search.noResults

search.searching

dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
PROCESS CHEMISTRY


Towards the


ammonia economy


Much has been made of the future use of hydrogen as a fuel for clean fuel cells that only emit water vapour as waste and are carbon neutral as long as the hydrogen is produced from renewables. The problem is, however, that hydrogen is quite difficult to store and transport safely. It would have to be either compressed or cooled to the liquid state, or adsorbed to a carrier material. In all these cases, the additional weight required would be a multiple of the usable amount of hydrogen. Thus, it turns out that ammonia, which contains 3/17 or


17.6 % hydrogen by weight, and can be transported much more safely, may be a better option for non-polluting fuel cells in transport applications. Other nitrogen compounds including urea and ammonium carbonate, or aqueous solutions of ammonia may also be good candidates. A range of options have to be considered, as Shanwen Tao and colleagues have outlined in a review.2


Ammonia


or similar substances could be carried as fuel precursor for hydrogen fuel cells. In this scenario, heat and/or a catalyst would release the hydrogen from the chemical as and when needed. This has the advantage that efficient hydrogen fuel cells already exist, as does the know- how of handling ammonia, which is used in industrial refrigeration, among other things. Related substances like urea and ammonium carbonate are even easier to handle. Drawbacks of this approach are that residual traces of ammonia could affect the performance of some fuel cells, and that in all of these scenarios, energy has to be invested first to liberate the hydrogen, meaning the fuel cell cannot start under its own power. Therefore, Tao and colleagues argue that it would


be preferable to use ammonia directly as the fuel in new types of fuel cells specifically developed for this purpose. The best approach would be solid oxide fuel cells. Nitric oxide (NO) can be formed as an unwanted byproduct under some conditions, but with the right set of parameters this can be reduced to zero. Remaining problems include the limited durability of the cells and the high operating temperatures, but progress in R&D is improving the prospects steadily. Another approach highlighted by Tao is the


development of fuel cells directly fuelled by urea. This has the advantage that urea solution, known as AdBlue, is already widely distributed from service stations, as it is required for the NOx


reduction in catalytic converters for


diesel vehicles. Thus, the fuel would be easy to distribute, but the fuel cell is still in development.11 In the end, to obtain a technology viable for mass use


in transport, a safe fuel and a robust fuel cell technology with low operating temperature and short start-up time, will have to come together. Drivers could in the future fill up their vehicles with ammonia, urea or ammonium carbonate produced from renewable energies, but more research is needed to find out which combination could be the winning team that could beat the dominating fossil fuels.


on a support made of praseodymium oxide. This setup works at atmospheric pressure but still requires temperatures of ca300°C. Its other drawback is the reliance on the relatively rare and expensive lanthanide element praseodymium, for which the authors are currently searching for a more readily accessible replacement.7 In another separate effort


reported earlier in 2017, the group of Yasushi Sekine at Waseda University, Tokyo, used a static electric field in combination with ruthenium/caesium catalysts, and achieved ammonia synthesis at ambient temperature and pressure.8


The production


rate is modest, so far, with just half a gram ammonia produced per gram of catalyst per hour, but the authors anticipate the method could serve in small-scale facilities to produce ammonia on demand. While most of these


efforts are based on electron donation and transition metal catalysis, Masakazu Iwamoto and colleagues at Chuo University in Tokyo, Japan, investigated the


discoveries, I find it hard to imagine that these improvements will be able to replace the current large-scale and fully optimised technology,’ he says. ‘In the fertiliser area, novel technology will at best become a niche market for very special situations. Also, the CO2


footprint is


Ruthenium catalysis is important in the search for more efficient methods of ammonia synthesis. One approach is to apply ‘super promoters’ to provide electrons that destabilise nitrogen by weakening the triple bond and making the molecule more reactive.


150bar


hardly diminished.’ In the long term, Niemantsverdriet has hope for the ammonia economy as championed by Tao and others, providing carbon-free hydrogen from renewable energies. ‘I strongly believe that there will be scope for large industrial parks where this technology can be cleverly integrated with gasification of coal in China, and perhaps biomass elsewhere, which both need O2


, a


New ammonia synthesis plants use novel iron catalysts and operate at lower pressures than in the past. Traditional methods of ammonia synthesis require temperatures of ca500°C and pressures of 200bar.


use of a non-thermal atmospheric- pressure plasma of nitrogen and hydrogen. In a non-thermal plasma, the free electrons display a much higher temperature than the matrix of neutral particles and ions, which can be at room temperature. In their initial report in 2016, the researchers used copper wool to produce the plasma and found that it acts as a catalyst.9


More recently, they have


shown that wool-like electrodes made of gold, palladium or silver also yield promising results.10 Hans Niemantsverdriet, director


of SynCat@Beijing in China, acknowledges the rapid progress being made, but also strikes a note of caution. ‘In spite of interesting


References 1 R. Lan et al., Sci. Reports, 2013, 3, 1145. 2 R. Lan et al., Int. J. Hydr. Energy, 2012, 37, 1482 3 M. Kitano et al., Nature Chem., 2012, 4, 934. 4 Y. Horiuchi et al., Chem. Lett., 2013, 42, 1282. 5 M. Hara, M. Kitano, H. Hosono, ACS Catal., 2017, 7, 2313.


very expensive gas if made from cryogenic separation, and production of clean fuels, ammonia and a range of chemicals,’ he says. ‘If dimensioned properly, this has the potential to reduce the carbon footprint in the future.’ As the world is


striving to reduce its carbon emissions, while maintaining food security for a growing population,


ammonia synthesis is a key process that will need rethinking or at least improving. At BASF, where the Haber Bosch


process was first implemented, experts say they are aware of the developments in Japan, but prefer to rely on improving the established process. The company points out that, in ammonia plants going online nowadays, improved iron catalysts and innovative process design mean that the pressure can be kept as low as 150bar.


Thus, while several new


approaches are in the race and may find their niches in the market, it is still too early to write off the Haber Bosch process.


6 Y. Inoue et al., ACS catal., 2016, 6, 7577. 7 K. Sato et al., Chem. Sci. 2017, 8, 674. 8 R. Manabe et al., Chem. Sci., 2017, 8, 5434. 9 K. Aihara et al., Chem. Commun., 2016, 52, 13560. 10 M. Iwamoto et al., ACS Catal., 2017, 7, 6924. 11 W. Xu et al., Energy Technol., 2016, 4, 1329.


38 10 | 2017


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52