Partial oxidation of methane to syngas on Rh/Al2O3 and Rh/Ce-ZrO2 catalysts

ARTICLE

Partial oxidation of methane to syngas on Rh/Al₂O₃ and Rh/Ce-ZrO₂ catalysts

Raquel L. Oliveira; Isabela G. Bitencourt; Fabio B. Passos^*

Departamento de Engenharia Química e Petróleo, Universidade Federal Fluminense, Rua Passo da Pátria, 156, 24210-240 Niterói-RJ, Brazil

ABSTRACT

The partial oxidation of methane with γ-Al₂O₃-, CeO₂-, ZrO₂- and Ce-ZrO₂-supported rhodium catalysts was investigated. DRIFTS (diffuse reflectance infrared spectroscopy) measurements of adsorbed CO showed the formation of different rhodium species on different supports, which influenced the dispersion of the metal. The effects of the metal dispersion, oxygen storage capacity on the activity of these catalysts for the partial oxidation of methane are discussed.

Keywords: methane partial oxidation, methane, ceria-zirconia, rhodium, DRIFTS

RESUMO

A oxidação parcial do metano sobre catalisadores de ródio suportados em γ-Al₂O₃, CeO₂, ZrO₂ e Ce-ZrO₂ foi investigada. Medidas de DRIFTS (espectroscopia no infravermelho com reflectância difusa) de CO adsorvido mostraram que houve a formação de diferentes espécies de ródio nos diferentes suportes, o que por sua vez influenciou na dispersão do metal. Os efeitos da dispersão metálica e da capacidade de armazenamento de oxigênio sobre a atividade dos catalisadores na oxidação parcial foram discutidos.

Introduction

The catalytic partial oxidation of methane is an alternative to steam reforming, the industrial process for the production of synthesis gas from methane. Partial oxidation is a more energy efficient and less expensive process than steam reforming. The partial oxidation of methane (POM) is a mildly exothermic reaction, in which methane reacts with a limited amount of oxygen to produce hydrogen and carbon monoxide:

The POM process is capable of producing syngas with a H₂/CO ratio of approximately 2, which makes it a favorable method for methanol and hydrocarbon synthesis.^1,2 Noble metal catalysts (Ir, Ru, Rh, Pt, Pd) exhibit high activity, along with long-term stability, while minimizing the levels of coking, particularly when compared to the non-noble metals (such as Fe, Co, Ni).^1,2 The lack of a hydrogen infrastructure, as well as disadvantages in hydrogen storage, has stimulated the development of compact fuel processors, which are able to produce a rich hydrogen gas mixture from hydrocarbons. Several studies have reported that Rh catalysts are especially appropriate for use in compact fuel processors because of their high activity, selectivity for syngas and resistance to carbon deposition.²

One of the main disadvantages of methane conversion is the deactivation due to coke formation. Thus, there is a great interest in designing catalysts that minimize the deleterious effect of coking. The presence of oxygen vacancies in supports that contain CeO₂ proved to be instrumental in keeping the metal particles free from coke deposition during the steam reforming,^3-5 partial oxidation,^6-9 CO₂ reforming and autothermal reforming of methane,^10-12 when using nickel or noble metals as catalysts.^13-15 Furthermore, ceria may also improve the catalytic performance by increasing the noble metal dispersion and stabilizing the support. Both the metal dispersion and support reducibility of Pt/CeO₂ and Pt/Ce_xZr_1–xO₂ were found to influence the catalytic activity and stability of the catalysts in the partial oxidation of methane.^6,10 An increase in the activity of Rh/CeZrO_2, for the partial oxidation of methane as a result of a higher Rh dispersion has also been reported.^11,12

In this work, the effects of the support on the partial oxidation of methane were investigated further. Rh/γ-Al₂O₃, Rh/CeO₂, Rh/ZrO₂ and Rh/Ce-ZrO₂ catalysts were prepared and characterized by diffuse reflectance UV-Vis spectroscopy (DR-UV-Vis spectroscopy), temperature programmed reduction (TPR), X-ray diffractometry (XRD), CO and H₂ chemisorption and diffuse reflectance infrared spectroscopy (DRIFTS) of adsorbed CO. The metal surface was also examined through the dehydrogenation of the cyclohexane model reaction. The partial oxidation of methane by these catalysts was investigated by temperature programmed surface reaction (TPSR) and the stability of the catalysts was probed in a continuous reactor.

Experimental

Catalyst preparation

The catalysts were prepared by incipient wetness using Rh(NO₃)₃ (final Rh content equal to 1.5%) as the precursor. γ-Al₂O₃, ZrO₂ and CeO₂ supports were obtained by calcining bohemite (Catapal), (NH₄)₂Ce(NO₃)₆ (Aldrich) and Zr(OH)₂ (Aldrich), respectively, at 800 ºC for 1 h. The Ce_xZr_1-xO₂ (x = 0.25, 0.5, 0.75) supports were obtained through a co-precipitation method.¹³ An aqueous solution of (NH₄)₂Ce(NO₃)₆ (Aldrich) and ZrO(NO₃)₂ (Aldrich) was prepared to obtain the desired amounts of CeO₂ and ZrO₂. Then, the ceria and zirconium hydroxides were co-precipitated by the addition of excess NH₄OH (Merck). Finally, the precipitate was washed with distilled water and calcined at 800 ºC for 1 h.

Catalyst characterization

BET specific surface areas were measured at –196 ºC in a Micromeritics ASAP 2010. The samples (1 g) were previously dried at 250 ºC under vacuum. TPR measurements were carried out in a micro-reactor coupled to a quadrupole mass spectrometer (Balzers, Omnistar). The samples (150 mg) were dehydrated at 150 ºC for 30 min under He flow prior to reduction. After cooling to room temperature, the samples were then subjected to 5% H₂/Ar gas flow (30 mL min^-1), and the temperature was raised to 1000 ºC at a rate of 10 ºC min^-1.

XRD data of the calcined catalysts, and their respective supports, were performed in a Rigaku Miniflex spectrometer using monochromatic Cu K_α (1.540 Å) radiation with a scan rate of 0.05 ºC min^-1 and a 2θ range of 2-90º.

In the literature, structure insensitive reactions have been reported for the characterization of catalysts.¹⁴ The rhodium active phase was probed through cyclohexane dehydrogenation as a structure insensitive reaction.¹⁴ This reaction was performed under atmospheric pressure in a continuous flow micro-reactor. The pretreatment of the samples (10 mg) consisted of drying them at 150 ºC under a stream of He (30 mL min^-1) for 30 min, followed by reduction at 500 ºC under a H₂ flow (10 ºC min^-1) and a final purge using He (30 mL min^-1) at 800 ºC for 30 min. The reactant mixture was obtained by bubbling hydrogen through a saturator containing cyclohexane at 12 ºC (H₂/C₆H₁₂ 13.6). The total flow rate was equal to 100 mL min^-1 and the temperature was 270 ºC. The effluent gas phase was analyzed with an on-line gas chromatograph (HP-5890) equipped with a flame ionization detector and an HP Innowax capillary column. Under these conditions, no significant deactivation of the catalysts was observed, and there were no diffusional or thermodynamic limitations.

H₂ and CO chemisorption measurements were conducted using a Micromeritics ASAP 2010 gas sorption analyzer. The mass of the samples used was 500 mg. The calcined samples were subjected to a standard procedure consisting of heating the sample under He (30 mL min^-1) at 150 ºC for 30 min, followed by reduction in a flow of H₂ (30 mL min^-1) at a rate of 10 ºC min^-1 up to 500 ºC. Then, the samples were heated in He (30 mL min^-1) to 800 ºC. Then, the samples were evacuated for 30 min at the final temperature to remove any residual H₂, before cooling under vacuum to 35 ºC for analysis. The irreversible H₂ and CO uptakes were obtained using the difference between total and reversible H₂ and CO uptakes. A chemisorption stoichiometry of H:Rh 1:1 and CO:Rh 1:1 was assumed.

DR-UV-Vis spectroscopy was carried out in a Varian model Cary 5.0 spectrometer equipped with a diffuse reflectance accessory (Harrick). To separate the contribution of the support, the reflectance R(λ) of the sample was made proportional to the reflectance of the respective support, and the Kubelka-Munk function F(R) was calculated.

In situ infrared spectra of the samples were conducted using a Bruker VERTEX 70 FTIR equipped with an LN-MCT detector, using a diffuse reflectance cell (Harrick, HVC-DRP-4) with ZnSe windows. The spectra were recorded at 4 cm^-1 resolution with 256 scans for each spectrum in the range from 2200 to 1700 cm^-1. The samples were pretreated ex situ as described in the chemisorption experiments, and then passivated under a flow of 5% O₂/He for 15 min at room temperature. Before each experiment, the samples were reduced in the cell under a H₂ flow (30 mL min^-1) at 500 ºC for 1 h. After purging with He (30 mL min^-1) for 30 min at the reduction temperature, background interferograms were obtained at the following temperatures: 300, 200, 100 and 30 ºC. The samples were subsequently subjected to a 5% CO/He gas stream (30 mL min^-1) at room temperature for 1 h. The samples were purged with He for 1 h. Then, interferograms of the adsorbed CO were collected at each temperatures at which a background interferogram was taken. The absorbance spectrum was calculated from the ratio of the sample interferogram and to that of the background interferogram.

Partial oxidation of methane

The partial oxidation of methane was performed in a quartz reactor at atmospheric pressure. Prior to each reaction, the catalyst was reduced under a flow of H₂ (30 mL min^-1) at 500 ºC for 1 h and then heated to 800 ºC under a He flow (30 mL min^-1). The reaction was carried out at 800 ºC and weight hourly space velocity (WHSV) of 520 h^-1 for all catalysts. A reactant mixture with a CH₄:O₂ ratio of 2:1 and a total flow rate of 100 mL min^-1 was used. To avoid temperature gradients, catalyst samples (10 mg) were diluted with inert SiC (18 mg). The transfer lines were kept at 140 ºC to avoid condensation. The exit gases were analyzed using a gas chromatograph (Varian CP3800) equipped with a thermal conductivity detector and a Carboxen 1010 capillary column (SUPELCO). The H₂ and CO selectivities were determined via the equations:

The TPSR experiments were performed in the same apparatus used for the TPR measurements. After drying under He at 150 ºC, the samples (150 mg) were reduced under a flow of H₂ at 500 ºC for 1 h, purged with a flow of He at 800 ºC for 30 min, and cooled to room temperature. The samples were subjected to a flow of CH₄/O₂/He (2:1:27) at 30 mL min^-1 while the temperature was raised to 800 ºC at rate of 20 ºC min^-1.

Results and Discussion

Catalyst characterization

BET specific surface areas of the several catalysts are shown in Table 1. As expected, the 1.5%Rh/γ-Al₂O₃ presented the highest surface area, followed by the mixed oxide supported Rh catalysts, which presented higher surface areas than the single CeO₂- and ZrO₂-supported catalysts.

TPR profiles of the supports are presented in Figure 1. The CeO₂ support showed H₂ consumption peaks at approximately 490 and 810 ºC; usually these are ascribed to the reduction of the capping and bulk CeO_2, respectively.¹⁵ No reduction was observed for γ-Al₂O₃ and ZrO₂.^16,17 Ce-ZrO₂ supports presented maximum temperatures of reduction that shifted from 602 to 549 ºC with increasing CeO₂ content. These peaks are related to the CeO₂ reduction in a solid Ce-ZrO₂ solution, which happened at a lower temperature than the reduction temperature observed for pure CeO₂.¹⁸

The TPR profile (Figure 2) for the 1.5%Rh/γ-Al₂O₃ catalyst showed a peak at 154 ºC, which may be attributed to the reduction of Rh₂O₃.^19,20 The profile for the 1.5%Rh/CeO₂ catalyst showed peaks at 96, 224 and 967 ºC. The first two peaks may be attributed to the reduction of well-dispersed Rh₂O₃ and of larger Rh₂O₃ particles,²¹ which occurs simultaneously with the reduction of the surface ceria, and the last peak results from the reduction of bulk ceria.²² For the 1.5%Rh/ZrO₂, there were peaks at 98 and 211 ºC corresponding to the reduction of two different types of Rh₂O₃, one with high dispersion and the other with low dispersion.²³ The 1.5%Rh/Ce₇₅Zr₂₅O₂ TPR profile presented peaks at 98, 135, 238 and 1003 ºC, which may be attributed to the reduction of well dispersed Rh₂O₃, larger Rh₂O₃ particles, surface ceria and the reduction of bulk ceria, respectively. Additionally, the TPR profiles of the 1.5%Rh/Ce₂₅Zr₇₅O₂ and 1.5%Rh/Ce₅₀Zr₅₀O₂ catalysts presented only two peaks, the first due to the reduction of well dispersed Rh₂O₃ and the second resulting from simultaneous reduction of Rh₂O₃ and the respective supports.²³ The hydrogen uptakes of the supported catalysts during the TPR experiments are displayed in Table 1 and are consistent with the previous peak assignments.

Figure 3 shows the XRD patterns of the Rh catalysts. Due to the low Rh content and high dispersion,² no Rh₂O₃ peaks were detected. Thus, all catalysts presented diffraction patterns essentially identical to their respective supports.

The 1.5%Rh/γ-Al₂O₃ catalyst showed the presence of peaks at approximately 2θ 45 and 70º, which are characteristic of γ-Al₂O₃ obtained through the calcination of bohemite.^24,25 For 1.5%Rh/CeO₂, it was possible to observe the diffraction patterns relative to that of CeO₂ with a fluorite structure (JCPDS 4-0593), while a monoclinic ZrO₂ phase (JCPDS 13-307) could be detected for the 1.5%Rh/ZrO₂ catalyst.^15,26 The diffraction pattern for 1.5%Rh/Ce₇₅Zr₂₅O₂ did not show separate phases for CeO₂ or ZrO₂. There was a shift in the corresponding CeO₂ peaks from 2θ 28.6, 33.1, 47.51 and 56.41º to 2θ 28.9, 33.3, 48.1 and 57.0º. This shift increased to higher Bragg angles as the ZrO₂ content was increased (1.5%Rh/Ce₂₅Zr₇₅O₂ and 1.5%Rh/Ce₅₀Zr₅₀O₂). This shift is related to the formation of a solid solution of CeO₂ and ZrO₂ with a cubic structure.²⁷

Figure 4 shows the DR-UV-Vis spectra of the catalysts. For 1.5%Rh/γ-Al₂O₃, a broad band centered at 320 nm was present. This band corresponds to the ¹A_1g→ ¹T_1g transition.^28,29 The spectra for the 1.5%Rh/ZrO₂, 1.5%Rh/CeO₂ and 1.5%Rh/Ce_-ZrO₂ catalysts displayed two broad bands, one centered at approximately 260 nm due to Rh-O charge transfer and the other at approximately 320 nm due to the ¹A_1g→ ¹T_1g transition.³⁵ These results are consistent with the TPR results as the broad bands are due to the inhomogeneity of the surface.

Table 2 displays the results of H₂ and CO chemisorption on the catalysts. The 1.5%Rh/γ-Al₂O₃ catalyst showed larger H/Rh and CO/Rh values due to the higher γ-Al₂O₃ surface area. Rh/ZrO₂ presented the lowest H/Rh and CO/Rh ratios, although these values increase with CeO₂ content for the Rh/Ce-ZrO₂ catalysts. Since H₂ and CO can adsorb on CeO₂, these gases are not considered adequate to evaluate the dispersion of the CeO₂ supported catalysts.³⁰

Additionally, cyclohexane dehydrogenation was used to probe the surface active sites of the investigated catalysts (Table 3). The initial dehydrogenation rate increased in the following order: 1.5%Rh/ZrO₂ < 1.5%Rh/Ce₅₀Zr₅₀O₂ < 1.5%Rh/CeO₂ < 1.5%Rh/Ce₇₅Zr₂₅O₂ < 1.5%Rh/Ce₂₅Zr₇₅O₂ < 1.5%Rh/γ-Al₂O₃.

DRIFTS absorption spectra of CO adsorbed on the catalysts at room temperature are displayed in Figure 5. The 1.5%Rh/γ-Al₂O₃ catalyst presented a major band with a maximum at 2090 cm^-1 and a smaller band at 2019 cm^-1. These bands are associated with the symmetric and asymmetric CO stretching modes of the gem-dicarbonyl species Rh^I-(CO)₂, which are generated by reaction of atomically dispersed rhodium particles with surface hydroxyl groups and CO.^37,38 An additional band at 2062 cm^-1 was observed for 1.5%Rh/CeO₂. This band results from the linear adsorption of CO on metallic rhodium crystallite particles.^31,32 This is consistent with the presence of both well dispersed and larger Rh particles indicated by the TPR experiments. Additionally, two broad bands at 1850 and 1940 cm^-1 were observed and are attributed to two types of bridge-bonded CO on the Rh⁰ sites.³³ With 1.5%Rh/ZrO₂, the absorbance intensities were lower than observed for the other catalysts, and the bands relative to gem-dicarbonyl species (2090 and 2020 cm^-1) were much smaller as expected because of the low CO adsorption capacity observed for this catalyst. A broad band between 2000 and 2050 cm^-1, attributed to the linear adsorption of CO on Rh metal, and a small band at 1850 cm^-1, attributed to bridge-bonded CO, were observed.³⁴ The spectra of the Rh/CeZrO₂ catalysts contained very broad bands, indicating a non-uniform distribution of Rh particle sizes. For the 1.5%Rh/Ce₇₅Zr₂₅O₂ catalyst, a main broad band was observed at 2055 cm^-1 due to linear CO-Rh adsorption, with a tail at 1950 cm^-1. A small band at 1850 cm^-1 due to the two types of bridge-adsorbed CO was also observed. A small shoulder was observed at 2020 cm^-1 indicating a superposition of the bands resulting from the gem-dicarbonyl species (2090 and 2020 cm^-1) and the broad band centered at 2055 cm^-1. Similar results were observed for the 1.5%Rh/Ce₅₀Zr₅₀O₂ and 1.5%Rh/Ce₂₅Zr₇₅O₂ catalysts.

Figure 6 shows the thermal desorption spectra for the 1.5%Rh/γ-Al₂O₃ catalysts. The intensity of the band relative to the gem-dicarbonyl species decreases as the temperature was raised. The frequencies of these bands were essentially invariant as is expected for CO adsorption on isolated sites.³⁷

Conversely, 1.5%Rh/Ce₂₅Zr₇₅O₂ shows a decrease (Figure 7) in the frequency of the linearly bound CO from 2055 to 2028 cm^-1 due to the decrease in the dipole-dipole interactions as CO coverage decreases. The bridge-bond CO was less stable to the temperature increase than the linearly adsorbed CO. Similar results were obtained for the other Rh/CeZrO₂ catalysts. These results confirm that the presence of Ce influences the state of Rh particles as reported by Erickson et al.¹¹

Partial oxidation of methane

Figure 8 shows the conversion of methane during the partial oxidation of methane using these catalysts. All of the catalysts showed an initial induction time, required to stabilize the actives sites, except for 1.5%Rh/γ-Al₂O₃. This may be because only 1.5%Rh/γ-Al₂O₃ presented isolated Rh atoms as indicated by the DRIFTS experiments. All of the catalysts were stable during the 24 h time-on-stream, and no coke could be detected by temperature programmed oxidation of the used catalysts. The order of activity (Table 4) approximated the same order observed for the H/Rh and CO/Rh chemisorption ratios, with 1.5%Rh/γ-Al₂O₃ presenting the highest conversion. Thus, the H₂ and CO chemisorption results were more reliable in predicting the order of methane conversion activity than the initial rate of cyclohexane dehydrogenation. This result is different than that observed for Pt/Ce-ZrO₂ catalysts, in which cyclohexane dehydrogenation provided good estimates for methane partial oxidation activity.¹⁰

The addition of cerium to ZrO₂ improved the activity of the Rh/Ce-ZrO₂ catalysts but only for molar ratios greater than 50%. The H₂ and CO selectivities for the catalysts followed the same trend as the activities (Figures 9 and 10).

The ratio of H₂/CO (Table 4) was close to a value of 2 for the catalysts, indicating that there was no influence from the reverse water gas shift reaction. X-ray Photoelectron (XPS) and IR spectroscopic studies have previously shown the complex nature of Rh/γ-Al₂O₃ for CPO, with Rh⁰, Rh⁺ and Rh³⁺ coexisting on the surface under the reaction conditions.¹¹

The TPSR results for all of the catalysts were consistent with an indirect mechanism for the partial oxidation of methane. Figure 11 shows a typical profile for the 1.5%Rh/Ce₂₅Zr₇₅O₂ catalyst, and the other samples showed similar profiles. In the beginning of the reaction, rhodium particles are covered by oxygen making the catalyst active for methane combustion. Then, the formation of CO and H₂ occurs through the CO₂ and H₂O reforming of methane. Several authors have also observed this indirect mechanism with Pt/ZrO₂,³⁵ Pt/Ce-ZrO₂ and Rh/Ce-ZrO₂ catalysts.^16,17 In fact, recent theoretical and experimental results showed that the indirect mechanism is most probable for the catalytic partial oxidation of CH₄ as CO and H₂ are predominantly formed upon complete O₂ depletion from the sequential reforming steps.⁴²

The advantage of using CeZrO₂ as a support has been explained by the continuous removal of carbonaceous deposits from the active sites at the metal-support interfacial perimeter.^6,11 However, under the reaction conditions used in this study, the catalysts have not been deactivated and the coke could not be detected by temperature programmed oxidation experiments. From this, an improvement in the dispersion for the Rh/CeZrO₂ catalysts when compared to Rh/ZrO₂ was the main observed effect.

Conclusions

Rhodium catalysts supported on γ-Al₂O₃, CeO₂, ZrO₂ and Ce-ZrO₂ were characterized and tested for the partial oxidation of methane to produce hydrogen. The formation of different rhodium species on the surface of different supports was evidenced by the DRIFTS measurements of adsorbed CO. The performance of Rh catalysts in the partial oxidation of methane was primarily dependent on the fraction of exposed metal. Rh/Ce-ZrO₂ showed better dispersion and consequently better catalytic activity compared to Rh/ZrO₂.

Acknowledgements

The authors acknowledge financial support from CNPq, MCT, FINEP and Petrobras.

Submitted: June 30, 2012

Published online: February 7, 2013

• 1. Tian, Z.; Dewaele, O.; Marin, G. B.; Catal. Lett. 1999, 57, 9.
• 2. Eriksson, S.; Nilsson, M.; Boutonnet, M.; Jaras, S.; Catal. Today 2005, 100, 447.
• 3. Laosiripojana, N.; Assabumrungrat, S.; Appl. Catal., A 2005, 290, 200.
• 4. Dong, W. S.; Roh, H. S.; Jun, K. W.; Park, S. E.; Oh, Y. S.; Appl. Catal., A 2002, 226, 63.
• 5. Purnomo, A.; Gallardo, S.; Abella, L.; Salim, C.; Hinode, H.; React. Kinet. Catal. Lett. 2008, 95, 213.
• 6. Mattos, L. V.; Oliveira, E. R.; Resende, P. D.; Noronha, F. B.; Passos, F. B.; Catal. Today 2002, 77, 245.
• 7. Xu, S.; Wang, X.; Fuel 2005, 84, 563.
• 8. Larrondo, S. A.; Kodjaian, A.; Fábregas, I.; Zimicz, M. G.; Lamas, D. G.; Int. J. Hydrogen Energy 2008, 33, 3607.
• 9. Salazar-Villalpando, M. D.; Reyes, B.; Int. J. Hydrogen Energy 2009, 34, 9723.
• 10. Passos, F. B.; de Oliveira, E. R.; Mattos, L. V.; Noronha F. B.; Catal. Lett. 2006, 110, 161.
• 11. Eriksson, S.; Rojas, S.; Boutonnet, M.; Fierro, J. L. G.; Appl. Catal., A 2007, 326, 8.
• 12. Boullosa-Eiras, S.; Zhao, T.; Chen, D.; Holmen, A.; Catal. Today 2011, 171, 104.
• 13. Hori, C. E.; Permana, H.; Simon, K. Y.; Brenner, A.; More, K.; Rahmoeller, M.; Belton, D.; Appl. Catal., B 1998, 16, 105.
• 14. Passos, F. B.; Frety, R.; Schmal, M.; Catal. Lett. 1992, 14, 57.
• 15. Roh, H. S.; Jun, K. W.; Dong, W. S.; Chang, J. S.; Park, S. E.; Joe, Y.; J. Mol. Catal. A: Chem. 2002, 181, 137.
• 16. Bozo, C.; Guilhaume, N.; Garbowski, E.; Primer, M.; Catal. Today 2000, 59, 33.
• 17. Dong, W. S.; Roh, H. S.; Jun, K. W.; Park, S. E.; Oh, Y. S.; Appl. Catal., A 2002, 226, 63.
• 18. Vidal, H.; Kapar, J.; Pijolat, M.; Colon, G.; Bernal, S.; Cordo'n; Appl. Catal., B 2001, 30, 75.
• 19. Yao, H. C.; Japar, S.; Shelef, M.; J. Catal. 1977, 50, 407.
• 20. Hwang, C. P.; Yeh, C. T.; Zhu, Q.; Catal. Today 1999, 51, 93.
• 21. Van't, J. C.; Blik, H. F. J.; Huizinga, T.; Van Grondelle, J. V.; Prins, R.; J. Catal. 1985, 95, 333.
• 22. Diagne, C.; Idriss, H.; Kiennemann, A.; Catal. Commun. 2002, 3, 565.
• 23. Wang, J. A.; Lopez, T.; Bokhimi, X.; Novaro, O.; J. Mol. Catal. A: Chem. 2005, 239, 249.
• 23. Ruckenstein, E.; Wang, H. Y.; J. Catal. 1999, 187, 151.
• 25. Wang, S.; Lu, G. Q. M.; Appl. Catal., B 1998, 16, 269.
• 26. Pengpanich, S.; Meeyoo, V.; Rirksomboon, T.; Bunyakiat, K.; Appl. Catal., A 2002, 234, 221.
• 27. Hori, C. E.; Permana, H.; Simon, N. K. Y.; Brenner, A.; More, K.; Rahmoeller, K.M.; Appl. Catal., B 1998, 16, 105.
• 28. Lin, Q.; Shimizu, K.; Satsuma, A.; Appl. Catal., A 2012, 419, 142.
• 29. Stoyanovskii, V. O.; Vedyagin, A. A.; Aleshina, G. A.; Volodin, A. M.; Noskov, A. S.; Appl. Catal., B 2009, 90, 141.
• 30. Rogemond, E.; Essayem, N.; Frety, R.; Perrichon, V.; Primet, M.; Mathis, F.; J. Catal. 1997, 166, 229.
• 31. Yang, A. C.; Garland, C. W.; J. Phys. Chem. 1957, 61,1504.
• 32. Rice, C. A.; Worley, S. D.; Curtis, C. W.; Guin, J. A.; Tarrer, A. R.; J. Chem. Phys. 1981, 74, 6487.
• 33. Kondarides, D. I.; Zhang, Z., Verykios, X. E.; J. Catal. 1998, 176, 536.
• 34. Gutierrez, A.; Karinen, R.; Airaksinen, S.; Kaila, R.; Krause, A. O. I.; Int. J. Hydrogen Energy 2011, 36, 8967.
• 35. Hegarty, M. E. S.; Catal. Today 1998, 42, 232.

Publication Dates

• Publication in this collection
  28 Feb 2013
• Date of issue
  Jan 2013

History

• Received
  30 June 2012
• Accepted
  07 Feb 2013