Can this center speaker be placed horizontal instead of vertical ?
The brochure only shows the In-Room Silver in a vertical orientation and the Center Silver in a horizontal orientation, so I would say, no, the preferred orientation is vertical. If this is a free standing in-room speaker, you are certainly welcome to try both but the dispersion pattern when set horizontally will not be ideal and may lead to a tonal difference between the center and the L+R, especially off axis.