H₂ Production from Oxygenated Hydrocarbons

Backgrounds:

The steam reforming of ethanol has been the focus of recent study as one of many promising technologies to supply portable hydrogen fuel. This interest is particularly due to the advent of the bioethanol industry, in which ethanol can be produced from biomass and delivered to existing infrastructures for automobiles. Supported cobalt has been reported to possess noble-metal like activity for the C-C bond cleavage around 400 ^oC to produce hydrogen from ethanol steam reforming. Several supporting materials have been used for cobalt including Al₂O₃, MgO, ZnO, SiO₂, CeO₂, ZrO₂, and CeZrO₄ to increase its catalytic activity for ESR. Cobalt is believed to be the main active species in various supported cobalt catalysts, although some findings reported that the surface cobalt oxide might play a role during the reaction.

Research Objectives:

The objective of this research is to investigate questions regarding (1) how different oxidation state of surface cobalt can affect the C-C bond cleavage, (2) how the oxidation state of this surface cobalt is affected by the different supports, crystalline structure of cobalt and operating conditions, (3) whether bulk cobalt undergoes phase transformation and surface cobalt undergoes oxidation state change during ethanol steam reforming, and (4) which phase is more active if phase transforms. By addressing these questions, we can gain insight into cobalt-support interactions for designing a better supported ESR catalyst system.

Effect of Different Supports on ESR:

Figure on the right shows the product composition of ethanol steam reforming over different supported cobalt (Co) catalysts. A steam to carbon (S/C) ratio is four and the weight hourly space velocity (WHSV) of ethanol is 0.63 h^-1. The ZrO₂ supported Co promotes high ethanol conversion while CeO₂ supported Co suppresses methanation. Based on this finding, we design the novel CeZrO₄ supported Co to combine the desired catalytic behaviors of both ZrO₂ and CeO₂ supported Co catalysts. This synergistic effect of CeZrO₄ support leads to a high hydrogen yield at low temperature for 10 mass % Co/CeZrO₄ catalysts.

Effect of Different Metallic Phase of Cobalt on ESR:

The catalytic activity of cobalt to produce hydrogen via ethanol steam reforming has been investigated in relating to the crystalline structure of metallic cobalt. In order to subtract the support effect, we have used unsupported cobalt for this study. At reaction temperature of 350 ^oC, the specific hydrogen production rates show that cobalt in hexagonal close-packed (hcp) structure possesses higher activity as compared with cobalt in face-centered cubic (fcc) structure. However, at typical reaction temperatures (400-500 ^oC) for ethanol steam reforming, hcp cobalt will transform to less active fcc cobalt, as confirmed by in situ X-ray diffractometry (XRD). The addition of CeO₂ promoter into cobalt stabilizes the hcp structure at reforming temperatures up to 600 ^oC. Moreover, CeO₂ promoter prevents the sintering of nanoparticles in the pre-reduction of Co₃O₄ to hcp cobalt at 350^oC. Both reforming experiments and in situ diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS) study showed that the surface reactions have been modified by CeO₂ promoter on 10% Ce-Co (hcp) to give a very low CO selectivity and a higher H₂ yield as compared with the unpromoted hcp Co.