Combined Temperature-Programmed Processes, Pulse Reactions

18646

J. Phys. Chem. C 2007, 111, 18646-18662

Combined Temperature-Programmed Processes, Pulse Reactions, and On-Line Mass Spectroscopy Study of CH4, CO, and H2 Interaction with Ni/Al2O3 Catalysts Qiangu Yan,* Hossein Toghiani,* and Mark G. White DaVe C. Swalm School of Chemical Engineering, Mail Stop 9595, Mississippi State UniVersity, Mississippi 39762 ReceiVed: May 27, 2007; In Final Form: September 24, 2007

The interactions of CH4, CO, and CH4/O2 with Ni/Al2O3 catalysts were examined by the pulse reaction technique and the transient response technique. The adsorption and dissociation of CH4, CO, CH4/CO2, and CH4/O2 on nickel alumina catalysts were also extensively investigated by temperature-programmed hydrogenation (TPH) and temperature-programmed desorption (TPD). The phase structure and nickel oxidation states of Ni/Al2O3 samples were examined by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. XRD, TPR, and XPS results demonstrated that nickel is mainly present as NiO and NiAl2O4 in the as-prepared catalyst, while around 83%-90% nickel is Ni0 after the catalyst is reduced at 700 °C. Methane pulse reactions and transient response analysis demonstrate that the methane oxidation mechanism changes as the nickel oxidation state changes over Ni/Al2O3 when there is no gaseous oxygen present. CH4 is efficiently oxidized into CO and H2 via a direct oxidation mechanism when Ni/Al2O3 is prereduced, while CH4 may be converted by a nonselective oxidation process over an oxidized Ni/Al2O3. CO pulse reaction results over the prereduced catalyst suggest that the CO disproportionation reaction occurs over Ni/Al2O3 catalyst under operation conditions, while CH4 is generated through the hydrogenation of the surface carbidic species from CO disproportionation. TPD and TPH studies show that the decomposition of methane results in the formation of at least three kinds of surface carbon species on supported nickel catalysts. TPD and TPH results also prove CO is converted to CO2 and surface carbidic CR through the disproportionation reaction.

1. Introduction Several techniques are available for hydrogen production from hydrocarbons including steam reforming, which is generally accompanied by a water gas shift conversion and a hydrogen purification process. The importance of the process of reforming of hydrocarbons to obtain syngas (H2 and CO) is well-known. As an alternative to the traditional steam reforming of hydrocarbons process, partial oxidation and autothermal reforming are promising processes from both the energetic and environmental points of view. Syngas is traditionally produced by highly endothermic steam reforming (SR) of hydrocarbons, especially natural gas1 (eq 1). However, this process provides a high H2/CO ratio (>3), which is not suitable for Fischer-Tropsch and methanol syntheses. Thus, research efforts have been directed toward obtaining syngas with a more suitable H2/CO ratio of 2 or lower via methane partial oxidation (POM) (eq 2) or CH4/CO2 reforming (MR) (eq 3).2-12 Partial oxidation of methane and CO2 reformCH4 + H2O f CO + 3H2

∆H298 ) 207 kJ/mol

(1)

CH4 + 1/2O2 f CO + 2H2

∆H298 ) -36.0 kJ/mol

(2)

CH4 + CO2 f 2CO + 2H2

∆H298 ) 247 kJ/mol

(3)

ing of methane have the potential to reduce the cost of syngas2 and can be applied in solar energy storage13 and/or CO2 utilization technologies.9 * To whom correspondence should be addressed. E-mail: qy8@ra. msstate.edu or [email protected].

Partial oxidation of methane to syngas offers the potential for fast, efficient, and economical production of syngas due to the high conversion of methane, high selectivity, suitable H2/ CO ratio, and a very short residence time. However, the POM process cannot be controlled easily due to the difficulty of removing the reaction heat from the reactor.14 Therefore, interest in methane conversion to syngas via CO2 reforming has grown. Reforming methane with carbon dioxide is attractive for two reasons: (1) the H2/CO ratio produced is relatively low (e1)2 and is suitable for Fischer-Tropsch synthesis to higher hydrocarbons; (2) CO2, a greenhouse gas, is consumed in a useful manner. CH4/CO2 reforming is strongly endothermic, so high temperatures (800-1000 °C) are required. Unfortunately, these extreme temperatures promote carbon deposition, deactivate the catalyst, and sinter the supported metals.15,16 The oxidation of CH4 has been interpreted by some researchers to proceed via a two-step reaction pathway.10,11 First, complete oxidation of a part of the CH4 to CO2 and steam occurs, followed by reforming of the remaining CH4 with CO2 and steam to syngas. However, direct partial oxidation of CH4 to syngas has also been suggested12 by Schmidt and co-workers. They proposed a mechanism where methane is catalytically pyrolyzed to carbon species on the catalyst surface followed by oxidation of the surface carbon oxidation and hydrogen desorption. Thus the mechanism of POM remains controversial. Therefore, a better understanding of the reaction mechanism and of the nature of the active catalytic sites is required. Mallens et al.17 found differences in the selectivity toward CO and H2 during POM using Rh versus Pt catalysts. These differences were attributed to the lower activation energy for methane decomposition on Rh versus that on Pt. Mallens et al.

10.1021/jp0740898 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

CH4, CO, and H2 Interaction with Ni/Al2O3 Catalysts suggested that the catalyst’s ability to activate methane determines (1) the product distribution and (2) the concentration of active surface species of oxygen, carbon, and hydrogen. Fathi et al.18 studied the partial oxidation of methane to syngas over platinum catalysts and proposed that the product distribution is determined by both the concentrations and the types of surface oxygen species present at the catalyst surface. Qin et al.19 suggested that the support might also influence the concentration of adsorbed oxygen and, as a consequence, the activation of methane and the product distribution. Li et al.20 studied the effect of gas-phase O2, reversibly adsorbed oxygen, and oxidation state of the nickel in the Ni/Al2O3 catalyst on CH4 decomposition and partial oxidation using transient response techniques at 700 °C. They concluded that the surface state of the catalyst affects the reaction mechanism and plays an important role in POM conversions and selectivity. Li et al.20 also argued that direct oxidation is the major POM route, and that the indirect oxidation mechanism cannot become dominant under their experimental conditions. Two groups of catalyst have been used for methane activation to generate syngas: Ni-based catalysts11 and noble metal based catalysts,5 particularly Rh and Ru.12,13 The high cost and limited availability of noble metals highlights the need to develop Nibased catalysts, which can exhibit stability for extended periods. Ni/Al2O3 catalysts have been reported to be very active catalysts for these reactions. They are the catalysts with the most potential used in fuel processing due to low cost and ready availability. However, they suffer a serious problem of deactivation due to carbon deposition and nickel sintering. Among them, carbon deposition is the most serious problem and appears unavoidable even under higher H2O/CxHy ratios on some catalysts. Carbon deposition on the catalyst might cause loss of active sites, whereas growth of filamentous carbon nanotubes can lead to reactor blocking. Catalyst deactivation is especially problematic for Ni-based catalysts.5,6 Coking of Ni surfaces is an important technological problem, and a number of experimental studies have addressed this process.21-25 Several types of carbon were detected, and the origin of the carbon has also been investigated. It is reported that hydrocarbon decomposition (CxHy f xC + y/2H2) and carbon monoxide disproportionation (2CO f C + CO2) are the main routes of carbon deposition; their contribution to carbon deposition depends on reaction conditions. Hydrocarbon dehydrogenation dominates at high temperatures, whereas the Boudouard reaction is a low-temperature pathway to carbon atoms.15 Various carbon species have been observed on the surface of catalysts, ranging from carbides to hydrogen-rich aliphatic polymers. It was reported that three kinds of carbonaceous species formed on the Ni/Al2O3 catalyst, designated as CR at 150-220 °C, a carbide-like structure where carbon atoms are dissolved in the Ni bulk; Cβ at 530-600 °C, a partially dehydrogenated surface carbonaceous species; and Cγ at >650 °C, a carbidic cluster species.26 At low reaction temperatures (500-600 °C), the catalyst surface is populated with the Cβ species (whose amount corresponds to several monolayers of equivalent carbon on Ni crystallites), along with small amounts of the CR species. However, the Cβ species can change into the Cγ species when the time of exposure is longer. The active CR species is responsible for formation of synthesis gas, while most of the Cγ species is responsible for catalyst deactivation. The Cβ species is a surface poison or spectator at low reaction temperature (600 °C). The present work concerns the interaction of H2, CO, and CH4 with a Ni/Al2O3 catalyst and the subsequent methane

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18647 activation mechanism over Ni/Al2O3. One of the key questions remaining to be answered for POM is whether the oxygen initiating the reaction is directly from the metal oxide (NiO) lattice or from the support Al2O3. The answer to this question was investigated by pulse experiments and by transient response analysis in the present work. Experiments carried out with CH4 pulses and transient responses provide some insight into the reaction mechanism. Special attention was given to the correlation between the nickel oxidation states and the methane activation mechanism over the Ni/Al2O3 catalyst. The phase structure and nickel oxidation states of the Ni/Al2O3 samples were examined by X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. To investigate carbon deposition and carbon species over Ni/Al2O3 catalyst by reaction conditions, temperature-programmed desorption (TPD) and temperature-programmed hydrogenation (TPH) are used to identify carbon species over Ni/Al2O3 catalyst through adsorbed CO and hydrocarbons. TPD and TPH experiments are demonstrated in this paper. 2. Experimental Section 2.1. Catalyst Preparation. Aluminum isopropoxide (AIP), 98% (19.40 g, from Alfa Aesar Inc.), was dissolved in 150 mL of water at 85 °C and hydrolyzed for 1 h; then a small amount of nitric acid (1.0 mL) was added to obtain a clear solution. The necessary amount of nickel nitrate (1.70 g) was dissolved in 33.5 g of 1,3-butanediol, and then this solution was added to the sol at room temperature. After 1 day of stirring at room temperature, the solvent was slowly evaporated to form a gel. The gel was dried at 100 °C for 24 h and then was calcined in air in 700 °C for 6 h. An 8 wt % Ni/Al2O3catalyst was obtained. 2.2. Temperature-Programmed Reduction (TPR), Temperature-Programmed Desorption (TPD), Pulsed Reactions, and Temperature-Programmed Hydrogenation (TPH). Temperature-programmed reduction (TPR), temperature-programmed desorption (TPD), pulse reactions, and temperature-programmed hydrogenation (TPH) were performed by means of an automated catalyst characterization system AutoChem II 2910, which incorporates a thermal conductivity detector (TCD); however, all the processes were monitored and measured by an on-line quadrupole mass spectrometer (MS). Before starting TPR runs, the sample, 0.02 g, was activated under flowing O2 (10 vol %)/He at 600 °C for 90 min. TPR and TPD experiments were carried out at heating rates of 10 °C/min and 30 °C/min, respectively. The reactive gas compositions were H2 (10.2 vol %)/Ar and CO (1.01 vol %)/ He for TPR. The TPR experiments were performed up to a temperature 800 °C, at which the sample was maintained for 5 min. TPD experiments were performed following TPR after cooling the samples in H2 (10 vol %)/Ar or CO (1.01 vol %) flow to 30 °C. The samples were then purged at 20 °C in flowing Ar or He to remove the residual hydrogen or CO. After purging, Ar or He was passed over the samples, which were heated to 800 °C and then held at 800 °C for 5 min. CH4, CO, and CH4/O2 interactions with Ni/Al2O3 catalyst (0.02 g) were investigated by pulse reaction techniques. The pulse reaction experiments were performed at 700 °C. A gas pulse containing 1 mL (STP) of CH4/Ar (1/20), CH4/O2/Ar (2/ 1/40), or CO/He was injected into helium carrier gas flow (50 mL/min), which continuously flowed through the reactor during the experiments. The transient responses of pulsing reactions were also investigated and analyzed at 700 °C under 1 atm.

18648 J. Phys. Chem. C, Vol. 111, No. 50, 2007

Figure 1. XRD patterns of (a) 8 wt % sol-gel Ni/Al2O3 calcined at 600 °C and (b) 8 wt % Ni/Al2O3 catalyst reduced at 700 °C.

Yan et al. pulse experiments: CH4/O2 (40 mL/min for 2 h), CH4/CO2, CO/ H2, or CO/He reaction. After terminating these reactions, the reactor was quickly cooled to room temperature under a helium flow and purged with He for 30 min. To determine the extent of carbonaceous residue deposited on the catalyst, the temperature was then ramped at 10 °C/min from room temperature to 800 °C while a 10% H2 (vol)-Ar mixture (50 mL/min) was passed through the catalyst bed. 2.3. Catalyst Characterization. X-ray powder diffraction (XRD) spectra were recorded in a Philips X’pert diffractometer. Cu KR (λ ) 0.154 nm, 40 kV, 20 mA) radiation was used as the X-ray source. Scanning was conducted over the range of 2θ ) 5-80°. XRD patterns of reduced catalysts were collected under air atmosphere. X-ray photoelectron spectra (XPS) were recorded by a Phi Model 1600 XPS spectrometer using Al KR radiation (1486.6 eV). Both the unreduced and reduced Ni/Al2O3 samples were investigated using XPS. The XPS data from the regions related to the C 1s, O 1s, Al 3d, and Ni 2p core levels were recorded for each sample. The binding energies were adjusted relative to C 1s at 284.5 eV. Metal dispersions on the catalysts were measured by H2 chemisorption at room temperature. The percentage dispersion of Ni metal was calculated assuming an H/M atomic ratio of 1,27 estimated by hydrogen chemisorption at room temperature. The catalyst sample was first reduced under hydrogen at 700 °C for 1 h and evacuated at 700 °C under high vacuum for 30 min, and then it was cooled to room temperature under vacuum for chemisorption. 3. Results

Figure 2. H2-TPR spectra of Ni-alumina catalyst prepared by solgel technique.

Temperature-programmed hydrogenation (TPH) was used to investigate carbonaceous deposits formed on used catalyst samples. The catalyst, 0.02 g, was first pretreated at 700 °C under air for 1 h, followed by helium for 30 min and then H2 for 30 min. The catalyst was then exposed to CH4 in methane

3.1. Characterization of Ni/Al2O3 Catalyst. To see the effect of catalyst structure on the activity, XRD measurements were performed for the Ni/Al2O3 catalysts in as-prepared state, and after 2 h of reduction. The XRD patterns of calcined and reduced 8 wt % Ni/Al2O3 catalysts are compared in Figure 1. The support calcined at 600 °C is relatively amorphous as evidenced by broad peaks that correspond to γ-Al2O3. The possible crystalline phases in the catalyst calcined at 600 °C are NiO, NiAl2O4, and γ-Al2O3. The XRD pattern of as-prepared Ni/Al2O3 shows main peaks at 2θ ) 37.3°, 45.5°, and 67.3°, corresponding to the reflections (311), (400), and (440) of cubic γ-Al2O3. The very weak reflections at 2θ ) 43.3° and 63.0° indicate that part of NiO was formed in the Ni/Al2O3 catalyst, while a possible

Figure 3. Ni 2p X-ray photoelectron spectra of (a) 8 wt % sol-gel Ni/Al2O3 calcined at 600 °C and (b) 8 wt % Ni/Al2O3 catalyst reduced at 700 °C.

CH4, CO, and H2 Interaction with Ni/Al2O3 Catalysts

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18649

Figure 4. Effluent gases from methane (CH4/Ar, 1/20 mole ratio) pulsing experiments at 700 °C over 10 wt % Ni(O)/Al2O3 in air at 700 °C for 6 h: (a) methane, (b) hydrogen, (c) carbon monoxide, (d) carbon dioxide, and (e) water.

reaction product of the sol-gel method is the NiAl2O4, which crystallizes to the γ-Al2O3 support in a face-centered-cubic oxygen packing with partial occupation of voids. XRD spectra of the samples display very weak observable Bragg reflections due to NiO, which suggests the presence of very small NiO particles or the formation of NiAl2O4. After reduction under 10% H2/Ar at 700 °C, some amounts of metallic nickel were formed as confirmed by the diffraction lines of a metallic nickel phase at 2θ ) 51.8° and 76.4°, respectively; the 2θ ) 44.5° peak was overlapped by a γ-Al2O3 peak.

TPR experiments were conducted from 25 to 800 °C to investigate the reducibility of nickel oxide on the Ni/Al2O3 catalyst. Figure 2 illustrates hydrogen by Ni/Al2O3 catalysts as a function of temperature during the reduction. The H2-TPR profile of Ni/Al2O3, shown in Figure 2, has a large peak centered at 648 °C. The high-temperature peak can be assigned to the reduction of Ni2+ ions in the lattice of the amorphous nickel aluminate. It was revealed that the Ni2+ ions incorporated in the crystalline lattice of nickel aluminate were more difficult to reduce than bulk NiO.28 However, the TPR result indicates


Yan et al.

Figure 5. Transient responses upon pulsing CH4/Ar into He flow at 700 °C over a oxidized Ni-Al2O3 catalyst: (a) first pulse, (b) fourth pulse, (c) 16th pulse, and (d) 22nd pulse.

that most of Ni species is reduced before 700 °C; i.e., 90% of Ni oxides are reduced according to peak area. XPS was used to obtain further information about the oxidation state of the elements and surface composition of Ni/ Al2O3 by inspecting the spectral line shape and the intensities of the O 1s and Ni 2p core-level electrons. The value of 855.5 eV can be assigned to NiO. XPS was conducted to study the nature of surface nickel species over the catalysts after reduction under 10% H2/Ar at 700 °C for 2 h. The Ni 2p spectra of the Ni/Al2O3 are shown in Figure 3. The Ni 2p spectra of the catalysts exhibit features that are assigned to Ni2+ with Ni 2p3/2 binding energy (BE) at 855.8 eV and Ni0 with Ni 2p3/2 binding energy at 852.7 eV. Ni 2p spectra indicate that the reduction of nickel species to the metallic state in the sol-gel catalysts is almost completed due to higher reduction temperature. Deconvolution of the Ni 2p spectra was performed by the Gaussian-Lorentzian curve-fitting method to determine the peak areas for obtaining relative surface concentrations of nickel species (Figure 3b). The ratio of peak areas ANi0/(ANi0 + ANi2+) is used to illustrate the degree of reducibility. It was found that ANi0/(ANi0 + ANi2+) was around 0.83. This is in agreement with TPR results. 3.2. H2 Chemisorption. H2 chemisorption was performed to determine the nickel dispersion and nickel surface area of the catalysts. The reduction was performed under 10% H2/Ar at 700 °C for 2 h. The nickel dispersion and nickel surface area were calculated assuming that one hydrogen atom is adsorbed on one surface nickel atom. The nickel dispersion is observed to be 8.5% for reduced Ni/Al2O3. The good nickel dispersion on the high surface area of sol-gel alumina could be due to high reducibility at higher temperature (700 °C). This observation is in good agreement with TPR results. When Ni/Al2O3 catalysts are calcined at 600 °C temperature, NiO interacts with

Al2O3 to form porous NiAl2O4, which is more difficult to reduce at lower temperature. However, it can be reduced at the temperature of 700 °C or higher. The catalysts exhibit higher nickel dispersion and nickel surface area when reduced at 700 °C. 3.3. Interaction of Methane and CO with the Ni/Al2O3 Catalyst by Pulsed Reactions. 3.3.1 Pulse Reactions and Transient Response of CH4/Ar oVer an Unreduced Ni/Al2O3 Catalyst. Pulse methane reactions were first conducted over an oxidized Ni/Al2O3 without added oxygen (Figure 4). Any methane oxidation in these experiments must be derived from oxygen originating from the solid Ni/Al2O3 catalyst system. Methane diluted in argon (CH4/Ar ) 1/20) was pulsed (1 mL gas, STP) over unreduced Ni/Al2O3 at 700 °C and 1 atm. Figure 4 shows typical methane pulse reaction results. During the first CH4 pulse, little CH4 was converted, and only small amounts of CO2, H2, and CO were detected by the online MS. The surface oxygen present on the catalyst was responsible for the formation of both CO and CO2, since no oxygen was present in the feed gas, and the catalyst was not prereduced. There was 2.7 × 10-5 mol of NiO in 0.02 g of 8 wt % Ni/Al2O3 sample, while there was 2.2 × 10-6 mol of CH4 in each pulse. Three pulses of CH4 would consume oxygen in NiO if CH4 was completely oxidized to CO2 and H2O (CH4 + 4NiO f CO2 + 2H2O + 4Ni). Oxygen in NiO would react with 13 pulses of CH4 if CH4 was partially oxidized to H2 and CO (CH4 + NiO f CO + 2H2 + Ni). Methane conversions increased over the first four pulses and then decreased. The yield of CO increased from the first to the fourth pulse and then decreased. Substantial tailing of the CO peaks was observed beginning in the 15th pulse, and the length of the tailing increased thereafter as the pulse number increased; this means surface oxygen species is insufficient to react with surface



Figure 6. Effluent gases from methane (CH4/Ar, 1/20 mole ratio) pulsing experiments at 700 °C over prereduced 10 wt % Ni/Al2O3 in air at 700 °C for 6 h: (a) methane, (b) hydrogen, (c) carbon monoxide, (d) carbon dioxide, and (e) water.

carbon species in a short period. The CO2 yield increased sharply over the first three pulses, and then decreased rapidly (Figure 4d). After the fourth pulse, only a small amount of CO2 was detected. The H2 evolution pattern was similar to that for CO. Transient responses of pulsing CH4/Ar reaction were conducted at 700 °C under 1 atm over oxidized Ni/Al2O3 catalyst. The responses of H2, CO, CH4, H2O, and CO2 to these pulsing experiments are shown in Figure 5. Figure 5a shows the responses to the first pulse over unreduced Ni/Al2O3. CO2

appears immediately and sharply increases to form a peak, and CO is detected 1 s after CO2, while H2 is found 3 s later than CO2. This result suggests that CH4 may be converted by a nonselective oxidation process over an oxidized Ni/Al2O3. CO2 is the direct product while CO and H2 are secondary products through CH4 reforming with the complete oxidation products: CO2 and H2O. Figure 5b shows the responses to pulsing CH4/ Ar in the fourth pulse over unreduced Ni/Al2O3. Carbon monoxide and carbon dioxide appear almost at the same time


Yan et al.

Figure 7. Transient responses upon pulsing CH4/Ar into He flow at 700 °C over a prereduced Ni-Al2O3 catalyst: (a) first pulse, (b) second pulse, and (c) seventh pulse.

after pulsing CH4/Ar. The carbon oxide signal increases sharply to a maximum. H2 is found 1 s later than CO and CO2. CH4 may be partially converted by selective oxidation process after Ni/Al2O3 is partially reduced. CO and H2 are primary products through CH4 direct oxidation, while there is still a little bit of CH4 converted through the nonselective process. H2 is 1 s lagging behind CO and CO2; this might due to H2 first spillover to alumina before the alumina surface is fully hydrated. The product responses to the 16th CH4/Ar pulse over unreduced Ni/ Al2O3 at 700 °C are shown in Figure 3c. CO appears sooner than CO2 in contrast to the behavior of the first pulse. H2 appears between CO and CO2. Figure 5d depicts the transient product response to this 22nd CH4/Ar pulse over the unreduced Ni/Al2O3 catalyst. The CO and H2 appear at almost the same time. A trace amount of CO2 is formed and appears after H2 and CO. Afterward, the nickel oxide is gradually reduced with subsequent pulsing CH4/Ar. CH4 is efficiently activated via a direct oxidation mechanism only when the nickel is reduced to Ni0 or when its surface oxygen coverage is very low. CO and H2 are the primary products through CH4 direct oxidation in the 22nd pulse, and a small part of CO is converted to CO2 through further oxidation or disproportionation reactions. 3.3.2. Pulse Reactions and Transient Response of CH4/Ar oVer a Prereduced Ni/Al2O3 Catalyst. Figure 6 presents the results of the methane pulsing over the prereduced Ni/Al2O3 catalyst, which had been reduced at 700 °C in hydrogen for 1 h. Methane appears to be activated even with the first pulse, and methane conversion decreased rapidly with subsequent pulsing. CO, H2, and a trace amount of H2O were detected as the gas-phase products from these methane reactions over the prereduced catalysts. The yields of these products were highest for the first pulse and decreased with increasing pulse number. No CO2 was detected for any of the pulses (Figure 6d). The

CO and H2 peaks were found to exhibit longer tailing. Readily observed tailing of the CO peak occurred after each CH4 pulse. CH4 was dissociated to CHx (x ) 0-3) and H2 over the prereduced nickel particles. These observations suggest that CHx reacts with oxygen species on the catalyst surface to form CO. Since there was no gas oxygen or NiO on the prereduced catalyst, the only oxygen sources may come from OH, adsorbed H2O on Ni/Al2O3 interface where surface oxygen on alumina is activated by a hydrogen atom, and spillover back to the nickel particle surface.29,30 It has been proven that an alumina surface is rich in variety of OH groups if it is hydrated and even alumina alone catalyzes several reactions.31,32 Transient responses of pulsing CH4/Ar reaction were conducted at 700 °C under 1 atm over a prereduced Ni/Al2O3 catalyst. The responses of H2, CO, CH4, H2O, and CO2 to these pulsing experiments are shown in Figure 7. Figure 7a plots the responses to this step change in the first pulse over prereduced Ni/Al2O3. H2, CO, and CO2 appear at almost the same time; CO and H2 increase sharply, while the CO2 concentration increases gradually. Figure 7b shows the responses to pulsing CH4/Ar in the second pulse over prereduced Ni/Al2O3. H2 elutes 3 s prior to CO and 6 s prior to CO2. The product responses to the seventh CH4/Ar pulse over prereduced Ni/Al2O3 at 700 °C are shown in Figure 7c. H2 is found 6 s prior to CO. Almost no CO2 is detected. The oxidation state of nickel changes with increasing methane pulse because oxygen species in NiO (including adsorbed and lattice oxygen)33 were consumed during the pulsing reaction, while the product distribution and generation sequence changed; this suggested that the methane activation mechanism changes with nickel oxidation state. 3.3.3. Pulse Reactions and Transient Response of CH4/O2/ Ar oVer a Prereduced Ni/Al2O3 Catalyst. The effluent gases of CH4/O2/Ar pulses over reduced Ni/Al2O3 at 700 °C for 1 h are



Figure 8. Effluent gases from methane (CH4/Ar, 1/20 mole ratio) pulsing experiments at 700 °C over prereduced 10 wt % Ni/Al2O3 in air at 700 °C for 6 h: (a) methane, (b) hydrogen, (c) carbon monoxide, (d) carbon dioxide, (e) water, and (f) oxygen.

shown in Figure 8. The amount of oxygen in these pulses is in stoichiometric proportion to form CO and H2. Figure 8 depicts the CO, H2, and CO2 formation responses during CH4/O2/Ar pulsing over Ni/Al2O3. Large amounts of H2 and CO were observed and only a trace amount of CO2 was detected; CO2 and H2O yields decrease with subsequent pulsing. This means that Ni/Al2O3 has good activity and selectivity to H2 and CO under these reaction conditions. It was also found that oxygen was consumed completely during the reaction (Figure 8f).

Transient responses of pulsing CH4/O2/Ar into He flow over prereduced Ni/Al2O3 catalyst were performed at 700 °C under 1 atm. The responses of the product distribution to these pulses, Figure 9, show the responses to the first pulse (Figure 9a) over prereduced Ni/Al2O3. CO and H2 appear at the same time and sharply increase with time. Only a trace of carbon dioxide was detected 2 s later than the appearance of CO and H2. Figure 9b plots the products versus time after the second CH4/O2/Ar pulse. CO and H2 appear at once and then increase sharply to form a


Yan et al.

Figure 9. Transient responses upon pulsing CH4/O2/Ar into He flow at 700 °C over a prereduced Ni-Al2O3 catalyst: (a) first pulse, (b) second pulse, and (c) eighth pulse.

peak after the pulsing. A trace amount of CO2 was observed 2 s after CO and H2. Figure 9c plots the responses to the eighth pulse of CH4/O2/Ar over prereduced Ni/Al2O3catalyst at 700 °C. H2 appears 1 s before CO and 2 s before CO2 after pulsing. Transient responses prove further that CH4 is efficiently converted to H2 and CO via a direct oxidation mechanism over prereduced Ni/Al2O3 catalysts. 3.3.4. Pulse Reactions of CO/He with the Prereduced Catalyst. To investigate the CO interaction with the Ni/Al2O3 catalyst, pulses of CO/He were passed over a prereduced Ni/ Al2O3 catalyst. Figure 10 presents the results of the CO/He pulsing over the prereduced Ni/Al2O3 catalyst, which had been reduced at 700 °C in hydrogen for 1 h. Figure 10a-d shows typical CO pulse reaction results. During CO pulses, CO was found to convert to CO2 and CH4 (Figure 10a,d); however only trace amounts of H2 and H2O were detected by the on-line MS during the CO pulse (Figure 10b,e). CO/He pulsing results suggest that the CO disproportionation reaction occurs over Ni/ Al2O3 catalyst under operation conditions, while CH4 is generated through the hydrogenation of the surface carbidic from CO disproportionation (Cs + 4Hs f CH4). Surface hydrogen may come from hydrogen species on the alumina surface which was the result of hydrogen spillover from the nickel surface during the catalyst reduction process.29,30 The product responses to pulsing CO/He over a prereduced Ni/Al2O3 catalyst at 700 °C are shown in Figure 11. Figure 11a plots the responses to the first pulse over prereduced Ni/ Al2O3. CO2 appears almost 3 s prior to CO and CH4 after the pulsing. Figure 11b and Figure 11c plot the responses to the second and 10th pulses over prereduced Ni/Al2O3. The responses are the same as that of first pulse. CO2 is both the direct and the major product over prereduced Ni/Al2O3 catalyst. CO is first converted to CO2 and surface carbidic CR through the dispro-

portionation reaction, and then CO2 is quickly desorbed from nickel surface and enters the gas phase since CO2 is weakly adsorbed over Ni/Al2O3 catalyst;34 while the surface carbon species react with the surface hydrogen atom or hydroxyl group to form methane. CH4 is a secondary product of the pulsing process; therefore, CH4 appears after CO2 in the product. 3.4. Temperature-Programmed-Desorption Processes over Ni/Al2O3 Catalysts. The temperature-programmed desorption of hydrogen from a Ni/Al2O3 catalyst shows one large peak at 758 °C (Figure 12a). The signal at 758 °C can be assigned to the H2 desorption from metallic Ni surface sites. No water desorption peak was found in the TPD spectra; therefore, H2 desorption cannot be attributed to the dissociation of H2O (Nis + H2O f NisOads + H2), which may be stored on the support in the H2-TPR experiments.29,30 No low-temperature H2 desorption was detected since the sample was pretreated under high temperature. Water was removed because of the maximum, heat pretreatment temperature of 800 °C. No desorbed CH4, CO, and CO2 were detected during the temperature evaluation process. The catalyst surface should be clean and no carbonaceous species should exist on fresh Ni/Al2O3 catalyst surface after H2-TPR. TPD profiles over the fresh Ni/Al2O3 catalyst subsequent to CO-TPR were obtained (Figure 12b). The responses of CO, H2, CH4, and CO2 formation, even water level, were recorded in order to monitor the occurrence of CO disproportionation, surface carbidic or carbonaceous hydrogenation, H2 desorption, and H2 spillover from alumina during the temperature-programming process. Three apparent CO desorption peaks appear at 200, 410, and 720 °C, respectively. There is only one CO2 peak, which appears at 180 °C. This CO2 peak is related to CO molecularly adsorbed on basal planes (low index planes) of Ni particles.35 The CO2 formation results probably from the



Figure 10. Effluent gases from carbon monoxide (CO/He, 1/20 mole ratio) pulsing experiments at 700 °C over prereduced 10 wt % Ni/Al2O3 in air at 700 °C for 6 h: (a) methane, (b) hydrogen, (c) carbon monoxide, (d) carbon dioxide, and (e) water.

Boudouard reaction (2CO f C + CO2). This indicates that the CO disproportionation reaction occurred at low temperature, and weakly adsorbed CO2 was formed. The CO desorbed at 410 °C may come from strongly adsorbed CO on the catalyst which begins to desorb from 300 °C. No H2 and H2O desorption peaks were found in the TPD profile. Two CH4 evolution peaks were observed in the regions of 150-250 °C and 650-800 °C. These two peaks resolved into two CH4 peaks occurring at 182 and 720 °C, respectively. The CH4 peak at low temperature

corresponds to the surface carbidic CR which was generated from the carbon monoxide disproportionation reaction. The CH4 peak at high temperature was due to surface carbidic cluster Cγ which was the precursor of inactive carbon deposits and was formed by the agglomeration and conversion of CR. TPD profiles of a Ni/Al2O3 catalyst after H2-TPR followed by CO adsorption are shown in Figure 12c. The response of CO, H2, CH4, and CO2 formation was recorded during the temperature-programming process. Compared with CO-TPD


Yan et al.

Figure 11. Transient responses upon pulsing CO/He into He flow at 700 °C over a prereduced Ni-Al2O3 catalyst: (a) first pulse, (b) second pulse, and (c) 10th pulse.

profiles over fresh Ni/Al2O3 and used catalysts, the TPD profiles on a Ni/Al2O3 catalyst after H2-TPR followed by CO adsorption are different. There are two CO2 peaks at 150 and 350 °C respectively. Two CO2 desorption peaks appear on TPD profiles over the coadsorbed catalysts at ca. 150 and 350 °C. They seem to correspond to desorbed CO2 which was produced by disproportionation of weakly adsorbed CO over surface nickel sites. There are two H2 desorption peaks. The H2 desorption peaks for Ni/Al2O3 catalyst after H2-TPR followed by CO adsorption were at 200 and 400 °C. The first peak is usually attributed to weakly adsorbed H2 from the metal particles and indicates the exposed fraction of Ni atoms. The peaks at the higher temperatures are attributed to hydrogen located in the subsurface layers or to H2 due to spillover on the alumina support.36 CH4 evolution was observed in the regions of 100200 °C and 200-400 °C. Two CH4 peaks occur at 150 and 350 °C, respectively. The CH4 peak at low temperature is very weak, and it should correspond to the surface carbidic, which was generated from the carbon monoxide disproportionation reaction. The CH4 peak at 350 °C is due to CO-H2 methanation rather than to hydrogenation of a carbon deposit, because this peak occurs concomitantly with both CO and CO2 evolution. TPD profiles of a Ni/Al2O3 catalyst after H2-TPR and H2TPD followed by CO adsorption are shown in Figure 12d. The responses of CO, H2, CH4, H2O, and CO2 formation were recorded during the temperature-programming process. The TPD profiles were very similar to the TPD profiles over the fresh Ni/Al2O3 catalyst subsequent to CO-TPR. Four CO desorption peaks appear at 250, 330, 410, and 668 °C, respectively. There is only one CO2 peak appearing at 200 °C. This CO2 peak results probably from the CO disproportionation reaction occurring at low temperature, and weakly adsorbed CO2 was

formed.Two CH4 peaks are observed at the temperatures of 200 and 670 °C, respectively; they are hydrogenation products of CR and Cγ. No H2O desorption peak was found in the TPD profile. TPD profiles of a used Ni/Al2O3 catalyst after CO-TPR followed by H2 adsorption are shown in Figure 12e. There are two CO desorption peaks including one small peak centered at 150 °C, and one large peak began at 350 °C and centered at 525 °C. There are two CO2 peaks at 150 and 440 °C, respectively. There is one H2 desorption peak beginning at 350 °C and centered at 480 °C. CH4 evolution was observed in a broad region between 100 and 550 °C. This was resolved into two CH4 peaks occurring at 150 and 450 °C, respectively. These CH4 peaks represent CO-H2 methanation rather than hydrogenation of a carbon deposit, because this peak occurs concomitantly with both CO and CO2 evolution. TPD profiles of a used Ni/Al2O3 catalyst after CH4 pulse reaction are shown in Figure 12f. There are two CO desorption peaks including one weak peak centered at 250 °C and one strong peak beginning at 450 °C and centered at 650 °C. There is one broad CO2 peak from 100 to 450 °C; another CO2 peak is found between 450 and 650 °C. The low-temperature, broad CO2 band may come from two sources: one corresponds to desorbed CO2 in the form of weakly adsorbed CO; another is surface carbon species CHx which may react with surface oxygen species (2CHx + 3Oad f CO + CO2 + xH2). There are three H2 peaks at 290, 400, and 600 °C. The H2 desorption peak at 290 °C is attributed to CHx reacting with surface oxygen species. CH4 evolution is observed in a broad region between 200 and 550 °C and a weak peak at 550 °C. The broad CH4 band is resolved into two CH4 peaks occurring at 250 and 400 °C, respectively. The CH4 peak at 250 °C is very weak,



Figure 12. TPD spectra of Ni-alumina catalysts after (a) H2-TPR reaction, (b) CO-TPR reaction, (c) H2-TPR reaction followed by CO adsorption, (d) H2-TPR and H2-TPD followed by CO adsorption, (e) CO-TPR reaction followed by H2 adsorption, (f) CH4 pulse reaction, (g) CH4/O2 reaction, and (h) CH4/CO2 reaction.


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Figure 13. Temperature-programmed-hydrogenation (TPH) spectra of 10 wt % Ni-Al2O3 after (a) pulse CH4/Ar at 700 °C 50 times, (b) catalyzing CH4/O2 reaction for 2 h at 700 °C, (c) catalyzing CH4/CO2 reaction for 2 h at 700 °C, (d) catalyzing CO/H2 at 700 °C for 2 h, and (e) catalyzing CO/He at 700 °C for 2 h.

and it should correspond to the surface carbide. The CH4 peak at 400 should be attributed to partially dehydrogenated CHx (1 < x < 3) species Cβ, and the 550 °C peak represents hydrogenation of carbidic clusters Cγ. Part of CO, H2, and CO2 at high temperature should be related with surface carbonaceous residue. Carbon species were converted to CO, H2, and CO2 through steam reforming (Cs + H2O f CO + H2 + CO2) while water was consumed. TPD profiles of a used Ni/Al2O3 catalyst after CH4/O2 reaction are shown in Figure 12g. Compared with TPD profiles

over a Ni/Al2O3 catalyst after a CH4 pulse reaction, on which two respective CO2 desorption peaks appear, the TPD profiles on a used catalyst are only a little different. Two CO2 desorption peaks appear on the CH4/O2 pretreated catalysts at ca. 280 and 470 °C. They seem to correspond to desorbed CO2 in the form of weakly and strongly chemisorbed on different sites in both catalysts. It is interesting to note that large amounts of CO and CO2 were desorbed at temperatures above 400 °C, but the increase in CO desorption lagged behind. This could be manifested by the surface carbon species and the reactivity of

CH4, CO, and H2 Interaction with Ni/Al2O3 Catalysts the oxygen species such as OH-, O2-, or even H2O on the nickel catalyst. The surface oxygen species can attack the neighboring carbon species to form CO and CO2. It is also possible that the CO2 desorbed from the catalyst readsorbed and then reacted with surface carbonaceous species to produce CO. Similar to the TPD profiles of a used Ni/Al2O3 catalyst after CH4/O2 reaction shown in Figure 12g, results of a TPD investigation of a Ni/Al2O3 catalyst subsequent to CH4/CO2 reaction are profiled in Figure 12h. There is an additional, weak CO2 peak at ca. 150 °C on the CH4/CO2 pretreated catalyst. The CO desorption also increases with temperature from ca. 400 °C. 3.5. Temperature-Programmed Hydrogenation (TPH) of Ni/Al2O3 Catalyst. TPH was performed to investigate possible carbon deposits on the catalyst after Ni/Al2O3 had employed CH4 pulsing at 700 °C 50 times or CH4/O2 or CH4/CO2 reforming or CO/H2 and CO/He pretreatment at 700 °C for 2 h followed by cooling in a He flow to room temperature. The effluent gases were analyzed with on-line quadrupole mass spectrometry. Figure 13a shows the temperature-programmed-hydrogenation (TPH) reaction of the surface carbon species formed on the Ni/ Al2O3 surface, which was prereduced by pulsing CH4 at 700 °C. There is one weak peak at 100-200 °C and two strong peaks at 250-400 and 400-700 °C, respectively. The TPH peak at the low hydrogenation temperature (100-200 °C) corresponds to the carbon species with completely dehydrogenated carbidic CR. The TPH peak at hydrogenation temperatures of 250400 °C corresponds to the carbon species with partially dehydrogenated CHx (1 < x < 3) species, namely Cβ. The concentration of Cβ was higher than that of CR, and also the result indicates that the Cβ species is more stable than CR on the catalyst surface. The TPH peak at hydrogenation temperatures of 400-700 °C was assigned to inactive carbidic clusters Cγ, which were converted from CR and Cβ. Figure 13b shows the curve of temperature-programmed-hydrogenation reaction of the surface carbon species formed on Ni/Al2O3 prereacted with CH4/O2 at 700 °C for 2 h. There are two peaks corresponding to Cβ between 200 and 310 °C and Cγ between 600 and 800 °C. The peak area of Cβ in Figure 13b is smaller than that in Figure 13a. It is well-known that C-H bond activation of CH4 on a catalyst is the rate-determining step, while the oxidation of CHx is a faster reaction. If there was no oxygen co-feeding, there should be more Cβ species converted to CR and Cγ. This is consistent with experimental results. However, if oxygen was co-fed with methane, Cβ species might be consumed quickly; the concentration of Cβ species should be low. This is also consistent with the reaction result. Figure 13c shows the curve of the temperature-programmed-hydrogenation reaction of the surface carbon species formed on Ni/Al2O3 prereacted with CH4/CO2 at 700 °C for 2 h. CH4 evolution is observed in a broad region between 150 and 800 °C during TPH of the catalyst used in the CH4/CO2 reaction (Figure 13c). This may be resolved into two CH4 peaks occurring at 250 and 700 °C, respectively. The broad CH4 peak at 250 °C could be attributed to hydrogenation of both surface carbons CR and Cβ. The CH4 peak near 700 °C represents hydrogenation of a carbon deposit over the catalyst. Figure 13d shows the curve of the temperature-programmedhydrogenation reaction of the surface carbon species formed on Ni/Al2O3 prereacted with H2/CO at 700 °C for 2 h. Two CH4 peaks were detected at 150 and 465 °C, respectively (Figure 13d). The peak at 150 °C was attributed to hydrogenation of surface carbide, CR, while the peak at 465 °C was due to COH2 methanation rather than to hydrogenation of a carbon deposit,

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18659 because this peak occurs concomitantly with both CO and CO2 evolution. No CH4 peak was found at higher temperatures under reaction conditions. Figure 13e shows the curve of the temperature-programmedhydrogenation reaction of the surface carbon species formed on Ni/Al2O3 prereacted with CO/He at 700 °C for 2 h. There are one weak peak at 350-450 °C and two strong peaks at 150200 °C and 550-800 °C, respectively. The TPH peak at low hydrogenation temperature (100-200 °C) should correspond to the surface carbide CR which was generated from the carbon monoxide disproportionation reaction. The TPH peak at hydrogenation temperatures of 350-450 °C corresponds to CO-H2 methanation. The TPH peak at hydrogenation temperatures of 550-800 °C was assigned to inactive graphite carbide clusters Cγ converted from surface carbide CR. 4. Discussion 4.1. Nickel Oxidation State and Its Effects on Methane Activation Mechanism over Ni/Al2O3. Different surface metal oxidation states can result in different reaction mechanisms.37-41 CH4 pulse experiments provide some mechanistic insight of methane oxidation over Ni/Al2O3 under different nickel oxidation states. During the first pulse of oxygen-free CH4 over unreduced Ni/Al2O3, little CH4 conversion occurred due to the absence of active Ni0 sites. The oxygen species on Ni/Al2O3 catalyst surface may include strongly adsorbed oxygen (O2), O2-. In the bulk catalyst, lattice oxygen (O2-) is present.33 Only traces of CO, H2, and CO2 were detected. Thus, the reaction over the unreduced catalyst takes place between gas-phase or weakly adsorbed CH4 and strongly adsorbed oxygen or lattice oxygen. As the number of CH4 pulses over the unreduced Ni/Al2O3 increased, CH4 conversion increased, because NiO was increasingly reduced to form Ni0 sites on the surface. Since there was no oxygen in the feed, the formation of CO and CO2 demonstrates that the catalyst must have been the oxygen source. Surface OH groups and/or NiO lattice oxygen and/or oxygen from the Al2O3 support participated in the activation of methane to COx. Buyevskaya et al.39 previously concluded that surface OH groups of the support were involved in the conversion of CHx to CO. Pure Al2O3 did not catalyze methane conversion at 700 °C. Therefore, reduced nickel sites are required for the surface OH groups and/or low coordination oxygen of the Al2O3 support to react with CH4 to form COx. NiO appears responsible for the formation of CO2 and H2O observed in the first five pulses. Haber40 pointed out that total oxidation of hydrocarbons is often observed over transition metal oxides even in the absence of gaseous oxygen. Hence, a redox reaction between methane and nickel oxide (CH4 + 4NiO f CO2 + 2H2O + 4Ni0) on the oxidized catalysts appears reasonable. This leads to the oxidation of methane and the reduction of nickel oxide. It was estimated by calculation according to the reaction CH4 + 4NiO f CO2 + 2H2O + 4Ni0 that three pulses of methane were sufficient to reduce the NiO in 8 wt % Ni/Al2O3 catalyst. Both unreduced and reduced Ni/Al2O3 catalysts were active for methane oxidation, indicating that methane C-H bond dissociation is indeed a key step for CO formation. This suggests that CO formation proceeds via CHx (x ) 0-3) species produced by methane dissociation at Ni0 sites. Then, during the first five pulses of methane over the oxidized catalysts, three types of reactions occurred: (1) complete oxidation to CO2 over nickel

18660 J. Phys. Chem. C, Vol. 111, No. 50, 2007 oxide, (2) partial oxidation to CO, and (3) decomposition to surface carbon species over the resulting metallic nickel. CH4 + 4NiO f CO2 + 2H2O + 4Ni0 CH4 + (5 - x)Ni f . . . f Ni-CHx + (4 - x)Ni-H Ni-CHx + Ni-O f Ni-CO + Ni-H Ni-CO f CO(g) + Ni Ni-CO + Ni-O f CO2(g) + 2Ni After reduction of Ni/Al2O3 at 700 °C for 1 h, the surface oxygen species concentration at active Ni sites was very low. Therefore, the unreduced NiO which has a strong interaction with Al2O3 or Al2O3 support itself was the main oxygen source. H2 spillover to the alumina surface has been demonstrated.29,30 Surface oxygen on alumina is activated by Hs to OH and, subsequently, OH back-spillover to Ni surface to react with surface carbonaceous and give CO and H2. CO subsequently can convert to secondary product CO2 through further oxidation or disproportionation reactions. Thus, H2, CO, CO2, and H2O were observed in the gas phase, when methane was pulsed over freshly reduced Ni/Al2O3. Methane conversion decreased upon continued pulsing over the reduced catalyst. Two factors could contribute: (1) the amount of oxygen species reaching nickel sites decreased; (2) metallic sites might become increasingly poisoned (covered) by carbon species. The methane pulse reactions reveal that lattice oxygen participates in the reaction over both unreduced and reduced catalysts. Ruckenstein and Hu found, using isotopic methods, that the lattice oxygen participates in the reaction even when the gas feed mixture contains oxygen.41,42 Methane pulse reactions demonstrated that the mechanism of methane oxidation changes at different stages as the oxidation state of nickel changes. CH4 does not decompose to surfacebound CHx and H species on oxidized Ni sites (e.g., NiO). However, CH4 can be oxidized by oxygen in NiO or by active oxygen within the alumina support via the Eley-Rideal mechanism. The oxidation of methane via the Eley-Rideal mechanism is nonselective. The products may be H2O and CO2 or H2O, CO2, H2, and CO depending on the temperature. Thus, if the catalyst surface has been fully oxidized before CH4 is pulsed into the reaction system, only nonselective oxidation of CH4 occurs. This process uses surface oxygen atoms of the solid catalyst via the Eley-Rideal mechanism and leads to rather low CH4 conversions and the poor selectivity to CO and H2. CH4 is efficiently activated via a direct oxidation mechanism only when the nickel is reduced to Ni0 or when its surface oxygen coverage is very low. This produces CO and H2 via the LangmuirHinshelwood mechanism in which adsorbed methane and oxygen species react with each other.42 Transient response results illustrate further methane activation mechanism changes with nickel oxidation evolution. In summary, the methane pulse reactions clearly demonstrated that the oxidation state of nickel controls the methane activation mechanism and the product distribution when there is no gaseous oxygen present. A remarkable tailing of the CO, H2, and CO2 peaks was observed from the ninth pulse onward during oxygen-free methane pulsing over unreduced Ni/Al2O3. However, tailing was not observed for CH4 peaks. Furthermore, independent pulsing of CO, CO2, or H2 over the Ni/Al2O3 catalyst bed (either reduced or oxidized) did not give any tailing. Only sharp peaks were observed. These observations imply that the rate-determining

Yan et al. step in syngas formation involves a carbon species reacting with oxygen species over the catalyst. The side reaction producing CO2 from CO and oxygen was fast. The CO2 peak tailing observed was due to the original CO tailing. This was confirmed by observing sharp CO2 peaks upon pulsing CO over the catalyst. Thus, the reaction between CHx and surface O species could be the rate-determining step in syngas formation over Ni/ Al2O3. CO and H2 exhibited longer tailing responses during the pulsing of CH4/Ar over reduced Ni/Al2O3 (Figure 6a-d). This indicates that CHx species are present on the reduced catalyst. In contrast, CHx species are not present on oxidized Ni/Al2O3 catalyst samples, showing that Ni0 is responsible for methane activation. Tailing peaks were also observed over the reduced catalyst. The CO peaks formed from the first CH4/Ar pulse over reduced Ni/Al2O3 exhibited long tailings (Figure 6c). The delay could result from a combination of the desorption of CO formed in the earlier pulses or CO formation by reactions between surface CHx and O species. However, peak tailing was not found for CO2. This absence of peak tailing rules out CO desorption and favors a surface reaction between the CHx and the O species. When CO is pulsed over the reduced catalyst, sharp peaks are obtained. Consequently, the reaction over the reduced Ni/Al2O3 catalyst takes place via the Langmuir-Hinshelwood mechanism, in which the adsorbed CHx and O species are involved. The pulsing of CH4 over an oxidized catalyst first gives fully oxygenated products, H2O and CO2, and the rate of CH4 adsorption is slow. H2O strongly adsorbs on the support and is not seen as a distinct peak, but can be deduced as present from the H2O background increase and the absence or H2 delay. The CH4 conversion increases as pulsing continues and the surface is depleted of adsorbed oxygen species, and after a maximum, the methane conversion decreases. Thus, oxidized nickel inhibits CH4 adsorption and this is evidence that CH4 and O2 adsorb on the same sites over a nickel-based catalyst. This feature has also been used by Wang et al.43 and Soick et al.44 but is in contrast to Hickman and Schmidt45 and Elmasides, Ioannides, and Verykios,46 who postulated different adsorption sites for CH4 and O2. During a CH4 pulse sequence that increasingly depletes the surface of adsorbed oxygen species, the relative amounts of CO2 and CO change. The product is CO2 only when the coverage in oxygen is high; therefore, the amount of CO2 begins to decrease with increase of pulsing CH4, while the CO amount increases. CO2 and CO are complements in that when the CO2 concentration is high, the CO concentration is small and vice versa; that is, their sum is about constant. This suggests that CO2 is produced by the further oxidation of CO; that is, CO is the primary product of the surface-catalyzed reaction. 4.2. Oxygen Species during CH4 Activation over Ni/Al2O3 Catalysts. The oxygen species on unreduced Ni/Al2O3 catalyst surface may include strongly adsorbed oxygen (O2), O2-.33 In the bulk catalyst, it is lattice oxygen (O2-). Thus, the reaction over the unreduced catalyst takes place between gas-phase or weakly adsorbed CH4 and strongly adsorbed oxygen or lattice oxygen. The surface oxygen species concentration at active Ni sites should be very low when Ni/Al2O3 is reduced by hydrogen at 700 °C for 1 h. The surface O(s) or OH(s) species could be derived from either surface or bulk oxygen atoms of the support. The surface OH(s) species are derived by two ways, i.e., (i) the dissociation of adsorbed H2O on the nickel surface and (ii) activation of surface oxygen on alumina by surface H atom. Surface or bulk oxygen (lattice) of the support may be activated

CH4, CO, and H2 Interaction with Ni/Al2O3 Catalysts by surface hydrogen, H(s), on nickel sites via hydrogen- or oxygen-spillover processes envisioned in the following elementary reaction steps. H(s)(Ni) f H(s)(Al2O3) H(s)(Al2O3) + O(s)(Al2O3) f HO(s)(Al2O3) HO(s)(Al2O3) f HO(s)(Ni) On a surface hydrated before the experiments, CO and H2 production continues for many more pulses in a CH4 pulse experiment and the amount of oxygen in the oxygenated products far exceeds the amount of adsorbed oxygen on the metal particles. This is evidence of another oxygen source and had also been reported by Wang et al.47,48 The similarity of the present results to those of Wang et al.47 suggests a similar interpretation that the additional oxygen source is OH spillover from the Al2O3 support. These results are that, with the gradual desorption of water from the support, partial oxidation products (CO and H2) appeared, which indicates that the oxidizing species contains both hydrogen and oxygen. That is, they are OH groups. This oxygen species can react with carbon species on Ni0 sites, thereby depressing carbon accumulation on the nickel surface. This interpretation explains the TPD and TPH results. 4.3. Carbon Species Formation and Carbon Removal over Ni/Al2O3 surface. There are three possible mechanisms for carbon formation, including the catalytic hydrocarbon decomposition reaction (CxHy f xC + y/2H2), the Boudouard reaction (2CO h CO2(g) + C(s)), and the heterogeneous water gas reaction (H2 + CO h H2O + C)). Both the Boudouard reaction and the heterogeneous water gas reaction are exothermic. Conversely, the hydrocarbon decomposition reaction is endothermic. Carbon formation occurs during the fuel reforming process, but surface carbon is also removed so that catalyst deactivation is not so fast by carbon buildup. The following elementary reaction steps are suggested for carbon formation on the catalyst as methane is converted to syngas during the partial oxidation of methane. These steps are based on the TPD and TPH results of the present work and the earlier studies of Vannice et al.49 CH4 f CHx(s) + (4 - x)H(s) CHx(s) f C(s) + xH(s) CHx(s) + O(s) f CO(s) + xH(s) 2CO h CO2 (g) + C(s) CO(s) h C(s) + O(s) The symbol “(s)” represents a site on the reduced nickel surface. Removal of carbon from the surface could take place by the following elementary steps: C(s) + OH(s) f CO(s) + H(s) C(s) + O(s) f CO(s) CO(g) + C(s) f 2CO(s) C(s) + 4H(s) f CH4(g) 5. Conclusions The interactions of CH4, CO, and CH4/O2 with Ni/Al2O3 catalysts prepared through sol-gel process were examined by

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18661 the pulse reaction and transient response techniques. The interactions of H2, CO, and CH4 with a Ni/Al2O3 catalyst were also investigated by temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), and temperatureprogrammed hydrogenation (TPH). XRD, TPR, and XPS results demonstrated that nickel is mainly present as NiO and NiAl2O4 in the as-prepared catalyst, while around 80-90% nickel is Ni0 after the catalyst is reduced at 700 °C for 2 h. Pulse reactions and transient response demonstrated that the mechanism of methane oxidation changes at different stages as the oxidation state of nickel changes. CH4 is oxidized by lattice oxygen in NiO or by active oxygen within the Al2O3 support via the EleyRideal mechanism over oxidized Ni/Al2O3. The oxidation of methane via the Eley-Rideal mechanism is nonselective. The products are H2O, CO2, H2, and CO. CH4 is efficiently converted to CO and H2 via a direct oxidation mechanism when Ni/Al2O3 is prereduced. The methane pulse reaction clearly demonstrated that the oxidation state of nickel controls the mechanism and the product distribution in the process of methane pulsing without gaseous oxygen. The occurrence of CO disproportionation, surface carbidic or carbonaceous hydrogenation, H2 desorption, and H2 spillover from alumina over Ni/Al2O3 was investigated by TPD processes. CO pulse reaction results over the prereduced catalyst suggest that CO disproportionation reaction occurs over Ni/Al2O3 catalyst under operation conditions, while CH4 is generated through the hydrogenation of the surface carbidic from CO disproportionation. TPD and TPH studies showed that the decomposition of methane results in the formation of at least three kinds of surface carbon species on supported nickel catalysts. Surface carbidic, surface carbonaceous, and surface carbidic clusters were formed over Ni/Al2O3 catalyst during CH4 and CO interacting with catalyst. Surface carbon species formed by the decomposition of methane and CO disproportionation demonstrated different reactivities and stabilities by TPD and TPH experiments. Carbidic CR is a very active and important intermediate in methane conversion, the partially dehydrogenated Cβ species can react with H2 or O2 or CO2 to form CH4 or CO, and the carbidic clusters Cγ species might be the precursor of surface carbon deposition. Acknowledgment. This work is generously supported by the Dave C. Swalm School of Chemical Engineering at Mississippi State University. References and Notes (1) van Hook, J. P. Catal. ReV.sSci. Eng. 1981, 21, 1. (2) Ashcroft, A. T.; Cheetham, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrel, A. J.; Veron, P. D. F. Nature 1990, 344, 319. (3) Wang, H. Y.; Ruckenstein, E. J. Catal. 1999, 186, 181. (4) Tsipouriari, V. A.; Zhang, Z.; Verykios, X. E. J. Catal. 1998, 179, 283. (5) Au, C. T.; Wang, H. Y. J. Catal. 1997, 167, 337. (6) Torniainen, P. M.; Chu, X.; Schmidt, L. D. J. Catal. 1994, 146, 1. (7) Vermeiren, W. J. M.; Blomsma, E.; Jacobs, P. A. Catal. Today 1992, 13, 427. (8) Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. J. Catal. 1993, 139, 326. (9) Nakamura, J.; Aikawa, K.; Sato, K.; Uchijima, T. Catal. Lett. 1994, 25, 265. (10) Prettre, M.; Eichner, Ch.; Perrin, M. Trans. Faraday Soc. 1946, 43, 335. (11) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117. (12) Hickman, D. A.; Schmidt, L. D. Science 1993, 259, 343. (13) Yan, Q. G.; Wu, T. H.; Weng, W. Z.; Toghiani, H.; Toghiani, R. K.; Wan, H. L.; Pittman, C. U., Jr. J. Catal. 2004, 226, 247. (14) Liu, S.; Xiong, G.; Dong, H.; Yang, W. Appl. Catal., A 2000, 202, 141. (15) Yan, Q.; Chao, Z.; Wu, T.; Weng, W.; Wan, H. Stud. Surf. Sci. Catal. 2000, 130D, 354.

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