Syngas Production Through H2O/CO2 Thermochemical Splitting Over Doped Ceria-Zirconia Materials

This study investigates the catalytic properties of K⁺ and Cu^{2 +} /Fe^{3 +} 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.

Introduction

The transformation of solar energy into synthetic fuels holds huge promise for sustainable approaches to harvesting renewable energy (Nguyen and Blum, 2015; Chuayboon and Abanades, 2020; Mao et al., 2020). Thermochemical splitting cycles have been proposed as a promising sustainable option, as this approach uses concentrated solar energy to convert H₂O and CO₂ into H₂ and CO (Costa Oliveira et al., 2018; Bhosale et al., 2019; Takalkar et al., 2019). These building blocks can be further reacted into gaseous and liquid fuels.

Among thermochemical cycles, two-step high temperature processes are the most promising (Rao and Dey, 2015), because they are less complex and can achieve a higher and more efficient theoretical solar-to-fuel energy conversion (η_{solar–to–fuel}) (Rao and Dey, 2015). The thermochemical splitting catalyst ceria is recognized as an important development (Bhosale et al., 2019) due to its thermal stability, high oxygen storage capacity (OSC), without any structural changes, and the fast kinetics of reduction and splitting reactions (Costa Oliveira et al., 2018; Bhosale et al., 2019; Chen et al., 2019; Takalkar et al., 2019). The thermochemical cycle can be schematically defined as follows:

$${\text{CeO}}_2\to {\text{CeO}}_{(2-\mathrm{\delta })}+\mathrm{\delta }/2O_2(1)$$

$${\text{CeO}}_{(2-\mathrm{\delta })}+\mathrm{\delta }{\mathrm{CO}}_2\to {\text{CeO}}_2+\mathrm{\delta }\mathrm{CO}(2)$$

$${\text{CeO}}_{(2-\mathrm{\delta })}+\mathrm{\delta }{\mathrm{H}}_2\text{O}\to {\text{CeO}}_2+\mathrm{\delta }{\mathrm{H}}_2(3)$$

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 (Kaneko et al., 2011; Petkovich et al., 2011; Gokon et al., 2013, 2015a; Agrafiotis et al., 2015; Jarrett et al., 2016; Muhich et al., 2016; Al-Shankiti et al., 2017; Cho et al., 2017; Zhou et al., 2017; Luciani et al., 2019; Bhosale and AlMomani, 2020; Costa Oliveira et al., 2020; Haeussler et al., 2020). Modifying ceria fluorite structure with metal cations has proved to be a successful strategy for improving both oxygen ion mobility and thermal stability, ultimately enhancing the redox activity of the material (Kang et al., 2013; Cooper et al., 2015; Gokon et al., 2015a,b; Takacs et al., 2015; Bhosale et al., 2016; Lin et al., 2016; Zhao et al., 2016; Mostrou et al., 2017; Muhich and Steinfeld, 2017; Bhosale and Takalkar, 2018; Takalkar et al., 2018, 2020a,b; Naghavi et al., 2020).

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 (Abanades et al., 2010; Kaneko et al., 2011; Le Gal and Abanades, 2011; Abanades and Le Gal, 2012; Call et al., 2013; Le Gal et al., 2013; Jacot et al., 2017; Muhich and Steinfeld, 2017). Moreover, the performance of this class of materials can be further enhanced by doping with bi- or trivalent cations, such as Cu^{2 +}, Mn^{3 +}, Fe^{3 +} (Gokon et al., 2015a,b; Lin et al., 2016).

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 (Pappacena et al., 2016, 2017), which is associated with a decrease of surface Ce/Zr ratio (Luciani et al., 2019). Good splitting performance has been related also to the formation of both bulk and surface oxygen vacancies, enhancing surface reactivity (oxygen evolution during self-reduction and splitting reactivity during oxidation) and oxygen diffusivity from the bulk to the surface and vice-versa (Luciani et al., 2019).

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 (Furler et al., 2012; Lorentzou et al., 2014, 2017; Falter and Pitz-Paal, 2018; de la Calle and Bayon, 2019; Tou et al., 2019; Hao et al., 2020). It is worth noting that co-splitting has been generally defined as concurrent or combined splitting, but it should be better defined as competitive splitting; as a matter of fact, CO₂ and H₂O compete for the same reduced sites, whereas the overall fuel production depends on the reduction extent of the material.

Recently, we proved that potassium cation doping improved Ce₀.₇₅Zr₀.₂₅O₂ splitting performance toward CO₂ or H₂O (Landi, 2019). Alkali metal ions doping as well as transition metals cations co-doping, such as Cu(II) and Fe(III), provided the formation of both bulk and surface oxygen vacancies, significantly enhancing the splitting performance. Interestingly, K-addition boosted the oxidation kinetics, in agreement with the results obtained in the three step cycles (Charvin et al., 2009). Recently, Takalkar et al. showed a positive effect of Li⁺ cation on the CO production during CO₂ splitting cycles (Takalkar et al., 2020b).

In this work, K-M-doped ceria-zirconia solid solutions (M = Cu^{2 +} and Fe^{3 +}), 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.

Materials and Methods

Materials Preparation

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 (Le Gal and Abanades, 2012).

Bare ceria-zirconia, as well as doped and co-doped materials, were prepared according to the co-precipitation method (Luciani et al., 2018;, Landi, 2019). Stoichiometric amounts of precursor salts were dissolved in 75 ml of bi-distilled water and stirred for 3 h. Then, solutions were heated in an MW oven (CEM SAM-155) until a homogeneous gel was obtained. The calcination in air at 1,100°C for 4 h was then carried out to decompose nitrates and to obtain the oxides. Table 1 summarizes the composition of produced materials as well as the label used.

TABLE 1

Table 1. Materials tested for the thermochemical splitting process, including nominal compositions (molar ratios), general formulas, and maximum oxygen evolutions (mmol/g).

H₂O/CO₂ Splitting and Co-splitting Tests

The experimental equipment and experimental details described above were used to assess the splitting ability of the samples under investigation (Luciani et al., 2018, 2019; Portarapillo et al., 2019). Temperature programmed reduction (TPR) and oxidation (TPO) were carried out on powdered samples (500 mg; 170–300 μm), placed in a tubular quartz reactor, and inserted into an electric tubular furnace (Lenton). The catalyst temperature was measured by a K-type thermocouple placed inside a tube co-axial with the reactor.

As previously reported (Luciani et al., 2018, 2019; Portarapillo et al., 2019), the samples were pre-treated in a hydrogen flow (TPR; 10 l(SLT)/h; 5 vol.% H₂/N₂) up to approximately 1,000°C, heating rate 10°C/min (Al-Shankiti et al., 2013; Pappacena et al., 2016, 2017) and cooled down to 60°C under a nitrogen flow. TPO was then carried out by flowing (i) a 3 vol.% H₂O/N₂ mixture (10 l(SLT)/h), (ii) a 6 vol.% CO₂/N₂ mixture (10 l(SLT)/h), (iii) a (6 vol.% H₂O + 6 vol.% CO₂)/N₂ mixture (10 l(SLT)/h); the temperature raised up to about 1,000°C (heating rate: 10°C/min; time at the maximum temperature: 20 min). We chose to use a diluted mixture according to the method outlined by Petkovich et al. (2011), to avoid a fast reaction, especially on H₂-reduced samples. The consistency of this approach has been demonstrated (Luciani et al., 2018, 2019; Portarapillo et al., 2019). The reduction and oxidation profiles were deconvoluted using OriginPro 8.5 software.

Oxygen, hydrogen, and carbon monoxide amounts (mol/g), n_O2, n_H2, and n_CO respectively, were calculated as follows:

$$n_{O_{2,i}}={\displaystyle \frac{2\cdot n_{H_2\mathrm{\_}\mathit{cons},i}}{m_{\mathit{cat}}}}={\displaystyle \frac{2\cdot \frac{{\mathit{area}}_{H_2\mathrm{\_}\mathit{cons},i}}{100}\cdot F_{\mathit{red}}\cdot \frac{P}{R\cdot T}}{m_{\mathit{cat}}}}(4)$$

$$n_{H_{2,i}}={\displaystyle \frac{n_{H_2\mathrm{\_}\mathit{prod},i}}{m_{\mathit{cat}}}}={\displaystyle \frac{\frac{{\mathit{area}}_{H_2\mathrm{\_}\mathit{prod},i}}{100}\cdot F_{\mathit{ox}}\cdot \frac{P}{R\cdot T}}{m_{\mathit{cat}}}}(5)$$

$$n_{{\mathit{CO}}_i}={\displaystyle \frac{n_{\mathit{CO}\mathrm{\_}\mathit{prod},i}}{m_{\mathit{cat}}}}={\displaystyle \frac{\frac{{\mathit{area}}_{\mathit{CO}\mathrm{\_}\mathit{prod},i}}{100}\cdot F_{\mathit{ox}}\cdot \frac{P}{R\cdot T}}{m_{\mathit{cat}}}}(6)$$

where m_cat (g) is the sample mass, P (atm) and T (K) are standard pressure and temperature, R (atm⋅l⋅mol⁻¹⋅K^–1) is the ideal gas constant, F_red and F_ox (l(STP)/s) are the flow rates of permanent gases during H₂ treatment and splitting test, respectively, n_{H2_cons,i} and n_{H2_prod,i} are consumed and produced hydrogen moles during the H₂ reduction treatment and splitting test, respectively. The area_{H2_cons,i}, and area_{H2_prod,i} (%⋅s) are the calculated areas of the reduction and oxidation H₂ profiles, respectively. n_{CO_prod,i} is the amount of carbon monoxide produced during the splitting test, and area_{CO_prod,i} (%⋅s) is the calculated area of the oxidation CO profile. The n_O2 does not represent a real molecular oxygen evolution but oxygen subtracted from the materials.

The reduction degree (x_red,i) after the current reduction or oxidation step (i) was calculated as follows:

$$x_{\mathit{red},i}=x_{\mathit{red},i-1}+{\displaystyle \frac{n_{O_2,i}}{n_{O_{2,\mathit{max}}}}}(\mathrm{7a})$$

and as in the following for oxidation steps:

$$x_{\mathit{red},i}=x_{\mathit{red},i-1}-{\displaystyle \frac{n_{H_2,i}}{2\cdot n_{O_{2,\mathit{max}}}}}(\mathrm{7b})$$

$$x_{\mathit{red},i}=x_{\mathit{red},i-1}-{\displaystyle \frac{n_{\mathit{CO},i}}{2\cdot n_{O_{2,\mathit{max}}}}}(\mathrm{7c})$$

where n_O2,max is the maximum O₂ amount (mol/g) that could be evolved, if reducible cations were reduced to their lowest oxidation state (Ce⁴⁺ to Ce^{3 +}, Fe^{3 +} to Fe^{2 +}, Cu^{2 +} to Cu⁰; corresponding values are reported in Table 1). x_red,i–1 is the reduction degree of the previous (oxidation or reduction) step. Therefore, x_red,i does not simply quantify the current fraction of Ce^{3 +} ions on the overall content of Ce atoms in the system.

Oxidation yield (α_i) is calculated through produced CO (mol/g) during the current re-oxidation step and the corresponding O₂ evolved amount (mol/g) during the previous reduction step:

$${\mathrm{\alpha }}_i=\frac{n_{\mathit{CO},i}}{2\cdot n_{O_2,i-1}}(8)$$

Reduction yield (β_i) is calculated through the overall O₂ amount (mol/g) produced during the current reduction step and CO evolved amount (mol/g) during the previous oxidation step:

$${\mathrm{\beta }}_i=\frac{2\cdot n_{O_2,i}}{n_{\mathit{CO},i-1}}(9)$$

Kinetic Model

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_red as the reduced form and σ_ox as the oxidized form.

According to these hypotheses, the reaction steps are:

$${\mathrm{\sigma }}_{red}+C{O}_2\stackrel{k_{c{o}_2}}{\to }{\mathrm{\sigma }}_{\text{ox}}+CO(10)$$

$${\mathrm{\sigma }}_{\text{r}ed}+H_{2}O\stackrel{k_{H_{2}O}}{\to }{\mathrm{\sigma }}_{\text{ox}}+H_2(11)$$

Sites fractions are defined by the following equations:

$${\mathrm{\theta }}_{\text{red}}={\displaystyle \frac{{\mathrm{\sigma }}_{red}}{{\mathrm{\sigma }}_{ox}+{\mathrm{\sigma }}_{red}}}(12)$$

$${\mathrm{\theta }}_{\text{ox}}={\displaystyle \frac{{\mathrm{\sigma }}_{ox}}{{\mathrm{\sigma }}_{ox}+{\mathrm{\sigma }}_{red}}}(13)$$

The reaction rates of steps (10–11) are:

$$r_{C{O}_2}=k_{C{O}_2}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}C{O}_2}}\cdot Y_{C{O}_2}^{n_{C{O}_2}}(14)$$

$$r_{H_{2}O}=k_{H_{2}O}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}{H}_{2}O}}\cdot Y_{H_{2}O}^{n_{H_{2}O}}(15)$$

where θ_red is the reduced sites fraction, n_i are the reaction orders, Y_i are the reactants molar fractions, and k_i are the kinetic constants which are evaluated as follows:

$$k_{C{O}_2}=k_{C{O}_2}^0\exp \left(-{\displaystyle \frac{E_{C{O}_2}}{RT}}\right)(16)$$

$$k_{H_{2}O}=k_{H_{2}O}^0\exp \left(-{\displaystyle \frac{E_{H_{2}O}}{RT}}\right)(17)$$

where E_i are the activation energies, and $k_{\text{i}}^0$ are the frequency factors.

The unsteady balance equations on reduced sites fraction for CO₂ splitting, H₂O splitting, and co-splitting are respectively:

$${\displaystyle \frac{d{\mathrm{\theta }}_{red}}{dt}}=-k_{C{O}_2}^0\exp \left(-{\displaystyle \frac{E_{C{O}_2}}{RT}}\right){\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}C{O}_2}}\cdot Y_{C{O}_2}^{n_{C{O}_2}}(18)$$

$${\displaystyle \frac{d{\mathrm{\theta }}_{red}}{dt}}=-k_{H_{2}O}^0\exp \left(-{\displaystyle \frac{E_{H_{2}O}}{RT}}\right){\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}{H}_{2}O}}\cdot Y_{H_{2}O}^{n_{H_{2}O}}(19)$$

$${\displaystyle \frac{d{\mathrm{\theta }}_{red}}{dt}}=-k_{C{O}_2}^0\exp \left(-{\displaystyle \frac{E_{C{O}_2}}{RT}}\right){\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}C{O}_2}}\cdot Y_{C{O}_2}^{n_{C{O}_2}}\mathrm{\hspace{1em}}-k_{H_{2}O}^0\exp \left(-{\displaystyle \frac{E_{H_{2}O}}{RT}}\right){\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}{H}_{2}O}}\cdot Y_{H_{2}O}^{n_{H_{2}O}}(20)$$

The unsteady balance equations on gaseous molar fractions are:

$${\displaystyle \frac{d{Y}_{C{O}_2}}{dt}}={\displaystyle \frac{Y_{C{O}_2}^{IN}-Y_{C{O}_2}}{\mathrm{\tau }}}-k_{C{O}_2}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}C{O}_2}}\cdot Y_{C{O}_2}^{n_{C{O}_2}}(21)$$

$${\displaystyle \frac{d{Y}_{H_{2}O}}{dt}}={\displaystyle \frac{Y_{H_{2}O}^{IN}-Y_{H_{2}O}}{\mathrm{\tau }}}-k_{H_{2}O}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}{H}_{2}O}}\cdot Y_{H_{2}O}^{n_{H_{2}O}}(22)$$

$${\displaystyle \frac{d{Y}_{CO}}{dt}}={\displaystyle \frac{Y_{CO}^{IN}-Y_{CO}}{\mathrm{\tau }}}-k_{C{O}_2}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}C{O}_2}}\cdot Y_{C{O}_2}^{n_{C{O}_2}}(23)$$

$${\displaystyle \frac{d{Y}_{H_2}}{dt}}={\displaystyle \frac{Y_{H_2}^{IN}-Y_{H_2}}{\mathrm{\tau }}}-k_{H_{2}O}{\mathrm{\theta }}_{red}^{n_{\mathrm{\theta }\mathrm{\_}{H}_{2}O}}\cdot Y_{H_{2}O}^{n_{H_{2}O}}(24)$$

where τ is the residence time equal to the experimental one.

The initial conditions are:

$$\text{t}=0\hspace{1em}{\mathrm{\theta }}_{\text{red}}={\mathrm{\theta }}^0(25)$$

$$\text{t}=0\hspace{1em}{\text{Y}}_{\mathrm{CO2}}=0.06(26)$$

$$\text{t}=0{\text{Y}}_{\mathrm{H2O}}=0.03\left({\text{H}}_2\mathrm{O}\mathrm{splitting}\right);0.06\left(\mathrm{co}-\mathrm{splitting}\right)(27)$$

$$\text{t}=0\hspace{1em}{\text{Y}}_{\text{CO}}=0(28)$$

$$\text{t}=0\hspace{1em}{\text{Y}}_{\mathrm{H2}}=0(29)$$

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:

$$SRMSE=\frac{1}{y_{MAX}}\sqrt{\frac{1}{N}\sum _{i=1}^N{\left(y_{exp}\left(i\right)-y_{mod}(i)\right)}^2}(30)$$

Model and experimental curves were considered in good agreements for SRMSE values ≤0.045.

Results

Experimental Results

Previous research has reported on H₂/H₂O cycling, outlining that it can be used to define the splitting properties of materials (Landi, 2019). Results of thermogravimetric (TG) measurements (reduction under inert Ar atmosphere; oxidation under 40 vol.% CO₂/Ar) and H₂O splitting tests on H₂-reduced samples were successfully compared (Al-Shankiti et al., 2013; Pappacena et al., 2016, 2017; Luciani et al., 2018).

Results by Landi (2019) showed that K-addition to bare ceria-zirconia and materials doped with transition metals (K-Fe-CeZr and K-Cu-CeZr) significantly enhance the evolved oxygen amount and lower the reduction onset temperature. These reduction profiles obtained for the co-splitting reaction are consistent with those reported by other studies (Landi, 2019) and will, therefore, not be further discussed.

Figure 1 (bottom) shows H₂ profiles measured in the H₂O splitting tests. The profiles refer to the stable performance obtained after the multiple reduction/oxidation cycle (Landi, 2019). One wide peak was detected over the K-Cu-CeZr sample, while two peaks were found over K-Fe-CeZr catalysts.

FIGURE 1

Figure 1. H₂ (blue solid lines) and CO (red dashed lines) concentration profiles during splitting tests under different oxidizing streams on K-Fe-CeZr (left) and K-Cu-CeZr (right).

Potassium doped samples show faster oxidation kinetics than K-free materials (Landi, 2019). In particular, H₂ production occurs at very low temperature (peak temperature at 160°C for K-Fe-CeZr and 200°C for K-Cu-CeZr sample, respectively). Furthermore, the K-Fe-CeZr sample shows a second oxidation phenomenon at a higher temperature (at about 590°C).

Table 2 reports the amount of produced O₂ and H₂, reduction degrees after each step (x_red), oxidation yields (α), and reduction yields (β) as obtained during H₂O splitting tests. Co-doped samples show higher oxygen and hydrogen production than the bare CeZr catalyst, as well as full re-oxidation (approximately 100%). Indeed, K-doping of M-doped ceria-zirconia (M = Fe^{3 +} and Cu^{2 +}) can have limited effects on redox performance with no significant changes in the oxygen released (Fe-CeZr: K-free 847 μmol/g, K-doped 747 μmol/g; Cu-CeZr: K-free 576 μmol/g, K-doped 559 μmol/g) and produced hydrogen (Fe-CeZr: K-free 1,437 μmol/g, K-doped 1,458 μmol/g; Cu-CeZr: K-free 1,070 μmol/g, K-doped 1,237 μmol/g) (Landi, 2019).

TABLE 2

Table 2. Amount (in μmol/g) of released oxygen (as O₂) and produced H₂ during H₂O splitting tests, reduction degrees after each step (x_red), oxidation yields (α), and reduction yields (β).

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.

Figure 1 shows H₂ and CO concentration profiles as obtained in CO₂/H₂O co-splitting tests and separate H₂O and CO₂ splitting experiments over co-doped materials. Figure 2 also shows the fuel amount produced over K-Fe-CeZr and K-Cu-CeZr catalysts.

FIGURE 2

Figure 2. Quantitative analysis of splitting tests under different oxidants. Partial and overall amounts of H₂ + CO on K-Fe-CeZr (A) and K-Cu-CeZr (B); H₂ (blue symbols) and CO (red symbols) amounts during H₂O splitting (•) co-splitting (▲), and CO₂ splitting (▼) on K-Fe-CeZr (C) and K-Cu-CeZr (D).

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 (Di Benedetto et al., 2013; Barbato et al., 2016).

Moreover, CO₂ feed results in a lower produced H₂ amount (Figure 2C). The second oxidation phenomenon occurs at the same temperature independently from the oxidizing stream (Figures 1, 2). As reported above, this peak is related to bulk oxidation. This reaction is controlled by the bulk-to-bulk oxygen diffusion and is thus not influenced by the chemical nature of the oxidant. However, during H₂O/CO₂ co-splitting only CO production is (unexpectedly) detected, suggesting a key role of gaseous molecules on the reaction pathway.

CO₂ splitting on K-Cu-CeZr (Figure 1) features one CO production peak at about 600°C, confirming lower activity toward CO₂ than the K-Fe-CeZr sample, which was also detected by TG analysis (Landi, 2019). In the H₂O/CO₂ co-splitting tests, the presence of CO₂ shifts H₂ production to higher temperatures (Figures 1, 2D) and reduces H₂ amounts (Figures 2B,D), as reported for the K-Fe-CeZr sample. Furthermore, CO production is negligible, probably because the material is fully oxidized at a temperature lower than that of CO₂ activation.

Kinetic Model of K-Fe-CeZr

The results of co-splitting tests (Figure 1) show that the H₂O splitting reaction is faster than CO₂. However, at high temperatures, the H₂O splitting reaction is prevented and CO₂ splitting is favored. To understand the observed behavior, we developed a kinetic model. As reported in section “Kinetic Model,” only the surface reactions have been modeled. The low temperature peaks of H₂O and CO₂ splitting tests were used as experimental data to calculate the kinetic parameters. Figures 3, 4 show the experimental and model curves as obtained in H₂O and CO₂ splitting tests, respectively. The results reported in Table 3, show a low value of the SRMSE, suggesting that the fitting is good.

FIGURE 3

Figure 3. (A) Experimental and model CO production (low temperature peak, bare CO₂ splitting) and (B) the corresponding surface site evolution calculated by the kinetic model.

FIGURE 4

Figure 4. (A) Experimental and model H₂ production (low temperature peak, bare H₂O splitting) and (B) the corresponding surface site evolution calculated by the kinetic model.

TABLE 3

Table 3. Kinetic parameters of surface splitting reactions.

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 (n_{θ _H2O} = 1); whereas the CO₂ oxidation rate exhibits an order lower than 1 with respect to active sites concentration (n_{θ _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:

$$R={\displaystyle \frac{r_{{\mathit{CO}}_2}}{r_{H_{2}O}}}(31)$$

Figure 5 shows H₂O and CO₂ splitting reaction rates of on K-Fe-CeZr as calculated by the kinetic model as a function of the active sites concentration (θ), at different values of temperature. In these figures, θ values along the x-axis are decreasing.

FIGURE 5

Figure 5. Surface reaction rates of H₂O and CO₂ splitting reaction (left) and their ratio R (right) as a function of decreasing reduced sites fraction on K-Fe-CeZr at different temperatures as calculated by the kinetic model.

Three regimes can be identified:

Regime I- H₂O driven regime: @T ≤ 560°C, the reaction rates do not intersect and R < 1 in the whole θ range. H₂ production prevails over CO production.

Regime II- mixed regime: @ 560 < T < 700°C, r_CO2 and r_H2O intersect. At low θ values R > 1, at high θ values R < 1. Reaction rates are of the same order of magnitude, suggesting a co-production of carbon monoxide and hydrogen.

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. Figure 6 shows the ratio between CO and H₂ produced amounts (F, Eq. 32) as a function of temperature.

FIGURE 6

Figure 6. Ratio between CO and H₂ production as a function of temperature during isothermal co-splitting simulations.

$$F=\frac{n_{\mathit{CO}}}{n_{H_2}}(32)$$

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.

Conclusion

In this work, ceria-zirconia was co-doped with transition metal (Cu^{2 +}, Fe^{3 +}) 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.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

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.

Conflict of Interest

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.

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