A combination of careful heating and hydrogen gas done in a quartz tube with a removable quartz frit on one end in a tube furnace.
Doing it this way makes it much easier to remove all of the ammonium chloride (or chlorine gas if you start from a halide). A separate tube for Pt, Pd, Ir, Rh. No acids, no filtration, simply load the tube, set the PID to its preprogrammed ramp/cycle and open the regulator when at temperature, close the regulator after a certain amount of time (material & compound dependent), and open the valve to the vacuum as it cools back to RT. In a large 15 cm tube, one can reduce many kilograms of ammonium hexachloroplatinate, palladate, rhodate, osmate, etc. (it is general). The vacuum is applied as it is cooled to ensure that there is no hydrogen left adsorbed onto the surface. Several years back, I had rhenium sponge take light after opening the cool tube. Quite an experience! I've done the whole thing with just straight calcination. Works fine for platinum if you go nice and easy, and to a lesser extent Pd and Rh (I usually let them cool under a soft H2 flame).
There are the most minimal PM losses this way, and all the platinum group can be reduced cleanly and very cheaply. Mole for mole, hydrogen is very cheap. The quartz tube is several hundred dollars, the regulator several hundred dollars, the vacuum pump, a thousand, the T cylinder is three hundred or so, and $28 to fill, the rest of the glass (which may be borosilicate), several hundred dollars, and the tube furnace, several thousand dollars. For about ten thousand dollars, you can have a setup that can reduce EVERY platinum group metal (as well as produce their halides, oxides, and various other salts and compounds for a value-added product). The versatility is what makes it so useful. Hydrazine and its salts work great, but handling hydrazine gets old, and even the relatively benign sulfate is still irksome. I've long since grown tired of aqueous work with the platinum group. Beaker and flask are fine for ~10-20 oz, but anymore, and it's easier to do it in a tube furnace rather than get a sealed glass-lined Pfaulder kettle (and much less reagent intensive). Being frank, the oxidation is easier on scrap material too (especially for rhodium).
Formic acid works quite well for platinum, palladium, and to a lesser extent rhodium if the proper precautions and care is taken. I usually take whatever salt, dissolve it in a minimum quantity of water, and then add ammonia. The solution may be clear or turbid. I then heat, and add small (50 mL) portions of conc. formic acid. You will see ebullition as CO, CO2 and H2 gas is produced. It should be pH 6.5 or above to work correctly. Boil away (80-90*C) and go until all trace of green is gone (or light brown for Rh). It will be inky black when all is reduced , but will gray as you boil and the particles stick together. Filter on a quartz frit.* Filtering PGM blacks is a pain in the butt, so avoid them at all costs. They quickly clog the frits, and vacuum filtration goes quite slow especially if you have gallons of solution. Filtering with a vacuum setup and fritted glass is a proposition only for very high purity metals in 10-30 oz. quantities, any more than that, and a dry method using the tube furnace is preferred.
Ammonium formate solutions are old hat insofar as organic and inorganic chemistry goes...
*Quartz frits are somewhat of an investment, but when I do not want contaminants or extra steps, they are useful. They may be cleaned with aqua regia, or with conc. sulfuric acid, and you should have one for each metal, clearly marked (ask the glassblower) to avoid cross contamination).
