Gas-Phase Reactions of Carbon Dioxide with Copper Hydride Anions

reduce CO2 into CO.22-29 Transition metal hydride ions MHq (such as FeD+, NiD+, ... The copper hydride cluster anions were generated by laser ablation...
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Cite This: J. Phys. Chem. C 2018, 122, 19379−19384

Gas-Phase Reactions of Carbon Dioxide with Copper Hydride Anions Cu2H2−: Temperature-Dependent Transformation Yun-Zhu Liu,†,‡ Li-Xue Jiang,†,‡ Xiao-Na Li,*,†,§ Li-Na Wang,†,‡ Jiao-Jiao Chen,†,‡ and Sheng-Gui He*,†,‡,§

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State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Beijing National Laboratory for Molecular Sciences, CAS Research/Education Center of Excellence in Molecular Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: The hydrogenation of carbon dioxide into value-added chemicals is of great importance for CO2 recycling. However, the underlying mechanism of CO2 hydrogenation remains elusive owing to the lack of experimental evidence for the formation of the C−H bond. Herein, the gas-phase reaction of copper hydride anion Cu2H2− with CO2 at variable temperatures (∼300−560 K) was investigated. Metal hydrides are the ideal models to study the nature of C−H bond formation in CO2 hydrogenation, while the related studies are scarcely reported, particularly for the hydrogenation reactions at temperatures above 300 K. The generation of formate (HCO2−) attached on product CuH2CO2− was identified by temperature-dependent mass spectrometric experiments and density functional theory calculations. Temperature played crucial roles to fine-tune the product selectivity, from Cu2H2CO2− that dominates the room-temperature reaction into CuH2CO2− at elevated temperatures. The nature behind the temperature-dependent product selectivity and the mechanism of CO2 hydrogenation has been interpreted by using theoretical calculations. The combined experimental and computational studies have provided solid evidence for the formation of formate attached in CuH2CO2−. metal hydride ions MHq (such as FeD+, NiD+, CoH−, NiH−, Cu2H−, and AgH2− 30−34) that have been widely studied in the gas phase are the ideal models to study the nature of C−H bond formation in hydrogenation reactions, while few studies on the reactivity of metal hydrides toward CO2 have been reported.35−38 The reactions of [Cp2TiH]+ 35 and PtH3− 36 with CO2 result in the insertion of CO2 into the M−H bond of metal hydrides to form metal formate complexes. The thermal activation of CO2 by CuH2− anions to generate the coordinated formate has been proposed.37 Recently, we have identified the direct generation of formate anion HCO2− as a product in the reaction of FeH− with CO2, providing convincing evidence for the generation of the C−H bond.38 It is noteworthy that these reported reactions were usually studied under room-temperature conditions. The activation and transformation of CO2 at elevated temperatures are important to narrow the gaps between the gas-phase reactions and the related condensed-phase reactions, while such studies have been scarcely reported.39 Temperature has been demonstrated to play pivotal roles to promote and tune the

1. INTRODUCTION The transformation of CO2 into value-added chemicals, such as formic acid and methanol, is greatly related to energy storage, fuel production, and the problem of global warming.1−3 CO2 is thermodynamically stable with the fully oxidized state of carbon (C4+), making its direct conversion challenging.4,5 Transition metals have been widely utilized to catalyze CO2 transformation,6−15 and several iron11−13 and copper14,15 based catalysts exhibit exceptional reactivity comparable to noble metals. The C−H bond formation has long been regarded as the core to transform CO2 into chemicals, while the fundamental insights into this elementary step are still under debate because of the lack of experimental evidence of C−H bond formation. Such knowledge on the structures of catalytically active sites and the reaction mechanisms is pivotal for rational design of efficient catalysts for CO2 conversion under ambient conditions. Gas-phase ion−molecule reactions performed under isolated conditions have been intensively investigated to reveal the mechanistic details of related condensed-phase systems.16−20 The activation and transformation of CO2 by gas-phase clusters have attracted a growing interest in recent years.21 Bare metal cations M+ and metal oxide cations MO+ (e.g., M = Y, Zr, Mo) generally reduce CO2 into CO.22−29 Transition © 2018 American Chemical Society

Received: May 31, 2018 Revised: July 23, 2018 Published: August 8, 2018 19379

DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384

Article

The Journal of Physical Chemistry C

Figure 1. Upon the interaction of Cu2H2− with CO2 at 298 K (Figure 1b), adsorption products Cu 2 H 2 CO 2 − and

product selectivity of condensed-phase CO2 transformation.40,41 Herein, we report the gas-phase reaction of the copper hydride anion Cu2H2− with CO2 at variable temperatures (298−559 K). The results show that the increased reaction temperature can switch the product formation of Cu2H2CO2− at room temperature to the formation of CuH2CO2− at 559 K, indicating the possible formation of stable formate species in CuH2CO2−. This temperaturedependent product selectivity and the mechanism of CO2 hydrogenation mediated by Cu2H2− have been interpreted by using theoretical calculations.

2. METHODS 2.1. Experimental Methods. The copper hydride cluster anions were generated by laser ablation of a rotating and translating copper disk in the presence of pure H2 (100%) with the backing pressure of 3.0 standard atmospheres. Different percentages (10, 20, 40, and 100%) of H2 seeded in He carrier gas have been tested to generate copper hydride (Figure S1, Supporting Information), and the experimental results show that pure H2 as carrier gas can generate the best signal of Cu2H2− for further mass-selection. The Cu2H2− cluster was mass-selected using a quadrupole mass filter, and then entered into a high-temperature linear ion trap (LIT) reactor, where they were confined and heated to a controlled temperature by collisions with a pulse of preheated He buffer gas for about 1.8 ms and then interacted with CO2 for about 3.2 ms. The instantaneous molecule density of He in the reactor was estimated to be 3.5 × 1015 molecule cm−3 at a temperature of 298−559 K, which means that the Cu2H2− anions had been thermalized (by a number of collisions around 5300) to the defined temperature of LIT before they reacted with CO2.42,43 The cluster ions ejected from the high-temperature LIT were detected by a reflectron time-of-flight mass spectrometer (TOF-MS). The details of running the TOF-MS,44 the quadrupole mass filter,45 and the high-temperature LIT46 can be found in our previous works. 2.2. Computational Methods. Density functional theory (DFT) calculations using Gaussian 09 software47 were carried out to investigate the structures of Cu2H2− clusters and the mechanisms of the reaction with CO2. The 6-311+g* basis sets48,49 for Cu, H, O, and C atoms were used. In order to find an appropriate functional, the bond dissociation energies of Cu−H, Cu−Cu, Cu−O, Cu−C, O−CO, and H−COOH were computed by various functionals and compared with available experimental data (Table S1). It turns out that the TPSS functional50 was the best overall. A Fortran code51 based on the genetic algorithm was used to generate initial guess structures of the clusters. The initial guess structures of transition states were obtained through relaxed potential energy surface scans and were optimized by using the Berny algorithm.52 Vibrational frequency calculations were performed to check that the intermediates or transition states have zero and only one imaginary frequency, respectively. Intrinsic reaction coordinate53,54 calculations were performed so that each transition state connects two appropriate local minima.

Figure 1. TOF mass spectra for the reactions of mass-selected Cu2H2− with He at 298 K (a), CO2 at 298 K (b), 351 K (c), 446 K (d), and 559 K (e) for around 3.2 ms. The maximum molecule density of reactant gases is about 1.8 × 1014 molecules cm−3.

Cu2H2(CO2)2− (reactions 1a and 1b) and the product CuH2CO2− corresponding to the signal with the evaporation of a single Cu atom (reaction 2) could be observed. At T = 351 K, the relative ion intensities of products Cu2H2(CO2)1,2− decreased greatly, while the intensity of product peak CuH2CO2− slightly increased (Figure 1c), indicating that most of the products Cu2H2(CO2)1,2− might desorb CO2 into the gas phase to regenerate Cu2H2− rather than transform into CuH2CO2−. With the further increase of temperatures, the relative ion intensity of CuH2CO2− increased gradually, while the adsorption products Cu2H2(CO2)1,2− disappeared at T = 559 K (Figure 1c−e). Cu 2H 2− + CO2 → Cu 2H 2CO2− −

Cu 2H2 + 2CO2 → Cu 2H2(CO2 )2 −



(1a) −

Cu 2H 2 + CO2 → CuH 2CO2 + Cu

(1b) (2)

The pseudo-first-order rate constants (k1, in unit of 10−12 cm3 molecule−1 s−1) for the reaction of Cu2H2− with CO2 at different temperatures can be well fitted (Figure S2, Supporting Information), and the determined values are shown in Figure 2a. It can be seen that the temperaturedependent rates are non-Arrhenius. The k1 value at T = 298 K is 1.7 × 10−12 cm3 molecule−1 s−1, while the k1 value decreases to 1.2 × 10−12 cm3 molecule−1 s−1 at T = 351 K and then increases gradually with the further increase of temperature. Adsorption product Cu2H2CO2− contributes mainly to the reaction at T = 298 K, while Cu2H2CO2− prefers to desorb CO2 when T > 298 K. This behavior rationalizes the decrease of reaction rate with the increase of temperature from 298 to 351 K. The gradually increased generation of CuH2CO2− contributes to the increased reaction rate at elevated temperatures. The theoretical collision rate constant (kcollision)55 upon the reaction of Cu2H2− with CO2 is 6.9 × 10−10 cm3 molecule−1 s−1; thus, the reaction efficiency (k1/

3. RESULTS 3.1. Experimental Results. The time-of-flight (TOF) mass spectra for the interaction of laser ablation generated, mass-selected, and thermalized Cu2H2− cluster anions with CO2 in the temperature range from 298 to 559 K are shown in 19380

DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384

Article

The Journal of Physical Chemistry C

path I), during the process of which CO2 is activated with a change of O−C−O angle from 180° in isolated CO2 to 143° in I1. At the same time, the related Cu−H bond elongates from 155 pm in free Cu2H2− to 163 pm in I1. The next step is accompanied by the rupture of one Cu−H bond and the final formation of the C−H bond with a tiny barrier of 0.03 eV (I1 → TS1 → I2) to form the Cu2H(HCO2)− complex. Then, the reaction is completed by the evaporation of a Cu atom from Cu2H(HCO2)− [P1: −0.09 eV at the CCSD(T) level]. In path II, the CO2 molecule can be trapped by the Cu site with a large binding energy of 1.05 eV (I3), and then, the simultaneous cleavage of Cu−C and Cu−H bonds to form Cu2H(HCO2)− (I2) has to overcome a barrier up to 1.43 eV (I3 → TS2 → I2), and this barrier can hardly be suppressed at room temperature. Moreover, the intermediate with the formation of a single Cu−O bond between CO2 and Cu2H2− has also been considered, while such a species with all positive vibrational frequencies cannot be located. On the basis of the equation of Maxwell−Boltzmann distribution,56,57 the percentage of CO2 molecules with enough kinetic energies to overcome TS2 can be estimated and the results are shown in Figure 4a. At T = 298 K, a negligible percentage (∼10−6 %) of CO2 molecules have enough kinetic energy to overcome TS2 to form Cu2H(HCO2)−. As can be seen in Figure 3, both I2 and I3 can be the possible candidates of the experimentally observed adsorption product Cu2H2CO2−. Note that the increased temperature disfavors the stabilization of an adsorption complex and leads to the formation of separated reactants or products, depending

Figure 2. Rate constants (a) and product branching ratios (b) at different temperatures on the reaction of Cu2H2− with CO2. The rate constant (k1) is in unit of 10−12 cm3 molecule−1 s−1.

kcollision) at room temperature is about 0.25%. The branching ratio (BR) of product CuH2CO2− (reaction 2) increases at higher temperatures (Figure 2b), while the BR of Cu2H2CO2− decreases, suggesting a switch of the product selectivity when the temperature increases. This temperature-dependent behavior reveals in most cases the fact that the adsorbed CO2 in Cu2H2(CO2)1,2− can desorb back into gas-phase CO2. 3.2. Theoretical Results. The DFT calculated structures of Cu2H2− and CuH2CO2− species are shown in Figure S3 (Supporting Information), and the mechanism of reaction Cu2H2− + CO2 is shown in Figure 3. The most stable Cu2H2− isomer is in the doublet state and contains a terminal H atom. As shown in Figure 3, the CO2 molecule can approach this terminal H atom directly with a binding energy of 0.21 eV (I1,

Figure 3. Density functional theory calculated potential energy profiles for reaction Cu2H2− + CO2 on the doublet state. The relative energies are given in eV, and bond lengths are given in pm. The zeropoint vibration corrected energies (ΔH0, eV) with respect to the separated reactants are given. The values in the square brackets are the single point energies calculated at the CCSD(T) level.

Figure 4. (a) Percentage of CO2 molecules with enough kinetic energy to overcome the barriers of TS2 (0.38 eV) and (b) the product (0.28 eV). (c) DFT calculated natural charges on the hydrogen atom, CO2, and the Cu2H unit during C−H bond formation in I1 (Figure 3, path I). The bond lengths are given in pm. 19381

DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384

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Cu2H2− anion. Natural bond orbital analysis (Figure 4c) indicates that, when CO2 approaches the H atom, significant negative charge (−0.57 e) is transferred from Cu2H2− to CO2 in I1 and then the formation of the Cu2H(HCO2)− complex is subject to a tiny barrier. This behavior parallels the mechanism of the gas-phase reaction of FeH− anion with CO238 and the mechanism of CO2 hydrogenation by iron based catalysts.68 The generation of isolated HCO2− through direct rupture of the Cu−H bond (I1) or the Cu−O bond (I2) has also been considered. The energy of the separate products (Cu2H + HCO2−) is high (0.94 eV, Figure S5) with respect to that of Cu + CuH2CO2− (0.28 eV, Figure 3). This can be due to the much stronger Cu−H (2.60 eV) and Cu−O (2.94 eV) bonds with respect to the Cu−Cu bond (2.05 eV).69 Furthermore, intermediates I1 and I2 have relatively large dipole moments (3.50 D for I1 and 3.19 D for I2) with respect to the isolate Cu2H2− (1.73 D). The larger dipole moment can promote electron flow70 from the negatively charged Cu atom (−0.46 e in I1 and −0.51 e in I2) that is on the other site of the HCO2 unit into product CuH2CO2−. Such electron flow facilitates the final evaporation of the neutral Cu atom from the reaction complex.

on their relative energies. I3 prefers to dissociate back into separated reactants (I3 → Cu2H2− + CO2) at higher temperatures because the overall positive barrier of 0.38 eV can hardly be overcome. In contrast, I2 can evaporate the Cu atom and transform to products (I2 → CuH2CO2− + Cu) because of the comparable thermodynamics of separated reactants and products. The adsorption of a second CO2 molecule on I2 and I3 has been calculated (Figure S4, Supporting Information), and the results indicate that the exposed Cu site in I2 or the terminal H atom in I3 can capture a second CO2 molecule to form the weak signal of product Cu2H2(CO2)2− (Figure 1b). Thus, the strong signal of product Cu2H2CO2− may be contributed by both I2 and I3. Figure 4b shows that, at T = 298 K, only a small number of CO2 molecules (∼10−3 %) can have enough kinetic energy to overcome the DFT calculated barrier of 0.28 eV to form CuH2CO2− (I2 → CuH2CO2− + Cu), indicating that most of the reaction intermediates will dissociate back into Cu2H2− + CO2. With the increase of temperature, CO2 has an increased possibility to react with Cu2H2− to form CuH2CO2−. The possibility increases to about 6.13% at T = 559 K (Figure 4b). This can well interpret the gradual increase of the rate in transforming Cu2H2− + CO2 to Cu + CuH2CO2− (Figure 2a) from T = 395 to 559 K. At higher temperatures, the possibility of transforming I3 to CuH2CO2− cannot be completely ruled out because an increased percentage of CO2 molecules (0.48%, T = 559 K) can overcome TS2.

5. CONCLUSIONS The reaction of the copper hydride anion Cu2H2− with CO2 under variable temperatures (∼300−560 K) has been studied. This study reports the first gas-phase reaction of metal hydride species with CO2 at high temperatures to the best of our knowledge. The elevated temperature could fine-tune the product selectivity gradually from Cu2H2CO2− to CuH2CO2−. This temperature-dependent experiment proposed that CO2 is hydrogenated in product CuH2CO2−, which was supported by the quantum chemistry calculations. Direct hydride transfer from Cu2H2− to the C atom of CO2 is the most favorable pathway for the C−H bond formation. This study provides the molecular-level insights into the effect of temperature on product selectivity and the mechanism of formate formation in the reaction of Cu2H2− with CO2.

4. DISCUSSION In the condensed-phase studies for CO2 utilization, the temperature effect on the product selectivity has been readily explored.40 In the gas-phase studies, the high-temperature experiments have been rarely reported and the available examples typically explored are the reaction systems of metal oxide clusters.58−62 Recently, the reaction of FeC3− anion with CH4 at high temperatures (300−610 K) was reported and the production of C2H2 via C−C coupling was identified.57 The reaction of CO2 on liquid Al100+ species that was preheated in the cluster source was reported by Jarrold and co-workers.39 Herein, we demonstrate that the elevated temperature (T > 300 K) can easily desorb CO2 into the gas phase from product Cu2H2CO2− and tune the reaction gradually to the formation of CuH2CO2−. This high-temperature experiment indicates that CO2 may be molecularly bonded to the cluster and Cu2H2CO2− does not readily to transform to product CuH2CO2−. The gradually increased intensity of product CuH2CO2− with the increase of temperature provides evidence for the formation of a hydrogenation species (COOH or HCO2) in CuH2CO2−. Several mechanisms of CO2 hydrogenation have been proposed for condensed-phase systems.63−67 The approach of the electrophilic CO2 to the vacant metal site through η2coordination followed by the migration of H atom to CO2 is a widely recognized mechanism to generate the formate species,35,63,66 and this mechanism can well account for the hydrogenation of CO2 by [Cp2TiH]+ 35 and PtH3−.36 In this study, we demonstrate that the direct hydride transfer from Cu2H2− to the C atom of CO2 is the most favorable pathway to form the formate species CuH(HCO2)−. Such a direct hydrogen transfer mechanism has been much less reported.65,68 To have a better understanding of the C−H bond formation, a relaxed potential energy curve scan was performed for the approach of CO2 to the H atom in the



ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b05216. Additional experimental and theoretical results (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-62536990. Fax: +8610-62559373. *E-mail: [email protected]. Phone: +86-10-62536990. Fax: +86-10-62559373. ORCID

Xiao-Na Li: 0000-0002-0316-5762 Sheng-Gui He: 0000-0002-9919-6909 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21573246 and 21773254) and the Beijing Natural Science Foundation (2172059). X.19382

DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384

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The Journal of Physical Chemistry C

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N.L. is thankful for the grant from the Youth Innovation Promotion Association, Chinese Academy of Sciences (2016030).



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DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.8b05216 J. Phys. Chem. C 2018, 122, 19379−19384