Oxidation of Soot over Bimetallic Noble Catalysts in the Presence of O 2 in Fluidized Bed Reactor

Oxidation of soot over a series of bi-noble metals Ru-X (X=Rh, Pd, Ag, Ir, Pt and Au) supported on SiO2 has been conducted in fluidized bed reactor with temperature programmed reaction (TPR) technique. The effects of molar ratio and combination of Ru and X on the Ru-X/SiO2 activity in the soot oxidation have been investigated. The Ru1−y-Pdy/SiO2 (0.1 ≤ y ≤ 0.9) exhibited the high catalytic activity and selectivity to CO2 when the molar ratio is 1:1 (y = 0.5). Among Ru-X/SiO2 (X=Rh, Pd, Ag, Ir, Pt and Au), Ru-Pd/SiO2 shows high activity for the soot oxidation in fluidized bed reactor. Compared to soot oxidation in fixed bed reactor, the oxidation of soot was accomplished at low temperature and this can be attributed to the promotion of mass transfer in fluidized bed reactor. [DOI: 10.1380/ejssnt.2011.142]


I. INTRODUCTION
Soot emitted from the diesel engines as an unwelcome byproduct can cause serious environmental problems.Soot also presents a respiratory hazard as it often contains adsorbed polycyclic aromatic hydrocarbon (PAH).In addition, unburned hydrocarbon molecules, several of which show mutagenic or carcinogenic characteristics, condense on the surface of the soot particulate.Because of these diverse implications, research on the removal of particulates (soot) from diesel exhaust gas has scattered.Among various technical approaches for the control of diesel engine particulate emission, the direct and much valid way to reduce soot is to place a diesel particulate filter (DPF) in the exhaust stream, which needs to clean the accumulation periodically by combustion or oxidation in order to prevent pressure increase.
It is verified by earlier work [2] that the contact between catalyst and soot is an important factor.When this contact is poor ('loose contact'), the catalytic activity of catalyst is much lower than when the contact is 'tight contact'.Unfortunately, the contact between catalyst and particulates is poor and comparable with loose contact under practical conditions in a classical filter.Thus, improvement of contact between soot and catalyst under practical operations is of great importance.However, most of experiments about simulated soot oxidation have been conducted in fixed bed reactor.The oxidation of soot in fluidized bed reactor which has excellent heat and mass transfer characteristics has not been reported so far.In addition, the contact between soot and catalysts will be improved in fluidized bed reactor which can evade the disadvantage of loose contact.
In this work, we investigate the soot oxidation over Rubased bimetallic catalysts in fluidized bed reactor, and carry out a research about the effects of molar ratio and combination of Ru and other noble metals on the activities of a series of Ru-based bimetallic catalysts for the soot oxidation.In addition, we also discuss why the oxidation reaction of soot in fluidized bed reactor is accomplished at lower temperature than fixed bed reactor.

A. Catalyst Preparation
The preparation of bimetallic Ru-X/SiO 2 (X=Rh, Pd, Ag, Ir, Pt and Au) with the constant sum of Ru and X loading (1%, w/w) and different Ru/X atomic ratios that is Ru 1−y -X y /SiO 2 (0.1 ≤ y ≤ 0.9) were obtained by a iso-volumetric co-impregnation technique of SiO 2 .The precursors used are RuCl The support used was SiO 2 (Qingdao Haiyang Chemical Co., Ltd, 0.125-0.425mm, 450-600 m 2 /g) which was impregnated with an aqueous solution of the precursors.After left one day at room temperature, they were dried in air at 110 • C for 12 h followed by calcination in air at 500 • C for 3 h.The catalysts were screened to get granules of 0.150-0.250mm (i.e.60-100 mesh) for measuring the catalytic reaction activity.

B. Activity Measurements
Catalytic activity was measured with temperature programmed reaction (TPR) technique.The experiments were carried out in a quartz microreactor (9 mm i.d.) that can be operated in either fluidized or fixed bed modes.The perforated plate and filter were added in the middle Printex-U (Degussa AG; 100 m 2 /g) was used as model soot for the activity measurement which is similar to literature [1].The catalyst granules (0.2 g) and model soot (0.02 g) or catalyst/soot (10/1, w/w, 0.22 g) were carefully mixed and held on the perforated plate in the mircroreactor.The reaction gas which contained O 2 (4.4 vol%) with Ar as the balance gas, was fed through the catalyst bed at a rate of 150 ml/min.
The catalytic activity was evaluated by temperatures of soot oxidation and selectivity to CO 2 throughout a TPR run.The values of T 10 , T 50 , and T 90 , were defined as the temperature of 10%, 50% and 90% soot oxidized during the TPR run.The selectivity to carbon dioxide (S co2 , %) was calculated by the ratio of CO 2 to the sum of CO and CO 2 concentrations in a TPR run.

A. Effect of Bimetallic Noble Catalysts
Soot oxidations in the presence of O 2 over SiO 2 and Ru-Pd/SiO 2 were carried out in fluidized bed reactor.From Fig. 1, there is large CO formation during the As for bimetallic catalysts, two factors will affect its activities during the soot oxidation.One is the optimum molar ratio of the two metals and the other is the best combination of the two kinds of metals for the soot oxidation in the fluidized bed reactor.
The catalytic activities of Ru 1−y -Pd y /SiO 2 (0.1 ≤ y ≤ 0.9) in fluidized bed reactor were tabulated in Table I.Ru 0.5 -Pd 0.5 /SiO 2 exhibits the lowest temperature and the highest activity for the soot oxidation in the fluidized bed reactor.Selectivity to CO 2 during soot oxidation is higher than others.This implies that the optimum molar ratio of Ru 1−y -Pd y /SiO 2 (0.1 ≤ y ≤ 0.9) is y = 0.5 that is molar ratio of Ru:Pd is 1:1.

C. Catalytic Activities of Ru/SiO2 Substituted by Different Noble Metal
In order to find the effect on the soot oxidation over Ru/SiO 2 substituted by different noble metal, the results of T 10 , T 50 , T 90 and S co2 over the Ru 1−y -X y /SiO 2 (X= Rh, Pd, Ag, Ir, Pt, Au and y = 0.5) were tabulated in Table II.Among the Ru-X/SiO 2 , Ru-Pd/SiO 2 exhibits the best performance for the soot oxidation in the fluidized bed reactor, due to its low temperature and high selectivity to CO 2 .Its T 50 is about 20-60  Ru-X/SiO 2 (X= Rh, Ag, Ir, Pt, Au and y = 0.5).

D. Comparison of soot oxidations in fluidized bed and fixed bed reactor
As for soot oxidation T 50 attracts more attention, here, T 50 of Ru-X/SiO 2 (X= Rh, Pd, Ag, Ir, Pt and Au) in flu-idized and fixed bed reactors were given in Fig. 2. T 50 of Ru-Pd/SiO 2 in fluidized bed reactor is about 60 • C lower than that in fixed bed reactor.Other Ru-X/SiO 2 (X= Rh, Ag, Ir, Pt and Au) also show a higher activity in fluidized bed reactor compared to fixed bed reactor.In order to explain why soot oxidation can be accomplished at lower temperature in fluidized bed reactor, here, comparison over Ru-Rh/SiO 2 was investigated in fluidized bed and fixed bed reactor for soot oxidation (Fig. 3).TPR profiles as a function of temperature in fixed bed reactor is broad and at high temperature, however, in fluidized bed reactor, profiles is sharp and the highest concentration of CO 2 is larger than those in fixed bed reactor.The sharp peak of CO 2 implies that soot oxidation at a narrow temperature window and the homogeneity of heat thanks to the high transmission in fluidized bed reactor.More CO formation during the soot oxidation in fluidized bed reactor can be attributed the quick oxidation of soot and lack of oxygen in some distinct.

IV. CONCLUSIONS
The oxidation of soot simulated from diesel exhaust over Ru-based bimetallic catalysts supported on SiO 2 was investigated in fluidized bed reactor and Ru-Pd/SiO 2 show a high activity for soot oxidation.The optimum molar ratio of Ru 1−y -Pd y /SiO 2 (0.1 ≤ y ≤ 0.9) is y = 0.5, that is molar ratio of Ru and Pd is 1:1.From the comparison among Ru-X/SiO 2 (X=Rh, Pd, Ag, Ir, Pt and Au), temperatures of soot oxidation over Ru-Pd/SiO 2 is the lowest and selectivity to CO 2 is the highest.Soot oxidation accomplished at low temperature in fluidized bed reactor can be attributed to the homogeneity of mass in fluidized bed reactor.
Figure 1 compares the performance of SiO 2 alone with that of Ru-Pd/SiO 2 .Soot began combustion over SiO 2 at about 480 • C, reached the peak at about 560 • C and burned out at about 600 • C.However, after loaded bimetallic Ru-Pd, T 10 , T 50 and T 90 are 427 • C, 470 • C and 486 • C, separately.They are 60 • C, 90 • C and 110 • C lower than those of SiO 2 .Therefore, Ru-Pd/SiO 2 can promote the soot oxidation at lower temperature actively.
oxidation of soot over SiO 2 , however, little CO formation for the soot oxidation over Ru-Pd/SiO 2 in fluidized bed reactor.In addition, TPR profiles of Ru-Pd/SiO 2 were sharper compared to those of SiO 2 .These showed that Ru-Pd/SiO 2 can promote soot oxidized to CO 2 completely and lower energy supplied for soot oxidation over Ru-Pd/SiO 2 .B.