Methanol steam reforming behavior of sol-gel synthesized nanodimensional Cu sub(x)Fe sub(1-x)Al sub(2)O sub(4) hercynites

Abstract

This work reports the outstanding catalytic activity behavior of sol-gel synthesized nanostructured Cu sub(x)Fe sub(1-x)Al sub(2)O sub(4) (0.3 less than or equal to x less than or equal to 0.8; named as CuFeAln, where n= 30, 40, 50, 60, 70 and 80) hercynites towards methanol steam reforming (MSR) for hydrogen generation. Based on the durability studies, we had categorized the higher Cu-doped hercynites (CuFeAl70 and CuFeAl80) as the more effective in regard to activity and stability (maintenance of a methanol conversion of approx. 80 percent with low CO selectivity of 2 percent after 50 h of continuous operation at 275 degrees C for CuFeAl80) when compared with the lower Cu-doped counterparts (CuFeAl30 and CuFeAl40). The specific surface area of all the materials was about 50 m sup(2) g sup(-1) and they had similar reduction characteristics as obtained from H sub(2)-TPR analysis. The lower reducibility below 280 degrees C of CuFeAl70 and CuFeAl80 was correlated with the higher stability of these samples during time on stream operation. The powder XRD analyses had shown pure phase hercynite formation with the gradual increase of Cu-doping, while there occurred a phase segregation in the reforming atmosphere leading to the formation of metallic copper. High resolution microstructural analyses had confirmed single phase hercynite formation at nanoscale and a reduction of copper subsequent to ageing as well as certain growth of the copper metal particles (from approx. 5 nm to approx. 8 nm) corroborating the XRD studies. The surface features from in-situ XPS had also suggested formation of reduced copper species, which was much lower for the higher Cu-doped samples. Cu K edge XANES spectral analyses also pointed to lower occurrence of reduced copper in the aged samples of higher Cu-doped hercynites. The experimental findings had been explained on the basis of partial breakdown of the spinel lattice structure leading to the formation of CuO, followed by its reduction to metallic copper nanocrystallites in the MSR atmosphere. A definite ratio of the reduced to oxidized copper species was maintained with time on stream and this caused nearly stable conversion behavior of the catalysts in methanol steam reforming.