Catalysts for Hydrogen Production by the Auto-Thermal Reforming of Glycerol
Due to a number of major environmental issues, essentially greenhouse gas emissions and fossil fuel reliance, the implications of hydrogen as a promising clean energy carrier have significantly increased. In this respect, the use of bio-renewable feedstock such as glycerol for hydrogen production is becoming important. The majority of natural glycerol is produced as a by-product of the bio-diesel industry. Presently, almost all crude glycerol is refined before its ultimate end use. It is the ultimate goal of the present study to develop an effective catalytic auto-thermal reforming process to convert glycerol into top value bio-based products. Glycerol can be converted to hydrogen-rich streams by steam reforming (SR), partial oxidation (POX), gasification, auto-thermal reforming (ATR), supercritical water reforming (SCWR) or aqueous-phase reforming (APR). Most of the studies have focused on SR processes for producing hydrogen over various costly noble metal-based catalysts. A portfolio of nickel-based catalysts with nominal composition 5% Ni/Ce0.5Zr0.33M0.16O2- [where M is the promoter element(s) selected from Mg, Ca, Y, La, or Gd] was prepared and examined for their catalyst activity for glycerol auto-thermal reforming. The catalysts’ activity was evaluated using a plug flow reactor at atmospheric pressure and in the temperature range of 450°C to 700°C, steam-to-glycerol ratio of 6, 9, and 12, and oxygen-to-glycerol ratio of 0.2, 0.5, and 0.8 at atmospheric pressure in a packed bed tubular reactor (PBTR). The preliminary screening studies were carried out for a time-on-stream (TOS) of 6 hours with sampling intervals of 1 hour. The physicochemical and textural characteristics of the catalysts were investigated by means of a variety of characterization techniques such as temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), nitrogen (N2) physisorption, hydrogen (H2) chemisorption, UV-Vis diffuse, thermogravimetry analysis (TGA), and X-ray diffraction (XRD). The gaseous products were analyzed by online gas chromatography equipped with a thermal conductivity detector (GC/TCD). The structural, textural, and physicochemical characteristics of the catalysts have been investigated with the help of different bulk and surface characterization techniques. Different ratios of steam/glycerol and oxygen /glycerol were employed to optimize the conditions to achieve the highest conversion as well as H2 selectivity. Based on the above analyses, glycerol conversion and H2 selectivity were calculated. Among all the catalyst formulations screened in the current study, the catalyst formulations prepared with Ca, Y, Mg, La, and Gd exhibit stable and steady activity even at 500°C. The catalyst formulation with Gd as a promoter element performed the best at all the investigated temperatures. Hence, it is a potential candidate for future commercialization and plausible membrane reactor applications. The thermal and catalytic effects on catalytic auto-thermal reforming were identified by performing a number of non-catalytic reaction runs and then comparing the results with the corresponding catalytic reactions.