I have already read Is adsorption exothermic, and if so, why? but it didn't answered my query.
To illustrate, let's take a hypothetical vacuum space. In that space, there is a solid and a gas, the gas is getting adsorbed over the solid. In that case, if we'll use momentum conservation, we'll get that the net kinetic energy of system has increased due to positive work done by the internal forces, but heat exchanged from system to surrounding or vice-versa is zero( as there is no outside interaction present. Therefore, dU>0 as U is sum of kinetic and potential energy of system( note that I haven't made the internal force as due to a field with a assigned potential function). We know dH=dU+PdV for processes in which the pressure outside the system is constant( which we have assumed). Therefore, as dV=0( because I had taken a fixed volume of space), therefore dH>0. But in books it is written dH due to adsorption is negative. Also, if I had assigned a potential function for internal forces, what change would have taken place? Please help me out.