Introduction
There has recently been a push from the commercial, as well as the military sectors, to develop small fuel cells to serve as mobile power sources to replace lithium-ion batteries in portable electronics [1]. Applications include long-lasting personal power systems for soldiers in the field as well as personal mobile electronics such as laptops. Research has shown that the energy density of small proton-exchange membrane (PEM) fuel cells is several times that of comparable lithium-ion power sources. Hydrogen-fed fuel cells represent a superior design for portable power supplies due to their high efficiency and negligible pollution issues. Therefore, on-demand production of hydrogen from a liquid fuel in a microreactor is much more desirable.
Due to their high energy density and low reforming temperatures, alcohols are often considered the best choice of liquid fuels to generate hydrogen for fueling portable proton-exchange membrane (PEM) fuel cells [2]. However, CO can be an unwanted by-product in the steam reforming of alcohols. Therefore it is important to develop active and selective catalysts towards H2 and CO2. Pd/ZnO has been shown in recent years to possess high selectivity towards CO2 during methanol and ethanol steam reforming [3, 4]. It is commonly assumed that PdZn bulk alloy formation is essential to achieve high selectivity towards CO2. Co/ZnO is also a well-known catalyst for the steam reforming of ethanol [5], although it has been shown that there is no bulk CoZn alloy formation. In this work, we analyze the effect of the ZnO support on the cobalt and palladium-based catalysts on the steam reforming of alcohols.
Materials and Methods
The catalysts were prepared via impregnation of metal precursors on the metal-oxide supports, followed by calcinations, and subsequent reduction in hydrogen. We used XRD and TEM to obtain estimates of average crystallite size and bulk composition. By using a STEM electron probe of 2.0nm, we can analyze the bulk and surface compositions of these alloy nanoparticles. Measurements of the steam reforming of both alcohols were performed in tubular packed-bed reactors of 1.75mm i.d.. The effluent of the reactors was analyzed using gas chromatography.
Results and Discussion
In the case of methanol reforming, we found that even without any pretreatment, PdZn alloy particles were formed after reaction at 250ºC, due to the facile reduction of ZnO in the presence of Pd and H2. Samples treated at low temperatures in H2 show the co-existence of monometallic Pd and PdZn alloy particles. On the other hand, high temperature reduction leads complete transformation of Pd to PdZn alloy. However, the selectivity towards CO2 was found not to increase monotonically with the extent of alloy formation. Samples with low alloy content also showed CO2 selectivity comparable to those where alloy formation was complete.
We have also performed experiments involving physical mixtures of ZnO with Pd/Al2O3. The presence of ZnO leads to enhanced selectivity of CO2 showing the vital role played by ZnO during the reforming of alcohols.
Significance
This study helps to further understand the fundamental role of ZnO in Pd/ZnO and Co/ZnO in the steam reforming of alcohols. We also provide insight into the role of bulk versus surface alloy formation in this catalyst system. This work has been funded by the U. S. Army Research Laboratory under the Collaborative Technology Alliance Program, Cooperative Agreement and the U.S. Department of Energy.
References
1. Palo, D.R., et al., Journal of Power Sources, 2002. 108(1-2): p. 28.
2. Browning, D., P. Jones, and K. Packer, Journal of Power Sources, 1997. 65(1-2): p. 187.
3. Casanovas, A., et al., Journal of Molecular Catalysis A-Chemical, 2006. 250(1-2): p. 44.
4. Iwasa, N. and N. Takezawa, Topics in Catalysis, 2003. 22(3-4): p. 215.
5. Llorca, J., et al., Applied Catalysis B-Environmental, 2003. 43(4): p. 355.