Linda L. Jewell

Autoreduction and Catalytic Performance of a Cobalt Fischer–Tropsch Synthesis Catalyst Supported on Nitrogen-Doped Carbon Spheres

Carbon materials have been investigated in a wide variety of applications due to their good mechanical stability and electrical conductivity. They have also been used as a catalyst support but in order to establish a uniform coverage of metal particles on the surface of pure carbon materials, it is necessary to activate the chemically inert surface. Generally, different acid or oxidizing treatments have been used to functionalize the carbon surface and to create carboxylic, carbonyl and hydroxy groups that are able to bind the carbon surface to metal clusters. Unfortunately, these treatments can considerably reduce the mechanical and electronic performance of the carbon due to the introduction of a large number of defects. Recently, the doping of heteroatoms into carbon materials has been used as an alternative procedure to successfully bind metals to carbon materials. Nitrogen-doped carbon materials contain sites that are chemically active and allow for the attachment of metal precursors onto the surface of carbon materials without functionalization by strong acid treatments. Following on from the discovery of fullerenes and later the seminal studies on carbon nanotubes by Iijima, the role of curved sp-hybridized carbon atoms was seen to play a key role in the new graphitic carbon structures. 6] This should also apply to carbon materials with other shapes, such as carbon spheres (CSs). Indeed, although carbon spheres have been known for decades, recent studies have paved the way for a reinvestigation of the synthesis, chemistry, and properties of spherical carbons. In the past few years, new synthesis methods have been reported to make a variety of carbon spheres (hollow, solid, core/shell) and these new carbon spheres are expected to exhibit excellent physical and chemical properties. Their use as a catalyst support has, however, hardly been studied. Co and Fe catalysts have been used in Fischer–Tropsch synthesis (FTS) 13] and NH3 decomposition studies. [14] The reduction of the metal oxide to the metal is an indispensable step in activating the catalyst, and is closely related to catalytic performance. 15] However, reduction is affected by the strong metal–support interaction (SMSI). This process typically inhibits the metal reduction process, leading to a lower catalytic activity. Herein, we report for the first time that cobalt oxide supported on nitrogen-doped carbon spheres (N-CSs) can be autoreduced completely by the carbon support. The autoreduced cobalt catalyst pretreated in Ar showed superior FTS catalytic performance to a Co catalyst reduced in H2. Nitrogen-doped carbon spheres (N-CSs) were prepared by chemical vapor deposition (CVD) through the pyrolysis of acetylene and NH3 at 900 8C. [9] This synthesis gave smooth, round carbon spheres with a uniform diameter (ca. 700 nm, BET surface area = 3.4 m g ; see the Supporting Information, Figure S1). Elemental analysis revealed that the nitrogen content of the as-prepared carbon spheres was approximately 2 wt % (see the Supporting Information, Table S1). The N-CSs-supported cobalt catalyst (Co/N-CSs) was prepared by a homogeneous deposition precipitation method using urea as the deposition agent at 90 8C. After filtration, the material was dried in an oven at 100 8C for 12 h and was found to contain 2.3 wt % Co (measured by ICP-AES). The Co/N-CSs had an average cobalt oxide particle size of approximately 13 nm and transmission electron microscope (TEM) images show that the cobalt species were retained on the surface of the carbon spheres even after sonication for 4–5 min (Figure 1 a). Addition of cobalt to CSs that did not contain nitrogen led to large Co particles, even at loadings below 1.5 wt % Co (Figure 1 b). The catalyst was characterized by thermogravimetric analysis (TGA, Perkin–Elmer STA 6000) under N2 using a heating rate of 10 8C min . Figure 2 displays the TGA curves of the nitrogendoped carbon spheres and the 2.3 wt % Co/N-CSs catalyst under N2. A weight loss of approximately 6 % was detected when the N-CSs were heated to 900 8C (Figure 2), owing to the loss of the nongraphitic carbon. The weight loss of greater than 10 % detected for 2.3 wt % Co/N-CSs in the temperature range 400–900 8C can be attributed to CO2 formation as the cobalt oxide is reduced. A possible cobalt-catalyzed loss of carbon from the matrix may also have contributed to the weight loss. The effect of the pretreatment temperature (prior to catalyst testing) on the reduction behavior of the resulting 2.3 wt % Co/N-CSs was monitored by hydrogen temperatureprogrammed reduction (TPR, Micromeritics Auto Chem II) under 5 % H2/Ar. Figure 3 presents the TPR profiles of 2.3 wt % Co/N-CSs pretreated in a flow of high purity Ar at different temperatures. As can be seen, the TPR profile of 2.3 wt % Co/N-CSs after pretreatment at 250 8C (Figure 3 a) has two reduction peaks, corresponding to the reduction of Co3O4 and a mixture of Co3O4 and CoO, respectively. [17] The first peak, [a] Dr. H. Xiong, Prof. D. G. Billing, Prof. N. J. Coville DST/NRF Centre of Excellence in Strong Materials University of Witwatersrand, Johannesburg 2050 (South Africa) Fax:(+27) 11-7176749 E-mail : neil.coville@wits.ac.za [b] Dr. H. Xiong, M. Moyo, M. K. Rayner, Prof. D. G. Billing, Prof. N. J. Coville School of Chemistry, University of the Witwatersrand Johannesburg 2050 (South Africa) [c] M. Moyo, Dr. L. L. Jewell School of Chemical and Metallurgical Engineering University of the Witwatersrand, Johannesburg 2050 (South Africa) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.200900309.

Analysis of Arsenic Species in Environmental Samples

Many analytical methods for determining arsenic in various forms in environmental samples have been developed in recent years. The main objective of this review article is the presentation and comparison of the three principal techniques for the determination arsenic compounds, namely: hydride generation (HG), voltammetry and chromatography (liquid chromatography including high-performance liquid chromatography (HPLC), and gas chromatography (GC). These techniques (with the exception of voltammetry) are usually coupled with numerous sensitive detectors, which gives almost unlimited possibilities for the identification of arsenic species in environmental samples. Unfortunately, every method has its own advantages and disadvantages, with the more sensitive techniques requiring complicated sample preparation involving extraction of one kind or another to concentrate the sample and eliminate the effects of the background matrix of the sample. Speciation is the most important requirement for analysis in environmental research and HPLC is the most powerful method for arsenic species.

Carbon Spheres Prepared by Hydrothermal Synthesis—A Support for Bimetallic Iron Cobalt Fischer–Tropsch Catalysts

Carbon spheres (CSs) synthesised by the hydrothermal approach were explored as a model support material for a bimetallic Fe–Co Fischer–Tropsch (FT) catalyst. The CSs were characterised by N2 adsorption–desorption, thermogravimetric analysis, FTIR spectroscopy and powder XRD. If annealed at 900 °C for 4 h, the CSs exhibited an improved surface area, thermal stability and crystallinity. A series of Fe–Co bimetallic FT catalysts supported on the annealed CSs were prepared by co‐precipitation. A variety of Fe‐to‐Co ratios were used with the total metal loadings maintained at 10 %. Catalyst reducibility studies were performed by H2 temperature‐programmed reduction and in situ powder XRD. Catalysts with a Fe/Co ratio of 5:5 (w/w) showed Co–Fe alloy formation upon reduction at >450 °C. Interestingly, the presence of this alloy did not correlate with high C5+ selectivities during FT synthesis; rather the Co‐rich/Fe‐poor catalyst gave the best selectivity. The CSs allowed the metal–metal interactions in the bimetallic systems to be monitored because of the weak interaction of the metals with the support.

Seasonal changes in organotin compounds in water and sediment samples from the semi-closed Port of Gdynia.

The effect of seasonal changes on the distribution of organotin compounds (OTC) in the sediments and seawater from the docks of the Port of Gdynia was investigated. Sediment and seawater samples were collected from four industrial docks in February (winter) and June (summer) in 2009. The samples were analyzed for butyltin, phenyltin, octyltin, and tricyclohexyltin (total of 9 OTC derivatives). The fine fraction (<0.063 mm) accumulated the highest concentration of OTC, although it was not the dominant fraction in the sediment samples from the Port of Gdynia. The average concentration of TBT, DBT and MBT in collected samples were as follows: 4400; 2188; 730 ng cation g⁻¹ d.w. (February) 3638; 1590; 474 ng cation g⁻¹ d.w. (June) in the fine sediment samples, 2805; 1266; 485 ng cation g⁻¹ d.w. (February) in <2.00 mm sediment fractions and 118.6; 39.2; 25.3 ng cation L⁻¹ (February) and 46.5; 12.6; 8.2 ng cation L⁻¹ (June) in the water samples. Higher concentrations of butyltin derivatives (BT) were observed in samples collected in February than in those collected in June. Seasonal changes in BT correlate well with changes in the water pH and concentrations of organic matter and can be attributed to sorption/desorption to sediments, photodegradation and biodegradation. Although the Port of Gdynia does not represent the natural features of a marine environment, seasonal variations recorded in the pH values as well as BT, organic carbon and biogenic element concentrations seem to be influenced by temperature and microbial activity.

The Speciation of Organotin Compounds in Sediment and Water Samples from the Port of Gdynia

Organotin compounds (OTC) are toxic towards all living organisms. The application of organitin-based antifouling systems is becoming the main source of OTC in the ocean. Harbor sediments and water contain large deposits of organotin compounds due to application of antifouling systems in the shipping industry. OTC contamination presents a potential risk to the marine environment. Sediment and water samples were collected in 2009 from Gdynia Harbor. For all the analyzed organotin compounds, the mean concentration values were determined: water samples monobutyltin (MBT): 13.2, dibutyltin (DBT): 16.7, tributyltin (TBT): 60.7 (ng cation dm−3), and sediment samples MBT: 261.4, DBT: 751.9, TBT: 2148.2 (ng cation g−1 d.w.). The estimated content of monophenyltin (MPhT), diphenyltin (DPhT), triphenyltin (TPhT), monooctyltin (MOT), dioctyltin (DOT), and tricyclohexyltin (TCHT) were below the detection limit of the applied method. It was found that the content of organic matter, the amount of fine fraction, and the pH all play a significant role in the distribution and sorption process of OTC between the water and the sediment on the bottom. Compared to an earlier study, the concentrations of all OTC are much lower, confirming that the applied legislation has had a positive impact.

Arsenic-Based Warfare Agents: Production, Use, and Destruction

Since the beginning of time, civilizations have looked for more creative ways to dominate and defeat their enemies. The rapid development of the chemical industry just before the Second World War started the era of modern chemical weapon production based on poisons, including toxic arsenic compounds. This paper provides a detailed overview of the production, usage and destruction of this dangerous chemical weapon. Milestones include: (i) the development of knowledge concerning the synthesis and decomposition of toxic warfare agents containing arsenic compounds, (ii) increased awareness of the influence of this poison on human life and the environment, (iii) the development of modern technology for the destruction of chemical weapons, (iv) implementation of legislation which prohibits the use of chemical weapons in combat, and (v) the development of analytical methods to detect arsenic compounds in the environment that was used in warfare. The article includes events before World War I and next focuses on World War II, the Vietnam War and the two Gulf Wars. It further details the development of specific arsenical chemical weapons (e.g. Lewisite, Clark I, Clark II, Adamsite), as well as some agents used as herbicides, like Agent Blue. Special attention is paid to the disarmament times and the challenges of implementing a world-wide plan to destroy chemical weapon stockpiles.

Neutral-Neutral Direct Hydroamination Reactions of Substituted Alkenes: A Computational Study on the Markovnikov Selection Rule

We have carried out a detailed computational study on the neutral-neutral direct hydroamination reactions between various substituted ethylene derivatives with ammonia, using the density functional theory based B3LYP/6-31++G(2df,2p) level of theory. Analysis of the potential energy surfaces for all these reactions shows that in all these cases thermodynamically the Markovnikov product is more stable than the anti-Markovnikov product. Analysis of the transition states for all these reactions shows that kinetically in some cases the Markovnikov product is preferable whereas other cases the anti-Markovnikov product is preferable. This gives a clear indication on how to control the selectivity in these reactions by mere alteration of the substituent. We observed that, in the case of ethylene with an electron withdrawing substituent (i.e. CH2=CH–NO2) reacting with ammonia, the barrier height is reduced by approximately 70 kJ/mol and at the same time the anti-Markovnikov product is preferred kinetically. This situation clearly mimics the general catalytic hydroamination reaction where the ethylene is being activated by the catalyst. Also, this suggests that a stronger electron withdrawing group than −NO2 will possibly be able to reduce the barrier height further and ultimately a non-catalytic neutral-neutral direct hydroamination reaction will be physically attainable.