions. H₂ addition studies showed that H₂ formed during CH₄/CO₂ reactions near the bed inlet led to inhibited catalyst deactivation in downstream catalyst regions, even after CO₂ co-reactants were depleted.     

Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method

The methane conversion and carbon yield of the chemical vapor deposition (CVD) reaction suggests that the optimum reaction conditions of the formation of multi-wall carbon nanotubes (MWCNTs) can be obtained by using a 50 mg of nano-MgNi alloy under pyrolysis of the pure CH₄ gas with the flow rate about 100-120 cm³/min at 650 °C for 30 min. Raman results indicate the CNTs are in multi-wall structure, since no single-wall characteristic features appearing in the 200-400 cm⁻¹ region. This is consistent with those of the XRD and TGA findings. Under selected condition, the carbon yield and the CNTs purity can reach up to 1231% and 92% in the presence of hydrogen. It is presumable that the presence of hydrogen in the pyrolysis of CH₄ prevents the deactivation of catalysts and enhances the graphitization degree of CNTs. In addition, the presence of Mg metal in the alloy can prevent the aggregation of the Ni metal and forms the active Mg₂Ni phase to enhance the CH₄ pyrolysis to form CNTs. After the purification procedures with both air oxidation at 550 °C and HCl treatments, the final purified yield and purity of CNT reach to 73.2% and (98.04 ± 0.2)% respectively.     

Potassium-enhanced stability of Ni/MgO catalysts in the dry-reforming of methane

The catalytic behaviour of the bare and K-promoted 19 wt% Ni/MgO catalyst in the CO₂-reforming of CH₄ at 650°C has been investigated. The effects of the K loading (1.5–2.5 wt%) on the catalytic activity, stability and coking rate have been addressed. Both sintering and formation of large amounts of whisker carbon lead to a marked deactivation of the bare Ni/MgO system. K addition depresses the reactivity of the catalyst strongly improving its resistance to both coking and sintering processes. A change in the electronic and geometric properties of the active phase, monitored by a rise in E_app from 50 to 70 kJ mol⁻¹, accounts for the effects of K addition on the catalytic behaviour of the Ni/MgO catalyst in the dry-reforming of CH₄.     

Influence of composition of MgAl2O4 supported NiCeO2ZrO2 catalysts on coke formation and catalyst stability for dry reforming of methane

Dry reforming of methane was studied over Ni catalysts supported on γAl₂O₃, CeO₂, ZrO₂ and MgAl₂O₄ (670 °C, 1.5 bar, 16–20 l CH₄ ml_catalyst⁻¹ h⁻¹). It is shown that MgAl₂O₄ supported Ni catalysts promoted with both CeO₂ and ZrO₂ are promising catalysts for dry reforming of methane with carbon dioxide. Within a certain composition range, the simultaneous promotion with CeO₂ and ZrO₂ has great influence on the amount of coke and the catalyst service time. XRD analyses indicate that formation of crystalline Ce_xZr_1−xO₂ mixed oxide phases occurs on double promotion. In particular, incorporation of low amounts of Zr in the CeO₂ fluorite structure provides stable dry reforming catalysis. As shown with TPR, promotion leads to a higher reduced state of Ni. SEM, XRD and TPR analyses demonstrate that highly dispersed, doubly promoted Ni catalysts with a strong metal-support interaction are essential for stable dry reforming and suppression of the formation of carbon filaments.     

Synthesis of Dimethyl Carbonate from Methanol and Carbon dioxide using Potassium Methoxide as Catalyst under Mild Conditions

Pd-supported on WO₃–ZrO₂ (W/Zr atomic ratio=0.2) calcined at 1073 K was found to be highly active and selective for gas-phase oxidation of ethylene to acetic acid in the presence of water at 423 K and 0.6 MPa. Contact time dependence demonstrated that acetic acid is formed via acetaldehyde formed by a Wacker-type reaction, not through ethanol by hydration of ethylene.     

Evidence for a cation intermediate during methanol dehydration on Pt(110)

NiB amorphous alloy and Ni catalysts supported on HMCM-22, HZSM-5, HY, -Al₂O₃ and SiO₂ were prepared, respectively, by the chemical reduction method and the standard incipient wetness impregnation method. These catalysts were examined for catalytic performance in the two-step conversion of CH₄ to produce hydrogen and higher hydrocarbons. All catalysts give similar methane conversion and yields of hydrogen and H-deficient carbon-containing species in step I. In the subsequent hydrogenation step (step II), they have similar carbon conversion, however, the yield of C₂ and C₃ hydrocarbons depends greatly on the nature of the metal particles and support acidity. Supported NiB amorphous alloy catalysts offer higher yield of C₂ and C₃ hydrocarbons than the corresponding Ni catalysts, due to their unique properties: nanoscale size, long-range disorder in structure, and electron-deficiency. Of the zeolite supported catalysts, HMCM-22 and HZSM-5 supported catalysts produce higher yield of C₂ and C₃ hydrocarbons than the zeolite HY supported catalyst because of stronger acidity of the supports. A NiB/HMCM-22 catalyst shows a rather slow deactivation during a multiple reaction cycles test. High temperature favors CH₄ decomposition and H₂ production in step I, but makes the subsequent hydrogenation of carbon formed from CH₄ decomposition difficult. The nature of the carbons formed from CH₄ decomposition was also studied by XPS and TEM combined with H₂-TPSR.     

Performance of Ni catalyst supported on La-hexaaluminate in CO<Subscript>2</Subscript> reforming of CH<Subscript>4</Subscript>

CO₂ reforming of CH₄ was performed using Ni catalyst supported on La-hexaaluminate which has been an well-known material for high-temperature combustion. La-hexaaluminate was synthesized by sol-gel method at various conditions where different amount of Ni (5–20 wt%) was loaded. Ni/La-hexaaluminate experienced 72 h reaction and its catalytic activity was compared with that of Ni/Al₂O₃, Ni/La-hexaaluminate shows higher reforming activity and resistance to coke deposition compared to the Ni/Al₂O₃ model catalyst. Coke deposition increases proportionally to Ni content. Consequently, Ni(5)/La-hexaaluminate(700) is the most efficient catalyst among various Ni/La-hexaaluminate catalysts regarding the cost of Ni in Ni(X)/La-hexaaluminate catalysts. BET surface area, XRD, EA, TGA and TPO were performed for surface characterization. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.