Important Palladium Chemical Information
Physical and chemical properties of selected palladium compounds
Processes for the production of important palladium compounds
Ammine complexes of palladium: Addition of ammonia to solutions of palladium(II) chloride first causes the formation of a pink precipitate of the binuclear complex Pd(NH3)4PdCl4, Vauquelin's salt, which is converted to soluble tetraammine palladium(II) chloride by further addition of ammonia.
Palladium(II) acetate: This compound is prepared from palladium sponge (or nitrate) and glacial acetic acid.
Palladium(II) chloride: Palladium(II) chloride is prepared by the careful evaporation of a solution of hydrogen tetrachloropalladate(II) in hydrochloric acid, preferably in a rotary evaporator.
Palladium(II) nitrate: This compound is prepared from palladium and nitric acid.
Palladium(II) oxide: Palladium(II) oxide is obtained by reaction of palladium black (powder) with oxygen or air at 750 °C. Decomposition occurs at 850 °C. A catalytically active palladium preparation analogous to platinum(IV) oxide (PtO2[H2O]x) can be obtained by evaporating a solution of hydrogen tetrachloropalladate(II) and sodium nitrate and fusing the product.
Tetrachloropalladic(II) acid: The metal is dissolved in hydrochloric acid/chlorine or hydrochloric acid/nitric acid. If dissolution occurs below about 50 °C, hexachloropalladic(IV) acid is formed first. Commercial solutions in hydrochloric acid contain 20% palladium.
Uses of palladium metal
Electronics and electrical technology: Silver-palladium alloys are used for electrical contacts, and other palladium alloys are used for electrical relays and switching systems in telecommunication equipment. In low-current technology, electrical contacts of palladium and its alloys are used. Large numbers of so-called reed contacts (silver-palladium-, rhodium- or ruthenium-coated contacts) have been used in telephone relays. Palladium can sometimes replace gold in coatings for electronics, electrical connectors and lead frames of semiconductors (Kroschwitz, 1996). The plating solutions contain palladium(II) diamminedinitrite [Pd(NH3)2(NO2)2], the tetraammine complex or palladium(II) chloride (Smith et al., 1978; Renner, 1992; Kroschwitz, 1996).
Automobile exhaust catalysts: For more than 20 years, automobile exhaust catalysts have been used to reduce levels of nitrogen oxides, carbon monoxide and hydrocarbons in automobile exhausts. In the last few years, catalysts employing precious metal combinations of platinum or palladium and rhodium in a ratio of 5 to 1 (1.4-1.8 g PGM/Liter catalyst volume) have been developed successfully (Abthoff et al., 1994; Degussa, 1995; Kroschwitz, 1996). Exhaust gas purification by equipping of passenger car diesel engines with palladium oxidation catalysts has been achieved only since about 1989 (Fabri et al., 1990), but more recent information shows that palladium is not used on diesel vehicles, which account for around 23% of the European market (Cowley, 1997). Concentrations of the precious metals vary and depend upon the specifications of the manufacturer (IPCS, 1991). Much of this information is proprietary.
Catalysts in chemical processes: Palladium has a strong catalytic activity for hydrogenation, dehydrogenation, oxidation and hydrogenolysis reactions. Industrial palladium catalysts are in the form of finely divided powder, wire or gauze or supported on substrates such as activated carbon, gamma-aluminium oxide or aluminium silicates. Often, two or more PGMs are combined (Table 9). In the petroleum industry, PGM catalysts are used to produce gasolines with high antiknock properties. Palladium(II) chloride and tetrachloropalladic(II) acid are important homogeneous catalysts used in the large-scale oxidation of ethylene to acetaldehyde in the Wacker process. Palladium catalysts are also used for the acetoxylation of ethylene to vinyl acetate (Fishbein, 1976) and in the manufacture of sulfuric acid and methanol (Smith et al., 1978; Kroschwitz, 1996).
Uses of important palladium compounds
Ammine complexes of palladium: The compounds and reactions are important in the industrial separation of palladium, i.e., chloropalladosamine is a precurser of metallic palladium sponge. It is also used in electroless plating and bright palladium plating. Ammonium hexachloropalladate(IV) is important in separation technology.
Palladium(II) acetate: Palladium(II) acetate is of some importance in preparative chemistry. It is used as a catalyst.
Palladium(II) chloride: Palladium(II) chloride is used in plating baths. Pellets or monoliths of oxidation catalysts are either immersed in an aqueous solution of palladium(II) chloride (impregnation technique) or sprayed with a solution of this chemical.
Palladium(II) nitrate: Palladium(II) nitrate is used as a catalyst in organic syntheses and in the separation of chlorine and iodine.
Palladium(II) oxide: Palladium(II) oxide is used as a hydrogenation catalyst in the synthesis of organic compounds.
Hydrogen tetrachloropalladate(II): The solution of hydrogen tetrachloropalladate(II) is an industrially important palladium preparation. It is the starting material for many other palladium compounds, particularly catalysts.
Tetraammine palladium hydrogen carbonate: Tetraammine palladium hydrogen carbonate is used as an intermediate in the production of automobile catalysts.
Examples of the catalytic activity of palladium