Advances in carbon nanotubes as efficacious supports for palladium-catalysed carbon–carbon cross-coupling reactions

Since the 1970s, palladium-catalysed carbon–carbon (C–C) bond formation has made a critical impact in organic synthesis. In early studies, homogeneous palladium catalysts were extensively used for this reaction with limitations such as difficulty in separation and recycling ability. Lately, heterogeneous palladium-based catalysts have shown promise as surrogates for conventional homogeneous catalysts in C–C coupling reactions, since the product is easy to isolate, while the catalyst is reusable and hence sustainable. Recently, a better part of these heterogeneous palladium catalysts are supported on carbon nanotubes (Pd/CNTs), that have shown superior catalytic performance and better recyclability since the CNT support imparts stability to the palladium catalyst. This review discusses the wide variety of surface functionalization techniques for CNTs that improve their properties as catalyst supports, as well as the methods available for loading the catalyst nanoparticles onto the CNTs. It will survey the literature where Pd/CNTs catalysts have been utilized for C–C coupling reactions, with particular emphasis on Suzuki–Miyaura and Mizoroki–Heck coupling reactions. It will also highlight some of the important parameters that affect these reactions.     

Studies on nanosized iron based modified catalyst for Fischer-Tropsch synthesis application

To improve catalytic performance iron based catalyst, the effects of some metal promoters, especially potassium, copper and other transition metal oxides as well as different supports have been reported. A series of Fe/K/Cu catalysts promoted with magnesium and ceria by precipitation method, followed by impregnation method; keeping Cu and K content same. The catalysts were characterized by XRD, N2 physisorption, TPR and TEM techniques. From XRD, the presence of hematite (Fe2O3) phase was detected in all precipitated iron catalysts and CFe2.5 phase in all used catalysts. TPR results showed that addition of Mg facilitated the reduction of Fe2O3 and decrease in reduction temperature. The catalytic performance was investigated in a fixed-bed reactor at 250 degrees C, 2 MPa pressure and H2/CO molar ratio of 2. Concentration of Mg was found to affect the CO conversion and product distribution. It was found that precipitated iron catalyst Fe/Mg/Cu/K with Mg/Fe ratio of 0.1 showed highest conversion (60.6%) and C5(+) selectivity (92.4%) among all catalysts tested.     

Bimetallic catalysts for continuous catalytic wet air oxidation of phenol.

Catalytic wet oxidation has proved to be effective at eliminating hazardous organic compounds, such as phenol, from waste waters. However, the lack of active long-life oxidation catalysts which can perform in aqueous phase is its main drawback. This study explores the ability of bimetallic supported catalysts to oxidize aqueous phenol solutions using air as oxidant. Combinations of 2% of CoO, Fe2O3, MnO or ZnO with 10% CuO were supported on gamma-alumina by pore filling, calcined and later tested. The oxidation was carried out in a packed bed reactor operating in trickle flow regime at 140 degrees C and 900 kPa of oxygen partial pressure. Lifetime tests were conducted for 8 days. The pH of the feed solution was also varied. The results show that all the catalysts tested undergo severe deactivation during the first 2 days of operation. Later, the catalysts present steady activity until the end of the test. The highest residual phenol conversion was obtained for the ZnO-CuO, which was significantly higher than that obtained with the 10% CuO catalyst used as reference. The catalyst deactivation is related to the dissolution of the metal oxides from the catalyst surface due to the acidic reaction conditions. Generally, the performance of the catalysts was better when the pH of the feed solution was increased.     

Characterization of pre-shaped zirconia bodies for catalytic applications

Literature indicates that application of zirconia in a supported dehydrogenation catalyst is viable. Textural and structural properties of commercial pre-shaped zirconia supports from various suppliers were characterized using electron microscopy, element analysis, nitrogen physisorption, mercury intrusion porosimetry and X-ray diffraction. Most zirconias are sufficiently pure (>97 %) and thermostable to be applied in supported catalysts. Specific surface areas as large as 10 m²g^–1 are stable at temperatures of about 850 °C. Specific surface areas up to about 30 m²g^–1 can be established by a thermal treatment in air at temperatures up to the operating temperatures of the dehydrogenation process. Steam treatment affects the texture differently from treatment in dry air: sintering proceeds more rapidly in the presence of steam. The preshaped supports show a porosity (about 50%) which is higher than that reported for zirconia powders with the same pore-size distribution (5%). This is advantageous, both in the catalyst preparation step and in the catalytic reaction. However, the pre-shaped supports exhibit some microporosity.     

Production of hydrogen by autothermal reforming of propane over Ni/delta-Al2O3

The performance of Ni/delta-Al2O3 catalyst in propane autothermal reforming (ATR) for hydrogen production was investigated in the present study. The catalysts were characterized using XRD, TEM, and SEM. The activity of the Ni/delta-Al2O3 catalyst manufactured by the water-alcohol method was better than those of the catalysts manufactured by the impregnation and chemical reduction methods. The Ni/delta-Al2O3 catalysts were modified by the addition of promoters such as Mg, La, Ce, and Co, in order to improve their stability and yield. Hydrogen production was the largest for the Ni-Co-CeO2/Al2O3, catalyst.     

Sulphur condensation influence in Claus catalyst performance

The Claus process is an efficient way of removing H(2)S from acid gas streams and this is widely practised in industries such as natural gas processing, oil refining and metal smelting. Increasingly strict pollution control regulations require maximum sulphur recovery from the Claus unit in order to minimise sulphur-containing effluent. The most widely used Claus catalyst in sulphur recovery units is non-promoted spherical activated alumina. Properties associated with optimum non-promoted Claus catalyst performance include high surface area, appropriate pore size distribution and enhanced physical properties. The objective of this paper is to outline a procedure in order to estimate Claus catalyst effectiveness after pore plugging due to sulphur condensation. Catalyst deactivation due to pore plugging by sulphur is modelled employing a Bethe lattice and its corresponding performance is described by means of a modified effectiveness factor. Model results show an improvement in the modified effectiveness factor due to modifications in catalyst porous structure.     

Reactivity,Selectivity, and Stability of Zeolite-Based Catalysts for Methane Dehydroaromatization

Non-oxidative dehydroaromatization is arguably the most promising process for the direct upgrading of cheap and abundant methane to liquid hydrocarbons. This reaction has not been commercialized yet because of the suboptimal activity and swift deactivation of benchmark Mo-zeolite catalysts. This progress report represents an elaboration on the recent developments in understanding of zeolite-based catalytic materials for high-temperature non-oxidative dehydroaromatization of methane. It is specifically focused on recent studies, relevant to the materials chemistry and elucidating i) the structure of active species in working catalysts; ii) the complex molecular pathways underlying the mechanism of selective conversion of methane to benzene; iii) structure, evolution and role of coke species; and iv) process intensification strategies to improve the deactivation resistance and overall performance of the catalysts. Finally, unsolved challenges in this field of research are outlined and an outlook is provided on promising directions toward improving the activity, stability, and selectivity of methane dehydroaromatization catalysts.     

改性碳纳米材料在低温燃料电池中的应用

碳纳米材料(如炭黑、介孔碳、碳纳米管、石墨烯、碳纳米纤维、碳纳米角等)因其优异的电学性能和结构特性(良好的导电性能和超大的比表面积),被研究者广泛用作低温燃料电池贵金属催化剂的载体.然而,作为催化剂载体的这类碳纳米材料通常都存在电化学腐蚀的问题,碳载体的腐蚀通常会导致贵金属纳米催化剂的聚集,这将使催化剂的性能降低.为了改善碳载体的抗腐蚀性能,提高金属纳米粒子的活性和稳定性,许多研究工作致力于制备特殊结构的碳纳米材料,或对碳纳米材料进行表面修饰、掺杂等.与此同时,为了取代价格昂贵的贵金属催化剂,非贵金属催化剂的研究也成为一大热点,掺杂碳纳米材料就是研究热点之一.对近几年来围绕碳纳米材料制备、改性,以及这些改性碳纳米材料作为金属纳米粒子载体等的研究工作做了较为详细的综述,同时介绍了掺杂碳纳米材料作为氧还原催化剂的研究进展.     

改性碳纳米材料在低温燃料电池中的应用

碳纳米材料(如炭黑、介孔碳、碳纳米管、石墨烯、碳纳米纤维、碳纳米角等)因其优异的电学性能和结构特性(良好的导电性能和超大的比表面积),被研究者广泛用作低温燃料电池贵金属催化剂的载体。然而,作为催化剂载体的这类碳纳米材料通常都存在电化学腐蚀的问题,碳载体的腐蚀通常会导致贵金属纳米催化剂的聚集,这将使催化剂的性能降低。为了改善碳载体的抗腐蚀性能,提高金属纳米粒子的活性和稳定性,许多研究工作致力于制备特殊结构的碳纳米材料,或对碳纳米材料进行表面修饰、掺杂等。与此同时,为了取代价格昂贵的贵金属催化剂,非贵金属催化剂的研究也成为一大热点,掺杂碳纳米材料就是研究热点之一。对近几年来围绕碳纳米材料制备、改性,以及这些改性碳纳米材料作为金属纳米粒子载体等的研究工作做了较为详细的综述,同时介绍了掺杂碳纳米材料作为氧还原催化剂的研究进展。     

Synthesis of carbon nanostructures--in-situ study of carbon deposition parameters

An in-situ study of carbon deposition parameters by the method of chemical vapor deposition (CVD) from methane is used in order to estimate the time of the deactivation catalyst (Fe/Mo catalyst supported on alumina (AI2O3)). The deposition process is studied by measuring the weight change of the sample using a microbalance. The effect of the operating parameters, such as temperature, feed concentration, and catalyst composition, on the deposition rate and morphology of the deposits is investigated. Based on the initial weight of the catalyst, the relative weight gain of the sample due to carbon deposition on the catalyst is higher than 300% in a few minutes of the deposition process but the deposition rate reduces in more than 100 min. The carbon material characterized with Scanning Electron Microscopy (SEM) and Raman Spectroscopy and it is found to consist of carbon nanostructures.     

Application of Ni:SiO2 nanocomposite to control the carbon deposition on the carbon dioxide reforming of methane

Stable Ni nanoparticles embedded in a mesoporous silica material were used as catalysts for the conversion of methane into synthesis gas. This catalyst has the singular properties of controlling the carbon deposition and deactivation of active sites. A comparative study of our nanocomposites with conventional catalysts showed that impregnation material presented a preferential encapsulation and growth of carbon nanotubes on the metal surface. The impregnated catalyst showed a higher tendency for carbon nanotube and whiskers formation.     

Fast and facile preparation of reduced graphene oxide supported Pt–Co electrocatalyst for methanol oxidation

In this study, we report a scalable, fast, and facile method for preparation of reduced graphene oxide (RGO) sheets supported Pt–Co electrocatalyst for methanol oxidation. Mixed reducing agents were used and the activity of the catalysts was studied. It was found that the presence of RGO leads to higher activity, which might be due to the increasing of electrochemically accessible surface areas and easier charge–transfer at the interfaces. Co can greatly enhance the electrocatalytic activity and moderate the poisoning of Pt catalyst. Under same Pt loading mass and experimental conditions, the RGO-Pt-Co catalyst shows the highest electro catalytic activity and improved resistance to carbon monoxide poisoning among the prepared catalysts.     

Oxidative dehydrogenation of n-butane over vanadium magnesium oxide catalysts supported on nano-structured MgO and ZrO2: effect of oxygen capacity of the catalyst

Vanadium-magnesium oxide catalysts supported on nano-structured MgO and ZrO2 (Mg3(VO4)2/MgO/ZrO2) were prepared by a wet impregnation method with a variation of Mg:Zr ratio (8:1, 4:1, 2:1, and 1:1). For comparison, Mg3(VO4)2/MgO and Mg3(VO4)2/ZrO2 catalysts were also prepared by a wet impregnation method. The prepared catalysts were applied to the oxidative dehydrogenation of n-butane in a continuous flow fixed-bed reactor. Mg3(VO4)2/MgO/ZrO2 (Mg:Zr = 4:1, 2:1, and 1:1) and Mg3(VO4)2/ZrO2 catalysts showed a stable catalytic activity during the whole reaction time, while Mg3(VO4)2/MgO/ZrO2 (8:1) and Mg3(VO4)2/MgO catalysts experienced a severe catalyst deactivation. Deactivation of Mg3(VO4)2/MgO/ZrO2 (8:1) and Mg3(VO4)2/MgO catalysts was due to their low oxygen mobility. Effect of oxygen capacity (the amount of oxygen in the catalyst involved in the reaction) of the supported Mg3(V04)2 catalysts on the catalytic performance in the oxidative dehydrogenation of n-butane was investigated. Experimental results revealed that oxygen capacity of the catalyst was closely related to the catalytic activity in the oxidative dehydrogenation of n-butane. A large oxygen capacity of the catalyst was favorable for obtaining a high catalytic activity in this reaction. Among the catalysts tested, Mg3(VO4)2/MgO/ZrO2 (4:1) catalyst with the largest oxygen capacity showed the best catalytic performance.     

Application of scanning electron microscopy to the study of the morphology of multicomponent catalyst systems

The morphology of multicomponent catalyst systems was investigated with the aid of the scanning electron microscope (SEM). The catalysts which were studied consisted of alumina, complex aluminates, and a mixture of both. These materials are used in the reduction of sulphur dioxide by methane to sulphur. Direct observations with the scanning electron microscope permitted the study of the effects of various thermal treatments on changes in the morphology with respect to pore geometry, topography and to both particle size and shape. It is shown that changes in the catalyst morphology — as resolvable by the SEM — are related to changes in catalytic activity.     

Single and bi-metallic silver modified ZSM-5 for waste gases neutralization

Single and bi-metallic silver modified ZSM-5 catalysts were synthesized using three methods of preparation, characterized by different techniques: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), nitrogen physisorption, X-ray fluorescence (XRF), inductively coupled plasma atomic emission spectrometry (ICP-AES) and tested in heterogeneous catalytic decomposition of ozone at ambient temperature. The highest degree of ozone decomposition was observed over 5Ag(Imp)-H(IE)-ZSM-5 catalyst, which exhibit 97% conversion and does not show any deactivation with the time. The method of preparation, amount of supported silver and the presence of modifiers as Cu, Ce, and Zr were found to be important for the ozone decomposition reaction.     

Reactivity of platinum nanoaggregates on various types of supports in catalytic decomposition of hydrazine in acid solutions

Platinized catalysts on various types of supports were tested in the catalytic decomposition of hydrazine in HClO₄ and HNO₃ solutions, where the process follows different pathways. In HClO₄, the activity of the catalysts supported on a Termoksid ceramic material is higher than that of the catalysts supported on amorphous silica gel. In nitric acid solutions, the trend is reverse. Peptization of the ceramic supports in acid solutions increasing in the order 75% TiO₂-25% SnO₂ < 75% TiO₂-25% ZrO₂ ? TiO₂ < ZrO₂ was observed. In perchloric acid solutions, the catalyst specific activity in the hydrazine adsorption-dissociative decomposition increases with decreasing size of platinum crystallites on the support. In nitric acid solutions, where the hydrazine decomposition proceeds as its catalytic oxidation with nitric acid, the catalyst specific activity decreases with a decrease in the size of the catalyst crystallites, i.e., the catalyst centers located on large crystallites are more active. The results obtained were attributed to the energetic heterogeneity of the surface Pt atoms and various mechanisms of the hydrazine catalytic decomposition in various media.     

Hydrogen production of nickel–scandia-stabilized zirconia and copper/nickel–scandia-stabilized zirconia catalysts through steam methane reforming for solid oxide fuel cell operation

In this study, the catalytic activities of the steam methane reforming (SMR) reactions with two catalysts, including nickel–scandia-stabilized zirconia (Ni–SSZ) and copper/nickel–scandia-stabilized zirconia (Cu/Ni–SSZ), were examined and compared. The microstructure and crystallinity of the as-prepared catalysts were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Mass spectrometer was applied in the outlet streams, in order to simultaneously monitor the time-dependent kinetics in the reactor for an activity test and conversion examination. Finally, thermogravimetric analysis (TGA) and Raman spectrometer were implemented for further verification of carbon residuals on the catalysts. It was found that the incorporation of Cu on Ni–SSZ imposed significant constraints on the growth of nickel crystallites from NiO during the annealing process in reducing atmospheres. The methane conversion of Ni–SSZ and Cu/Ni–SSZ catalysts (annealed at 300 °C for 2 h) was 36.2 and 26.0%, respectively. However, the amount of carbon residuals on Cu/Ni–SSZ catalyst (300 °C for 2 h) was 18.6%, which is lower than that of the Ni–SSZ catalysts (33.2%) from TGA results. Further Raman experiments revealed that more graphite-like carbon residuals and less defects or amorphous carbons (I_G/I_D?=?2.0) were found in the case of Cu/Ni–SSZ catalysts (300 °C for 2 h). Among the catalysts in this study, the Cu/Ni–SSZ catalyst (300 °C for 2 h) is considered as a promising catalyst for SMR reaction, since it has a fair methane conversion, and characterized higher CO₂ selectivity and lower CO selectivity without compromising the hydrogen purity. More importantly, the least amount of carbon residuals was found in Cu/Ni–SSZ catalyst (300 °C for 2 h), which assured a better lifetime.     

Influence of physicochemical treatments on spent palladium based catalyst for catalytic oxidation of VOCs

To recycle the spent catalyst for the removal of VOCs, the benzene, toluene, and xylene (BTX) complete oxidations were studied over pretreated palladium based spent catalyst in a fixed bed flow reactor system at atmospheric pressure. Two different pretreatment methods with gas (air and hydrogen) and acid aqueous solution (HCl, H(2)SO(4), HNO(3), H(3)PO(4) and CH(3)COOH) were used to investigate the catalytic activity of spent catalyst. The properties of the spent and pretreated Pd based catalyst were characterized by XRD, BET, TEM, ICP, and XPS. The results of light-off curves indicate that the catalytic activity of toluene oxidation for pretreated samples is in the order of hydrogen>air>HNO(3)>CH(3)COOH>H(2)SO(4)>H(3)PO(4)>HCl. In addition, the air and the acid aqueous pretreated catalyst activities were significantly decreased compared to that of the spent (or parent) catalyst. Moreover, hydrogen pretreated (or reduced) catalysts having mainly metallic form show the best performance in removing the toluene vapours compared to other pretreated samples. The reduction temperature made a significant difference in the catalytic performance of the spent catalyst pretreated with hydrogen. XPS results clearly supported that the palladium state of the spent catalysts pretreated at 300 degrees C was shifted more toward metallic form than other reduced catalysts. Furthermore, the results of a long-term test and catalytic activity of aromatic hydrocarbons also supported that the hydrogen pretreated spent catalyst was a good candidate for removing toxic compounds.     

Physicochemical characterization of WO3/ZrO2 and WO3/Nb2O5 catalysts and their photoactivity for 4-nitrophenol photooxidation in aqueous dispersion

Two different photo-catalysts consisting of tungsten oxide supported on zirconia and niobia have been studied. The photoactivity of the samples has been investigated in both the absence and presence of H2O2 for the photo-oxidation of 4-nitrophenol in aqueous suspensions and compared to that of WO3 and of the pure ZrO2 and Nb2O5 supports. Both catalysts were found to be photoactive, although no beneficial influence of the presence of tungsten oxide on the reaction rate was observed in the absence of H2O2. The presence of hydrogen peroxide was observed to be beneficial for all of the samples. Scanning electron microscopy, X-ray diffraction, diffuse reflectance and laser Raman spectroscopies, surface area and porosity determination, as well as Fourier transform infra-red spectroscopy monitoring of surface acidity were used to characterize the catalysts. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mn-HAP基低温SCR催化剂的制备及抗硫中毒性能

低温选择性催化还原(SCR)脱硝是工业烟气末端治理的重要技术, 强化催化剂硫抗性是低温SCR领域内亟待解决的问题。本研究以羟基磷灰石(HAP)为载体、Mn为活性组分通过共沉淀法成功合成了Mn-HAP低温(100~200℃)脱硝催化剂, 探究了其脱硝性能及金属硫酸盐和硫酸铵的中毒特性。结果表明: 以HAP作为活性组分Mn的载体能一定程度上提高催化剂的抗硫性。当反应温度为140 ℃时, SCR催化剂脱硝效率达到100%, 金属硫酸盐相较硫酸铵对催化剂低温脱硝活性的影响更显著, 120 ℃时脱硝效率分别降低37.40%和8.83%。不同手段分析表明, 不同表面硫物种均会不同程度地降低催化剂比表面积并改变活性Mn氧化态。金属硫酸盐显著降低Mn⁴⁺/Mn比例是造成催化剂失活的主要原因。