Nature of Acid Sites in Silica-Supported Zirconium Oxide: A Combined

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Nature of Acid Sites in Silica-Supported Zirconium Oxide: A Combined Experimental and Periodic DFT Study Juan-Manuel Arce-Ramos,† Lars C. Grabow,*,‡ Brent E. Handy,† and María-Guadalupe Cárdenas-Galindo*,† †

CIEP Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava No. 6, San Luis Potosí, SLP C.P. 78210, México ‡ Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, United States S Supporting Information *

ABSTRACT: A combined experimental and DFT study was carried out to investigate the acidity of silica-supported zirconium oxide synthesized via the grafting method. XRD results show the crystal phase of ZrO2 is tetragonal. Sample acidity was characterized via ammonia adsorption microcalorimetry and DRIFTS pyridine desorption. Theoretical results predict that on a vacancy-free surface of t-ZrO2 at low coverage NH3 binds preferentially to Lewis acid sites with an adsorption energy up to 120 kJ/mol and that ammonia adsorption energies below 100 kJ/mol calculated at higher coverage can be well explained by lateral NH3−NH3 interactions. Higher differential heats of adsorption at low coverage (up to 190 kJ/mol) on both the (101) and (201) crystal planes of tZrO2 are in agreement with dissociative adsorption of ammonia near an oxygen vacancy site. The theoretical results are in good agreement with our findings in ammonia adsorption microcalorimetry and DRIFTS pyridine desorption on the ZrO2/SiO2 sample.

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INTRODUCTION Acidity and basicity of catalysts are two properties that affect conversion and selectivity not only in acid−base catalytic reactions but also in many other reactions involving redox transformations.1−3 This is the case of ZrO2-based catalysts (ZrO2 as active phase or catalytic support) which, due to their acid−base properties coupled with their chemical and thermal stability, have been investigated for numerous catalytic applications that include ethanol coupling to 1-butanol,4 dehydration of glycerol,5−7 steam reforming,8,9 and production of olefins from alcohols,10−13 to mention only a few examples. In these and in other reported cases, acid strength is often cited as an important factor in determining activity and selectivity behavior. Acid strength differences can be attributed to the introduction of defects when zirconia is incorporated with silica, although an understanding of structural surface details responsible for increased acidity is lacking.14 Even though experimental studies based on pure metal oxides and single crystals are of significant help in understanding chemical phenomena on surfaces, from an application point of view supported catalysts cover the broadest spectra. Hence, studying materials of this type is naturally important. In particular, one must consider that some features and properties of materials can be altered when the preparation method changes. This is particularly true for catalyst acidity, which depends not only on the local composition but also on coverage effects.15,16 © XXXX American Chemical Society

With respect to the characterization of acid sites on catalytic surfaces, several experimental methodologies have been developed. The most common ones are based on amine, pyridine, or ammonia titrations, adsorption microcalorimetry measurements,16,17 thermal desorption,16,18 and spectroscopic techniques.18 However, these methodologies are governed by different physical and chemical principles; therefore, the direct comparison of acidity is often difficult.19 Adsorption microcalorimetry is useful to determine the strength and quantity of acid sites, while adsorption modes of the probe molecule can be studied with spectroscopic measurements. The complementary information obtained from these two techniques helps to elucidate the nature of acid sites present on the catalyst. In the literature there are several examples in which a combination of experimental work and theoretical calculations can be used to study the acidity of solid materials or merely the adsorption of a probe molecule. To this end, Niwa et al. have studied the Brønsted acidity in different zeolites by means of combining ammonia infrared/mass spectroscopy (IRMS) temperature-programmed desorption with density functional theory (DFT) results.20−22 Mino et al. studied the adsorption of CO on anatase TiO2 and through periodic DFT calculations were able to associate the experimental Fourier transform infrared spectroscopy (FTIR) features to CO adsorbed on Received: March 11, 2015 Revised: June 9, 2015

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DOI: 10.1021/acs.jpcc.5b02394 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C different crystal faces.23 Finally, Hornebecq et al. performed carbon dioxide adsorption microcalorimetry experiments over mesoporous zirconia in combinations with modeling using a multi-Langmuir model and DFT calculations on a ZrO2 cluster in an attempt to determine the kind and strength of the different sites evidencing surface heterogeneity from an energetic point of view; however, the cluster-model approach was not effective at modeling stronger adsorption sites.24 As shown in the examples briefly described above, DFT calculations are a powerful complementary tool to characterize surfaces of materials that can provide data that otherwise could be very difficult, if not impossible, to obtain experimentally. Ammonia is a common base probe molecule for the titration of acid sites of surfaces, bonding via its nitrogen lone pair of electrons. In particular, ammonia can bond to the surface of a transition metal oxide in three different adsorption modes,25 as is shown in Figure 1. The first of them involves the transfer of a

Nitrogen Physisorption. A homemade volumetric adsorption apparatus equipped with precision capacitance manometers (Baratron MKS Instruments, Inc.) was used to determine the specific surface area of the supported oxide via nitrogen physisorption at 77 K. First, the sample was treated under high vacuum (10−3 Torr) at 523 K for at least 3 h, in order to desorb any impurity or water on surface that could interfere with the experiment. X-ray Diffraction (XRD). X-ray powder diffraction data were recorded with a PANalytical X-ray diffractometer model X’pert PRO MPD with Cu Kα radiation (λ = 0.154 05 nm, 35 kV, 30 mA) and collecting data in the 10° < 2θ < 70° range. Diffuse Reflectance Infrared Fourier Transform Spectroscopy-Pyridine Desorption (DRIFTS-Py). Infrared spectroscopic studies were performed on an IR Vector22 (BRUCKER) spectrometer at a resolution of 4 cm−1, allowing 120 sample scans per spectrum. The catalyst samples were placed in a Harrick Praying Mantis cell with environmental chamber to collect diffuse reflectance data. The sample was initially pretreated at 773 K under Ar gas flow and evacuated at that temperature. After it was cooled to room temperature, pulses of pyridine were admitted in flowing helium over 20 min to ensure gas equilibrium with surface acid sites The sample then was heated at a constant rate of 10 K/min, and IR spectra were recorded every 50 K until 823 K was reached. Microcalorimetric Measurements. NH3 adsorption microcalorimetry for the direct measurement of the differential heat of adsorption (Qdiff = ΔHads) was carried out at 473 K using a differential heat-flow microcalorimeter of the TianCalvet type coupled to a diffusion-pumped high-vacuum system. Detailed methodology description and equipment design are mentioned elsewhere.26 Approximately 0.2 g of sample was loaded in a quartz cell and degassed under vacuum at 823 K for several hours to ensure that physisorbed water and other impurities are evacuated. At 823 K subsequent cycles of O2 exposure (∼80 Torr of O2) and degasification were done at least three times, until no O2 uptake was observed by pressure drop; hence, the supported oxide has a near recently calcined condition. After outgassing at the same temperature and cooling, the sample cell was inserted into the microcalorimeter, and the baseline of heat flux was stabilized overnight. Successive ammonia doses of 1.5−10 μmol were admitted to the sample, allowing sufficient time between doses to achieve thermal equilibrium, occurring when the recorded heat signal baseline was re-established. Dosing was continued to equilibrium pressures ≥5 Torr, after which the sample was evacuated. The desorption heat signal was integrated to assess the density of reversible acid sites titrated during the adsorption experiment. Density Functional Calculations. Theoretical calculations were done using the Vienna Ab Initio Simulation Program (VASP)27−30 with the projected augmented wave (PAW) method31,32 in combination with the PW91 form of the GGA exchange-correlation functional.33,34 By expanding a series of plane waves with an energy cutoff of 400 eV, the wave functions were constructed, and a Gaussian smearing with kbT = 0.1 eV was used. The tetragonal zirconia slab structure was built from the optimized bulk structure, for which we calculated the lattice constants a = 3.63 Å and c = 5.24 Å in good agreement with experimental values.35 We used supercell sizes of 2 × 2, 2 × 3, 3 × 4, and 3 × 5 units of the primary cell for the (101) t-ZrO2 model (shown in Figure 4) with a k-point mesh of 2 × 2 × 1, except for the smallest 2 × 2 supercell in which a mesh of 2 × 4

Figure 1. Schematic representation of main ammonia adsorption modes on a metal oxide. From left to right, a Brønsted acid site, a Lewis acid site, and adsorption by means of hydrogen bonding, respectively.

proton from the surface acting as a Brønsted acid. The second mode involves the formation of a bond by sharing the lone pair of electrons from the nitrogen of ammonia with an electron deficient surface metal atom, acting as a Lewis acid site. Finally, the third adsorption mode takes place via hydrogen bonding, either nitrogen interaction with hydrogens on surface hydroxyl groups or hydrogen from ammonia with surface anions. As the adsorption modes and strength of probe molecules can be predicted by computational methods, such as DFT, theoretical results may then be used to relate the binding strength to binding modes, which further aids in the characterization of surface acidity. In this work, we studied experimentally the acidity of a silica-supported zirconium oxide catalyst in combination with DFT simulations of ammonia adsorption on the tetragonal phase of ZrO2, which was determined to be the predominant phase of our catalyst using X-ray diffraction (XRD). We show how DFT adsorption calculations on the crystalline phase of the metal oxide can be used to interpret experimental acidity results and determine the nature and strength of the acid sites on a supported oxide, ZrO2/SiO2.

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EXPERIMENTAL AND THEORETICAL METHODS Catalyst Preparation. Silica-supported zirconium oxide material was prepared via grafting of zirconium(IV) n-butoxide (Aldrich, 80%) on silica (Degussa Aerosil, 380 m2/g). First, silica was mixed with sufficient toluene at 333 K, and then the corresponding volume of zirconium n-butoxide was added to get the mass ratio of 3:10 for Zr:SiO2 under an inert atmosphere. The solution was continuously stirred for 12 h at constant temperature. Finally, the solids produced were dried at room temperature and calcined at 823 K for 8 h in a horizontal quartz reactor under 100 mL/min of flowing oxygen. B


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The Journal of Physical Chemistry C × 1 was used. For the calculations over the (201) t-ZrO2 surface, a supercell of 1 × 3 is used with a k-point mesh of 2 × 2 × 1. Convergence with respect to the k-point mesh was confirmed for the primary unit cell, and the larger supercells use an equivalent or better k-point mesh. Four layers of the (101) and (201) facets in t-ZrO2 were chosen, since no significant changes in the adsorption energy of a probe molecule (NH3) are obtained increasing the number of layers. The atomic positions of the two bottom layers were kept fixed to the optimized bulk positions, while the two top layers were fully relaxed. Adsorption was studied on the upper layer only, and the electrostatic potential was adjusted accordingly.36 All relaxed geometries were optimized until the forces were below 0.05 eV/Å, a value that has been used frequently elsewhere.37−39 The molecular graphics were prepared with CrystalMaker for Windows version 9.0.3.40

A depiction of bulk t-ZrO2 is shown in Figure 3. When the crystal structure of t-ZrO2 (space group P42/nmc) is built from

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RESULTS AND DISCUSSION Catalyst Characterization. Nitrogen physisorption revealed a significant decrease of surface area due to zirconium modification of silica support (Aerosil 380, 380 m2/g). The final ZrO2−SiO2 supported oxide shows a specific surface area of 218 m2/g. The characterization by XRD clearly shows the presence of tetragonal ZrO2 with peaks at 2θ ∼ 30.1°, 34.5°, 50.3°, and 60.0°, as can been seen in Figure 2. Line broadening

Figure 3. Unit cell of bulk t-ZrO2 crystal. Zr atoms are shown in blue, O atoms linked strongly to the central Zr atom in dark red, and O atoms linked weakly to the central Zr atom in red.

its experimentally determined lattice parameters a = 3.64 Å and c = 5.27 Å,35 every zirconium atom is linked to eight oxygen atoms, four of them with a bond length of 2.065 Å, while the other four have a bond length of 2.455 Å, which implies a weaker bond than the former. As we will discuss later, these differences lead to the formation of two types of zirconium and oxygen surface atoms. Table 1 presents the computed lattice Table 1. Comparison between Experimental and Calculated Bulk Parameters and Bond Lengths for Bulk t-ZrO2a parameter/length

expb [Å]

theor [Å]

deviation [%]

a c dc Zr−O (w) Zr−O (s)

3.64 5.27 0.065c 2.463 2.065

3.63 5.24 0.047c 2.391 2.104

0.27 0.57 27.7 2.92 −1.89

a

Figure 2. X-ray diffraction pattern of the ZrO2−SiO2 supported oxide.

dc denotes the displacement of oxygen atoms in the c-direction with respect to the position in the ideal cubic phase. bExperimental values were extracted from Teufer’s study.35 cValues in fractional coordinates.

analysis with the Scherrer equation on the (101) peak indicates an average crystallite size of 30 nm. The broad signal centered at ∼22° in 2θ is due to the amorphous silica used as support. The predominance of the tetragonal phase over the thermodynamically more stable monoclinic phase has been reported previously for this type of synthesis and crystallite size under 30 nm.41 Moreover, some studies shown that only lowindex crystal planes, primarily the most favored (101) facet, are the most frequently exposed.42,43 Theoretical Surface Models. Given that the amorphous silica support is practically inactive, the model surface for our calculations is composed of a periodic slab of zirconium oxide in its tetragonal form (t-ZrO2). On the basis of the experimental evidence presented above, we mainly modeled the (101) facet of t-ZrO2 in our DFT calculations. The SiO2 support is mesoporous, which should not restrict molecular orientation, hinder molecular diffusion, or introduce confinement effects for small molecules, as it occurs in microporous catalysts. Therefore, we anticipate the zirconia surface to be freely accessible to small molecule adsorption and not influenced by curvature of the support pore structure.

constants and main bond lengths for tetragonal zirconia crystal. As can be seen in this table, a very good agreement was obtained between the values calculated in this work and those determined experimentally. Cleaving the bulk of t-ZrO2 along the (101) plane yields the primary t-ZrO2 (101) unit cell shown in Figure 4. The surface dimensions of this cell are 6.374 × 3.63 Å. The t-ZrO2 (101) plane contains two different types of Zr and O surface atoms. The first type, labeled Zrs and Os, refers to atoms lacking one strong (short) Zr−O bond. The second type, labeled Zrw and Ow, refers to the lack of one weak (long) Zr−O bond.44 These different atoms are thought to have different capacity to bind an external molecule and hence different acidic properties. When the t-ZrO2 (101) structure is optimized from its bulk structure, the atomic positions of the relaxed layers remain close to their original positions. However, a further rearrangement of the surface occurs after ammonia adsorption. These atomic positions were used as the initial guess for a subsequent optimization of the clean surface. Figure 5 shows the configuration of the optimized 2 × 3 supercell after this procedure. In this model there are 12 Zr and 12 O atoms C


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Figure 4. Primary unit cell t-ZrO2 (101). Zrs in dark blue, Zrw in light blue, Os in dark red, Ow in pink, and subsurface Zr and O atoms in gray and red, respectively.

exposed. As can be seen in Figure 5c, the most disrupted atoms on surface are the Os atoms, reducing the strength of one Zrs− Os bond by elongating this bond to 3.013 Å, which is significantly longer than Zr−O bonds in the bulk t-ZrO2 crystal. The implication is that some Zrs and Os atoms are expected to show an increase in reactivity. In addition to sites residing on flat surfaces, real catalysts almost invariantly present a considerable amount of sites exposed at edges and steps. In order to test the ability of edge Zr atoms to attract NH3, we also generated a slab with the (201) facet of t-ZrO2 exposed. The primary cell of this surface has the dimensions 11.09 × 3.63 Å. Figure 6 shows a representation of the 1 × 3 supercell of the t-ZrO2 (201) slab, where three edge zirconium atoms are 6-coordinated with four Zr−O bonds within the range 2.21−2.28 Å and two strong Zr− O bonds within 2.03−2.05 Å. Besides these zirconium atoms, the slab is composed of six other zirconium atoms 7coordinated, where three of them have lost one weak Zr−O bond and the three others have lost one strong Zr−O bond. Surface Oxygen Vacancy Formation. Although the ZrO2−SiO2 is calcined with oxygen at high temperatures to oxidize the surface, it is possible to have oxygen vacancy defects at the surface, which can alter the surface properties even in low concentrations. For this reason we have considered computational models with oxygen vacancies on the flat (101) facet and the (201) stepped surface as shown in Figures 7 and 8, respectively. The vacancy formation energies in each case are shown in Table 2 and calculated according to the equation 1 ΔEvac = Evac + EO2 − Eslab 2

Figure 5. Top (a) and side (b) view of the 2 × 3 supercell of t-ZrO2 (101) facet. Zrs atoms in dark blue, Zrw in light blue, Os in dark red, Ow in pink, and subsurface Zr and O atoms in gray and red, respectively. Supercell axes are shown in dotted blue lines. Depicted Zr−O bonds are limited to a length of 2.5 Å. Surface atom displacements with respect to bulk positions in t-ZrO2 (101) facet are shown in (c), in which the displacement vectors are scaled by a factor of 3 for increased emphasis.

where Evac, EO2, and Eslab are the total energy of the slab with a vacancy, the gas phase oxygen molecule, and the clean slab, respectively. We note that the formation energy of an Os vacancy (544.9 kJ/mol) requires less energy than the formation of an O w vacancy (615.8 kJ/mol). While this seems counterintuitive at first, it is consistent with the surface reconstruction shown in Figure 5c; on the reconstructed surface one Zrs−Os is bond is significantly elongated, rendering the involved Zrs and Os atoms noticeably more reactive. Figure 7a shows the final configuration of such an Os surface vacancy, leaving three 6-coordinated zirconium surface atoms behind. This result is in good agreement with previous work. Safonov et al.45 performed embedded cluster calculations for oxygen vacancy formation on the t-ZrO2(101) surface using Hartree−

Fock and DFT (B3LYP) levels, obtaining a vacancy formation energy of 550 kJ/mol, and Ganduglia-Pirovano obtained 529 kJ/mol using periodic boundaries with the PAW method and a 400 eV basis set.46 Surface relaxation effects are not limited to the topmost layer only. We find that upon Os vacancy formation subsurface oxygen atoms may diffuse to the surface to partially heal the surface defect and further decrease the required vacancy formation energy. Two examples of this scenario are depicted in Figures 7b and 7c where the subsurface oxygen atom is highlighted in yellow. While the diffusing oxygen atom loses one bond with a subsurface zirconium atom, it forms two new bonds to the closest Zr surface atoms, thereby lowering the vacancy formation energy to 488.8 and 515.8 D


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Figure 6. Side view of the optimized (201) facet model slab with four layers. Edge zirconium atoms are shown in dark gray; surface atoms follow the same color code as in Figure 5.

kJ/mol for configurations in Figures 7b and 7c, respectively. Similar considerations on the t-ZrO2 (201) surface (Figure 8) show that the oxygen vacancy formation at the coordinatively unsaturated step edge requires only 469.2 kJ/mol. While not directly observed in our calculations, it may be postulated that partial vacancy healing with subsurface oxygen may also occur on the step sites, which would result in even lower energy requirements for the defect formation. Characterization of ZrO2−SiO2 Acidity. Experimental ammonia adsorption microcalorimetric results are shown in Figure 9, where the differential heat of adsorption Qdiff is plotted vs the amount of ammonia adsorbed on the catalyst sample. Differential heats of adsorption and computed energies of adsorption are exothermic but will be shown in this work as

Figure 8. Top (a) and side (b) views of the 1 × 3 supercell of the tZrO2 (201) surface with one oxygen vacancy. Atoms are shown in the same colors as in Figure 6.

positive values for ease. Figure 9 shows an initial heat of adsorption of ∼165 kJ/mol, which drops to 50 kJ/mol when a coverage of ∼1.1 μmol/m2 is achieved. This is the region where ammonia is expected to adsorb exclusively on the surface of ZrO2, since it is well-known that amorphous SiO2 has very

Figure 7. The 2 × 3 supercell of the t-ZrO2(101) surface showing (a) a surface Os vacancy, (b) and (c) diffusion of one subsurface oxygen atom (yellow atom) to the surface layer due to presence of a surface vacancy. Zrs appears in dark blue, Zrw in light blue, Os in dark red, Ow in pink, and subsurface Zr and O atoms in gray and red, respectively. E


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by the bands at 1445 and 1608 cm−1, while pyridine adsorbed on Brønsted acid sites (1540 and 1640 cm−1; labeled P−B in figure) has bands of considerably lower intensity that practically disappear at 550 °C. This is in agreement with other experimental reports,52−54 where the acid sites in t-ZrO2 are mainly of the Lewis type. The band at ∼1595 cm−1 is sometimes interpreted as pyridine linked to weak Lewis acid sites.18 It can be observed that the bands due to pyridine bonded to Lewis acid sites are still present after heating to 550 °C, presumably due to the presence of strong Lewis acid sites. The adsorption of NH3 at different coverages on t-ZrO2 (101) was modeled including slabs of 3 × 5, 3 × 4, 2 × 3, and 2 × 2 times the primary unit cell slab. Regardless of the size of the supercell, the molecular adsorption of ammonia is preferred on a Lewis acid site near a Zrs site (see Supporting Information). In the case of the defect-free 2 × 3 supercell, the most stable configuration results in NH3 bound to a Zrs (Lewis acid site) atom via the electron lone pair of nitrogen, releasing 115.3 kJ/ mol of energy. The dissociative adsorption of ammonia forming both Zrs−N and Os−H bonds is also possible, yet is with an energy release of only 88.7 kJ/mol less likely. Brønsted acid sites were also tested for ammonia adsorption, but only a very weak interaction was observed (less than 40 kJ/mol, data not shown). This is consistent with our DRIFTS-Py results on the ZrO2−SiO2 sample, in which primarily strong Lewis acid sites are occupied.

Table 2. Vacancy Formation Energies on (101) and (201) Facets facet

oxygen type

Figure

ΔEvac [kJ/mol]

101 101 101 101 201

Os Os Os Ow edge

5a 5b 5c n/a 6

+544.9 +488.8 +515.8 +615.8 +469.2

Figure 9. Ammonia adsorption microcalorimetry results of ZrO2− SiO2 sample at 473 K. Dotted lines mark the position of Qdiff = 50 kJ/ mol and nads = 1.1 μmol/m2 as reference.

weak acidity (2 μmol NH3/m2). However, the measured initial heat values (0−2 μmol/m2 coverage) are significantly higher than the calculated values. This can be explained by the presence of O defect sites as discussed below. The decrease in the adsorption energy due to lateral NH3− NH3 interactions is consistent with the charge density difference analysis displayed in Figure 13, where we compare the adsorption of one, two, and three ammonia molecules on the 2 × 3 supercell of the clean t-ZrO2 (101) facet. In Figure 13a (one NH3 in the supercell), the electronic redistribution due to NH3 adsorption does not extend far enough to affect the adsorption of neighboring NH3 molecules. In the case of two NH3 molecules adsorbed on the 2 × 3 slab (Figure 13b) every adsorbate has two nearest-neighbor NH3 molecules, and the electron density response extends somewhat to neighboring electronic clouds. However, the shape of the electron density difference cloud remains essentially unaltered. On the other hand, with three ammonia molecules coadsorbed on the 2 × 3

Figure 12. Comparison between experimental ammonia adsorption microcalorimetry results (●, where x = exposed ZrO2 area/total BET area of catalyst) and DFT results on the t-ZrO2(101) facet at different coverage in open symbols; adsorption tested on the 3 × 5 (◇), 3 × 4 (△), 2 × 3 (○), and 2 × 2 (□) supercells.

DFT simulation results, we assumed that only 15% of the total surface area is accessible ZrO2. Without overemphasizing the quantitative agreement we rationalize this assumption by calibrating the theoretical and experimental results based on the transition of NH3 adsorption on Zrs sites to Zrw sites or SiO2. At the transition, the NH3 binding energy is ca. 50 kJ/ mol, which occurs at approximately 1.1 μmol/m2 in the microcalorimetry experiments (Figure 9) and 7.2 μmol/m2 on the ideal ZrO2 (101) DFT model surface (Figure 11). This calibration changes the number of available sites with a given energy; i.e., it rescales the x-axis of Figure 9, but not the binding energies themselves. Given the experimental noise and flat slope of the differential binding energy change near 50 kJ/mol in Figure 9, the determination of 1.1 μmol/m2 as critical

Figure 13. Electron density difference plots upon ammonia adsorption on the t-ZrO2 (101) facet at different coverages seen from the side and top. One, two, and three ammonia molecules are adsorbed in the 2 × 3 supercell in images labeled a, b and c, respectively. Dashed black lines represent the limits of the repeated supercell. Blue and red contours represent areas of depleted and accumulated electron density, respectively. The contours are chosen to be 0.002 e/Å3. G


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The Journal of Physical Chemistry C slab (Figure 13c) there is a clear perturbation of neighboring adsorbates in the electron density response associated with a decrease in adsorption energy. The corresponding adsorption energies calculated from the configurations mentioned above are shown in the first three points (open circle markers) of Figure 12. The electron density difference analysis strongly suggests that the abrupt decrease in adsorption energy going from two (112 kJ/mol) to three (88 kJ/mol) NH3 molecules adsorbed on the 2 × 3 slab is consistent with the electroninduced perturbation due to lateral interactions. As mentioned above, the DFT and experimental results agree well for moderate and weak acidity (in the range of 115−50 kJ/ mol in NH3 adsorption energy), and the decrease in binding energy is well explained by electronic perturbations of neighboring adsorbates. Yet, the experimentally observed adsorption energies higher than 115 kJ/mol are not reproduced on the O defect-free ZrO2 (101) surface. To resolve this discrepancy, we first considered stepped surfaces, which are generally more reactive than flat surfaces. But our test calculations using the stepped t-ZrO2 (201) surface yield a NH3 binding energy to a 6-coordinated Zr edge atom of only 116 kJ/mol, similar to the value obtained for the flat (101) facet. Alternatively, oxygen vacancy defect sites may be present on the ZrO2−SiO2 catalyst, and we considered NH3 adsorption to Zr sites adjacent to an oxygen vacancy. For this configuration we calculated that the dissociative adsorption of ammonia releases up to 190 kJ/mol, over either the terrace (101) or the stepped (201) surfaces. Over the (101) facet with Os oxygen vacancy, NH2 bridges two zirconium atoms (one Zrs and one Zrw) via nitrogen, while the remaining hydrogen binds to a Zrs in the most stable configuration as shown in Figure 14. On the

Figure 15. Top view of most stable configuration of NH3 dissociatively adsorbed on the stepped (201) surface. N in green, H in white, edge Zr in dark blue, the rest of surface Zr atoms in light blue, and O in red.

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CONCLUSIONS The nature of the acid sites present on the SiO2-supported ZrO2 catalyst was characterized using XRD, DRIFTS-Py, microcalorimetry, and DFT simulations. The initial heat of NH3 adsorption from microcalorimetry measurements is 165 kJ/mol and rapidly decreases for increasing coverage. DRIFTSPy characterization using pyridine as probe molecule shows that the surface acidity is primarily of Lewis nature. DFT calculations indicate that NH3 binds molecularly with ΔEads up to 120 kJ/mol (zero coverage limit) to Lewis sites on the defect-free t-ZrO2 (101) and t-ZrO2 (201) surfaces. The experimentally observed decrease in binding energy is quantitatively reproduced for higher coverages and is consistent with lateral NH3−NH3 interactions as supported by electron density difference analyses. Higher experimentally obtained differential heats of adsorption at low coverage (>120 kJ/mol) are in agreement with the dissociative adsorption of ammonia near an oxygen vacancy site as supported by DFT calculations. Overall, the experimental and theoretical evidence strongly suggests that the strongest Lewis acid sites are associated with oxygen vacancies in t-ZrO2. These results demonstrate that carefully conducted DFT calculations in combination with experimental validation are necessary to provide a detailed description of local bonding relationships and energetics of acid sites on supported zirconia.

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Figure 14. Top view of most stable configuration of NH3 dissociatively adsorbed on the (101) terrace surface. N in green, H in white, Zrs in dark blue, Zrw in light blue, Os in dark red (only shown in bonds), Ow in pink (only shown in bonds), and subsurface O in red.

ASSOCIATED CONTENT

S Supporting Information *

Total energies of gas phase ammonia, slabs, and stable adsorption configurations; an alternative superposition of computational and experimental adsorption energies. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02394.

(201) surface the most stable configuration for the dissociative adsorption of ammonia (Figure 15) involves the binding of NH2 to an edge Zr atom adjacent to an oxygen vacancy, and hydrogen binds to a contiguous edge Zr atom, which is also adjacent to the vacancy. Dissociative adsorption at strong acid sites is consistent with our microcalorimetry results, which indicated irreversible adsorption of NH3 as described earlier. Overall, the DFT results show good agreement with experimental measurements and aid in the interpretation of our experimental observations. The combination of theory and experiment suggests that highly acidic sites on ZrO2 can be attributed to oxygen vacancies present on surface.

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AUTHOR INFORMATION

Corresponding Authors

*(M.-G.C.-G.) E-mail [email protected]; Ph +52(444)826 2440 ext 6558. *(L.C.G.) E-mail [email protected]; Ph +1 (713) 743 4326. Notes

The authors declare no competing financial interest. H


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ACKNOWLEDGMENTS Financial support for this work was provided by C13-FAI-0305.05 (UASLP). Juan M. Arce is grateful for support from CONACYT scholarship 317312. The authors acknowledge the use of the Maxwell/Opuntia Cluster and the advanced support from the Center of Advanced Computing and Data Systems (CACDS) at the University of Houston to carry out the research presented here. Additional resources were provided by the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy, under Contract DEAC02-05CH11231.

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