990
J. Phys. Chem. 1995, 99, 990-998
Characterization of Li-Doped Ni/MgO Catalysts Francesco Arena,? Andrej L. Chuvilin,* and Adolfo Parmaliana*a Dipartimento di Chimica Industriale, Universitb degli Studi di Messina, Salita Sperone c.p. 29, I-981 66 S. Agata (Messina), Italy, Boreskov Institute of Catalysis, Prospekt Akademika Luvrentieva 5, 630090 Novosibirsk, Russia, and Istituto CNR-TAE, Salita S. Lucia 39, I-98126 S. Lucia (Messina), Italy Received: April 15, 1994; In Final Form: October 1, 1994@
The effects of the Li-doping on the reducibility and morphological properties of a N W g O catalyst have been investigated by TPR,BET surface area, TEM,XPS, H2, and CO chemisorption measurements. The roles of both the doping method (pre- and postimpregnation) and lithium loading (0.67-2.50 wt %) have been assessed. Lithium addition before calcination (preimpregnation) yields sintering of NiO, affecting the overall “distribution” of the Ni precursor across the MgO matrix. The promoting effect of Li’ on the reducibility of the N W g O system has been discussed. At high reduction temperatures (Tr L 600 “C), Li causes a collapse in the catalyst structure, resulting also in a dramatic sintering of the metal phase.
Introduction Although metal-support interactions are crucial in controlling the reactivity of supported Ni catalysts,1-4 the catalytic properties of such systems can be timely tuned by the addition of alkali promoter^.^-'^ Indeed, the electron-donor character of alkali oxides can either neutralize the surface acidic sites of steam reforming catalysts, thus enhancing their resistance to coking,5J1J3 or modify the electron density of metal particles, altering the activity-selectivity pattern of Ni catalyst^.^^^^' 1,12 Such a peculiar influence of the alkali metals on the catalytic behavior of Ni catalysts has attracted a great deal of both fundamental and applied research interest. It has been ascertained that, depending on the nature of the support and on the extent of the loading, the alkali’s addition may result in either a promoting or poisoning Besides, it is decided that (i) the interaction of alkali moieties with acidic conventional carriers (i.e., A1203, Si02, A1203Si02, zeolite, etc.) affects both the reducibility of supported Ni precursors and the dispersity of the metal phase8-10 and (ii) the alteration of the catalytic pattern arises from the “decoration” of Ni crystallites by alkali However, the origin of the effects produced by alkali metals on the chemisorption properties and the catalytic behavior of Ni-based systems (Le., geometric and/or electronic efsect) still remains a topic of debate.1°-12 Previous studies dealing with the characterization and catalflc activity of NilMgO have shown the potential suitability of such a system as a “real” catalyst for the conventional steam reforming reaction.13-15 In fact, in spite of the marked tendency of the Ni/MgO system to form “ideal” and nonreducible Ni,Mg(l-,)O solid solution^,^^'^-^^ which could limit the availability of the active Ni p h a ~ e , ~ we . ~ ,have ’ ~ pointed out that the selection of adequate preparation conditions is crucial to hinder the structural rearrangement linked with the “dissolution” of NiO into the MgO l a t t i ~ e .Then, ~ the suitability of the title system for C& steam ref~rming’~ along with the basic character and nonporous (“open”) structure of the ~ u p p o r t , adequate ~~’~ to prevent structure degradations (Le., pore blocking andor chemical interaction) induced by alkali vapors coming from the electrolytic Li2C03-K2C03 eutectic mixture, renders NilMgO
* Author to whom correspondence
should be addressed.
’ Universith degli Studi di Messina. @
Boreskov Institute of Catalysis. Istituto CNR-TAE. Abstract published in Advance ACS Abstracts, November 15, 1994.
0022-365419512099-0990$09.0010
a promising catalyst also for steam reforming inside molten carbonate fuel cells (i.e., internal reforming).20 However, despite this renewed interest in NiJMgO catalyst^,'^-^^^^^ the effects of alkali metals on the physicochemical and catalytic properties of the title system have been till now scarcely i n v e ~ t i g a t e d . ’ ~ , ~ ~ . ~ ~ A preliminary evaluation of the influence of K+ addition on the reducibility and morphology of FeJMgO and Ni/MgO catalysts22 has shown that the alkali promoter inhibits the reduction of both Fe and Ni precursors and causes a moderate decrease in metal dispersion. Moreover, a high unsteadiness of alkali-doped MgO-based systems resulting both in a loss of promoter^^^.^^ and in strong modifications of the morphology of the MgO support23at Tin excess of 500 “C has been pointed out. However, in spite of a poor reactivity of basic oxides toward alkali species,23lithium exhibits some affinity for either the Mg024or Ni025-27lattices, since its ionic radius is similar to that of Mg2+ and Ni2+. In particular, it has been ascertained that small amounts of Lif (0.2 wt %) can be incorporated into the surface regions of the MgO lattice, creating electron holes,24 whilst Li+ and NiO form LixNi(l-x)Osolid solution in a wide range (0 < x < 0.6) of atomic c o m p ~ s i t i o n . ~Therefore, ~-~~ this study is aimed at shedding light on the influence of the Li addition on the reducibility and surface properties of a “model” 19% Ni/MgO catalyst,4J5 attempting also to rationalize the complex interactions of the “Li20-NiO-MgO” ternary system occurring in Li-doped NilMgO catalysts. Experimental Section
(A) Catalysts. Ni/MgO catalyst (MPF16) was prepared by the incipient wetness method according to the procedure elsewhere described, l 8 using a toluenic solution of nickel acetylacetonate (Ni(C5H702)2) and MgO “smoke powder” (UBE Ind. Ltd., Japan, high purity grade product; BET surface area (BET S.A.), 34 m2.g-’; pore volume, 0.230 cm3*g-’) as support. After impregnation, the catalyst was dried overnight at 120 “C and atmospheric pressure and then air-calcined at 400 “C for 16 h (MPF16). Li-doped N W g O catalysts were prepared by a “wet impregnation’’ of the uncalcined (preimpregnation) and calcined (postimpregnation) catalyst with an isopropanolic solution of lithium acetate (CH3COOLi.113H20). The slurry was continuously stirred and the solvent removed by heating at 80 “C under a nitrogen stream, After the Li addition, all the samples 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 3, 1995 991
Characterization of Li-Doped Ni/MgO Catalysts TABLE 1: List of SamDlee sample
chemical composition
doping method
BET S A . (m2g-')
Ni loading (wt %)
MPF 16 MPF 16-L1 MPF16-L3 L1-MPF16
NiMgO Li-Ni/MgO Li-NilMgO Li-NiMgO
postimpregnation postimpregnation preimpregnation
32 27 23 26
19.0 19.0 16.5 18.1
Li loading (wt %) 0.67 2.50 0.83
All the samples have been calcined at 400 "C.
(MPF16-L1, MPF16-L3, and L1-MPF16) were dried overnight at 80 "C and then calcined for 16 h at 400 "C. The calcined samples were pressed (-400 bar), crushed, and sieved, and the 40-70 mesh fraction was used for all the experimental tests with the exception of BET S.A., apparent density (d), and TEM analyses, for which pressed powder (< 150 mesh) was used. Ni and Li loadings, as respectively determined by AAS and AES spectroscopies,with the relative notation used to identify all the investigated samples, are summarized in Table 1. In order to evaluate the influence of the reduction treatment on the BET S.A. and apparent density (d), several aliquots of the MPF16 and MPF16-L1 samples were treated in a H2 flow at 400, 600, and 800 "C for half an hour and at 1000 "C for 5 min, respectively. (B) BET Surface Area (BET S.A.). BET S.A. measurements were carried out in a volumetric apparatus (Sorptomatic 1900, Carlo Erba Instruments) by using nitrogen as adsorbate at -196 "C. (C) X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed with an ESCA LAB-200X spectrometer (VG Instruments) operating in a fixed analyzer transmission mode. XPS areas were calculated using the total integrated area of the Mg(2p), Ni(2p3/2), and Li(1s) regions. Peak positions were calibrated using the C( 1s) photoelectron peak at 285.0 eV (B.E.) as reference. (D) Temperature-Programmed Reduction (TPR). TPR measurements in the T range 50-1000 "C were carried out in a conventional linear quartz gradientless microreactor (id. = 4 mm; 1 = 200 mm) using a 5.2% H2/N2 mixture flowing at 60 STP cm3min-' and a heating rate Cp) of 10 "Cmin-l. Mass samples (%0.05g ) had been selected so as to avoid mass- and heat-transfer limitations.la The H2 consumption was monitored by a thermal conductivity detector (TPR-TCD) connected to a personal computer for data storage and processing. A series of TPR experiments was also carried out using a Thermolab quadrupole mass spectrometer (Fisons Instruments) connected on line with the TPR reactor (TPR-QMS) by a heated (-180 "C) inlet capillary system (transit time < 0.5 s). TPRQMS tests were performed in the T range 50- 1000 "C with /? equal to 10 "C-min-' using a 5.2% H2/Ar reducing mixture flowing at 60 STP cm3min-l. Mass spectra were recorded in multiple ion monitoring (MIM) mode using the SEM amplifier operating at 1250 V and an ionization potential of -70 V. TPRQMS spectra have been obtained by acquiring the signals relative to the following mass to charge ( d z ) ratios: 2 (Hz), 15 (CH3), 17 (OH), 18 (H20), 28 (CO), 40 (Ar),and 44 (C02). The abundances of the CH3 ( d z , 15), H20 ( d z , 18), CO (4 z , 28), and C 0 2 (dz,44) fragments in methane, water, carbon monoxide, and carbon dioxide molecules are 0.411,0.74,0.95, and 0.81, respectively. Besides, in order to normalize the QMS signals of CO, C02, and C& to that of H20, the response factors 0.98, 1.00, 1.51, and 1.59 for d z 15, 18, 28, and 44, respectively, were determined. The calibration of the H2 consumption (TPR-TCD) and H2O signal (TPR-QMS) in the whole range of T was performed by monitoring the reduction
of known amounts (0.005-0.01 g) of Nin, Cun, and Sn" oxides. Under the above experimental conditions, TPR-TCD and TPRQMS proved to be in good agreement in both peak position ( f 5 "C) and hydrogen consumption or amount of water formed
(f5%). Before the TPR tests, the catalyst samples were treated in situ at 400 "C in an 0 2 flow for half an hour to remove any
adsorbed H20 and C02 and then were cooled to 50 "C and purged in a He flow. The reduction degree of NiO has been estimated by integrating the TPR-QMS peak area of H20 up to 1000 "C (al). After each TPR run, the catalyst samples were cooled in flowing He at 450 "C and then 0 2 pulses were injected onto the sample in order to check the degree of NiO reduction (a2). lv4 (E) Hydrogen Chemisorption. H2 chemisorption data were obtained by flow thermal desorption tests4 using the above flow apparatus according to the experimental procedure suggested by Jones and Bartholomew.28 Catalyst samples (0.1-0.2 g) were placed in a tubular quartz U-microreactor and reduced for half an hour in a H2 flow (25 STP crn3min-l) at a temperature (Tr) ranging between 400 and 800 "C. After the reduction treatment, the sample was cooled in flowing H2 to room temperature, equilibrated for 30 min, and then further cooled in a dry ice/ethanol bath to -72 "C, equilibrating for 15 min. Then the H2 was shut off and the sample was purged with the nitrogen carrier stream (30 STP cm3min-') until stabilization of the baseline was reached (%20 min). After purging, the dry ice/ethanol bath was removed and the U-reactor was quickly placed into a furnace preheated at 530 "C (this temperature was selected in order to obtain a symmetric desorption peak). The H2 desorption process, lasting about 5 min, was monitored and quantified by using a TCD connected to a DP 700 Data Processor (Carlo Erba Instruments). After every desorption run, a calibration test was made by injecting in the carrier gas a known amount of H2. Metal dispersion (D,%) was calculated from the following experimental ratio, by assuming a chemisorption stoichiometry of m i s u r f = 1:
D = 100[XH2/Xo2] where X H is ~ the hydrogen uptake @molgCat-') and XO,is the oxygen uptake @mol.gCat-') at 450 "C, corresponding to the fraction (a) of NiO reduced to Ni0.1.4328The metal surface area (MSA, mNiLgcat-l) was calculated assuming a Ni site density of 0.065 nm2/atom$~28 while the surface average Ni particle size (& nm) was derived from the equation suggested by Smith et al.? assuming a spherical shape for metal particles: d, = 101/D
(F) CO Chemisorption. Flow CO chemisorption measurements were performed in the above experimental apparatus using He as carrier gas flowing at 30 STP cm3min-'. Catalyst samples (0.1-0.2 g) were reduced under a H2 flow (25 STP cm3min-l) for half an hour at T,ranging between 400 and 800
992 J. Phys. Chem., Vol. 99, No. 3, 1995 35 I
Arena et al. I2
0
.a
.2
400
800 Tr ('C)
600
1000 '
.o
Figure 1. Effect of the reduction temperature on the BET S.A. (A, 0) and apparent density, 6, (A,0 ) of unpromoted MPF16 (A, A) and 0 ) catalysts. MPF16-Ll (0,
"C and then flushed for 30 min at 500 "C in He carrier flow. After the samples were cooled to room temperature in flowing He and CO, pulses (10% C o m e ; Vpulse, 1.0 STP cm3) were injected in the carrier until saturation coverage of the catalyst sample was attained.6 ( G ) Transmission Electron Microscopy (TEM). The morphology and the nickel particle size distributions (PSDs) of the catalysts were investigated by a JEOL 100 C Transmission Electron Microscope (point-to-point resolution 0.5 nm) using a finely ground and ultrasonically dispersed catalyst sample (in ethanol), deposited over a thin carbon film supported on a standard copper grid. Prior to TEM analyses, supported nickel catalysts were reduced in a H2 flow according to the above procedure for half an hour at T, ranging between 600 and 800 "C. The Ni particle size distribution (PSD) was obtained by considering at least 500 Ni particles for each catalyst sample, while the surface average Ni particle size (& nm) was calculated according to the normal statistical formula:'
Temperature ('C)
Figure 2. TPR-TCDspectra of MPF16 (a), MPF16-L1 (b), MPF16L3 (c). and L1-MPF16 (d) catalysts.
light on the origin of such unlikely reducibility values, a series of TPR-QMS experiments on the above catalysts has been performed. The TPR-QMS spectra of the bare MPF16 sample (a) and MPF16-L1- (b), MPF 16-L3- (c), and L1-MPF16- (d) doped catalysts, shown in Figure 3, consist of a H20 trace spanning the Trange 200-1000 "C and overlapping signals of CHq (300 < T < 550 "C), CO (400 < T < 900 "C), and CO;? (500 < T 800 "C) for the Li-doped catalysts (Figure 3b-d). Further, temperature-programmed experiments under flowing He of the Li-modified systems (results not reported here) showed a characteristic CO2-desorptionprofile at T > 550 "C, the intensity of which was proportional to the lithium content. Then, it can be argued that the TPR profile of the Li-Ni/MgO systems involves the occurrence of the following set of elementary reactions:
+ H, - Ni + H,O (1) Li2C0, - Li,O + CO, (2) CO, + H, CO + H20 (3) CO, + 4H2 CH, + 2 H 2 0 (4) CO + 3H, CH, + H,O (5) Li,CO, + ( b + 4c)H2 - Li,O/LiOH + aCO, + bCO + cCH, + (b + 2c)H,O (6) where (a + b + c) = 1. NiO
Results (A) BET S.A. and Apparent Density (6). The influence of Tr, in the range 400-1000 "C, on the specific surface area (BET S.A.) and apparent density (6) of MPF16 and MPF16L1 catalysts is shown in Figure 1. It can be observed that the reduction treatment up to 800 "C does not significantly affect the initial BET S.A. (m32 rn2-g-') and 6 (1.13 g-cmF3)values of the unpromoted MPF16 catalyst, while at TI > 800 "C an incipient sintering yields a slight decrease in BET S.A. along with a parallel growth in 6. On the contrary, the MPF16-L1 sample behaves quite differently, since we note a dramatic drop in BET S.A. on increasing T, from 400 ( ~ 2 rn2.g-l) 6 to 1000 "C ( ~ 0 . 3m2-gg1). Such a strong loss in S.A. is accompanied by a considerable growth of 6. Indeed, a sudden increase in the apparent density value is found upon TI rises from 400 (6, 1.13 g - ~ m - ~to) 600 "C (6, 1.66 g - ~ m - ~ )Afterward, . 6 slightly grows at 800 "C (1.72 g ~ r n - ~remaining ), unchanged up to 1000 "C (Figure 1). (B) Temperature-Programmed Reduction (TPR). The TPR-TCD profiles of the bare MPF16 catalyst (a), and lithiumdoped MPF16-L1 (b), MPF16-L3 (c), and L1-MPF16 (d) samples are comparatively shown in Figure 2. It is evident that the lithium addition induces substantial modifications in the reduction pattern of the Ni/MgO system, giving rise to "apparent" reducibility values higher than 100%. In order to shed
-
-
According to reaction 1, the extent of NiO reduction and the NiO reduction profile can be obtained from the amount of H20 formed and the H20 signal, respectively. However, from the inspection of the above set of reactions it arises that also the formation of CHq and CO is accompanied by the formation of 1 and 2 water molecules, respectively. Then, the real NiO reduction profiles of the studied catalysts, obtained by subtracting from the H20 profile resulting from the contribution of water formed in reactions 1 and 3-6 the normalized levels of CO and CH4 multiplied by a stoichiometric factor 1 and 2, respectively, are shown in Figure 4. The temperature of peak maxima ( T M ~along ) with the relative intensity of peaks (Zi, %) and the NiO reduction degree values (a1and a,) is listed in Table 2.
J. Phys. Chem., Vol. 99,No. 3, 1995 993
Characterization of Li-Doped Ni/MgO Catalysts
(b)
Temperature pC)
1.mPr.trr. ("c)
Figure 4. NiO reduction profile of MPF16 (a), MPF16-Ll (b), MPF16-L3 (c), and L1-MPF16 (d) catalysts. TABLE 2: TPR-QMS Characterizationof Pure and Li-Doped MPF16 Catalysts intensity of peak (%) T of peak maximum" ("C) catalyst TMI TMZ Th13 T M ~ TMS 11 12 13 14 MPF16 MPF16-L1 MPF16-L3 L1-MPF16
256 235 255 244
315(350) 344 347 365
598 600 594 596
764 895 899 891
1.3 1.2 1.1 1.8
16.5 14.9 14.1 41.7
49.3 76.1 79.7 50.2
NiO reduction degreeb (%) 15
32.9 7.8 5.1 6.3
a1
a2
80
80 17 7 14
89 89 88
a In parentheses is reported the value of the secondary maximum. al,NiO reduction degree calculated from the integrated HzO peak area; a2. NiO reduction degree calculated from 0 2 uptake measurements at 450 "C after TPR up to 1000 O C .
The TPR profile of the MPF16 catalyst (Figure 4a), appearing as a continuous and convoluted H20 signal in the range 2001000 "C, consists of four reduction peaks with maxima at 256, 315, 598, and 764 "C, respectively, the reduction degree a1 results equal to 80% being in full agreement with the value (a2)obtained by 0 2 uptake measurements (see Table 2). Although Li markedly affects the reduction pattern of the Ni/ MgO system, it is evident that the TPR spectra of MPF16-L1
(Figure 3b), MPF16-L3 (Figure 3c), and L1-MPF16 (Figure 3d) catalysts still retain the typical features of the undoped Ni/MgO system (Figure 4a). In particular, considering two temperature regions in the TPR-QMS spectra, (i) a low-T (LTR)region from 50 to 500 "C and (ii) a high-T region (HTR)from 500 to 1000 "C,the following main findings can be pointed out: LTR. The addition of Li to the uncalcined Ni/MgO catalyst (Ll-MPF16) causes a considerable growth of the "more easily"
Arena et al.
994 J. Phys. Chem., Vol. 99, No. 3, 1995
TABLE 3: H2 Chemisorption on Pure and Li-Doped MPF16 Catalysts Hz uptake catalyst Tr ("C) a (%) OlmolgCat-') D (a) MPF16 MPF16-L1 MPF16-L3 L 1-MPF16
400 600 800 400 600 800 400 600 800 400 600 800
22.5
50.5 74.6 18.1 80.0 83.8 21.0 84.0
n.d. 35.3 83.9 87.0
71.4 103.1 122.2 46.7 98.5 64.5 33.4 69.3 8.9 60.4 66.6 42.7
19.6 12.6
10.0 15.9 7.6 4.8 11.3 5.8 11.0 5.1 3.2
I
600 800 1,('C) Figure 5. Effect of Tron the MSA (mNl~gca-l) of MPF16 (0),MPF16L1 (A), MPF16-L3 (O), and L1-MPF16 (A) catalysts. 400
reducible "NiO f0ms",4,18J9respectively denoted by T M ~ and T M ~leading , also to a marked sharpening of the TMZreduction peak (Figure 4d) and a shift of the TM2 value (Table 2). By contrast, doping of precalcined catalyst (MPF16-L1 and MPF16L3) does not substantially affect either the shape or the intensity of T M and ~ TM2 peaks (Figure 4b and c). HTR. Li addition gives rise to (a) a strong enhancement in the reduction rate of the TM3 "NiO displaying then a well resolved peak shape, (b) the disappearance of the TM4 peak, and (c) the appearance of a resolved TMSpeak at T 900 "C (Table 2). As a consequence of the enhanced reduction rate, NiO reducibility values higher than that of the unpromoted system have been experienced for all the Li-promoted catalysts (see Table 2). However, after the TPR run all the Li-promoted catalysts exhibit a limited 0 2 uptake at 450 "C, which results in a great difference between the values of NiO reduction degree (Table 2 ) estimated on the basis of the integrated H20 peak area (al)and 0 2 uptake (ad. (C) H2 Chemisorption. Hydrogen uptakes of the undoped and Li-promoted MPF16 catalysts reduced at 400,600, and 800 "C along with the NiO reduction degree (a)and metal dispersion (D)values are listed in Table 3. With the exception of the MPF16-L3 sample reduced at 800 "C, doped catalysts reduced in the T range 400-800 "C are quite reactive toward oxygen at 450 "C, and therefore reproducible and reliable estimates of a have been obtained (Table 3). Significant differences in the NiO reduction degree with respect to the bare MPF16 system (a,50.5 and 74.8% at 600 and 800 "C, respectively) have been experienced for all the promoted samples at T, 1 600 "C (a,ca. 80% at 600 "C). At 400 "C, MPF16-L1 and MPF16-L3 present a reduction degree similar to that of the bare MPF16 system (a,ca. 20%), while for the L1-MPF16 catalyst a considerably higher value of a (-35%) has been found. Li addition lowers the H2 chemisorption capability of the MPF16 system. This evidence along with the promoting effect exerted by Li on the NiO reducibility, mainly at T, 1. 600 "C, results in a dramatic decrease of D (Table 3). For both L1MPF16 and MPF16-L3 samples a considerable decrease in metal dispersion (D, ca. 1l%), also at T, = 400 "C, has been observed. A comprehensive view of the effects of T, on the metal surface area (MSA, mNi2gCat-')of the investigated systems is reported in Figure 5 . Even if increasing Trleads to a progressive decrease in the metal dispersion of the MPF16 catalyst (see Table 3), a monotonous increase in MSA up to 800 "C is observable. By contrast, a volcano-shape trend, with the maximum at 600 "C, features the relationship between MSA and T, for the promoted catalysts. For low-loaded Li- ( e1% Li) doped catalysts, the negative effect of the promoter on the MSA is stronger on the preimpregnated system (Ll-MPF16),
-
TABLE 4: CO Chemisorption on Pure and Li-Doped MPF16 Catalysts catalyst MPF16 MPFl6-L1 MPF16-L3 L 1-MPF16
Tr ("C)
CO uptake @mo1.gCat-')
XCdXH~
400 600 800 400 600 800 400 600 800 400 600 800
62.8 93.8 111.2 16.2 46.4 42.1 0.2 0.4 1.5 4.4 30.8 25.4
0.88 0.91 0.91 0.35 0.47 0.66 0.01 0.01 0.13 0.08 0.46 0.61
whereas an increase in the Li concentration on precalcined Ni/ MgO catalyst (MPF16-L3) produces the most dramatic decrease in MSA in the whole range of T, (Figure 5). (D) CO Chemisorption. Carbon monoxide chemisorption for pure and doped MPF16 catalysts, reduced at T, ranging between 400 and 800 "C, together with the ratio between the amount of CO and H2 uptakes (XCdXHJ is reported in Table 4. It is evident that for the MPF16 catalyst the stoichiometry of the CO adsorption is unaffected by T,, resulting thus in a trend of CO uptake with T, analogous to that found for hydrogen chemisorption measurements. In particular, a xcdxH2ratio value equal to ca. 0.9 signals that under the adopted experimental conditions CO adsorbs on the bare MPF16 sample according to the stoichiometry of one CO molecule per two surface Ni atoms. By contrast, CO chemisorption data of doped systems show that the availability of surface Ni atoms to CO is markedly depressed by lithium addition. The extent of such an inhibiting effect is a function of both the Li loading and doping method. A rise in T, from 400 to 800 "C yields in any case a gradual increase in the Xc&H2 value. In particular, the slight co adsorption capability of the preimpregnated L1-MPF16 system reduced at 400 "C and the negligible chemisorption of MPF16L3 sample at any investigated T, (Table 4) are noteworthy. On this account, the following scale of surface metal availability to CO can be drawn: MPF16-L3 < L1-MPF16 < MPF16-L1 < MPF16
(E) X-ray Photoelectron Spectroscopy. The influence of the reduction treatment on the surface atomic composition of pure and doped MPF16 catalysts has been investigated by means of X P S measurements. The surface atomic concentration of
Characterization of Li-Doped Ni/MgO Catalysts
J. Phys. Chem., Vol. 99,No. 3, 1995 995
TABLE 5: Influence of the Reduction Treatment at 600 “C on the Surface Atomic Composition (XpS)of Pure and Li-Doped MPF16 Catalysts? X P S atomic intensity ratio
MPF 16 0.87 0.63 MPF 16-L1 0.79 n.d. MPF16-L3 0.66 0.46 L 1-MPF16 0.51 0.45 a b , before reduction; a, after reduction.
0.00 0.04 0.00
n.d.
0.23 0.15
Ni, Mg, and Li, expressed as Ni(2p3/:!)/Mg(2p) and Li(1s)Ng(2p) ratios, before and after H2 treatment at 600 “C, is reported in Table 5 . The untreated MPF16 and MPF16-L1 catalysts show similar values of the Ni(2p3n)/Mg(2p) atomic ratio, while such a ratio is considerably lower for both MPF16-L3 and L1MPF16 samples. Upon reduction at 600 “C the Ni/Mg ratio decreases for all the studied catalysts and a significant growth of the Li(ls)/Mg(2p) atomic ratio occurs for L1-MPF16 and MPF16-L3 systems (Table 5). (F) Transmission Electron Microscopy. The changes induced by Li addition in the morphology and Ni particle size distributions (PSDs) of the Ni/MgO catalysts have been further probed by TEM analysis. Typical micrographs of MPF16, MPF16-Ll, and L1-MPF16 systems, reduced at 600 and 800 “C, are shown in Figure 6. It is clearly evident that Li addition results in a marked sintering of the catalyst structure already at TI = 600 “C so as to render quite irregular and smoothed (Figure 6b and c) the cubic habit of the smoked MgO (Figure 6a). At Tr = 800 “C such a process is more evident (Figure 6b’ and c’), as lithium causes an almost complete destroying of the regular catalyst structure, giving rise to agglomerates containing large amounts of catalyst particles. This structure rearrangement produces a sensitive growth and a concomitant lowering in the “surface density” of the Ni particles (cf. Figure 6b and b’, and Figure 6c and c’) with respect to the undoped MPF16 catalyst. In fact, for the MPF16 sample a peculiar abundance of “small” Ni particles regularly displaced on the surface of the MgO crystals can be observed (Figure 6a and a’). The Ni particle size distributions (PSDs) of MPF16 (Figure 6a and a’), MPF16-L1 (Figure 6b and b’), and L1-MPF16 (Figure 6c and c’) samples, reduced at 600 (a-c) and 800 “C (a’-c’), are presented in Figure 7. It can be observed that the PSD of the MPF16 catalyst reduced at 600 “C (Figure 7a) consists of a very narrow histogram with a well resolved maximum at 5 nm which slightly broadens upon T, increases to 800 “C (Figure 7a’). Such evidence is supported by the values of the surface average Ni particle size (dJ reported in Table 6. In fact, dsslightly increases from 8 to 10 nm upon TI rises from 600 to 800 “C. Li addition, besides inducing dramatic changes in the morphology of the Ni/MgO catalyst, causes the sintering of the metal phase, giving rise to the appearance of quite larger Ni particles (15-40 nm) with a consequent marked broadening in the PSD (Figure 7b and c) of MPF16-L1 and L1-MPF16 systems reduced at 600 “C. At T, = 800 “C (Figure 7b’ and c’) a further broadening in the PSD, more pronounced in the preimpregnated catalyst (Ll-MPF16), is observed. These data agree well with H2 chemisorption results, indicating for MPF16L1 and L1-MPF16 catalysts a considerable growth in ds upon TI rises from 600 to 800 “C (Table 6). Discussion Effect of the Li-Doping on the Reducibility of the Ni/MgO Catalyst. The solid-state interaction occurring between NiO
and MgO support during the calcination treatment is crucial in controlling both the reducibility and the dispersion of Ni/MgO ~ a t a l y s t s ? J ~ Indeed, - ~ ~ in earlier TPR studies17-19 the role of Ni2+diffusion across the MgO lattice in stabilizing several “NiO forms” characterized by a different interaction strength with the magnesia lattice has been pointed out. On this account, the existence of a close relationship between the reducibility and the “location” of Ni2+ ions on the MgO structure has been claimed.17-19 In particular, the typical TPR pattern of the Ni/ MgO-supported system air calcined at 400 “C (Figure 4a) has been rationalized as follow^:'^^'^ (i) The T M and ~ TM2 peaks, lying in the LTR, have been ascribed to the reduction of “surface” (non-interacting with the MgO lattice) NiO species, respectively the ‘‘Nim)” and “free NiO” forms. (ii) The TM3 and Th14 peaks (HTR) were associated with the reduction of NiO forms having progressively higher interaction strength with magnesia support. (iii) The residual amount of nonreducible NiO (up to 1000 “C) was assumed to represent the fraction of Ni2+ (-20%) “dissolved” into the subsurface layers of the MgO lattice and thus forming a substitutional NiO/MgO solid solution. Therefore, this complex interaction pattern, arising from the marked tendency of NiO and MgO to intermix: points to the unusual distribution of the Ni precursor across the MgO support resulting in the stabilization of several “interaction states” between the host MgO lattice and the guest Ni0.18 Then, the strong enhancement in the reduction rate of the T M NiO ~ form brought about by the Li-addition (Figure 4b-d) indicates that the promoter influences the intrinsic properties of the Ni/MgO system, enabling a more effective reduction mechanism which enhances the reducibility of the MPF16 catalyst (Table 2). The data presented in Figure 3b-d indicate that on Li-doped Ni/ MgO catalysts the surface Li2C03 reacts in HZ atmosphere according to reactions 2-6, yielding CHq at lower T ( 400 “C become partially mobile, “spreading” on the surface of the catalyst particles, as indicated by the sudden increase in the Li/Mg X P S ratio of the Li-doped catalysts reduced at 600 “C (Table 5 ) . The occurrence of these processes, causing a melting in the utmost layers of the MgO l a t t i ~ e , ~gives ~ , ~rise ~ , ~to~ the agglomeration and sintering (see Figure 6b and c) of the support grain^,^^^^^ thus favoring the “extraction” and the consequent reduction of Ni2+ ions located in the near-surface regions of the MgO lattice (i.e., TM4 “NiO f0rm”).’*3’~Besides, a more effective H:!spillover across the basic Li2O/LiOH/Li2CO3 mediurn3l could concur to improve the above reduction mechanism. On the other hand, the negligible reactivity of the basic magnesia toward alkali compounds23allows us to disregard the occurrence of competitive reactions between promoter and carrier, which in conventional catalytic systems render easier the reduction of the supported Ni0.9J0 Yet, the similar values of the TM3 maximum for the unpromoted and promoted systems (see Table 2) indicate that lithium does not affect the interaction strength of this NiO form with the MgO support. This finding, appearing as probatory evidence of the catalytic effect exerted by Li+ on the reduction of the T M “NiO ~ form”, allows us to disregard the occurrence during TPR (Le., reducing conditions) of any appreciable diffusion of Li+ ions into the MgO l a t t i ~ e ~in~ , ~ ~ , ~ ~ competition with Ni2+ions (scavenging efsect). This statement is further strengthened by the light changes induced by the promoter in the LTR of postimpregnated systems (Figure 4b and c). Indeed, taking into account that up to 400 “C the Li2C03/LiOH/Li20 phase is rather stable,29 it can be argued
996 J. Phys. Chem.. Vol. 99,No.3, 1995
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respectively. Magnification: x2ooO00. that in the LTR any interaction between the promoter and the N M g O system is hindered. The TPR profile of the L1-MPF16 sample displays remarkable differences in both the LTR and HTR, denoting that Li addition before calcination affects more deeply the overall pattern of the NiO-MgO interaction in the N m g O system. In particular, the increased reduction rate of the TM2"NiO form" along with its larger intensity (see Table 2) proves the presence of an increased extent of the "free" NiO phase in such a system. In this case, the shift of T M to ~ a higher value (Table 2) should indicate a more difficult reduction process linked with the presence of sintered NiO particles with a more regular structure.18 The lower NMg X P S ratio (0.51) of the untreated L1MPF16 sample with respect to that of the undoped MPF16 catalyst (0.87) well supports the hypothesis on the formation of such larger NiO particles (Table 5). The explanation of such a finding probably lies in the fact that Li+ ions, interacting with the forming NiO lattice during air calcination (formation of surface Li-Ni-0 mixed phase).lo,zs,26favor the agglomeration (sintering) of NiO, hindering then the distribution of NiZ+across the MgO l a t t i ~ e . "A~rise ~ ~in~ the ~ ~amount ~ ~ ~ of Ni(I") species (11)parallels the increased concentration of the '%e" NiO form (12). even if the lower intensity ratio of TMIand TMZpeaks (Ill Iz) with respect to the other investigated systems (Table 2) also signals the formation of less "defective" NiO particles.l8J9
Besides, such a different "distribution" of the Ni precursor across the MgO matrix is further confirmed by the lower extent of the TM3 NiO form (Table 2). even if the relative TPR peak. still looking similar to that of the other doped systems (MPF16-LI and MPF16-L3), accounts for the occurrence of the same reduction mechanism (Figure 4). Finally, the well resolved TMypeak, appearing as a peculiar feature in the TPR spectra of all the doped catalysts (Figure 4b-d), signals a promoting effect of Li also on the reducibility of Ni*+ ions belonging to the NiO-MgO solid solution located in the subsurface layers of the MgO l a t t i ~ e . ' ~Namely, .~~ this enhanced reduction rate could be a consequence of the rearrangement in the structure of the catalyst which enables the surface segregation and a consequent easier reduction of NiZ+ ions lying below the surface of the s ~ p p o r t . ~ ~ . ~ ~ Effect nf the Li-Doping on the Morphology of the NilMgO Catalyst. The lower Hz uptake values of both MPF16-L1 and MPF16-L3 reduced at 400 "C with respect to the corresponding value of the bare MPF16 catalyst indicate that Li addition exerts a negative effect on the metal dispersion of the N m g O system, the extent of which depends on the Li loading. This negative influence of the Li on the MSA of the MPF16 catalyst (Figure 5) can be ascribed to a poisoning effect of the Li-containing phase which masks a portion of the surface Ni sites for HZ chemisorption. This lower accessibility of the Ni phase in
J. Phys. Chem.. Vol. 99, No. 3, 1995 997
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Figure 7. Ni particle size distribution lPSD) on MPF16 la. a?. MPFl6-L1 (b, b’), and LI- MPF16 (c, c’) catalysts reduced at 600 (a, b, c) and 800 “C (a’, b’, c’), respectively. TABLE 6 Comparison between TEM and HI Chemisorption Estimates of the Surface Average Ni Particle Size (d.) for Pure and Li-DoDed MPF16 Catalysts
the value of ds obtained by TEM and Hz chemisorption measurements (Table 6) confirms that the lower chemisorption capability of doped catalysts reduced at T, 2 600 “C mainly arises from the sintering of the metal phase. Although the lower catalyst Tr ?C) d S , m(om) ds,ckm (nm) D of the Ll-MPF16 sample at 4M) “C allows a flatter trend in MPF16 600 8 8 MSA with T, (Figure 5), the marked broadening in PSDs of 800 IO IO MPFl6-Ll 600 16 13 L1-MPF16 and MPF16-Ll samples (cf. Figure 7b and b’ and 800 21 21 Figure 7c and c’) is diagnostic of the higher sintering rate which 20 leads to the formation of larger Ni particles and on the whole 31 to lower MSA values (Figure 5). Our previous IR spectroscopy results have indicated that CO postimpregnated systems reduced at 400 “C is paralleled by a adsorbs on the hare NilMgO catalysts forming both mono- and decrease of the Ni/Mg XPS ratio in the untreated samples (Table polycarhonyl species.15 However, it is likely that the formation 5). while the stronger decrease in metal dispersion of the L1of monocarbonyl species under the adopted experimental MPF16 catalyst with respect to the same loaded MPF16-L1 conditions (Le., flow chemisorption) should be very limited (Table 3) mainly arises from the aforementioned NiO sintering because of kinetic constraints (e&, short contact time) which (Table 5 ) . preferentially allow the stabilization of bridged carbonyl species The reduction of the MPF16 catalyst at 600 and 800 “C does on the bare MPF16 catalyst, as indicated by a X&XH, value not affect the regular morphology of the MgO support (Figure equal to 0.9 (Table 4). The considerable decrease in the CO 6a and a‘),15 while the T, increase from 600 to 800 “C yields a uptake observed especially at lower T, on Li-doped catalysts slight broadening in the PSD and a concomitant little increase (Table 4) can not be ascribed either to a Ni particle size increase of d, from 8 to 10 nm (Table 6). However, in spite of such or to a weakening in the Ni-CO bond caused by electronic sintering of the Ni particles, the increase of T, from 600 to 800 effects of the alkali species.6,l2 This lowering in the CO OC allows an increase in the MSA (Figure 5 ) because of the chemisorption capability likely reflects the fact that a portion positive effect of Tr on the degree of NiO reduction! By of surface Ni sites, masked by the Li-adlayer phase, are contrast, the remarkable decrease in D experienced at 600 OC inaccessible to CO molecules, being on the contrary accessible for all the doped catalysts (Table 3) arises from the ongoing to small hydrogen atoms which are able to diffuse between the structural rearrangement induced by the melting of Li-moieties. metal surface and such a Li-adlayer phase. In this respect, In particular, this process yields the epitaxial growth of MgO Praliaud et al.32 firstly pointed out such a phenomenon on grains and a recrystallization p r o ~ e s swhich ~ ~ . cause ~ ~ ~both ~ ~a K-doped NdSiOz catalysts, claiming the occurrence of an alkalilowering in BET surface area30and a concomitant increase in induced “sieving effect” which limits the accessibility of the the apparent density (Figure 1). This structural rearrangement metal surface to larger probe molecules. Therefore, the deviance of the MgO crystallites (Figure 6), further to the formation of of the XCdX,, ratio from 0.9, which in our experimental local liquid phase (“cement effect”)F3 enhances the sintering conditions represents the full accessibility of Ni sites to CO, of the Ni crystallites. All these processes are strongly enhanced will be taken as a measure of the Li-induced sieving effect. On by a further rise in T,. Indeed, at 800 ‘C the BET S.A. of the this account, the experimental results reported in Table 4 lead MPF16-L1 system further decreases ( ~ mLg-’) 3 likely because of the spreading of the undecomposed liquid LizC03 phasez3~z9~30 us to draw out the following remarks: over magnesia, leading to a progressive collapse in the catalyst (i) At T, = 400 “C the very low CO adsorption capacity of structure (Figure 6 b’ and c’). Therefore, in light of this evidence the Ll-MPF16 system indicates that the preimpregnation allows it can be inferred that the support collapse plays a predominant a preferential contact between Li and NiO phases, favoring the role in determining the sudden drop of MSA at 800 “C for the NiO sintering during calcination. Upon reduction at 400 OC, Li-doped systems (Figure 5). In fact, the full agreement between the Li-containing phase still remains located on the surface of ~~
Arena et al.
998 J. Phys. Chem., Vol. 99, No. 3, 1995 Ni particles, strongly hindering the CO chemisorption, while the higher XCO/XH* of MPF16-L1 system seems to reflect a statistical distribution of the promoter on the surface of the precalcined system. Accordingly, the higher Li content of the MPF16-L3 sample implies a “homogeneous” coverage of the catalyst surface which inhibits at all any CO chemisorption (Table 4). (ii) On increasing T, from 400 to 800 “C, Li-containing moieties in contact with the NiO phase decompose, giving rise to surface liquid phases which spread onto the support surface, as indicated by the increase in the Li/Mg X P S ratio (Table 5 ) . As a result, a larger fraction of the Ni surface becomes accessible to CO, leading hence to an increase in the X C ~ X ratio. H~ (iii) A further rise in the reduction temperature up to 1000 “C (TPR) causes the final collapse in the catalyst structure, sticking Ni and MgO particles in a sort of conglomerate (glass) which hinders the reoxidation of the reduced Ni crystallites (Table 2 ) . However, the occurrence of such collapse of the catalyst structure is controlled also by the Li loading, since for the MPF16-L3 sample (2.5% Li) it has already been observed after reduction at 800 “C (Table 3). Conclusions Addition of lithium yields dramatic modifications in the physicochemical properties of the Ni/MgO catalyst, altering also the “distribution” of Ni precursor across the MgO matrix. In particular, the experimental results reported in this study point out that (i) Li enhances the reducibility of the Ni/MgO system, favoring the “extraction” of Ni2+dispersed into the MgO lattice; (ii) Li strongly affects the morphology of the Ni/MgO system inducing significant sintering phenomena; and (iii) Li exerts a negative effect on the chemisorption properties of the Ni/MgO system. The extent of the phenomena i-iii is controlled by the doping method, Li loading, and reduction temperature. Acknowledgment. The financial support of this work by Consiglio Nazionale delle Ricerche and M.U.R.S.T. is gratefully acknowledged. References and Notes (1) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J. Catal. 1980, 65, 335. (2) Smith, J. S.; Thrower, P. A,; Vannice, M. A. J. Catal. 1981, 68, 270. (3) Narayanan, S.; Sreekanth, G. J. Chem. SOC.,Faraday Trans. I 1989, 85 ( l l ) , 3785. (4) Arena, F.; Horrell, B. A,; Cocke, D. L.; Parmaliana, A.; Giordano, N. J. Catal. 1991, 132, 58.
(5) Martin, G. A. In Studies in Surface Science and Catalysis. MetalSupport and Metal-Additive Efects; Imelik, B., Naccache, C., Coudurier, C., Praliaud, H., Meriaudeau, P., Gallezot, P., Martin, G. A., Vedrhe, J. C., Eds.; Elsevier: Amsterdam, The Netherlands, 1982; Vol. 11, p 315. (6) Bailey, K. M.; Campbell, T. K.; Falconer, J. L. J. Catal. 1989, 54, 159. (7) Chen, I.; Chen, F.-L. Ind. Eng. Chem. Res. 1990, 29, 534. (8) Coughlan, B.; Keane, M. A. J. Mol. Catal. 1990, 63, 193. (9) Houalla, M.; Lemaitre, J.; Delmon, B. J. Chem. SOC., Faraday Trans. I 1982, 78, 1389. (10) Narayanan, S.; Uma, K. J. Chem. SOC.,Faraday Trans. 1 1987, 83, 733. (11) Mross, W. D. Catal. Rev.-Sci. Eng. 1983, 25 (4), 591. (12) Praliaud, H.; Dalmon, J. A.; Mirodatos, C.; Martin, G. A. J. Catal. 1986, 97, 344. (13) Rostrup-Nielsen, J. R. Steam Reforming Catalysts;Danish Technical Press: Copenhagen, 1975; p 81. (14) Parmaliana, A.; Frusteri, F.; Arena, F.; Mondello, N.; Giordano, N. In Studies in Surface Science and Catalysis. Structure and Reactivity of Surfaces; Morterra, C., Zecchina, A., Costa, G., Eds.; Elsevier: Amsterdam, The Netherlands, 1989; Vol. 48, p 739. (15) Parmaliana, A,; Arena, F.; Frusteri, F.; Coluccia, S.;Marchese, L.; Martra, G.; Chuvilin, A. J. Catal. 1993, 141, 34. (16) Hagan, A. P.; Lofthouse, M. G.;Stone, F. S.; Trevethan, M. A. In Studies in Surface Science and Catalysis. Preparation of Catalysts II; Delmon, B., Grange, P., Jacobs, P. A,, Poncelet, G., Eds.; Elsevier: Amsterdam, The Netherlands, 1979; Vol. 3, p 417. (17) Bond, G. C.; Sarsam, A. P. Appl. Catal. 1988, 38,429. (18) Parmaliana, A,; Arena, F.; Frusteri, F.; Giordano, N. J. Chem. Soc., Faraday Trans. 1990, 86 (14). 2663. (19) Arena, F.; Licciardello, A.; Parmaliana, A. Catal. Lett. 1990, 6, 131. (20) Pannaliana, A.; Frusteri, F.; Tsiakaras, P.; Giordano, N. Int. J. Hydrogen Energy 1988,13, 729. (21) Parmaliana, A.; Arena, F.; Frusteri, F.; Mondello, N.; Giordano, N. In Studies in Surface Science and Catalysis. Catalyst Deactivation; Bartholomew, C.H., Butt, J. B., Eds.; Elsevier: Amsterdam, The Netherlands, 1991; Vol. 68, p 489. (22) Moral, P.; Praliaud, H.; Martin, G. A. React. Kinet. Catal. Lett. 1987, 34, 1. (23) Pemchon, V.; Durupty, M. C. Appl. Catal. 1988, 42, 217. (24) Catlow, C. R. A.; Jackson, R. A,; Thomas, J. M. J. Phys. Chem. 1990, 94, 7889. (25) Bielanski, A.; Deren, J.; Haber, J.; Sloczynski, J. Trans. Faraday SOC.1966, 58, 166. (26) Marini, A,; Berbenni, V.; Massarotti, V.; Flor, G.;Riccardi, R.; Leonini, M. Solid State Ionics 1989, 32/33, 398. (27) Cochran, S. J.; Larkins, F. P. Aust. J. Chem. 1985, 38, 1293. (28) Jones, R. D.; Bartholomew, C. H. Appl. Catal. 1988, 39, 77. (29) Andersen, A. G.; Norby, T. Catal. Today 1990, 6, 575. (30) Mirodatos, C.; Pemchon, V.; Durupty, M. C.; Moral, P. In Studies in Surface Science and Catalysis. Catalyst Deactivation; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, The Netherlands, 1987; Vol. 34, p 183. (31) Dalmon, J. A.; Mirodatos, C.; Turlier, P.; Martin, G. A. In Studies in Surface Science and Catalysis. Spillover of Adsorbed Species; Pajonk, G. M., Teichner, S. J., German, J. E., Eds.; Elsevier: Amsterdam, The Netherlands, 1982; Vol. 17, p 169. (32) Martin, G. A.; Praliaud, H. Catal. Lett. 1991, 9, 151. JP940971Z
