Part II Of Other Things 16. The Haste-Makers
When I first began writing about science for the general public-far back in medieval times-I coined a neat phrase about the activity of a "light-fingered magical catalyst. "
My editor stiffened as he came across that phrase, but not with admiration (as had been my modestly confident expectation). He turned on me severely and said, "Nothing in science is magical. It may be puzzling, mysterious, in expbeable-but it is never magical. "
It pained me, as you can well imagine, to have to learn a lesson from an editor, of all people, but the lesson seemed too good to miss and, with many a wry grimace, I learned
That left me, however, with the problem of describing the workings of a catalyst, without calling upon magical power for an explanation.
Thus, one of the first experiments conducted by any beginner in a high school chemistry laboratory is to pre pare oxygen by heating potassium chlorate. If it were only potassium chlorate he were heating, oxygen would be evolved but slowly and only at comparatively high temper atures. So he is instructed to add some manganese dioxide first. When he heats the mixture, oxygen comes off rapidly at comparatively low temperatures.
What does the manganese dioxide do? It contributes no oxygen. At the conclusion of the reaction it 'is all still there, unchanged. Its mere presence seems sufficient to hasten the evolution of oxygen. It is a haste-maker or, more properly, a catalyst.
And how can one explain influence by mere presence?
Is it a kind of molecular action at a distance, an extra sensory perception on the part of potassium chlorate that the influential aura of manganese dioxide is present? Is it telekinesis, a para-natural action at a distance on the part of the manganese dioxide? Is it, in short, magic?
Well, let's see. . .
To begin at the beginning, as I almost invariably do, the first and most famous catalyst in scientific history never existed.
The alchemists of old sought methods for turning base metals into gold. They failed, and so it seemed to them that some essential ingredient was missing in their recipes. The more imaginative among them conceived of a substance which, if added to the mixture they were heating (or what ever) would bring about the production of gold. A small quantity would suffice to produce a great deal of gold and it could be recovered and used again, no doubt.
No one had ever seen this substance but it was de scribed, for some reason, as a drv, earthy material. The ancient alchemists therefore called it xenon, from a Greek word meaning "dry. "
In the eighth century the Arabs took over alchemy and called this gold-making catalyst "the xerion" or, in Arabic, at-iksir. When West Europeans finally learned Arabic alchemy in the thirteenth century, at-iksir became "elixir. "
As a further tribute to its supposed dry, earthy prop erties, it was commonly called, in Europe, "the philos opber's stone. " (Remember that as late as 1800, a "natural philosopher" was what we would now call a "scientist. ")
The amazing elixir was bound to have other marvelous properties as well, and the notion arose that it was a cure for all diseases and might very well confer immortality.
Hence, alchemists began to speak of "the elixir of life. "
For centuries, the philosopher's stone and/or the elixir of life was searched for but not found. Then, when finally a catalyst was found, it brought about the formation not of lovely, shiny gold, but messy, dangerous sulfuric acid.
Wouldn't you know?
Before 1740, sulfuric acid was hard to prepare. In the* That's all right, though. Sulfuric acid may not be as costly as gold, but it is conservatively speaking-a trillion times as in trinsically useful.
ory, it was easy. You bum sulfur, combining it with oxygen to form sulfur dioxide (SO2)- You burn sulfur dioxide further to make sulfur trioxide (SO3)- You dissolve sulfur trioxide in water to make sulfuric acid, (H2SO4) - The trick, though, was to make sulfur dioxide combine with oxygen.
That could only be done slowly and with difficulty.
In the 1740s, however, an English sulfuric acid man ufacturer named Joshua Ward must have reasoned that saltpeter (potassium nitrate), though nonflammable itself, caused carbon and sulfur to burn with great avidity. (In fact, carbon plus sulfur plus saltpeter is gunpower. ) Con sequently, he added saltpeter to his burning sulfur and found that he now obtained sulfur tri'oxide without much trouble and could make sulfuric acid easily and cheaply.
The most wonderful thing about the process was that, at the end, the saltpeter was still present, unchanged. It could be used over and over again. Ward patented the process and the price of sulfuric acid dropped to 5 per cent of what it was before.
Magic? - Well, no.
In 1806, two French chemists, Charles Bernard Ddsormes and Nicholas C16ment, advanced an explanation that contained a principle which is accepted to this day.
It seems, you see, that when sulfur and saltpeter bum together, sulfur dioxide combines with a portion of the saltpeter molecule to form a complex. The oxygen of the saltpeter portion of the complex transfers to the sulfur dioxide portion, which now breaks away as sulfur tri oxide.
What's left (the saltpeter fragment minus oxygen) pro ceeds to pick up that missing oxygen, very readily, from the atmosphere. The saltpeter fragment, restored again, is ready to combine with an additional molecule of sulfur dioxide and pass along oxygen. It is the saltpeter's task simply to pass oxygen from air to sulfur dioxide as fast as it can. It is a middleman, and of course it remains un changed at the end of the reaction.
In fact, the wonder is not that a catalyst hastens a re action while remaining apparently unchanged, but that anyone should suspect even for a moment that anything "magical" is involved. If we were to come across the same phenomenon in the more ordinary affairs of life, we would certainly not make that mistake of assuming magic.
For instance, consider a half-finished brick wall and, five feet from it, a heap of bricks and some mortar. If that were all, then you would expect no change in the situation between 9 A. m. and 5 P. m. except that the mortar would dry out.
Suppose, however, that at 9 A. M. you observed one fac tor in addition-a man, in overalls, standing quietly be tween the wall and the heap of bricks with his hands empty. You observed matters again at 5 P. m. and the same man is standing there, his hands still empty. He has not changed. However, the brick wall is now completed and' the heap of bricks is gone.
The man clearly fulfills the role of catalyst. A reaction has taken place as a result, apparently, of his mere pres ence and without any visible change of diminution in him.
Yet would we dream for a moment of saying "Magic!"?
We would, instead, take it for granted that had we ob served the man in detail all day, we would have caught him transferring the bricks from the heap to the wall one at a time. And what's not magic for the bricklayer is not magic for the saltpeter, either.
With the birth and progress of the nineteenth century, more examples of this sort of thing were discovered. In 1812, for instance, the Russian chemist Gottlieb Sigis mund Kirchhoff. . .
And here I break off and begin a longish digression for no other reason than that I want to; relying, as I always do, on the infinite patience and good humor of the Gentle Readers.
It may strike you that in saying "the Russian chemist, Gottlieb Sig7ismund Kirchhoff" I have made a humorous error. Surely no one with a name like Gottlieb Sigismund Kirchhoff can be a Russian! It depends, however, on whether you mean a Russian in an ethnic or in a geographic sense.
To explain what I mean, let's go back to the beginning of the thirteenth century. At that time, the regions of our land and Livonia, along the southeastern shores of the Baltic Sea (the modem Latvia and Estonia) were in habited by virtually the last group of pagans in Europe. It was the time of the Crusades, and the Germans to the southeast felt it a pious duty to slaughter the poorly armed and disorganized pagans for the sake of their souls.
The crusading Germans were
of the "Order of the Knights of the Sword" (better known by the shorter and more popular name of "Livonian Knights"). They were joined in 1237 by the Teutonic Knights, who had first established themselves in the Holy Land. By the end of the thirteenth century the Baltic shores had been conquered, with the German expeditionary forces in control.
The Teutonic Knights, as a political organization, did not maintain control for more than a couple of centuries.
They were defeated by the Poles in the 1460s. The Swedes, under Gustavus Adolphus, took over in the 1620s, and in the 1720s the Russians, under Peter the Great, replaced the Swedes.
Nevertheless, however the political tides might shift and whatever flag flew and to whatever monarch the loyal in habitants might drink toasts, the land itself continued to belong to the "Baltic barons" (or "Balts") who were the German-speaking descendants of the Teutonic Knights.
Peter the Great was an aggressive Westernizer who built a new capital, St. Petersburg* at the very edge of the Livonian area, and the Balts were a valued group of sub jects indeed.
This remained true all through the eighteenth and nine teenth centuries when the Balts possessed an influence within the Russian Empire out of all proportion to their numbers. Their influence in Russian science was even more lopsided.
The trouble was that public education within Russia lagged far behind its status in western Europe. The Tsars saw no reason to encourage public education and make trouble for themselves. No doubt they felt instinctively that The city was named for his name-saint and not for himself.
Whatever Tsar Peter was, a saint he was not a corrupt and stupid government is only really safe with an uneducated populace.
This meant that even elite Russians who wanted a secular education had to go abroad, especially if they wanted a graduate education in science. Going abroad was not easy, either, for it meant leaming a new language and new ways. What's more, the Russian Orthodox Church viewed all Westerners as heretics and little better than heathens. Contact with heathen ways (such as science) was at best dangerous and at worst damnation. Consequently, for a Russian to travel West for an education meant the overcoming of religious scruples as well.
The Balts, however, were German in culture and Lu theran in religion and had none of these inhibitions. They shared, with the Germans of Germany itself, in the,height ening level of education-in particular, of scientific educa tion-through the eighteenth and nineteenth centuries.
So it follows that among the great Russian scientists of the nineteenth century we not only have a man with a name like Gottlieb Sigismund Kirchhoff, but also others with names like Friedrich Konrad Beilstein, Karl Ernst von Baer, and Wilhelm Ostwald.
This is not to say that there weren't Russian scientists in this period with Russian names. Examples are Mikhail Vasilievich Lomonosov, Aleksandr Onufrievich Kovalev ski, and Dmitri Ivanovich Mendel6ev.
However, Russian officialdom actually preferred the Balts (who supported the Tsarist government under which they flourished) to the Russian intelligentsia itself (which frequently made trouble and had vague notions of reform).
In addition, the Germans were the nineteenth-century scientists par excellence, and to speak Russian with a German accent probably leiit distinction to a scientist.
(And before you sneer at this point of view, just think of the American stereotype of a rocket scientist. He has a thick German accent, nicht wahr?-And this despite the fact that the first rocketman, and the one whose experi ments started the Germans on the proper track [Robert Goddard], spoke with a New England twang. )
So it happened that the Imperial Academy of Sciences of the Russian Empire (the most prestigious scientific organization in the land) was divided into a "German party" and a "Russian party," with the former dominant.
In 1880 there was a vacancy in the chair of chemical technology at the Academy, and two names were proposed.
The German party proposed Beilstein, and the Russian party proposed Mende]6ev. There was no comparison really. Beilstein spent years of his life preparing an encyclo pedia of the properties and methods of preparation of many thousands of organic compounds which, with nu merous supplements and additions, is still a chemical bible.
This is a colossal monument to his thorough, hard-work ing competence-but' it is no more. Mendel6ev, who worked out the periodic table of the elements, was, on the other hand, a chemist of the first magnitude-an un doubted genius in the field.
Nevertheless, government officials threw,their weight be bind Beilstein, who was elected by a vote of ten to nine.
It is no wonder, then, that in recent years, when the Russians have finally won a respected place in the scientific sun, they tend to overdo things a bit. They've got a great deal of humiliation to make up for.
That ends the digression, so I'll start over As the nineteenth century wore on, more examples of baste-making were discovered. In 1812, for instance, the Russian chemist Gottlieb Sigismund Kirchhoff found that if he boiled starch in water to which a small amount of sulfuric acid had been added, the starch broke down to a simple form of sugar, one that is now called glucose. This would not happen in the absence of acid. When it did happen in the presence of acid, that acid was not consumed but was still present at the end.
Then, in 1816, the English chemist Humphry Davy found that certain organic vapors, such as those of alcohol, combined with oxygen more easily in the presence of metals such as platinum. Hydrogen combined more easily with oxygen in the presence of platinum also.
Fun and games with platinum started at once. In 1823 a German chemist, Johann Wolfgang Debereiner, set up a hydrogen generator which, on turning an appropriate stop cock, would allow a jet of hydrogen to shoot out against a strip of platinum foil. The hydrogen promptly burst into flame and "Dbbereiner's lamp" was therefore the first cigarette lighter. Unfortunately, impurities in the hydrogen gas quickly "poisoned" the expensive bit of platinum and rendered it useless.
In 1831 an English chemist, Peregrine Phillips, reasoned that if platinum could bring about the combination of hydrogen and of alcohol with oxygen, why should it not do the same for sulfur dioxide? Phillips found it would and patented the process. It was not for years afterward, how ever, that methods were discovered for delaying the poisoning of the metal, and it was only after that that a platinum catalyst could be profitably used in sulfuric acid manufacture to replace Ward's saltpeter.
In 1836 such phenomena were brought to the attention of the Swedish chemist J6ns Jakob Berzelius who, during the first half of the nineteenth century, was the uncrowned king of chemistry. It was he who suggested the words "catalyst" and "catalysis" from Greek words meaning "to break down" or "to decompose. " Berzelius had in mind such examples of catalytic action as the decomposition of the large starch molecule into smaller sugar molecules by the action of acid.
But platinum introduced a new glamor to the concept of catalysis. For one thing, it was a rare and precious metal. For another, it enabled people to begin suspecting magic again.
Can platinum be expected to behave as a middleman as saltpeter does?
At first blush, the answer to that would seem to be in the negative. Of all substances, platinum is one of the most inert. It doesn't combine with oxygen or hydrogen under any normal circumstances. How, then, can it cause the two to combine?
If our metaphorical catalyst is a bricklayer, then plati num can only be a bricklayer tightly bound in a strait jacket.
Well, then, are we reduced to magic? To molecular action at a distance?
Chemists searched for something more prosaic. The suspicion grew during the nineteenth century that the inert ness of platinum is, in one sense at least, an illusion. In the body of the metal, platinum atoms are attached to each other in all directions and are satisfied to remain so. In bulk, then, platinum will not react with oxygen or hydro gen (or most other chemicals, either).
On the surface of the platinum, however, atoms on the metal boundary an
d immediately adjacent to the air have no other platinum atoms, in the air-direction at least, to attach themselves to. Instead, then, they attach themselves to whatever atoms or molecules they find handy oxygen atoms, for instance. This forms a thin film over the surface, a film one molecule thick. It is completely invisible, of course, and all we see is a smooth, shiny, platinum sur face, which seems completely nonreactive and inert.
As parts of a surface film, cixygen and hydrogen react more readily than they do when making up bulk gas.
Suppose, then, that when a water molecule is formed by the combination of hydrogen and oxygen on the platinum surface, it is held more weakly than an oxygen molecule would be. The moment an oxygen molecule struck that portion of the surface it would replace the water molecule in the film. Now there would be the chance for the forma tion of another water molecule, and so on.
The platinum does act as a middleman after all, through its formation of the monomolecular gaseous film.
Furthermore, it is also easy to see how a platinum catalyst can be poisoned. Suppose there are molecules to which the platinum atoms will cling even more tightly than to oxygen. Such molecules will replace oxygen wherever it is found on the film and will not themselves be replaced by any gas in the atmosphere. They are on the, platinum sur face to stay, and any catalytic action involving hydrogen or oxygen is killed.
Since it takes very little substance to form a layer merely one molecule thick over any reasonable stretch of surface, a catalyst can be quickly poisoned by impurities that are present in the working mixture of gases, even when those impurities are present only in trace amounts. . If this is all so, then anything which increases the amount of surface in a given weight of metal will also increase the catalytic efficiency. Thus, powdered platinum, with a great deal of surface, is a much more effective catalytic agent than the same weight of bulk platinum. It is perfectly fair, therefore, to speak of "surface catalysis. "
But what is there about a surface film that hastens the process of, let us say, hydrogen-oxygen combination? We still want to remove the suspicion of magic.
To do so, it helps to recognize what catalysts can't do.
For instance, in the 187Ws, the American physicist Josiah Willard Gibbs painstakingly worked out the applica tion of the laws of thermodynamics to chemical reactions.
He showed that there is a quantity called "free energy" which always decreases in any chemical reaction that is spontaneous-that is, that proceeds without any input of energy.
Thus, once hydrogen and oxygen start reacting, they keep on reacting for as long as neither gas is completely used up, and as a result of the reaction water is formed. We explain this by saying that the free energy of the water is less than the free energy of the hydrogen-oxygen mixture.
The reaction of hydrogen and oxygen to form water is analogous to sliding down an "energy slope. "
But if that is so, why don't hydrogen and oxygen mole cules combine with'each other as soon as they are mixed.
Why do they linger for indefinite periods at the top of the energy slope after being mixed, and react and slide down ward only after being heated?
Apparently, before hydrogen and oxygen molecules (each composed of a pair of atoms) can react, one or the other must be pulled apart into individual atoms. That requires an energy input. It represents an upward energy slope, before the downward slope can be entered. It is an "energy hump," so to speak. The amount of energy that must be put into a reacting system to get it over that energy hump is called the "energy of activation," and the con 207 cept was first advanced in 1889 by the Swedish chemist Svante August Arrhenius.
When hydrogen and oxygen molecules are colliding at ordinary temperature, only the tiniest fraction happen to possess enough energy of motion to break up on collision.
That tiniest fraction, which does break up and does react, then liberates enough energy, as it slides down the energy slope, to break up additional molecules. However, so little energy is produced at any one-time that it is radiated away before it can do any good. 'ne net result is that hydrogen and oxygen mixed at room temperature do not react. ff the temperature is raised, molecules move more rapidly and a larger proportion of them possess the nec essary energy to break up on collision. (More, in other words, can slide over the energy hump. ) More and more energy is released, and there comes a particular tempera ture when more energy is released than can be radiated away. The temperature is therefore further raised, which produces more energy, which raises the temperature still further-and hydrogen and oxygen proceed to react with an explosion.
In 1894 the Russian chemist Wilhelm Ostwald pointed out that a catalyst could not alter the free energy relation ships. It cannot really make a reaction go, that would not go without it-though it can make a reaction go rapidly that in its absence would prciceed with only imperceptible speed.
In other words, hydrogen and oxygen combine in the absence of platinum but at an imperceptible rate, and the platinum baste-maker accelerates that combination. For water to decompose to hydrogen and oxygen at room tem perature (without the input of energy in the form of an electric current, for instance) is impossible, for that would mean spontaneously moving up an energy slope. Neither platinum nor any other catalyst could make a chemical reaction move up an energy slope. If we found one that did so, then that would be magic.
Or else we would have to modify the laws of thermodynamics.
But how does platinum hasten the reaction it does hasten? What does it do to the molecules in the film?
Ostwald's suggestion (accepted ever since) is that cata lysts hasten reactions by lowering the energy of activation of the reaction-flattening out the hump. At any given tem perature, then, more molecules can cross over the hump and slide downward, and the rate of the reaction increases, sometimes enormously.
For instance, the two oxygen atoms m an oxygen mole cule hold together with a certain, rather strong, attachment, and it is not easy to split them apart. Yet such splitting is necessary if a water molecule is to be formed.
When an oxygen atom is attached to a platinum atom and forms part of a surface film, however, the situation changes. Some of the bond-forming capabilities of the oxygen molecule are used up in forming the attachment to the platinum, and less is available for holding the two oxygen atoms together. The oxygen atom might be said to be "strained. "
If a hydrogen atom happens to strike such an oxygen atom, strained in the film, it is more likely to knock it apart into individual oxygen atoms (and react with one of them) than would be the case if it collided with an oxygen atom free in the body of a gas. The fact that the oxygen molecule is strained means thaf it is easier to break apart, and that the energy of activation for the hyqrogen-oxygen combination has been lowered.
Or we can try a metaphor again. Imagine a brick resting on the upper reaches of a cement incline. The brick should, ideally, slide down the incline. To do so, however, it must overcome frictional forces which hold it in place against the pull of gravity. The frictional forces are here analogous to the forces holding the oxygen molecule together.
To overcome the frictional force one must give the brick an initial push (the energy of activation), and then it slides down.
Now, however, we will try a little "surface catalysis. " We will coat the slide with wax. If we place the brick on top of such an incline, the merest touch will start it moving downward. It may move downward without any help from us at all.
In waxing the cement incline we haven't increased the force of gravity, or added energy to the system. We have merely decreased the frictional forces (that is, the energy, hump), and bricks can be delivered down such a waxed incline much more easily and much more rapidly than down an unwaxed incline.
So you see that on inspection, the magical clouds of glory fade into the light of common day, and the wonderful word "catalyst" loses all its glamor. In fact, notlfing is left to it but to serve as the foundation for virtually all of chem
ical industry and, in the form of enzymes, the founda tion of all of life, too.
And, come to think of it, that ought to be glory enough for any reasonable catalyst.