Deactivation of platinum catalysts

Promotion and Deactivation of Platinum Catalysts in Liquid-Phase Oxidation of Secondary Alcohols... 

Chemical deactivation by adsorbed impurities or reaction products was identified as a primary cause of catalyst deactivation [42,43,45-48,50]. Deactivation of platinum catalysts in l-methoxy-2-propanol oxidation was attributed to polymeric species formed by aldol-dimerization and detected by chromatographic... 

B. Mendyka, A. Musialik-Piotrowska, K. Syczewska. Effects of chlorine compounds on the deactivation of platinum catalysts. Catalysis Today, 11 597-6810, 1992. 

V. Hlavacek and O. Mikus, "Deactivation of Platinum Catalysts by Poisons a Study of Behavior of afterburner Converters", Chem. Eng. Commun.. 1982, IS, 1-14. 

Platinum is generally acknowledged as the most effective catalyst for the electroreduction of oxygen in a wide range of conditions (e.g. fuel cells). In the instance of aqueous HC1 electrolysis, the basic drawback is corrosion or deactivation of the catalyst during cell shutdown, owing to chemical attack from HC1 and chlorine that diffuse across the membrane. 

Catalyst poisoning. The more or less permanent deactivation of a catalyst by chemical reaction with a contaminant. Sulfur will poison platinum catalysts vanadium will poison zeolyte catalysts. 

The Pt-Re system has been studied extensively since the 1970s because adding Re to AhOs-supported platinum catalysts increases the resistance to deactivation of the catalysts used in naphtha reforming by preventing coke deposition. By using carbonyl precursors, well-defined bimetalhc species have been prepared. A proper characterization of these species allowed a relationship to be established between their structure and their catalytic behavior. Table 8.3 shows several Pt-Re bimetaUic catalytic systems prepared using different carbonyl species in which Pt-Re interactions determine the catalytic behavior. 

To improve the contact of reacting components with the catalyst, the reactor is filled with a carrier, pumice, which is pre-processed with hydrochloric acid to eliminate traces of iron, which deactivates the platinum catalyst. 

Whilst the ability of platinum-based catalysts to effect the dehydrogenation of alkanes to the corresponding alkenes is well established [1-4], carbon laydown and consequential deactivation of the catalyst during the dehydrogenation reactions is a well known phenomenon... 

The paraffins dehydrogenation on platinum-alumina catalysts proceeds with constant rate up to some time-on-stream after which a slow deactivation of the catalysts takes place Since relative changes of the catalyst activity ( characterized by reaction rate) are proportional to relative amounts of the deposited coke it can suppose that coke formation is the main reason of deactivation. Deactivation can be related with an attainment of a threshold in coke concentration (Co) on catalysts. The threshold amounts are 1.8 wt.% for A-I, 6,8% and 2.2% for A-II and A-IXI catalysts respectively. The isobutane dehydrogenation in non-stationary region (C > Co) is described by the following kinetic equation ... 

To investigate the deactivation of these catalysts, a Co-containing catalyst was prepared by adding ca. 1000 ppm of Co to a cobalt and platinum supported on... 

By using close-to-neutral conditions for the catalytic oxidation of glyceric and tartronic acids on platinum-bismuth catalysts to their respective keto derivatives, deactivation of the catalyst by adsorbed acids may be reduced, leading to higher degrees of conversion and improved yields. In addition, higher loadings of bismuth promoter may also serve to reduce deactivation by adsorbed acids. 

The lower rate of deactivation of platinum-rhenium catalysts relative to platinum catalysts cannot be attributed to a lower rate of accumulation of carbonaceous residues on the surface. For a given time on stream, the amount of such residues on the surface is not decreased by the presence of the rhenium. This point is interesting because metallic rhenium, like metallic iridium, has much higher hydrogenolysis activity than platinum (26). It is possible that the difference between platinum-rhenium and platinum-iridium catalysts is due to the strong retention of sulfur by the former. The inhibiting effect of sulfur on hydrogenolysis activity is well known. The improved activity maintenance of a platinum-rhenium catalyst relative to a platinum catalyst is due to better tolerance of the carbonaceous residues. 

Butane has been isomerized to isobutane in 95% yield (at 24.5% conversion) with platinum/Cs2 5H0.5PW12O40 at 200-300X. under hydrogen at 0.05 atm.294 The hydrogen reduced deactivation of the catalyst. The oxidation of isobutane to isobutylene is needed for the synthesis of tert-butyl methyl ether to put into gasoline. Isobutylene, in turn, can be oxidized to methacrylic acid for conversion to methyl methacrylate, an important monomer (6.56). Making methyl methacrylate this way avoids the use of toxic hydrogen cyanide in the present commercial process. 

AI2O3 was modified with MgO and was used as support in the preparation of platinum catalysts. The prepared catalysts were characterised by Scaning electron microscope (SEM), X-ray diffraction (XRD), chemisorption ofH2and BET surface area and were tested for their activity in hydrodechlorination of chlorobenzene. The investigations show that support modification reduces catalyst deactivation remarkably. The catalysts are also found to be active and selective for the hydrodechlorination of chlorobenne reaction. 

Despite the uncertain namre of the catalyst and mechanistic pathways, the simplicity of the system using commercially available NiBr2 makes it attractive for some laboratory hydrogenations without the need for H2. A further advantage is that the system operates xmder aerobic conditions, while the often used platinum metal, phosphine-containing conqtlexes are usually air-sensitive in solution (1, 17). Further, the solid inorganic residue obtained after a reduction of C5H10O could be used for repeat conversions, when only slow deactivation of the catalyst was noted 1st run, 99.9% conversion after 30 min 2nd, 95% (3 h) 3rd, 92% (7 h) 4th, 63% (3 h). 

The rate of deactivation of platinum oxidation catalyst leads to a first order dependence on lead concentration at low lead levels (see Figure 3). 

PEP experiments are performed using either NHs or [ 0]-02 to obtain further insight in the reaction mechanism and deactivation behaviour. Transient ammonia pulse experiments are performed to study the adsorption and dissociation of ammonia on pure platinum catalysts, followed by the focus on the deactivation of platinum. Finally, we will briefly discuss the influence of the alumina support. 

However, above 413 K and also on the pre-oxidised catalyst, the high activity and selectivity towards nitrogen sustains. The presence of oxygen at the platinum surface apparently does not cause a permanent deactivation of the catalyst. Above 413 K, the catalyst is reduced by ammonia. 

The PEP and TPD experiments indicated that after the deactivation of the catalyst, the surface of platinum is fully covered with NH and NH2 species. This means that the endothermic reactions between NHx and OH are not proceeding very fast. The production of water through the reaction of two hydroxyls is much faster... 

Deactivation of the catalyst is not observed at these temperatures and only N2 and H2O are formed. A different reaction mechanism depending on the surface coverage is suggested by the reactivation experiment (Fig. 15), in which N2O formation rapidly decreases. This decrease cannot be explained only by the decomposition of N2O at platinum just low amounts of N2O can be decomposed... 

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