Promotion of Copper on the Reduction Of Cobalt Ions in Perovskite-Type Oxides

Promotion of Copper on the Reduction Of Cobalt Ions in Perovskite-Type Oxides

Promotion Of Copper On The Reduction Of Cobalt Ions In Perovskite-Type Oxides

Nguyen Tien Thao

Faculty of Chemistry, VNU University of Science, Vietnam National University-Hanoi 19 Le Thanh Tong ST, Hoan Kiem, Hanoi, VIETNAM

Abstract

Ground LaCo1-xCuxO3-, and mixed oxide CuO+LaCoO3 were characterized by several physical means as X-ray diffraction, BET, SEM, and H2-TPR. LaCo1-xCuxO3- samples show a well crystallized rhombohedral structure and uniform nanocrystalites. The substitution cobalt by copper in perovskite lattice has no significant changes in the structure, but remarkably affects the reducibility of perovskite up to x

=0.3. Under hydrogen treatment, the reduction temperature of cobalt ions in LaCo1-xCuxO3, varies with the intra-lattice copper. A lower cobalt reduction temperature observed for LaCo1-xCuxO3- in comparison to LaCoO3 produces a finely dispersed bimetal on the La2O3 matrix. A close distance between cobalt and copper sites in the perovskite framework facilitates the reducibility of cobalt ions and improves the metallic dispersion of metals in the reduced form.

Keywords: Nanoperovskite, promotion, intra-lattice, copper, cobaltite, LaCoCuO3, reducibility.

1. Introduction

   Perovskite-type mixed oxides have the cubic structure with the general formula of ABO3 and the space-group Pm3m-Oh [1]. In the cubic structure, cation A is at the body center while the transition-metal cations

   (B) occupy at the cube corners. All oxygen anions stay at the midpoint of the cube edges. Thus, both A and B positions can be partially replaced by a di- or trivalent element to produce some more complicated perovskites [1,2]. For example, La can be replaced by either Sr or Th to produce a series of La1-xSr(Th)xCoO3 perovskites which played as active catalysts for carbon dioxide hydrogenation [3] and methane oxidation [4]. Nevertheless, the replacement of the cation at A-position is much less attractive than that at B-site because the nature of cation A is usually less attractive because of

   inactive nature of cations A [5]. In other context, the substitution of cation B by another transition element could produce various active catalysts for many potential reaction applications [1,2,6] and/or numerous supported bimetallic catalysts after tailored reductions [1,2,7-9]. For instance, Bedel et al. [5] prepared a Fe-Co alloy after reduction of LaFe0.75Co0.25O3 orthorhombic perovskite at

   600oC. Lima and Assaf [9] found that the partial

   substitution of Ni by Fe in the perovskite lattice leads to a decreased reduction temperature of Fe3+ ions and the formation of Ni-Fe alloy. In the latter cases, the intercalation of another transition metal (Fe3+, Ni3+) into the perovskite lattice always leads to modify the catalyst morphology and the catalytic behavior [1,7-10]. In addition, this modification also causes dilution of the transition metal sites and also may affect the reducibility of the parent perovskite.

   This article is to report the role of intra-lattice copper in the reduction of cobalt ions in the perovskite lattice. The result may also suggest a simple way to prepare a well-homogenized supported bimetal catalyst system.

2. Experimental Section

   1. Perovskite preparation

      Three nominal formulae of LaCo1-xCuxO3 perovskite-type mixed oxides were synthesized by the reactive grinding method [10,11]. In brief, the stoichiometric proportions of commercial lanthanum, copper, and cobalt oxides (99%) were mixed together with three hardened steel balls in a hardened steel crucible (50 mL). Then, the resulting powder was mixed to 50% sodium chloride and further milled for 12 hours before washing the additives with distilled water. The slurry was dried in oven at 60-80oC before calcination at 250oC for 150 min.

      A reference sample, LaCoO3 + CuO, was prepared by milling a mixture of the ground perovskite LaCoO3 having a specific surface area of 43 m2/g with cupric

      oxide (CuO/LaCoO3 = 1.5 molar ratio) at ambient temperature before drying at 120oC overnight in oven.

      2.2. Characterization

      The specific surface area of all obtained samples was measured from nitrogen adsorption equilibrium isotherms at -196 oC measured using an automated gas sorption system (NOVA 2000; Quantachrome). Phase analysis and particle size determination were performed by powder X-ray diffraction (XRD) using a SIEMENS D5000 diffractometer with CuK radiation ( = 1.54059 nm). Temperature programmed characterization (TPR) was examined using a multifunctional catalyst testing (RXM-100 from Advanced Scientific Designs, Inc.). Prior to each test analysis, a 50 mg sample was calcined

      at 500 oC for 90 min under flowing 20% O2/He (20 ml/min, ramp 5 oC/min). The sample was then cooled down to room temperature under flowing pure He (20

      mL/min). TPR of the catalyst was then carried out by ramping under 5 vol% of H2/Ar (20 mL/min) from room temperature up to 800oC (5 oC/min). The effluent gas

      was passed through a cold trap (dry ice/ethanol) in order to remove water prior to a thermal conductivity detector (TCD).

3. Results and Discussion

   1. Physical characteristics

      A major drawback of grinding method is usually to produce a low chemical homogeneity and yields a small amount of impurities of unreacted constituent oxides in the final product mixture [2,10]. To investigate the formation of perovskite phase, all perovskite-type mixed oxides are recorded X-ray diffraction. Figure 1 presents XRD patterns for all prepared samples.

      LaCoO3+CuO

      Figure 1. XRD patterns for all ground samples XRD diffractograms of three perovskites and a mixture of LaCoO3 and CuO are displayed in Figure 1. In general, it is observed that the ground LaCoO3 appears a set of sharp reflection peaks at 23.1, 33.2, 40.6, 47.3,

      52.8 and 58.6o representing for the typically crystalline

      rhombohedral perovskite, in accordance with the JSCDS card Nos. 48-0123, 46-0059, 51-1511) [10, 12,13]. The

      substitution of Co3+ by Cu2+ in LaCo1-xCuxO3 perovskite mixed oxides is possible up to x = 0.3, and the perovskite

      is still preserved without changing the crystal symmetry [9,12], substantiated by no observable change in XRD diffractograms between LaCoO3 and LaCoCuO3 patterns. No appearance of other peaks indicates that copper ions are fully incorporated into the perovskite lattice [6,8,9,14]. For the physical mixture, XRD pattern shows the presence of CuO reflected by the appearance of 2- theta values of 35.4 and 38.6o in addition to the LaCoO3 perovskite phase (Fig. 1). In all cases, the reflection lines are rather broadening, implying the formation of nanocrystalites of perovskite [12,13]. The crystal domains of LaCoO3, LaCo0.9Cu0.1O3 and LaCo0.7Cu0.3O3, estimated from the Full Width at Half Maximum (FWHM) of the (102) diffraction peak (2 = 33.2o) using Scherrers equation are around 10 nm, in good agreement with SEM observation [6,11,14].

      1. Morphology and Surface area

         Morphological perovskite was examined by SEM technique. Microphotographs of two representative samples are depicted in Figure 2. It is observed a similar surface morphology of SEM micrographs between two selected ground perovskite samples, indicating a high producibility of the grinding synthesis [6,10,11,14]. Moreover, the perovskite powder is constituted by the aggregation of various spherical particles with the average particle size in the range of numerous nanometers, in coincident with the reults estimated from

         1500

         Counts (a.u)

         Counts (a.u)

         1000

         CuO CuO

         LaCo0.7Cu0.3O3 LaCo0.9Cu0.1O3

         LaCoO3

         Scherrers equation.

         The arrangement of such spherical particles yields many slit-shaped spaces between nanometric particles and thus external surface area increases [11,12,14]. Indeed, the specific surface area of the ground perovskites is in the range of 25 – 56 m2/g as reported in Table 1 using the BET (Brunauer-Emmet-Teller)

         500

         0

         20 25 30 35 40 45 50 55 60

         2-theta(o)

         equation. The nitrogen adsorption/desorption isotherms are displayed in Figure 3. Adsorption curves of all examined samples are classified into type of II isotherm which is characteristic of a nonporous or possibly micro- and macro-porous material having a high energy of adsorption [15]. However, the isothermal curves are not parallel to the horizontal axis and appears a hysteresis loop at high relative pressure in the narrow range of 0.8-

         1.0. This observation may suggest the existence of micropores and some slit-like mesopores in the ground perovskite samples. Indeed, the ground perovskite is consisted of uniform spherical particles [10,14]. There are considerable spaces between the nanometric

         240

         LaCoO3 LaCo0.9Cu0.1O3 LaCo0.7Cu0.3O3

         perovskite grains. As indicated by SEM images, the

         particles are perfect spheres, the pore space or porosity is rather high and the pore sizes are more uniform. The porous structure of the ground perovskite would facilitate the reducibility of transition metal ions in perovskite lattice [12,13].

         A

         180 LaCoO3+CuO

         Quantity adsorbed (cm3/g)

         Quantity adsorbed (cm3/g)

         120

         60

         0

         0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

         Relative pressure P/Po

         Figure 3. Nitrogen adsorption/desorption isotherms of ground perovskites and reference sample

      2. Reducibility

      The reducibility of the ground perovskites is investigated by the temperature programmed reduction of

      hydrogen (H2-TPR). All experiments are carried out from room temperature to 800 oC under H2/Ar flowrate

      temperature.

      48

      42

      36

      TCD signal (a.u

      TCD signal (a.u

      30 LaCoO3+CuO

      Figure 2. SEM photographs of LaCoO3 (A) and LaCo0.9Cu0.1O3 (B)

      24

      18

      12

      LaCo0.9Cu0.1O3

      6

      0 LaCoO3

      LaCo0.7Cu0.3O3

      Table 1. Specific surface area of samples

      Nominal BET surface area composition (m2/g)

      LaCoO3 56

      LaCo0.9Cu0.1O3 24

      100 150 200 250 300 350 400 450 500 550 600 650 700 750 800

      Reduction temperature (o)

      Figure 4. H2-TPR profiles of ground samples

      The reduction of cobaltite perovskites takes place in two consecutive steps as described in Figure 4. H2-TPR profiles of cobaltite perovskite (LaCoO3) shows two

      LaCo0.7Cu0.3O3 25

      LaCoO3 + CuO 21

      broad bands at 411 and 713 oC. The first peak is

      attributed to the reduction of Co3+ to Co2+ in the perovskite lattice [1,2,11,17]. At this step, the perovskite

      structure is still remains, evidenced by the XRD patterns collected at different reduction temperatures. As seen in Figure 5, no significant changes in X-ray diffractograms of LaCoO3 are observed in the reduction temperature

      range of 300-450 oC. In the temperature window of 450-

      500 oC, the corresponding XRR patterns of LaCoO3 are essentially unchanged, but the noise to signal ratio dramatically increases, indicating the slightly modification of the perovskite structure at high reduction temperature [16]. This is in good agreement with H2- TPR result displayed in Figure 4. Furthermore, H2-TPR signal for the ground LaCoO3 perovskite sample does not reach the baseline after the first reduction step, implying the commencement of second reduction step at 460 oC (Fig. 4). Thus, it is suggested that the second step starts at 460 oC and lasts through a maximum at 730 oC [17,18]. In practice, the XRD pattern of LaCoO3 significantly changes and some new peaks are observed at 550 oC, demonstrating the gradual modification in the perovskite structure [2,17,18]. Therefore, this step describes the complete reduction of Co2+ to Co0 accompanied by the gradual destruction of the perovskite structure.

      LaCo0.7Cu0.3O3 in Fig. 4). Meanwhile the broad peak at high temperature in the latter sample LaCo0.7Cu0.3O3 becomes more broadening and reaches nearly flat in the range of 400- 650 oC. Therefore, it is suggested that the reduction temperatures of both Co3+/Co2+ and Co2+/Co0 in LaCoCuO3 are much lower than those in LaCoO3 (Fig. 1). To demonstrate this hypothesis, we have collected some XRD patterns of a representative sample LaCo0.7Cu0.3O3 reduced at different temperatures [17]. In line with H2-TPR interpretation, the first reduction step

      almost terminates at 350 oC [19,20] and thus the

      reduction of LaCo0.7Co0.3O3 at 450 oC leads to the deformation of perovskite structure (Fig. 6). This is different from the copper-free perovskite sample (LaCoO3) (Fig. 5) [10,14,15,17]. Moreover, the

      appearance of week diffraction peaks at 43.6 and 50.6o is

      characteristics of metallic copper (JCPDS card No. 04- 0836) [11] in addition to the reflection signals of La2O3 (JCPDS card No. 74-2430) and Co (JCPDS card 15- 0806) and intermediate perovskite phases. These diffraction line intensities increase as increasing reduction temperature to 550oC [2,9].

      550C

      500C

      450C

      400C

      300C

      Unreduced

      800C

      550C

      450C

      Unreduced

      20 25 30 35 40 45 50 55 60

      2-Theta

      Figure 5. XRD patterns for LaCoO3 reduced at different temperatures in H2/Ar flowrate

      H2-TPR profiles of LaCo1-xCuxO3 perovskites show a slight change in shape as compared with that of cobaltite perovskite. Indeed, H2-TPR curve of LaCo0.9Cu0.1O3 sample displays a broad peak at lower temperatures along with a visible shoulder at 370oC. These peaks are ascribed to the reduction of Co3+ and Cu2+ to lower oxidation states [8,11-13,18]. These signal peaks in LaCo0.9Cu0.1O3 combines into a single peak as increased copper content to x = 0.3 (sample

      20 25 30 35 40 45 50 55 60

      2-theta(o)

      Figure 6. XRD patterns for LaCo0.7Cu0.3O3 reduced at different temperatures in H2/Ar flowrate

      Thus, the presence of copper in the perovskite lattice has strongly promoted the reducibility of cobalt ions in the framework while extra-lattice copper does not significantly affect the reduction ability of cobaltite as

      analyzed by H2-TPR traces of CuO/LaCoO3 mixture. The reduction temperatures of Co3+ and Cu2+ in the blend are almost on par with those in LaCoO3 and single CuO oxide (Fig. 4). The facilitation of intra-lattice copper on

      the reducibility of cobalt ions in the perovskite lattice is

      interpreted by the corresponding extraction of Cu0 at low temperatures which may act as an active site for dissociation of hydrogen [9,14,18-20]. Therefore, hydrogen molecules chemisorbed on Cu0 sites who are proximate to cobalt ions. Accordingly, these cobalt ions should be attacked by atomic hydrogen and easily convert to metallic clusters at significantly lower temperatures [9,11,19]. As a consequence, the reduction of LaCoCuO3 leads to the formation of high dispersion of bimetallic copper-cobalt at lower temperatures.

4. Conclusions

1. L. Bedel, A.C. Roger, C. Estournes, A. Kiennemann, Co0 from partial reduction of La(Co,Fe)O3 perovskites for Fischer- Tropsch synthesis, Catal. Today 85 (2003) 207-218.

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3. L. Lisi, G. Bagnasco, P. Ciambelli, S. D. Rossi, P. Porta, G. Russo, and M. Turco, Perovskite-type oxides: II. Redox properties of LaMn1-xCuxO3 and LaCo1-xCuxO3 and methane catalytic combustion, J. Solid State Chem., 146, 176-183, 1999.

4. S. Kaliaguine, A. Van Neste, V. Szabo, J.E. Gallot, M. Bassir, R. Muzychuk, Perovskite-type oxides synthesized by reactive grinding, Appl. Catal. A, 209, 345-358, 2001.

5. Nguyen Tien Thao, Effects of alkali additives on the

Three ground LaCo

1-x

CuxO3

(x = 0 – 0.3) oxides and

surface properties of La(Co,Cu)O3 perovskites, VN Journal of Chemistry, 50 (5B), 6-10, 2012.

a blend of LaCoO3 with CuO are prepared by milling

method. All LaCo1-xCuxO3 samples show well rhombohedral perovskite structure. The perovskite structure is preserved up to x = 0.3. The ground solids are composed of nanometric particles with the average diameter around 10 nm. They possess a moderate surface area and consist of slit-like micro and mesopores. The presence of intra-copper lattice has strongly promoted the reducibility of cobalt in the perovskite framework while extra-lattice copper has negligibly impacted on the thermal stability of LaCoO3 perovskite. All Cu- containing perovskite are reduced at lower temperature as compared with the copper-free perovskite sample and a blend of CuO-LaCoO3.

Acknowledgement. This research is funded by Vietnam National University – Hanoi (VNU) under the Type B of Project, code number QG.12.08.

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*Corresponding author: Nguyen Tien Thao, Ph.D Department of Petrochemistry, VNU University of Science, Vietnam National University in Hanoi,

19 Le Thanh Tong ST, Hoan Kiem, Hanoi, VIETNAM Tel.: +84.043.933 1605; Fax.: +84.043.824.1140

Email: ntthao@vnu.edu.vn / nguyentienthao@gmail.com