Many metallic catalysts, particularly the precious-metal ones, are often deposited as very thin films on a substance of high surface area per unit mass, such as alumina \(\left(\mathrm{Al}_{2} \mathrm{O}_{3}\right)\) or silica \(\left(\mathrm{SiO}_{2}\right) .(\mathbf{a})\) Why is this an effective way of utilizing the catalyst material compared to having powdered metals? (b) How does the surface area affect the rate of reaction?

Short Answer

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(a) Depositing metallic catalysts as thin films on high surface area materials like alumina or silica is effective because it provides better dispersion of the catalyst, maximizing the number of active sites for reactants to interact with. This not only enhances the reactivity of the catalyst but also improves its stability, prolonging its lifespan. (b) A larger surface area results in more available active sites on the catalyst surface, leading to an increased probability of reactant-catalyst interactions. As the number of active sites and the frequency of reactant-catalyst interaction increase, the rate of reaction is boosted, speeding up the overall chemical process.

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(a) Advantages of thin films compared to powdered metals

The effectiveness of a catalyst is largely determined by its capacity to provide an increased number of active sites where reactant molecules can interact, thus promoting the desired reaction to proceed at a faster rate. Depositing the catalyst material as a thin film on a substrate with a high surface area per unit mass, like alumina or silica, helps maximize its efficiency in several ways: 1. Improved Dispersion: Creating a thin film allows for a better dispersion of the catalyst material over the surface of the support. This ensures the optimal use of the catalytic material since it becomes more evenly distributed and provides a larger number of active sites. In contrast, using powdered metals could limit the dispersion of the catalyst, deactivating some of its molecules by preventing them from being accessible to reactants. 2. High Surface Area: A high surface area support, such as alumina or silica, increases the number of available active sites for reactants to interact with. This occurs because a larger surface area means that there is more surface for the catalyst to cover, creating more potential reaction sites and enhancing the reactivity of the catalyst. 3. Stability: The precious-metal catalysts are often expensive and rare, making it important to use them sparingly and prolong their lifespan. Depositing a thin film of the catalyst on a stable material can help preserve the material's properties and degradation due to environmental factors, thus enhancing its long-term effectiveness.
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(b) Effect of surface area on the rate of reaction

The surface area of a catalyst has a direct impact on the rate of reaction. As previously discussed, the effectiveness of a catalyst lies in its ability to provide active sites for reactant molecules to interact. A larger surface area means more available active sites on the catalyst surface, which leads to an increased probability of reactant molecules encountering and interacting with the catalyst. As the number of active sites and the frequency of reactant-catalyst interaction increase, the rate of reaction is boosted. The reaction takes place more rapidly, producing the desired products faster and speeding up the overall chemical process. Therefore, using catalysts with a high surface area or depositing catalysts on high surface area supports can greatly enhance the efficiency and rate of reactions.

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Most popular questions from this chapter

Consider the following reaction: $$ 2 \mathrm{NO}(g)+2 \mathrm{H}_{2}(g) \longrightarrow \mathrm{N}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g) $$ (a) The rate law for this reaction is first order in \(\mathrm{H}_{2}\) and second order in NO. Write the rate law. \((\mathbf{b})\) If the rate constant for this reaction at \(1000 \mathrm{~K}\) is $6.0 \times 10^{4} \mathrm{M}^{-2} \mathrm{~s}^{-1}\(, what is the reaction rate when \)[\mathrm{NO}]=0.035 \mathrm{M}\( and \)\left[\mathrm{H}_{2}\right]=0.015 \mathrm{M} ?(\mathbf{c})$ What is the reaction rate at \(1000 \mathrm{~K}\) when the concentration of \(\mathrm{NO}\) is increased to \(0.10 \mathrm{M},\) while the concentration of \(\mathrm{H}_{2}\) is \(0.010 \mathrm{M} ?\) (d) What is the reaction rate at \(1000 \mathrm{~K}\) if \([\mathrm{NO}]\) is decreased to \(0.010 \mathrm{M}\) and \(\left[\mathrm{H}_{2}\right]\) is increased to \(0.030 \mathrm{M} ?\)

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