Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 & Hydrothermal Synthesis Synergistic Effect of Simultaneous Doping of Ceria Nanorods with Cu and Cr on CO Oxidation and NO Reduction Shawn C. Rood,[a] Oriol Pastor-Algaba,[b] Albert Tosca-Princep,[b] Bruno Pinho,[c] Mark Isaacs,[d] Laura Torrente-Murciano,*[c] and Salvador Eslava*[a, e] Abstract: Ceria particles play a key role in catalytic applica- H2-TPR and XPS analysis show that the simultaneous Cu and tions such as automotive three-way catalytic systems in Cr co-doping results in a catalyst with a higher surface Cu which toxic CO and NO are oxidized and reduced to safe concentration and a much-enhanced surface reducibility, in CO2 and N2, respectively. In this work, we explore the incor- comparison with either undoped or singly doped (Cu or Cr) poration of Cu and Cr metals as dopants in the crystal struc- ceria nanorods. While single Cu doping enhances catalytic ture of ceria nanorods prepared by a single-step hydrother- CO oxidation and Cr doping improves catalytic NO reduc- mal synthesis. XRD, Raman and XPS confirm the incorpora- tion, co-doping with both Cu and Cr enhances the benefits tion of Cu and Cr in the ceria crystal lattices, offering ceria of both dopants in a synergistic manner employing roughly nanorods with a higher concentration of oxygen vacancies. a quarter of dopant weight. XPS also confirms the presence of Cr and Cu surface species. Introduction the creation of additional oxygen vacancies within the lattice structure to compensate for the charge difference, improving Ceria is widely used in catalytic applications across a broad redox capabilities.[12] Additionally, the presence of elements range of fields, such as automotive three-way catalysis, due to with an ionic radius very different from that of cerium can its oxygen storage capability.[1–8] Doping ceria by replacing create lattice defects and distortions, another contributor to cerium atoms with other readily available metals has the po- oxygen vacancy concentration.[9] Co-doping ceria with multiple tential to improve their low-temperature performance, other- elements has been recently investigated for various applica- wise typically achieved by the addition of expensive Pt-group tions. With regard to CO oxidation, DFT experiments show metals.[9–11] In particular, the replacement of cerium atoms with promising results for Mn/Fe-doped ceria, and experimental re- other elements, particularly those of lower valence, can lead to sults have been reported for Mn/Cu and Ag/Cu-doped ceria.[13–16] A co-doped Mn/Co-ceria catalyst has also been re- [a] Dr. S. C. Rood, Dr. S. Eslava ported with enhanced performance for the reduction of NO by Centre for Sustainable Chemical Technologies Department of Chemical Engineering, University of Bath CO.[17] On the other hand, not all dopant combinations provide Claverton Down, Bath, BA2 7AY (UK) better catalytic properties compared with similarly prepared [b] O. Pastor-Algaba, A. Tosca-Princep singly doped ceria materials, demonstrating the need to care- Departament d’Enginyeria Qu&mica, Biolkgica i Ambiental fully consider dopant properties and their effect on catalytic Universitat Autknoma de Barcelona, Bellaterra, 08193 (Spain) activity.[18] [c] Dr. B. Pinho, Dr. L. Torrente-Murciano While the suitability of doped ceria for CO oxidation has Department of Chemical Engineering and Biotechnology University of Cambridge, Philippa Fawcett Drive, Cambridge, CB3 0AS (UK) been widely examined, investigation into NOx reduction has E-mail: lt416@cam.ac.uk been less exhaustive.[19–27] While doping of ceria is generally a [d] Dr. M. Isaacs good strategy for catalytic oxidation reactions because it im- Department of Chemistry, University College London proves reducibility and oxygen vacancy formation, this can London, WC1H 0AJ (UK) lead to a trade-off of reducing the favorability of oxygen va- [e] Dr. S. Eslava cancy healing, necessary to complete the catalytic cycle for Department of Chemical Engineering, Imperial College London London, SW7 2AZ (UK) NOx reduction.[28, 29] For this reason, both reactions must be ex- E-mail: s.eslava@imperial.ac.uk amined simultaneously. Supporting information and the ORCID identification number(s) for the In this work, a template-free hydrothermal method is used author(s) of this article can be found under: to synthesize Cu and Cr-doped ceria nanorods with improved https://doi.org/10.1002/chem.202004623. activities as low-temperature automotive catalysts. Ceria nano- T 2020 The Authors. Chemistry - A European Journal published by Wiley- rods are chosen for their preferential exposure of the (110) VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and repro- ceria facet that contains more surface oxygen vacancies and duction in any medium, provided the original work is properly cited. offers higher catalytic activity than other crystal facets.[30–32] Chem. Eur. J. 2021, 27, 2165 – 2174 2165 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 The doped ceria nanorods are tested as catalysts for both CO oxidation and NO reduction. Due to improved reducibility, it is shown that doping ceria with Cu improves CO oxidation con- versions, but is not so helpful for the reduction of NO. Con- versely, doping ceria with Cr does not improve the CO oxida- tion conversions but does result in a catalyst with better low- temperature performance for NO reduction. Co-doping ceria with both Cu and Cr provides a catalyst with enhanced per- formance for both CO oxidation and NO reduction. Co-doping synergistic effects moreover significantly decrease the amount of doping needed for an enhanced performance. The use of doped ceria nanorods offers an attractive route to producing automotive catalysts with better low-temperature per- formance. Results and Discussion Undoped and doped ceria nanorods were hydrothermally syn- thesized, filtered and washed, and dried under vacuum. The undoped ceria synthesis produced a pale-yellow powder prod- uct, while Cu and/or Cr-doped ceria produced varying shades of brown. The co-doped Cu/Cr-ceria sample was synthesized Figure 2. Powder XRD patterns of undoped, Cu-doped, and Cr-doped ceria nanorods. Ceria diffraction peaks are labelled with their Miller indices; ~ rep- with a 1 % loading of each dopant for ease of comparison with resents copper oxide diffraction peaks. singly doped ceria samples with equivalent or higher levels of Cu or Cr. Transmission electron microscopy (TEM) representa- tive micrographs of undoped ceria, 7 % Cu-ceria, 5 % Cr-ceria, (JCPDS 34-0394). For copper-doped ceria between 1 and 7 % and 1 % Cu/1 % Cr-ceria are shown in Figure 1. In all cases, Cu and chromium-doped ceria at all dopant loadings synthe- there is a range of nanorod sizes: with nanorod widths gener- sized (1 to 9 % Cr), only ceria diffraction peaks are seen. While ally below 50 nm and nanorod lengths ranging from 100 to this is expected for the 1 % Cu-ceria due to a detection limit of several hundred nm. In the Cu-doped ceria sample, a minority approximately 1 %, for the samples with higher copper levels, of small nanoparticles and nanocubes can be seen in addition this is an indication of dopant substitution into the ceria lattice to nanorods. In all cases, there is no evidence of formation of and formation of a homogeneous fluorite structure.[33] For the Cu and/or Cr particles outside the rod structure at these levels 1 % Cu/1 % Cr-ceria, only ceria can be identified in the diffrac- of doping below 7 %. tion pattern as well. Anyway, the exclusive diffraction of ceria Powder X-ray diffraction (XRD) patterns of doped and un- does not eliminate the possibility of highly dispersed copper doped ceria nanorods are shown in Figure 2. The main diffrac- oxide or chromium oxide species on the surface of the ceria. tion peaks of fluorite-type ceria are observed in all the samples For 9 % Cu-ceria, two additional diffraction peaks are seen at 36.9 and 43.58 2q—likely Cu2O.[34] Therefore, it is likely that at loadings below 7 % Cu, copper atoms are introduced within the ceria lattice, substituting cerium with copper ions, but above 7 %, additional copper atoms are unable to be substitut- ed into the lattice and agglomerate into copper oxide parti- cles. A small shift to higher 2q values can be seen in the diffrac- tion patterns with both dopants, shown in Figure 3. For Cu- ceria and Cr-ceria, a higher level of dopant corresponds to a larger peak shift, indicative of increasing levels of incorporation into the crystal lattice.[33] In the case of copper, this shift only continues up to 7 % Cu; at higher copper dopant levels, there is no further shift which further support the formation of exter- nal copper oxide particles at higher loading levels. The Ce4 + ionic radius is 0.97 a and Ce3 + is 1.14 a, compared with for ex- ample, 0.73 a for Cu2 + and 0.44 a for Cr6 + .[35] Therefore, it is expected that increasing levels of dopant will decrease the unit cell size if the dopant is successfully integrated into the Figure 1. Representative TEM images of (a) undoped ceria nanorods, (b) 7 % Cu-ceria nanorods, (c) 5 % Cr-ceria nanorods, and (d) 1 % Cu/1 % Cr-ceria lattice structure. Correspondingly, unit cell values calculated nanorods. using Bragg’s Law for the doped ceria materials show a de- Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2166 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 Figure 5. (a) Raman spectra of undoped and doped with 1 % Cu, 1 % Cr, and 1 % Cu/1 % Cr ceria nanorods, and (b) F2g Raman band for the same ceria samples. Figure 3. (111) Diffraction peak shift for (a) Cu-doped and (b) Cr-doped ceria nanorods prepared with different dopant concentrations. ceria samples it is shifted. There are two main contributions to this shift. First, the substitution of Ce ions with the dopant ions changes the lattice parameters and the oxygen–metal crease in the size of ceria’s unit cell with increasing levels of bond length, affecting lattice vibration. Second, differences in dopant (Figure 4 a). These results indicate that the Cu and Cr oxygen vacancy concentration affect the vibrational mode.[38, 39] dopants substitute Ce ions in the crystal structure rather than Additionally, a Raman peak broadening is visible in the doped occupy interstitial or surface sites. Anyway, as explained ceria materials, which is associated with changes in crystallite before, the substitution of Ce ions in the crystal structure does size.[40] This correlates well with the coherent diffraction not eliminate the possibility of highly dispersed copper oxide domain sizes calculated from the XRD spectra in Figure 2, or chromium oxide species on the surface of the ceria. which showed a domain size decrease upon doping. An addi- tional band at approximately 600 cm@1 is visible in the doped ceria samples, more prominently than in the case of undoped ceria. This band is generally attributed either to the quantity of oxygen vacancies in the sample or point defects related to the presence of Ce3 + ions.[37] Therefore, it can be concluded that the use of copper or chromium dopants increases the quantity of oxygen vacancies in the ceria nanorod samples. In addition to the characteristic ceria Raman bands, chromi- um-doped ceria samples had a prominent band at 860 cm@1 and a weak one at 1010 cm@1. Both of these are indicative of Figure 4. (a) Unit cell parameter ’a’ vs. dopant wt % and (b) coherent diffrac- the presence of CrVI oxide species interacting with the CeO2 tion domain sizes (crystallite size) vs. dopant wt % for Cu and Cr-doped surface, instead of being incorporated into ceria’s lattice struc- ceria. ture.[41] This contrasts with the powder XRD spectrum of Cr- ceria (Figure 2), in which only ceria diffraction peaks are visible; In addition to unit cell dimensions, coherent diffraction however, very small (< 2 nm) Cr particles may be present that domain (crystallite) size calculations using the Scherrer equa- cannot diffract X-rays. However, EDS analysis (Figures S1–S4) tion show that increasing the amount of dopant decreases the shows that Cr is still highly dispersed across the catalyst. domain size, shown in Figure 4 b. This is potentially related to To understand how Cu and Cr dopants affect the reducibility the preparation method used for the nanorod synthesis. When of the ceria nanorod catalysts, temperature-programmed re- the cerium precursor is added to the NaOH solution, Ce(OH)3 duction (TPR) experiments were performed on the undoped nuclei are formed. During hydrothermal synthesis, these nuclei and doped materials (Figure 6). For nanosized ceria materials, dissolve and then recrystallize into nanorods, growing aniso- two areas of reduction are typically seen: a low-temperature tropically.[36] High levels of additional ions in the solution can area (200 to 500 8C) representing consumption of hydrogen interfere with this process, which would explain the nanoparti- due to readily available surface oxygen, and a higher-tempera- cles and small nanocubes seen in Figure 1 b in addition to ture area (> 600 8C) representing reduction of the bulk lattice ceria nanorods. However, Cr-doped ceria does not follow this oxygen.[36, 42] In most cases, the deconvolution of the lower trend of decreasing crystallite size past a dopant level of 5 %. temperature reduction area (between 200 and 500 8C) can be Representative Raman spectra of undoped and doped ceria represented with two Gaussian peaks (percentage peak fitting nanorod samples are shown in Figure 5. Ceria has a character- error inferior to 3 %). For Cu-ceria nanorods, the position of istic Raman band at approximately 465 cm@1 (F2g mode), which these two low-temperature peaks shifts to the left, from 359 is attributed to the symmetric vibrational breathing of oxygen and 467 8C corresponding to undoped ceria nanorods to 275 surrounding the cerium ions in the fluorite-type crystal lat- and 338 8C for the 5 % Cu-ceria, indicating an increase in sur- tice.[37] This band is prominent in all spectra, but for the doped face reducibility at lower temperatures with increasing levels Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2167 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 relative to the surface, the bulk structure becomes more re- ducible. However, for chromium-doped ceria, the opposite trend is seen: the bulk becomes less reducible relative to the surface, with a ratio of 0.70 to 0.86. This is potentially due to improvement of the surface reducibility and not the bulk or an overall decrease in bulk reducibility. Two additional sharp and intense reduction peaks appear in the 7 % Cu-ceria nanorods, centered at 197 and 222 8C. These peaks can be attributed to the reduction of copper particles, segregated from the ceria phase due to the high dopant con- tent.[46–48] Because separate copper oxide diffraction peaks were not seen until dopant levels reached 9 % (Figure 2), these particles observed at 7 % Cu must be too small and dispersed to significantly diffract X-rays. The TPR reduction profile of the simultaneous Cu- and Cr- doped ceria nanorods differs from the singly doped materials. Although there are still distinct low-temperature and high-tem- perature areas of reduction, the low-temperature reduction area is wider and extends further to lower temperature values in the case of the Cu@Cr doped ceria. Unlike the singly doped nanorod samples, to keep the fitting error inferior to 3 %, the Figure 6. TPR analysis of undoped, Cu, Cr, and Cu/Cr-doped ceria nanorods. deconvolution of this region is represented by three Gaussian peaks instead of two. The lowest-temperature peak is centered at 192 8C. This is a strong indication that the two-dopant mate- of copper dopant (Table S1). For 1 % Cr-ceria nanorods, there is rial has much-improved surface reducibility relative to the also some shift to the left of both peaks, to 303 and 418 8C, single copper-doped ceria nanorods—a synergistic improve- but increasing the quantity of Cr dopant does not continue to ment that cannot be explained by the cumulative or average improve surface reducibility at lower temperatures. effect of single-dopant contributions. The ratio of low-temperature to high-temperature areas in Additionally, the ratio of the surface to bulk oxygen reduc- Figure 6, representing the total consumption of hydrogen in tion areas is 1.2, much higher than that of undoped or singly surface and bulk reduction, respectively, can be used as a qual- doped ceria materials. This could also indicate an improvement itative assessment of the reducibility of the bulk lattice struc- in surface reducibility, or instead, similarly to Cr-doped ceria ture, relative to the surface by showing the ratio of readily re- materials, the two-dopant system might not have improved ducible surface to bulk oxygen.[30, 43] Doping ceria with other the oxygen bulk reducibility but only that of the surface. metals generally changes the bulk properties, not just those of X-ray photoelectron spectroscopy (XPS) analysis was used to the surface.[17] While reactions such as CO oxidation and NO re- gain further insight into the surface state of the ceria nano- duction occur on the surface of ceria catalysts, improvements rods. XPS spectra of Ce 3d, O 1s, Cu 2p, and Cr 2p are shown in bulk properties can also improve catalyst performance due in Figures 7 and 8. In Figure 7 a, the Ce 3d XPS spectra consist to the mobility of oxygen vacancies and vacancy hopping of five peaks (Uo, U, U’, U’’, and U’’’) representing 3d3/2 and five mechanisms.[44, 45] For undoped ceria the ratio of low-tempera- peaks (Vo, V, V’, V’’, and V’’’) representing 3d5/2. According to ture to high-temperature areas is 0.47 (Table 1). With increasing previously published methods, Vo, V’, Uo, and U’ can be as- levels of copper dopant, this ratio shifts in favor of the high- signed to Ce3 + , while the remaining peaks (V, V’’, V’’’, U, U’’, temperature area (with a ratio of 0.24 to 0.34), indicating that and U’’’) can be assigned to Ce4 + , and the relative ratio of Table 1. Surface concentration of Ce3 + in ceria nanorods, calculated from the Ce 3d XPS spectra, and relative peak areas of Oa, Ob and Og in O 1s XPS spectra. H2 TPR result. Material Ce3 + [at. %] Oa [at. %] Ob [at. %] Og [at. %] Low T/High T TPR peak area ratio Undoped CeO2 22 50.8 28.9 20.2 0.47 1 wt % Cu 23 37.9 40.9 21.3 0.24 3 wt % Cu 23 41.4 36.0 22.6 0.27 5 wt % Cu 26 39.8 38.5 21.7 0.31 7 wt % Cu 27 32.5 38.3 29.2 0.34 1 wt % Cr 16 34.2 50.1 15.8 0.70 3 wt % Cr 15 35.5 49.0 15.5 0.86 5 wt % Cr 16 40.5 42.8 16.7 0.76 1 wt % each Cu/Cr 33 42.5 34.4 23.1 1.20 Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2168 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 work on similarly prepared ceria nanorods demonstrated (via DRIFTS experiments) that hydroxy and carbonate species were present on the surface of the ceria nanorods and that the pres- ence of hydroxy-containing groups was correlated with catalyt- ic activity for oxidation reactions of naphthalene and tolu- ene.[30] it was tentatively proposed that these OH sites act as adsorption points in the first reaction steps at low tempera- tures. No shift in the O 1s binding energy is evident when compar- ing the undoped and doped ceria samples. However, the un- doped ceria spectrum has a larger contribution from stoichio- metric lattice oxygen (Oa) than the doped materials: approxi- mately 50 % of the total area of the three peaks compared with, for example, 38, 34, and 43 % for 1 % Cu, 1 % Cr, and 1 % Cu/1 % Cr-ceria, respectively. As shown in Figure 8 a, no Cu 2p peaks are observable in Figure 7. (a) Ce 3d and (b) O1s XPS spectra for Cu, Cr, and Cu/Cr-doped the 1 % Cu-doped ceria sample by XPS. Weak Cu 2p1/2 and Cu ceria nanorods. 2p3/2 peaks are visible in the 3 % and 5 % Cu-ceria, while the 7 % Cu- ceria shows strong Cu 2p1/2 and Cu 2p3/2 peaks at 933.0 and 952.8 eV, respectively. In contrast to 1 % Cu-ceria, peak areas can be used to determine the percentage concen- the 1 % Cu/1 % Cr-ceria has observable Cu 2p1/2 and Cu 2p3/2 tration of Ce3 + and Ce4 + at the surface.[30, 37, 42] This is shown in peaks, indicating that the co-doped ceria has a higher concen- Table 1. This analysis reveals that undoped ceria nanorods tration of Cu at the surface than the singly doped 1 % Cu-ceria. have a nonstoichiometric surface, with 22 % of Ce 3d photo- Analysis of both the Cu 2p and Cu LMM region (Figure 9) was emission due to Ce3 + and 78 % due to Ce4 + . The addition of employed to reliably determine copper speciation, using the low levels of Cu dopant (1 and 3 %) only slightly increases the modified auger parameter (a’). The Cu LMM feature was de- concentration of Ce3 + , but higher levels of Cu (5 and 7 %) convoluted and kinetic energy determined and a’ was found result in a higher level of surface reduction, with the percent- to range between 1850.12 and 1847.7 eV, consistent with ionic age of Ce3 + increasing to 26 and 27 %, respectively. In contrast, copper species.[53] Furthermore, there appeared to be an inver- the addition of Cr dopant decreases the percentage of Ce3 + at the surface. Both results corroborate the TPR analysis above; in summary, Cu improves ceria’s surface reducibility, whereas Cr does not have a relevant effect. Cu/Cr-doped ceria nanorods demonstrate the highest level of surface reduction, with the concentration of Ce3 + calculated to be 33 %. This phenomenon of enhanced surface reducibility with low levels of both Cu and Cr dopant also supports the TPR analysis in Figure 6. XPS spectra in the O 1s region are shown in Figure 7 b for the undoped and doped ceria nanorods, while relative peak areas are reported in Table 1. The O 1s spectrum is broad and Figure 8. (a) Cu 2p and (b) Cr 2p XPS spectra for Cu, Cr, and Cu/Cr-doped consists of peak contributions from the various oxygen species ceria nanorods. at the surface of ceria. These spectra were resolved with three Gaussian-Lorentzian peaks. The peak at approximately 529 eV (Oa) can be attributed to the lattice oxygen of Ce4 + , while the peak at 531 eV (Ob) can be attributed to oxygen vacancies or the lower-coordination lattice oxygen of Ce3 + . This is signifi- cant because reduced Ce3 + defects and oxygen vacancies are understood to be the active sites on ceria-catalyzed surface re- actions.[49] CO oxidation on ceria surfaces proceeds via the Mars van Krevelen mechanism: CO molecules adsorb onto the ceria surface and react with a surface oxygen atom to form an intermediate. This intermediate desorbs as CO2, leaving behind an oxygen vacancy, which is compensated by the reduction of Ce4 + to Ce3 + .[50–52] However, Ob can also be associated with surface adsorbed oxygen and hydroxy groups. The broad peak at 533 eV (Og) is Figure 9. Cu LMM spectra for (a) Cu-doped ceria nanorods (from top to also associated with these surface oxygen species.[30, 37] Prior bottom; 1, 3, 5, 7, 9 % Cu-ceria) and (b) 1 % Cu/1 % Cr ceria nanorods. Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2169 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 sely proportional relationship between a’ and copper loading, with lower loadings reporting a lower modified auger parame- ter. This lowering of a’ can be attributed to decreased average screening due to a decrease in copper particle size and the in- teraction between the copper and the highly polarizable sup- port.[54] Due to the lack of a clear shakeup satellite peak in the Cu 2p spectra at 942 eV, characteristic of Cu2 + , the samples ex- posure to air, and the Cu2O XRD peaks observed for 9 % Cu- ceria, Cu 2p XPS peaks can be assigned to Cu + . Figure 8 b shows the Cr 2p XPS spectra for Cr and Cu/Cr-doped ceria nanorods. The visible Cr 2p peaks at 576 and 586 eV are as- signed to Cr3 + and the shoulder peaks at 580 and 589 eV to Cr6 + .[55] Intensity is higher for the 3 % and 5 % Cr-ceria. Unlike Cu, the XPS spectrum for 1 % Cu/1 % Cr-ceria shows a weaker Cr intensity than 1 % Cr-ceria. Nitrogen adsorption experiments show that the ceria nano- rods, both undoped and doped, are non-porous but meso- porosity is observed due to interparticle voids, with a type IV isotherm in all cases (Figure S6).[56] BET surface area, measured at low relative pressures, is in the range 34 to 55 m2 g@1 (Figure 10). The presence of Cu has a negligible effect on the surface area, with no clear overall trend. For Cr-doped ceria, all doped samples had higher surface areas than undoped ceria nanorods. Figure 11. CO oxidation catalytic conversions for (a) Cu-doped and (b) Cr- doped ceria nanorods. NO reduction catalytic conversions for (c) Cu-doped and (d) Cr-doped ceria nanorods. Conversion of two-dopant Cu/Cr doped ceria nanorods for (e) CO oxidation and (f) NO reduction. lyst.[60, 61] However, copper and other base metals are not used in automotive catalysis due to sulfur poisoning and thermal Figure 10. BET surface area vs. dopant level for Cu and Cr doped ceria. durability issues, so despite the enhanced CO conversion, a separate copper phase is not desirable.[3] A further increase in copper loading from 7 to 9 % did not result in an improvement CO oxidation and NO reduction on the doped ceria nano- in low-temperature CO conversion. This could be attributed to rods were tested over cycles of six steps between room tem- the doping limit being reached somewhere between 5 and perature and 500 8C and representative stability data is shown 7 % Cu, and the additional copper oxide particles in the 9 % in Figure S7 and S8. The CO oxidation at various temperatures sample (visible in XRD) not providing more catalytic CO con- was stable. The NO reduction was stable for the first three version than the copper oxide particles present in the 7 % Cu- steps, and then slightly decreased for the following steps, but ceria nanorod material (as confirmed by TPR). In contrast to samples were still active. Figure 11 shows the averaged catalyt- copper, doping ceria nanorods with 1 % Cr slightly increases its ic conversion for the first three steps. Copper doping of ceria catalytic CO oxidation at low temperatures but decreases it at significantly improves the low-temperature catalytic CO oxida- all temperatures for higher Cr percentages, compared with un- tion with respect to undoped ceria rods, with conversion in- doped ceria nanorods (Figure 11 b). This also correlates with creasing as the Cu loading increases up to 7 % Cu above which TPR and XPS results, which suggest that chromium doping of no further improvements are observed (Figure 11 a). Similar ceria mostly decreased the reducibility of the material and the trends in improved CO conversion on ceria catalysts have been concentration of Ce3 + at the surface. previously observed with transition metal doping.[57–59] The Regarding NO reduction (Figure 11 c,d), Cr-doped ceria nano- large jump in CO conversion between the 5 % Cu and 7 % Cu rods are effective at improving conversions beyond that of ceria catalysts can be explained with the appearance of the pure ceria, while Cu-doped ceria do not show improvement separate copper oxide phase at 7 % Cu loading (and at 9 % in compared with undoped ceria nanorods. This is expected in powder XRD), seen in the TPR analysis. Ceria-supported copper view of the characterization: Cu doping improves ceria’s oxida- is known to be an effective low-temperature oxidative cata- tive capability by increasing the reducibility of ceria, but this Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2170 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 can also make the healing of oxygen vacancies somewhat less oxygen vacancy concentration and surface reducibility, NO ad- favorable.[29] In contrast, chromium demonstrates the opposite sorption on the catalyst surface is not necessarily improved, so effect. However, chromium doping at levels above 1 % do not no overall improvement in catalytic conversion is seen. show further improvement in NO reduction conversions. As Therefore, while doping ceria with copper improves CO oxi- with CO oxidation, this activity matches the analysis of the Ce dation conversions but inhibits NO reduction and doping ceria 3d XPS spectra—ceria nanorods doped with 1, 3, and 5 % Cr with chromium has the opposite effect, the simultaneous co- show very similar levels of surface reduction. doping of ceria nanorods with both Cu and Cr retains the ben- Catalyst performance of the 1 % Cu/1 % Cr-ceria nanorod efits of each single dopant while significantly enhancing CO material is shown in Figure 11 e for CO oxidation and Fig- oxidation, a phenomenon that has not yet been reported in ure 11 f for NO reduction. Despite containing only 1 % each Cu the literature for this doped system. These results also show and Cr, the two-dopant ceria nanorods show CO oxidation that that the improved reducibility provided by copper doping is similar to the 7 and 9 % Cu ceria nanorods, as well as slightly does not necessarily have a negative effect on the healing of improved NO reduction compared with 1 % Cr-ceria. Therefore, oxygen vacancies. Co-doping is necessary for this synergistic co-doped Cu@Cr ceria appears to provide a unique synergistic effect—a physical mixture of Cu-ceria and Cr-ceria shows effect, with a CO oxidation close to the one of 9 % Cu-ceria much lower catalytic CO oxidation (Figure S9). nanorods, despite only containing 1 % Cu (2 % total dopant Related studies of NO reduction with CO on supported content). At the same time, the co-dopant catalyst retains NO metals on ceria (MOx/CeO2) prepared via incipient wetness im- reduction conversions similar to that of the Cr-ceria nanorods, pregnation have shown the importance of controlling the with a slightly improved T10 value. “Synergistic” in this case can loading amount to maximize the number of metal–oxygen– be defined as showing improvement in reactant conversions support interfacial bonds.[72–77] Maximum conversions and ac- beyond what an equivalent level of single-metal dopant could tivities are typically found at approximately the monolayer cov- be responsible for. erage, avoiding the presence of separate crystallites phases The ceria nanorods of this work preferentially expose (110) that would be detectable by for example XRD. Such results are ceria facets.[30] Assuming Cu and Cr atoms are incorporated similar to these presented here, in which small and controlled similarly on (110) and (111) ceria surfaces, DFT simulations by amounts of dopants lead to optimal results, especially in 1 % Yoshida et al. can shed some light on such synergy in 1 % Cu/ Cu/1 % Cr-ceria that has a higher concentration of Cu at the 1 % Cr-ceria nanorod.[64, 66] They report that when Ce on the surface than the singly doped 1 % Cu-ceria according to XPS. (111) surface of ceria is substituted with Cu, the dominant spe- Various other ceria catalysts doped with copper or chromi- cies is Cu2 + . However, the substitution of Ce with Cu and Cr re- um have recently been reported.[14, 21, 22, 61, 62] Direct comparisons sults in the dominant Cu species being Cu + . The presence of with other articles is difficult due to differences in reaction Cu + in doped ceria is known to enhance CO chemisorption on conditions such as catalyst loading and reactant flowrates or the catalyst surface and catalytic activity for CO oxidation.[67] In ratios, which widely vary between different research groups. this case, CO adsorbs on the surface at Cu + sites and react Nevertheless, conversions can be normalized in terms of rate, with neighboring oxygen to form CO2 and create oxygen va- shown in Table 2 for CO oxidation and NO reduction along cancies, which are balanced by the reduction of Ce4 + to Ce3 + . with T10 and T50 values (the temperature at which 10 % and Conversely, for the reduction of NO, the presence of Cr enhan- 50 % conversion of CO or NO is achieved). While the rates of ces electron density on neighboring Ce sites and lowers the our catalyst is similar to or better than many other reported barrier for oxygen vacancy formation, both of which improve doped ceria materials, even those with higher dopant content, NO adsorption on Ce sites near Cr on the doped ceria sur- the low-temperature activity for CO oxidation is not as good face.[68, 69] This corroborates the enhanced reducibility of the as that of the 3.9 % Cu-doped ceria nanorods reported in Cu/Cr-ceria seen in TPR and XPS analysis as well as the signifi- Table 2.[21] This can be attributed to the higher copper loading cantly improved catalytic performance for CO oxidation com- and improvements in BET surface area (72 m2 g@1), and porosity pared with singly doped ceria. due to their MOF-templated synthesis, compared with our hy- For NO reduction, the 1 % Cu/1 % Cr-ceria catalyst showed drothermally prepared Cu/Cr-doped nanorods. In contrast, for improvement compared with 1 % Cr-ceria, even though the ad- NO reduction, our Cu/Cr-doped ceria nanorods performed dition of copper alone did not improve conversions over un- slightly better at 150 8C but somewhat worse at 200 8C com- doped ceria. This can be attributed to the combination of the pared with a Cr-doped ceria catalyst (Ce20Cr1Ox) or a 0.07 % influence of both dopants on the properties of the catalyst. A each Cu/Cr deposited on ceria (Table 2).[22] In any case, the higher concentration of oxygen vacancies is known to enhance latter uses a pulsed cathodic arc plasma technique in vacuum, the reduction of NO to N2 by promoting the disassociation of much more complicated than our hydrothermal approach. NO.[70, 71] However, improved NO adsorption is also necessary to improve overall catalytic activity. In the 1 % Cu/1 % Cr-ceria cat- Conclusions alyst, Cu provides additional oxygen vacancies while Cr enhan- ces NO adsorption on Ce3 + sites on the catalyst surface, and in Copper and chromium have been used as dopants for ceria combination this provides additional catalytic conversions nanorod catalysts produced via a hydrothermal synthesis beyond that of either singly doped catalyst. Conversely, for the method. Doping does not significantly affect the nanorods singly doped Cu-ceria catalysts, while Cu will still enhance shape. Metal particles different to ceria and large enough to Chem. Eur. J. 2021, 27, 2165 – 2174 www.chemeurj.org 2171 T 2020 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202004623 Table 2. Comparison of copper-ceria and chromium-ceria catalysts for CO oxidation and NO reduction. Catalyst Reaction T10 [8C] T50 [8C] Rate [mmol g@1 min@1] Ref. At 50 8C At 100 8C 1 % each Cu/Cr co-doped ceria nanorods CO ox. 100 206 8.2 27.3 this work 3.9 % Cu-doped ceria nanorods CO ox. 65 122 61.3 352.7 [21] 3.9 % Cu-doped ceria nanospheres CO ox. 142 232 0.0 13.1 [61] 2.0 % Cu-doped ceria nanopolyhedra CO ox. 113 161 0.0 1.2 [14] 8.5 % Cu deposited on ceria nanorods CO ox. 40 73 16.4 90.0 [62] 0.07 % each Cu/Cr deposited on ceria CO ox. 86 120 0.0 18.0 [63] Catalyst Reaction T10 [8C] T50 [8C] Rate [mmol g@1 min@1] Ref. At 150 8C At 200 8C 1 % each Cu/Cr co-doped ceria nanorods NO red. 194 243 6.1 26.6 this work Ce20Cr1Ox NO red. 202 216 0.0 40.9 [22] 0.07 % each Cu/Cr deposited on ceria NO red. 143 300 17.2 45.4 [64] diffract X-rays in our conditions are only observed for copper dopant (for example, 0.448 mL of CuSO4 solution was added to levels above 7 %. The experimental characterization confirms achieve 1 % Cu-ceria). The mixture was stirred vigorously for 20 s. the incorporation of Cu or Cr in the ceria crystal lattice, in- The autoclave was sealed and heated at 100 8C in an air-circulating oven at a rate of 5 8C min@1, held at this temperature for 10 h, after crease of surface oxygen vacancies, and changes in the surface which it was allowed to cool naturally to room temperature. Post- and bulk reducibility. When tested as catalysts, copper doping synthesis, the product was separated from the sodium hydroxide significantly improves the CO oxidation conversions but not solution via vacuum filtration and washed with approximately for NO reduction, while the opposite is true for chromium- 500 mL of deionized water. The wet product was dried in a doped ceria. However, simultaneously doping a ceria nanorod vacuum oven at 80 8C overnight. The resulting dry powder was catalyst with both Cu and Cr dopants results in a catalyst gently grounded in an agate mortar and pestle. Highly crystalline which shows improved conversions for both CO oxidation and ceria nanorods, free of organic matter, were obtained following NO reduction compared with undoped ceria nanorods. Addi- this approach. No calcination was, therefore, carried out. Samples were labelled according to the wt % of metal dopant used in the tionally, the use of co-dopants appears to provide a synergistic synthesis. effect, with only 1 % Cu and 1 % Cr required to provide CO oxi- dation conversions like 7 % Cu-doped ceria nanorods. This be- XRD was performed with a Bruker D8-Advance using a CuKa radia- havior is supported by TPR and XPS analyses which show im- tion source. Measurements were taken from 20 to 908 2q with a step size of 0.01648 2q and 0.25 sec per step. Coherent diffraction proved low-temperature surface reducibility in co-doped Cu/ domain sizes were calculated with the Scherrer equation. Catalyst Cr-ceria compared with either Cu or Cr singly doped ceria ma- surface areas were calculated with nitrogen adsorption experi- terials. Cu XPS moreover shows a higher surface Cu concentra- ments using a Micromeritics 3Flex Surface Characterization Ana- tion, which based on DFT calculations in literature are expected lyzer. Samples were degassed under vacuum for 90 minutes at to be mainly Cu + , known to enhance CO chemisorption on 200 8C. The surface areas were calculated using the BET. XPS analy- the catalyst surface and therefore catalytic CO oxidation. In a sis was performed using a Kratos SUPRA XPS instrument with mon- nutshell, Cu/Cr-doped ceria nanorods show promise as an im- ochromated AlKa X-rays (1486.69 eV). Survey scans were recorded proved catalyst for both oxidative and reductive reactions. This with a pass energy of 160 eV, while high resolution spectra were recorded with a pass energy of 20 eV. TEM was performed using a research demonstrates the potential effectiveness of co-doping JEOL JEM2100Plus microscope. Raman microscopy was performed with transition metals as a strategy for improving the effective- using a Renishaw inVia Raman Microscope, using a 532 nm green ness of ceria-based catalysts. Future research should be fo- laser with 10 s exposure time and 1 % laser power. TPR experi- cused on the optimization of dopant levels, examination of ments were performed with a Micromeritics AutoChem II 2920 other dopant metals, and testing these catalysts under more equipped with a thermal conductivity detector; 0.1 g of each typical real-world conditions such as after oxidative pre-treat- sample was degassed under helium and consequently heated from ments. It would also be useful to research the N2 selectivity for room temperature to 900 8C at a rate of 10 8C min@1, under a flow the NO reduction with CO in these doped materials. of 50 mL min@1 of 5 % hydrogen in argon. The catalytic CO oxidation and NO reduction were performed with a U-shaped quartz tube reactor (10 mm ID). In a typical experi- ment, a catalytic bed of 4 cm3 silicon carbide particles was used, Experimental Section with 15 mg ceria nanorod catalyst dispersed throughout. Within A facile hydrothermal method was used to synthesize ceria nano- the quartz tube reactor, the catalytic bed was secured at both rods, both undoped and doped.[36] 120 mL of a 15 m aqueous ends with high-temperature quartz wool. For CO oxidation, the re- sodium hydroxide solution and 1.8 g of Ce(NO3)3·6 H2O were added actant feed consisted of 2000 ppm CO and 2000 ppm O2 in nitro- to a 150 mL Teflon-lined stainless-steel autoclave. For doped ceria, gen. The total flow rate was 50 mL min@1, achieving a weight a necessary amount of the respective dopant precursor in aqueous hourly space velocity (WHSV) of 200 L g@1 h@1. 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