Unless you ride in dirt, knobby tires slow you down and increase pedaling effort. The bumps really are not what protects you against glass and goat heads. Good tires do.
You need good
road tires, just in a 26" size. Look
here for a selection of commuter tires for MTBs.
The Schwalbe Marathon Supremes are a top rated tire in goat head land, although they are very expensive. A search in this forum will bring up other offerings. I'm lucky enough to have decent roads and can get away with the cheap stuff.
A nice set of smooth tires will be the best thing you could do for your bike.
They will get you riding at the speed of happy.
