Vibrational spectroscopy of adsorbed species on nickel by neutron inelastic scattering

CHEMICAL

VIBRATIONAL

PHYSICS

LETTERS

15 Seprcmber

1979

SPECTROSCOPY OF ADSORBED SPECIES ON NICKEL

BY NEUTRON INELASTIC SCATTERING

Rccciked -I 51.1) 1979

he

In ;1 text of the utility of neutron inelz.tic spectroscopy (NE) for stud> ing n~olccular vibrations in s.~rfacc rrxctions. ae c\amincd the adsorption, co-adsorption, and reaction of hydro~cn .md carbon monoxide on Raney nickel cat&sts.

1. Introduction Studies of the vibr3tion31 spectra of surface species have provided 3 great deal of insight into the structure imd dynamics of adsorbed ato,ms and molecules_ Such methods as electron energy loss spectroscopy (EELS) [I 1, reflection-adsorption infrared spectroscopy (RAIS) [2,3], infrared tmnsmission [4], inelastic electron tunneliing spectroscopy (IETS) [S] and Raman spectroscopy [6] are 311being used to characterize a wide range of surf3ce molecular structures_ Another method which can provide information not obtainable by the other vibration31 techniques is neutron inelastic scattering (NIS) [7]. In NIS, there are no dipole or polarizability selection ru!es to be obeyed_ so that in principle all vibration31 modes 3re active [S]_ In practice, the scattered intensity is proportion31 inter alia to the scattering cross sections of the vibrating atoms; the incoherent scattering cross section for H is ~20 times greater than most other atoms. In studies of adsorbed hydrogencontaining species, scattering from 1-Iwih be a domin3nt feature of the NIS spectra even when the number of surf3cc H atoms is much less than the total number of substrate or other surface atoms present. NIS is particularly well suited for vibrational studies of optically-opaque samples, and has the potential for characterizing surfaces in situ, under hi& temperature and pressure conditions (i.e., typical operating conditions for heterogeneous cakdysis)_ The utility of NIS for surface studies has been demonstrated in studies of

hydrogen chemisorption on metal powders [9-l Z] _ and in studies of the adsorption of small organic molecules on carbon [Is] _ The useful range of NIS for vibrational studies is typically 2 to 100 meV (16 to 1600 cm-t), but this c3n, in principle_ be extended as high as 500 meV by the use of high temperature moderators or pulsed neutron sources. In the present effort, we have applied NIS to a study of the adsorbed reactants, Hz and CO, involved in an important surface reaction, i.e.. the hydrogenation of CO over Rsney Ni to produce methane [ 14, Is]_ In particular, we !rave addressed the questions: can NIS be used to detect the effects of chemical interaction between adsorbed reactants_ and can NIS detect stable reaction intermediates and contribute to an understanding of the mechanism of a surface reaction? This preliminary study of the vibrational spectra for hydrogen on Ni, hydrogen + CO on Ni, and hydrogen adsorbed on 3 carbon overla)er on Ni provides new insights into the structure of reactants and the mechanism of the methrtnation reaction over Ni, as well as the usefulness of the XIS method-

3. Experimental

2 I_ Procedure 2 I. I. Sample A commercial Raney nickel (W’_R_ Grace Co., type Raney 4100) was used in these studies. As supplied by 159

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CHEMICAL

I

PHYSICS

the manufacturer, the material is activated and stored in water_ The chemical composition of this Raney nickeI is typically 92-94 wt % Ni, S-6 wt G AI, and L~ntains a pxticuhrly low level (a.05 wt s) of A1203_ A high resolution neutron diffraction study of the samples used (after all proceainy:

15 September

LETTERS

1979

trapped from the gas stream and measured 370 cm3 (STP) when the reaction was stopped. The sampIe was then exposed to Hz? at 300 K and slowIy (over several hours) took up 275 cm3 (SIP) H,_ The cell was isolated with pH_ = 100 Torr for the NIS measurement-

and after the

spectra were recorded) confirmed tIlat the materid wzs essentklly a dilute poIycryst;rlline nickelatuminum a11oy (
The cell ms filIed \i-ith wet Rancy nickel (~70 g, dry basis) and dried in flowing argon at 373 K_ Following a reduction in flowing II2 at 473 K, the surface area of the sampIe was measured by physisorption (BEI-. N, at 77 K. surface are7, ~50 m’/g) and by Hz chemisorption (300 K. surface area ==I 5 ml/g)_ One sample cell was used for NIS measurements of Haney nickel (a) with monohyer_coverage of hydrogen. (b) with one quarter monolayer corerage of CO in addition to the monolayer of Hz. and (c) following the removal of chemisorbed species_ Another (essentially identic21) sample cell was used for a NIS measurement of hydrogen

chemisorbed

on ztcarbon covered

Rancy

nickel surface_ D&ails of the proc&g of the umples are ;1s follows: (a) Ccli I (containing a 72 g sample) was evxuated fo 1 X 10m6 Torr. mounted in the neutron scattering apparatus and aIlowed to chemisorb 256 cm3 (STP) Ii2 (equilibrium Prt, = 55 Torr) which is approsimateiy a monohyer. based on the adsorption isotherm_ FoIlowing the S1S measurement of hydrogen on nickel, the sample was allowed to adsorb 136 cm3 (STP) CO at 77 c(. then equilibrated at 300 K before another SlS measurc’mr’nt. After thrw measurements, sample cell 1 wzs h-=;ltrd in flatting Hz aft 525 K for t 2 h (to remove CO). evacuated to I X ! 0-6 Torr and reinstalled in the NIS apparatus for the bxkgound sc-attering rucasur~%xxtr _ tb) Cell 11(containing

;16S 8 sample which xisorb-

cm3 (STP) H-, at monohyer coverage at 300 K) was tlrGliCCiin flowing, high purity CO at 500 ii_ CO1 (produced by rhc reacrion ZCO * C + CO1) was Set 215

160

The schematic ISIS experiment [S] is shown in fig_ I _ The thermal neutron flux from the 10 MW NBS reactor is incident upon a Cu crystal (230 planes) monochromatcr_ The monochromatic neutron beam (energy Eo and momentum k,-,) then impinges upon the Haney Ni sample, and the inelastically scattered neutrons are analyzed using a 20 cm, room temperature Be filter (bandpass O-5 meV) and a 5.1 cm X 5.1 cm 3He detector_ In these measurements, NIS is analogous fo Rarnan Stokes scattering: we detect neutrons which have iost energy due to excitation of specific vibrational modes, or multiphonon processes_ -AlI spectra presented in the following discussion were obtained with the sample cooled to 77 K in a cryostat: this served to minimize multiphonon scattering_ Spectra1 features obtained at room temperature were similar to the Iow temperature data, except for spectral changes related to desorption- The data reported here ;Ire for the ener,T range 40 to 170 meV; at lower energies, the spectra were dominated by phonon scattering from the i\l’i, and at higher energies. xxx-rements

were not practical due ro rapidty

creasing background.

The experimental

in-

energy resolu-

Sample

!iYiF

1-i~ I_ A schematic view of a neutron inriastic scattering spectrometer_ E,,. ko and E, k are the neutron energy nod momentum before zmd after scattering by the sample_ In the general eae, k-0 and ko cm Lx varied and the distribution of scattered neutron energies and momenta can be .maIyzed selectively for each Eo and ko_ In the present experiment E, and k. uere vzuied x\hik E was “fLxd* at ;m awzmge enera of 3

mcV by p.!ssing the scattered neutrons through 2 beryllium titter \xhich allowed aI1 scattered neutrons botwxn 0 and 5 meV to be detected

Volume 66. number 1

CHEMICAL

PHYSICS

tion was. 6 and 9 meV at 60 and 140 meV, respectively. The neutron scattering angle (0) was SO”. Typical measurement time for all data shown was 36 h_

3_ Results The experimental data are shown in fig_ 2 as piots of scattered neutron intensity as a function of the energy transfer (meV). The complete spectra of fig. 2 have been smoothed using a three point smoothing routine_ The Iow energy repime of fig. 2 is replotted without smoothing in fig. 3. The presence of a peak in these spectra indicates that neutrons have lost energy due to excitation of a particular type of vibrational mode. Fig. 2a is the thermal neutron ‘-background“

15 September

LETTERS

1979

spectrum (after subtraction of a smooth epithermal neutron background) obtained from the reduced Raney Ni sample after desorption of adsorbed species. The dominant feature in spectrum 2a is a peak at 95 meV due to an OH bending mode presumably associated with the presence of a smal1 amount of oxide phase of the activated Raney alloy; the increasing background below 50 meV is due to the high energy tail of the phonon spectrum of Ni. Ail of the spectra in figs. 2 and S have been background-corrected. Figs. 2b and 3a are the NIS spectra associated with a monolayer of hydrogen on Raney Ni. Fig. 2b is in excellent agreement with the results of Renouprez et al. [9] who reported the features at -120 and z-140 meV, and associated them with H atoms interacting with several substrate Ni atoms.

.Bpeak at 78

In addition,

meV is clearly resolved in the presenr darx Andersson

60

80

100

120

140

160

180

iI24 c&

[ 161 reported a vibrational loss feature at 75 meV in an EELS study of hydrogen on NI( 100); he associated this peak with the normal vibration of an H atom bonded in the four-fold hollow site.

Inelasf~c S~IXWS

’

/: ‘._

_,

‘.‘.

<=--z .

.

80

100

60

Energv Trawfer I

400

I

600

I

800 Eneqv

Fig.

7.

140

Raney

Scattermg Nickel

from Adsorbed

LOW Energy

Regrme

Carbon-covere, Nickel

(a) Background

-.__,-_

. 120

Neutron

/bJ Adsorbed Hydrogen

,.--.. -__.--

on

180

160

200

6meV

,me”, .

1000

t

1200

-rlansfer (cm

,

40

60 Energy

,400 -’ ,

IKS spedra from adsorbed species on Raney nickel_

J&h spectrum was smoothed xsith a 3 point smooting routine; srarisrinl error bars are indicn-ted at various data points; the lower vertical scale marker refers fo spectra a, b. and c.

L 400

Transfer

80 (meV\

100

t

1

600

800

[cm ‘1

Fis 3. INS spectra from adsorbed species on Raney nickel: unsmoolhed data for the low energy regime_

161

Volume 66. number 1

CHEMICAL PZiYSICS LET-l-ES

Figs_ 2cand 3b are the spectra associated with a monohtyyer of H2 on Raney Ni which was exposed (at 77’ K) tu enough CO to adsorb as an additiona 0.25 monolayer- The ampie wrts then warmed to 300 K for equilibration, and cooled to 77 K for the NIS measurement_ The co-adsorption of hydrogen and CO on a Ni(100) surface [ 171 is known to result in disphtcement of a fraction of the hydrogen, as well as an interactional effect which produces new binding modes of both hydrogen and CO, as revealed using temperrtture programmed desorption (TPD) methods_ The data of tig_ 3 are remarkably similar to those of fig_ 3b, indicating that 3 substantird fraction of the adsorbed hydrogen is neither disphtced nor vibmtionally perturbed by the co-adsorbed CO_ The most notimbfe change is a small peak which appears at 6S meV, which may be zt manifestation ofthe CO-hydrogen intemction seen in the TPD measurements [ 17]_ Since the caraiytic methrtnstion reaction is believed to proceed via the hydrogenstion of an active carbideIike layer [ SSJS] produced by the disporportiona:ion of CO (X0 - C&is) + CO&)), we simuhtted these conditions in smnple cell II for the NIS measurements. The XIS spectra for this hydrogen-on+.rbide Iayer are shown in tigs_ 3d and 3c. The vibrational spectra seen here are substrtntir.tI~y different from figs_ 26 and 3, and suggest the presence of hydrogenated forms of carbon on the surface- lhe broad band with rt maximum at I21 meV is in the region of CH bends or CH, scissors vibrations, possibly from several species, and the pertks at 53 and 73 meV may be due to a N-C stretching vibration made pisible because of the hydrogen motion in Xi-CH, compIcxes_ Modes, attributed to bending anti siretching vibrations of CH and CH2 species, have been observed with EELS in this frequency mnge on Ni( 11 I) [ IS] _ The scattering intensity in the range I IO--I40 meV may aiso inchtde hydrogen-nickel vibrations

?_ Discussion NIS is p-,rticulariy wetl suited to the observation of the vibmtions of hydrogenic species adsorbed on the surface of a heavy metal since the incoherent cross section for hydrogen dominates the scattering from such species_ The differential cross section for onephonon, incoherent, inelastic neutron scattering by

15 September 1979

an hydrogenic species into soiid angle a and with energy tmnsfer fiu is given by [S]

x

[h(K - c~)‘/4x.di&f~i]

csch(hp7)6(wi-

cd) ,

where a,, is the known hydrogen scattering amplitude, AfH is the hydrogen mass, M is the number of hydrogen atoms in the moIecule, wi and $ are the frequency and hydrogen displacement vectors for the 11thhydrogen atom in thejth normal mode, exp(-2itJ,t) is the Debye-WdIer factor for the rzth hydrogen, S is a deha function and ko and k are the incident and scattered momenta, and K= k- ko_ In the present context the most interesting feature of eq. (1) is that for a given temperature and normal mode frequency the scattering cross section (2nd thus the observed spectral peak intensity) is roughIy propor-tional to the mean square amplitude of the H atom displacement in that particuhtr mode, (c-;)~. In the case of hydrogen adsorbed on Ni, the vibrational amplitudes of the low frequency (70 to 150 meV) vibrational modes are not expected to differ by more than 50%. Thus. within a factor of two, the integrated scattered neutron intensity in a given peak (e-g. figs. 2 and 3) is a direct measure of the fractional hydrogen coverage which populates that vibrationa mode. Assignment of NIS spectra: Hydrogen adsorbs on a ckm nickel surface dissociatively, i.e., 3s atomic hydrogen_ The hydrogen atom may be located on a top site, bonded to one nick1 atom or in a variety of sites involving multipIe nickel atoms. The metal-hydrogen stretching vibration in the top site would be expected to occur at a higher frequency than that of the muhiply bonded sites_ The vibrational fundamental of diatomic NiH is 1927 cm-t [ IS]_ A number of transition metal carbonyl hydrides have been synthesked and shown shown to hzve 3 metal-hydrogen stretch at =2000 cm-1 (e-g_* H hlr~(CO)~_ where zt 17S2 cm-l band is assigned to the Mn-H stretch [7_0]). Infrared spectm of hydrogen chemisorbed on supported platinum catalysts has shown bands =?I00 cm-r, attributdbIe to the metal-hydrogen stretch [?-I]_ E&hens [22] reported that, despite many attempts with IR, no bands due to hydrogen on supported nickel catalysts could

be detected above 1200 L~-I _ Andersson [ 161, in a high resolution EELS experiment, detected a loss peak

VoIume

CHEMICAL PHYSICS LETTERS

66, number 1

at 600 cm-t but no features at higher frequency for hydrogen chemisorbed on a Ni(100) crystal. Ibach [23], in a prcIiminary acccum of an EELS study of monolayer coverage of hydrogen on a stepped Ni(I 11) crystal_ reports the observation of five low frequency loss features, at 370 cm-l, SO0 cm-t, 940 cm-I, 1 X50 cm-l and 1300 cm-l, again with no evidence for features 3t higher frequency. In contrast to these results, Nakata [2-l], in an IR study of high area supported nicke!, assigns a very weak bank at 1880 cm-t to the Xi-H stretch (deuterium substitution gave the apprcpriate frequency shift). Also. a recent iaser Raman study [151 of hydrogen on nickel (supported on Si02) reports bands at 202s cm-t , 1999 cm-t, and a number of bands from 950 cm- 1 to 6S3 cm-t, all of which are assigned to vibrations cfchemisorbed hydrogen on nickel. However, spectra and experimental detaiIs were not included in the published report. Two recent theoretical approaches have L~lculared vibrational energies for a hydrogen atom adsorbed on 2 Ni surface_ Fassaert and van der Avcird (see Andersson [ 161). using an cstended Hiickel calculation, predict the kibraticnal frequencies for H on Ni(l00). Ni( 1 IO). and Ni(i 11). On each facet the calculated frequencies for the atop site, the bridge (two-fold)site, and the center (three- or four-fold) site arc ~1350, =XiO_ and ~600 cm-* _ respectively_ In another approach, Upton et al_ [261 have applied ab iniric merhcds in caIcularing the properties of hydrogen chemisorbed on smatl nickel clusters (up to 25 Ni atoms). They find that three-fold and four-fold sites are cntrgeti~xtlly the most favored for adsorption. The calculated vibrational frequencies correspond to symmetric strerching vibrations, and are 1430 cm-’ (tlxc-fcfd site). 1212 cm- ! (three-fold site), and 597 cm- * (fourfold site)_ They did not compute the asymmetric stretching vibrational frequencies for the multiply bonded hydrogen_ The qualitative agreement between the theoretical and experimental frequency ranges suggests that the wbraticns detected by NIS in the present work on pclycrys~alline Raney nickel (cf_ fig_ Zb) at 1130 cm-t (14 1 meV).

952

cm-l

(119

meV),

and 623 cm-l

(78

meV) can be assigned to multipIy-bonded hydrogen on Ni. The lack of quantitative agreement with rheory may be due. in part, to contributions in the NIS spectra from asymmetric vibrations not treated thcoretically_ The assignment of the vibrational mode at 624 cm-t

IS September 1979

fc a hydrogen chemiscrbed on a four-foId site is suggested by neutron scattering and neutron diffraction studies of hydrogen in tmnsiricn merals. A neutron diffraction study [27] established that nickel hydride is smlilar to palladium hydride in that :he hydrogen atoms are situated in the octahedral interstices of the face centered cubic lattice. Neutron scattering studies (see for example ref. [2X]) of palladium hydride show the hydrogen vibrational frequencies to be centered‘ at about 60 meV (4S0 cm-t). For niche1 hydride (smaller lattice parameter). hydrogen vibrations in these sites would be expected to have a somewhat higher frequency consistent with the assignment of a fourfold site (the surface anslogue to the bulk octahedral site) for the 624 cm- 1 !I\ _ drogen mode. A more definitive assignment must await further e1perimcnts including the hydrogen coverage dependence of the peak intensities - work which is in progress_ We hope to be able to answer the question as to whether or not the peaks at 119 and 141 mcVgrow in together in the same ratio (indicating thar they probably originate from asymmetric and symmetric vibrations at a single type of site) or whether the relative intensities of these features change nith coLerage (indicating different sites). it is interesting to note that while the 600 cm-l (75 meV) peak is the only H-X vlbraricn detected by EELS on the (100) facet of Ni. the relative intensities of the NlS pe&sclearly indicate that the H-bound species represented by the 78 me\’ peak is a small ccmponent (
A clear assignment of the vibrations derected by SIS when Hj and CO are cc-adsorbed (figs. Ic and 3b) and when H2 is adsorbed on a carbon covered nickel surface (figs. 2d and 3cj is not possible at present. However, in the latter case the data clearly show evidence for hydrogenic deformation and bending modes in the upper frequency range PI00 meV), while the

163

Volume 66, number 1

CHEMICAL PHYSICS LERERS

Iower frequency peaks are most IikeIy associated with nickeLcarbon vibrations_ Progress can be made in this area by varying the surface coverage of the hydrogenic species, by increasing the NIS spectral resolution and by studying the adsorption of molecules believed to fragment into adsorbed -Cl-& species - work is underway in all of these areas- The detection

of vibrationrd

modes

due to a hydrogen-CO interaction and due to

-CH,

species on the surface of a catalyst clearly in-

the potential for NE spectroscopy in identifying intermediates and in mechanistic studies of important cataiytic processes_ dicwzs

Acknowledgement We ack.nowIedge with pIeawe of this work by the Departmest the Division of Basic &ergy

the partial support of Energy,

throttgh

Sciences_

References 1L1 IL Ibach, K Hopster and B_ Sextoo, AppL Surface Sei l(1977) I_ 121 A.M. Bradshav and FAL HoKmm, Surface Sci 72 (1978) 513131 J- Pritchard, in: Proceedings of an lnteroational Conference, Jiikb, June 1978, eds. H_ lbxb snd S_ Lehwdd, p_ 1l4_ [4I X_ Sheppard and T-T- Nguyen. in: Adxmces in infrared mtd R;lmm~mtroxopy, VoL 5, ed~_ RE_ Ilaier and RJ.Ii_ Chrk (Heyden, London, i97S)_ 151 T_\volfram.ed_,Ine&s ILL -- eIecI:run tunneling spectra* copy (Springer, Bedim, 1978). 161 F-W- Kirtz, RP_ van Du, ne and G-C_ Schatz, J_ Chea Phys. 69 (1978) 4.?72_ [71 J_ Houxd znd T_C_ Waddington, AppL Surface Sci 2 (1978) IO?, and references therein.

15 September 1979

[ 81 J-J. Rush, in: Critical evaluation of chemical and physical structural information (National Academy of Sciences, Washington. 1974) p_ 369. [S] A.J. Renouprez. P. Fouillo~~\, G. Coudurier. D. Tocchetti and R Stockmeyer, J_ Chem Sot. Faraday Tmnr 173 (1977) 1. [IO] R. Stockmeyer, KM. Conrad. A_ Renouprez and P_ Fouillou.., Surface Sci 49 (1975) 549_ [I I] J_ Howrd, T-C_ Waddington and C_J_Wright, Chem. Phys_ Letters 56 (1978) 258_ [I21 H_ Asada, I_ Toya, H Motohasbi, BL Srtkamoto and Y_ Hamagehi. J. Chem Phyr 63 (1975) 4078_ [ 131 H_ Taub, 1-l-R Danner, Y.P. Sharma, H.L. Mcblurray and P.M. Brugger, Phyr Rev. Letters 39 (1977) 215. [ 141 V- Ponec, CataL Rev. Sci Eng 18 (1978) 151, and references therein. [ I.51 T-E_ Madey, D-W_ Goodm;m and RD. Kelley, J_ Vacuum Sci TechnoL (March-April, 1979). to be published_ [ 161 S. Andcrsson, Chem. Phys. Letters 55 (i978) 185. [ 171 J.T. Yates Jr., D.W. Goodman and T.E. Mxiry. Procecdings of the 7th International Vacuum Congcss and 3rd International Conference on Solid Surfaces (Dobrozemsky, Vienna, 1977) p_ I133_ [iSI J-E_ Debluth and II. lbach, Surfxe Sci. 78 (i978) L238. [ 191 G_ llerzbergg Spectra of dhtomic molecules, 2nd Ed_ (Van Nostrand, Princeton, 1965) p_ 557. [201 W-F_ Fdgdl, G. Asato, W. Wilsun and C. Angel!, J. Am. Chem Sot_ Sl(l939) 2011. [ 21 J W_A_ Pliskin and RP_ Eischens, Z. Physik Chrm_ (Frankfurt am Main) 24 (1960) I I. 1221 R-P_ Eischens and %‘_A_ Pliskin. Advzm. Cat. 10 (1959) 1. [23] H_ lbach, private communication_ [r-l1 T. Naktta, J. Chem. Ph)s_ 65 (1976) 487. [25 J \V_Crasser and A_ Renouprez, in: Proceedings of an InternationaI Conference, Jiilich, June 1978, edr H_ lbach and S_ Lehxrald. p_ 17% [261 T&L Upton, N’_A Goddard III and C-F_ Nefius, J_Vacuum Sci TechnoL. to be published; Solid Srate Commun_ 28 (1978) 501_ [271 B. Siqel and G-G. Liboxiitz, in: Mekd hydrides, edr W_ ;\I_hfueller, J_P_BIacWed~e and G-G. Liboltitz (Academic Press, New York, 1968) p_ 63 1. I281 %I_C~IOW&IW and D_ Ross, Solid state Commun 13 (1973) 229.