Gas-Phase Reaction Studies of Dipositive Hafnium and Hafnium

Article pubs.acs.org/JPCA

Gas-Phase Reaction Studies of Dipositive Hafnium and Hafnium Oxide Ions: Generation of the Peroxide HfO22+ Célia Lourenço,† Maria del Carmen Michelini,*,‡ Joaquim Marçalo,§ John K. Gibson,∥ and Maria Conceiçaõ Oliveira*,† †

Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal Dipartimento di Chimica, Università della Calabria, 87030 Arcavacata di Rende, Italy § Unidade de Ciências Químicas e Radiofarmacêuticas, IST/ITN, Instituto Superior Técnico, Universidade Técnica de Lisboa, 2686-953 Sacavém, Portugal ∥ Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡

ABSTRACT: Fourier transform ion cyclotron resonance mass spectrometry was used to characterize the gas-phase reactivity of Hf dipositive ions, Hf2+and HfO2+, toward several oxidants: thermodynamically facile O-atom donor N2O, ineffective donor CO, and intermediate donors O2, CO2, NO, and CH2O. The Hf2+ ion exhibited electron transfer with N2O, O2, NO, and CH2O, reflecting the high ionization energy of Hf+. The HfO2+ ion was produced by O-atom transfer to Hf2+ from N2O, O2, and CO2, and the HfO22+ ion by Oatom transfer to HfO2+ from N2O; these reactions were fairly efficient. Density functional theory revealed the structure of HfO22+ as a peroxide. The HfO22+ ion reacted by electron transfer with N2O, CO2, and CO to give HfO2+. Estimates were made for the second ionization energies of Hf (14.5 ± 0.5 eV), HfO (14.3 ± 0.5 eV), and HfO2 (16.2 ± 0.5 eV), and also for the bond dissociation energies, D[Hf2+−O] = 686 ± 69 kJ mol−1 and D[OHf2+−O] = 186 ± 98 kJ mol−1. The computed bond dissociation energies, 751 and 270 kJ mol−1, respectively, are within these experimental ranges. Additionally, it was found that HfO22+ oxidized CO to CO2 and is thus a catalyst in the oxidation of CO by N2O and that Hf2+ activates methane to produce a carbene, HfCH22+.

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INTRODUCTION Transition metal oxides play vital roles in catalytic, biological, and atmospheric processes. The study of the energetics of elementary metal oxide species in the gas-phase is essential to fully understand their properties. Many studies have been made by Bohme and co-workers during the past decade, examining the reactivity of many metal ions in the gas phase, including Hf+, with several oxidants, namely, O2,1 N2O,2 NO,3 CO2,4 and D2O.5 Armentrout and co-workers have used guided ion-beam tandem mass spectrometry to investigate the bond dissociation energies for M+−O species, including for M = Hf, following gas-phase reactions with O2 and CO.6 Recently, the unusual triply charged oxide ion HfO3+ was produced by O− ion-beam sputtering.7 Several matrix isolation infrared spectroscopic studies of the reactions of Hf metal atoms with oxidants have also been performed, as well as a number of spectroscopic studies of neutral and ionic Hf oxides.8−13 For doubly charged Hf2+ and HfO2+, there are no reports of gas-phase oxidation studies, and there are no thermodynamic values available for HfO2+ or HfO22+. We have previously used Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) to study the gas-phase oxidation reactions of several doubly charged actinide and lanthanide cations,14−17 © 2012 American Chemical Society

experiments that provided estimates of the ionization energies and bond dissociation energies of monoxides and dioxides. Recently, we reported a combined experimental and computational study of the synthesis and properties of TaO2+ and TaO22+ resulting from the investigation of gas-phase oxidation reactions of Ta2+.18 In the present work, we use a similar approach to examine the gas-phase reactivity of Hf2+ with oxidants and the thermochemistry of doubly charged Hf oxides. Density functional theory (DFT) calculations were performed to evaluate the structures, bonding, and energetics of HfO2+ and HfO22+.

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EXPERIMENTAL DETAILS The experimental procedures have been previously described,15 and a summary is presented here. The experiments were performed with an Extrel-Finnigan FT/MS 2001-DT 3-T FTICR mass spectrometer, controlled by a Finnigan Venus Odyssey data system. Doubly charged hafnium cations were produced by laser desorption/ionization using a SpectraReceived: September 5, 2012 Revised: November 21, 2012 Published: November 28, 2012 12399

dx.doi.org/10.1021/jp3088385 | J. Phys. Chem. A 2012, 116, 12399−12405

The Journal of Physical Chemistry A

Article

Table 1. Thermodynamics of Oxidants, XOa

a

XO

N2O

O2

CO2

NO

CH2O

CO

D[X−O] IE[XO]

167.1 ± 0.1 12.889 ± 0.004

498.4 ± 0.1 12.0697 ± 0.0002

532.2 ± 0.2 13.777 ± 0.001

631.6 ± 0.4 9.2642 ± 0.00002

751.5 ± 0.1 10.88 ± 0.01

1076.4 ± 0.5 14.014 ± 0.0003

From ref 31. D[X−O], 298.15 K, in kJ mol−1; IE[XO], 0 K, in eV.

Table 2. Efficiencies, Rate Constants, and Product Distributions at 298 K for Reactions of Hf2+a Hf2+

N2O

O2

CO2

NO

CH2O

0.51 [0.72] HfO2+/65% Hf+/5% HfN+/15% HfO+/15%

0.42 [0.47] HfO2+/65% HfO+/15% Hf+/20%

0.47 [0.63] HfO2+/95% HfO+/5%

0.55 [0.70] Hf+/100%

0.22 [1.07] Hf+/60% HfH+/10% HfO+/30%

The reaction efficiencies, k/kcol, are given; the values in brackets are the pseudo first-order reaction rate constants, k, in units of 10−9 cm3 molecule−1 s . The absolute values are considered accurate to ±50%; the relative values for comparative purposes are considered accurate to ±20%. The product distributions have uncertainties of ±10%. a

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relative rate constants for different reactions are estimated to be accurate to ±20%, while typical precisions for replicate measurements of the same reaction are ±10%. The background in the spectrometer mainly consisted of water and air, with base pressures of (1−2) × 10−8 Torr, that is, approximately an order of magnitude lower than the reagent pressures used. Care was taken to minimize the interference of residual water and oxygen in the oxidation reactions and all the reactions were compared with those occurring under background conditions.

Physics Quanta-Ray GCR-11 Nd:YAG laser operated at the fundamental wavelength (1064 nm). The reagent gases (N2O, O2, CO2, NO, CO, CH4, and H2) were commercial products (>99.9% purity) and used as supplied. Dry, gaseous CH2O was prepared by thermal decomposition under vacuum of commercial paraformaldehyde. The neutral reagents were introduced into the mass spectrometer through leak valves to pressures in the range of 5 × 10−8 to 5 × 10−7 Torr, and their purities confirmed from electron ionization mass spectra. The pressures were measured with a Bayard−Alpert type ionization gauge calibrated using standard reactions and corrected for the relative sensitivities of the gases. Isolation of the Hf2+ ions was achieved using single-frequency, frequency-sweep, or SWIFT excitation.19 The hafnium oxide ions, HfO2+ and HfO22+, when formed in sufficient amounts with a particular reagent, were also isolated using the same techniques. In some cases, the Hf oxide ions were produced by reaction of the Hf2+ with N2O introduced into the spectrometer through pulsed valves, to study their subsequent reactions with other reagents. The reactant ions were thermalized by collisions with argon introduced into the spectrometer through pulsed valves to pressures of ca. 10−5 Torr or through a leak valve to pressures in the range of (1−5) × 10−6 Torr. Experiments were performed at ∼298 K; thermalized reactants possess 3/2kT of relative translational energy and 3/2kT of relative rotational energy and are thus at ∼8 kJ mol−1 above zero. The reproducibility of the product distributions and reaction kinetics for different collisional cooling periods and/or collision gas pressures, as well as the linearity of the kinetics plots, indicated the thermalization of the reactant ions. Nonobservation of electron transfer from Ar to the reactant ions is another indication of the effectiveness of the thermalization procedure. The product distributions have uncertainties of ±10%. Rate constants, k, were determined from the pseudofirst-order decay of the relative signals of the reactant ions as a function of time at constant neutral pressures. The decays were followed until the relative intensity of the reacting dipositive ion had reached less than 10% of its initial intensity. Reaction efficiencies, k/kcol, were obtained using the collisional rate constants, kcol, from the theory of Su and Chesnavich,20 calculated using experimental molecular polarizabilities and dipole moments of the neutral reagents.21 The main source of uncertainty in the absolute rate constants is the pressure measurement, and errors of ±50% are assigned to them;

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COMPUTATIONAL DETAILS DFT computations were performed using the B3LYP22,23 hybrid functional. The LANL2TZ(f) basis set24−26 was used for Hf, whereas the triple-ζ with diffuse and polarization 6311+G(3df) basis set27,28 was used for O atoms. No symmetry constraints were imposed during geometry optimizations. Analytical frequencies were computed to verify that the optimized structures attained a minimum on the potential energy surface of the system and to evaluate the zero-point vibrational energy corrections to the electronic energies, which are included in all reported energies. Ultrafine pruned grids for numerical integration were employed. Restricted DFT was used for closed-shell molecules, whereas open-shell species were computed using unrestricted (spin-polarized) DFT. Different spin multiplicities were tested in order to establish the ground spin-state of the studied molecules. All calculations were carried out using Gaussian09 (revision B.01) package.29 Natural bond orbital (NBO) and natural population analysis (NPA) were performed using NBO 3.1 as implemented in Gaussian09.30

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RESULTS AND DISCUSSION Reactions of Hf2+ with Oxidants: Formation of HfO2+. The O-atom affinities (OA ≡ D[X−O]) and the ionization energies (IE) of the oxidants (XO) used in this study are presented in Table 1. Values at 298.15 K have been used throughout the work with the exception of spectroscopically determined IEs, which correspond to 0 K values. No thermal corrections were applied when combining 298.15 and 0 K values as the differences of a few kJ mol−1 are significantly smaller than the uncertainties involved in estimates of thermodynamics quantities. The results obtained for the reactions of Hf2+ with these oxidants are summarized in Table 2 as reaction efficiencies, k/kcol, absolute reaction rate constants, k, and product 12400



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distributions. Figure 1 shows, as an example, the kinetics of the reaction of Hf2+ with CO2. The Hf2+ ions are efficiently

Figure 2. Kinetics for the reaction HfO2+/O2 at 298 K (1.0 × 10−7 Torr O2; ca. 3 × 10−6 Torr Ar). Figure 1. Kinetics for the reaction Hf /CO2 at 298 K (1.5 × 10 Torr CO2; ca. 3 × 10−6 Torr Ar). 2+

product in the reaction of Hf2+ with N2O is presented in Figure 3. It was further demonstrated that isolated and thermalized

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oxidized to HfO2+ by N2O, O2, and CO2, which have IEs above 12 eV (see Table 1). With NO, the oxidant with the lowest IE, electron transfer to give Hf+ was the only reaction channel observed. Electron transfer from a neutral molecule to a dipositive ion requires substantial exothermicity (i.e., −ΔHrxn > 1−2 eV),32−34 and therefore, the absence of electron transfer from CO2 is in agreement with its IE, only ca. 1.1 eV below IE[Hf+] = 14.9 ± 0.5 eV,35 the latter value being revised slightly downward below. Charge separation channels were observed with N2O, leading to HfN+ and HfO+, and with O2, CO2, and CH2O to produce HfO+. In addition, hydride transfer to form HfH+ (10%) was also observed in the reaction of Hf2+ with CH2O. The observed oxidation reactions of the Hf2+ ions, summarized in Table 2, bear a general similarity with those previously observed in the case of Ta2+,18 with the main differences arising from the higher second ionization energy of Ta (IE[Ta+] = 15.8 ± 0.3 eV)18 and thus a greater propensity for electron transfer to Ta2+. Oxidation of HfO2+: Formation of HfO22+. The reactions of HfO2+ with N2O, O2, and CO2 were studied to evaluate the oxidation of HfO2+. With O2 electron transfer was the only reaction channel observed, as indicated in Table 3, while CO2 was unreactive. However, O-atom transfer was observed for N2O, yielding the dipositive ion HfO22+. Figure 2 displays, as an example, the kinetics of the reaction of HfO2+ with O2 . A mass spectrum showing the formation of HfO22+ as a secondary

Figure 3. Mass spectrum for the Hf2+/N2O reaction showing the formation of HfO2+ and HfO22+ (2.5 × 10−7 Torr N2O; ca. 3 × 10−6 Torr Ar; reaction time 0.1 s); the insert shows a mass spectrum in the absence of N2O (ca. 3 × 10−6 Torr Ar, reaction time 0.7 s).

HfO2+ reacts with N2O to produce HfO22+; as shown in Table 3, the sole product of the HfO2+/N2O reaction is HfO22+. The Hf+, HfN+, and HfO+ peaks in Figure 3 are from the Hf2+/N2O reaction, whereas the HfO2+ peak is due to electron transfer from N2O to HfO22+ (see below). From the study of the reactions of Hf+ and HfO+ ions with N2O, it was established that the HfO+ and HfO2+ peaks also arise from the oxidation of the singly charged ions, with efficiencies k/kcol = 0.20 (k = 0.14 × 10−9 cm3 molecule−1 s−1) and k/kcol = 0.19 (k = 0.13 × 10−9 cm3 molecule−1s−1), for Hf+ and HfO+, respectively; these results are in agreement with those previously reported by Bohme and co-workers.2 Furthermore, the secondary products HfO+ and HfO2+ react with background water to yield HfO2H+. In the recently reported reaction of TaO2+ ions with N2O,18 electron transfer was the major channel (70%) but TaO22+ was also formed. This ion was shown to be a bent dioxide with an oxygen-centered radical structure by DFT computations.18 In the case of HfO22+, the most plausible structure is a peroxide, {Hf-(η2-O2)}2+, in which Hf(IV) is in its most stable oxidation state. This structure prevails for late transition metals, which do not support high oxidation states.12,36 Several neutral and anionic Hf peroxide species have recently been described based

Table 3. Efficiencies, Rate Constants, and Product Distributions at 298 K for Reactions of HfO2+a HfO2+

N2O

O2

0.31 [0.44] HfO22+/100%

0.31 [0.34] HfO+/100%

a The reaction efficiencies, k/kcol, are given; the values in brackets are the pseudo first-order reaction rate constants, k, in units of 10−9 cm3 molecule−1 s−1. The absolute values are considered accurate to ±50%; the relative values for comparative purposes are considered accurate to ±20%.

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on spectroscopy studies.11,13 Both neutral HfO2 and anionic HfO2− have bent dioxide structures as indicated by spectroscopy and computations.12 Computational Studies: Geometry and Electronic Structure of HfO2+ and HfO22+. Our computations indicate that the HfO2+ ground state (GS) has a 1Σ+ electronic state and a bond distance of 1.639 Å. The natural partial charge on the metal atom is 2.68 and the natural electron configuration is [core]6s0.015d1.326p0.03. NBO analysis of the HfO2+ GS shows a triple bond formed from the interaction of almost pure metal dorbitals. In particular, the σ-type orbital is formed from an s(2%)−p(3%)−d(95%) Hf hybrid orbital and an s(8%)− p(92%) O hybrid orbital, whereas the π-type orbitals are formed from the interaction of pure d metal atomic orbitals and pure p oxygen atomic orbitals. Oxygen contributes the largest percentage to these NBOs (up to 79%). Geometry optimizations for HfO22+ were performed taking into account three possible structural isomers, namely, the sideon (η2) and end-on (η1) bonded metal dioxygen complexes, and the inserted metal dioxide molecule. Different initial structures were chosen for each of the considered conformations; singlet and triplet spin states were analyzed for each of them. The HfO22+ GS has an η2 structure with the C2v point group symmetry and the 1A1 electronic state. The computed O−O bond distance (dO−O = 1.516 Å) is typical of peroxides. The HfO22+ GS optimized structure is shown in Figure 4 together

from pure p oxygen orbitals and pure d metal orbitals. NPA analysis indicates a charge of 2.68 and a [core]6s0.085d1.236p0.03 natural population on the Hf atom. A comparison between the NBO of HfO2+ and HfO22+ shows that the contribution of Hf to the bonding is noticeably lower in the latter; i.e., the contribution to the σ-bonds is 24% in the first and 17% in the second. Reactions of HfO22+: Oxidation of CO and Catalytic Oxidation of CO by N2O. HfO22+ reacted by electron transfer with N2O, CO2, and CO to give HfO2+, as indicated in Table 4. Table 4. Efficiencies, Rate Constants, and Product Distributions at 298 K for Reactions of HfO22+a HfO22+

N2O

CO2

CO

0.33 [0.46] HfO2+/100%

0.24 [0.32] HfO2+/100%

0.32 [0.44] HfO2+/60% HfO2+/40%

a The reaction efficiencies, k/kcol, are given; the values in brackets are the pseudo first-order reaction rate constants, k, in units of 10−9 cm3 molecule−1 s−1. The absolute values are considered accurate to ±50%; the relative values for comparative purposes are considered accurate to ±20%. The product distributions have uncertainties of ±10%.

Figure 5. Kinetics for the reaction HfO22+/N2O at 298 K (2.0 × 10−7 Torr N2O; ca. 3 × 10−6 Torr Ar). Figure 4. Ground-state (GS) and low-energy HfO22+ isomers. Relative energies (RE) are given with respect to the ground-state isomer. Bond lengths are given in angstroms and angles in degrees.

Figure 5 shows the kinetics of the reaction of HfO22+ with N2O. The HfO22+/CO2 and HfO22+/CO reactions were studied by isolating Hf2+, introducing N2O through pulsed valves to produce HfO22+, isolating it, and allowing it to react with CO2 or CO present at constant pressure. It was also found that, concomitantly with electron transfer (40% branching ratio), HfO2+ was formed in the reaction of HfO22+ with CO (60% branching ratio), demonstrating the oxidation of CO to CO2 by HfO22+. Combining the oxidation of CO to CO2 with the oxidation of HfO2+ to HfO22+ by N2O, it becomes evident that the HfO2+/HfO22+ couple can act as a catalyst in the oxidation of CO by N2O. Several examples of metal−ion mediated gas-phase catalytic oxidations, mainly involving N2O, have been reported in the past decade.37,38 We have previously shown that several doubly charged metal oxides, namely, LaO2+, GdO2+, LuO2+, CmO2+,17 PaO22+,16 and

with two low-energy structural isomers: a 3B1 dioxide cation having a folded structure (bond angle 86.8 degrees) that is 147 kJmol−1 higher in energy than the GS peroxide isomer, and a superoxide isomer (dO−O = 1.327 Å), which is 151 kJmol−1 higher in energy than the GS structure. NBO analysis of the HfO22+ GS structure confirms the presence of a single O−O bond formed from almost pure (96%) oxygen p orbitals. The bonding between Hf and O is very polarized toward oxygen, as shown by high contribution of oxygen (up to 87%) to the NBOs. The Hf−O σ-type bonds are formed from s(10%)−d(90%) Hf hybrid orbitals and s(7%)− p(93%) oxygen hybrids, whereas the π-type bonds are formed 12402



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TaO22+,18 mediate the catalytic oxidation of CO by N2O. The overall catalytic cycle for the HfO2+/HfO22+ couple is presented in Figure 6, where the O-transfer processes are indicated in

Table 5. Electron-Transfer Efficiencies for Dipositive Metal Ionsa M2+/IE[M+] (eV)

N2O (12.89 eV)

CO2 (13.78 eV)

CO (14.01 eV)

O2 (12.07 eV)

Sn2+/14.63 Pb2+/15.03 Mn2+/15.64 Ge2+/15.93 Bi2+/16.69

0.010 0.11 0.44 0.55 0.40

Gas-Phase Reaction Studies of Dipositive Hafnium and Hafnium

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