Dear Professor Gräslund, ladies and gentlemen, it’s a great honour for me to be the first speaker in this outstanding symposium.
100 years ago not only Count Lennart Bernadotte was born but also the Nobel Prize in chemistry was awarded to Wilhelm Ostwald,
the founder of physical chemistry and he got the prize for his discovery of the principle of catalysis.
Catalysis was introduced in 1835 and throughout the 19th century there was a heavy debate about the nature of catalysis.
Ostwald found that catalysis has something to do with the kinetics of a chemical reaction.
A catalyst is a substance which affects the velocity of chemical reactions without being part of the final products.
And what Ostwald had in mind was essentially heterogeneous catalysis
where the catalyst is a solid body interacting with particles from the gas phase and in this way a new reaction path is offered.
Particles from the gas phase are absorbed, bonded to surfaces because the surface atoms have free valences
and in this way particles moving across the surface may react with each other and leave back into the gas phase.
And the sequence of all these reaction steps offers a path with a higher reaction rate than without the catalyst.
Catalysis can also be done with substances in the same phase and it’s called homogeneous catalysis
and throughout the symposium there will be more lectures about homogeneous catalysis
but here we will talk about the processes taking place at solid surfaces.
Chemistry is faced with challenges nowadays concerning energy, climate or environment
and for all these challenges heterogeneous catalysis will be used to find solutions.
100 years ago it was another challenge, the exhaust of the natural supply of nitrogen fertiliser
was a serious danger for the whole of mankind.
And there was a demand for finding a process in which nitrogen can be fixated.
Nitrogen molecules split up into another component from where reaction can take place.
And this challenge was solved by Fritz Haber who got the Nobel Prize in 1918.
And again 100 years ago with this apparatus Haber was able to form ammonia from nitrogen and hydrogen.
It was exactly on 2nd of July of 1909, just about 100 years ago.
And immediately after this discovery which was observed also by people from BASF,
the great industrial chemist Carl Bosch took over the task to transform this laboratory scale process
into a large scale industrial production of the Haber-Bosch process for industrial production of ammonia.
And this is one of the modern plants where ammonia is fabricated.
And if we look at the development of the world population and the production of ammonia
it’s quite clear that our world would look quite different if it would not be possible to form ammonia in this way,
nitrogen fertilisers.
An estimate says about one third of the world population would starve without this reaction.
The first plant started operation at the BASF Company in 1913.
Nevertheless it took about 70 years to find out what is the real mechanism of this reaction.
This was because of the surface chemistry involved in this process which is rather complicated.
And if we look at the catalyst which was developed at the BASF Company by Mittasch, it becomes clear why this is so complicated.
This is an electron micrograph from the ammonia catalyst, 100 nanometres.
These parts here, these particles, these are the catalytically active scenes on the surface.
The composition of the catalyst, bulk composition is essentially every 3 or 4 magnitude
but under reaction conditions these particles become reduced and the catalytically active spots
are essentially metallic iron covered with a thin mono layer, sub mono layer of potassium plus oxygen.
The reaction takes place on the iron surfaces but potassium and oxygen act as a so called electronic promoter
to facilitate the rate limiting step of nitrogen dissociation.
While the other promoters, aluminum and calcium form a framework which prevents small iron particles from cindering,
they are structure promoters.
So it looks rather complicated and that’s why it took quite some time to find out what is the mechanism of this reaction.
This is summarised here, we start from nitrogen and hydrogen and form ammonia.
This is the overall reaction enthalpy.
And without the catalyst we would have to first dissociate the nitrogen molecule and the hydrogen molecules,
this would require a very large amount of energy.
Once we would have the atoms they would recombine step wise but this energy is prohibitive.
And this is why the catalyst offers an alternative reaction path
by absorbing the nitrogen molecule as a molecule from where it dissociates to atoms
and this is a rate limiting step of the overall reaction.
Hydrogen is also dissociating and then we are on the bottom of this energy well here
and then step wise recombination of nitrogen and hydrogen takes place to NH, NH2, NH3 which finally desorb.
This whole sequence of reaction steps marks the overall reaction mechanism.
It looks complicated, that’s why I will omit further discussion of this mechanism and now go to a much simpler reaction
which is also of quite some importance for environment, that’s carbon monoxide oxidation in your car exhaust catalyst.
The car exhaust contains toxic gasses like nitric oxides, unburned hydro carbons and carbon monoxide
and the exhaust is flowing through this ceramic wash coat covered with very small particles of platinum metals.
And these small particles expose their crystal planes to the molecules arriving from the gas phase.
Carbon monoxide is absorbed through the carbon atom on the surface,
oxygen is dissociatively absorbed and then the 2 recombine on the surface to CO2 which goes back into the gas phase.
The elucidation of these reaction steps is complicated by the fact that this catalyst expose different crystal planes,
they are usually very small particles and this shows an electron micrograph of such a catalytic active substance,
small ruthenium particles.
And an oxide support.
And this is a high resolution image showing the individual atoms of the ruthenium particles and even crystal defects.
And if you make a model with these small particles, expose different crystal planes here and here and defect zones
and all these different configurations of course will have different reactivities.
And that’s why the overall reactivity is a composition of all the different contributions
which makes the situation so complicated.
The solution to this problem is found if one uses just well defined single crystal surfaces
and looks at the individual steps separately.
Such a single crystal surface 111 for example, can be used to study the elementary processes.
And one of the decisive steps obviously is the dissociation of molecules like with the ammonia synthesis
and all this kind of carbon monoxide oxidation, the oxygen molecule has to dissociate.
And we can understand the mechanism of this dissociation by this picture here.
Illustrating the energy change when a diatomic molecule approaches a surface,
makes a weak bond to the surface whereby the bond between the 2 atoms is already weakened
so the formation of bonds to the surface.
And this diagram is a 2 dimensional energy plot of the energy as a function of the separation X of the molecule
from the surface and the separation Y between the 2 atoms.
So if the molecule approaches the surface we find a weak minimum for molecular absorption
by overcoming an activation barrier it can dissociate and the 2 atoms fly apart from each other.
This is a one dimensional diagram due to Lennard Jones.
Now, the secret of the catalyst now consists in the fact
that this barrier should be as low as possible to enable high probability for dissociative absorption.
But on the other hand the bond to the surface should be not too strong because these particles have to react further.
So it’s always a compromise between these energetic considerations which determine the choice of a suitable catalyst.
We can do such an experiment now with a single crystal surface, expose it to a small amount of oxygen molecules
and look at the surface with atomic resolution, with this scanning tunnelling microscope afterwards.
These are the platinum atoms of the top most layer and if we expose the surface to a small amount of oxygen at 165 Kelvin,
we find afterwards these pairs of new structures.
These bright dots surrounded by dark areas.
These are the oxygen atoms resulting from the dissociation of the oxygen molecule.
And there are several aspects that come into play, first of all the 2 atoms are never on neighbouring sides,
they’re always separated by 5 to 8 angstroms from each other.
That’s because the energy which is released during the dissociation process has to be transferred to the solid
and this takes some time.
And if we estimate this time from the separation and the energy which is released,
we get a time scale of about 3 times 10 to the minus 13 seconds, point 3 of a pico second.
That’s a typical time scale for the energy relaxation in the exchange of energy between molecules and surfaces.
And these processes can be studied separately by modern methods of femtosecond laser spectroscopy.
Secondly we see that these bright spots are surrounded by dark areas because in this working mode of the STM
not only is the topography but also the local electronic structure is probed.
Oxygen atoms are electro negative, so this draws an electronic charge from the substrate
and that’s why we have this depletion of electronic charge around the oxygen atom.
And as a consequence the local electronic structure of the substrate is affected,
giving rise to interactions between absorbed particles and the surface through the substrate.
Furthermore at 165 Kelvin these oxygen atoms are always located in 3 fold coordinated sites,
that’s the absorption site and they don’t move, they are fixed there.
If you go to higher temperature they start to move because now we can overcome the activation barrier for surface diffusion.
If we approach a particle, an atom to the surface the energy will change in such a way we gain the absorption energy
and the lifetime of a particle on the surface will be given by the absorption energy and the temperature.
If you move now a particle along the surface from one side to the other one,
the energy will vary periodically and the activation barrier is much smaller than the absorption energy.
The activation energy for surface diffusion and that’s why the residence time at a certain side is much smaller,
it jumps from one side to the other one and only after many, many jumps it desorbs back into the gas phase.
So if you go to higher temperatures, this lifetime will become shorter
and this is a snap shot now from oxygen atoms on a ruthenium surface at room temperature.
This snap shot was taken in less than a 10th of a second.
And the STM in this case is so fast that we can really follow the motion of the absorbed particle on the surface in real time.
And in this light Brownian motion in 2 dimensions, the atoms jump around across the surface randomly.
And if you look closer the mean residence time of such an atom at a certain site is about 60 milliseconds.
But as soon as 2 atoms come close to each other they stay together a little bit longer.
Because there’s a weak attractive interaction caused by this electronic interaction through the substrate.
And this has consequences, if you go to higher coverages
the distribution of the absorbed particles will no longer be random as in this case here
but because of these attractive interactions you’ll have a phase separation.
You have now 2 phases:
a 2-dimensional gas phase and a 2-dimensional solid phase
with long range order configuration of the absorbed atoms on the surface.
That’s a true equilibrium because we have continuous condensation and evaporation of the particles
from the gas phase to the solid.
We have the defects we know from 3 dimensional chemistry also here, the vacancies
and we have continuous nucleation and dissolution of the nuclei.
This long range order of the absorb particles is more the rule than the exception
and then we can also use diffraction techniques to determine the actual structure of the absorbed over layers.
And if you go back to our problem of carbon monoxide oxidation and look at the structures formed by oxygen and carbon monoxide
we see that the oxygen atoms prefer the 3 fold coordinated sites from a relatively open unit cell.
This is a saturation which can be reached at room temperature.
While the carbon monoxide is absorbed either on top or in bridge positions
like in carbon compounds and forms a relatively densely packed over layer.
And this has consequences, if we expose such a surface to carbon monoxide it can be absorbed inside the unit
and forms this mixed phase.
While on the other hand this over layer of carbon monoxide prevents absorption of oxygen.
And if you look at the mechanism of the reaction, this is depicted again here,
carbon monoxide forms a relatively densely packed layer and if its coverage exceeds a certain critical value,
oxygen can no longer absorb, carbon monoxide is a poison.
And that’s a reason why your car exhaust catalyst doesn’t work in the cold,
you have to go to higher temperatures in order to desorb part of the carbon monoxide
so that the oxygen can now absorb dissociatively and the CO then can be absorbed between the oxygen atoms.
And from this configuration CO2 can be formed.
This is the sequence of steps, carbon monoxide is absorbed, oxygen absorbs through the molecular state,
dissociatively and then oxygen and CO recombine to CO2, that’s the energetics.
CO and oxygen, that’s the overall reaction enthalpy.
Most of it is obtained by absorption of the particles.
If you have it both on the surface they can recombine by overcoming a relatively small activation barrier
and the CO2 is so weakly added that it’s immediately released into the gas phase.
So if we operate such a catalyst as shown before in a reactor which is packed with a catalyst
and we have a steady state flow of the reacting molecules, we have our partial pressures constant,
our temperature constant in a fixed catalyst, we will get a steady state reaction rate.
Everything will be constant, the outflow of product.
That’s the way in industry also catalytic reactors are operating.
Of course nature is full of other reactors and living systems behave in a different way.
We always admit continuous flow of free energy of food and air
and nevertheless the response of a living system is not a steady reaction rate,
living systems show structure, temporal structure and also spatial structure.
And we ask ourselves is there something different which we cannot detect with our inorganic systems.
The answer is: If we look closer, we find similar effects also with the inorganic systems, this simple reaction.
If we look again in our reaction, carbon monoxide and oxygen on a platinum 110 surface, as a structure of the surface
and we keep everything constant, temperature, CO pressure
and at this point we change the oxygen pressure from 2 to 2.7 times 10 to the minus 4 milibar,
we see that the reaction rate, formation of CO2 continuously increases and then starts to oscillate.
It oscillates in this large amplitude over hours.
So there is a temporal self organisation of our system despite the fact that we expose it to constant flow of reactants
at a constant temperature, the response is not constant.
And this is against intuition.
You have from thermo dynamic system always tries to reach the state of maximum entropy.
The point is that in this case it’s not a closed system, it’s an open system, an open system far from equilibrium.
And these systems can show effects of this kind.
You can find these effects in quite different fields.
One very famous example is shown here, the number of furs from hares and lynxes
delivered to the Hudson’s Bay Company as a function of time, it was not constant,
it’s periodic and whenever the hares have a high concentration they go down and the lynxes go up a little bit faster.
And then it goes up and down.
The explanation is quite straightforward, there is a reaction between hares and lynxes like with carbon monoxide and oxygen.
And if there is enough supply of hares the lynxes will increase their population,
they eat more and more so that the birth rate of the hares can no longer provide the support,
the hares start to die out and with some time delay also the lynxes don’t have any longer enough food,
so they will decrease their population and the hares can recover again.
It’s a very simple system from population dynamics which behaves very similar to our reaction.
And already in the 20’s mathematicians tried to explain this effect in terms of a set of differential equations.
In the language of chemical kinetics the concentration of hare changes this time proportional to their birth rate
minus the collision between hare and lynxes, this is a temptational state.
The concentration of lynxes changes with time proportional to the supply by hares and that’s a natural death rate.
This is a set of 2 coupled ordinary differential equations which are non-linear, they are non-linear here and here.
And if these equations are solved for certain parameters, alpha and beta,
the results, the solutions are indeed of this oscillatory type, the hares and the lynxes up and down.
So the secret is the non-linear corrector of these equations and that’s why this whole field is called non-linear dynamics.
This, of course, is not the whole explanation because here the equations, ordinary differential equations,
the variables X and Y only depend on time.
But if we have an extended surface where we have hares and lynxes there must be some communication,
some synchronisations, otherwise not the whole system would go up and down at the same time.
So you need some coupling between the variables across the surface.
And this gives rise to the general formulation of these problems.
Open systems far from equilibrium which gives rise to spatio-temporal self-organisation.
The lateral variation of the state variables or surface concentrations
can be obtained by coupling between different parts by surface diffusion or by heat conductance or other effects at all.
If you have diffusion then the equations are of this type, these are partial differential equations.
This is the kinetics and that’s a diffusion of the particles across a surface,
if you have a high concentration here and a low concentration there, then there will be diffusion.
And this coupling of reaction and diffusion gives rise to the spatio-temporal self-organisation.
One example, the heat conduction, this decisive is shown here,
it’s a very thin platinum foil, 200 nanometre thick over a 5 millimetre diameter hole, subject to oscillatory kinetics.
And if the reaction rate goes up then also the heat released will go up and the temperature will rise.
In this case by the periodic variation of the reaction rate, the temperature may change by up to 30 degrees.
This gives rise to a thermal expansion of the platinum and the thermal expansion will give rise to deformation.
And so if you can really follow the progress of the reaction by the deformation of this foil
and this is a periodic deformation where the propagation of these heat waves through heat conduction
is a synchronising mechanism, we call it heart beats.
Everything is known with this reaction, nevertheless the overall behaviour was dilatory type.
Under conditions where the temperature is kept constant, alpha thermal conditions, concentration patterns will appear.
And there’s a very famous example of this kind of concentration patterns in literature,
that’s Belousov-Zhabotinsky reaction taking place in a Petri dish where very complicated reaction with the different colours
just reflect the different concentration of the reacting species.
The length scale 5 centimetres.
It’s no longer atomic, it’s given by the coupling of reaction and diffusion.
Now going back to our surface reaction we have to use a technique
which is able to image the lateral distribution of the absorbed particles on the surface, oxygen and carbon monoxide.
And this photo image in electron microscopy which is based on the different dipole moments of the absorbed particles.
In this case dark areas are predominantly oxygen covered, bright areas are predominantly carbon monoxide covered.
And you see these are no longer circular waves like with the Belousov-Zhabotinsky reaction, they are elliptical.
Because this is the structure of the surface which is un-isotropic,
diffusion along this direction is faster than along this direction, this gives rise to this elliptic wave patterns.
And we can now also follow the propagation of these waves in real time with this microscope
and we see how these waves propagate
and the whole structure switches between predominantly carbon monoxide covered bright
and predominantly oxygen covered was dilatory.
These kind of target patterns are very common for these systems
and also an artist had the vision of this kind of target patterns,
this was Paul Klee where he just point out the changing character of this target patterns.
Even if the reaction rate is not oscillatory we can have partial patterns across the surface
and one of the most frequently absorbed patterns are spirals, spiral waves.
Again with this PEEM technique, 500 micrometer diameter, so again the length scale given by the diffusion coupling to reaction.
And if you look again at the propagating spiral waves we see how these spirals expand.
There are different wavelengths because a core of such a spiral is usually determined by the local defect structure.
They propagate with the speed of several microns per second while these dark areas are predominantly oxygen covered.
And here each site catalyses about 10 times per second the reaction to CO2.
So these are completely different time scales.
Coupling of reaction and diffusion to the spatial order and the reaction itself as this locally oxygen covered areas.
If spirals collide, you see characteristic patterns here emerging and spirals of course
are observed in very different regions, the largest object in nature,
the galaxies, are spirals, and also the weather very often gives rise to formation of spirals.
The oldest picture of a spiral I could find is found at the small isle of Malta.
It’s a stone which is more than 4,000 years old and it shows the spirals
and all these parts here which I have shown to you before, very strange.
With our reaction, it’s a very simple reaction between carbon monoxide and oxygen,
we know everything, we know the reaction rates, we know the diffusion coefficients.
So we can also write down the equations, the partial differential equations which are responsible for this pattern formation.
And now we can use a computer to solve these differential equations and look what kind of pattern formation the theory predicts.
We start with a random distribution of oxygen and carbon monoxide on the surface
and let the whole system run now on the computer and we see the formation of these kind of structures leading to the spirals.
It is very sensitively effected by the other parameters,
so if we change the parameters a little bit, we again get the formation of spirals which then break up and become disordered.
It’s then a state of turbulence or spatio temporal chaos.
So this is really complexity and of course this has nothing to do with atoms anymore, but it’s from atoms to complexity.
The whole is more than the sum of the constituents.
Vincent Van Gogh once wrote to his brother: "The great things are never made by pure impulse
but by bringing small particles, small things together to form a new body."
And his vision of the world is shown here with his spirals, we see the patterns, we see the turbulent state, everything is here.
With this I thank you very much.
(applause).