Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

Development of residential PEFC cogeneration systems: Ru catalyst for CO preferential oxidation in reformed gas

The performance of the novel Ru catalyst in a single-stage CO preferential oxidation removal reactor was investigated for residential polymer electrolyte fuel cell (PEFC) cogeneration systems. The outlet CO concentration of the CO removal reactor was reduced to less than 1 ppm even at [O₂]/[CO]=1.5. The natural gas fuel processor equipped with the CO removal reactor achieved the target thermal efficiency of 77% (LHV). Moreover, the durability of the Ru catalyst has been confirmed for more than 16,000 h at a micro-reactor and for more than 8000 h at an actual CO removal reactor. Because of the low O₂/CO molar ratio, the high CO removal conversion and the long-term durability, the Ru catalyst contributes to the development of residential PEFC cogeneration systems.     

A catalytic NOx reduction system using periodic steps, lean and rich operations

Catalytic lean NO reduction system in periodic two steps, lean/rich operations has been investigated over Rh-based catalysts. The investigation was done using a pulse reaction. In the reaction, after H₂ or CO was pulse-injected for a moment to achieve reducing conditions, the highly dispersed Rh catalyst could catalyze NO reduction at 200–400 °C in lean conditions for 1 min. Furthermore, NO was effectively reduced over the highly dispersed Rh/β-zeolite after exposure to the gas composed of 3% of O₂, 40 ppm of SO₂ and balance of He at 300 and 400 °C for 24 h.     

Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO–H2–O2 reactions

The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H₂ in the presence of excess O₂ (NO–H₂–O₂ reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H₂, 10% O₂, and He balance yielded N₂ with less than 10% selectivity, which could not be increased by changing Pt loading or H₂ concentration in the gas feed. Co-impregnation of NaHCO₃ and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO_x conversion (>90%) and the N₂ selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N₂ selectivity was attained by the post-impregnation of NaHCO₃ onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO₂⁻-type species, which would play a role of an intermediate to yield N₂. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H₂–O₂ reaction at low temperatures.     

Performance of CO preferential oxidation reactor with noble-metal catalyst coated on ceramic monolith for on-board fuel processing applications

On-board fuel processors are being developed to provide hydrogen-rich gas to the polymer electrolyte fuel cell automotive propulsion systems. Whereas the anode catalyst in the fuel cell has low tolerance for carbon monoxide, 10–100 ppm, reforming of gasoline and other hydrocarbon fuels generally produces 1–2% of CO. Of the many methods of removing CO from the reformer gas, preferential oxidation (PrOx) of CO over noble-metal catalysts is practiced most frequently. In this paper, we present experimental data for CO conversion on a Pt-based catalyst that is active at room temperature and was coated on a ceramic monolith. The data is used to develop an empirical correlation for selectivity for CO oxidation as a function of CO concentration and oxygen stoichiometry at 30,000–80,000/h space velocity. The selectivity correlation is used in a model to analyze the performance of multi-stage, adiabatic PrOx reactors with heat exchange between the stages to cool the reformate to 100 °C. An optimization algorithm is used to determine the operating conditions that can reduce CO concentration to 10 ppm while minimizing parasitic loss of H₂ in the reformate stream. It is found that the 10 ppm constraint limits the maximum inlet CO concentration to 1.05% in a single-stage reactor and to 3.1% in a two-stage reactor. The results clearly show the incremental reduction in parasitic H₂ loss by addition of second and third stages.     

Preferential oxidation of CO over CuO/CeO2 and Pt-Co/Al2O3 catalysts in micro-channel reactors

Micro-channel plates with dimension of 1 mm × 0.3 mm × 48 mm were prepared by chemical etching of stainless steel plates followed by wash coating of CeO₂ and Al₂O₃ on the channels. After coating the support on the plate, Pt, Co, and Cu were added to the plate by incipient wetness method. Reaction experiments of a single reactor showed that the micro-channel reactor coated with CuO/CeO₂ catalyst was highly selective for CO oxidation while the one coated with Pt-Co/Al₂O₃ catalyst was highly active for CO oxidation. The 7-layered reactors coated with two different catalysts were prepared by laser welding and the performances of each reactor were tested in large scale of PROX conditions. The multi-layered reactor coated with Pt-Co/Al₂O₃ catalyst was highly active for PROX and the outlet concentration of CO gradually increased with the O₂/CO ratio due to the oxidation of H₂ which maintained the reactor temperature. The multi-layered reactor coated with CuO/CeO₂ showed lower catalytic activity than that coated with Pt catalyst, but its selectivity was not changed with the increase of O₂/CO ratios due to the high selectivity. In order to combine advantages (high activity and high selectivity) of the two individual catalysts (Pt-Co/Al₂O₃, CuO/CeO₂), a serial reactor was prepared by connecting the two multi-layered micro-channel reactors with different catalysts. The prepared serial reactor exhibited excellent performance for PROX.     

Development of novel Ru catalyst of preferential CO oxidation for residential polymer electrolyte fuel cell systems

CO preferential oxidation on a novel Ru catalyst greatly improved in activity and selectivity over a wide temperature range by the pre-treatment of H₂ reduction was characterized. The high performance was obtained by increasing the population of surface Ru(0) which improved O₂ activation at low temperatures. Methanation of CO on the catalyst can also contribute to the final CO clean-up from ca. 100 to <1 ppm at low temperatures where the influence of CO₂ methanation can be ignored.     

Partial oxidation of methane to synthesis gas over iridium–nickel bimetallic catalysts

Partial oxidation of methane to synthesis gas was carried out using supported iridium–nickel bimetallic catalysts, in order to reduce loading levels of iridium and nickel, and to avoid carbon deposition on nickel-based catalysts by adding iridium. The performance of supported iridium–nickel bimetallic catalysts in synthesis gas formation depended strongly upon the support materials. La₂O₃ gave the best performance among the support materials tested. Ir(0.25 wt%)–Ni(0.5 wt%)/La₂O₃ afforded 36% conversion of methane (CH₄/O₂=5) to give CO and H₂ with the selectivities of above 90% at 800°C, and those at 600°C were 25.3% conversion of methane and CO and H₂ selectivities of about 80%, respectively. Reduced monometallic Ir(0.25 wt%)/La₂O₃ and Ni(0.5 wt%)/La₂O₃ catalysts did not produce synthesis gas at 600°C. A higher conversion of methane was obtained by synergistic effects. The product concentrations of CO, H₂, and CO₂, and CH₄ conversion were maintained in high values, even increasing the space velocity of feed gas over Ir–Ni/La₂O₃ catalyst, indicating that rapid reaction takes place. As a by-product, a small amount of carbon deposition was observed, but carbon formation decreased with increasing the space velocity. On the other hand, with reduced monometallic Ni(10 wt%)/La₂O₃ catalyst, yield of synthesis gas and carbon decreased with increasing the space velocity.     

High combustion activity of methane induced by reforming gas over Ni/Al2O3 catalysts

During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH₄/H₂O/O₂/Ar = 20/10/20/50 than under CH₄/H₂O/O₂/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH₄/H₂O/O₂/Ar = 20/10/20/50, although it is mainly oxidized to NiAl₂O₄ under CH₄/H₂O/O₂/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH₄/H₂O = 10/10) to the combustion gas (CH₄/O₂ = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity.     

NO removal by CH4 on Co-NaX-CO and Ag-NaX catalysts in a dual-bed system

NO removal using CH₄ as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co²⁺-, Ag⁺-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co₃O₄, which promotes NO oxidation to NO₂ by excess O₂ at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N₂ by CH₄ in the presence of excess O₂ and H₂O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N₂ in a deNO bed at 523 K and a deNO₂ bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H₂O 10% gas mixture after 160 min.     

Hydrogen production by autothermal reforming of sulfur-containing hydrocarbons over re-modified Ni/Sr/ZrO2 catalysts

The catalytic autothermal reforming (ATR) of liquid hydrocarbons to provide hydrogen for mobile or stationary fuel cells was carried out over a Ni/Sr/ZrO₂ catalyst that is active for steam reforming (SR). The catalyst system was found to be active for the ATR reaction, although the hydrogen concentration obtained by ATR, under the conditions employed, was a little lower than that for SR. Addition of sulfur, introduced in the form of thiophene, reduced the catalytic stability of Ni/Sr/ZrO₂, even at 1073 K. The catalyst lifetime decreased with increasing sulfur concentration between 0 and 100 ppm. Additives for improving the sulfur-tolerance of Ni/Sr/ZrO₂ were examined, and additions of Re or La were found to be effective in improving the stability of the catalysts. The best catalyst was 5 wt.% Re–Sr/Ni/ZrO₂. This catalyst was used in the ATR of liquid hydrocarbon fuels such as commercial premium gasoline, hydrotreated FCC gasoline, reagent mixtures, and methylcyclohexane. For premium gasoline, the activity remained unchanged during 30 h, but then diminished rapidly. With the other fuels, however, the catalyst showed a much improved performance, indicating that the presence of sulfur could be associated with catalyst stability. ATR coupled with the water–gas shift reaction led to a reduction in the CO concentration by up to 2800 ppm. The catalyst's activity remained constant even after cold-start runs with 853–423–853 K temperature cycles under H₂O/O₂/N₂ conditions. Thus, the Re–Sr/Ni/ZrO₂ catalyst is effective for ATR of liquid hydrocarbon fuels. Further work is currently under way to extend the catalyst life.     

Catalytic reduction of sulfur dioxide using hydrogen or carbon monoxide over Ce1−xZrxO2 catalysts for the recovery of elemental sulfur

This study focuses on the direct sulfur recovery process (DSRP), in which SO₂ can be directly converted into elemental sulfur using a variety of reducing agents over Ce_1−xZr_xO₂ catalysts. Ce_1−xZr_xO₂ catalysts (where x = 0.2, 0.5, and 0.8) were prepared by a citric complexation method. The experimental conditions used for SO₂ reduction were as follow: the space velocity (GHSV) was 30,000 ml/g_-cat h and the ratio of [CO (or H₂, H₂ + CO)]/[SO₂] was 2.0. It was found that the catalyst and reducing agent providing the best performance were the Ce_0.5Zr_0.5O₂ catalyst and CO, respectively. In this case, the SO₂ conversion was about 92% and the sulfur yield was about 90% at 550 °C. Also, a higher efficiency of SO₂ removal and elemental sulfur recovery was achieved in the reduction of SO₂ with CO as a reducing agent than that with H₂. In the reduction of SO₂ by H₂ over the Ce_0.5Zr_0.5O₂ catalyst, SO₂ conversion and sulfur yield were about 92.7% and 73%, respectively, at 800 °C. Also, the reduction of SO₂ using synthetic gas with various [CO]/[H₂] molar ratios over the Ce_0.5Zr_0.5O₂ catalyst was performed, in order to investigate the possibility of using coal-derived gas as a reducing agent in the DSRP. It was found that the reactivity of the SO₂ reduction using the synthetic gas with various [CO]/[H₂] molar ratios was increased with increasing CO content of the synthetic gas. Therefore, it was found that the Ce_1−xZr_xO₂ catalysts are applicable to the DSRP using coal-derived gas, which contains a larger percentage of CO than H₂.     

Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation

In this study, a novel bifunctional catalyst IrFe/Al₂O₃, which is very active and selective for preferential oxidation of CO under H₂-rich atmosphere, has been developed. When the molar ratio of Fe/Ir was 5/1, the IrFe/Al₂O₃ catalyst performed best, with CO conversion of 68% and oxygen selectivity towards CO₂ formation of 86.8% attained at 100 °C. It has also been found that the impregnation sequence of Ir and Fe species on the Al₂O₃ support had a remarkable effect on the catalytic performance; the activity decreased following the order of IrFe/Al₂O₃ > co-IrFe/Al₂O₃ > FeIr/Al₂O₃. The three catalysts were characterized by XRD, H₂-TPR, FT-IR and microcalorimetry. The results demonstrated that when Ir was supported on the pre-formed Fe/Al₂O₃, the resulting structure (IrFe/Al₂O₃) allowed more metallic Ir sites exposed on the surface and accessible for CO adsorption, while did not interfere with the O₂ activation on the FeO_x species. Thus, a bifunctional catalytic mechanism has been proposed where CO adsorbed on Ir sites and O₂ adsorbed on FeO_x sites; the reaction may take place at the interface of Ir and FeO_x or via a spill-over process.     

SIMULTANEOUS REACTIONS OF CO, NO, O2 AND NH3 ON Pt/γA12O3 CATALYST IN A TUBULAR REACTOR

A reliable method to continuously monitor NH₃ in a gas stream containing CO—NO—O₂ and H₂O has been developed. The method is based on a quantitative oxidation of NH₃ to NO on a Pt catalyst. The extent of this reaction is affected by temperature, excess oxygen present, and space-velocity. There is a significant effect of inlet O₂ concentration on extent of various reactions in the CO—NO—O₂—H₂O system on a Pt/γAl₂O₃ catalyst. At fixed space-velocity and catalyst temperature, and for fixed reactor inlet concentrations of CO and NO. there is negligible CO—NO reaction either in the absence of oxygen or in the presence of excess oxygen. However, short of the stoichiometric amount of O₂ required for CO oxidation, there is appreciable CO—NO (and possibly also CO—NO—H₂O) reaction whose extent increases with increasing oxygen concentration. This increase is especially dramatic in a narrow window of O₂: concentrations near the stoichiometric point. Interestingly enough, near the stoichiometric point, self-sustained isothermal oscillations in the outlet CO and NO concentrations are also observed (Subramaniam and Varma. submitted for publication)     

铜基催化剂还原过程研究综述

铜基催化剂广泛应用于工业生产中,催化剂还原是催化剂生产的最后一道工序,也是工业使用前的第一个步骤,对几种铜基催化剂的还原过程进行综述。铜基催化剂主要应用于CO与H_2合成甲醇和CO低温变换,也可用于CO_2与H_2合成甲醇以及脂肪酯加氢制脂肪醇。铜基催化剂的还原方法主要有液相还原法和气相还原法,其中,气相还原法用途较广。对影响还原的条件(H_2浓度、温度、压力和空速等)及杂质(H_2O、O_2和CO_2等)进行总结,并以甲醇合成催化剂为例对低氢还原法和高氢还原法作了介绍。     

Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH₄ traces under lean-burn conditions was studied in the presence or the absence of water or H₂S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H₂S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H₂S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H₂S-poisoned Pd/Al₂O₃ catalyst was studied by TPD in O₂/He. Poisoning species decompose above 873 K as SO₂ and O₂ in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O₂/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H₂S-poisoned Pd/Al₂O₃. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.     

富氧条件下金属催化CO还原NO的研究进展

CO广泛存在于燃煤烟气及汽车尾气中,利用未完全燃烧的CO催化还原NO可同时脱除NO和CO,过程中催化剂起着决定性作用。本文对近年来含氧条件下CO催化还原NO的研究成果进行了系统梳理,重点关注了Pd系、Ir系、Cu系、其他贵金属及金属氧化物催化剂的研究进展,分析了催化剂制备方法、掺杂改性及反应条件对催化性能的影响,同时考察了O₂浓度、H₂O以及SO₂对催化反应的影响,总结并对比了不同体系催化剂的活性位点及其催化机理,指明了O₂在催化还原过程中的抑制机理,得出了几种体系催化剂催化CO还原NO的活性顺序。最后,针对富氧条件下CO催化还原NO所存在的问题和难点,提出深入研究O₂抑制机理、降低贵金属用量、添加活性助剂是今后的研究方向。     

Mechanistic study of partial oxidation of methane to synthesis gas over supported rhodium and ruthenium catalysts using in situ time-resolved FTIR spectroscopy

In situ time-resolved FTIR spectroscopy was used to study the reaction mechanism of partial oxidation of methane to synthesis gas and the interaction of CH₄/O₂/He (2/1/45) gas mixture with adsorbed CO species over SiO₂ and γ-Al₂O₃ supported Rh and Ru catalysts at 500–600°C. It was found that CO is the primary product for the reaction of CH₄/O₂/He (2/1/45) gas mixture over H₂ reduced and working state Rh/SiO₂ catalyst. Direct oxidation of methane is the main pathway of synthesis gas formation over Rh/SiO₂ catalyst. CO₂ is the primary product for the reaction of CH₄/O₂/He (2/1/45) gas mixture over Ru/γ-Al₂O₃ and Ru/SiO₂ catalysts. The dominant reaction pathway of CO formation over Ru/γ-Al₂O₃ and Ru/SiO₂ catalysts is via the reforming reactions of CH₄ with CO₂ and H₂O. The effect of space velocity on the partial oxidation of methane over SiO₂ and γ-Al₂O₃ supported Rh and Ru catalysts is consistent with the above mechanisms. It is also found that consecutive oxidation of surface CO species is an important pathway of CO₂ formation during the partial oxidation of methane to synthesis gas over Rh/SiO₂ and Ru/γ-Al₂O₃ catalysts.     

添加WO_3对铈钴催化剂CO氧化性能的影响

采用共沉淀法制备xWO_3-Ce O2-Co_3O_4复合型非贵金属CO低温催化剂,考察不同WO_3添加量和空速对催化剂催化活性的影响,并考察催化剂的抗硫性能。通过孔隙结构测试、H2-TPR、FT-IR和SEM等对催化剂进行表征。结果表明,WO_3添加质量分数1%时,催化剂具有最佳的低温活性。在CO进口体积分数0.12%、O2进口体积分数5%和空速15 000 h-1条件下,50℃时,CO转化率即可达到99.6%,60℃时,CO转化率达100%。添加WO_3,催化剂氧化能力增强,催化效率提高。随着空速升高,CO转化率下降。WO_3的加入可有效提高催化剂的比表面积,抑制硫酸盐在催化剂表面聚集,提高催化剂的抗硫性能。     

Investigations of sulphur deactivation of NOx storage catalysts: influence of sulphur carrier and exposure conditions

The influence of SO₂, H₂S and COS in low concentrations on the deactivation of Pt/Rh/BaO/Al₂O₃ NO_x storage catalysts was investigated. Different samples of the catalyst were exposed to synthetic gas mixtures mimicking lean/rich engine cycling in a mixed lean application at 400 °C. The lean gas mixture contained 8 vol.% O₂, 500 vol-ppm C₃H₆ and 400 vol-ppm NO balanced to 100 vol.% with Ar. The rich excursions were performed by switching off the oxygen supply. Sulphur, 25 vol-ppm of either SO₂, H₂S or COS, was added to the gas flow either during the lean, the rich or both periods. This procedure aimed at investigating the influence of the exposure conditions and therefore the lean and rich periods were kept equally long (5 min). In addition, thermodynamical calculations for the prevailing conditions were performed.

It was concluded that all sulphur compounds investigated, i.e. SO₂, H₂S and COS, had similar, negative impact on the NO_x storage ability of the catalyst and that they all showed increased deactivation rates during rich exposure compared to lean. During lean exposure, all sulphur carriers showed similar behaviour, while H₂S and COS caused severe loss of noble metal activity during rich exposure.