A ligand-free heterogeneous metal catalyst system (represented as Pd/SiO2 (O)) derived by calcination of Pd(acac)(2)/SiO2 in air and its catalytic properties toward the Heck coupling of bromobenzene (PhBr) and styrene have been studied. X-ray photoelectron spectroscopy (XPS) and catalytic results demonstrate that most of Pd2+ is reduced to Pd-0 on SiO2 by N,N-dimethylacetamide (DMA) during the Heck reaction and that the resulting Pd-0/SiO2 is highly active for the Heck reaction, the remaining Pd2+/SiO2 is not responsible for the high activity. Pd/SiO2 (O) possesses incomparable advantages over a heterogeneous homolog (represented as Pd/SiO2 (H)) prepared by reduction of Pd(acac)(2)/SiO2 in H-2 as a pre-catalyst in both activity and catalyst recycling. The activity over Pd/SiO2 (O) is comparable to that over a homogeneous Pd system. Transmission electron microscopy (TEM) analysis illustrates that the high activity over Pd/SiO2 (O) consists in the small size of supported Pd particles generated in-situ with gentle reducing agents at a mild temperature.