ZrO2 Catalysts as Determined by

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J. Phys. Chem. B 1999, 103, 9967-9977

9967

The Surface Composition of CuOx/ZrO2 Catalysts as Determined by FTIR, XPS, ESR Spectroscopies and Volumetric CO Adsorption Valerio Indovina,*,† Manlio Occhiuzzi,† Daniela Pietrogiacomi,† and Simonetta Tuti‡ Dipartimento di Chimica, UniVersita` di Roma “La Sapienza”, P.le A. Moro 5, 00185 Rome, Italy, and Dipartimento di Ingegneria Meccanica e Industriale, UniVersita` Roma Tre, Rome, Italy ReceiVed: June 23, 1999; In Final Form: September 7, 1999

CuOx/ZrO2 samples prepared by adsorption from copper solutions or by impregnation were characterized by means of FTIR, XPS, ESR, DRS, volumetric CO adsorption, and redox cycles with H2 and O2. In samples prepared by adsorption, the maximum copper uptake corresponded to an extended plateau at 2.5 atoms nm-2. In as-prepared samples, isolated CuII species were in a distorted octahedral configuration, and in samples heated in dry O2 at 773 K, in a square-pyramidal configuration. Water vapor adsorption transformed the latter species into distorted octahedral complexes. In all samples, heating in O2 at 773 K anchored copper to the zirconia surface. All copper was present as CuII. Evacuation of these samples at 773 K caused no copper reduction, whereas heating with H2 above 450 K reversibly reduced CuII to metal copper. Evacuation of as-prepared samples differed according to how samples were prepared. In particular, evacuation of samples prepared from Cu-acetylacetonate or Cu-acetate reduced CuII to CuI at 473 K and to copper metal at higher temperature. Evacuation reduced copper because acetylacetonates and acetates underwent oxidation during desorption. Evacuation up to 773 K of samples prepared from Cu-nitrate caused no copper reduction. In samples heated in O2 at 773 K, CO adsorption at RT yielded CuI-CO and carbonates. Volumetric CO adsorption combined with FTIR showed that copper was highly dispersed on the ZrO2 surface up to 2.5 atoms nm-2.

Introduction Copper-based catalysts have been intensely studied because of their activity in the selective catalytic reduction of NOx with various hydrocarbons in the presence of O2. Two reviews have addressed the structural features, redox properties, and catalytic activity of copper exchanged in the zeolite framework or supported on oxide matrices.1,2 Zeolite-based catalysts are generally more active than the relevant supported transition metal ions. A possible reason why copper is more active in ZSM5 than in supported oxides is that the zeolite disperses the active metal at the atomic level.1-4 Many papers have centered on the characterization of CuZSM5, CuOx/Al2O3, and CuOx/SiO2 and on copper dispersion.1,2 Using volumetric adsorption combined with DRIFT, Dandekar and Vannice5 determined the dispersion and oxidation state of copper on various supports, all samples containing 5 Cu wt %. Few papers have reported the characterization of CuOx/ZrO2. By means of DRS, XRD, DTA, and H2-TPR, Shimokawabe et al.6 found that in CuOx/ZrO2, isolated species were prevalent up to 3 wt %, whereas at higher copper loading CuO formed. More recently, by means of H2-TPR, STEM/EDX, XRD, and XPS, Kundakovic and Flytzani-Stephanopoulos7 identified in CuOx-ZrO2 (Y-doped), as the Cu content increased, isolated ions and dispersed clusters, small clusters, and CuO particles. By means of XANES, EXAFS, FTIR, and TPR with CO, Okamoto * Corresponding author: Prof. Valerio Indovina, Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, BOX n°34 - Roma 62, 00185 Roma, Italy. Phone: 06-49913381. Fax: 06-490324. E-mail: [email protected]. † Universita ` degli Studi di Roma “La Sapienza”. ‡ Dipartimento di Ingegneria Meccanica e Industriale, Universita ` Roma Tre, Rome, Italy.

et al.8 found that CuOx/ZrO2 samples with Cu content up to 3 atoms nm-2 contained highly dispersed copper species, whereas samples with higher copper loading contained CuO particles of increasing size. At 373 K dispersed CuII was reduced to CuI, and at 443 K to metallic copper. By means of EPR, FTIR, and DRS, Morterra et al.9 found two CuII types in dilute CuOx/ZrO2 (0.2 and 0.8 Cu atoms nm-2) prepared by copper nitrate impregnation of zirconia obtained from Zr(O-iso-C3H7)4. Owing to the presence of (CH)n surface contaminants, on evacuation at increasing temperature, up to 773 K, CuII was reduced to CuI and segregated metallic copper. CuI was oxidized by water exposure at RT, whereas copper metal required heating in O2. In a recent study, using propene or ammonia as the reducing agent in the presence of oxygen, we found that the activity for NO abatement of CuOx/ZrO2 depended on copper dispersion. With propene and with ammonia, the turnover frequency (NO molecules converted s-1 Cu atom-1) was nearly independent of the Cu content, up to about 2.5 atoms nm-2. Interestingly, for the selective catalytic reduction with propene, turnover frequency values on CuOx/ZrO2 were close to those on CuZSM5.10 These results prompted us to characterize, by means of FTIR, XPS, DRS, and EPR spectroscopies, CuOx/ZrO2 catalysts with Cu content from 0.1 to 8.4 atoms nm-2 prepared by adsorption from copper acetylacetonate in toluene or from copper acetate in water solutions, and by impregnation with copper acetate or nitrate aqueous solutions. The samples used for the present study were portions of those previously used for the catalytic investigation.10 Our aim was to investigate the structure, stability, and dispersion of the copper species supported on zirconia. To assess the dispersion and oxidation state of copper, in addition to XPS, ESR, and redox cycles with H2

10.1021/jp9920904 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/23/1999

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and O2, we used volumetric CO adsorption combined with FTIR analysis of the relevant copper carbonyl species.

TABLE 1: Catalysts CuOx/ZrO2 and Methods for Their Characterization

Experimental Section

ZCu0.1(i nit) ZCu0.1(a acac) ZCu0.2(i nit) ZCu0.3(a acac) ZCu0.3(i ac) ZCu0.7(a acac) ZCu0.8(a acac) ZCu0.9(a acac) ZCu0.9(i ac) ZCu1.0(a acac) ZCu1.2(a acac) ZCu1.4(i nit) ZCu1.5(a acac) ZCu1.6(a acac) ZCu1.7(i ac) ZCu2.0(a acac) ZCu2.3(a acac) ZCu2.3(a ac) ZCu2.4(i nit) ZCu2.5(a acac) ZCu2.5(i ac) ZCu3.1(a ac) ZCu4.3(i nit) ZCu4.6(i ac) ZCu8.4(i ac)

catalysts

Catalyst Preparation. The zirconia support was prepared by hydrolysis of zirconium oxychloride with ammonia, as already described.11 The precipitate hydrous zirconium was washed with water until the Cl- test with AgNO3 gave no visible opalescence. Before its use as a support, the material was calcined at 823 K. The CuOx/ZrO2 catalysts were prepared as already reported in detail.10 The following methods were used: (i) adsorption from solutions of copper acetylacetonate, Cu(acac)2, in toluene, (ii) adsorption from solutions of copper acetate, Cu(ac)2, in water, (iii) dry impregnation with Cu(ac)2 aqueous solutions, and (iv) dry impregnation with Cu(NO3)2 aqueous solutions. All specimens were then dried at 383 K for 24 h and ground into fine powder. The CuOx/ZrO2 catalysts are designated as ZCux(p c), where x gives the analytical Cu content (atoms nm-2), p indicates the preparation method (a for adsorption, and i for impregnation), and c the copper compound used in the preparation (acac for Cu(acac)2, ac for Cu(ac)2, and nit for Cu(NO3)2). A few samples were also prepared as mechanical mixtures by grinding ZrO2 and CuO in an agate mortar and are designated as ZCux(mec). Cu(acac)2 was purchased from Merck-Schuchardt, Cu(ac)2, Cu(NO3)2, and CuO from Carlo Erba (R.P.). After dissolving a known amount of ZCu solid specimen in a concentrated (40%) HF solution, the copper content was determined by atomic absorption (Varian Spectra AA-30). In the adsorption method, the copper per unit area of zirconia (atoms nm-2) progressively increased as a function of the number of Cu atoms available in the solution used for the preparation. Zirconia adsorbed all copper, up to 1.5 atoms nm-2. As the copper in solution increased further, uptake reached an extended plateau, corresponding to about 2.5 Cu atoms nm-2. Samples with higher Cu contents were prepared by the dry impregnation method. The x range studied was 0.1 to 8.4 Cu atoms nm-2. BET surface areas (SA/m2g-1) were measured by N2 adsorption at 77 K. Pure ZrO2 (58 m2g-1) and ZCu samples (55 to 60 m2g-1) had similar SA. Table 1 lists the catalysts with the methods used for their characterization. Procedures and Characterization Techniques. Apparatus. Specimens were placed in a silica reactor equipped with two tubes for XPS and ESR measurements and connected to an allglass circulation apparatus. The apparatus was equipped with a magnetically driven pump, a pressure transducer (10-2 to 1000 Torr, MKS Baratron), and a trap placed downstream from the reactor and kept at RT or 77 K, as specified. The catalysts were characterized as prepared (a.p.), after evacuation at increasing temperature up to 773 K, after heating in air at 773 K, and after heating in dry oxygen (50 Torr) at 773 K (standard oxidation, s.o.). In the s.o. treatment, the downstream trap was kept at 77 K to condense the products desorbed from the surface (mostly CO2 and H2O). Before measurements, s.o. samples were evacuated at RT or 773 K, as specified. CO Adsorption. Samples ZCu treated by s.o. were contacted with CO (30 Torr) at RT. Adsorption was considered complete when two successive pressure readings at 5 min intervals differed by less than 0.05 Torr. From the pressure decrease we calculated the total amount of adsorbed CO (molecules nm-2), (CO)tot. Samples were then evacuated at RT for 30 min before re-admission of CO at RT, to determine RT-reversible CO adsorption, (CO)rev. The RT-irreversible CO adsorption was

FTIR

XPS

+ + + +

+

redox cycles

adsorption

+ + + +

+ + + + + +

ESR

+ + + + + + + + + + + + + + + + + +

+ + +

+ +

+ +

+

+ +

+

+ + +

+

+

+

+

+

+

+

calculated as (CO)irr ) (CO)tot - (CO)rev. In other experiments, after determining (CO)tot, catalysts were evacuated at RT and subsequently contacted with O2 (30 Torr) at RT. The decrease in pressure gave the O2 reacted, (O2)tot. Finally, samples were heated in O2 at 773 K, maintaining the adsorption chamber in contact with the downstream trap kept at 77 K. In O2 treatment at 773 K, the desorbed CO2 (CO32- f CO2 + O2-) was condensed in the trap and then measured by evaporating it into a known volume maintained at RT, (CO32-)tot. From (O2)tot and (CO32-)tot, electrons acquired or lost per copper atom, e/Cu, were calculated, as reported in detail under Results and Discussion, along with the relevant surface reactions. Redox Cycles. ZCu s.o. samples evacuated at RT for 30 min were reduced in H2 (50 Torr) at temperatures in the range 323 K to 653 K, evacuated at the reduction temperature for 30 min, and heated in O2 at 773 K. The amount of H2 consumed gave the reduction extent, (e/Cu)H2, and the O2 reacted gave the oxidation extent, (e/Cu)O2. After reduction, the average oxidation number of copper ((0.1) was calculated as nav ) 2 - (e/Cu)H2. XPS Measurements. XPS spectra were recorded with a Leybold-Hereaus LHS 10 spectrometer operating in FAT mode and interfaced to a 2113 HP computer, using Mg KR radiation (1253.6 eV). Because the intense Zr3p3/2 peak would have obscured the Cu(L3M45M45) Auger transition, this was recorded using Al KR radiation (1486.6 eV). The spectrometer was equipped with a special vacuum-tight device attached to the preparation chamber and was operated from the outside. This allowed samples previously treated in the gas circulation apparatus to be put into the spectrometer without being exposed to air.12 The analysis chamber was evacuated at pressures lower than 10-8 Torr. The computer collected the peaks sequentially: Cu2p1/2, Cu2p3/2, Cu(L3M45M45), Zr3d3/2, and Zr3d5/2. Binding energy values (Eb/eV) were referenced to the Zr3d5/2 peak, taken as 182.5 eV. Spectra analyses involved smoothing, inelastic background removal by a linear integral profile, curve-fitting by the least-squares method (using a mixed Gaussian-Lorentzian function), and determination of the peak area by integration. The O1s plasmon band of pure zirconia was subtracted from the Cu(L3M45M45) transition.

Surface Composition of CuOx/ZrO2 Catalysts The CuII to CuI photoreduction occurring during XPS measurements is well documented.13-15 To minimize this effect, low X-ray flux was used (anode operating at 12 KV and 10 mA). However, to obtain a signal-to-noise ratio g 100, dilute ZCu specimens (0.8 Cu atoms nm-2) had to be irradiated for 80 min and, for a significant comparison, the same irradiation time was used with all samples. For all ZCu samples, this irradiation dose yielded a similar photoreduction extent. Therefore, under Results and Discussion, changes in the Cu2p3/2 Eb and in its satellite to main peak intensity ratio, Is/Im, after a specific chemical treatment will be entirely attributed to the treatment itself. X-ray Diffraction. The XRD patterns were obtained with a Philips PW 1729 diffractometer equipped with an IBM computer (software APD-Philips). The Ni-filtered Cu KR radiation was used. Diffuse Reflectance Spectroscopy. The DRS spectra were recorded in the wavelength range 200-2500 nm using a Cary 2300 spectrometer equipped with an IBM PS2 computer for data acquisition and analysis (software Spectra calc.-Galactic Industries Corp.). ESR Measurements. The ESR spectra of ZCu samples were recorded at RT or 77 K on a Varian E-9 spectrometer (X band), equipped with an on-line computer for data analysis. SpinHamiltonian parameters (g and A values) were obtained from spectra calculated with the program SIM14 A.16 The g values were computed taking as reference the sharp peak at g ) 2.0008 of the E′1 center (marked with an asterisk in Figures 4 and 5); the center was formed by UV irradiation of the silica dewar used as a sample holder.17 Absolute concentrations of CuII species (spins nm-2, (10%) were obtained using Cu(acac)2 and CuSO4‚5H2O in polycrystalline state as standards. The spectra of both Cu standards consisted of an exchange narrowed signal (giso ) 2.13, ∆Hpp) 200 G for Cu(acac)2 and giso ) 2.18, ∆Hpp) 188 G for CuSO4‚5H2O). The linear spin density, f/spins cm-1, was calculated as f ) Nd/M, where N is the Avogadro number, M the molecular weight, and d the linear density (d/g cm-1). The number of spins, Na/spins nm-2, was calculated as Na ) (fb/da SA) (areaa/areab), where a refers to ZCu samples and b to standards. FTIR Measurements. FTIR spectra were recorded at RT on a Perkin-Elmer 2000 spectrometer, collecting 4-20 scans at a resolution of 4 cm-1. Powdered materials were pelleted (pressure 1.5 × 104 kg cm-2) in self-supporting disks of 40-50 mg cm-2 and 0.1-0.2 mm thickness. Disks were placed into an IR quartz cell allowing thermal treatments in vacuo or in a controlled atmosphere. During thermal treatment, a condensation trap placed close to the activation chamber was kept at 77 K. Absorbance spectra were obtained by subtracting the relevant background. Results and Discussion XPS Characterization. In s.o. ZCu samples with a copper content e 2.5 atoms nm-2, removal of oxygen by evacuation at RT or 773 K yielded nearly identical XPS spectra (compare spectra a and b in Figure 1). Because RT removal of oxygen is highly unlikely to reduce copper, this result indicates that evacuation at 773 K left the copper oxidation state in s.o. ZCu samples unchanged. Evacuation at 773 K also failed to reduce copper in a.p. ZCu(i nit) samples (spectrum c). The relevant spectra of all these samples showed the following XPS parameters for the Cu2p3/2 region: Eb ) 933.8 ( 0.2 eV and Is/Im, ) 0.25 ( 0.05. Conversely, because acetylacetonates and acetates were oxidized during desorption, evacuation at increasing temperature

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Figure 1. XPS spectra of ZCu samples after various treatments. Sample s.o. ZCu2.3(a ac) evacuated at RT (spectrum a) or at 773 K (spectrum b). Sample a.p. ZCu2.4(i nit) evacuated at 773 K (spectrum c). Sample a.p. ZCu2.3(a ac) evacuated at RT (spectrum d), at 473 K (spectrum e), or at 473 K and exposed to O2 (spectrum f), or evacuated at 773 K (spectrum g). Sample s.o. ZCu2.3(a ac) reduced with H2 at 523 K (spectrum h). Sample s.o. ZCu4.6(i ac) reduced with H2 at 523 K: XPS (spectrum i) and Auger (spectrum in the inset).

of a.p. ZCu obtained from Cu(acac)2 or Cu(ac)2 caused copper reduction. The reduction extent increased with the evacuation temperature, 473 K yielding CuI and 773 K yielding clustered metallic copper. Specifically, evacuation at 473 K shifted the main peak of the a.p. ZCu2.3(a ac) sample evacuated at RT (spectrum d) from Eb ) 934.5 eV to Eb ) 933.6 eV and markedly decreased the Is/Im ratio from Is/Im ) 0.51 to Is/Im ) 0.23, both changes indicating reduction to CuI (Figure 1, compare spectra d and e). Subsequent addition of water vapor at RT left the spectrum unchanged, whereas the addition of O2

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Figure 2. Spreading of copper on the ZrO2 surface. Intensity ratio, Cu2p/Zr3d, as a function of the copper content (atoms nm-2) in samples as prepared (open symbols) or calcined at 773 K (full symbols). Samples: (O) ZCu(a acac), (3) ZCu(i nit), (]) ZCu(mec), (4) ZCu(i ac), (0) ZCu(a ac) and (+) ZCu(a ac) after reduction with H2 at 523 K. The full line was calculated according to the “spherical model” proposed by Cimino et al.21,22

at RT restored the original spectrum, indicating oxidation to CuII (spectrum f, Eb ) 934.4 eV and Is/Im ) 0.40). Evacuation at 773 K shifted the main peak to Eb ) 932.9 eV, close to the value reported for copper metal,18-19 caused the satellite peak to disappear, and markedly lowered the spectrum intensity (spectrum g). The position of the bands, the disappearance of the satellite peak, and the decreased spectrum intensity indicated that copper metal particles had formed. Reduction with H2 at 523 K generated spectra nearly identical to spectrum g for both this sample (spectrum h) and for a more concentrated one, ZCu4.6(i ac) (spectrum i). The ZCu4.6(i ac) sample showed the Cu(L3M45M45) Auger transition (inset in Figure 1) very close to that reported for Cu metal (918.9 eV vs 918.7 eV).20 All of these findings indicate that evacuation at 773 K of a.p. ZCu samples obtained from Cu(acac)2 or Cu(ac)2 causes the formation of copper metal. On all ZCu samples, after the various treatments under vacuum or in H2, a subsequent s.o. treatment yielded XPS spectra nearly identical to those obtained on fresh s.o.-treated samples, showing the spreading and the oxidation state of copper to be fully reversible. Irrespective of the preparation method, the intensity ratio Cu2p/ Zr3d increased proportionally to the Cu content up to about 3 atoms nm-2 in a.p. samples and up to 4.6 Cu-atoms nm-2 in s.o. samples (Figure 2). The Cu2p/Zr3d experimental values approached those calculated with the “spherical model” proposed by Cimino et al.,21,22 suggesting that copper species were uniformly spread on the zirconia surface. In samples with higher Cu content and in mechanically mixed samples, the experimental Cu2p/Zr3d values were much lower than the calculated values, indicating that copper had been segregated. In s.o. samples reduced with H2 at 523 K, the Cu2p/Zr3d ratios were markedly below the theoretical line, indicating that metallic copper had been segregated (Figure 2). DRS and XRD Characterization. The UV-vis spectra of ZCu samples calcined at 773 K with a Cu content e 3 atoms nm-2 consisted of an asymmetric band at 600-900 nm, centered at 760 nm due to dispersed CuII (Figure 3, spectra a and b). Analogous bands have been detected on CuOx/ZrO2 containing

Indovina et al.

Figure 3. DRS spectra of ZCu samples calcined at 773 K. ZCu1.6(a acac) (spectrum a), ZCu3.1(a ac) (spectrum b), ZCu4.6(i ac) (spectrum c), and ZCu8.4(i ac) (spectrum d).

0.5 to 3 Cu wt %6 and dilute CuOx/Al2O323,24 and have been assigned to CuII ions in axially distorted octahedral sites. The spectrum of the ZCu8.4 (i ac) sample consisted of a broad adsorption band in the region 400-800 nm due to CuO (spectrum d). The ZCu4.6(i ac) sample showed the combined spectral features of dispersed CuII and CuO particles (spectrum c). In particular, the d-d transition band shifted to a lower wavelength and lost its asymmetry, suggesting that small CuO particles had formed. The XRD pattern of ZCu5.5(mec) showed the most prominent CuO peak (38.8°), whereas those of ZCu4.6(i ac) and ZCu8.4(i ac) did not, indicating that the CuO particles were smaller than about 400 nm. ESR Characterization. Spectra at both RT and 77 K of a.p. ZCu samples evacuated at 393 K consisted of two overlapping signals: an axial signal (species A) with hyperfine structure in the parallel region, and a broad, unresolved and nearly isotropic band (giso ) 2.160, ∆Hpp ) 200 G). As the Cu content increased to more than 0.3 Cu atoms nm-2, the resolution decreased and the broad band became prevalent (Figure 4). The broad band was assigned to magnetically interacting CuII ions, whose spectrum therefore showed no hyperfine structure. Spectra of s.o. samples evacuated at RT or 773 K, in addition to the broad band, showed an axial signal with a composite hyperfine structure. As in a.p. samples, increasing the Cu content caused the broad band to prevail. In samples with Cu content e 0.3 atoms nm-2, the hyperfine structure was resolved in both the parallel and the perpendicular region, showing the concomitant presence of three copper species, B, C, and D (Figure 5, spectrum a). The a.p. samples evacuated at T g 450 K also contained these three copper species. Addition of water vapor at RT reversibly transformed B, C, and D into A. Addition of carbon dioxide at RT transformed C into B and D (Figure 5, spectrum b). From computer calculated Spin-Hamiltonian parameters, the ratio ∆gII/∆g⊥ ) (ge - gII)/(ge - g⊥), and the AII value of B, C, and D were higher than those of A, indicating that B, C, and D had a higher crystal-field axial component (Table 2). The addition of H2O or NH3 to chromyl and vanadyl species on ZrO2 causes analogous changes that have been attributed to the reversible transformation of these square-pyramidal pentacoordinate complexes into octahedral esa-coordinates.17,25 Hence we

Surface Composition of CuOx/ZrO2 Catalysts

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Figure 4. EPR spectra at RT of ZCu as prepared samples. ZCu0.1(a acac) (spectrum a), ZCu0.2(i nit) (spectrum b), ZCu0.7(a acac) (spectrum c), and ZCu1.2(a acac) (spectrum d). The asterisk indicates the marker at g ) 2.0008.

assign B, C, and D to isolated copper complexes in a squarepyramidal configuration, CuII5c (the three species differing possibly in their crystal-field symmetry) and A to an isolated copper complex in a distorted octahedral symmetry, CuII6c. Irrespective of the preparation method, in a.p. samples with a Cu content e 0.3 atoms nm-2, containing mostly isolated CuII, the ESR-detected copper, 100(Na/x), accounted for 80-100% of the analytical copper. In more concentrated a.p. samples, containing mostly magnetically interacting CuII, the copper detected by ESR progressively diminished (to about 20% in the sample with 2.5 Cu atoms nm-2). In s.o. samples as well, ESR-detected CuII depended on Cu content. At all copper content, ESR detected 10 to 20% less CuII in s.o. than in the relevant a.p. samples. In samples with a Cu content e 0.3 atoms nm-2, we studied the stability of CuII species during evacuation at increasing temperature. Evacuation up to 773 K of all s.o. samples and a.p. ZCu(i nit) samples left the signal intensity practically unchanged (Figure 6), thus showing that no CuII had been reduced. Conversely, evacuation at increasing temperature of a.p. samples obtained from Cu(acac)2 or Cu(ac)2 caused a sharp and progressive decrease in the signal intensity. In these samples, evacuation reduced copper because acetylacetonates and acetates underwent oxidation during desorption. After evacuation up to 500 K, a subsequent exposure to O2 at RT restored the CuII signal, whereas at higher evacuation temperature, to restore the signal, the sample had to be heated in O2 at 773 K. This behavior indicates that evacuation up to 500 K generates CuI while evacuation at higher temperature yields copper metal. IR Characterization. ZCu a.p. and s.o. Samples EVacuated at Increasing Temperature. Evacuation at RT of a.p. ZCu(a

Figure 5. EPR spectra at RT of the ZCu0.1(a acac) sample after various treatments. Sample after heating in oxygen at 773 K and evacuation at 773 K (spectrum a) and same sample after CO2 addition at RT (spectrum b). The asterisk indicates the marker at g ) 2.0008.

TABLE 2: ESR Parameters of CuII Surface Species in ZCu Catalysts species

signal

gII

g⊥

∆gII/∆g⊥a

AII/Gauss

(Cu2+)6c

A

2.333

2.066

5.19

139

(Cu2+)5c

B C D

2.307 2.298 2.249

2.052 2.052 2.045

6.13 5.95 5.78

153 141 161

a

A⊥/Gauss 24 17 24

∆g ) g - ge ) g - 2.0023

acac), a.p. ZCu(a ac), and a.p. ZCu(i ac) samples containing the same amount of copper, gave similar IR spectra (spectra not shown). Spectra consisted of (i) a broad band at 3700-

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Figure 6. Percentage of CuII detected by ESR as a function of the evacuation temperature (T/K). Samples as prepared (open symbols) or heated in oxygen at 773 K (full symbols): (O) ZCu0.1(a acac), and (0) ZCu0.1(i nit).

2600 cm-1 (stretching νOH of interacting hydroxyls and of adsorbed H2O), (ii) weak absorptions at 2960, 2928, 2855 cm-1 (stretching νC-H of methyl and methylene groups), (iii) a shoulder at about 1620 cm-1 (deformation δHOH of coordinated H2O), (iv) very intense bands at about 1560 cm-1 (asymmetric stretching vibrations νCOO- of carboxylate ions) and 1445 cm-1 (symmetric νCOO- and deformation δCH3), (v) weak absorption at 1348 cm-1 (scissoring δCH3) and very weak absorptions at 1050 and 1022 cm-1 (rocking FCH3) and 924 cm-1 (stretching νC-C). Bands under (ii), (iv), and (v) resembled those observed on various oxides after exposure to acetic acid26 or acetaldehyde,27 and were close to those of the acetate ion in aqueous solution,28 and were therefore assigned to carboxylate species. In addition to the bands from interacting hydroxyls and adsorbed H2O, the IR spectra of a.p. ZCu (i nit) consisted of broad bands at about 1570 cm-1 (asymmetric stretching vibration νONO) and 1320 cm-1 (symmetric stretching vibration νONO), with a weak band at about 1050 cm-1 (stretching vibration νNO) assigned to unidentate nitrates.29 Evacuation at increasing temperature, from RT to 773 K, gradually removed all adsorbed species. Nitrates disappeared at 623 K from a.p. ZCu(i nit) and carboxylate species at 773 K from a.p. ZCu(a acac), a.p. ZCu(a ac), and a.p. ZCu(i ac) samples. After evacuation at 423 K, weak bands at 3780 and 3670 cm-1 appeared, superimposed on the broad band at 37002600 cm-1, and were assigned to terminal (3780 cm-1) and bridged (3670 cm-1) free hydroxyls of monoclinic zirconia, in agreement with previous reports.8,9,30-32 After evacuation at 623 K, the broad band at 3700-2600 cm-1 disappeared. IR spectra of all s.o ZCu samples evacuated at RT or 773 K showed terminal and bridged hydroxyls alone. As the Cu content of the sample increased, the intensity of both hydroxyl bands decreased, preferentially that of terminal OH. The decreasing intensity of the hydroxyl bands is attributed to the formation of Zr-O-Cu species in the copper anchoring process. As the Cu content increased, the preferential decreasing of the terminal OH intensity indicated that copper preferentially anchored to this hydroxyl type, as other investigators have observed for CuOx/ZrO2.8,9 The preferential anchorage to terminal rather than to bridged OH of ZrO2 has been also observed with Mo31 and V.32

Figure 7. IR spectra of copper carbonyls and carbonate species formed by CO adsorption at RT on ZCu samples, after various treatments. Section a (carbonyls region): ZrO2 evacuated at 773 K (spectrum 1); ZCu1.6(a acac) a.p. evacuated at 523 K (spectrum 2) or at 773 K (spectrum 3); ZCu1.6(a acac) s.o. evacuated at 773 K (spectrum 4), reduced with H2 at 373 K (spectrum 5), or reduced with H2 at 573 K (spectrum 6). Section b (carbonates region): ZrO2 evacuated at 773 K (spectrum 1), ZCu1.6(a acac) s.o. evacuated at 773 K (spectrum 2), ZCu0.3(i ac) s.o. evacuated at 773 K (spectrum 3).

Surface Species Formed on CO Adsorption. On ZrO2 evacuated at 773 K, the CO adsorption at RT yielded a band consisting of two components at 2191 and 2185 cm-1 (Figure 7, section a, spectrum 1) both reversible on evacuation at RT, assigned to σ-coordinated CO on two ZrIV Lewis acid sites of different strengths, in agreement with previous reports.33-35 In the region 1800-1000 cm-1, no bands were detected (Figure 7, section b, spectrum 1). On ZCu a.p. samples evacuated at RT, CO adsorption generated no IR bands. After evacuation at 523 K, CO adsorption yielded a band at 2097 cm-1, assigned to σ-coordinated CO on CuI sites (Figure 7, section a, spectrum 2).5,9,23,36-42 The 1800-1000 cm-1 region showed no carbonate bands or extremely weak carbonate bands (spectrum not shown). After evacuation at 773 K, CO adsorption yielded a composite and exceedingly weak band centered at about 2115 cm-1, assigned to CuI-CO, with a shoulder at 2102 cm-1, possibly due to Cu0CO,5,40,42-44 and intense bands of ZrIV-CO (Figure 7, section a, spectrum 3). The 1800-1000 cm-1 region contained no

Surface Composition of CuOx/ZrO2 Catalysts carbonate bands. A subsequent evacuation at RT removed all ZrIV-CO and 75% of copper carbonyls (CuI-CO and Cu0CO). Notably, the shoulder at 2102 cm-1 disappeared. The higher surface hydration extent explains the lower wavenumber of CuI-CO carbonyls in a.p. ZCu evacuated at 523 K than in a.p. ZCu evacuated at 773 K (2097 cm-1 vs 2115 cm-1). Other investigators have already observed an analogous red-shift for CuI-CO carbonyls due to the surface hydration extent.9,36,45a On all ZCu s.o. samples, after removal of O2 at 773 K, CO adsorption yielded IR spectra nearly identical to those obtained on the relevant s.o. samples cooled in O2 to RT and evacuated at RT. The 2250-2000 cm-1 region showed an intense band centered at about 2120 cm-1, due to copper carbonyls CuICO, and weak bands of ZrIV-CO (Figure 7, section a, spectrum 4). In the 1800-1000 cm-1 region, we observed intense bands assigned to bidentate carbonates (1554 cm-1, νCdO, 1308 cm-1, asymmetric νOCO, and 1058 cm-1, symmetric νOCO),37,46-48 and weak bands assigned to zirconium-CO2 adducts (1770 cm-1, asymmetric νCO2 and 1136 cm-1, symmetric νCO2)49 and to bidentate bicarbonates (1620 cm-1, asymmetric νCO2, 14201369 cm-1, symmetric νCO2, 1252-1178 cm-1, δCOH)46-48 (Figure 7, section b, spectra 2 and 3). A subsequent evacuation at RT removed all ZrIV-CO and 20% of copper carbonyls and left carbonates unchanged. Evacuation completely removed CuI-CO at 498 K and carbonates at 773 K. On ZCu s.o. reduced with H2 at 373 K and evacuated at the same temperature, CO adsorption at RT yielded CuI-CO at 2098 cm-1, with a pronounced tail at a lower wavenumber, possibly arising from Cu0-CO (Figure 7, section a, spectrum 5), and extremely weak bands from carbonates (spectrum not shown). Also in this sample, the high hydration extent of the ZrO2 surface, due to the water produced in the reduction with H2, accounts for the low wavenumber of CuI-CO. Evacuation at RT removed 80% of carbonyls, mostly those corresponding to the tail. On ZCu s.o. reduced with H2 at 573 K and evacuated at the same temperature, CO adsorption at RT yielded ZrIVCO carbonyls at 2185 cm-1 and an extremely weak band centered at about 2105 cm-1 (Figure 7, section a, spectrum 6). Notably, after the adsorption of CO on reduced samples no carbonates formed (spectrum not shown). A subsequent evacuation at RT completely removed all carbonyls. Because no CuIICO formed at RT, the stability of copper carbonyls on ZCu samples was CuII-CO , Cu0-CO < CuI-CO. On all ZCu samples after the various treatments under vacuum or in H2, a subsequent s.o. treatment yielded, on CO adsorption, copper carbonyls identical to those obtained on fresh s.o.-treated samples. When s.o. ZCu samples evacuated at RT or 773 K were exposed to CO at RT, CuI-CO and carbonates formed simultaneously. In these samples, as the adsorption equilibrium pressure increased, the integrated band intensities of CuI-CO and carbonates increased in parallel (Figure 8, curve 1). On increasing the CO pressure, both the integrated intensity of carbonyls and carbonates plateaued at the highest values of curve 1 (Figure 8). This result strongly suggests as the cause of CuII to CuI reduction the oxidation of CO to carbonates during the adsorption of CO at RT. On a subsequent evacuation for 30 min at RT, the intensity of carbonates remained unchanged, whereas that of carbonyls decreased by about 20% and remained unchanged after an additional evacuation at RT for 1 h (Figure 8, curve 2). The CO adsorption at RT indicates the following reduction process:

J. Phys. Chem. B, Vol. 103, No. 45, 1999 9973

Figure 8. Integrated band intensity (cm-1) of carbonates as a function of that of copper carbonyls. Sample: ZCu0.9(i ac) s.o. Integrated bands intensity after various treatments at RT (in sequence): CO adsorption at increasing pressure (curve 1), evacuation for 30 min (curve 2), O2 adsorption at increasing pressure (curve 3), and a second CO adsorption at increasing pressure (curve 4).

2CuII + 2O2-+ 3CO f 0.4(CuI-CO)rev + 1.6(CuI-CO)irr +(CO32-)rid (R.1) where the total amount of copper carbonyls formed, (CuICO)tot, consisted of CuI-(CO)rev removed on evacuation at RT (20% of total carbonyls) and (CuI-CO)irr not removed at this temperature. A subsequent treatment with O2 at RT transformed (CuI-CO)irr into CuII and carbonates (Figure 8, curve 3), so that after O2 removal at RT, a second CO adsorption restored CuI-CO to 70%, and caused the amount of carbonates to rise again (Figure 8, curve 4). The high carbonate coverage reached at this stage explains why CuI-CO carbonyls formed in smaller amounts during the second CO adsorption. The O2 adsorption at RT indicates the following overall oxidation process:

1.6(CuI-CO)irr + 0.8O2- + 1.2O2 f 1.6CuII + 1.6(CO32-)ox (R.2) 0.4CuI + 0.1O2 f 0.4CuII + 0.2O2-

(R.3)

where CuI species in R.3 (20% of total carbonyls) arose from the decomposition under vacuum of (CuI-CO)rev. Conversely, when a.p. ZCu obtained from Cu(acac)2 or Cu(ac)2 and evacuated at 523 to 773 K were exposed to CO at RT, carbonyls CuI-CO, and possibly Cu0-CO, formed but carbonates did not. This result strongly suggests as the cause

9974 J. Phys. Chem. B, Vol. 103, No. 45, 1999

Indovina et al. little as the Cu content increased further, indicating that a copper content up to 2.5 atoms nm-2 is highly dispersed on the zirconia surface. Characterization by CO Adsorption: Volumetric Measurements. On ZCu s.o. samples, both the total CO amount adsorbed at RT, (CO)tot, and the irreversibly adsorbed amount, (CO)irr, increased with copper content (Table 3). From (CO)irr we calculated the CuI-CO total amount, (CuI-CO)tot, as

(CuI-CO)tot ) (CuI-CO)irr + (CuI-CO)rev ) (1.6 + 0.4)(CO)irr/(1 + 1.6) where the CuI-CO irreversibly adsorbed, (CuI-CO)irr, and that reversibly adsorbed, (CuI-CO)rev, were derived from the stoichiometry of R.1

(CO)irr ) (CuI-CO)irr + (CO32-)rid Figure 9. Integrated intensity of copper carbonyls and carbonates after heating in CO at various temperatures. Integrated intensity (cm-1) of copper carbonyls (O, scale on the left) and carbonates (0, scale on the right) as a function of temperature (T/K). The ZCu0.3(i ac) s.o. sample was heated in CO at the various temperatures and then cooled to RT in the presence of CO.

of CuII reduction, the oxidation of acetylacetonates and acetates, during their desorption. To evaluate the copper reduction extent caused by CO adsorption at RT, s.o. samples were heated in CO at various temperatures, from RT to 593 K. Before recording the spectrum, samples were maintained for 5 min at each temperature in the presence of CO and then cooled to RT, always in the presence of CO. Up to 473 K, the band intensities of copper carbonyls and carbonates progressively increased in parallel, whereas, at higher temperature, the intensity of the copper carbonyl band markedly decreased and that of carbonates increased further (Figure 9). This behavior indicates that the heating in CO up to 473 K progressively reduced CuII to CuI, whereas the heating at higher temperature caused reduction to copper metal. Assuming that heating with CO at 473 K reduced all CuII to CuI, namely (e/Cu)473 ) 1, and that every CuI ion yielded CuI-CO, the reduction extent at RT, (e/Cu)RT, was evaluated from

(e/Cu)RT ) (e/Cu)RT/(e/Cu)473 ) IRT/I473 ) 0.6 where IRT was the integrated intensity of the CuI-CO band after CO adsorption at RT and I473 was that after heating with CO at 473 K and cooling to RT in the presence of CO. Copper Dispersion. In s.o. ZCu samples, at a given copper content, irrespective of the preparation method, CuI-CO bands had nearly identical shapes and positions (Figure 10). On the ZCu0.1(a acac) sample, they centered at about 2110 cm-1 (section a), and as the copper content increased, gradually shifted to about 2120 cm-1 (section b-d). On all s.o ZCu samples with a Cu content up to 2.5 atoms nm-2, the band consisted of various (generally three) poorly resolved components showing surface heterogeneity. Conversely, the ZCu8.4(i ac) sample had an intense band at 2118 cm-1 with a shoulder at 2130 cm-1 and a pronounced tail at a lower wavenumber, with shoulders at 2010 and 2098 cm-1. All of these bands resembled those reported for massive CuO (band at 2115 cm-1 with a shoulder at 2140 cm-1) and were assigned to CuI-CO on the CuO surface.45b On s.o. ZCu samples, the integrated band intensity of copper carbonyls, at the same CO equilibrium pressure, was proportional to the Cu content up to 2.5 atoms nm-2, and increased

(CO32-)rid ) (CuI-CO)irr/1.6 (CuI-CO)rev ) 0.4(CuI-CO)irr/1.6. In ZCu samples with a copper content up to 2.5 atoms nm-2, (CuI-CO)tot was nearly proportional to the copper content, thus paralleling the CuI-CO integrated band intensity, as assessed by IR. The volumetric analysis specified that copper carbonyls corresponded to 50-60% of the analytical copper content. In contrast, in the most concentrated sample, ZCu8.4(i ac), (CuICO)tot still increased, but little, again in agreement with the IR result, and corresponded to 20% of the analytical copper (Table 3). We assessed the extent of copper reduction caused by CO adsorption at RT with two independent procedures, as follows. With the first procedure, from the total oxygen consumed in a subsequent oxidation at RT, (O2)tot, we calculated the oxygen amount consumed for copper oxidation, (O2)Cu, as

(O2)Cu ) 0.5(O2)tot/1.3 and the copper reduction extent as

(e/Cu)O2 ) 4(O2)Cu/Cu where (O2)Cu was derived from the stoichiometry of R.2 and R.3

(O2)tot ) (O2)Cu + (O2)CO (O2)Cu ) (0.4 + 0.1)(O2)tot/(1.2 + 0.1) and (O2)CO corresponded to the oxygen amount yielding (CO32-)ox in R.2. With the second procedure, from the total amount of carbonates formed during a subsequent heating in O2 at 773 K, (CO32-)tot, we calculated that produced in the copper reduction, (CO32-)rid, as

(CO32-)rid ) (CO32-)tot/(1 + 1.6) and the copper reduction extent as

(e/Cu)CO32- ) 2(CO32-)rid/Cu

Surface Composition of CuOx/ZrO2 Catalysts

J. Phys. Chem. B, Vol. 103, No. 45, 1999 9975

Figure 10. Copper and zirconium carbonyls in ZCu s.o. samples evacuated at 773 K. IR spectra after CO adsorption at RT at increasing CO pressure. Sample ZCu0.1(a acac) (section a): after adsorption of a small dose of CO (0.1 µmol) (spectrum 1), after adsorption of a second dose (spectrum 2), CO at equilibrium pressure 0.02 Torr (spectrum 3), 0.2 Torr (spectrum 4), 1.1 Torr (spectrum 5), 5.8 Torr (spectrum 6), 17 Torr (spectrum 7), and 54 Torr (spectrum 8). Sample ZCu0.3(i ac) (section b): CO at equilibrium pressure 0.02 Torr (spectrum 1), 1.3 Torr (spectrum 2), 6.0 Torr (spectrum 3), 17 Torr (spectrum 4), 38 Torr (spectrum 5), and 78 Torr (spectrum 6). Sample ZCu0.9(i ac) (section c): CO at equilibrium pressure 0.01 Torr (spectrum 1), 0.07 Torr (spectrum 2), 0.12 Torr (spectrum 3), 0.6 Torr (spectrum 4), 2.4 Torr (spectrum 5), 16 Torr (spectrum 6), 38 Torr (spectrum 7), and 53 Torr (spectrum 8). Sample ZCu1.6(a acac) (section d): CO at equilibrium pressure 0.01 Torr (spectrum 1), 0.02 Torr (spectrum 2), 0.18 Torr (spectrum 3), 0.27 Torr (spectrum 4), 0.5 Torr (spectrum 5), 0.8 Torr (spectrum 6), and 2.9 Torr (spectrum 7).

TABLE 3: Reduction Extent of Copper (e/Cu) after CO Adsorption at RT ZCu 0.3(i ac) ZCu 0.9(i ac) ZCu 2.5(i ac) ZCu 8.4(i ac) a

(CO)tota

(CO)irra

(CuI-CO)tota

(CuI-CO)tot/(Cu)b

(e/Cu)O2c

(e/Cu)CO32- c

0.5 1.0 1.8 3.3

0.2 0.6 1.4 2.6

0.2 0.5 1.1 1.9

0.6 0.6 0.5 0.2

0.6 0.6 0.6 0.3

0.6 0.5 0.5 0.3

Molecules nm-2. b Copper carbonyls per Cu atom. c Electrons per Cu atom.

where (CO32-)rid was derived from the stoichiometry of R.1 and R.2

(CO32-)tot ) (CO32-)rid + (CO32-)ox (CO32-)ox/(CO32-)rid ) 1.6. The (e/Cu)O2 and the relevant (e/Cu)CO32- values agreed well (Table 3). In all samples with a Cu content up to 2.5 atoms

nm-2, both values were nearly constant (0.5-0.6), but the most concentrated sample, ZCu8.4(i ac), yielded substantially lower values (0.3). The finding that e/Cu values agreed well with (CuI-CO)/(Cu) ratios indicates that every CuI ion formed on CO adsorption at RT yielded CuI-CO (Table 3). In ZCu samples with a copper content up to 2.5 atoms nm-2, the e/Cu value determined by volumetric measurements agreed well with that evaluated from IR, (e/Cu)RT ) 0.6. The agreement validates the assumption adopted in the calculation of the reduction extent from IR data, namely that heating these samples

9976 J. Phys. Chem. B, Vol. 103, No. 45, 1999

Indovina et al. The finding on copper dispersion is in line with evidence that when CuOx/ZrO2 are used for the selective catalytic reduction of NO with ammonia or propene, the turnover frequency per Cu atom is independent of the Cu content, up to 2.5 atoms nm-2.10 Acknowledgment. Financial support was provided by the Ministero dell’ Universita` e della Ricerca Scientifica e Tecnologica (Programmi di ricerca scientifica di rilevante interesse nazionale). References and Notes

Figure 11. Reduction of ZCu s.o. samples with H2 at various temperatures. Reduction extent (e/Cu) as a function of the reduction temperature (T/K). Samples: (O) ZCu0.7(a acac), (") ZCu0.9(i ac), (])ZCu1.2(a acac), (3) ZCu2.0(a acac), (!) ZCu2.3(a ac), and (0) ZCu2.5 (a acac).

with CO at 473 K reduced all CuII to CuI, and that on CO adsorption at RT every CuI ion produced CuI-CO. Redox Cycles with H2 and O2. ZCu s.o. samples evacuated at 773 K consumed no oxygen on a subsequent heating in O2 at 773 K, (e/Cu)O2 ) 0. Hence evacuation of s.o. samples at 773 K does not reduce CuII. Regardless of how samples were prepared and at Cu contents up to 2.5 atoms nm-2, heating in H2 up to 450 K caused the reduction extent to increase sharply. At higher temperature, the reduction extent leveled off to (e/Cu)H2 ) 2.0 (Figure 11). ZCu s.o. samples therefore contained CuII alone and after heating in H2 above 450 K copper metal alone. Heating with H2 at 323 to 653 K and a subsequent heating with O2 at 773 K gave identical e/Cu values (within the experimental error, (0.1, (e/Cu)H2 ) (e/Cu)O2), showing that the copper oxidation state remained fully reversible. Conclusions In all CuOx/ZrO2 samples, irrespective of the preparation method (adsorption or impregnation), heating in O2 at 773 K causes the anchoring of stable CuII species to the zirconia surface. Evacuation at 773 K causes no reduction, whereas heating with H2 or CO up to 450 K reversibly reduces CuII to CuI and at higher temperature to metal copper. In as-prepared evacuated CuOx/ZrO2 samples, stability, dispersion, and the oxidation state of copper differ according to how samples are prepared. In particular, in samples prepared from Cu-acetylacetonate or Cu-acetate evacuation at 473 K reduces CuII to CuI and at higher temperature to copper metal. The reduction is caused by the oxidation of acetylacetonates and acetates during desorption. Conversely, evacuation up to 773 K of samples prepared from Cu-nitrate causes no copper reduction. In samples heated in O2 at 773 K, CO adsorption at RT yields CuI-CO and carbonates. Volumetric CO adsorption combined with FTIR shows that copper is highly dispersed on the ZrO2 surface up to 2.5 atoms nm-2: every copper atom yields carbonyls. Notably, up to the copper uptake limit found in the preparation of catalysts by adsorption from solution (2.5 atoms nm-2), copper atoms are well dispersed on the ZrO2 surface.

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