Why enthalpy of adsorption is negative?
I will point out another equation:

$$\mathrm{d}G = \mathrm{d}H - T\,\mathrm{d}S$$

For adsorption $\mathrm{d}S&lt;0$, as molecules go more organized.    

Therefore also $\mathrm{d}H &lt; 0$ must be true, otherwise $\mathrm{d}G &lt; 0$, needed for a spontaneous process, would not be achieved.