In either dc power converters, there must be a diode. I don't understand the need for it? Would a buck/boost converter function without a diode? Also, what is the maximum reverse voltage for a diode in both cases? Does it have to be higher than the output voltage?
If you look at literature, you will find that switching converters operate by charging and discharging an inductor. The main building block is composed by an inductor and two switches, connected in such a way that one switch charges the inductor and the other discharges it.
Now, you have to give a signal to those switches, so one of them must be a controllable switch (aka transistor; usually MOSFETs are used).
The other switch is usually substituted by a diode, because its usual behavior (allowing current to flow in just one direction) is enough to guarantee the proper functioning of the device.
Anyway of course you can remove the diode and substitute it with another MOSFET. Your control logic then becomes more complicated (it needs to control two switches), bit the performances increase (the voltage drop on the diode, usualli 0.3V, is now reduced to almost 0). These converters are called synchronous converters (see here for more infos).
The maximum reverse voltage depends on the device. For buck converters the reverse voltage on the diodes is roughly Vin, while on boost ones it is Vout.