Morphology effects of Co₃O₄ nanocrystals catalyzing CO oxidation in a dry reactant gas stream

Abstract

Nanocrystals of Co₃O₄ having different shapes (plates, rods, cubes, and spheres) were synthesized by using a hydrothermal method and their catalytic activities for CO oxidation were measured in dry reactant gas. The plate-like NCs mainly exposing {111} planes showed the highest catalytic activity among the four kinds of NCs.

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Nano-sized materials that we call nanocrystals (NCs) have attracted interest because their properties differ from those of bulk crystals,^1–4 and such properties have been found to depend on the shape as well as the size of the NCs.^5–11

Co₃O₄ NCs exhibit catalytic activity so high that they could possibly replace noble metal catalysts. Their catalytic activity in CO oxidation has been recently reported to strongly depend on the NC morphology.^12,13Co₃O₄ nanorods, for example, show much higher catalytic activity than do conventional nanoparticles of this material.¹²

In the present study we further investigated the effects of morphology on catalytic activity by using a hydrothermal method to synthesize Co₃O₄ NCs having different shapes—plate-like (P-Co₃O₄), rod-like (R-Co₃O₄), cubical (C-Co₃O₄), and roughly spherical (S-Co₃O₄)—and comparing their catalytic activities in CO oxidation. Synthesis conditions were changed as little as possible in order to minimize the influence of surface defects and impurities on catalytic properties.^14–18 In the catalytic activity measurements special care was taken to supply dry reactant gas in order to decrease the influence of the several ppm H₂O vapor usually present in reactant feed gas,^19–21 and H₂O adsorbed on the surface of Co₃O₄ NCs^21–24 was removed by pre-treatment at 573 K.

X-Ray diffraction (XRD) patterns for the four kinds of samples prepared are shown in ESI†, Fig. S1. All the patterns obtained could be indexed with the spinel structure of Co₃O₄ without any unidentified peaks. We thus conclude that the samples were Co₃O₄ NCs.

TEM images of the four kinds of samples are shown in Fig. 1 along with selected-area electron diffraction (SAED) patterns and illustrations. Fig. 1(a1–a3) shows results for P-Co₃O₄ NCs with an average length (edge to edge) of 80 nm and an average thickness of 8 nm. Some cracks and interspaces in the plate can be seen and most of the plate-shaped sample consists of small crystals with sizes from 10 to 17 nm. Electron diffraction (ED) results revealed that each plate exposes the {111} plane. Fig. 1(b1–b3) shows results for R-Co₃O₄ NCs with a mean diameter of 13 nm and lengths ranging from several nanometres to 100 nm (average length = 40 nm). The rods consist of bead-like small NCs. Strong ED intensity from the (110) of Co₃O₄ with weak diffraction spots was observed, suggesting that the rod-like NCs were mainly formed by the {110} facets. Fig. 1(c1–c3) shows results for C-Co₃O₄ NCs having 30 nm edges and partly round corners. Its exposed plane is the (100) plane. Fig. 1(d1–d3) shows results for S-Co₃O₄ NCs. Although their morphologies were not well defined, most of the NCs were roughly spherical and had an average diameter 42 nm. Many facets can be confirmed by the ED patterns.

Fig. 1 TEM images, selected-area electron diffraction (SAED) patterns, and illustrations for Co₃O₄ NCs with different shapes: (a1)–(a3), plate; (b1)–(b3), rod; (c1)–(c3), cube; (d1)–(d3), sphere.

As summarized in Table 1, the sizes of the NCs estimated from diffraction peak widths in the XRD patterns were 11 nm for P-Co₃O₄ NCs, 17 nm for R-Co₃O₄ NCs, 31 nm for C-Co₃O₄NCs, and 29 nm for S-Co₃O₄ NCs. These estimated sizes are fairly consistent with the sizes seen in the TEM observations. The specific surface areas obtained by BET measurements for the corresponding samples were, respectively, 53, 62, 36, and 30 m² g⁻¹. Note that the specific surface area of R-Co₃O₄ NCs is larger than that of P-Co₃O₄ NCs.

Table 1 The morphology parameters of differently shaped Co₃O₄ NCs and the kinetic parameters for CO oxidation

Shape	Average size by TEM/nm	Crystalline size by XRD/nm	Specific surface area/m^2 g^−1	Predominant plane	E a/kJ mol^−1	A/molecule m^−2s^−1	Reaction rate at 200 K/10^−2 μmol CO m^−2s^−1
Plate	8 (thickness)	11	53	{111}	21	1.3 × 10^22	9.5
	80 (contour)
Rod	13 (diameter)	17	62	{110}	40	1.1 × 10^26	0.8
	40 (length)
Cube	30 (edge)	31	36	{100}	33	1.9 × 10^24	0.7
Sphere	42 (diameter)	29	30	Multi planes	27	6.3 × 10^22	0.7

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Fig. 2 shows light-off curves of CO oxidation obtained for the four kinds of Co₃O₄ NCs. All samples had the same surface area (3.6 m²) but their masses differed. The oxidation of CO occurs at temperatures below 273 K, and the T₅₀ (reaction temperature at 50% conversion) values for P-, R-, C- and S-Co₃O₄ NCs were, respectively, 190, 218, 225, and 230 K (Table 1). These values are much lower than those reported in a normal gas stream (T₅₀ > 323 K). The lowest T₅₀ among those of the four kinds of samples was that for P-Co₃O₄, and that value was much lower than the value reported for Co₃O₄ NCs (S = 52.1 m² g⁻¹, SV = 20 000 h⁻¹ ml g⁻¹_cat) in a dry gas stream (T₅₀ = 219 K).¹⁹ It was also confirmed that the sample morphologies of the present Co₃O₄ NCs were the same after the catalytic measurements.

Fig. 2 Conversion as a function of reaction temperature for CO oxidation over differently shaped Co₃O₄ NCs: (▲) plate, ([thick line, graph caption]) rod, (■) cube, (●) sphere. The amount of each sample corresponded to a surface area of 3.6 m²: 68 mg for P-Co₃O₄ NCs, 58 mg for R-Co₃O₄ NCs, 100 mg for C-Co₃O₄ NCs, and 120 mg for S-Co₃O₄ NCs. The stream consisted of 1.0 vol% CO and 21 vol% O₂ in He, and the gas flow rate was 50 ml min⁻¹. Hourly space velocities for P-, R-, and S-Co₃O₄ NCs were, respectively, 4.4, 5.2, 3.0, and 2.5 × 10⁴ h⁻¹ ml g⁻¹_cat.

Fig. 3 shows the results of Arrhenius plots for CO oxidation over the Co₃O₄ NCs. The apparent activation energies (E_a) obtained from the plots are also listed in Table 1. The E_a for P-, R-, C-, and S-Co₃O₄ NCs were, respectively, 21, 40, 33, and 27 kJ mol⁻¹, all of which were much lower than the reported E_a in a normal reactant gas stream containing more than 1 ppm H₂O (50–54 kJ mol⁻¹).¹⁶P-Co₃O₄ had the lowest E_a among the four. The E_a of S-Co₃O₄ NCs (27 kJ mol⁻¹) is similar to that of usual Co₃O₄ NCs with a smaller diameter (11 nm).¹⁶R-Co₃O₄ NCs had the highest E_a values. The different E_a values show that reaction pathways differ somewhat depending on the morphology in dry gas stream. Reaction rates per unit specific surface area at 200 K were, respectively, 9.5, 0.8, 0.7, and 0.7 × 10⁻² μmol m⁻²s⁻¹, for P-, R-, C-, and S-Co₃O₄ NCs (Table 1). Note that the value for P-Co₃O₄ NCs is an order of magnitude higher than the values for R-, C-, and S-Co₃O₄ NCs.

Fig. 3 Arrhenius plots for the rate of CO oxidation over differently shaped Co₃O₄ NCs: (▲) plate, ([thick line, graph caption]) rod, (■) cube, (●) sphere. Apparent activation energy (E_a) is shown for each catalyst. The stream consisted of 1.0 vol% CO and 21 vol% O₂ in He, and the gas hourly space velocity was changed in the range of 3.0 × 10⁴ to 2.4 × 10⁵ h⁻¹ ml g⁻¹_cat in order to obtain CO conversion below 15% for differential reactor assumption.

It was reported that for CO oxidation under a normal gas stream containing more than 3 ppm H₂O the catalytic activity of the {110} crystal planes of Co₃O₄ NCs was higher than that of either the {100} or {111} planes.¹² Indeed, rod-shaped Co₃O₄ NCs, which mainly consisted of {110} exposed planes, showed high catalytic activity at around 200 K.¹² On the other hand, calculation results showed that {100} planes are more stable than {111} and {110} planes that expose the active sites of Co³⁺ in Co₃O₄ NCs.^25–27 Moreover, {111} planes expose a stable outmost surface after relaxation, while surface relaxation of the {110} plane is not significant.^26,27 In our study the catalytic activities were measured under dry stream, which is closer to the ideal condition assumed in computational calculation. We then suppose that {111} planes may keep their active surfaces in dry reactant gas to give a high catalytic activity. In addition, our R-Co₃O₄ NCs exposed {110} planes together with other crystal planes to lower their catalytic activity. These lead to a total result that our P-Co₃O₄ NCs showed the highest catalytic activity. The stability of surface and/or surface energy is related to catalytic activity, for example, the surface energy of anhydrous Co₃O₄ NCs has been reported to be twice that of hydrated Co₃O₄ NCs,²⁸ which is consistent with the difference of catalytic activity between anhydrous and hydrated Co₃O₄ NCs.¹⁹

In summary, Co₃O₄ NCs having different shapes—plate-like, rod-like, cubical, and roughly spherical—were synthesized in aqueous solution by a hydrothermal reaction. Plate-like Co₃O₄ NCs mainly exposing {111} planes exhibited the highest catalytic activity for CO oxidation: T₅₀ = 190 K and E_a = 21 kJ mol⁻¹. At 200 K their catalytic activity was an order of magnitude higher than that of the other NCs.

This work was partly supported by a Grant-in-Aid for Scientific Research (19GS0207) and by the Joint Project of Chemical Synthesis Core Research Institutions from the Japanese Ministry of Education, Culture, Sports, Science and Technology.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details. See DOI: 10.1039/c1cy00113b

‡ Current address: Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan.

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