Tandem Base-Metal Oxide Catalyst for Automotive Three-way Reaction: MnFe2O4 for Preferential Oxidation of Hydrocarbon

A combination of two base-metal oxides in tandem configuration can realize three-way reaction without platinum group metals. For this purpose, catalysts for hydrocarbon preferential oxidation (HC-PROX) and for NO reduction by CO are required. For the design of HC-PROX catalysts, competitive oxidation of propene and CO on spinel-type MFe2O4 (M = Co, Cu, Mg, Mn, Ni, Zn) was investigated. MnFe2O4 preferentially oxidized propene in the co-presence of CO showing the best propene oxidation activity. Among the series of MFe2O4, the activity controlling factor was correlated to the M-O bond energy of the second metal oxides, and the preference for HC oxidation was dependent on the electronegativity of the second ion. A tandem catalyst using MnFe2O4 for HC-PROX and CuCo2O4 for NO-CO reaction showed TWC activity comparable to a Rh/CeO2.

We have previously clarified that the low NO reduction activity of base metal oxide catalysts is caused by the surface poisoning with hydrocarbon derived species [27,28]. In the oxidation of unburned hydrocarbons, oxygenated catalyst, we have made preliminary screening of a competitive oxidation of propene and CO over the 4th period transient metal oxide catalysts. Cr 2 O 3 showed the highest propene oxidation activity and the lowest CO oxidation activity. The second-best catalyst was g-Fe 2 O 3 , which was active for propene oxidation with low CO oxidation. Among various iron oxides, spinel, corundum, and perovskite-type structures, spinel type γ-Fe 2 O 3 showed the best activity.
In this study, we focus the promotion effect of the second element on spinel type iron oxide catalysts for HC-PROX. The controlling factors for the HC oxidation activity and the preference for HC oxidation were also discussed to find a strategy for the design of HC-PROX catalysts.

Experimental
MFe 2 O 4 catalysts were prepared by a reverse strike coprecipitation method (M = Co, Cu, Ni, Zn) or a sol-gel method (M = Mn, Mg). Metal nitrate salts, Fe(NO 3 were used as precursors. For the reverse strike coprecipitation method, aqueous solutions of Fe(NO 3 ) 3 •9H 2 O and a second metal (M) nitrate were dissolved in a 100 mL distilled water in an atomic ratio of Fe:M = 2:1. The obtained aqueous nitrate solution was added dropwise to a 1M NaOH aqueous solution (pH = 13), and a brown precipitate was obtained. After a 1h of stirring of the slurry, the precipitate was filtered and washed with hot water at 60°C for several times until the pH of the filtrate became neutral to completely remove NaOH in the precipitate. The sample was dried overnight at 80°C and calcined at 500°C for 3h in air. For the sol-gel method, Fe(NO 3 ) 3 •9H 2 O and the second metal (M) nitrate were dissolved in a tiny amount of water (5 mL) in an atomic ratio of Fe:M = 2:1. The aqueous solution was stirred with heating temperature at 100°C until it forms a gel. The gel was calcined at 500°C for 3h in air.
The structure of MFe 2 O 4 were evaluated by using a laboratory-scale XRD (Rigaku MiniFlex diffractometer with Cu Kα radiation at 30kV and 15 mA). Propene-Temperature Programmed Reduction (Propene-TPR) was conducted by a flow apparatus connected with a NOx/CO/CO 2 (HORIBA VIA-3100) analyzer. A 50.0mg of a catalyst was placed in a glass tube, the catalyst was pretreated in a flow of 0.6%O 2 /Ar at 400 ºC for 15min, cooled to 100 ºC, purged in a flow of Ar, and finally the temperature ramped from 100 to 500 ºC in a flow of 1000 ppm propene/Ar at a rate of 5 ºC/min. The light-off temperature was defined as the temperature at which the CO 2 concentration increased by 2 ppm from the baseline. CO-TPR profiles were measured in a similar manner to propene-TPR in a flow of 4000 ppm CO/Ar. The HC-PROX performance was evaluated by a conventional fixed bed flow reactor under a flow of 1000 ppm NO, 1000 ppm propene, 4000 ppm CO, 6000 ppm O 2 and Ar balance. A catalyst (17.5mg) was placed into a U-shaped quartz tube with 4mm inner diameter and 150mm length. A model exhaust gas at stoichiometric composition was fed to the catalysts with a total flow rate of 60 mL min − 1 (gas hourly space velocity (GHSV) ≈ 200,000 mL g − 1 h − 1 ). In the tandem TWC tests, a 17.5mg of HC-RPOX catalyst was placed upstream and a 17.5mg of NO-CO reaction catalyst was placed downstream of the U-shaped quartz tube (GHSV ≈ 100,000 mL g − 1 h − 1 ). The activity test was carried out from 200 to 500 ºC in 50 ºC increments. The steady-state activity was measured using a NOx/CO/CO 2 (HORIBA VIA-3100) analyzer for NO, CO and CO 2 detection, and a micro gas chromatograph (Agilent 490 Micro GC equipped with MS-5A and PoraPLOT Q columns, and TCD detector) for N 2 O and propene. The propene conversion and the CO conversion were calculated according to Eqs. (1) and (2). The specific rates of propene and CO were measured at 350 ºC below 30% conversion (< 30%) by adjusting the catalyst weight. As an indicator of preferential oxidation of HC, a parameter Rate(HC)/Rate(CO) at 350 ºC was used.

Results and Discussion
The bulk crystal structure of the obtained catalysts was confirmed by XRD patterns shown in Fig. 1 The catalytic activity for competitive oxidation of propene and CO was tested on these iron-spinel metal oxides in a flow of NO-propene-CO-O 2 . Figure 2 shows the propene and CO conversions as a function of reaction temperature. It should be noted that the light-off temperatures of NO reduction were around 350 ºC for these iron spinels ( Figure  S1), indicating NO reduction activity is far lower than those of PGM-based catalysts. For γ-Fe 2 O 3 , propene conversion increased with the reaction temperature from 10% to 250 ºC to 60% at 300 ºC, while the conversions of CO were 0% and 15% at the same temperatures. CuFe 2 O 4 showed higher propene oxidation activity than γ-Fe 2 O 3 , while the conversion of CO reached to 100% at 250 ºC. Since the aim of HC-PROX catalyst is to consume hydrocarbons and to supply unreacted CO to a downstream NO-CO reaction catalyst, CuFe 2 O 4 is not suitable for HC-PROX catalyst. On the other  Fig. 3. The red bars show the specific rate of propene oxidation, and the blue bars show the specific rate of CO oxidation. The ratios of these rates, Rate(HC)/ Rate(CO), are also indicated by green circles as an indicator of propene oxidation preference. MnFe 2 O 4 showed the highest propene oxidation rate and Rate(HC)/Rate(CO), indicating propene is preferentially oxidized on MnFe 2 O 4 in the co-presence of CO. Propene oxidation also preferentially proceeded on MgFe 2 O 4 and ZnFe 2 O 4 ; however, these catalysts showed insufficient activity for propene oxidation at hand, MnFe 2 O 4 showed higher propene oxidation activity and lower CO oxidation activity than γ-Fe 2 O 3 , MnFe 2 O 4 is preferable for HC-RPOX. Although the suppression of CO oxidation activity was more significant on MgFe 2 O 4 and ZnFe 2 O 4 , the propene oxidation activities of these catalysts were very low. Except CuFe 2 O 4 , spinel ferrite catalysts showed higher propene oxidative activity than CO oxidative activity.
For quantitative comparison of the catalytic activity, the specific reaction rate per surface area at 350 ºC are  In order to obtain the strategy for the design of better HC-PROX catalysts, the controlling factors for the propene oxidation activity and the preference for HC oxidation were investigated. Since the redox property of metal oxides is one of the critical factors for catalytic oxidations, the redox property of the catalyst was evaluated with Temperature Programmed Reduction using propene as a reductant (Propene-TPR). Figure 5 shows Propene-TPR profiles of the series of MFe 2 O 4 catalysts. The total amount of produced CO 2 was in the order of Cu > Ni > Mn > > Mg, Co, Zn. Since the total amount of produced CO 2 includes catalyst reduction at higher temperatures above 400 ºC, at which most of the catalyst showed 100% propene conversion, it cannot be reflected the redox activity of ferrites in HC-PROX. For example, NiFe 2 O 4 having a big reduction peak mainly above 400 ºC showed lower oxidation activity than MnFe 2 O 4 having a smaller reduction peak around 300 ºC. Therefore, the light-off temperature of propene-TPR is focused in this study. The light-off temperature was in the order of MnFe 2 O 4 (140 ºC) < CuFe 2 O 4 (195 ºC) < NiFe 2 O 4 and CoFe 2 O 4 (242 ºC) < ZnFe 2 O 4 (264 ºC) < MgFe 2 O 4 (280 ºC). Figure 6 shows correlation between the rate of propene oxidation activity at 350 ºC and the light-off temperature in propene-TPR. The good correlation in the figure indicates that redox activity of the ferrites is the determining factor the oxidation activity.
What is the controlling factor for the redox activity of MFe 2 O 4 ? Since the oxidation reaction on iron oxide-based catalysts proceeds via Mars-van-Krevelen mechanism 350 ºC. Co, Ni and Cu-doped ferrites are not preferable for HC-PROX because the reaction rates of both propane and CO were almost comparable. It was found that MnFe 2 O 4 is the best HC-PROX catalyst among the series of ferrites.
Using MnFe 2 O 4 as a HC-PROX catalyst and CuCo 2 O 4 as a NO-CO reaction catalyst, TWC activity of a tandem catalyst was tested. Cu and Co are well known comportment for the effective NO reduction by CO [30][31][32][33][34]. MnFe 2 O 4 was put upstream and CuCo 2 O 4 was put downstream of the reactor in a tandem layout. Figure 4 shows the conversions of NO over the tandem base metal oxide catalysts and Rh/CeO 2 as a reference of PGM catalyst. The tandem MnFe 2 O 4 (17.5mg) + CuCo 2 O 4 showed comparable NO conversion to Rh/CeO 2 below 300 ºC, while the NO conversion decreased at 350 ºC. The decrease in the NO conversion can be assigned to incomplete oxidation of propene. Actually, the propene conversion on MnFe 2 O 4 was 98.4%, as shown in Fig. 2a. It can be expected that the incomplete oxidation of propene produced residual amount of oxygenated hydrocarbons, such as acetic acid, which poisoned the downstream CuCo 2 O 4 catalyst for NO-CO reaction. In the case of ZnCr 2 O 4 , which showed 100% propene conversion, such decrease in the NO conversion at 350 ºC was not observed [28]  These plots indicate that the metal-oxygen bond energy is the controlling factor for the oxidation activity.
Although overall oxidation activity can be rationalized by the metal-oxygen bond energy, the reaction rates of propene and CO showed different somewhat dependence. As shown in Figure S2, the rate of propene was the highest on MnFe 2 O 4 , while CuFe 2 O 4 showed the highest rate of CO. The order of the CO oxidation activity of MFe 2 O 4 agreed well with the light-off temperature of CO-TPR ( Figure S3). The light-off temperature of CO-TPR was the lowest for CuFe 2 O 3 which showed the highest reaction rate of CO. The result suggests that the reaction rates of the individual substrates are not dependent only on oxidation activity but also on substrate preference. Then, the controlling factor for the substrate preference is discussed.
The factor for the preference is rationalized by the electronegativity of the second ions. Figure 8 shows the dependence of the ratio of R(HC)/R(CO) as a function of the electronegativity of second ions (c i ). The electronegativity of second ions (c i ) is defined as c i = (1 + 2i) c 0 ; where i is the valence of ion, and c 0 is the Pauling electronegativity of the element [38]. The higher c i indicates higher acidity. When the electronegativity of ion is low, propene oxidation preferentially proceeds. As increase in the electronegativity of ion, the relative activity of CO oxidation increases. The result suggests the acid-base property of catalyst influences the preferential oxidation of propene in the presence of CO. [35,36], the lattice oxygen release rate should be the key factor for the oxidation activity. Thus, in Fig. 7, the specific rate of CO 2 formation, as an indicator of oxidation activity of MFe 2 O 4 , is plotted as a function of the metal-oxygen bond energy. The metal-oxygen bond energy was estimated from the metal oxide formation enthalpy per oxygen atom [37].

Conclusion
For the design of PGM-free and Cr-free TWC, a series of spinel type iron ferrites (MFe 2 O 4 ) was examined as catalysts for hydrocarbon preferential oxidation (HC-PROX). MnFe 2 O 4 selectively oxidized propene in the co-presence of CO with the best propene oxidation activity. A tandem catalyst using twice amount of MnFe 2 O 4 as an upstream HC-PROX catalyst and CuCo 2 O 4 as a downstream NO-CO reaction catalyst showed comparable NO reduction activity to Rh/CeO 2 . The possibility of PGM-free and Cr-free TWC was demonstrated. Among the series of MFe 2 O 4 , the oxidation activity was correlated to the M-O bond energy as an indicator of oxygen release. The electronegativity of second metal ions was suggested to be a controlling factor of the preference for propene oxidation.