  This paper addresses the origin of the silicate emission observed in PG QSOs,
based on observations with the Spitzer Space Telescope. Scenarios based on the
unified model suggest that silicate emission in AGN arises mainly from the
illuminated faces of the clouds in the torus at temperatures near sublimation.
However, detections of silicate emission in Type 2 QSOs, and the estimated cool
dust temperatures, argue for a more extended emission region.To investigate
this issue we present the mid-infrared spectra of 23 QSOs. These spectra, and
especially the silicate emission features at ~10 and ~18 mu can be fitted using
dusty narrow line region (NLR) models and a combination of black bodies. The
bolometric luminosities of the QSOs allow us to derive the radial distances and
covering factors for the silicate-emitting dust. The inferred radii are 100-200
times larger than the dust sublimation radius, much larger than the expected
dimensions of the inner torus. Our QSO mid-IR spectra are consistent with the
bulk of the silicate dust emission arising from the dust in the innermost parts
of the NLR.
