The catalytic activation of primary alcohols on niobium oxidesurfaces unraveled: the gas phase reactions of NbxO

ÿy clusters

with methanol and ethanol

Phillip Jackson *, Keith J. Fisher, Gary D. Willett 1

The School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia

Received 12 September 2000

Abstract

The reactions of oligomeric niobium oxide anions (up to Nb6Oÿ15), generated by laser ablation and studied using a

Fourier transform ICR mass spectrometer, have been used to deduce the roles of (i) Nb(III,IV,V) centers, (ii) Nb/O

double bonds and (iii) proximal Nb centers, in the catalytic activation of methanol and ethanol. The most important

recurring mechanism involves initial alcohol condensation at a cluster metal-oxygen double bond to yield

Nb(OH)(OCH3). There is no change in the oxidation state of the cluster during this step. The so-formed niobium-

hydroxyl bond is the new reactive site in the cluster, and undergoes ligand switching in a follow-up collision to yield a

bis-methoxy cluster and neutral water. Dehydrogenation is only observed to occur with clusters possessing two Nb/O

double bonds at a single metal center, and involves reduction of the participating Nb(V) center to Nb(III). An ion

ejection/selection step was used to monitor the activity of a number of the ionic reaction products towards the alcohols,

and in most instances spontaneous or kinetically-activated decomposition resulted in regeneration of the parent cluster

from the substituted species. Ó 2000 Elsevier Science B.V. All rights reserved.

Keywords: Metal oxide anion clusters; Catalysis; Ion±molecule; Fourier transform ICR mass spectrometry

1. Introduction

As the techniques of nanoscale engineering areimproved, the reality of tailoring particle sizesfor optimal performance in catalytic processes isnearer to fruition. The cost bene®ts that this mighto�er to large-scale operations are potentially enor-mous. Given that the importance of such devel-

opments is widely recognized, the study of metallicclusters in the gas phase by physical scientistshas exploded over the last 15 years [1±3]. Some-what surprisingly though, most research activity ofcatalytic relevance has concentrated on bare orligated metal cations [4±8]. Recently, with the ad-vent of commercial cluster sources [9], researchersare now probing much larger positively chargedmetallic clusters via ion±molecule chemistry [10±12], but still there are only a handful of articlesdealing with the ion±molecule chemistry of metal-containing cluster anions [13±20]. Notably, therecent work of Shi and Ervin [21] has demon-strated that anionic platinum clusters will transfer

Chemical Physics 262 (2000) 179±187

www.elsevier.nl/locate/chemphys

* Corresponding author. Present address: Research School of

Chemistry, Australian National University, Canberra, ACT

0200, Australia.

E-mail address: g.willett@unsw.edu.au (G.D. Willett).1 Also corresponding author. Fax: +61-2-9385-6141.

0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.

PII: S03 0 1-0 1 04 (0 0 )0 03 0 4- 9

a chemisorbed oxygen atom to an incoming COmolecule.

The dearth of metal anion studies is undoubt-edly due to the di�culty of synthesizing ``lowelectron a�nity'' species in the gas phase. Anadded complication with gas phase metal studies is®nding a suitably volatile precursor. This problemis exempli®ed for the refractory d1±d6 elements, forwhich researchers often resort to high tempera-tures and chemically extreme conditions to gener-ate a su�cient particle ¯ux for spectroscopicanalysis. These are exactly the conditions underwhich anions, particularly those that might con-tain metals, are rarely observed.

In the laser ablation Fourier transform ion cy-clotron resonance [22] (LA-FT/ICR) mass spec-trometer employed in this study, violent/rapidnon-equilibrium heating, by means of a focusedlaser beam, is used to generate a ¯ux of NbxOy

material from a pressed Nb2O5 disk. The crucialdi�erence between this approach and other vari-ants is that the solid target is ablated in the ICRcell of a FT/ICR mass spectrometer [23], so thata fraction of the photoejected electrons, which ina conventional cluster source are deposited onthe walls of the source, are instead trapped by thecombination of electric and magnetic ®elds in theICR cell. This renders the charges available forcapture by any gaseous species with a positiveelectron a�nity. We have demonstrated that suchin cell laser ablation is particularly useful forgenerating anionic transition metal chalcogenideclusters [16,17], and has recently been applied tothe study of anionic homonuclear main-groupmetal clusters [19,20].

2. Experimental

In the LA-FT/ICR mass spectrometry experi-ments, powdered niobium oxide (Nb2O5) fromAldrich was compressed to a thickness of 2 mm ina detachable cylindrical (r � 5 mm, h � 10 mm)stainless steel FT/ICR probe tip. Experiments wereperformed on a Spectrospin CMS-47 mass spec-trometer equipped with a 4.7 T superconductingmagnet and a 128 K, 24 bit Aspect 3000 computer[23,24]. Typical, for the LA-FT/ICR MS experi-

ments, the 1064 nm Q-switch fundamental of aspectra-physics DCR-11 Nd-YAG laser was fo-cused on to an area of �0.1 mm2 on the surface ofthe sample. The laser power was measured with aScientech ED-500 power meter and Schott glassneutral density ®lters were used to vary the powerdensities at the sample. Typically it was greaterthan 15,000 MW cmÿ2.

Pulse programs similar to those shown previ-ously were used for the collection of the massspectra in the di�erent LA-FT/ICR mass spect-rometry experiments and are not discussed anyfurther here [25,26].

3. Results and discussion

By LA-FT/ICR-MS it was possible to generatethe anion clusters NbOÿ3 , Nb2Oÿ5 , Nb2O6Hÿ,Nb3Oÿ8 , Nb4Oÿ10, Nb5Oÿ13, Nb6Oÿ15, etc. with gen-eral stoichiometries �Nb2O5�ÿn and �Nb2O5�n ±NbOÿ2 , n � 1±4, (Fig. 1). A very similar series ofcationic niobium oxide clusters have been gener-ated using ¯owing afterglow, and their reactionswith a number of neutral reagents studied [27]. Inthis present study we focus on the chemistries ofNbOÿ3 , Nb2Oÿ5 , Nb2O6Hÿ, Nb3Oÿ8 and Nb4Oÿ10

with the alcohols methanol and ethanol.Model Nb(V)-oxide monomers and dimers

dispersed on silica supports have previously beenstudied using extended X-ray absorption ®nestructure [28], and the role of Nb(IV) centers inalcohol activations proposed on the basis of elec-tron paramagnetic resonance measurements [29].In both instances, only circumstantial evidenceconcerning the roles of particular Nb-moieties wasobtained, and no information regarding reactionmechanisms was available. Due to the intrinsicmolecular nature of the MS experiment, mecha-nistic information and variations in the activitywith cluster size can be discerned. Moreover, dueto the deceptively simple nature of the clusterstructures, representative anions with the featuresimplicated in Nb±O surface activity can be inde-pendently examined to test the spectroscopic par-adigms. Due to the high strength of the Nb±Obond [30] and high oxygen: metal ratio, with fewexceptions, notably Nb2Oÿ5 , all Nb atomic centers

180 P. Jackson et al. / Chemical Physics 262 (2000) 179±187

will exist in the �5 oxidation state and the Nb±Obonding will dominate the cluster structure.

We have recently reported the activity of NbOÿ3towards a variety of neutral molecules, and it isimportant to recount the results for the reactionsof this ion with methanol and ethanol [26]. NbOÿ3dehydrogenates methanol in an e�cient, concertedreaction to yield a relatively non-reactive Nb(III)product ion NbO(OH)ÿ2 . Dehydration was a veryminor reaction, which is probably due to the spinrestrictions of the ground state product 3CH2. Theaforementioned situation is reversed for the largeralcohol because the C2H4 is a thermochemicallystable closed shell neutral. Dehydrogenation wasobserved to compete with the dehydration reac-tion, but overall the activation of ethanol is far lesse�cient. An ion ejection/selection step was used toisolate the Nb(V) product ion from this reaction,NbO4Hÿ2 , which was observed to undergo an ex-tremely ine�cient ligand switching reaction withethanol to form NbO4C2Hÿ6 (less than three reac-tions per one hundred collisions). Given that slow

or ine�cient activity is an indicator of a poor orpoisoned catalyst, NbO4Hÿ2 was re-isolated andkinetically activated using RF irradiation. Thisresulted in regeneration of the more catalyticallyactive parent ion NbOÿ3 . Prior to interrogating thision using gas phase chemistry, we were equippedwith an a priori knowledge of its likely structurefrom local density functional calculations. In ad-dition, O-atom loss was dominant in CID experi-ments, so a peroxo-type structure NbO�O2�ÿ wasimmediately discounted.

It is already clear that the MS experiments,even for our model monomer system, o�ers im-mense insights into Nb±O/alcohol surface chem-istry, so we now investigate the reactions of amodel Nb(IV) system, Nb2Oÿ5 . Simple empiricalelectron counting, and niobiumÕs preference forthe �5 oxidation state, dictates that this clusterpossesses a radical Nb(IV) center, as the elec-trophilicity of oxygen is much greater than thatof niobium. (EA�Nb� � 0:89� 0:03 eV [31] cf.EA�O� � 1:461 eV [32]). Predictably, it is the

Fig. 1. Laser ablation Fourier transform negative-ion mass spectrum obtained from the laser-irradiation of a solid Nb2O5 pellet. The

measured laser power density was >15 000 MW cmÿ2. The ®rst number in the peak labels corresponds to the number of niobium

atoms, while the second number corresponds to the number of oxygen atoms.

P. Jackson et al. / Chemical Physics 262 (2000) 179±187 181

radical site and not the charge site that is domi-nant in the ®rst reaction of this cluster with bothmethanol and ethanol. That is, hydroxyl abstrac-tion and liberation of the respective alkyl frag-ment in e�cient, radical propagation reactionsare observed �/ADO;EtOH � 0:47, and /ADO;MeOH �0:81�. 2 Moreover, this is the sole reaction ob-served for Nb2Oÿ5 , and from general bond strengthconsiderations D�Nb2Oÿ5 ±OH� > D�CH3±OH� �92:3� 0:7 kcal molÿ1 [38±40]. The primary prod-uct ion Nb2O6Hÿ then undergoes a ligandswitching reaction with methanol to yield meth-oxydiniobium pentoxide anion �/ADO;MeOH � 0:46�and the neutral by-product water. Although li-gand switching is also observed for ethanol, thispathway competes less e�ciently with the proton-transfer reaction �/ADO;EtOH � 0:75� that yieldswater, neutral diniobium pentoxide and presum-ably the ethoxy anion. Due to the nature of theexperiment, in which only charged species aredetected, we are left to speculate about the natureof the neutrals that are generated, and bis-hydroxydiniobium tetroxide could indeed be theundetected product of this reaction. Utilizing anion ejection/selection step, two minor reactions ofthe secondary product were observed that eitheroccur spontaneously, or require slight kinetic ac-tivation, and both are representative of diniobiumpentoxide catalysis. The ®rst of these reactions isa rearrangement/elimination that yields neutralformaldehyde and hydroxydiniobium tetroxideanion in a formal redox process. This reactionmay or may not require collision-induced activa-tion with an additional methanol molecule. Thesecond pathway is either a C±C or C±O couplingreaction that regenerates the parent ion Nb2O6Hÿ

and produces either ethanol (chain propagation)

or dimethyl ether. Given that ethers have beenobserved in the product gases of Nb2O5 activatedalcohols, the latter pathway is probably operative.

After ion ejection/selection, the monomethoxy-diniobium pentoxide anion was also observed toundergo an exothermic condensation reaction withmethanol, followed by a further ligand switchingreaction to ®nally yield a tris-methoxydiniobiumtetroxide anion. This quaternary reaction productis either inert towards methanol, or further reac-tion requires thermal activation. At this point itwould probably be prudent to regenerate theparent ion as the e�ciency of the alkoxydiniobiumpentoxide reactions is reduced to 0.31 and 0.38 formethanol and ethanol respectively.

Due to the presence of small amounts of ad-sorbed moisture on the precursor pellet, it was alsopossible to study the alcohol reactions of laser-generated Nb2O6Hÿ. Interestingly, in addition toligand switching, methanol oxidation was alsofacile for the primary parent ion. Assuming thatalcohol oxidation reactivity is indicative of proxi-mal metal-oxygen double bonds, geometric isom-erism is a likely cause of this e�ect (Fig. 2). Thebranching ratio for laser-generated Nb2O6Hÿ re-acting with methanol is dehydration (60%) andoxidation (40%). The reactions and rates of theisolated product ions are su�ciently close to thosegenerated from Nb2Oÿ5 to suggest they are geo-metrically and electronically equivalent. Thus noelaboration of these results is necessary, althoughit should be mentioned that the yields of the oxi-dation product Nb2O6Hÿ3 were signi®cantly lowerfor the ion±molecule reaction product Nb2O6Hÿ

reacting in a further collision with CH3OH. This isnot unreasonable considering the possibility ofgenerating a mixture of geometric isomers by thelaser ablation/resonant electron capture method ishigh.

We now extend our gas phase study to a clustercontaining three niobium atoms, Nb3Oÿ8 , forwhich no information is available from earlierspectroscopic studies. It will be interesting tocompare these results with those obtained for thesmaller clusters, particularly for the appearance ofsize e�ects and any unexpected reactions thatmight be induced by the presence of the additionalmetal atom.

2 The / values are calculated by dividing the measured

second-order rate constant by the calculated collision fre-

quency. They are a measure of the reaction probability per col-

lision. We have adopted the Ôaverage dipole orientationÕ (ADO)

theory of Bowers and coworkers [33±36] as a suitable model for

the prediction of collision frequencies. The molecular param-

eters used in these calculations (molecular dipole moments and

polarisabilities) were taken from CRC Handbook of Chemistry

and Physics [37].

182 P. Jackson et al. / Chemical Physics 262 (2000) 179±187

In total, four molecules of CH3OH are ac-tivated by Nb3Oÿ8 , and a recurrent mechanismappears to be operative which involves (i) a con-densation reaction at an Nb@O bond to yieldNb(OH)(OCH3)ÿ and (ii) a follow-up ligandswitching reaction in the ensuing collision to yieldNb(OCH3)ÿ2 and water as the neutral by-product.The reactions with ethanol essentially follow thesame scheme, but with niobium ethoxy speciesbeing the products of the reactions. We note thatno methanol or ethanol oxidation is observed withthis cluster, and reactivity terminates with the ®nal(slow) ligand switching reactions. The e�cienciesfor each reaction step with CH3OH are as follows:

first condensation : /ADO � 0:61;kexp � 5:3� 10ÿ10 cm3 sÿ1;

first ligand switch : /ADO � 0:32;kexp � 2:8� 10ÿ10 cm3 sÿ1;

second condensation : /ADO � 0:34;kexp � 3:0� 10ÿ10 cm3 sÿ1;

second ligand switch : /ADO � 0:04;kexp � 3:3� 10ÿ11 cm3 sÿ1:

The last ligand switching reaction is notablyslower than the preceding reactions, which all ap-pear to follow statistical behavior for the proba-bility of encountering a reactive or activatedcluster site. That is to say, the ®rst condensationcan take place at two of the three metal centers(probability � 66%), whereas the follow-up ligandswitching reaction can only take place at the ac-tivated cluster site with a probability of one inthree (�33%). We could extend this argument tothe second condensation reaction, so the ®rstcondensation/switching paci®es one of the metalcenters towards further alcohol activation, leavingonly one site available for further reaction. What issurprising is the ine�ciency of the ®nal switchingreaction, for which we can cite hydroxyl ligandocclusion as a possible explanation (Fig. 3).

Another interesting aspect of the reaction ofNb3Oÿ8 with both alcohols was the almost simul-taneous appearance of the condensation productNb3Oÿ8 �ROH and the primary hydration productNb3Oÿ8 �H2O. Using an ion ejection/selection step,

Fig. 2. Geometric isomers, reaction pathways and branching ratio for laser-generated Nb2O6Hÿ.

P. Jackson et al. / Chemical Physics 262 (2000) 179±187 183

we were able to isolate the primary condensationproduct and examine subsequent reactions of thision with the respective alcohols. Interestingly, thehydration product does indeed result from furtherreaction of the condensation product with the al-cohols. This is probably via C±O coupling to form

ROR, where R is either methyl or ethyl. In addi-tion, the hydration product decomposes to yieldthe parent ion without activation. This is a re-markable result, although we cannot completelyexclude some kinetic energy being imparted to theparent ion by the ion ejection/selection process.

Fig. 3. Geometric isomers derived from the observed cluster reactions of Nb3Oÿ8 , and the chemical fate of isolated Nb3O9CHÿ4 upon

further reaction with methanol: an example of molecular catalysis.

184 P. Jackson et al. / Chemical Physics 262 (2000) 179±187

Nevertheless, the direct observation of catalysis ona small, mass-selected cluster has been realized.Two structural possibilities for the Nb3Oÿ8 clusterappear in Fig. 3, together with the branching ratiofor the reaction of the isolated ®rst condensationproduct. Catalysis reactions involving cluster ca-tions by FT/ICR mass spectrometry have beenreported previously by Irion and coworkers in thegas phase study of the trimerization of ethylene byFe�4 clusters [41,42] and further discussed in a re-view by Irion on size e�ects in metal cluster-ionchemistry [43].

Continuing with our analysis of the higher massclusters, we proceed to examine Nb4Oÿ10, which we®nd to be totally inert towards both alcohols, anda number of other reactive molecules includingH2S and N2O. The high stability of this cluster, atleast towards the alcohols, can be attributed to aclosed-shell structure probably containing noNb@O bonds, and a large HOMO±LUMO gap.We rely on the previous spectroscopic analyses ofShira et al. [28] and Wada et al. [29] as well as theestablished chemistry of the constituent elementsfor deductions regarding the cluster structure. Al-though the arguments presented pertaining to thecluster structure of Nb4Oÿ10 are compelling, theyare not proven without recourse to high level abinitio calculations. At this point it appears that astructural bottleneck for further alcohol activationhas been reached, as the higher clusters are eitheretched to produce smaller NbxO

ÿy daughter clus-

ters, or simply undergo slow condensation reac-tions, or both. For example, Nb5Oÿ13 undergoes aslow condensation reaction with CH3OH, and theparent ion can be retrieved from this product bycollision-induced decomposition.

The relative ADO e�ciencies of some selectedparent and product ions with both alcohols arepresented in Fig. 4. It is clear that the mono-meric and dimeric Nb clusters are the most reac-tive, with the only selectivity discrepancies beingthe Nb2O6Hÿ reaction with ethanol (acid±base)and NbOÿ3 , which does not add H2O and eliminateCH2 because of either unfavorable thermochem-istry or a spin barrier.

Finally, to ensure that many of the product ionswere not irreversibly poisoned, a number of thesewere ®rst isolated using an ion ejection/selection

step, and then accelerated with resonant radiofrequency radiation to induce collisional frag-mentation, thus replicating thermal reduction. Forinstance, isolation and resonant irradiation of thesecondary product ion of the reaction betweenNb2O6Hÿ and C2H5OH, Nb2O7C4Hÿ11 (bis-eth-oxyhydroxydiniobium tetroxide anion) for 40 lsyields Nb2O6Hÿ and Nb2O6C2Hÿ5 in the approxi-mate ratio 0.4:0.6. Thus, ligand coupling is takingplace with a minimum of e�ort, but it appears thatthis reaction does require some heating to proceed.Increasing the irradiation time to 60 ls (therebyincreasing the center-of-mass collision energy) re-sults in a greater degree of parent ion fragmenta-tion as outlined below:

This scheme suggests that a much richer clusterchemistry is accessible at higher temperatures, andthat dehydration, some oxidation reactions and

Fig. 4. Relative ADO reaction e�ciencies for the selected

cluster/cluster products reacting with methanol and ethanol:

R�CH3 for methanol, R�C2H5 for ethanol.

P. Jackson et al. / Chemical Physics 262 (2000) 179±187 185

ligand switching are all exothermic or ther-moneutral for most of the clusters under theconditions of our experiment. We present thecollisionally activated regeneration of Nb3Oÿ8 fromthe tertiary product ion Nb3O9C4Hÿ10 in Fig. 5.This ®gure clearly illustrates there are no inter-mediate decomposition products, so it is apparentthat ligand coupling proceeds on the cluster beforeactivation. If it takes place during activation, sta-tistical arguments could be used to rationalize avery small activation energy for this reaction.

We are currently exploring other anionic metaloxide systems for analogous behavior.

Acknowledgements

We gratefully acknowledge ®nancial supportfrom the Australian Research Council, the Aus-tralian Institute of Nuclear Science and Engi-neering, and the Nuclear Medicine ResearchFoundation, Concord Hospital, Sydney, Australia.

Fig. 5. Kinetic activation of Nb3O9C4Hÿ10, derived from Nb3Oÿ8 reacting with ethanol, after a resonant radio-frequency irradiation

time of (A) 40 ls and (B) 60 ls. The parent ion is the only decomposition product. The decrease in the signal to noise ratio in (B) is

attributed to increased competition from electron detachment at higher center-of-mass collision energies.

186 P. Jackson et al. / Chemical Physics 262 (2000) 179±187

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