Pt agglomeration

For a zeohtic catalyst where Pt, Pd or other transition metal might be present to provide metal activity, STEM can be used to determine whether the metal is agglomerated and to what extent, whether the metal is in the zeolite or present on the geometric exterior or whether the metal is associated with the zeolite or binder. As an example of the utility of the technique. Figure 4.15 shows the growth of Pt clusters for fresh and spent faujasite zeolite catalyst. After time under reaction conditions, the Pt clusters have grown from Inm to 2nm. The clusters have remained in the channels of the faujasite. Pt agglomeration can be concluded as the deactivation mechanism. 

Cathode lifetime durability presents a special challenge in PEM fuel cells. The major problems associated with the catalysts are Pt agglomeration, sintering, dissolution, and redistribution. The cathode environment is highly oxidative and corrosive due to high voltage (e.g., 0.6 to 1.0 V), low pH, elevated temperature, and the presence... 

Rodriguez-Reinoso et al. [121,124,125] claim that the necessity of decomposing supported precursor ammonia complexes stems from the fact that in the presence of H2 they produce an intermediate labile hydride [Pt(NH3)4]H2 [131], which leads to Pt agglomeration. However, the existence of that on a carbon surface is to a large extent speculative. In addition, this interpretation does not explain the fact that the calcination procedure is not needed to form fine Pt particles from adsorbed hydroxide [Pt(NH3)4](OH)2 and does not cause any changes if protons of the surface acidic sites are substituted by Ca ions prior to supporting [Pt(NH3)4]Cl2 [126]. 

Moreover, the presence of Pt agglomerates in Pt/C electrocatalysts brings about an additional peak in a COads stripping voltammogram, which is shifted 100 mV negative versus CO oxidation on Pt particles 3.3 mn in size... 

Time-on-Stream Behavior of Pt/KL-zeolite and Pt/y-AfOs in n-C7 Aromatization — Pt agglomeration is the major cause of Pt/KL-zeolite deactivation... 

Fig. 16.2 Diagrams representing the changes within an HT-PEMFC MEA during cell operation. The Pt agglomeration occurs at a higher rate initially and slower rate in latter stage. The acid loss occurs at a...

The addition of some metal oxides into the cathode Pt/C system has been studied to facilitate the supply of oxygen species and prevent Pt agglomeration, leading to enhanced fuel cell performance and durability. For example, CeOj was used as a promoter for Pt/C catalyst. Xu et al. (2003) reported that its... 

There have been also reports where catalyzers are said to be more stable if supported by CNTs. For example, it has been reported (Lee et al, 2012) the loss of oxygen-related functional groups which are known to accelerate carbon corrosion and Pt agglomeration. In that study, authors observed an effect of catalytic cleavage of hydrogen molecules followed by spillover and a dehydration reaction. 

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