While praying congregation prayers behind an Imam, What am I supposed to recite in Zuhr and Asar prayers when the Imam is silently reciting the Surahs? Till now I have been reciting 'Subhanallah' verbally in small voice like whispering while the Imam is reciting surahs in first 2 rakats of zuhr and asar silently. Am I wrong, or can I be silent?

Yes it is right,you are not allowed to recite Surah either concentrate in meanings if hard to concentrate then recite zikr silently like Subhanallah