Catalysts for Hydrogen Production by the Auto-Thermal Reforming of Glycerol
Abstract
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.