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Zr precursors
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Role of ZrO2 in Cu/ZnO/ZrO2 catalysts prepared from the precipitated Cu/Zn/Zr precursors Cheonwoo Jeong, Young-Woong Suh ∗ Department of Chemical Engineering, Hanyang University, Seoul 133–791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 June 2015 Received in revised form 28 July 2015 Accepted 29 July 2015 Available online xxx Keywords: Cu/ZnO/ZrO2 Precursor structure Amorphous zirconia Specific copper surface area Support
a b s t r a c t In Cu/Zn/Zr precursors prepared by coprecipitation, Zr ions showed no effect on a substitution level of Cu2+ by Zn2+ in the zincian malachite (zM) and were not incorporated into zM lattice upon precipitation and subsequent ageing. As hydroxyl-rich Zr aggregates were precipitated independent of mixed Cu/Zn precipitates, they served as a barrier to formation of larger Cu/Zn particles during precipitation and transformation into Cu/Zn needles during ageing. After calcination of Cu/Zn/Zr precursors with higher Zr contents, smaller CuO/ZnO particles were produced in between which amorphous ZrO2 was situated. Upon reduction of CuO/ZnO/ZrO2 with more Zr, this type of ZrO2 resulted in formation of smaller Cu0 particles yielding the higher Cu dispersion. However, the specific copper surface area and catalytic activity of the reduced Cu/ZnO/ZrO2 exhibited a volcano-type relationship with the Zr/(Cu + Zn) ratio. This could be a combined result between the lower amount of total Cu and the higher Cu dispersion. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The industrial Cu/ZnO/Al2 O3 catalyst for methanol synthesis has recently attracted renewed interest because of potential of the socalled “methanol economy” [1]. Although it is widely accepted that the addition of Al improves the performance of the binary Cu/ZnO catalyst and Al2 O3 serves as a structural promoter [2–4], the nature of its promoting effect has been poorly understood until a report of Behrens et al. [5,6]. They demonstrated a structural and electronic promoting effect from the fact that the highest Cu dispersion could be attained by desired incorporation of both Zn2+ and Al3+ into the catalyst precursor. In Cu/ZnO system, Cu2+ in malachite is substituted by Zn2+ yielding the zincian malachite (zM) structure. ¯ Calculated from the d-spacing of (2 0 1)-reflection of the zM, the maximal Zn2+ substitution is around 28% in Cu/ZnO catalyst [7,8]. In Cu/ZnO/Al2 O3 system, simultaneous coincorporation of Zn2+ and Al3+ (up to approximately 3.3%) is possible because the chemical potential of Al is appropriately low in this compositional window. The dispersion of Cu in the final catalysts is thus promoted by a geometric effect. An electronic promotion effect by Al3+ incorporation was also elucidated by analogy to Ga- and Cr-promoted Cu/ZnO systems. It is unfortunate that the above concept derived from Cu/ZnO and Cu/ZnO/Al2 O3 has not been extended yet to other ternary
∗ Corresponding author. Tel.: +82 2 2220 2329; fax: +82 2 2220 2294. E-mail address: [email protected] (Y.-W. Suh).
Cu/ZnO-based systems. Among a variety of the third elements our attention was particularly paid to ZrO2 , since the Handbook of Heterogeneous Catalysis ranks the supports as in the order ZrO2 > (Al2 O3 ) > TiO2 > SiO2 where Al2 O3 also acts as a promoter [9]. Arena et al. [10–12] intensively studied the performance of ternary Cu/ZnO/ZrO2 catalysts and identified ZrO2 as an excellent support, where ZrO2 loading was fixed around 43 wt.%. The structure and chemistry of Cu/Zn/Zr precursors were not discussed in their study. In a recent report of Frei et al. [13], the influence of precipitation and ageing temperatures on the preparation of an active Cu/ZnO/ZrO2 catalyst was investigated. They characterized a series of Cu/Zn/Zr precursors, calcined CuO/ZnO/ZrO2 and reduced Cu/ZnO/ZrO2 samples in detail. Nevertheless, it was found that the structural differences of the precursors affected the catalytic performances only slightly due to structural reorganization happening upon reduction and activity test. Therefore, a more understanding of the precursor chemistry and Zr speciation is crucial to facilitate knowledge-based optimization of Cu/ZnO/ZrO2 catalyst and also elucidate the role of ZrO2 in the catalyst, like the recent progress of Cu/ZnO/Al2 O3 , which is the focus of this study. We herein prepared Cu/Zn/Zr precursors by a coprecipitation method, in which the Zr/(Cu + Zn) ratio varied from 0/100 to 60/100. In the preparation of all precursors, the Cu/Zn ratio was fixed at 70/30 since this is a preferred ratio for binary Cu/ZnO and ternary Cu/ZnO/Al2 O3 catalysts, and the zM structure is preferentially produced [7,8,14]. The chemical and structural identity of Cu/Zn/Zr precursors was investigated in detail by several characterization techniques. For comparison, pure Zr precursor and binary
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Table 1 Metal composition and TG weight loss of Cu/Zn/Zr samples prepared in this study. Sample code
CZZr-0 CZZr-5 CZZr-10 CZZr-40 CZZr-60
Nominal metal ratio (%)
Metal composition (at.%)a
Zr/(Cu + Zn)
Cu/(Cu + Zn)
Cu
Zn
Zr
measured
calculatedb
accuracyc
0 5 10 40 60
70 70 70 70 70
70.0 (69.1) 66.7 (64.2) 63.6 (63.6) 50.0 (51.2) 43.8 (44.9)
30.0 (30.9) 28.6 (31.1) 27.3 (27.3) 21.4 (26.9) 18.8 (17.5)
0.0 (0.0) 4.8 (4.7) 9.1 (11.7) 28.6 (21.9) 37.5 (37.6)
34.8 32.2 31.1 23.8 22.3
33.3 32.4 31.6 27.8 26.1
0.985 1.002 1.004 1.040 1.038
TG weight loss (wt.%)
Calculation
a
The metal compositions of the calcined samples were determined by EDX spectroscopy. The calculation was based upon the assumption that (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O is physically mixed with ZrO(OH)1.82 (CO3 )0.09 at the nominal Zr/(Cu + Zn) ratios in which each component is thermally decomposed to (Cu0.7 Zn0.3 )2 O2 and ZrO2 . c Calculation accuracy = [(weight of residual sample after TG analysis) + (weight of calculated loss)]/(weight of original sample before analysis)]. b
Zn-free Cu/Zr precursors were prepared analogously. The characteristics of the calcined CuO/ZnO/ZrO2 and reduced Cu/ZnO/ZrO2 samples were also studied in order to identify if the precursor structure influences their structure and morphology. After the reduced catalyst was tested in the hydrogenolysis of butyl butyrate, the catalytic performance was correlated with the Cu dispersion and specific Cu surface area. From these results, an explanation was formulated for the role of ZrO2 in Cu/ZnO/ZrO2 showing a volcano-type relationship between the activity and the Zr/(Cu + Zn) ratio. 2. Experimental 2.1. Catalyst preparation A series of Cu/Zn/Zr precursors with different Zr/(Cu + Zn) ratios were prepared by a reverse precipitation method. The ratio of Cu/(Cu + Zn) was fixed at 70% because both Cu/ZnO and Cu/ZnO/ZrO2 catalysts with this ratio showed the best performance in preliminary activity tests (Fig. S1). The nominal Cu/Zn/Zr ratio is 70/30/x and the x value is 100 × [Zr]/{[Cu] + [Zn]}, thus the sample being labeled as CZZr-x with x of 0, 5, 10, 40 and 60. Sample details are summarized in Table 1. In the preparation of CZZr-10, a mixed metal solution (total 1.2 M, 25 mL) containing Cu(NO3 )2 ·3H2 O (4.66 g), Zn(NO3 )2 ·6H2 O (2.48 g) and ZrO(NO3 )2 (0.63 g) was added dropwise into an aqueous NaHCO3 (0.1 M, 600 mL) at 70 ◦ C for 15 min under vigorous stirring. After complete addition (final pH of 7), the resulting precipitate suspension was aged at the same temperature for 90 min. Then, the precipitate was filtered and washed with deionized water (ca. 1 L). The washing step was repeated at least three times in order to assure the complete removal of sodium and nitrate ions. The so–obtained filter cake was dried in a convection oven at 105 ◦ C overnight. For comparison, the synthetic malachite (Cu/Zn/Zr = 100/0/0), Zr precursor (Cu/Zn/Zr = 0/0/100), binary Zn-free Cu/Zr precursors (Zr/Cu = 2.5/70 and 5/70), and ternary Cu/Zn/Zr precursors with the Cu/Zn ratios of 30/70 and 50/50 (x = 0, 10, 40 and 60) were prepared using the above recipe. The dried precursor was crushed and sieved for obtaining the particle size of smaller than 200 m. Finally, the calcination was carried out in a static air at 400 ◦ C for 3 h. 2.2. Characterization Powder XRD (PXRD) analysis was conducted with a Rigaku D/Max RINT 2000 diffractometer using Cu K␣ ( = 0.1541 nm) as a radiation source (40 kV and 60 mA). XRD patterns were collected with a step increment of 0.02◦ and scan speed 4◦ /min in the 2 range of 10◦ –80◦ . For obtaining XRD patterns of reduced Cu/ZnO/ZrO2 catalysts, a calcined sample was pretreated in the reactor with H2 , where the temperature was raised to 350 ◦ C at a ramping rate of 5 K/min and kept at the same temperature for 3 h, and purged with
N2 for 1 h. The reduced samples were immediately analyzed to measure a metallic state of the sample. TGA analysis (TA instrument, SDT Q600) was carried out using a sample, where the temperature was raised from room temperature to 800 ◦ C at a ramping rate of 10 K/min in an air flow (100 ml/min). ATR-IR spectroscopy (Thermo Scientific IS50 spectrometer) with a scan number of 32 and a resolution of 4 cm−1 was used to monitor functional groups of dried Cu/Zn/Zr precursors along with the synthetic malachite and Zr precursor, and commercial Zr(OH)2 (CO3 )·ZrO2 (Sigma–Aldrich). SEM images of Pt-coated samples were taken in a FEI Nova NanoSEM 450 microscope operated at 5 kV. For TEM-EDS analysis coupled with electron diffraction, a JEM 2100F microscope (JEOL) was used with a Gatan DigitalMicrograph imaging filter. For BET measurement, a calcined sample (ca. 0.1 g) was first pretreated measured at 200 ◦ C for 2 h under vacuum in an ASAP 2010 (Micrometrics) to remove adsorbed species. N2 physisorption was then conducted at a liquid nitrogen temperature to measure the specific BET surface area. Prior to TPR measurement performed with an AutoChem 2910 (Micrometrics), the stable TCD baseline was acquired at 90 ◦ C at all runs. Then, 10% H2 in Ar at a flow rate of 50 ml/min was introduced into a quartz U-tube containing the sample, where the temperature was raised from 90 to 350 ◦ C at a ramping rate of 2.5 ◦ C/min. The same instrument for TPR measurement was used for N2 O chemisorption to determine a specific copper surface area (As,Cu ). It was assumed that the reaction stoichiometry between copper and oxygen is two (Cu/O = 2/1) and the copper surface density is 1.46 × 1019 Cu atom/m2 . In a typical experiment, about 50 mg of calcined sample was reduced with 10% H2 in Ar at 350 ◦ C for 3 h. After the reactor was cooled to 90 ◦ C in pure He and maintained for 10 min, N2 O titration was started. Although N2 O titration result cannot be directly related to only the exposed Cu surface due to oxidation of reduced Zn species by N2 O studied in recent reports [15,16], the measured copper surface area still remains the best indicator for the catalytic performance of Cu/ZnO-based catalysts. 2.3. Activity test Prior to the activity test, the calcined CuO/ZnO/ZrO2 samples (ca. 0.4 g) was reduced at 20 bar H2 (99.99%) at 350 ◦ C for 3 h. After cooling to ca. 220 ◦ C at the same pressure, the catalytic activity on the hydrogenolysis of butyl butyrate was examined in a stainless steel reactor. Experimental details were described in our previous report [17]. 3. Results The Cu/Zn/Zr precursors obtained after precipitation, ageing and drying were characterized by PXRD, TG-MS and ATR-IR. Fig. 1a shows the typical reflections of the zincian malachite
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Fig. 1. (a) PXRD patterns of Cu/Zn/Zr precursors. At the bottom are given the reflections of the malachite structure (41–1390) as indexed from ICDD database. (b) Magnification ¯ and (2 1 1)-reflections ¯ induced by incorporation of Zn2+ into the malachite lattice was not affected by Zr4+ ions present in Cu/Zn/Zr of 2 = 30–35◦ , where 2 shift of (2 0 1)precursors.
(Cu1−x Znx )2 (OH)2 (CO3 ) in all precursors, independent of the Zr content. This phase is a result of Cu2+ substitution by Zn2+ (i.e., lattice contraction) in the malachite structure Cu2 (OH)2 (CO3 ), which was ¯ and (2 1 1)-reflections ¯ as indicated identified by 2 shift of (2 0 1)in Fig. 1b. This originates from the Cu/Zn molar ratio of 70/30 chosen for the synthesis. The more important finding was no shift of the two reflections in all Cu/Zn/Zr precursors while the peak intensities were lower with increasing the Zr content. In other words, the chemical structure of the zincian malachite (zM) was not affected by Zr ions added for coprecipitation of a mixed metal solution. This
is the first report to clearly observe no change of the zM lattice by Zr ions. In PXRD patterns of all Cu/Zn/Zr precursors, no reflections belonging to any Zr phase were detected. This implies that Zr precursor may exist in a form of amorphous and non-diffracting phase. Since IR spectroscopy is capable of probing both the amorphous and crystalline parts of the sample, ATR-IR spectra of the Cu/Zn/Zr precursors were thus recorded at room temperature (Fig. 2). The spectrum of the synthetic malachite was compared. In the range 4000–2500 cm−1 for O H stretching bands, the spectrum of the
Fig. 2. IR spectra of Cu/Zn/Zr precursors in the ranges 4000–2500 and 1800–400 cm−1 . For comparison, IR spectra of the synthetic malachite, pure Zr precursor and commercial Zr(OH)2 (CO3 ) are given as top and bottom traces.
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Fig. 4. PXRD patterns of CuO/ZnO/ZrO2 samples calcined at 400 ◦ C. At the bottom are given the reflections of CuO (05–0661; blue bars) and ZnO (36–1451; red bars). The reflection of CuO(1¯ 1 1) at 2 = 35.6◦ was used for calculating the crystal size of CuO.
Fig. 3. (Top two) TG and DTG curves of Cu/Zn/Zr precursors in the temperature range 50–800 ◦ C. (Bottom two) MS traces for H2 O (m/z = 18) and CO2 (m/z = 44) evolved during TG experiments.
synthetic malachite showed a splitting into two modes at 3318 and 3402 cm−1 like that of the mineral malachite [18], which is attributed to the two crystallographically different OH groups [19]. The splitting was enhanced to 171 cm−1 for CZZr-0, due to Zn2+ incorporation into the malachite structure [20]. However, a change of the splitting was not noticed in the spectra of all Cu/Zn/Zr precursors, though the band became broader with increasing the Zr content. This feature holds for the asymmetric C O stretching bands of the carbonate anions in the region 1700–1100 cm−1 , and OH deformation vibrations and further carbonate bands in the region 1200–600 cm−1 . It could be proposed again that the chemical structure of the zM was hardly changed by Zr ions added into the starting metal solution, although we failed to observe fingerprint bands of Zr-related precursors by IR spectroscopy. The prepared Cu/Zn/Zr precursors were additionally characterized by TG-MS from 50 to 800 ◦ C (Fig. 3). The measured weight loss of CZZr-0 was 34.8%, which is close to the calculated loss of 33.3% corresponding to thermal decomposition of (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O into (Cu0.7 Zn0.3 )2 O2 accompanied by emission of H2 O and CO2 gases. The chemical formula of CZZr0 is assumed from the zincian malachite, (Cu1−x Znx )2 (OH)2 (CO3 ) with x = 0.3, with a water adduct. In the case of Cu/Zn/Zr precursors, as the Zr content increased the weight loss decreased in the following order: 32.2% for CZZr-5 > 31.1% for CZZr-10 > 23.8% for CZZr-40 > 22.3% for CZZr-60 (Table 1). Although accurate discussion of the result is possible when the chemical structure of Zr precursor will be determined, release of lighter gaseous molecules (i.e., H2 O rather than CO2 ) is expected upon decomposition of Cu/Zn/Zr precursors with higher Zr contents. This was confirmed by normalized MS traces of H2 O (m/z = 18) and CO2 (m/z = 44). In CZZr-0,
decarbonylation events occurred below and above 400 ◦ C, where the latter represents decomposition of “high-temperature carbonate” often observed in the zM [7]. The two events were not shifted as a function of the temperature in Cu/Zn/Zr precursors, while they were lessened with increasing the Zr content. In the range 250–400 ◦ C showing a simultaneous dehydroxylation and decarbonylation step, emission of water molecules was reduced in all Cu/Zn/Zr precursors compared to CZZr-0 but the reduction extent was independent of the Zr content. In contrast, an additional dehydroxylation event appeared in the range 100–250 ◦ C for CZZr-40 and CZZr-60. Similarly, higher values of derivative TG (DTG) in the same temperature range were observed in Cu/Zn/Zr precursors with higher Zr contents. The TG-MS results suggest that Zr ions present in a mixed metal hydroxycarbonate precursor do not influence intrinsic decomposition of the zM and they exist as a hydroxy-rich Zr precursor decomposed at low temperatures. This type of Zr precursor will be discussed later. Next, CuO/ZnO/ZrO2 samples obtained after calcination at 400 ◦ C were characterized by PXRD, BET and SEM. The reflections of CuO and ZnO were only detectable in all PXRD patterns, and these reflections were sharper with decreasing the Zr content (Fig. 4), alike PXRD patterns of Cu/Zn/Zr precursors. Calculated using the most intense reflection of the samples, that is, CuO(1¯ 1 1) at 2 = 35.6◦ , the crystal size of CuO decreased from 61.3 to 56.6 A˚ with increasing the Zr/(Cu + Zn) ratio from 0% to 60% (Table 2). This is due to a decrease of Cu loading in CuO/ZnO/ZrO2 with a higher Zr content. For confirmation, the specific surface areas of the calcined samples (As,BET ) were measured; the binary CZZr-0 showed only 31 m2 /g and the ternary samples high surface areas of 58–102 m2 /g (Table 2). As the Zr/(Cu + Zn) ratio increased to 40%, the specific surface area increased to 102 m2 /g and then maintained around this value. It was noted that the margin of the increase is greater in the specific surface area than in the CuO crystal size. This explains that all individual oxide particles become smaller in CuO/ZnO/ZrO2 samples with more Zr content. SEM micrographs of calcined CuO/ZnO/ZrO2 samples were also taken (Fig. S2). Obviously, the size of overall particles decreased with increasing the Zr content. EDS mapping was additionally conducted for identifying the presence of Zr, since no ZrO2 -related reflections
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Table 2 Properties of the calcined CuO/ZnO/ZrO2 samples, and properties and performance of the reduced Cu/ZnO/ZrO2 catalysts. Butanol productivityh
Sample code
Calcined rP,XRD a (Å)
As,BET b (m2 /g)
rP,XRD c (Å)
rP,N2O d (Å)
As,Cu e (m2 /g)
NH2 f (mmol g−1 )
DCu g (%)
(mmol gcat −1 h−1 )
CZZr-0 CZZr-5 CZZr-10 CZZr-40 CZZr-60
61.3 59.1 58.3 58.0 56.6
31 58 68 102 100
183.5 153.8 139.1 139.1 138.9
168.9 124.3 102.9 100.6 83.3
21.8 26.8 30.3 24.3 23.9
8.24 7.55 6.85 5.24 4.70
6.5 8.7 10.8 11.3 12.4
43.3 53.5 65.4 37.5 36.2
a b c d e f g h
Reduced
Crystal size of CuO calculated from the reflection of CuO(1¯ 1 1) at 2 = 35.6◦ . Specific BET surface area from N2 physisorption. Crystal size of Cu0 calculated from the reflection of Cu(1 1 1) at 2 = 43.3◦ . Particle size of Cu0 calculated from N2 O titration and metal composition. Specific Cu surface area from N2 O titration at 363 K, as outlined in Section 2. Amount of consumed H2 from TPR measurement. Cu dispersion = 2 × (N2 amount used for calculation of As,Cu )/(total amount of reduced CuO). Reaction condition: 493 K, 20 bar, H2 /butyl butyrate = 20, weight hourly space velocity = 9 h−1 .
were visible in all PXRD patterns. As summarized in Table 1, the measured atomic percentages of Cu, Zn and Zr were close to the nominal ones in each sample, which is an indication of successful coprecipitation of all metal components. Therefore, it is believed that Zr exists in the form of an amorphous ZrO2 phase due to a low calcination temperature of 400 ◦ C. This seems to be valid since tetragonal and monoclinic ZrO2 phases were found after calcination at 550, 650 and 850 ◦ C (Fig. S3). PXRD patterns were collected for Cu/ZnO/ZrO2 samples obtained after reduction at 350 ◦ C (Fig. 5). All measurements showed only Cu0 and ZnO as detectable phases, and the intensities of all reflections became lower with increasing the Zr content, similar to calcined CuO/ZnO/ZrO2 samples. When the copper crystal size was calculated using the reflection of Cu(1 1 1) at 2 = 43.3◦ , it decreased from 183.5 (CZZr-0) to 138.9 A˚ (CZZr-60), as summarized in Table 2. Since CZZr-10, CZZr-40, and CZZr-60 catalysts exhibited similar Cu0 sizes due to lower and broader PXRD reflections, we additionally calculated Cu0 particle size using N2 O titration results and total Cu content measured by EDX spectroscopy. The value rP,N2O also decreased from 168.9 (CZZr-0) to 83.3 A˚ (CZZr-60). This is in accordance with the finding that Cu dispersion (DCu ) increased from 6.5% to 12.4% with the Zr content (Table 2). Therefore, smaller Cu0 particles were formed upon reduction of CuO/ZnO/ZrO2
Fig. 5. PXRD patterns of Cu/ZnO/ZrO2 samples reduced at 350 ◦ C. At the bottom are given the reflections of Cu (04–0836; blue bars) and ZnO (36–1451; red bars). The reflection of Cu(1 1 1) at 2 = 43.3◦ was used for calculating the crystal size of Cu0 .
samples with higher Zr contents, possibly due to a decrease of Cu loading. The catalytic activities of Cu/ZnO/ZrO2 samples were tested in the hydrogenolysis of butyl butyrate at 20 bar H2 and 220 ◦ C (Table 2). The butanol productivity showed a volcano-type relationship with the Zr content; as the Zr/(Cu + Zn) ratio increased from 0% to 10%, the productivity was improved from 43.3 to 65.4 mmol gcat −1 h−1 , and then decreased to 36.2 mmol gcat −1 h−1 (CZZr-60). It was also found that the productivity had a strong dependence on the specific copper surface area, of which more details will be discussed later.
4. Discussion We investigated the characteristics of the prepared Cu/Zn/Zr samples in the precursor, oxide and reduced states. Understanding the chemical and structural identity of the coprecipitated precursor is of great importance since it can help predicting the structure of the calcined oxide and, furthermore, the property and activity of the final catalyst (i.e., reduced state), which is the so-called “chemical memory”. From this standpoint, the XRD result of Fig. 1 revealed that Zr ions present in the starting precipitate solution had no effect on a substitution level of Cu2+ by Zn2+ in the zincian malachite (zM). Also, no incorporation of Zr4+ itself into the zM lattice could be corroborated. This is in sharp contrast to the Cu/ZnO/Al2 O3 catalyst system investigated by Behrens et al. [5,6]. They revealed that variation of the Al content in the starting solution had a direct influence on the concentration of Cu2+ in the zM and simultaneous coincorporation of Zn2+ and Al3+ was possible at a certain Al content, based on ¯ spacing of the zM with respect to the Al content. a change of d(2 0 1) To ensure no incorporation of Zr4+ , we additionally prepared Cu/Zn/Zr precursors with different Cu/Zn ratios of 50/50 and 30/70, and found that all reflections of the binary Cu/Zn precursors were not shifted at all by adding a higher amount of Zr source into the precipitating solution (Fig. S4). This suggests that Zr ions cannot be incorporated into any lattice of mixed Cu/Zn hydroxycarbonate precursors. The question then arises regarding whether such a phenomenon can happen in the pure Cu precursor (i.e., the malachite). Thus, Zn-free Cu/Zr precursors with the Zr/Cu ratios of 2.5/70 and 5/70 were synthesized under the same condition as used in the preparation of Cu/Zn/Zr precursors. PXRD patterns of these pre¯ and cursors are presented in Fig. 6. Surprisingly, 2 shift of (2 0 1)¯ (2 1 1)-reflections of the malachite was negligible, although there was a very slight shift of 0.04◦ corresponding to a d-spacing dif˚ This means no incorporation of Zr4+ into the ference of 0.004 A. malachite lattice. Therefore, we strongly believe that both the malachite and zincian malachite structure are not able to incorporate even small amounts of Zr4+ .
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¯ Fig. 6. (a) PXRD patterns of Cu/Zr precursors. At the bottom are given the reflections of the malachite structure (41–1390). (b) Magnification of 2 = 30–35◦ , where (2 0 1)¯ of the malachite structure was shifted very negligibly in Cu/Zr precursors (2 difference () = 0.04◦ ). and (2 1 1)-reflections
The Zn incorporation into the malachite lattice is principally due to the chemical similarity of Cu2+ and Zn2+ concerning cation charge and size in the precursor state [14]. Therefore, no Zr incorporation is presumed to result from the different size and structure of Zr4+ species in aqueous solution. In general, Zr4+ cations are very easily hydrolyzed in H2 O and form the complex exhibiting the tetrameric structure of a slightly distorted square followed by ageing into larger polynuclear complexes containing a number of Zr atoms linked by hydroxyl bridges [21]. Kanazhevskii et al. [22] reported that the complexes formed from aqueous zirconium oxynitrate (ZrO(NO3 )2 ·nH2 O) and zirconium oxychloride solutions were structurally similar and they were polynuclear complexes of a square-planar configuration. These polynuclear complexes are susceptible to condensation reaction to form larger oligomers and, further, colloidal particles and gelatinous precipitates. The precipitates can be described as an amorphous mixed oxide/hydroxide expressed by the general formula ZrOx (OH)4−2x with x of 0 or 1 [23]. Since zirconium oxynitrate was used as a Zr source and NaHCO3 as a precipitating agent in this work, the Cu/Zn-free Zr precursor was prepared analogously. The TG weight loss of the synthetic Zr precursor was 14.2% at 800 ◦ C, while H2 O and CO2 were evolved simultaneously at the temperature centered around 200 ◦ C (Fig. 7). This is very different from the decomposition behavior of commercial Zr(OH)2 (CO3 )·ZrO2 ; the weight loss was measured to be 21.1% close to the calculated value (20.1%) for decomposition into ZrO2 while the maximum H2 O evolution was observed around 127 ◦ C and CO2 was emitted at 117 and 580 ◦ C. Therefore, it was considered that the Zr precursor was precipitated as ZrOx (OH)4−2x with a small amount of CO3 adduct. This is valid since the IR spectrum of the synthetic Zr precursor exhibited very weak carbonate bands at 1514 and 1385 cm−1 , and broad O H stretching bands in the range 4000–2500 cm−1 (Fig. 2). The feature of such bands is the reason why we failed to observe any Zr-related IR bands in the spectra of Cu/Zn/Zr precursors. In contrast, distinct carbonate bands were observed at 1629, 1549 and 1338 cm−1 in the spectrum of Zr(OH)2 (CO3 )·ZrO2 . Thus, the chemical formula of the synthetic Zr precursor would be assumed ZrO(OH)2−y (CO3 )y/2 with y of 0.18 (Table S1 for detailed calculation procedure). When the assumption that (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O is “physically”
mixed with ZrO(OH)1.82 (CO3 )0.09 at nominal Zr/(Cu + Zn) ratios used for Cu/Zn/Zr precursors was made, the calculation of the Cu/Zn/Zr precursor weight showed quite a good accuracy in the range 0.985–1.040 (Table 1). This may possibly explain that during coprecipitation a separated mixture of Cu/Zn and Zr precursors is produced rather than a Zr-incorporated Cu/Zn precursor. Since it was reported that an amorphous precipitate of zincian georgeite is transformed into crystalline needles of the zM by dissolution and re-precipitation events occurring upon ageing [8], we investigated the effect of ageing on Zr4+ incorporation into the zM lattice. Thus the precursors, CZZr-10 and CZZr-60, aged for different periods of 10 and 90 min were compared by TEM-EDS (Fig. 8). The electron micrograph of CZZr-10-10 min showed large spherical particles and randomly-dispersed aggregates of small particles. More interestingly, large particles consisted of Cu (red) and Zn (blue) atoms whereas small ones were composed of Zr atoms (yellow). This suggests that Cu/Zn precursor is precipitated independent of Zr precursor, which is in accordance with the above explanation. Upon further ageing of CZZr-10, large Cu/Zn particles in the early stage were transformed into Cu/Zn needles while small Zr
Fig. 7. TG curves (black), and MS traces for evolved H2 O (m/z = 18; red) and CO2 (m/z = 44; green) of pure Zr precursor (solid lines) and Zr(OH)2 CO3 (dotted lines) in the temperature range 50–800 ◦ C.
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Fig. 8. TEM and electron mapping images (Cu: red, Zn: blue, Zr: yellow) of CZZr-10 and CZZr-60 precursors obtained after ageing for 10 and 90 min. The arrows indicate Cu/Zn needles with no Zr atoms.
aggregates were maintained more or less. Particularly, Zr atoms were not found in Cu/Zn needles (see the points indicated by arrows in the micrograph of CZZr-10-90 min). The similarity was observed in the micrograph of CZZr-60-90 min also showing no Zr atoms in Cu/Zn needles. It is therefore believed that no incorporation of Zr4+ into the zM lattice occurs during ageing of Cu/Zn/Zr precipitates. Additionally it was noticed that there were more aggregates of small particles in CZZr-60-90 min than in CZZr-10-90 min, which supports the result of Fig. 1 that the intensities of PXRD peaks were lower with increasing the Zr content. This is associated with the particle size of pristine Cu/Zn/Zr precipitates. As shown in the micrograph of CZZr-60-10 min, the early aged product looked aggregates of small particles where Cu, Zn and Zr atoms were well distributed. After ageing of this precursor for 90 min, Cu/Zn needles were formed in part by a series of dissolution and re-precipitation of the pristine CZZr-60 precipitate, and also a significant amount of small Cu/Zn aggregates was formed. In the case of CZZr-10, Cu/Zn needles mainly observed in the 90 min-aged product originate from
transformation of Cu/Zn spheres of several tens to hundreds of nanometers size seen in the early aged product. The comparison of these four precursors explains that the precipitated Zr aggregates inhibit formation of larger Cu/Zn particles in the precipitation stage and transformation into Cu/Zn needles in the ageing stage, and that such an effect of Zr aggregates is pronounced in the preparation of Cu/Zn/Zr precursor with higher Zr contents. Therefore it can be claimed that Zr precipitates act as a so-called “structural inhibitor”. Since the above results revealed that Zr4+ ions were not incorporated into the zM lattice, Zr aggregates should be placed between Cu/Zn needles and, furthermore, they will be changed upon calcination into Zr-related oxide that can spatially separate mixed Cu/Zn oxide particles. Thus HR-TEM images of the calcined CuO/ZnO/ZrO2 samples were taken and electron diffraction patterns were obtained in specific areas of the particles (Fig. 9). The binary CZZr-0 was crystalline all around the particles whereas the ternary samples exhibited a mixture of amorphous and crystalline parts as marked with the numbers 1 and 2 in the picture of
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Fig. 9. TEM images of the calcined CuO/ZnO/ZrO2 samples. Electron diffraction patterns are given in specific areas of each image. In the image of CZZr-10, the amorphous and crystalline parts are marked with the numbers 1 and 2, respectively.
CZZr-10, respectively. It should be noted here that Zr atoms were detected only in amorphous parts. This indicates that amorphous ZrO2 is formed upon calcination at 400 ◦ C. Also, amorphous parts were more visible in CuO/ZnO/ZrO2 with higher Zr contents, possibly producing smaller CuO/ZnO particles in these samples. The decrease of the particle size of CuO was already evidenced by PXRD results of Fig. 4. Therefore, the role of amorphous ZrO2 is presumed to be a so-called “nano-spacer” between CuO/ZnO particles. This is caused by Zr aggregates dispersed between Cu/Zn needles in the aged Cu/Zn/Zr precursors.
Recalling the meaning of “chemical memory”, we tried to infer the property and activity of the reduced Cu/ZnO/ZrO2 from the structural property of the calcined CuO/ZnO/ZrO2 . The first indication was the higher Cu dispersion found from Cu/ZnO/ZrO2 with higher Zr contents (Fig. 10b). The fact that the presence of ZrO2 in higher amounts led to formation of smaller Cu0 particles stems from the role of ZrO2 in the calcined CuO/ZnO/ZrO2 , because sintering of Cu0 particles upon reduction will be prevented thanks to a “nano-spacer” ZrO2 . The second was that the amount of H2 consumption (NH2 ) decreased with the increase of the Zr/(Cu + Zn)
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precipitates, inhibited both the formation of larger Cu/Zn particles in the precipitation stage and the transformation into Cu/Zn needles in the ageing stage. They were then transformed by calcination into amorphous ZrO2 playing a role of “nano-spacer” between CuO/ZnO particles. Even after reduction, amorphous ZrO2 was maintained and behaved as a support to improve the Cu dispersion. The specific Cu surface area and catalytic activity were correlated in a volcano-type relationship with the Zr/(Cu + Zn) ratio, where the maximum was at the ratio of 10%. This was a combined consequence of the lower amount of total Cu and the higher Cu dispersion in Cu/ZnO/ZrO2 with higher ZrO2 contents. Consequently, all results explain the so-called “chemical memory” of the Cu/Zn/Zr precipitates when they are converted into the final Cu/ZnO/ZrO2 catalysts. We therefore suggest that the precursor structure need to be examined in other metal oxides previously recognized as a support, alike this work. Acknowledgements
Fig. 10. (a) Butanol productivity and specific copper surface area of the reduced Cu/ZnO/ZrO2 catalysts. (b) Amount of H2 consumed and Cu dispersion. The values are plotted as a function of the Zr/(Cu + Zn) ratio. The grey symbol represents the catalytic activity of Cu/ZnO/Al2 O3 (RP-CZA0.66) prepared in our previous work [17].
ratio (Fig. 10b). Since NH2 measured by TPR corresponds to the total amount of reduced CuO, it can be compared with the measured Cu composition of the calcined CuO/ZnO/ZrO2 . As a result, a linear relationship was obtained between these two values (Fig. S5). This suggests that ZrO2 has no influence on the reduction behavior of CuO due to no structural interaction between CuO/ZnO and ZrO2 in the calcined material, and that the decrease of NH2 simply results from the lower amount of total CuO in CuO/ZnO/ZrO2 with the higher ZrO2 content. Therefore, the specific copper surface area (As,Cu ) followed a volcano-type relationship with the Zr/(Cu + Zn) ratio (Fig. 10a), because As,Cu is a product of the Cu dispersion (DCu ) and the total Cu amount (i.e., Cu loading) equivalent to NH2 . The very similar relationship was obtained in the plot of the butanol productivity versus the Zr/(Cu + Zn) ratio (Fig. 10a). In other words, the size of Cu0 particles decreased (namely, increases of As,Cu and DCu ) and hence the catalytic activity was enhanced as the Zr/(Cu + Zn) ratio increases up to 10%. When the ratio was higher than 10%, both As,Cu and the activity declined due to the lower amounts of total Cu present in Cu/ZnO/ZrO2 even if smaller Cu0 particles were formed. Consequently, ZrO2 is believed to serve as a support material in Cu/ZnO/ZrO2 catalysts. Additionally, Cu/ZnO/Al2 O3 catalyst with the Cu/Zn = 67/33 (RPCZA0.66) prepared in our previous work [17] was tested under the same reaction condition. As presented in Fig. 10, the measured butanol productivity was 57.6 mmol h−1 gcat −1 that was in line between the activities of CZZr-10 and CZZr-40. Therefore, it can be addressed that the catalytic activity of Cu/ZnO/ZrO2 is comparable to that of Cu/ZnO/Al2 O3 . 5. Conclusion It was elucidated above that the chemical and structural identity of the prepared Cu/Zn/Zr precursor determined the structure of the calcined CuO/ZnO/ZrO2 , and the property and activity of the reduced Cu/ZnO/ZrO2 as well. All consequences shown in both the calcined and reduced states came from no incorporation of Zr4+ ions into the zM lattice observed in the precursor. Hydroxyrich Zr aggregates, precipitated independent of mixed Cu/Zn
This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2012M3C1A1054501), as well as Converging Technology Project (ARQ201206071) funded by the Korea Ministry of Environment. We also thank Ms. Yoon Jung Kang at the Hanyang Center for Research Facilities (Seoul) for her help with HR-TEM analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.07. 053 References [1] G.A. Olah, A. Goeppert, G.K. Surya Prakash, Beyond Oil and Gas: The Methanol Economy, Wiley-VCH, Weinheim, 2006. [2] M. Kurtz, H. Wilmer, T. Genger, O. Hinrichsen, M. Muhler, Deactivation of supported copper catalysts for methanol synthesis, Catal. Lett. 86 (2003) 77–80. [3] M.V. Twigg, M.S. Spencer, Deactivation of copper metal catalysts for methanol decomposition, methanol steam reforming and methanol synthesis, Top. Catal. 22 (2003) 191–203. [4] M. Kurtz, N. Bauer, C. Büscher, H. Wilmer, O. Hinrichsen, R. Becker, S. Rabe, K. Merz, M. Driess, R.A. Fischer, M. Muhler, New synthetic routes to more active Cu/ZnO catalysts used for methanol synthesis, Catal. Lett. 92 (2004) 49–52. [5] M. Behrens, S. Zander, P. Kurr, N. Jacobsen, J. Senker, G. Koch, T. Ressler, R.W. Fischer, R. Schlögl, Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al, J. Am. Chem. Soc. 135 (2013) 6061–6068. [6] J. Schumann, T. Lunkenbein, A. Tarasov, N. Thomas, R. Schlögl, M. Behrens, Synthesis and characterisation of a highly active Cu/ZnO:Al catalyst, ChemCatChem 6 (2014) 2889–2897. [7] B. Bems, M. Schur, A. Dassenoy, H. Junkes, D. Herein, R. Schlögl, Relations between synthesis and microstructural properties of copper/zinc hydroxycarbonates, Chem. Eur. J. 9 (2003) 2039–2052. [8] M. Behrens, Meso- and nano-structuring of industrial Cu/ZnO/(Al2 O3 ) catalysts, J. Catal. 267 (2009) 24–29. [9] G. Ertl, H. Knöezinger, F. Schüth, J. Weitkamp, Handbook of Heterogeneous Catalysis, 2nd ed., Wiley-VCH, Weinheim, 2008. [10] F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro, F. Frusteri, Synthesis, characterization and activity pattern of Cu–ZnO/ZrO2 catalysts in the hydrogenation of carbon dioxide to methanol, J. Catal. 249 (2007) 185–194. [11] F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura, L. Spadaro, F. Frusteri, Solid-state interactions, adsorption sites and functionality of Cu-ZnO/ZrO2 catalysts in the CO2 hydrogenation to CH3 OH, Appl. Catal. A 350 (2008) 16–23. [12] F. Arena, G. Italiano, K. Barbera, G. Bonura, L. Spadaro, F. Frusteri, Basic evidences for methanol-synthesis catalyst design, Catal. Today 143 (2009) 80–85. [13] E. Frei, A. Schaadt, T. Ludwig, H. Hillebrecht, I. Krossing, The influence of the precipitation/ageing temperature on a Cu/ZnO/ZrO2 catalyst for methanol synthesis from H2 and CO2 , ChemCatChem 6 (2014) 1721–1730. [14] M. Behrens, R. Schlögl, How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts, Z. Anorg. Allg. Chem. 639 (2013) 2683–2695.
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Role of ZrO2 in Cu/ZnO/ZrO2 catalysts prepared from the precipitated Cu/Zn/Zr precursors Cheonwoo Jeong, Young-Woong Suh ∗ Department of Chemical Engineering, Hanyang University, Seoul 133–791, Republic of Korea
a r t i c l e
i n f o
Article history: Received 29 June 2015 Received in revised form 28 July 2015 Accepted 29 July 2015 Available online xxx Keywords: Cu/ZnO/ZrO2 Precursor structure Amorphous zirconia Specific copper surface area Support
a b s t r a c t In Cu/Zn/Zr precursors prepared by coprecipitation, Zr ions showed no effect on a substitution level of Cu2+ by Zn2+ in the zincian malachite (zM) and were not incorporated into zM lattice upon precipitation and subsequent ageing. As hydroxyl-rich Zr aggregates were precipitated independent of mixed Cu/Zn precipitates, they served as a barrier to formation of larger Cu/Zn particles during precipitation and transformation into Cu/Zn needles during ageing. After calcination of Cu/Zn/Zr precursors with higher Zr contents, smaller CuO/ZnO particles were produced in between which amorphous ZrO2 was situated. Upon reduction of CuO/ZnO/ZrO2 with more Zr, this type of ZrO2 resulted in formation of smaller Cu0 particles yielding the higher Cu dispersion. However, the specific copper surface area and catalytic activity of the reduced Cu/ZnO/ZrO2 exhibited a volcano-type relationship with the Zr/(Cu + Zn) ratio. This could be a combined result between the lower amount of total Cu and the higher Cu dispersion. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The industrial Cu/ZnO/Al2 O3 catalyst for methanol synthesis has recently attracted renewed interest because of potential of the socalled “methanol economy” [1]. Although it is widely accepted that the addition of Al improves the performance of the binary Cu/ZnO catalyst and Al2 O3 serves as a structural promoter [2–4], the nature of its promoting effect has been poorly understood until a report of Behrens et al. [5,6]. They demonstrated a structural and electronic promoting effect from the fact that the highest Cu dispersion could be attained by desired incorporation of both Zn2+ and Al3+ into the catalyst precursor. In Cu/ZnO system, Cu2+ in malachite is substituted by Zn2+ yielding the zincian malachite (zM) structure. ¯ Calculated from the d-spacing of (2 0 1)-reflection of the zM, the maximal Zn2+ substitution is around 28% in Cu/ZnO catalyst [7,8]. In Cu/ZnO/Al2 O3 system, simultaneous coincorporation of Zn2+ and Al3+ (up to approximately 3.3%) is possible because the chemical potential of Al is appropriately low in this compositional window. The dispersion of Cu in the final catalysts is thus promoted by a geometric effect. An electronic promotion effect by Al3+ incorporation was also elucidated by analogy to Ga- and Cr-promoted Cu/ZnO systems. It is unfortunate that the above concept derived from Cu/ZnO and Cu/ZnO/Al2 O3 has not been extended yet to other ternary
∗ Corresponding author. Tel.: +82 2 2220 2329; fax: +82 2 2220 2294. E-mail address: [email protected] (Y.-W. Suh).
Cu/ZnO-based systems. Among a variety of the third elements our attention was particularly paid to ZrO2 , since the Handbook of Heterogeneous Catalysis ranks the supports as in the order ZrO2 > (Al2 O3 ) > TiO2 > SiO2 where Al2 O3 also acts as a promoter [9]. Arena et al. [10–12] intensively studied the performance of ternary Cu/ZnO/ZrO2 catalysts and identified ZrO2 as an excellent support, where ZrO2 loading was fixed around 43 wt.%. The structure and chemistry of Cu/Zn/Zr precursors were not discussed in their study. In a recent report of Frei et al. [13], the influence of precipitation and ageing temperatures on the preparation of an active Cu/ZnO/ZrO2 catalyst was investigated. They characterized a series of Cu/Zn/Zr precursors, calcined CuO/ZnO/ZrO2 and reduced Cu/ZnO/ZrO2 samples in detail. Nevertheless, it was found that the structural differences of the precursors affected the catalytic performances only slightly due to structural reorganization happening upon reduction and activity test. Therefore, a more understanding of the precursor chemistry and Zr speciation is crucial to facilitate knowledge-based optimization of Cu/ZnO/ZrO2 catalyst and also elucidate the role of ZrO2 in the catalyst, like the recent progress of Cu/ZnO/Al2 O3 , which is the focus of this study. We herein prepared Cu/Zn/Zr precursors by a coprecipitation method, in which the Zr/(Cu + Zn) ratio varied from 0/100 to 60/100. In the preparation of all precursors, the Cu/Zn ratio was fixed at 70/30 since this is a preferred ratio for binary Cu/ZnO and ternary Cu/ZnO/Al2 O3 catalysts, and the zM structure is preferentially produced [7,8,14]. The chemical and structural identity of Cu/Zn/Zr precursors was investigated in detail by several characterization techniques. For comparison, pure Zr precursor and binary
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Table 1 Metal composition and TG weight loss of Cu/Zn/Zr samples prepared in this study. Sample code
CZZr-0 CZZr-5 CZZr-10 CZZr-40 CZZr-60
Nominal metal ratio (%)
Metal composition (at.%)a
Zr/(Cu + Zn)
Cu/(Cu + Zn)
Cu
Zn
Zr
measured
calculatedb
accuracyc
0 5 10 40 60
70 70 70 70 70
70.0 (69.1) 66.7 (64.2) 63.6 (63.6) 50.0 (51.2) 43.8 (44.9)
30.0 (30.9) 28.6 (31.1) 27.3 (27.3) 21.4 (26.9) 18.8 (17.5)
0.0 (0.0) 4.8 (4.7) 9.1 (11.7) 28.6 (21.9) 37.5 (37.6)
34.8 32.2 31.1 23.8 22.3
33.3 32.4 31.6 27.8 26.1
0.985 1.002 1.004 1.040 1.038
TG weight loss (wt.%)
Calculation
a
The metal compositions of the calcined samples were determined by EDX spectroscopy. The calculation was based upon the assumption that (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O is physically mixed with ZrO(OH)1.82 (CO3 )0.09 at the nominal Zr/(Cu + Zn) ratios in which each component is thermally decomposed to (Cu0.7 Zn0.3 )2 O2 and ZrO2 . c Calculation accuracy = [(weight of residual sample after TG analysis) + (weight of calculated loss)]/(weight of original sample before analysis)]. b
Zn-free Cu/Zr precursors were prepared analogously. The characteristics of the calcined CuO/ZnO/ZrO2 and reduced Cu/ZnO/ZrO2 samples were also studied in order to identify if the precursor structure influences their structure and morphology. After the reduced catalyst was tested in the hydrogenolysis of butyl butyrate, the catalytic performance was correlated with the Cu dispersion and specific Cu surface area. From these results, an explanation was formulated for the role of ZrO2 in Cu/ZnO/ZrO2 showing a volcano-type relationship between the activity and the Zr/(Cu + Zn) ratio. 2. Experimental 2.1. Catalyst preparation A series of Cu/Zn/Zr precursors with different Zr/(Cu + Zn) ratios were prepared by a reverse precipitation method. The ratio of Cu/(Cu + Zn) was fixed at 70% because both Cu/ZnO and Cu/ZnO/ZrO2 catalysts with this ratio showed the best performance in preliminary activity tests (Fig. S1). The nominal Cu/Zn/Zr ratio is 70/30/x and the x value is 100 × [Zr]/{[Cu] + [Zn]}, thus the sample being labeled as CZZr-x with x of 0, 5, 10, 40 and 60. Sample details are summarized in Table 1. In the preparation of CZZr-10, a mixed metal solution (total 1.2 M, 25 mL) containing Cu(NO3 )2 ·3H2 O (4.66 g), Zn(NO3 )2 ·6H2 O (2.48 g) and ZrO(NO3 )2 (0.63 g) was added dropwise into an aqueous NaHCO3 (0.1 M, 600 mL) at 70 ◦ C for 15 min under vigorous stirring. After complete addition (final pH of 7), the resulting precipitate suspension was aged at the same temperature for 90 min. Then, the precipitate was filtered and washed with deionized water (ca. 1 L). The washing step was repeated at least three times in order to assure the complete removal of sodium and nitrate ions. The so–obtained filter cake was dried in a convection oven at 105 ◦ C overnight. For comparison, the synthetic malachite (Cu/Zn/Zr = 100/0/0), Zr precursor (Cu/Zn/Zr = 0/0/100), binary Zn-free Cu/Zr precursors (Zr/Cu = 2.5/70 and 5/70), and ternary Cu/Zn/Zr precursors with the Cu/Zn ratios of 30/70 and 50/50 (x = 0, 10, 40 and 60) were prepared using the above recipe. The dried precursor was crushed and sieved for obtaining the particle size of smaller than 200 m. Finally, the calcination was carried out in a static air at 400 ◦ C for 3 h. 2.2. Characterization Powder XRD (PXRD) analysis was conducted with a Rigaku D/Max RINT 2000 diffractometer using Cu K␣ ( = 0.1541 nm) as a radiation source (40 kV and 60 mA). XRD patterns were collected with a step increment of 0.02◦ and scan speed 4◦ /min in the 2 range of 10◦ –80◦ . For obtaining XRD patterns of reduced Cu/ZnO/ZrO2 catalysts, a calcined sample was pretreated in the reactor with H2 , where the temperature was raised to 350 ◦ C at a ramping rate of 5 K/min and kept at the same temperature for 3 h, and purged with
N2 for 1 h. The reduced samples were immediately analyzed to measure a metallic state of the sample. TGA analysis (TA instrument, SDT Q600) was carried out using a sample, where the temperature was raised from room temperature to 800 ◦ C at a ramping rate of 10 K/min in an air flow (100 ml/min). ATR-IR spectroscopy (Thermo Scientific IS50 spectrometer) with a scan number of 32 and a resolution of 4 cm−1 was used to monitor functional groups of dried Cu/Zn/Zr precursors along with the synthetic malachite and Zr precursor, and commercial Zr(OH)2 (CO3 )·ZrO2 (Sigma–Aldrich). SEM images of Pt-coated samples were taken in a FEI Nova NanoSEM 450 microscope operated at 5 kV. For TEM-EDS analysis coupled with electron diffraction, a JEM 2100F microscope (JEOL) was used with a Gatan DigitalMicrograph imaging filter. For BET measurement, a calcined sample (ca. 0.1 g) was first pretreated measured at 200 ◦ C for 2 h under vacuum in an ASAP 2010 (Micrometrics) to remove adsorbed species. N2 physisorption was then conducted at a liquid nitrogen temperature to measure the specific BET surface area. Prior to TPR measurement performed with an AutoChem 2910 (Micrometrics), the stable TCD baseline was acquired at 90 ◦ C at all runs. Then, 10% H2 in Ar at a flow rate of 50 ml/min was introduced into a quartz U-tube containing the sample, where the temperature was raised from 90 to 350 ◦ C at a ramping rate of 2.5 ◦ C/min. The same instrument for TPR measurement was used for N2 O chemisorption to determine a specific copper surface area (As,Cu ). It was assumed that the reaction stoichiometry between copper and oxygen is two (Cu/O = 2/1) and the copper surface density is 1.46 × 1019 Cu atom/m2 . In a typical experiment, about 50 mg of calcined sample was reduced with 10% H2 in Ar at 350 ◦ C for 3 h. After the reactor was cooled to 90 ◦ C in pure He and maintained for 10 min, N2 O titration was started. Although N2 O titration result cannot be directly related to only the exposed Cu surface due to oxidation of reduced Zn species by N2 O studied in recent reports [15,16], the measured copper surface area still remains the best indicator for the catalytic performance of Cu/ZnO-based catalysts. 2.3. Activity test Prior to the activity test, the calcined CuO/ZnO/ZrO2 samples (ca. 0.4 g) was reduced at 20 bar H2 (99.99%) at 350 ◦ C for 3 h. After cooling to ca. 220 ◦ C at the same pressure, the catalytic activity on the hydrogenolysis of butyl butyrate was examined in a stainless steel reactor. Experimental details were described in our previous report [17]. 3. Results The Cu/Zn/Zr precursors obtained after precipitation, ageing and drying were characterized by PXRD, TG-MS and ATR-IR. Fig. 1a shows the typical reflections of the zincian malachite
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Fig. 1. (a) PXRD patterns of Cu/Zn/Zr precursors. At the bottom are given the reflections of the malachite structure (41–1390) as indexed from ICDD database. (b) Magnification ¯ and (2 1 1)-reflections ¯ induced by incorporation of Zn2+ into the malachite lattice was not affected by Zr4+ ions present in Cu/Zn/Zr of 2 = 30–35◦ , where 2 shift of (2 0 1)precursors.
(Cu1−x Znx )2 (OH)2 (CO3 ) in all precursors, independent of the Zr content. This phase is a result of Cu2+ substitution by Zn2+ (i.e., lattice contraction) in the malachite structure Cu2 (OH)2 (CO3 ), which was ¯ and (2 1 1)-reflections ¯ as indicated identified by 2 shift of (2 0 1)in Fig. 1b. This originates from the Cu/Zn molar ratio of 70/30 chosen for the synthesis. The more important finding was no shift of the two reflections in all Cu/Zn/Zr precursors while the peak intensities were lower with increasing the Zr content. In other words, the chemical structure of the zincian malachite (zM) was not affected by Zr ions added for coprecipitation of a mixed metal solution. This
is the first report to clearly observe no change of the zM lattice by Zr ions. In PXRD patterns of all Cu/Zn/Zr precursors, no reflections belonging to any Zr phase were detected. This implies that Zr precursor may exist in a form of amorphous and non-diffracting phase. Since IR spectroscopy is capable of probing both the amorphous and crystalline parts of the sample, ATR-IR spectra of the Cu/Zn/Zr precursors were thus recorded at room temperature (Fig. 2). The spectrum of the synthetic malachite was compared. In the range 4000–2500 cm−1 for O H stretching bands, the spectrum of the
Fig. 2. IR spectra of Cu/Zn/Zr precursors in the ranges 4000–2500 and 1800–400 cm−1 . For comparison, IR spectra of the synthetic malachite, pure Zr precursor and commercial Zr(OH)2 (CO3 ) are given as top and bottom traces.
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Fig. 4. PXRD patterns of CuO/ZnO/ZrO2 samples calcined at 400 ◦ C. At the bottom are given the reflections of CuO (05–0661; blue bars) and ZnO (36–1451; red bars). The reflection of CuO(1¯ 1 1) at 2 = 35.6◦ was used for calculating the crystal size of CuO.
Fig. 3. (Top two) TG and DTG curves of Cu/Zn/Zr precursors in the temperature range 50–800 ◦ C. (Bottom two) MS traces for H2 O (m/z = 18) and CO2 (m/z = 44) evolved during TG experiments.
synthetic malachite showed a splitting into two modes at 3318 and 3402 cm−1 like that of the mineral malachite [18], which is attributed to the two crystallographically different OH groups [19]. The splitting was enhanced to 171 cm−1 for CZZr-0, due to Zn2+ incorporation into the malachite structure [20]. However, a change of the splitting was not noticed in the spectra of all Cu/Zn/Zr precursors, though the band became broader with increasing the Zr content. This feature holds for the asymmetric C O stretching bands of the carbonate anions in the region 1700–1100 cm−1 , and OH deformation vibrations and further carbonate bands in the region 1200–600 cm−1 . It could be proposed again that the chemical structure of the zM was hardly changed by Zr ions added into the starting metal solution, although we failed to observe fingerprint bands of Zr-related precursors by IR spectroscopy. The prepared Cu/Zn/Zr precursors were additionally characterized by TG-MS from 50 to 800 ◦ C (Fig. 3). The measured weight loss of CZZr-0 was 34.8%, which is close to the calculated loss of 33.3% corresponding to thermal decomposition of (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O into (Cu0.7 Zn0.3 )2 O2 accompanied by emission of H2 O and CO2 gases. The chemical formula of CZZr0 is assumed from the zincian malachite, (Cu1−x Znx )2 (OH)2 (CO3 ) with x = 0.3, with a water adduct. In the case of Cu/Zn/Zr precursors, as the Zr content increased the weight loss decreased in the following order: 32.2% for CZZr-5 > 31.1% for CZZr-10 > 23.8% for CZZr-40 > 22.3% for CZZr-60 (Table 1). Although accurate discussion of the result is possible when the chemical structure of Zr precursor will be determined, release of lighter gaseous molecules (i.e., H2 O rather than CO2 ) is expected upon decomposition of Cu/Zn/Zr precursors with higher Zr contents. This was confirmed by normalized MS traces of H2 O (m/z = 18) and CO2 (m/z = 44). In CZZr-0,
decarbonylation events occurred below and above 400 ◦ C, where the latter represents decomposition of “high-temperature carbonate” often observed in the zM [7]. The two events were not shifted as a function of the temperature in Cu/Zn/Zr precursors, while they were lessened with increasing the Zr content. In the range 250–400 ◦ C showing a simultaneous dehydroxylation and decarbonylation step, emission of water molecules was reduced in all Cu/Zn/Zr precursors compared to CZZr-0 but the reduction extent was independent of the Zr content. In contrast, an additional dehydroxylation event appeared in the range 100–250 ◦ C for CZZr-40 and CZZr-60. Similarly, higher values of derivative TG (DTG) in the same temperature range were observed in Cu/Zn/Zr precursors with higher Zr contents. The TG-MS results suggest that Zr ions present in a mixed metal hydroxycarbonate precursor do not influence intrinsic decomposition of the zM and they exist as a hydroxy-rich Zr precursor decomposed at low temperatures. This type of Zr precursor will be discussed later. Next, CuO/ZnO/ZrO2 samples obtained after calcination at 400 ◦ C were characterized by PXRD, BET and SEM. The reflections of CuO and ZnO were only detectable in all PXRD patterns, and these reflections were sharper with decreasing the Zr content (Fig. 4), alike PXRD patterns of Cu/Zn/Zr precursors. Calculated using the most intense reflection of the samples, that is, CuO(1¯ 1 1) at 2 = 35.6◦ , the crystal size of CuO decreased from 61.3 to 56.6 A˚ with increasing the Zr/(Cu + Zn) ratio from 0% to 60% (Table 2). This is due to a decrease of Cu loading in CuO/ZnO/ZrO2 with a higher Zr content. For confirmation, the specific surface areas of the calcined samples (As,BET ) were measured; the binary CZZr-0 showed only 31 m2 /g and the ternary samples high surface areas of 58–102 m2 /g (Table 2). As the Zr/(Cu + Zn) ratio increased to 40%, the specific surface area increased to 102 m2 /g and then maintained around this value. It was noted that the margin of the increase is greater in the specific surface area than in the CuO crystal size. This explains that all individual oxide particles become smaller in CuO/ZnO/ZrO2 samples with more Zr content. SEM micrographs of calcined CuO/ZnO/ZrO2 samples were also taken (Fig. S2). Obviously, the size of overall particles decreased with increasing the Zr content. EDS mapping was additionally conducted for identifying the presence of Zr, since no ZrO2 -related reflections
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Table 2 Properties of the calcined CuO/ZnO/ZrO2 samples, and properties and performance of the reduced Cu/ZnO/ZrO2 catalysts. Butanol productivityh
Sample code
Calcined rP,XRD a (Å)
As,BET b (m2 /g)
rP,XRD c (Å)
rP,N2O d (Å)
As,Cu e (m2 /g)
NH2 f (mmol g−1 )
DCu g (%)
(mmol gcat −1 h−1 )
CZZr-0 CZZr-5 CZZr-10 CZZr-40 CZZr-60
61.3 59.1 58.3 58.0 56.6
31 58 68 102 100
183.5 153.8 139.1 139.1 138.9
168.9 124.3 102.9 100.6 83.3
21.8 26.8 30.3 24.3 23.9
8.24 7.55 6.85 5.24 4.70
6.5 8.7 10.8 11.3 12.4
43.3 53.5 65.4 37.5 36.2
a b c d e f g h
Reduced
Crystal size of CuO calculated from the reflection of CuO(1¯ 1 1) at 2 = 35.6◦ . Specific BET surface area from N2 physisorption. Crystal size of Cu0 calculated from the reflection of Cu(1 1 1) at 2 = 43.3◦ . Particle size of Cu0 calculated from N2 O titration and metal composition. Specific Cu surface area from N2 O titration at 363 K, as outlined in Section 2. Amount of consumed H2 from TPR measurement. Cu dispersion = 2 × (N2 amount used for calculation of As,Cu )/(total amount of reduced CuO). Reaction condition: 493 K, 20 bar, H2 /butyl butyrate = 20, weight hourly space velocity = 9 h−1 .
were visible in all PXRD patterns. As summarized in Table 1, the measured atomic percentages of Cu, Zn and Zr were close to the nominal ones in each sample, which is an indication of successful coprecipitation of all metal components. Therefore, it is believed that Zr exists in the form of an amorphous ZrO2 phase due to a low calcination temperature of 400 ◦ C. This seems to be valid since tetragonal and monoclinic ZrO2 phases were found after calcination at 550, 650 and 850 ◦ C (Fig. S3). PXRD patterns were collected for Cu/ZnO/ZrO2 samples obtained after reduction at 350 ◦ C (Fig. 5). All measurements showed only Cu0 and ZnO as detectable phases, and the intensities of all reflections became lower with increasing the Zr content, similar to calcined CuO/ZnO/ZrO2 samples. When the copper crystal size was calculated using the reflection of Cu(1 1 1) at 2 = 43.3◦ , it decreased from 183.5 (CZZr-0) to 138.9 A˚ (CZZr-60), as summarized in Table 2. Since CZZr-10, CZZr-40, and CZZr-60 catalysts exhibited similar Cu0 sizes due to lower and broader PXRD reflections, we additionally calculated Cu0 particle size using N2 O titration results and total Cu content measured by EDX spectroscopy. The value rP,N2O also decreased from 168.9 (CZZr-0) to 83.3 A˚ (CZZr-60). This is in accordance with the finding that Cu dispersion (DCu ) increased from 6.5% to 12.4% with the Zr content (Table 2). Therefore, smaller Cu0 particles were formed upon reduction of CuO/ZnO/ZrO2
Fig. 5. PXRD patterns of Cu/ZnO/ZrO2 samples reduced at 350 ◦ C. At the bottom are given the reflections of Cu (04–0836; blue bars) and ZnO (36–1451; red bars). The reflection of Cu(1 1 1) at 2 = 43.3◦ was used for calculating the crystal size of Cu0 .
samples with higher Zr contents, possibly due to a decrease of Cu loading. The catalytic activities of Cu/ZnO/ZrO2 samples were tested in the hydrogenolysis of butyl butyrate at 20 bar H2 and 220 ◦ C (Table 2). The butanol productivity showed a volcano-type relationship with the Zr content; as the Zr/(Cu + Zn) ratio increased from 0% to 10%, the productivity was improved from 43.3 to 65.4 mmol gcat −1 h−1 , and then decreased to 36.2 mmol gcat −1 h−1 (CZZr-60). It was also found that the productivity had a strong dependence on the specific copper surface area, of which more details will be discussed later.
4. Discussion We investigated the characteristics of the prepared Cu/Zn/Zr samples in the precursor, oxide and reduced states. Understanding the chemical and structural identity of the coprecipitated precursor is of great importance since it can help predicting the structure of the calcined oxide and, furthermore, the property and activity of the final catalyst (i.e., reduced state), which is the so-called “chemical memory”. From this standpoint, the XRD result of Fig. 1 revealed that Zr ions present in the starting precipitate solution had no effect on a substitution level of Cu2+ by Zn2+ in the zincian malachite (zM). Also, no incorporation of Zr4+ itself into the zM lattice could be corroborated. This is in sharp contrast to the Cu/ZnO/Al2 O3 catalyst system investigated by Behrens et al. [5,6]. They revealed that variation of the Al content in the starting solution had a direct influence on the concentration of Cu2+ in the zM and simultaneous coincorporation of Zn2+ and Al3+ was possible at a certain Al content, based on ¯ spacing of the zM with respect to the Al content. a change of d(2 0 1) To ensure no incorporation of Zr4+ , we additionally prepared Cu/Zn/Zr precursors with different Cu/Zn ratios of 50/50 and 30/70, and found that all reflections of the binary Cu/Zn precursors were not shifted at all by adding a higher amount of Zr source into the precipitating solution (Fig. S4). This suggests that Zr ions cannot be incorporated into any lattice of mixed Cu/Zn hydroxycarbonate precursors. The question then arises regarding whether such a phenomenon can happen in the pure Cu precursor (i.e., the malachite). Thus, Zn-free Cu/Zr precursors with the Zr/Cu ratios of 2.5/70 and 5/70 were synthesized under the same condition as used in the preparation of Cu/Zn/Zr precursors. PXRD patterns of these pre¯ and cursors are presented in Fig. 6. Surprisingly, 2 shift of (2 0 1)¯ (2 1 1)-reflections of the malachite was negligible, although there was a very slight shift of 0.04◦ corresponding to a d-spacing dif˚ This means no incorporation of Zr4+ into the ference of 0.004 A. malachite lattice. Therefore, we strongly believe that both the malachite and zincian malachite structure are not able to incorporate even small amounts of Zr4+ .
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¯ Fig. 6. (a) PXRD patterns of Cu/Zr precursors. At the bottom are given the reflections of the malachite structure (41–1390). (b) Magnification of 2 = 30–35◦ , where (2 0 1)¯ of the malachite structure was shifted very negligibly in Cu/Zr precursors (2 difference () = 0.04◦ ). and (2 1 1)-reflections
The Zn incorporation into the malachite lattice is principally due to the chemical similarity of Cu2+ and Zn2+ concerning cation charge and size in the precursor state [14]. Therefore, no Zr incorporation is presumed to result from the different size and structure of Zr4+ species in aqueous solution. In general, Zr4+ cations are very easily hydrolyzed in H2 O and form the complex exhibiting the tetrameric structure of a slightly distorted square followed by ageing into larger polynuclear complexes containing a number of Zr atoms linked by hydroxyl bridges [21]. Kanazhevskii et al. [22] reported that the complexes formed from aqueous zirconium oxynitrate (ZrO(NO3 )2 ·nH2 O) and zirconium oxychloride solutions were structurally similar and they were polynuclear complexes of a square-planar configuration. These polynuclear complexes are susceptible to condensation reaction to form larger oligomers and, further, colloidal particles and gelatinous precipitates. The precipitates can be described as an amorphous mixed oxide/hydroxide expressed by the general formula ZrOx (OH)4−2x with x of 0 or 1 [23]. Since zirconium oxynitrate was used as a Zr source and NaHCO3 as a precipitating agent in this work, the Cu/Zn-free Zr precursor was prepared analogously. The TG weight loss of the synthetic Zr precursor was 14.2% at 800 ◦ C, while H2 O and CO2 were evolved simultaneously at the temperature centered around 200 ◦ C (Fig. 7). This is very different from the decomposition behavior of commercial Zr(OH)2 (CO3 )·ZrO2 ; the weight loss was measured to be 21.1% close to the calculated value (20.1%) for decomposition into ZrO2 while the maximum H2 O evolution was observed around 127 ◦ C and CO2 was emitted at 117 and 580 ◦ C. Therefore, it was considered that the Zr precursor was precipitated as ZrOx (OH)4−2x with a small amount of CO3 adduct. This is valid since the IR spectrum of the synthetic Zr precursor exhibited very weak carbonate bands at 1514 and 1385 cm−1 , and broad O H stretching bands in the range 4000–2500 cm−1 (Fig. 2). The feature of such bands is the reason why we failed to observe any Zr-related IR bands in the spectra of Cu/Zn/Zr precursors. In contrast, distinct carbonate bands were observed at 1629, 1549 and 1338 cm−1 in the spectrum of Zr(OH)2 (CO3 )·ZrO2 . Thus, the chemical formula of the synthetic Zr precursor would be assumed ZrO(OH)2−y (CO3 )y/2 with y of 0.18 (Table S1 for detailed calculation procedure). When the assumption that (Cu0.7 Zn0.3 )2 (OH)2 (CO3 )·H2 O is “physically”
mixed with ZrO(OH)1.82 (CO3 )0.09 at nominal Zr/(Cu + Zn) ratios used for Cu/Zn/Zr precursors was made, the calculation of the Cu/Zn/Zr precursor weight showed quite a good accuracy in the range 0.985–1.040 (Table 1). This may possibly explain that during coprecipitation a separated mixture of Cu/Zn and Zr precursors is produced rather than a Zr-incorporated Cu/Zn precursor. Since it was reported that an amorphous precipitate of zincian georgeite is transformed into crystalline needles of the zM by dissolution and re-precipitation events occurring upon ageing [8], we investigated the effect of ageing on Zr4+ incorporation into the zM lattice. Thus the precursors, CZZr-10 and CZZr-60, aged for different periods of 10 and 90 min were compared by TEM-EDS (Fig. 8). The electron micrograph of CZZr-10-10 min showed large spherical particles and randomly-dispersed aggregates of small particles. More interestingly, large particles consisted of Cu (red) and Zn (blue) atoms whereas small ones were composed of Zr atoms (yellow). This suggests that Cu/Zn precursor is precipitated independent of Zr precursor, which is in accordance with the above explanation. Upon further ageing of CZZr-10, large Cu/Zn particles in the early stage were transformed into Cu/Zn needles while small Zr
Fig. 7. TG curves (black), and MS traces for evolved H2 O (m/z = 18; red) and CO2 (m/z = 44; green) of pure Zr precursor (solid lines) and Zr(OH)2 CO3 (dotted lines) in the temperature range 50–800 ◦ C.
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Fig. 8. TEM and electron mapping images (Cu: red, Zn: blue, Zr: yellow) of CZZr-10 and CZZr-60 precursors obtained after ageing for 10 and 90 min. The arrows indicate Cu/Zn needles with no Zr atoms.
aggregates were maintained more or less. Particularly, Zr atoms were not found in Cu/Zn needles (see the points indicated by arrows in the micrograph of CZZr-10-90 min). The similarity was observed in the micrograph of CZZr-60-90 min also showing no Zr atoms in Cu/Zn needles. It is therefore believed that no incorporation of Zr4+ into the zM lattice occurs during ageing of Cu/Zn/Zr precipitates. Additionally it was noticed that there were more aggregates of small particles in CZZr-60-90 min than in CZZr-10-90 min, which supports the result of Fig. 1 that the intensities of PXRD peaks were lower with increasing the Zr content. This is associated with the particle size of pristine Cu/Zn/Zr precipitates. As shown in the micrograph of CZZr-60-10 min, the early aged product looked aggregates of small particles where Cu, Zn and Zr atoms were well distributed. After ageing of this precursor for 90 min, Cu/Zn needles were formed in part by a series of dissolution and re-precipitation of the pristine CZZr-60 precipitate, and also a significant amount of small Cu/Zn aggregates was formed. In the case of CZZr-10, Cu/Zn needles mainly observed in the 90 min-aged product originate from
transformation of Cu/Zn spheres of several tens to hundreds of nanometers size seen in the early aged product. The comparison of these four precursors explains that the precipitated Zr aggregates inhibit formation of larger Cu/Zn particles in the precipitation stage and transformation into Cu/Zn needles in the ageing stage, and that such an effect of Zr aggregates is pronounced in the preparation of Cu/Zn/Zr precursor with higher Zr contents. Therefore it can be claimed that Zr precipitates act as a so-called “structural inhibitor”. Since the above results revealed that Zr4+ ions were not incorporated into the zM lattice, Zr aggregates should be placed between Cu/Zn needles and, furthermore, they will be changed upon calcination into Zr-related oxide that can spatially separate mixed Cu/Zn oxide particles. Thus HR-TEM images of the calcined CuO/ZnO/ZrO2 samples were taken and electron diffraction patterns were obtained in specific areas of the particles (Fig. 9). The binary CZZr-0 was crystalline all around the particles whereas the ternary samples exhibited a mixture of amorphous and crystalline parts as marked with the numbers 1 and 2 in the picture of
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Fig. 9. TEM images of the calcined CuO/ZnO/ZrO2 samples. Electron diffraction patterns are given in specific areas of each image. In the image of CZZr-10, the amorphous and crystalline parts are marked with the numbers 1 and 2, respectively.
CZZr-10, respectively. It should be noted here that Zr atoms were detected only in amorphous parts. This indicates that amorphous ZrO2 is formed upon calcination at 400 ◦ C. Also, amorphous parts were more visible in CuO/ZnO/ZrO2 with higher Zr contents, possibly producing smaller CuO/ZnO particles in these samples. The decrease of the particle size of CuO was already evidenced by PXRD results of Fig. 4. Therefore, the role of amorphous ZrO2 is presumed to be a so-called “nano-spacer” between CuO/ZnO particles. This is caused by Zr aggregates dispersed between Cu/Zn needles in the aged Cu/Zn/Zr precursors.
Recalling the meaning of “chemical memory”, we tried to infer the property and activity of the reduced Cu/ZnO/ZrO2 from the structural property of the calcined CuO/ZnO/ZrO2 . The first indication was the higher Cu dispersion found from Cu/ZnO/ZrO2 with higher Zr contents (Fig. 10b). The fact that the presence of ZrO2 in higher amounts led to formation of smaller Cu0 particles stems from the role of ZrO2 in the calcined CuO/ZnO/ZrO2 , because sintering of Cu0 particles upon reduction will be prevented thanks to a “nano-spacer” ZrO2 . The second was that the amount of H2 consumption (NH2 ) decreased with the increase of the Zr/(Cu + Zn)
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precipitates, inhibited both the formation of larger Cu/Zn particles in the precipitation stage and the transformation into Cu/Zn needles in the ageing stage. They were then transformed by calcination into amorphous ZrO2 playing a role of “nano-spacer” between CuO/ZnO particles. Even after reduction, amorphous ZrO2 was maintained and behaved as a support to improve the Cu dispersion. The specific Cu surface area and catalytic activity were correlated in a volcano-type relationship with the Zr/(Cu + Zn) ratio, where the maximum was at the ratio of 10%. This was a combined consequence of the lower amount of total Cu and the higher Cu dispersion in Cu/ZnO/ZrO2 with higher ZrO2 contents. Consequently, all results explain the so-called “chemical memory” of the Cu/Zn/Zr precipitates when they are converted into the final Cu/ZnO/ZrO2 catalysts. We therefore suggest that the precursor structure need to be examined in other metal oxides previously recognized as a support, alike this work. Acknowledgements
Fig. 10. (a) Butanol productivity and specific copper surface area of the reduced Cu/ZnO/ZrO2 catalysts. (b) Amount of H2 consumed and Cu dispersion. The values are plotted as a function of the Zr/(Cu + Zn) ratio. The grey symbol represents the catalytic activity of Cu/ZnO/Al2 O3 (RP-CZA0.66) prepared in our previous work [17].
ratio (Fig. 10b). Since NH2 measured by TPR corresponds to the total amount of reduced CuO, it can be compared with the measured Cu composition of the calcined CuO/ZnO/ZrO2 . As a result, a linear relationship was obtained between these two values (Fig. S5). This suggests that ZrO2 has no influence on the reduction behavior of CuO due to no structural interaction between CuO/ZnO and ZrO2 in the calcined material, and that the decrease of NH2 simply results from the lower amount of total CuO in CuO/ZnO/ZrO2 with the higher ZrO2 content. Therefore, the specific copper surface area (As,Cu ) followed a volcano-type relationship with the Zr/(Cu + Zn) ratio (Fig. 10a), because As,Cu is a product of the Cu dispersion (DCu ) and the total Cu amount (i.e., Cu loading) equivalent to NH2 . The very similar relationship was obtained in the plot of the butanol productivity versus the Zr/(Cu + Zn) ratio (Fig. 10a). In other words, the size of Cu0 particles decreased (namely, increases of As,Cu and DCu ) and hence the catalytic activity was enhanced as the Zr/(Cu + Zn) ratio increases up to 10%. When the ratio was higher than 10%, both As,Cu and the activity declined due to the lower amounts of total Cu present in Cu/ZnO/ZrO2 even if smaller Cu0 particles were formed. Consequently, ZrO2 is believed to serve as a support material in Cu/ZnO/ZrO2 catalysts. Additionally, Cu/ZnO/Al2 O3 catalyst with the Cu/Zn = 67/33 (RPCZA0.66) prepared in our previous work [17] was tested under the same reaction condition. As presented in Fig. 10, the measured butanol productivity was 57.6 mmol h−1 gcat −1 that was in line between the activities of CZZr-10 and CZZr-40. Therefore, it can be addressed that the catalytic activity of Cu/ZnO/ZrO2 is comparable to that of Cu/ZnO/Al2 O3 . 5. Conclusion It was elucidated above that the chemical and structural identity of the prepared Cu/Zn/Zr precursor determined the structure of the calcined CuO/ZnO/ZrO2 , and the property and activity of the reduced Cu/ZnO/ZrO2 as well. All consequences shown in both the calcined and reduced states came from no incorporation of Zr4+ ions into the zM lattice observed in the precursor. Hydroxyrich Zr aggregates, precipitated independent of mixed Cu/Zn
This research was financially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2012M3C1A1054501), as well as Converging Technology Project (ARQ201206071) funded by the Korea Ministry of Environment. We also thank Ms. Yoon Jung Kang at the Hanyang Center for Research Facilities (Seoul) for her help with HR-TEM analysis. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.07. 053 References [1] G.A. Olah, A. Goeppert, G.K. Surya Prakash, Beyond Oil and Gas: The Methanol Economy, Wiley-VCH, Weinheim, 2006. [2] M. Kurtz, H. Wilmer, T. Genger, O. Hinrichsen, M. Muhler, Deactivation of supported copper catalysts for methanol synthesis, Catal. Lett. 86 (2003) 77–80. [3] M.V. Twigg, M.S. Spencer, Deactivation of copper metal catalysts for methanol decomposition, methanol steam reforming and methanol synthesis, Top. Catal. 22 (2003) 191–203. [4] M. Kurtz, N. Bauer, C. Büscher, H. Wilmer, O. Hinrichsen, R. Becker, S. Rabe, K. Merz, M. Driess, R.A. Fischer, M. Muhler, New synthetic routes to more active Cu/ZnO catalysts used for methanol synthesis, Catal. Lett. 92 (2004) 49–52. [5] M. Behrens, S. Zander, P. Kurr, N. Jacobsen, J. Senker, G. Koch, T. Ressler, R.W. Fischer, R. Schlögl, Performance improvement of nanocatalysts by promoter-induced defects in the support material: methanol synthesis over Cu/ZnO:Al, J. Am. Chem. Soc. 135 (2013) 6061–6068. [6] J. Schumann, T. Lunkenbein, A. Tarasov, N. Thomas, R. Schlögl, M. Behrens, Synthesis and characterisation of a highly active Cu/ZnO:Al catalyst, ChemCatChem 6 (2014) 2889–2897. [7] B. Bems, M. Schur, A. Dassenoy, H. Junkes, D. Herein, R. Schlögl, Relations between synthesis and microstructural properties of copper/zinc hydroxycarbonates, Chem. Eur. J. 9 (2003) 2039–2052. [8] M. Behrens, Meso- and nano-structuring of industrial Cu/ZnO/(Al2 O3 ) catalysts, J. Catal. 267 (2009) 24–29. [9] G. Ertl, H. Knöezinger, F. Schüth, J. Weitkamp, Handbook of Heterogeneous Catalysis, 2nd ed., Wiley-VCH, Weinheim, 2008. [10] F. Arena, K. Barbera, G. Italiano, G. Bonura, L. Spadaro, F. Frusteri, Synthesis, characterization and activity pattern of Cu–ZnO/ZrO2 catalysts in the hydrogenation of carbon dioxide to methanol, J. Catal. 249 (2007) 185–194. [11] F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura, L. Spadaro, F. Frusteri, Solid-state interactions, adsorption sites and functionality of Cu-ZnO/ZrO2 catalysts in the CO2 hydrogenation to CH3 OH, Appl. Catal. A 350 (2008) 16–23. [12] F. Arena, G. Italiano, K. Barbera, G. Bonura, L. Spadaro, F. Frusteri, Basic evidences for methanol-synthesis catalyst design, Catal. Today 143 (2009) 80–85. [13] E. Frei, A. Schaadt, T. Ludwig, H. Hillebrecht, I. Krossing, The influence of the precipitation/ageing temperature on a Cu/ZnO/ZrO2 catalyst for methanol synthesis from H2 and CO2 , ChemCatChem 6 (2014) 1721–1730. [14] M. Behrens, R. Schlögl, How to prepare a good Cu/ZnO catalyst or the role of solid state chemistry for the synthesis of nanostructured catalysts, Z. Anorg. Allg. Chem. 639 (2013) 2683–2695.
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Please cite this article in press as: C. Jeong, Y.-W. Suh, Role of ZrO2 in Cu/ZnO/ZrO2 catalysts prepared from the precipitated Cu/Zn/Zr precursors, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.07.053