Question

A steam-hydrocarbon reforming process is one method for producing hydrogen from fossil fuels for use in a fuel cell. In the first step, steam reacts with hydrocarbons, such as \(\mathrm{CH}_{4}\), at high temperatures in the presence of a catalyst, yielding \(\mathrm{H}_{2}\) and CO. In the second step, the reaction of \(\mathrm{CO}\) and \(\mathrm{H}_{2} \mathrm{O}\), called the water- gas shift reaction, removes toxic carbon monoxide and produces more hydrogen. Step \(1: \mathrm{H}_{2} \mathrm{O}(g)+\mathrm{CH}_{4}(g) \stackrel{1100^{\circ} \mathrm{C} \mathrm{N}_{1} \text { cathmst }}{\longrightarrow} \mathrm{CO}(\mathrm{g})+3 \mathrm{H}_{2}(g)\) Step 2: \(\mathrm{CO}(g)+\mathrm{H}_{2} \mathrm{O}(g) \stackrel{400^{\circ} \mathrm{C}, \text { cualyst }}{\longrightarrow} \mathrm{CO}_{2}(g)+\mathrm{H}_{2}(g)\) (a) In Step 1 , which element is oxidized and which is reduced? (b) In Step 2, which element is oxidized and which is reduced? What is the oxidizing agent and reducing agent? (c) What are drawbacks of steam reforming in the production of hydrogen?

Short answer

Step 1: Carbon oxidized, hydrogen reduced. Step 2: Carbon oxidized, hydrogen reduced; oxidizing agent: water, reducing agent: CO. Drawbacks include greenhouse gas emissions and energy intensity.

Step-by-step solution

Analyze Step 1 Reaction

In Step 1, the reaction is \(\mathrm{H}_{2} \mathrm{O}(g) + \mathrm{CH}_{4}(g)\longrightarrow \mathrm{CO}(g) + 3 \mathrm{H}_{2}(g)\).First, identify the oxidation states of elements involved. Carbon in \(\mathrm{CH}_4\) starts with \(-4\), while in \(\mathrm{CO}\), carbon has an oxidation state of \(+2\). The carbon is oxidized. In water, hydrogen's oxidation number is \(+1\) and stays \(+1\) in \(\mathrm{H}_{2}\), while oxygen is \(-2\) in water and remains \(-2\) in \(\mathrm{CO}\): no overall change.

Identify Oxidized and Reduced Element in Step 1

In Step 1, carbon undergoes oxidation because its oxidation state increases from \(-4\) in \(\mathrm{CH}_4\) to \(+2\) in \(\mathrm{CO}\). The element that is reduced is hydrogen from \(\mathrm{H}_2\mathrm{O}\), as it ends up as hydrogen gas \(\mathrm{H}_2\). No reduction for carbon or oxygen.

Analyze Step 2 Reaction

In Step 2, the reaction is \(\mathrm{CO}(g) + \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow \mathrm{CO}_{2}(g) + \mathrm{H}_{2}(g)\).Carbon in \(\mathrm{CO}\) moves from \(+2\) to \(+4\) in \(\mathrm{CO}_2\) indicating oxidation, while oxygen remains \(-2\), and hydrogen \(+1\). The reducing element is carbon.

Identify Oxidized and Reduced Element in Step 2

In Step 2, carbon is oxidized (from \(\mathrm{CO}\) to \(\mathrm{CO}_2\)), and hydrogen is reduced (from \(\mathrm{H}_2\mathrm{O}\) to \(\mathrm{H}_2\)). The oxidizing agent (accepting electrons) is \(\mathrm{H}_2\mathrm{O}\) and the reducing agent (donating electrons) is \(\mathrm{CO}\).

Discuss Drawbacks of Steam Reforming

The drawbacks of steam reforming include the release of greenhouse gases, particularly \(\mathrm{CO}_2\); dependency on non-renewable fossil fuels; and high operational temperatures and costs, making it an energy-intensive process.

Key concepts

Steam Reforming

Steam reforming is a method used to produce hydrogen from hydrocarbons like methane (\(\mathrm{CH}_4\)). It involves a chemical reaction where water (in the form of steam) interacts with hydrocarbons at high temperatures, usually around 1100°C, in the presence of a catalyst. The catalyst helps to accelerate the reaction without being consumed in the process. This reaction results in the formation of carbon monoxide (\(\mathrm{CO}\)) and hydrogen gas (\(\mathrm{H}_2\)).
Steam reforming is widely used because it is currently one of the more cost-effective methods for hydrogen production compared to other methods. However, there are environmental concerns since the process also produces significant amounts of \(\mathrm{CO}_2\), a greenhouse gas, as a by-product.

Water-Gas Shift Reaction

Following the steam reforming step, the water-gas shift reaction plays a crucial role. This reaction involves converting carbon monoxide (\(\mathrm{CO}\)) with water vapor (\(\mathrm{H}_2\mathrm{O}\)) into carbon dioxide (\(\mathrm{CO}_2\)) and more hydrogen (\(\mathrm{H}_2\)). The reaction can be represented by the equation:
\[ \mathrm{CO}(g) + \mathrm{H}_2\mathrm{O}(g) \longrightarrow \mathrm{CO}_2(g) + \mathrm{H}_2(g) \]
The water-gas shift reaction occurs at a lower temperature than steam reforming, around 400°C, and also requires a catalyst. This step is essential because it increases the yield of hydrogen and helps reduce the amount of toxic carbon monoxide in the product gas mix. In addition to producing hydrogen, it also assists in removing carbon monoxide, which can be harmful if not properly managed.

Oxidation-Reduction Reactions

The reactions involved in hydrogen production via steam reforming and water-gas shift are classified as oxidation-reduction (redox) reactions. Redox reactions involve the transfer of electrons between chemical species, which results in changes in their oxidation states.
In the steam reforming step, methane (\(\mathrm{CH}_4\)) goes through oxidation as its carbon atom loses electrons, going from an oxidation state of \(-4\) to \(+2\) in \(\mathrm{CO}\). Concurrently, hydrogen in water reduces from \(+1\) to forming \(\mathrm{H}_2\) gas.
For the water-gas shift reaction, carbon in \(\mathrm{CO}\) is further oxidized from \(+2\) to \(+4\) in \(\mathrm{CO}_2\), while the hydrogen from \(\mathrm{H}_2\mathrm{O}\) is reduced to \(\mathrm{H}_2\). Identifying which elements undergo oxidation or reduction and which act as reducing or oxidizing agents is key to understanding the underlying chemical processes.

Greenhouse Gases

One of the primary drawbacks of steam reforming is the emission of greenhouse gases, especially carbon dioxide (\(\mathrm{CO}_2\)). Greenhouse gases trap heat in the atmosphere, contributing to global warming and climate change.
As steam reforming relies on fossil fuels, it inherently results in substantial \(\mathrm{CO}_2\) emissions. Despite being an efficient method for hydrogen production, steam reforming's environmental impact is significant. This is due to both the high \(\mathrm{CO}_2\) output and reliance on non-renewable resources like natural gas, which raises concerns about its sustainability and ecological footprint.
Efforts are being made to mitigate these environmental impacts through carbon capture and storage technologies and finding renewable sources for hydrogen production.

Catalysis in Chemistry

Catalysts are essential in the chemical reactions involved in hydrogen production, such as steam reforming and the water-gas shift reaction. A catalyst is a substance that speeds up a chemical reaction without being consumed in the process. This is crucial in industrial processes as it reduces energy costs and increases reaction efficiency.
In steam reforming, catalysts such as nickel are commonly used to facilitate the breaking of \(\mathrm{C-H}\) bonds in methane. For the water-gas shift reaction, typically a mix of iron and chromium oxides serve as catalysts. These catalysts optimize the reaction conditions, ensuring high yields of hydrogen.
Understanding how catalysts work can provide insights into improving efficiency and reducing energy consumption, making hydrogen production more sustainable.