An experiment of hydrogen preparation from steam-carbon reaction catalyzed by K2CO3 was carried out at 700°C, which was driven by the solar reaction system simulated with Xenon lamp. It can be found that the rate of reaction with catalyst is 10 times more than that without catalyst. However, for the catalytic reaction, there is no obvious change for the rate of hydrogen generation with catalyst content range from 10% to 20%. Besides, the conversion efficiency of solar energy to chemical energy is more than 13.1% over that by photovoltaic-electrolysis route. An analysis to the mechanism of catalytic steam-carbon reaction with K2CO3 is given, and an explanation to the nonbalanced [H2]/[CO + 2CO2] is presented, which is a phenomenon usually observed in experiment.
Hydrogen is an important energy material to fuel cell and various chemicals such as methanol, dimethyl ether, and synthetic gasoline. The traditional way to obtain hydrogen is to decompose the methane and water, which consumes a lot of other powers such as electricity. Therefore, the technical improvements to make hydrogen generation a cost-effective reality have become challenging subjects.
The sunlight is an inexhaustible and cost-free source of energy. The direct use of solar energy to obtain hydrogen is the most desirable way that has drawn wide attention from energy scientists all over the world [
The energy conversion efficiency (ECE) of solar-to-hydrogen is defined as [
In fact, there are more simple ways to produce hydrogen by solar energy, for example, the endothermic reactions of
In this study, the hydrogen was prepared by catalytic steam-carbon reaction at 700°C, which was driven by a concentrated solar reactor. The generation rate of hydrogen has been investigated with different contents of potassium carbonate. A special attention was paid to the impact of catalyst content and the reaction mechanism onto the energy conversion efficiency over the process.
The feedstock was prepared with equivalent-volume impregnation method. The preweighed potassium carbonate was dissolved with deionized water to obtain the impregnation solution; then the preweighed granular activated carbon with the size of 20~40 meshes (0.38 mm~0.83 mm) was then impregnated in the configured solution for four hours. After that, it was transpired by water bath at 80°C and then dried at 110°C for more than ten hours to thoroughly remove the moisture. A series of feedstock was prepared with potassium carbonate contents of 0%, 10%, 15%, and 20% in weight. The phase and morphology of the typical feedstock were characterized by X-ray diffraction (XRD, Dandong, China, Cu/K
Figure
Schematic of the main part in experiment.
Figure
Experimental flow diagram.
Each experiment with different potassium carbonate was carried out at the same conditions: temperature of 700°C, water rate of 0.27 mL/min, feedstock of 1.2 g, and reaction time of 45 minutes.
The main reaction of steam with active carbon is
In order to evaluate the catalytic performance of potassium carbonate in steam-carbon reaction, the experiments with different catalyst contents of 0, 10%, 15%, and 20% were carried out; the results are shown in Figure
The generation rate versus time in experiments with different catalyst contents. (a) Hydrogen, (b) carbon monoxide, and (c) carbon dioxide.
It can be found that there is an obvious difference between noncatalytic steam-carbon reaction and catalytic reaction. In the initial stage, the generation rate of hydrogen and carbon monoxide is about ten times to that without potassium carbonate, but there is less difference among that with potassium carbonate contents of 10%, 15%, and 20%. Similar results were also found in steam gasification of coal char catalyzed with K2CO3 by Wang et al. [
By Figures
It is worth noticing the ratio of [H2]/[CO + 2CO2] from experimental results. Assuming that all H2 is generated via reactions (
Figures
(a) Carbon conversation; (b) hydrogen production; (c) energy conversation efficiency during reaction.
The TEM and XRD results were shown in Figures
TEM images of feedstock.
XRD patterns of the feedstock with K2CO3 of 10%: (a) before reaction; (b) after reaction.
Moulijn and Kapteijn [
The calculation of quantum mechanics by Wjgmans et al. [
Considering from the above, the catalytic reaction of steam-carbon by K2CO3 can be conjectured. During the reactor bed at 700°C, the K2CO3 firstly decomposed into K2O and CO2; K2O then functioned according to the route shown in Figure
H2O–C catalytic reaction schematic diagram.
Therefore, the catalytic process can be expressed as follows:
By this way, the catalytic effects of K2O on steam-carbon reaction could be concluded in two aspects: one is to provide the amount of active sites on carbon surface; another is to decrease the activated energy to form Cr(O) intermediates.
The steam-carbon gasification is a heterogeneous catalytic reaction. It involves the adsorption, diffusion, reaction, and desorption on gas-solid surface. In the initial stage, the bigger precursor surface can provide many active sites. It makes chemical adsorption for H2O molecular at these sites to release oxygen atoms and H2. The Cr(O) intermediates were formed by these oxygen atoms and left from surface by their thermal motion. If the reaction temperature is not high enough, the Cr(O) intermediates could not leave in time. Therefore, the ratio of [H2]/[CO + 2CO2] is larger than 1 in the initial stage. When the Cr(O) intermediates reached saturation on carbon surface, the ratio is less than 1 in the latter stage. This can be the reason why the Cr(O) intermediates run away from surface in the controlling step of steam-carbon reaction, which was observed in many experiments.
The hydrogen preparation experiment was carried out with catalytic steam-carbon reaction driven by simulated solar energy, and the observations in experiments were analyzed. There is something that can be concluded: with the K2CO3 catalyst, the reaction is achieved at temperature of 700°C, and the conversion efficiency of optical energy to chemical energy is more than 13.1% over that by photovoltaic-electrolysis route. The catalytic mechanism of steam-carbon reaction with K2CO3 was that the K2CO3 firstly decomposed into K2O at higher temperature; then the K2O−C+ clusters were formed by the charge center displacement which was caused by electron interaction between K2O and neighboring C atom. The H2O was chemically adsorbed at K2O−C+ sites to form Cr(O) intermediates and released H2 at the same time, and the Cr(O) intermediates were broken loose from carbon chain by their thermal motion; then a newborn site was left. This cycle repeated itself until the end of reaction.
This study is sponsored by the National Nature Science Foundation of China (Grant no. 11075113). The authors express thanks to the program and thanks for Analysis and Testing Center of Sichuan University for supporting this study.