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Thermally induced sintering and redispersion of Au nanoparticles supported on Ce1-xEuxO2 nanocubes and their influence on catalytic CO oxidation
Catalysis Communications 131 (2019) 105798
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Thermally induced sintering and redispersion of Au nanoparticles supported on Ce1-xEuxO2 nanocubes and their influence on catalytic CO oxidation
T
⁎
Oleksii Bezkrovnyia, , Piotr Kraszkiewicza, Igor Krivtsovb, Jorge Quesadab, Salvador Ordóñezb, Leszek Kepinskia a b
W. Trzebiatowski Institute of Low Temperature and Structure Research Polish Academy of Sciences, Wroclaw, Poland Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceria nanocubes Eu doping Gold nanoparticles Sintering Redispersion CO oxidation
The effect of Eu doping of ceria nanocubes on the morphology, oxidation state of deposited Au particles and their activity in CO oxidation was studied. The decisive role of doping of ceria support with Eu in the sintering of deposited Au nanoparticles has been established and discussed for the first time. The redispersion of gold nanoparticles is responsible for the formation of small, ca. ~1 nm Au nanoclusters, and as a consequence the increase of the number of active sites participating in CO-oxidation reaction.
1. Introduction CeO2 is a well-known “active support” of metals owing to its ability to supply active labile oxygen species to the metal nanoparticles (e.g., Au) [1]. The efficiency of this process depends on the rate of oxygen diffusion over the ceria surface/sub-surface, which is facilitated by the presence of oxygen vacancies. Since the energy of oxygen vacancy formation is surface sensitive and follows the sequence {100} < {110} < {111} [2], ceria shaped as nanocubes and nanorods is preferable as active support of metal catalysts [3–5]. In the present work, we choose ceria nanocubes as a model support of Au nanoparticles, active in CO oxidation, for two main reasons, namely, the relatively high activity and the simple structure of ceria nanocubes, e.g. single crystals terminated by {100} faces, with a small contribution of {110} and {111} faces at the edges and corners, respectively. Recent studies showed that the atoms of Au nanoparticles which have direct contact with both ceria substrate and the gas phase play the primary role as active sites in oxidation reactions [6,7]. Therefore, in general, high dispersion of metal is required, and sintering of Au during reaction conditions will decrease its catalytic activity. It was shown [7] that the rate of sintering of supported gold nanoparticles may be strongly sensitive to the concentration of oxygen vacancies at the surface of ceria support. It was proved that Au nanoparticles are firmly attached to the ceria support through an intimate contact with the surface oxygen vacancies. The number of oxygen vacancies at the surface of ceria nanoparticles directly depends on the concentration of
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aliovalent dopant ions (e.g., Eu3+). Thus, Eu3+ incorporation into the crystal lattice of ceria support should have a pronounced impact on the thermally induced sintering process of Au nanoparticles. Though such effect can significantly affect the catalytic activity of Au/ceria system, this effect remains unexplored until now. The present study aims to determine the effect of Eu doping on the processes of redispersion and oxidation of Au nanoparticles on ceria nanocubes. The influence of these processes on the catalytic activity of Au/Ce1-xEuxO2 nanoparticles in the CO oxidation reaction has also been investigated and discussed. 2. Experimental Cube-shaped nanocrystals of ceria were synthesized by applying the microwave-assisted hydrothermal method (MAHT), as described in our previous works [8,9]. In short, Ce(NO3)3·6H2O and Eu(NO3)3·5H2O were first dissolved in distilled water. The obtained solution was mixed with an appropriate amount of aqueous NaOH solution and then stirred for 60 min. The final solution was treated at 200 °C for 3 h under autogenous pressure in an autoclave. The obtained precipitate powder was washed and dried at 60 °C for 12 h. Au nanoparticles were deposited on the ceria nanocubes using a deposition-precipitation method, similar to that used by Lin et al. [10]. 100 mg of ceria nanocubes, 4 mg HAuCl4, 256 mg (NH2)2CO and 12 ml H2O were mixed to form a suspension. The suspension was stirred and kept at 80 °C in a silicone oil bath for 24 h. Au/ceria particles were
Corresponding author. E-mail address: [email protected] (O. Bezkrovnyi).
https://doi.org/10.1016/j.catcom.2019.105798 Received 30 April 2019; Received in revised form 16 August 2019; Accepted 20 August 2019 Available online 20 August 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 131 (2019) 105798
O. Bezkrovnyi, et al.
S3, Fig. 1-b and Figs. S3-S5). It should be noted that in the as-prepared samples (dried at 60 °C), the size distribution of Au particles on CeO2 and Ce0.70Eu0.30O2 nanocubes were unimodal, and typical particle sizes were in the range of 2–4 and 1–3 nm, respectively (see Table S3 and Figs. S6 and S7). The differences in the dispersion of Au NP over pure and Eu-doped ceria nanocubes after treatment at 300 °C can be explainedas follows. There are two primary mechanisms of the thermally driven growth process of noble metal nanoparticles. In the first (particle migration), individual particles migrate over the support surface and coalesce with other particles after collision [12,13]. The second mechanism is Ostwald ripening (OR), where single atoms detach from small particles and diffuse towards larger ones, either over the support surface or via the gas phase [13]. Akita et al. [14,15] showed that Au nanoparticles are immobile at the surface of CeO2 support in the air at temperatures up to 600 °C, and atomic transport is responsible for the observed particle growth. Thus, in the present case (at temperatures up to 300 °C), OR mechanism should be a dominant one responsible for the sintering of Au nanoparticles. The efficiency of the OR mechanism depends directly on the rate of surface diffusion of gold atoms over the ceria support, which, in turn, depends on the number of oxygen vacancies on the surface of ceria support. Ta et al. [7] showed that Au nanoparticles are firmly attached, through intimate contact with the surface oxygen vacancies, to ceria nanorods. We increased the number of oxygen vacancies at the surface of ceria by its doping with aliovalent Eu3+ ions, so that doped ceria nanocubes have more anchoring sites for gold nanoparticles than pure CeO2 nanocubes [9]. Therefore, the surface diffusion of gold atoms over ceria decreases with its doping with Eu3+, which inhibits sintering of Au nanoparticles. In effect, the size of Au nanoparticles, which in the as-prepared Au/CeO2 and Au/ Ce0.70Eu0.30O2 was in the 2–4 and 1–3 nm range, respectively, changed after annealing at 300 °C to bimodal for sizes < 1 nm and 3–6 nm, or to unimodal for sizes 2–4 nm, respectively (see Fig. 1, Figs. S3-S7 and Table S3). It appears that Eu3+ dopant which induced anchoring of Au nanoparticles prevents the “redispersion” process responsible for the formation of Au NPs with sizes < 1 nm on the undoped CeO2. This result has a significant implication on the reducibility and catalytic activity of these materials.
washed, dried at 50 °C for 12 h and annealed in air at 300 °C for 2 h. Crystal structure and morphology of the samples were determined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The adsorbed phase composition, catalyst surface composition and the oxidation state of surface atoms in the samples were measured by DRIFTS and X-ray photoelectron spectroscopy (XPS). Specific surface area (SSA) was measured by N2 adsorption/desorption isotherms at 77 K. The reducibility of the samples was studied by the H2-TPR (hydrogen temperature-programmed reduction) technique. The catalytic activity of the samples in CO oxidation. A detailed description of the experimental methods is given in the electronic supplementary materials.
3. Results and discussion 3.1. Ceria structure and morphology Previously, we showed that ceria nanocubes, synthesized by the MAHT method, are single crystals terminated mainly by the {100} faces and with a small contribution only of {110} and {111} faces at the edges and corners, respectively [9,11]. Eu distribution in the ceria nanocubes structure was found to be homogeneous [9]. Powder XRD results (Fig. S1) show that Au/Ce1-xEuxO2 (x = 0, 0.1 and 0.20) samples contain fluorite-type ceria as a support. In addition, weak reflections of monoclinic EuOOH appeared in the patterns of Au/Ce0.70Eu0.30O2 and Au/Ce0.60Eu0.40O2 solids. The whole pattern fitting analysis for these samples revealed the presence of ~ 3 and 26.5 wt% EuOOH, respectively. The true Eu content (x) in ceria nanocubes in the Au/ Ce0.60Eu0.40O2 sample is thus ~ 0.3 (see Fig. S1 and Table S1 in ESI). EDS data also show that the overall Eu content in the samples agree with the nominal value set in the synthesis, and the Au content in all samples is ~ 1 at.% (see Table S2 in ESI). Fig. 1 presents STEM images of the CeO2 and Ce0.70Eu0.30O2 nanocubes decorated with gold nanoparticles after pretreated at 300 °C in air for 3 h. FTIR spectra obtained (see Fig. S2 in ESI) show that annealing at 300 °C almost completely cleans the surface of Au/Ce1-xEuxO2 catalysts from –OH, –CO3 and -NO3 groups, that could eventually influence CO oxidation catalytic tests. Interestingly, the Au particles supported on pure CeO2 have bimodal size distribution. Both nanoclusters (NC), smaller than 1 nm, and 3–6 nm nanoparticles (NP) are present (see Fig. 1-a and Table S3). On the contrary, Au NPs supported on Eu-doped nanocubes appear more homogeneous. Typical sizes of Au nanoparticles on Eu-doped (x = 0–0.4) nanocubes after treatment at 300 °C are 2–4 nm (See Table
3.2. XPS study The oxidation of gold nanoparticles plays a critical role in the Au catalyzed CO oxidation reaction [16,17]. The XPS spectrum of Au 4f region of the Au/CeO2 and Au/Ce1-xEuxO2 samples is presented in Fig. 2
Fig. 1. STEM images of the Au/CeO2 (a) and Au/Ce0.70Eu0.30O2 (b) samples pre-treated at 300 °C for 3 h in air. HR-STEM images of the Au nanoparticles (NP) and nanoclusters (NC) are shown as insets. 2
Catalysis Communications 131 (2019) 105798
O. Bezkrovnyi, et al.
Fig. 2. Au 4f region of XPS spectra of Au/CeO2 sample treated at 300 °C in air for 3 h. Measured spectrum (black), its deconvolution into three doublets: Au0 (red), Au+ (green) and Au3+ (blue), as well as the sum (dash red) are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 Au0 and Auδ+ content in Au/Ce1-xEuxO2 samples. Sample
Au0, %
Auδ+, %
Au/CeO2 Au/Ce0.90Eu0.10O2 Au/Ce0.80Eu0.20O2 Au/Ce0.70Eu0.30O2
84 69 72 79
16 31 28 21
Fig. 3. Low-temperature region of H2-TPR profiles of the Au/Ce1-xEuxO2 (x = 0; 0.1; 0.20; 0.30; 0.40) catalysts (pre-treated at 300 °C in air for 3 h).
35 °C) to the reduction of Ce-O-Au species on the perimeter of Au nanoparticles, which have a direct contact with both ceria substrate and the gas phase, and the second peak (at ~ 90 °C) to the surface reduction of ceria support (see ESI for details). The intensity of the first peak strongly declines with increasing concentration of europium, while that of the second peak (at 90 °C) increases. We suppose that the reduction of Ce-O-Au species in small (< 1 nm) gold clusters makes the main contribution to the first peak (at 35 °C). Taking into account that doping of ceria support with Eu inhibits the formation of Au sub-nanometer clusters in size (see Fig. 1), the lowering of Ce-O-Au related peak becomes clear.
and Fig. S8 (ESI), respectively. As seen in Table 1, the Auδ+ content depends on the europium content in a rather complicated way. It increases up to x = 0.10, then starts to decline. The observed trend reflects the changes in the morphology of Au NPs in the samples. For Au/CeO2, a bimodal size distribution of Au NPs was observed, with both very small (≤ 1 nm) and large (3–6 nm) particles detected in the HAADF (High-angle annular dark-field) images. According to the literature [17], single Au atoms on ceria are fully oxidized, gold clusters with sizes < 2 nm have mixed cationic and metallic states, while Au NPs with sizes in the 3–4 nm range appear as reduced Au structures. We may expect, therefore, certain content of cationic Auδ+ in the Au/CeO2 sample. Doping with Eu3+ increases initially the net positive charge of Au particles, (Table 2), though according to TEM, it decreases the number of the smallest particles (< 1 nm). An explanation for this apparent inconsistency could be based on the assumption that Au3+ contribution mainly comes from individual Au atoms adsorbed at the ceria surface [18]. Such species can not, however, be detected in HAADF images of the present samples due to the large thickness of the ceria particles. A large number of surface defects in the Eu-doped ceria causes Au atoms to be strongly bonded at the surface, and do not aggregate during pretreatment at 300 °C, as it happens for pure ceria support. At the highest Eu content (x = 0.3), there is a net decline in the amount of cationic gold (see Table 1). A possible reason is the aggregation of oxygen vacancies with a simultaneous decrease in the number of single atoms at the surface [9]. It should be noted that the above offered explanation of the effect of Eu doping of ceria support on the gold nanoparticles charge (oxidation stte) is presumptive and needs more experimental confirmation.
3.4. Catalytic performance in CO oxidation Fig. 4 shows CO-oxidation light-off curves for the Au/Ce1-xEuxO2 samples (x = 0, 0.10, 0.20, 0.30 and 0.40). Since the specific surface area of all the ceria samples studied is comparable, ca. 10 m2/g, it is admissible to conclude that the catalytic activity of the present Au/ceria samples is determined mainly by the morphology of the Au metal phase and the defect concentration in the ceria support phase and not by differences in the specific surface area. As shown in Fig. 4, the introduction of europium to ceria lattice at
3.3. H2-TPR study Since the easily reducible metal cationic species at the Au-ceria interface are vital to the rate of catalytic oxidation reaction, we will focus on the low-temperature region of the H2-TPR profiles depicted in Fig. 3. The full temperature range of H2-TPR profiles of the Au/Ce1xEuxO2 samples and the associated discussion are given in ESI (Fig. S9). Two reduction peaks centered at ~ 35 and ~ 90 °C can be distinguished in all samples (Fig. 3). We attribute the first peak (at ~
Fig. 4. CO oxidation light-off curves for the Au/Ce1-xEuxO2 samples (x = 0, 0.10, 0.20, 0,30 and 0.40). The dependence of T50 on the Eu content in ceria support is also shown in the inset graph. 3
Catalysis Communications 131 (2019) 105798
O. Bezkrovnyi, et al.
Principado de Asturias (GRUPIN-ID2018-170; GRUPIN-ID2018-116).
the level up to x = 0.2 decreases visibly the catalytic activity in terms of the temperature at 50% conversion (inversely proportional to T50). We attribute this decline to the decreased population of small (≤1 nm) Au nanoclusters deposited on the Eu-doped ceria support (see Fig. 1). This result also agrees with the H2-TPR studies (see Fig. 3), which reveal the drop of the intensity of Ce-O-Au reduction peak with the increase of europium content. However, by further increasing the content of europium to the value of x = 0.3, the activity of the catalyst is enhanced (see Fig. 4, inset graph), where the activity of Au/Ce0.70Eu0.30O2 is higher than that of Au/Ce0.80Eu0.20O2 in the whole T-range (0–125 °C) and higher than Au/CeO2 in the 0–50 °C range and similar at higher T's (Fig. 4). We attribute the relatively high catalytic activity of Au/Ce0.70Eu0.30O2 to the presence of a large number of Eu-induced oxygen vacancies, favoring the CO oxidation according to the Mars-van Krevelen mechanism [3]. Futher increase of the Eu content (x = 0.40) results in a strong decrease of the catalytic activity, most likely due to the formation of bi-phasic ceria –EuOOH support composition. As we showed earlier, the highest ceria doping level with Eu3+ is achieved at x ≈ 0.3, and thus no gain in oxygen vacancies concentration occurs for x = 0.4. However, a noticeable amount of EuOOH phase is formed, which apparently is not an appropriate support composition for Au compared to ceria. Another factor that determines the catalytic activity of Eu-doped ceria samples is the electronic structure of deposited Au nanoparticles. According to the XPS data (Table 1), the activity of the catalysts correlates well with the concentration of cationic gold species present, i.e., the least active sample contains the highest concentration of cationic Au. It should be stated at this point that there is no consensus in the literature which state of gold must be considered as active in the CO oxidation reaction. Some authors assigned the high activity of supported gold catalysts to the presence of cationic Au [19–21]. Other researchers reported that cationic gold is not active in the low-temperature CO oxidation reaction, and instead the metallic gold is the active one. [22,23]. The results of the present work show that there is a negative correlation between the content of cationic gold species and the activity in CO oxidation reaction.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105798. References [1] M.M. Schubert, S. Hackenberg, A.C. van Veen, M. Muhler, V. Plzak, R.J. Behm, CO oxidation over supported gold catalysts—“inert” and “active” support materials and their role for the oxygen supply during reaction, J. Catal. 197 (2001) 113–122. [2] J.C. Conesa, Computer modeling of surfaces and defects on cerium dioxide, Surf. Sci. 339 (1995) 337–352. [3] A. Trovarelli, J. Llorca, Ceria catalysts at Nanoscale: how do crystal shapes shape catalysis? ACS Catal. 17 (2017) 4716–4735. [4] L. Liu, Z. Yao, Y. Deng, F. Gao, B. Liu, L. Dong, Morphology and crystal-plane effects of nanoscale ceriaon the activity of CuO/CeO2 for NO reduction by CO, ChemCatChem 3 (2011) 978–989. [5] M. Lykaki, E. Pachatouridou, S.A.C. Carabineiro, E. Iliopoulou, C. Andriopoulou, N. Kallithrakas-Kontose, S. Boghosian, M. Konsolakis, Ceria nanoparticles shape effects on the structural defects and surface chemistry: implications in CO oxidation by Cu/CeO2 catalyst, Appl. Catal. B Environ. 230 (2018) 18–28. [6] T. Fujitani, I. Nakamura, Mechanism and active sites of the oxidation of CO over au/ TiO2, Angew. Chem. Int. Ed. 50 (2011) 10144–10147. [7] N. Ta, J. Liu, S. Chenna, P.A. Crozier, Y. Li, A. Chen, W. Shen, Stabilized gold nanoparticles on ceria Nanorods by strong interfacial anchoring, J. Am. Chem. Soc. 134 (2012) 20585–20588. [8] O.S. Bezkrovnyi, R. Lisiecki, L. Kepinski, Relationship between morphology and structure of shape-controlled CeO2 nanocrystals synthesized by microwave assisted hydrothermal method, Cryst. Res. Technol. 51 (2016) 554–560. [9] O. Bezkrovnyi, M.A. Małecka, R. Lisiecki, V. Ostroushko, A.G. Thomas, S. Gorantla, L. Kepinski, The effect of Eu doping on the growth, structure and red-ox activity of ceria nanocubes, CrystEngComm 20 (2018) 1698–1704. [10] Y. Lin, Z. Wu, J. Wen, K. Ding, X. Yang, K.R. Poeppelmeier, L.D. Marks, Adhesion and atomic structures of gold on ceria nanostructures: the role of surface structure and oxidation state of ceria supports, Nano Lett. 15 (2015) 5375–5381. [11] O.S. Bezkrovnyi, P. Kraszkiewicz, M. Ptak, L. Kepinski, Thermally induced reconstruction of ceria nanocubes into zigzag {111}-nanofacetted structures and its influence on catalytic activity in CO oxidation, Catal. Commun. 117 (2018) 94–98. [12] E. Ruckenstein, B. Pulvermacher, Kinetics of crystallite sintering during heat treatment of supported metal catalysts, AICHE J. 19 (1973) 356–364. [13] F. Behafarid, B.R. Cuenya, Towards the understanding of sintering phenomena at the Nanoscale: geometric and environmental effects, Top. Catal. 56 (2013) 1542–1559. [14] T. Akita, K. Tanaka, M. Kohyama, TEM and HAADF-STEM study of the structure of au nano-particles on CeO2, J. Mater. Sci. 43 (2008) 3917–3922. [15] T. Akita, K. Tanaka, M. Kohyama, M. Haruta, Analytical TEM study on structural changes of Au particles on cerium oxide using a heating holder, Catal. Today 122 (2007) 233–238. [16] C. Zhang, A. Michaelides, D.A. King, S.J. Jenkins, Positive charge states and possible polymorphism of gold Nanoclusters on reduced ceria, J. Am. Chem. Soc. 132 (2010) 2175–2182. [17] L.-W. Guo, P.-P. Du, X.-P. Fu, C. Ma, J. Zeng, R. Si, Y.-Y. Huang, C.-J. Jia, Y.W. Zhang, C.-H. Yan, Contributions of distinct gold species to catalytic reactivity for carbon monoxide oxidation, Nat. Commun. 7 (2016) 13481. [18] J.H. Carter, X. Liu, Q. He, S. Althahban, E. Nowicka, S.J. Freakley, L. Niu, D.J. Morgan, Y. Li, J.W.H. Niemantsverdriet, S. Golunski, C.J. Kiely, G.J. Hutchings, Activation and deactivation of gold/ceria–zirconia in the low-temperature water–gas shift reaction, Angew. Chem. Int. Ed. 56 (2017) 16037–16041. [19] P. Concepcion, S. Carrettin, A. Corma, Stabilization of cationic gold species on Au/ CeO2 catalysts under working conditions, Appl. Catal. A Gen. 307 (2006) 42–45. [20] J. Guzman, B.C. Gates, Catalysis by supported gold: correlation between catalytic activity for CO oxidation and oxidation states of gold, J. Am. Chem. Soc. 126 (2004) 2672–2673. [21] G.J. Hutchings, M.S. Hall, A.F. Carley, P. Landon, B.E. Solsona, C.J. Kiely, A. Herzing, M. Makkee, J.A. Moulijn, A. Overweg, J.C. Fierro-Gonzalez, J. Guzman, B.C. Gates, Role of gold cations in the oxidation of carbon monoxide catalyzed by iron oxide-supported gold, J. Catal. 242 (2006) 71–81. [22] S. Wei, X.-P. Fu, W.-W. Wang, Z. Jin, Q.-S. Song, C.-J. Jia, Au/TiO2 catalysts for CO oxidation: effect of gold state to reactivity, J. Phys. Chem. C 122 (2018) 4928–4936. [23] B.M. Reddy, L. Katta, G. Thrimurthulu, Novel Nanocrystalline Ce1-xLaxO2-δ (x = 0.2) solid solutions: structural characteristics and catalytic performance, Chem. Mater. 22 (2010) 467–475.
4. Conclusions It was shown that doping of ceria nanocubes with Eu3+ results in the “anchoring” of deposited Au nanoparticles. However, at the same time it prevents the “redispersion” process responsible for the formation of Au nanoparticles with < 1 nm in size observed for the case of pure CeO2 nanocubes support. Such nanoclusters play a key role in the lowtemperature reducibility of Au/Ce1-xEuxO2 system, the latter determining the concentration of surface active Ce-O-Au sites with proper metal oxidation states in favor of CO oxidation reaction rate. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements The authors thank Silesian Association of Electron Microscopy, Wrocław Research Centre EIT+, Wroclaw, Poland for access to the Titan microscope, Dr. K. Adamska for BET measurements and Dr. W. Mista for the CO oxidation tests. This work was financially supported by NCN (UMO-2017/27/N/ST5/02731) and the Spanish MINECO (MAT2016-78155-C2-1-R; CTQ2017-89443-C3-2-R) and Gobierno del
4
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Thermally induced sintering and redispersion of Au nanoparticles supported on Ce1-xEuxO2 nanocubes and their influence on catalytic CO oxidation
T
⁎
Oleksii Bezkrovnyia, , Piotr Kraszkiewicza, Igor Krivtsovb, Jorge Quesadab, Salvador Ordóñezb, Leszek Kepinskia a b
W. Trzebiatowski Institute of Low Temperature and Structure Research Polish Academy of Sciences, Wroclaw, Poland Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain
A R T I C LE I N FO
A B S T R A C T
Keywords: Ceria nanocubes Eu doping Gold nanoparticles Sintering Redispersion CO oxidation
The effect of Eu doping of ceria nanocubes on the morphology, oxidation state of deposited Au particles and their activity in CO oxidation was studied. The decisive role of doping of ceria support with Eu in the sintering of deposited Au nanoparticles has been established and discussed for the first time. The redispersion of gold nanoparticles is responsible for the formation of small, ca. ~1 nm Au nanoclusters, and as a consequence the increase of the number of active sites participating in CO-oxidation reaction.
1. Introduction CeO2 is a well-known “active support” of metals owing to its ability to supply active labile oxygen species to the metal nanoparticles (e.g., Au) [1]. The efficiency of this process depends on the rate of oxygen diffusion over the ceria surface/sub-surface, which is facilitated by the presence of oxygen vacancies. Since the energy of oxygen vacancy formation is surface sensitive and follows the sequence {100} < {110} < {111} [2], ceria shaped as nanocubes and nanorods is preferable as active support of metal catalysts [3–5]. In the present work, we choose ceria nanocubes as a model support of Au nanoparticles, active in CO oxidation, for two main reasons, namely, the relatively high activity and the simple structure of ceria nanocubes, e.g. single crystals terminated by {100} faces, with a small contribution of {110} and {111} faces at the edges and corners, respectively. Recent studies showed that the atoms of Au nanoparticles which have direct contact with both ceria substrate and the gas phase play the primary role as active sites in oxidation reactions [6,7]. Therefore, in general, high dispersion of metal is required, and sintering of Au during reaction conditions will decrease its catalytic activity. It was shown [7] that the rate of sintering of supported gold nanoparticles may be strongly sensitive to the concentration of oxygen vacancies at the surface of ceria support. It was proved that Au nanoparticles are firmly attached to the ceria support through an intimate contact with the surface oxygen vacancies. The number of oxygen vacancies at the surface of ceria nanoparticles directly depends on the concentration of
⁎
aliovalent dopant ions (e.g., Eu3+). Thus, Eu3+ incorporation into the crystal lattice of ceria support should have a pronounced impact on the thermally induced sintering process of Au nanoparticles. Though such effect can significantly affect the catalytic activity of Au/ceria system, this effect remains unexplored until now. The present study aims to determine the effect of Eu doping on the processes of redispersion and oxidation of Au nanoparticles on ceria nanocubes. The influence of these processes on the catalytic activity of Au/Ce1-xEuxO2 nanoparticles in the CO oxidation reaction has also been investigated and discussed. 2. Experimental Cube-shaped nanocrystals of ceria were synthesized by applying the microwave-assisted hydrothermal method (MAHT), as described in our previous works [8,9]. In short, Ce(NO3)3·6H2O and Eu(NO3)3·5H2O were first dissolved in distilled water. The obtained solution was mixed with an appropriate amount of aqueous NaOH solution and then stirred for 60 min. The final solution was treated at 200 °C for 3 h under autogenous pressure in an autoclave. The obtained precipitate powder was washed and dried at 60 °C for 12 h. Au nanoparticles were deposited on the ceria nanocubes using a deposition-precipitation method, similar to that used by Lin et al. [10]. 100 mg of ceria nanocubes, 4 mg HAuCl4, 256 mg (NH2)2CO and 12 ml H2O were mixed to form a suspension. The suspension was stirred and kept at 80 °C in a silicone oil bath for 24 h. Au/ceria particles were
Corresponding author. E-mail address: [email protected] (O. Bezkrovnyi).
https://doi.org/10.1016/j.catcom.2019.105798 Received 30 April 2019; Received in revised form 16 August 2019; Accepted 20 August 2019 Available online 20 August 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 131 (2019) 105798
O. Bezkrovnyi, et al.
S3, Fig. 1-b and Figs. S3-S5). It should be noted that in the as-prepared samples (dried at 60 °C), the size distribution of Au particles on CeO2 and Ce0.70Eu0.30O2 nanocubes were unimodal, and typical particle sizes were in the range of 2–4 and 1–3 nm, respectively (see Table S3 and Figs. S6 and S7). The differences in the dispersion of Au NP over pure and Eu-doped ceria nanocubes after treatment at 300 °C can be explainedas follows. There are two primary mechanisms of the thermally driven growth process of noble metal nanoparticles. In the first (particle migration), individual particles migrate over the support surface and coalesce with other particles after collision [12,13]. The second mechanism is Ostwald ripening (OR), where single atoms detach from small particles and diffuse towards larger ones, either over the support surface or via the gas phase [13]. Akita et al. [14,15] showed that Au nanoparticles are immobile at the surface of CeO2 support in the air at temperatures up to 600 °C, and atomic transport is responsible for the observed particle growth. Thus, in the present case (at temperatures up to 300 °C), OR mechanism should be a dominant one responsible for the sintering of Au nanoparticles. The efficiency of the OR mechanism depends directly on the rate of surface diffusion of gold atoms over the ceria support, which, in turn, depends on the number of oxygen vacancies on the surface of ceria support. Ta et al. [7] showed that Au nanoparticles are firmly attached, through intimate contact with the surface oxygen vacancies, to ceria nanorods. We increased the number of oxygen vacancies at the surface of ceria by its doping with aliovalent Eu3+ ions, so that doped ceria nanocubes have more anchoring sites for gold nanoparticles than pure CeO2 nanocubes [9]. Therefore, the surface diffusion of gold atoms over ceria decreases with its doping with Eu3+, which inhibits sintering of Au nanoparticles. In effect, the size of Au nanoparticles, which in the as-prepared Au/CeO2 and Au/ Ce0.70Eu0.30O2 was in the 2–4 and 1–3 nm range, respectively, changed after annealing at 300 °C to bimodal for sizes < 1 nm and 3–6 nm, or to unimodal for sizes 2–4 nm, respectively (see Fig. 1, Figs. S3-S7 and Table S3). It appears that Eu3+ dopant which induced anchoring of Au nanoparticles prevents the “redispersion” process responsible for the formation of Au NPs with sizes < 1 nm on the undoped CeO2. This result has a significant implication on the reducibility and catalytic activity of these materials.
washed, dried at 50 °C for 12 h and annealed in air at 300 °C for 2 h. Crystal structure and morphology of the samples were determined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The adsorbed phase composition, catalyst surface composition and the oxidation state of surface atoms in the samples were measured by DRIFTS and X-ray photoelectron spectroscopy (XPS). Specific surface area (SSA) was measured by N2 adsorption/desorption isotherms at 77 K. The reducibility of the samples was studied by the H2-TPR (hydrogen temperature-programmed reduction) technique. The catalytic activity of the samples in CO oxidation. A detailed description of the experimental methods is given in the electronic supplementary materials.
3. Results and discussion 3.1. Ceria structure and morphology Previously, we showed that ceria nanocubes, synthesized by the MAHT method, are single crystals terminated mainly by the {100} faces and with a small contribution only of {110} and {111} faces at the edges and corners, respectively [9,11]. Eu distribution in the ceria nanocubes structure was found to be homogeneous [9]. Powder XRD results (Fig. S1) show that Au/Ce1-xEuxO2 (x = 0, 0.1 and 0.20) samples contain fluorite-type ceria as a support. In addition, weak reflections of monoclinic EuOOH appeared in the patterns of Au/Ce0.70Eu0.30O2 and Au/Ce0.60Eu0.40O2 solids. The whole pattern fitting analysis for these samples revealed the presence of ~ 3 and 26.5 wt% EuOOH, respectively. The true Eu content (x) in ceria nanocubes in the Au/ Ce0.60Eu0.40O2 sample is thus ~ 0.3 (see Fig. S1 and Table S1 in ESI). EDS data also show that the overall Eu content in the samples agree with the nominal value set in the synthesis, and the Au content in all samples is ~ 1 at.% (see Table S2 in ESI). Fig. 1 presents STEM images of the CeO2 and Ce0.70Eu0.30O2 nanocubes decorated with gold nanoparticles after pretreated at 300 °C in air for 3 h. FTIR spectra obtained (see Fig. S2 in ESI) show that annealing at 300 °C almost completely cleans the surface of Au/Ce1-xEuxO2 catalysts from –OH, –CO3 and -NO3 groups, that could eventually influence CO oxidation catalytic tests. Interestingly, the Au particles supported on pure CeO2 have bimodal size distribution. Both nanoclusters (NC), smaller than 1 nm, and 3–6 nm nanoparticles (NP) are present (see Fig. 1-a and Table S3). On the contrary, Au NPs supported on Eu-doped nanocubes appear more homogeneous. Typical sizes of Au nanoparticles on Eu-doped (x = 0–0.4) nanocubes after treatment at 300 °C are 2–4 nm (See Table
3.2. XPS study The oxidation of gold nanoparticles plays a critical role in the Au catalyzed CO oxidation reaction [16,17]. The XPS spectrum of Au 4f region of the Au/CeO2 and Au/Ce1-xEuxO2 samples is presented in Fig. 2
Fig. 1. STEM images of the Au/CeO2 (a) and Au/Ce0.70Eu0.30O2 (b) samples pre-treated at 300 °C for 3 h in air. HR-STEM images of the Au nanoparticles (NP) and nanoclusters (NC) are shown as insets. 2
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Fig. 2. Au 4f region of XPS spectra of Au/CeO2 sample treated at 300 °C in air for 3 h. Measured spectrum (black), its deconvolution into three doublets: Au0 (red), Au+ (green) and Au3+ (blue), as well as the sum (dash red) are shown. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Table 1 Au0 and Auδ+ content in Au/Ce1-xEuxO2 samples. Sample
Au0, %
Auδ+, %
Au/CeO2 Au/Ce0.90Eu0.10O2 Au/Ce0.80Eu0.20O2 Au/Ce0.70Eu0.30O2
84 69 72 79
16 31 28 21
Fig. 3. Low-temperature region of H2-TPR profiles of the Au/Ce1-xEuxO2 (x = 0; 0.1; 0.20; 0.30; 0.40) catalysts (pre-treated at 300 °C in air for 3 h).
35 °C) to the reduction of Ce-O-Au species on the perimeter of Au nanoparticles, which have a direct contact with both ceria substrate and the gas phase, and the second peak (at ~ 90 °C) to the surface reduction of ceria support (see ESI for details). The intensity of the first peak strongly declines with increasing concentration of europium, while that of the second peak (at 90 °C) increases. We suppose that the reduction of Ce-O-Au species in small (< 1 nm) gold clusters makes the main contribution to the first peak (at 35 °C). Taking into account that doping of ceria support with Eu inhibits the formation of Au sub-nanometer clusters in size (see Fig. 1), the lowering of Ce-O-Au related peak becomes clear.
and Fig. S8 (ESI), respectively. As seen in Table 1, the Auδ+ content depends on the europium content in a rather complicated way. It increases up to x = 0.10, then starts to decline. The observed trend reflects the changes in the morphology of Au NPs in the samples. For Au/CeO2, a bimodal size distribution of Au NPs was observed, with both very small (≤ 1 nm) and large (3–6 nm) particles detected in the HAADF (High-angle annular dark-field) images. According to the literature [17], single Au atoms on ceria are fully oxidized, gold clusters with sizes < 2 nm have mixed cationic and metallic states, while Au NPs with sizes in the 3–4 nm range appear as reduced Au structures. We may expect, therefore, certain content of cationic Auδ+ in the Au/CeO2 sample. Doping with Eu3+ increases initially the net positive charge of Au particles, (Table 2), though according to TEM, it decreases the number of the smallest particles (< 1 nm). An explanation for this apparent inconsistency could be based on the assumption that Au3+ contribution mainly comes from individual Au atoms adsorbed at the ceria surface [18]. Such species can not, however, be detected in HAADF images of the present samples due to the large thickness of the ceria particles. A large number of surface defects in the Eu-doped ceria causes Au atoms to be strongly bonded at the surface, and do not aggregate during pretreatment at 300 °C, as it happens for pure ceria support. At the highest Eu content (x = 0.3), there is a net decline in the amount of cationic gold (see Table 1). A possible reason is the aggregation of oxygen vacancies with a simultaneous decrease in the number of single atoms at the surface [9]. It should be noted that the above offered explanation of the effect of Eu doping of ceria support on the gold nanoparticles charge (oxidation stte) is presumptive and needs more experimental confirmation.
3.4. Catalytic performance in CO oxidation Fig. 4 shows CO-oxidation light-off curves for the Au/Ce1-xEuxO2 samples (x = 0, 0.10, 0.20, 0.30 and 0.40). Since the specific surface area of all the ceria samples studied is comparable, ca. 10 m2/g, it is admissible to conclude that the catalytic activity of the present Au/ceria samples is determined mainly by the morphology of the Au metal phase and the defect concentration in the ceria support phase and not by differences in the specific surface area. As shown in Fig. 4, the introduction of europium to ceria lattice at
3.3. H2-TPR study Since the easily reducible metal cationic species at the Au-ceria interface are vital to the rate of catalytic oxidation reaction, we will focus on the low-temperature region of the H2-TPR profiles depicted in Fig. 3. The full temperature range of H2-TPR profiles of the Au/Ce1xEuxO2 samples and the associated discussion are given in ESI (Fig. S9). Two reduction peaks centered at ~ 35 and ~ 90 °C can be distinguished in all samples (Fig. 3). We attribute the first peak (at ~
Fig. 4. CO oxidation light-off curves for the Au/Ce1-xEuxO2 samples (x = 0, 0.10, 0.20, 0,30 and 0.40). The dependence of T50 on the Eu content in ceria support is also shown in the inset graph. 3
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Principado de Asturias (GRUPIN-ID2018-170; GRUPIN-ID2018-116).
the level up to x = 0.2 decreases visibly the catalytic activity in terms of the temperature at 50% conversion (inversely proportional to T50). We attribute this decline to the decreased population of small (≤1 nm) Au nanoclusters deposited on the Eu-doped ceria support (see Fig. 1). This result also agrees with the H2-TPR studies (see Fig. 3), which reveal the drop of the intensity of Ce-O-Au reduction peak with the increase of europium content. However, by further increasing the content of europium to the value of x = 0.3, the activity of the catalyst is enhanced (see Fig. 4, inset graph), where the activity of Au/Ce0.70Eu0.30O2 is higher than that of Au/Ce0.80Eu0.20O2 in the whole T-range (0–125 °C) and higher than Au/CeO2 in the 0–50 °C range and similar at higher T's (Fig. 4). We attribute the relatively high catalytic activity of Au/Ce0.70Eu0.30O2 to the presence of a large number of Eu-induced oxygen vacancies, favoring the CO oxidation according to the Mars-van Krevelen mechanism [3]. Futher increase of the Eu content (x = 0.40) results in a strong decrease of the catalytic activity, most likely due to the formation of bi-phasic ceria –EuOOH support composition. As we showed earlier, the highest ceria doping level with Eu3+ is achieved at x ≈ 0.3, and thus no gain in oxygen vacancies concentration occurs for x = 0.4. However, a noticeable amount of EuOOH phase is formed, which apparently is not an appropriate support composition for Au compared to ceria. Another factor that determines the catalytic activity of Eu-doped ceria samples is the electronic structure of deposited Au nanoparticles. According to the XPS data (Table 1), the activity of the catalysts correlates well with the concentration of cationic gold species present, i.e., the least active sample contains the highest concentration of cationic Au. It should be stated at this point that there is no consensus in the literature which state of gold must be considered as active in the CO oxidation reaction. Some authors assigned the high activity of supported gold catalysts to the presence of cationic Au [19–21]. Other researchers reported that cationic gold is not active in the low-temperature CO oxidation reaction, and instead the metallic gold is the active one. [22,23]. The results of the present work show that there is a negative correlation between the content of cationic gold species and the activity in CO oxidation reaction.
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4. Conclusions It was shown that doping of ceria nanocubes with Eu3+ results in the “anchoring” of deposited Au nanoparticles. However, at the same time it prevents the “redispersion” process responsible for the formation of Au nanoparticles with < 1 nm in size observed for the case of pure CeO2 nanocubes support. Such nanoclusters play a key role in the lowtemperature reducibility of Au/Ce1-xEuxO2 system, the latter determining the concentration of surface active Ce-O-Au sites with proper metal oxidation states in favor of CO oxidation reaction rate. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements The authors thank Silesian Association of Electron Microscopy, Wrocław Research Centre EIT+, Wroclaw, Poland for access to the Titan microscope, Dr. K. Adamska for BET measurements and Dr. W. Mista for the CO oxidation tests. This work was financially supported by NCN (UMO-2017/27/N/ST5/02731) and the Spanish MINECO (MAT2016-78155-C2-1-R; CTQ2017-89443-C3-2-R) and Gobierno del
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