The Influence of Phosphorus Poisoning

2 The Influence of Phosphorus Poisoning. - Catalytic oxidation using noble metal catalysts has been used to reduce the concentration of unburned hydrocarbons, carbon monoxide pollutants released from internal combustion engines, and similar applications. It is well known that contaminants arising from lubricants, (P, Ca, and Zn) deactivate these catalysts. Phosphorus compounds in printing processes are the source of decay of noble metal catalysts used to control these emissions. 

Tzou and Weller (1994) have studied the catalytic oxidation of dimethyl methylphosphonate (DMMP), a nerve gas stimulant, over laboratory-prepared Pt/AbOs as a function of Pt loading (0.5 or 2.0%) and temperature (150, 250, or 400 °C).A conventional flow system was used for the activity studies. They identified intermediate phosphorous-containing compounds in the reactor effluent after catalyst activity decline and looked for evidence of aluminum phosphate in the catalyst, which could be produced by reaction of alumina support with phosphoric acid. 

The deactivated catalyst was studied by several methods scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS), infrared spectroscopy (IR), and by extracting water-insoluble phosphorus. The SEM-EDS studies gave no useful results. IR absorption was measured on samples that were mulled in mineral oil. Comparisons of IR spectra were made with samples of y -alumina and aluminum phosphate. Determination of total P in the deactivated sample, presumed to be present as water-insoluble aluminum phosphate, was made by standard wet chemical analysis dissolution in hot, dilute HCl followed by colorimetric determination of phosphate.  

As expected, the 2.0% Pt/AbOB catalyst showed less rapid deactivation than the 0.5% catalyst, as measured by breakthrough time in the fixed bed reactor at 250 °C (75 vs. 50 h). At 150°C the catalyst showed a very short breakthrough time of only 5-6 h and the CO2 production never exceeded 50% of theoretical, suggesting incomplete oxidation even on the fresh catalyst. At 400 °C, a decline in CO2 concentration with time, coupled with a lack of unreacted DMMP in the effluent, suggested that the phosphorous was retained within the catalyst. 

An earlier study using this same compound, DMMP, led to a mathematical model of the deactivation process. Graven et a/. studied the oxidation of DMMP vapor in a stream of air, or nitrogen, over platinum-alumina catalysts. A commercial catalyst and a number of laboratory-prepared catalysts were investigated over a range of temperatures from 573-773 K, residence times from 0.15 to 2.7 seconds. The average catalyst particle sizes varied from 0.31 to 2.4 mm. They found that the fresh catalyst showed a very high activity, but after a few hours on stream it deactivated to the point that measurable quantities of DMMP vapor appeared in the effluent.. The reaction products over the deactivated catalyst were methanol and phosphorus acid. 

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