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CO2 reforming of methane over zeolite-Y supported ruthenium catalysts
Applied Catalysis A: General 193 (2000) 173–183
CO2 reforming of methane over zeolite-Y supported ruthenium catalysts U.L. Portugal Jr. a , C.M.P. Marques b , E.C.C. Araujo a , E.V. Moralesc,1 , M.V. Giotto a , J.M.C. Bueno a,∗ a
DEQ/UFSCar, Caixa Postal 676, 13565-905 São Carlos, SP, Brazil DQ/UFSCar, Caixa Postal 676, 13565-905 São Carlos, SP, Brazil c Universidad Central de Las Villas, Cuba
b
Received 12 August 1998; received in revised form 3 September 1999; accepted 7 September 1999
Abstract Ru/HY and Ru/NaY catalysts were prepared by ion-exchange from an aqueous solution of [Ru(NH3 )6 ]Cl3 . The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR). The results showed that Si/Al ratio of the zeolite framework after use depends on the catalyst preparation procedure. Upon activation in He, Si/Al ratio in Ru/NaY is similar to that of initial zeolite NaY. After exposure to reaction conditions, NMR data show dealumination, whose extent increases with Ru loading. Samples activated in He followed by ion exchange with NaNO3 (neutralization) and reactivated in CH4 /CO2 /N2 undergo only slight dealumination. The results suggest that during reaction the charge compensating protons, generated during Ru reduction, cause dealumination, accompanied by local destruction of the zeolite structure. The structure of zeolite and Ru dispersion was therefore, stabilized and the catalyst specific activity increased by neutralizing the protons prior to use in CO2 reforming. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium catalyst; Synthesis gas; Methane reforming; Zeolite Y; NMR
1. Introduction
replacing steam by CO2 according to the following equation:
Due to the existence of large reserves of natural gas, transformation of methane into more valuable products has acquired great economic importance. At present, the main use of natural gas in the petrochemical industry is to produce hydrogen and synthesize gas by means of steam reforming with supported Ni catalysts [1]. One of the routes for obtaining chemicals and fuels from methane is hydrogenation of CO from synthesis gas. Reforming of CH4 can also be done by
CH4 +CO2 ⇔ 2CO+2H2
∗ Corresponding author. E-mail address: [email protected] (J.M.C. Bueno). 1 Currently on leave from Universidad Central de Las Villas, Cuba.
1H = 247 kJ mol−1 (1)
Steam reforming of CH4 leads to a high ratio of H2 /CO > 3, whereas reforming of CH4 with CO2 yields a H2 /CO ratio close to 1. The low H2 /CO ratio suffices for producing liquid hydrocarbon fuels, whereas the excess of hydrogen obtained in steam reforming increases formation of light hydrocarbons [2]. The CO2 reforming of CH4 is normally accompanied by secondary reactions. Of these, the reverse water–gas shift, Eq. (2), seems the most important. CO2 + H2 ⇔ CO + H2 O
0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 4 2 6 - 3
1H = 41.2 kJ mol−1 (2)
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Recently, numerous studies have appeared on reforming of CH4 with CO2 [3–17]. One of the greatest challenges is catalyst stability, since catalysts based on non-noble metals deactivate rapidly, due to carbon deposition, when water is replaced by CO2 . Carbon may be formed by the Boudouard reaction or methane cracking, Eqs. (3) and (4), respectively; depending on experimental conditions, these reactions may be thermodynamically favorable. 2CO ⇔ C + CO2
(3)
CH4 ⇔ C + 2H2
(4)
CO2 reforming of CH4 catalysts based on noble metals, e.g., Rh and Ru, on a variety of supports, are usually described as stable, i.e., they do not catalyze carbon formation [12]. However, Zhang et al. [10] showed that with Rh catalysts the deactivation rate depends on the support and particle size of the metal. Deactivation generally occurs as a consequence of three factors: carbon deposition, sintering of metal crystallites, and blocking of metal sites by carbon residue originating from the carrier [10]. Deactivation is observed over Ru/TiO2 and Ru/Al2 O3 [18], or Rh/TiO2 and Rh/MgO catalysts; low deactivation rates are observed over Rh/YSZ, ␥-Al2 O3 , LaO3 and SiO2 catalysts [10]. Bhat and Sachtler [13] who studied Rh supported on crystalline oxide NaY zeolites, obtained catalysts combining extraordinary stability with high activity and selectivity, and turnover frequency depending on proton concentration on NaY. Ru catalysts have always been studied supported on amorphous oxides [3,12,16,17]. The support exerts a great influence on the turnover frequency, specific activity and stability [17,18]. The present paper reports results on catalytic CO2 reforming of CH4 , with NaY-supported Ru and characterization of these Ru/NaY catalysts after a variety of activation procedures. Ru/NaY catalyst activity was compared with Al2 O3 and SiO2 -supported Ru and Rh/NaY catalysts.
2. Experimental 2.1. Catalyst preparation NaY and NH4 Y were prepared from LZ Y-52 (Si/Al = 2.6, Linde Molecular Sieves) by two succes-
sive exchanges with NaNO3 or NH4 NO3 , respectively, according to a method previously described [19,20]. Ruthenium was ion-exchanged into NaY or NH4 Y, from aqueous solutions of [Ru(NH3 )6 ]Cl3 , to achieve a desired weight loading between 0.5 and 3.0 wt.% of Ru. The usual procedure involves slow addition of a 0.002 M [Ru(NH3 )6 ]Cl3 solution to stirred zeolite slurry (1.5 g/l) at room temperature over a 4 h period, followed by additional stirring for 20 h. Samples were filtered and washed at room temperature with doubly deionized water until Cl− free, they were dried in air at room temperature and in a vacuum desiccator for 3 h and resulting samples were stored in a desiccator over a saturated NH4 Cl solution. To prevent [Ru(NH3 )6 ]3+ [21] hydrolysis, samples were used within 24–28 h after preparation. Rhodium was ion-exchanged into NaY from aqueous solutions of [Rh(NH3 )5 Cl]Cl2 , to achieve desired weight loading between 0.5 and 3.0 wt.% of Rh. The usual procedure involves two steps. The first includes the addition of a solution of 0.002 M [Rh(NH3 )5 Cl]Cl2 to a zeolite slurry (3.0 g/l) at 353 K over a 12 h period; secondly, the slurry is stirred for an additional 60 h [22]. After samples were filtered and washed by similar procedures described above for RuNaY samples. Silica-supported Ru was prepared by impregnation of SiO2 (Aerosil 200, Degussa) with an aqueous solution of [Ru(NH3 )6 ]Cl3 and resulting samples were dried in a rough vacuum at room temperature.
2.2. Catalyst activation Ru-supported on zeolite Y was activated by different procedures: (A) Samples of Ru/NaY or Ru/HY activated in He (UHP-5.0, AGA, Brazil) were heated to 773 K at 2 K/min in a total flow of 100 ml/min. These samples were labeled Ru/NaY-A or Ru/HY-A. (B) Sample activated under reaction conditions was heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature at 2 K/min in a total flow of 100 ml/min. This sample was labeled Ru/NaY-B. (C) Samples of Ru/NaY or Ru/HY were activated in He by a procedure similar to that described for sample Ru/NaY-A. They were subsequently submitted to heating from room temperature to 973 K at 2 K/min in flow of CH4 /CO2 /N2 = 20/20/60
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with a total flow of 100 ml/min. These samples were labeled Ru/NaY-C or Ru/HY-C. (D) Sample was activated in He by a procedure similar to that described for the Ru/NaY-A sample. Protons formed during Ru ions reduction in Ru/NaY sample were neutralized by ion exchange in two successive treatments with a 1 M aqueous solution of NaNO3 at room temperature. This sample was labeled RuN /NaY-D. (E) Sample was activated by a procedure similar to that described for sample RuN /NaY-D. Subsequently the sample was heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature at 2 K/min in a total flow of 100 ml/min and labeled RuN /NaY-E. (F) Sample was activated in He by a procedure similar to that described for Ru/NaY-A sample, and then submitted to steam action by heating from room temperature to 973 K at 2 K/min in H2 /H2 O/Ar (5/2.6/92.4 ) at a total flow of 20 ml/min. Afterwards the sample was purged in Ar and labeled Ru/NaYS -F. (G) Sample was first activated by a procedure similar to that described for RuN -NaY-D sample. It was subsequently exposed first to 5% H2 /Ar and heated to 973 K at 2 K/min, then to steam in H2 /H2 O/Ar (5/2.6/92.4) at a total flow of 20 ml/min. This sample was labeled RuN /NaYS -G. Ru/SiO2 sample was activated by a procedure similar to that described for Ru/NaY-C sample. Rh/NaY-B sample was activated by procedure similar to that described for Ru/NaY-B sample. RhN /NaY-E sample was calcined in a flow of O2 at 723 K for 12 h, activated in H2 at 673 K, neutralized and heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature by a procedure similar to that described for RuN /NaY-E sample. 2.3. Catalyst characterization Catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP), X-ray diffraction (XRD), transmission electronic microscopy (TEM), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR). XRD diffraction patterns were collected with a Siemens D-5000 diffractometer using Cu k␣ radia-
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tion (40 kV, 40 mA) with a Ni filter. Data were collected for fresh samples after activation in He and for reaction-used samples. Surface analysis by X-ray photoelectron spectroscopy (XPS) was performed using a XSAM HS spectrometer from Kratos Analytical. Radiation source used to excite the photoelectrons was Mg K␣ (power given by 15 kV and 15 mA). Analysis pressure chamber was in the low 10−9 Torr range. Samples were flooded with low energy electrons from a flood gun to avoid charging effects. 29 Si MAS NMR spectra were carried out using a Varian Unity Plus 400 MHz spectrometer operating at 9.4 T using 7 mm diameter rotors spinning at 5 kHz. Quantitative spectra were measured at 79.5 MHz with 50◦ radiofrequency pulses and 30 s recycle delays. Chemical shifts were given in relation to tetramethylsilane (TMS) adjusted for a single 29 Si resonance at −91.5 ppm of kaolinite, consistent with the presence of a single Q3 (OAl) silicon environment [23]. TEM analyses were performed using a Philips CM-120 microscope equipped with an EDAX CM-120 elemental analysis probe. Microanalytical registers were recorded inclining the sample 15–18◦ according to the specimen holder axis, with X-ray detector axis inclined at a constant 20◦ angle. The incident beam in each area analyzed had a diameter of 20–30 nm; analysis count time was 100 s. Samples for TEM analysis were prepared by deposition of an ultrasonic suspension of catalyst particles in isoamylacetate on grids with carbon film. TPD analyses were performed using a Micromeritics Pulse Chemisorb 2705. After activation, samples were exposed to 5% H2 /Ar for 20 min and purged in Ar at 20 ml/min. The samples were cooled to 253 K, then heated from 253 to 873 K at 10 K/min followed by a 30 min temperature hold. The Ru/NaY and Rh/NaY samples (Table 1) after use at 993 K were submitted to two consecutive TPD analyses. The H2 /Ru ratio was estimated from results of second TPD analyses. 2.4. Reaction Reaction studies were performed in a tubular, fused silica reactor at 1 atm pressure with a CH4 /CO2 /N2 feed of compositions varying between 10–20/20–30/60 or CH4 /CO2 of 50/50 at a total flow rate between 100 and 400 ml/min, with 0.05–0.2 g of
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Table 1 H/M ratio obtained from TPD data, samples M/supported with 1 and 3 wt.% M (M = Rh or Ru) Sample
Activation procedure
TPD (H/M)- sample 1 wt.% M
TPD (H/M)- sample 3 wt.% M
Ru/NaY-A Ru/NaYS -F RuN /NaY-D RuN /NaYS -G Ru/NaY-C Ru/NaY-B RuN /NaY-E Rh/NaY-B RhN /NaY-E Ru/ SiO2 -B
Autored. Only Autored. and treat. in H2 -H2 O Autored., neutr. and activ. in H2 Autored., neutr., activ. in H2 and treat. in H2 -H2 O Autored., used at 993 K Used sample at 993 K Used sample at 993 K Used sample at 993 K Used sample at 993 K Autored. and activ. in H2
0.60 0.31 0.69 0.68 0.20 0.28 0.65 0.28 0.68 –
0.53 0.20 0.53 0.45 – 0.18 0.43 0.18 0.90 0.39
catalyst and temperature range of 723–973 K. Catalysts were maintained in reaction conditions at each test temperature for 8 h. Ru/NaY-C and RuN /NaY-E samples were also maintained in reaction conditions for 300 h. The catalyst bed was supported on quartz wool and the reactor was filled with fused silica chips. Reaction products were analyzed by on-line gas chromatography, employing a combination of two GCs (Varian-3400) having a TCD detector. One chromatograph employed He and the other N2 as carrier gas. For reaction product analysis, two columns of Porapak N and Molecular Sieve 13 X, were used in a series-bypass arrangement. Turnover frequencies (TOFi ) and rates (moli /s) per number of Ru on surface (molRu ) were determined under different CH4 conversions obtained by variation of total flow rate (Table 2). 3. Results and discussion Cl absence in NaY has been verified by XPS for Ru/NaY obtained by encapsulating of [Ru(NH3 )6 ]3+ . Similar results were obtained for Rh/NaY obtained from [Rh(NH3 )5 OH]2+ [22].
Diffraction patterns of Ru/NaY-A samples with Ru loading between 0.5 and 3 wt.% ruthenium after activation in He corresponded to zeolite Y with crystallinity similar to that of the initial NaY zeolite (Figs. 1(a) and 2(a)). XRD pattern analysis of Ru/NaY-B and Ru/NaY-C samples, corresponding to activation in He and then in CH4 /CO2 /N2 or heated directly in CH4 /CO2 /N2 ‘in situ’, respectively, showed a decrease in the intensity of diffraction bands of zeolite Y with increasing Ru-loading (Fig. 1). Similar results were obtained for Ru/NaY-B and Ru/NaY-C samples, suggesting that partial destruction of zeolite matrix increase with increasing Ru-loading. However, with the Ru/HY-C sample a much stronger decrease in intensity of XRD bands characteristic of zeolite structure was registered. XRD pattern analyses of the RuN /NaY-E samples, corresponding to activation in He and neutralization with NaNO3 followed by heating in CH4 /CO2 /N2 (Fig. 2(c)) showed diffraction band intensity similar to that of initial zeolite Y. High resolution solid state 29 Si MAS NMR was used to determine Si/Al ratio of the framework for
Table 2 Turnover frequencies and kinetic parameter for CO hydrogenation to CH4 on supported Ru catalysts Catalysts
H/Ru
γ
δ
kr (Torr−(γ +δ) s−1 )
TOFfor,CO (s−1 )
Reference
1.0% 1.0% 1.6% 0.5% 4.8%
0.28 0.69 0.19 0.051 0.085
1.7 1.7 1.6 2.0 1.8
−0.55 −0.55 −0.6 −0.5 −0.55
0.0175 0.0065 0.087 0.026 0.002
0.65 0.28 3.5 9.1 0.24
This study This study [18] [18] [18]
Ru/NaY-B RuN /NaY-D Ru/-Al2 O3 Ru/TiO2 Ru/C
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Fig. 1. XRD: (a) NaY; (b) 1 wt.% Ru/NaY-B; (c) 3 wt.% Ru/NaY-B.
Fig. 2. XRD: (a) 3 wt.% Ru/NaY-A; (b) 3 wt.% Ru/NaY-C; (c) 3 wt.% RuN /NaY-E.
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Fig. 3. 29 Si MAS NMR of (a) NaY; (b) 1 wt.% Ru/NaY-B; (c) 3 wt.% Ru/NaY-B.
Ru/NaY samples treated by the various activation procedures. The initial NaY 29 Si MAS spectrum presented four resonances occurring at −87.9, −93.5, −99.4, −104.5 ppm corresponding, respectively, to Si(3Al), Si(2Al), Si(1Al) and Si(0Al) environments with Si/Al = 2.6, Fig. 3(a). Si/Al ratio accuracy calculated by integrating Gaussian curves was around 5% [24,25]. Ru/NaY-B and Ru/NaY-C samples with different Ru loading showed that Si/Al ratio increase is higher for samples with higher Ru loading. See Fig. 3(b–c). 29 Si MAS NMR spectra of Ru/NaY with Ru loading between 0.5 and 3 wt.% Ru activated only in He according to procedure A (Ru/NaY-A) showed a Si/Al ratio similar to that of the initial NaY (see Figs. 3(a) and 4(a)). Dealumination of Ru/NaY samples with 3 wt.% Ru, can be minimized, from Si/Al = 3.2 to 2.7, in the case of samples neutralized after activation in He, which were labeled RuN /NaY-E (Figs. 3(c) and 4(b)). The 3% Ru/HY sample activated by procedure B or C (Ru/HY-B or Ru/HY-C) showed stronger dealumination, from Si/Al = 2.6 (NH4 Y initial) to Si/Al = 3.7, Fig. 4(c). However, some silanes may have been
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Fig. 4. 29 Si MAS NMR of 3% Ru/NaY at different conditions: (a) Ru/NaY-A; (b) RuN /NaY-E; (c) 3% Ru/HY-B.
present in the 3% Ru/HY zeolite framework, affecting spectra deconvolution accuracy. McCarthy et al. [19] showed that auto-reduction of Ru is complete upon decomposing [Ru(NH3 )6 ]3+ /NaY by heating in He to 773 K. The metal clusters formed are smaller than the Y-supercage and invisible in TEM. Results derived from EXAFS suggest a coordination number of 0.6–0.8 [19]. Similar TEM results were obtained by us for the sample auto-reduced in He (Ru/NaY-A). TEM results of Ru/NaY-B and Ru/NaY-C samples, activated, respectively, in situ or in He after use in the reforming reaction, show that Ru particles are highly dispersed, visible in TEM, and with similarly sized from about 10–30 Å, regardless of Ru loading varying between 0.5 and 3.0 wt.%. Fig. 5(a and b) shows the bright field images of catalysts containing 1 and 3 wt.% of Ru and EDAX elemental analysis reveals the presence of Ru. Fig. 6 shows the micrographs in a dark field exhibiting a large amount of illuminated ruthenium particles inside the zeolites, with the same orientation for diffraction. TEM results of neutralized RuN /NaY-E used under reaction conditions, show small particles (10–30 Å) similar to those shown in the results observed for the sample activated in situ (Ru/NaY-B). However, some particles
can be visualized in RuN /NaY-E, although the number of oriented particles was very much smaller than that observed for Ru/NaY-B and Ru/NaY-C samples. The samples that were submitted to neutralization (Ru/NaY-A) presented only Ru particles smaller than the zeolite Y supercage (13 Å), results similar to those described by McCarthy et al. [19]. TPD results of 1% Ru/NaY auto-reduced in He (Ru/NaY-A) showed a higher H/Ru ratio of 0.60, similar to the H/Ru ratio of 0.69 obtained for samples submitted to neutralization (RuN /NaY-D). TPD results of 1% Ru/NaY auto-reduced in He, when submitted to steam action (Ru/NaYS -F), showed a marked decrease in H/Ru ratio from 0.60 to 0.31. On the other hand TPD results of 1% RuN /NaYS -G auto-reduced and neutralized, when submitted to H2 O/H2 steam action, showed a smaller decrease in H/Ru ratio from 0.69 to 0.68. TPD results of neutralized 1% RuN /NaY-E after use in reaction at 973 K for 300 h, showed a H/Ru ratio of 0.65 similar to sample before use. While the 1% Ru/NaY non-neutralized samples when submitted to reaction showed a Ru agglomeration with strong decrease of H/Ru ratio from 0.60 (Ru/NaY-A) to 0.28 (Ru/NaY-C). TPD results of 3% Ru/NaY submitted to steam action showed a tendency similar to that presented by 1% Ru/NaY samples (see Table 1). 29 Si MAS NMR spectra of 3% RuN /NaYS -G, auto-reduced and neutralized when submitted to steam action (H2 O/H2 ) showed a Si/Al ratio similar to initial NaY. The auto-reduced 3% Ru/NaYS -F sample, when submitted to steam action demonstrated an increase of Si/Al ratio in the zeolite framework structure (Si/Al = 3.0), similar to that of the samples Ru/NaY-B and Ru/NaY-C used in the reaction. Thermodynamic equilibrium was not reached in experimental conditions adopted over the temperature range of 723–973 K, but for runs with 0.200 g of catalyst at temperatures around 973 K data of the equilib2 × rium constant Kp were close to the ratio Q = PCO −1 −1 2 PH2 ×PCH4 ×PCO2 . Ru/HY-B or Ru/NaY catalysts activated according to different procedures (Ru/NaY-B, Ru/NaY-C or RuN /NaY-E) resulted in lower H2 /CO ratios at lower temperatures, when equilibrium was not reached. These results suggest that at lower temperatures the reverse water–gas shift reaction (WGSR) prevails (see Table 3), in agreement with thermodynamics [13] and reaction of CO hydrogenation to CH4 can be present [18].
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179
Fig. 5. TEM bright field images: (a) 1 wt.% Ru/NaY-B 420 000X (after reaction), (b) 3 wt.% Ru/NaY 320 000X (sample used).
Although NMR results for Ru/HY-B or Ru/NaY-B catalysts showed a strong increase in Si/Al ratio in the zeolite framework structure, these catalyst samples show a remarkable stability in activity and selectiv-
ity in reaction conditions. Independent of activation method adopted. Ru/NaY-B or RuN /NaY-E catalyst samples were maintained for 300 h on stream, with different partial pressures of CH4 between 0.2 and
Table 3 Reaction dataa at temperature of 773 K for samples M/supported (M = Rh or Ru) prepared by different procedures Catalysts
M (wt.%)
Wt (g)
H/Mb
Conv.% (CH4 )
Conv.% (CO2 )
H2 /CO
r (mol CH4 /g Ru.h)c
Ru/NaY-B
0.5 1.0 3.0 3.0 1.0 3.0 3.0 3.0 1.0 3.0 3.0 3.0
0.202 0.201 0.050 0.200 0.202 0.050 0.050 0.050 0.201 0.200 0.200 0.050
0.28 0.18 0.17 0.68 0.45 0.20 0.45 n.d. 0.18 0.90 0.39
6.8 9.9 7.5 13.3 9.8 8.2 5.8 8.5 8.0 5.4 9.0 1.6
10.8 14.8 10.5 19.2 14.6 10.6 7.3 13.4 11.6 6.6 9.6 3.57
0.54 0.60 0.66 0.63 0.61 0.74 0.76 0.55 0.63 0.79 0.93 0.22
3.30 2.44 2.51 1.11 2.43 2.76 1.94 2.84 1.87 0.47 0.74 0.53
Ru/NaY-C RuN /NaY-E Ru/NaYS -F RuN /NaYS -G Ru/HY-B Rh/NaY-B Rh/NaY-E Ru/SiO2 -B
Reaction condition: 100 ml/min, CH4 /CO2 /N2 (20/20/60), PCH4 = 200 Torr. Ratio H/M determinates as shown in Table 1. c Reaction rate average. a
b
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Fig. 6. TEM dark field image of 1 wt.% Ru/NaY-B 250 000X (sample used).
0.5 atm, with molar ratio CH4 /CO2 = 1 operating in a cycle of increasing and decreasing temperature. Within this period, catalyst activity and selectivity were constant. TPO results of Ru/NaY-B and RuN /NaY-E catalyst samples cooled in feed, followed
Fig. 7. Observed TOF for CO formation at 723 K as a function −0.55 PCO for (䊏) 1% Ru/NaY-B and (䊊) 1% Ru/NaY-E. of PH1.7 2 Reaction conditions: CH4 : CO2 : N2 = 1 : 1 : 3 and P = 1 atm.
by purging in Ar, showed low signals due to CO and CO2 , independent of time-on-stream. These results suggest that dealumination of the samples occurs at the initial stage of heating in CH4 /CO2 /N2 . Catalyst stabilization happens at temperatures below 773 K. Similar results were obtained previously for Rh/NaY catalyst [13]. Under similar reaction conditions for Ru/NaY, Ru/Al2 O3 [17,18] and Ru/SiO2 -B catalysts, Ru/NaY showed higher stability than supported Ru over amorphous materials like a SiO2 and Al2 O3. A lower H2 /CO ratio was initially obtained for Ru/SiO2 -B (H2 /CO = 0.22, at 773 K) and the H2 /CO ratio increases with time-on-stream followed by carbon deposition. For CO2 reforming, high temperatures are required, and one might wonder whether the CO2 in the reaction mixture can oxidize Ru to one of its oxides. However, in view of known volatility of RuO2 and RuO4 , the high stability of the present catalysts used indicates that no such oxides were formed under the reaction conditions employed. Ru catalysts supported on amorphous materials such as Al2 O3 or MgO have been utilized in methane reforming with CO2 in mixtures of CO2 and H2 O [3,12,16]. In these studies, however, no mention has been made about Ru loss. High stability Ru particles in O2 atmosphere were found when stabilized within SiO2 framework structure, with Ru particle size reported between 10 and 25 Å [26]. With the different activation procedures and preparations, activity (molCH4 conv./h g-Ru) decreases in the order RuN /NaY-E > Ru/NaY-B > Ru/NaY-C (Table 3), a similar activity order having been reported for Rh/NaY catalysts activated by different procedures [13]. Within experimental conditions for studying the reactions in this work, the interfacial gradient of temperature and concentration [27], confirms absence of external mass and heat transfer resistances. However, TOF values for 1.0% Ru/NaY-B and 1.0% RuN /NaY-D samples at 723 K were very dependent on methane conversion. Supported Ru catalysts are very active for CO [18] and CO2 [28] hydrogenation. As discussed previously by Bradford and Vannice [18] the CO hydrogenation reaction coupled with the water–gas shift reaction, which is quasi-equilibrated in these conditions, constitutes the overall reverse reaction. Similar to that described for Ru over amorphous support [18], TOFCH4 or TOFCO observed
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181
Table 4 Turnover frequencies of catalysts at temperature of 723 Ka Catalysts Catalysts
GHSV Conv. Conv. TOFCH4 TOFfor CH4 Catalysts (h−1 ) (%) CH4 (%) CO2 (s−1 × 103 ) (s−1 × 103 )
Ru/NaY-B 3.0 wt.% Ru Ru/NaY-B 1.0 wt.% Ru Ru/NaY-B 0.5 wt.% Ru Ru/NaYS -F 3.0 wt.% Ru Ru/-Al2 O3 1.6 wt.% Rub
72 000
5.2
7.4
160
263c
120 000 2.1
3.5
202
226c
28 000
3.4
5.4
155
200c
37 500
5.6
7.3
80.0
210c
12.3
970
970
595 500 6.2
RuN /NaY-E 1.0 wt.% Ru RuN /NaY-E 3.0 wt.% Ru RuN /NaYS -Ga 3.0 wt.% Ru Rh/NaY-B 3 wt.% Rh Rh/NaY-E 3 wt.% Rh Ru/SiO2 -B 3 wt.% Ru
GHSV Conv. Conv. TOFCH4 TOFfor CH4 (h−1 ) (%) CH4 (%) CO2 (s−1 × 103 ) (s−1 × 103 ) 120 000 2.3
3.1
90.4
100d
75 000
6.1
8.0
78.2
109d
75 000
5.9
9.3
75.8
96.6d
25 000
2.0
2.6
14.0
–
50 000
2.2
2.6
9.4
–
25 000
2.2
4.9
12.6
–
PCH4 = 200 Torr; CH4 /CO2 = 1.0. Ref. [18]. c Calculated from Eq. (5) with parameters from Table 2, where k = 0.0175 (Torr−(γ +δ) s−1 ) for samples series Ru/NaY-B. r d Calculated from Eq. (5) with parameters from Table 2, where k = 0.0065 (Torr−(γ +δ) s−1 ) for samples series RuN /NaY. r a
b
(TOFobs ) at 723 K for Ru/NaY-B and RuN /NaY-E γ δ (Fig. 7) and show a linear correlation with PH2 PCO TOFobs can be represented approximated by expression power rate law expression γ
δ TOFobs = TOFfor − TOFrev = TOFfor − kr PH2 PCO
(5) Table 2 shows TOFfor (turnover frequency for the forward CO during CO2 –CH4 reforming reaction at 723 K) and parameters γ and δ values obtained for TOFCO over 1% Ru/NaY-B and 1% RuN /NaY-E. This table also presents results described in literature [18] for Ru/-Al2 O3 , Ru/TiO2 and Ru/C catalysts, data for which were correlated in Eq. (5). Higher TOF values for Ru/NaY-B corresponds to samples showing dealumination. Ru/NaY-B or -D presented higher activity than Rh/NaY-B and RhN /NaY-D (Tables 2 and 4). Ru/-Al2 O3 catalyst showed higher TOF than both Ru/NaY, TOF values for Ru over different supports, showed the TOF order Ru/-Al2 O3 Ru/NaY Ru/SiO2 . Ru/NaY samples were free of Cl− while Ru/-Al2 O3 [18], showed an initial 2.5 Cl/Ru ratio. Bradford and Vannice [18] showed that, although the additional studies are necessary, the literature data suggest the possibility that Cl presence could promote CO2 –CH4 reforming over Ru catalysts.
For the Ru/NaY-B catalyst activated in situ and non-neutralized, TOFs showed slight increases with Ru loading increases and consequently with Ru dispersion decreases (Tables 1 and 4). The present NMR results for samples activated in situ show that dealumination also increases with Ru loading. Although for neutralized protons (RuN /NaY-E) TOFs were lower than for non-neutralized samples. For a given Ru loading, neutralized protons (RuN /NaY-E) lead to a slight enhanced specific activity (molCH4 conv./h g-Ru). This neutralization suppresses dealumination, as the NMR results show. Dealumination enhances Ru particles agglomeration, although this is not evident for the TEM data which only show the effect of Ru loading on particle size, which is evident for the TPD of hydrogen (Table 1). Most of the literature shows that protons can act as ‘chemical anchors’ for metal particles in zeolite cages, thus, stabilizing high metal dispersion [29,30]. However, the present results indicate that in the presence of H2 O and at very high temperature protons destabilize metal dispersion, because they favor zeolite dealumination [31]. The dealumination creates voids that are only partially filled by Al2 O3 . Although the aluminum extra framework was not extracted from NaY structure, TEM and TPD results show that Ru particles inside such voids are likely to agglomerate. Under such conditions where dealumination is likely, catalyst neutralization before activation in He will exhibit
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higher steady-state activity and higher specific activity for reactions catalyzed by the zeolite supported metal. Dispersion of RuN /NaY obtained after activation in He will be stable under reaction conditions. Ru/NaY-A sample when neutralized and activated in H2 (RuN /NaY-A) show similar Ru dispersion. Consequently this particle should present similar free energy for possible growth of particles by migration inside zeolyte structures, but Ru particles easily agglomerate in non-neutralized Ru/NaY under reaction conditions (Ru/NaY-B). McCarthy [19] has shown that a small amount of protons, present in Ru/NaY activated in He, are consumed in the supercage region when exposed to CO, forming Run + carbonyl species. Similar results was found for Rh/NaY [32], forming Rh+ (CO)2 . However, dicarbonyl cations might exist during the period in which Ru/NaY catalysts are heating in contact with reactants and this carbonyl species can increase Ru mobility, consequently promoting Ru agglomeration. Results of experiments heating Ru/NaY non-neutralized sample under an H2 /H2 O stream free of CO, show that at high temperatures protons destabilize metal dispersion (Ru/NaYS -F — Table 1). However, Ru/NaY neutralized under an H2 /H2 O stream show a stable Ru dispersion, independent of CO presence, because protons favor dealumination with destruction of the encaging zeolite framework. Ru/NaY samples obtained by different preparation methods were grouped as following: (a) neutralized samples and (b) non-neutralized samples. TOF’s value for neutralized samples were corrected from kinetics parameters obtained for 1% Ru/NaY-D neutralized samples and calculated TOFfor for unidirectional forward rates for the CO2 /CH4 reforming reaction on Ruthenium in ‘standard’ conditions (see Table 4). Similar procedure was utilized for non-neutralized samples, utilizing kinetics parameter from 1% Ru/NaY-B non-neutralized samples. The ratio between TOFfor,CH4 for Ru/NaY-B and TOFfor,CH4 for RuN /NaY-D samples was approximately 2 (see Table 4). Ru loading caused dealumination, creating voids in the crystalline structure. However, Ru loading did not change those TOF ratios. Apparent activation energy for CH4 (ECH4 ) over 1% Ru/NaY-B and 1% RuN /NaY-D samples, obtained from TOFfor values, were 27.4 and 25.5 kcal/mol, respectively. These ECH4 values were similar to those
obtained for Ru/Al2 O3 [18]. From these results, it is reasonable to assume a reaction rate not limited by pore diffusion. The dealumination cause a minor modification of encaging zeolite framework of NaY and the possibility that slight increase of activity for Ru/NaY-B was caused by modification of diffusion effects inside microporous particle cannot be excluded.
4. Conclusions Under experimental conditions of reaction utilized, Ru/NaY prepared by ion-exchange from an aqueous solution of [Ru(NH3 )6 ]Cl3 was a potential catalyst for CO2 reforming of CH4 . Ru/NaY showed higher stability than Ru/SiO2 and, unlike Ru/SiO2 , did not deactivate during a period of 300 h on stream. The Si/Al ratio in the zeolite framework, and stability of Ru dispersion on NaY depends on the preparation procedures. The protons formed by reduction of Ru in the supercage destabilized zeolite structure in the presence of reaction products, favoring dealumination of the zeolite. Dealumination under reforming reaction conditions was minimized by neutralizing the protons formed during auto-reduction of [Ru(NH3 )6 ]3+ in He. The TEM micrographs showed uniform Ru particles having sizes of ∼ =10–30 Å. Higher Ru dispersion and stability were found for samples whose protons were neutralized before use as catalysts.
Acknowledgements The authors gratefully acknowledge financial support of FAPESP (Fundação para o Amparo a Pesquisa do Estado de São Paulo), proc 96/7853-1 and 96/9434-6 (Brazil). We wish to thank Professor Wolfgang M.H. Sachtler, V.N. Ipatieff Laboratory, Center for Catalysis and Surface Science, Department of Chemistry, Northwestern University, IL, USA, for helpful discussions and manuscript suggestions and Professor Hans-Jürgen Kestenbach, Department of Materials Engineering, Universidade Federal de São Carlos, SP, Brazil, for his permission to use an electron microscope. U. Portugal Jr acknowledges support from CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior — Brazil).
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CO2 reforming of methane over zeolite-Y supported ruthenium catalysts U.L. Portugal Jr. a , C.M.P. Marques b , E.C.C. Araujo a , E.V. Moralesc,1 , M.V. Giotto a , J.M.C. Bueno a,∗ a
DEQ/UFSCar, Caixa Postal 676, 13565-905 São Carlos, SP, Brazil DQ/UFSCar, Caixa Postal 676, 13565-905 São Carlos, SP, Brazil c Universidad Central de Las Villas, Cuba
b
Received 12 August 1998; received in revised form 3 September 1999; accepted 7 September 1999
Abstract Ru/HY and Ru/NaY catalysts were prepared by ion-exchange from an aqueous solution of [Ru(NH3 )6 ]Cl3 . The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and nuclear magnetic resonance (NMR). The results showed that Si/Al ratio of the zeolite framework after use depends on the catalyst preparation procedure. Upon activation in He, Si/Al ratio in Ru/NaY is similar to that of initial zeolite NaY. After exposure to reaction conditions, NMR data show dealumination, whose extent increases with Ru loading. Samples activated in He followed by ion exchange with NaNO3 (neutralization) and reactivated in CH4 /CO2 /N2 undergo only slight dealumination. The results suggest that during reaction the charge compensating protons, generated during Ru reduction, cause dealumination, accompanied by local destruction of the zeolite structure. The structure of zeolite and Ru dispersion was therefore, stabilized and the catalyst specific activity increased by neutralizing the protons prior to use in CO2 reforming. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium catalyst; Synthesis gas; Methane reforming; Zeolite Y; NMR
1. Introduction
replacing steam by CO2 according to the following equation:
Due to the existence of large reserves of natural gas, transformation of methane into more valuable products has acquired great economic importance. At present, the main use of natural gas in the petrochemical industry is to produce hydrogen and synthesize gas by means of steam reforming with supported Ni catalysts [1]. One of the routes for obtaining chemicals and fuels from methane is hydrogenation of CO from synthesis gas. Reforming of CH4 can also be done by
CH4 +CO2 ⇔ 2CO+2H2
∗ Corresponding author. E-mail address: [email protected] (J.M.C. Bueno). 1 Currently on leave from Universidad Central de Las Villas, Cuba.
1H = 247 kJ mol−1 (1)
Steam reforming of CH4 leads to a high ratio of H2 /CO > 3, whereas reforming of CH4 with CO2 yields a H2 /CO ratio close to 1. The low H2 /CO ratio suffices for producing liquid hydrocarbon fuels, whereas the excess of hydrogen obtained in steam reforming increases formation of light hydrocarbons [2]. The CO2 reforming of CH4 is normally accompanied by secondary reactions. Of these, the reverse water–gas shift, Eq. (2), seems the most important. CO2 + H2 ⇔ CO + H2 O
0926-860X/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 4 2 6 - 3
1H = 41.2 kJ mol−1 (2)
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Recently, numerous studies have appeared on reforming of CH4 with CO2 [3–17]. One of the greatest challenges is catalyst stability, since catalysts based on non-noble metals deactivate rapidly, due to carbon deposition, when water is replaced by CO2 . Carbon may be formed by the Boudouard reaction or methane cracking, Eqs. (3) and (4), respectively; depending on experimental conditions, these reactions may be thermodynamically favorable. 2CO ⇔ C + CO2
(3)
CH4 ⇔ C + 2H2
(4)
CO2 reforming of CH4 catalysts based on noble metals, e.g., Rh and Ru, on a variety of supports, are usually described as stable, i.e., they do not catalyze carbon formation [12]. However, Zhang et al. [10] showed that with Rh catalysts the deactivation rate depends on the support and particle size of the metal. Deactivation generally occurs as a consequence of three factors: carbon deposition, sintering of metal crystallites, and blocking of metal sites by carbon residue originating from the carrier [10]. Deactivation is observed over Ru/TiO2 and Ru/Al2 O3 [18], or Rh/TiO2 and Rh/MgO catalysts; low deactivation rates are observed over Rh/YSZ, ␥-Al2 O3 , LaO3 and SiO2 catalysts [10]. Bhat and Sachtler [13] who studied Rh supported on crystalline oxide NaY zeolites, obtained catalysts combining extraordinary stability with high activity and selectivity, and turnover frequency depending on proton concentration on NaY. Ru catalysts have always been studied supported on amorphous oxides [3,12,16,17]. The support exerts a great influence on the turnover frequency, specific activity and stability [17,18]. The present paper reports results on catalytic CO2 reforming of CH4 , with NaY-supported Ru and characterization of these Ru/NaY catalysts after a variety of activation procedures. Ru/NaY catalyst activity was compared with Al2 O3 and SiO2 -supported Ru and Rh/NaY catalysts.
2. Experimental 2.1. Catalyst preparation NaY and NH4 Y were prepared from LZ Y-52 (Si/Al = 2.6, Linde Molecular Sieves) by two succes-
sive exchanges with NaNO3 or NH4 NO3 , respectively, according to a method previously described [19,20]. Ruthenium was ion-exchanged into NaY or NH4 Y, from aqueous solutions of [Ru(NH3 )6 ]Cl3 , to achieve a desired weight loading between 0.5 and 3.0 wt.% of Ru. The usual procedure involves slow addition of a 0.002 M [Ru(NH3 )6 ]Cl3 solution to stirred zeolite slurry (1.5 g/l) at room temperature over a 4 h period, followed by additional stirring for 20 h. Samples were filtered and washed at room temperature with doubly deionized water until Cl− free, they were dried in air at room temperature and in a vacuum desiccator for 3 h and resulting samples were stored in a desiccator over a saturated NH4 Cl solution. To prevent [Ru(NH3 )6 ]3+ [21] hydrolysis, samples were used within 24–28 h after preparation. Rhodium was ion-exchanged into NaY from aqueous solutions of [Rh(NH3 )5 Cl]Cl2 , to achieve desired weight loading between 0.5 and 3.0 wt.% of Rh. The usual procedure involves two steps. The first includes the addition of a solution of 0.002 M [Rh(NH3 )5 Cl]Cl2 to a zeolite slurry (3.0 g/l) at 353 K over a 12 h period; secondly, the slurry is stirred for an additional 60 h [22]. After samples were filtered and washed by similar procedures described above for RuNaY samples. Silica-supported Ru was prepared by impregnation of SiO2 (Aerosil 200, Degussa) with an aqueous solution of [Ru(NH3 )6 ]Cl3 and resulting samples were dried in a rough vacuum at room temperature.
2.2. Catalyst activation Ru-supported on zeolite Y was activated by different procedures: (A) Samples of Ru/NaY or Ru/HY activated in He (UHP-5.0, AGA, Brazil) were heated to 773 K at 2 K/min in a total flow of 100 ml/min. These samples were labeled Ru/NaY-A or Ru/HY-A. (B) Sample activated under reaction conditions was heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature at 2 K/min in a total flow of 100 ml/min. This sample was labeled Ru/NaY-B. (C) Samples of Ru/NaY or Ru/HY were activated in He by a procedure similar to that described for sample Ru/NaY-A. They were subsequently submitted to heating from room temperature to 973 K at 2 K/min in flow of CH4 /CO2 /N2 = 20/20/60
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with a total flow of 100 ml/min. These samples were labeled Ru/NaY-C or Ru/HY-C. (D) Sample was activated in He by a procedure similar to that described for the Ru/NaY-A sample. Protons formed during Ru ions reduction in Ru/NaY sample were neutralized by ion exchange in two successive treatments with a 1 M aqueous solution of NaNO3 at room temperature. This sample was labeled RuN /NaY-D. (E) Sample was activated by a procedure similar to that described for sample RuN /NaY-D. Subsequently the sample was heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature at 2 K/min in a total flow of 100 ml/min and labeled RuN /NaY-E. (F) Sample was activated in He by a procedure similar to that described for Ru/NaY-A sample, and then submitted to steam action by heating from room temperature to 973 K at 2 K/min in H2 /H2 O/Ar (5/2.6/92.4 ) at a total flow of 20 ml/min. Afterwards the sample was purged in Ar and labeled Ru/NaYS -F. (G) Sample was first activated by a procedure similar to that described for RuN -NaY-D sample. It was subsequently exposed first to 5% H2 /Ar and heated to 973 K at 2 K/min, then to steam in H2 /H2 O/Ar (5/2.6/92.4) at a total flow of 20 ml/min. This sample was labeled RuN /NaYS -G. Ru/SiO2 sample was activated by a procedure similar to that described for Ru/NaY-C sample. Rh/NaY-B sample was activated by procedure similar to that described for Ru/NaY-B sample. RhN /NaY-E sample was calcined in a flow of O2 at 723 K for 12 h, activated in H2 at 673 K, neutralized and heated in CH4 /CO2 /N2 = 20/20/60 to reaction temperature by a procedure similar to that described for RuN /NaY-E sample. 2.3. Catalyst characterization Catalysts were characterized by inductively coupled plasma atomic emission spectroscopy (ICP), X-ray diffraction (XRD), transmission electronic microscopy (TEM), temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance (NMR). XRD diffraction patterns were collected with a Siemens D-5000 diffractometer using Cu k␣ radia-
175
tion (40 kV, 40 mA) with a Ni filter. Data were collected for fresh samples after activation in He and for reaction-used samples. Surface analysis by X-ray photoelectron spectroscopy (XPS) was performed using a XSAM HS spectrometer from Kratos Analytical. Radiation source used to excite the photoelectrons was Mg K␣ (power given by 15 kV and 15 mA). Analysis pressure chamber was in the low 10−9 Torr range. Samples were flooded with low energy electrons from a flood gun to avoid charging effects. 29 Si MAS NMR spectra were carried out using a Varian Unity Plus 400 MHz spectrometer operating at 9.4 T using 7 mm diameter rotors spinning at 5 kHz. Quantitative spectra were measured at 79.5 MHz with 50◦ radiofrequency pulses and 30 s recycle delays. Chemical shifts were given in relation to tetramethylsilane (TMS) adjusted for a single 29 Si resonance at −91.5 ppm of kaolinite, consistent with the presence of a single Q3 (OAl) silicon environment [23]. TEM analyses were performed using a Philips CM-120 microscope equipped with an EDAX CM-120 elemental analysis probe. Microanalytical registers were recorded inclining the sample 15–18◦ according to the specimen holder axis, with X-ray detector axis inclined at a constant 20◦ angle. The incident beam in each area analyzed had a diameter of 20–30 nm; analysis count time was 100 s. Samples for TEM analysis were prepared by deposition of an ultrasonic suspension of catalyst particles in isoamylacetate on grids with carbon film. TPD analyses were performed using a Micromeritics Pulse Chemisorb 2705. After activation, samples were exposed to 5% H2 /Ar for 20 min and purged in Ar at 20 ml/min. The samples were cooled to 253 K, then heated from 253 to 873 K at 10 K/min followed by a 30 min temperature hold. The Ru/NaY and Rh/NaY samples (Table 1) after use at 993 K were submitted to two consecutive TPD analyses. The H2 /Ru ratio was estimated from results of second TPD analyses. 2.4. Reaction Reaction studies were performed in a tubular, fused silica reactor at 1 atm pressure with a CH4 /CO2 /N2 feed of compositions varying between 10–20/20–30/60 or CH4 /CO2 of 50/50 at a total flow rate between 100 and 400 ml/min, with 0.05–0.2 g of
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Table 1 H/M ratio obtained from TPD data, samples M/supported with 1 and 3 wt.% M (M = Rh or Ru) Sample
Activation procedure
TPD (H/M)- sample 1 wt.% M
TPD (H/M)- sample 3 wt.% M
Ru/NaY-A Ru/NaYS -F RuN /NaY-D RuN /NaYS -G Ru/NaY-C Ru/NaY-B RuN /NaY-E Rh/NaY-B RhN /NaY-E Ru/ SiO2 -B
Autored. Only Autored. and treat. in H2 -H2 O Autored., neutr. and activ. in H2 Autored., neutr., activ. in H2 and treat. in H2 -H2 O Autored., used at 993 K Used sample at 993 K Used sample at 993 K Used sample at 993 K Used sample at 993 K Autored. and activ. in H2
0.60 0.31 0.69 0.68 0.20 0.28 0.65 0.28 0.68 –
0.53 0.20 0.53 0.45 – 0.18 0.43 0.18 0.90 0.39
catalyst and temperature range of 723–973 K. Catalysts were maintained in reaction conditions at each test temperature for 8 h. Ru/NaY-C and RuN /NaY-E samples were also maintained in reaction conditions for 300 h. The catalyst bed was supported on quartz wool and the reactor was filled with fused silica chips. Reaction products were analyzed by on-line gas chromatography, employing a combination of two GCs (Varian-3400) having a TCD detector. One chromatograph employed He and the other N2 as carrier gas. For reaction product analysis, two columns of Porapak N and Molecular Sieve 13 X, were used in a series-bypass arrangement. Turnover frequencies (TOFi ) and rates (moli /s) per number of Ru on surface (molRu ) were determined under different CH4 conversions obtained by variation of total flow rate (Table 2). 3. Results and discussion Cl absence in NaY has been verified by XPS for Ru/NaY obtained by encapsulating of [Ru(NH3 )6 ]3+ . Similar results were obtained for Rh/NaY obtained from [Rh(NH3 )5 OH]2+ [22].
Diffraction patterns of Ru/NaY-A samples with Ru loading between 0.5 and 3 wt.% ruthenium after activation in He corresponded to zeolite Y with crystallinity similar to that of the initial NaY zeolite (Figs. 1(a) and 2(a)). XRD pattern analysis of Ru/NaY-B and Ru/NaY-C samples, corresponding to activation in He and then in CH4 /CO2 /N2 or heated directly in CH4 /CO2 /N2 ‘in situ’, respectively, showed a decrease in the intensity of diffraction bands of zeolite Y with increasing Ru-loading (Fig. 1). Similar results were obtained for Ru/NaY-B and Ru/NaY-C samples, suggesting that partial destruction of zeolite matrix increase with increasing Ru-loading. However, with the Ru/HY-C sample a much stronger decrease in intensity of XRD bands characteristic of zeolite structure was registered. XRD pattern analyses of the RuN /NaY-E samples, corresponding to activation in He and neutralization with NaNO3 followed by heating in CH4 /CO2 /N2 (Fig. 2(c)) showed diffraction band intensity similar to that of initial zeolite Y. High resolution solid state 29 Si MAS NMR was used to determine Si/Al ratio of the framework for
Table 2 Turnover frequencies and kinetic parameter for CO hydrogenation to CH4 on supported Ru catalysts Catalysts
H/Ru
γ
δ
kr (Torr−(γ +δ) s−1 )
TOFfor,CO (s−1 )
Reference
1.0% 1.0% 1.6% 0.5% 4.8%
0.28 0.69 0.19 0.051 0.085
1.7 1.7 1.6 2.0 1.8
−0.55 −0.55 −0.6 −0.5 −0.55
0.0175 0.0065 0.087 0.026 0.002
0.65 0.28 3.5 9.1 0.24
This study This study [18] [18] [18]
Ru/NaY-B RuN /NaY-D Ru/-Al2 O3 Ru/TiO2 Ru/C
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Fig. 1. XRD: (a) NaY; (b) 1 wt.% Ru/NaY-B; (c) 3 wt.% Ru/NaY-B.
Fig. 2. XRD: (a) 3 wt.% Ru/NaY-A; (b) 3 wt.% Ru/NaY-C; (c) 3 wt.% RuN /NaY-E.
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Fig. 3. 29 Si MAS NMR of (a) NaY; (b) 1 wt.% Ru/NaY-B; (c) 3 wt.% Ru/NaY-B.
Ru/NaY samples treated by the various activation procedures. The initial NaY 29 Si MAS spectrum presented four resonances occurring at −87.9, −93.5, −99.4, −104.5 ppm corresponding, respectively, to Si(3Al), Si(2Al), Si(1Al) and Si(0Al) environments with Si/Al = 2.6, Fig. 3(a). Si/Al ratio accuracy calculated by integrating Gaussian curves was around 5% [24,25]. Ru/NaY-B and Ru/NaY-C samples with different Ru loading showed that Si/Al ratio increase is higher for samples with higher Ru loading. See Fig. 3(b–c). 29 Si MAS NMR spectra of Ru/NaY with Ru loading between 0.5 and 3 wt.% Ru activated only in He according to procedure A (Ru/NaY-A) showed a Si/Al ratio similar to that of the initial NaY (see Figs. 3(a) and 4(a)). Dealumination of Ru/NaY samples with 3 wt.% Ru, can be minimized, from Si/Al = 3.2 to 2.7, in the case of samples neutralized after activation in He, which were labeled RuN /NaY-E (Figs. 3(c) and 4(b)). The 3% Ru/HY sample activated by procedure B or C (Ru/HY-B or Ru/HY-C) showed stronger dealumination, from Si/Al = 2.6 (NH4 Y initial) to Si/Al = 3.7, Fig. 4(c). However, some silanes may have been
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Fig. 4. 29 Si MAS NMR of 3% Ru/NaY at different conditions: (a) Ru/NaY-A; (b) RuN /NaY-E; (c) 3% Ru/HY-B.
present in the 3% Ru/HY zeolite framework, affecting spectra deconvolution accuracy. McCarthy et al. [19] showed that auto-reduction of Ru is complete upon decomposing [Ru(NH3 )6 ]3+ /NaY by heating in He to 773 K. The metal clusters formed are smaller than the Y-supercage and invisible in TEM. Results derived from EXAFS suggest a coordination number of 0.6–0.8 [19]. Similar TEM results were obtained by us for the sample auto-reduced in He (Ru/NaY-A). TEM results of Ru/NaY-B and Ru/NaY-C samples, activated, respectively, in situ or in He after use in the reforming reaction, show that Ru particles are highly dispersed, visible in TEM, and with similarly sized from about 10–30 Å, regardless of Ru loading varying between 0.5 and 3.0 wt.%. Fig. 5(a and b) shows the bright field images of catalysts containing 1 and 3 wt.% of Ru and EDAX elemental analysis reveals the presence of Ru. Fig. 6 shows the micrographs in a dark field exhibiting a large amount of illuminated ruthenium particles inside the zeolites, with the same orientation for diffraction. TEM results of neutralized RuN /NaY-E used under reaction conditions, show small particles (10–30 Å) similar to those shown in the results observed for the sample activated in situ (Ru/NaY-B). However, some particles
can be visualized in RuN /NaY-E, although the number of oriented particles was very much smaller than that observed for Ru/NaY-B and Ru/NaY-C samples. The samples that were submitted to neutralization (Ru/NaY-A) presented only Ru particles smaller than the zeolite Y supercage (13 Å), results similar to those described by McCarthy et al. [19]. TPD results of 1% Ru/NaY auto-reduced in He (Ru/NaY-A) showed a higher H/Ru ratio of 0.60, similar to the H/Ru ratio of 0.69 obtained for samples submitted to neutralization (RuN /NaY-D). TPD results of 1% Ru/NaY auto-reduced in He, when submitted to steam action (Ru/NaYS -F), showed a marked decrease in H/Ru ratio from 0.60 to 0.31. On the other hand TPD results of 1% RuN /NaYS -G auto-reduced and neutralized, when submitted to H2 O/H2 steam action, showed a smaller decrease in H/Ru ratio from 0.69 to 0.68. TPD results of neutralized 1% RuN /NaY-E after use in reaction at 973 K for 300 h, showed a H/Ru ratio of 0.65 similar to sample before use. While the 1% Ru/NaY non-neutralized samples when submitted to reaction showed a Ru agglomeration with strong decrease of H/Ru ratio from 0.60 (Ru/NaY-A) to 0.28 (Ru/NaY-C). TPD results of 3% Ru/NaY submitted to steam action showed a tendency similar to that presented by 1% Ru/NaY samples (see Table 1). 29 Si MAS NMR spectra of 3% RuN /NaYS -G, auto-reduced and neutralized when submitted to steam action (H2 O/H2 ) showed a Si/Al ratio similar to initial NaY. The auto-reduced 3% Ru/NaYS -F sample, when submitted to steam action demonstrated an increase of Si/Al ratio in the zeolite framework structure (Si/Al = 3.0), similar to that of the samples Ru/NaY-B and Ru/NaY-C used in the reaction. Thermodynamic equilibrium was not reached in experimental conditions adopted over the temperature range of 723–973 K, but for runs with 0.200 g of catalyst at temperatures around 973 K data of the equilib2 × rium constant Kp were close to the ratio Q = PCO −1 −1 2 PH2 ×PCH4 ×PCO2 . Ru/HY-B or Ru/NaY catalysts activated according to different procedures (Ru/NaY-B, Ru/NaY-C or RuN /NaY-E) resulted in lower H2 /CO ratios at lower temperatures, when equilibrium was not reached. These results suggest that at lower temperatures the reverse water–gas shift reaction (WGSR) prevails (see Table 3), in agreement with thermodynamics [13] and reaction of CO hydrogenation to CH4 can be present [18].
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179
Fig. 5. TEM bright field images: (a) 1 wt.% Ru/NaY-B 420 000X (after reaction), (b) 3 wt.% Ru/NaY 320 000X (sample used).
Although NMR results for Ru/HY-B or Ru/NaY-B catalysts showed a strong increase in Si/Al ratio in the zeolite framework structure, these catalyst samples show a remarkable stability in activity and selectiv-
ity in reaction conditions. Independent of activation method adopted. Ru/NaY-B or RuN /NaY-E catalyst samples were maintained for 300 h on stream, with different partial pressures of CH4 between 0.2 and
Table 3 Reaction dataa at temperature of 773 K for samples M/supported (M = Rh or Ru) prepared by different procedures Catalysts
M (wt.%)
Wt (g)
H/Mb
Conv.% (CH4 )
Conv.% (CO2 )
H2 /CO
r (mol CH4 /g Ru.h)c
Ru/NaY-B
0.5 1.0 3.0 3.0 1.0 3.0 3.0 3.0 1.0 3.0 3.0 3.0
0.202 0.201 0.050 0.200 0.202 0.050 0.050 0.050 0.201 0.200 0.200 0.050
0.28 0.18 0.17 0.68 0.45 0.20 0.45 n.d. 0.18 0.90 0.39
6.8 9.9 7.5 13.3 9.8 8.2 5.8 8.5 8.0 5.4 9.0 1.6
10.8 14.8 10.5 19.2 14.6 10.6 7.3 13.4 11.6 6.6 9.6 3.57
0.54 0.60 0.66 0.63 0.61 0.74 0.76 0.55 0.63 0.79 0.93 0.22
3.30 2.44 2.51 1.11 2.43 2.76 1.94 2.84 1.87 0.47 0.74 0.53
Ru/NaY-C RuN /NaY-E Ru/NaYS -F RuN /NaYS -G Ru/HY-B Rh/NaY-B Rh/NaY-E Ru/SiO2 -B
Reaction condition: 100 ml/min, CH4 /CO2 /N2 (20/20/60), PCH4 = 200 Torr. Ratio H/M determinates as shown in Table 1. c Reaction rate average. a
b
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Fig. 6. TEM dark field image of 1 wt.% Ru/NaY-B 250 000X (sample used).
0.5 atm, with molar ratio CH4 /CO2 = 1 operating in a cycle of increasing and decreasing temperature. Within this period, catalyst activity and selectivity were constant. TPO results of Ru/NaY-B and RuN /NaY-E catalyst samples cooled in feed, followed
Fig. 7. Observed TOF for CO formation at 723 K as a function −0.55 PCO for (䊏) 1% Ru/NaY-B and (䊊) 1% Ru/NaY-E. of PH1.7 2 Reaction conditions: CH4 : CO2 : N2 = 1 : 1 : 3 and P = 1 atm.
by purging in Ar, showed low signals due to CO and CO2 , independent of time-on-stream. These results suggest that dealumination of the samples occurs at the initial stage of heating in CH4 /CO2 /N2 . Catalyst stabilization happens at temperatures below 773 K. Similar results were obtained previously for Rh/NaY catalyst [13]. Under similar reaction conditions for Ru/NaY, Ru/Al2 O3 [17,18] and Ru/SiO2 -B catalysts, Ru/NaY showed higher stability than supported Ru over amorphous materials like a SiO2 and Al2 O3. A lower H2 /CO ratio was initially obtained for Ru/SiO2 -B (H2 /CO = 0.22, at 773 K) and the H2 /CO ratio increases with time-on-stream followed by carbon deposition. For CO2 reforming, high temperatures are required, and one might wonder whether the CO2 in the reaction mixture can oxidize Ru to one of its oxides. However, in view of known volatility of RuO2 and RuO4 , the high stability of the present catalysts used indicates that no such oxides were formed under the reaction conditions employed. Ru catalysts supported on amorphous materials such as Al2 O3 or MgO have been utilized in methane reforming with CO2 in mixtures of CO2 and H2 O [3,12,16]. In these studies, however, no mention has been made about Ru loss. High stability Ru particles in O2 atmosphere were found when stabilized within SiO2 framework structure, with Ru particle size reported between 10 and 25 Å [26]. With the different activation procedures and preparations, activity (molCH4 conv./h g-Ru) decreases in the order RuN /NaY-E > Ru/NaY-B > Ru/NaY-C (Table 3), a similar activity order having been reported for Rh/NaY catalysts activated by different procedures [13]. Within experimental conditions for studying the reactions in this work, the interfacial gradient of temperature and concentration [27], confirms absence of external mass and heat transfer resistances. However, TOF values for 1.0% Ru/NaY-B and 1.0% RuN /NaY-D samples at 723 K were very dependent on methane conversion. Supported Ru catalysts are very active for CO [18] and CO2 [28] hydrogenation. As discussed previously by Bradford and Vannice [18] the CO hydrogenation reaction coupled with the water–gas shift reaction, which is quasi-equilibrated in these conditions, constitutes the overall reverse reaction. Similar to that described for Ru over amorphous support [18], TOFCH4 or TOFCO observed
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181
Table 4 Turnover frequencies of catalysts at temperature of 723 Ka Catalysts Catalysts
GHSV Conv. Conv. TOFCH4 TOFfor CH4 Catalysts (h−1 ) (%) CH4 (%) CO2 (s−1 × 103 ) (s−1 × 103 )
Ru/NaY-B 3.0 wt.% Ru Ru/NaY-B 1.0 wt.% Ru Ru/NaY-B 0.5 wt.% Ru Ru/NaYS -F 3.0 wt.% Ru Ru/-Al2 O3 1.6 wt.% Rub
72 000
5.2
7.4
160
263c
120 000 2.1
3.5
202
226c
28 000
3.4
5.4
155
200c
37 500
5.6
7.3
80.0
210c
12.3
970
970
595 500 6.2
RuN /NaY-E 1.0 wt.% Ru RuN /NaY-E 3.0 wt.% Ru RuN /NaYS -Ga 3.0 wt.% Ru Rh/NaY-B 3 wt.% Rh Rh/NaY-E 3 wt.% Rh Ru/SiO2 -B 3 wt.% Ru
GHSV Conv. Conv. TOFCH4 TOFfor CH4 (h−1 ) (%) CH4 (%) CO2 (s−1 × 103 ) (s−1 × 103 ) 120 000 2.3
3.1
90.4
100d
75 000
6.1
8.0
78.2
109d
75 000
5.9
9.3
75.8
96.6d
25 000
2.0
2.6
14.0
–
50 000
2.2
2.6
9.4
–
25 000
2.2
4.9
12.6
–
PCH4 = 200 Torr; CH4 /CO2 = 1.0. Ref. [18]. c Calculated from Eq. (5) with parameters from Table 2, where k = 0.0175 (Torr−(γ +δ) s−1 ) for samples series Ru/NaY-B. r d Calculated from Eq. (5) with parameters from Table 2, where k = 0.0065 (Torr−(γ +δ) s−1 ) for samples series RuN /NaY. r a
b
(TOFobs ) at 723 K for Ru/NaY-B and RuN /NaY-E γ δ (Fig. 7) and show a linear correlation with PH2 PCO TOFobs can be represented approximated by expression power rate law expression γ
δ TOFobs = TOFfor − TOFrev = TOFfor − kr PH2 PCO
(5) Table 2 shows TOFfor (turnover frequency for the forward CO during CO2 –CH4 reforming reaction at 723 K) and parameters γ and δ values obtained for TOFCO over 1% Ru/NaY-B and 1% RuN /NaY-E. This table also presents results described in literature [18] for Ru/-Al2 O3 , Ru/TiO2 and Ru/C catalysts, data for which were correlated in Eq. (5). Higher TOF values for Ru/NaY-B corresponds to samples showing dealumination. Ru/NaY-B or -D presented higher activity than Rh/NaY-B and RhN /NaY-D (Tables 2 and 4). Ru/-Al2 O3 catalyst showed higher TOF than both Ru/NaY, TOF values for Ru over different supports, showed the TOF order Ru/-Al2 O3 Ru/NaY Ru/SiO2 . Ru/NaY samples were free of Cl− while Ru/-Al2 O3 [18], showed an initial 2.5 Cl/Ru ratio. Bradford and Vannice [18] showed that, although the additional studies are necessary, the literature data suggest the possibility that Cl presence could promote CO2 –CH4 reforming over Ru catalysts.
For the Ru/NaY-B catalyst activated in situ and non-neutralized, TOFs showed slight increases with Ru loading increases and consequently with Ru dispersion decreases (Tables 1 and 4). The present NMR results for samples activated in situ show that dealumination also increases with Ru loading. Although for neutralized protons (RuN /NaY-E) TOFs were lower than for non-neutralized samples. For a given Ru loading, neutralized protons (RuN /NaY-E) lead to a slight enhanced specific activity (molCH4 conv./h g-Ru). This neutralization suppresses dealumination, as the NMR results show. Dealumination enhances Ru particles agglomeration, although this is not evident for the TEM data which only show the effect of Ru loading on particle size, which is evident for the TPD of hydrogen (Table 1). Most of the literature shows that protons can act as ‘chemical anchors’ for metal particles in zeolite cages, thus, stabilizing high metal dispersion [29,30]. However, the present results indicate that in the presence of H2 O and at very high temperature protons destabilize metal dispersion, because they favor zeolite dealumination [31]. The dealumination creates voids that are only partially filled by Al2 O3 . Although the aluminum extra framework was not extracted from NaY structure, TEM and TPD results show that Ru particles inside such voids are likely to agglomerate. Under such conditions where dealumination is likely, catalyst neutralization before activation in He will exhibit
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higher steady-state activity and higher specific activity for reactions catalyzed by the zeolite supported metal. Dispersion of RuN /NaY obtained after activation in He will be stable under reaction conditions. Ru/NaY-A sample when neutralized and activated in H2 (RuN /NaY-A) show similar Ru dispersion. Consequently this particle should present similar free energy for possible growth of particles by migration inside zeolyte structures, but Ru particles easily agglomerate in non-neutralized Ru/NaY under reaction conditions (Ru/NaY-B). McCarthy [19] has shown that a small amount of protons, present in Ru/NaY activated in He, are consumed in the supercage region when exposed to CO, forming Run + carbonyl species. Similar results was found for Rh/NaY [32], forming Rh+ (CO)2 . However, dicarbonyl cations might exist during the period in which Ru/NaY catalysts are heating in contact with reactants and this carbonyl species can increase Ru mobility, consequently promoting Ru agglomeration. Results of experiments heating Ru/NaY non-neutralized sample under an H2 /H2 O stream free of CO, show that at high temperatures protons destabilize metal dispersion (Ru/NaYS -F — Table 1). However, Ru/NaY neutralized under an H2 /H2 O stream show a stable Ru dispersion, independent of CO presence, because protons favor dealumination with destruction of the encaging zeolite framework. Ru/NaY samples obtained by different preparation methods were grouped as following: (a) neutralized samples and (b) non-neutralized samples. TOF’s value for neutralized samples were corrected from kinetics parameters obtained for 1% Ru/NaY-D neutralized samples and calculated TOFfor for unidirectional forward rates for the CO2 /CH4 reforming reaction on Ruthenium in ‘standard’ conditions (see Table 4). Similar procedure was utilized for non-neutralized samples, utilizing kinetics parameter from 1% Ru/NaY-B non-neutralized samples. The ratio between TOFfor,CH4 for Ru/NaY-B and TOFfor,CH4 for RuN /NaY-D samples was approximately 2 (see Table 4). Ru loading caused dealumination, creating voids in the crystalline structure. However, Ru loading did not change those TOF ratios. Apparent activation energy for CH4 (ECH4 ) over 1% Ru/NaY-B and 1% RuN /NaY-D samples, obtained from TOFfor values, were 27.4 and 25.5 kcal/mol, respectively. These ECH4 values were similar to those
obtained for Ru/Al2 O3 [18]. From these results, it is reasonable to assume a reaction rate not limited by pore diffusion. The dealumination cause a minor modification of encaging zeolite framework of NaY and the possibility that slight increase of activity for Ru/NaY-B was caused by modification of diffusion effects inside microporous particle cannot be excluded.
4. Conclusions Under experimental conditions of reaction utilized, Ru/NaY prepared by ion-exchange from an aqueous solution of [Ru(NH3 )6 ]Cl3 was a potential catalyst for CO2 reforming of CH4 . Ru/NaY showed higher stability than Ru/SiO2 and, unlike Ru/SiO2 , did not deactivate during a period of 300 h on stream. The Si/Al ratio in the zeolite framework, and stability of Ru dispersion on NaY depends on the preparation procedures. The protons formed by reduction of Ru in the supercage destabilized zeolite structure in the presence of reaction products, favoring dealumination of the zeolite. Dealumination under reforming reaction conditions was minimized by neutralizing the protons formed during auto-reduction of [Ru(NH3 )6 ]3+ in He. The TEM micrographs showed uniform Ru particles having sizes of ∼ =10–30 Å. Higher Ru dispersion and stability were found for samples whose protons were neutralized before use as catalysts.
Acknowledgements The authors gratefully acknowledge financial support of FAPESP (Fundação para o Amparo a Pesquisa do Estado de São Paulo), proc 96/7853-1 and 96/9434-6 (Brazil). We wish to thank Professor Wolfgang M.H. Sachtler, V.N. Ipatieff Laboratory, Center for Catalysis and Surface Science, Department of Chemistry, Northwestern University, IL, USA, for helpful discussions and manuscript suggestions and Professor Hans-Jürgen Kestenbach, Department of Materials Engineering, Universidade Federal de São Carlos, SP, Brazil, for his permission to use an electron microscope. U. Portugal Jr acknowledges support from CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior — Brazil).
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