铬氧化物团簇负离子%28CrO_%283%29%29_%281-4%29O%5E%28-%29活化烷烃的尺寸效应.pdf

CHINESE JOURNAL OF CHEMICAL PHYSICSVOLUME 36,NUMBER 3JUNE 27,2023ARTICLESize-Dependent Reactivity of Chromium Oxide Cluster Anions(CrO3)14Otowards AlkanesGong-Ping Weia,b,c,Yan-Xia Zhaoa,c,Sheng-Gui Hea,b,ca.State Key Laboratory for Structural Chemistry of Unstable and Stable Species,Institute of Chemistry,Chinese Academy of Sciences,Beijing 100190,Chinab.University of Chinese Academy of Sciences,Beijing 100049,Chinac.Beijing National Laboratory for Molecular Sciences,CAS Research/Education Center of Excellencein Molecular Sciences,Beijing 100190,China(Dated:Received on March 30,2023;Accepted on May 15,2023)The reactivity of chromium oxide clus-ter anions(CrO3)14Otowards low car-bon(C1C4)alkanes has been experi-mentally investigated at 298 K by em-ploying a homemade ship-lock-type re-actor coupled with a time-of-flight massspectrometer.The results demonstratethat while CrO4and Cr2O7clusterscould abstract a hydrogen atom fromC2H6and CH4,respectively,Cr3O10and Cr4O13clusters were found to beinert towards n-C4H10within the de-tection limit.Theoretical results revealthat CrO4and Cr2O7clusters pos-sess delocalized chromium-bonded oxy-gen radicals(CrO),which rationalizes the hydrogen atom abstraction reactions between(CrO3)1,2Oclusters and alkanes.However,the active sites of(CrO3)3,4Oclusters evolveto peroxide species(O22),which exhibit inferior activity compared to Oradicals.Theincrease of Cr-3d orbital energy driven by the more negative charge around CrOyunit formedvia downsizing the cluster size has been proposed to account for favorable reduction of O22and selective generation of reactive Oradicals in small-sized(CrO3)1,2Oclusters.Thisstudy not only enriches the chemistry of metal-bonded Oradicals toward alkane activationunder mild conditions,but also provides a new insight into the conversion between O22andOradicals over metal oxides.Key words:Reactive oxygen species,Alkane activation,Chromium oxide cluster,Massspectrometry,Density functional theory calculationPart of the special topic for“the Chinese Chemical Societys17th National Chemical Dynamics Symposium”Author to whom correspondence should be addressed.E-mail:I.INTRODUCTIONAlkanes originating from biogenic and fossil sourcesrepresent important raw materials in the chemical in-dustry 1,2.It is extremely challenging to activatethe CH bonds under mild conditions in contempo-rary catalysis due to the inherent thermodynamic sta-bility and kinetic inertness of alkanes 37.MetalDOI:10.1063/1674-0068/cjcp2303027279c 2023 Chinese Physical Society280Chin.J.Chem.Phys.,Vol.36,No.3Gong-Ping Wei et al.oxides are extensively used as both catalysts and cat-alytic support materials for alkane conversion 710.The catalytic role of metal oxide surfaces lies in theirability to form and provide oxygen in activated states.Reactive oxygen species(ROS)such as O2(super-oxide radicals)1114,O22(peroxide species)15,16,and O(oxygen-centered radicals)1720 thatcan be formed during the dissociation of molecular oxy-gen on the metal oxide surfaces via the general schemeof O2O2O222O2O2(lattice oxygen)areconsidered as key intermediates for CH activation.The Oradicals could be reactive to initiate hydro-gen atom transfer(HAT)from alkanes,which was em-phasized as a crucial step for the oxidative coupling oflower alkanes to higher hydrocarbons 10,2124.Bycontrast,the O2and O22species were proposed tobe much less reactive than Oradicals 17.It is essen-tial to discover factors that can manipulate the selectivegeneration of Oradicals rather than O2radicals andO22species on metal oxides.However,it is difficult todifferentiate the local active sites with different struc-tures of metal oxides due to the complexity of condensedphase systems.Investigation on the atomically precise metal ox-ide clusters in gas phase provides a distinctive ap-proach to generalize the structure-activity relationshipof metal oxides with different compositions and sizes2530.Previous investigations found that the metaloxide clusters(MxOyq)featuring with 2y+qnx=1usually contain Oradical sites 31,in which n is thehighest oxidation state of the metal M and q is thecharge number of the clusters.The 3d5d metals lo-cated on the bottom-left side of the V-Mo-Re diago-nal line in the periodic table are classified as metals ofwhich the MxOyq(=1)clusters contain Oradicals31.For example,the=1 clusters of(Sc2O3)122+32,(TiO2)15+33,34,(V2O5)111+35,36,(Nb2O5)114+37,(Sc2O3)11838,(V2O5)131O39,40,and(MoO3)16O41 have been characterizedto possess Oradical sites and can abstract a hydro-gen atom from methane or other low-carbon alkanes.By contrast,the theoretical study about the oxide clus-ters of transition metals on the up-right side of the V-Mo-Re diagonal line predicted that the CrxOyqclusterswith=1(except CrO4)do not contain Oradicalsites 31,but there is no decisive experimental evidenceto support the proposal.In this context,the reactions of chromium oxide clus-ter anions(CrO3)14O(=1)towards C1C4alka-nes have been experimentally investigated by utilizingour newly-developed ship-lock type reactor,which canmeasure the cluster reactions with a low detection limitof 1018cm3molecule1s1.While the CrO4andCr2O7clusters can abstract a hydrogen atom fromethane and methane,respectively,the(CrO3)3,4Oclusters are unreactive with n-butane within the de-tection limit.Such a dramatically size-dependent reac-tivity reveals that the active sites of the(CrO3)1,2Oand(CrO3)3,4Oclusters are entirely different.Fur-ther DFT calculations show that the active sites of the(CrO3)1,2Oclusters could be characterized as Orad-icals,while the(CrO3)3,4Oclusters possess the less re-active ROS of O22,which could be attributed to lowerCr-3d orbital energies affected by less delocalized nega-tive charge within larger clusters.It is noteworthy thatsupported chromium oxide systems are active catalystsin a number of hydrocarbon reactions such as polymer-ization,dehydrogenation,and dehydrocyclization 42,43.II.METHODSA.Experimental methodsThe negatively charged chromium oxide clusters(CrxOy)were generated by using a laser ablationmethod.A pulsed 532 nm laser(second harmonic ofNd3+:YAG)with an energy of about 58 mJ and arepetition rate of 10 Hz was focused onto a chromiumdisk,the process of which could produce chromiummetal plasma that reacted with 1%5%O2/He to formCrxOycluster ions with different sizes and composi-tions.The clusters of interest(CrO3)14Owere mass-selected by using a quadrupole mass filter(QMF)andthen entered into the ship-lock type reactor(SLTR).Af-ter the ions were thermalized by collisions with a pulseof He gas(about 2 Pa)for about 20 ms,two separateelectromagnet valves were triggered to close the inletand the outlet of the SLTR for a gas-tight condition.A pulse of reactant gas including C1C4alkanes andtheir deuterated compounds(up to around 100 Pa)ex-cept for C2D6was then injected into the SLTR and in-teracted with the cluster ions.After reacting for about100 ms or 700 ms,the valves were moved to open theinlet and outlet of the SLTR to expel the high-pressurereactant gas until the pressure was low enough to ejectthe ions from the SLTR.The ions were then guided byDOI:10.1063/1674-0068/cjcp2303027c 2023 Chinese Physical SocietyChin.J.Chem.Phys.,Vol.36,No.3Reactivity of(CrO3)14Otowards Alkanes281a hexapole system to enter into a reflectron time-of-flight mass spectrometer(Re-TOF-MS).The details forrunning the Re-TOF-MS,QMF,and the SLTR can befound in our previous work 44.The rate constants(k1)of the pseudo-first-order reac-tions between(CrO3)14Oclusters and alkanes wereevaluated by the following equation:IR=exp(k1tR)(1)where IRis the relative intensity of the reactant clusterions,tRis the reaction time,and is the molecular den-sity of alkanes that can be calculated by=P/(kBT),in which P is the effective pressure of alkanes 44,kBis the Boltzmann constant,and T is the temperature(298 K).The systematic deviations of tR(3%),T(2%),P(20%),and the uncertainty of IRdeter-mined during fitting the k1values by Eq.(1)have beenconsidered to calculate the errors of k1values.For ki-netic isotope effect(KIE)values,both the systematicdeviations of tR(3%)and T(2%)can be neglected,thus,only the uncertainties of IRand P(20%)con-tribute to the errors of KIE values 45.B.Computational methodsDensity functional theory(DFT)calculations usingGaussian 16 program 46 were carried out to investi-gate the structures of reactant clusters(CrO3)14Oand the reaction mechanisms of(CrO3)1,2Owith CH4.Two different functionals of B3LYP 4749 and TPSS50 were tested(Table SI in Supplementary materials,SM)and adopted to inspect the ground state struc-tures of(CrO3)14O.The TZVP basis set 51 wasused for all the atoms.The isomeric structures ofCrO4and Cr2O7were obtained from the literatures31,52,53.A Fortran code based on a genetic al-gorithm 54 was used to search the global minimumstructures of(CrO3)3,4O.More than 500 structuresfor(CrO3)3,4Owere generated with coarse thresholds,among which more than 10 low-lying energy isomerswere reoptimized with strict thresholds.The lowest-lying optimized structures were considered as the iso-mers reacting with alkanes in the experiment.The re-action mechanism calculations involved geometry op-timization of reaction intermediates(Is)and transitionstates(TSs)through which the Is transfer to each other.The initial structures of Is and TSs along the reactionpathways were obtained by performing the relaxed po-tential energy surface scans.Vibrational frequency cal-culations were conducted to check that the Is and TSshave zero and only one imaginary frequency,respec-tively.Each TS connecting two appropriate local min-ima was confirmed through intrinsic reaction coordinatecalculations 55,56.The energies with zero-point vi-bration corrections(H0)and in unit of eV are reportedin this work 45.Natural charge analysis was calculatedby NBO 5.9 57.The Rice-Ramsperger-Kassel-Marcus(RRKM)the-ory and RRKM-based variational transition-state the-ory(VTST)58 were used to calculate the rate oftraversing transition states and CH4desorption fromadsorption complexes,respectively.For these calcula-tions,the energy(E)of the initially formed adsorptioncomplexes and the energy barrier(E=)were needed.The E value included the vibrational energies(Evib)of CrxOyand CH4,the center-of-mass kinetic energy(Ek),and the binding energy(Eb)between CrxOyand CH4.The values of Eviband Ebwere determinedby the DFT calculations and Ek=v2/2,in which is the reduced mass and v is the velocity(550 m/s)59.The densities and the numbers of states requiredfor RRKM and VTST calculations were obtained bythe direct count method 60 with the DFT calculatedvibrational frequencies under the approximation of har-monic vibrations.Graphical structures are presented by using VisualMolecular Dynamics(VMD)61.All spin density dis-tributions,orbitals,and surface electrostatic potentialswere plotted by using Multiwfn 62 and VMD.III.RESULTS AND DISCUSSIONA.Reactivity of(CrO3)14OclustersThe TOF mass spectra for the reactions of mass-selected cluster anions(CrO3)14Owith n-C4H10at298 K are plotted in FIG.1.Upon the interactionof CrO4with 0.93 Pa n-C4H10for about 100 ms(FIG.1(a2),the product peak assigned as CrO4Hbecomes more intense relative to the background sig-nal generated from the reaction with water impurity(FIG.1(a1),suggesting the occurrence of hydrogenatom abstraction(HAA)from n-C4H10to produce neu-tral C4H9radicals.Such a reaction channel also pre-vails in the reaction of Cr2O7with n-C4H10.More-over,the product ions of Cr2O7Hgenerated under alower n-C4H10gas pressure of 0.10 Pa(FIG.1(b2)have a similar abundance to CrO4Hproduced inDOI:10.1063/1674-0068/cjcp2303027c 2023 Chinese Physical Society282Chin.J.Chem.Phys.,Vol.36,No.3Gong-Ping Wei et al.FIG.1 TOF mass spectra for the reactions of(a,b)mass-selected(CrO3)1,2Owith n-C4H10/n-C4D10and(c,d)(CrO3)3,4Owith n-C4H10.The pressure of reactant gases and the reaction time are shown.The CrxOyX(X=H,D)species are labeled as+X.The peaks marked with asterisks are originated from the reactions with water impurities.the reaction of CrO4with a higher pressure of n-C4H10,indicating that Cr2O7is more reactive thanCrO4.The identification of evident deuterium atomabstraction(DAA)products(CrO3)1,2ODin the re-actions of(CrO3)1,2Owith n-C4D10(FIG.1(a3)and(b3)confirms the HAA reaction channel and alsosupports their relative activity.Differently,the reac-tions with much higher n-C4H10gas pressures(40 and38 Pa)for a longer reaction time(700 ms)reveal thatboth Cr3O10and Cr4O13clusters are unreactive to-wards n-C4H10(FIG.1(c2)and(d2).The exper-iments thus indicate a significant size-dependent re-activity of the(CrO3)14Oclusters in the order of(CrO3)2O(CrO3)O(CrO3)3,4O.In addition to the activation of n-C4H10,the ad-ditional experiments demonstrate that the reactive(CrO3)1,2Oclusters could also abstract an H atomfrom C2H6(7.5 and 4.5 Pa)and C3H8(4.4 and 0.34 Pa)under a reaction time of 100 ms(FIG.2(a2,a3)and(b2,b3).The HAA product Cr2O7Hwas even ob-served in the reaction of Cr2O7with the most inertalkane,CH4,under the conditions of a CH4gas pres-sure of 75 Pa and a reaction time of 700 ms(FIG.2(c2),which was confirmed by the characterization ofDAA product Cr2O7Din the reaction of Cr2O7with142 Pa CD4(FIG.2(c3).The reaction of CrO4withCH4has also been tested,however,it is difficult to ob-serve the product due to the strong scattering of CrO4ions with small mass by the high pressure(e.g.,10 Pa)CH4gas pulsed into the reactor.The experimentallyobserved reaction channels can be summarized as fol-lows:(CrO3)O+CmH2m+2(CrO3)OH+CmH2m+1,m=2 4(2)(CrO3)2O+CmH2m+2(CrO3)2OH+CmH2m+1,m=1 4(3)(CrO3)3,4O+n-C4H10 no reaction(4)Note that an evident peak assigned as Cr2O6alsoappears in the reactions of Cr2O7with CH4andCD4,which is supposed as collision induced dissocia-tion(CID)product confirmed by replacing CH4withDOI:10.1063/1674-0068/cjcp2303027c 2023 Chinese Physical SocietyChin.J.Chem.Phys.,Vol.36,No.3Reactivity of(CrO3)14Otowards Alkanes283FIG.2 TOF mass spectra for the reactions of(a)mass-selected CrO4with C2H6,C3H8,and C3D8,and(b,c)Cr2O7with C2H6,C3H8,C3D8,CH4,CD4,and Ar.The pressure of reactant gases and the reaction time are shown.TheCrxOyX(X=H,D)species are labeled as+X.The peaks marked with asterisks are originated from the reactions withwater impurities.Ar as the reactant gas(FIG.2(c4).Kinetic analysis of the depletion of(CrO3)1,2Oandthe formation of corresponding HAA products as a func-tion of alkane pressure was performed to quantify therate constants(k1)of the pseudo-first-order HAA re-actions(FIG.S1 in SM).As shown in Table I,the k1value for the reaction of CrO4with n-C4H10is deter-mined as 3.81014cm3molecule1s1,which is lowerthan that of the reaction between Cr2O7and n-C4H10(2.6 1013cm3molecule1s1).Such a reactivity dif-ference between CrO4and Cr2O7is also observedin the reactions with C2H6and C3H8(Table I).Thek1value for the reaction of Cr2O7with CH4is eval-uated as 3.71017cm3molecule1s1.The kineticisotope effects(KIE)fo