摘要:

Broader contextCO2is the most abundant waste gas produced by human activities and one of the greenhouse gases. On the other hand, CO2provides a nontoxic, cheap, and highly functional carbon source. How to convert this carbon resource into useful products is a significant issue in the environmental field. To realize that, chemical fixation of CO2with epoxides to form cyclic carbonates under mild conditions by using ionic liquid-based heterogeneous catalysts offers a promising alternative. In our current study, we have demonstrated an economic and efficient synthetic strategy to fabricate a series of heterogeneous catalysts by means of grafting ionic liquids onto the commercial silica surfaces. These heterogeneous catalysts exhibited high catalytic activities and selectivities in the coupling reaction of allyl glycidyl ether and CO2. The factors such as molecular structure of grafted ionic liquids (alkyl chain length and anions), reaction conditions (temperature and pressure) as well as the texture properties (pore size and surface area) of commercial silica supports on the efficiency of coupling reaction have been investigated in detail.

ISSN:0046-225X    

DOI:10.1039/b910763k

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

IntroductionIn the region of 3 billion years ago biology developed the capacity to efficiently capture solar energy and use it to power the synthesis of organic molecules. This photosynthetic process set into motion an unprecedented explosion in biological activity allowing life to prosper on an enormous scale as witnessed by the fossil records and by the extent and diversity of living organisms on our planet today. Indeed, it was the process of photosynthesis over eons of time which has provided us with the oil, gas and coal needed to power our technologies, heat our homes and produce the wide range of chemicals and materials that support everyday life.Today, it is estimated that photosynthesis produces more than 100 billion tons of dry biomass annually, which would be equivalent to a hundred times the weight of the total human population on our planet at the present time and equal to a mean storage rate of about 100 TW. The success of this energy generating and storage system stems from the fact that the raw materials and energy needed to drive the synthesis of biomass are available in almost unlimited amounts; sunlight, water and carbon dioxide. At the heart of the reaction is the splitting of water by sunlight into dioxygen and reducing equivalents. Prior to the evolution of the water splitting reaction, photosynthetic organisms had relied on hydrogen/electron donors such as H2S, NH3, organic acids and Fe2+, which were in limited supply compared with the vast quantities of water available on our planet.The appearance of the water splitting reaction of photosynthesis had dramatic consequences, converting the atmosphere of Earth from anoxygenic to oxygenic and at the same time allowing the ozone layer to be established. With oxygen available, the efficiency of metabolism increased significantly since aerobic respiration provides in the region of twenty times more cellular energy than anaerobic respiration. It was probably this substantial improvement in cellular efficiency, due to aerobic metabolism, which drove the subsequent evolution of eukaryotic cells and multicellular organisms. The establishment of the ozone layer provided a shield against harmful UV radiation allowing organisms to explore new habitats and especially to exploit the terrestrial environment.The photosynthetic enzyme that uses light energy to split water is known as Photosystem II (PSII). It is a multiprotein complex contained within the thylakoid membranes of all types of plants, algae and cyanobacteria.1In contrast to chemical and electrochemical water splitting, which are thermodynamically highly demanding, the PSII-catalyzed biological water-splitting mechanism is truly remarkable since it proceeds with very little over-potential.2–4The processes underpinning this reaction are initiated by the absorption of visible light by chlorophyll (Chl) and other pigments that act as an antenna system for the reaction centre (RC) where energy is initially stored by charge transfer. The primary electron donor consisting of Chlais located within the RC, composed of the D1 and D2 proteins, and is called P680. Excited P680 (P680*) donates an electron to the primary electron acceptor, pheophytin (Pheo)a. The formation of a radical pair P680&z.rad;+Pheo&z.rad;−takes place in a few picoseconds and occurs across the membrane. Stabilisation of the charge transfer state is accomplished by electron transfer from Pheo&z.rad;-to a plastoquinone acceptor QBin the microsecond to millisecond time domain according to the redox state of QB. This electron transfer is aided by an intermediate plastoquinone molecule QA. Unlike QB, QAplastoquinone is tightly bound within the RC, functions as a single electron acceptor and does not undergo protonation. The QBplastoquinone, however, accepts two electrons and is fully protonated prior to its departure from the RCviathe hydrophobic lipid phase of the membrane. In this way reducing equivalents leave PSII and with the aid of a second light reaction occurring in Photosystem I (PSI) are used to reduce NADP+and ultimately CO2.On the oxidising side of the PSII RC, which is localised towards the lumenal surface of the thylakoid membrane, P680&z.rad;+is used to split water. Four oxidising equivalents are needed to form dioxygen. Since each water splitting reaction occurs at a single catalytic centre, the formation of dioxygen requires four photochemical turnovers of the PSII RC and the storage of four oxidising equivalents. This storage of oxidising potential is driven by electron transfer to P680&z.rad;+viaa redox active tyrosine YZand occurs in a catalytic centre containing four Mn ions and a Ca2+. The S-state cycle of Joliot and Kok5,6provides a framework for describing the chemical intermediates of the catalytic cycle, where the energy of each photon absorbed by PSII drives the conversion from S0to S1, S1to S2, S2to S3and S3to S4, where the S4to S0transition is a dark reaction giving rise to the formation of dioxygen. In this cycle the S1-state is stable in the dark so that maximum oxygen emission occurs from dark-adapted cells on the third, single turnover, flash followed by a subsequent period of four. The details of the S-state cycle and the chemistry of the water splitting reaction have been emerging over many years through the application of a wide range of techniques (see various articles inref. 7 and 8) being particularly spurred by the recent structural analyses of PSII by X-ray absorption spectroscopy9–12and X-ray crystallography.13–17These studies, coupled with quantum mechanical analyses have provided detailed schemes for the water splitting chemistry leading to O–O bond formation18–20and refinement of the structure of the oxygen evolving centre (OEC).21,22However, one factor which has been difficult to consider is the apparent involvement of chloride in the chemistry of the water splitting reaction.The recognition that chloride is required as a cofactor for photosynthetic oxygen evolution stems from the pioneering studies of Arnon and Whatley,23Boveet al.24and Izawaet al.25Since then this requirement has been investigated in great depth as detailed in the recent review of van Gorkom and Yocum.26However the role and function of Cl−within the S-state cycle is not understood and remains the subject of a good deal of debate.26–28In plant PSII, spectroscopic and enzymological studies have led to proposed models with either a single Cl−-site with two different binding affinities29or two independent binding sites (ref 30–32, see alsoref. 33for a recent discussion).The location of the Cl−−binding site (or sites) is also uncertain, with some authors favouring a Cl−directly liganded to one of the Mn ions of the cluster34or to Ca2+,35,36and others preferring the Cl−to be outside the first coordination sphere of the metal cluster.37–39To date the resolutions of the crystal structures available are insufficient to assign electron density to this anion. Bromide is known to substitute for chloride in the OEC and give almost complete enzyme function.40Indeed, a recent X-ray absorption study, using Br−-containing PSII,39has provided what the authors describe as “tentative evidence” for a Br−(and hence Cl−) at a distance of ≈5 Å from a metal (presumably Ca or Mn): too far to be a direct ligand.X-Ray anomalous dispersion, a specific and sensitive method to detect heavy atoms within crystals,41has been successfully used, even at low resolution, to confirm the localisation of the Sr ion in the Mn4Sr cluster of the OEC after a biosynthetic Ca2+/Sr2+exchange.42Chloride has no accessible absorption edges, but bromide does at 13.47 keV, a near-optimal wavelength for many synchrotron beam lines and therefore ideal for X-ray anomalous diffraction studies.In the present work we have prepared two kinds of 3-dimensional crystals of PSII isolated fromThermosynechococcus elongatus: (i) standard PSII crystals infiltrated with bromide and (ii) PSII crystals obtained from cells grown in a culture medium in which the chloride content was replaced by bromide. In both cases, anomalous X-ray diffraction analyses were conducted to detect bromide binding sites at or close to the OEC. We find similar results in both types of sample: the presence of two bromide binding sites close to the Mn4Ca cluster but too far to be ligands to the metals.

ISSN:0046-225X    

DOI:10.1039/b810067p

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Broader contextPhotocatalysis potentially can provide solutions for many of the environmental challenges facing the modern world because it provides a simple way to use light to induce chemical transformations. Pollution control, either in aqueous solutions or air, is very likely the most studied application of photocatalysis, although commercial uses relate mainly to self-cleaning surfaces. Besides this, photocatalysis can be also applied to the production of fuels like hydrogen or as a green route to obtain fine chemicals. Currently, TiO2is by far the most widely used photocatalyst because it comprises the best balance of properties among the known or assayed semiconductors. However, it still presents some disadvantages such as limited activity and reduced sensitivity to sunlight. Therefore, in the last few years significant effort has been devoted to the search for new materials that may overcome the limitations of TiO2. This review gives an overview of photocatalysts, different to TiO2, that have been tested for the most relevant photocatalytic applications: water splitting, detoxification and disinfection, and organic synthesis.

ISSN:0046-225X    

DOI:10.1039/b907933e

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Xue-Ping GaoXue-Ping Gao is Professor at Nankai University of China, where he has served as the Director of the Institute of New Energy Materials Chemistry since 2004. He received his doctorate at the Department of Chemistry from Nankai University in 1995. He worked as a visiting research fellow at Kogakuin University in Japan from 1997 to 1999, and at the University of Queensland in Australia in 2001. Currently, his main research is focused on new materials for energy storage and conversion.

ISSN:0046-225X    

DOI:10.1039/b916098a

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

1IntroductionHydrogen has the potential to play an important role in creating large-scale changes to the mix of energy sources currently used by our global society, particularly in transport applications that currently are entirely dependent on liquid hydrocarbons. In the most utopian setting, hydrogen could be produced by electrolysis of water from a truly renewable energy source (e.g.solar). In the nearer term, a time still likely to be measured in decades, large-scale hydrogen production will involve gasification of hydrocarbon resources, including coal and biomass. If the syngas produced from these gasification processes can be effectively separated into (nearly pure) hydrogen and concentrated CO2, then an overall process that is carbon-neutral (for coal) or carbon-negative (for biomass) can be envisioned.1,2This concept of course assumes that the resulting CO2is sequestered in some long term manner. As with any scenario for large-scale energy use, this scheme has a number of negative aspects and faces serious technological, infrastructure and policy hurdles.3One of the key technological challenges associated with large-scale production of hydrogenviagasification is the need to economically purify H2from syngas and other feeds. If the resulting H2is to be used in fuel cell applications, extremely high levels of purity are desirable.4,5It is of course important to perform this separation economically. In gasification applications, feed streams are typically at high temperatures; temperatures of 300–500 °C.6,7Membrane-based separations provide an excellent general strategy for achieving separations with these requirements.8Ockwig and Nenoff recently produced an extensive review of membrane technologies for hydrogen purification, which include a large range of materials.9For high temperature applications, dense metal membranes are in many cases the most promising membrane materials.Dense metal membranes function by allowing the diffusion of individual H atoms through a dense membrane layerviainterstitial diffusion; a process that effectively excludes atoms of all elements except H.9This allows these membranes to have potentially “perfect” selectivity for H2. This observation has led to a large body of work aimed at developing durable, cost-effective dense membranes with large permeabilities for H2. Multiple reviews of this field have appeared in recent years.9–14The materials used for these membranes can be broadly divided into elemental metals, crystalline metal alloys and amorphous metal alloys. The fundamental properties of essentially all elemental metal membranes are known, with Pd being by far the most widely studied. When alloys are considered, it becomes difficult to systematically consider all the possible compositions that can be considered as potential membranes. Large numbers of binary crystalline alloys and a smaller number of ternary and other multi-component alloys have been examined experimentally, but the search for materials that are durable, cost-effective, poison tolerant and exhibit high permeability for H continues to be an active area.Using thin films of amorphous metals, that is, metals without long range structural order, offers significant opportunities for fabrication of membranes that outperform existing Pd-based metal membranes based on crystalline alloys. Dolanet al.have recently provided a detailed review of hydrogen-selective amorphous alloy membranes,11and Ockwig and Nenoff highlighted recent progress with these materials as part of their comprehensive review of membranes for hydrogen production.9Perhaps the single most important observation from these two reviews is that amorphous metal membranes have been tested by multiple groups, so the methods required for creating pinhole-free membranes are now well developed. As is the case with crystalline membranes, the number of distinct alloy compositions that can be considered as candidates for amorphous film membranes is vast. Ockwig and Nenoff noted that development of these membranes is “still an entirely open field”, and this is in large part due to the huge number of materials that could be used. Among the limited number of materials that have been tested as membranes, a number have shown promising permeabilities relative to well known crystalline membranes. Inoue and coworkers have reported several films involving Zr, Ni and other metals that have H2permeabilities comparable to pure Pd.15–18The use of amorphous metal films as membranes can address several of the difficulties associated with crystalline membranes. In general, concerns about hydrogen embrittlement and sintering are greatly reduced in amorphous materials when compared to crystalline materials.11The cost of the membrane materials in amorphous films can potentially be far less than in Pd-based alloys. Many of the amorphous films studied to date have been based on low-cost materials such as Zr, Ni, Cu and Al. Another positive feature of these films is that the glass-forming requirements of materials for amorphous films are much less stringent than for bulk samples of the same materials. The glass forming ability of an alloy is typically characterized by the minimum cooling rate that must be used to avoid crystallization.19,20Flow casting methods exist for films as thin as 20 μm that give cooling rates up to 1010°C s−1;21this cooling rate is much larger than the critical cooling rate for most alloys.In this paper, we consider how the development of amorphous metal membranes for hydrogen purification could be accelerated by making quantitative theoretical predictions about the performance of these membranes. Although the properties of interstitial H in amorphous metals has been studied for at least 30 years,22–24previous theoretical work on these materials has been phenomenological rather than quantitative. Below, we introduce a new approach based on quantum chemistry calculations that gives quantitative information about the flux of hydrogen through amorphous metal films. These methods should be useful in seeking amorphous alloys that are worth intensive experimental investigation and device development.The paper is structured as follows. In section 2, we discuss several general principles regarding the use of theoretical models to screen materials for practical applications, and describe why metal membranes are one area to which theoretical calculations are well suited. In section 3, we introduce our new methods for using quantum chemistry and statistical mechanics to predict the solubility, diffusion and permeation of H through amorphous metals. We illustrate our approach for a specific material, amorphous Fe3B, that can be directly compared to a crystalline material with the same composition. Comparing these two materials provides an example of how an amorphous structure can lead to permeation rates through a membrane that are greatly enhanced relative to a crystalline material. In section 4, we summarize the methods we have introduced in relation to the principles listed in section 2, with a particular emphasis on discussing the limitations of our current approach and areas where experimental efforts are required to provide a complete description of membrane performance.

ISSN:0046-225X    

DOI:10.1039/b806909n

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Xiaoyu YanDr Xiaoyu Yan is a post-doctoral researcher at Smith School of Enterprise and the Environment at the University of Oxford. He received his PhD in mechanical engineering from Queen Mary, University of London. His research interests include: energy demand and supply, environmental impacts and policies related to the road transportation sector; life cycle assessment of road vehicle fuel/propulsion options.

ISSN:0046-225X    

DOI:10.1039/b915801d

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

Kurt Zenz HouseKurt Zenz House received his PhD in Geosciences from Harvard University in 2008 for workOn the Physics & Chemistry of Carbon Dioxide Capture & Storage in Terrestrial & Marine Environments. House studies and develops methods for large-scale capture and storage of human-made carbon dioxide. He recently patented electrochemical weathering, a novel process that expedites the ocean's natural ability to absorb carbon dioxide, and cofounded a venture-capital-backed alternative-energy company. Additionally, he cofounded the Harvard Energy Journal Club to facilitate cross-disciplinary discussions about energy technology; in 2008Esquiremagazine featured him among its “Best and Brightest”.

ISSN:0046-225X    

DOI:10.1039/b811608c

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Reinhold KneerReinhold Kneer was born in Ehingen (Donau) on July 15, 1959. He graduated from Karlsruhe University (TH) with a Diploma in Mechanical Engineering in 1985 and a Doctorate in 1993 (thesis on the mixture preparation for gas turbine combustors). In 1994, he joined Delphi Corp. in Luxembourg as an application engineer in the fuel injection group and ended up as the innovation center manager. Since August 2004, Reinhold Kneer has been the head of the Institute of Heat and Mass Transfer at RWTH Aachen University. His current research focuses on coal combustion, liquid fuel injection and falling film phenomena.

ISSN:0046-225X    

DOI:10.1039/b908501g

出版商:RSC    

年代:2009

数据来源: RSC

摘要:

 Dr Velmurugan Thavasi (left) is a Research Fellow at the NUS Nanoscience and Nanotechnology Initiative. He obtained a MEng in Chemical Engineering and a PhD in Chemistry from National University of Singapore. His research interests include synthesis of 1-dimensional nanostructured materials and design of excitonic solar cells and bio-solar cells.

ISSN:0046-225X    

DOI:10.1039/b809074m

出版商:RSC    

年代:2008

数据来源: RSC

摘要:

Nuria de las Heras studies the development and evaluation of metal bipolar plates for polymer electrolyte membrane (PEM) fuel cells. She has developed experimental techniques for characterising the corrosion and contact resistance characteristics of plate materials.

ISSN:0046-225X    

DOI:10.1039/b813231n

出版商:RSC    

年代:2008

数据来源: RSC