Noble metals, such as rhodium, rhenium, palladium and platinum, are used in a range of applications, from catalysts, through solar cells, to electronic devices such as smart phones. But the scarcity of these metals – some of which can only be sourced from China or Russia – and their potential toxicity to the environment are causing a headache for European manufacturers, who are now actively researching cheaper, more readily available alternatives. And what could be more abundant than carbon?
A pan-European research project, FreeCats, aims to identify carbon compounds that could replace rare metals and potentially toxic materials, such as vanadium pentoxide, used as catalysts. The partnership comprises academics from the universities of Warwick in the UK and Porto in Portugal, CNRS in France, CSIC in Spain and CNR in Italy, with industrial partners Sicat (France), Prototech (Norway) and Advantech (Portugal). The project is coordinated by Magnus Ronning, professor of chemistry at the Norwegian University of Science and Technology (NTNU) in Trondheim, whose expertise lies in the use of carbon nanotubes (CNTs) in catalysis. The other project partners have expertise in noble metal catalysis.
FreeCats will focus on the potential use of CNTs in three different areas: alkene (olefin) production, water treatment and fuel cells. ‘This is a good opportunity to test some of these materials in new processes, to see how they could replace noble metals,’ Ronning says. The CNTs are ‘doped’ with other substances to change their electron density. This helps them to mimic the behaviour of noble metal catalysts, he explains. The materials are most commonly doped with nitrogen but boron and phosphorus are also used, depending on the nature of the chemical reaction being catalysed.
Alkenes are used widely across the chemicals industry, most notably as precursors for plastics like polyethylene. They are made from natural gas by converting alkanes, such as ethane, into alkenes like ethene, which is then polymerised into polyethylene. Ronning says that the doped CNTs have three potential advantages over traditional catalysts: they are more selective, potentially cheaper and they work at lower temperatures.
‘Existing catalysts go via alkane dehydrogenation, which is a high temperature process’, he says. A phosphorus-doped CNT goes via oxidative dehydrogenation, which takes place at lower temperatures. The difference in temperature is around 200°C lower, which could mean enormous energy savings, he says. The process also produces fewer unwanted byproducts – such as higher alkanes or carbon deposits – and more of the desired compound. This reduces the need for separation technologies.
Despite the potential benefits of the new materials, Ronning acknowledges that they must be able to show distinct advantages, and not simply be comparable with their predecessors. ‘We are competing with mature technologies,’ he says. ‘Our materials will need to be better than the existing ones, otherwise the chemical companies will not take a chance on them.’
Another key area for noble metal catalysts is in the automotive industry, where catalytic converters – commonly based on platinum, ruthenium and rhodium – clean exhaust gases. But despite this potentially enormous market, Ronning and his team have their sights set further in the future. ‘The European Union is planning to phase out the internal combustion engine by 2050,’ he says. ‘We’re looking at how our materials could be used in their replacement – fuel cells.’
Fuel cells operate by converting a fuel such as hydrogen into electrical energy via catalysed chemical reactions. In a proton exchange membrane (PEM) cell, hydrogen is oxidised at the anode, and oxygen is reduced at the cathode. The oxygen reduction reaction is currently driven by platinum catalysts. Ronning believes that nitrogen-doped CNT catalysts could do this in place of platinum. The amount of platinum used in fuel cells could gradually be reduced and eventually completely replaced by carbon, he says.
The third potential use of these carbon catalysts is in water treatment. However, as Ronning points out, not for drinking water – the volumes are far too high – but for wastewater treatment to produce environmentally safe waters and sludge. The most common method of treatment is wet-air oxidation, in which pesticides, bacteria and other undesirables are oxidised, rendering them harmless. Several methods are employed: a blast of heat, or UV light; chemical sorption; and catalytic oxidation, using materials such as cerium oxide. Again, he believes carbon-based catalysts could be used as replacements.
When FreeCats comes to an end in three years, Ronning hopes to have made catalysts that work better than the existing platinum-based materials. So far, the team has synthesised some of these catalysts, such as N-doped CNTs, and has testing facilities in place. ‘In each case, we would replace the noble metal catalyst with our own, and run the reaction,’ he says. They would be used as ‘drop-in’ replacements.
The likely speed of commercialisation will depend partly on the project’s results and partly on the attitude of the relevant industry. ‘If we can demonstrate a better solution, I think commercialisation should be possible within 10 to 15 years,’ he says.
Meanwhile, researchers at the University of Basel in Switzerland have proposed a new way of making solar cells using a cheap, available and – in their words – downright ‘boring’ material.
A dye-sensitised solar cell (DSC) consists of a semiconductor – titanium dioxide – that is coated with a coloured dye. The dye, which is traditionally based on ruthenium, absorbs sunlight and transfers an electron into the semiconductor, setting up a photocurrent. However, the researchers comment that an efficient DSC based on such ‘extremely rare materials’ is unlikely to enter the mainstream (Chem. Commun., doi:10.1039/c2cc31729).
The researchers are currently developing dye compounds based on zinc, which is around 2000 times cheaper than ruthenium. At the same time, they have devised a way of simultaneously making and attaching the dye to the surface of the titanium dioxide nanoparticles, which makes for an efficient process.
Edwin Constable, who oversees the dye research, says that his team discovered new organic compounds that bind to zinc to give coloured materials in the course of researching next-generation lighting devices. ‘Nobody in their right mind would use zinc because it is boring and colourless,’ he says. ‘We were very surprised, and it has worked better than anybody expected.’
The devices are currently inefficient, but could eventually lead to DSCs that use new – and much cheaper – dyes. While ruthenium costs around €3000/kg, zinc is priced around €1.5/kg. The team is also investigating dyes based on copper, which costs around €6/kg.
‘This is a significant step towards our dream of coupling photovoltaics and lighting in an “intelligent curtain”, which can store solar energy during the day and function as a lighting device at night,’ says Constable. While price is undoubtedly an issue here, he says that material availability is probably a more important factor driving the search for alternatives. ‘We are starting to see it in FP7 research calls,’ he says. ‘There are many themes to replace platinum by more sustainable materials in the electronics industry.’
Carbon might also be used as a replacement in one of the most voracious modern consumers of rare metals – touch screens.
Researchers at the Fraunhofer Institute in Germany have synthesised conductive carbon-based inks and are using these to produce transparent electrodes for use in touch screens. The carbon-based ink can be applied to the surface of an inexpensive plastic substrate, such as polyethylene terephthalate (PET) or polyacrylonitrile (PAN), to create a transparent conductive film. Usually, electrodes used in touch screens rely on the expensive, and relatively rare, indium tin oxide.
Ivica Kolaric, project manager at the Fraunhofer Institute for Manufacturing Engineering and Automation (IPA) in Germany, believes that carbon – in the form of both nanotubes and graphene – could be used as a replacement for ITO in some applications.
The carbon-based materials are unlikely to compete head-on with ITO – while transparency is comparable, ITO is still some three times more conductive. So applications like iPads, iPhones etc will continue to use ITO, but applications that do not need the high conductivity provided by ITO could benefit. ‘Many home appliances, like telephones or fridges, could incorporate touch screens quite cheaply using our materials,’ he says.
Moreover, Kolaric explains that his carbon-based materials have other advantages, compared with indium tin oxide. They are flexible – indium tin oxide is notoriously brittle – and so might find use in flexible photovoltaic devices, and they are more resistant to UV light than conductive polymers, such as Pedot (poly(3,4-ethylenedioxythiophene)), so outdoor vending machines are another possible application of the technology.
According to Kolaric, effort now needs to be put into developing mass production processes. ‘We can make these coatings in the laboratory, but making them on a larger scale is challenging,’ he says. It is difficult, for example, to transfer a single layer of graphene from the copper surface – where it is produced – to the plastic substrate. ‘Transferring this monatomic layer – without wrinkling it – is very difficult,’ he says. Kolaric expects to have a working prototype ready within two or three years. ‘It will probably be another 18 months before we get to mass production,’ he says.
In common with other researchers of ‘alternative’ materials, he says that his research is motivated by the need to reduce reliance on rare materials. Touch screen production is dominated by Asian giants LG and Samsung, and there is no production in Europe. Kolaric believes his material could help to reverse that situation. ‘We think we can help to revitalise the electronics industry in Europe,’ he says.
For now, most of these alternative materials are still at the laboratory stage. But the researchers are confident that they could soon be used in place of the rare – and expensive – materials that may one day be impossible to access.
Replacing one chemical or material with another extends beyond the replacement of rare metals, and it is not always about cost saving, or economics. Sometimes, manufacturers are forced to change their materials in order to comply with legislation or customer demand. A number of heavy metals – chiefly lead, mercury and chromium – are already restricted, or even outlawed. Many other substances are also under scrutiny, with many due to be phased out, so manufacturers will need to replace them in their formulations – whether they are plastics, shampoos or foodstuffs.
Subsport (Substitution Support Portal) is a collaborative European project that has built a database that contains more than 100 case studies to help companies in their own efforts to substitute hazardous chemicals for more benign ones. ‘Substitution is the most preferred risk reduction measure,’ says Steffen Brenzel of Kooperationsstelle Hamburg IFE, one of the project partners in the Subsport project. ‘Before you look at using hazard protection equipment, you should first look to use less hazardous substances.’
Examples of successful substitutions, which are on the Subsport database, include the replacement of two allergenic preservatives methylisothiazoline and methylchloroisothiazoline by the more benign phenoxymethanol in cosmetics formulations; a study identifying potential alternatives to musk xylene in fragrance formulations, including the macrocyclic musks civetone and ethylene brassylate; and the replacement of ethylene oxide with carbon dioxide for sterilisation in the food industry.
There are links to the many specific directives and laws, which are most likely to drive the need for substitution. This includes everything from EU directives to specific country legislation.
There are also a few specific company ‘blacklists‘. Nokia, for example, has a long list of banned substances, and anybody supplying to the company must be aware of this. Similarly, car companies like Volvo have their own list of restricted substances.
These lists – and the database of case studies – should help companies to supply these companies with more confidence. However, the ‘companies’ list is not exhaustive because, as Brenzel points out, entering the data is time-consuming and expensive. ‘We hope we can at the very least keep it updated – but for it to get any bigger it needs more funding,’ he says.
Lou Reade is a freelance science writer based in Kent, UK.