Oxygen, reaction with platinum

This overview is organized into several major sections. The first is a description of the cluster source, reactor, and the general mechanisms used to describe the reaction kinetics that will be studied. The next two sections describe the relatively simple reactions of hydrogen, nitrogen, methane, carbon monoxide, and oxygen reactions with a variety of metal clusters, followed by the more complicated dehydrogenation reactions of hydrocarbons with platinum clusters. The last section develops a model to rationalize the observed chemical behavior and describes several predictions that can be made from the model. 

Platinum has also had its share of attention in recent years. The effect of phosphoric acid concentration on the oxygen evolution reaction kinetics at a platinum electrode using 0-7 m-17-5 m phosphoric acid at 25°C has been studied with a rotating disc electrode . The characteristics of the ORR are very dependent on phosphoric acid concentration and H2O2 is formed as an intermediate reaction. Also, platinum dissolution in concentrated phosphoric acid at 176 and 196°C at potentials up to 0-9 (SHE) has been reported . 

In 1962, the English chemist Neil Bartlett overturned the conventional wisdom. Bartlett was exploring the reactions of platinum hexafluoride, an extremely reactive molecule. He found that PtFg reacted cleanly and rapidly with molecular oxygen O2 FPIFg —> O2 FPlFg ... 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

The reaction is exothermic (AH298 = -320.91 kj). Hydrogen is present as a contaminant in hydrogen fluoride, and this results in the formation of plutonium trifluoride (PuF3). Oxygen is added to avoid this. Fluorination is carried out in Hastealloy-C boats lined with platinum. 

Co-limited kinetics with a significant utilization region. As with platinum, the model predicts that the chemical portion of the reaction will be co-limited by molecular dissociation and transport. Values of k calculated from the model for the analyzed conditions vary from 0.4 to 20 gvc depending on Pq2 temperature, and electrode surface area, with typical values in the 3—5 gm range. This result indicates that a significant portion of the electrode surface is active for oxygen reduction, which explains Takeda s (and other s) observation that the performance of LSG electrodes on YSZ improves with thickness up to a... 

The hexafluoride is a very powerful oxidizing agent reacting violently with most oxidizable substances. Reaction with liquid water is violent forming HF, oxygen, lower fluorides of platinum, and other products. In vapor phase hydrolysis occurs more smoothly. 

Another issue with platinum catalysts is that their capacity sometimes fades over time. Several factors are responsible, including a phenomenon similar to the side effects described for medications in chapter 3. Side effects occur when a medication acts on healthy tissue instead of the intended target. With platinum electrodes, the problem is that sometimes unwanted reactions occur at the electrodes. In the oxygen reactions taking place at the cathode, for example, hydroxide (OH) and other molecules sometimes form and bind to the platinum atoms. These molecules cover the platinum atoms and block access to the desired reactant, thereby reducing the catalytic activity. Sometimes the molecules even pull platinum atoms away from the surface, causing serious electrode degradation. 

Beyer and coworkers later extended these reactions to platinum clusters Ptn and have demonstrated that similar reaction sequences for the oxidation of carbon monoxide can occur with larger clusters [70]. In addition, they were able to demonstrate poisoning effects as a function of surface coverage and cluster size. A related sequence for Pt anions was proposed by Shi and Ervin who employed molecular oxygen rather than N2O as the oxidant [71]. Further, the group of Bohme has screened the mononuclear cations of almost the entire transition metal block for this particular kind of oxidation catalysis [72,73]. Another catalytic system has been proposed by Waters et al. in which a dimolybdate anion cluster brings about the oxidation of methanol to formaldehyde with nitromethane, however, a rather unusual terminal oxidant was employed [74]. 

Oxidation of Elaidic Acid Ozonization Products. Aliquots of the unseparated ozonization products from elaidic acid were autoxidized at 95 °C. uncatalyzed and in acetone over reduced platinum oxide as before. Total yields of acids and esters were determined by titration and were found to be 74.6 and 19.2%, respectively, in the catalyzed reaction with uptake of 63% of the theoretical volume of oxygen. Time required for uptake of half this volume was 4 hours at 21 °C. Uncatalyzed oxidation at 95°C. of the other fraction gave 27.4% yield of esters and 74.5% yield of acids, calculated on the assumption that one original olefinic linkage can produce one ester function or two acid functions. When elaidic acid was ozonized in methyl acetate and the catalyzed oxidation performed in the same solvent, acid yield was 80.8%, and ester yield was 7.3% with a half-uptake time of 5.6 hours and 88% of the theoretical quantity of... 

The ease with which the hexosyl-4-ulose derivative 109 undergoes dehydration was demonstrated in a non-enzymic, model reaction. Oxidation of methyl /8-D-galactopyranoside with oxygen and a platinum catalyst, followed by hydrogenation over the same catalyst, resulted in a 35% yield of methyl /8-D-fucopyranoside,423 presumably formed through reactions analogous to those in Fig. 3. 

The mechanism of this strongly inhibited reaction has not yet been explained in detail, even with platinum, the most intensively studied of all catalysts 26>. The results obtained to date show that the course of the reaction is not the same on all catalysts and that other factors, such as oxygen absorption on platinum metal catalysts, play an important part. In most of the reaction mechanisms hitherto formulated and discussed H2O2 occurs as an intermediate stage 26> ... 

---