The exhaust contains principally three primary pollutants, unburned or partially burned hydrocarbons (HCs), carbon monoxide (CO) and nitrogen oxides (NOx), mostly NO, in addition to other compounds such as water, hydrogen, nitrogen, oxygen, etc. Sulphur oxides, though polluting, are normally not removed by the post-combustion treatments, since the only effective way is to reduce them to elemental sulphur, which would accumulate in the system. Accordingly, it is preferred to minimise sulphur emissions by diminishing the sulphur content in the fuel.
J. Kašpar et al, Automotive catalytic converters: current status and some perspectives, Catal. Today, 2003, 77, 419, doi:10.1016/S0920-5861(02)00384-X
For a chemist who likes cars the tailpipe is the best place to be. One day we will fill our clothes irons with water from our fuel-cell powered cars (and how soon it happens it partly depends on white coats like me…what a weighty responsibility!); for the present, the chemistry of catalytic converters is already interesting enough. So, let us take inspiration from the recent news and have a (very) quick look at it. Luckily, days in the laboratory are dotted with time-outs between experiments, which gives me a handful of snippets of time that I can devote to writing.
Recently, there has been extensive media coverage of a test-rigging scandal involving a major German car manufacturer, which has admitted to equipping its Diesel cars in the US and Europe with an illegal software to cheat during emission tests. As a car enthusiast since the ripe age of 3 (ask my parents), I have often read the road tests published in car magazines, regularly reporting a higher fuel consumption than that advertised by manufacturers in their sleek brochures. However, this is not particularly surprising, because consumption (and emission) tests are defined by standard procedures based on stationary measurements performed on a test rig. Yet, this time the car manufacturer seems to have stepped up its game: after all, when competition is cut-throat, such as in the automotive market, cutting corners is the only way to up the ante and survive, right? I have grasped a rough idea of how the system actually worked by listening to a science programme on the Italian public radio, and so I warn you that what follows is not an exhaustive (sorry for the silly pun) explanation: official enquiries have been launched to uncover the details. In a nutshell, the software running on the electronic control unit featured, along with several ‘legal’ operating modes (those options with catchy names like “sports”, “eco”, “city”) a low-nitrogen oxides (NOx) routine which activated when only two wheels were revolving (exactly as on the test rig). Under these conditions, the emissions of NOx dramatically decrease, falling below the pass threshold of the test. However, low emissions mean higher fuel consumption, and so this operating mode would never be practical while driving on the road, because low consumption is the main asset of a car, at least from the point of view of parsimonious buyers. At any rate, the whole affair will at the very least sap the consumer’s confidence in this car make, and in this case it is going to be a headlong crash from a high reputation. Cars, like friends, come in two different categories: those you go out with just to have fun, and those who you can always trust and rely on regardless of what life throws at you: German cars definitely (used to?) belong to the latter.
As a chemist, it is somewhat flattering to think that nowadays all motor vehicles are chemical reactors on four wheels, equipped as they are with catalytic converters. First of all, mind your words: catalysis is a learned combination from Ancient Greek, meaning ‘dissolution’, among other things1, and when it comes to cars we will be talking about heterogeneous catalysis, because the characters of this play belong to different phases of matter. For hetero-, have a look at a previous post on heterolysis, a word which, yes, recycles once more that Greek root that chemists like so much, λύω, ‘I release, I set free, I unharness’. Maybe chemists unconsciously think of their discipline as a means to ‘unbind’ a Promethean power which lay locked away. A dangerous idea. More practically, when referring to things λύω means ‘I break up something into its component parts; I resolve’, which would be analysis in Lavoisier’s terms, one of the two poles of chemistry along with synthesis. Anyway, as Gerhard Ertl reminded in his 2007 Nobel Lecture, Jöns Jacob Berzelius was the first to use of the word to describe a chemical phenomenon in 1835; another ‘first’ for this pole star of chemistry rising from the north to shine over the discipline in the first decades of the 19th century, as we saw elsewhere.
For the casual (and brave!) non-scientist who might have landed on this blog by chance, catalysis is the chemists’ equivalent of the fast-forward button in old video players: a videotape has its own normal speed at which we can play it, but what if we cannot afford to wait forever and we really want to watch that particular scene of our favourite film? In this case, catalysis fast-forwards the chemical tape taking us there where we want to go, and fast. In other words, there are reactions that just will not happen unless a fast-forward agent, a catalyst, is introduced in the system, and the catalyst’s secret is its ability to engage in a special relationship with the molecules taking part in the reaction. To visualise this “physical touch”, remember that the video recorder controls the playing speed with a spindle that plugs into a socket on the videocassette, but this connection is by no means permanent, because the cassette can be ejected when we wish to. So, a catalyst must somehow bind the reagents, but unlike diamonds (while sadly similar to most romantic relationships), this bond does not last forever.
As for the type of catalysis operative in catalytic converters, heterogeneous catalysis as we said above, there are three main ingredients:
- a solid support coated with the catalyst (think of gilding a piece of wood or baser metal)
- gases emitted by the engine during combustion
- energy, usually in the form of thermal energy (say: heat)
Imagine an unusual ball game: there is a wall covered with the hooky fabric of touch fastener (“Velcro”), and two players with two different sets of tennis balls, one with the same hooky fabric, the other with a (more tennis-like) hairy fabric. The players start throwing their tennis balls, aiming to bind two balls of different sets. The normal tennis balls will stick to the wall, while the “hooky” counterparts will bounce off unless they hit a hairy ball in the right way and bind to it. When a ball-ball couple (a ‘dimer’) is formed, it is either heavy enough to fall off or light enough to keep hanging onto the wall, which means that we need to go and pull it away. As you can see, the players spend some sort of energy at least once (throwing the ball), and maybe a second time (separating the ball-ball couple from the wall).
Is this just a far-fetched metaphor? Maybe, but Prof. Ertl introduces automotive catalysts in his Nobel Lecture by talking about the following reaction
2CO + O2 → 2CO2
CO and O2 bang against the catalyst (Pt or Pd), CO sticks, O2 hits hard enough for it to fall apart into two O atoms, one of which can bind to CO if it finds some of it in the surroundings, and CO2 falls off as soon as it is formed. A harmful combustion product, CO, is thus transformed into CO2, which can be regarded as harmless. In this respect, a greenhouse gas can be seen as the lesser evil, but this turns up global warming another tiny notch. Bottom line: use the bike instead if you want to go green.
A burning issue
Let’s start from the basics, which reminds me of my undergraduate course in environmental and atmospheric chemistry. It feels such a long time ago. It was the autumn term of 2004 and I was looking forward to exploring this branch of chemistry, and learning how Crutzen, Sherwood Rowland and Molina had won their Nobel Prize in 1995 by unravelling the chemical underpinnings of ozone depletion in the stratosphere … unfortunately the course turned out to be the most boring ever: the lecturer delivering the classes did his best to confuse the audience, and so we hoped that the textbook would come to our rescue. To no avail: the book itself, seemingly typed on an old typewriter, was a dry collection of reaction cycles to be learned by rote. I remember long hours spent scribbling the reaction pathways of atmospheric chemistry on a small blackboard. Funnily, the OH radical rampaging all around, reacting with this and that molecule, is one of the few concepts that has stuck into my memory: for me it was, and is, a poignant metaphor of the ultimate embodiment of life’s wear and tear. After all, “free radicals” purportedly play a key role in ageing.
Memories aside, if one wants to have a rough idea of automotive catalysts it is good to point out a few concepts to start with. Car engines burn fuel to extract energy from it; combustion is a combination of three actors (the good old “triangle”), fuel, oxidant and “heat”, or any suitable source of energy (the spark in petrol engines, for example). Combustion is just yet another chemical reaction, and like all of them, it is wise to weigh out the reactants and make sure we can control their relative proportions. That’s stoichiometry, which reads as the “measure of the elements” 2. The relative proportion of reactants which satisfies the stoichiometry of the reaction can be named stoichiometric ratio. For a simple combustion like:
CH4 + O2 → CO2 + 2H2O
the stoichiometric ratio between methane and oxygen is one molecule to one molecule. In other words, if there is a molecule of oxygen available for every molecule of methane, all fuel is expected to become CO2, while no O2 is left at the end of the reaction. (This in a real world where there are no practical issues with the actual combustion). Put too much O2, and you will end up with some of it in the combustion exhausts; on the other hand, if you are economical with O2, some CH4 will survive the combustion unscathed, or burn to incomplete stage, CO, that requires half as much oxygen:
2CH4 + O2 → 2CO + 2H2O
Similarly, one can define stoichiometric ratios for internal combustion engines. The exact values of these ratios will of course depend on the type of fuel being burned: petrol (gasoline across the Atlantic, and poetic, ephemeral essence in Francophone lands) or Diesel fuel, but the difference between the two types of fuel is not massive. A good idea is to define the air/fuel ratio with respect to the stoichiometric optimal, and call it λ: if λ > 1, there is more air than required, which is called lean mixture in car jargon, if λ < 1 there is more fuel than determined by stoichiometry, leading to a rich mixture. The parameter is not carved in stone, it keeps changing as we drive around. Roughly speaking, if we are cruising in our petrol-powered car on a motorway, λ should be slightly higher than 1, while as we want to go flat out and we put our foot down during hard acceleration λ will be less than 1 to achieve maximum power.
If we now have a look at the combustion products, for two typical engines (Table 1 in a specialised review3, here reported in a modified and shortened version):
|Exhaust components and conditions
|Unburned hydrocarbons, HC
||50–330 ppm C
||500–5000 ppm C
The exhaust gases contain several nasty fellows, for example CO and NOx; the catalytic converter needs to remove them, which is quite challenging considering that exhausts look like a hotchpotch. The following plot (reproduced here by myself with low-tech tools but originally appeared as a figure in a specialised review3) shows how typical pollutants vary with respect to air/fuel ratios:
The classical example of an automotive catalytic converter is the so-called ‘three-way’ catalyst for petrol engines, which is a fascinating, ingenious device. Its three tasks are:
- convert NOx into N2 (mostly done by rhodium)
- convert CO into CO2 (mostly done by platinum and palladium)
- convert unburned fuel hydrocarbons into CO2 (mostly done by platinum and palladium)
Task 1, the conversion to N2 is in chemical terms a reduction, which can be accomplished with the proper catalyst and a reactant that could act as the reducing agent. The simplest example is H2,which steals the oxygen becoming H2O in turn. As for the actual mixture of exhaust gas leaving the combustion chamber of the engine, one has to make do with the potential reducing agents already present in this exhaust mixture. Possible candidates are CO and hydrocarbons that have escaped complete combustion:
2NO + 2CO → 2CO2 + N2
NO + HC → CO2 + N2 + H2O (not balanced)
Going back to our image of balls hooking onto a Velcro wall, NO sticks onto the catalyst, and two NO molecules nearby will engage in what they think to be a fleeting liaison. Yet, this encounter will change them forever, as they shed their useless, acidic partner oxygen to make perfectly homogeneous pair, and they fly away together, looking forward an eternal, ethereal life as N2. Oxygen is left onto the catalyst, effectively taking up space for other NO molecules to stick, split and click. Individual oxygen atoms yearn for new partners as well, and they are ready for a getaway with any oxygen-loving molecule which happens to fly past. CO is a familiar match, small and thin, similar as it is to NO in many respects; partially burned hydrocarbons are instead burly blokes with charcoal on their faces, who look forward to a life-changing metamorphosis into CO2 by abducting as many stranded oxygens as possible. In a sense, a catalyst surface is the dance floor where couples meet, flirt and swap. Is this image, seemingly taken straight from Zygmunt Bauman’s essay Liquid Love, out of place on a scientific blog? Not really…lysis (λύσις) also means ‘divorce’ in Ancient Greek.
Promiscuity aside, reactions in task 1 help to remove CO and HC, which the pollutants that are to be tackled by tasks 2 and 3, showing a synergy in catalytic conversion. However, let us also note that tasks 2 and 3 involve oxidations: antipodal reactions with respect to the NOx reduction in task 1.
CO + O2 → CO2
HC + O2 → CO2 + H2O (not balanced)
Oxidation reactions like these ideally revel in the presence of extra oxygen. So, now we understand that a catalytic converter treads a narrow line in terms of contrasting demands, and the optimisation of its three tasks at the same time is a tall order achieved by continuous fine-tuning. It is the electronic control unit that calls the shots, finding a delicate trade-off is found. Another simple plot (again reproduced here by myself with chalk and blackboard but originally appeared as a figure in a specialised review3) will show how the efficiency of pollutant removal can plummet quickly as λ moves away from 1.
Managing oxygen is tricky also at a microscopic level, and for this reason catalytic converters also include oxides, for example CeO2 or ZrO2, which have several beneficial effects apart from physically “supporting” the catalyst; in particular, oxides can store extra oxygen when there is plenty, stockpiling a back-up supply of oxygen to keep running tasks 2 and 3 when, for example, a sudden surge in HC floods the catalytic converter as the driver goes at full throttle.
Diesel engines pose different challenges. The air/fuel ratio is higher than petrol engines, which makes it relatively straightforward to burn off CO and hydrocarbons to CO2 (functions 2 and 3 of the three-way catalyst) with a so-called Diesel Oxidation Catalyst (DOC), usually platinum on aluminium oxide (alumina). However, the DOC cannot remove the particulate matter4 produced by Diesel engines. On the other hand, more oxygen means a paucity of CO and unburned hydrocarbons in the exhaust and ready to be used to reduce NOx. This makes a traditional three-way catalyst ineffective for NOx abatement. To make matters worse, lots of oxygen means a very oxidising enviroment and, consequently, more NOx – which, as I narrated in more poetical terms in my previous post on the nitrogen cycle, forms whenever sparks fly in air.
Particulate matter and NOx have to be tackled simultaneously. There is the rub. The following excerpt from a 2011 specialised review sounds like a “writing on the wall”, and its take-home message (valid at the time of publication, but most likely still true today) is: when it comes to reducing particulate and NOx emissions of Diesel engines, striking a balance between the two is very challenging: “The performances of commercial catalytic post-treatment systems are not optimized to fulfill the forthcoming U.S.standard legislation and those that will be implemented in Europe near 2014, particularly the low limit of NOx emissions from diesel engines. The lean-burn engine is actually the most attractive solution combining low consumption and low CO2 emission, recognized for its greenhouse gas behavior. There are also apparent advantages in Europe that might explain a continuous expansion of the diesel car market related to the implementation of a favorable tax system. However, the suitability of this technology, from an environmental point of view implies the minimization of atmospheric pollutants, particularly nitric oxide emissions, which actually represent a serious drawback with no practical solution commercially available. Hence, while CO and unburned hydrocarbons can be easily removed, the simultaneous abatement of NOx and particulates from diesel exhaust gas represents an outstanding issue. The current three-way technology used near stoichiometric conditions is unable to meet upcoming regulations in Europe, United States, and Japan. The existing technical solutions […] involving an exhaust gas recirculation to get an optimal NOx/particulates compromise by controlling the recirculated gas rate or modifying the distribution channel will likely be unable to fulfill the next Euro 6 standard regulation. […] the implementation of an optimal strategy is not an easy task because a reduction of NOx induces an increase in particulate emission […] and reversibly subsequent reduction of particulate matter will induce an increase in NOx emission […]” 5.
What a catch-22…
But I feel I am beating about the bush as I promised to take inspiration from the test-rigging affair to talk about catalysis and nitrogen oxides. Here we step into my own turf, and you can read more in a previous post on the nitrogen cycle.
There are various possible strategies to reduce the amount of NOx emitted by a Diesel engine:
- Special NOx removal catalysts able to tolerate the excess oxygen in the exhausts
- NOx “traps”, which exploit acid/base chemistry, using oxides or carbonates such as BaCO3 as alkaline counterparts to immobilise acidic NOx as nitrate, which is released and reduced as the engine briefly operates with a rich mixture.
- Selective Catalytic Reduction, my favourite
Selective catalytic reduction is a reincarnation of a class of reactions called comproportionations, which involve two reactants containing the same element but in two different oxidation states. Most people will remember comproportionations from general chemistry courses as outlandish chimeras standing out from the crowd of ‘normal’ reactions. As for the nitrogen cycle, think of the violent comproportionation that accounts for the explosive decomposition of ammonium nitrate. This salt is intrinsically unstable right because, in its two moieties, it combines the extreme oxidation states of nitrogen, nitrate (+5) and ammonium (−3), which liberates a powerful entropic ‘kick’ by releasing a large amount of gases in a veritable free-for-all which ends in both nitrogen atoms being triply married to each other in a multiple wedlock.
2NH4NO3→ 4H2O+ O2+ 2N2
Selective catalytic reductions are the milder (and more useful) version of this explosion. Developed for stationary sources of NOx (say, fossil-fuel power plants), they require a suitable catalyst and a nitrogen-containing molecule willing to take the lift from a lower oxidation state to the stability of ground floor. Why not ammonia, I hear you say, and so let us take ammonia on board our Diesel cars…but how? Ammonia could for instance attack and corrode the walls of a reservoir, and spill out while we are driving around in our eco-friendly (but leaky, as my favourite burette) cars.
Yet, storing ammonia is just one of the many challenges posed by automotive SCR. Running the reaction at top conversion is another tall order. When looking at the catalyst, the image of the Velcro wall and hairy tennis balls can be conjured up once more. In fact, ammonia sticks very well onto the surface of these catalysts as long as the temperature is not too high (below 473 K, as reported in a topical review); when the exhaust heats up, for example when a heavy lorry goes at full throttle, ammonia can become loose again and do business. It is exactly under these hotter conditions that SCR can occur. Implementing SCR is by no means an easy task, as it requires continuous monitoring of engine and exhaust parameters to avoid, for instance, injecting an excessive amount of ammonia that would end up being released at the tailpipe. Stoichiometry shows up again, and a NH3/NOx ratio less than 1 is often preferred for operations (after all, a short foray into rich-burn conditions could provide extra reducing agents to remove NOx leftovers). The catalyst, once more a thin layer ‘gilded’ on top of a suitable support, is vanadia (V2O5), which is a crowded nightclub where molecules flirt and hook onto each other in all possible ways 5. There is some true SCR going on, like
4NH3 + 4NO +O2 → 6H2O + 4N2
8NH3+ 6NO2 → 7N2 + 12H2O
But the latter could turn into an acid-base neutralisation leading to an old acquaintance of ours, ammonium nitrate:
2NH3+ 2NO2 →N2 + NH4NO3 + H2O
And, last but not least, there is enough oxygen for ammonia to be oxidized on its own:
4NH3 + 3O2 → 2N2 + 6H2O
Vanadium is the DJ playing the tunes for this wild night out, cycling his records between an oxidised form (V=O) that is catalytically active and a reduced one (V-OH) that consumes oxygen to bounce back to action. A complete reaction equation would then be5
NO + NH3 + V(+5)=O → N2 + H2O + V(+4)-OH
Instead of storing a tankful of ammonia, someone had a brilliant idea that would make Friedrich Wöhler revel in his grave: take urea, instead, and hydrolyse it on board when its needed. Controlling the amount of urea, hence ammonia, injected in the exhaust would also offer another degree of freedom to adjust SCR to engine performance in real time. A concentrated urea solution is vaporised prior to the actual SCR catalyst, and Wöhler’s most beloved molecule first breaks up into ammonia and isocyanic acid5
NH2CONH2 → HNCO (gas) + NH3
In the best-case scenario, the latter acid gives off a second ammonia molecule and CO2 in a following step:
HNCO (gas) + H2O → NH3 + CO2
Things could in principle also go awry: isocyanic acid could start one of those funny and silly versions of the ‘conga line’ in which (tipsy) people line up one after the other in a moving ‘train’. This polymerization could for example lead to melamine (yes, the one that ended up in babies’ powdered milk some time ago in China), which is not great news because it could clog the catalyst (think of no space anymore for single club-goers to move on the dance floor when the snake is around). Luckily, this will not happen if a catalyst for urea hydrolysis and isocyanic acid decomposition is chosen, for example TiO2.
To sum up, the sequence of treatment stages tackling Diesel exhausts is:
- Diesel Oxidation Catalyst
- Urea injection and hydrolysis
- Selective Catalytic Reduction
- Diesel Particulate Filter
Fed up with noxious car exhausts? Too bad, you deserve some stinky sewage then, but I promise, it will be just a quick dive.
The shortcut to heaven
Miracles happen, or at least sometimes those who seem to be misfits do become famous, eventually end up in the spotlight. This is probably the case for some bacteria, which have had their fair share of fame, rising from complete obscurity to gain a certain notoriety5 in 1999, while taking researchers by surprise. Their metabolism features a reaction called anammox (anaerobic ammonium oxidation), which plays an important role in removing nitrogen species from the oceans:
NH4+ + NO2− → N2 + 2H2O.
This time, no explosive metaphor is exaggerate: these bacteria succeed in handling an anion and a cation which, as a salt, few people would like to work with for fear of explosions. Like desperate lovers imprisoned in nearby cells, nitrite and ammonium will pull all the stops to break free, flee and fly away as N2, tearing down all walls in the process. When swallowed by these bacteria, their separation ends quietly without dramatic escapes. The story of the discovery of these bacteria is fascinating and deserves a post of its own. I happen to know it because I read some of the literature when I was working for my PhD research project at Leiden University. It was my supervisor the one to point out that other Dutch researcher, part of an international team, had identified the elusive bacteria responsible for anammox reaction6. This reaction is exploited in a patented bacterial process (having the same name as the reaction) for the treatment of ammonium-rich sewage or wastewater.
It was to my great surprise that, while I was doing research as a PhD student, we observed that the same recombination between nitrite and ammonia occurs under certain conditions on some surfaces of Pt catalysts when an electrochemical potential is applied7. This time, the catalytic Velcro wall must be of a very special shape for the molecules to stick: the most external Pt atoms must be arranged as if they were located on the corners of a square, and this for the entire surface. No matter how unrelated electrochemistry is to microbiology, the overall reaction
NH3 + NO2− → N2 + H2O + OH−
takes place, and, for the most curious among you, I will also mention that the ‘stuck’ (e.g. adsorbed) hairy tennis balls seem to be in this case NO and NHx. The nitrogen cycle perhaps has another surprise in store: a research group based in Québec has proposed that the very same intermediates are involved in the oxidation of NH3 to N2 on the atomically square-shaped Pt surface, and once more performed by help of an electrochemical potential8, but conceptually equivalent to the oxidation of NH3 to N2 by O2 as seen previously in the discussion of the SCR process. Is this really an universal process common to a special Pt surface, ‘anammox’ bacteria and catalysts for NOx removal from Diesel exhausts? Only time will tell, but at the moment it really seems so.
In conclusion, whatever happens in my future career as a scientist, I can already look back and feel somewhat proud, because my own investigation of the nitrogen cycle has followed Roald Hoffmann’s guiding philosophy for chemistry research: “look at hundreds of small(er) problems in chemistry, and keep in mind the connections that must be there” and you “will see the chemical universe” 9. A series of experiments part of a PhD project in electrocatalysis started unravelling a skein of yarn, pulling a thin Arianna’s thread that would lead to the world of SCR for Diesel exhausts and some puzzling bacteria. Eventually, the yarn has been rolled into a ball, and a single line connects all the dots in an elegant common pattern.
A star-studded sky waiting for curious eyes to recognise and trace more new constellations: such is the chemical universe.
Just look above you, and follow the exhausts.
- The verb καταλύω encompasses a range of meanings which look quite nasty, such as (in the order reported by this online dictionary) “I destroy”, “I dissolve” (of a political system), “I bring to an end” (of life as well). The name κατάλῠσις follows suit, meaning “dissolution” (of a political system), “disbanding” (a crew, a group of men). If I miss any meaning, forgive me: I used to know someone who had studied Ancient Greek but we are not in touch anymore.
- Jeremias Richter (1762-1807), starting from a Kantian perspective and following the “dynamist” approach to chemistry, tried to quantify properties such as acidity and basicity. Stoichiometry was developed with a view to establishing a “neutralisation” law for the formation of salts (see Chemistry, the Impure Science, Bernadette Bensaude-Vincent and Jonathan Simon, Imperial College Press, 2012).
- J. Kašpar et al., Automotive catalytic converters: current status and some perspectives, Catal. Today, 2003, 77, 419, doi:10.1016/S0920-5861(02)00384-X
- Defined in J. Kašpar et al., Automotive catalytic converters: current status and some perspectives, as “the most complex of diesel emissions. Diesel particulates, as defined by most emission standards, are sampled from diluted and cooled exhaust gases. This definition includes both solids, as well as liquid material which condenses during the dilution process. The basic fractions of DPM are elemental carbon, heavy HCs derived from the fuel and lubricating oil, and hydrated sulphuric acid derived from the fuel sulphur. DPM contains a large portion of the polynuclear aromatic hydrocarbons (PAH) found in diesel exhaust. Diesel particulates contains (sic) small nuclei with diameters below 0.04 μm, which agglomerate forming particles as large as 1 μm. The non-gaseous diesel emissions are grouped into three categories: soluble organic fraction (SOF), sulphate and soot“
- P. Granger and V. I. Parvulescu, Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies, Chem. Rev., 2011, 111, 3155–, doi: 10.1021/cr100168g
- M. Strous et al., Missing lithotroph identified as new planctomycete, Nature, 1999, 400, 446-, doi:10.1038/22749
- M. Duca et al., Selective catalytic reduction at quasi-perfect Pt(100) domains: a universal low-temperature pathway from nitrite to N2, J. Am. Chem. Soc., 2011, 133, 10928-, doi: 10.1021/ja203234v
- D.A. Finkelstein et al., Mechanistic Similarity in Catalytic N2 Production from NH3 and NO2– at Pt(100) Thin Films: Toward a Universal Catalytic Pathway for Simple N-Containing Species, and Its Application to in Situ Removal of NH3 Poisons, J. Phys. Chem. C, 2015, 118, 9860-, doi: 10.1021/acs.jpcc.5b00949
- As reported in Q & A Roald Hoffmann: Chemical connector, Nature, 2011, 480, 176, doi:10.1038/480179a