Chemisorption platinum supported catalysts

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

There is a wealth of information available on CO chemisorption over single-crystal and polycrystalline platinum surfaces under ultrahigh-vacuum conditions research efforts in this area have gained a significant momentum with the advent of various surface analysis techniques (e.g., 2-8). In contrast, CO chemisorption on supported platinum catalysts (e.g., 9, 10, 11) is less well understood, due primarily to the inapplicability of most surface-sensitive techniques and to the difficulties involved in characterizing supported metal surfaces. In particular, the effects of transport resistances on the rates of adsorption and desorption over supported catalysts have rarely been studied. 

The application of selective chemisorption to supported Pt catalysts is well established but there have been valuable additional studies of the use of hydrogen in the pulse-flow technique and of CO adsorption using TPD and carbon monoxide. Recently the usual assumption about the stoicheiometry for hydrogen adsorption, Ptg/H = 1 has been questioned. For the Council of Europe Pt-Si02 catalyst, where a weak metal-support interaction was postulated, 1.75 hydrogen atoms per surface metal atom were found at 300 K in two adsorbed forms (the formation of jSa was activated). Recent work on selective chemisorption applied to metals of catalytic interest other than platinum will now be examined. 

We obtained initial rates for the reaction of neopentane on supported platinum and platinum powder catalysts at 300°, 1 atm total pressure, and a hydrogen-to-neopentane ratio equal to 10. As before, surface platinum atoms were titrated by selective chemisorption of hydrogen (27). Before discussing the results, it is important to stress the reproducibility of the results on samples of different origin but nearly identical dispersion and pretreatment. Thus, the same value of the selectivity to isomerization was found on two catalysts an experimental catalyst containing 2% platinum on t -alumina and a commercial sample with 0.6% platinum on y-alumina. Percentage dispersion of the metal was 64 and 73, respectively, and the selectivity was 1.5. Both samples were reduced at 500° under identical standard conditions. 

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

Mossbauer spectra at 25°C are shown in Figure 4.32 for alumina-supported platinum, iridium, and bimetallic platinum-iridium catajysts containing 57Fe (samples B, C, and D, respectively) (3,4 V. The platinum-iridium catalyst contained 1.75 wt% each of platinum and iridium, while the other two catalysts contained 1.75 wt% of either platinum or iridium. All of the catalysts had metal dispersions (as determined by chemisorption) in the range 0.7-1. Also... 

In considering the nature of platinum-rhenium catalysts, we begin with a comparison of the chemisorption properties of alumina-supported rhenium, platinum, and platinum-rhenium catalysts (40). Data on the chemisorption of carbon monoxide and hydrogen at room temperature are given in Table 4.4 for catalysts with platinum and/or rhenium contents in the range of interest for reforming applications. 

A 1 %-wt platinum supported on alumina catalyst was used throughout this study. This catalyst was prepared by a proprietary incipient wetness method with tetraammine platinum (II) hydroxide, Pt(NH3)4(OH)2, as precursor. After drying in air at 393 K for 2 hours the catalyst was calcined in air for 3 hours at 623 K and reduced at 803 K in flowing hydrogen for 3 hours. The average platinum particle diameter, measured by both CO-chemisorption and TEM, amounted to 2 nm. The total exposed surface area of the platinum amounts to 1.4 m /g. The BET surface area of the used AI2O3 was 92 m /g. 

The calculated dispersion of Pt atoms at the surface of alumina was 54%, considering the reduction degree of 82% of platinum oxide (Table 13.8). On the other hand, the Pt/Ti02 catalyst has two times more hydrogen consumption than the alumina-supported catalyst, indicating reduction of Ti02 and thus interaction with metallic Pt°. The H2 chemisorption is three times less (17%) than the Pt/Al203 catalyst and confirms SMSI effect after reduction at 500°C. 

Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts by Volumetric Vacuum Method... 

Perrichon, V., Retailleau, L., Bazin, P. et al. (2004) Metal dispersion of Ce02—Zr02 supported platinum catalysts measured by H2 or CO chemisorption, Appl. Catal. A, 260, 1. 

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

---