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Ind. Eng. Chem. Res. 2007, 46, 4444-4450
CO2 Reforming of CH4 over Xerogel Ni-Ti and Ni-Ti-Al Catalysts Haijun Sun,† Jian Huang,‡ Hui Wang,* and Jianguo Zhang Department of Chemical Engineering, UniVersity of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9
CO2 reforming of CH4 reaction was studied over the Ni based xerogel catalysts. The Ni-Ti and Ni-Ti-Al xerogel composites were prepared using the sol-gel method and reduced in H2-N2 mixture gas. The reaction was conducted at 973 K in a micropacked-bed reactor. Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), and thermogravimetric/differential thermal analysis (TG/DTA) were used to characterize the fresh and spent (after reaction) catalyst samples. Compared with the published data, it has been found that the carbon deposition on both Ni-Ti and Ni-Ti-Al xerogel catalysts is effectively suppressed. Ni-Ti-Al xerogel catalyst reveals higher activity and longer stability than Ni-Ti xerogel catalyst does, due to its larger surface area and more resistance to Ti reduction. However, further study is anticipated to eliminate the carbon deposition on these xerogel catalysts. 1. Introduction Steam reforming, carbon dioxide reforming (dry reforming), and partial oxidation of methane are three important reactions, each of which is a constituent of the chemical route that converts natural gas into liquid fuels and chemicals. Recently, carbon dioxide (CO2) reforming of methane (CH4) is attracting more research attention because it consumes CO2, the major component of greenhouse gases whose emission results in global warming and extreme weather events.1 If successful, the application of CO2 reforming of CH4 in chemical engineering may provide a route of converting this emitted greenhouse gas into value-added products. Although CO2 reforming of CH4 was first investigated as early as 1888, no success has been achieved in commercializing this reaction because of the lack of effective and economic catalysts.2 Over 100 years, the research has been focused on the discovery of an active, stable, and cheap catalyst with good selectivity. There are also studies on reaction mechanism and kinetics.3 In the recent decades, most reported research has been directed on nonprecious metal catalyst, especially nickel, because it has comparable activity and selectivity with precious metals but a much cheaper price.2 However, the great challenge of using nickel-based catalysts is to suppress the carbon deposition to which the catalysts are prone. The objective of this research is to develop a novel catalyst using the sol-gel method. The catalyst should be able to suppress carbon deposition such that it facilitates not only high activity but also long stability for the reforming reaction. The sol-gel method has been a common procedure of catalysts preparation. Most research using the sol-gel method is focused on aerogel catalyst preparation. Xerogel preparation, an easier procedure, on the other hand, has nearly been neglected because it is believed that xerogel cannot support fine structures and thus provides small surface area and low activity. We have prepared Ni-Ti and Ni-Ti-Al xerogel catalysts using the solgel method4 and compared their physical properties with those of Ni-based aerogel catalysts in literature. The comparison shows that the calcination temperature has significant impacts on the properties of both aerogel and xerogel catalysts. When calcina* Corresponding author. Tel.: +306 966 2685. Fax: +306 966 4777. E-mail:
[email protected]. † Present address: Fluor Canada Ltd, Calgary, AB, Canada T2X 3R4. ‡ Present address: Yuncheng University, Yuncheng, Shanxi, China 044000.
Figure 1. Schematic diagram of the reactor system.
tion temperature increases from 453 to 973 K, the surface area of the Ni-Ti aerogel and xerogel catalysts dramatically declines. However, the Ni-Ti-Al xerogel catalyst calcined at 973 K still maintains a surface area as high as 300 m2/g. CO2 reforming of CH4 must be carried out at around 1000 K to achieve an acceptable conversion,3 which means that the catalyst for this reaction must be calcined at the same temperature or higher so as to facilitate the thermal stability. Therefore, the Ni-Ti-Al xerogel catalyst should be a good candidate for this reaction. Our previous paper focuses on Ni-Ti and Ni-Ti-Al xerogel catalysts preparation and characterization.4 This paper reports the CO2 reforming of CH4 reaction performance over the NiTi and Ni-Ti-Al xerogel catalysts. 2. Experimental Section Xerogel catalysts were prepared using the sol-gel method; however, the gel was dried without applying supercritical CO2 protection.4 To prepare the Ni-Ti composite xerogel catalyst, the precursors, nickel nitrate (Ni(NO3)2‚6H2O) (98%, Lancaster) and titanium n-butoxide (Ti(C4H9O)4) (>99%, Alfa Aesar), were dissolved in methanol (MeOH) (>99.9%, Burdick & Jackson). Then hydrogen nitrate (HNO3) solution (69-70%, EDM Chemicals, Inc.) as hydrolyzant was added into the alkoxide solution, and the solution formed a green transparent gel. The gel was aged and then dried without supercritical CO2 or inert gas protection. The dried sample was then calcined at a higher temperature. This catalyst is denoted as x% Ni-Ti, in which x% represents the mass percentage of NiO in the catalyst. The Ni-Ti-Al xerogel catalyst with certain mass ratios of
10.1021/ie070049e CCC: $37.00 © 2007 American Chemical Society Published on Web 05/23/2007
Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007 4445 Table 1. Analysis Result for a Typical Run of Catalyst Testa Component inlet flow, mmol/h outlet flow, mmol/h 0.25 h 1h 2h 3h 4h 7h 10 h
N2
CH4
CO2
H2
CO
469
260
260
0
0
469 469 469 469 469 469 469
138 113 110 109 111 110 120
115 113 110 109 111 110 120
235 151 252 148 252 244 245
274 292 293 290 297 291 293
carbon balance across the reactor for the 10 h run (including carbon deposition 1.73 mmol) total carbon consumed, mmol a
2730
total carbon produced, mmol
2900
Catalyst ) 5% Ni-Ti-Al(0.8); WHSV ) 90 000 mL/gcat‚h; reaction temperature ) 973 K.
Al2O3 to TiO2 or atomic ratios of Al/Ti was prepared by adding various amounts of hydrated aluminum nitrate (Al(NO3)3‚9H2O) in the well-mixed MeOH solution of Ti(C4H9O)4 and Ni(NO3)2‚ 6H2O when the gel is made. The Ni loading was controlled to be 5%. The other procedures were the same as described in our previous publication.4 Samples were made with different Al/Ti atomic ratios of 0.8, 1.6, and 4.7, and denoted as 5%Ni-TiAl (0.8), 5%Ni-Ti-Al (1.6), and 5%Ni-Ti-Al (4.7), respectively. Following the same procedure described in ref 4, BrunauerEmmett-Teller (BET) surface area and pore-size distribution measurements, X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS) analysis, and thermogravimetric/ differential thermal analysis (TG/DTA) were conducted to characterize the spent catalysts in this study. The schematic diagram of the reactor system used for testing the catalyst performance is shown in Figure 1. The flow of the feed gases was controlled by mass flow controllers made by Brooke Instruments, Inc. (New Jersey). The BTRS-jr benchtop microreactor system is the product of Autoclave Engineers (Pennsylvania). The reactor tube was made of quartz that is 225 mm in length and 5.6 mm in inner diameter. The gas supplier was Praxair Canada, Inc. (Ontario, Canada). The purities of CH4, CO2, N2, and H2 were 99.2, 99.9+, 99.9+, and 99.9%, respectively. Ultrahigh-purity helium used as the carrier gas for the gas chromatograph was also from Praxair. Catalyst (0.1-0.5 g, 45-80 mesh) diluted with 0.5 g of quartz sand (45-60 mesh) was packed in the reactor for each run. The catalyst bed with a length of about 2.2 cm was supported by quartz wool. The position of the catalyst bed in the tube was measured to ensure that it was located in the middle section of the furnace. A thermocouple was inserted from the top of the reactor tube and embedded in the middle of the catalyst bed. Before reaction, the catalyst was reduced by passing a H2N2 mixture (1:2) through the catalyst bed at a flow rate of 90 mL/min (standard temperature and pressure, STP). The reduction condition was studied by varying the reduction temperature from 823 to 1073 K and the reduction time from 2 to 6 h. The reaction temperature was 973 K. The reactants CH4 and CO2 were mixed with N2 in the ratio of 1:1:1.8. The flow rate varied according to the preset weight hourly space velocity (WHSV), of which the unit mL/gcat‚h is calibrated to the values at the standard temperature and pressure (STP). An Agilent 6890N gas chromatograph (GC) was used for gas composition analysis. The column was the GS-GASPRO capillary column (J&W Scientific) of 60 m in length and 0.32 mm in inner diameter, and the detector was the thermal conductivity detector (TCD). The GC oven temperature was programmed as maintaining at 213 K for 3 min, ramping to 303 K at a rate of 25 K/min, and staying at 303 K until the last effluent component was detected (about
5 min). Liquid CO2 was used to cool the oven when its temperature was required to be below the room temperature. Further detailed experimental description is found in a thesis by Sun.5 The analysis results for a typical run of catalyst test is shown in Table 1. The difference in carbon balance across the reactor for this run is 5.8%, which is the maximum for all runs. The carbon balance also indicates that CO is the predominant C-containing product. The carbon that is deposited on the catalyst surface is
