How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol

G Model CATTOD-8355; No. of Pages 8

ARTICLE IN PRESS Catalysis Today xxx (2013) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Catalysis Today journal homepage: www.elsevier.com/locate/cattod

How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol F. Arena a,b,∗ , G. Mezzatesta a , G. Zafarana a , G. Trunfio a , F. Frusteri b , L. Spadaro a,b a b

Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università degli Studi di Messina, Salita Sperone 31 c.p. 29, I-98166 S. Agata, Messina, Italy Istituto CNR-ITAE “Nicola Giordano”, Salita S. Lucia 5, I-98126 S. Lucia, Messina, Italy

a r t i c l e

i n f o

Article history: Received 31 August 2012 Received in revised form 5 February 2013 Accepted 12 February 2013 Available online xxx Keywords: Cu–ZnO catalyst Oxide carrier CO2 hydrogenation Methanol Surface functionality Reaction mechanism

a b s t r a c t The reactivity pattern of Al2 O3 (CuZnAl), CeO2 (CuZnCe) and ZrO2 (CuZnZr) supported Cu–ZnO systems in the synthesis of methanol via CO2 hydrogenation in the range of 453–513 K at 3.0–5.0 MPa has been addressed. The CuZnCe system shows superior surface methanol productivity, though textural and chemical effects of zirconia carrier account for the better performance of CuZnZr catalyst. Characterization data of “steady-state” catalysts show significant surface coverage by CO2 irrespective of metal surface area (MSA). Direct relationships among activity, CO2 uptake and oxides surface area (OSA) point out a dualsite Langmuir–Hinshelwood reaction mechanism, involving hydrogenation and CO2 adsorption sites at the surface of both metal and oxide phases. The influence of space–velocity on selectivity signals the occurrence of a parallel-consecutive path leading to methanol and CO, while higher reaction rate and methanol selectivity with lowering contact time signal a negative influence of water formation on the catalyst performance. © 2013 Published by Elsevier B.V.

1. Introduction Environmental and economic advantages deriving from extensive recycle of carbon dioxide emissions and exploitation of fuels alternative to LPG, gasoline and gasoil pressed in the recent years a big scientific concern onto the CO2 -hydrogenation to methanol and/or dimethylether [1–4]. In this respect, many studies documented a superior performance of Cu–ZnO/ZrO2 catalysts [4–8], generally ascribed to a lower water-affinity than commercial alumina-based ones [3,4,8,9]. However, the lack of probative evidences on the CO2 -hydrogenation mechanism of oxide promoted Cu systems represents to date the major drawback for catalyst development [3,4,7,10–12]. Indeed, though methanol synthesis reaction on Cu catalyst relies on a classical “one-site” Langmuir–Hinshelwood (L–H) model [13,14], a mix of structural and/or electronic effects determines the positive influence of many oxide promoters on the CO2 -hydrogenation pattern of the Cu catalyst [3,7,10]. In particular, ZnO plays an essential promoting influence on the catalytic functionality of Cu owing to enhanced metal dispersion and indefinite electronic effects of ZnO patches, leading the stabilization of Cu–Zn alloy(s) and electron-deficient Cuı+ sites at the Cu–ZnO interface [3,5,14–17]. Moreover, according

to Bell and Fischer who pointed out an active role of zirconia carrier on the reactivity of Cu/ZrO2 catalyst [18], the lack of direct relationships between metal exposure (MSA) and activity lead us to ascribe the superior functionality of Cu–ZnO/ZrO2 catalysts to the synergism of active sites at the surface of both metal and oxide phases [5,6]. Further, in the light of the peculiar catalytic functionality of CeO2 -based systems [7,12], we argued that oxide carriers and promoters control the reactivity of the active Cu phase by determining texture, exposure of active sites and interaction pattern with reagents, products and reaction intermediates [3–7,10]. Therefore, this study is aimed at providing a comparative assessment of the CO2 -hydrogenation pattern of Al2 O3 , ZrO2 and CeO2 supported Cu–ZnO systems in the range of 453–513 K at 3.0 and 5.0 MPa. Characterization of steady-state catalysts indicates a fundamental role of CO2 adsorption on the catalytic functionality, while textural and chemical effects of zirconia carrier determine the superior performance to Cu–ZnO/ZrO2 catalyst, fixed by methanol −1 at 10% CO2 conversion space–time–yield of 1.2 kgCH3 OH kg−1 cat h per pass.

2. Experimental ∗ Corresponding author at: Dipartimento di Chimica Industriale e Ingegneria dei Materiali, Università degli Studi di Messina, Italy. Tel.: +39 0906765484; fax: +39 090391518. E-mail address: [email protected] (F. Arena).

2.1. Materials Ceria (CuZnCe) and zirconia (CuZnZr) supported Cu–ZnO systems with a Cu/Zn atomic ratio of ca. 3 were prepared by the reverse

0920-5861/$ – see front matter © 2013 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.cattod.2013.02.016

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

2

Table 1 Physico-chemical properties of the calcined and reduced catalysts. SA (m2 /g)

PV (cm3 g−1 )

MSA (mCu 2 g−1 )

D (%)

–

105 79

0.23 0.29

– 36

– 21

5

42.3

–

154 132

0.34 0.28

– 73

– 32

3

–

43.6

47 34

0.24 0.17

– 10

– 5

21

Catalyst

Sample

CuO

ZnO

Al2 O3

ZrO2

CeO2

CuZnAl

Calcined Reducedb

35.0

33.1

31.9

–

CuZnZr

Calcined Reducedb

43.2

14.5

–

CuZnCe

Calcined Reducedb

42.7

13.7

–

a b

Chemical Composition (wt %)

dCu a (nm)

Average Cu particle size obtained from the formula dCu = 104/D(%) [4]. Reduced samples at 573 K (1 h) in H2 flow.

co-precipitation under ultrasounds irradiation route of Cu2+ , Zn2+ , Ce3+ and ZrO2+ nitrate precursors, according to the procedure elsewhere described [4], keeping constant loading of CuO (≈42 wt%), ZnO (≈14 wt%) and oxide carrier (≈44 wt%). After precipitation, catalysts were filtered, dried at 373 K (16 h) and further calcined in air at 623 K (4 h). A commercial Cu–ZnO/Al2 O3 (CuZnAl) methanol synthesis catalyst (G66A, Sud Chemie AG) was taken as reference. Powdered catalysts were pressed at 40 MPa and then crushed and sieved to 40–70 mesh particle size fraction. The list of the catalysts, with their relative formulation and main physico-chemical properties, is reported in Table 1.

was controlled by a thermocouple in contact with catalyst bed. Before testing, catalysts were reduced in situ at 573 K in H2 flow (100 stp mL/min) at atmospheric pressure (1 h). Thereafter, the reactor was cooled down to 453 K, admitting the reaction mixture and raising the pressure to 3.0–5.0 MPa. Heating–cooling cycles at any pressure were performed to ascertain the lack of significant deactivation phenomena. The reaction stream was analyzed by a GC equipped with a two-column analytical system connected to FID (CH3 OH, CH3 OCH3 ) and TCD (CO, N2 , CO2 , H2 ) [4]. Methanol and CO were the only detected reaction products, methane and dimetylether being always at trace levels, if any.

2.2. Methods Surface area (SA) and pore volume (PV) values were obtained by the standard BET and BJH method elaboration respectively of nitrogen adsorption/desorption isotherms (77 K), taken by an automated gas adsorption device (ASAP 2010, Micromeritics Instrument). Before the adsorption measurements, samples were outgassed at 423 K under vacuum for 2 h. Metal surface area (MSA) and dispersion (D) were determined by “single-pulse” (0.65 mL) N2 O titration measurements at 363 K [4–7]. Before measurements, catalysts were reduced at 573 K in H2 flow (100 stp mL/min) for 1 h and, then “flushed” in a N2 carrier flow at 583 K for 15 min and further cooled down to 363 K, for the N2 O pulse injection. A chemisorption stoichiometry of N2 O:Cu = 1:2 and a surface concentration of 1.46 × 1019 Cu atom/m2 were assumed for metal surface area (MSA) and dispersion (D) calculations, respectively [4]. Temperature programmed surface reaction (TPSR) tests in the range of 293–573 K were performed at heating rate of 6 K/min using Ar or 75% H2 /Ar mixture as carriers (30 stp mL/min). Before measurements catalysts (50 mg) were reduced for 1 h at 573 K in H2 flow (50 stp mL/min), then cooled down to 493 K admitting a stoichiometric CO2 /H2 /Ar (1/3/2) mixture (60 stp mL/min) and keeping stationary conditions for 30 min. Thereafter, catalysts were quenched to 298 K still under reaction mixture flow, and then flushed by the carrier until stabilization of signals baseline (≈20 min). Profiles of reagents and products were obtained by a Quadrupole Mass Spectrometer (Hiden Analytical, HPR 20), equipped with a heated (453 K) inlet capillary system, monitoring the following mass-to-charge signal ratios: 2 (H2 ), 40 (Ar), 18 (H2 O), 28 (CO), 31 (CH3 O), 44 (CO2 ). Catalyst testing was carried out in the range of 453–513 K at 3.0–5.0 MPa using a gas–liquid feed automated plant, equipped with an Inconel micro-reactor (7–10 mm) jacketed with a stainlesssteel round to ensure isothermal reaction conditions. The reactor was loaded with 0.5 g of catalyst diluted (1/1, wt/wt) with same-sized ␣-Al2 O3 and fed with a stoichiometric CO2 /H2 /N2 (23/69/8) mixture at rate varying between 80 and 500 stp mL/min (GHSV, 8800–55,000 NL kgcat −1 h−1 ), while temperature

3. Results and discussion 3.1. Physico-chemical properties The data in Table 1 show a different chemical composition of the studied catalysts, as reference CuZnAl catalyst includes similar loadings of CuO (≈34 wt%), ZnO (≈38 wt%) and Al2 O3 (≈29 wt%), while lab-made CuZnZr and CuZnCe contain comparable higher loading (≈43 wt%) of CuO and oxide carrier at the expense of ZnO (≈14 wt%). This corresponds to a Zn/Cu atomic ratio of 0.3–0.4 (Table 1), previously indicated as an optimum for Cu–ZnO/ZrO2 catalyst composition [4]. Then, reference CuZnAl system features a SA of 105 m2 /g lowering to 79 m2 /g after the reduction treatment (−SA≈25%), while the calcined CuZnZr system displays the largest SA (154 m2 /g) and the smallest decay (−SA≈14%) further to reduction, confirming the most effective role of zirconia as textural promoter of the Cu–ZnO system [4–7]. Whereas, the lowest SA (47 m2 /g), decreasing to 34 m2 /g on the reduced sample (−SA≈28%), denotes a poorer efficiency of ceria carrier to promote surface texture and resistance to sintering of CuZnCe system [7]. On the whole, N2 O-titration data (Table 1) indicate the occurrence of a close relationship between metal sites exposure (MSA) and SA, as shown in Fig. 1. The CuZnCe catalyst is characterized by the lowest metal dispersion (D, 5%) and surface area (MSA, 10 m2 /g), though a MSA/SA ratio considerably lower (≈0.3) than CuZnZr and CuZnAl systems (Fig. 1) denotes an enhanced tendency to sintering, probably induced by an incipient reduction of ceria carrier [7]. Higher dispersion (32%) and MSA (73 m2 /g) mirror the largest surface development of the CuZnZr system (Fig. 1), while a MSA/SA ratio of 0.55 signals a very strong promoting influence of zirconia carrier also on texture of metal phase [4,7]. The CuZnAl sample features intermediate dispersion (21%) and MSA (36 m2 /g) values (Fig. 1A), accounting for a MSA/SA ratio of 0.46 (Fig. 1). These data imply that the oxide-surface-area (i.e., OSA = SA − MSA) to SA ratio (OSA/SA) depicts an opposite trend

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

MSA (mCu2/gcat) - OSA (mOx2/gcat)

0.8

OSA/SA

60

40

0.6

0.4

MSA/SA

20 OSA

0

0.2

MSA

30

0.0 70

110

MSA/SA (mCu2/mcat2) - OSA/SA (mOx2/mcat2)

80

3

150

SA (mcat2/g) Fig. 1. Influence of surface area (SA) on the extent of metal (MSA) and oxide surface area (OSA) and MSA/SA and OSA/SA ratios.

to the MSA/SA one (Fig. 1), resulting larger for CuZnCe system (0.71). Determining the texture of the final catalysts [3–7,11], it is expected that oxide carriers control the CO2 -hydrogenation functionality of the Cu–ZnO system. 3.2. Effect of temperature and pressure on activity and surface functionality The activity pattern of the studied catalysts (F, 80 stp mL/min; GHSV, 8800 NL h−1 kgcat −1 ) at 3.0 and 5.0 MPa is shown in Fig. 2, comparing CO2 conversion (XCO2 ), CH3 OH selectivity (SCH3 OH ) and yield (YCH3 OH ) in the range of 453–513 K with relative thermodynamic equilibrium values. These were obtained from the equation system of equilibrium constants KMS and KWGSR (see Appendix A of Supplementary Information) relative to methanol synthesis (MS) and reverse water gas shift (RWGS) reactions respectively, as

methanol decomposition (MD) is a linear combination of previous reactions [4]: CO2 + 3H2  CH3 OH + H2 O CO2 + H2  CO + H2 O, CH3 OH  CO + 2H2 .

(MS) (RWGS) (MD)

As a rule, conversion increases with both pressure and temperature, while selectivity rises with pressure and decreases with temperature. Moreover, though all catalysts exhibit their performance within the limits of thermodynamics, conversion-yield values higher than 20–30% of corresponding equilibrium values suggest that at 493 K, and mostly 513 K, the reactivity pattern of CuZnAl and CuZnZr catalysts is controlled by the approach to equilibrium composition (Fig. 2). Namely, the commercial CuZnAl system (Fig. 2A and A ) shows conversion values increasing from 3.2 and 4.8% (453 K) to 13.5 and

Fig. 2. Conversion, selectivity and yield data (dotted lines) in the range of 453–513 K at 3.0 (A–C) and 5.0 MPa (A –C ) of CuZnAl (A, A ), CuZnZr (B, B ) and CuZnCe (C, C ) catalysts. Continuous lines refer to equilibrium values (experimental conditions: Fmix , 80 stp mL/min; CO2 /H2 /N2 , 23/69/8; wcat , 0.5 g).

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

•

0.20

•

0.15

CuZnAl CuZnZr CuZnCe

•

•

0.10 0.05 3.0 MPa

0.00 453

0.25

SSA (mmol CO2 mcat -2 h-1)

0.25

A)

•

0.20

•

0.15

473 493 Temperature (K)

0.05 5.0 MPa

453

0.05

473 493 Temperature (K)

3.0 MPa

453

•

0.00

0.10

0.00

•

0.10

513

−1

A’)

0.15

0.25

B)

CuZnAl CuZnZr CuZnCe

CuZnAl CuZnZr CuZnCe

0.20

513

SSY (mmol CH3OH mcat-2 h-1)

SSA (mmol CO2 mcat -2 h-1)

0.25

SSY (mmol CH3OH mcat -2 h-1)

4

473 493 Temperature (K)

513

B’)

CuZnAl CuZnZr CuZnCe

0.20 0.15 0.10 0.05

5.0 MPa

0.00 453

473 493 Temperature (K)

513

−1

−2   Fig. 3. (A, B) Specific surface rate of CO2 conversion (SSA, mmolCO2 m−2 cat h ) and (A , B ) methanol formation (SSY, mmolCH3 OH mcat h A ) and 5.0 MPa (B, B ) (experimental conditions: Fmix , 80 stp mL/min; CO2 /H2 /N2 , 23/69/8; wcat , 0.5 g).

19.5% (513 K) at 3.0 and 5.0 MPa, respectively. Although selectivity lowers from 90–93% (453 K) to 44–61% (513 K), the yield rises steadily with temperature and pressure from minimum values of 2.9–4.5% (453 K) to 5.9–11.9% (513 K) at 3.0 and 5.0 MPa, respectively. With comparable or higher methanol selectivity levels and CO2 conversion values rising from 4.4–5.7% (453 K) to 19.3–22.4% at 3.0 and 5.0 MPa respectively, the CuZnZr catalyst (Fig. 2B and B ) shows the highest yields, increasing from 3.8–5.1% (453 K) to 9.7–14.3% (513 K). At least, with a conversion increasing from 2.0–2.2% (453 K) to 7.2 (3.0 MPa) and 8.5% (5.0 MPa) at 513 K (Fig. 2C and C ), the CuZnCe catalyst shows the lowest activity and selectivity levels considerably higher than the previous systems (81–97%). Then, methanol selectivity higher than equilibrium values (Fig. 2C and C ) might depend on the poor activity of the system, resulting in the occurrence of kinetic conditions in the whole range of temperature. Overall, these data provide the following reactivity scale in terms of methanol yield CuZnZr > CuZnAl > CuZnCe. Despite such data confirm the superior process performance of the CuZnZr system [3–8], yet a definitive assessment of the effects of carrier on the catalytic functionality of the Cu–ZnO system must take into account the total and metal surface development of the catalysts (Table 1). Then, the specific surface rates of CO2 con−1 version (SSA, mmolCO2 m−2 cat h ) and methanol production (SSY, −1 mmolCH3 OH m−2 cat h ) in the range of 453–513 K at 3.0 and 5.0 MPa are compared in Fig. 3. Despite the lower activity, the CuZnCe catalyst features SSA values higher than both CuZnAl and CuZnZr at 3.0 MPa (Fig. 3A), while at 5.0 MPa CuZnCe and CuZnAl catalysts show a comparable SSA in the range of 453–513 K, slightly higher than CuZnZr catalyst (Fig. 3B). Consequence of much higher selectivity levels, yet, the CuZnCe catalyst ensures SSY significantly higher than CuZnAl and CuZnZr systems in the whole range of temperature (Fig. 3A and B ). Besides, as a consequence of the lowest MSA/SA ratio (Fig. 1), the CuZnCe catalyst features an even higher reactivity in terms of specific metal rate of CO2 conversion (SMSA,

) in the range of 453–513 K at 3.0 (A,

mmol mCu −2 h−1 ) and CH3 OH production (SMSY, mmol mCu −2 h−1 ), as shown in Fig. 4. Therefore, while the poor process performance of the CuZnCe catalyst relies on the low efficiency of ceria as structural promoter of the Cu–ZnO system, such findings lead to ascribe its enhanced surface functionality to the peculiar reactivity of the ceria carrier [7,12]. 3.3. Effect of contact time on the activity–selectivity pattern The potential constraints of thermodynamic equilibrium on reaction kinetics and catalyst performance prompted us to inspect the influence of CO2 conversion on the reactivity pattern of the CuZnZr system. Conversion–selectivity-yield data, summarized in Table 2, disclose a decreasing trend of conversion with flow rate while methanol selectivity rises steadily. Moreover, conversion data disclose an increasing reaction rate with space–velocity in the whole range of temperature (Fig. 5A and B), denoting a general negative effect of CO2 conversion on the reaction kinetics [3,19,20]. Then, at T > 473 K increasing reaction rate data are consistent with the aforesaid negative influence of the approach to equilibrium composition, while at T ≤ 473 K this effect might depend mostly on kinetic factors. However, the Weisz–Prater criterion signals the lack of diffusional resistances on reaction kinetics at any temperature (see Appendix B of Supplementary Information). Therefore, the positive influence of flow rate on reaction kinetics must be linked to a negative influence of CO2 conversion and, particularly, to the consequent increase of water partial pressure across the catalyst bed, which would determine an incipient oxidation of metal Cu sites (e.g., Cu + H2 O → CuO + H2 ), depressing reaction rate at higher contact times. In fact, a selectivity pattern strongly dependent on flow rate at T < 493 K (Table 2) signals a marked influence of conversion on the rate of product formation, likely linked also to the state of the catalyst surface under steady-state conditions [21]. Namely, for differential (<10%) extents of CO2 conversion, in the absence of any thermodynamic constraints, parallel paths of CH3 OH (MS) and CO formation (RWGS) should result in a constant CH3 OH-to-CO selectivity ratio (SCH3 OH /SCO ), according to the fact that the forward

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

CuZnAl CuZnZr CuZnCe

0.6

A)

0.4 0.2 3.0 MPa

0.0 453

SMSA (mmol CO2 m Cu-2h-1)

1.0

473 493 Temperature (K)

CuZnAl CuZnZr CuZnCe

0.8

B)

0.6 0.4 0.2 5.0 MPa

0.0 453

0.8

473 493 Temperature (K)

5

CuZnAl CuZnZr CuZnCe

0.6

A’)

0.4 0.2 3.0 MPa

0.0

513

453

SMSY (mmol CH3OH m Cu-2h-1)

SMSA (mmol CO2 mCu-2h-1)

0.8

SMSY (mmol CH3OH m Cu-2h-1)

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

0.8

473 493 Temperature (K)

513

CuZnAl CuZnZr CuZnCe

0.6

B’)

0.4 0.2 5.0 MPa

0.0

513

453

473 493 Temperature (K)

513

−1

−1

Fig. 4. (A, B) Specific metal surface rate of CO2 conversion (SMSA, mmolCO2 m−2 h ) and (A , B ) methanol formation (SMSY, mmolCH3 OH m−2 h Cu Cu at 3.0 (A, A ) and 5.0 MPa (B, B ) (experimental conditions: Fmix , 80 stp mL/min; CO2 /H2 /N2 , 23/69/8; wcat , 0.5 g).

rates depend only on the partial pressure of reagents (see Appendix C of Supplementary Information) SCH3 OH SCO

=

rateCH3 OH rateCO

kMS · pCO2

=

kRWGS · pCO2

=

kMS , kRWGS

(1)

where kMS and kRWGS are the pseudo-first order constants of parallel MS and RWGS reactions, including the hydrogen dependence. On the other hand, the SCH3 OH /SCO ratio should result a direct function of conversion for a consecutive path. In particular, assuming that at differential (<10%) extents of CO2 conversion the average partial pressure of reagents and products across catalyst bed can be roughly considered equal to the arithmetic average and taking into account only forward formation rates, far away from thermodynamic constraints, the SCH3 OH /SCO ratio results a straight function of the reciprocal of CO2 conversion (1/XCO2 ), according to the following equation (see Appendix C of Supplementary Information) SCH3 OH SCO

kMS =2· · kMD



1 XCO2



− 1,

(2)

where kMS and kMD are the pseudo-first order constants of consecutive MS and MD reactions. Then, experimental selectivityconversion data of the CuZnZr catalyst at different space–velocity were inspected by Eq. (2), as shown in Fig. 5A and B . A satisfactory fit of experimental SCH3 OH /SCO data (r2 , 0.90–0.99) proves the consecutive path of CO formation via the methanol decomposition

) in the range of 453–513 K

(MD) route at T ≤ 493 K. Moreover, despite SCH3 OH /SCO values of ca. 40 suggest that methanol is a primary product of the CO2 hydrogenation reaction (453 K) [20], the steep decrease of the SCH3 OH /SCO ratio proves that the MD reaction is favoured at higher temperature and conversion. While, the progressive flattening of the SCH3 OH /SCO ratio with temperature (Fig. 5A and B ) mirrors the increasing contribution of the parallel RWGS path [20]. Then, the network of MS, RWGS and MD reactions catches the main clues of the surface CO2 -hydrogenation pattern of supported Cu–ZnO catalysts, since the catalytic functionality is dominated by MS and MD paths at T ≤ 473 K, while the parallel RWGS, characterized by higher activation barrier than MS and MD reactions, becomes important at high temperature [20]. Consequence of the strong influence of process conditions on catalyst performance, the CuZnZr features a steady rise of the methanol space–time-yield with flow rate (Fig. 6), up to the remarkable value of 1.2 kg kgcat −1 h−1 at ≈10% CO2 conversion per pass (T, 513 K; P, 5.0 MPa).

3.4. Catalyst steady-state, active sites and surface functionality In spite of the extensive literature on the reactivity pattern of oxide-promoted Cu catalyst, the nature of active sites responsible of the various surface functionalities is still matter of debate [3–7,11,12,15–18,21]. For instance, Klier et al. proposed [22] that

Table 2 Activity data of CuZnZr catalyst in the range of 453–513 K at 3.0–5.0 MPa and different feed flow rate. Fmix (stp mL/min)

 (s)

P (MPa)

TR , 453 K

TR , 473 K

TR , 493 K

TR , 513 K

XCO2 − SCH3 OH − YCH3 OH (%) 80

0.4

160

0.2

330

0.1

500

0.06

3.0 5.0 3.0 5.0 3.0 5.0 3.0 5.0

4.4 5.7 3.6 4.3 2.5 3.1 2.0 2.7

87 90 94 94 95 96 97 97

3.8 5.1 3.4 4.1 2.4 3.0 1.9 2.6

8.4 10.0 5.0 6.2 3.3 4.7 2.3 3.5

74 78 84 84 88 90 90 91

6.2 7.8 4.2 5.2 2.9 4.2 2.1 3.2

13.9 17.4 7.9 11.0 5.0 7.0 4.3 5.6

57 65 64 71 70 74 77 78

7.9 11.3 5.1 7.8 3.5 5.2 3.3 4.4

19.3 22.4 13.3 16.9 9.5 11.0 7.5 9.7

50 64 48 60 51 60 55 62

9.6 14.3 6.4 10.1 4.8 6.6 4.1 6.0

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

reaction rate (molCO2 gcat-1 s-1)

2.0×10 - 5

•

3.0 MPa

A)

•

1.5×10 - 5 1.0×10 - 5 493K 473K 453K

2.0×10 - 5

1.0×10 - 5

493K 473K

5.0×10 - 6

453K

0

0

40

200 400 flow rate ( stp mL/min)

0

600

40

3.0 MPa

A’)

30 20 10 493K

0

20

40

5.0 MPa 453K

20

0 60

1/XCO2

600

B’)

473K

10

473K 513K

200 400 flow rate (stp mL/min)

30

453K

SCH3OH/SCO

SCH3OH/SCO

513K

1.5×10 - 5

0

0

5.0 MPa

B)

•

•

513K

5.0×10 - 6

reaction rate (molCO2 gcat-1 s-1)

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

6

493K

513K

0

10

20 1/XCO2

30

40

Fig. 5. (A, B) Influence of flow rate on the rate of CO2 conversion at 3.0 (A) and 5.0 (B) MPa and different temperature. (A , B ) Fitting of methanol-to-CO selectivity ratio (SCH3 OH /SCO ) as a function of the reciprocal CO2 conversion at 3.0 (A ) and 5.0 (B ) MPa by Eq. (2) (experimental conditions: CO2 /H2 /N2 = 23/69/8; wcat , 0.5 g).

oxidized Cu is active and reduced Cu is not active in methanol synthesis, arguing that copper incorporated into the Zn lattice as Cu+ was the active site for reactant adsorption [15,22]. In the case of methanol decomposition, both Cu+ and Cu0 seem to be active; however, because of the reducing environment of methanol decomposition, Cu0 was thought to be the active site on Cu–ZnO/Al2 O3 catalyst [23]. Moreover, while Fisher and Bell suggest that metal Cu serves as a sink for atomic hydrogen during MD on Cu/ZrO2 /SiO2 catalyst [24], Choi and Stenger come to conclusion that Cu2+ centres drive both MD and methanol steam reforming (MSR) reactions [21]. Despite the similar physico-chemical characteristics of the catalysts driving MS, WGS, MSR and MD reactions, the big uncertainty affecting the nature of active sites depends also on the different composition and redox potential of the reacting atmosphere, deeply influencing the state of surface Cu atoms [21]. Indeed, MSR and WGS reactions imply the occurrence of prevalently oxidative conditions in the presence of huge amounts of 1.5

STY (kg CH3OH kg cat-1 h -1)

5.0 MPa •

1.0 3.0 MPa

•

0.5

0.0 0

5

10

15

-1 -1 • flow rate (stp mLg cat • s ) Fig. 6. Influence of flow rate on methanol space-time-yield (STY) at 513 K and 3.0 and 5.0 MPa (experimental conditions: reaction mixture composition, CO2 /H2 /N2 = 23/69/8; wcat , 0.5 g).

water, at variance of MS and MD reactions proceeding in a strongly reducing environment [21]. This determines a different surface state of metal sites and, in turn, significant changes in the reactivity towards MS–RWGS and MSR–WGS processes. In addition, dispersion and morphology of metal particles determine affinity and interaction strength of the various reaction intermediates with surface Cu sites [4,5]. All these evidences suggest that the activityselectivity pattern depends on different functionalities driven by different active sites. In this context, according to Choi and Stenger [21], the negative effect of contact time on rate and methanol selectivity would mostly reflect the influence of an increasing water partial pressure across the catalyst bed, leading to an incipient oxidation of Cu sites (CuO) [21]. This is consistent with the fact that the consecutive MD path is much more evident for CuZnZr and CuZnAl catalysts, since their high dispersion would enhance the oxidation of Cu sites by water [4,5,20]. Then, to shed lights into the surface “steady-state”, reduced catalysts (573 K) were kept under “stationary” reaction conditions at 493 K for 30 min in a CO2 /H2 (1/3) mixture flow and, then, quenched to 293 K (<10 s). Temperature programmed analyses in the range of 293–573 K under Ar disclose abundance and interaction pattern of adsorbed reagents and products with the catalyst surface, whereas the reactivity of adsorbed CO2 species was probed by analyses in H2 stream of catalyst samples subjected to the above saturation treatment. Using proper response factors for the various m/z values monitoring reagents and products, mass-balance calculations (C.M.B.) on the released CO2 , CH3 OH and CO both in Ar and H2 streams result always in a satisfactory agreement (100 ± 5%), as observable from Table 3. Under Ar stream all the catalysts display a slight release rate of CO2 and CO in the range of 293–413 K, both rising suddenly at higher temperature (Fig. 7A–C; solid lines), along with trace amounts of methanol in the range of 353–423 K (Table 3), “frozen” on the catalyst surface. Quantitative data in Table 3 indicate that the extent of released CO2 and CO varies from 125 (CuZnCe) to 306 ␮mol gcat −1 (CuZnZr) and from 10 (CuZnAl) to 62 ␮mol gcat −1 (CuZnZr), respectively. In particular, the CO2 profile of CuZnAl

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

A)

H2O

CO2 H2O

CH3OH

C)

CO

pressure (a. u.)

CO2

B)

CO

pressure (a. u.)

pressure (a. u.)

CO

7

CH3OH

CO2 H2O

CH3OH

293 333 373 413 453 493 533 573

293 333 373 413 453 493 533 573

293 333 373 413 453 493 533 573

Temperature (K)

Temperature (K)

Temperature (K)

Fig. 7. Temperature programmed profiles of CO2 , CO, CH3 OH (black lines) and H2 O (grey lines) of “steady-state” CuZnAl (A), CuZnZr (B) and CuZnCe (C) catalysts in Ar (solid lines) and H2 (dotted lines).

30

Surface COx release (µmol•m-2 )

(Fig. 7A) and CuZnZr (Fig. 7B) samples consists of a broad release with maxima at 443–453 K and 503–513 K, while the CuZnCe catalyst shows only a main component centred at 513 K (Fig. 7C). Further, CuZnZr and CuZnCe systems show a CO profile with a broad maximum at ca. 513 K, while for the CuZnAl one it is much featureless showing a poorly resolved maximum at ca. 433 K. Characterized by a poor interaction strength with Cu0 sites [4,5], adsorbed CO is expected to desorb at much lower temperature and, then, it should arise from the splitting of CO2 to CO and oxygen atoms, causing an incipient oxidation of metal particles, and CO strongly adsorbed on Cuı+ sites [4,5]. Although a poor water release indicates that products do not arise from surface reactions between adsorbed CO2 and H2 , a larger water signal seems to account for the higher water-affinity of CuZnAl catalyst, induced both by alumina support and higher ZnO load [3,4]. Referred to total, metal and oxide surface area values (Table 1) the released amount of COx against dispersion result in the trends shown in Fig. 8. These document an apparent negative influence of dispersion on metal surface COx uptakes, similar to TOF of Cu sites (see infra) [4–6,20]. The SA and OSA uptakes decrease with dispersion according to analogous trends (Fig. 8), resulting still larger on CuZnCe system exposing a larger fraction of oxide phases (Fig. 1). Indeed, no contribution of promoter and carrier to surface adsorption would account for a metal surface adsorption capacity decreasing from ≈30 (CuZnCe) to 4–6 ␮mol mCu −2 while, with reference to OSA, the surface uptakes are comprised between 12 and 3–6 ␮mol mOx −2 , respectively (Fig. 8). In any case, these document a strong promoting effect of ceria carrier on the CO2 adsorption capacity of the Cu–ZnO system, ascribable to its peculiar ability to form surface and bulk oxygen vacancies at metal–oxide interface under reducing atmosphere [25]. The profiles of reagents and products under H2 stream (Fig. 7A–C; dotted lines) are quite different both in qualitative and quantitative terms, since they mirror the ongoing hydrogenation of adsorbed CO2 , probed by the strong decrease (60–80%) of its signal

MSA 20

OSA

10

SA 0 5

10

15

20

25

30

35

D (%) Fig. 8. Influence of metal dispersion on the specific “steady-state” adsorption capacity referred to total (SA), metal (MSA) and oxide surface area (OSA).

and the corresponding rise of CH3 OH, CO and water ones (Table 3). Then, the CO2 profile monitors spectator CO2 moieties, while the missing part of the profile recorded in Ar highlights concentration and type of reactive species. Spanning the range of 333–573 K and with broad maxima at 373–413 K and 483–503 K, respectively, methanol profiles look similar for all the catalysts. An integral area ca. two orders of magnitude larger than that recorded in Ar (Table 3) substantiates the prevalent MS functionality of the studied systems, confirming also that methanol is a primary product of the CO2 -hydrogenation reaction at very low temperature (<373 K). The lack of CO at lower temperature probably depends on the inhibiting effect of H2 on CO2 splitting, while the steady rise at T > 373 K to a much larger extent than in Ar mirrors instead the increasing contribution of MD and RWGS reactions [21].

Table 3 Temperature programmed data of “steady-state” catalysts. Catalyst

CuZnAl CuZnZr CuZnCe

Ar stream amount desorbed (␮mol/gcat )

H2 stream amount desorbed (␮mol/gcat )

CO2

CO

CH3 OH

products

CO2

CO

CH3 OH

products

125 306 259

10 62 29

2 2 3

137 370 291

35 104 104

45 196 143

59 69 51

139 369 298

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016

ARTICLE IN PRESS

G Model CATTOD-8355; No. of Pages 8

F. Arena et al. / Catalysis Today xxx (2013) xxx–xxx

Specific surface rate (µmol CO2 m-2 s-1)

8

4×10-0 1

•

A)

In particular, the main results of the work can be summarized in the followings:

B)

• Oxide carrier controls texture and adsorption properties of the Cu–ZnO system; • Methanol is a primary reaction product at low temperature and extent of CO2 conversion; • CO formation occurs via a consecutive-parallel reaction network, mostly at T ≥ 473 K; • Steady-state CO2 adsorption is not related to metal surface exposure (MSA); • The catalytic activity depends on a synergism of metal and basic oxide sites, pointing to a dual-site reaction path; • Textural and chemical effects of zirconia carrier confer a superior process performance to ZrO2 -supported system, ensuring −1 methanol space time yields up to 1.2 kgCH3 OH kg−1 at ≈10% cat h of CO2 conversion per pass.

•

3×10 -0 1

MSA

2×10 -0 1 OSA

1×10 -0 1 SA

0

5

10

15

20

25

30

35

Specific surface rate (µmolCO2 m-2 s-1)

D (%) 4×10 -0 1

•

MSA

•

3×10 -0 1

Appendix A. Supplementary data 2×10

-0 1

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. cattod.2013.02.016.

OSA

1×10 -0 1

SA

References 0

5

10

15

20

25

30

35

D (%) Fig. 9. Influence of metal dispersion on the specific rate of CO2 conversion (473 K) referred to total (SA), metal (MSA) and oxide surface area (OSA) at 3.0 (A) and 5.0 (B) MPa.

Therefore, the oxide carrier controls the catalytic functionality of the Cu–ZnO system by influencing its steady-state CO2 adsorption pattern, as it is evident by comparing kinetic activity data (473 K) at 3.0 and 5.0 MPa normalized to the SA, MSA and OSA of the studied catalysts (Fig. 9). Once more the MSA specific activity as a function of dispersion results in typical decreasing trends, pointing out an apparent higher specific activity of larger Cu particles [3–5,20]. However, it is consequence of assuming a competitive one-site L–H mechanism, disregarding any contribution of promoter and carrier on the catalytic functionality of the system. In fact, activity data normalized to the OSA result in constant specific rate values, irrespective of dispersion and catalyst composition (Fig. 9), stressing the fundamental contribution of the oxide carrier on the CO2 -hydrogenation functionality of the Cu–ZnO system. Hence, chemical effects of the oxide carrier [3,5–7,11,12,25] substantiate the dual-site nature of the CO2 -to-methanol hydrogenation reaction on supported Cu–ZnO systems. 4. Conclusions The effects of temperature, pressure and space–velocity on the activity–selectivity pattern of Al2 O3 , ZrO2 and CeO2 supported Cu–ZnO systems in the synthesis of methanol via CO2 hydrogenation have been addressed.

[1] G.A. Olah, A. Goeppert, G.K. Surya Prakash, Journal of Organic Chemistry 74 (2009) 487. [2] C. Song, Catalysis Today 115 (2006) 2. [3] X.-M. Liu, G.Q. Lu, Z.-F. Yan, J. Beltramini, Industrial and Engineering Chemistry Research 42 (2003) 6518. [4] F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro, F. Frusteri, Journal of Catalysis 249 (2007) 185. [5] F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura, L. Spadaro, F. Frusteri, Applied Catalysis A General 350 (2008) 16. [6] F. Arena, G. Italiano, K. Barbera, G. Bonura, L. Spadaro, F. Frusteri, Catalysis Today 143 (2009) 80. [7] G. Bonura, F. Arena, G. Mezzatesta, C. Cannilla, L. Spadaro, F. Frusteri, Catalysis Today 171 (2011) 251. [8] C. Yang, Z. Ma, N. Zhao, W. Wei, T. Hu, Y. Sun, Catalysis Today 115 (2006) 222. [9] S.-I. Fujita, M. Usui, H. Ito, N. Takezawa, Journal of Catalysis 157 (1995) 403. [10] W. Wang, S. Wang, X. Ma, J. Gong, Chemical Society Reviews 40 (2011) 3703. ´ [11] J. Słoczynski, R. Grabowski, P. Olszewski, A. Kozłowska, J. Stoch, M. Lachowska, J. Skrzypek, Applied Catalysis A General 310 (2006) 127. [12] K.A. Pokrovski, A.T. Bell, Journal of Catalysis 241 (2006) 276. [13] S. Askgaard, J.K. Nørskov, C.V. Ovesen, P. Stoltze, Journal of Catalysis 156 (1995) 229. [14] I. Chorkendorff, J.W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, WILEY-VCH GmbH & Co. KGaA, Weinheim, 2005. [15] J. Nakamura, I. Nakamura, T. Uchijima, Y. Kanai, T. Watanabe, M. Saito, T. Fujitani, Journal of Catalysis 160 (1996) 65. [16] V.E. Ostrovskii, Catalysis Today 77 (2002) 141. [17] D. Granjean, V. Pelipenko, E.D. Batyrev, J.C. van de Heuvel, A.A. Khassin, T.M. Yurieva, B.M. Weckhuysen, Journal of Physical Chemistry C 115 (2011) 20175. [18] I.A. Fisher, A.T. Bell, Journal of Catalysis 172 (1997) 222. [19] Q. Sun, Y.-L. Zhang, H.-Y. Chen, J.-F. Deng, D. Wu, S.-Y. Chen, Journal of Catalysis 167 (1997) 92. [20] A. Karelovic, A. Bargibant, C. Fernàndez, P. Ruiz, Catalysis Today 197 (2012) 109. [21] Y. Choi, H.G. Stenger, Applied Catalysis B Environmental 38 (2002) 259. [22] K. Klier, V. Chatikavanij, R.G. Herman, G.W. Simmons, Journal of Catalysis 74 (1982) 343. [23] W.H. Cheng, Applied Catalysis A General 130 (1995) 13. [24] I.A. Fisher, A.T. Bell, Journal of Catalysis 184 (1999) 357. [25] J.B. Wang, H.-K. Lee, T.-J. Huang, Catalysis Letters 83 (2002) 79.

Please cite this article in press as: F. Arena, et al., How oxide carriers control the catalytic functionality of the Cu–ZnO system in the hydrogenation of CO2 to methanol, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.02.016