Mechanistic Study of Hydrogen Production By Aqueous Phase Reforming of Ethanol and Acetic Acid over Various Supported Ru Catalysts
Authors: Naito Shuichi , Nozawa Toshiaki , Hikichi Shiro 
Published: February 2020

The production of hydrogen as a clean energy carrier from bio-renewable sources is considered to be one of the most promising ways to minimize the global environmental problems that are associated with fossil fuel combustion. Among bio-renewable liquid feedstock for producing hydrogen, ethanol (EtOH) and acetic acid (AcOH) are the most promising candidates because of their non-toxicity, facile storage, safe handling and possibility as model compounds of bio-oil or bio-ethanol. Liquid phase reforming of oxygenates have been reported fewer than steam reforming, although they have advantages as compared to steam reforming. It is possible to reduce the cost of the process since it can be operated at lower temperatures, generating H2 and CO2 with lower level of CO.

   In the present work, we have investigated the loading amount dependency of Ru upon H2 production by aqueous phase reforming (APR) of EtOH and AcOH over Ru/TiO2 and Ru/Na-Y-zeolite catalysts.  In the case of EtOH-APR (473K) over 5wt% Ru/TiO2, the formation of H2 and CO2 showed a plateau at the later stage of the reaction, accompanied with a large increase of CH4 formation.  On the other hand, over 0.5wt% Ru/TiO2 main product was H2 with a smaller amount of CH4 and CO2 (1:1 ratio), indicating the possibility of the prevention of undesirable methanation by employing the catalysts with smaller particle sizes. 

Similar loading amount dependency was observed in the cases of AcOH-APR (473K) over 5 and 0.5wt% Ru/TiO2 catalysts. In both cases, H2, CH4 and CO2 are the major products in the gas phase with small amount of liquid phase products.  Over 5wt% catalyst, H2 was rapidly formed only at the beginning of the reaction and its formation rate became very slow at the later stage. On the other hand, 1:1 ratio of CH4 and CO2 were formed continuously, suggesting the occurrence of simple decomposition of acetic acid. Over 0.5wt% catalyst, the formation of H2 continued for longer period and the amount of formed CH4 was much less than CO2, clearly indicating the occurrence of the complete reforming process to form H2 and CO2.

It is generally accepted that dehydrogenation of ethanol gives acetaldehyde (AcH), which goes to three different reaction pathways as follows, (1) decomposition to form CH4 and CO followed by the water gas shift to form H2 and CO2, (2) hydration to form acetic acid (AcOH) or (3) complete reforming to only H2 and CO2.  Once CH4 is formed, it is rather difficult to reform it into H2 and CO2 at lower temperatures. In the case of 0.5wt% Ru/TiO2, EtOH-APR reaction proceeded through the decomposition of AcH forming 1:1 ratio of CO2 and CH4. Successive methanation reaction of CO2 with H2 was not observed at all, which was the predominant undesirable reaction in the case of 5wt% catalyst. However, no reforming of AcH seems to be operating on both catalysts, suggesting that particle size may not be the selectivity controlling factor for EtOH-APR reaction. On the contrary, in the case of AcOH-APR reaction over 0.5wt% catalyst, Ru metals with smaller particle sizes can participate in the reforming process of AcH, probably because of their stronger ability for H2O activation.  The existence of such positively charged small Ru clusters was confirmed by in-situ XPS and FT-IR spectra of CO adsorption.

We have also studied Ru/NaY-zeolite catalysts for APR reaction of AcOH (473K).  NaY-zeolite has a unique three-dimensional ordered porous structure as well as cation-exchangeable ability. Such characteristics would be efficient for controlling the structural and electronic properties of supported Ru species. As we have expected, Ru/NaY-zeolite catalysts exhibited high selectivity for complete reforming.  In this work, we have tried to characterize the efficient active site formed on NaY-zeolite. Catalytic activities of the supported ruthenium species on NaY-zeolite depended on immobilization conditions of [Ru(NH3)6]3+ and oxidation state controlled by treatment procedures. The catalysts NaY(exc), of which the ruthenium precursor was supported through cation-exchanging process, produced H2 continuously until 10 hrs. Reducing the loading amounts of ruthenium led to improve the selectivity toward complete reforming of AcOH giving H2 and CO2. Also, contents of sodium in the catalysts affected the reaction selectivity. The NaY(exc) catalysts, of which the part of sodium was exchanged to proton, exhibited relatively higher CH4 forming activity and that led to decrease the selectivity toward complete reforming.