Question

How can I make PdCl₂ from Pd? I did once but I forgot. Looks like I used H₂O₂ and aqua regia or HCl.

Answer

Hydrogen peroxide isn't a necessary component to dissolve the palladium metal.  Glancing at Hawley's ⁽¹⁾ the derivation is listed as "solution of palladium in aqua regia and evaporation".  Aqua regia in this case is the mixture of 1 part nitric acid (approx. 70%) with 3 or 4 parts hydrochloric acid (aprox. 38%), less concentrated solutions are also used but this is the traditional mixture in my experience.  Remember that solutions of aqua regia should be used fresh as they loose their potency over time.

Ammen's book ⁽²⁾ which covers the precious metals in great detail from a practical perspective does not list explicitly how to make palladium chloride but does not mention any difficulty in dissolving palladium in aqua regia.  Palladium is one of the least noble of the metals of the platinum group and will also dissolve in hot nitric and boiling sulfuric acid. 

Once you have your palladium dissolve in your aqua regia it is evaporated to a small volume, then hydrochloric acid is added, the process is repeated.  This drives off the nitric acid and leaves behind the palladium chloride.  The anhydrous material is stable to 675ºC ⁽¹⁾ so there should be no worries about decomposition of the product during evaporation.

(1)  Lewis, Richard J. Sr. Hawley's Condensed Chemical Dictionary.  13th ed.  New York, NY: John Wiley & Sons, Inc.  1997

(2) Ammen, C.W. Recovery and Refining of Precious Metals. 2nd ed. Alberta, Canada: Deep Rock Resources Inc. 1993