Effect of Pt dispersion

Figure 10.8. Effect of Pt dispersion on CO oxidation rate over aged CeO] and Ceo75Zro2502 supports with differeni degrees of reduction. [ 17]...

The aim of this work is to study the effect of Pt dispersion and the addition of a second metal (Au) in the selective carvone hydrogenation. 

Bhatia D, Harold MP, Balakotaiah V (2010) Modeling the effect of Pt dispersion and temperature during anaerobic regeneration of a lean NOx trap catalyst, Catal. Today 151 314-329... 

Ceria-based OSC compounds may have an impact on oxidation reactions especially when the catalysts are working around the stoichiometry (as this is the case under TW conditions). One of the first systematic studies was reported by Yu Yao [53,54], Most results were obtained in 02 excess (0.5% CO + O.5% 02 or 0.1% HC+ 1% 02). Several series of Pt, Pd and Rh/Al203 of various dispersion, as well as metal foils, were investigated in CO, alkane and alkene oxidation. The effect of metal dispersion in CO and the propane oxidation are shown in Figure 8.5. 

Figure 6.25 Effect of Pt surface density (pmol/m2) and calcination on the dispersion of reduced Pt/silica catalysts prepared by SEA. x, literature, calcined between 373 and 573 K. This study solid triangle and solid line — dried at 373 K small open triangle and dotted line — calcined at 523 K and large open triangle and dashed line — calcined at 573 K. (From Miller, J.T., Schreier, M., Kropf, A.J., and Regalbuto, J.R., J. Catal. 225, 2004, 203.)...

This difference may be explained by a higher specific activity of Pd, compared with Pt, for oxidation of CO and alkenes [13]. Furthermore, the CO-TPD measurements (see Table 1) show that Pd adsorbs more CO than Pt when supported on alumina which probably is an effect of higher dispersion. This may also contribute to the higher activity of Pd/AbOs than Pt/AbOs. 

The reason for this is the relative instability of Pt oxide and its tendency to decompose at temperatures <550 C even under oxidizing conditions. Platinum oxide does not penetrate into the subsurface of any support material. To achieve a high dispersion additives are used in which the oxygen ions are more reactive than in 7-AI2O3 as for instance CeC. Thus the addition of 2.6% ceria to 7-AI2O3 increases the surface concentration of PtO from 2.2 /imole Pt/m to 4.2 /imole Pt/m (BET) [3]. As will be shown below other more reactive metal oxide additives have a similar effect on Pt dispersion. This, in turn affects the behavior of the catalysts with respect to several structure sensitive reactions. 

Fig. 27. Effect of Ce02 addition on stability of Pt dispersion in 0.9 wt% Pt/Al203 (squares) and 0.9 wt% Pt/Ce02(25 wt%)/ A1203 (circles). (Diwell et al. 1991.)...

Effect of Pt and PtPd or Ptir on their Dispersion in H-MOR and H-BEA Zeolites... 

In many studies of oxidation reactions in our laboratory, dating back to the early 80s, and more recently in our smdy of the role of Cl on Pt supported catalysts, we have carefully measured reaction rates independent of mass transfer effects. In these studies, we have observed that the conversion and the reaction rate per unit mass were independent of Pt dispersion. Consequently, when reaction rates are calculated per Pt area, i.e., as turnover frequency, the catalysts with the larger Pt area (high dispersion) have the lowest TOP. The presence of oxygen makes it difficult to determine uniquely the cause of this result because it is not clear if the surface is metallic, oxidized, or a mixture of oxide and metallic phases. Whereas this is true for supported Pd and Rh catalysts, in the case of Pt catalysts, we found that, as suggested by Burch and Loader, - platinum particles seem to have a strong memory of previous reduction pretreatments, as they remain in metallic state under oxidizing conditions. Because most surface analysis techniques are conducted under vacuum or ex situ, it is not always clear if the same oxidation state is valid under reaction conditions at atmospheric pressure. Controlled atmosphere EXAFS and... 

Particle size effects of highly dispersed supported catalysts (Pt) on the hydrogen oxidation reaction were evaluated by the same authors using this technique (97). Moreover, electrocatalysts for oxidation of methanol were screened using a technique called scanning differential electrochemical mass spectrometry (98, 99). This method uses a capillary probe scanned over the array that allows the intake and detection by mass spectrometry of products generated locally on each electrode. 

Belt JA, Kinoshita K, Stonehart P (1976) Crystallite growth of Pt dispersed on graphitized carbon black ii effect of liquid environment. J Catal 41 124-133... 

Bimetallic catalysts have been the subject of great interest for a long time because of their exceptional properties compared to the monometallic catalysts, yet the reason behind their improved activity is still a question of debate and they are subject of many recent studies. Tanskale et al. [39] have studied the promoting effect of Pt and Pd in bimetallic Ni-Pt and Ni-Pd catalysts supported on alumina nanofibre (Alnf) for the liquid phase reforming of sorbitol to produce hydrogen. Fig. 4.19 shows TPD profiles for CO desorption for several monometallic and bimetallic catalysts dispersed on alumina nanofibre. 

Figure 12.4. Transient effect of an applied negative current (I=-20mA) on the reaction rate r of CyT, oxidation on Pt finely dispersed on Au supported on YSZ (solid curve) and on the catalyst potential Uwr (dashed curve). Conditions catalyst C2, T=42l°C, po2=14.8 kPa, pc2H4=0.l kPa, fiow=411 ml/min, open circuit rate ro=0.037xl0 6 mol/s.7 Reprinted with permission from Academic Press.

Figure 12.5. Ethylene oxidation on Pt finely dispersed on Au supported on YSZ.7 Effect of the current 1 on x 1, where x is the time constant measured during a galvanostatic transient experiment with I as the applied current x is obtained by fitting either r/r0=exp(-t/x) or l-exp(-t/x) to the experimental data depending on the sign of the current and whether the reaction is electrophilic or electrophobic, (a) Positive values of I for electrophilic (squares, T=371°C, pO2=18.0 kPa, Pc2H4=0-6 kPa) and electrophobic behavior (circle, T=421°C, p02=l 4.8 kPa, Pc2H4 CU kPa) (b) negative currents, electrophilic behavior (T=421°C, p02=14.8 kPa, pC2H4=0.1 kPa. Reprints with permission from Academic Press.

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

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