512
J. Phys. Chem. B 1997, 101, 512-518
Bimetallic Nb-Mo Carbide Hydroprocessing Catalysts: Synthesis, Characterization, and Activity Studies C. Charles Yu,† S. Ramanathan,† B. Dhandapani,† J. G. Chen,‡ and S. Ted Oyama*,† Department of Chemical Engineering, Virginia Polytechnic Institute and State UniVersity, Blacksburg, Virginia 24061, and Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 ReceiVed: August 15, 1996; In Final Form: NoVember 13, 1996X
A series of Nb1.0MoxOC (x ) 0.67-2.0) catalysts were prepared by a temperature-programmed reaction technique. The catalysts were synthesized from oxide precursors in a flow of 20% CH4/H2 reactant gas mixture, while the temperature was increased linearly at 5 K/min (8.3 × 10-2 K s-1). The samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), elemental analysis, CO chemisorption, surface area measurements, and temperatureprogrammed reduction. XRD patterns of the fresh catalysts indicated that Nb1.0Mo1.5OC and Nb1.0Mo1.75OC consisted of pure bimetallic carbide phases, while the other compositions showed impurity phases of NbO2 or Mo2C at high concentrations of Nb and Mo, respectively, in the starting oxide. The hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activity of these materials was studied in a high-pressure reactor system. The reactions were carried out at 3.1 MPa and 643 K using model liquid compounds containing moderate concentrations of sulfur, nitrogen, oxygen, and aromatics. All the catalysts were found to be active for quinoline HDN, and the activity did not show much variation with changes in the ratio of the two metals (Mo/Nb). However, the HDS activity was found to be more sensitive to the composition (Mo/Nb) and Nb1.0Mo1.75OC showed the highest HDS activity among the catalysts tested. The bimetallic compounds showed enhancement in the activity and stability compared to the corresponding monometallic carbides. X-ray diffraction patterns of the spent catalysts did not show any sulfide, oxide, or metal peaks, indicating that the catalysts were stable and tolerant of sulfur.
Introduction Transition metal carbides and nitrides are well-known for their refractory, electronic and magnetic properties.1,2 They have already found applications in cutting tools, wear-resistant parts, and hard coatings and as electronic and magnetic components and superconductors. Not yet developed are their applications as catalysts. Early studies involving transition metal carbides and nitrides as catalysts for ammonia synthesis3,4, FischerTropsch synthesis5, hydrogenation6-9, and oxidation10 have demonstrated their resemblance in activities to those of the group 8-10 metals (Pt, Pd, Rh, etc.). Recently, these compounds have been found to be active for the removal of nitrogen11-15 and sulfur16,17 from model compounds and coal-derived liquids.18,19 In some cases they even show superior selectivity, stability, and resistance to poisoning. Carbide and nitride phases can exist over broad composition ranges with appreciable vacancy concentrations, and their physical properties are quite sensitive to composition. Most of the catalytic studies on transition metal carbides and nitrides reported so far have focused on binary compounds (monometallic) because of the complexity of the multimetallic systems. Since changes in valence electron density are probably a major cause of the sensitivity to composition, it is possible to induce positive interactions among metal and nonmetal elements by introducing other elements into binary carbides and nitrides. It has been successfully demonstrated that simultaneous substitution of metal and nonmetal elements in the binary nitride Mo2N can substantially improve its catalytic performance.20 * To whom correspondence should be addressed. † Virginia Polytechnic Institute and State University. ‡ Exxon Research and Engineering Company. X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S1089-5647(96)02496-0 CCC: $14.00
In the current investigation a new bimetallic oxycarbide system, Nb-Mo-O-C (Mo/Nb ) 0.67-2.0), was synthesized by the same temperature-programmed reaction method. The surface characteristics of these Nb-Mo bimetallic oxycarbides, including CO chemisorption, active sites, and reactivity in hydrodenitrogenation and hydrodesulfurization, were studied. Experimental Section Materials used in the current study were molybdenum(VI) oxide (MoO3, 99.95%, Johnson Matthey) and niobium(V) oxide (Nb2O5, 99.9%, Johnson Matthey). Chemicals used in this study were dibenzothiophene (Aldrich, 99.5%), quinoline (Aldrich, 99.9%), benzofuran (Aldrich, 99.9%), tetralin (Aldrich, 99.5%), amylbenzene (Aldrich, 99.5%), and tetradecane (Jansen Chimica, 99%). The gases employed were 20% CH4/H2 (Airco, UHP Grade), He (Airco, Grade 5.0), CO (Linde Research Grade, 99.97%), and 0.5% O2/He (Airco, UHP Grade). Sample Preparation. The preparation of the catalysts was carried out in two stages. The first stage involved the preparation of the bimetallic oxide precursors of desired composition. In the second stage, precursors were carburized in a reactive stream of 20% methane/hydrogen gas. The bimetallic oxide precursors were prepared by the solid state fusion of two monometallic oxides. Two monometallic oxides, at a prechosen metal ratio, were first ground together using a mortar and pestle with added ethanol to achieve better dispersion. They were then partially dried and pressed at 8000 psi in a 1/2 in. diameter hard steel die. Since the remaining ethanol in the mixture facilitated compacting, no binder was needed for pellet pressing. The oxide pellets were subsequently fired at 1058 K for 6 h and were finally cooled to room temperature and pulverized into fine powders. © 1997 American Chemical Society
Nb-Mo Carbide Catalysts Bimetallic oxycarbides were prepared by the temperatureprogrammed reaction of the bimetallic oxides using a methane/ hydrogen gas mixture. The bimetallic oxide powders synthesized, as described above, were transferred to a quartz reactor, which was placed inside a tubular resistance furnace (Applied Test Systems, Inc. Series 3210) controlled by a temperature programmer (Omega Model CN2000). The amount of the bimetallic oxide loading was 4 g/batch. A 20% CH4/H2 gas mixture was passed through the oxide powders at a flow rate of 1.1 × 10-3 µmol s-1 (1600 cm3/min). The temperature was increased at a linear rate of 8.3 × 10-2 K s-1 (5 K/min.) to the final temperature (1063 K), which was held for 1.5 h. The gas flow was switched from 20% CH4/H2 to helium, and the samples were quenched to room temperature. After cooling, the pure He gas was switched to a gas mixture containing 0.5% O2 in He. The time of passivation was determined to be sufficient by monitoring the oxygen signal with a mass spectrometer (Ametek/Dycor, Model MA100) until it reached steady state, approximately 15 h. Sample Characterization. NEXAFS experiments were carried out at the U1 beamline of the National Synchrotron Light Source at the Brookhaven National Laboratory. Details about the experimental setup for studying powder materials have been described previously.21 In the current study, NEXAFS spectra were recorded by measuring the electron-yield intensity by a Channeltron electron multiplier located near the sample holder. To make the electron-yield method more surface sensitive, the entrance of the Channeltron was biased by a negative voltage of 100 eV to repel low-energy secondary electrons.21 Under these conditions the probing depth of the near-edge X-ray absorption fine structure (NEXAFS) technique, in the carbon and oxygen K-edge regions, is generally in the range 1.0-1.5 nm.22 As described previously,21 powder samples were pressed into a stainless steel sample holder of 6 mm diameter and 1 mm depth. The sample could be heated by resistively heating two tungsten wires, which were spot-welded onto the back of the sample holder. The chemisorbed surface oxygen on NbMo bimetallic oxycarbides was removed by heating the sample holder to 723 K in a stream of H2 at 8 kPa pressure and maintaining the temperature and pressure for 0.5 h. The chamber was then evacuated to below 10-5 Pa for NEXAFS measurements. In addition to the Nb-Mo bimetallic oxycarbide samples, monometallic oxycarbides of Nb and Mo were also studied in the NEXAFS measurements. These oxycarbides were prepared by mildly treating NbC and Mo2C samples (99.5%, Johnson Matthey) in oxygen (10-4 Pa, 600 K, 0.2 h). XRD analysis of both bimetallic oxides and bimetallic oxycarbides was carried out using a powder diffractometer (Scintag, Model ASC0007 with a Cu KR monochromatized radiation source) operated at 45 kV and 40 mA. XPS analysis (Perkin-Elmer, Model 5600 ci XPS/Auger, Mg source) was carried out to study the surface composition of the fresh and spent catalysts. The C 1s peak was taken as the reference for XPS. The bimetallic oxycarbides were analyzed by inductively coupled plasma (ICP) for elemental composition. The samples were also characterized by temperature-programmed reduction (TPR) to 723 K, by CO chemisorption, and by N2 physisorption. A sample of approximately 0.2 g of the bimetallic oxynitride samples was loaded into a quartz microreactor, which was then placed inside a tubular resistance furnace. A 10% H2 in He gas mixture was passed through the sample at a rate of 20 µmol s-1 (30 cm3/min). The temperature was increased at a linear rate of 0.16 K s-1(10 K/min) to 723 K, where it was held for 2 h. The temperature of the sample was measured by a chromel-alumel thermocouple placed in a well located close
J. Phys. Chem. B, Vol. 101, No. 4, 1997 513 to the center of the reactor bed. During the TPR process, the effluent gas stream was sampled into a mass spectrometer (Ametek/Dycor Model MA100) chamber through a variable leak valve (Granville Phillips Model 203). A computer recorded the mass signals of the effluent gas and the sample temperatures through a RS232 interface. At the end of the TPR process, the H2/He gas mixture was switched to pure He and the samples were cooled to room temperature for CO chemisorption measurements. CO chemisorption was used to titrate the surface metal atoms in the sample. After the 2 h reduction described above, pulses of CO gas were introduced through a sampling valve with the He carrier gas stream passing over the samples. The total uptake was calculated by referencing the areas under the CO mass signal 28 peaks to the known quantity of 12 µmol CO for a single peak. Surface areas were determined immediately after the CO uptake measurements by a similar flow technique using a 30% N2 in He gas mixture passed over the samples maintained at liquid nitrogen temperature. The amount of physisorbed N2 was obtained by comparing the area of the desorption peaks to the area of calibrated N2 pulses corresponding to 38 µmol N2/ pulse. The surface area was then calculated from the single point BET equation. Catalytic Testing. The reactivity of the series of Nb-MoO-C catalysts was tested in a high-pressure trickle-bed reactor. The catalysts were tested for their hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) activities at 3.1 MPa and 643 K using model liquid compounds. The liquid feed composition consisted of 3000 ppm sulfur (dibenzothiophene), 2000 ppm nitrogen (quinoline), 500 ppm oxygen (benzofuran), 20 wt % aromatics (5 wt % amylbenzene and 15 wt % tetralin), and balance aliphatics (tetradecane). Typically, a sample of about 1.0-2.0 g of the catalyst was loaded in a 316 SS tubular reactor, which was immersed in a fluidized sand bath (Techne, Model SBL2). Detailed description of the high-pressure trickle-bed reactor system is presented elsewhere.23 Hydrogen flow to the reactors was regulated by mass flow controllers (Brooks, Model 5850E). Liquid was fed to the reactors using high-pressure liquid pumps. Prior to catalytic testing, the oxycarbides were reduced in situ at atmospheric pressure in hydrogen at 723 K for 3 h. After the reduction, the reactors were brought to the reaction temperature (643 K), hydrogen was pressurized to 3.1 MPa, and the liquid feed was started. Hydrogen and liquid passed over the catalyst bed and out to the liquid sampling valve. Liquid samples were collected at regular intervals for a period of 60 h and were analyzed off-line using a gas chromatograph (HP 5890 Series II) equipped with a capillary column (CPSIL5CB, Chrompack, Inc.) and flame ionization detector. Once the reaction was completed, the catalysts were washed in hexane to remove residual reactant liquid from the surface and were stored for postreaction characterization. Results and Discussion Synthesis and Characterization. Parts b-g of Figure 1 are the XRD patterns of the bimetallic oxides prepared by solidstate reaction. The XRD patterns of the parent mono-oxides, MoO3 and Nb2O5, are also included (parts a and h of Figure 1) for comparison. It is obvious that the major phases of these Nb-Mo oxides have similar XRD patterns corresponding to a ternary oxide. However, a small amount of Nb2O5 is found in the Nb-rich compositions (Nb1.0Mo0.67-O and Nb1.0Mo1.0-O), and a small amount of MoO3 is found in the Mo-rich composition (Nb1.0Mo1.75-O and Nb1.0Mo2.0-O). Figure 2 shows the mass spectrometer traces following the synthesis of one of the compounds (Nb1.0Mo1.5OC). Similar
514 J. Phys. Chem. B, Vol. 101, No. 4, 1997
Yu et al.
Figure 1. X-ray diffraction patterns of Nb-Mo oxides. Figure 3. X-ray diffraction patterns of Nb-Mo oxycarbides.
TABLE 1: Characteristics of Nb-Mo Oxycarbides sample
CO uptake (µmol g-1)
SA (m2 g-1)
site density (×1015 cm-2)
Dc (nm)
Dp (nm)
NbC Nb1.0-Mo0.67 Nb1.0-Mo1.0 Nb1.0-Mo1.25 Nb1.0-Mo1.5 Nb1.0-Mo1.75 Nb1.0-Mo2.0 Mo2C
28 0 0 14 54 32 70 99
42 60 61 72 98 88 119 42
0.31 0 0 0.012 0.033 0.022 0.035 0.14
16 11 7.4 7.9 11 8.8 8.2 11
23 12 12 9.8 7.2 8.0 5.9 14
TABLE 2: Molar Composition of Nb1.0MoxOC by Elemental Analysis
Figure 2. Temperature-programmed reaction synthesis traces of Nb1.0Mo1.5OC.
traces are obtained with the other compounds. The mass signals 2, 18, 28, and 44 represent hydrogen, water, carbon monoxide, and carbon dioxide, respectively. To avoid interference from oxygen (M ) 16), the mass signal 15 is used to represent methane. The synthesis reaction proceeds in two stages. First, there is an initial reduction of the precursor oxide and then a further reduction and carburization. Reduction is manifested as a decrease in the hydrogen mass spectrometer signal (M ) 2) accompanied by simultaneous water (M ) 18) evolution. Carburization is indicated by the increase of CO (M ) 28) and CO2 (M ) 44) signals. Evidently, methane (M ) 15) also acts as a reducing agent. There is considerable amount of water
compound
Nb
Mo
O
C
Nb1.0Mo0.67OC Nb1.0Mo1.0OC Nb1.0Mo1.5OC Nb1.0Mo2.0OC
1.0 1.0 1.0 1.0
0.64 0.92 1.4 1.6
4.3 4.2 6.2 5.0
0.34 0.17 1.48 0.5
formation during the initial reduction. The leak valve suffered a decrease in gas conduction because of condensation. This is seen as a shift of the base line for the hydrogen (M ) 2), methane (M ) 15), and carbon dioxide (M ) 44) signals. The XRD patterns of the bimetallic oxycarbides, prepared by temperature-programmed reaction, are shown in Figure 3. For comparison, the XRD patterns of NbO2, NbC, and Mo2C are also included. Obviously, a NbO2 phase exists in high Nb content samples (Nb1.0Mo0.67OC, Nb1.0Mo1.0OC, and Nb1.0Mo1.25OC), but its proportion decreases as the Mo content increases. The Nb1.0Mo1.5OC and Nb1.0Mo1.75OC are single phase materials whose XRD peaks are consistent with a facecentered cubic metallic arrangement. The line-broadening of the X-ray peaks indicates the presence of small crystallites in these materials. A minor phase of Mo2C exists in the Mo-rich compound (Nb1.0Mo2.0OC). XPS analysis of the Nb1.0Mo1.5OC and Nb1.0Mo1.75OC are summarized in Table 3. It can be seen that most of the oxide present in the sample is associated preferentially with niobium.
Nb-Mo Carbide Catalysts
J. Phys. Chem. B, Vol. 101, No. 4, 1997 515
TABLE 3: Summary of XPS Analysis of Fresh Nb1.0MoxOC (x ) 1.5 and 1.75) Catalysts Nb
c
a
Mo
fresh catalyst
oxide
carbide
Nb1.0Mo1.5OC Nb1.0Mo1.75OC
80 69
20 31
b
oxide 35 25
c
carbided 65 75
a Oxide 3d b 5/2 peak at 207.5 eV. Carbide 3d5/2 peak at 202.5 eV. Oxide 3d5/2 peak at 232.5 eV. d Carbide 3d5/2 peak at 228.4 eV.
Figure 5. NEXAFS C K-edge features of Nb-Mo oxycarbides.
Figure 4. Temperature-programmed reduction traces of Nb1.0Mo1.5OC after passivation.
Figure 4 shows the typical mass spectrometer traces of the temperature-programmed reduction of Nb1.0Mo1.5OC immediately following chemisorption of CO. The evolved gases during temperature-programmed reduction to 723 K in 10% H2/He include CO, CO2, H2O, and CH4. The appearance of CO2 indicates strong interactions between chemisorbed CO and surface oxygen. Table 1 summarizes the CO uptakes, surface areas, active site densities, and crystallite and particle sizes of the oxycarbides. There is a systematic trend in the characteristics of these Nb-Mo oxycarbide compounds. High Nb content compounds (Nb1.0Mo0.67OC and Nb1.0Mo1.0OC) have low CO uptakes and surface areas. With increase in Mo content, both the CO uptake and surface area increase initially, reaching a maximum at a composition of Nb1.0Mo1.5OC, then decrease with further additions of Mo. The high CO uptake of Nb1.0Mo2.0OC is most likely associated with the presence of molybdenum. The crystallite size (Dc) was estimated from the highest intensity peak using the Scherrer equation24
Dc ) (Kλ/β)cos θ where K is a constant taken to be 0.9, λ is the wavelength of the X-ray radiation, and β is the width of the XRD peak at half-maximum corrected for instrumental broadening (0.1°). The particle size (Dp) was calculated using the equation25
Dp ) 6/(FSg)
where F is the density of the particle and Sg the surface area of the catalyst. The crystallite size agrees very well with the particle size obtained from surface area measurements. Table 2 lists the elemental analysis data of the Nb1.0MoxOC compounds. It can be seen that the Mo content deviates from the starting conditions for the Mo-rich compound (Nb1.0Mo2.0OC), and this could be due to the sublimation of MoO3 during the bimetallic oxide synthesis. Surface compositions of Nb-Mo bimetallic oxycarbides were also characterized using NEXAFS. Figure 5 shows a comparison of the C K-edge spectra of several Nb-Mo bimetallic oxycarbides. For comparison, the C K-edge features of monometallic oxycarbides, Nb-O-C and Mo-O-C, are also shown in Figure 5. As demonstrated in previous studies,26-28 the NEXAFS technique is very sensitive in probing the local structural and electronic properties of transition metal carbides. For example, NEXAFS measurements and band structure calculations for both NbC29 and Mo2C26,30 have been carried out. In general, C K-edge features of early transition metal carbides are characterized by two sharp features at 285.5-286.6 and 287.5-289.0 eV and by a broad peak at g295 eV.26-31 By comparing the band-structure calculations to the NEXAFS data, the two sharp resonances at
