- D1 is not needed if you are using the LM324, because it is capable of accepting up to 50mA negative input current.
I agree with most of dc42's comments. If VC = 9V and you want to limit Vout to
5V, you can use a voltage divider instead, or else add an R between the opApp
and D2 to limit current, as dc42 mentioned.
What I would do is use another zener in place of D1 to protect the opAmp input
terminal from large voltage spikes out of the Pz.