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Methane Dry Reforming Catalysts Based on Pr-doped Ceria-Zirconia Synthesized in Supercritical Propanol

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08 May 2023

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09 May 2023

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Abstract
This work is devoted to the study of active and stable nickel catalysts for methane dry reforming based on Pr-doped ceria-zirconia obtained by solvothermal continuous method. The studies of physicochemical and catalytic properties of the 5%Ni\Ce0.75Zr0.25-xPrxO2 series showed that Pr introduction leads to an increase in the amount of highly reactive oxygen of the oxide lattice. Praseodymium-based catalysts showed significantly higher reactant conversions. In addition to the nature of support, the method of nickel introduction was also studied - Ni was added both by impregnation and one-pot with mixed oxide preparation. The method of Ni addition was shown to have significant effect on the morphology of the supported active component and, respectively, on the catalytic activity. The 5%Ni/Ce0.75Zr0.15Pr0.1O2 catalyst prepared by one-pot method showed stable operation in the MDR reaction for 30 hours at CO2 and CH4 conversions of ~40% and an H2 yield of ~18% (Т=700°С, τ=10ms).
Keywords: 
dry reforming
;  
methane
;  
ceria-zirconia
;  
supercritical synthesis
;  
oxygen vacancies
;  
stability
Subject: 
Chemistry and Materials Science  -   Nanotechnology

1. Introduction

Methane dry reforming (MDR) is a promising way to utilize two greenhouse gases - CH4 and CO2 - to convert them into synthesis gas with its subsequent use as a feedstock for the chemical industry or a fuel [1,2,3]:
СН4 + СО2 = 2CO + 2H2,    ∆H0298 = 247.3 kJ/mole
In addition to direct use, it can be effectively integrated into existing hydrogen production chains, taking into account current environmental requirements [4]. MDR catalysts are representatives of typical catalysts for hydrocarbons reforming processes, which are a transition metal, primarily nickel, deposited on an oxide support [5]. Studies over the past decades show that the poor stability due to sintering and carbonization of the active species remains a major problem for the reforming catalysts [6]. Developing a thermally stable catalyst with superior activity and enhanced resistance to carbon formation and metal sintering is among the major challenges faced by industries and researchers [7].
The carbonization problem begins at the stage of competition between simultaneously occurring routes of CHx intermediates transformation on metal particles. If CHx oxidation occurs rapidly, the formation of the desired CO is observed. But if the rate of CHx formation is higher than its oxidation, carbon deposits grow. A well-known, highly efficient approach to increase the rate of the desired route is to involve the lattice oxygen of the support into the catalytic cycle [8]. Fuels transformation over catalysts with high oxygen reactivity occurs by the redox reaction mechanism. During the reaction, the substrate (CH4, CO) is oxidized by the lattice oxygen, accompanied by the formation of a vacancy and its replenishment through the activation of the oxidizer (CO2, H2O or O2) on the oxide surface cations and its fast surface transfer to the direct catalytic act zone (reverse oxygen spillover) [9,10].
However, establishing an unambiguous relationship between the structural characteristics of the oxide and the catalytic activity in reforming reactions is still a debatable issue: steady-state reaction conditions are differed from equilibrium, while the properties of as-prepared catalysts are more likely defined by thermodynamic equilibrium during the preparation process [11]. Nevertheless, huge number of works has been done and described in the literature based on a complex of catalytic, structural studies and DFT calculations, which showed correlations of catalytic activity with the so-called «support oxygen activity» [7,12,13].
The term “support oxygen activity” is complex, including several correlated characteristics of the oxide’s lattice oxygen, such as oxygen storage capacity (OSC), surface and bulk mobility, the number and formation energy of oxygen vacancies. For example, according to many reports, the redox properties of catalysts were believed to relate to the amount of oxygen vacancies (regarded as active sites) in oxide catalysts. However, just presence of vacancies is not enough: if the vacancy formation barrier is too high, this stage becomes rate-limiting, which casts doubt on the probability of the entire process proceeding along such a route. That phenomenon, for example, was shown and confirmed by DFT calculations for nickel-containing undoped cerium oxide [14]. Moreover, the steady-state concentration of oxygen vacancies is related to the rates of vacancy formation and vanishing. In other words, a real active oxygen vacancy should have a high abilities of adsorbing the reactant molecules and desorbing the product molecules [11,15]. A decrease in the energy of vacancy formation can be achieved, for example, by increasing the length of at least one oxygen bond in the lattice, or by creating so-called asymmetric vacancies [16,17].
Oxides capable of providing such cycles of lattice vacancies formation with charge compensation due to a cation with a variable valence are mainly transition-metal oxides and are called reducible, or oxides with the active oxygen [18]. Among oxides with the lattice oxygen taking part in the reaction, the most common are perovskites, spinels and fluorites. The group of oxides based on ceria doped with a wide range of cations is deeply studied [12,19,20,21]. Detailed kinetic and computational studies have shown that the activity of oxygen in such oxides depends on the concentration and formation energy of these vacancies [22,23]. Moreover, it was also shown that the presence and energy of the vacancies on the surface of cerium oxide can determine the overoxidation of the surface intermediate -CO into -CO2, thus largely determining the H2/CO ratio in the resulting synthesis gas [24,25]. One of the most popular doping cations which also increases thermal stability of CeO2 oxide is Zr [26,27]. The effective generation of asymmetric vacancies in the CeO2-based oxides upon doping with Pr increasing the oxygen catalytic activity has been shown in a number of works [14,28,29,30]. Furthermore, substitution of no more than 25% of cerium has been shown to be effective, since the Pr extent would hamper the Ce4+/Ce3+ redox pair by exclusively locating electrons generated by the vacancy formation at praseodymium centers and prevent completion of the catalytic cycle [16,18].
The synthesis method of these polycation oxides for catalytic application should provide a high chemical uniformity and oxide dispersion. Known to use methods are: coprecipitation [31], hydrothermal method [32], solvothermal method [33], sol–gel–citrate [34,35] and ester polymer precursors (Pechini) [36], microemulsions [37], microwave method [38,39], sonochemical method [40], solution combustion [41,42], spray pyrolysis reactions [43]. The use of the solvothermal method for the synthesis of such polycationic oxides provides structural homogeneity, as well as improved textural characteristics and tunable particle size distribution [44].
It is important to emphasize the decisive influence of the nickel introduction method. In an oxide-metal system, the dispersion of the supported metallic particles and the strength of its interaction with the support are the key properties that determine catalyst stability in reforming reactions on a par with the support oxygen activity [45,46,47]. It was shown that the traditional method of wet impregnation is less efficient than the method of nickel introducing into the oxide structure at the stage of support synthesis. In the second case, during the reduction pretreatment, formation of finely dispersed metal particles strongly bound to the carrier is observed [48,49]. At the same time, the main disadvantage of this method is the possible loss of the active component in the volume of the support blocked for the reaction.
Hence, the use of zirconium as a doping cation to ceria increases stability of the structure and increases the structure defectiveness, while doping with praseodymium makes it possible to increase the number of oxygen vacancies and obtain more reactive surface oxygen. Accordingly, in this work, our goal was the synthesis of a series of complex metal-oxide catalysts 5%Ni\Ce0.75Zr0.25-xPrxO2 obtained by the solvothermal method with Ni added by both impregnation and one-pot method. This work present results of the study of 5%Ni\Ce0.75Zr0.25-xPrxO2 textural and redox properties and their influence on the catalytic performance in the MDR reaction.

2. Materials and Methods

2.1. Synthesis Methods

The catalyst supports were synthesized in supercritical alcohol media using original installation shown in the Figure 1. Cerium nitrate Ce(NO3)3·6H2O (Vekton, Russia), praseodymium nitrate Pr(NO3)3·6H2O and zirconium butoxide (80 wt% in n-butanol, Alfa Aesar, Germany) were dissolved in required proportions in isopropanol (Reakhim, Russia). Then the solution was fed into the reactor and synthesis was carried out according to the method described earlier [50]. For impregnated catalysts, nickel was supported by the incipient wetness impregnation of supports with water solution of Ni(NO3)2 [49].
For one-pot catalysts, nickel nitrate Ni(NO3)2·6H2O (Vecton) was dissolved together with Ce, Zr (Pr) salts, and synthesis in a supercritical medium was carried out according to the procedure described above. All catalysts were dried at 200°C and calcined at 700°C for 2 h. The loading amount of Ni was 5 wt% both for impregnated and one-pot catalysts. The abbreviations of the prepared samples are presented in the Table 1.

2.2. Characterizations

Specific surface area (SSA) was defined by the BET method using a Quadrasorb evo (Quantachrome Instruments, USA) installation. Pore volumes and pore size distribution were determined from the desorption branch of the isotherm using the BJH method.
XRD analysis was performed on a Thermo X’tra diffractometer, in the angle range of 20-85° with a step of 2ϴ = 0.02° and a speed of 1°/min, using a Mythen2R 1D linear detector (Decstris, Switzerland). The CuKα radiation (λ = 1.5418A) was used. The average sizes of coherent scattering regions (CSRs) were calculated using the Scherrer formula from the 111 fluorite reflection. The description of the diffraction profile was performed using the Fityk program with the Lorentz function.
XRD with H2 in situ analysis was made on a Bruker D8 Advance diffractometer (Germany), in the angle range of 20-55° with a step of 2Ө = 0.05° and an accumulation time of 3 s at each point using a LynxEye (1D) line detector. The monochromatic CuKα radiation (λ = 1.5418Å) was used. The measurements were carried out using an XRK-900 high-temperature reactor chamber (Anton Paar, Austria). The 10%H2/90%He mixture at a flow rate of 100 mL/min was passed through the chamber during heating and cooling to room temperature. Heating was carried out according to the following scheme: 250, 315, 400, 500, 700, 30°C with the heating rate 12°/min. Refinement of lattice parameters and phase relationships was performed by the Rietveld method [51]. In the refinement, the CeO2 structure was used; only Ce atoms were in the cationic position.
TEM with EDX. TEM (transmission electron microscopy) micrographs were obtained with a Themis-Z 3.1 instrument (TFS, USA) equipped with X-FEG-monochromator and CS/S double corrector, accelerating voltage 200 kV, and with a JEM-2200FS transmission electron microscope (JEOL Ltd., Japan, acceleration voltage 200 kV, lattice resolution ~ 1 A) equipped with a Cs corrector. Elemental analysis was performed with a Super-X EDS detector (energy resolution about 120 eV) in HAADFSTEM mode. Samples for the TEM study were prepared by ultrasonic dispersing in ethanol and subsequent deposition of the suspension upon a “holey” carbon film supported on a copper grid.
TPR-H2. Temperature-programmed reduction by hydrogen was carried out from 25 °C to 900°C in a flow installation with a Tsvet 500 (JSC Tsvet, Russia) thermal conductivity detector using 10 vol% H2 in Ar feed at the flow rate 40 ml/min.

2.3. Catalytic Tests

The catalysts were preliminary reduced in a stream of 5% H2 in He at 600 ◦C for 1 hour. The catalytic activity in methane dry reforming reaction was studied using a tubular quartz plug flow reactor and gas analyzer with IR sensors for CO, CO2 and CH4, and electrochemical sensor for H2 (Boner LLC, Novosibirsk, Russia). Studies were conducted with the initial mixture of 15% CH4 + 15% CO2 + 70 % N2 in the temperature range of 600–750 ◦C and contact time 10 ms. Long-term stability tests were carried out for 30h at 700 ◦C.

3. Results

3.1. Textural and Structural Features

Table 1 lists designation, chemical composition and specific surface area (SSA) of samples obtained.
Table 1. Sample’s designation, composition, synthesis methods, SSA and Fm3m lattice parameter.
Table 1. Sample’s designation, composition, synthesis methods, SSA and Fm3m lattice parameter.
Designation Ni addition method Composition SSA, m2/g a, Å
CZP - Ce0.75Zr0.15Pr0.1O2 14 5.403(1)
N/CZP impregnation 5%Ni/Ce0.75Zr0.15Pr0.1O2 9 5.397(1)
N+CZP one-pot 5%Ni+Ce0.75Zr0.15Pr0.1O2 14 5.407(1)
CZ - Ce0.75Zr0.25O2 29 5.369(1)
N/CZ impregnation 5%Ni/Ce0.75Zr0.25O2 21 5.373(1)
N+CZ one-pot 5%Ni+Ce0.75Zr0.25O2 14 5.372(1)
As can be seen, samples obtained do not have a very high specific surface area: the values of SSA for the entire series of samples are about 10-30 m2/g. The introduction of praseodymium reduces the specific surface area of the initial support from 29 m2/g for CZ to 14 m2/g for CZP. It correlates well with the data of pore’s size distribution (Figure 2), where the number of micropores for praseodymium-doped samples is significantly reduced. In contrast, for samples prepared by the one-pot method (N+CZ and N+CZP) there is no difference in the SSA values.
According to XRD data (Figure 3), for the entire series main intense peaks located at 2θ = 28.7, 47.7, 56.6, 59.4, 69.8, 77.0, 79.4º correspond to 111, 200, 220, 311, 222, 331, 420, 422 reflections of the fluorite structure Fm3m (PDF 81-0792). There are no reflections of Zr and Pr individual oxides for all series. For Pr-containing samples, the absence of double peaks and a shift of the fluorite peaks to the lower angles’ region (Figure 2, Table 1) suggest that praseodymium is embedded into the СZ cubic structure, which is consistent with literature data [1,2]. The ability of praseodymium to replace cerium without destroying the fluorite structure is ensured by close radii of cations of the same valencies (Ce4+ = 1.110 Å; Pr4+ = 1.100 Å; Ce3+ = 1.283 Å; Pr3+ = 1.266 Å), and a slightly higher Pr4+/Pr3+ redox potential leads to the formation of a structure with more active oxygen compared to undoped ones [52,54].
The method of the nickel introducing has a strong effect on the oxide’s morphology. XRD (Figure 3) and TEM data (Figure 4) show that the deposition of nickel by wet impregnation on the formed oxide resulted in the formation of large well-crystallized NiO particles about 20 nm in size, weakly bound to the support. For this samples, there are peaks at 2θ = 37.4, 43.3, 62.8 º, which can be associated with 111, 200 and 220 reflections of nickel oxide NiO (Fm3m). The introduction of nickel at the stage of support synthesis leads to the incorporation of a significant amount of nickel into the fluorite structure and formation of residual finely dispersed NiO particles firmly fixed on the oxide surface or encapsulated into it.

3.2. Reducibility and Oxygen Reactivity

The oxide’s oxygen activity was estimated by a combination of complementary methods – TPR-Н2 and XRD with in situ H2-reduction. The TPR-H2 results are presented in the Figure 5. The curves of two supports (CZ and CZP) represent two reduction regions, classical for these oxides: the medium-temperature one, related to the reduction of the more reactive surface oxygen, and the high-temperature reduction of bulk oxygen, limited by the internal diffusion. It can be seen that for the support with praseodymium, both the start of reduction and the maximum of the first peak are shifted by about 200 °C towards lower temperatures, which indicates a significant facilitation of the reduction of surface oxygen with doping.
All curves of Ni-containing samples have a complex contour with peaks that are difficult to separate from each other. The calculation of the total amount of hydrogen consumed over the entire temperature range shows approximately the same values for all samples: 2.0–2.15 mmol/gcat. However, significant changes in the nature of reduction are noted with the introduction of praseodymium and nickel to CZ. For the supported N/CZP, a large sharp peak is observed with a maximum at 250 °C, which corresponds to the reduction of significant amount of NiO and indicates its weak interaction with the oxide support. For the one-pot N+CZP sample, the reduction occurs more uniformly in the range of 200-400 °C. The XRD with in situ H2 reduction (Figure 6a–c) confirms that, after reduction at 400 °C, a significant amount of metallic nickel (2θ=44.2 °) is detected for all nickel-containing samples. Moreover, in the temperature range of 400–500 °C, the intensity of nickel reflections for supported catalysts is higher compared to one-pot samples, which is consistent with the TPR-H2 data and the assumption that nickel is partially incorporated into the fluorite lattice in one-pot samples.
The XRD with in situ H2 reduction shows also (Figure 6d) non-linear change in the lattice parameter in the temperature range of 200–400 °C, where hydrogen interaction with the most reactive oxide’s oxygen occurs. Stronger parameter changes in this region for Pr-containing catalysts compared to undoped ones confirms facilitation of the lattice oxygen release obtained by the TPR-H2 method with Pr doping. A noticeable overstating value of the lattice parameter at 300-400 °C coincides with the appearance of the nickel metallic reflections on the XRD patterns and indicates a greater depth of reduction due to a well-known phenomenon of hydrogen spillover described in the literature [55,56,57]. It is also important to note that after returning to the room temperature, the Fm3m fluorite monophase with an increased parameter is preserved, hence, the release of oxygen occurs without destroying the structure.

3.3. Catalytic Activity in MDR

The results of MDR catalytic experiments are shown in Figure 7. For all samples, an increase in the conversions of methane and carbon dioxide is observed with increasing temperature in accordance with the endothermicity of the reaction.
The activity of Pr-containing catalysts N/CZP and N+CZP exceeds the activity of samples based on undoped CZ support, and the difference rises significantly with the temperature increasing. This is consistent with literature data describing increase in the contribution of support to the reaction with the temperature rise due to more efficient activation of CO2 and transfer of active oxygen forms over the oxide surface to the metal-support interface [24,25].
At the same time, the method of nickel introduction does not noticeably affect the activity in experiments with temperature variation, but strikingly determines stability in long-term tests. For the N/CZP sample obtained by wet impregnation, reactor plugging due to carbon formation occurs within 1 hour of a long test at 700 °C (not shown on Figures). Figure 7, f, shows the data on the H2 and CO yields in long-term tests for N+CZP one-pot catalyst. For 30 hours of the reaction, the hydrogen yield decreased from 35 to 32%, and the CO yield – from 19 to 17%, and the main drop occurred for the first 3 hours of reaction. Previously, for such systems, we have shown that this decrease in first hours is associated with the rapid carbon formation, that comes to steady state with reaction of carbon oxidation after 3 hours of reaction and further the carbon formation rate (CFR) decreases almost linearly with increasing time [58].
The TEM studies of catalysts after”reac’Ion showed that the N/CZP sample is characterized by whiskers carbon deposits and weak bond between nickel and the support resulted in detachment of nickel particles from the surface, while the N+CZP sample demonstrates metal particles firmly fixed on the oxide surface and an insignificant number of thin layers of carbon directly on the metal particles that do not block active catalytic centers (Figure 8).
Hence, the catalytic activity of samples modified with praseodymium is significantly higher than that of the unmodified ones. An increase in the conversion of both methane and CO2 correlates with an increase in the activity of the carrier’s highly reactive oxygen, as shown by the TPR-H2 method. At the same time, stability of the catalyst is dramatically affected by the strength of the interaction of resulting nickel metal particles with the oxide support. Thus, for samples obtained by wet impregnation, the weak bond with the support, shown by TEM and TPR-H2 methods, eliminates all the advantages of the active support and prevents the effective occurrence of the redox mechanism, that leads to rapid deactivation with formation of a significant amount of fibrous carbon deposits, confirmed by TEM images. For samples with nickel introduced at the stage of support formation, high stability for 30 hours of reaction at 700°C and the absence of carbon deposits in TEM images were shown.

5. Conclusions

In this work, catalysts series 5%Ni\Ce0.75Zr0.25-xPrxO2 (x=0; 0.1) were successfully synthesized by the solvothermal method in a flow-through setup in supercritical isopropanol. It was shown that all samples are crystallized in the structural type Fm3m of cubic fluorite with the incorporation of praseodymium and zirconium cations into the lattice. Addition of Pr to mixed ceria-zirconia leads to increased reducibility during TPR-H2 experiments and stronger parameter changes during in situ XRD in hydrogen which together confirm higher reactivity of lattice oxygen. The nickel introduction into the catalyst composition directly at the stage of support synthesis (one-pot) allows a significant amount of nickel to be incorporated into the fluorite structure, which leads both to an increase in the defectiveness of the oxide and to the formation of highly dispersed particles of metallic nickel, strongly bound to the support oxide, during reduction pretreatment. The Pr introduction into the oxide support significantly increases catalysts activity in syngas production, which is most likely associated with facilitating the oxidant molecule activation, thus promoting the redox mechanism of MDR. The use of Pr-doped ceria-zirconia as a support makes it possible to achieve excellent stability due to the high dispersion of metal particles, the optimal strength of the metal-support interaction, and lattice oxygen activity in reactions of oxidant activation and gasification of carbon precursors. The 5%Ni/Ce0.75Zr0.15Pr0.1O2 catalyst showed stable performance in the MDR reaction for 30 hours at a CH4 conversion of 40% and an H2 yield of 20% (Т=700°С, τ=10ms).

Author Contributions

Methodology, M.A., E.S., Y.B. and M.S.; investigation, K.V., V.F., A.H. and O.B.; writing—original draft preparation, M.A.; writing—review and editing, E.S., Y.B. and M.S.; supervision, V.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Russian Science Foundation, grant number 18-73-10167.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the Shared Research Center “VTAN” of the Novosibirsk State University supported by Ministry of Science and Higher Education of the Russian Federation by agreement #075-12-2021-697 for HRTEM investigation. Authors would like to thank personally A.V. Ishchenko for carrying out HRTEM studies and A.A. Leonova for experiments of nitrogen adsorption/desorption.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental setup for solvothermal continuous synthesis. 1 – vessel with isopropanol, 2 – vessel with a solution of metal salts, 3 – plunger pump, 4 – syringe pump, 5 – reactor, 5a – isopropanol pre-heater, 6 – mixer, 7 – oven, 8 – heat exchanger, 9 – back pressure valve, 10 – storage tank.
Figure 1. Experimental setup for solvothermal continuous synthesis. 1 – vessel with isopropanol, 2 – vessel with a solution of metal salts, 3 – plunger pump, 4 – syringe pump, 5 – reactor, 5a – isopropanol pre-heater, 6 – mixer, 7 – oven, 8 – heat exchanger, 9 – back pressure valve, 10 – storage tank.
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Figure 2. The pore size distribution for the supports and Ni-containing samples.
Figure 2. The pore size distribution for the supports and Ni-containing samples.
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Figure 3. Diffraction pattern of supports and Ni-containing samples. Reflections of the fluorite structure are marked by (hkl) indices, the arrows indicate the NiO reflections.
Figure 3. Diffraction pattern of supports and Ni-containing samples. Reflections of the fluorite structure are marked by (hkl) indices, the arrows indicate the NiO reflections.
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Figure 4. TEM: (a, c) impregnated Ni/CZP; (b, d) one-pot N+CZP.
Figure 4. TEM: (a, c) impregnated Ni/CZP; (b, d) one-pot N+CZP.
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Figure 5. TPR-H2 patterns for the supports and Ni-containing samples.
Figure 5. TPR-H2 patterns for the supports and Ni-containing samples.
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Figure 6. Diffraction patterns recorded during in situ reduction: (a) CZP; (b) N/CZP; (c) N+CZP. The reflections of the fluorite structure are labeled; the positions of the NiO peaks are indicated by solid lines, and Ni by dotted lines. (d) Fluorite lattice parameters versus temperature.
Figure 6. Diffraction patterns recorded during in situ reduction: (a) CZP; (b) N/CZP; (c) N+CZP. The reflections of the fluorite structure are labeled; the positions of the NiO peaks are indicated by solid lines, and Ni by dotted lines. (d) Fluorite lattice parameters versus temperature.
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Figure 7. Results of catalytic tests for the N/CZ(P) and N+CZ(P) catalysts in methane dry reforming: temperature dependences of a) CH4 conversion b) CO2 conversion; c) CO yield, d) H2 yield, e) H2/CO; and f) H2 and CO yields in long-term tests for N+CZP one-pot catalyst at 700 °C. Catalyst grain size 0.25-0.5 mm, contact time 10 ms, C0(CH4) = C0(CO2) = 15%.
Figure 7. Results of catalytic tests for the N/CZ(P) and N+CZ(P) catalysts in methane dry reforming: temperature dependences of a) CH4 conversion b) CO2 conversion; c) CO yield, d) H2 yield, e) H2/CO; and f) H2 and CO yields in long-term tests for N+CZP one-pot catalyst at 700 °C. Catalyst grain size 0.25-0.5 mm, contact time 10 ms, C0(CH4) = C0(CO2) = 15%.
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Figure 8. TEM of Ni/CZP and Ni+CZP catalysts after reaction. Nickel particles are marked with circles.
Figure 8. TEM of Ni/CZP and Ni+CZP catalysts after reaction. Nickel particles are marked with circles.
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