To measure the evolved hydrogen gas during the reaction, you can use a gas-tight syringe to collect gas samples at different time intervals. The gas samples can then be transferred to sample vials for GC analysis. The GC instrument can detect and quantify the amount of hydrogen gas in the samples. By analyzing the samples collected at different time intervals, you can determine the rate of hydrogen gas evolution during the photocatalytic reaction. It is important to ensure that the syringe and sample vials are clean and free from any contaminants that may interfere with GC analysis.

Evgeny Kabachkov, Can I use Tedlar gas sampling bags to collect gas.

Also, give your insight regarding the capacity of gas syringe to collect gas

Yes, Tedlar gas sampling bags should work well for H2. You want to use a material that has a high barrier performance so that you can keep H2 inside for a few days. Re the gas syringe to collect gas, any gas-tight 1-2ml syringe may be a good start. Our lab uses the one from Hamilton. Also, you might need to check if there may be adaptivity/compatibility issues between your GC-TCD injector and the syringe. Hope this helps.