Questions on Catalysts

1. Catalytic converters used in automobiles typically contain platinum, palladium and/or rhodium metal coated onto a honeycomb ceramic surface. They convert oxides of nitrogen, NOx, carbon monoxide and volatile organic compounds, VOCs, such as unburned octane, into less harmful gases. They operate in three different stages. Oxidation, reduction and the monitoring of oxygen content of the exhaust gases to feed back to air-to-fuel input.

(a) Identify the particular property that the three transition metals, platinum, rhodium and palladium, possess that makes them particularly efficient heterogeneous catalysts.

(b) Explain why the ceramic surface of the catalytic converter is honeycombed.

(c) Using octane as a typical example of a volatile organic compound, state the equation for the reaction it undergoes during the oxidation stage.

(d) State two equations for reactions taking place during the reduction stage.

(e) Discuss why the air-to-fuel ratio is critical for the correct operation of an internal combustion engine.

2. Zeolites are hydrated aluminosilicate minerals. They consist of interlinked tetrahedrons of alumina, Al₂O₃, and silica, SiO₂. They
also contain group 1 or group 2 metal ions and water trapped in the gaps between them. Some zeolites are natural substances and others are man-made. Explain why zeolites are good catalysts.

3. Oxalic acid, (COOH)₂ can be titrated with an acidified solution of potassium manganate(VII).
The redox reaction, which converts the oxalic acid into carbon dioxide, is initially very slow at room temperature. When the first few drops of acidified potassium manganate(VII) are added from the burette into the conical flask containing the oxalic acid solution it is necessary to warm the oxalic acid solution to about 60 ^oC. During the reaction the purple colour of the potassium manganate(VII) solution disappears and bubbles of a gas are seen to be evolved.
After the first few drops of the acidified potassium manganate(VII) solution have been added it is no longer necessary to keep the reactants above room temperature as further amounts of acidified potassium manganate(VII) solution are added as the reaction is now much faster. The end-point occurs when one drop of the titrant causes a faint pink colour to remain.

(a) Deduce the half-equation for the oxidation of oxalic acid.

(b) Deduce the half-equation for the reduction of manganate(VII) ions, MnO₄⁻(aq), in acid solution.

(c) Deduce the overall equation for the reaction that takes place.

(d) Suggest why it is no longer necessary to heat the reaction mixture after some of the product has been formed.

(e) Explain why the end-point is taken when one drop of titrant causes a faint pink colour to remain.

Catalyst questions