Improvement of coke resistance of Ni/Al2O3 catalyst in CH4/CO2 reforming by ZrO2 addition

This work investigates the improvement of Ni/Al₂O₃ catalyst stability by ZrO₂ addition for H₂ gas production from CH₄/CO₂ reforming reactions. The initial effect of Ni addition was followed by the effect of increasing operating temperature to 500–700 °C as well as the effect of ZrO₂ loading and the promoted catalyst preparation methods by using a feed gas mixture at a CH₄:CO₂ ratio of 1:1.25. The experimental results showed that a high reaction temperature of 700 °C was favored by an endothermic dry reforming reaction. In this reaction the deactivation of Ni/Al₂O₃ was mainly due to coke deposition. This deactivation was evidently inhibited by ZrO₂, as it enhances dissociation of CO₂ forming oxygen intermediates near the contact between ZrO₂ and nickel where the deposited coke is gasified afterwards. The texture of the catalyst or BET surface area was affected by the catalyst preparation method. The change of the catalyst texture resulted from the formation of ZrO₂–Al₂O₃ composite and the plugging of Al₂O₃ pore by ZrO₂. The 15% Ni/10% ZrO₂/Al₂O₃ co-impregnated catalyst showed a higher BET surface area and catalytic activity than the sequentially impregnated catalyst whereas coke inhibition capability of the promoted catalysts prepared by either method was comparable. Further study on long-term catalyst stability should be made.     

Supported Rh catalysts for methane partial oxidation prepared by OM-CVD of Rh(acac)(CO)2

The organometallics chemical vapour deposition (OM-CVD) technique, using Rh(acac)(CO)₂ as a precursor, was employed for the preparation of heterogeneous Rh catalysts supported on low surface area refractory oxides (α-Al₂O₃, ZrO₂, MgO and La₂O₃). Prepared systems were tested in the methane catalytic partial oxidation (CH₄-CPO) reaction in a fixed bed reactor and compared to a reference catalyst prepared from impregnation of Rh₄(CO)₁₂.Catalysts supported on Al₂O₃, ZrO₂ and MgO show better or comparable performances with respect to the reference system.Complete decomposition of Rh precursor during formation of the metal phase under reductive conditions was investigated by TPRD and confirmed by infrared and mass spectrometry data.Supported Rh phase was characterized by CO and H₂ chemisorption, CO-DRIFT spectroscopy and HRTEM microscopy in fresh and aged selected samples. Rh(I) isolated sites and Rh(0) metal particles were found on fresh catalysts; after ageing an extensive reconstruction occurs mainly consisting in a sintering of Rh isolate sites to metal particles but without large increase in mean particles size.Catalytic performances and Rh species balance were found to be dependent on the support material.     

Highly active and stable Rh/MgOAl2O3 catalysts for methane steam reforming

Highly active and coke-resistant Rh catalysts were developed for methane steam reforming in microchannel chemical reactors. Rh loading was optimized on a stable MgOAl₂O₃ support to improve the volumetric productivity for methane conversion. Catalyst activities were stable over a wide range of steam/carbon ratios. In particular, experimental results demonstrated that Rh/MgOAl₂O₃ catalysts are extremely active for methane steam reforming and are resistant to coke formation at stoichiometric steam/carbon ratio of 1 for over 14 h time-on-stream with no sign of deactivation. Methane steam reforming activities on this catalyst is compared in both a microchannel reactor and a conventional micro-tubular reactor. Significant performance enhancement was observed in microchannel reactors owing to improved heat and mass transfer.     

Catalytic syngas production from greenhouse gasses: Performance comparison of Ru-Al2O3 and Rh-CeO2 catalysts

In this work, 3% Ru-Al₂O₃ and 2% Rh-CeO₂ catalysts were synthesized and tested for CH₄-CO₂ reforming activity using either CO₂-rich or CO₂-lean model biogas feed. Low carbon deposition was observed on both catalysts, which negligibly influenced catalytic activity. Catalyst deactivation during temperature programmed reaction was observed only with Ru-Al₂O₃, which was caused by metallic cluster sintering. Both catalysts exhibited good stability during the 70 h exposure to undiluted equimolar CH₄/CO₂ gas stream at 750 °C. By varying residence time in the reactor during CH₄-CO₂ reforming, very similar quantities of H₂ were consumed for water formation. Reverse water-gas shift (RWGS) reaction occurred to a very similar extent either with low or high WHSV values over both catalysts, revealing that product gas mixture contained near RWGS equilibrium composition, confirming the dominance of WGS reaction and showing that shortening the contact time would actually decrease the H₂/CO ratio in the syngas produced by CH₄-CO₂ reforming, as long as RWGS is quasi equilibrated. H₂/CO molar ratio in the produced syngas can be increased either by operating at higher temperatures, or by using a feed stream with CH₄/CO₂ ratio well above 1.     

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.     

NOx adsorption and diesel soot combustion over La2O3 supported catalysts containing K, Rh and Pt

In this work we report results of NOx adsorption and diesel soot combustion on a noble metal promoted K/La₂O₃ catalyst. The fresh-unpromoted solid is a complex mixture of hydroxide and carbonate compounds, but the addition of Rh favors the preferential formation of lanthanum oxycarbonate during the calcination step. K/La₂O₃ adsorbs NOx through the formation of La and K nitrate species when the solid is treated in NO + O₂ between 70 and 490 °C. Nitrates are stable in the same temperature range under helium flow. However, they become unstable at ca. 360 °C when either Rh and/or Pt are present, the effect of Rh being more pronounced. Nitrates decompose under different atmospheres: NO + O₂, He and H₂. The effect of Rh might be to form a thermally unstable complex (Rh–NO⁺) which takes part both in the formation of the nitrates when the catalyst is exposed to NOx and in the nitrates decomposition at higher temperatures. Regarding soot combustion, nitrates react with soot with a temperature of maximun reaction rate of ca. 370 °C, under tight contact conditions. This temperature is not affected by the presence of Rh, which indicates that the stability of nitrates has little effect on their reaction with soot.     

Nanodispersed Au/Al2O3 catalysts for low-temperature CO oxidation: Results of research activity at the Boreskov Institute of Catalysis

This paper presents some important results of the studies on preparation and catalytic properties of nanodispersed Au/Al₂O₃ catalysts for low-temperature CO oxidation, which are carried out at the Boreskov Institute of Catalysis (BIC) starting from 2001. The catalysts with a gold loading of 1–2 wt.% were prepared via deposition of Au complexes onto different aluminas by means of various techniques (“deposition-precipitation” (DP), incipient wetness, “chemical liquid-phase grafting” (CLPG), chemical vapor deposition (CVD)). These catalysts have been characterized comparatively by a number of physical methods (XRD, TEM, diffuse reflectance UV/vis and XPS) and catalytically tested for combustion of CO impurity (1%) in wet air stream at near-ambient temperature. Using the hydroxide or chloride gold complexes capable of chemical interaction with the surface groups of alumina as the catalyst precursors (DP and incipient wetness techniques, respectively) produces the catalysts that contain metallic Au particles mainly of 2–4 nm in diameter, uniformly distributed between the external and internal surfaces of the support granules together with the surface “ionic” Au oxide species. Application of organogold precursors gives the supported Au catalysts of egg shell type which are either close by mean Au particle size to what we obtain by DP and incipient wetness techniques (CVD of (CH₃)₂Au(acac) vapor on highly dehydrated Al₂O₃ in a rotating reactor under static conditions) or contain Au crystallites of no less than 7 nm in size (CLPG method). Regardless of deposition technique, only the Cl-free Au/Al₂O₃ catalysts containing the small Au particles (d_i ≤ 5 nm) reveal the high catalytic activity toward CO oxidation under near-ambient conditions, the catalyst stability being provided by adding the water vapor into the reaction feed. The results of testing of the nanodispersed Au/Al₂O₃ catalysts under conditions which simulate in part removal of CO from ambient air or diesel exhaust are discussed in comparison with the data obtained for the commercial Pd and Pt catalysts under the same conditions.     

Influence of the oxygen pretreatment on the CO2 reforming of methane on Ni/β-SiC catalyst

The influence of the O₂ pretreatment on the CO₂ reforming of methane to synthesis gas has been investigated with Ni catalysts supported on β-SiC extrudate. The structure and properties of the catalysts were characterised by SEM, TEM and XRD techniques. The pretreatment of the catalyst by a mixture of CO₂ and O₂ significantly improves the catalytic activity for the CO₂ reforming. On the Ni 5 wt.% supported on β-SiC catalyst, the CH₄ conversion has reached 90% with the O₂ pretreatment instead of 80% by direct activation under CO₂/CH₄ mixture. The oxygen pretreatment seems to stabilize the metallic nickel phase instead of NiSi₂.     

Activity and characterization of Rh/Al2O3 and Rh-Na/Al2O3 catalysts for the SCR of NO with CO in the presence of SO2 and HCl

SO₂ and HCl are major pollutants emitted from waste incineration processes. Both pollutants are difficult to remove completely and can enter the catalytic reactor. In this work, the effects of SO₂ and HCl on the performance of Rh/Al₂O₃ and Rh-Na/Al₂O₃ catalysts for NO removal were investigated in simulated waste incineration conditions. The characterizations of the catalysts were analyzed by BET, SEM/EDS, XRD, and ESCA. Experimental results indicated the 1%Rh/Al₂O₃ catalyst was significantly deactivated for NO and CO conversions when SO₂ and HCl coexisted in the flue gas. The addition of between 2 and 10 wt.% Na promoted the activity of the 1%Rh/Al₂O₃ catalyst for NO removal, but decreased the CO oxidation and BET surface area. The catalytic activity for NO removal was inhibited by HCl as a result of the formation of RhCl₃. Adding Na to the Rh/Al₂O₃ catalyst decreased the inhibition of SO₂ because of the formation of Na₂SO₄, which was observed in the XRD and ESCA analyses. SEM mapping/EDS showed that more S was residual on the surface of the Rh-Na/Al₂O₃ catalyst than Cl.     

Steady State Isotopic Transient Kinetic Analysis of Flameless Methane Combustion over Pd/Al2O3 and Pt/Al2O3 Catalysts

The SSITKA measurements were performed in the steady state of complete methane oxidation on the Pd/Al₂O₃ and Pt/Al₂O₃ catalysts. It was found that the number of intermediates and their average life-time on the catalyst surface changes with the increase of reaction temperature. On the Pd/Al₂O₃ catalyst there is larger number of active centres than on Pt/Al₂O₃ catalyst which permits the course of methane oxidation at lower temperatures.     

Partial oxidation of ethanol over Pd/CeO2 and Pd/Y2O3 catalysts

The effect of the support nature on the performance of Pd catalysts during partial oxidation of ethanol was studied. H₂, CO₂ and acetaldehyde formation was favored on Pd/CeO₂, whereas CO production was facilitated over Pd/Y₂O₃ catalyst. According to the reaction mechanism, determined by DRIFTS analyses, some reaction pathways are favored depending on the support nature, which can explain the differences observed on products distribution. On Pd/Y₂O₃ catalyst, the production of acetate species was promoted, which explain the higher CO formation, since acetate species can be decomposed to CH₄ and CO at high temperatures. On Pd/CeO₂ catalyst, the acetaldehyde preferentially desorbs and/or decomposes to H₂, CH₄ and CO. The CO formed is further oxidized to CO₂, which seems to be promoted on Pd/CeO₂ catalyst.     

Electrochemical promotion of CO<Subscript>2</Subscript> hydrogenation on Rh/YSZ electrodes

The electrochemical promotion of the CO₂ hydrogenation reaction on porous Rh catalyst–electrodes deposited on Y₂O₃-stabilized-ZrO₂ (or YSZ), an O²⁻ conductor, was investigated under atmospheric total pressure and at temperatures 346–477 °C, combined with kinetic measurements in the temperature range 328–391 °C. Under these conditions CO₂ was transformed to CH₄ and CO. The CH₄ formation rate increased by up to 2.7 times with increasing Rh catalyst potential (electrophobic behavior) while the CO formation rate was increased by up to 1.7 times with decreasing catalyst potential (electrophilic behavior). The observed rate changes were non-faradaic, exceeding the corresponding pumping rate of oxygen ions by up to approximately 210 and 125 times for the CH₄ and CO formation reactions, respectively. The observed electrochemical promotion behavior is attributed to the induced, with increasing catalyst potential, preferential formation on the Rh surface of electron donor hydrogenated carbonylic species leading to formation of CH₄ and to the decreasing coverage of more electron acceptor carbonylic species resulting in CO formation.     

Performance of Pd catalysts supported on nanocrystalline α-Al2O3 and Ni-modified α-Al2O3 in selective hydrogenation of acetylene

Nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ have been prepared by sol–gel and solvothermal methods and employed as supports for Pd catalysts. Regardless of the preparation method used, NiAl₂O₄ spinel was formed on the Ni-modified α-Al₂O₃ after calcination at 1150 °C. However, an addition of NiO peaks was also observed by X-ray diffraction for the solvothermal-made Ni-modified α-Al₂O₃ powder. Catalytic performances of the Pd catalysts supported on these nanocrystalline α-Al₂O₃ and Ni-modified α-Al₂O₃ in selective hydrogenation of acetylene were found to be superior to those of the commercial α-Al₂O₃ supported one. Ethylene selectivities were improved in the order: Pd/Ni-modified α-Al₂O₃–sol–gel > Pd/Ni-modified α-Al₂O₃-solvothermal ≈ Pd/α-Al₂O₃–sol–gel > Pd/α-Al₂O₃-solvothermal  Pd/α-Al₂O₃-commerical. As revealed by NH₃ temperature program desorption studies, incorporation of Ni atoms in α-Al₂O₃ resulted in a significant decrease of acid sites on the alumina supports. Moreover, XPS revealed a shift of Pd 3d binding energy for Pd catalyst supported on Ni-modified α-Al₂O₃–sol–gel where only NiAl₂O₄ was formed, suggesting that the electronic properties of Pd may be modified.     

Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts

Supported nickel catalysts of composition Ni/Y₂O₃–ZrO₂ were synthesized in one step by the polymerization method and compared with a nickel catalyst prepared by wet impregnation. Stronger interactions were observed in the formed catalysts between NiO species and the oxygen vacancies of the Y₂O₃–ZrO₂ in the catalysts made by polymerization, and these were attributed to less agglomeration of the NiO during the synthesis of the catalysts in one step. The dry reforming of ethanol was catalyzed with a maximum CO₂ conversion of 61% on the 5NiYZ catalyst at 800 °C, representing a better response than for the catalyst of the same composition prepared by wet impregnation.     

Effect of CO2 in the feed stream on the deactivation of Co/γ-Al2O3 Fischer–Tropsch catalyst

The influence of CO₂ on the deactivation of Co/γ-Al₂O₃ Fischer–Tropsch (FT) catalyst in CO hydrogenation has been investigated. The presence of CO₂ in the feed stream reveals a negative effect on catalyst stability and in the formation of heavy hydrocarbons. The CO₂ acts as a mild oxidizing agent on cobalt metal during Fischer–Tropsch synthesis. During FT synthesis on Co/γ-Al₂O₃ of 70 h, the CO conversion and C₅₊ selectivity in the presence of CO₂ decreased more significantly than in the absence of CO₂. CO₂ is found to be responsible for the partial oxidation of surface cobalt metal at FT synthesis environment with the co-existence of generated water.     

Synthesis of DME from syngas on the bifunctional Cu–ZnO–Al2O3/Zr-modified ferrierite: Effect of Zr content

The catalytic activity on the coprecipitated Cu–ZnO–Al₂O₃/Zr-ferrierite (CZA–ZrFER) with different Zr content from 0 to 5 wt.% was investigated for the direct synthesis of dimethylether (DME) from H₂-deficient and biomass-derived model syngas (H₂/CO molar ratio = 0.93). The catalytic functionalities, such as CO conversion and DME selectivity, showed their maxima on the bifunctional catalyst with 3 wt.% Zr-modified ferrierite. Detailed characterization studies were conducted on the catalysts to measure their properties such as surface area, acidity by temperature-programmed desorption of ammonia (NH₃-TPD), reducibility of Cu oxide by temperature-programmed reduction (TPR), copper surface area measurements by N₂O titration method, electronic states of copper by IR analysis and particle size measurement by XRD and TEM analysis. The number of acid sites measured by NH₃-TPD on the bifunctional catalysts decreased monotonously with the increase of Zr content, meanwhile, the acidic strength is found to be minimal on the catalyst showing best performance. The reducibility of copper oxide and the surface area of metallic copper also exhibited their maximum values at the same Zr composition indicating that these are responsible for the optimum functionality of the bifunctional CZA–ZrFER catalyst. The role of easily reducible copper species with small particle size and the suppressed strong acidic sites is also emphasized in the consecutive reaction from syngas to DME on the bifunctional catalyst. The different behavior of intrinsic rate of the bifunctional catalysts is also well correlated with the metallic surface area of copper and the amount of acidic sites with their acidic strength.     

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.     

Direct synthesis of acetic acid from CH4 and CO2 by a step-wise route over Pd/SiO2 and Rh/SiO2 catalysts

The theoretical and experimental feasibility of direct conversion of CH₄ and CO₂ to acetic acid by an isothermal step-wise route over Pd/SiO₂ and Rh/SiO₂ catalysts was investigated. The methyl radical formation from CH₄ dissociation and CO₂ inserting into the intermediate are regarded as two limiting steps. Preliminary experimental results have shown that the following step-wise route can circumvent the thermodynamic limitation of this direct synthesis at low temperatures. Pd catalysts are more active than Rh catalysts at 170 °C and 200 °C, while formic acid is only produced on Pd catalysts. The optimum contact time of CH₄ and CO₂ with catalysts is 1 min under the experimental conditions. And there is no apparent deactivation resulting from carbon deposition for catalysts during the successive reaction cycles.     

The effect of sodium on the catalytic activity of ZnO–Al2O3/ZSM-5 and SnO–Al2O3/ZSM-5 for the transesterification of vegetable oil with methanol

In order to elucidate the effect of sodium on the activity of ZSM-5 supported metal oxides catalysts (ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5) for the transesterification of soybean oil with methanol, ZSM-5 supported metal oxides were prepared with and without sodium hydroxide by impregnation. The metal compositions of the ZSM-5 supported metal oxide catalysts and the metal concentrations dissolved from the catalysts to the methylester phase were measured by SEM-EDS and inductive coupled plasma spectroscopy, respectively. The catalytic activity of ZnO–Al₂O₃/ZSM-5 and SnO–Al₂O₃/ZSM-5 containing sodium did not originate from surface metal oxides sites, but from surface sodium sites or dissolved sodium leached from the catalyst surface.     

Reduction and oxidation properties of Fe2O3/ZrO2 oxygen carrier for hydrogen production

Fe₂O₃ is a promising oxygen carrier for hydrogen production in the chemical-looping process. A set of kinetic studies on reduction with CH₄, CO and H₂ respectively, oxidation with water and oxygen containing Ar for chemical-looping hydrogen production was conducted. Fe₂O₃ (20 wt.%)/ZrO₂ was prepared by a co-precipitation method. The main variables in the TGA (thermogravimetric analyzer) experiment were temperatures and gas concentrations. The reaction kinetics parameters were estimated based on the experimental data. In the reduction by CH₄, CO and H₂, the reaction rate changed near FeO. Changes in the reaction rate due to phase transformation were observed at low temperature and low gas concentration during the reduction by CH₄, but the phenomenon was not remarkable for the reduction by CO and H₂. The reduction rate achieved using CO and H₂ was relatively faster than achieved using CH₄. The Hancock and Sharp method of comparing the kinetics of isothermal solid-state reactions was applied. A phase boundary controlled model (contacting sphere) was applied to the reduction of Fe₂O₃ to FeO by CH₄, and a different phase boundary controlled model (contacting infinite slab) was fit well to the reduction of FeO to Fe by CH₄. The reduction of Fe₂O₃ to Fe by CO and H₂ can be described by the former phase boundary controlled model (contacting sphere). This phase boundary controlled model (contacting sphere) also fit well for the oxidation of Fe to Fe₃O₄ by water and FeO to Fe₂O₃ by oxygen containing Ar. These kinetics data could be used to design chemical-looping hydrogen production systems.