Hi Ali, Hydrogen as known as reducting agent. It means H2 will reduce the oxidation state of metal ion by taking out oxygen atom which was bonded to metal ion to form water and reduced metal ion (For example : Fe2O3 ---> 2FeO  + H2O). To follow the mechanism, you can run the TPR (Temperature Program Reduction) and other analytical method (XPS, and a simple one: FTIR). The idea to reduce with hydrogen is actually to know in what temperature is metal ion will play in role in certain reaction.