This is a red-ox reaction, and we can definitely crack it. But why? It doesn't answer your jargon: Sensitizer/Activator/Brightener are all jargon for sometimes proprietary agents used to alter the plating process
Finding the Answer for the Jargon is not really my aim as they have been used for such a long time that the term Sensitizer and Activator is almost guaranteed to include Stannous Chloride as sensitizer and Palladium Chloride combo for the metallization of non conductive surfaces, I was just trying understand the science behind the reaction and by doing that maybe find alternatives.
Sn2+ ions that are absorbed and left on the surface of the piece after the sensitizer immersion and later rinse will reduce the Pd2+ ions on the Palladium chloride activator(PdCI), to elemental Palladium which leaves a catalytic surface that will allow the piece to be coated with nickel.
While this process has been proven to be very effective at least in the last half century, one gram of palladium is quite expensive, other precious metals have been proven to work well, but as precious metals, their price is expensive too, but I believe that only precious metals have been tested because no other sensitizer(Stannous Chloride) has been used before and due to Tin location on the metal reactivity series no other metals that are proven to be catalytic to electroless nickel have been tested as an “Activator”, non-precious Metals that are catalytic to electroless nickel(that will initiate the nickel deposition) are only a few, Iron, Nickel and Cobalt.
My Theory is that a Zinc Chloride based sensitizer(proven by me to be effective for Silvering) will reduce a Cobalt Chloride base activator to elemental Cobalt and the surface will prove to be catalytic to electroless nickel, in any case I have made the necessary order for the test procedures(nickel/cobalt electroless solution and Stannous/Palladium activator, Zinc and Cobalt chloride), I will post my findings here when I get them.