Langmuir 1991, 7 , 2642-2648
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Characterization of a Ru/Ti02 Catalyst for Selective Methanation at Room Temperature and Atmospheric Pressure K. Ravindranathan Thampi, L. Lucarelli,+and John Kiwi* Imtitut de Chemie Physique, Ecole Polytechnique, Fkdbrale de Lausanne, CH 1015, Lausanne, Switzerland Received March 27, 1991. In Final Form: June 10, 1991 The characterization of an efficient and selective catalyst for C02 methanation at room temperature and atmospheric pressure is reported here. T h e most efficient catalyst, a 3.8% Ru-RuOJTiOz (Degussa P25), x I2, is characterized in detail by elemental analysis, mercury porosimetry, physisorption, BET area, and chemisorption of H2. A systematic variation of the hysteresis as a function of Ru loading on Ti02 was observed. Catalyst characteristics such as B E T area, monolayer volume, and specific pore volume are affected by the preparation method.
Introduction Hydrogenation of carbon dioxide to useful fuels is an important reaction, from an energy point of view:
C02 + 4H2
+
CH4 + 2H2O(g)
ACo,K
= -27 kcal/mol (1)
Although this reaction (Sabatier reaction) is thermodynamically downhill, it is difficult to achieve, because of the eight-electron reduction involved and the large kinetic barriers necessary to generate the reaction intermediates. Our laboratory112 has recently reported about C02 methanation (nearly 100% selective toward CH4) over Ru/ TiOz, at room temperature and atmospheric pressure. For ecological reasons, recycling CO2 from waste gas streams would be beneficial, provided the H2 used for the hydrogenation is obtained from clean resources such as water. In this paper, we report experiments related t o the characterization of a solid-gas interface of a Ru/TiO2 room-temperature methanation catalyst. The difficulties and limitations encountered in obtaining a quantitative determination for t h e observed adsorption will also be discussed. Several techniques, as complementary as possible, have been used to allow a pragmatic and reliable characterization of the Ru/TiOz catalyst under consideration. The experimental results reported were based on elemental analysis, mercury porosimetry, N2 physisorption, and H2 chemisorption. Catalytic reaction studies have showed that a catalyst prepared b y a deposition precipitation technique (preferably with 3.8% Ru loading) is superior in activity to catalysts prepared by other techniques such as impregnation. T h i s s t u d y presents a n evaluation of t h e surface characteristics of such Ru/TiOz catalysts.
Experimental Section In order to obtain small particles of the active component Ru, its hydroxide was hydrolyzed and precipitated (depositionprecipitation technique), from a solution of RuC&-nHzOat various pH values. RuC13.3Hz0 (0.1 g) was dissolved in 100 mL of 0.1 M HCl and the temperature of the dispersion was maintained t Microstructure Laboratory,CarloErba Strumentazione,Rodano, Milan, Italy.
(1)Thampi, K. R.; Kiwi, J.; Griitzel, M. Nature 1987, 327, 506, and references therein. (2)Thampi, K. R.; Kiwi, J.; Griitzel, M. In Proceedings of 9th International Congress on Catalysis; Phillips, M. J., Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, ON, Canada, 1988; Vol. 2, p 837.
0743-7463/91/2407-2642$02.50/0
at 343 K for 5 h, during which the final pH was repeatedly readjusted to the desired value by adding 0.1 M NaOH very slowly. Since the porosity of the support Ti02 (Degussa P25) is relatively small, the transport problems are kept minimal and the nucleation proceeds in a controlled manner. In order to set the best preparative conditions, the final pH was varied between 2 and 11 for different preparations. Water was subsequently evaporated off, and the powder was calcined for 18 h at 443 K and another 18 h at 643 K. The C1- and Na+ added during the precipitation stage was removed by an extended dialysis after calcination. This method has been recently reported in detail by our laborat~ry.'-~Impregnated catalysts were prepared by the usual wet techniques using aqueous RuC18 solution in 0.01 M HCl. They were not dialyzed after calcination. Catalysts prepared via the deposition-precipitation technique will be designated as deposited catalysts throughout this paper. Mercury porosimetry was carried out with a Carlo-Erba 2000 mercuryporosimeter equipped with an IBM PC 2 dataproceseor. The physisorption and chemisorption properties of the catalysts were characterized by a Carlo-Erba Sorptomat 1900 system. Surface area, cumulative pore volume, and pore size distribution were evaluated with the help of this fully computerized system. Statistical analysis of the results showed that the adsorbates like Nz and Hz behaved as real gases, rendering isotherms with a 2 % deviation at pressures around 1bar. The sample was maintained at various temperatures in the range from -196 to 100 "C by cooling in liquid Nz or by immersing them in a thermostat maintained at the desired temperature. Up to 1-g samples were used for each experiment. The adsorption isotherm was constructed by the admission of successivequantities of gas with the aid of a volumetric dosing technique and application of the gas laws. The dead space volume was previously calibrated for each cell. During nitrogen adsorption isotherm measurements, large quantities of gas were adsorbed at the boiling point of nitrogen. However, at ambient temperature, the quantity of gas adsorbed was relatively small and this made the measurements more difficult at such temperatures. The value of B(T), the second virial coefficient for Nz, Hz, and COz at a given temperature T has been calculated according to the equation B(T) = A
+ c exp(D/T)
cm3mol-' (2) The values for A, C, and D are taken from the literature.' The different samples under study were preheated for 2 h in Ar at 493 K and then outgassed for another 2 h at the same temperature to remove moisture and air, prior to physisorption measurements. BET surface area measurements were carried out on a Micromeritics surface area analyzer (Model 2205). (3) Baltzer, P.; Davidson, R.; Tseung, A.; Gratzel, M.; Kiwi, J. J. Am. Chem. SOC.1984,106,1505. (4) Dymond, J. H.; Smith, B. E. The Virial Coefficients of Gases - A Critical Compilation; Calendron Press: Oxford, England, 1969.
0 1991 American Chemical Society
Langmuir, Vol. 7, No.11,1991 2643
A RulTiOz Catalyst for Selective Methanation Pore a i z e distribution
putation of pore volume as a function of pore radii has been carried out for the P25 blank, as well as for deposited (Figure lb) and impregnated catalysts (Figure IC).Two regions of porosity on the P25 blank are seen in Figure la. On the left-hand side supermesoporesare observed. The right-hand side in Figure l a shows the resence of interparticle porosity at 100 000-500 000 It is therefore evident that the pore size distribution, cumulative pore volume, and pore radii are influenced by the catalyst preparation technique. The shape of the envelope in Figure l a reflects disaggregation of particles in the samplearound 100 000 A where inter- and intraparticle macroporosity is shown. In the bimodal distribution observed in Figure la, intraporosity up to 600 A is observed with mesopores of dimensions of 250 A. Sample collapse under pressure took place below l00A, the lower limit of mercury porosimetry. If the pores are assumed to be cylindrical, the relationship between pore size and the exerted pressure is established by the Washburn equation+
1
0
-
t
"
0
2000
00
0
---
--
-
a
A-
8
.
---
--
\ -
-
For hydrogen chemisorption experimenta, the samples were pretreated in a stream of Hz (150 mL/min) for 1.5 h at 473 K. The samples were then evacuated to lod Torr for 3 h, while the temperature was raised to 573 K at a rate of 3 K/min. Ru content and the level of impurities in the catalysts were estimated by elemental analysis. The level of impurities were as follows: C = 0.13 f 0.02%; C1- = 0.06 f 0.01%; Na+ = 84 f 8 pgjg; and K+ 25 f 5 pg/g.
Results and Discussion Mercury Porosimetry Studies. Figure la-c shows the range of pore sizes available and the cumulative pore volume as a function of pore radii for various catalysts as evaluated by mercury porosimetry using pressures up to 2000 bar. The P25 Ti02 blank was prepared by the same procedure as that of deposition-precipitation, except that Ru was not added to the solution. Assuming a cylindrical pore model" and the absence of micropores, the com(5) Sing, W. K. Pure Appl. Chem. 1980,54, 2201. (6) Carli, F.; Motta, A. J . Phiarm. Sci. 1984, 73, 197. (7) Gfegg, S.; Sing, W. K. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982.
r = 29 cos Q / P (3) where r is the pore radius (A), s is the surface tension of mercury (dyn/cm2), Q is the contact angle, and P is the pressure exerted (kg/cm2). The volume of pores penetrated at each intrusion is registered. By using the Washburn equation, these data are transformed into pore sizes for the entire range. This model assumes constancy of the pore shape geometry throughout the whole range of pore size. Gas adsorpion measurementsare presented later in this study to understand in a more detailed way the pore size distribution of these materials. The pressure required depends on the pore size, and if the pores are cylindrical, the relationship between pore size and the pressure exerted holds well. When parts a and b of Figure 1are compared, the cumulative pore volume for these two samples are the following: 820.2 mm3/g for the P25 blank and 924.5 mm3/g for the 3.8% Ru/TiOz deposited sample. The cumulative pore volume for an impregnated 3.8% Ru/TiOz sample is shown to be 1580 mm3/g. This value reveals that mesoporosity itself is not a fundamental parameter which affects activity since the impregnated samples have shown to be less active in our studies than the deposited Ru The highly dispersed Ru particles, therefore, have little influence on catalyst's pore volume. The 0.95, 1.9, and 7.6% Ru/TiO2 deposited catalysts gave the following cumulative pore volumes: 864.2, 847.2, and 1232.3 mm3/g, respectively. The particle diameter for Ti02 blank particles up to 200 pm is shown in Figure 2a. Figure 2b shows agglomeration in size up to 200 pm for a 3.8 % Ru/TiO2 deposited sample as studied via mercury porosimetry. These aggregates are ensembles of loosely coherent particles." By transmission electron microscopy observations (not reported in this paper), the basic polycrystalline Ti02 unit was found to be 100-500 A (0.01-0.05 pm) in size. What is changed, therefore, is the distribution in the aggregate size of the particle ensemble, as studied by mercury porosity and reported in Figure 2a and b. The hysteresis observed during capillary condensation is shown in Figure 3a-e. It is readily seen from these traces that hysterisis depends on the nature of the catalyst used. The hysteresis curves show that, for Ti02 and for Ru loadings of 0.95, 1.9, 3.8, and 7.6 96, the adsorption and desorption curves deviate from one another. In Figure 3a (8)Kiwi, J.; Italiano, P. J. Mol. Catal. 1989,50, 131. (9) Mercury Pressure Macro pore unit, Carlo Erba, Rodano, Milan, Italy, 1986.
2644 Langmuir, Vol. 7, No.11,1991 100
CY..
2
VOl.
IDm3/01
-
r 0
. 1 I
-
bp
4
0
> 0
U
L 100
0 2000
t
,r-
i
->
bp
0
P)
a
/=
D Particle ( D )
p
Figure 2. (a) Cumulative pore volume vs particle diameter for Ti02 blank (Degussa P25) obtained via mercury porosimetry. (b) Same as in (a) for a 3.8% Ru-RuOJTiOz deposited catalyst.
the two branches are almost vertical and nearly parallel, suggesting the existence of regular agglomerates with narrow pore size distributions.1° Parts b-d of Figure 3 show a progressive change in the two branches to a nearly horizontal and parallelgeometry. In the latter case, the distribution of pore size and shape is not well-defined. Indeed, this type of loop is very difficult to interpret.@ Parts b-d of Figure 3 show an intermediate situation between the two extreme hysteresis loops shown in a and e. Over the last few years, this has been attributed to a difference in the mechanism between condensation and evaporation processes occurring in pores with narrow necks and wide bodies? But this is an oversimplified picture. In Figure 3a (TiO2, Degussa P25) we have practicdly a nonpolar species. Here the pore filling process would involve the formation of a cylindrical meniscus at high pressure. Figure 3e shows the effect of Ru+&polar species on the surface of Ti02 on a Ru-RuO,/TiOa catalyst. The loop observed with a near-horizontal desorption branch would reflect the profound modification of this surface in
the presence of a high concentration of Ru ionic (+a) species. The driving force for the observed hysteresis would be the interaction between Ru ionic species and the adsorbate. For 7.6 76 Ru-RuO,/TiOo, the desorption occurs by the release of the condensate from the whole pore, while in Ti08 (Figure 3a),the desorption will proceed from a heqispherical meniscus.lo Gas Adsorption Studies.The adsorption of a gas on a solid gives valuable information about the surface area and pore structure of the solid. Figure 4 shows N2 adsorption data at 77 K from a 3.8% Ru-RuO,/TiOz deposited sample. Figure 4 indicates that the limiting uptake takes place over a range of high P/Po. The availability of macropores makes the isotherm rise rapidly at PIP0 = 1and confirms the results obtained by mercury porosimetry (reporting macropores) in Figure la,b. The characteristic feature of this isotherm is the hysteresis loop which, associated with capillary condensation, takes place in mesopores. The initial part of the isotherm shows the N2 monolayer whereas the second part shows multilayer adsorption.l&l' Since the adsorption-desorption loopsare so close in Figure
(10) Parfitt, D. G.;Sing,W . K.Characterization of Powder Surfaces; Academic Press: New York, 1976.
(11) Andereon,J. TheStructweofMetollic Catalysts;AcademicPn: New York, 1975.
A RulTi02 Catalyst for Selective Methanation
Langmuir, Vol. 7, No. 11,1991 2645
4, capillary condensation begins in the region prior to the lower closure point of the hysteresis loop. The linear part of the isotherm extrapolated to the adsorption axis gives a small positive intercept. This is equivalent to the micropore v0lume.1~The results of our measurements are presented in Table I. The error factor for the values reported in Table I is &8%. Table I presents values for the monolayer volume, Nz BET area, c point values, and specific pore volumes for the various catalysts under study. This type of isotherm is susceptible to BET analysis if the value of c is not too high (
