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Methanol partial oxidation over palladium-, platinum-, and rhodium-integrated LaMnO3 perovskites
Applied Catalysis B: Environmental 107 (2011) 284–293
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Methanol partial oxidation over palladium-, platinum-, and rhodium-integrated LaMnO3 perovskites Chia-Liang Li, Yu-Chuan Lin ∗ Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 21 July 2011 Accepted 23 July 2011 Available online 30 July 2011 Keywords: Perovskite Noble metals Methanol Oxidation Redox
a b s t r a c t This study investigates methanol partial oxidation over LaMnO3 and LaMn0.95 B0.05 O3 (B = Pd, Pt, and Rh) perovskites. Their surface and bulk properties were characterized by appropriate physicochemical techniques including BET surface area measurement, XRD, EDS, XPS, H2 -TPR, TPO, O2 -TPD, and CH3 OHTPD. Results show significant differences in reducibility and oxygen activity between promoted and untainted LaMnO3 . Promoted perovskites showed higher reactivity than pure LaMnO3 . Results suggest that a redox pathway enhances partial oxidation to formaldehyde while oxygen activity, particularly surface chemisorbed oxygen, plays a central role in combustion. This study also reveals a plausible reaction mechanism of methanol partial oxidation over LaMnO3 -based perovskites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Perovskite (ABO3 ) has attracted a lot of attention in the last few decades [1]. It is a versatile catalyst with numerous applications in fields such as CO oxidation [2], pollution abatement [3], and photocatalysis [4]. This is due to perovskite’s unique crystalline structure, nonstoichiometric chemistry, and adsorption properties [1]. In some cases, perovskites show superior performances and stability than noble metal supported catalysts, such as in automotive emission control [5–7]. Daihatsu Motors recently announced a successful industrial practice by implementing a series of platinum group metal (pgm) promoted perovskites in exhaust converters [8–12]. The self-regeneration behavior of pgm-doped perovskites in oxidative and reductive environments provides them with higher activity and longer durability than conventional noble metal catalysts. This breakthrough has earned pgm-doped perovskites the name “intelligent catalysts [13].” Another important application of perovskite is the partial oxidation of hydrocarbons, especially light alkanes. The partial oxidation of methane is mainly used for syngas preparation [1,14,15]. The chemistry of perovskite’s A-site and B-site elements plays a key role in varying catalytic performance and product distribution. Slagtern and Olsbye investigated the B-site effects of LaMO3 (M = Rh, Ni, and Co) and attributed the outstanding reactivity and durability to highly dispersed Rh ions [16]. Lago et al. studied A-site effects
∗ Corresponding author. Tel.: +886 3 463 8800x3554; fax: +886 3 455 9373. E-mail address: [email protected] (Y.-C. Lin). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.026
by a series of LnCoO3 (Ln = La, Pr, Nd, Sm, and Gd). The activity and syngas selectivity can be improved by increasing the lanthanum ionic radius [17]. Ethane partial oxidation (also called oxidative dehydrogenation) is a potential replacement for steam cracking in ethylene production [18,19]. Donsi et al. extensively worked on this field using LaMnO3 [20–22] and lately by Pt/LaMnO3 [23]. They proposed that perovskite is intrinsically active because ethyl species, a major intermediate of ethylene, can be formed on its surface [20]. Conner et al. investigated propane partial oxidation to acetaldehyde, acrolein, and methanol over complex perovskites, e.g., Ba2 (Bi2/3 1/3 Te)O6 ( represents a vacancy) [24]. They suggested that the synergistic effects of A-site vacancies and B-sites cations were indispensible to its catalytic performance. Compared to alkanes partial oxidation, however, relatively few efforts have been dedicated to methanol partial oxidation (MPO) using perovskites [25,26]. MPO can be applied to either formaldehyde (Eq. (1)) or hydrogen (Eq. (2)) production. CH3 OH + 1/2O2 → HCOH + H2 O H ◦ = −159 kJ/mol CH3 OH + 1/2O2 → 2H2 + CO2
H ◦ = −192 kJ/mol
(1) (2)
De and Balasubramanian used SrVO3 in MPO and showed that the lattice oxygen in perovskite’s framework can assist methanol conversion [27]. The catalyst is highly selective to formaldehyde (90%) at moderate temperature (320 ◦ C), but the activity is low (∼44% conversion). Kuhn and Ozkan studied Sr- and Co-doped LaFeO3 in MPO for hydrogen-rich gas production [25]. The same group also presents a detailed survey of oxygen transfer kinetics and how B-site elements mediate reaction pathways [28–30].
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Galenda et al. used two methods of La4/5 Sr1/5 Ga4/5 Fe1/5 O3 perovskite synthesis and employed them in MPO [31]. Methanol combustion, partial oxidation, and decomposition could proceed simultaneously. In addition, different perovskite synthesis methods can vary the product distribution. This is due to the differences in catalytic surface caused by preparation [31]. Our group has recently reported that doping trace palladium into LaMO3 (M = Mn, Fe, and Co) can achieve attracting conversion (80% and up) and formaldehyde selectivity (90%) in MPO [32]. This enhancement is correlated to the improved reducibility and oxygen mobility of Pd-doped perovskites, which in turn promote formaldehyde yield through MPO and methanol dehydration. This study investigates the effects of B-site-ion replacement by pgms (i.e., Pd, Pt, and Rh) in MPO. LaMnO3 was selected as the host because of its attracting activity and oxygen-excess nature [32,33]. A survey of the physical and chemical properties of these catalysts reveals that characteristics of perovskites are closely related to their catalytic behaviors. Moreover, the reaction scheme under partial oxidation environments was elucidated. This study is a continuation of our previous work on lanthanum-transition metal perovskites for MPO. 2. Experimental 2.1. Catalyst preparation A series of LaMnO3 -based perovskites, including LaMnO3 , LaMn0.95 Pd0.05 O3 , LaMn0.95 Pt0.05 O3 , and LaMn0.95 Rh0.05 O3 were synthesized by a sol-gel method [32,34]. For convenience, LaMnO3 and LaMn0.95 B0.05 O3 (B = Pd, Pt, and Rh) are henceforth referred to as LaMn and LaMnB (e.g., LaMnPd), respectively. Stoichiometric proportions of metal acetylacetonates (acac) (La-, Mn-, Pd-, Pt-, and Rh-(acac)3 ) were dissolved in a 1/4 volumetric ratio mixture of ethylene glycol and methanol (260 mL). Approximately 10 mmol of A-site and B-site metal ions were used. The solution was subjected to vigorous stirring for 1 h. Excess solvents were subsequently removed in a rotary evaporator at 80 ◦ C. The remaining paste was dried in air at 150 ◦ C for 10 h using a heating rate of 10 ◦ C/min. The dried sample was then calcined at the same heating rate from 150 to 700 ◦ C for 2 h. 2.2. Catalyst characterization Surface area was estimated by the Brunauer, Emmet, and Teller (BET) method, using an automated N2 adsorption apparatus (Micromeritics, ASAP 2010). Powder X-ray diffraction (XRD) patterns were recorded using a Shimadzu Labx XRD-6000 with Cu K␣ radiation (0.15418 nm). Scans were performed at a scanning rate of 4◦ /min in 20–80◦ angle range (2). The voltage and current were set to 40 kV and 30 mA. Crystallite sizes were measured using the Scherrer relation. The instrumental broadening effect was found to be negligible using the Warren’s correction [35]. Surface elemental analysis was carried out using an energy dispersive spectrometer (EDS, Oxford ISIS 310) attached to the scanning electron microscopy (SEM, JEOL JSM-5600). The INCA Xsight software package (Oxford Instrument) was used to quantify surface compositions. Three trials with different locations were averaged for each sample. The acquisition time was 10 min per location. The SEM accelerating voltage was 10 kV. The scanned areas were approximately 0.12 mm × 0.09 mm. Elements including O, La, Mn, Pd, Pt, and Rh were determined by EDS from their K-lines. X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific K-Alpha system equipped with a 180◦ hemispherical sector analyzer and Al monochromator (Al K␣ X-ray
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sources, 1486.6 eV). The X-ray spot size was 400 m. The C 1s signal (284.4 eV) of adventitious carbon was employed for energy shift correction. Hydrogen temperature programmed reduction (H2 -TPR), temperature programmed oxidation (TPO), oxygen temperature programmed desorption (O2 -TPD), and methanol temperature programmed desorption (CH3 OH-TPD) were all conducted in the same system [32,36]. The system consisted of a U-shaped cell, a temperature-controlled furnace, and a thermal conductivity detector (TCD). Approximately 100 mg of sample was used per trial. Before the experiment, each sample was pretreated in an Ar stream (30 mL/min) at 150 ◦ C (10 ◦ C/min) for 30 min to remove physisorbed water. In H2 -TPR analysis, a 7.4% H2 /Ar stream (25 mL/min) was used to reduce the sample by heating up to 900 ◦ C at 5 ◦ C/min. For TPO study, the sample was pre-reduced in a 7.4% H2 /Ar stream (25 mL/min) from room temperature to 450 ◦ C at 5 ◦ C/min. After cooling and purging with Ar, the sample was re-oxidized in a 2.5% O2 /He stream (30 mL/min) and ramped to 900 ◦ C at 5 ◦ C/min. O2 TPD was operated in a He stream (25 mL/min) from 100 to 950 ◦ C at 5 ◦ C/min followed by an isotherm at 950 ◦ C for 1 h. The amount of desorbed O2 was quantified by executing the aforementioned program in a thermal gravimetric analyzer (TGA, PerkinElmer Pyris 1). The volatile portion was regarded as liberated O2 during O2 TPD. CH3 OH-TPD was performed by first saturating the sample with methanol at 60 ◦ C, which suppressed physisorbed methanol and oxidation reactions [25]. Pulses of methanol vapor in a He stream were repeatedly passed through a bubbler and a 6-port valve (VICI) design at room temperature. All tubing was heated to 100 ◦ C to prevent methanol condensation. After cooling and attaining a stable TCD baseline level, CH3 OH-TPD was carried out from ambient temperature to 450 ◦ C (5 ◦ C/min) in a He stream. To inspect oxygen evolution of methanol-exposed perovskite, a series of O2 -TPD was performed immediately after CH3 OH-TPD with the abovementioned O2 -TPD program. 2.3. Catalytic activity evaluation Catalytic performance was measured in a continuous fixed bed reactor (i.d. = 10 mm). The system outlet was connected inline to a gas chromatograph (GC, SRI 8610) equipped with a TCD, a methanizer, and a flame ionization detector (FID). Ultra-high purity He was served as the carrier gas. Two packed columns, 5 A˚ molecular sieve and Porapak Q, were employed to separate reactants and products. The sample was pretreated in a 10% O2 /N2 stream at 700 ◦ C for 1 h prior to the experiment. After cooling down in the same stream, the catalytic testing was conducted at desired temperature. Identified effluents included N2 , CO, CO2 , H2 , HCOH, and CH3 OH. Nitrogen served as the internal standard. All products excluding water were calculated relative to GC calibration standards. The amount of water was estimated based on the oxygen atom balance. Approximately 10 mg of catalyst with a 40–80 mesh size was used in each trial. To maintain an isothermal condition, the catalyst was diluted with 150 mg SiO2 (40–80 mesh). SiO2 was inert in each testing. A K-type thermocouple was placed in the middle of the catalyst bed to record the reaction temperature. Methanol was introduced into the system through a syringe pump (KDS-100) at a 1 mL/h injection rate. All tubing of the system was wrapped with heating tape and kept at 150 ◦ C to preclude condensation. The molar ratio of the feed for MPO was CH3 OH/O2 /N2 = 11/5/84 with a fixed residence time of 0.4 g catalyst h/mol of CH3 OH. To provide insights into the MPO mechanism, a set of experiments of methanol decomposition (MD, Eqs. (3) and (4)) and steam reforming (SRM, Eq. (5)) was conducted under the same reaction platforms as MPO. The
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Fig. 1. XRD patterns of LaMnO3 -based perovskites.
feed’s molar composition of MD was CH3 OH/N2 = 11/89; SRM, CH3 OH/H2 O/N2 = 6/10/84. CH3 OH → HCOH + H2 CH3 OH → CO + 2H2
H ◦ = +84 kJ/mol ◦
H = +91 kJ/mol
CH3 OH + H2 O → CO2 + 3H2
◦
H = +50 kJ/mol
(3) (4) (5)
All reported data had carbon and hydrogen atom mass balances within ±10% error, and mostly less than ±5%. Two or three trials were recorded after the first steady-state value (about 30 min after starting the inline GC system). These multiple trials were applied to estimate 95% confidence intervals. The methanol conversion was calculated as moles of methanol reacted divided by moles of methanol injected. The conversion of oxygen was calculated as the same way as that of methanol. The selectivity of product Xi was calculated as 100 × (moles of methanol converted to product Xi )/(moles of methanol converted). 3. Results and discussion 3.1. Physicochemical characterizations Fig. 1 shows the XRD patterns of perovskite catalysts. The index reflections of LaMn ([JCPDS: 35-1353]) were clearly defined, while diffractions of noble metal oxides were absent. Up to 5 mol% of Pd, Pt, or Rh could be implemented in the B-site position without damaging the perovskite framework. Another possible explanation may be the small cluster size of pgm oxides, which is below the detection limit (∼4 nm). A close inspection of XRD patterns within 32.5–33.5◦ showed that the peaks of precious metal-doped perovskites, especially LaMnPt, shifted slightly downward to lower 2. This indicates that increasing the B-site cation radius increased the d-spacing. Table 1 presents the surface areas, crystallite sizes, and elemental composition values of studies catalysts obtained by EDS analysis. Doping Pd, Pt, and Rh ions into LaMn generated positive effects for the first two criteria. LaMnPt showed the highest surface area and smallest particle size. This is probably because of Pt’s ionic radius, which is the largest among noble metals used in this study. According to Goldschmidt’s rule [37], LaMnPt should have the lowest tolerance factor, thereby yielding the highly disordered perovskite structure. This agrees with the shift of XRD peak of LaMnPt in Fig. 1. The elemental analysis obtained by EDS showed that the Mn-to-La ratios were all less than unity, ranging from 0.83 to 0.86. This trend can be correlated to the surface enrichment of La. As for contents of noble metal ions, LaMnPd and LaMnPt were closely matched to their stoichiometric ratios while LaMnRh was about half of its designated value.
Fig. 2 displays the XPS profiles of LaMnPd (Pd 3d), LaMnPt (Pt 4f), and LaMnRh (Rh 3d). For LaMnPd, two major peaks of oxidized Pd+2 at 336.7 (Pd 3d5/2 ) and 341.9 eV (Pd 3d3/2 ) were recognized [38,39]. The signals of both metallic Pd (335.2 eV) [40] and high-oxidation-state Pd+3 and Pd+4 (339.0 eV) [38,39] were insignificant. Therefore, it is highly probable that Pd ions exist in LaMnPd in the +2 state. The Pt 4f spectrum showed a doublet at 74.5 and 77.7 eV, known as the characteristic binding energies of Pt+4 [40,41]. The contributions of Pt+2 (72.9 eV) [42] and Pt0 (71.0 eV) [40] were negligible. That is, oxidic Pt species existed in LaMnPt in the +4 state. Regarding the Rh 3d profile, signals of Rh+3 ions appeared at 308.7 and 313.5 eV [43]. Metallic Rh (307.1 eV) [43] was undetectable. This emphasizes the presence of Rh+3 ions in LaMnRh. Fig. 3 shows the XPS patterns of La (3d), Mn (2p), and O (1s) of perovskites in this study. Two doublets at 830–840 and 845–860 eV were identified in the La 3d region. The former can be assigned to the index peak of La 3d5/2 ; the latter, La 3d3/2 . Both of these two are associated with the trivalent lanthanum oxide state [44]. The responses at ∼641.8 and 653.3 eV are corresponded to Mn 2p3/2 and Mn 2p1/2 levels [44]. Although it is difficult to differentiate Mn+2 (640.9 eV), Mn+3 (641.8 eV) and Mn+4 (642.5 eV) in the Mn 2p3/2 region [45], we may exclude Mn+2 ions due to the lack of satellite peak at ∼648.8 eV [46]. The O 1s state displays two main peaks at ∼529.1 and 530.7 eV. The former is known to be lattice oxygen ions in perovskite skeleton while the latter can be ascribed to adsorbed oxygen, OH− and CO3 −2 groups [25,47,48]. Generally, the shifts caused by noble metal substitution were insignificant, mainly within 1 eV. Table 2 summarizes the detailed peak positions and surface elemental contents. The surface compositions again displayed high concentrations of La, in agreement with EDS analysis. Fig. 4 compares the H2 -TPR results of LaMn-based perovskites. Each sample displayed two separated peaks, suggesting a consecutive reduction process. It is generally accepted that a complete reduction of Mn ions to metallic Mn does not happen [49,50]. Therefore, the first peak can be correlated to Mn+4 to Mn+3 while the second peak is associated with the deep reduction of Mn+3 to Mn+2 [51]. Some reduction of Mn+3 to Mn+2 at low temperature region cannot be ruled out [52]. To ascertain this, XPS and XRD characterizations of exhausted samples from TPR at 600 and 950 ◦ C are shown in Figs. S1 and S2 (see Supplementary data), respectively. The XPS of post TPR samples at 600 ◦ C displayed similar profiles as those of freshly prepared catalysts in Fig. 3. However, a downward shift to low energy level of Mn 2p3/2 could be observed for LaMnPd and LaMnPt. A shoulder at ∼639.6 eV was also identified. This implied the increase of Mn+2 concentrations after the first reduction step. The XRD patterns showed the characteristic peaks of La2 O3 and MnO while the reflection of LaMnO3 perovskite was no longer existed. This indicates the destruction of perovskite structure with lanthanum and manganese ions as +3 and +2 states at this stage. Both of the H2 consumption peaks shifted to lower temperatures after doping Pd, Pt, and Rh ions into LaMn. The first reduction peak declined following the trend of LaMn (425 ◦ C) > LaMnRh (322 ◦ C) > LaMnPt (310 ◦ C) > LaMnPd (224 ◦ C); the second, LaMn (855 ◦ C) > LaMnRh (833 ◦ C) > LaMnPd (821 ◦ C) ∼ LaMnPt (820 ◦ C). This indicates that catalytic reducibility can be enhanced by Bsite-ion substitution. Among them, LaMnPd is the most reducible catalyst. A probable explanation may be due to the small crystallites and disordered lattice structure [53,54] or hydrogen spillover [55–57]. The reduction of PdO and Rh2 O3 , both occur at approximately 100 ◦ C [55,58], were not observed on LaMnPd and LaMnRh. It is also possible that PtO2 reduction, which usually occurs below 0 ◦ C [59], was absent in LaMnPt. This implies well-embedded Pd, Pt, or Rh ions in perovskite structure.
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Table 1 Physicochemical properties of catalysts used in this study. Catalyst
SBET (m2 /g)
dp (nm)
La (%)a
Mn (%)a
LaMn LaMnPd LaMnPt LaMnRh
6.3 14.1 14.5 11.1
16.6 16.1 15.8 16.2
33.6 35.4 33.8 36.9
29.0 29.4 28.7 30.5
a
Pd (%)a
Pt (%)a
Rh (%)a
O (%)a
Mn/La
0.6
37.4 33.4 36.2 32.0
0.86 0.83 0.85 0.83
1.8 1.3
Pd/La
Rh/La
0.05
Obtained by EDS analysis.
Fig. 2. X-ray photoelectron spectra of (a) Pd 3d, (b) Pt 4f, and (c) Rh 3d of LaMnPd, LaMnPt, and LaMnRh, respectively.
Fig. 3. X-ray photoelectron spectra of (a) La 3d, (b) Mn 2p, and (c) O 1s of LaMnO3 -based perovskites.
Pt/La
0.04 0.02
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Table 2 Surface compositions estimated based on XPS analysis. LaMn
La 3d
LaMnPt
LaMnRh
Surface content (%)
Position (eV)
Surface content (%)
Position (eV)
Surface content (%)
Position (eV)
Surface content (%)
834.1 837.5
22.8 24.5 47.3
833.9 837.4
25.2 23.0 48.2
833.3 836.9
19.6 23.4 43.0
833.8 837.0
23.9 24.6 48.5
641.8 653.3
13.7 9.0 22.7
642.0 653.4
12.2 7.5 19.7
642.0 653.6
13.3 9.9 23.2
641.0 652.5
13.8 7.1 20.9
336.7 341.9
1.2 1.3 2.5 74.5 77.7
0.8 – 0.8 308.7 313.5
1.7 1.1 2.8
529.3 531.0
14.5 13.3 27.8
La total Mn 3p
LaMnPd
Position (eV)
Mn total Pd 3d Pd total Pt 4f Pt total Rh 3d Rh total O 1s O total
529.1 530.7
12.0 18.0 30.0
528.9 530.7
14.7 14.9 29.6
529.3 531.1
18.1 14.9 33.0
Fig. 5 shows the TPO analysis. This experiment was designed to assess the re-oxidation activity of reduced perovskites. The profile was composed of two portions: a major hump at 200–600 ◦ C and a shoulder at 600 ◦ C and above. The maximum re-oxidation temperature for LaMnPd was located at 482 ◦ C, while the other catalysts were at ∼388 ◦ C. LaMnPd consumed the highest amount of O2 due to the largest area of its TPO response. Moreover, the onset of LaMnPd was at about 108 ◦ C, which was lower than all perovskites. This indicates that LaMnPd possesses the greatest re-oxidation activity in the reduced state. Fig. 6 depicts the O2 -TPD profiles of freshly prepared samples. The onset temperatures of oxygen desorption increased with the order LaMnPt (495 ◦ C) < LaMnPd (508 ◦ C) ∼ LaMn (509 ◦ C) < LaMnRh (590 ◦ C). Moreover, the sum of desorbed oxygen increased in the order of LaMn < LaMnRh < LaMnPd < LaMnPt, as summarized in Table 3. This suggests that LaMnPt has greater oxygen mobility and capacity than the other catalysts.
Fig. 5. TPO of LaMnO3 -based perovskites.
Fig. 6. O2 -TPD of LaMnO3 -based perovskites. Fig. 4. H2 -TPR of LaMnO3 -based perovskites.
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Table 3 Oxygen desorbed from freshly prepared and post CH3 OH-TPD LaMnO3 -based perovskites. Catalyst
Amount of oxygen desorbed (mol/g catalyst) Freshly prepared
LaMn LaMnPd LaMnPt LaMnRh
Post CH3 OH-TPD
␣-O2
-O2
Total
␣-O2
-O2
Total
53.0 87.8 111.2 66.5
492.3 622.1 637.5 633.5
545.1 709.9 748.7 700.0
157.7 302.8 243.4 302.4
237.7 258.5 241.7 209.3
395.4 561.3 485.1 511.7
In general, the O2 -TPD signal can be categorized into low (␣O2 ) and high (-O2 ) regions, using 700–800 ◦ C as the demarcation interval [52,60]. This study uses 750 ◦ C to divide these two regions. ␣-O2 is known to be the weakly bonded oxygen species on the surface, while -O2 is assigned to the lattice oxygen in the perovskite skeleton. The former is responsible for combustion; the latter, partial oxidation [61]. Table 3 lists the amounts of ␣- and -O2 estimated by O2 -TPD signals. Again, LaMn possessed the least of these two oxygen species, while LaMnPt held the most. A close inspection showed a similar -O2 concentration in precious metal integrated perovskites. This may account for the B-site replacement, which improves the diffusivity of lattice oxygen from the bulk to the surface, and thereby liberating more -O2 than LaMn [51]. Fig. 7 illustrates the outcomes of CH3 OH-TPD. Two broad desorption peaks were identified at 50–150 ◦ C and 175–325 ◦ C. This corresponds well with earlier studies by Ozkan and collaborators [25,30], who investigated key desorption species based on their MS fragments. They assigned the former to Lewis-bound methanol adducts [25,30,62]; the latter, oxidative products, e.g. CO2 and H2 O [25,30]. An additional signal was detected at 325–375 ◦ C for LaMn. This indicates stronger chemisorbed species, such as carbon oxides or formaldehyde [30]. The difference of CH3 OH-TPD patterns between promoted and un-promoted perovskites again reveals the varying chemistry caused by B-site ion substitution. Fig. 8 shows the O2 -TPD after CH3 OH-TPD and Table 3 lists the amount of liberated oxygen. This study evaluates the oxygen activity and storage of methanol-reduced perovskites. Compared to Fig. 6, the onset of the TPD curve in Fig. 8 shifts downward to lower temperature and exhibits irregular evolving profile. This suggests an increase of surface inhomogeneity after methanol oxidation (perovskite reduction). This figure also reveals a significant growth of ␣-O2 accompanying decreases in -O2 . This indicates that the lattice oxygen (-O2 ) replenished surface chemisorbed
Fig. 7. CH3 OH-TPD of LaMnO3 -based perovskites.
oxygen (␣-O2 ), part of which was spent in methanol oxidation during CH3 OH-TPD [25]. Among the catalysts, LaMnPd liberated the highest amounts of ␣-, -, and overall oxygen. Presumably, it should have the greatest oxygen mobility and availability in the reduced form caused by oxidation of chemisorbed methanol.
Fig. 8. O2 -TPD of LaMnO3 -based perovskites following CH3 OH-TPD.
Fig. 9. (a) Methanol and (b) oxygen conversions as a function of temperature.
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Fig. 10. Product selectivity of (a) HCOH, (b) CO2 , (c) CO, (d) H2 , and (e) H2 O as a function of methanol conversion.
3.2. Catalytic performance Fig. 9 depicts the temperature-dependence of methanol and oxygen conversions, revealing a strong, positive correlation between these two. The conversions increased steeply within narrow temperature intervals, and then steadily increased with growing temperature. The initiation temperature decreased as LaMn (260–310 ◦ C) > LaMnPd (160–210 ◦ C) ∼ LaMnRh (160–180 ◦ C). As for LaMnPt, the initiation interval was unclear, possibly at lower temperatures than the others. Oxygen was nearly depleted at 285 ◦ C for LaMnRh, 335 ◦ C for LaMnPd, and 360 ◦ C for LaMnPt and LaMn, respectively. At these points, methanol conversions declined as follows: LaMnPt (68%) > LaMnPd (56%) > LaMn (51%) ∼ LaMnRh (50%). LaMnPt was the most active at low (<175 ◦ C) and high temperatures (>300 ◦ C). LaMn was the least reactive in almost all the trials. The S-shaped dependence of methanol conversion on temperature implies a stepwise reaction pathway. Based on the feed stoichiometry, a 5/11 molar ratio of O2 -to-CH3 OH indicates that a total consumption of oxygen could yield ∼91% methanol conversion by following the partial oxidation route (Eq. (1)). This was not the case, as the maximum methanol conversion for each catalyst was less than 70%. Moreover, a detailed survey of ratios of spent O2 divided by converted methanol results mainly in the range of 1.2–0.6 from 135 to 360 ◦ C. In another words, different amounts of
O2 were converted per mole of expensed CH3 OH, and decreased with increasing temperature. Fig. 10 illustrates product selectivities as a function of methanol conversion. To extrapolate this data, the selectivity patterns were divided into low (<20%), moderate (20–45%), and high (>45%) conversion regions. Grossly speaking, HCOH increased with enhancing methanol conversion. The highest HCOH selectivity (∼22%) was achieved over LaMnPd at 60% conversion, where HCOH selectivity decreased as follows: LaMnPd > LaMnRh > LaMnPt > LaMn. In the moderate conversion region, LaMnRh became the least selective to HCOH while the other catalysts still followed this order. In the low conversion region, trace amounts of HCOH formed over each catalyst. With increasing conversion, CO2 declined from 31 to 17% for LaMnPt and from 21 to 12% for LaMnPd. LaMn displayed a reverse trend, increased from 12 to 19% then level off. LaMnRh exhibited an inverted U-shape curve. Its highest CO2 selectivity was 28% at a methanol conversion of about 36%. Comparing CO profiles shows that LaMn produced greater CO than promoted perovskites, declining from 18 to 5%. Less than 2% CO was formed by LaMnPd, LaMnPt, and LaMnRh. Interestingly, LaMnPd and LaMnRh displayed an upward and then downward trend. LaMnPt showed a constant CO selectivity (∼0.5%) at high conversions. Precious metal doped perovskites generated two to three times as much H2 as LaMn. Not until methanol conversion exceeded
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Fig. 11. (a) Methanol conversion and (b) HCOH (solid), CO (dash), and H2 (dot) selectivities as a function of temperature in the absence of oxygen.
40% could LaMn produce detectable H2 . In contrast, LaMn yielded the highest H2 O in moderate and high conversion regions. At low methanol conversions, LaMnRh and LaMnPd showed higher H2 O selectivity than LaMn and LaMnPt. The differences of H2 selectivities were insignificant for promoted catalysts at a methanol conversion below 45%, while H2 O selectivities were quite close at 50% and above. Fig. 11 shows the methanol conversion and HCOH and CO selectivities as a function of temperature in oxygen-free environments. Precious metal incorporated perovskites displayed growing methanol conversions as the temperature increased, whereas LaMn remained inert. All catalysts displayed less than 5% conversions. Among them, LaMnPd and LaMnRh were more active than LaMnPt. Identified products were HCOH, CO, and H2 . At all temperatures, HCOH selectivity was high with trace CO detected at 300 ◦ C and above. CO receded following the order of LaMnPd > LaMnPt > LaMnRh. Fig. 12 depicts the outcomes of methanol steam reforming. Again, LaMn was inactive while other catalysts displayed low activities. The activities of perovskites were very close with respect to their performances in methanol decomposition. The major products were HCOH and H2 ; trace CO2 was generated at high temperatures. According to Eq. (5), converting a mole of methanol through steam reforming should yield a mixture of CO2 and H2 at a 1/3 ratio. This suggests that methanol decomposition still dominated the system under steam reforming conditions, yielding HCOH and H2 as primary products. In another words, methanol steam reforming was trivial under MPO conditions.
291
Fig. 12. (a) Methanol conversion and (b) HCOH (solid), CO2 (dash), and H2 (dot) selectivities as a function of temperature in methanol steam reforming.
As methanol conversion increased, relatively less oxygen was available in the moderate conversion region. According to the stoichiometric values of methanol-to-oxygen, the system should gradually transit from combustions to partial oxidation. This is supported by growing HCOH and H2 together with declining CO2 and H2 O. Surprisingly, LaMnRh and LaMn exhibited increasing CO2 in this regime. Fig. 13 shows that the CO2 generation could be attributed to various reactions, including methanol partial oxidation to CO2 and H2 , HCOH oxidation, CO oxidation, water-gas shift, and steam reforming. This effectively rules out the influences of steam reforming based on the observation of Figs. 11 and 12. LaMn is highly active in CO oxidation [64]. Moreover, a closely related molar ratio of (increased CO2 )/(lost CO), which nearly equals to unity, suggests that CO oxidation should be more important than the remaining reactions. This is not the case for LaMnRh: significant low CO selectivity implied less CO could be converted to CO2 via CO oxidation or water-gas shift. Therefore, methanol and HCOH oxidation may be prominent for LaMnRh at this stage. Note that methanol dehydrogenation may proceed under oxidative environments, thereby contributing a certain amounts of HCOH and H2 . Oxygen was limited or even depleted in high methanol conversions. HCOH and H2 accumulated while CO2 and H2 O declined in this region. That is, contributions of partial oxidation and dehydrogenation of methanol became more and more important. Note that promoted perovskites exhibited different CO trends: declining CO for LaMnPd and LaMnRh, but constant CO for LaMnPt. The CO distribution pattern in Fig. 11 shows that LaMnPd produced the highest CO under oxygen-free environments. This suggests appreciable CO oxidation reactivity of LaMnPd.
3.3. Reaction mechanism 3.4. Influences of surface chemistry and oxygen activity Fig. 13 presents possible reactions occurring in methanol oxidation [25,26,63]. At low methanol conversions, oxygen is still plentiful at this stage. Therefore, full or incomplete combustion of methanol should be prominent. Thus, CO2 , CO, and H2 O are essential products. However, these two reactions evolved diversely over tested catalysts. LaMn exhibited almost twice as much CO as CO2 , suggesting incomplete combustion was favored. In contrast, LaMnPd, LaMnPt, and LaMnRh exhibited great amounts of CO2 , indicating full combustion.
The results above confirm the relationships between perovskite’s physicochemical properties and catalytic behaviors. The substitution of Pt ions into LaMnO3 results in a more defective surface than other perovskites. This leads LaMnPt to possess the highest surface area and lowest O2 desorption temperature. Furthermore, the electronic state of Pt+4 induced a plethora of surface cations. This allows LaMnPt to host excess chemisorbed oxygen species, i.e., ␣-O2 , to fulfill the electroneutrality. ␣-O2 is very active
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Fig. 13. Reaction pathways of methanol conversion under oxidative environments.
Fig. 14. Redox mechanism of MPO over perovskite catalysts.
in combustion [51,61], yielding the highest activity and CO2 selectivity for LaMnPt in low reaction temperatures. The reactivity of ␣-O2 can probably be sustained when O2 is still available, resulting in great methanol conversions of LaMnPt at high temperatures. The Pd+2 ion plays a different role in LaMnO3 . It substantially improves the reducibility and re-oxidation activity. The O2 -TPD after CH3 OH-TPD of LaMnPd also showed the lowest desorption temperature and greatest O2 capacity among all catalysts. This emphasizes the redox chemistry of LaMnPd. Fig. 14 shows the plausible redox cycle on perovskites. This mechanism is critical to the HCOH production in methanol partial oxidation [65]. ␣-O2 seems to play a central role in this mechanism. This is because HCOH was formed at low methanol conversion region, where operating temperatures were quite low. Under this circumstance, influences of -O2 , which requires greater thermal inputs than ␣-O2 to be activated, are limited. The great oxygen storage and mobility of methanol-reduced LaMnPd suggests that lattice oxygen can be efficiently transferred from the bulk (-O2 ) to the surface (␣-O2 ), and subsequently participated in the redox cycle. Therefore, substantial high selectivity of HCOH can be obtained over LaMnPd. The Rh replaced LaMnO3 seems to be the least reactive and reducible of the promoted catalysts. Due to the relatively close radius of Rh to Mn compared to the pairs of Pd to Mn and Pt to Mn,
LaMnRh should have the highest Goldschmidt tolerance factor. In addition, the electronic state of Rh+3 , which equals to the designated charge of B-site ions in LaMnO3 , could yield a more stable phase for LaMnRh than LaMnPt and LaMnPd.
4. Conclusions Substitution of trace Pd, Pt, and Rh ions into LaMnO3 can significantly alter the physicochemical properties of perovskite. This doping improves catalyst reducibility, oxygen mobility, and oxygen storage. All these factors allow promoted perovskites to have greater activity than untainted catalyst. Moreover, the catalytic behavior in MPO showed different reaction pathways between promoted and un-promoted perovskites. Methanol and CO combustions are major pathways with minor contributions of partial oxidation over pure LaMnO3 . LaMnPd shows surpassing reactivity of partial oxidation to HCOH compared to other catalysts. This trend is correlated to its redox property. Besides, LaMnPd also displays attracting reactivity in CO oxidation. LaMnPt mediates the reaction pathway mainly toward methanol combustion. This is likely due to the great amount of combustionprompt ␣-O2 on its surface. LaMnRh seems to be the least reactive
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of the promoted catalysts in this study, possibly because of its low reducibility and oxygen activity. The current work shows that it is possible to manipulate the reaction pathways of MPO with selected promoters in perovskites. Future research should focus on a molecular-level understanding of the surface chemistry and reaction mechanism to facilitate the design of LaMnO3 -based catalysts. Acknowledgements Valuable suggestions of Prof. C.-T. Yeh (Yuan Ze University) and anonymous reviewers are gratefully appreciated. This work was sponsored by the National Science Council (Taiwan) under grant #NSC 98-2218-E-155-006. Appendix A. Supplementary data
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Methanol partial oxidation over palladium-, platinum-, and rhodium-integrated LaMnO3 perovskites Chia-Liang Li, Yu-Chuan Lin ∗ Department of Chemical Engineering and Materials Science, Yuan Ze Fuel Cell Center, Yuan Ze University, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 11 May 2011 Received in revised form 21 July 2011 Accepted 23 July 2011 Available online 30 July 2011 Keywords: Perovskite Noble metals Methanol Oxidation Redox
a b s t r a c t This study investigates methanol partial oxidation over LaMnO3 and LaMn0.95 B0.05 O3 (B = Pd, Pt, and Rh) perovskites. Their surface and bulk properties were characterized by appropriate physicochemical techniques including BET surface area measurement, XRD, EDS, XPS, H2 -TPR, TPO, O2 -TPD, and CH3 OHTPD. Results show significant differences in reducibility and oxygen activity between promoted and untainted LaMnO3 . Promoted perovskites showed higher reactivity than pure LaMnO3 . Results suggest that a redox pathway enhances partial oxidation to formaldehyde while oxygen activity, particularly surface chemisorbed oxygen, plays a central role in combustion. This study also reveals a plausible reaction mechanism of methanol partial oxidation over LaMnO3 -based perovskites. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Perovskite (ABO3 ) has attracted a lot of attention in the last few decades [1]. It is a versatile catalyst with numerous applications in fields such as CO oxidation [2], pollution abatement [3], and photocatalysis [4]. This is due to perovskite’s unique crystalline structure, nonstoichiometric chemistry, and adsorption properties [1]. In some cases, perovskites show superior performances and stability than noble metal supported catalysts, such as in automotive emission control [5–7]. Daihatsu Motors recently announced a successful industrial practice by implementing a series of platinum group metal (pgm) promoted perovskites in exhaust converters [8–12]. The self-regeneration behavior of pgm-doped perovskites in oxidative and reductive environments provides them with higher activity and longer durability than conventional noble metal catalysts. This breakthrough has earned pgm-doped perovskites the name “intelligent catalysts [13].” Another important application of perovskite is the partial oxidation of hydrocarbons, especially light alkanes. The partial oxidation of methane is mainly used for syngas preparation [1,14,15]. The chemistry of perovskite’s A-site and B-site elements plays a key role in varying catalytic performance and product distribution. Slagtern and Olsbye investigated the B-site effects of LaMO3 (M = Rh, Ni, and Co) and attributed the outstanding reactivity and durability to highly dispersed Rh ions [16]. Lago et al. studied A-site effects
∗ Corresponding author. Tel.: +886 3 463 8800x3554; fax: +886 3 455 9373. E-mail address: [email protected] (Y.-C. Lin). 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.07.026
by a series of LnCoO3 (Ln = La, Pr, Nd, Sm, and Gd). The activity and syngas selectivity can be improved by increasing the lanthanum ionic radius [17]. Ethane partial oxidation (also called oxidative dehydrogenation) is a potential replacement for steam cracking in ethylene production [18,19]. Donsi et al. extensively worked on this field using LaMnO3 [20–22] and lately by Pt/LaMnO3 [23]. They proposed that perovskite is intrinsically active because ethyl species, a major intermediate of ethylene, can be formed on its surface [20]. Conner et al. investigated propane partial oxidation to acetaldehyde, acrolein, and methanol over complex perovskites, e.g., Ba2 (Bi2/3 1/3 Te)O6 ( represents a vacancy) [24]. They suggested that the synergistic effects of A-site vacancies and B-sites cations were indispensible to its catalytic performance. Compared to alkanes partial oxidation, however, relatively few efforts have been dedicated to methanol partial oxidation (MPO) using perovskites [25,26]. MPO can be applied to either formaldehyde (Eq. (1)) or hydrogen (Eq. (2)) production. CH3 OH + 1/2O2 → HCOH + H2 O H ◦ = −159 kJ/mol CH3 OH + 1/2O2 → 2H2 + CO2
H ◦ = −192 kJ/mol
(1) (2)
De and Balasubramanian used SrVO3 in MPO and showed that the lattice oxygen in perovskite’s framework can assist methanol conversion [27]. The catalyst is highly selective to formaldehyde (90%) at moderate temperature (320 ◦ C), but the activity is low (∼44% conversion). Kuhn and Ozkan studied Sr- and Co-doped LaFeO3 in MPO for hydrogen-rich gas production [25]. The same group also presents a detailed survey of oxygen transfer kinetics and how B-site elements mediate reaction pathways [28–30].
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Galenda et al. used two methods of La4/5 Sr1/5 Ga4/5 Fe1/5 O3 perovskite synthesis and employed them in MPO [31]. Methanol combustion, partial oxidation, and decomposition could proceed simultaneously. In addition, different perovskite synthesis methods can vary the product distribution. This is due to the differences in catalytic surface caused by preparation [31]. Our group has recently reported that doping trace palladium into LaMO3 (M = Mn, Fe, and Co) can achieve attracting conversion (80% and up) and formaldehyde selectivity (90%) in MPO [32]. This enhancement is correlated to the improved reducibility and oxygen mobility of Pd-doped perovskites, which in turn promote formaldehyde yield through MPO and methanol dehydration. This study investigates the effects of B-site-ion replacement by pgms (i.e., Pd, Pt, and Rh) in MPO. LaMnO3 was selected as the host because of its attracting activity and oxygen-excess nature [32,33]. A survey of the physical and chemical properties of these catalysts reveals that characteristics of perovskites are closely related to their catalytic behaviors. Moreover, the reaction scheme under partial oxidation environments was elucidated. This study is a continuation of our previous work on lanthanum-transition metal perovskites for MPO. 2. Experimental 2.1. Catalyst preparation A series of LaMnO3 -based perovskites, including LaMnO3 , LaMn0.95 Pd0.05 O3 , LaMn0.95 Pt0.05 O3 , and LaMn0.95 Rh0.05 O3 were synthesized by a sol-gel method [32,34]. For convenience, LaMnO3 and LaMn0.95 B0.05 O3 (B = Pd, Pt, and Rh) are henceforth referred to as LaMn and LaMnB (e.g., LaMnPd), respectively. Stoichiometric proportions of metal acetylacetonates (acac) (La-, Mn-, Pd-, Pt-, and Rh-(acac)3 ) were dissolved in a 1/4 volumetric ratio mixture of ethylene glycol and methanol (260 mL). Approximately 10 mmol of A-site and B-site metal ions were used. The solution was subjected to vigorous stirring for 1 h. Excess solvents were subsequently removed in a rotary evaporator at 80 ◦ C. The remaining paste was dried in air at 150 ◦ C for 10 h using a heating rate of 10 ◦ C/min. The dried sample was then calcined at the same heating rate from 150 to 700 ◦ C for 2 h. 2.2. Catalyst characterization Surface area was estimated by the Brunauer, Emmet, and Teller (BET) method, using an automated N2 adsorption apparatus (Micromeritics, ASAP 2010). Powder X-ray diffraction (XRD) patterns were recorded using a Shimadzu Labx XRD-6000 with Cu K␣ radiation (0.15418 nm). Scans were performed at a scanning rate of 4◦ /min in 20–80◦ angle range (2). The voltage and current were set to 40 kV and 30 mA. Crystallite sizes were measured using the Scherrer relation. The instrumental broadening effect was found to be negligible using the Warren’s correction [35]. Surface elemental analysis was carried out using an energy dispersive spectrometer (EDS, Oxford ISIS 310) attached to the scanning electron microscopy (SEM, JEOL JSM-5600). The INCA Xsight software package (Oxford Instrument) was used to quantify surface compositions. Three trials with different locations were averaged for each sample. The acquisition time was 10 min per location. The SEM accelerating voltage was 10 kV. The scanned areas were approximately 0.12 mm × 0.09 mm. Elements including O, La, Mn, Pd, Pt, and Rh were determined by EDS from their K-lines. X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific K-Alpha system equipped with a 180◦ hemispherical sector analyzer and Al monochromator (Al K␣ X-ray
285
sources, 1486.6 eV). The X-ray spot size was 400 m. The C 1s signal (284.4 eV) of adventitious carbon was employed for energy shift correction. Hydrogen temperature programmed reduction (H2 -TPR), temperature programmed oxidation (TPO), oxygen temperature programmed desorption (O2 -TPD), and methanol temperature programmed desorption (CH3 OH-TPD) were all conducted in the same system [32,36]. The system consisted of a U-shaped cell, a temperature-controlled furnace, and a thermal conductivity detector (TCD). Approximately 100 mg of sample was used per trial. Before the experiment, each sample was pretreated in an Ar stream (30 mL/min) at 150 ◦ C (10 ◦ C/min) for 30 min to remove physisorbed water. In H2 -TPR analysis, a 7.4% H2 /Ar stream (25 mL/min) was used to reduce the sample by heating up to 900 ◦ C at 5 ◦ C/min. For TPO study, the sample was pre-reduced in a 7.4% H2 /Ar stream (25 mL/min) from room temperature to 450 ◦ C at 5 ◦ C/min. After cooling and purging with Ar, the sample was re-oxidized in a 2.5% O2 /He stream (30 mL/min) and ramped to 900 ◦ C at 5 ◦ C/min. O2 TPD was operated in a He stream (25 mL/min) from 100 to 950 ◦ C at 5 ◦ C/min followed by an isotherm at 950 ◦ C for 1 h. The amount of desorbed O2 was quantified by executing the aforementioned program in a thermal gravimetric analyzer (TGA, PerkinElmer Pyris 1). The volatile portion was regarded as liberated O2 during O2 TPD. CH3 OH-TPD was performed by first saturating the sample with methanol at 60 ◦ C, which suppressed physisorbed methanol and oxidation reactions [25]. Pulses of methanol vapor in a He stream were repeatedly passed through a bubbler and a 6-port valve (VICI) design at room temperature. All tubing was heated to 100 ◦ C to prevent methanol condensation. After cooling and attaining a stable TCD baseline level, CH3 OH-TPD was carried out from ambient temperature to 450 ◦ C (5 ◦ C/min) in a He stream. To inspect oxygen evolution of methanol-exposed perovskite, a series of O2 -TPD was performed immediately after CH3 OH-TPD with the abovementioned O2 -TPD program. 2.3. Catalytic activity evaluation Catalytic performance was measured in a continuous fixed bed reactor (i.d. = 10 mm). The system outlet was connected inline to a gas chromatograph (GC, SRI 8610) equipped with a TCD, a methanizer, and a flame ionization detector (FID). Ultra-high purity He was served as the carrier gas. Two packed columns, 5 A˚ molecular sieve and Porapak Q, were employed to separate reactants and products. The sample was pretreated in a 10% O2 /N2 stream at 700 ◦ C for 1 h prior to the experiment. After cooling down in the same stream, the catalytic testing was conducted at desired temperature. Identified effluents included N2 , CO, CO2 , H2 , HCOH, and CH3 OH. Nitrogen served as the internal standard. All products excluding water were calculated relative to GC calibration standards. The amount of water was estimated based on the oxygen atom balance. Approximately 10 mg of catalyst with a 40–80 mesh size was used in each trial. To maintain an isothermal condition, the catalyst was diluted with 150 mg SiO2 (40–80 mesh). SiO2 was inert in each testing. A K-type thermocouple was placed in the middle of the catalyst bed to record the reaction temperature. Methanol was introduced into the system through a syringe pump (KDS-100) at a 1 mL/h injection rate. All tubing of the system was wrapped with heating tape and kept at 150 ◦ C to preclude condensation. The molar ratio of the feed for MPO was CH3 OH/O2 /N2 = 11/5/84 with a fixed residence time of 0.4 g catalyst h/mol of CH3 OH. To provide insights into the MPO mechanism, a set of experiments of methanol decomposition (MD, Eqs. (3) and (4)) and steam reforming (SRM, Eq. (5)) was conducted under the same reaction platforms as MPO. The
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Fig. 1. XRD patterns of LaMnO3 -based perovskites.
feed’s molar composition of MD was CH3 OH/N2 = 11/89; SRM, CH3 OH/H2 O/N2 = 6/10/84. CH3 OH → HCOH + H2 CH3 OH → CO + 2H2
H ◦ = +84 kJ/mol ◦
H = +91 kJ/mol
CH3 OH + H2 O → CO2 + 3H2
◦
H = +50 kJ/mol
(3) (4) (5)
All reported data had carbon and hydrogen atom mass balances within ±10% error, and mostly less than ±5%. Two or three trials were recorded after the first steady-state value (about 30 min after starting the inline GC system). These multiple trials were applied to estimate 95% confidence intervals. The methanol conversion was calculated as moles of methanol reacted divided by moles of methanol injected. The conversion of oxygen was calculated as the same way as that of methanol. The selectivity of product Xi was calculated as 100 × (moles of methanol converted to product Xi )/(moles of methanol converted). 3. Results and discussion 3.1. Physicochemical characterizations Fig. 1 shows the XRD patterns of perovskite catalysts. The index reflections of LaMn ([JCPDS: 35-1353]) were clearly defined, while diffractions of noble metal oxides were absent. Up to 5 mol% of Pd, Pt, or Rh could be implemented in the B-site position without damaging the perovskite framework. Another possible explanation may be the small cluster size of pgm oxides, which is below the detection limit (∼4 nm). A close inspection of XRD patterns within 32.5–33.5◦ showed that the peaks of precious metal-doped perovskites, especially LaMnPt, shifted slightly downward to lower 2. This indicates that increasing the B-site cation radius increased the d-spacing. Table 1 presents the surface areas, crystallite sizes, and elemental composition values of studies catalysts obtained by EDS analysis. Doping Pd, Pt, and Rh ions into LaMn generated positive effects for the first two criteria. LaMnPt showed the highest surface area and smallest particle size. This is probably because of Pt’s ionic radius, which is the largest among noble metals used in this study. According to Goldschmidt’s rule [37], LaMnPt should have the lowest tolerance factor, thereby yielding the highly disordered perovskite structure. This agrees with the shift of XRD peak of LaMnPt in Fig. 1. The elemental analysis obtained by EDS showed that the Mn-to-La ratios were all less than unity, ranging from 0.83 to 0.86. This trend can be correlated to the surface enrichment of La. As for contents of noble metal ions, LaMnPd and LaMnPt were closely matched to their stoichiometric ratios while LaMnRh was about half of its designated value.
Fig. 2 displays the XPS profiles of LaMnPd (Pd 3d), LaMnPt (Pt 4f), and LaMnRh (Rh 3d). For LaMnPd, two major peaks of oxidized Pd+2 at 336.7 (Pd 3d5/2 ) and 341.9 eV (Pd 3d3/2 ) were recognized [38,39]. The signals of both metallic Pd (335.2 eV) [40] and high-oxidation-state Pd+3 and Pd+4 (339.0 eV) [38,39] were insignificant. Therefore, it is highly probable that Pd ions exist in LaMnPd in the +2 state. The Pt 4f spectrum showed a doublet at 74.5 and 77.7 eV, known as the characteristic binding energies of Pt+4 [40,41]. The contributions of Pt+2 (72.9 eV) [42] and Pt0 (71.0 eV) [40] were negligible. That is, oxidic Pt species existed in LaMnPt in the +4 state. Regarding the Rh 3d profile, signals of Rh+3 ions appeared at 308.7 and 313.5 eV [43]. Metallic Rh (307.1 eV) [43] was undetectable. This emphasizes the presence of Rh+3 ions in LaMnRh. Fig. 3 shows the XPS patterns of La (3d), Mn (2p), and O (1s) of perovskites in this study. Two doublets at 830–840 and 845–860 eV were identified in the La 3d region. The former can be assigned to the index peak of La 3d5/2 ; the latter, La 3d3/2 . Both of these two are associated with the trivalent lanthanum oxide state [44]. The responses at ∼641.8 and 653.3 eV are corresponded to Mn 2p3/2 and Mn 2p1/2 levels [44]. Although it is difficult to differentiate Mn+2 (640.9 eV), Mn+3 (641.8 eV) and Mn+4 (642.5 eV) in the Mn 2p3/2 region [45], we may exclude Mn+2 ions due to the lack of satellite peak at ∼648.8 eV [46]. The O 1s state displays two main peaks at ∼529.1 and 530.7 eV. The former is known to be lattice oxygen ions in perovskite skeleton while the latter can be ascribed to adsorbed oxygen, OH− and CO3 −2 groups [25,47,48]. Generally, the shifts caused by noble metal substitution were insignificant, mainly within 1 eV. Table 2 summarizes the detailed peak positions and surface elemental contents. The surface compositions again displayed high concentrations of La, in agreement with EDS analysis. Fig. 4 compares the H2 -TPR results of LaMn-based perovskites. Each sample displayed two separated peaks, suggesting a consecutive reduction process. It is generally accepted that a complete reduction of Mn ions to metallic Mn does not happen [49,50]. Therefore, the first peak can be correlated to Mn+4 to Mn+3 while the second peak is associated with the deep reduction of Mn+3 to Mn+2 [51]. Some reduction of Mn+3 to Mn+2 at low temperature region cannot be ruled out [52]. To ascertain this, XPS and XRD characterizations of exhausted samples from TPR at 600 and 950 ◦ C are shown in Figs. S1 and S2 (see Supplementary data), respectively. The XPS of post TPR samples at 600 ◦ C displayed similar profiles as those of freshly prepared catalysts in Fig. 3. However, a downward shift to low energy level of Mn 2p3/2 could be observed for LaMnPd and LaMnPt. A shoulder at ∼639.6 eV was also identified. This implied the increase of Mn+2 concentrations after the first reduction step. The XRD patterns showed the characteristic peaks of La2 O3 and MnO while the reflection of LaMnO3 perovskite was no longer existed. This indicates the destruction of perovskite structure with lanthanum and manganese ions as +3 and +2 states at this stage. Both of the H2 consumption peaks shifted to lower temperatures after doping Pd, Pt, and Rh ions into LaMn. The first reduction peak declined following the trend of LaMn (425 ◦ C) > LaMnRh (322 ◦ C) > LaMnPt (310 ◦ C) > LaMnPd (224 ◦ C); the second, LaMn (855 ◦ C) > LaMnRh (833 ◦ C) > LaMnPd (821 ◦ C) ∼ LaMnPt (820 ◦ C). This indicates that catalytic reducibility can be enhanced by Bsite-ion substitution. Among them, LaMnPd is the most reducible catalyst. A probable explanation may be due to the small crystallites and disordered lattice structure [53,54] or hydrogen spillover [55–57]. The reduction of PdO and Rh2 O3 , both occur at approximately 100 ◦ C [55,58], were not observed on LaMnPd and LaMnRh. It is also possible that PtO2 reduction, which usually occurs below 0 ◦ C [59], was absent in LaMnPt. This implies well-embedded Pd, Pt, or Rh ions in perovskite structure.
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287
Table 1 Physicochemical properties of catalysts used in this study. Catalyst
SBET (m2 /g)
dp (nm)
La (%)a
Mn (%)a
LaMn LaMnPd LaMnPt LaMnRh
6.3 14.1 14.5 11.1
16.6 16.1 15.8 16.2
33.6 35.4 33.8 36.9
29.0 29.4 28.7 30.5
a
Pd (%)a
Pt (%)a
Rh (%)a
O (%)a
Mn/La
0.6
37.4 33.4 36.2 32.0
0.86 0.83 0.85 0.83
1.8 1.3
Pd/La
Rh/La
0.05
Obtained by EDS analysis.
Fig. 2. X-ray photoelectron spectra of (a) Pd 3d, (b) Pt 4f, and (c) Rh 3d of LaMnPd, LaMnPt, and LaMnRh, respectively.
Fig. 3. X-ray photoelectron spectra of (a) La 3d, (b) Mn 2p, and (c) O 1s of LaMnO3 -based perovskites.
Pt/La
0.04 0.02
288
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Table 2 Surface compositions estimated based on XPS analysis. LaMn
La 3d
LaMnPt
LaMnRh
Surface content (%)
Position (eV)
Surface content (%)
Position (eV)
Surface content (%)
Position (eV)
Surface content (%)
834.1 837.5
22.8 24.5 47.3
833.9 837.4
25.2 23.0 48.2
833.3 836.9
19.6 23.4 43.0
833.8 837.0
23.9 24.6 48.5
641.8 653.3
13.7 9.0 22.7
642.0 653.4
12.2 7.5 19.7
642.0 653.6
13.3 9.9 23.2
641.0 652.5
13.8 7.1 20.9
336.7 341.9
1.2 1.3 2.5 74.5 77.7
0.8 – 0.8 308.7 313.5
1.7 1.1 2.8
529.3 531.0
14.5 13.3 27.8
La total Mn 3p
LaMnPd
Position (eV)
Mn total Pd 3d Pd total Pt 4f Pt total Rh 3d Rh total O 1s O total
529.1 530.7
12.0 18.0 30.0
528.9 530.7
14.7 14.9 29.6
529.3 531.1
18.1 14.9 33.0
Fig. 5 shows the TPO analysis. This experiment was designed to assess the re-oxidation activity of reduced perovskites. The profile was composed of two portions: a major hump at 200–600 ◦ C and a shoulder at 600 ◦ C and above. The maximum re-oxidation temperature for LaMnPd was located at 482 ◦ C, while the other catalysts were at ∼388 ◦ C. LaMnPd consumed the highest amount of O2 due to the largest area of its TPO response. Moreover, the onset of LaMnPd was at about 108 ◦ C, which was lower than all perovskites. This indicates that LaMnPd possesses the greatest re-oxidation activity in the reduced state. Fig. 6 depicts the O2 -TPD profiles of freshly prepared samples. The onset temperatures of oxygen desorption increased with the order LaMnPt (495 ◦ C) < LaMnPd (508 ◦ C) ∼ LaMn (509 ◦ C) < LaMnRh (590 ◦ C). Moreover, the sum of desorbed oxygen increased in the order of LaMn < LaMnRh < LaMnPd < LaMnPt, as summarized in Table 3. This suggests that LaMnPt has greater oxygen mobility and capacity than the other catalysts.
Fig. 5. TPO of LaMnO3 -based perovskites.
Fig. 6. O2 -TPD of LaMnO3 -based perovskites. Fig. 4. H2 -TPR of LaMnO3 -based perovskites.
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289
Table 3 Oxygen desorbed from freshly prepared and post CH3 OH-TPD LaMnO3 -based perovskites. Catalyst
Amount of oxygen desorbed (mol/g catalyst) Freshly prepared
LaMn LaMnPd LaMnPt LaMnRh
Post CH3 OH-TPD
␣-O2
-O2
Total
␣-O2
-O2
Total
53.0 87.8 111.2 66.5
492.3 622.1 637.5 633.5
545.1 709.9 748.7 700.0
157.7 302.8 243.4 302.4
237.7 258.5 241.7 209.3
395.4 561.3 485.1 511.7
In general, the O2 -TPD signal can be categorized into low (␣O2 ) and high (-O2 ) regions, using 700–800 ◦ C as the demarcation interval [52,60]. This study uses 750 ◦ C to divide these two regions. ␣-O2 is known to be the weakly bonded oxygen species on the surface, while -O2 is assigned to the lattice oxygen in the perovskite skeleton. The former is responsible for combustion; the latter, partial oxidation [61]. Table 3 lists the amounts of ␣- and -O2 estimated by O2 -TPD signals. Again, LaMn possessed the least of these two oxygen species, while LaMnPt held the most. A close inspection showed a similar -O2 concentration in precious metal integrated perovskites. This may account for the B-site replacement, which improves the diffusivity of lattice oxygen from the bulk to the surface, and thereby liberating more -O2 than LaMn [51]. Fig. 7 illustrates the outcomes of CH3 OH-TPD. Two broad desorption peaks were identified at 50–150 ◦ C and 175–325 ◦ C. This corresponds well with earlier studies by Ozkan and collaborators [25,30], who investigated key desorption species based on their MS fragments. They assigned the former to Lewis-bound methanol adducts [25,30,62]; the latter, oxidative products, e.g. CO2 and H2 O [25,30]. An additional signal was detected at 325–375 ◦ C for LaMn. This indicates stronger chemisorbed species, such as carbon oxides or formaldehyde [30]. The difference of CH3 OH-TPD patterns between promoted and un-promoted perovskites again reveals the varying chemistry caused by B-site ion substitution. Fig. 8 shows the O2 -TPD after CH3 OH-TPD and Table 3 lists the amount of liberated oxygen. This study evaluates the oxygen activity and storage of methanol-reduced perovskites. Compared to Fig. 6, the onset of the TPD curve in Fig. 8 shifts downward to lower temperature and exhibits irregular evolving profile. This suggests an increase of surface inhomogeneity after methanol oxidation (perovskite reduction). This figure also reveals a significant growth of ␣-O2 accompanying decreases in -O2 . This indicates that the lattice oxygen (-O2 ) replenished surface chemisorbed
Fig. 7. CH3 OH-TPD of LaMnO3 -based perovskites.
oxygen (␣-O2 ), part of which was spent in methanol oxidation during CH3 OH-TPD [25]. Among the catalysts, LaMnPd liberated the highest amounts of ␣-, -, and overall oxygen. Presumably, it should have the greatest oxygen mobility and availability in the reduced form caused by oxidation of chemisorbed methanol.
Fig. 8. O2 -TPD of LaMnO3 -based perovskites following CH3 OH-TPD.
Fig. 9. (a) Methanol and (b) oxygen conversions as a function of temperature.
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Fig. 10. Product selectivity of (a) HCOH, (b) CO2 , (c) CO, (d) H2 , and (e) H2 O as a function of methanol conversion.
3.2. Catalytic performance Fig. 9 depicts the temperature-dependence of methanol and oxygen conversions, revealing a strong, positive correlation between these two. The conversions increased steeply within narrow temperature intervals, and then steadily increased with growing temperature. The initiation temperature decreased as LaMn (260–310 ◦ C) > LaMnPd (160–210 ◦ C) ∼ LaMnRh (160–180 ◦ C). As for LaMnPt, the initiation interval was unclear, possibly at lower temperatures than the others. Oxygen was nearly depleted at 285 ◦ C for LaMnRh, 335 ◦ C for LaMnPd, and 360 ◦ C for LaMnPt and LaMn, respectively. At these points, methanol conversions declined as follows: LaMnPt (68%) > LaMnPd (56%) > LaMn (51%) ∼ LaMnRh (50%). LaMnPt was the most active at low (<175 ◦ C) and high temperatures (>300 ◦ C). LaMn was the least reactive in almost all the trials. The S-shaped dependence of methanol conversion on temperature implies a stepwise reaction pathway. Based on the feed stoichiometry, a 5/11 molar ratio of O2 -to-CH3 OH indicates that a total consumption of oxygen could yield ∼91% methanol conversion by following the partial oxidation route (Eq. (1)). This was not the case, as the maximum methanol conversion for each catalyst was less than 70%. Moreover, a detailed survey of ratios of spent O2 divided by converted methanol results mainly in the range of 1.2–0.6 from 135 to 360 ◦ C. In another words, different amounts of
O2 were converted per mole of expensed CH3 OH, and decreased with increasing temperature. Fig. 10 illustrates product selectivities as a function of methanol conversion. To extrapolate this data, the selectivity patterns were divided into low (<20%), moderate (20–45%), and high (>45%) conversion regions. Grossly speaking, HCOH increased with enhancing methanol conversion. The highest HCOH selectivity (∼22%) was achieved over LaMnPd at 60% conversion, where HCOH selectivity decreased as follows: LaMnPd > LaMnRh > LaMnPt > LaMn. In the moderate conversion region, LaMnRh became the least selective to HCOH while the other catalysts still followed this order. In the low conversion region, trace amounts of HCOH formed over each catalyst. With increasing conversion, CO2 declined from 31 to 17% for LaMnPt and from 21 to 12% for LaMnPd. LaMn displayed a reverse trend, increased from 12 to 19% then level off. LaMnRh exhibited an inverted U-shape curve. Its highest CO2 selectivity was 28% at a methanol conversion of about 36%. Comparing CO profiles shows that LaMn produced greater CO than promoted perovskites, declining from 18 to 5%. Less than 2% CO was formed by LaMnPd, LaMnPt, and LaMnRh. Interestingly, LaMnPd and LaMnRh displayed an upward and then downward trend. LaMnPt showed a constant CO selectivity (∼0.5%) at high conversions. Precious metal doped perovskites generated two to three times as much H2 as LaMn. Not until methanol conversion exceeded
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Fig. 11. (a) Methanol conversion and (b) HCOH (solid), CO (dash), and H2 (dot) selectivities as a function of temperature in the absence of oxygen.
40% could LaMn produce detectable H2 . In contrast, LaMn yielded the highest H2 O in moderate and high conversion regions. At low methanol conversions, LaMnRh and LaMnPd showed higher H2 O selectivity than LaMn and LaMnPt. The differences of H2 selectivities were insignificant for promoted catalysts at a methanol conversion below 45%, while H2 O selectivities were quite close at 50% and above. Fig. 11 shows the methanol conversion and HCOH and CO selectivities as a function of temperature in oxygen-free environments. Precious metal incorporated perovskites displayed growing methanol conversions as the temperature increased, whereas LaMn remained inert. All catalysts displayed less than 5% conversions. Among them, LaMnPd and LaMnRh were more active than LaMnPt. Identified products were HCOH, CO, and H2 . At all temperatures, HCOH selectivity was high with trace CO detected at 300 ◦ C and above. CO receded following the order of LaMnPd > LaMnPt > LaMnRh. Fig. 12 depicts the outcomes of methanol steam reforming. Again, LaMn was inactive while other catalysts displayed low activities. The activities of perovskites were very close with respect to their performances in methanol decomposition. The major products were HCOH and H2 ; trace CO2 was generated at high temperatures. According to Eq. (5), converting a mole of methanol through steam reforming should yield a mixture of CO2 and H2 at a 1/3 ratio. This suggests that methanol decomposition still dominated the system under steam reforming conditions, yielding HCOH and H2 as primary products. In another words, methanol steam reforming was trivial under MPO conditions.
291
Fig. 12. (a) Methanol conversion and (b) HCOH (solid), CO2 (dash), and H2 (dot) selectivities as a function of temperature in methanol steam reforming.
As methanol conversion increased, relatively less oxygen was available in the moderate conversion region. According to the stoichiometric values of methanol-to-oxygen, the system should gradually transit from combustions to partial oxidation. This is supported by growing HCOH and H2 together with declining CO2 and H2 O. Surprisingly, LaMnRh and LaMn exhibited increasing CO2 in this regime. Fig. 13 shows that the CO2 generation could be attributed to various reactions, including methanol partial oxidation to CO2 and H2 , HCOH oxidation, CO oxidation, water-gas shift, and steam reforming. This effectively rules out the influences of steam reforming based on the observation of Figs. 11 and 12. LaMn is highly active in CO oxidation [64]. Moreover, a closely related molar ratio of (increased CO2 )/(lost CO), which nearly equals to unity, suggests that CO oxidation should be more important than the remaining reactions. This is not the case for LaMnRh: significant low CO selectivity implied less CO could be converted to CO2 via CO oxidation or water-gas shift. Therefore, methanol and HCOH oxidation may be prominent for LaMnRh at this stage. Note that methanol dehydrogenation may proceed under oxidative environments, thereby contributing a certain amounts of HCOH and H2 . Oxygen was limited or even depleted in high methanol conversions. HCOH and H2 accumulated while CO2 and H2 O declined in this region. That is, contributions of partial oxidation and dehydrogenation of methanol became more and more important. Note that promoted perovskites exhibited different CO trends: declining CO for LaMnPd and LaMnRh, but constant CO for LaMnPt. The CO distribution pattern in Fig. 11 shows that LaMnPd produced the highest CO under oxygen-free environments. This suggests appreciable CO oxidation reactivity of LaMnPd.
3.3. Reaction mechanism 3.4. Influences of surface chemistry and oxygen activity Fig. 13 presents possible reactions occurring in methanol oxidation [25,26,63]. At low methanol conversions, oxygen is still plentiful at this stage. Therefore, full or incomplete combustion of methanol should be prominent. Thus, CO2 , CO, and H2 O are essential products. However, these two reactions evolved diversely over tested catalysts. LaMn exhibited almost twice as much CO as CO2 , suggesting incomplete combustion was favored. In contrast, LaMnPd, LaMnPt, and LaMnRh exhibited great amounts of CO2 , indicating full combustion.
The results above confirm the relationships between perovskite’s physicochemical properties and catalytic behaviors. The substitution of Pt ions into LaMnO3 results in a more defective surface than other perovskites. This leads LaMnPt to possess the highest surface area and lowest O2 desorption temperature. Furthermore, the electronic state of Pt+4 induced a plethora of surface cations. This allows LaMnPt to host excess chemisorbed oxygen species, i.e., ␣-O2 , to fulfill the electroneutrality. ␣-O2 is very active
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Fig. 13. Reaction pathways of methanol conversion under oxidative environments.
Fig. 14. Redox mechanism of MPO over perovskite catalysts.
in combustion [51,61], yielding the highest activity and CO2 selectivity for LaMnPt in low reaction temperatures. The reactivity of ␣-O2 can probably be sustained when O2 is still available, resulting in great methanol conversions of LaMnPt at high temperatures. The Pd+2 ion plays a different role in LaMnO3 . It substantially improves the reducibility and re-oxidation activity. The O2 -TPD after CH3 OH-TPD of LaMnPd also showed the lowest desorption temperature and greatest O2 capacity among all catalysts. This emphasizes the redox chemistry of LaMnPd. Fig. 14 shows the plausible redox cycle on perovskites. This mechanism is critical to the HCOH production in methanol partial oxidation [65]. ␣-O2 seems to play a central role in this mechanism. This is because HCOH was formed at low methanol conversion region, where operating temperatures were quite low. Under this circumstance, influences of -O2 , which requires greater thermal inputs than ␣-O2 to be activated, are limited. The great oxygen storage and mobility of methanol-reduced LaMnPd suggests that lattice oxygen can be efficiently transferred from the bulk (-O2 ) to the surface (␣-O2 ), and subsequently participated in the redox cycle. Therefore, substantial high selectivity of HCOH can be obtained over LaMnPd. The Rh replaced LaMnO3 seems to be the least reactive and reducible of the promoted catalysts. Due to the relatively close radius of Rh to Mn compared to the pairs of Pd to Mn and Pt to Mn,
LaMnRh should have the highest Goldschmidt tolerance factor. In addition, the electronic state of Rh+3 , which equals to the designated charge of B-site ions in LaMnO3 , could yield a more stable phase for LaMnRh than LaMnPt and LaMnPd.
4. Conclusions Substitution of trace Pd, Pt, and Rh ions into LaMnO3 can significantly alter the physicochemical properties of perovskite. This doping improves catalyst reducibility, oxygen mobility, and oxygen storage. All these factors allow promoted perovskites to have greater activity than untainted catalyst. Moreover, the catalytic behavior in MPO showed different reaction pathways between promoted and un-promoted perovskites. Methanol and CO combustions are major pathways with minor contributions of partial oxidation over pure LaMnO3 . LaMnPd shows surpassing reactivity of partial oxidation to HCOH compared to other catalysts. This trend is correlated to its redox property. Besides, LaMnPd also displays attracting reactivity in CO oxidation. LaMnPt mediates the reaction pathway mainly toward methanol combustion. This is likely due to the great amount of combustionprompt ␣-O2 on its surface. LaMnRh seems to be the least reactive
C.-L. Li, Y.-C. Lin / Applied Catalysis B: Environmental 107 (2011) 284–293
of the promoted catalysts in this study, possibly because of its low reducibility and oxygen activity. The current work shows that it is possible to manipulate the reaction pathways of MPO with selected promoters in perovskites. Future research should focus on a molecular-level understanding of the surface chemistry and reaction mechanism to facilitate the design of LaMnO3 -based catalysts. Acknowledgements Valuable suggestions of Prof. C.-T. Yeh (Yuan Ze University) and anonymous reviewers are gratefully appreciated. This work was sponsored by the National Science Council (Taiwan) under grant #NSC 98-2218-E-155-006. Appendix A. Supplementary data
[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apcatb.2011.07.026.
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