We have performed two-dimensional angular correlation of electron-positron annihilation radiation (2D-ACAR) measurements on a series of porous silicon with photoluminescence (PL) peak energy in the range from 1.6 to 2.0 eV. The electron-positron momentum spectra of porous silicons can be well resolved into two peaks with different line width. The results shows two distinct momentum spectra for the studied samples. The samples with PL peak energy below 1.8 eV exhibit a strong positronium signal in the momentum spectra, and the obtained result shows a good correlation with PL spectra as predicted by quantum confinement. However, the samples with PL peak energy above 1.8 eV show little amount of positronium signal and the width of positronium peak remains almost constant. This behavior can be explained by the formation of silicon compounds on the surface of pores. With the result of these measurements, we suggest that the PL mechanism is different for the porous silicon with PL peak below and above 1.8 eV. For the PL peak below 1.8 eV, the emission arises from silicon nano-crystal, so that the PL spectrum highly depends on the nano-crystal size. However, for the PL above 1.8 eV, we suggest that the luminescence is dominated by the silicon compounds on the surface of the pores which block the free volume for positronium annihilation. Our suggested model can be used to resolve several difficulties in the existing PL spectra.