Q Copyright 1991 American Chemical Society
AUGUST 1991 VOLUME 7,NUMBER 8
Letters Influence of the Calcination Treatment on the Surface Chemical Properties of Ni/MgO Catalyst: A CO2 Temperature Programmed Desorption Approach F. Arena,? A. Parmaliana,*J N. Mondello,? F. Frusteri,? and N. Giordanot Istituto CNR-TAE, Salita S. Lucia 39,I-98126 S. Lucia (Messina), Italy, and Dipartimento di Chimica Industriale, Universith di Messina, Salita Sperone c.p. 29,148166 S. Agata (Messina),Italy Received November 29,1990. In Final Form: April 23,1991 The modifications induced by the calcination temperature (400I TCI lo00 "C) on the surface chemical properties of a 19wt 96 Ni MgO catalyst have been systematically evaluatedby means of C02 temperature programmed desorption ( PD). The TPD profiles both of the MgO support and Ni/MgO catalyst reveal the presence of several adsorbed COz forms well accounting for the surface heterogeneity. Ni2+ ions, lying on the most reactive surface sites, exert a general weakening effect on the basicity of the MgO support. The enhancement in the "C02 adsorption capability"of Ni/MgO catalyst calcined at progressively higher Tc reflectsthe parallel diffusion of Ni2+ions into the MgO matrix. The steep increase in both the apparent density and surface basicity observed at TC > 800 "C has been associated with the formation of a substitutional Ni,Mg(l,)O solid solution.
1
Introduction The solid-state reactions occurring between active phase and support during the calcination treatment are crucial in controlling either the structural properties or the reactivity of supported oxide catalysts. The rationalization of these interactions is further complicated when the matrix of the support is able to "incorporate" transitionmetal ions in solid solution. In this respect, the NiOMgO system represents avery interesting model since NiO and MgO exhibit the peculiarity to form "ideal" solid solutions over the whole molar fraction range.' Indeed, the bulk and the surface structures of the NiO-MgO system as well as its reactivity have been the subject of numerous papers.2+' In particular, it has been stated that the
* Author to whom the correspondence should be addressed.
+ Istituto
CNR-TAE.
UniversitA di Messina. (1) k h i n a , A,; Spoto, G.;Coluccia, S.; Guglielmiiotti, E. J. Chem. SOC.,Faraday "RIM. 1 ISM,80,1891. (2)P d i a n a , A,; Arena,F.; Fmtari, F.; Giordano,N. J.Chem. SOC., t
Faraday T r c r ~1990,86,2663. (3)Arena, F.;Licciardello, A,; Parmaliana, A. Catal.Lett. 1990,6,139. (4)Arena, F.; Horrell,B. A.; Cocke, D. L.; Parmaliana, A.; Giordano, N.J. Catal., in press.
calcination temperature (Tc)markedly affects the diffusion of Ni2+ ions into the MgO lattice controlling the reducibility and the reactivity of Ni/MgO catalysts.S#6*6In addition, we demonstrated that high Tc's cause the dissolution of NiO into the MgO matrix leading to a system "quasi-atomically" dispersed upon the reducing treatment.4 However, despite the knowledge achieved on this system, the chemical nature of the Ni/MgO surface and its modifications induced by the formation of NiO/MgO solid solutions deserve more scrutiny. Therefore, the present investigation aims to correlate, by means of a systematic characterization (COrTPDand BET S.A.), the physical and surface chemical properties of a typical Ni/MgO catalyst with its structural rearrangements promoted by Tc.Besides, these results allow the gaining of further insights into the NiO-MgO interaction mechanism proposed in our earlier investigations.% Experimental Section Ni/MgO catalyst (MPF 16) was prepared by the incipient wetness impregnationmethod. MgO 'smoke powder" (UBEInd. Ltd., High Purity Grade,averageparticle size, 500A; pore volume, (5) Bond, G. C.; Sarsam, S. P. Appl. Catol. 19811,38,366.
(6)Borowiecki, T.Appl. Catal. 1984,10,273.
0743-7463I9112407-1555%02.50/0 0 1991 American Chemical Societv
Letters
1556 Langmuir, Vol. 7, No. 8,1991 Table I. Characterization of MgO Support and MPF 16 Catalyst (19 wt W Ni/MgO) apparent SA, density, T of peak maxima, OC sample Tc,O C m a g 1 gem" TMI T m T m TM4" M4 400 37 1.00 123 257 357 M6 600 29 1.00 126 257 360 23 1.04 M8 800 122 252 363 lo00 19 1.17 122 252 361 M 10
MPF 16/4 400 MPF 16/6 600 800 MPF 16/8 MPF 16/10 lo00
36 28 22 11
1.10 1.10 1.27 1.63
116 121 122 121
245 248 252 251
317 319 327 350
471 464
Oxygen desorption peak.
0.230cm3 gl; BET surface area, 34 m2g-l), pretreated at 400 "C in air for 16 h, was contacted under continuous stirring at 100
"C with a toluene solutionof nickel acetylacetonateNi(C&02)2. The toluene was removed by evaporation under vacuum. Several aliquota of the dried MPF 16 catalyst and of the MgO support (M) were calcined overnight in air flow at TCranging from 400 (MPF 16/4,M 4) to lo00 "C (MPF 16/10,M 10). The nickel content of MPF 16 catalyst, determined by AAS, is 19.0 wt % . The 40-70 mesh fraction was used for all the measurements. The specific surface areas were determined according to the BET method by using Ns as adsorbate at -196 "C. TPD testa were carried out by the flow apparatus previously described) using He as carrier gas flowing at 60 STP cm3min-l and a heating rate of 10 "C m i d . Sample weight was kept to less than 0.1 g. The C02 desorption process was monitored by a thermal conductivity detector connected to a DP 700 Data Processor (Carlo Erba Instruments). Quantitative calibration of COrTPDpeak area was made by monitoring the decompoeition of known amounta of CaCOg.
Results and Discussion COz-TemperatureProgrammed Desorption. COzTPD spectra of the differently air calcined M (a) and MPF 16 (b) samples are shown in Figure 1, while the relative peak maxima (TM), the BET surfacearea and the apparent density values of the investigated samples are reported in Table I. MgO Support (M). From the analysis of Figure l a immediately emerges that Tc does not substantially affect the C02-TPD pattern of MgO neither in the peaks shape nor in TMvalues (see Table I). The 'wide" range of temperatures spanned by the desorption process well reflects the heterogeneity of the MgO surface characterized by Lewis basic sites with different electron-pair donor (EPD) strength.' Then, the three peak maxima can be assigned to several "forms" of adsorbed C02. In particular, the T Mpeak, ~ centered at about 120 "C, is ascribable to the "bicarbonate" form,' while TMZ and TMSpeaks are associated with the desorption of more stable "monodentate" carbonate species anchored on EPD sites with different coordinative u n ~ a t u r a t i o n . ~ ~ ~ Besides, Figure l a shows that by increasing the Tc from 400 to loo0 "C, there is a general lowering in the C02 adsorption capability which cannot be attributed to the surface arealoss induced by sintering (seeTable I). Indeed, Figure 2, outlining the relationship between surface basicity (pmol of adsorbed COz m 3 and Tc, shows a progressive reduction of the surface reactivity likely due to the lattice restructuring that involves a lower coordinative unsaturation of the surface (Le., less steps, kinks, and cavities). In fact, this is strongly supported by the ( 7 ) Lercher, J. A.; Colombier, C.; Noller, H.J. Chem. SOC.,Faraday Tram 1 1984,80,949. (8) Zecchina,A,; Lofthouee,M. G.;Stone, F. S.J.Chem. SOC., Faraday Trans 1 1975,71,1476.
150
-
450
150
TC'C)
-
450
Figure 1. C02-TPD profiles of differently air calcined (a) M (MgO)and (b) MPF 16 (19wt% Ni/MgO) systems(TPDprofiles of the MPF 16 system have been multiplied by 2).
600
400
800
1000
Tc ( % 1
(w)
Figure 2. Effect of TCon surface basicity (SB)of (A)M and (A)MPF 16 (19w t % Ni/MgO) systems. ( 0 )Effect of Tc on the 'normalized" surface basicity of the MPF 16 system (0= SBMPFI~SBM).
enhanced shortening of the T w peak at high Tc, which accounts for the decreased amount of the most reactive sites of the MgO lattice. This structural rearrangement involves also an intraparticle crystal growth? which gives rise, at loo0 "C, to a considerable increase in the apparent density (see Table I). 19 wt % Ni/MgO Catalyst (MPF 16). The TPD profiles of the MPF 16 system (Figure lb) outline a strong negative effect of NiO on the "COz adsorption capability" of the MgO carrier determining both a general lowering in the amount of adsorbed COz and a shift of T Mtoward lower values. Besides, 'bulk NiO" did not show any COz desorption exhibiting only a slight and broad 02 desorption peak with T Mat 465 "C. Therefore, all of this evidence leads us to state that the "acidic NiOm6partially masks the ~
~~
~~
(9) Chino, A.; Porta,P.; Valid, M.J. Am. Ceram. SOC. 1966,49,152.
Letters basic sites of the MgO surface depressing its reactivity toward C02. However, the presence of NiO seems to affect particularly the extent and the maximum value of T M ~ peak. This could be explained by inferring that Ni2+ ions occupy preferentially a fraction of the most reactive sites of the MgO lattice' acting as strong electron acceptor species1°which lower the EPD strength of the neighboring "unsaturated" sites. In other words, surface Ni2+ ions perturb the effective ionic charge of the MgO matrix,10 the change of which implies an enhancementof the "acidic" properties of the guest Ni2+ions that renders on the whole "less basic" the outermost layer of the MgO structure.ll Therefore, the consequent weakening in the CO2-MgO interaction well accounts for the observed shift of T M ~ peak. The broad and less pronounced T Mpeak ~ (464-471"C), experiencedin the TPD spectra of bulk NiO, of MPF 16/4 and MPF 16/6samples, is correlated with the desorption of "nonstoichiometric oxygen" arising from thermal decomposition of surface higher Ni(1II) oxides?J2 Influence of Tc on BET Surface Area (SA) and Surface Basicity (SB)of Ni/MgO Catalysts (MPF 16). The BET surface areas and the apparent density values of the MPF 16 catalyst, calcined in the range 40010oO OC, are reported in Table I. It is remarkable the negative influence of TCon the specific surface area, the decrease of which becomes more evident for Tc > 800 "C. This and the peculiar and substantial increase in the apparent density are the unambiguous proofs of the complex rearrangement in the "multilayer" Ni/MgO structure connected with the formationof the "bulk" NiOMgO solid s o l ~ t i o n .In~ ~fact, ~ Tc, controlling the Ni2+ diffusion through the MgO l a t t i ~ e causes ,~ progressive dissolution of the supported NiO up to stabilize, at 10oO "C,a substitutional Ni,Mg(l-,)O solid s 0 1 u t i o n ~ which ~J~ gives rise to a strong decrease in the MgO lattice parameter (ao).13 Therefore, the closer "lattice packing", arising from (10) Kung, H.H. J. Catal. 1982, 73,387. (11) Stone, F. S. J. Mol. Catal. 1990,59, 147. (12) Moroney, L. M.; Smart, R. C.; Roberta, M. W. J. Chem. SOC., Faraday Trans 1 1983, 79, 1769. (13) Hagan,A. P., Lofthouse, M. G.; Stone, F. S.; Trevethan, M. A. In
Preparation of CatalystsI4 Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Ede.; Elsevier: Amstardam, 1979; p 417.
Langmuir, Vol. 7, No.8, 1991 1557 the homogeneous dispersion of Ni2+ ions into the MgO lattice, well accounts for the observed marked increase in the apparent density of MPF 16/10 sample. Moreover, the dramatic lowering of surface area in the MPF 16/10 system is in agreement with previous observationsof Hagan et al.,13since they found a significant SA decrease for high surface area "ideal" NiO-MgO solid solutions with 15-20 mol 7% Ni:Mg. Hence, this evidence is diagnostic of the substantial structure modifications from a typical supportedsystem (MPF 16/4)to a "bulk" oxide solid solution (MPF 16/10)that implies also a significant change in the surface chemical properties. In this respect, on the basis of previous X P S findings, we preliminarly associated the tendency of MgO and NiO to "intermix" with an enhanced basic character of supported Ni/MgO system calcined at This hypothesis is well confirmed by the higher T C . ~ relationship between the surface basicity (pmol of adsorbed C02 m-2) and Tc, shown in Figure 2, which outlines a characteristic trend explainable in terms of a different surface chemical composition (at Tc > 6000C).3 However, since the reactivity of MgO decreases with Tc, a more accurate evaluation of the surface chemical properties of Ni/MgO system can be obtained by "normalizing" the SB of the MPF 16 system to that of the bare MgO support (0 = S B ~ F ~ ~ / S B Indeed, M ) . the trend of 0 with Tc, also shown in Figure 2, undoubtedlyproves the increasing basic nature of the Ni/MgO system having a higher degree of NiO-MgO solid solution.2-" Namely, this trend suggests that the surface NiO depletion, occurring at Tc > 600 "C, determines a parallel MgO enrichment4that enhances the reactivity of Ni/MgO system toward carbon dioxide. Besides, as TCrises, owing to the diffusion of surface Ni2+ ions into the core of MgO structure, EPD sites are progressively less affected by the interaction with the electron-acceptor Ni2+ions. This involves a stronger C02MgO interaction which accounts for the shift of T Mto~ higher T (see Table I). In conclusion, the similar T M ~ values of MPF 16/10and M10 samplestogether with their comparable surface basicity (0 = 0.67) further confirm that at 10oO "C the Ni/MgO system has definetively evolved into a "homogeneous" and "ideal" NiO-MgO solid solution.l-3J3 Registry No. Ni,7440-02-0; MgO, 1309-48-4; COZ,124-38-9.
