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CHINESE JOURNAL OF CATALYSIS Volume 28, Issue 4, April 2007 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2007, 28(4): 293–295.

Received date: 2007-01-22. * Corresponding author. Tel/Fax: +86-411-84581234; E-mail: xuhy@dicp.ac.cn Foundation item: Supported by the National Basic Research Program of China (973 Program, 2005CB221401). Copyright © 2007, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

The Effect of Supports on the Activity of Methane Dissociation over Rh Catalysts

WANG Rui1,2, XU Hengyong1,*, CHEN Yanxin1, LI Wenzhao1 1 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: The Rh–CeO2 interaction and the activity of CH4 dissociation were investigated on Al2O3- and SiO2-supported Rh catalysts by the pulse reaction technique, infrared spectroscopy of adsorbed CO, and infrared spectroscopy of adsorbed pyridine. The Rh–CeO2 inter-action was strongly influenced by the acidity of support. Addition of CeO2 to Rh/Al2O3 increased the number of Lewis acid sites on Al2O3, which in turn enhanced the ability of Al2O3 to accept electrons. This decreased the electron density of Rh and consequently facili-tated the activation and dissociation of CH4 on Rh. On the contrary, the addition of CeO2 to Rh/SiO2 decreased both the number and the strength of Lewis acid sites on SiO2, which weakened the electron-accepting ability of SiO2. As a result, it brought out an increase in the electron density of Rh and inhibited CH4 dissociation on Rh.

Key Words: methane dissociation; rhodium; alumina; silica; supported catalyst; ceria; interaction; acidity

Ceria has been extensively studied as a promoter or catalyst support because of its oxygen storage and release capacity, redox behavior, and electronic effects on metals [1–3]. These unique functions are associated with the interaction between the metal and ceria, which is strongly influenced by the type of metal and support. Yang et al. [4] found that the addition of CeO2 into Ni/γ-Al2O3 resulted in a Ni–CeO2 interaction, which enhanced the d-electron density of Ni0 and inhibited CH4 de-composition.

We have recently studied the Rh–CeO2 interaction in a Rh-CeO2/γ-Al2O3 catalyst [5], which caused the coexistence of the redox couples of Rh0/Rhδ+ and Ce4+/Ce3+ that facilitated the activation of CH4 and CO2 molecules. In the present work, the Rh–CeO2 interaction and the catalytic activity of CH4 dis-sociation on Al2O3- and SiO2-suppported Rh catalysts were investigated by the pulse reaction technique, infrared spec-troscopy of adsorbed CO (CO-FTIR), and infrared spectros-copy of adsorbed pyridine (Py-FTIR).

The Rh-based catalysts were prepared by impregnation fol-lowed by drying at 110ºC for 12 h and calcination at 500ºC for 2 h in air. All Rh contents were 1% (Rh/Ce molar ratio = 1.2). Prior to the pulse reactions, the catalysts were reduced in H2 (20 ml/min) at 500ºC for 0.5 h and then heated to 700ºC in

Ar. A CH4 pulse (300.58 µl) was passed over the catalyst and the effluents were analyzed by a gas chromatograph equipped with a TCD. In situ CO-FTIR and Py-FTIR experiments were carried out on a Bruker Equinox 55 Fourier spectrometer.

Table 1 displays the results obtained on the different Rh-based catalysts after CH4 pulses at 700ºC. The CH4 pulses resulted in the production of both H2 and CO. The H2 yield was much higher than the CO yield, which indicated that the dissociation of CH4 into CHx species (CH4 → CHx + (4 − x) /2 H2) on Rh was dominant. Meanwhile, the partial oxidation of CHx species by mobile oxygen present on the support pro-duced CO and H2 (CHx + Osupport → CO + x/2H2). When CeO2 was added to Rh/Al2O3, the CH4 conversion increased,

Table 1 Amount of products over Rh-based catalysts in CH4 pulse at 700ºC

Catalyst CH4 conversion

(%) H2 yield (μmol)

CO yield (μmol)

Rh/Al2O3 68.71 17.30 0.58 Rh-CeO2/Al2O3 71.93 17.80 1.63

Rh/SiO2 63.08 15.51 0.42 Rh-CeO2/SiO2 58.17 14.24 1.34

WANG Rui et al. / Chinese Journal of Catalysis, 2007, 28(4): 293–295

and the amount of H2 and CO produced was also enhanced. This indicated a higher activity for methane dissociation on Rh-CeO2/Al2O3 compared to the Rh/Al2O3 catalyst. In addi-tion, the partial oxidation of CHx species by the mobile oxy-gen of ceria also occurred to form CO and H2. In contrast, when CeO2 was added to Rh/SiO2, the CH4 conversion de-creased, which suggested a decrease in the catalytic activity of the Rh-CeO2/SiO2 catalyst for methane dissociation. Indeed, a lower amount of H2 was produced. However, the increase in CO yield can be attributed to the partial oxidation of CHx spe-cies by the mobile oxygen available on CeO2. Based on the above results, it is concluded that the support of the catalysts has a significant influence on the activity of CH4 dissociation on Rh.

The FT-IR spectra of CO adsorbed on the various reduced catalysts are shown in Fig. 1. For the Rh/Al2O3 catalyst, the strong peak at 2023 cm−1 was assigned to the linear CO spe-cies adsorbed on metallic Rh0 sites. The broad band centered at about 1850 cm−1 corresponded to the bridged CO species adsorbed on Rh0 sites. When CeO2 was added to Rh/Al2O3, the linear adsorbed CO band shifted from 2023 to 2033 cm−1. Blyholder [6] has postulated that the CO to metal bond results from an electron transfer from the HOMO of CO (5σ) to a free σ symmetry d orbital of the metal and an electron back-donation from an occupied π symmetry d orbital of the metal to the empty 2π-antibonding orbital of CO. This metal back-donation to the CO antibonding orbital weakens the C–O bond. The blue shift of the linear CO frequency of the Rh-CeO2/Al2O3 catalyst indicates a decrease of electron den-sity on rhodium (Rhδ+), which weakens the electron back-donation from Rh to the antibonding orbital of CO and decreases the Rh–CO bond strength. Thus, Rh0 and Rhδ+ spe-cies coexist on the Rh-CeO2/Al2O3 catalyst.

Fig. 1 FT-IR spectra of CO adsorbed on Rh/Al2O3 (1), Rh-CeO2/ Al2O3 (2), Rh/SiO2 (3), and Rh-CeO2/SiO2 (4)

For the Rh/SiO2 catalyst, two peaks appeared at 2031 and 1843 cm−1, corresponding to linear and bridged CO species adsorbed on Rh0 sites, respectively. For Rh-CeO2/SiO2, the

linear adsorbed CO band shifted from 2031 to 2021 cm−1. This red shift suggested an increase in electron density on Rh0 species (Rhδ–), which enhanced the back-donation of electrons from Rh to the antibonding orbital of CO and increased the Rh–CO bond strength, demonstrating the coexistence of both Rh0 and Rhδ– species on the Rh-CeO2/SiO2 catalyst. We con-clude that the addition of CeO2 has different electronic effects on Al2O3- and SiO2-supported Rh catalysts, which depends on the properties of the support itself.

It has been proposed that the metal–support interaction is associated with the acidity/alkalinity of the support [7, 8]. With increasing support alkalinity, electron transfer between the support oxygen atoms and the nearby metal particles can increase the electron density on the metal particles. Fig. 2 shows the FT-IR spectra of adsorbed pyridine on the different catalysts after evacuation at 50, 150, and 300ºC. For bare Al2O3 (Fig. 2(a)), the existence of Lewis acid sites was shown by the peaks at 1450 and 1614 cm−1 at 50ºC, while no adsorp-tion band characteristic of Brønsted acid sites at 1540 cm−1 was observed. After the evacuation of pyridine at 150ºC, the band intensity was significantly decreased. After heating to 300ºC, only the very weak band at 1450 cm−1 was present. It is clear that Al2O3 possessed two types of Lewis acid sites with different acid strength, namely, the weak acid sites did not retain pyridine upon heating to 300ºC, while for the strong acid sites the characteristic band (1450 cm−1) was still visible at 300ºC.

Fig. 2 FT-IR spectra of pyridine adsorbed on Al2O3 (a) and CeO2/Al2O3 (b) at different temperatures

(1) 50ºC, (2) 150ºC, (3) 300ºC

In the case of Al2O3 doped with CeO2 (Fig. 2(b)), the bands associated with Lewis acidity (1447 and 1614 cm−1) were still present up to 300ºC, and their intensities were much higher

WANG Rui et al. / Chinese Journal of Catalysis, 2007, 28(4): 293–295

than that of unpromoted Al2O3. This result indicated that CeO2 increased the number of Lewis acid sites on the Al2O3 sup-port, which enhanced the ability of Al2O3 to accept electrons from CeO2 and consequently decreased the electron density on the Rh0 species (Rhδ+). This is consistent with the results of the literature [9, 10].

For bare SiO2 (Fig. 3(a)), only the existence of Lewis acid sites (1445 and 1598 cm−1) at 50ºC was detected, which was similar to Al2O3. However, their intensity was much lower than that on Al2O3, and the bands disappeared after evacuation at 300oC. Therefore, SiO2 possessed a smaller number of Lewis acid sites and weaker acid strength than Al2O3. When SiO2 was doped with CeO2 (Fig. 3(b)), the peak intensity of the Lewis acid sites was decreased at 50ºC, and the peaks disappeared completely after evacuation at 150ºC. This indi-cated that the addition of CeO2 had a negative effect on both the abundance and the strength of the acid sites on SiO2. The degradation of acidity weakened the electron-accepting ability of SiO2 and inhibited the transfer of free electrons of CeO2 to SiO2.

The combined results of CO-FTIR and Py-FTIR showed that the addition of CeO2 modified the Lewis acidity of Al2O3 and SiO2. For the Rh/Al2O3 catalyst, the addition of CeO2 increased the number of Lewis acid sites on Al2O3 and thus enhanced the ability of Al2O3 to accept electrons. This facili-tated the transfer of electrons from Rh to CeO2–x and Al2O3 at the Rh–CeO2–x–Al2O3 interface, which resulted in a decrease in electron density on Rh0 species (Rhδ+). While for the Rh/SiO2 catalyst, CeO2 remarkably decreased the amount and the strength of Lewis acid sites, which in turn weakened the ability of SiO2 to accept electrons. As a result, the transfer of free electrons of CeO2–x to Rh increased the electron density of Rh0 species (Rhδ–) on SiO2.

Based on the above discussion, the decrease in electron density of Rh (Rhδ+) on the Rh–CeO2/Al2O3 catalyst makes it easier to accept σ electrons from the cleavage of C–H bonds, which facilitates the activation and dissociation of CH4 on Rh into CHx species and H2 over the Rh/Al2O3 catalyst. For the Rh-CeO2/SiO2 catalyst, the increase in electron density on Rh (Rhδ–) inhibits the cleavage of C–H bonds to release σ elec-trons, which suppresses the dissociation of CH4 on Rh into CHx species and H2 over the Rh/SiO2 catalyst. In addition, CO can be formed through the partial oxidation of CHx species by mobile oxygen present in CeO2.

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Fig. 3 FT-IR spectra of pyridine adsorbed on SiO2 (a) and

CeO2/SiO2 (b) at different temperatures (1) 50ºC, (2) 150ºC, (3) 300ºC