Platinum ammonia effects

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

MRR) to recover platinum. The extraction of palladium by the sulfide is slow with 2-3 hr to achieve equilibrium, which inhibits flow sheet design. However, it has been shown that tertiary amines can act as a kinetic accelerator, reducing equilibrium times to minutes, but no information has been published on the effect of these amines on palladium-platinum separation. High loading of the sulfides is possible, and stripping is easy with aqueous ammonia. The overall extraction and stripping equations are as follows ... 

A support-effect such as demonstrated in the classical case of platinum asbestos is of negligible importance with ammonia catalysts except with osmium and ruthenium. 

Currently, nitric acid is manufactured exclusively by catalytic oxidation of ammonia. Platinum or platinum-rhodium is an effective catalyst of this oxidation (Ostwald process). Three basic steps in such ammonia oxidation process are (1) oxidation of ammonia to form nitric oxide ... 

The importance of catalysts in chemical reactions cannot be overestimated. In the destruction of ozone previously mentioned, chlorine serves as a catalyst. Because of its detrimental effect to the environment, CFCs and other chlorine compounds have been banned internationally. Nearly every industrial chemical process is associated with numerous catalysts. These catalysts make the reactions commercially feasible, and chemists are continually searching for new catalysts. Some examples of important catalysts include iron, potassium oxide, and aluminum oxide in the Haber process to manufacture ammonia platinum and rhodium in the Ostwald synthesis of nitric... 

It appears that one of these rate constants (kc or kt) is 10 times as large as the other and anyone who has worked in platinum chemistry will say, of course, that chloride has a greater trans effect than ammonia. These are both weak trans directors, but one would expect the fast reaction site to be the cw-chlorides in other words, the chloride which is trans to a chloride, the cis-diaquo isomer, is the one which should be formed rapidly. 

Another instance of the same curious phenomenon can be cited. Hydrogen has a most pronounced inhibiting effect on the thermal decomposition of ammonia at the surface of a heated platinum wire. On the other hand, it is found to be almost without influence on the decomposition of hydrogen iodide on the surface of the same wire, Compt. Rend., 1922, 175, 277. 

E. C. C. Baly and H. M. Duncan studied the decomposition of ammonia by means of a hot platinum wire. Two types of ammonia may be prepared—an inactive and an active modification—which are decomposed to different extents by the same quantity of energy. The active form is obtained by the slow withdrawal of ammonia from a cylinder containing the compressed gas by warming the cone. aq. soln. and drying the gas by quicklime and by isothermal evaporation of the liquefied gas at its b.p. The inactive form is obtained by the rapid evaporation of the liquefied gas. The inactive gas slowly recovers its activity on remaining in contact with the liquefied gas. The same effect can be produced by gently warming the gas by means of a platinum wire heated at 200°. In order to observe these phenomena, the platinum wire must be activated in the same way as is customary in W. Ostwald s process for the catalytic oxidation of ammonia in air to nitric acid. Alternatively,... 

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