Catalysts, alloys and interstitial compounds

A car, a spoon and a bell

A catalytic converter cleans the smoke of a car. A steel spoon does not rust like an iron nail. A bell rings with a clear note because it is made of bronze. Each of these depends on a d-block metal in some special form. We look at three of them: the metal as a catalyst, as an alloy, and as a host for small atoms.

Why d-block metals make good catalysts

A catalyst speeds up a reaction and is found unchanged at the end. The d-block metals and their compounds are the best catalysts we have, for two reasons.

1. They have several oxidation states. A metal ion can take electrons from one reactant and become one state lower. Then it gives the electrons to the next reactant and returns to where it began.
2. They have a surface with free d orbitals. Reactant molecules stick to the surface of the solid metal. This is adsorption. On the surface the molecules are held close together, and their bonds are weaker, so they react faster. The products then leave.

Here are the catalysts you should know.

• Vanadium(V) oxide, $\text{V}_2\text{O}_5$: the contact process, $2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3$.
• Iron with a little molybdenum: the Haber process, $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$.
• Platinum and rhodium gauze: the oxidation of ammonia, $4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}$, the first step to nitric acid.
• Finely divided nickel: hydrogenation of a vegetable oil into a solid fat.
• Titanium(IV) chloride with an aluminium compound: the making of polythene from ethene at low pressure.
• Platinum, palladium and rhodium: the converter, $2\text{CO} + 2\text{NO} \rightarrow 2\text{CO}_2 + \text{N}_2$.
• Manganese(IV) oxide: $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$ and $2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$.

A catalyst changes only the rate. It reaches the same equilibrium sooner. It does not change the position of the equilibrium.

How the oxidation states do the work

In the contact process, vanadium(V) oxide oxidises sulfur dioxide and is itself reduced. Then oxygen oxidises it back.

Vanadium goes from $+5$ to $+4$ in the first step. Sulfur goes from $+4$ to $+6$. Use the first step twice and the second once. The vanadium oxides cancel and only $2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$ is left.

The iron(III) ion catalyses a slow reaction between two negative ions: $2\text{I}^- + \text{S}_2\text{O}_8^{2-} \rightarrow \text{I}_2 + 2\text{SO}_4^{2-}$. Two negative ions repel each other, so they meet rarely. With iron(III), each step is between ions of opposite charge:

Add the two steps. The iron ions cancel and the net equation is the one above.

Try it in Python

The program adds the two steps of the contact process. It uses the first step twice, cancels what has no net change, and prints the net equation.

Alloys

An alloy is a homogeneous mixture of a metal with other metals or with non-metals, made by melting them together. The atoms of one metal take places in the lattice of the other. Its composition can change smoothly, so an alloy has no fixed formula.

Two metals mix well when their atoms are of similar size. The rule of thumb: the radii differ by no more than 15 per cent of the smaller radius. The metals should also have a similar crystal structure.

The atoms from vanadium to zinc are very nearly the same size, from about $125\ \text{pm}$ to $137\ \text{pm}$. So these metals mix freely in alloys. Take the difference between radii:

• Iron, $126\ \text{pm}$, and chromium, $129\ \text{pm}$: $\dfrac{129 - 126}{126} \times 100 = 2.4\ \%$. They mix.
• Copper, $128\ \text{pm}$, and zinc, $137\ \text{pm}$: $7.0\ \%$. They mix, and the alloy is brass.
• Copper and lead, $175\ \text{pm}$: $36.7\ \%$. They do not mix well.

Alloys are harder and stronger than the pure metals. In a pure metal, the atoms are in neat rows, and one layer slips over the next easily. An atom of another size sits in a row and blocks the slip.

• Brass: copper with 30 to 40 % zinc. Used for taps and musical instruments.
• Bronze: copper with about 10 % tin. Used for bells and statues.
• Stainless steel: iron with at least 10.5 % chromium, often with nickel. Used for cutlery and sinks.
• Nichrome: about 80 % nickel and 20 % chromium. Used for heating wire.

Chromium makes stainless steel resist rust. A thin, tight film of $\text{Cr}_2\text{O}_3$ forms on the surface, and it protects the iron under it. Ordinary steel is iron with a little carbon, less than about 2 per cent. Carbon makes iron harder.

Interstitial compounds

An interstitial compound forms when small atoms of hydrogen, carbon or nitrogen are trapped in the gaps of a metal lattice. The gaps are called interstices. Examples are $\text{TiC}$, $\text{Mn}_4\text{N}$, $\text{Fe}_3\text{H}$, $\text{TiH}_{1.7}$ and $\text{VH}_{0.56}$.

The two kinds of gap in a close-packed lattice are the octahedral gap and the tetrahedral gap. For a host atom of radius $R$, the octahedral gap can hold a sphere of radius $0.414R$, and the tetrahedral gap one of radius $0.225R$. For a simple interstitial lattice, the guest radius divided by the host radius should be below about 0.59. This is a rule of Hägg.

Carbon fits titanium and vanadium. It sits just above the limit in chromium and iron, and those metals form more complicated carbides, such as $\text{Fe}_3\text{C}$. Hydrogen is so small that it fits every one of them.

Practise with new numbers

Interstitial compounds have properties that are different from the pure metal.

• Very hard. The small atoms lock the layers, so that they cannot slip. Cutting tools are tipped with tungsten carbide.
• High melting points, higher than those of the pure metals. Titanium melts at about $1940\ \text{K}$, and titanium carbide at about $3430\ \text{K}$. The extra bonds between the metal and the small atoms make the lattice stronger.
• Metallic conduction is kept. The metal lattice is still there.
• Chemically inert.
• Non-stoichiometric. The gaps need not all be full, so the formula is not a whole-number ratio. $\text{TiH}_{1.7}$ is an example.

Common mistakes

• Saying that a catalyst is used up. It is changed during the reaction and then returns to its first state.
• Saying that a catalyst moves the equilibrium. It speeds up both directions equally.
• Treating an alloy as a compound. An alloy has a variable composition and no fixed formula.
• Dividing by the larger radius in the size rule. Divide the difference by the smaller radius.
• Calling an interstitial compound brittle and ionic. It is hard but it conducts, and it has metallic bonding.

Remember these

• The d-block metals catalyse through several oxidation states and through a surface that holds reactants.
• A catalyst gives the same equilibrium sooner. It does not change the position.
• Two metals alloy well when their radii differ by no more than 15 per cent.
• Brass is copper and zinc, bronze is copper and tin, and stainless steel is iron with chromium.
• Interstitial compounds have small atoms (H, C, N) in the gaps. They are hard, have high melting points and conduct.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the catalyst, and say why it works, in the contact process.

2. A catalyst is added to a reaction that is at equilibrium. What happens to the amounts of reactants and products?

3. Why are copper and zinc able to form brass?

4. Why is an alloy usually harder than the pure metal in it?

5. Name three properties of an interstitial compound such as $\text{TiC}$.