catalysts

Article Copper-Containing Mixed Metal Oxides (Al, Fe, Mn) for Application in Three-Way Catalysis

Tim Van Everbroeck 1 , Radu-George Ciocarlan 1, Wouter Van Hoey 1 , Myrjam Mertens 2 and Pegie Cool 1,*

1 Laboratory of Adsorption and Catalysis, Department of Chemistry, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium; [email protected] (T.V.E.); [email protected] (R.-G.C.); [email protected] (W.V.H.) 2 VITO Flemish Institute for Technological Research, Boeretang 200, B-2400 Mol, Belgium; [email protected] * Correspondence: [email protected]



Received: 8 October 2020; Accepted: 17 November 2020; Published: 19 November 2020


Abstract: Mixed oxides were synthesized by co-precipitation of a Cu source in combination with Al, Fe or Mn corresponding salts as precursors. The materials were calcined at 600 and 1000 ◦C in order to crystallize the phases and to mimic the reaction conditions of the catalytic application. At 600 ◦C a mixed spinel structure was only formed for the combination of Cu and Mn, while at 1000 ◦C all the materials showed mixed spinel formation. The catalysts were applied in three-way catalysis using a reactor with a gas mixture containing CO, NO and O2. All the materials calcined at 600 ◦C displayed the remarkable ability to oxidize CO with O2 but also to reduce NO with CO, while the pure oxides such as CuO and MnO2 were not able to. The high catalytic activity at 600 ◦C was attributed to small supported CuO particles present and imperfections in the spinel structure. Calcination at 1000 ◦C crystallized the structure further which led to a dramatic loss in catalytic activity, although CuAl2O4 and CuFe2O4 still converted some NO. The materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, H2-Temperatrue Programmed Reduction (H2-TPR), N2-sorption and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX).

Keywords: spinel; copper oxide; three-way catalysis; NO reduction; CO oxidation

1. Introduction Even with the most recent advances in alternatively-powered vehicles 86.8% of all the new-car registrations in the EU were fueled by diesel or petrol. The largest share of these accounted to the petrol cars (57.3% of the EU market) [1]. So even though most research nowadays is focused on technology for alternatively-powered vehicles, there is still a lot of fundamental research necessary to cope with the ever-increasing demand in petrol cars. Petrol cars are using three-way catalytic convertors to abate the release of CO, volatile organic compounds (VOCs) (hydrocarbons) and NOx to the environment. This technology uses critical raw materials (CRMs) such as Pt, Pd and Rh as the main catalytic materials [2,3]. The reason why these materials are considered critical is related to their great economic importance, while their supply is at risk. This means that the existing technologies need to be adapted in order to reduce the amount of CRMs. One way to reduce the use of CRMs is to replace them with alternative catalytic materials. A comparison between the specific reaction rates for the oxidation of CO shows that the most interesting materials to replace Pd are Co3O4 (having a reaction rate 20 times lower) or CuO (having 50 times lower reaction rate) [4]. Since cobalt is also considered a CRM [3], the logical choice is to study 2 CuO. To compensate for the low specific reaction rate (e.g., mL CO2/(min. m ), larger amounts of CuO

Catalysts 2020, 10, 1344; doi:10.3390/catal10111344 www.mdpi.com/journal/catalysts Catalysts 2020, 10, 1344 2 of 20

(tens of percent) than are regularly used of the precious materials (<1%) can be used. Moreover, the amount of available catalytic surface area can be increased by using porous support materials. Other ways of increasing the reaction rate consider the generation of synergies between different materials by using mixtures of oxides [5,6]. By treatment at high temperatures, these mixtures can grow into new crystal structures of mixed oxides such as spinels and perovskites [7]. Another important factor in defining a valuable catalyst is related with the stability in order to be reused in several catalytic cycles. Here, in particular, it is important to investigate if these mixed oxides are also effective for real-world applications, as the materials will have to endure long exposures to high temperatures and many cycles of work. In this work, we chose to investigate mixtures of CuO with aluminum-, iron- and manganese oxides. All of these elements are abundant and therefore ideal to be used as a catalyst in large productions. The other aspect they all have in common is that they commonly occur in a trivalent state in oxides (Al2O3, Fe2O3, Mn2O3). However, they differ in the existence of other stable oxidation states of the metals. The difference is that Al3+ cannot be reduced to Al2+, Fe3+ is reducible to Fe2+, and Mn3+ can be reduced to Mn2+ and oxidized to Mn4+. This is important with respect to their role in the catalytic conversion mechanism. The mixture of copper and manganese oxides is quite known for the oxidation of CO under the name “hopcalite”. Originally, this was developed during the first World War to remove CO from air in gasmasks [5,8,9]. One of the most active forms of the mixture consists of 60% MnO2 and 40% CuO [5,9]. Most literature that feature hopcalite-like mixtures focus on oxidation reactions only such as CO [6,10–13] or VOCs [14–16] and not on the simultaneous removal of NO as in the case of three-way catalysis. Al, Fe and Mn oxides can co-exist in the same structure with CuO and form different spinel-type structures. The formation of these types of mixed oxides enhances the dispersion factor of elements, when compared with composite materials (separate oxide forms). In order for this to happen, most of the times a specific ratio (1:2) of the elements must be applied and thermal treatments must be used. 2+ 3+ The classical spinel structure follows the general formula A B 2O4, which can also be noted as the 2+ 3+ two types of oxides (A O)(B 2O3). The A and B represent the two sites of the spinel structure, namely tetrahedral—A and octahedral—B, respectively. With regards to the cation sites occupancy, one can define three different types of spinel structures: (I) Normal spinel—in which the divalent cation (A2+) is fully occupying the tetrahedral site, (II) inverse spinel—where the divalent (A2+) cation is totally occupying the octahedral site, while half of the trivalent (B3+) is occupying the tetrahedral site and (III) spinel structures with a different inversion degree—where the divalent (A2+) and trivalent (B3+) are occupying both sites of the structure [17]. In other words, a more general formula for spinels would be tet oct (A(1 x)Bx) [AxB(2 x)] O4 with x being the inversion parameter. This means that if x is 0, the spinel is − − normal, while if x is 1 it is inverse [18]. Cu and Mn are quite peculiar elements to be incorporated in the spinel structure because they display the Jahn–Teller effect [19]. This means that the octahedral Cu2+, Mn2+ and Mn3+ [20] are tetragonally distorted. While these distortions make both elements more difficult to incorporate in a mixed oxide, this seems to be compensated when they are used together in copper manganite [21]. CuFe2O4 occurs mostly as an inverse spinel with only about 6–24% Cu in the A sites [22–24]. For CuMn2O4 reports vary between 25 and 80% inversion [25–27]. In bulk CuAl2O4 x is around 0.4 which means about 60% of Cu is in the tetrahedral sites [18,28,29]. In this research work, we synthesized mixtures of oxides by co-precipitation of Cu-salts with Al, Fe or Mn salts. A 1:2 ratio between Cu and the other element was used to encourage the formation of a spinel form. In the case of Mn, also 1:1 ratio was used to see if there was an influence on the catalytic performance. These materials were calcined at 600 and 1000 ◦C which are representing the temperatures of the catalytic converter in normal and extreme conditions. In order to keep the testing conditions simple, the materials were tested as powders in a flow of CO, NO and O2 instead of a full mixture for three-way catalysts consisting of CO, H2, different hydrocarbons, O2, CO2,H2O and NO. This way it is easier to evaluate the ability of the materials to oxidize CO and to reduce NO. The materials CatalystsCatalysts2020 2020, 10, 10, 1344, x 33 of of 20 20 materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, H2-Temperatrue wereProgrammed characterized Reduction by X-ray (H di2-TPR),ffraction N2 (XRD),-sorption Raman and spectroscopy,Scanning Electron H2-Temperatrue Microscopy Programmed with Energy ReductionDispersive (H X-ray2-TPR), Spectroscopy N2-sorption (SEM-EDX). and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX). 2. Results 2. Results 2.1. Physicochemical Properties of the Catalysts 2.1. Physicochemical Properties of the Catalysts Figure 1 displays the X-ray diffractograms (XRD) of CuAl 600 and CuAl 1000. After the Figure1 displays the X-ray di ffractograms (XRD) of CuAl 600 and CuAl 1000. After the calcination calcination at 600 °C, the aluminum is not detected as a crystal structure of Al2O3 or CuAl2O4, at 600 ◦C, the aluminum is not detected as a crystal structure of Al2O3 or CuAl2O4, suggesting the suggesting the formation of amorphous alumina or small particles of CuAl2O4. On the other hand, formation of amorphous alumina or small particles of CuAl O . On the other hand, Cu is clearly Cu is clearly present as CuO (tenorite), in a monoclinic crystal2 arrangement.4 The detailed crystallites present as CuO (tenorite), in a monoclinic crystal arrangement. The detailed crystallites size data are size data are presented in Table 1. The approximate size of the CuO crystallites, calcined at 600 °C presented in Table1. The approximate size of the CuO crystallites, calcined at 600 C and calculated and calculated from the (111) reflection at 38.7°, is 10 nm. Noticeably, after the calcination◦ at 1000 °C, from the (111) reflection at 38.7◦, is 10 nm. Noticeably, after the calcination at 1000 ◦C, the Bragg the Bragg reflections of CuAl2O4 appear, suggesting the formation of a well-defined spinel structure reflections of CuAl O appear, suggesting the formation of a well-defined spinel structure at this at this elevated temperature2 4 when compared to 600 °C. The CuO average crystallites size is around elevated temperature when compared to 600 ◦C. The CuO average crystallites size is around 44 nm, 44 nm, while the CuAl2O4 average crystallites size is roughly 56 nm, taking into account the (113) while the CuAl O average crystallites size is roughly 56 nm, taking into account the (113) reflection reflection at 36.9°2 4 for the latter component. SEM-EDX (Supplementary Table S1) proved that in both at 36.9 for the latter component. SEM-EDX (Supplementary Table S1) proved that in both cases the cases ◦the Cu-Al ratio is the same; about 36 wt% Al and 64 wt% Cu, which means a molar ratio of Cu-Al ratio is the same; about 36 wt% Al and 64 wt% Cu, which means a molar ratio of about 1.3:1 about 1.3:1 Al:Cu instead of 2:1. This leads to the conclusion that the sample contains more Cu and Al:Cu instead of 2:1. This leads to the conclusion that the sample contains more Cu and explains the explains the presence of large CuO crystallites. presence of large CuO crystallites.

♦ = CuO CuAl 600 ♦ A = CuAl2O4 CuAl 1000 ♦

♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦

A

intensity (a.u.) intensity A A A A A A

10 20 30 40 50 60 70 2θ(°)

Figure 1. XRD of CuAl calcined at 600 and 1000 ◦C. Figure 1. XRD of CuAl calcined at 600 and 1000 °C. Table 1. Estimated average crystallite size based on XRD. Table 1. Estimated average crystallite size based on XRD. Catalyst/Size (nm) CuO Spinel CuAl 600Catalyst/Size 10(nm) CuO Spinel - CuFe 600CuAl 600 17 10 - - CuMn 600CuFe 600 11 17 - 22 CuMn2 600 - 33 CuMn 600 11 22 CuAl 1000CuMn2 600 44 - 33 56 CuFe 1000 46 62 CuAl 1000 44 56 CuMn 1000 26 63 CuMn2 1000CuFe 1000 - 46 62 56 CuMn 1000 26 63 CuMn2 1000 - 56 The XRD patterns of the CuFe samples are shown in Figure2. Here, after the thermal treatment at 600 C, a mixture of oxides is observed, with hematite (Fe O ), magnetite (Fe O ) and tenorite (CuO) ◦ The XRD patterns of the CuFe samples are shown in2 Figure3 2. Here, after3 4the thermal treatment phases present. On the other hand, at 1000 ◦C only the spinel structure of CuFe2O4 and tenorite (CuO) at 600 °C, a mixture of oxides is observed, with hematite (Fe2O3), magnetite (Fe3O4) and tenorite (CuO)

Catalysts 2020, 10, x 4 of 20 Catalysts 2020, 10, x 4 of 20 Catalysts 2020, 10, 1344 4 of 20 phases present. On the other hand, at 1000 °C only the spinel structure of CuFe2O4 and tenorite (CuO) phasesare observed. present. CuO On thephase other shows hand, an at increase 1000 °C of only the crysthe spineltallites structure size from of 17 CuFe nm to2O 464 and nm tenorite when calcined (CuO) areat 600 observed. °C and CuO1000 phase°C, respectively shows an increase (Table 1). of the crystallitescrystallites sizesize from 1717 nm to 46 nm when calcined at 600 °C and 1000 °C, respectively (Table 1). at 600 ◦C and 1000 ◦C, respectively (Table1).

M H = Fe2O3 CuFe 600 H = Fe O M CuFe CuFe 600 1000 M = Fe23O34 ♦ CuFe 1000 M = = Fe CuO3O4 ♦• == CuOCuFe2O4 • = CuFe2O4 ♦ ♦ ♦ ♦ M M H H H M MH MH ♦M H M M ♦H HH MH ♦H ♦ ♦ H ♦ ♦ H M MH MH ♦ H M ♦ HH MH ♦H ♦ ♦ ♦ ♦ • • • • • intensity (a.u.) intensity • • • • • intensity (a.u.) intensity ♦• ♦ • • • •• • • • • • ♦ • • • • ♦ • ♦ ♦ •♦ ♦ • • • • ♦• • ♦ • ♦ ♦ ♦• •♦

10 20 30 40 50 60 70 10 20 302 40θ (°) 50 60 70 θ 2 (°) Figure 2. XRD of CuFe calcined at 600 and 1000 °C. FigureFigure 2.2. XRD of CuFe calcined at 600 and 10001000 ◦°C.C. Figure 3 shows the diffractograms of different Cu-Mn samples (Cu:Mn = 1:1 = CuMn, Cu:Mn = Figure3 3 shows shows the the di diffractogramsffractograms of of di differentfferent Cu-Mn Cu-Mn samples samples (Cu:Mn (Cu:Mn= 1:1 = 1:1= CuMn, = CuMn, Cu:Mn Cu:Mn= 1:2 = =1:2CuMn2). = CuMn2). In bothIn both cases, cases, at 600at 600C °C the the crystal crystal structure structure of of Cu Cu1,5MnMn1,5OO4is is presentpresent inin thethe composites.composites. 1:2 = CuMn2). In both cases, at 600◦ °C the crystal structure of Cu1.51,5Mn1.51,5O44 is present in the composites. ThisThis structure structure has has been been reported reported in in previous previous studies studies in in literature literature [ 10[10,14,19,30,31].,14,19,30,31]. AfterAfter thethe calcinationcalcination Thisat 1000 structure °C, the has structure been reported is slightly in previouschanged studiesin both inratios literature of the [10,14,19,30,31]. Cu:Mn systems. After A clear the evidencecalcination of at 1000 ◦C, the structure is slightly changed in both ratios of the Cu:Mn systems. A clear evidence atthis 1000 modification °C, the structure is observed is slightly for the changed reflections in both at 57.6° ratios and of 63.3°, the Cu:Mn which systems. represent A the clear (115) evidence and (044) of ofthis this modification modification is observed is observed for forthe thereflections reflections at 57.6° at 57.6 and◦ and63.3°, 63.3 which◦, which represent represent the (115) the (115)and (044) and (044)plane, plane, respectively respectively (Figure (Figure 4). For4). CuMn For CuMn 1000, an 1000, extra an tenorite extra tenorite (CuO) phase, (CuO) with phase, a size with of a 26 size nm, of is plane,detected respectively in the XRD. (Figure In the 4). Forcase CuMn of CuMn2, 1000, an where extra moretenorite Mn (CuO) is present phase, than with necessarya size of 26 to nm, form is 26detected nm, is detectedin the XRD. in the In XRD. the Incase the of case CuMn2, of CuMn2, where where more more Mn Mn is present is present than than necessary necessary to to form CuCu1.5Mn1.5OO4, ,a a segregated segregated bixbyite bixbyite phase phase (Mn (Mn2OO3) is) isobtained obtained from from the the additional additional Mn. Mn. At At1000 1000 °C, theC, Cu1.51.5Mn1.51.5O4,4 a segregated bixbyite phase (Mn2O23) is3 obtained from the additional Mn. At 1000 °C, the◦ Mn phase formed is hausmannite, which is written as Mn2+2+Mn233++O422−, and its crystal structure the Mn phase formed is hausmannite, which is written as Mn 2+Mn23+ O24− −, and its crystal structure Mnarrangement phase formed is similar is hausmanni to a tetragonallyte, which deformed is written spinel as [32]. Mn Mn2 O4 , and its crystal structure arrangement isis similarsimilar toto aa tetragonallytetragonally deformeddeformed spinelspinel [[32].32].

Δ Δ = Cu1,5Mn1,5O4 CuMn 600 Δ Δ ∇ = Cu1,5Mn1,5O4 CuMn CuMn 600 1000 = Cu1,4Mn1,6O4 ∇ CuMn CuMn2 1000 600 ♦ = = Cu CuO1,4Mn1,6O4 CuMn2 CuMn2 600 1000 ♦  == CuOMn O 2 3 CuMn2 1000 ♠ = = Mn Mn2O3O3 4 ♠ = Mn3O4 Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ Δ ∇ ∇ Δ ∇ ∇♦ ∇ ∇ Δ intensity(a.u.) ∇ ∇ ∇ ∇ ♦ ∇♦ ∇ ♦ ∇ ♦ ♦ ♦ intensity(a.u.) ∇ Δ ♦ ♦ ♦ ♦ ♦  Δ Δ Δ Δ Δ Δ Δ Δ  Δ Δ   Δ Δ Δ   Δ  Δ Δ∇ ∇ΔΔ ∇ Δ  ∇ ∇ Δ  ∇ ∇    Δ ♠ ♠ ∇ ∇ ♠∇ ♠∇ ♠ ∇ ♠ ∇ ♠ ♠∇ ♠ ♠ ♠ ♠∇ ∇ ♠ ♠ ♠ ♠ ♠ ♠ ♠ ♠

10 20 30 40 50 60 70 10 20 302 40θ (°) 50 60 70 2θ (°) Figure 3. XRD of CuMn and CuMn2 calcined at 600 and 1000 C. Figure 3. XRD of CuMn and CuMn2 calcined at 600 and 1000◦ °C. Figure 3. XRD of CuMn and CuMn2 calcined at 600 and 1000 °C.

Catalysts 2020, 10, x 5 of 20 Catalysts 2020, 10, 1344 5 of 20 Catalysts 2020, 10, x 5 of 20

9000 CuMn 600 CuMn 1000 8000 9000 CuMn CuMn2 600 600 7000 CuMn CuMn2 1000 1000 8000 CuMn2 600 6000 7000 CuMn2 1000

5000 6000

4000 5000 intensity(a.u.) 3000 4000

intensity(a.u.) 2000 3000

1000 2000

0 1000 55 56 57 58 59 60 61 62 63 64 65 0 2θ (°) 55 56 57 58 59 60 61 62 63 64 65 2θ (°) Figure 4. Detail of the XRD of CuMn and CuMn2 calcined at 600 and 1000 °C. Figure 4. Detail of the XRD of CuMn and CuMn2 calcined at 600 and 1000 C. Figure 4. Detail of the XRD of CuMn and CuMn2 calcined at 600 and 1000 ◦°C. Raman spectroscopy is used to identify the phases that are not distinguished from XRD analysis. Raman spectroscopy is used to identify the phases that are not distinguished from XRD analysis. The Ramanspectra spectroscopythat are presented is used here to identify are obtained the phas by esfocusing that are on not areas distinguished where CuO from is less XRD present, analysis. so The spectra that are presented here are obtained by focusing on areas where CuO is less present, so Thethat spectra the spectra that wouldare presented mostly displayhere are the obtained other phases. by focusing on areas where CuO is less present, so that the spectra would mostly display the other phases. that theFigure spectra 5 shows would the mostly Raman display spectra the of other CuAl phases. 600 and CuAl 1000. From the microscopy images, Figure5 shows the Raman spectra of CuAl 600 and CuAl 1000. From the microscopy images, greyFigure areas with5 shows black the spots Raman were spectra visible. of For CuAl this 600 reason, and theCuAl beam 1000. was From focused the microscopy on the light images, colored grey areas with black spots were visible. For this reason, the beam was focused on the light colored greyareas, areas where with the black typical spots signals were of visible. CuO are For very this low. reason, For theCuO, beam only was three focused optical on phonon the light modes colored (Ag areas, where the typical signals of CuO are very low. For CuO, only three optical phonon modes + 2 Bg) out of nine (4Au + 5Bu + Ag + 2Bg) are Raman active. These are found at 288 cm−1 (Ag), 330 and areas, where the typical signals of CuO are very low. For CuO, only three optical phonon modes (Ag (A + 2 B ) out of nine (4A + 5B + A + 2B ) are Raman active. These are found at 288 cm 1 (A ), 621g cm−1 g(Bg), with a weaku intensityu ing the presentedg spectra [33]. The remaining bands match− veryg + 2 Bg) out of nine (4Au + 5Bu + Ag + 2Bg) are Raman active. These are found at 288 cm−1 (Ag), 330 and 330 and 621 cm 1 (B ), with a weak intensity in the presented spectra [33]. The remaining bands match well with those− predictedg for γ-Al2O3 by Liu et al. [34]. In the mentioned study, the phonon 621 cm−1 (Bg), with a weak intensity in the presented spectra [33]. The remaining bands match very very well with those predicted for γ-Al O by Liu et al. [34]. In the mentioned study, the phonon frequencies were calculated for a spinel2 (as3 γ-Al2O3 is usually described as a defective spinel) and a well with those predicted for γ-Al2O3 by Liu et al. [34]. In the mentioned study, the phonon frequencies were calculated for a spinel (as γ-Al O is usually described as a defective spinel) and a non-spinel model, based on the dehydration of 2boehmite3 γ-Al2O3. There are 57 Raman active modes frequencies were calculated for a spinel (as γ-Al2O3 is usually described as a defective spinel) and a non-spinel model, based on the dehydration of boehmite γ-Al O . There are 57 Raman active modes for the spinel model: 34 Ag and 23 Bg, while for the non-spinel2 mode3 there are 60 (34 Ag and 26 Bg). non-spinel model, based on the dehydration of boehmite γ-Al2O3. There are 57 Raman active modes forThe the peaks spinel in model:the spectra 34 A atg 210,and 370, 23 B g410,, while 490, for560, the 640 non-spinel and 670 cm mode−1 match there well are with 60 (34 the A gmostand intense 26 Bg). for the spinel model: 34 Ag and 23 Bg, while for the non-spinel mode1 there are 60 (34 Ag and 26 Bg). The peaks in the spectra at 210, 370, 410, 490, 560, 640 and 670 cm− match well with the most intense Thepeaks peaks observed in the inspectra the theoretical at 210, 370, study 410, for490, the 560, spin 640el andmodel, 670 whilecm−1 match the relative well with intensity the most and intense Raman peaksshift of observed the peaks in at the 740 theoretical and 810 cm study−1 match for the rather spinel with model, those while of the the non-spinel relative intensity model. and Raman peaks observed in the theoretical study1 for the spinel model, while the relative intensity and Raman shift of the peaks at 740 and 810 cm− match rather with those of the non-spinel model. shift of the peaks at 740 and 810 cm−1 match rather with those of the non-spinel model.

γ y = -Al2O3 y CuAl 600 t = CuO CuAl 1000 y = γ-Al O s = CuAl2 23O4 y y CuAl 600 t = CuO y y CuAl 1000 s = CuAl2O4 yt y y y y y y t y t y y s y s s y t y s intensity (a.u.) s s s s s intensity (a.u.) s s s t t s s 100 200t 300t 400 500 600 700 800 900 Raman shift (cm-1) 100 200 300 400 500 600 700 800 900 Figure 5. Raman spectraRaman of shift CuAl (cm-1 600) and CuAl 1000. Figure 5. Raman spectra of CuAl 600 and CuAl 1000. On the other hand, theFigure Raman 5. spectrumRaman spectra of CuAl of CuAl 1000 600 is completely and CuAl 1000. different from that of CuAl 600. 1 Again,On the the signals other ofhand, CuO the are Raman not very spectrum intense atof 288CuAl and 100 3300 is cm completely− . All the different other bands from represent that of CuAl the −1 contribution600. OnAgain, the otherthe of CuAlsignals hand,2O 4of the. AccordingCuO Raman are not spectrum to very literature, intense of CuAl only at 288100 five 0and modesis completely 330 cancm be. All observeddifferent the other from (A bands1g +thatE grepresent of+ 3FCuAl2g). 1 2 4 1 1 1g g 2g The600.the bandcontributionAgain, at the 790 signals cm of −CuAlcan of CuOO be. assignedAccording are not tovery to A 1gliterature, intense, at 270, at 590,only 288 710 andfive cm 330modes− cmto − Fcan1.2g All andbe the observed 475 other cm −bands (AtoE + grepresent E[35 + –3F37].). The band at 790 cm−1 can be assigned to A1g, at 270, 590, 710 cm−1 to F2g and 475 cm−1 to Eg [35–37]. the contributionFor CuFe 600, of theCuAl Raman2O4. According modes (Figure to literature,6) are clearly only assignable five modes to can hematite be observed (Fe 2O3 (A). This1g + E materialg + 3F2g). For CuFe 600, the Raman modes (Figure 6) are clearly assignable to hematite (Fe2O3). This1 isThe characterized band at 790 cm by− seven1 can be Raman assigned active to A modes,1g, at 270, (I) 590, two 710 A1g cmmodes−1 to F are2g and found 475 cm at 220−1 toand Eg [35–37]. 490 cm − , material is characterized by seven Raman active modes, (I) two A1g modes are found at 220 and 490 For CuFe 600, the Raman modes (Figure 6) are clearly assignable to hematite (Fe2O3). This material is characterized by seven Raman active modes, (I) two A1g modes are found at 220 and 490

Catalysts 2020, 10, x 6 of 20 Catalysts 2020, 10, 1344 6 of 20 Catalysts 2020, 10, x 6 of 20 cm−1, (II) three of the five Eg modes are found at 290, 400 and 600 cm−1, while (III) the remaining two −1 are−1 very weak in intensity and are normally found at 250 and 300 cm−1 [38]. cm , (II) three of the five Eg modes are found at 290, 400 and 6001 cm , while (III) the remaining two (II) three of the five Eg modes are found at 290, 400 and 600 cm− , while (III) the remaining two are −1 are very weak in intensity and are normally found at 250 and 300 1cm [38]. very weak in intensity and are normally found at 250 and 300 cm− [38]. h = Fe2O3 CuFe 600 CuFe 1000 s = CuFe2O4 h h = Fe2O3 CuFe 600 h CuFe 1000 s = CuFe2O4 h h h

h h h h intensity (a.u.) h s s s

intensity (a.u.) s s s s s s s s s

100 200 300 400 500 600 700 800 900 Raman shift (cm-1) 100 200 300 400 500 600 700 800 900 Raman shift (cm-1) Figure 6. Raman spectra of CuFe 600 and CuFe 1000. Figure 6. Figure 6. Raman spectra ofof CuFeCuFe 600600 andand CuFeCuFe 1000.1000. The bands of CuFe 1000 are very broad which makes it difficult to distinct between the separate The bands of CuFe 1000 are very broad which makes it difficult to distinct between the separate structures’The bands modes. of CuFe The 1000typical are bands very broadof the whichcubic comakespper itferrite difficult structure to distinct can betweenstill be distinguished. the separate structures’ modes. The typical bands of the cubic copper ferrite structure can still be distinguished. structures’There are five modes. Raman The active typical modes bands (A of1g the+ E gcubic + 3Fg )co forpper cubic ferrite copper structure ferrite. can Bands still at be 220, distinguished. 460 and 560 Therecm−1 can are be five assigned Raman activeto F2g modes, modes while (A1g + theEg band+ 3Fg at) for 280 cubic and 660 copper cm−1 ferrite. can be Bandsassigned at 220,to the 460 Eg and and There are five Raman active modes (A1g + Eg + 3Fg) for cubic copper ferrite. Bands at 220, 460 and 560 1 −1 1 560A1g−cm1 band− can [39,40]. be assigned Another to band F2g modes, can be found while thebetween band at300 280 and and 400 660 cm− cm1 . − can be assigned to the Eg cm can be assigned to F2g modes, while the band at 280 and 660 cm can be1 assigned to the Eg and and AFinally,1g band in [39 Figure,40]. Another 7 the Raman band canspectra be found of the between manganese 300 containing and 400 cm compounds− . are presented. A1g band [39,40]. Another band can be found between 300 and 400 cm−1. Finally, in Figure7 the Raman spectra of the manganese containing compounds are presented. SimilarFinally, to the in previous Figure 7 spinelthe Raman compound, spectra fiveof the Raman manganese active modescontaining (A1g compounds+ Eg + 3F2g) areare expectedpresented. to Similarappear. to The the band previous at 650 spinel cm−1 compound,can again be five assigned Raman to active A1g and modes at 430 (A cm1g +−1 Etog E+g.3F Around2g) are expected350, 470 and to Similar to the previous spinel1 compound, five Raman active modes (A1g + E1 g + 3F2g) are expected to appear.580 cm− The1 theband F2g modes at 650 are cm found− can [41]. again The be Raman assigned active to A optical1g and modes at 430 cmof Mn− to2O E3 g(bixbyite). Around are 350, found 470 appear. The band at 650 cm−1 can again be assigned to A1g and at 430 cm−1 to Eg. Around 350, 470 and 1 −1 −1 andat 309, 580− 1500 cm− andthe 640 F2g cmmodes which are foundis close [41 to]. those The Ramanof CuO active (280, 330 optical and modes 621 cm of) Mn [42].2O 3For(bixbyite) this reason, are 580 cm the F2g modes are found1 [41]. The Raman active optical modes of Mn2O3 (bixbyite)1 are found found at 309, 500 and 640−1 cm which is close to those of CuO (280, 330 and 621 cm )[42]. For this atonly 309, the 500 mode and at640 500 cm cm−1 which is −definite is close evidence to those for of theCuO presence (280, 330 of andMn2 O6213. The cm− Raman1) [42].− For active this modes reason, of 1 −1 reason,Mn3O4 only(hausmannite) the mode at(310,− 5001 365, cm− 472,isdefinite 650 cm evidence) are in this for case the presence also difficult of Mn to2 Odistinguish3. The Raman from active those only the mode at 500 cm is definite evidence for the presence1 of Mn2O3. The Raman active modes of modesof the copper of Mn3 Omanganite4 (hausmannite) spinel structure (310, 365, as 472, this 650 mate cmrial− ) also are inhas this the case spinel also structure difficult and to distinguish most of the Mn3O4 (hausmannite) (310, 365, 472, 650 cm−1) are in this case also difficult to distinguish from those from those of the copper manganite spinel structure as this material also has the spinel structure and ofbands the copper overlap. manganite spinel structure as this material also has the spinel structure and most of the most of the bands overlap. bands overlap.

b = Mn2O3 s CuMn 600 CuMn 1000 h = Mn3O4 s b = Mn O s CuMn CuMn2 600 600 t = CuO2 3 s CuMn CuMn2 1000 1000 hs = = Mn Cu3OMn4 O s s s x 3-x 4 s CuMn2 600 t = CuO s s s s s CuMn2 1000 s = CuxMn3-xO4 s t s s b s t s b

intensity(a.u.) t b t b s

intensity(a.u.) t b t b s s h s s s s h s 100 200 300 400 500 600 700 800 900 Raman shift (cm-1) 100 200 300 400 500 600 700 800 900

Figure 7. Raman spectra of CuMnRaman 600, CuMnshift (cm-1 1000,) CuMn2 600 and CuMn 1000. Figure 7. Raman spectra of CuMn 600, CuMn 1000, CuMn2 600 and CuMn 1000. H2-TPR providesFigure 7. informationRaman spectra about of CuMn the reducibility600, CuMn 1000, of the CuMn2 catalysts 600 and and CuMn the technique 1000. is used to compareH2-TPR thedi providesfferent samples. information The about reduction the reducibility profiles are of represented the catalysts as and a function the technique of temperature is used to compare the different samples. The reduction profiles are represented as a function of temperature (FigureH28-TPR). The provides profile of information bulk CuO isabout added the as reducibility a reference of material. the catalysts For CuO, and the the technique onset of reduction is used to compare the different samples. The reduction profiles are represented as a function of temperature

Catalysts 2020, 10, x 7 of 20 Catalysts 2020, 10, 1344 7 of 20 (Figure 8). The profile of bulk CuO is added as a reference material. For CuO, the onset of reduction appears around 275 °C with the maximum being at 330 °C. The peaks for the as-prepared materials appears around 275 ◦C with the maximum being at 330 ◦C. The peaks for the as-prepared materials are in all the cases very broad, which makes it difficult to conclude any statements on speciation. One are in all the cases very broad, which makes it difficult to conclude any statements on speciation. noticeable observation about these results is that all the materials calcined at 600 °C have their onset One noticeable observation about these results is that all the materials calcined at 600 ◦C have their temperatures at significantly lower temperatures than bulk CuO, which means that the CuO particles onset temperatures at significantly lower temperatures than bulk CuO, which means that the CuO are very small and in a dispersed state in the samples or have become more reducible, due to particles are very small and in a dispersed state in the samples or have become more reducible, due to interactions with the other metal oxide [6,13,43–45]. The lowest onset temperature can be found for interactions with the other metal oxide [6,13,43–45]. The lowest onset temperature can be found for CuMn 600 although all the synthesized materials show a peak under 250 °C, probably due to highly CuMn 600 although all the synthesized materials show a peak under 250 ◦C, probably due to highly dispersed CuO present on the other oxide but not bound to it. Both CuMn 600 and CuMn2 600 already dispersed CuO present on the other oxide but not bound to it. Both CuMn 600 and CuMn2 600 already 2+ show reduction above 350 °C due to Cu2+ in the spinel phase. After the calcination at 1000 °C, the show reduction above 350 ◦C due to Cu in the spinel phase. After the calcination at 1000 ◦C, the difference between the materials becomes larger with the onset temperature being under 350 °C for difference between the materials becomes larger with the onset temperature being under 350 ◦C for CuFe 1000 and CuAl 1000 while it’s closer to 400 °C for CuMn2 1000. In this case, the difference is CuFe 1000 and CuAl 1000 while it’s closer to 400 ◦C for CuMn2 1000. In this case, the difference is probably due to the fact that CuO is reduced easier than Cu in the spinel structure. In all the cases, probably due to the fact that CuO is reduced easier than Cu in the spinel structure. In all the cases, 2+ there is a more than 100 °C shift between the calcination at 600 °C and 1000 °C, proving that Cu2+ in there is a more than 100 ◦C shift between the calcination at 600 ◦C and 1000 ◦C, proving that Cu in the spinel structure is less reducible when the crystallinity is increased for this mixed oxide structure. the spinel structure is less reducible when the crystallinity is increased for this mixed oxide structure.

CuAl 600 CuFe 600 CuMn 600 CuMn2 600 CuAl 1000 CuFe 1000 CuMn 1000 CuMn2 1000 CuO signal (a.u.) signal

100 200 300 400 500 600 700 800 900 1000 temperature (°C)

Figure 8. H2-TPR of the synthesized materials and CuO. Figure 8. H2-TPR of the synthesized materials and CuO. 2.2. Catalytic Activity 2.2. Catalytic Activity In this research work, the catalytic performance test is done with an O2 content much lower than In this research work, the catalytic performance test is done with an O2 content much lower than stoichiometrically necessary to oxidize all the CO. In this way, the O2 conversion is used to compare the stoichiometrically necessary to oxidize all the CO. In this way, the O2 conversion is used to compare ability of the catalysts to oxidize CO with O2 (Reaction 1), while at the same time, the NO conversion is usedthe ability to compare of the the catalysts ability to to reduce oxidize NO CO with with CO (ReactionO2 (Reaction 2). Figures1), while9–11 at present the same the CO,time, NO the and NO conversion is used to compare the ability to reduce NO with CO (Reaction 2). Figures 9–11 present O2 conversion for each of the catalysts calcined at 600 ◦C on the left and calcined at 1000 ◦C on the the CO, NO and O2 conversion for each of the catalysts calcined at 600 °C on the left and calcined at right side. Figure 12 shows the conversions for the pure oxides CuO and MnO2, while Table2 presents 1000 °C on the right side. Figure 12 shows the conversions for the pure oxides CuO and MnO2, while the light-off temperatures (onset temperature of significant catalytic activity) [46]. In case of O2, we Table 2 presents the light-off temperatures (onset temperature of significant catalytic activity) [46]. In take the temperature at which 50% is converted (T50 O2), which is typically used, and for NO the case of O2, we take the temperature at which 50% is converted (T50 O2), which is typically used, and temperature at which 20% NO is converted (T20 NO), as 50% is almost never achieved in the current for NO the temperature at which 20% NO is converted (T20 NO), as 50% is almost never achieved in experiments. Moreover, Table2 depicts the reaction rates for CO at 200 ◦C and for NO at 550 ◦C, in termsthe current of mol experiments. of the reactant Moreover, converted Table per 2 second, depicts forthe areaction gram of rates catalyst. for CO As at most200 °C of and the for catalytic NO at reactions550 °C, in take terms place of atmol the of surface the reactant of a catalyst, converted one mustper second, take into for account a gram theof catalyst. corresponding As most surface of the area,catalytic in order reactions to accurately take place assess at the the surface performance. of a catalyst, For thisone reason,must take Table into2 accountalso lists the the corresponding BET specific surface area, in order to accurately assess the performance. For this reason, Table 2 also lists the BET surface area and specific reaction rate at 200 ◦C for CO and at 550 ◦C for NO. The specific reaction rate describesspecific surface the converted area and amount specific of reactantreaction perrate second at 200 per °C mfor2 ofCO catalyst and at surface 550 °C area. for NO. The The supporting specific 2 informationreaction rate includes describes the the graphs converted of the specificamount reaction of reactant rates per (Supplementary second per m Figures of catalyst S9 and surface S10), area. as a functionThe supporting of temperature, information in the includes regions the where graphs it is of not th limitede specific by reaction the reactants. rates (Supplementary Due to the low surfaceFigures S9 and S10), as a function of temperature, in the regions where it is not limited by the reactants. Due

Catalysts 2020, 10, x 8 of 20 Catalysts 2020, 10, 1344 8 of 20 Catalysts 2020, 10, x 8 of 20 to the low surface area of the catalysts calcined at 1000 °C (even under 1 m2/g), the calculated rates 2 areto theincredibly low surface large area and of extremely the catalyst sensitives calcined to at experimental 1000 °C2 (even errors, under compared 1 m /g), the to calculated the materials rates area of the catalysts calcined at 1000 ◦C (even under 1 m /g), the calculated rates are incredibly large calcinedare incredibly at 600 °C.large Taking and extremelythis into account,sensitive it tois experimentalvery likely that errors, the performancecompared to per the m materials2 of the and extremely sensitive to experimental errors, compared to the materials calcined at 600 C. Taking materialscalcined calcinedat 600 °C. at 1000Taking °C isthis overestimated, into account, leading it is very to the likely conclusion that the that performance it is possible per more m◦2 layersof the this into account, it is very likely that the performance per m2 of the materials calcined at 1000 C is ofmaterials the material calcined than at just1000 the °C issurface overestimated, are involved leading in tothe the reaction. conclusion For thatthis it reason, is possible the morecalculated layers◦ overestimated, leading to the conclusion that it is possible more layers of the material than just the turnoverof the material frequencies than based just the on thesurface total are mol involved of Cu present in the in reaction. each sample For this(Table reason, 2) provide the calculated a better surfaceviewturnover of are the involvedfrequencies results. inThe based the corresponding reaction. on the total For thismolgraphs reason,of Cu are present thealso calculated presented in each sample turnoverin the (Tablesupporting frequencies 2) provide information based a better on the total(Supplementaryview mol of of the Cu results. present Figures The in S11 each corresponding and sample S12). (Table graphs2) provide are also a better presented view ofin the the results. supporting The corresponding information graphs(Supplementary are also presented Figures inS11 the and supporting S12). information (Supplementary Figures S11 and S12). 100 100 CuAl 1000 CuFe 1000 100 100 CuAl 1000 CuMn 1000 80 80 CuFe 1000 CuMn2 1000 80 80 CuMn 1000 60 60 CuMn2 1000 60 60 40 40 CuAl 600 40 CuFe 600 40 CO conversion (%) conversion CO CuAl 600 (%) conversion CO 20 CuMn 600 20 CuFe 600 CO conversion (%) conversion CO 20 CuMn2 600 (%) conversion CO 20 0 CuMn 600 0 CuMn2 600 00 100 200 300 400 500 600 00 100 200 300 400 500 600 Temperature (°C) Temperature (°C) 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temperature (°C) Temperature (°C) Figure 9. CO conversion of the materials calcined at 600 °C (left) and 1000 °C (right). FigureFigure 9. 9.CO CO conversion conversion of of the the materialsmaterials calcinedcalcined at 600 °C◦C (left) (left) and and 1000 1000 °C◦ C(right). (right). 100 CuAl 600 100 CuAl 1000 CuFe 600 CuFe 1000 100 CuAl 600 100 CuAl 1000 CuMn 600 CuMn 1000 80 CuFe 600 80 CuFe 1000 CuMn2 600 CuMn2 1000 80 CuMn 600 80 CuMn 1000 60 CuMn2 600 60 CuMn2 1000 60 60 40 40 40 40 NO conversion (%) conversion NO 20 (%) conversion NO 20

NO conversion (%) conversion NO 20 (%) conversion NO 20 0 0 00 100 200 300 400 500 600 00 100 200 300 400 500 600 Temperature (°C) Temperature (°C 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temperature (°C) Temperature (°C Figure 10. NO conversion of the materials calcined at 600 °C (left) and 1000 °C (right). FigureFigure 10. 10.NO NO conversion conversion of of the the materials materials calcined calcined atat 600600 ◦°CC (left) and and 1000 1000 °C◦C (right). (right). Figure 10. NO conversion of the materials calcined at 600 °C (left) and 1000 °C (right). 100 100 CuAl 1000 CuFe 1000 100 100 CuAl 1000 CuMn 1000 80 80 CuFe 1000 CuMn2 1000 80 80 CuMn 1000 60 60 CuMn2 1000 60 60 40 40

conversion (%) CuAl 600 conversion (%) 2 40 2 40 O CuFe 600 O

conversion (%) 20 CuAl 600 conversion (%) 20 2 CuMn 600 2

O CuFe 600 O 20 CuMn2 600 20 0 CuMn 600 0 CuMn2 600 00 100 200 300 400 500 600 00 100 200 300 400 500 600 Temperature (°C) Temperature (°C) 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Catalysts 2020, 10, x Temperature (°C) Temperature (°C) 9 of 20 FigureFigure 11. 11.O O22 conversionconversion of the the materials materials calcined calcined at at 600 600 °C◦ C(left) (left) and and 1000 1000 °C ◦(right).C (right). Figure 11. O2 conversion of the materials calcined at 600 °C (left) and 1000 °C (right). 100 CuO CO CuO NO

CuO O2 80 MnO2 CO

MnO2 NO MnO O 60 2 2

40 conversion (%) conversion

20

0

0 100 200 300 400 500 600 Temperature (°C)

Figure 12. CO, NO and O2 conversion for CuO and MnO2. Figure 12. CO, NO and O2 conversion for CuO and MnO2.

Table 2. Temperatures of 50% O2 (T50 O2) conversion and 20% NO conversion (T20 NO), BET specific

surface area (SSA), seaction rates (RR), specific reaction rates (SRR) and turnover frequencies (TOF).

- T50 O2 T20 NO SSA RRCO Spec. RRCO TOFCO RRNO Spec. RRNO TOFNO - - - - at 200 °C at 200 °C at 200 °C at 550 °C at 550 °C at 550 °C

- (°C) (°C) (m2/g) (mol. s−1. gcat−1) (mol. s−1 (m2)−1) (h−1) (mol. s−1. gcat−1) (mol. s−1 (m2)−1) (h−1) CuAl 600 178 390 129 1.56 × 10−6 1.21 × 10−8 1.21 1.97 × 10−6 1.53 × 10−6 0.152 CuFe 600 217 384 49 5.71 × 10−7 1.17 × 10−8 0.82 2.88 × 10−6 5.90 × 10−6 0.413 CuMn 600 140 377 36 1.32 × 10−6 3.61 × 10−8 1.12 3.23 × 10−6 8.86 × 10−6 0.276 CuMn2 600 141 412 47 1.24 × 10−6 2.63 × 10−8 1.76 2.35 × 10−6 4.98 × 10−6 0.334 CuAl 1000 221 543 3 4.60 × 10−7 1.32 × 10−7 0.36 1.61 × 10−6 4.63 × 10−6 0.125 CuFe 1000 293 483 1 7.44 × 10−8 1.11 × 10−7 0.11 2.15 × 10−6 3.21 × 10−6 0.308 CuMn 1000 370 - 0.2 0 0 0.00 8.35 × 10−6 4.72 × 10−6 0.007 CuMn2 1000 516 - 0.1 4.20 × 10−9 3.02 × 10−8 0.01 7.85 × 10−6 5.64 × 10−6 0.011 CuO 182 - 1 1.55 × 10−6 1.69 × 10−6 0.44 0 0 0.000 MnO2 360 - 4 4.41 × 10−8 1.06 × 10−8 0.01 2.09 × 10−6 5.04 × 10−6 0.007

From these results it can be seen that pure CuO is already able to oxidize CO with O2 at rather low temperatures (T50 O2 = 182 °C). Also MnO2, supposedly the most active manganese oxide for CO oxidation [47,48], manages to do this although at a much higher temperature (T50 O2 = 360 °C). None of these simple oxides is however able to reduce NO in this catalytic test. The as-prepared materials calcined at 600 °C show an excellent catalytic performance able to convert CO with O2 at even lower 1 temperatures than CuO, with the exception of CuFe 600 (T50 O2 = 217 °C). Moreover, all these as-

prepared materials are able to convert also NO, in contrast to the reference oxides CuO and MnO2. It is surprising that the performance of NO conversion is very much alike in all cases. The CuAl 600 catalyst stands out, due to the appearance of two volcano plots at low temperatures, under 350 °C. This takes place because N2O is selectively formed through Reaction 3 instead of Reaction 2 [49,50]. Furthermore, CuAl 600 has a much higher surface area than the other catalysts, which can result in to an advantage over the other materials. To compensate for this effect, one can compare the specific reaction rates for each catalyst. The values (Table 2) show that the performance of CuAl 600 for CO oxidation is rather comparable with that of CuFe 600 instead of CuMn 600 and CuMn2 600. At the same time, its NO performance is lower than the other samples at the higher temperature region. This is even more clear from the TOFNO.

2 CO + O → 2 CO (1)

2 CO + 2 NO → 2 CO +N (2)

CO + 2 NO → CO +NO (3)

The materials calcined at 1000 °C display much more variations in their catalytic performances. In all the cases, the O2 conversion takes place at higher temperatures than the reference material CuO. CuAl 1000 shows the lowest T50 O2, followed by CuFe 1000, CuMn 1000 and CuMn2 1000. After calcination at 1000 °C NO conversion is not detected for CuMn 1000 and CuMn2 1000. On the contrary, CuAl 1000 and CuFe 1000 still manage to convert 26 and 32% at 600 °C, which is still higher

Catalysts 2020, 10, 1344 9 of 20

Table 2. Temperatures of 50% O2 (T50 O2) conversion and 20% NO conversion (T20 NO), BET specific surface area (SSA), seaction rates (RR), specific reaction rates (SRR) and turnover frequencies (TOF).

-T50 O2 T20 NO SSA RRCO Spec. RRCO TOFCO RRNO Spec. RRNO TOFNO

- - - - at 200 ◦C at 200 ◦C at 200 ◦C at 550 ◦C at 550 ◦C at 550 ◦C 2 1 1 1 2 1 1 1 1 1 2 1 1 -(◦C) (◦C) (m /g) (mol. s− . gcat− ) (mol. s− (m )− ) (h− ) (mol. s− . gcat− ) (mol. s− (m )− ) (h− ) CuAl 600 178 390 129 1.56 10 6 1.21 10 8 1.21 1.97 10 6 1.53 10 6 0.152 × − × − × − × − CuFe 600 217 384 49 5.71 10 7 1.17 10 8 0.82 2.88 10 6 5.90 10 6 0.413 × − × − × − × − CuMn 600 140 377 36 1.32 10 6 3.61 10 8 1.12 3.23 10 6 8.86 10 6 0.276 × − × − × − × − CuMn2 600 141 412 47 1.24 10 6 2.63 10 8 1.76 2.35 10 6 4.98 10 6 0.334 × − × − × − × − CuAl 1000 221 543 3 4.60 10 7 1.32 10 7 0.36 1.61 10 6 4.63 10 6 0.125 × − × − × − × − CuFe 1000 293 483 1 7.44 10 8 1.11 10 7 0.11 2.15 10 6 3.21 10 6 0.308 × − × − × − × − CuMn 1000 370 - 0.2 0 0 0.00 8.35 10 6 4.72 10 6 0.007 × − × − CuMn2 1000 516 - 0.1 4.20 10 9 3.02 10 8 0.01 7.85 10 6 5.64 10 6 0.011 × − × − × − × − CuO 182 - 1 1.55 10 6 1.69 10 6 0.44 0 0 0.000 × − × − MnO2 360 - 4 4.41 10 8 1.06 10 8 0.01 2.09 10 6 5.04 10 6 0.007 × − × − × − × − Catalysts 2020, 10, 1344 10 of 20

From these results it can be seen that pure CuO is already able to oxidize CO with O2 at rather low temperatures (T50 O2 = 182 ◦C). Also MnO2, supposedly the most active manganese oxide for CO oxidation [47,48], manages to do this although at a much higher temperature (T50 O2 = 360 ◦C). None of these simple oxides is however able to reduce NO in this catalytic test. The as-prepared materials calcined at 600 ◦C show an excellent catalytic performance able to convert CO with O2 at even lower temperatures than CuO, with the exception of CuFe 600 (T50 O2 = 217 ◦C). Moreover, all these as-prepared materials are able to convert also NO, in contrast to the reference oxides CuO and MnO2. It is surprising that the performance of NO conversion is very much alike in all cases. The CuAl 600 catalyst stands out, due to the appearance of two volcano plots at low temperatures, under 350 ◦C. This takes place because N2O is selectively formed through Reaction 3 instead of Reaction 2 [49,50]. Furthermore, CuAl 600 has a much higher surface area than the other catalysts, which can result in to an advantage over the other materials. To compensate for this effect, one can compare the specific reaction rates for each catalyst. The values (Table2) show that the performance of CuAl 600 for CO oxidation is rather comparable with that of CuFe 600 instead of CuMn 600 and CuMn2 600. At the same time, its NO performance is lower than the other samples at the higher temperature region. This is even more clear from the TOFNO.

2 CO + O 2 CO (1) 2 → 2 2 CO + 2 NO 2 CO + N (2) → 2 2 CO + 2 NO CO + N O (3) → 2 2 The materials calcined at 1000 ◦C display much more variations in their catalytic performances. In all the cases, the O2 conversion takes place at higher temperatures than the reference material CuO. CuAl 1000 shows the lowest T50 O2, followed by CuFe 1000, CuMn 1000 and CuMn2 1000. After calcination at 1000 ◦C NO conversion is not detected for CuMn 1000 and CuMn2 1000. On the contrary, CuAl 1000 and CuFe 1000 still manage to convert 26 and 32% at 600 ◦C, which is still higher than the yield achieved for pure CuO. Moreover, all the materials have their surface area reduced by more than 97% after the calcination at 1000 ◦C, which brings it to a comparable level with the pure CuO sample. As an exception, only CuAl 1000 manages to keep a surface area above 1 m2/g, compared to all other samples calcined at 1000 ◦C.

3. Discussion

The characterization study of the catalysts shows that after the calcination at 600 ◦C, only the manganese containing materials featured crystals of a copper containing spinel structure. In the case of CuAl 600, the Raman study confirmed that this sample contains γ-Al2O3, which is also described as having a defective spinel structure. Certain bands in the Raman spectrum imply not only the formation of a spinel type alumina, but also of non-spinel type alumina. The calcination at 1000 ◦C is forcing copper into the alumina structure, forming a CuAl2O4 spinel structure. A large amount of copper remains outside of this structure in the form of CuO (as SEM-EDX pointed out), since the amount of aluminum is lower than stoichiometrically required to form spinel. Part of the Al is washed away during the washing step, in the form of the soluble Al(OH)4−, which is the dominant species at high pH [51]. This is the reason why the synthesis method is adapted in this case, so that the pH is kept as low as possible, while still being able to precipitate the Cu species. However, as the results show, part of the Al is inevitably washed away. In CuFe 600 the CuFe2O4 structure is not crystallized but magnetite (Fe3O4), having an inverse spinel structure, is present [52]. Just like in the case of alumina, the magnetite will incorporate copper in its structure at higher temperatures, to form CuFe2O4. Because Fe3O4 and CuFe2O4 are very alike in structure, it is possible that CuFe 1000 still contains some Fe3O4, which explains the extra CuO phase. Catalysts 2020, 10, 1344 11 of 20

In the Cu-Mn materials the spinel-like phase matches with the structure that is reported as Cu1.5Mn1.5O4 instead of CuMn2O4. This structure has a simple cubic system (P 41 3 2), but after calcination at 1000 ◦C the structure shifts towards the familiar face-centered cubic spinel structure (Fd3m). Many literature reports state that pure CuMn2O4 samples could not be prepared [19,21,30,53]. Rapid quenching from sinter temperature results in a tetragonal structure, while slowly cooling, as is the case here, leads to cubic spinel phases contaminated with extra phases [19,21]. CuMn 1000 shows an increase in the size of CuO which could also be evidence that the copper manganite is eliminating copper to transform from Cu1.5Mn1.5O4 towards CuMn2O4. It is not surprising that at 600 ◦C highly crystalline phases of spinel are not present, as often very high temperatures (above 1000 ◦C) with very long sintering times (e.g., 48 h) are used [19,54,55]. In this study, the calcination temperatures are chosen to pursue the conditions of the automotive catalyst, under normal conditions, between 400 and 600 ◦C and about 900 ◦C for high power operations, which are normally only met briefly [4]. The Raman spectra of CuAl 1000, CuFe 1000 and CuMn 1000 (Figures5–7) confirm the formation of spinel structures with the apparition of broad bands in the specific regions. The broadening pattern of the bands could possibly be due to defects in the structure, which leads to deviations from the perfectly cubic structure. Another reason for the broad bands’ formation can be correlated with the modification of the inversion degree in the spinel structure. This fact is strictly depending on the preference of the cations to occupy one of the two sites, with regards to the synthesis method and calcination temperature. As a result, if the Cu2+ and Fe3+ cations are occupying both the tetrahedral 1 sites, the Raman mode around 700 cm− characteristic to A1g symmetry of the tetrahedral site is split in two different energy bands, due to the different atomic masses of the cations [56]. Bands broadening in Raman spectroscopy of solids is often associated with the materials being amorphous or containing very small crystals [57]. However, here it is not the case, since one would expect a higher crystallinity after the thermal treatment at 1000 ◦C compared to 600 ◦C. In the spectra of CuFe 1000 (Figure6), most of the bands are attributed to the cubic copper ferrite structure, while other do not match this structure. Chatterjee et al. report the appearance of some extra 1 1 bands at 388 cm− together with a weak band at 625 cm− , which is likely also the case here. In their study, the reason for the extra bands is related to the exciting radiation of the Raman laser (532 nm), being larger than the particle diameter [40]. 1 For all the materials, the presence of maxima under 200 cm− is still under debate. Lazarevic et al. suggest that translational movement of the whole tetrahedron is the reason of these modes [58]. Saravanakumar et al. support the idea of a Jahn–Teller effect of Cu(II) ions, which leads to a lowering 1 lattice symmetry of F2g mode [59]. Pekov et al. correlate the presence of the strong band at 125 cm− with the lattice modes [60]. The catalytic tests prove that the composites calcined at 600 ◦C are indeed better for CO oxidation and NO reduction reactions than the separate oxides. After the calcination at 1000 ◦C, CuAl 1000 and CuFe 1000 are still showing NO conversion at higher temperature, while pure CuO does not, which is impressive for materials with such a low surface area. From the characterization of the materials calcined at 600 ◦C, one would expect a different performance for the manganese containing compounds in comparison to the others, as copper manganite is crystallized at much lower temperatures than the other spinels. Both mechanism of CO oxidation and NO reduction are based on the Mars-Van Krevelen mechanism, which means the metal oxide surface has an active part in the reaction [45,61–63]. For the oxidation of CO, CO adsorbs first to the metal cation and abstracts an O from the surface (Os), leaving behind an oxygen vacancy (Vo) as CO2 desorbs. If O2 is present, it will bind to the surface filling the vacancy with one O while the other O can bind to a new CO molecule to form CO2, which leaves the surface restored to its original state, so that the catalytic cycle can restart [63]. This mechanism is depicted below: Os + CO (g) Vo + CO (g) (4) → 2 Catalysts 2020, 10, 1344 12 of 20

Vo + O (g) [Os O] (5) 2 → − [Os O] + CO (g) Os + CO (g) (6) − → 2 For NO reduction it is required that the oxygen vacancies are formed first by interaction with CO, as displayed in Reaction (4), so that NO can adsorb there with its oxygen. Different mechanisms are proposed for NO reduction on metal oxides, but always they involve the presence of two neighboring oxygen vacancies [62,64,65]. This means that there is a competition between O2 and NO for the oxygen vacancies in tests like this, while the NO reduction is having the great disadvantage of needing two neighboring vacancies in order to take place. For brevity reasons, we are not showing all the possible mechanisms here or make any statements on which of the possible mechanisms (Eley–Rideal or Langmuir–Hinshelwood) take place in our studied systems, since this is not the purpose of the study. Below, an Eley–Rideal mechanism (Reaction 7–10) is presented. A first NO adsorbs to a first vacancy and a second NO reacts from the gas phase, forming an adsorbed N2O2. The oxygen of the second NO gets abstracted by the second vacancy leaving behind an adsorbed N2O which decomposes to N2 filling the first vacancy with oxygen.

V + NO [V ON] (7) O → O − [V ON] + NO (g) [V N O ] (8) O − → O − 2 2 [V N O ] + V [V N O] + Os (9) O − 2 2 O → O − 2 [V N O] N (g) + Os (10) O − 2 → 2 In case there is no adjacent vacancy the adsorbed N2O2 can also lose oxygen to the surface and desorb as N2O (Reaction 11). This can explain the N2O formation at low temperature, as stated before.

[V N O ] N O (g) + Os (11) O − 2 2 → 2 Catalysts 2020, 10, x 12 of 20 In a possible Langmuir–Hinshelwood mechanism, the second NO adsorbs to the second vacancy and forms a bidentate N2O2 with the other adsorbed NO. N2 is obtained, restoring the oxygen at the surface. 2 V +2 NO →2 V −ON (12) 2 V + 2 NO 2 [V ON] (12) O → O − 2 V −ON → V −NO −V (13) 2 [V ON] [V N O V ] (13) O − → O − 2 2 − 0 [V V N−NOO V−V] →N N( g(g))++2 2 O Os (14) O − 2 2 − 0 → 2 From the mechanisms it can be concluded that the most important factor for NO reduction is providing enoughenough vacanciesvacancies forfor NONO toto adsorbadsorb to.to. PatelPatel etet al.al. claimclaim thatthat CuAlCuAl22O4 isis able able to reduce NO better than CuO,CuO, duedue toto thethe dissimilarly dissimilarly charged charged metal-oxygen metal-oxygen configuration configuration (Cu (Cu2+2+-O-Al-O-Al33++), which makes the structure more susceptible for electron transfertransfer [[66].66]. The structure remains stable during the changing ofof thethe oxidationoxidation statesstates andand onon toptop ofof that,that, bulkbulk CuAlCuAl22OO44 is more conductiveconductive thanthan AlAl22O3,, which makes it easiereasier toto shareshare freefree electronselectrons inin thethe wholewhole structure.structure. In other words, the vacanciesvacancies created by the reduction withwith COCO cancan bebe betterbetter stabilized.stabilized. This pathway is envisionedenvisioned inin FigureFigure 1313..

FigureFigure 13.13. ReductionReduction byby COCO onon CuAlCuAl22O4.

Although thisthis reasoningreasoning explains explains the the NO NO reduction reduction at highat high temperatures temperatures for CuAl for CuAl 1000 and1000 CuFe and 1000CuFe as 1000 opposed as opposed to CuO to where CuO therewhere is there no activity is no atacti all,vity this at doesall, this not does explain not the explain catalytic the resultscatalytic of theresults materials of the calcinedmaterials at calcined 600 ◦C. at XRD 600 and°C. RamanXRD and clearly Raman show clearly that show CuAl that 600 CuAl consists 600 of consists CuO and of γ CuO-Al2 Oand3 in γ clear-Al2O contrast3 in clear with contrast the CuAl with2O the4 of CuAl CuAl2O 1000.4 of CuAl It is very 1000. likely It is thatvery some likely Cu that diff someuses into Cu diffuses into the Al2O3 matrix in CuAl 600 forming “surface aluminate” as it is often reported [66–69]. However, in the work of Patel et al. [66,67] it is stated that as opposed to bulk aluminate, this surface aluminate is a lot less effective in NO reduction because the reduced copper ions do not have the conductive field around them as in bulk aluminate. They order the species from most effective to least effective for NO reduction as bulk CuAl2O4 > CuO > surface CuAl2O4. Our results show however that the enhanced performance for NO reduction and CO oxidation as well, is rather due to the effect of supporting smaller crystals of CuO on the other oxide than due to the formation of spinel. The low onset temperature in H2-TPR is also characteristic for highly dispersed copper oxide and is certainly related to the high CO oxidation [43]. The estimated CuO crystallites size is also smaller for CuAl 600 than for CuFe 600, which correlates with the lower T50 O2 and H2-TPR onset temperature. The extra CuO in CuMn 1000 compared to CuMn2 1000 also explains the better CO conversion. The energy cost for the generation of oxygen vacancies is much lower on steps and corners than on a perfect surface of a metal oxide, so smaller crystals will have a larger surface to bulk ratio and more low- coordinated ions at edges, which explains the lower reduction temperature [45,70]. The mixture of CuO and Fe2O3 in CuFe 600 did not show any improvement for CO oxidation compared to CuAl 600, which indicates there is probably also no spill-over effect of oxygen from Fe2O3 to CuO that improves the reducibility [6]. Now, a very interesting observation is that the copper-manganese materials formed a totally different system than the other materials. At 600 °C, they did not consist of crystals of the separate oxides, but mostly of the non-stoichiometric spinel Cu1.5Mn1.5O4, while at 1000 °C the materials consist of a spinel together with an extra phase of CuO or Mn3O4. Thus, the catalytic behavior cannot be explained by the same reasoning as before. As mentioned in the introduction, Mn3+ can also be oxidized to Mn4+, and studies are showing that in copper manganese spinels a solid state charge transfer redox system exists that oxidizes Mn3+ to Mn4+ and reduces Cu2+ to Cu1+[71]. Accordingly, in this spinel system, the most stable configuration has Cu1+ in the tetrahedral spots and Mn4+ in the octahedral.

Cu + Mn ⇌ Cu + Mn (15)

In this way, the mechanism for CO oxidation is explained by the adsorption of CO on Mn4+ and the adsorption of O2 onto Cu1+ [6,72].

Catalysts 2020, 10, 1344 13 of 20

the Al2O3 matrix in CuAl 600 forming “surface aluminate” as it is often reported [66–69]. However, in the work of Patel et al. [66,67] it is stated that as opposed to bulk aluminate, this surface aluminate is a lot less effective in NO reduction because the reduced copper ions do not have the conductive field around them as in bulk aluminate. They order the species from most effective to least effective for NO reduction as bulk CuAl2O4 > CuO > surface CuAl2O4. Our results show however that the enhanced performance for NO reduction and CO oxidation as well, is rather due to the effect of supporting smaller crystals of CuO on the other oxide than due to the formation of spinel. The low onset temperature in H2-TPR is also characteristic for highly dispersed copper oxide and is certainly related to the high CO oxidation [43]. The estimated CuO crystallites size is also smaller for CuAl 600 than for CuFe 600, which correlates with the lower T50 O2 and H2-TPR onset temperature. The extra CuO in CuMn 1000 compared to CuMn2 1000 also explains the better CO conversion. The energy cost for the generation of oxygen vacancies is much lower on steps and corners than on a perfect surface of a metal oxide, so smaller crystals will have a larger surface to bulk ratio and more low-coordinated ions at edges, which explains the lower reduction temperature [45,70]. The mixture of CuO and Fe2O3 in CuFe 600 did not show any improvement for CO oxidation compared to CuAl 600, which indicates there is probably also no spill-over effect of oxygen from Fe2O3 to CuO that improves the reducibility [6]. Now, a very interesting observation is that the copper-manganese materials formed a totally different system than the other materials. At 600 ◦C, they did not consist of crystals of the separate oxides, but mostly of the non-stoichiometric spinel Cu1.5Mn1.5O4, while at 1000 ◦C the materials consist of a spinel together with an extra phase of CuO or Mn3O4. Thus, the catalytic behavior cannot be explained by the same reasoning as before. As mentioned in the introduction, Mn3+ can also be oxidized to Mn4+, and studies are showing that in copper manganese spinels a solid state charge transfer redox system exists that oxidizes Mn3+ to Mn4+ and reduces Cu2+ to Cu1+ [71]. Accordingly, in this spinel system, the most stable configuration has Cu1+ in the tetrahedral spots and Mn4+ in the octahedral. Cu2+ + Mn3+ Cu1+ + Mn4+ (15)

In this way, the mechanism for CO oxidation is explained by the adsorption of CO on Mn4+ and 1+ the adsorption of O2 onto Cu [6,72].

CO + Mn4+ CO+ + Mn3+ (16) → ads

1 + 2+ O (g) + Cu O− + Cu (17) 2 2 → ads + CO + O− CO (g) (18) ads ads → 2 This mechanism is quite peculiar as it suggests the involvement of two sites, while the previous one (Reaction 4 to 6) is explained with only one site. This ability to exchange electrons between Cu and Mn can also be seen as a way to stabilize the vacancies for NO reduction, similar to the before mentioned theory (Figure 13). The experiments of Spassova et al. show that CO interacts first with the oxygen of the catalysts surface and that NO only starts to reduce afterwards [73]. This means the mechanism is similar to the ones mentioned before (Reaction 4 to 14). The research of Spassova et al. also pointed out that in the presence of O2, the mechanism for NO reduction can first go through the oxidation of NO to NO2, as follows: 2 NO + O 2 NO (19) 2 → 2 2 NO + 4 CO 4 CO + N (20) 2 → 2 2 In this way, O2 is prevented from re-oxidizing the surface, so that the catalytic cycle will not restart and none of the NO will be reduced, but instead NO2 covers the surface and forms nitrites that can decompose through Reaction 20. On top of this, NO2 is reduced faster with CO than NO [74]. Catalysts 2020, 10, 1344 14 of 20

The question arising is why the catalytic performance of the copper manganites is not very good here? These materials apparently perform best when they are still amorphous [6,72]. As shown also here, the materials have already formed spinel crystals by calcination at 600 ◦C, while this was not the case for copper aluminate and copper ferrite. Crystallization causes a great reduction in surface area and the concentration of surface-active centers. The spinel crystal structure formed at 600 ◦C is not perfect, which explains also why it still changed by the calcination at 1000 ◦C and its defects in the Cu1.5Mn1.5O4 structure together with the remaining mixture of more amorphous or small CuO and Mn2O3 particles are actually responsible for the catalytic activity. The generally underwhelming performance of all the spinels calcined at 1000 ◦C is related to the lower amount of defects present in the structure, which are representing the sites where oxygen vacancies require less energy to be formed. The surface areas of all the materials has been reduced by more than 97% after calcination at 1000 ◦C. The tested mixed oxides represent a very good alternative for three-way catalysis after some small modifications. They will have to be loaded onto a supporting material with a high surface area, via a strong interaction in order to prevent the crystal structures from growing too much during the exposure to the real working conditions of three-way catalysis.

4. Materials and Methods

4.1. Chemicals All chemicals were purchased from Sigma-Aldrich/Merck, Darmstadt, Germany and used without further purification. The used chemicals are: Cu(NO3)2.3H2O (99.0%), Al(NO3)3.9H2O (100%), Fe(NO3)3.9H2O (99.0%), MnCl2 (97.0%), sodium hydroxide (NaOH) (100%), CuO (100%), MnO2 (100%).

4.2. Catalyst Preparation Spinel type materials were prepared by a simple co-precipitation reaction between the corresponding metal nitrate salts, or chloride in the case of manganese, with NaOH. In a first step, the appropriate volume of Cu(NO3)2.3H2O solution (0.2 M) is mixed with the required volume of Al/Fe/Mn-salt solution (0.2 M). This mixture is refluxed at 80 ◦C for 30 min and 3 M NaOH solution is added. The pH of the mixture is maintained around 14, in order to precipitate the metals. In case of Al(NO3)3 precipitation, the sodium hydroxide solution was added very carefully and slowly, until the metals were precipitated (pH~11). This was visually observed by the change of color, from light blue to black. Further, the mixture is stirred at 80 ◦C for 1 h. Finally, the precipitate was separated and washed thoroughly with water until neutral pH is reached and calcined at 600 or 1000 ◦C for 4 h. In the case of 600 ◦C thermal treatment, the ramping speed was 1◦C/min, while in the case of 1000 ◦C the ramping speed was 2 ◦C/min. For simplicity, the samples were denoted as follows, according to the ratio between the metals: Cu:Al = 1:2 = CuAl, Cu:Fe = 1:2 = CuFe, Cu:Mn = 1:1 = CuMn, Cu:Mn = 1:2 = CuMn2. The number after the code indicates the calcination temperature.

4.3. Physicochemical Characterisation The structural analysis of the annealed samples was performed by powder X-ray Diffraction (XRD) using an X-ray diffractometer (PANalytical X’Pert PRO MPD) equipped with CuKα radiation (λ = 0.15406 nm); measurements were done in the 2θ range from 20◦ to 80◦ using a bracket sample holder with a scanning speed of 0.04◦/4 s in continuous mode. The crystallite size distribution was approximated using the Scherrer equation:

Kλ d = (21) β cos(θ) where d = crystallite size, λ = the wavelength of the X-ray source (0.15406 nm), β = the full width at half maximum of the chosen diffraction peak (2θ), θ the Bragg angle and K = the shape constant chosen Catalysts 2020, 10, 1344 15 of 20 as 0.9. Using this equation, morphological effects and disorder to reflection widths were not taken into account so the obtained values were merely used for relative comparison of the materials [75]. Raman spectra were obtained on a Micro-Raman Horiba (Xplora Plus Microscope) equipped with 1 a 532 nm laser. The range was between 50–1000 cm− Raman shift. The instrument is equipped with a 100 magnification microscope objective. × Temperature programmed reduction measurements with hydrogen (H2-TPR) were performed on a Quantachrome iQ. About 20 mg of the sample was outgassed at 200 ◦C for 16h prior to the measurements and then weighed again after cooling. The sample was then pre-treated under a He flow for 1 h. Reduction was done with a 5% H2/Ar flow a rate of 25 mL/min. The temperature was raised between 100 and 800 ◦C at 10 ◦C/min. The hydrogen consumption was monitored with a thermal conductivityCatalysts 2020, 10 detector, x (TCD). These signals were normalized by the outgassed catalyst weight. 15 of 20 SEM-EDX was used to determine the surface composition using a FEG-ESEM-EDX, FEI Quanta from the conversion of CO, assuming that all NO conversion was due to the reaction between CO 250 by scanning the whole field instead of discrete points. and NO to form N2 (Reaction 2). The following formula was used for the O2 conversion: N2-physisorption measurements were carried out on a Quantachrome Quadrasorb SI automated C, gas adsorption system. Prior to the measurements,X −X . the samples were outgassed at 200 ◦C for 16 h. C C The specific surface area (SSA) was calculated using the, Brunauer–Emmet–Teller, (BET) equation.(22) X = . 2 C, 4.4. Catalytic Activity Measurements With X being the conversion and Cin the input concentration. TheThe catalyticconcentrations performance of the wasgases tested in the in mixtures a fixed bedwere continuous as follows: flow CO: quartz 3000 ppm, reactor, NO: with 1000 a ppm, total gasO2: flow1050 ppm rate ofwith 200 N mL2 being/min. the The balance tested gas. powders were pelletized using an IR pelletizer, crushed and sievedThe reaction in order rate to obtain(RR) of aCO particle and NO size was between calculated 150 and as follows 300 µm. (example 200 mg offor the CO): sieved powder was loaded in the reactor and pre-treated under the same gas flow as during the test. The speed C,.X.F.p RR = . ramping from room temperature to 600 ◦C was 10 ◦C/min, maintaining this temperature for 1 h(23) and T. R. g then cooling back to room temperature. The catalytic test was done by ramping with 10 ◦C/min from F is the total flow rate, p the pressure (1.013 bar), T the temperature, R the gas constant. gcat the room temperature to 600 ◦C and holding the temperature about every 50 ◦C for at least 10 min, in orderweight to of have the acatalyst. constant conversion value of the measurement. Figure 14 illustrates the protocol in a relevantThe specific timescale. reaction The levelsrate (Spec. of CO RR) and is NO,calculated from whichas: the conversions were calculated, were measured using a Dräger polytron 7000 CO-detector andRR a 7Solutions GDS-15JP1-CA-NO NO-detector, Spec. RR = . (24) respectively. The experimental set-up is illustrated in FigureSSA 15. The conversion of O2 was calculated from theWith conversion SSA being of the CO, BET assuming specific that surface all NO area. conversion was due to the reaction between CO and NO toThe form turnover N2 (Reaction frequency 2). The (TOF) following is calculated formula as wasfollows: used for the O2 conversion:    CRRin,NO TOFXCO = XNO. C . (25) − # mol CuOin,CO⁄ gCin,CO XO2 = . (22) 2 Cin,O The # mol CuO/ gcat was calculated from the SEM-EDX results.2

FigureFigure 14.14. TemperatureTemperature protocolprotocol of of the the catalytic catalytic test test in in a a relevant relevant timescale. timescale.

Figure 15. Experimental set-up for the catalytic tests.

Catalysts 2020, 10, x 15 of 20

from the conversion of CO, assuming that all NO conversion was due to the reaction between CO and NO to form N2 (Reaction 2). The following formula was used for the O2 conversion:

C, X −X. C, C, (22) X = . 2 C,

With X being the conversion and Cin the input concentration. The concentrations of the gases in the mixtures were as follows: CO: 3000 ppm, NO: 1000 ppm, O2: 1050 ppm with N2 being the balance gas. The reaction rate (RR) of CO and NO was calculated as follows (example for CO):

C,.X.F.p RR = . (23) T. R. g

F is the total flow rate, p the pressure (1.013 bar), T the temperature, R the gas constant. gcat the weight of the catalyst. The specific reaction rate (Spec. RR) is calculated as: RR Spec. RR = . (24) SSA With SSA being the BET specific surface area. The turnover frequency (TOF) is calculated as follows: RR TOF = . (25) # mol CuO⁄ g

The # mol CuO/ gcat was calculated from the SEM-EDX results.

Catalysts 2020, 10, 1344 16 of 20 Figure 14. Temperature protocol of the catalytic test in a relevant timescale.

Figure 15. Experimental set-up for the catalytic tests. Figure 15. Experimental set-up for the catalytic tests.

With X being the conversion and Cin the input concentration. The concentrations of the gases in the mixtures were as follows: CO: 3000 ppm, NO: 1000 ppm, O2: 1050 ppm with N2 being the balance gas. The reaction rate (RR) of CO and NO was calculated as follows (example for CO):

Cin,CO.XCO.F.p RRCO = . (23) T.R.gcat

F is the total flow rate, p the pressure (1.013 bar), T the temperature, R the gas constant. gcat the weight of the catalyst. The specific reaction rate (Spec. RR) is calculated as:

RR Spec. RR = . (24) SSA With SSA being the BET specific surface area. The turnover frequency (TOF) is calculated as follows:

RR TOF = . (25) # mol CuO/gcat

The # mol CuO/ gcat was calculated from the SEM-EDX results.

5. Conclusions Mixed oxides were synthesized by co-precipitation of a Cu source in combination with Al, Fe or Mn corresponding salts as precursors. The materials were calcined at 600 and 1000 ◦C in order to crystallize the phases. Of the synthesized materials calcined at 600 ◦C only the manganese containing materials formed the crystal structure of a mixed spinel. The other materials consisted of mixtures of CuO with Al2O3 or iron oxides (Fe2O3 and Fe3O4). After the calcination at 1000 ◦C, all the materials featured a copper spinel structure, often accompanied by a separate CuO phase which shows that the formation of the spinel was not yet fully complete. Instead of the anticipated stoichiometric spinel CuMn2O4, a variant richer in copper, Cu1.5Mn1.5O4, was formed both times at 600 ◦C. At 1000 ◦C the structure slightly changed probably towards a variant with less copper. The as-synthesized materials show excellent performance in three-way catalysis. The prepared catalysts calcined at 600 ◦C could oxidize CO at lower temperatures than CuO, except for CuFe 600. All these materials were able to convert NO in these tests, while pure CuO was not able to do so. After calcination at 1000 ◦C, CO oxidation only occurred at higher temperatures. CuAl 1000 and CuFe 1000 still showed NO conversion although at higher temperatures. The improved CO oxidation and NO reduction were mainly due to highly dispersed CuO or small CuO particles supported on the other Catalysts 2020, 10, 1344 17 of 20 oxide, while the spinel formation was useful for NO reduction. The underwhelming performance of the spinels can be attributed to the growth of the crystals, which reduces the amount of available sites and defects in the structure. The research proves that the mixed oxides tested here have great potential to be applied in three-way catalysis but that crystal growth will need to be prevented to ensure a long lifetime of the catalyst. Copper oxide—aluminum oxide combinations or copper oxide—iron oxide combinations are more promising than copper oxide—manganese oxide combinations, due to the formation of large crystals at lower temperatures for the latter one.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4344/10/11/1344/s1, Supplementary Table S1: wt% and ratio of the second element X (Al, Fe, Mn) to Cu, as determined by SEM-EDX, Supplementary Figure S1: SEM-EDX spectrum for CuAl 600, Supplementary Figure S2: SEM-EDX spectrum for CuAl 1000, Supplementary Figure S3: SEM-EDX spectrum for CuFe 600, Supplementary Figure S4: SEM-EDX spectrum for CuFe 1000, Supplementary Figure S5: SEM-EDX spectrum for CuMn 600, Supplementary Figure S6: SEM-EDX spectrum for CuMn 1000, Supplementary Figure S7: SEM-EDX spectrum for CuMn2 600, Supplementary Figure S8: SEM-EDX spectrum for CuMn2 1000, Supplementary Figure S9: Specific reaction rates for CO, Supplementary Figure S10: Specific reaction rates for NO, Supplementary Figure S11: TOF for CO, Supplementary Figure S12: TOF for NO. Author Contributions: Conceptualization, T.V.E. and R.-G.C.; methodology, T.V.E., R.-G.C.; formal analysis, T.V.E., W.V.H., M.M.; investigation, T.V.E., W.V.H.; resources, P.C.; data curation, T.V.E.; writing—original draft preparation, T.V.E.; writing—review and editing, R.-G.C., P.C.; visualization, T.V.E.; supervision, P.C.; project administration, P.C.; funding acquisition, P.C. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Union’s Horizon 2020 Project Partial-PGMs (H2020-NMP-686086) and the FWO-Flanders (project no. G038215N.) Acknowledgments: The authors like to thank K. De Wael and G. Nuyts from the AXES research group, Department of Chemistry, UA, for performing the SEM-EDX measurements. Conflicts of Interest: The authors declare no conflict of interest.

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