Why does chemisorption decrease with increasing temperatures? Why is it that chemisorption decreases with higher temperatures?
The explanations I found online were quite vague and one of them cited Le Chatelier's principle, but I thought chemisorption is an irreversible process (because covalent bonds are formed between the adsorbate and adsorbent) so there is no reversible equilibrium.
Explanations I found online:

&quot;Further increase will increase the energy of the molecules adsorbed and will also increase the rate of desorption. Therefore, the extent of adsorption decreases.&quot; - why does the rate of desoprtion increase?

&quot;The initial increase is due to the fact that heat supplied acts as activation energy. The decrease afterward is due to the exothermic nature of adsorption equilibrium.&quot;
Let's answer this question with the help of some thermodynamics and kinetics. Chemisorption is a chemical process with $\Delta S&lt;0$ and $\Delta H&lt;0$. Thus, in simpler words it is an enthalpy driven exothermic equilibrium
Every chemical process is an equilibrium with varying extent of completion. So your idea of chemisorption not being an equilibrium is wrong. Every chemical reaction is theoretically reversible in nature. What we call irreversible reactions in general are such reactions which are heavily favoured on one side of the equilibrium.
Now,
$\ln K = -\frac{\Delta H}{RT}  + \frac{\Delta S}{R}$
It is evident from this above expression that the $K$ will decrease when we increase $T$ when $\Delta H&lt;0$ and thus the aforementioned conclusion.
But this is only half the story. Like all other chemical reactions, chemisorption also needs some activation energy to overcome the transition state. At lower temperatures there are very few molecules which have the kinetic energy enough to overcome the activation energy. The fraction of these molecules increases with increase in temperature and thus increasing the extent of chemisorption.
Conclusion
Due to the combination of thermodynamic and kinetic effects the rate of chemisorption first increases with the increase in temperature but after a certain saturation point the thermodynamic effects start dominating and thus the extent of chemisorption starts to decrease.
The graph below might help you understand how the extent of chemisorption changes with change in temperature.

Here,
$x$ = Mass of adsorbate
$m$ = Mass of adsorbent