When I have received free night certificates for category 1 - 4 hotels, I have been able to view them in my account.
Log onto your account in marriott.com. go to my account overview, then account activity.
Down the bottom of the page is transactions. If you do not see the reward certificate there then adjust the dates.
The actual free night certificate is listed below the transactional activity and it remains there (and shows the expiration date to the right) until you use it or it expires. When you book a free night and want to use this certificate book using 'Rewards Points' and the process will show you that you have an unused certificate that can be used for the reservation and will ask you if you want to use it.