I mapped Ctrl-Alt-Up/Down to open web-browser and email client but it didn't take effect. Ctrl-Alt-Up/Down still switch workspaces as the default setting of GNOME 3.8.

 

In Ubuntu 18.04 these shortcuts were shown as "Super+Down" and "Super+Up" but were actually Ctrl+Alt+Down and Up. In this case disabling these "Super" shortcuts was enough. –  

 

solution:

默认是Super+Down/Up的两个切换工作区间的按键给reset（按Backspace按键）。