Catalysts for hydrogen production by the steam reforming of mixtures of oxygenated hydrocarbons.

摘要

The implications of hydrogen as a promising clean energy carrier have increased significantly due to a number of pertinent environmental concerns, mainly greenhouse gas emissions and fossil fuel reliance. In this respect, the utilization of bio-renewable feedstock such as oxygenated hydrocarbon(s), like bio-ethanol, glycerol, pyrolysis bio-oils, etc., for hydrogen generation are becoming significant, owing to the carbon neutral nature of the above feedstocks. In the current study, steam reforming of an equimolar liquid mixture of six oxygenated hydrocarbons (ethanol, 1-propanol, 1-butanol, lactic acid, ethylene glycol, and glycerol) was conducted at temperatures of 700°C, 600°C, and 500°C and at atmospheric pressure in a packed bed tubular reactor (PBTR). A portfolio of mixed oxide supports with nominal composition Ce0.5Zr0.33M0.17Ox (where, M = Ca, Gd, La, Mg and Y) and Ce0.5Zr0.33Ca 0.8Mg0.08Ox were prepared by a surfactant-assisted method using a surfactant/metal (S/M) ratio 1.25 for this purpose. To optimize the surfactant/metal (S/M) ratio for preparing supports, some Ce0.5Zr 0.33Mg0.17Ox supports were prepared using different surfactant/metal (S/M) ratios. A nominal amount of nickel (Ni) was impregnated over the supports by conventional wet-impregnation method. Some bi-metallic catalysts were prepared by simultaneous impregnation of nickel (Ni) and copper (Cu) over Ce0.5Zr0.33M0.17Ox support. The physico-chemical and textural characteristics of the catalysts were investigated by means of various characterization techniques such as: temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), nitrogen (N 2) physisorption, hydrogen (H2) chemisorption, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The preliminary screening studies were carried out for a time-on-stream (TOS) of 6 hours with sampling intervals of 0.5 hours. The gaseous products were analyzed online by gas chromatography equipped with a thermal conductivity detector (GC/TCD). Based on the above analyses, OxyHC conversion and H2 selectivity were calculated. Among all the catalyst formulations screened in the current study, the catalyst formulations prepared with Ca, CaMg, Mg, and Gd exhibit stable and steady activity even at 500°C. The catalyst formulation with Mg as a promoter element performs the best at all the investigated temperatures. Therefore, it is a potential candidate for future commercialization and plausible membrane reactor applications. The thermal and catalytic effects on catalytic steam reforming were identified by performing a number of non-catalytic reaction runs and compared with the corresponding catalytic reactions. However, the addition of copper (Cu) in the best catalyst formulation significantly decreased the catalytic activity. To reduce the catalyst production cost, the surfactant/metal (S/M) was optimized. Valuable correlations have been established between the catalytic performance and the catalyst characterization data to find out the parameters that influence the catalytic behaviour for further development of the catalyst. Some important correlations are that catalytic activity increases with increasing active metal reducibility, pore volume/surface area (PV/SA), and active metal dispersion and decreases with increasing carbon propensity factor (CPF).