Embedded structure catalyst: a new perspective from noble metal supported on molybdenum carbide

Abstract

An embedded structure of noble metal (Pt) in situ supported on molybdenum carbide was found for the first time, which hindered Pt sintering at high temperature, and promoted the interaction between Pt and molybdenum carbide. This structure exhibited an excellent and stable catalytic activity for water gas shift reaction at low temperature.

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Transition-metal carbides (e.g. molybdenum carbide) have attracted considerable attention due to their unique surface properties and outstanding physicochemical properties.^1–6 To date, due to its unique catalytic properties (Pt-like behaviour),^4,7 the molybdenum carbide catalyst has been applied to various reactions, such as methane dry reforming, steam reforming of methanol, methanol decomposition, CO₂ reduction, hydrocarbon isomerization and hydrogenation reactions.^8–15 On the other hand, molybdenum carbide can serve as a catalyst support, and in this case, some unique catalytic properties resulting from the interaction between the doped metal and the molybdenum carbide were generated, which benefit the supported metal dispersion. A large number of metal supported molybdenum carbide catalysts, such as Ni/MoC,^16–18 Co/MoC,¹⁹ Cu/MoC²⁰ and Pt/MoC²¹ have been reported.

Recently, Thompson et al.²¹ developed a synthesis process for the preparation of metal modified molybdenum carbide catalysts, in which they used an aqueous wet impregnation method in order to deposit Pt on molybdenum carbide surface directly (i.e., not on the oxidized surface). This kind of catalyst showed high activity for the Water Gas Shift (WGS) reaction. They found that the supported Pt showed special morphology (raft-like), which could provide more active sites around the interface between metal and molybdenum carbide surface. This aqueous wet impregnation method avoided the exposure of the carbide to oxygen, however, it is difficult to apply to industry.

In this communication, Pt was doped on molybdenum carbide using the Temperature Program Reaction (TPRe) method. Based on STEM, SEM and HR-TEM analyses, a new embedded structure was found for the first time, and when it was used for the WGS reaction, excellent and stable catalytic properties were exhibited at very low temperatures.

XRD patterns of 3 wt% Pt modified molybdenum carbide catalysts synthesized at different carburization temperatures (see ESI† (ref. 22)) are shown in Fig. 1(a). Herein, the typical peaks corresponding to MoO₃ are those at 2θ of 12.7°, 25.6° and 38.9°. For the carburization at 300 °C (Pt–Mo-300 °C), there were no peaks for molybdenum oxide or molybdenum carbide, the peaks at 2θ of 39.6° and 67.4° were attributed to Pt metal. In this case, the carbon atom could enter into the molybdenum oxide phase with hydrogen reduction, and replace the oxide atom so that the molybdenum oxide phase became disordered. When the carburization temperature was increased to 500 °C, the peaks of 25.9°, 36.9° and 53.5° at 2θ are attributed to the intermediate phase of MoO₂ and the peak at 2θ of 43.9° should be attributed to the intermediate phase of MoO_xC_y. When the carburization temperature was increased to 700 °C (Pt–Mo-700 °C), the peaks at 2θ of 34.8° and 39.8° are attributed to the presence of a hexagonal β-Mo₂C phase with hcp crystal structure while the peaks at 2θ of 37.1° and 42.8° are attributed to a cubic α-MoC_1−x phase with fcc crystal structure. The presence of α-MoC_1−x was due to the topotactic phase transformation involved during the carburization process. Jung et al.²³ reported that the β-Mo₂C phase, formed by the transformation of MoO₃ into an intermediate phase of MoO₂ during the carburization process followed a non-topotactic route (MoO₃–MoO₂–β-Mo₂C), and the α-MoC_1−x was produced by driving the MoO₃ reduction into a topotactic transformation through the MoO_xC_y intermediate phase (MoO₃–MoO_xC_y–α-MoC_1−x). Thus, at a carburization temperature of 700 °C, the molybdenum carbide with coexisting phases was finally produced. On the other hand, the peaks of 39.6° and 67.4° at 2θ were attributed to the metal Pt phase. It should be noted that comparing with the XRD pattern of Pt–MoO₃, the intensity of the Pt peak became weaker after the carburization process, indicating that the size of Pt particles decreased or the morphology of Pt particles became more platelet-like.

Fig. 1 XRD patterns for the Pt modified molybdenum carbides at different carburization temperatures. Inset in (b) shows Pt peak for Pt–Mo-700 °C.

Pt peaks in the XRD patterns (2θ = 39.6°) at different carburization temperatures are shown in Fig. 1(b). One can see that with the increase in the carburization temperature, the Pt position shifted from 39.6° to 40°. This indicated that with the increase in the degree of carburization, the Pt crystal structure was changed or a solid mixture was formed during the carburization process.

STEM and HR-TEM were carried out for the investigation of the nano-structure of the fresh 3% Pt/Mo_xC_y as well as the oxide precursor (Pt/MoO₃). As shown in Fig. 2(a) and (b) and S1 (see ESI†), Pt particles seem to locate on the surface of MoO₃ while Pt particles inserted into the surface of Mo_xC_y after the carburization process. From the Pt diffraction patterns (see in Fig. S1(a) and (b), ESI†), it is found that the Pt particle is a single crystal. Furthermore, from the insets in Fig. 2(a) and (b), one can see that the morphology of MoO₃ is in the platelet-like form, and after the carburization, some parts of the molybdenum carbide still remained in its parent MoO₃ shape, i.e., platelet-like form, indicating that the topotactic transformation process occurred during the TPRe process; on the other hand, it should be noted that some parts of molybdenum carbide were changed to the agglomerated isotropic particles by breaking the platelet so that the morphology was also different with that of the parent MoO₃, indicating that the non-topotactic transformation process had also taken place. These two morphologies of molybdenum carbide should correspond to the cubic α-MoC_1−x phase and the hexagonal β-Mo₂C phase, respectively.

Fig. 2 Morphologies of Pt/MoO₃ and 3% Pt/Mo_xC_y: (a) STEM morphology of Pt/MoO₃ (inset: SEM image), (b) STEM morphology of 3% Pt/Mo_xC_y (inset: SEM image); (c) HR-TEM morphology of Pt/MoO₃ (magnification of circle 1 in (a)), (d) HR-TEM morphology of 3% Pt/Mo_xC_y (magnification of circle 2 in (b)).

HR-TEM images of Pt/MoO₃ and Pt/Mo_xC_y are shown in Fig. 2(c) and (d), respectively. It is obvious that Pt particles are located on the surface of MoO₃ with the largest particle size of about 17 nm, and the contact angle between the Pt and MoO₃ surface is greater than 90°, indicating that the interaction between Pt and MoO₃ surface should be very weak. However, after the carburization process, as shown in Fig. 2(d), it is obvious that almost all Pt particles are surrounded by the molybdenum carbide, and the Pt particle size decreases to some extent and the largest particle size is about 13 nm, indicating that Pt particles were not sintered during the carburization process at high temperature. This might be due to that with the increasing of the carburization degree, more molybdenum carbide phase was formed and the noble metal-like property of the support increased. Herein, Pt might have some catalytic activity for the formation of molybdenum carbide, and thus, at high temperature, the noble metal-like molybdenum carbide could be formed soon around the Pt particle. As such, Pt particles could permeate in the molybdenum carbide crystal lattice and be surrounded by molybdenum carbide, like a bimetal alloy. This should be the main reason why the Pt particle did not sinter during the high temperature carburization process. Furthermore, such a kind of nanostructure could enhance the interaction between the Pt and molybdenum carbide. On the other hand, it should be noted that compared with the Pt morphology on Pt/MoO₃ (Fig. 2(c)), the morphology of Pt particles on molybdenum carbide became more flat, which is also consistent with the XRD result (Fig. 1). It is possible that the Pt particle could spread along the molybdenum carbide surface and disperse better on the surface at high carburization temperature.

In order to investigate the interaction between Pt and the molybdenum carbide in more detail, XPS analyses of 3% Pt/Mo_xC_y and oxide precursor (Pt/MoO₃) were carried out. Pt 4f spectra of the fresh Pt modified molybdenum carbide catalysts by carburization at different temperatures were measured by XPS. As shown in Fig. S2 (see ESI†), the double peaks of Pt should have a splitting of ∼3.33 eV with a Pt 4f_7/2 to Pt 4f_5/2 ratio of 4 : 3. The peak in the banding energy range of 64–70 eV is attributed to Mo 4s. For the Pt/MoO₃ sample, there are two Pt species: the one with Pt 4f_7/2 binding energy (BE) of 70–72 eV is attributed to a Pt⁰ species involved in metal Pt–Pt bonding; the other with Pt 4f_7/2 binding energy of 72–74 eV is identified as Pt²⁺ (PtO). In contrast, after being carburized under CH₄/H₂ atmosphere, only a Pt⁰ species was found on the surface of the catalyst, as indicated in Fig. S2(b–d). Herein, for Pt/MoO₃ sample, after calcination under air, Pt particles were oxidized by air. However, the Pt particles will be reduced by methane and hydrogen during the carburization process. From the binding energy results as shown in Fig. S2 (see ESI†), one can see that compared with the Pt/MoO₃ sample, the binding energy of Pt⁰ shifted to the higher binding energy after the carburization process, and with the increase in the degree of carburization, the binding energy of Pt⁰ shifted from 71.48 to 71.87 eV. Here, the binding energy shift of Pt⁰ species is indicative of charge transfer between the matrix and Pt metal, indicating that the Pt and the molybdenum carbide are interacting with each other, and with the increase of the degree of carburization, the interaction between Pt and matrix should become stronger. The Mo 3d XPS results (see ESI Fig. S3 and Table S1†) also indicated that the binding energy of Mo²⁺ (Mo₂C) shifted from 228.4 to 228.9 eV when Pt was doped on the molybdenum carbide surface. Based on the XPS results, one can see that with the increase in the degree of carburization, the embedded structure was formed, and the Pt particles were surrounded by molybdenum carbide, and the electro-structure of the Pt particle was changed by the surrounding molybdenum carbide.

Correlated with the above results, the carburization process can be assumed as in Fig. 3, in which an embedded structure was formed during the carburization process.

Fig. 3 Schematic embedded structure of Pt on molybdenum carbide surface during the carburization process (blue ball: molybdenum, red ball: oxygen, green ball: carbon, grey ball: platinum).

This material was used as the catalyst for the WGS reaction. As shown in Fig. 4(a), comparing with others, 3% Pt/Mo_xC_y carburized at 700 °C catalyst exhibited a much higher catalytic activity. Especially, at a reaction temperature as low as 150 °C, the CO conversion reached 100%. Correlated with the above, one can see that with the increase in the degree of the carburization process, the embedded structure was formed, resulting in the increase in the catalytic activity. This indicates that the interaction between Pt and molybdenum carbide and the embedded structure are an important factor to promote the catalytic activity. On the other hand, it should be noted that for 3% Pt/Mo_xC_y (700 °C), with the increase in the reaction temperature, the CO conversion decreased. It is possible that the WGS reaction is an exothermic reaction, and at the higher reaction temperature the conversion rate is mainly controlled by thermodynamics.

Fig. 4 (a) Catalytic performances of 3% Pt/Mo_xC_y carburized at different temperatures (cyan ball: 300 °C, blue ball: 500 °C, red ball: 700 °C) and Mo₂C (black ball) catalysts for WGS reaction at different temperatures; (b) time-on-stream testing of 3% Pt/Mo_xC_y (700 °C) catalyst for WGS reaction at 150 °C and 400 °C.

A long-term stability test was also conducted at 150 °C and 400 °C for 3% Pt/Mo_xC_y (carburization at 700 °C) catalyst. As shown in Fig. 4(b), one can see that the CO conversion remained at 100% and 90% in the 50 h test at reaction temperatures of 150 °C and 400 °C, respectively. This indicated that as-prepared 3% Pt/Mo_xC_y had excellent catalytic stability for the WGS reaction. As shown in Fig. S4 (see ESI†), the XRD pattern of the spent catalyst had no change after the reaction.

In conclusion, the Pt/Mo_xC_y catalyst with the embedded structure can be simply prepared by the in situ carburization of Pt doped molybdenum oxide (Pt/MoO₃) using a TPRe method. Such a new structure can hinder the sintering of a noble metal at high temperature, and exhibit excellent and stable catalytic properties. This embedded structure could be attractive for the practical applications of noble metal-based catalysts.

Acknowledgements

This work is supported by Japan Science and Technology Agency (JST), Japan and Aomori City Government. Ma thanks the State Scholarship Fund of China Scholarship Council (2012). P. Phanthong gratefully acknowledges the scholarship from Ministry of Education, Culture, Sport, Science, and Technology (MEXT) of Japan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15226c

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