Promotion of N2O decomposition by Zr4+-doped CeO2 used as support of Rh catalyst

Accepted Manuscript Promotion of N2O decomposition by Zr4+-doped CeO2 used as support of Rh catalyst

Min-Jae Kim, Hye Jeong Kim, Seung-Jae Lee, In-Soo Ryu, Hyung Chul Yoon, Ki Bong Lee, Sang Goo Jeon PII: DOI: Article Number: Reference:

S1566-7367(19)30226-2 https://doi.org/10.1016/j.catcom.2019.105764 105764 CATCOM 105764

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

1 April 2019 26 July 2019 27 July 2019

Please cite this article as: M.-J. Kim, H.J. Kim, S.-J. Lee, et al., Promotion of N2O decomposition by Zr4+-doped CeO2 used as support of Rh catalyst, Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.105764

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ACCEPTED MANUSCRIPT Promotion of N 2O decomposition by Zr4+-doped CeO2 used as support of Rh catalyst

Min-Jae Kima,c, Hye Jeong Kimb, Seung-Jae Leec, In-Soo Ryuc, Hyung Chul Yoonc, Ki Bong

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Leea,* , Sang Goo Jeonc,*

Department of Chemical and Biological Engineering, Korea University, Anam-ro 145,

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Seongbuk-gu, Seoul, Republic of Korea b

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Graduate School of Energy Science and Technology Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea

Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic

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c

*Corresponding Author:

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of Korea

42-860-3134

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- Sang Goo Jeon (S. G. Jeon), E- mail: [email protected], Tel: +82-42-860-3387; Fax: +82-

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- Ki Bong Lee (K. B. Lee), E- mail: [email protected], Tel: +82-2-3290-4851; Fax: +822-926-6102

Abstract 1

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A series of Ce1-x Zrx O 2 -supported Rh catalysts was prepared and their catalytic efficiencies for N 2 O decomposition was determined in the 200-400 o C range. The activity of this catalytic system was found to be largely influenced by the Ce:Zr weight ratio. The Rh/Ce-ZrO 2 catalyst with a small amount of ZrO 2 showed a significantly improved activity performance

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than the Rh/CeO 2 catalyst, particularly when O 2 or H2O were present in the feed gas of N 2 O. H2 temperature-programmed reduction and X-ray photoelectron spectroscopy studies showed

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that the incorporation of Zr4+ in the CeO 2 lattice used as support of Rh catalyst improved Rh

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dispersion and redox ability, and stabilized Rh3+ species on the catalyst surface.

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Keywords: N 2 O decomposition, Rh catalyst, Ce–ZrO 2 support. H2 O-tolerance in deN 2 O.

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1. Introduction Nitrous oxide (N 2 O) is considered as a greenhouse gas because it contributes to the ozone layer destruction and consequently to global warming [1, 2]. However, N 2 O concentration in the atmosphere has been increasing continuously due to natural and

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anthropogenic activities at the level of 0.2–0.3% per year [2]. Two major techniques can be applied to restrict the amount of N 2 O emitted at point sources, ca. combustion processes in

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power plants, namely the selective catalytic reduction (SCR) and catalytic decomposition.

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The former shows high activity at low temperatures and employs a reductant such as a hydrocarbon or ammonia, but the use of a reducing agent makes it more expensive and less

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attractive than other techniques [1-3]. In contrast, catalytic decomposition (2N 2 O → 2N2 + O2 )

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is a very simple and cost-effective method. On the other hand, current commercial catalysts for N 2 O decomposition are still operated at temperatures higher than those encountered in the

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downstream of exhaust gas, thus the development of catalysts that exhibit high activity at low temperatures is considered very important. Thus, many researchers have devoted large efforts

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in developing various types of catalysts, including noble metals [4-8], metal oxides [9-11], and ion-exchanged zeolites [12, 13] for practical N2 O decomposition technology at low

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temperatures (ca. below 400 ºC).

Among these catalysts, Rh/CeO 2 is considered particularly promising because the CeO 2 support shows good resistance in the formation of large Rh metal particles, thus promoting metal dispersion [14, 15]. However, the catalytic performance of the Rh/CeO 2 catalyst is not yet sufficient for an industrial application. To develop highly active Rh/CeO 2 -based catalysts for N 2 O decomposition, several studies have focused on the crucial roles of the support and the promoter in the performance of the catalysts [14, 16]. It was reported that when Zr 4+ cations are introduced into the CeO 2 lattice, a change in the surface area, redox ability and 3

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strong metal–support interactions are obtained with Ce-ZrO 2 -supported Rh catalysts to exhibit improved Rh dispersion and structure stability [15, 17]. In this work, the catalytic activity and performance of a series of Rh/Ce-ZrO 2 catalysts with different Ce:Zr weight ratio were tested for the N2 O decomposition in the absence and

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presence of H2 O and O 2 in the feed stream. The Rh/Ce-ZrO 2 catalysts were characterized for their textural properties, redox behavior via H2 temperature-programmed reduction (H2 -TPR),

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and oxidation state of Rh species by X-ray photoelectron spectroscopy (XPS), for the better

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understanding of their de-N2O performance. For strict comparison, Rh/CeO 2 catalyst was also

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prepared and used as reference catalyst.

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2. Experimental 2.1 Catalysts preparation

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A series of Ce1-x Zrx O2 mixed oxides with different Ce:Zr weight ratio (x = 0.1, 0.3, 0.5, 0.7, 0.9) were prepared by the co-precipitation method. Appropriate quantities of cerium

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nitrate (99%, Sigma-Aldrich) and zirconium oxynitrate (99%, Sigma-Aldrich) were dissolved in distilled water so as to obtain a given Ce:Zr weight ratio. The precipitating agent (KOH, 15%

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w/w) was slowly added dropwise (2 mL/min) to the constantly stirred solution until its pH was 10.1, and the slurry was then digested for 3 h at room temperature. The precipitate was thoroughly washed several times with distilled water, dried at 110 ºC for 24 h, and then calcined in air at 550 ºC for 4 h. CeO 2 was synthesized exactly by the same process as described above. Rhodium was added onto the CeO 2 and Ce-ZrO 2 supports by the incipient wetness impregnation method (nominal amount of Rh: 1 wt%). Rh- impregnated materials were then dried at 110 ºC for 24 h and calcined in air at 550 ºC for 4 h. 4

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2.2 Catalysts characterization Powder X-ray diffraction (XRD) spectra were recorded in the 2θ range of 10–90º using CuKα radiation (Rigaku D/MAX-2500 instrument). The BET specific surface area of the

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catalysts samples was estimated using a BELSORP- max apparatus (BEL Japan, Inc). H2 -TPR experiments were conducted using an Autochem 2920 apparatus (Micromeritics). The

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loading of Rh after wet impregnation was estimated using inductively coupled plasma optical

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emission spectroscopy (ICP-OES, iCAP 6300, Thermo Scientific, UK). XPS was conducted in an ESCALAB Mark II spectrometer (Vacuum Generators, UK) using Al Kα radiation (hν

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= 1486.6 eV). The detailed procedure has been described in our previous publications [6, 18].

2.3 Catalytic performance

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The catalytic activity of supported Rh solids for N 2 O decomposition was tested in a fixed-bed flow reactor at atmospheric pressure. Prior to each measurement, the catalyst was

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pretreated to a gas stream of 10% O 2 /N2 at 400 o C for 1 h. The simulated feed gas compositions used were 500 ppm N 2 O, 500 ppm N 2 O + 3 vol% O 2 , 500 ppm N 2 O + 3 vol%

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H2 O and 500 ppm N 2 O + 3vol% O 2 + 3 vol% H2 O with N 2 as balance gas. The gas product stream from the reactor was analyzed on- line using an infrared gas analyzer (Madur Polska, Sensonic IR-1).

3. Results and discussion 3.1 Catalytic activity measurements Figure 1 shows results of the N 2 O conversion over a series of Rh/Ce-ZrO 2 catalysts 5

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with different Ce:Zr weight ratio at 250 and 300 ºC tested under the conditions of 500 ppm N2O/N2 at 45,000 h-1 . With increasing ZrO 2 content, the activity of the Rh/Ce-ZrO 2 catalysts for N 2 O decomposition was improved and strongly dependent on the Ce:Zr weight ratio. When a small amount of ZrO 2 was added to CeO 2 (Ce0.9 Zr0.1O2 and Ce0.7 Zr0.3O2 ), the

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catalytic performance of the Rh/Ce-ZrO 2 catalyst was improved compared to that of Rh/CeO 2 catalyst. However, further increase of the ZrO 2 content (Ce0.5 Zr0.5O2 , Ce0.3 Zr0.7 O2 , and

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Ce0.1 Zr0.9O2 ) leads to deterioration of the activity performance of the catalyst. Based on this

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result, an optimal Ce:Zr weight ratio of 7:3 for the Rh/Ce-ZrO 2 catalyst was established, and further evaluation experiments using the Rh/Ce0.7 Zr0.3 O2 catalyst were conducted.

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Figure 2 shows the temperature dependence of N 2 O conversion for the Rh/CeO 2 and

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Rh/Ce0.7 Zr0.3O2 catalysts. The latter catalyst shows a highly improved N 2 O conversion (66.1% at 225 ºC), two times higher than that exhibited by the Rh/CeO 2 catalyst. Rico-Pérez et al. [14]

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reported the de-N2 O catalytic performance of RhO x /Ce0.9 Pr0.1 O2 catalyst, where an N2 O conversion value of 100% at 325 ºC was obtained.

Parres-Esclapez et al. [16] tested a

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Rh/CeO 2 catalyst using 1000 ppm N 2 O/He feed gas composition at 10,000 h-1 and reported complete N 2 O decomposition at 350 ºC.

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Generally, inhibition gases of N 2 O decomposition, like O 2 and H2 O are produced at nitric and adipic acid plants [2]. These gases are competitively adsorbed on the active sites of the catalyst, causing deterioration of its activity performance. Thus, it is necessary to investigate the effect of such inhibition gases in de-N2 O industrial applications. Gas-switching experiments for N 2 O decomposition were performed as a function of time-on-stream, as shown in the inset of Figure 2. An excess amount of inhibition gas was introduced into the fixed-bed microreactor at 350 ºC (3% O 2 and/or 3% H2 O). As clearly shown the catalytic activity decreases when the inhibition gas is added in the feed stream, but 6

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it is immediately recovered as soon as O 2 and/or H2 O were removed from the feed gas stream. The N 2 O conversion obtained by the Rh/Ce0.7 Zr0.3 O2 catalyst did not have any detrimental effect regardless of the O2 addition. On the other hand, that offered by the Rh/CeO 2 catalyst decreased slightly in the presence of O 2 .

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Experiments were conducted under more severe conditions (GHSV = 100,000 h-1 ) to confirm that the Rh/Ce0.7 Zr0.3O2 catalyst exhibits resistance to O 2 . The N2 O conversion was

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noted to be affected by O 2 (data not shown). When 3 vol% H2 O was introduced in the feed

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gas stream, N 2 O conversion over the Rh/CeO 2 catalyst decreased to 48%, which is much lower than that (ca. 71%) obtained over the Rh/Ce0.7 Zr0.3 O2 catalyst. As seen in the stated

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experiments, the N2 O conversion over the catalysts examined deteriorated owing to the O2

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and/or H2 O inhibition gas, but the Rh/Ce0.7 Zr0.3O2 catalyst showed much improved N 2 O conversion compared to the Rh/CeO 2 catalyst. It is evident that the addition of a small

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amount of ZrO 2 to the Rh/CeO 2 catalyst can significantly influence the deN 2 O catalytic activity performance.

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Rhodium oxide can directly decompose N 2 O into N 2 and O2 [4]. However, since all the catalysts tested used the same amount of Rh (1 wt%), it might be considered that the large

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differences in deN 2 O activity performance (Fig. 1) are likely not due to Rh particle size effects (same Rh loading was used, ca. 1 wt%; see also Section 3.2) but very likely to support effects. In order to investigate the effect of ZrO 2 addition on deN 2 O catalytic activity perofrmance, XRD, BET, H2 -TPR and XPS analyses were conducted.

3.2 Catalysts structural and chemical properties As reported in Table 1, the BET areas were found to be 37.3 and 78.2 m2 g-1 and the total pore volume 3.6×10-2 and 8.1×10-2 cm3 g-1 for the Rh/CeO 2 and Rh/Ce0.7 Zr0.3O2 catalysts, 7

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respectively. Thus, the textural properties of the supported Rh changed with the addition of ZrO 2 . Table 1 also summarizes CO chemisorption data. The Rh particles were better dispersed on the Ce0.7 Zr0.3 O2 than CeO 2 support. The Rh particle size can be influenced by the support-dependent Rh dispersion [6, 19]. In the present work, the Rh mean particle size

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on CeO 2 was relatively larger than that on the Ce0.7 Zr0.3 O2 support at the same Rh loading.

1.4 nm for Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O2 , respectively.

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After assuming a stoichiometry of CO/Rh = 1, the Rh particle size was found to be 1.9 and

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The powder XRD spectra for Rh/CeO 2 and Rh/Ce0.7 Zr0.3O2 catalysts are shown in Figure 3. Diffraction peaks for RhO x phases were not observed in the spectra of any of the

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prepared catalysts, confirming the high dispersion of Rh (or small mean Rh particle size).

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All the characteristic reflections obtained for the Rh/CeO 2 catalyst belong to the cubic fluorite oxide-type structure (JCPDS 34-0394). The powder XRD spectra for the

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Rh/Ce0.7 Zr0.3O2 catalyst were similar to those for the Rh/CeO 2 catalyst. ZrO 2 crystallites were not found based on the XRD spectra of the Rh/Ce0.7 Zr0.3 O2 catalyst. This suggests that either

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some ZrO 2 existed as amorphous phase or crystalline one with mean primary crystal size less than 4 nm (detection limit) in the Rh/Ce0.7 Zr0.3 O2 catalyst [15, 20] or even most of Zr4+ was

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introduced in the crystal lattice of ceria forming a solid solution of unknown atom composition (Ce1-y Zry O2 ) [21].

On the other hand,

broader peaks and a small shift towards

Rh/Ce0.7 Zr0.3 O2 catalyst exhibited

higher 2θ values, which could be related to the

shrinkage of the crystal lattice of Ce-Zr-O due to the smaller size of Zr4+ compared to Ce4+ cation [21] (see inset in Figure 3). Although the support of both catalysts showed strongly similar diffraction peaks of the cubic fluorite structure, the broader peaks obtained for the Rh/Ce0.7 Zr0.3O2 catalyst indicates also a smaller support primary crystallite size; the crystallite size of both catalysts was estimated by the Scherrer’s equation using the diffraction 8

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peak at 28.5º and the results obtained are listed in Table 1. The catalyst redox property usually plays an important role in its activity for N2 O decomposition. Hence, H2 -TPR experiments were carried out to investigate the reduction behavior of the as-prepared samples before and after Rh impregnation. The H2 -TPR profiles

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of supports are shown in Figure 4 (dashed line). Two reduction peaks at 380 and 735 ºC were observed for the CeO 2 solid (total H2 consumption = 1.561 mmol g-1 ), which are attributed to

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the reduction of surface CeO 2 and bulk CeO 2 , respectively [16, 22]. For the Ce0.7 Zr0.3 O2

O (solid solution)

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support, the former peak could be ascribed to concurrent reduction of surface Ce4+ in Ce-Zrand the latter peak could be attributed to the reduction of bulk CeO 2

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and/or Ce1-x Zrx O2 (total H2 consumption = 1.373 mmol g-1 ) [16, 22]. The reduction profile of

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Ce0.7 Zr0.3O2 is similar to that of CeO 2 but the former appears to higher temperatures with broader reduction peaks, possibly due to the support phase distortion, which changes the

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metal–oxygen (e.g. Ce-O-Ce vs Ce-O-Zr) bond strengths [23]. Figure 4 also exhibits the reduction profiles of Rh- loaded catalysts (solid line). The

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reduction peaks below 250 ºC for all the catalysts were assigned to the reduction of welldispersed rhodium oxides on the support [15, 16, 22]. Cao et al. [15, 16] reported that the H2

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uptake observed for Rh2 O3 was much larger than the theoretical value (0.015 mmol/g) due to the concurrent reduction of Rh2 O3 and the support. Similar results were observed in the present experiments. In particular, the reduction peak of the Rh/Ce0.7 Zr0.3O2 (0.158 mmol/g) catalyst was larger than that of Rh/CeO 2 (0.076 mmol/g) catalyst. After impregnation of support with Rh, it was noted that the presence of Rh on the support changes the H2 -TPR profile of the support alone toward lower temperatures [22]. Comparison of the catalyst and the support alone showed that the support’s reduction peaks at 300–500 ºC almost disappeared and moved to lower temperatures (below 250 ºC), which may be due to reduction 9

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by H species spilt over the support [15, 22]. A decrease in the total H2 consumption between the support and the Rh-supported catalyst was observed. This phenomenon is probably related to changes in the electronic state of the catalyst, as suggested by XPS analysis. Obalova et al. [24] reported similar results, wherein the addition of an alkali promoter to

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Co4 MnAlOx resulted in a decrease in H2 consumption due to changes in the electron density. XPS analysis was performed to compare the electronic states and binding e nergies (BEs)

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of the Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O2 catalysts. A curve- fitting analysis was performed after

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Shirley-type background subtraction using a combination of Gauss ian and Lorentzian functions. The Rh 3d5/2 photoelectron spectra of the as-prepared catalysts are shown in Figure

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5. The peak for Rh 3d5/2 is located at 308.7–308.9 eV [5, 6]. The surface Rh species existed

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as Rh0 3d5/2 (307.9–308.2 eV), Rh3+ 3d5/2 (308.6–309.0 eV), and Rh4+ 3d5/2 (309.4–310.0 eV) on all catalysts, in good agreement with values quoted in the literature [5, 25]. The peaks for

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Rh 3d5/2 shifted slightly to a lower BE on the addition of ZrO 2 . The XPS results shown in Table 1 provide the surface Rh composition in three oxidation states, which was calculated

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after deconvolution of the Rh 3d5/2 spectra. Catalysts having a high value of surface Rh3+ concentration could generate more free

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electrons produced from Rh3+, and this could lead to an improved catalytic performance during N 2 O decomposition as noted in the present work. This phenomenon is due to a negative charge density transfer from Rh to ceria in the RhO x /CeO 2 catalyst, as reported by Rico-Pérez et al. [26]. They stated that Rh3+ closely interacts with Ce4+, forms oxygen vacant sites, improves oxygen mobility and transfers also charge density from Rh to ceria, consequently leading to an enhanced catalytic activity in N2 O decomposition [26]. Furthermore, an electric double layer created on the surface of Rh particles due to O 2spillover, either electrochemically by an externally imposed bias or spontaneously by 10

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thermally driven diffusion of O2- ions, invokes strong metal–support interactions [28-30]. The as-created [O δ-, δ +] Rh double layer enhances the intrinsic activity of Rh particles supported on CeO 2 -based supports with high oxygen lability [30]. XPS analysis indicated that the surface electronic state of rhodium oxide changed on the addition of ZrO 2 and that the Rh3+

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species on Rh/CeO 2 is a dominant factor that determines decomposition of N 2 O [15, 24]. According to the literature [3, 4], N 2 O decomposition follows an oxidation–reduction

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mechanism. The well-known mechanism proposed for the decomposition of N 2 O is as

N2 O + * → N2 + O*

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follows [31]:

(1)

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N2 O + O* → N 2 + O2 + * (2) (3)

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2 O* ↔ O2 * → O2 + 2*

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According to the above three elementary steps, N2 O molecules are adsorbed on the active sites (*) and N 2 O dissociates into N 2 gas and O* (reaction 1). N2 O molecules react also with

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adsorbed oxygen species (O*) towards the formation of N 2 and O 2 by reinstating the active sites (*) (reaction 2). The recombination of two adjacent adsorbed oxygen species leads to the

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formation of gaseous oxygen (reaction 3). The adsorption of the oxygen species can be described using the Langmuir–Hinshelwood mechanism, in which the oxygen species adsorbed on the surface migrates until two oxygen species combine to form molecular oxygen that desorbs [31]. According to the literature [27-29], Rh particles in contact with a support having labile lattice oxygen ions could result in weakening of the adsorption bond of oxygen (Rh–O bond). It facilitates the removal of adsorbed O from the surface, easily recovering free active sites necessary for the N 2 O decomposition cycle [29, 30]. Considering the results obtained from the present experimental work and their 11

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correspondence with literature on the N2 O decomposition mechanism, rhodium oxide was considered the active site for N 2 O decomposition, which is initiated when the N–O bond is cleaved and coordinated oxygen atoms were released [3, 31]. As shown in the present work, the composition of support changed the physicochemical properties of the catalyst and

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affected the catalytic reaction for N 2 O decomposition. Parres–Esclapez et al. [25] reported similar results in that CeO 2 had a significant effect on the catalytic reaction and they

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proposed the following reaction mechanism. Ce* –O on a ceria-based support takes part in the

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reaction and labile oxygen atoms from the ceria support are transferred to the noble metal surface. Thus, the catalytic activity over a Rh-based ceria support could be improved. Our

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observations suggest that oxygen atoms present on the Ce0.7 Zr0.3O2 support with improved

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performance of N2 O decomposition.

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redox properties moved more actively on the catalyst surface and improved the catalytic

Conclusions

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A series of Ce1-x Zrx O2-supported Rh catalysts were prepared and tested for their catalytic activity towards N2 O decomposition. The catalytic activity was strongly influenced

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by the Ce:Zr weight ratio. The 1 wt% Rh/Ce-ZrO 2 catalyst with a small amount of ZrO 2 showed better catalytic performance than the Rh/CeO 2 catalyst. An optimal ratio of Ce:Zr (7:3 w/w) was observed in terms of activity in the presence of O2 and/or H2 O inhibition gas. BET, H2 -TPR and XPS analyses showed that the incorporation of ZrO 2 into the Rh/CeO 2 catalyst improved the specific surface area, Rh dispersion and redox ability, and stabilized Rh3+ species on the catalyst surface. It was inferred that adsorbed oxygen species on the surface of Rh/Ce0.7 Zr0.3O2 catalyst were less strongly bound and able to desorb as dioxygen

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molecular species with higher rate as opposed to the case of Rh/CeO 2 , thus leading to an enhanced catalytic performance for N 2 O decomposition.

Acknowledgements

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the Korea Institute of Energy Research (KIER) (B9-2451-13).

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This work was conducted under the framework of the Research and Development Progra m of

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for low-temperature CO oxidation, Appl. Catal. B 83 (2008) 240-255. [24] L. Obalová, K. Karásková, A. Wach, P. Kustrowski, K. Mamulová-Kutláková, S. Michalik, K. Jirátová, Alkali metals as promoters in Co-Mn-Al mixed oxide for N 2 O decomposition, Appl. Catal. A 462-463 (2013) 227-235. [25] S. Parres-Esclapez, I. Such-Basañez, M.J. Illán-Gómez, C. Salinas-Martínez de Lecea, A. Bueno-López, Study by isotopic gases and in situ spectroscopies (DRIFTS, XPS and Raman) of the N 2 O decomposition mechanism on Rh/CeO 2 and Rh/γ-Al2 O3 catalysts, J. Catal. 276 (2010) 390-401. 15

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[26] V. Rico-Pérez, C. Salinas-Martínez de Lecea, A. Bueno-López, Preparation of RhO x /Cey Pr1-yO2 N 2O decomposition catalysts by rhodium nitrate impregnation with different solvents, Appl. Catal. A 472 (2014) 134-142. [27] C. Pliangos, I.V. Yentekakis, V.G. Papadakis, C.G. Vayenas, X.E. Verykios, Supported-

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induced promotional effects on the activity of automotive exhaust catalysts: 1. The case of oxidation of light hydrocarbons (C 2 H4 ), Appl. Catal. B 14 (1997) 161-173.

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[28] P. Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A.D. Lucas-Consuegra, J.-L.

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Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Ionically conducting ceramics as active catalyst supports, Chem. Rev. 113 (2013) 8192-8260.

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[29] I.V. Yentekakis, G. Goula, S. Kampouri, I. Betsi- Argyropoulou, P. Panagiotopoulou, M.J.

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Taylor, G. Kyriakou, R.M. Lambert, Ir-catalysed nitrous oxide (N 2 O) decomposition: Effect of Ir particle size and metal-support interactions, Catal. Lett. 148 (2018) 341-347.

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[30] I.V. Yentekakis, G. Goula, M. Hatzisymeon, I. Betsi- Argyropoulou, G. Botzolaki, K. Kousi, D.I. Kondarides, M.J. Taylor, C.M.A. Parlett, A. Osatiashtiani, G. Kyriakou, J.P.

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Holgado, R.M. Lambert, Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO 2 reforming of methane, Appl. Catal. B 243 (2019)

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490-501.

[31] T. Franken, R. Palkovits, Investigation of potassium doped mixed spinels Cux Co3−x O4 as catalysts for an efficient N 2 O decomposition in real reaction conditions, Appl. Catal. B 176177 (2015) 298-305.

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ACCEPTED MANUSCRIPT Table 1. Textural, structural and redox properties of Rh/CeO2 and Rh/Ce0.7 Zr0.3 O2 catalysts.

BET surface

Total pore

area

volume

Rh

Rh particle

Total H2

dispersion

size

consumption

[%]c

[nm]c

1.9

Crystallite Samples

size [nm]b [m2 g-1 ]a

[cm3 g-1 ]a

Rh/CeO 2

37.3

3.6×10-2

10.9

35.6

Rh/Ce0.7 Zr0.3 O2

78.2

8.1×10-2

7.5

47.3

a

Estimated from N 2 adsorption at -196 ºC.

b

Estimated from Scherrer’s equation at the 28.5 º diffraction peak.

c

Estimated from CO chemisorption at 50 ºC.

d

Estimated from XPS analysis.

D E

A M

T P

E C

C A

17

I R

C S

U N

1.4

T P

Rh speciesd (%)

[mmol g-1 ]

Rh0

Rh3+

Rh4+

1.088

0.16

0.66

0.18

0.794

0.13

0.73

0.14

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250 oC 300 oC

PT

80

60

RI

40

SC

N2O conversion (%)

100

NU

20

0 Rh/Ce0.9Zr0.1O2

Rh/Ce0.7Zr0.3O2 Rh/Ce0.5Zr0.5O2

MA

Rh/CeO2

Rh/Ce0.3Zr0.7O2

Rh/Ce0.1Zr0.9O2

Figure 1. N 2 O conversion over Rh/CeO 2 and Rh/Ce1-x Zrx O2 catalysts. Reaction conditions:

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500 ppm N 2 O in N2 balance; GHSV 45,000 h-1 .

18

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100

Rh/CeO2 Rh/Ce0.7Zr0.3O2

PT

60 100

20

N 2O

60

N 2O

N 2O N2O+O2

40

2

4

6

NU

250

N2O+O2+H2O

Rh/CeO2 Rh/Ce0.7Zr0.3O2

20

0

200

N 2O

N2O+H2O

0

0

RI

40

80

SC

N2O conversion (%)

N2O conversion (%)

80

300

T=350oC

8

10

12

14

Time (h)

350

400

o

MA

Temperature ( C)

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Figure 2. N2 O conversion over Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O2 . Reaction conditions: N 2 O 500 ppm in N2 balance; GHSV = 45,000 h-1 . Inset: Gas-switching experiments for the

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decomposition of N 2 O: 500 ppm N 2 O/N2 switching to 500 ppm N 2 O/3% O 2 /N2 or 500 ppm

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N2 O/3% H2 O/N2 or 500 ppm N 2 O/3%O 2 /3%H2 O/N 2 at 350 ºC; GHSV = 45,000 h-1 .

19

Relative intensity (a.u.)

Rh/Ce0.7Zr0.3O2 Rh/CeO2

28

2

30

32

PT

26

SC

RI

Rh/Ce0.7Zr0.3O2

Rh/CeO2

20

40

NU

Relative intensity (a.u.)

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60

80

MA

2

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Figure 3. Powder XRD spectra of Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O2 catalysts. Inset:

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spectra of Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O 2 catalysts in the 25-32º 2θ range.

20

XRD

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Rh/Ce0.7Zr0.3O2 H2 uptake

RI

=0.158mmol/g

SC

Relative intensity (a.u.)

support catalyst

H2 uptake

100

200

NU

=0.076mmol/g

300

400

500

600

Rh/CeO2

700

800

MA

Temperature (oC)

AC C

EP T

supported Rh catalysts.

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Figure 4. H2 -TPR traces of CeO 2 and Ce0.7 Zr0.3 O2 supports alone and their corresponding

21

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PT

Relative intensity (a.u.)

Rh 3d5/2

SC

RI

Rh/Ce0.7Zr0.3O2

311

310

NU

Rh/CeO2

309

308

307

MA

Binding energy (eV)

AC C

EP T

ED

Figure 5. Rh 3d5/2 photoelectron spectra of Rh/CeO 2 and Rh/Ce0.7 Zr0.3 O2 catalysts.

22

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Highlights



Ce-ZrO 2 -supported Rh catalysts were used in N 2 O decomposition



Ce:Zr weight ratio effected N 2 O decomposition efficiency of Rh, with Ce0.7 Zr0.3 O2 to be the optimal one Performance superiority of the optimal Rh/Ce0.7 Zr0.3 O2 catalyst is remaining even in

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ED

MA

NU

SC

RI

PT

the presence of inhibition gas

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

23