Edited by: Alfonso Chinnici, University of Adelaide, Australia
Reviewed by: Mahyar Silakhori, University of Adelaide, Australia; Gang Liu, Central South University, China
This article was submitted to Process and Energy Systems Engineering, a section of the journal Frontiers in Energy Research
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
This study investigates the catalytic properties of K+ and Cu2 + /Fe3 + co-doped ceria-zirconia (CeZr) toward water and carbon dioxide co-splitting. These materials can convert separate feeds of CO2 and H2O into CO and H2. In co-splitting tests, water reacts faster on the K-Cu-CeZr catalyst with negligible CO production. The reduction of the K-Fe-CeZr catalyst occurs over two broad temperature ranges: at low temperature, only H2 is produced; whereas CO is the most abundant product at high temperature. A kinetic model was developed to get insights into the reasons of the observed selectivity toward H2 at low temperature and CO at a higher temperature. The different reaction orders in the sites fraction were evaluated for CO2 and H2O reactions, highlighting that H2 production requires a larger number of adjacent reduced sites than CO production. Three regimes were identified through the model: Regime I- H2O driven regime @T ≤ 650°C; Regime II- mixed regime @ 560 < T < 700°C and Regime III: CO2 driven regime @ T > 700°C. These results indicate the appropriate conditions for tuning H2/CO selectivity, depending on the feed composition.
The transformation of solar energy into synthetic fuels holds huge promise for sustainable approaches to harvesting renewable energy (
Among thermochemical cycles, two-step high temperature processes are the most promising (
Reaction 1 corresponds to high temperature self-reduction under concentrated solar power, whereas reactions 2 and 3 are the splitting reactions, which restore the oxidized state of the materials and producing fuels.
Despite the huge CeO2 potential in thermochemical cycles, high reduction temperature (step1), compromising long-term thermal stability over multiple cycles as well as small fuel production still limits its technological application. To date, a number of other studies have addressed these issues (
It is widely accepted that ceria-zirconia solid solutions, in particular those with a Ce/Zr molar ratio ∼ 3, show improved splitting properties, especially in terms of self-reducibility (
The improved splitting activity of ceria-zirconia solid solutions has been related to the formation of a Zr2ON2 like-phase at the nanoscale level during thermal treatment in N2 atmosphere (
While a huge amount of literature has been devoted to the thermochemical splitting of bare H2O or CO2, little is known about their simultaneous splitting (
Recently, we proved that potassium cation doping improved Ce0.75Zr0.25O2 splitting performance toward CO2 or H2O (
In this work, K-M-doped ceria-zirconia solid solutions (M = Cu2 + and Fe3 +), containing 5% mol/mol of both K and M cations, were tested as catalysts for carbon dioxide and water co-splitting. Furthermore, a kinetic model was developed and used to get insights into the active sites of the catalysts and their activity toward H2O and CO2 splitting, to simulate the splitting performance as well as the composition of produced gases at different oxidation temperatures.
Reagents were purchased from Sigma-Aldrich and used as received. The material synthesis was carried out by employing the following reagents: cerium (III) nitrate hexahydrate, Ce(NO3)3⋅6H2O; zirconium (IV) oxynitrate hydrate, ZrO(NO3)2⋅xH2O; iron (III) nitrate non ahydrate, Fe(NO3)3⋅9H2O; potassium nitrate, KNO3; copper (II) nitrate tetrahydrate, Cu(NO3)2⋅4H2O; and manganese (II) nitrate tetrahydrate, Mn(NO3)2⋅4H2O. The Ce/Zr molar ratio was kept constant at 3 (Ce0.75Zr0.25O2 as general formula) since it ensured the highest reducible performance among CeO2-ZrO2 solutions (
Bare ceria-zirconia, as well as doped and co-doped materials, were prepared according to the co-precipitation method (
Materials tested for the thermochemical splitting process, including nominal compositions (molar ratios), general formulas, and maximum oxygen evolutions (mmol/g).
Sample | Ce/Zr | M/(Ce + Zr) | K/(Ce + Zr) | General formula | |
K-CeZr | 3 | – | 0.05 | K0.05Ce0.71Zr0.24O1.93 | 1.16 |
K-Fe-CeZr | 3 | 0.05 | 0.05 | K0.045Fe0.045Ce0.68Zr0.23O1.91 | 1.20 |
K-Cu-CeZr | 3 | 0.05 | 0.05 | K0.045Cu0.045Ce0.68Zr0.23O1.88 | 1.28 |
The experimental equipment and experimental details described above were used to assess the splitting ability of the samples under investigation (
As previously reported (
Oxygen, hydrogen, and carbon monoxide amounts (mol/g),
where
The reduction degree (
and as in the following for oxidation steps:
where
Oxidation yield (α
Reduction yield (β
A kinetic model was developed to get insights into the role of the superficial reaction on the selectivity in co-feed splitting.
The model is based on the Polanyi-Wigner equation under the Redhead approximation, which states that catalytic sites do not interact. The reactor is modeled as a continuous stirred tank reactor (CSTR).
The reaction mechanism is a redox pathway involving active sites σ: σ
According to these hypotheses, the reaction steps are:
Sites fractions are defined by the following equations:
The reaction rates of steps (10–11) are:
where θ
where
The unsteady balance equations on reduced sites fraction for CO2 splitting, H2O splitting, and co-splitting are respectively:
The unsteady balance equations on gaseous molar fractions are:
where τ is the residence time equal to the experimental one.
The initial conditions are:
Differential equations were numerically solved using the Eulero explicit method.
To analyze the fitting quality, the differences between the experimental data and the model were evaluated through the root mean square error (SRMSE), normalized for the maximum value of the curve of the flow test for each step:
Model and experimental curves were considered in good agreements for
Previous research has reported on H2/H2O cycling, outlining that it can be used to define the splitting properties of materials (
Results by
H2 (blue solid lines) and CO (red dashed lines) concentration profiles during splitting tests under different oxidizing streams on K-Fe-CeZr
Potassium doped samples show faster oxidation kinetics than K-free materials (
Amount (in μmol/g) of released oxygen (as O2) and produced H2 during H2O splitting tests, reduction degrees after each step (x
Sample | n |
x |
β (%) | n |
x |
α (%) |
CeZr | 450 | 56.3 | 206.4 | 509 | 34.6 | 56.5 |
K-CeZr | 549 | 67.4 | 191.6 | 1380 | 7.7 | 125.7 |
K-Fe-CeZr | 747 | 49.1 | 98.5 | 1458 | −4.8 | 97.6 |
K-Cu-CeZr | 559 | 81.0 | 105.2 | 1237 | 32.6 | 110.5 |
The low temperature oxidation peak could be addressed to the contribution of surface and sub-surface layers and/or less ordered bulk structures to splitting reactions, while the high temperature peak could be related to the oxidation of bulk ceria-zirconia.
Quantitative analysis of splitting tests under different oxidants. Partial and overall amounts of H2 + CO on K-Fe-CeZr
K-Fe-CeZr sample shows two oxidation peaks independently from the oxidizing stream. In CO2 splitting, the former peak (450°C) occurs at a higher temperature than that found in H2O splitting. This result suggests that the H2O splitting reaction is faster than the CO2 reaction.
In H2O/CO2 co-splitting tests, the H2O was reduced first. However, H2O started reducing at 200°C reaching the peak at 330°C. This peak temperature is higher than bare H2O splitting conditions. This behavior can be addressed to the CO2 adsorption on the surface, inhibiting H2O adsorption and further reaction. Actually, in our previous papers, we showed that CO2 desorption on ceria-based materials occurs at about 200°C (
Moreover, CO2 feed results in a lower produced H2 amount (
CO2 splitting on K-Cu-CeZr (
The results of co-splitting tests (
Kinetic parameters of surface splitting reactions.
Parameter | r |
r |
E |
4.61⋅104 | 1.50⋅105 |
k0i | 1.00⋅105 | 3.50⋅1010 |
θ0 | 0.95 | 0.95 |
nθ |
1 | 0.75 |
n |
0 | 0 |
SRMSE | 0.043 | 0.038 |
From the fitting results, we conclude that the activation energy of the CO2 splitting reaction is higher than that of H2O splitting. This enables the H2O splitting reaction to proceed at a lower temperature than CO2, as indicated by the results of this experiment.
We can also note that the H2O oxidation reaction is in the first order for active sites concentration (nθ _H2O = 1); whereas the CO2 oxidation rate exhibits an order lower than 1 with respect to active sites concentration (nθ _CO2 = 0.75). This result suggests that CO2 needs a lower number of nearby reduced sites, as expected due to the different structure of the CO2 molecule.
To study the competition between the two reaction rates we defined the parameter R:
Surface reaction rates of H2O and CO2 splitting reaction
Three regimes can be identified:
Regime I- H2O driven regime: @T ≤ 560°C, the reaction rates do not intersect and
Regime II- mixed regime: @ 560 < T < 700°C, r
Regime III: CO2 driven regime @ T > 700°C reaction rates do not intersect and R > 1 in the whole θ range. CO production prevails over H2 production.
Isothermal simulations were performed to compute the amount of H2 and CO produced as a function of temperature.
Ratio between CO and H2 production as a function of temperature during isothermal co-splitting simulations.
In this figure, the three regimes are also identified. It is worth noting that F = 1 at 670°C and inversion occurs at higher temperatures.
From these results, we conclude that by changing the temperature, the content of H2 and CO in the output current can be modulated.
In this work, ceria-zirconia was co-doped with transition metal (Cu2 +, Fe3 +) and potassium cations. We studied the co-doped ceria-zirconia to determine whether they act as catalysts for water and carbon dioxide co-splitting. All the investigated materials were able to convert separate feeds of CO2 and H2O into CO and H2, respectively. A single oxidation peak was detected on the K-Cu-CeZr catalyst, independently from the oxidant. Furthermore, the water splitting reaction rate appeared faster. Two oxidation peaks were detected on the K-Fe-CeZr catalyst for both the oxidants; the water reaction rate appeared faster at low temperature, while the high temperature peak seemed independent from the oxidant, suggesting that the reaction rate is limited by the oxygen diffusion from the surface to the bulk.
Co-splitting tests were carried out as temperature programmed oxidations. On the K-Cu-CeZr catalyst, water reacted faster and a negligible CO production was detected. the H2 production peak shifted to a higher temperature, probably due to CO2 adsorption on the catalyst surface, thus blocking active sites. The K-Fe-CeZr catalyst behavior was more complex. Two oxidation peaks were detected at low and high temperatures. For the low temperature, only H2 was produced in agreement with the faster H2O splitting kinetics measured during the separated H2O and CO2 splitting. In contrast, almost 100% selectivity to CO2 splitting was detected at high temperatures.
A kinetic model was developed to understand this behavior. This model revealed that the H2O splitting reaction featured a higher reaction order in terms of sites fraction compared to CO2. This suggests that H2 production requires a larger number of adjacent reduced sites. By comparing the calculated surface reaction rates at different temperatures, three regimes were identified. At temperatures below 560°C H2 production is faster than CO production independently from the surface reduced sites fraction; accordingly, isothermal co-splitting simulations showed a preeminent hydrogen production. At temperatures between 560 and 700°C, CO2 and H2O reaction rates are comparable and the pre-eminence depends on both temperature and surface reduced sites fraction. In this temperature range, co-production of CO and H2 occurs and their ratio can be tuned by an opportune choice of the co-splitting temperature. At temperatures higher than 700°C CO production is faster than H2 production independently from the surface reduced sites fraction, accordingly, isothermal co-splitting simulations showed a preeminent carbon monoxide production.
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
GLa and GLu prepared the materials and performed the experiments. AD developed the model and performed the simulations. GLa, GLu, and AD contributed to the planning and interpretation of results, and the writing of the manuscript. All authors contributed to the article and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.