Platinum catalyst degradation

Aindow TT, Haug AT, Jayne D (2011) Platinum catalyst degradation in phosphoric acid fuel cells for stationary applications. J Power Sources 196 4506 514... 

Key words fuel, hydrogen, methane, methanol, biogas, alkaline fuel cell (AFC), polymer electrolyte fuel cell (PEFC), phosphoric acid fuel cell (PAFC), platinum, catalyst, degradation, sulphur, carbon monoxide, poisoning, particulates. 

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

Photolytic. Water containing 2,000 ng/pL of dibromochloromethane and colloidal platinum catalyst was irradiated with UV light. After 20 h, dibromochloromethane degraded to 80 ng/pL bromochloromethane, 22 ng/pL methyl chloride, and 1,050 ng/pL methane. A duplicate experiment was performed but 1 g zinc was added. After about 1 h, total degradation was achieved. Presumed transformation products include methane, bromide, and chloride ions (Wang and Tan, 1988). 

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. 

EMDE DEGRADATION. Modification of the Hofmann degradation method for reductive cleavage of the carbon-nitrogen bond by treatment of an alcoholic or aqueous solution of a quaternary ammonium halide with sodium amalgam. Also used as a catalytic method with palladium and platinum catalysts. The method succeeds with ring compounds not degraded by the Hofmann procedure. 

Yu X, Ye S, (2007). Recent advances in activity and durability enhancement of Pt/C catalytic cathode in PEMFC. Part II Degradation mechanism and durability enhancement of carbon supported platinum catalyst. Journal of Power Sources 172 145-154... 

The two metals that have been found to give encouraging conversions and selectivities for the hydroformylation of styrene are platinum and rhodium. The platinum-based catalytic system uses tin chloride as a promoter. It also uses triethyl orthoformate as a scavenger that reacts with the aldehyde to form the acetal. By removing it as soon as it is formed, any further degradative reactions of the aldehyde are avoided. The chirality in these reactions is induced by the use of optically active phosphorus ligands. With the best platinum catalyst, branched and linear aldehydes are produced in about equal proportion, but the former has an e.e. of >96%. 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

T he successful use of platinum monolithic oxidation catalysts to control automobile emissions over many thousands of miles requires an intimate understanding of the many factors which contribute to catalyst degradation. Contamination of the active catalyst by lead and phosphorus compounds present in fuel and lubricating oil is a major factor in catalyst deterioration. 

The activities of fresh, supported platinum and base metal oxidation catalysts are evaluated in vehicle tests. Two catalysts of each type were tested by the 1975 FTP in four 600-4300 cm3 catalytic converters installed on a vehicle equipped with exhaust manifold air injection. As converter size decreased, base metal conversions of HC and CO decreased monotonically. In contrast, the platinum catalysts maintained very high 1975 FTP CO conversions (> 90% ) at all converter sizes HC conversions remained constant 70% ) at volumes down to 1300 cm3. Performance of the base metal catalysts with the 4300-cm3 converter nearly equalled that of the platinum catalysts. However, platinum catalysts have a reserve activity with very high conversions attained at the smallest converter volumes, which makes them more tolerant of thermal and contaminant degradation. 

Thermal Stability. Our initial work on catalyst degradation processes was designed to answer qualitatively several simple yet vital questions. This report is organized similarly. The question naturally arises whether platinum or palladium is best for automotive emission control. One aspect... 

It is perhaps not immediately obvious that the precious-metal catalysts that are employed for use in PEM fuel cells will be subject to degradation, agglomeration, and even dissolution. Most of us are familiar with platinum as an example of a noble metal, which, according to its definition, means that it resists chemical action and does not corrode. Yet there is compelling evidence that platinum can degrade under conditions experienced in the fuel cell operating environment. Within the catalyst and separator of the fuel cell, the conditions are quite acidic, and the presence of oxygen results in an environment that is extremely oxidative. 

DMFC performance loss due to catalyst degradation has been attributed to several factors a decrement of the electrochemically active surface area (ECSA) of the platinum electrocatalyst supported on a high-surface-area carbon, a loss of cathode activity towards the ORR by surface oxide formation, and ruthenium crossover [83, 85, 116, 117]. 

Much of the knowledge in Pt/C durability derives from the experience with phosphoric acid fuel cells (PAFCs) at operating temperatures of about 200°C. Catalyst degradation is witnessed as an apparent loss of platinum electrochemical surface area over time, " associated with platinum crystal growth. These changes are ascribed to different processes which include... 

Platinum catalyst Toluene-induced cathode degradation Li et al., 2008... 

Chen S, Gasteiger HA, Hayakawa K, Tada T, Shao-Hom Y (2010) Platinum-alloy cathode catalyst degradation in proton exchange membrane fuel cells nanometer-scale compositional and morphological changes. J Electrochem Soc 157(1) A82-A97... 

Various degradation mechanisms based on undesired side reactions of the platinum catalyst and carbon support were introduced in Section 20.1. In this section, we focus on degradation under dynamic operation at standard conditions. 

The results for constant potential holds over 400 h show that the loss of electrochemically active platinum surface area is neghgible at 0.87 and 1.2 V, respectively, whereas it is significant at 1.05 V [72]. The reason is that at 0.87 V the reaction kinetics are slow whereas at 1.2 V a PtO monolayer is formed, blocking any dissolution or precipitation. When the electrode is held at intermediate potentials, significant catalyst degradation occurs. 

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