For liquefying hydrogen by the Linde process the gas must first be pre-cooled with liquid nitrogen. The reason is that

1. Ahydrogen has a very high critical pressure
2. Bthe inversion temperature of hydrogen (about $193\mathrm{K}$) is below room temperature, so throttling at room temperature would heat it
3. Chydrogen reacts with the throttle valve at room temperature
4. Dnitrogen dissolves in hydrogen and lowers its boiling point