ARTICLE pubs.acs.org/JPCC
Adsorption of CO2 on a PdO(101) Thin Film Jose A. Hinojosa, Jr.,† Abbin Antony,† Can Hakanoglu,† Aravind Asthagiri,‡ and Jason F. Weaver*,† † ‡
Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611, United States William G. Lowrie Department of Chemical & Biomolecular Engineering, The Ohio State University, Columbus, Ohio 43210, United States
bS Supporting Information ABSTRACT: We investigated the adsorption of CO2 on PdO(101) using temperature-programmed desorption (TPD) and dispersion-corrected density functional theory calculations (DFT-D3). We find that CO2 desorbs from PdO(101) in two main TPD peaks (ε1 and ε2) centered at ∼176 and 120 K and estimate that the average binding energies in the ε1 and ε2 states are 54 ( 8 and 37 ( 5 kJ/mol, respectively. The CO2 layer saturates on PdO(101) at a total coverage of ∼0.47 monolayer (ML), with the CO2 molecules distributed nearly evenly between the two adsorbed states. TPD experiments of CO2 adsorption onto preadsorbed layers of pure 18O2 and H2O provide evidence that the ε1 and ε2 states correspond to CO2 molecules adsorbed on the coordinatively unsaturated (cus) vs saturated (4f) Pd sites of the PdO(101) surface, respectively. DFTD3 calculations predict CO2 saturation coverages and binding energies on PdO(101) that agree well with our experimental estimates. The calculations suggest that CO2 prefers to adsorb initially on a cus-Pd site in an upright, linear configuration and that CO2 dimers and trimers form on the cus-Pd row as the CO2 coverage increases. The dimers and trimers consist of CO2 molecules adsorbed in alternating bent and linear geometries. The calculations predict that CO2 trimers saturate the cus-Pd row at a coverage of 0.26 ML and that further increases in coverage cause CO2 to physisorb on the 4f-Pd sites until the CO2 layer saturates at a total coverage of 0.52 ML. DFT-D3 indicates that dispersion and electrostatic interactions govern the binding of CO2 on the cus-Pd rows of PdO(101).
’ INTRODUCTION The surface chemistry of late-transition-metal oxides is central to many applications of oxidation catalysis, including the catalytic combustion of natural gas, exhaust gas remediation in automobiles, and fuel cell catalysis. Under oxygen-rich conditions, various types of oxide phases can develop on transition-metal surfaces which can cause significant changes in catalytic behavior. Of particular relevance to the present study are reports that bulklike PdO is highly active toward the complete oxidation of methane and that the formation of PdO is responsible for the favorable performance of Pd as an oxidation catalyst under oxygen-rich conditions.1 Investigations with supported Pd catalysts also demonstrate that the combustion products can have deleterious effects on the catalytic activity,24 which motivates further study of the adsorption of CO2 and H2O on PdO surfaces. We have recently undertaken studies of the surface chemistry of a high-quality PdO(101) thin film on Pd(111) and find that the complete oxidation of n-alkanes (>C2) is facile on this PdO surface, with initial CH bond cleavage occurring below 200 K.57 An implication of these findings is that the PdO(101) surface is representative of active surfaces which develop on supported-Pd catalysts in oxidizing environments. Investigations of the surface chemistry of PdO(101) thus have potential to provide fundamental insights into applications of Pd-catalyzed r 2011 American Chemical Society
oxidation chemistry. In the present study, we extend our investigations with PdO(101) to the adsorption of CO 2 in an effort to further clarify the binding of small molecules on this surface. The adsorption characteristics of CO2 on well-defined transitionmetal oxides share common features among the systems that have been investigated in detail. For example, experiments using temperature-programmed desorption (TPD) commonly reveal multiple CO2 desorption peaks below 200 K arising from physisorbed and weakly bound chemisorbed species.812 Characterization using surface vibrational spectroscopy has provided detailed information about the nature of CO2 adsorbed on oxide surfaces. For example, vibrational spectra obtained from CO2 adsorbed on both TiO2(110)8 and Cr2O3(0001)9 reveal that the weakly bound adsorbates remain in linear geometries, implying limited charge transfer between the molecules and the surface. CO2 also adsorbs on Cr2O3(0001) as a carboxylate species (CO2δ) and achieves stronger binding to the surface compared with the linear species. The carboxylate species constitute a significant fraction of the CO2 desorption yield from Cr2O3(0001) and remain stable to above room temperature. Received: October 30, 2011 Revised: December 14, 2011 Published: December 20, 2011 3007
dx.doi.org/10.1021/jp2104243 | J. Phys. Chem. C 2012, 116, 3007–3016
The Journal of Physical Chemistry C Investigations with RuO2(110) also report weakly bound adsorbed states of CO2 that desorb below 200 K during TPD as well as a small fraction of CO2 which desorbs at 315 K.10,11 In this case, vibrational spectroscopy reveals that two CO2 desorption features between 175 and 190 K arise from a chemisorbed CO2δ species and CO2:CO2δ dimers. The carboxylate species which form on RuO2(110)10 are bound more weakly than carboxylate on Cr2O3(0001)9 and exhibit binding energies that are comparable to those of the linear CO2 species identified on TiO2(110).8,12 The CO2 desorption feature at 315 K obtained from RuO2(110) has been attributed to a carbonate species,10,11 in good agreement with computational predictions.13 In contrast, only small quantities of carbonate have been observed following CO2 adsorption on TiO2(110),8 and carbonate formation was not observed on Cr2O3(0001) in the work of Seiferth et al.9 Taken together, these prior studies demonstrate that similar configurations of adsorbed CO2 can exhibit wide variations in binding energies among different transition-metal oxides and that the tendency for carbonate formation depends strongly on the nature of the oxide surface. In this study, we investigated the adsorption of CO2 on a PdO(101) surface using both TPD experiments and dispersioncorrected DFT-D3 calculations. We find that CO2 prefers to adsorb on the cus-Pd row as dimers and trimers and begins to physisorb on the 4f-Pd sites after the cus-Pd row saturates. The DFT-D3 predictions agree well with our experimental estimates of the binding energies and saturation coverages of CO2 adsorbed on the cus-Pd and 4f-Pd sites of PdO(101). The good quantitative agreement between experiment and computation demonstrates that the DFT-D3 method is capable of accurately describing coverage effects in molecular adsorption.
’ EXPERIMENTAL DETAILS Previous studies14,15 provide details of the three-level ultrahigh vacuum (UHV) chamber utilized for the present experiments. The Pd(111) crystal employed in this study is a circular disk (8 mm ∼1 mm) spot-welded to W wires and attached to a copper sample holder that is held in thermal contact with a liquid nitrogen cooled reservoir. A type K thermocouple spot-welded to the backside of the crystal allows for sample temperature measurements. Resistive heating, controlled using a PID controller that varies the output of a programmable dc power supply, supports maintaining or linearly ramping the sample temperature from 85 to 1250 K. Initially, sample cleaning consisted of sputtering with 600 eV Ar+ ions at a surface temperature of 900 K, followed by annealing at 1100 K for several minutes. Subsequent cleaning involved routinely exposing the sample held at 856 K to an atomic oxygen beam for several minutes, followed by flashing the sample to 923 K to desorb oxygen and carbon oxides. As discussed previously,16 we limited the sample temperature to 923 K to maintain oxygen saturation in the subsurface reservoir and thereby ensure reproducibility in preparing the PdO(101) thin films used in this study. We considered the Pd(111) sample to be clean when we could no longer detect contaminants with X-ray photoelectron spectroscopy (XPS) and did not detect CO production during temperature-programmed desorption after oxygen adsorption. A two-stage differentially pumped chamber attached to the UHV chamber houses the inductively coupled RF plasma source (Oxford Scientific Instruments) utilized to generate beams containing
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oxygen atoms for this study. We refer the reader to prior work for details of the differentially pumped beam system.14,15 To produce a PdO(101) thin film, we expose a Pd(111) sample held at 500 K to an ∼22 ML dose of gaseous oxygen atoms supplied in a beam, where we define 1 ML as equal to the Pd(111) surface atom density of 1.53 1015 cm2. This procedure generates a high-quality PdO(101) film that has a stoichiometric surface termination, contains ∼4.0 ML of oxygen atoms, and is ∼16 Å thick. We refer the reader to the Computational Details section for further details pertaining to the structure of the PdO(101) surface. The PdO(101) films employed in the present study exhibit identical structural and chemical properties as those investigated previously17,18 but are about one layer thicker (∼0.7 ML of O atoms). We note that the Pd(111) sample is positioned ∼50 mm from the final collimating aperture and oriented at a 45° angle from the axis of the atomic oxygen beam during PdO film preparation. For these dosing conditions, we observe negligible gradients in the oxygen concentration across the oxidized Pd(111) sample, which suggests that the PdO films are uniformly thick across the substrate. CO2 (Airgas, 99.99%) was delivered to the PdO(101) surface at 85 K using a calibrated beam doser with the sample located ∼50 mm from the end of the doser to ensure uniform impingement of the gas across the surface. After the CO2 exposures, we collected TPD spectra by positioning the sample in front of a shielded quadrupole mass spectrometer at a distance of ∼10 mm followed by heating at a constant rate of 1 K s1 until the sample temperature reached 500 K. Limiting the sample temperature to 500 K prevents the PdO(101) thin films from thermally decomposing and allows a single film to be used in several CO2 TPD experiments. To confirm that the PdO(101) surface structure remains unaltered, we routinely collected TPD spectra after preparing a given CO2 coverage on a fresh PdO(101) film and those used in multiple TPD experiments and observe negligible differences. We also investigated CO2 adsorption onto preadsorbed layers of 18O2 (Isotec, 99 at. %) and H2O using TPD, the goal being to selectively block CO2 adsorption onto specific surface sites with the preadsorbed species. We used filtered and deionized water for the experiments and further purified the H2O using several freezepumpthaw cycles prior to use. We used the 18O2 at the purity supplied by the vendor. For these experiments, we initially adsorbed 18O2 or H2O in varying quantities on PdO(101) and subsequently exposed the surface to a saturation dose of CO2, with the surface held at 85 K. After preparing a coadsorbed layer, we conducted a TPD measurement in the manner described above. We calibrated CO2 desorption yields by monitoring the production of CO2 during CO oxidation on oxygen-covered Pd(111) and using a limiting amount of adsorbed atomic oxygen such that the CO2 yield is equal to the initial atomic oxygen coverage of 0.25 ML. We have previously found that this approach provides accurate and consistent estimates of the CO2 desorption yields.5,6,19 We estimate 18O2 desorption yields by equating a saturation 18O2 coverage prepared at 85 K on the PdO(101) surface to a value of 0.29 ML, as previously reported.20 We estimate H2O coverages from the TPD data by assuming that the first layer of H2O on PdO(101) saturates at a coverage of 0.70 ML, consistent with prior work.21
’ COMPUTATIONAL DETAILS The dispersion-corrected DFT (DFT-D3) calculations presented in this paper were performed using the Vienna ab initio 3008
dx.doi.org/10.1021/jp2104243 |J. Phys. Chem. C 2012, 116, 3007–3016
The Journal of Physical Chemistry C simulation package (VASP)2225 integrated with the recently reported “D3” method of treating dispersion in DFT calculations as developed by Grimme and co-workers.26 In the DFT-D3 method, dispersion forces and energies (Fdisp and Edisp) are empirically determined and added to the ionic forces (Fdft) and energy (Edft) determined by conventional DFT. Initial calculations show that DFT-D3 provides a more accurate description of molecular adsorption on metal surfaces than the earlier DFT-D1 and D2 methods.26 Key improvements included in the DFT-D3 method are more accurate estimates of the C6 parameters from time-dependent DFT calculations and the use of fractional coordination numbers to allow the C6 parameters to vary with the local chemical environment. We use the projector augmented wave (PAW) pseudopotentials27,28 provided in the VASP database and the Perdew BurkeErnzerhof (PBE) exchange-correlation functional29 with a plane wave cutoff of 400 eV to determine Fdft and Edft in our calculations. Parameters for the calculations are the same as those used in our earlier studies of the PdO(101) surface.6,21,30,31 Calculation of Edft is done using a residual minimization method with direct inversion in the iterative subspace for electronic relaxations accelerated using MethfesselPaxton Fermi-level smearing with a Gaussian width of 0.1 eV.32 The dispersion energy Edisp is determined using eq 1 where the pairwise interactions are summed over all of the atoms located within a cutoff radius of 50 Å. The additive term (Edisp) is a function of nth-order internuclear distances (rABn), dispersion coefficients (CAB n ) that have been computed ab initio by time-dependent DFT, a scaling factor (Sn) that ensures asymptotic exactness, and a damping function (fd,n(r)). These parameters have been directly taken from the most recent implementation of dispersion in DFT calculations by Grimme et al.26 " # 1 CAB CAB 6 8 Edisp ¼ S6 6 fd, 6 ðrAB Þ þ S8 8 fd, 8 ðrAB Þ ð1Þ 2 AB rAB rAB
∑
The total ionic forces (Fdft + Fdisp) are relaxed using a limited memory BroydenFletcherGoldfarbShanno optimization method33 until the forces on all unconstrained atoms are less than 0.03 eV/Å. Figure 1 illustrates the stoichiometric PdO(101) surface that is investigated in this study. Bulk crystalline PdO has a tetragonal unit cell and consists of square-planar units of Pd atoms 4-fold coordinated with oxygen atoms. The bulk-terminated PdO(101) surface is defined by a rectangular unit cell, where the a and b lattice vectors coincide with the [010] and [101] directions of the PdO crystal, respectively. The stoichiometric PdO(101) surface consists of alternating rows of 3-fold or 4-fold coordinated Pd or O atoms that run parallel to the a direction shown in Figure 1. Thus, half of the surface O and Pd atoms are coordinatively unsaturated (cus). The side view of PdO(101) shows that the coordinative environment associated with each cus-Pd atom resembles a square-planar Pd complex with a coordination vacancy directed away from the surface and three oxygen ligands, one of which is a cus-O atom. The areal density of each type of coordinatively distinct atom of the PdO(101) surface is equal to 35% of the atomic density of the Pd(111) surface. Hence, the coverage of cus-Pd atoms is equal to 0.35 ML, and each PdO(101) layer contains 0.7 ML of Pd atoms and 0.7 ML of O atoms. The PdO(101) model used in this computational study is the same as that used in earlier studies and consists of four layers corresponding to a thickness of ∼9 Å. Our earlier work has
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Figure 1. Model representation of the stoichiometric PdO(101) surface identifying coordinatively saturated (4f) and coordinatively unsaturated (cus) Pd and O atoms. The a and b directions correspond to the [010] and [101] crystallographic directions of PdO.
shown that this thickness is sufficient to accurately model the experimental PdO(101) film on Pd(111). The PdO(101) slab is strained to match the structure reported by LEED experiments17,18 (a = 3.057 Å, b = 6.352 Å). A vacuum spacing of 20 Å is maintained normal to the surface, and a 4 2 1 (2 4 1) MonkhorstPack k point mesh has been used for the 1 4 (2 2) cell size. Finer mesh sizes or increasing the number of layers does not result in significant changes in the adsorption energy (
