Catalyst agglomeration

The Sonogashira reaction of 2-chloropyrazine 1-oxide gave only recovered starting material. Pentylation and octylation of 2-chloropyrazine 1-oxide also failed [9]. Possible explanations for these results were either catalyst agglomeration or metal formation from pyrazinylpalladium... 

After ARCO patents issued, Stille and coworkers published on butadiene oxycarbonylation(14-16). Palladium was utilized as the oxidative carbonylation catalyst and copper(II) chloride was employed as a stoichiometric reoxidation agent for palladium. Although the desired hex-3 -enedioate is the exclusive product, commercial technology which uses stoichiometric copper is not practical. Once the copper(Il) is consumed, the monoatomic palladium spent catalyst agglomerates affording polymeric palladium which is not easily reoxidized to an active form. 

Figure 28.4 shows a typical image of the primary particles that make up the catalyst agglomerates. These particles are single crystals of a-Fe203 as... 

In a hydrophobic system catalyst agglomerates form channels which are wetted with the electrolyte solution, most of the internal surface is not wetted. 

Will the electrocatalytic activity be maintained in spite of (a) catalyst poisoning phenomena, particularly on the anode side, and (b) catalyst agglomeration phenomena, particularly on the cathode ... 

Use of these materials can cause operating problems like reduced life of heat exchanger tubes, masking of active surfaces of catalyst, agglomeration of catalyst particles, and escape of acid mist from the stack exit gases. 

Circulating fluidized-bed reactors have been operated successfully at Sasol for many years. The major disadvantages for them are the need to operate at high temperature to obtain sufficient productivity with the lower surface area catalysts that are needed to obtain attrition resistance, the low alpha operation where only vapor-phase products are produced as is required to eliminate catalyst agglomeration, the energy required for catalyst... 

Part B of Fig. 4 shows the present state a significant part of the catalyst particles is not separated by PTFE fibers (gas pores). Thus, catalyst particles are clustered to agglomerates and flooded by electrolyte. Exclusively the outside surfaces of these catalyst agglomerates, which are covered by a thin electrolyte film, can operate as three-phase zones. All internal catalyst surfaces within the agglomerates are nearly inactive due to insufficient gas transport through the long distance inside of electrolyte [9]. 

Surface area loss of eathode platinum due to catalyst agglomeration was also detected by transmission electron microscopy (TEM) analysis and cyclic voltammetry. The behaviors of both eleetrodes were measured and anode degradation eould be attributed to the high anode potential. Fuel starvation eaused severe and permanent damage to the eleetroeatalysts of the PEMFC. 

A colloid method such as this should be more suitable for applying catalyst ink onto a porous GDL because the larger catalyst agglomerates do not easily penetrate deeply into the GDL. Improved cell performance was observed with the catalyst layer prepared using this colloid method, which could be attributed to the increased reaction area, the reduced internal resistance, and the enhanced reactant mass transport. 

Fig. 16.3 (a) Schematic representation of the spherical catalyst agglomerate model, (b) Modified agglomerate model with distribution of discrete Pt particles. Reproduced from [82] with permission of The Electrochemical Society... 

In summary, the core-shell catalyst can be used in the catalyst layer to reduce the Pt loading without sacrificing the cell performance of HT-PEMFC ME As. To maximize the O2 concentration at the catalyst particles, the modified catalyst agglomerates, as shown in Fig. 16.3, where the catalyst particles are placed in the outer part of agglomerate sphere can be adapted. The reported works indicate that the agglomerate size of 200-500 nm optimizes the diffusion of O2 in the gas phase and dissolved phase [85, 86]. 

To reduce the cost and improve the durability of HT-PEMFC MEAs, the schematic of catalyst layer shown in Fig. 16.5 is proposed. To reduce the amount of Pt used in the catalyst layer, the catalyst agglomerate configuration in which the ft particles are placed in the outer periphery of the... 

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