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
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1、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 Ch
2、emistry,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
3、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 Cr2O7clust
4、erscould 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 ato
5、m 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
6、 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 insig
7、ht 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”Autho
8、r 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 inhere
9、nt 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 cat
10、alytic 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 th
11、e 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
12、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 todifferentia
13、te 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 diffe
14、rent 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
15、 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 Oradic
16、al 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,b
17、ut 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 wi
18、th 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
19、 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 aff
20、ected 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
21、 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/
22、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
23、 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 clust
24、er 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
25、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-ord
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