Debentures have no collateral. Bond buyers generally purchase debentures based on the belief that the bond issuer is unlikely to default on the repayment. An example of a government debenture would be any government-issued Treasury bond (T-bond) or Treasury bill (T-bill). T-bonds and T-bills are generally considered risk free because governments, at worst, can print off more money or raise taxes to pay these type of debts.
Investment dictionary. Academic. 2012.