How may hydrogen gas cause fire/explosion during synthesis of carbon nanotubes by acetylene and hydrogen?

The synthesis of carbon nanotubes (CNTs) often involves catalytic chemical vapor deposition (CVD) using hydrocarbons like acetylene (C₂H₂) and hydrogen (H₂) as precursors. While hydrogen itself is generally not the main reactant, it can still pose a fire or explosion hazard during the synthesis process due to the following reasons:

**Flammability of Hydrogen**: Hydrogen is a highly flammable gas. It has a wide range of flammable concentrations in air (about 4% to 75% by volume) and can ignite easily with a small spark or heat source. When used in the synthesis process, if there is a leak or accidental release of hydrogen into the atmosphere of the reactor or surrounding area, it can create a potentially explosive mixture.

**Reactivity with Acetylene**: In the presence of heat (which is often used to decompose acetylene and promote carbon nanotube growth), there is a possibility of a violent reaction occurring if hydrogen gas is mixed with acetylene in certain proportions. The combination of these gases at high temperatures can lead to combustion, potentially causing fire or explosion if not properly controlled.

**Oxygen Contamination**: If there is any inadvertent introduction of oxygen into the synthesis environment, the risk of fire or explosion increases tremendously. An oxygen-rich environment combined with hydrogen and hot surfaces can lead to rapid combustion events.

**Pressure Build-up**: The synthesis process can generate significant amounts of gas. If the system is not adequately designed to vent off excess pressure, a build-up of hydrogen and other gases can occur, leading to potential ruptures or explosive decompression.

**Catalyst Interaction**: The catalysts used in the CVD process can also influence the reactivity of gases. For instance, the presence of certain metals as catalysts can facilitate the production of reactive radicals and intermediates that may enhance the risk of combustion under certain conditions.

To mitigate these risks, laboratory protocols should include measures such as ensuring proper ventilation, employing gas leak detection systems, using explosion-proof equipment, and routinely monitoring gas concentrations in the synthesis environment. Proper safety training and protocols are critical to ensure a safe working environment during the synthesis of carbon nanotubes with acetylene and hydrogen.