Novel Pd/TiO2-Al2O3 Catalysts for Methane Total Oxidation at Low Temperature and Their 18O-Isotope...

Chinese Journal of Chemistry, 2005, 23, 1333—1338 Full Paper

* E-mail: [email protected] Received December 8, 2004; revised June 5, 2005; accepted June 21, 2005. Project supported by the Major State Basic Research Development Program (No. G2000077503), the National Natural Science Foundation of China

(No. 20173002), and Cooperation Project between NSFC and DFG (No. 20411130104).

© 2005 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Novel Pd/TiO2-Al2O3 Catalysts for Methane Total Oxidation at Low Temperature and Their 18O-Isotope Exchange Behavior

LIN, Weia(林伟） LIN, Lia(林莉) ZHU, Yue-Xiang*,a(朱月香) XIE, You-Changa(谢有畅) SCHEURELL, K.b KEMNITZ, E.b

a State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

b Institute of Chemistry, Humboldt University, Brook Taylor Street 2, Berlin D-12489, Germany

Pd supported on TiO2-Al2O3 binary oxides prepared by coprecipitation method has been investigated for the to-tal oxidation of methane. All Pd/TiO2-Al2O3 catalysts show higher activity than Pd/Al2O3 and Pd/TiO2. Among them, Pd/2Ti-3Al with a Ti/Al ratio of 2 to 3 has a T90% of 395 ℃ at a gas hourly mass velocity of 33000 mL/(h•g), which is at least 50 ℃ lower than that of Pd supported on single metal oxide Al2O3 or TiO2. The results of TPR and 18O-isotope exchange experiments demonstrated that the excellent activity of Pd/2Ti-3Al was due to its high oxygen mobility and moderate reducibility, which is in accordance with our previous work. XPS results indicated that the dispersion of Pd was not the key factor to influence the catalytic activity.

Keywords methane total oxidation, palladium, TiO2-Al2O3 composite, reducibility, oxygen mobility

Introduction

Catalytic total oxidation of methane is an effective way to use methane as an environment-friendly fuel.1-4 Nevertheless, methane contains only C—H bonds, which makes the molecule much more stable than compounds containing C—C bonds.4 So far, most of the researches have been focused on noble metal cata-lysts5-14 because of their high activity. Among various precious metals, palladium-based catalysts show the highest activity at low temperatures.

As reported in literatures, complexities of Pd states and catalytic properties were introduced when Pd was dispersed on different supports.15-21 γ-Alumina has been widely used as a support for the palladium catalyst due to its high specific surface area and low cost. The Pd/Al2O3 catalyst is active at medium temperature above 400 ℃, though its activity is insufficient for the low temperature ignition, e.g. at 300 ℃.22 Furthermore, the carbon dioxide and water produced during the reac-tion, serve as poisons for the Pd/Al2O3 catalysts, espe-cially at low conversions and low temperatures, making the activity deteriorated.23 Therefore, in order to get more active catalyst for low temperature catalytic com-bustion of methane, various kind of modified Al2O3 systems have been investigated as supports,17,18,22,24-28 such as SnO2, LaO, MgO, NiO, SiO2, CeO2, etc. How-ever, not all additives could benefit the activity of Pd/Al2O3. For example, the addition of SnO2 and LaO to Pd/Al2O3

24 improved the thermal stability of catalyst

but did not bring any benefit to the activity for methane combustion, while the catalytic activity and stability of Pd catalyst supported on KIT-1 mesoporous materials were considerably enhanced by titania loading.29,30 Kang29 reported that titania chemically bonded with the skeleton of mesoporous material interacted with co-loaded palladium to suppress the decomposition of palladium oxide, which is the more active phase in the methane combustion than palladium, thus resulting in the improvement of the catalytic performance. Our pre-vious work31-33 showed that the addition of titania into alumina by the fluidization method enhanced the reduc-tion as well as the oxygen mobility of palladium oxide, hence generating catalyst with higher activity for low temperature catalytic combustion of methane. In this paper, TiO2-Al2O3 binary oxides with different Ti/Al atomic ratios were prepared by coprecipitation method and used as supports for palladium oxide. The total oxidation of methane over these Pd/TiO2-Al2O3 cata-lysts was investigated and X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature- programmed reduction (TPR) and temperature-pro-grammed 18O-isotope exchange techniques were em-ployed for the characterization of the catalysts. Despite the difference of TiO2-Al2O3 texture, the results further confirmed that the moderate reducibility and high oxy-gen mobility were the critical factors to influence the novel catalytic activity as reported in our previous work.31-33

1334 Chin. J. Chem., 2005, Vol. 23, No. 10 LIN et al.


Experimental

Catalyst preparation

TiO2-Al2O3 binary oxide was prepared by coprecipi-tation method. Ti(C4H9O)4 and C2H5OH were mixed by volume ratio of 1∶4 at first, and then the mixture solu-tion and Al(NO3)3 solution were dropped simultane-ously into 5 mol/L NH3•H2O at room temperature. Af-terwards, the precipitate was vacuum filtered and dried at 120 ℃ followed by calcination at 600 ℃ in air for 8 h. Catalysts of 2 wt% Pd/TiO2-Al2O3 were prepared by impregnation method using palladium nitrate solu-tion, dried at 120 ℃ overnight and calcined in air at 600 ℃ for 4 h.

Catalytic activity tests

Activity evaluation was carried out in a U-shaped fixed-bed microreactor (i.d.＝10 mm) with a continuous flow at atmospheric pressure. The catalysts were pressed to pellets, and then crushed and sieved to 40—60 mesh. The catalyst (120 mg) was put in the microre-actor and pretreated in flowing air at 500 ℃ for 1 h and then cooled down to room temperature before the catalytic test. A K-type thermocouple was fixed to the middle of the catalyst bed to measure the reaction tem-perature and control the furnace temperature. The feed gas was 1% CH4 in air. The total feed flow rate was 66 mL•min－1, corresponding to a gas hourly mass velocity of 33000 mL/(h•g). The reactants and products were analyzed with an on-line SQ-206 gas chromatogram equipped with hydrogen flame ionization detector (FID). A methanator of nickel catalyst was used to convert CO2 and CO to CH4 for FID analysis. A 4-meter long Porapak Q column was employed to separate CH4, CO and CO2. The peaks of CH4, CO and CO2 were collected with a computer and the conversion of methane was calculated automatically. It should be noticed that dur-ing the measurement, no CO was detected, CO2 was the only production. Methane conversion was measured from 275 to 500 ℃ at an interval of 25 ℃. Each reac-tion temperature was kept stable for 30 min before ana-lyzing the effluent gas. The temperatures corresponding to 10%, 50%, and 90% methane conversion, T10%, T50%, and T90%, were obtained from the temperature dependent plot of methane conversion.

Characterization techniques

BET surface area of the catalysts was determined by N2 adsorption-desorption with a Micromeritics ASAP 2010 Analyzer. The samples were degassed in vacuum (10－3 Pa) for 2 h at 300 ℃ prior to adsorption meas-urements.

Phase composition of the catalysts was recorded on a Rigaku D/MAX-200 X-ray powder diffractometer with Ni-filtered Cu Kα radiation at 40 kV and 100 mA.

Surface composition of the samples was measured by a Kratos Axis Ultra System with monochromatic Al Kα X-ray (1486.71 eV) operated at 15 kV and 15 mA (emission current) in a chamber pressure of approximate

10－8 Pa. Energy step 1 and 160 eV pass energy were used for survey scan, while energy step 0.1 and 20 eV pass energy were used for element scan.

The reducibility of the catalysts was measured with fixed bed reactor. About 16 mg of 40—60 mesh catalyst were put in a U-shape reactor and kept at －15 ℃ for certain time, then a gas stream of 5% H2 in Ar was in-troduced into the reactor at a flow rate of 30 mL•min－1. H2-TPR profile was recorded when the temperature was raised from －10 to 80 ℃ at a constant rate of 2 ℃• min－1. The rate of hydrogen consumption during the reduction was monitored by a thermal conductivity de-tector (TCD).

Oxygen mobility of the catalysts was investigated adopting a temperature-programmed 18O-isotope ex-change method.21,30 The measurements were carried out in a quartz reactor with an on-line-coupled quadruple mass spectrometer QMG421 I (Pfeiffer Vacuum GmbH). For each test, about 300 mg of catalyst were introduced into the reactor and pretreated at 450 ℃ for 4.5 h in air. After cooling down to 100 ℃, a gas mixture of Ar, 16O2 and 18O2 with a pressure ratio of 4∶1∶1 and a total pressure of 100 Pa was introduced into the reaction sys-tem. All measurements were performed in the tempera-ture range 100—700 ℃ with a heating rate of 10 ℃ min－1. Between the gas phase of 16O2 and 18O2 and the catalyst, several processes such as oxygen uptake/release, homogeneous gas phase 18O-isotope exchange, partial heterogeneous 18O-isotope exchange and complete het-erogeneous 18O-isotope exchange may take place sepa-rately or simultaneously depending on the nature of the compound and/or the temperature range investigated. The temperature dependence of the ionic currents (IC) of 16O2,

18O2, and 16O18O provided the information about the isotope exchange reaction. In order to differ-entiate between simultaneous processes, four different coefficients, s, c, y and v, were derived from the meas-ured ionic currents.31-33 s represents the oxygen partial pressure of the gas phase standardized by the oxygen partial pressure at the beginning of the measurement. It changes only when oxygen uptake/release occurs. c ex-presses the proportion of 18O relative to the total oxygen content in the gas phase. It decreases when oxygen is released, and partial or complete heterogeneous ex-change processes take place. y describes the deviation of the actual partial pressure from the equilibrium partial pressure of the mixed isotope 16O18O. A decrease in y shows the occurrence of the partial heterogeneous or homogeneous 18O isotope exchange, while an increase in y indicates the release of 16O2. v represents the molar fraction of 18O in the gas phase that originates from the 16O18O molecules. It increases when any of the three types of 18O isotope exchange processes takes place.

Results and discussion

Activity evaluation

The results of the catalytic tests for Pd/TiO2, Pd/

Methane total oxidation Chin. J. Chem., 2005 Vol. 23 No. 10 1335


Al2O3 and Pd/TiO2-Al2O3 catalysts are shown in Table 1. Obviously, TiO2-Al2O3 composite supports, which were synthesized by coprecipitation method, can also pro-mote the catalytic performance of palladium oxide as pointed out in our previous paper. Among the Pd/TiO2-Al2O3 catalysts, Pd/2Ti-3Al has the highest activity for methane total oxidation. Its T90% was de-creases by at least 50 ℃ in comparison with those of Pd/TiO2 and Pd/Al2O3. Pd/2Ti-3Al with a Ti/Al ratio of 2 to 3 and about 51 wt% TiO2 in Ti-Al composite showed the highest activity, while in our previous work, TiO2/Al2O3 with a TiO2 content of 16 wt% supported palladium showed the highest activity.31 This may be attributed to the different preparation method of the support. In order to understand the behavior of different catalysts, XRD, XPS, TPR and 18O-isotope exchange techniques were used to characterize selected catalysts.

XRD and XPS measurements

XRD was employed to identify the bulk phase com-positions of the samples, with the patterns shown in Figure 1. Similar to our previously reported catalysts with TiAl support prepared by the fluidization method,31 palladium was present in the phase of PdO, the peak of which was located at 2θ＝33.6°. Pure Al2O3 and TiO2 turned out to be amorphous and anatase phase, respec-tively. In the XRD pattern of Pd/1Ti-4Al, no anatase peaks were shown up even though the TiO2 content was about 28 wt%, while in our previous work,31 TiAl2 with a TiO2 content of about 12 wt% displayed obvious peaks characteristic of anatase in its XRD pattern. This indicates that the states of titania in TiO2-Al2O3 com-posites prepared by different methods were different. It can also be seen that the peak intensity of PdO in Pd/2Ti-3Al was much lower than that of other catalysts, indicating the highest dispersion of PdO.

The specific surface area and surface atomic ratio are listed in Table 1. As well known, surface atomic ratio is closely correlated to the dispersion. The greater the sur-face atomic ratio, the higher the dispersion. It can be seen that Pd/2Ti-3Al with the best activity has the greatest surface atomic ratio, and this is in accordance with the XRD results, which can imply that the disper-

Figure 1 XRD patterns of catalysts Pd/TiO2-Al2O3.

sion may be closely correlated to the activity. However, the activities of Pd supported on pure Al2O3 and TiO2 were close, while the surface atomic ratios were differ-ent from each other, so were the samples Pd/3Ti-2Al and Pd/4Ti-1Al. Pd/1Ti-4Al was more active than either Pd/3Ti-2Al or Pd/4Ti-1Al, but the Pd dispersion of Pd/1Ti-4Al was between those of the two catalysts, meaning that the dispersion of palladium species is not the key factor to the catalytic activity of methane com-bustion, in agreement with our previous work.31-33

H2-TPR measurements

The reducibility of supported palladium oxide is an important factor influencing its catalytic perform-ance.31-34 Temperature-programmed reduction technique was used to study the reducibility of the catalysts. The TPR profiles of the catalysts are shown in Figure 2. For all samples investigated, there was a large peak below 30 ℃ corresponding to the reduction of PdO, and a small reverse peak between 50—60 ℃ attributable to the desorption of hydrogen adsorbed on the metallic palladium. To be noted, the supports themselves show no reduction peak in this temperature range.

The reduction temperatures of the Pd/TiO2-Al2O3 catalysts ranged from 7 ℃ of Pd/TiO2 to 25 ℃ of Pd/Al2O3, and the PdO reduction peaks were shifted to the lower temperature side with the increased contents

Table 1 The BET surface areas, surface atom ratio and activity of different catalysts

Temperature/℃ Surface atomic ratio Catalyst Ti∶Al BET surface area/(m2•g－1)

T10% T50% T90% Pd Ti Al O

Pd/Al2O3 193 312 386 447 1.42 36.1 62.5

Pd/1Ti-4Al 1∶4 163 311 377 425 2.52 5.42 27.4 64.7

Pd/2Ti-3Al 2∶3 175 300 352 395 4.87 7.36 23.8 63.9

Pd/3Ti-2Al 3∶2 149 328 389 437 3.40 12.0 19.1 65.5

Pd/4Ti-1Al 4∶1 159 330 389 437 1.91 21.3 10.3 66.5

Pd/TiO2 17.8 333 390 450 0.86 34.9 64.3



Figure 2 H2-TPR patterns of different catalysts.

of titania, which exhibits the same trend as our previous work. Addition of titania to alumina increased the re-ducibility of supported palladium oxide. Catalyst with higher reducibility would generate more metallic Pd for methane conversion,31-33 and hence the addition of TiO2 would improve the catalytic activity. But for catalyst Pd/TiO2, the reducibility is too high, too much metallic Pd will be formed, and there will be not enough PdO sites for the oxidation of CHx,

31-33 which inhibits the catalytic methane combustion.

Temperature-programmed 18O-isotope exchange

Oxygen isotope exchange is a common method to study the uptake/release properties of oxygen and the participation of oxygen from the catalyst in oxidation reactions.31-33,35-38 Temperature-programmed 18O-isotope exchange measurements of selected samples were carried out to investigate the oxygen mobility of se-lected catalysts. The samples selected were Pd/2Ti-3Al with the highest activity for methane combustion, Pd supported on the single oxide Pd/Al2O3 and Pd/TiO2.

The results of 18O-isotope exchange and the tem-perature dependence of the four coefficients of Pd/2Ti- 3Al is presented in Figure 3. When the temperature is below 442 ℃, all the ionic currents of 16O2,

18O2, and 16O18O as well as the four coefficients are constant. At above 442

℃, 18O2 began to decrease and continued up to 700 ℃. At the same time, the 16O2 and the coeffi-cients, s and v, were increased with temperature from 442 to 700 ℃, while the coefficient c was decreased. But the rates of increase or decrease were different de-pending on the temperature range. The y coefficient was first decreased with reaction temperature, demonstrating partial heterogeneous isotope exchange, and reached a minimum at 550 ℃, after which it kept constant and was increased at last. When the temperature exceeded 615 ℃, oxygen was released at a much higher rate, in-

Figure 3 Ionic current curves of 16O2, 16O18O and 18O2 and

variations of the coefficients for catalyst Pd/2Ti-3Al.

dicating the bulk decomposition of palladium oxide. According to the variation of the ionic currents and

the coefficients, the reaction process can be divided into three regions. The first region, between 442 and 550 ℃, a decrease in 18O2 ionic current is accompanied by in-creasing 16O18O and 16O2 ionic currents. The coeffi-cients v and s was increased, while coefficients c and y decreased. From these results, it can be deduced that both partial heterogeneous isotope exchange (PHE) and oxygen release take place in this temperature range, PHE predominating over the oxygen release. In the second region 550—615 ℃, 18O2 and 16O2 remained decreasing and increasing respectively, while 16O18O turned to move down after reaching a maximum. The corresponding coefficients, s, v and c, showed variations similar to those in the first region, but y became in-creased. Complete heterogeneous isotope exchange (CHE), desorption and partial heterogeneous isotope exchange (PHE) took place in this temperature range, with CHE and desorption predominating over PHE. In the third region, between 615 and 700 ℃, 18O2 and 16O18O continued to decrease with a rapid increase of 16O2. The coefficients v and s were also increased at a much higher rate than in the first and second regions. The decrease in coefficient c slowed down, but the co-efficient y was increased slowly. Therefore, CHE and oxygen release as predominant process took place at an

Methane total oxidation Chin. J. Chem., 2005 Vol. 23 No. 10 1337


increased rate in this temperature range, and PHE oc-curred simultaneously.

Similarly, the four coefficients of other samples can be obtained from the ionic currents of 16O2,

18O2, and 16O18O (Figures 4—6). Compared with Pd/2Ti-3Al, they show more or less the same behavior in isotope ex-change reaction, except for variation due to different temperature regions. The processes taking place on each sample during temperature programmed 18O-isotope exchange measurements are summarized in Table 2. Pd/2Ti-3Al displays the lowest onset temperature for 18O-isotope exchange, indicating the highest oxygen mobility. Pd/TiO2 and Pd/Al2O3 have close oxygen mo-bility, but both lower than Pd/2Ti-3Al.

Figure 4 Ionic current curves of 16O2 for different catalysts.

Figure 5 Ionic current curves of 16O18O for different catalysts.

Our previous paper31-33 proved that the reducibility and oxygen-exchange activity of PdO were two factors directly related to the overall catalytic activity of palla-dium catalysts. A moderate reducibility and high oxy-gen mobility will lead to high methane conversion. The reducibility of Pd/2Ti-3Al was between those of Pd/TiO2 and Pd/Al2O3, but its oxygen mobility was much higher. Therefore, Pd/2Ti-3Al illustrated excellent catalytic activity for methane combustion at low tem-perature. For Pd/TiO2 and Pd/Al2O3, the too high re-ducibility of the former and too low one of the latter were both bad for catalytic activity of methane combus-tion. In addition, the low oxygen mobility of Pd/TiO2 and Pd/Al2O3 further led to their low activity.

Figure 6 Ionic current curves of 18O2 for different catalysts.

Conclusions

Pd supported on TiO2-Al2O3 composite oxide showed very high activity for methane total oxidation. Pd/2Ti-3Al with the highest activity presented a T90% of 395 ℃ at a gas hourly mass velocity of 33000 mL/ (h•g), which is more than 50 ℃ lower than that of Pd supported on single metal oxide Al2O3 and TiO2. The 18O-isotope exchange measurements demonstrated that both partial and complete heterogeneous exchange as well as oxygen release were observed for all catalysts. Pd/2Ti-3Al has much higher activity for oxygen ex-change reaction than Pd/Al2O3 and Pd/TiO2. The excel-lent activity of Pd/2Ti-3Al is due to its high oxygen mobility and moderate reducibility, while the dispersion of Pd has little influence on the catalytic activity.

Table 2 Temperature of 18O2 isotope exchange and oxygen release processes for selected catalysts

Temperature range/℃ Catalyst

PHEa and Rb; PHEa dominates over Rb CHEc, Rb and PHEa, CHEc and Rb predominant over PHEa PHEa, Rb and CHEc; more CHEc and Rb

Pd/TiO2 456—572 572—622 622—700

Pd/2Ti-3Al 442—550 550—615 615—700

Pd/Al2O3 452—517 522—600 600—700 a Partial heterogeneous isotope exchange. b Oxygen release. c Complete heterogeneous isotope exchange.



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Novel Pd/TiO2-Al2O3 Catalysts for Methane Total Oxidation at Low Temperature and Their 18O-Isotope...

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