Co catalysts supported on ceria supports with two different particle sizes, one in the micro-and the other in the nano-range, were investigated for their ethanol and ethylene steam reforming performance. Pre-and post-reaction characterization techniques, including high-resolution transmission electron microscopy, temperature-programmed oxidation, dispersion, pore size measurements, in situ X-ray diffraction (XRD) and X-ray absorption fine structure spectroscopy (XAFS) studies were performed to examine the reducibility of the catalysts. Steady-state-activity testing has shown nanoparticles to have a higher reforming activity for ethanol, but also high ethylene yields. In spite of the high ethylene yields, catalysts supported on nanoparticles proved to be highly resistant to coking while the catalysts supported on larger ceria particles suffered from coke formation. Reforming experiments performed with ethylene showed significant differences in activity and stability. Bare supports were also tested for activity and the nanoparticle support was seen to have high dehydration activity. Operando DRIFTS experiments performed during ESR showed differences in surface species. Pulse experiments performed to use methanol oxidation as a probe reaction suggested differences in the relative abundance of redox sites and basic sites. The bare ceria supports also exhibited significant activity for ethanol dehydration, but not for C−C cleavage. The superior performance of the catalysts supported on nanoparticles is thought to be due to a combination of factors, including increased reducibility, improved metal dispersion, and a difference in relative abundance of redox sites on the surface. All of these properties and, in turn, the catalytic performance, appear to be affected by the particle size of the support.

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Reducibility of ceria under steam reforming conditions and the effect of particle size on its reducibility was examined using two ceria samples with distinctly different mean particle sizes (3.5 nm versus 120 nm), but with similar polyhedral morphologies. The degree of reduction from Ce 4+ to Ce 3+ was characterized by temperature programmed reduction (TPR) and in situ X-ray absorption near edge structure spectroscopy (XANES) where the nanopolyhedra were observed to reduce much more readily compared to the larger particle-size sample. There was also significant reduction of the nanopolyhedra under ethanol steam reforming conditions. Ceria nanopolyhedra exhibited significantly more Ce 3+ sites which contributed to a lower occurrence of surface acidic sites. The acidic/basic sites were probed by probe molecules such as pyridine and CO 2 through in situ diffuse reflectance infrared spectroscopy (DRIFTS). The particle size also showed major differences in the steam reforming activity of ceria, with nanopolyhedra with a 3.5-nm mean particle size exhibiting significantly higher carbon cleavage and ethanol dehydration activity than its counterpart of 120 nm mean particle size.

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The surface and bulk reduction characteristics of bare ceria and ceria with supported cobalt nanoparticles were investigated under ethanol steam reforming conditions using AP-XPS and XANES techniques. Ceria particles were prepared in two different particle sizes, one in nano and the other in micron size (termed CeO 2 -NP and CeO 2 -MP), with average particle sizes of 4 and 120 nm, respectively. It was found that particle size affects surface reducibility of ceria particles; smaller particle size leads to a higher extent of surface reduction. Supported cobalt nanoparticles have a significant effect on the surface reducibility of both CeO 2 -NP and CeO 2 -MP. Compared to bare ceria particles, the presence of fully oxidized cobalt nanoparticles on the surface of ceria support retards surface reducibility of ceria since reduction of the cobalt oxide phases (Co 3 O 4 and CoO) takes precedence over that of ceria. The degree of reduction of the cobalt phase during ethanol steam reforming determines the effect of cobalt on the reduction process of ceria, i.e., whether it retards or facilitates the reduction of ceria support. AP-XPS studies show that the surface of cobalt nanoparticles consists of both metallic Co and CoOx. The reduction of the surface region of CoOx to metallic Co forms a metallic Co-based shell and CoOx-based core. The anchor of metallic Co on CoOx make metallic Co shell well dispersed on CeO 2 without sintering. In addition, the reforming reaction takes place primarily at the interface of metallic Co and CeO 2 . The much larger difference between Co/CeO 2 -NP and Co/CeO 2 -MP than the difference between CeO 2 -NP and CeO 2 -MP suggests the significance of metallic Co in catalyzing the reforming reaction, although bare ceria support shows some dehydration activity in its own right.