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Hydrogen (H2) production via catalytic steam reforming of propane and liquefied petroleum gas (LPG)

Kokka Aliki

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Summary

In the present study, the development and optimization of catalytic materials, which are characterized by high activity, selectivity and stability, as well as the determination of optimal operating conditions for hydrogen (H2) production via steam reforming of propane and liquefied petroleum gas (LPG) were investigated. For this purpose, several parameters that may influence catalytic behavior, such as the nature (Ni, Ru, Rh, Ir, Re), loading (0.1-5 wt.%) and morphological characteristics of the metallic phase, the nature of the support (Al2O3, TiO2, YSZ, ZrO2, SiO2, CeO2), the use of composite metal oxides MxOy-Al2O3 (M: Ti, Y, Zr, La, Ce, Nd, Gd) as supports and the promotion of the support with a small amount of alkali metals (Li, Na, K, Cs) were examined. The prepared catalysts were characterized by various techniques in order to determine the physicochemical characteristics that affect catalytic performance. Due to the complexity of the LPG steam reforming reaction and given that the main component of the LPG mixture is propane, it was initially chosen to study the catalytic performance of the investigated materials under propane steam reforming conditions. In addition, in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were conducted to determine the active surface species and the elementary steps that constitute the mechanism of the propane steam reforming reaction. Moreover, the effect of reaction temperature, H2O/C ratio and Gas Hourly Space Velocity (GHSV) on catalytic performance as well as the dynamic response of catalyst to abrupt changes in the reaction conditions were studied both in the absence and presence of butane in the feed in an attempt to determine the optimal reaction conditions. The most active catalysts were prepared in the form of pellets and tested for their activity and stability under realistic reaction conditions, using a propane/butane mixture similar to that met in a real LPG mixture.Results showed that the catalytic activity significantly depends on the metal-support combination employed, as well as the loading and the particle size of the dispersed metallic phase, with 0.5%Rh/TiO2, 0.5%Rh/YSZ, 5%Ni/ZrO2 and 5%Ni/YSZ catalysts exhibiting high catalytic activity and selectivity. DRIFTS studies showed that the propane steam reforming reaction proceeds via intermediate formation of CHx species which are either hydrogenated to CH4 or interacted with OH groups yielding formates which are further decomposed to H2 and COx. The most active catalysts were found to be able to convert the intermediate produced CHx species to the desired reaction products. In contrast, CHx species were found to be less reactive on the less active catalysts leading to a lower reaction rate. In addition to the high activity of 0.5%Rh/TiO2 and 5%Ni/ZrO2 catalysts, they were also found to be stable for about 14 h and 30 h on stream, respectively. The addition of MxOy on Al2O3 surface resulted in a significant increase of the turnover frequency (TOF) of propane conversion for the 0.5%Rh/MxOy-Al2O3 catalysts, compared to that obtained for 0.5%Rh/Al2O3 catalyst. Catalytic activity was found to be varied in a manner which depends on the nature of MxOy, with the La2O3-containing sample exhibiting optimum performance. TOF of propane conversion for the 0.5%Rh/x%La2O3-Al2O3 and 0.5%Rh/x%Gd2O3-Al2O3 (where x= 0-20 wt.%) catalysts goes through a maximum for La2O3 or Gd2O3 content equal to 10 wt.%. Results of H2-TPR experiments and DRIFTS studies conducted following interaction of the pre-oxidized catalysts with 1%CO/He mixture showed that the reducibility of RhOx species and MxOy-Al2O3 support depends on the nature and loading of MxOy and is related to catalytic activity. In situ DRIFTS experiments demonstrated that the intermediate produced CHx species are hydrogenated to CH4 and/or interacted with OH groups yielding formates that are further decomposed to H2 and COx. Results indicated that catalytic activity becomes optimum for intermediate support reducibility and population of the produced formate species.Catalytic activity of 0.5%Ru/TiO2 catalysts for the propane steam reforming reaction can be significantly improved by addition of small amount (0.2 wt.%) of alkali metals (Li, Na, K, Cs). The catalyst promoted with Li exhibited higher catalytic activity, compared to K-, Cs- and Na-promoted samples. TOF goes through a maximum for Li content equal to 0.2 wt.% (investigated in the range of 0.0-0.4 wt.% Li). The observed catalytic activity improvement with the addition of alkali on TiO2 is related to the increased reducibility of RuOx species that are strongly interacting with the support, which was found to be enhanced for the most active 0.5%Ru/0.2%Li-TiO2 catalyst. DRIFTS experiments conducted following interaction of the pre-oxidized catalysts with 1%CO/He mixture showed that RuOx reduction occurs at lower temperatures with increasing Li content from 0.0 to 0.4 wt.%, whereas new adsorption sites are created at the metal-support interface. Based on the results of in-situ DRIFTS experiments carried out under reaction conditions it was found that the intermediate produced CHx species are either hydrogenated to CH4 or interacted with the lattice oxygen of the support producing formyl species and, finally, H2 and COx. The addition of alkalis leads to a decrease in the relative population of multicarbonyl species adsorbed on partially oxidized Ru sites and an increase in the population of linearly adsorbed CO species on Ru0 indicating that under reaction conditions (a) catalyst reduction takes place which is facilitated in the presence of alkalis and (b) the active sites for the propane steam reforming reaction are the reduced Ru sites.Results obtained based on the investigation of the effect of operating parameters on the catalytic performance of 0.5%Rh/TiO2 catalyst, which was among the most active catalysts, showed that the catalytic performance is improved by increasing the reaction temperature, steam content in the feed, and/or by decreasing GHSV both in the absence and presence of butane in the feed. It was, also, found that neither catalytic activity nor product selectivity is varied with time following abrupt changes of the H2O/C ratio between 2 and 7 both in the absence and presence of butane in the feed. The catalyst exhibited excellent stability with time-on-stream at 500 and 650 °C under propane steam reforming. However, a reversible catalyst deactivation seems to be operable when the reaction occurs at 600 °C, resulting in a progressive decrease of propane conversion, which, however, can be completely restored by increasing the temperature to 650 °C in He flow.The performance of catalysts tested in the form of pellets under realistic reaction conditions of LPG steam reforming using GHSV= 9000 h-1 and H2O/C=3.25, is improved following the order 0.5%Rh/TiO2 < 0.5%Rh/Al2O3 < 0.5%Rh/10%La2O3-Al2O3~ 0.5%Rh/10%Gd2O3-Al2O3. The optimum 0.5%Rh/10%La2O3-Al2O3 and 0.5%Rh/10%Gd2O3-Al2O3 catalysts are able to achieve propane and butane conversions higher than 95 and 97% respectively, at 410 oC. 0.5%Rh/TiO2 and 0.5%Rh/10%La2O3-Al2O3 catalysts were, also, found to exhibit excellent stability under realistic LPG steam reforming conditions and, therefore, can be considered as suitable catalysts for LPG reformers in fuel cells based energy production plants.

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