Rhodium Finely divided

The uncatalyzed addition of hydrogen to an alkene although exothermic is very slow The rate of hydrogenation increases dramatically however m the presence of cer tain finely divided metal catalysts Platinum is the hydrogenation catalyst most often used although palladium nickel and rhodium are also effective Metal catalyzed addi tion of hydrogen is normally rapid at room temperature and the alkane is produced m high yield usually as the only product... 

The conditions for hydrogenation of alkynes are similar to those employed for alkenes In the presence of finely divided platinum palladium nickel or rhodium two molar equivalents of hydrogen add to the triple bond of an alkyne to yield an alkane... 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

The resistance of rhodium to chemical attack is remarkable, and surpasses that of platinum. Its domain of stability (as seen from Fig. 6.4) is extremely wide, and in the absence of complexing agents it is stable in aqueous solutions of all pH values. In the massive form it is unattacked by caustic alkalis, acids and oxidising agents, including aqua regia. When finely divided, however, it is attacked by concentrated sulphuric acid and aqua regia. 

The most commonly used catalysts for hydrogenation (finely divided platinum, nickel, palladium, rhodium, and ruthenium) apparently serve to adsorb hydrogen molecules on their surface. 

The most active catalyst is platinum applied in finely divided form, for example platinised asbestos. Certain elements, especially arsenic and mercury, have a powerful effect in reducing the activity of the platinum, a quantity of arsenic equal to 0-2 per cent, of the weight of the platinum reducing the activity by 50 per cent.5 These poisons, as they are termed, also include less harmful substances such as antimony, lead, bismuth, etc. The presence of small quantities of rhodium, iridium or osmium in the platinum also causes diminished yields of trioxide, but the presence of palladium or ruthenium has the opposite effect.6... 

Rhodium Arsenide, E.hAs2, has been prepared in a pure form by heating rhodium chloride with an excess of arsenic in an atmosphere of hydrogen.8 If finely divided rhodium is heated with excess of arsenic in an atmosphere of an indifferent gas, the arsenide produced is not pure.9... 

In research al the Institute or Radiochemistry. Karlsruhe, West Germany during Ihe early 1970s. investigators prepared alloys of Curium with iridium, palladium, plalinum. and rhodium. These alloys were prepared by hydrogen reduction of the curium oxide or fluoride in the presence of finely divided noble metals. The reaction is called a coupled reaction because the reduction of the metal oxide can be done in the presence of noble metals. The hydrogen must be extremely pure, w ith an oxygen content of less than 10 -s Inrr. 

Rhodium was discovered by Wollaston (England) in 1803, Compact Rh is almost insoluble in all acids at 100cC, including aqua regia. Hot concentrated HiSCLi will slowly dissolve the finely divided metal. When alloyed with 90% or more of Pt, it is soluble in aqua regia. The metal is attacked by fused bisulfates. Rh is soluble in molten Pb, This is the basis of the classic separation of Rh and Ir. 

Rh compounds exhibit valences of 2, 3, 4, and 6. The tnvalent form is by far the most stable. When Rh is heated in air, it becomes coated with a film of oxide. Rhodium(III) oxide, Rh Os, can be prepared by heating the finely divided metal or its nitrate in air or O2. The rhodium IV) oxide is also known. Rhodium trihydroxide may be precipitated as a yellow compound by adding the stoichiometric amount of KOH to a solution of RhCb. The hydroxide is soluble in adds and excess base. When the freshly precipitated Rh(OH) is dissolved in HC1 at a controlled pH, a yellow solution is first obtained in which the aquochloro complex of Rh behaves as a cation. The hexachlororhodatetHI) anion is formed when the solution is boiled for 1 hour with excess HC1. The solution chemistry of RI1CI3 is often very complex. Two trichlorides of Rh aie known The trichloride formed by high-temperature combination of the elements is a red, crystalline, nonvolatile compound, insoluble in all aads. When Rh is heated in molten NaCl and treated with Clo, Na RJiClg is formed, a soluble salt that forms a hydrate in solution. Rhodium(III) iodide is formed by the addition of KI to a hot solution of tnvalent Rh. 

Platinum in a finely divided form is obtained by the in situ reduction of hydrated platinum dioxide (Adams catalyst) finely divided platinum may also be used supported on an inert carrier such as decolourising carbon. Finely divided palladium prepared by reduction of the chloride is usually referred to as palladium black. More active catalysts are obtained however when the palladium is deposited on decolourising carbon, barium or calcium carbonate, or barium sulphate. Finely divided ruthenium and rhodium, usually supported on decolourising carbon or alumina, may with advantage be used in place of platinum or palladium for some hydrogenation reactions. 

As an example, we want to improve the chemical synthesis of a drug. It is already known that finely divided platinum is a good catalyst for the reaction, but we want to investigate the potential use of other related metals. We therefore perform the synthesis of the drug under fixed conditions of pressure, temperature, etc., but vary the catalyst added. The metals investigated are platinum, palladium, iridium and rhodium and additionally an alloy of palladium and iridium. Five replicate... 

Explosive Ruthenium is obtained by dissolving an alloy of the metal with excess of zinc in hydrochloric acid. The zinc passes into solution, leaving metallic ruthenium as a finely divided, explosive residue. Unlike rhodium and iridium, ruthenium is explosive even when prepared in the entire absence of air. It seems hardly possible, therefore, that the same explanation for the explosivity can apply as for the first two metals (see pp. 156, 239). Perhaps Bunsen s original explanation is the correct one, namely, that an unstable modification or allotrope is first formed, and that this is converted into the stable variety with considerable heat evolution.7... 

Explosive Rhodium.—In 1868 Bunsen8 accidentally discovered that several of the platinum metals can be obtained in an explosive form. Rhodium is a case in point. If alloyed with excess of zinc or cadmium, and the product treated with hydrochloric acid, the zinc (or cadmium) passes into solution, leaving an insoluble residue of finely divided explosive rhodium. 

It appears probable, therefore, that rhodium trichloride undergoes partial dissociation at dull red heat, i.e. at about 550° C. The reaction between chlorine and finely divided rhodium begins 3 at about 250° C. 

Potassium Pentachlor-rhodite, K2RhCl5, is obtained in the anhydrous condition when finely divided rhodium is heated to redness with potassium chloride in a current of chlorine (Berzelius). 

Rhodium Tribromide, RhBr3.—Bromine begins to act on finely divided rhodium at about 250° C, but the product has an-uncertain composition attributable to partial dissociation at the temperature of formation. Thus 3 ... 

Potassium Pentabromrhodite, K RhBrg, is obtained by heating finely divided rhodium with potassium bromide in a stream of bromine... 

On heating to dull redness a mixture of finely divided rhodium and phosphoric acid, a phosphate is obtained which is soluble in water,3 whilst a basic phosphate results on treating rhodium sesquioxide with phosphoric acid. 

Detection.—Metallic Iridium, like rhodium, is insoluble in all acids, save that in a very finely divided condition it is slowly attacked by aqua regia. Fusion with potassium hydrogen sulphate oxidises the metal but does not effect its solution (contrast ruthenium and rhodium). When fused with a mixture of potassium nitrate and hydroxide an insoluble residue containing the sesquioxide, lr203, with alkali is obtained. 

The massive metal is inert, but very finely divided forms are more reactive, absorbing large quantities of either CO or H2. While rhodium black or rhodium sponge dissolves in aqua regia or aqueous chlorine under pressure, larger pieces must be fused with sodium hydrogen sulfate to effect dissolution. 

---