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Article

Highly Active Cerium Oxide Supported Solution Combustion Cu/Mn Catalysts for CO-PrOx in a Hydrogen-Rich Stream

by
Sbusiso Motha
,
Abdul S. Mahomed
,
Sooboo Singh
and
Holger B. Friedrich
*
Catalysis Research Group, School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000, South Africa
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(9), 603; https://doi.org/10.3390/catal14090603
Submission received: 30 June 2024 / Revised: 29 August 2024 / Accepted: 31 August 2024 / Published: 7 September 2024
(This article belongs to the Special Issue Catalytic CO Oxidation and Preferential CO Oxidation (PROX) II)
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Versions Notes

Abstract

:
Mono- and di-substituted cerium oxide catalysts, viz. Ce0.95Cu0.05O2-δ, Ce0.90Cu0.10O2-δ, Ce0.90 Cu0.05Mn0.05O2-δ, Ce0.85Cu0.10Mn0.05O2-δ, and Ce0.80Cu0.10Mn0.10O2-δ, were synthesized via a one-step urea-assisted solution combustion method. The elemental composition and textural and structural properties of the catalysts were determined by various physical, electronic, and chemical characterization techniques. Hydrogen temperature-programmed reduction showed that co-doping of copper and manganese ions into the CeO2-δ lattice improved the reducibility of copper. Powder XRD, XPS, HR-TEM, and Raman spectroscopy showed that the catalysts were a singled-phased, solid-solution metal oxide with a cerium oxide cubic fluorite (cerianite) structure, and evidence of oxygen vacancies was observed. Catalytic results in the preferential oxidation of CO in a hydrogen-rich stream showed that complete CO conversion occurred between 150 and 180 °C. Furthermore, at 150 °C, Ce0.90Cu0.05Mn0.05O2-δ, Ce0.90 Cu0.10O2-δ, and Ce0.85Cu0.10Mn0.05O2-δ catalysts were the most active, achieving complete CO conversion and CO2 selectivity of 81, 79, and 71%, respectively. The catalysts performed moderately in the presence of CO2 and water, with the Ce0.90Cu0.05Mn0.05O2-δ catalyst giving a CO conversion of 80% in CO2, which decreased to about 60% when water was added.

1. Introduction

Although hydrogen has wide industrial applications, such as in the transformation of syngas into hydrocarbons with different chain lengths or ammonia production, its use in fuel cell technology has been gaining momentum in recent years [1]. Hydrogen is generally produced through steam reforming; however, the reformate stream contains levels of carbon monoxide (CO) that are not suitable for direct use in fuel cells, such as proton exchange membrane fuel cells (PEMFCs). There is a need to reduce the concentration of CO to acceptable levels for use in fuel cells. The water gas shift reaction initially addresses this concern; however, this is not enough to reduce the levels to less than 10 ppm, which can be tolerated by PEM fuel cells [2,3,4]. Some methods, such as pressure swing gas adsorption, selective membrane separation, and selective CO methanation, can reduce CO to acceptable levels; however, preferential oxidation (PrOx) has been the preferred choice in recent years [5]. One of the drawbacks of using this method is the simultaneous oxidation of valuable hydrogen, leading to poor selectivity. The challenge is to develop materials that can efficiently promote CO oxidation but, at the same time, retard the oxidation of hydrogen.
There are several types of catalysts developed for the CO-PrOx application, prepared using various techniques, which are either supported or unsupported [6,7,8]. The use of cerium oxide in CO-PrOx catalytic systems has received considerable attention due to its ability to transform between oxidation states [9,10]. However, at high temperatures, CeO2 tends to have poor thermal stability due to sintering. There is a significant loss in its oxygen storage capacity, resulting in catalyst deactivation. To overcome this, many researchers have investigated the possibility of doping cerium oxide with another metal, predominantly Cu and sometimes Mn [11]. Copper–cerium oxide catalysts were prepared and tested by numerous researchers and have shown good PrOx activity [10,12,13].
Similarly, Mn–cerium oxide systems showed enhanced activity for CO oxidation at lower temperatures due to highly dispersed Mn2+/Mn3+ redox couples in the cerium oxide matrix [14]. Murugan et al. [15] used solution combustion synthesis to incorporate Mn into the CeO2 lattice and compared it to other methods of synthesis. They concluded that manganese was only present in the +2 and +3 oxidation states, and increasing the Mn content of the catalyst decreased the unit cell parameter. Furthermore, a uniform distribution of Mn, as well as the formation of a Ce1-xMnxO2-δ solid solution, was observed, and Mn incorporation was thought to prevent CeO2 from sintering by occupying the defect sites of CeO2.
As a result of these studies, several researchers considered materials incorporating Cu, Mn, and cerium oxide, which were found to be beneficial for the PrOx reaction. Elmhamdi et al. prepared a CuMn2O4 spinel as support for cerium oxide using a microemulsion method. They found that a relatively high content of cerium oxide was beneficial to generate a Cu–cerium oxide interface to enable the formation and preservation of Cu+, which favorably affected the CO-PrOx activity [11]. Li et al. found that forming a solid solution of Mn, Cu, and Ce by the hydrothermal method, amongst others, created oxygen vacancies, and the presence of more Cu+ and Mn4+ species enhanced the catalytic performance [16]. They maintained a constant Mn:Cu mole ratio of 1:5 and kept the Cu weight percent constant at 5%. Jin et al. reported that a Cu-Mn-Ce-O solid solution synthesized via a co-precipitation method yielded materials with a higher concentration of Cu+ species beneficial for CO oxidation, as opposed to fully reduced Cu, which is selective to H2 oxidation [17]. Xing et al. found that when they achieved partial incorporation of Cu and Mn into the cerium oxide matrix, a high catalytic activity for CO-PrOx was realized [18]. They used a simple impregnated method for low Cu loadings.
In line with the aforementioned work, we looked at the potential benefits of bimetallic substituted cerium oxide catalysts, incorporating Cu and Mn via a urea-assisted solution combustion method, paying special attention to particle size, metal dispersion, and metal–cerium oxide interaction. These new catalysts were tested for both dry and wet CO-PrOx.

2. Results and Discussion

2.1. Chemical Composition

The chemical composition of the catalysts, determined by inductively coupled plasma–optical emission spectrometry (ICP-OES), is reported in Table 1. There is a slight difference between the actual and nominal metal loading of the catalysts, probably due to the hygroscopic nature of the metal precursors. Furthermore, high temperatures reached during the solution combustion synthesis stage using an organic fuel, such as urea, could result in partial evaporation of the metal precursor, thus reducing the theoretical amounts [19,20]. However, the actual loading is in good agreement with the nominal values.

2.2. Structural and Morphological Properties

Figure 1A shows the powder XRD diffractograms of all the catalysts. These catalysts showed diffraction peaks that are indexed to a cerium oxide cubic fluorite structure, indicating that the fluorite structure was maintained, even after the substitution of the guest atoms [15,21,22]. The lattice parameter, a, decreased with the addition of copper and/or manganese (Table S1, Supplementary Information), with the exception of Ce0.95Cu0.05O2-δ. A slight peak shift towards higher Bragg angles was observed with the addition of Mn, whereas the addition of Cu shows no conclusive peak shifts. The shift to a higher Bragg angle is usually associated with lattice shrinkage [22].
No additional diffraction peaks corresponding to CuO, MnO, MnO2, and Mn2O3 were observed, indicating that the metals were incorporated with high probability into the CeO2 lattice [19,20,22,23,24], as also indicative of the change in the CeO2 lattice constant. This is supported by the decrease in peak intensity, lattice parameter, as well as crystallite size. The substitution of the smaller metal ions, Mn2+ and Cu2+, into the cerium oxide lattice increases the lattice strain. The contraction of the cell volume is induced by Mn doping into the cerium oxide lattice and a decrease in the lattice cell parameter (a) may be due to the formation of a Ce-metal-O solid solution [25]. The intensity of the peaks of the Mn-containing catalysts decreased, attributed to decreasing crystallite size (Table S1, Supplementary Information) or a possible coexistence of another amorphous manganese oxide phase [23].
The crystallite size of the catalysts was in the range of ~8 to 16 nm (see Table S1, Supplementary Information). CeO2-δ and Ce0.90Cu0.10O2-δ had the highest crystallite size of 15.6 and 14.1 nm, respectively, while Ce.80Cu0.10Mn0.10O2-δ had the smallest crystallite size of 7.9 nm. The crystallite size of the Mn-containing catalysts decreased with an increase in Mn loading. This could be due to Mn hindering the crystallization of the cerium oxide, thus the higher the Mn loading, the smaller the crystallite size [26]. The BET surface area data have been included in Table S1 in the Supplementary Information, together with the BET isotherms and pore size distribution data (Figures S1 and S2). The BET data show that the surface area for CeO2 and the 5at.% Mn catalysts were very similar. The 5% Cu-CeO2 catalyst had a lower surface area of about 37 m2g−1. However, the catalyst having a high loading of Cu and Mn (Cu0.10Mn0.10Ce0.80O2-δ) had the highest surface area of 51 m2g−1. The 10%Cu-CeO2 catalyst had the lowest surface area of 13 m2g−1, which deviated from the rest. It was surmised that pore blockage was a possible reason for the lower SA.
Raman spectroscopy was also used to study the structural properties of the catalysts and determine the presence of structural defects induced by the doping of Cu or Mn into the cerium oxide lattice. The oxygen vacancies are quantified using the Ov/F2g area ratio for the cubic fluorite CeO2 structure. The Raman spectra of CeO2-δ and the various cerium oxide-based catalysts are presented in Figure 1B. The spectral pattern of the catalysts shows evidence of a strong and sharp vibrational band around 466 cm−1, which is associated with the triply degenerate F2g Raman-active mode. The F2g band is commonly known to occur because of the symmetrical stretching vibration of the oxygen atoms surrounding the Ce4+ ions within the cubic CeO2 fluorite phase [9,27,28,29,30,31]. This F2g Raman-active band is a common characteristic band for all the cerium oxide catalysts, which indicates the presence of the Ce4+-O2−-Ce4+ arrangement. Furthermore, when doped with Cu or Mn ions, the F2g Raman-active band becomes shorter and broader and shifts to lower wavenumbers (Table S2, Supplementary Information). The F2g band position is sensitive to the incorporation of Cu or Mn into the cerium oxide lattice, and the evidence is shown by the red-shift for the various cerium oxide catalysts [22]. The progressive shift of the F2g band to lower energy is associated with decreasing crystallite/particle size, as well as the incorporation of cations into the cerium oxide matrix. On the lower energy side, the F2g band progressively becomes broader and asymmetric. This is due to inhomogeneous strain broadening, which is associated with particle size dispersion and phonon confinement [28]. There is a presence of weak overtone bands around 250 cm−1, 500 to 600 cm−1, and 1000 to 1300 cm−1 due to second-order transverse acoustic (2TA) modes, defect-induced (D) modes, and second-order longitudinal optical (2LO) modes, respectively. However, in contrast, Filtschew et al. [32] suggested that the Raman band observed around 250 cm−1 is associated with structural surface defects, rather than being related to the 2TA lattice vibrational modes, and further indicated that this band should be assigned to the hydroxyl group (Ce-OH) vibrations instead of the 2TA lattice vibration. The F2g Raman-active band shift is not only associated with crystallite size but also with defect sites in the cerium oxide lattice, e.g., oxygen vacancies, which occur due to a charge imbalance. The increase in the amount of defect sites induces a F2g band shift to a lower energy. This is consistent with the F2g band position and Ov/F2g area ratio (Table S2, Supplementary Information). Defect site-induced oxygen vacancies in the cerium oxide lattice are attributed to a broad band between 550 to 650 cm−1 (D band or defect site). The defective sites or weak D band for cerium oxide is observed around 600 cm−1, and broad and weaker D bands are observed for Cu- and Mn-doped cerium oxide at around 550 to 600 cm−1, associated with oxygen vacancies, denoted as Ov. This Ov band occurs due to disorder-induced components (Ce3+-Ov2−-Ce4+) or if the Ce ion is replaced by a dopant ion. The Ce0.85Cu0.10Mn.05O2-δ catalyst showed the highest Ov/F2g area ratio of 0.590, which was not significantly different from that of the Ce0.80Cu0.10Mn0.10O2-δ catalyst, indicating that these catalysts have the most oxygen vacancies or structural defects compared to the other catalysts. Comparing the Ov/F2g area ratio of the catalysts, it is clear that the pure cerium oxide and the Cu–cerium oxide catalysts have a smaller ratio compared to the Mn-doped catalysts. This suggests that the Mn-doped catalysts have the most defects, proposed to be due to the presence of Mn3+ ions in the cerium oxide matrix. The XPS data (discussed later) show evidence of the predominance of Mn3+ ions in the catalyst.
The morphology of the catalysts was studied using scanning electron microscopy (SEM), and the images are shown in Figure S3, Supplementary Information. From these images, all the catalysts are shown to have a spherical morphology of irregular-sized particles. The corresponding SEM-SAED images, EDX mapped images, and graphs for all the catalysts are shown in Figure S4, Supplementary Information. The SEM-EDX images and graphs confirm the presence of the metals, as well as a uniform metal dispersion/distribution. TEM images for the catalysts are shown in Figure 2. The images show that the catalysts have a cuboidal morphology but vary in size and particle density, with the particles appearing smaller upon the addition of Mn. High-resolution transmission electron microscopy (HR-TEM) was used for the determination of dominant lattice fringes, as well as to carry out electron diffraction analyses. Figure 3 shows the high-resolution bright-field images for the various catalysts and the corresponding electron diffraction patterns. The dominant lattice fringes for the catalysts were measured to be 0.32, 0.27, 0.20 nm, and 0.16 nm, corresponding to the most intense d(111), d(200), d(220), and d(311) lattice planes of CeO2-δ, respectively [9,33]. Lattice fringes corresponding to Miller planes for CuO, Cu0, and MnO2 were not observed, supporting the view of the incorporation of Cu and Mn into the cerium oxide lattice. It was found that the crystallite size, determined by TEM analysis (Table S1, Supplementary Information), matched the size and trend obtained by XRD analysis.

2.3. Electronic and Catalyst Reducibility Studies

X-ray photoelectron spectroscopy (XPS) was used to quantitatively estimate and investigate the nature of the surface and the effect of doping on the cerium oxide catalysts. Table S3, Supplementary Information, shows the metal surface atomic concentration. The surface Mn and Cu concentrations of the catalysts were slightly lower compared to the theoretical values, suggesting that more of the metals were incorporated into the CeO2 lattice [34]. The incorporation of these metals into the CeO2 lattice allows for the formation of solid solutions, while still maintaining the fluorite structure. Figure 4 shows the deconvoluted XPS spectra of Cu (2p3/2) for the catalysts. The spectra of Cu (2p3/2) show two characteristic peaks with binding energies of 930.5 eV and 932.2–933.1 eV. These peaks are associated with the presence of Cu1+ and Cu2+ species, respectively. The slightly lower-than-usual binding energy for Cu1+ is possibly due to Cu-Ce interactions. A satellite shake-up peak at approximately 940.5 eV is also present and is also due to the presence of Cu2+. Two Mn (2p) peaks for the Ce0.90Cu0.05Mn0.05O2-δ, Ce0.85Cu0.10Mn0.05O2-δ, and Ce0.80 Cu0.10Mn0.10O2-δ catalysts can be observed in their de-convoluted XPS spectra (Figure 5). The three oxidation states for manganese that have been reported are Mn2+, Mn3+, and Mn4+, which have binding energies of 640.4 to 640.9, 641.5 to 642.0, and 642.5 eV, respectively [20,34,35]. The Mn 2p3/2 binding energy peaks for Ce0.90Cu0.05Mn0.05O2-δ, Ce0.85Cu0.10Mn0.05O2-δ, and Ce0.80Cu0.10Mn0.10O2-δ are located at 641.3, 641.4, and 641.5 eV, respectively. These binding energies correspond to MnO, Mn3O4, Mn2O3, and Mn(OH)O, confirming the presence of both Mn2+ and Mn3+ ions, where Mn3+ could be the dominant species. A binding energy of 642.2 eV, which is associated with the existence of Mn4+, was not observed [34]. The broadening of the Mn (2p3/2) binding energy peaks further supports the possibility of the co-existence of both Mn2+ and Mn3+ ions since there is no evidence for the presence of Mn4+ species on the surface of the catalysts [23].
The Ce (3d) spectra of the catalysts after de-convolution are shown in Figure 6. The Ce3+ concentration is a measure of the defect sites/oxygen vacancies present in cerium oxide materials [36,37,38,39]. Table 2 gives a summary of the atom percent of Ce3+ in the catalysts. With the minor exception of Ce0.80Cu0.10Mn0.10O2-δ and Ce0.85Cu0.10Mn0.05O2-δ, where the sequence is inverted, the results correlate well with the Raman data, and the Mn containing materials clearly show more defect sites, likely due due to the presence of Mn3+ ions in the cerium oxide matrix.
Temperature-programmed hydrogen reduction (H2-TPR) was used to investigate the reducibility of the catalysts (Figure S5, Supplementary Information). Ceria itself showed no peaks in the temperature range studied. Ce0.95Cu0.05O2-δ and Ce0.90Cu0.10O2-δ showed two overlapping peaks between 155 °C and 185 °C, which are ascribed to the reduction of Cu2+ to Cu+ at the lower temperature and Cu+ to Cu0 at a higher temperature. The peak for the catalyst with the higher loading of copper and Mn shifted towards a lower temperature, suggesting a synergistic relationship between Mn and Cu that aids in the reduction of copper. According to reports, MnOx would show two strong overlapping reduction peaks around ~400 °C and ~500 °C, associated with a two-step reduction of MnO2/Mn2O3 to Mn3O4 and the reduction of Mn3O4 to MnO. It is clear that the incorporation of appropriate amounts of Cu and/or Mn ions in cerium oxide decreases the onset reduction temperature for the catalysts. It is determined that the overall reducibility order of the catalysts, based on the onset temperature, is Ce0.85Cu0.10Mn0.05O2-δ < Ce0.80Cu0.10Mn0.10O2-δ < Ce0.90Cu0.05Mn0.05O2-δ < Ce0.90Cu0.10O2-δ < Ce0.95Cu0.05O2-δ.

2.4. Catalytic Studies

Figure 7 shows CO conversion and selectivity over the copper-containing catalysts. Blank reactions using the synthesized cerium oxide as the catalyst showed little to no CO conversion below 150 °C; however, at temperatures above 180 °C, some CO conversion was observed. For the copper–cerium oxide catalysts, the conversion of CO increases significantly from 80 °C to about 200 °C, after which a slight decrease in conversion is observed for the Ce0.95Cu0.05O2-δ catalyst, probably due to catalyst deactivation.
In general, total conversion is obtained at 150 °C; however, the CO2 selectivity decreases from 100 °C. The decrease in selectivity to CO2 is due to the onset of hydrogen oxidation to form water from about 120 °C, and it becomes more pronounced as the temperature increases. This is supported by an increase in O2 conversion relative to CO, where an increase in O2 conversion above stoichiometric amounts to form CO2 is observed, suggesting oxidation of both CO and H2 to form CO2 and H2O, respectively. At 150 °C, Ce0.95Cu0.05O2-δ, Ce0.90Cu0.10O2-δ, Ce0.90Cu0.05Mn0.05O2-δ, and Ce0.85Cu0.10Mn0.05O2-δ showed a 100% conversion of CO, with selectivities towards CO2 of 64%, 73%, 81%, and 70%, respectively.
It is evident that incorporating manganese in the copper–cerium oxide catalysts improved the selectivity towards CO2, simultaneously preventing over-oxidation of hydrogen. The significant improvement in the oxidation of CO can be attributed to the improved reducibility of the copper catalysts upon the addition of manganese ions, suggesting that there is a synergistic interaction between manganese, copper, and cerium ions. However, in the temperature range from 80 to 180 °C, the catalyst containing the highest amount of Mn (Ce0.80Cu0.10Mn0.10O2-δ) showed the lowest conversion of CO. So, although Mn was beneficial to the activity, it is observed that this benefit is lost if too much Mn is present either in the bulk or on the surface of the cerium oxide support. The possible reason for this is the formation of an inactive phase of Mn that formed when the Mn concentration was increased. However, this was not evident from the XRD data likely due to the low concentration or high dispersion of this phase. The presence of any surface Mn phases would also suggest a lower Mn content in the solid solution incorporating Mn, Cu, and Ce.
In view of the importance of CO-PrOx, we looked at comparative activity data to other Cu-Mn-CeO2 catalytic systems reported in the literature. This is shown in Table S4 in the Supplementary Material section. Taking into consideration the various parameters, as shown in the table, as well as the preparation methods of the catalysts, it is clear that the Ce0.90Cu0.05Mn0.05O2-δ catalyst compared favorably with the published data. Indeed, the catalyst in this work showed the highest selectivity to CO2 at complete CO conversion, albeit at a higher temperature.
In addition to CO, the product stream after steam reforming also contains a substantial amount of CO2 and water. It is reported that the presence of both CO2 and H2O in the feed gas steam results in a decrease in the performance of CuO/CeO2 catalysts [12]. The Ce0.90Cu0.05Mn0.05O2-δ catalyst was chosen for an initial time-on-stream study. The catalyst maintained almost complete CO conversion, as well as a high CO2 selectivity for the duration of the experiment (Figure S6A, Supplementary Information).
The CO-PrOx reaction was then investigated by adding varying amounts of CO2, using Ce0.90Cu0.05Mn0.05O2-δ to assess its performance in the presence of CO2 (Figure 8). From the data presented, it is observed that the CO and O2 conversion decreases when CO2 is added to the feed. However, CO2 selectivity increases slightly with each increase in CO2. Furthermore, increasing the CO2 concentration from 5% to 15%, had a minimal effect on CO conversion, as well as CO2 selectivity. Importantly, at temperatures higher than 150 °C, the catalyst is capable of totally converting CO (see Figure S6B, Supplementary Information) but at the expense of CO2 selectivity.
To assess the influence of water, 7 vol.% water was added to the feed stream, also containing 15 vol.% CO2 (Figure 9). The conversion was about 60% for the first hour and remained at this value for the first 26 h of testing. The selectivity towards CO2 was approximately 70%, with the O2 conversion below 50%. Once the addition of H2O was stopped, the CO conversion immediately increased to almost 100% and remained at this conversion for the next 6 h. The O2 conversion also increased, but the selectivity towards CO2 decreased to about 55%. After this time, water was re-introduced. The CO conversion decreased quite rapidly to about 40%. This trend was maintained throughout the study (64 h). The sudden decrease in CO conversion upon the addition of water indicates that the adsorbed H2O molecules seem to induce a blocking effect on the Cu active sites [16] and promote the formation, as well as accumulation, of hydroxyls or carbonate species that could cause the temporary deactivation of the catalysts. XRD analysis was performed on the used catalysts, and no additional phases or species were detected.

3. Materials and Methods

For the preparation of Ce0.90Cu0.05Mn0.05O2-δ, 9.8680 g of ceric ammonium nitrate (H8CeN8O18), 0.2416 g of Cu(NO3)2.6H2O, 0.2510 g of Mn(NO3)2.6H2O (Sigma-Aldrich, Johannesburg, South Africa), and 4.5285 g of urea (Sigma-Aldrich, Johannesburg, South Africa) were weighed into a borosilicate dish containing 50 mL deionized water. The mixture was continually stirred at room temperature for 20 to 30 min for complete dissolution. The borosilicate dish was then placed in a programmable muffle furnace at room temperature under static air. The muffle furnace was programmed to sequentially ramp the temperature from room temperature to a temperature of 80 °C, held for 2 h, then ramped to 450 °C at approximately 3 °C min−1. The temperature was maintained at 450 °C for 3 h to facilitate combustion. After combustion, the furnace was cooled to room temperature. The solid formed was ground to a fine powder using a mortar and pestle. Catalysts denoted by Ce0.95Cu0.05O2-δ, Ce0.90Cu0.10O2-δ, Ce0.90Cu0.05Mn0.05O2-δ, Ce0.85Cu0.10Mn0.05O2-δ, and Ce0.80Cu0.10Mn0.10O2-δ were prepared by the method described above.
The catalysts were characterized by various physical, chemical, and electronic techniques. An Optima 5300 DV PerkinElmer Optical Emission Spectrometer (PerkinElmer, Shelton, CT, USA) was used to determine the content of the active metals of the catalysts. A detailed account of the different procedures is described in the Supplementary Information. Powder X-ray diffraction (P-XRD) diffractograms were obtained using a Bruker D8 Advance diffractometer equipped with a Cu Kα (λ = 0.1540 nm) radiation source, XRK900 in situ cell, and a Bruker VANTEC detector (Bruker, Karlsruhe, Germany). Diffracplus XRD commander software was used to interpret the data. XRD patterns with a 2θ range of 20 to 90° were obtained at a scan rate of 0.5° min−1 and a step width of 0.02°. The Scherrer equation was used to estimate the average crystallite size of the catalysts, whereas Raman analysis was performed on a Renishaw InVia Raman Microscope coupled with a class 3B laser beam and charge-coupled device (CCD) detector. The spectra were acquired using an excitation wavelength of 514 nm, and the sample was scanned between 100 to 3000 cm−1 wavenumbers (Renishaw, New Mills, UK). A single scan measurement was generated by the WiRE2 spectral acquisition. The laser strength/power was set at 10 mW, and the sample was exposed for 30 s at ambient temperature. Temperature-programmed studies were conducted using a Micromeritics AutoChem II 2920 chemisorption analyzer instrument equipped with a thermal conductivity detector (Micromeritics, Norcross, GA, USA). A Zeiss FEG-SEM Ultra Plus instrument (Zeiss, Jena, Germany) and Jeol JEM-1010 Electron Microscope (JEOL, Tokyo, Japan ) were used for scanning and transmission electron microscopy studies, respectively. X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos Axis Ultra DLD spectrometer, with an achromatic Al Kα source operated at 120 W (Kratos, Manchester, UK). The base pressure in the analysis chamber was maintained at about 1 × 10−9 mbar. Pass energies of 160 eV and 40 eV were used for the acquisition of survey and detailed regional scans, respectively. Binding energies were calibrated against the C (1s) signal from adventitious carbon contamination, which was assumed to have a binding energy of 284.7 eV.
Catalytic testing was conducted in a continuous flow fixed bed reactor. Two 500 mm reactor tubes enclosed in four-zone heating blocks were used at atmospheric pressure. The temperature was recorded using a thermocouple inserted in a tube coaxial with the reactor tube. Prior to any temperature measurements, the isothermal zone was determined, which was the area with consistent temperature, and the location of the catalyst bed. The feed and products were analyzed using an online Agilent gas chromatograph (GC) equipped with a thermal conductivity detector (TCD). The feed gas comprised 1 vol.% CO, 1 vol.% O2, and the balance, N2 for total oxidation. For CO-PrOx, the feed composition was 1 vol.% CO, 1 vol.% O2, 50 vol.% H2, and balance, N2. A total of 1 mL of catalyst with a particle size of 600 to 1000 µm was loaded by first mixing with 2 mL of 24 grit carborundum. The gas hourly space velocity varied from 48,000 to 60,000 h−1. The temperature range for catalytic testing was between 80 and 220 °C. Reactions were also carried out using a feed stream that contained CO2 and water, the details of which are reported elsewhere [40].

4. Conclusions

From structural characterization, it was established that the catalysts exist in a single phase, with a cubic fluorite structure or cerianite crystal phase. ICP-OES results showed that the elemental composition was close to the theoretical values. The EDX elemental line scan confirmed the presence of Cu, Mn, and Ce in close proximity to each other, indicating good metal–metal interaction. From XRD and Raman analyses, it was concluded that both Mn and Cu were incorporated to a high degree into the cerium oxide matrix, generating oxygen vacancies. The Cu0.10Mn0.05Ce0.90O2-δ catalyst had the highest concentration of oxygen vacancies or structural defect sites compared to other cerium oxide catalysts. The relative abundance of Ce3+ in the catalysts correlates with oxygen vacancies and follows the decreasing trend: Ce0.85Cu0.10Mn0.05O2-δ > Ce0.90 Cu0.05Mn0.05O2-δ > Ce0.95Cu0.05O2-δ > Ce0.90Cu0.10O2-δ > Ce0.80Cu0.10Mn0.10O2-δ.
In the preferential oxidation of the CO (CO-PROX) reaction, conversion increased with an increase in temperature; however, the selectivity towards CO2 decreased. At 150 °C, Ce0.90Cu0.05Mn0.05O2-δ gave total CO conversion and a high selectivity of 81% towards CO2. Time-on-stream experiments were conducted with this catalyst. The catalyst maintained total conversion over 24 h, after which the addition of CO2 (5 to 15 vol.%) resulted in a decrease in conversion. The conversion was further decreased when water was added; however, once the addition of water was stopped, the catalyst was able to regenerate and gave total conversion of CO, even in the presence of CO2.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14090603/s1, Figure S1: Combined N2 adsorption-desorption isotherms of the catalysts; Figure S2: Pore size distribution for the selected cerium oxide-based catalysts; Figure S3: SEM images of the catalysts; Figure S4: (a) SEM micrographs, as well as corresponding (b) SEM-SAED images, EDX mapped images, and (c) EDX graphs for all the cerium oxide catalysts; Figure S5: H2-TPR profiles for all the catalysts; Figure S6: CO conversion and CO2 selectivity, as a function of time (A) Feed composition: 1 vol. % CO, 1 vol. % O2, 50 vol. % H2 and balance, N2; GHSV = 48,000 h−1; Temperature = 150 °C) and as a function of temperature; (B) Feed composition: 1 vol. % CO, 1 vol.% O2, 50 vol.% H2, 15 vol. % CO2 and balance, N2; GHSV = 48,000 h−1) for CO-PrOx over Ce0.90Cu0.05Mn0.05O2-δ; Table S1: Lattice parameter, a, and crystallite size of the catalysts; Table S2: F2g peak position, FWHM, DRaman, and Ov/F2g area ratio results for the cerium oxide catalysts; Table S3: Metal surface atomic concentration (%) for the cerium oxide catalysts; Table S4: Comparison of activity data reported in this article with literature; References [11,16,18,25,41] were cited in Supplementary Materials.

Author Contributions

Conceptualization, S.M.; Methodology, S.M.; Validation, S.M., A.S.M. and S.S.; Formal Analysis, S.M.; Investigation, S.M.; Resources, H.B.F.; Data Curation, S.M.; Writing–Original Draft Preparation, S.M. and S.S.; Writing–Review and Editing, S.S., A.S.M. and H.B.F.; Visualization, S.M., A.S.M., S.S. and H.B.F.; Supervision, S.S., A.S.M. and H.B.F.; Project Administration, S.S., A.S.M. and H.B.F.; Funding Acquisition, H.B.F.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by NRF (grant number 118527) and HySA (grant number H977).

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

We thank A Folkard (UKZN) for assistance with the replotting of some figures and V. D. B. C. Dasireddy for recording the XPS spectra.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Catalysts 14 00603 g001
Figure 1. (A) Powder X-ray diffractograms and (B) Raman spectra of the catalysts.
Figure 1. (A) Powder X-ray diffractograms and (B) Raman spectra of the catalysts.
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Figure 2. TEM images of the catalysts.
Figure 2. TEM images of the catalysts.
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Figure 3. HR-TEM images with lattice fringes and corresponding electron diffraction patterns of the catalysts.
Figure 3. HR-TEM images with lattice fringes and corresponding electron diffraction patterns of the catalysts.
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Figure 4. De-convoluted Cu (2p) core-level spectra of the catalysts.
Figure 4. De-convoluted Cu (2p) core-level spectra of the catalysts.
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Figure 5. De-convoluted Mn (2p) core-level spectra of the catalysts.
Figure 5. De-convoluted Mn (2p) core-level spectra of the catalysts.
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Figure 6. De-convoluted Ce (3d) core-level spectra of the catalysts.
Figure 6. De-convoluted Ce (3d) core-level spectra of the catalysts.
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Figure 7. CO conversion (A), selectivity towards CO2 (B), and O2 conversion (C) as a function of temperature for CO-PrOx over Ce0.95Cu0.05O2-δ (▲), Ce0.90Cu0.10O2-δ (■), Ce0.90Cu0.05Mn0.05O2-δ (●), Ce0.85Cu0.10Mn0.05O2-δ (✕), and Ce0.80Cu0.10Mn0.10O2-δ (◆) (Feed composition: 1 vol. % CO, 1 vol. % O2, 50 vol. % H2 and balance, N2; GHSV = 60,000 h−1).
Figure 7. CO conversion (A), selectivity towards CO2 (B), and O2 conversion (C) as a function of temperature for CO-PrOx over Ce0.95Cu0.05O2-δ (▲), Ce0.90Cu0.10O2-δ (■), Ce0.90Cu0.05Mn0.05O2-δ (●), Ce0.85Cu0.10Mn0.05O2-δ (✕), and Ce0.80Cu0.10Mn0.10O2-δ (◆) (Feed composition: 1 vol. % CO, 1 vol. % O2, 50 vol. % H2 and balance, N2; GHSV = 60,000 h−1).
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Figure 8. CO conversion (▲), selectivity towards CO2 (▲), and O2 conversion (▲) as a function of time for CO-PrOx over the Ce0.90Cu0.05Mn0.05O2-δ catalyst (Feed composition: 1 vol. % CO, 1 vol. % O2, 50 vol.% H2, 0–15 vol.% CO2 and balance, N2; GHSV = 60,000 h−1, Temperature = 150 °C).
Figure 8. CO conversion (▲), selectivity towards CO2 (▲), and O2 conversion (▲) as a function of time for CO-PrOx over the Ce0.90Cu0.05Mn0.05O2-δ catalyst (Feed composition: 1 vol. % CO, 1 vol. % O2, 50 vol.% H2, 0–15 vol.% CO2 and balance, N2; GHSV = 60,000 h−1, Temperature = 150 °C).
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Figure 9. CO conversion (◆), selectivity towards CO2 (◆), and O2 conversion (◆) as a function of time for CO-PrOx over the Ce0.90Cu0.05Mn0.05O2-δ catalyst (Feed composition: 1 vol.% CO, 1 vol.% O2, 50 vol.% H2, 15 vol.% CO2, 7 vol.% H2O and balance, N2; GHSV = 60,000 h−1, Temperature = 150 °C).
Figure 9. CO conversion (◆), selectivity towards CO2 (◆), and O2 conversion (◆) as a function of time for CO-PrOx over the Ce0.90Cu0.05Mn0.05O2-δ catalyst (Feed composition: 1 vol.% CO, 1 vol.% O2, 50 vol.% H2, 15 vol.% CO2, 7 vol.% H2O and balance, N2; GHSV = 60,000 h−1, Temperature = 150 °C).
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Table 1. The metal content of the catalysts.
Table 1. The metal content of the catalysts.
CatalystCe Cu Mn
(atomic %)
CeO2-δ100--
Ce0.95Cu0.05O2-δ94.45.6-
Ce0.90Cu0.10O2-δ89.710.3-
Ce0.90Cu0.05Mn0.05O2-δ89.05.65.4
Ce0.85Cu0.10Mn0.05O2-δ84.110.95.0
Ce0.80Cu0.10Mn0.10O2-δ78.611.110.3
Table 2. Relative amounts of oxygen and Ce3+ species estimated from de-convoluted XPS spectra.
Table 2. Relative amounts of oxygen and Ce3+ species estimated from de-convoluted XPS spectra.
CatalystsCe3+ (at.%)
Ce0.95Cu0.05O2-δ31.3
Ce0.90Cu0.10O2-δ29.4
Ce0.90Cu0.05Mn0.05O2-δ33.6
Ce0.85Cu0.10Mn0.05O2-δ37.9
Ce0.80Cu0.10Mn0.10O2-δ43.1
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Motha, S.; Mahomed, A.S.; Singh, S.; Friedrich, H.B. Highly Active Cerium Oxide Supported Solution Combustion Cu/Mn Catalysts for CO-PrOx in a Hydrogen-Rich Stream. Catalysts 2024, 14, 603. https://doi.org/10.3390/catal14090603

AMA Style

Motha S, Mahomed AS, Singh S, Friedrich HB. Highly Active Cerium Oxide Supported Solution Combustion Cu/Mn Catalysts for CO-PrOx in a Hydrogen-Rich Stream. Catalysts. 2024; 14(9):603. https://doi.org/10.3390/catal14090603

Chicago/Turabian Style

Motha, Sbusiso, Abdul S. Mahomed, Sooboo Singh, and Holger B. Friedrich. 2024. "Highly Active Cerium Oxide Supported Solution Combustion Cu/Mn Catalysts for CO-PrOx in a Hydrogen-Rich Stream" Catalysts 14, no. 9: 603. https://doi.org/10.3390/catal14090603

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