Methanol steam reforming promoted by molten salt-modified platinum on alumina catalysts.
Bottom Line: Moreover, the hygroscopic nature and the basicity of the salt modification contribute to the considerable enhancement in catalytic performance.Too high catalyst pore fillings with salt introduce a considerable mass transfer barrier into the system as indicated by kinetic studies.Thus, the optimum interplay between beneficial catalyst modification and detrimental mass transfer effects had to be identified and was found on the applied platinum-on-alumina catalyst at KOH loadings around 7.5 mass%.
Affiliation: Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen (Germany), Fax: (+49) 9131-8527421 www.crt.cbi.uni-erlangen.de.Show MeSH
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Mentions: The dispersion D is determined by the number of platinum adsorption sites measured through CO uptake divided by the total amount of platinum on the catalyst (D=nads/nPt). Concerning the stoichiometry of CO adsorption at platinum catalysts, stoichiometric factors from 0.7 to 1.0 can be found in the literature. According to the DRIFTs measurements, even for the neat Pt/Al2O3 catalyst a small amount of CO is coordinated in bridge-bonded conformation. For the calculation we initially assumed a stoichiometry of 1:1. But for the KOH-coated catalysts, the dispersion was additionally calculated for a stoichiometry of 0.8:1. The mean platinum cluster size can be calculated through the dispersion D. For the uncoated catalyst, the dispersion was estimated to be 37 % (1:1 stoichiometry), which implies that 37 % of the platinum atoms are located at the cluster surface. From this, a mean platinum cluster diameter of 3 nm can be calculated. Assuming a stoichiometry of 0.8, the dispersion was estimated to be 29.8 % resulting in a mean particle diameter of 3.8 nm (see also TEM image in the Supporting Information, Figure S2). If the catalyst samples were coated with a potassium-containing molten salt, the dispersion values were enhanced for low salt loadings (up to w=10 wt %). Through the modification of the platinum adsorption sites with potassium (alkali doping), the binding sites of CO, the binding energy, and the stoichiometry of CO adsorption are altered.[17–23] For low salt loadings, potassium-coated Pt/Al2O3 catalysts bind more CO than in the unmodified state. This seems to be in conflict with the DRIFTS results on the first view. The shift of CO coordination from the terminal to the bridge-bonded configuration observed in the DRIFTS measurements should result in a lower number of active platinum sites; consequently, the dispersion should be reduced. One possible explanation is the formation of platinum aluminate on the nanoparticle surface resulting in a higher CO accessibility of the remaining Pt0 adsorption sites and thus an enhanced platinum dispersion at low salt loadings. However, a strong decline in dispersion was observed for the KOH- and the K[OAc]-coated samples with higher salt loadings. This can be confirmed by the DRIFT spectra. The higher the potassium loading, the more CO is bound in bridge-bonded conformation. A loading of w=30 wt % resulted in a dispersion below 17 wt %. Consequently, less than half of the originally available platinum adsorption sites take part in CO adsorption processes at these high loadings. Additionally, at higher salt loadings (w=15–75 wt %) occupation of CO adsorption sites by the solid salt takes place under the water-free conditions of this experiment (in contrast to methanol steam reforming). For a loading of w=75 wt % the dispersion was nearly zero (D=2 %), which suggests that the platinum clusters are almost completely covered by the solid and nonporous salt. With a density of KOH in the dry state of 2.04 g cm−3 and a catalysts’ Brunauer–Emmett–Teller (BET) surface area of 120 m2 g−1, the thickness of the salt film (given for KOH in Figure 6) can be calculated for a certain salt loading w. Accordingly, with salt loadings higher than 75 wt % the film becomes thicker than 3 nm and a full coverage of the platinum clusters (mean diameter 3–4 nm) can be expected. Apparently, for catalyst samples coated with K2CO3 a different mechanism applies. Here, with low salt loadings, the dispersion was also enhanced compared to the uncoated reference catalyst. But for higher loadings, the drop in dispersion was by far not as pronounced as that of the KOH- and K[OAc]-modified catalysts. With a loading of 75 wt % K2CO3 on the catalyst, a dispersion of nearly 30 % was retained. Clearly, the K2CO3 coating behaves differently in the pores, leaving platinum adsorption sites uncovered and reachable for CO probably due to the porous nature of the solid salt.
Affiliation: Lehrstuhl für Chemische Reaktionstechnik, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstrasse 3, 91058 Erlangen (Germany), Fax: (+49) 9131-8527421 www.crt.cbi.uni-erlangen.de.