In 1964, Khoobiar reported in his seminal paper that the normally green WO3 is reduced by H2 gas to blue HxWO3 when in contact with a platinum metal catalyst through simple mixing, thus being the first to establish the possibility of hydrogen spillover. This process was later characterized as hydrogen spillover by Boudart et al. which describes the migration of hydrogen atoms from a hydrogen-rich platinum surface, spilling over to a hydrogen-poor surface and reducing the WO3. While the exact definition of HSPE is theoretical, the most commonly accepted explanation is that a hydrogen molecule (H2) attaches to the heterogeneous platinum catalyst, dissociating into atomic hydrogen on the platinum surface, and finally hydrogen spills over from the catalyst onto its support toward the substrate . In addition to the work done by Khoobiar and Boudart on their fundamental contributions to understanding spillover as a concept, progress has continuously been made in applications utilizing spillover for hydrogen storage and advances in catalysis. On the other hand, hydrogen atom transfer with homogeneous transition metal catalysts consists of a hydrogen source dissociating on the metal complex, followed by a radical reaction with the coordinated hydrogen.