- Email: [email protected]
RuCuZSM5 catalysts: characterization by FT-IR spectroscopy
,..., ~_,‘,,,, ,...,...... ...~...:.::.:.:.:ii ..:.::.a~.~:.:..:~.~::~:~:::,: ..i “..,:,..:_ in,.,.,) :,., i,,:,, ~~:~~::~~~~.~,~:~:::i ‘i.x.‘;,>:.>
..‘.’ . .
.
. . . . >...~:~_,~,+$
;::.
applied
surface science ELSEVIER
Applied Surface Science 99 ( 1996) 401-409
Ru-Cu/ZSMS
catalysts: characterization by FT-IR spectroscopy
Carmelo Crisafulli a, Salvatore Scir2 a3*, Simona Mini& a, Rosario Maggiore a, Signorino Galvagno b * Diparfimento b Dipartimento
di Scienze Chimiche,
di Chimica
Industriale,
lJniversit&
Uni~ersitir
di Catania,
di Messina,
Viale A. Doria
6, I-95125
Cas. Post. 29, I-98166
Catania.
Sant’Agata
Ital!
di Messinu.
It&
Received 9 November 1995; accepted 7 February 1996
Abstract H-ZSMS supported Ru-Cu bimetallic catalysts have been studied by FI’-IR spectroscopy. Experiments have been carried out on samples prepared from Ru(NO)(NO,), and RuCl,. On the monometallic Ru/ZSMS samples adsorption of CO led to a band at 2046 cm-‘, due to CO linearly adsorbed on Ru’, and several bands at higher frequencies related to CO on Ru’+ species. A higher amount of Rus+ species is formed on the ex-Ru(NOXNO,), sample, due to the smaller metal particle size of this sample compared to the ex-RuCl 3 one. On monometallic Cu/ZSMS two bands at 215 1 and 2158 cm- ’are observed, both related to CO adsorbed on Cu+ ’ sites, formed through a reaction with the OH groups of the zeolite. On the bimetallic Ru-Cu samples, besides all the characteristic bands of the Ru and Cu monometallic samples, a band at 2138 cm- ‘, attributed to CO adsorbed on Cu bound to Ru has been detected, denoting the formation of Ru-Cu bimetallic particles. The fl-IR spectra indicate that on the samples prepared from RuCI, the extent of bimetallic Ru-Cu particles is higher than on those prepared from Ru(NOXNOJ,. This behaviour, attributed to the presence of chlorine on the ex-RuCl, samples, is in agreement with the catalytic performance of these systems in the hydrogenolysis of propane. Keywords:
FT-IR; Bimetallic; Copper; Ruthenium;
ZSMS; Carbon monoxide
1. Introduction
bimetallic
In recent years supported received
growing
attention
bimetallic because
catalysts
have
of their capabil-
ity in controlling
activity, selectivity and stability in many reactions of industrial interest. Among the bimetallic catalysts, Ru-Cu samples have been found of particular interest as model systems in that the two metals are practically immiscible in bulk. The large influence of copper on the activity of ruthenium indicates, however, that the two metals form
* Corresponding 580138.
Tel.:
Copyright SOl69-4332(96)00458-S
0169-4332/96/$15.00 PI/
author.
+39-95.330533;
fax:
+39-95-
aggregates
[l]. A large number
of investi-
gations carried out on powder samples [l-6] and on single crystals [7,8] have shown that the surface composition and therefore the chemisorption properties and the catalytic activity are strongly influenced by the precursor salts used and by the nature of the support [3,4,9- 111. FT-IR spectroscopy has been successfully used in order to obtain more insight on the surface characteristics of mono and bimetallic supported systems [12-171. A previous FT-IR study of CO adsorbed on RuCu/SiO, samples prepared from Ru(NOXNO,), and RuCl, has demonstrated the formation of Ru-Cu
0 1996 Elsevier Science B.V. All rights reserved.
402
C. Crisafidli
ef al. /Applied
Surface Science 99 (19961401-409
particles having a surface covered mainly by Cu [ 151. The amount of Cu on the surface has been found to be higher on the samples prepared from RuCl 3. H-ZSM5 supported copper catalysts have been extensively studied by FI’-IR [18-201 because of the importance of this system in NO., abatement. A few data are instead available on Ru/ZSMS systems 121,221. No FT-IR paper is available, to our knowledge, on Ru-Cu bimetallic systems supported on ZSM5 zeolite. The aim of this work was to verify the formation of bimetallic Ru-Cu particles on H-ZSMS and to correlate this parameter with the catalytic performance of the bimetallic samples.
Table
I
Chemical composition Code
ZlOON ZOSON 2040N Z020N
2.0 I.7 1.o 0.6
-
ZIOOCI ZO8OCl ZO4OCl ZO2OCI
2.0 1.7 1.0 0.6
-
ZOO0
and CO uptakes of Ru-Cu/ZSMS
Ru Cu (wt%z) (w&b)
-
Ru/(Ru (at%)
+ Cu)
CO uptake (cc6TP) g cat-’ )
samples CO/Ru
0.3 1.0 1.4
100 80 40 20
2.44 I .47 2.23 0.92
0.55 0.69 1.00 0.72
0.3 I.0 1.4
100 80 40 20
0.44 0.65 0.31 0.12
0.10 0.17 0.09 0.14
2.0
0
n.d.
-
n.d.: not detectable.
2. Experimental Ru-Cu samples were prepared by incipient wetness impregnation of the support with aqueous solutions of Ru(NOXNO,), or RuCl, (Johnson Matthey) and Cu(NO,), (Carlo Erba), having an appropriate concentration of metals. The salt(s) concentration in the solution was adjusted to yield a total (Cu + Ru) metal content of about 2 wt%. The support used (supplied as powder by Conteka) was a ZSMS zeolite with a BET surface area of 410 m’ g-’ and a SiO,/Al,O, ratio of 30. After impregnation catalyst samples were dried at 393 K for about 24 h, reduced in a flow reactor at 673 K for 1 h with a pure H, stream and then cooled at room temperature CRT) in flowing H,. Chemisorption of CO was measured in a conventional pulse system operating at RT. Pulses of a mixture of 10 ~01% CO in He were used. In order to avoid CO chemisorption on Cu/ZSMS all samples have been reduced in H, for 1 h at 673 K, cooled to 443 K in flowing H,, held in He at this temperature for 1 h to permit H, desorption, and then cooled in flowing He down to RT. Under these conditions negligible amounts of CO were found to chemisorb on the support and on Cu/ZSMS. Chemical composition and CO/Ru ratio of the Ru-Cu/ZSMS samples are reported in Table 1. Chemisorption of CO, instead of H,, was used to avoid spillover phenomena. Spillover of hydrogen from Ru to Cu has been previously reported [5]. The sample code used has the following meaning:
the first letter indicates the support used (Z = ZSMS), while the three digits indicate the atomic percentage of ruthenium in the metallic phase. The last letters indicate the precursor used (Cl = RuCl,, N = Ru(NOXNO,),). For IR studies the powdered samples were pressed into thin self-supporting discs of about 25 mg cm-* and 0.1 mm thick using a pressure of 18 X 1O3 bar. Pellets were evacuated at RT and reduced in pure H,, raising slowly the temperature to 673 K over a period of 2 h and held there for another 1 h. The sample was then cooled in H, at 443 K and evacuated for 1 h at 10m6 mbar and finally cooled at RT. On this reduced sample, CO was admitted at a pressure of 30 mbar, unless otherwise specified. Subsequent evacuations were performed at room temperature or at higher temperatures. The IR spectra were collected on a IT-IR spectrophotometer Perkin-Elmer System 2000 with a resolution of 2 cm-’ and equipped with a MCT detector. Data are reported as difference spectra obtained subtracting the spectrum on the sample recorded before the interaction with CO and are normalized to the same amount of catalyst per cm’ (25 mg/cm*).
3. Results 3. I. Monometallic
Ru and Cu catalysts
Fig. 1A shows the infrared spectra of CO adsorbed at room temperature on the monometallic
C. Crisafulli
22al
et al. /Applied
2m wavsnumbar(cm-l)
Fig. I. (A) Fl-IR spectra of CO adsorbed on the ‘reduced’ ZlOON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO: curve c. after 15 min outgassing at RT. (B) FT-IR spectra of CO adsorbed on the ‘oxidized’ ZIOON sample: curve a. 30 mbar of CO; curve b, 2 mbar of CO: curve c. after IS min outgassing at RT.
Ru/ZSM.5 sample prepared from Ru(NOXNO,), (sample ZIOON). After admission of CO the FT-IR spectrum (spectrum a> shows an asymmetric band of large intensity centred at 2046 cm- ’, attributed to CO linearly adsorbed on metallic Ru [21], and a band of smaller intensity at 2138 cm-‘, assigned to a tricarbonyl species on RuS+ atoms located inside the pores [21,23]. A not well-resolved peak at 2081 cm-’ and several shoulders (region 2 100-2 120 cm _’ ) are also present. The peak at 2081 cm- ’ has been reported to be related to CO linearly adsorbed on Ru’+ and/or to be partially associated to Ru tricarbonyl species [23]. The bands in the spectral region 2100-2120 cm-’ can be attributed to multicarbonyl CO species on RuS+ [24]. A band at 2 166 cm- ’ has been also observed in the spectrum of Fig. 1A. This band could be probably attributed to Ru’+(CO),, species [24] which were formed as a results of an ionic exchange with zeolite protons. The attribution of this peak to CO adsorbed
Sueace Science 94, C19%)40-409
JO3
on the zeolitic support can be excluded due to the high stability to outgassing of this peak (Fig. IA). Upon evacuation at 298 K the band at 2046 cm- ’ (Fig. IA) is quite stable and shifts slightly to lower frequency (2043 cm _ ’). The very low shift observed as the coverage decreases can be related to the very high Ru dispersion of this sample as confirmed by chemisorption results. The intensities of all other bands decrease very slightly upon evacuation. Fig. 1B shows the IR spectra of CO adsorbed on the ZlOON sample after interaction of the sample with O2 at room temperature (‘oxidized’ sample). After evacuation of the sample previously used to record the spectra of CO adsorbed on the ‘reduced’ catalyst, oxygen (20 mbar) was contacted with the sample at RT. After equilibration, CO was admitted. in the presence of Oz. into the cell. The same procedure was used for all the ‘oxidized’ samples. The spectra show a very intense band at 2081 cm _’ and another relevant one at 2 138 cm ’ Minor features are the shoulders at 2046 cm- ’ and 2020 cm-‘. The 2138, 2081 and 2046 cm _ ’ bands have also been observed on the ‘reduced’ sample and have already been discussed. The 2020 cm-’ band can be attributed to CO adsorbed on isolated Ru” atoms. This band is also present (as a shoulder) on the ‘reduced’ sample but it is covered bq the broad 2046 cm- ’ band. The comparison of these spectra with those of the ‘reduced’ sample shows clearly that in the presence of oxygen the intensity of Ru’+ bands (208 I and 2 138 cm-’ 1 strongly increases. with a corresponding decrease of the Ru” band (2046 cm-‘). showing a high affinity of Ru” towards oxygen. The spectra of CO adsorbed on the Ru/ZSMS sample prepared from RuCI, (ZlOOCI) are reported in Fig. 2A. The main feature of the spectrum is the peak at 2046 cm- ’ with a shoulder at 2088 cm- ’. These two bands have also been observed on the ZlOON sample of Fig. 1 and attributed to CO linearly adsorbed respectively on Ru” and Ru”. It is important to note that in this case the 2088 cm--’ /2046 cm-’ intensity ratio. i.e. the Ru”’ fraction. is much lower than that observed on ZIOON. This behaviour could be related to the bigger particle size of ZIOOCI (see chemisorption data of Table 1). which leads to a lower ruthenium-ZSM5 interaction. The larger particle size is confirmed by the stronger shift (IO cm ’)
C. Crisajdli et al. /Applied Surface Science 99 (1996) 401-409
404
2200
2uoo wubwlumba(cm-l)
Fig. 2. (A) FI-IR spectra of CO adsorbed on the ‘reduced’ ZlOOCl sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT. (B) FT-IR spectra of CO adsorbed on the ‘oxidized’ ZlOOCl sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT.
this is in contrast with the absence of this band on the samples prepared from Ru(NOXNO,), suggesting that the band at 2186 cm-’ is related to the higher acidity of the samples containing chlorine. The spectra of the ‘oxidized’ samples (Fig. 2B) show that also on ZlOOCl in the presence of oxygen the intensity of the Rus+ bands (2088, 2138 and 2152 cm-’ bands) strongly increases with a corresponding decrease of the Rue band (2046 cm-’ >. Fig. 3A shows the IR spectra of CO chemisorbed at room temperature on Cu/ZSMS (ZOO0 sample). After admission of CO two main bands (sharp and near) at 2158 and 2151 cm-’ are observed. The 2 158 cm -’ band, attributed to CO adsorbed on Cur+, is relatively stable to outgassing, whereas the 2151 cm-’ band disappears easily upon evacuation at RT. According to Sarkany et al. 1181 this latter band can be attributed to CO adsorbed on Cu’+ sites slightly perturbed by the CO present in the gas phase. A small band at 2180 cm-’ is also present and it has been assigned to CO on aluminium sites of
to lower frequency of the 2046 cm-’ band by evacuation treatments (Fig. 2A). Moreover, on ZlOOCl the intensity of Ru peaks is quite somewhat lower than on ZlOON. Formation of smaller metal particles on samples prepared from Ru(NOXNO,), with respect to those prepared from RuCl, has been previously reported on silica supported samples 14,251. Fig. 2A also shows the presence of a well defined band at 2152 cm-‘, just present as a shoulder in the spectrum of ZlOON sample. This band has been attributed to a Rus+ tricarbonyl species located outside the zeolite pores [21]. As reported in the literature [26], the presence of chlorine strongly reduces the mobility of Ru atoms, so hindering their migration inside the zeolite pores. A small peak at 2186 cm-’ is also present in the spectrum of Fig. 2A. This band easily disappears by RT evacuation and can be attributed to CO adsorbed on aluminium species of the zeolite [28]. However,
Fig. 3. (A) FI-IR spectra of CO adsorbed on the ZOO0 sample: curve a, 30 mbar of CO: curve b, 2 mbar of CO; curve c, after 15 mitt outgassing at RT. (B) FI-IR spectra of CO adsorbed on the ‘oxidized’ ZOO0 sample: curve a, 30 mbar of CO: curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT.
C. Crisafidli
et al. /Applied
305
Surface Science 99 (1996) 401-409
In the presence of oxygen (Fig. 3B) no important changes in the spectrum of CO on Cu/ZSMS are observed. Anyway, the appearance of a small band (shoulder) at 2117 cm-‘, attributed to CO adsorbed on Cu’+ [ 181, can be noted. 3.2. Bimetallic
Fig. 4. Influence of pretreatment conditions on IT-IR spectra of CO adsorbed on the ZOO0 sample after admission of 30 mbar di CO. Pretreatment: reduction in Hz at 673 K and curve a, evacuation at 773 K; curve b. evacuation at 673 K; curve c, cooling in Hz at 443 K and evacuation at 443 K.
the zeolite [28] and/or [311. It is important
associated
to Cu’+
species
to note that the intensity of the bands of CO on Cu is relatively small compared to Ru peaks. However outgassing the copper monometallic sample at 673 K (instead of cooling in H ? at 443 K and then outgassing) the 2 1.51 and 2 158 cm-’ bands (related to Cu” ) become far more intense (Fig. 4). As reported in the literature [ 18,201 this behaviour can be explained with the strong tendency of the Cu atoms (faster at higher temperature) to be oxidized to Cu’+ when supported on H-ZSM5 zeolite through a reaction with the OH groups of the zeolite. On the contrary on Cu/SiO,, Cu” is quite stable to oxidation (at least at 10d6 mbar and 673 K) as confirmed by the presence of just a small band at 2 117 cm-‘, attributed to CO adsorbed on Cu”, which disappears upon evacuation anyway [ 15,291. It has been widely accepted, in fact, that only Cu’+ can irreversibly absorb CO at RT while Cu” and Cu” cannot [19,30]. On the Cu/ZSMS spectrum it is possible to observe also a very small and relatively weak band at 2108 cm- ’ attributed to CO on Cu’. The very small intensity of this peak must be attributed both to the low relative response of the CO on the Cue band and/or to a large Cu metal particle size. A relatively intense peak attributed to CO on Cue has been in fact reported for highly dispersed silica supported copper [3 I I.
Ru-Cu
catalysts
The spectra of bimetallic Ru-Cu samples prepared both from Ru(NOXNO,), and from RuCI, are quite complicated. They present practically all characteristic bands of Ru and Cu monometallic samples with some important differences which will be discussed below. Fig. 5 shows the spectra of bimetallic Ru-Cu samples, prepared from Ru(NOXNO,),, at different Cu/Ru ratios and recorded at 2 mbar of CO. It can be noted that on increasing the Cu/Ru ratio there is a progressive decrease of the broad band at 2046 cm-‘, due to CO linearly adsorbed on Ru”. Also the band at 2081 and the shoulders at higher frequency, related to Ru’+. decrease in intensity on addition of Cu, even if at a less extent. Therefore on increasing the amount of copper the Ru”/Ru’ ratio increases continuously. A different behaviour is instead observed for the
H
1
2000
2200 wovrnumbrr
1600
(cm-l)
Fig. 5. ITIR spectra of CO adsorbed on Ru-Cu/ZSMS prepared from Ru(NOXNO,),. 2 mbar of CO.
samples
406
C. Crisqfulli et al. /Applied
Surface Science 99 (IYYri) 40/-409
2138 cm- ’ band. The intensity of this band at first increases with the Cu/Ru ratio, reaching a maximum on Z080N, and then decreases at higher Cu contents. This band is in the same spectral region of the band of CO adsorbed on Ru6+. However it cannot be assigned to Rus+-CO species in so far it shows a quite low stability to outgassing as shown in Fig. 6, where the behaviour of the CO bands to evacuation on the monometallic ZlOON (Fig. 6A) and on the bimetallic Z080N samples (Fig. 6B) is reported. In agreement with previous results of Knozinger et al. [16] this band can be attributed to CO adsorbed on Cu surface atoms bound to Ru atoms. A frequency of 2 138 cm-’ also has been reported on single Cu atoms adsorbed on Ru(OOl> [32]. A similar behaviour also has been observed on Ru-Cu silica supported catalysts and correlated with bimetallic formation [ 151. It can be therefore concluded that the band at 2138 cm-’ is made of two components. One is related to CO adsorbed on RuGi which is stable upon evacuation. The other component is assigned to CO adsorbed on Cu atoms bound to Ru and it is easily eliminated by evacuation at room temperature. The relative amount of the two species can be calculated by recording the spectra before and after evacuation. The attribution of the 2138 cm-’ band is further confirmed by the spectra of the Ru-Cu samples
2200 wav.numbrr
2000 (om-
2200 1)
wavmumbrr
2000 2200 wovwumbor
2000 (cm-
1 BOO 1)
Fig. 7. FT-IR spectra of CO adsorbed on Ru-Cu/ZSM5 prepared from R&I,. 2 mbar of CO.
samples
prepared from RuCl, (Fig. 7) and recorded at 2 mbar of co. On increasing the Cu/Ru ratio, in fact, a behaviour similar to the ex-Ru(NOXNO,), samples is observed. In this case the attribution of the 2138 cm -’ band to CO adsorbed on Cu bound to Ru is more simple due to the fact that on the ex-RuCl, monometallic Ru sample the 2138 cm-’ band is very small. The intensity of the band at 2 158 cm-’ (CO on Cu’+ > measured on the Ru-Cu bimetallic samples is higher than that observed on ZOO0 (monometallic Cu/ZSMS sample), notwithstanding the lower amount of Cu present on the bimetallic samples. Even though this result needs further investigations it is possible that on the Ru-Cu samples ruthenium catalyses the oxidation of copper by the OH groups of the zeolite at low temperature, thus increasing the fraction of Cu’ + present on the sample.
Ia00 (cm-
1)
Fig. 6. (A) Influence of outgassing at RT on the ZlOON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT. (B) Influence of outgassing at RT on the ZO8ON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO: curve c, after I5 min outgassing at RT.
4. Discussion To correlate the ET-IR spectra of the bimetallic Ru-Cu/ZSMS samples and their catalytic performance, let us summarize briefly the results of cat-
.u~ouyun
II!IS
s!
saw3aJ33v
3g
-1elauuq ns-na 40 uo~lw_i~o~ aql saxanuu! au!lolq3 qxq~ qBnolq1uyueqsaw aqL ‘[EE’ 1[ ‘11 pasn lJod -dns aql JO ssalpla3al uogcw~alu! ns-nx 01 pauodal
.saldwes asaql uo UO~ELLI~OJ
laq3!q G 3uy.1yuo~
-aId slsiCpwz aql uo doe
"‘Iya
uroy
Lq
aql 3ugzwqns
paled
u! aseal3ap laq%!q q3t-w
l2lCp &!h!lX
u! air? silnsal asaql
3!1/ClElt?:, aql ql!M
'pueq
aqi JO /(i!suaiu!leioi aqi u101~ (+yna lti IL' uo!ianDeAa
luauodwos
aql 1noAEJ
i(lsno!Aald uaaq seq au!lolqD 3u~uy~uo~
slos.uwald JO asn aqL 3qlwaw!q
t?MOqS L/3!ljM ‘(8 %J) iuakuaal3v
03)
paAolua1
,_um 8EIz uo paqlospr!
1OU S!
Lq pauyqo
qD!qM
uaaq wq
nx
01 punoq n3 uo 03 JO pueq aqi JO Qsuaiu! aqi ieqi palou aq isnlu 11 ‘(6 .3y> (KON)(()N)n’d-Xa aqi UO usql saldues 's! (,+v
uo
'13nx-xa
aql uo laq3!q qzcnu 'IDVJ u!
03) , -m3 5x70~.1~ pucq w JO ieqi pue
( , ~3 8~ I z) na ol punoq n3 uo 03 ~0 puw aql ~0 iCl!sualu!aql uaawaq ns-na
oyl
aql
'UO~RLLI.IOJsaisgled
~!1lvIau_uqJO iuaixa aqi sawanuu!
pasn loslnaa.td aql leql aDuap!Aa aA! s! y saldlues na-na
JO ewads
43~011s
01 aIq!ssod asp
81-u
aql LLIOLJ
'[go] yr?aM /(laAgeIal s! uo!i3waiu! D!uoliJala aqi ieqi papnlwo3 na/03
uaaq seq i! ‘pahlasqo IOU s! pueq
aqi JO slaqwnuaAvm
'OJ'4 3wpuodsauo3
aqi wqi
lam01
sple~oi
aql SE '1aAaMOH
-DeJalu! aql i(q pazylod n3
sno!,taJd
iyqs
'IQ ql!M UO!l
i([aAysod air!surole azzpns
saldum
pavoddns
S~SZ-H
uo
oslr! ieqi su~lyuo~
laded s!ql u! paluasald uope3psaAu! '[Lz] ysLloua3olpLq
auedold
x1-u
aqL
104 alq!suodsal
‘salqmasua na aApze aqiJ0 laqwnu aqi aloJalaqi puv stole na luaze[ppe aql JO Jaqumu aql aseaJ3ap p[noM um!uaqlnJ JO aDepns aql uo Jaddo3 tlau! JO ama -saJd aqL ‘[I] ylnq aql u! sp.?laLu 0~1 aqi ~0 Ll!l!q!D -syu~ aql alldsap pauuoJ aJtz sale3aJ33e zqlclauuq *saldu_m na-na aql ug .saIqluasua qms SKI! -pug JO AlyqeqoJd aql 01 paleIa1 s! h!l\p2 3!1@?1~?3 aql pue surole na lua3rz[ppI?I( JO salqwasua ~0 apwu salis aym uo sJnmo auedoJd ~0 s!sL[oua%oJpAq leql palsa%%ns uaaq svq l! Jno!mqaq s!ql u!eIdxa OJ. ‘[ 1I] ~001~ ueql ai\!m ssal Japio au0 KIUO SBM NOZOZ seaiaqm [3()()[Z 3gpwuouom aqi ueq1 an!lX SsaI apnl!U%k?lu JO slap10 aalql SBM aldures
130202 aqL “(‘ON)(ON)nX uoJ.4 ueql ‘13nIl u’0J.I paJedaJd sa1du.m aql uo a+lDaga aJow qmw *IXJ u! ‘SBM n3 ~0 uop!ppe aql .pasn JosJnsald aql uo luapuadap ~~3uoJls SBM aseaJDap s!qi JO luaixa aqL .slsiCp2le;, ~0 sa!Jas qloq JOJ aseaJ3ap 01 pun03 SBM ‘aDt+ns aql uo ruyuaqlru 40 mole pm puoDas Jad palJam auedoJd JO sa[otu se passaidxa pm &OOZ le pamwarn ‘(8 ‘3g) ysbloua3oJpLq auedoJd JO (SOL) huanbay Jar\ouJnl aql n3 JO uoyppa uo ‘[I I] auedold JO s!slc -1oua3oJpdq aql u! pauyqo
@nO!Aald
di!lzp3e 3pL1v
io!s/n3-n~ uo uoy23!lsahu! luawaaJ3e u! ‘aleJ!pu! plnos ‘( , ~13
'[g1'g1]
qly
8())1 1”) ,$Q/o3 Jo Wql ql!M paledmo:, ‘pueq s!ql JO Lmanbaq Jaqd!q aql wuolr! na JO mnomr! aql uo luapuadap oslr! s! I! lnq su101e n3 01 paleIaJ s! puzq s!ql laql salmpu! i(jJeaj3 (saseaiaap uaqi pue aldmes 080~ aql uo uInLu!xelu t! 3wqXaJ SawaiD -u! lsig /Ci!sualu! aqi) oyi na/ns aql Bu!svaJm! uo pai\Jasqo / .W3 8EIZ le puCq aql Jo h!Wlu! uoy~l~~ aql ‘JaAoaJoI/\I .3u!sse%no 01 icl!l!qw
‘>I EL@= 1 ,mddoD JO uoyppo JO uopmJ e se SNSZ-H uo pawd -dns saldues (NXXXz) (( LON)(ON)ll~-Xa puk!(131-I'xz) '[311x -xa uo s!s/(loua%o~pLq auedoJd JO sa!suanba.g DAOUXIJ ‘8 %J (x ruo)l, ) [(w + n3) I wl
oa
09
02
w
0
$0
~01 SI!JO asnwaq aq iouuw
+,na
uo paqlospc 03 ‘iy na
i! 1aAaMoq
aql sr! uo!daJ
JO puvq
jellDads awes
oipau3!sse
ILuoqlezypu_u
aql u! y
pueq
s!qL
'[g1’s I] saledaJ33e s!lIelauuq n3-nx Jo uo!lw~~o~ aql 01 palelaJJo3 ApzyJis s! l! aiojaiaql pur! ““8 01 punoq SLLIO~I! Jaddoa uo paq.Iospe 03 01 pawq!Jlll? s! saldum yp?iaw!q [[e uo , _u13 8~ IZ $13pueq aqL ma pue na JO spueq aql JO uopeu!qluo3 aIdLu!s B IOU aie qJ!qM eJlDads 81 ‘13ej u! ‘WA!8 sls/Cpm?~ gysz/n3-n~ uo 03 Jo uoylJospe aqL .aDkyns 1sLl -8183 aql uo paluloj am sale3aJ33v y-n8 ~qpzlauuq
C. Crisafulli et al. /Applied
408
Q g
0.4
2
f0.* 0
0 0
20
[cl’ 1(CU
60
+Iu), atom (
60
X)
Fig. 9. Ratio between the intensity of the evacuable 2138 cm-’ band and the 2046 cm-’ band (Cq,,, ,/Ru) as a function of addition of copper on ex-R&l, (ZxxxCl) and ex-Ru(NOXNO1), (Z.vcxN) samples supported on H-ZSMS. 30 mbar of CO.
However, it is noteworthy that the higher bimetallic formation on the ex-RuC1, samples cannot be attributed to their lower metal dispersion (Table 1). In fact, previous investigations carried out on Ru-Cu samples prepared by using the same ruthenium precursor have shown that the highest degree of interaction between the two metals and the highest Cu surface coverage are obtained on the samples with higher dispersion [25]. The FT-IR data reported in this paper do not allow to draw any conclusion on the nature of the interaction occurring between the two metals. However taking into account that no decrease in the CO/Ru ratio has been observed in addition of Cu both on ex-RuCl, and ex-Ru(NOXNO,), samples (Table l), it seems likely that the amount of copper interacting with ruthenium is relatively small. However, due to the large number of Ru atoms required to form an active ensemble for the hydrogenolysis of propane [1,9,27], this small amount of copper diluting the Ru aggregates is enough to strongly decrease the catalytic activity. A comparison between the FT-IR data presented in this paper and those reported previously on RuCu/SiO, samples [15] shows that the Ru-Cu interaction on H-ZSMS is lower than on SiO,. In fact, whereas on Ru-Cu/ZSMS samples pre-
Surface Science 99 (1996) 401-409
pared from RuCl, at the highest Cu/Ru ratio (sample ZO2OCl) the bands of CO on Ru are still quite evident (Fig. 7), on the contrary on silica, at the same Cu/Ru ratio (ex-RuCl, samples), only the band at 2 138 cm-’ (CO on Cu bound to Rul has been detected. In the spectral region where the band of CO adsorbed on Ru is expected, only a very weak and broad peak has been observed [ 151. On the basis of this observation it can be concluded that on ZSMS bimetallic supported samples the extent of Ru-Cu interaction is lower than on silica, where a complete coverage of the Ru surface atoms by Cu occurs [15,16]. In order to explain the different behaviour observed on silica and zeolite it can be suggested that the degree of interaction between Ru and Cu (i.e. the extent of bimetallic formation) is strongly influenced by the strength of the metal-support interaction: the higher the interaction with the support, the lower is the formation of bimetallic particles [ 10,341. A higher metal-support interaction of the samples supported on ZSMS, compared to the silica ones, has been confirmed by TPR experiments which have shown that the reduction temperature of RuCl,/ZSMS is considerably higher than RuCl JSiO,. On ZSMS the reduction of the Ru precursor starts at 165°C with several peaks at higher temperature, whereas on SiO, the reduction peak is sharp with a maximum at 130°C. A direct correlation between the reduction temperature of the precursor salts used and the strength of the metal-support interaction has been previously reported [ 10,341. The strong metal-support interaction on ZSMS is further confirmed by the FT-IR spectra which show that Ru and Cu metal particles undergo a positive polarization (high extent of Ru6+ or formation of positive ions of copper) through the participation of the OH groups of the zeolite [l&21].
5. Conclusions On the basis of the FT-IR results discussed in this paper the following conclusions can be drawn: 1. On H-ZSMS supported samples, bimetallic RuCu aggregates are formed on the catalyst surface, as confirmed by the 2138 cm-’ band detected on
C. Crisufulli
et al. /Applied
Sutface
the bimetallic samples and attributed to CO adsorbed on Cu bound to Ru’. The degree of Ru-Cu bimetallic formation strongly depends on the precursor used for catalyst preparation. On ex-RuCl, samples the extent of bimetallic Ru-Cu particles is higher than on ex-Ru(NOXNO,),, according to that previously reported on other supports [1.5,33]. These results are in good agreement with the catalytic performance of these systems in the hydrogenolysis of propane. A strong metal-support interaction has been observed on ZSM5 supported samples, leading to a positive polarization of Ru and Cu metal particles. This behaviour can account for the lower degree of Ru-Cu interaction observed on these samples compared to silica supported ones.
Acknowledgements This work was partially supported by a financial contribution from MURST and CNR.
References [II J.H. Sinfelt, J. Catal. 29 (1973) 308. 01 S.Y. Lai and J.C. Vickerman, J. Catal. 90 (1984) 337. i.31D.E. Damiani, E.D.P. Millan and A.J. Rouco, J. Catal. IO1 (1986) 162.
II A.J. Hong. A.J. Rouco. D.E. Resasco and G.L. Haller. J. Phys. Chem. 91 (1987) 2665.
El X. Wu. B.C. Gerstein and T.S. King, J. Catal. 121 (1990) 271.
161G. Ghtotti, F. Boccuzzi, A. Chiorino, S. Galvagno
and C. Crisafulli, J. Catal. 142 (1993) 437. [71 J.E. Houston, C.H.F. Peden, D.S. Blair and D.W. Goodman, Surf. Sci. 167 (1986) 427. 181 B. Sakakini. A.J. Swift, J.C. Vickerman, C. Harendt and K. Christman. J. Chem. Sot. Faraday Trans. I, 83 (1987) 1975.
Science
99 (19961
401-409
409
[9] C. Crisafulli. R. Maggiore, G. Schembari. S. Scire and S. Galvagno. J. Mol. Catal. 50 (1989) 67. [IO] C. Crisafulli. S. Galvagno. R. Ma&ore. S. Scire and A. Saeli. Catal. Lett. 6 (1990) 77. [Ii] R. Ma&ore, C. Criaafulli. S. Scire, L. Solarino and S. Galvagno, React. Kinet. Catal. Len. 48(2) (1992) 367. [12] N. Sheppard and T.T. Nguyen. in: Advances in Infrared and Raman Spectroscopy. Vol. 5 (Heyden and Son. London. 1978). [I31 M. Primet, J. Catal. 88 (1984) 273. [l4] F.J.C.M. Toolenar. F. Stoop and V. Ponec. J. Catal. 82 (1983) I. [l5] C. Crisafulli. R. Maggiore. S. Sciri- and S. Galvagno. J. Chem. Sot. Faraday Trans. 90 (I 994) 2809. 1161R. Liu. B. Tesche and H. Knotinger. J. Catal. 129 (1991) 402. 1171 K. Balakrishnan and J. Schwank, J. Catal. 138 (1992) 491. [I81 J. Sarkany. J.L. d’Itri and W.M.H. Sachtler. Catal. Lett. I6 (1992) 241. 1191 M. Iwamoto, H. Yahiro. K. Tanda. N. Mizuno, Y. Mine and S. Kagawa. J. Phys. Chem. 95 (1991) 3727. G.S. McDougall. A. Guerrero-Ruiz and DO1 C. Marquez-Alvarez. I. Rodriguez-Ramoa. Appl. Surf. Sci. 78 t 1994) 477. 1211 J.Y. Shen. A. Sayari and S. Kaliaguine. Appl. Spectrosc. 46 (1992) 1279. [22] J.Y. Shen. A. Sayari and S. Kaliaguine. Appl. Spectrosc. 16 (1992) 1288. I231 G.H. Yokomizo. C. Louis and A.T. Bell, J. Catal. I20 (1989) I. [241 J.G. Goodwin and C. Naccache, J. Catal. 64 (1980) 482. I251 C. Criaafulli. R. Maggiore. S. Scire. C. Milone and S. Galv*agno, J. Mol. Catal. 83 (1993) 237. [261 J.A. Mieth and A. Schwarz. J. Catal. I I8 (1989) 203. I271 J.A. Dalmon and G.A. Martin, J. Catal. 66 (1980) 214. t281 L. Kustov. V.B. Kazansky, S. Berdn, L. Kuhelkova and P. Jiru, J. Phys. Chem. 91 (1987) 5247. 1291G.J. Millar. C.H. Rochester and K.C. Waugh. J. Chem. Sot. Faraday Trans. 1. 87 (1991) 1367. [301 Y.Y. Huang. J. Catal. 30 (1973) 187. [3ll M.A. Kohler. N.W. Cant, MS. Wainwrtght and D.L. Trimm, J. Catal. 1 I7 (1989) 188. ]32] F.M. Hoffmann and J. Paul. J. Chem. Phys. 87 (1987) 1857. S. Scire. L. Solarino and S. [331 C. Crisafulli. R. Maggiorr. Galvagno. J. Mol. Catal. 63 (1990) 55. ]341 S. Galvagno. C. Crisafulli, R. Maggiore. G.R. Tauszik and A. Giannetto. J. Therm. Anal. 30 (1985) 61 I.
..‘.’ . .
.
. . . . >...~:~_,~,+$
;::.
applied
surface science ELSEVIER
Applied Surface Science 99 ( 1996) 401-409
Ru-Cu/ZSMS
catalysts: characterization by FT-IR spectroscopy
Carmelo Crisafulli a, Salvatore Scir2 a3*, Simona Mini& a, Rosario Maggiore a, Signorino Galvagno b * Diparfimento b Dipartimento
di Scienze Chimiche,
di Chimica
Industriale,
lJniversit&
Uni~ersitir
di Catania,
di Messina,
Viale A. Doria
6, I-95125
Cas. Post. 29, I-98166
Catania.
Sant’Agata
Ital!
di Messinu.
It&
Received 9 November 1995; accepted 7 February 1996
Abstract H-ZSMS supported Ru-Cu bimetallic catalysts have been studied by FI’-IR spectroscopy. Experiments have been carried out on samples prepared from Ru(NO)(NO,), and RuCl,. On the monometallic Ru/ZSMS samples adsorption of CO led to a band at 2046 cm-‘, due to CO linearly adsorbed on Ru’, and several bands at higher frequencies related to CO on Ru’+ species. A higher amount of Rus+ species is formed on the ex-Ru(NOXNO,), sample, due to the smaller metal particle size of this sample compared to the ex-RuCl 3 one. On monometallic Cu/ZSMS two bands at 215 1 and 2158 cm- ’are observed, both related to CO adsorbed on Cu+ ’ sites, formed through a reaction with the OH groups of the zeolite. On the bimetallic Ru-Cu samples, besides all the characteristic bands of the Ru and Cu monometallic samples, a band at 2138 cm- ‘, attributed to CO adsorbed on Cu bound to Ru has been detected, denoting the formation of Ru-Cu bimetallic particles. The fl-IR spectra indicate that on the samples prepared from RuCI, the extent of bimetallic Ru-Cu particles is higher than on those prepared from Ru(NOXNOJ,. This behaviour, attributed to the presence of chlorine on the ex-RuCl, samples, is in agreement with the catalytic performance of these systems in the hydrogenolysis of propane. Keywords:
FT-IR; Bimetallic; Copper; Ruthenium;
ZSMS; Carbon monoxide
1. Introduction
bimetallic
In recent years supported received
growing
attention
bimetallic because
catalysts
have
of their capabil-
ity in controlling
activity, selectivity and stability in many reactions of industrial interest. Among the bimetallic catalysts, Ru-Cu samples have been found of particular interest as model systems in that the two metals are practically immiscible in bulk. The large influence of copper on the activity of ruthenium indicates, however, that the two metals form
* Corresponding 580138.
Tel.:
Copyright SOl69-4332(96)00458-S
0169-4332/96/$15.00 PI/
author.
+39-95.330533;
fax:
+39-95-
aggregates
[l]. A large number
of investi-
gations carried out on powder samples [l-6] and on single crystals [7,8] have shown that the surface composition and therefore the chemisorption properties and the catalytic activity are strongly influenced by the precursor salts used and by the nature of the support [3,4,9- 111. FT-IR spectroscopy has been successfully used in order to obtain more insight on the surface characteristics of mono and bimetallic supported systems [12-171. A previous FT-IR study of CO adsorbed on RuCu/SiO, samples prepared from Ru(NOXNO,), and RuCl, has demonstrated the formation of Ru-Cu
0 1996 Elsevier Science B.V. All rights reserved.
402
C. Crisafidli
ef al. /Applied
Surface Science 99 (19961401-409
particles having a surface covered mainly by Cu [ 151. The amount of Cu on the surface has been found to be higher on the samples prepared from RuCl 3. H-ZSM5 supported copper catalysts have been extensively studied by FI’-IR [18-201 because of the importance of this system in NO., abatement. A few data are instead available on Ru/ZSMS systems 121,221. No FT-IR paper is available, to our knowledge, on Ru-Cu bimetallic systems supported on ZSM5 zeolite. The aim of this work was to verify the formation of bimetallic Ru-Cu particles on H-ZSMS and to correlate this parameter with the catalytic performance of the bimetallic samples.
Table
I
Chemical composition Code
ZlOON ZOSON 2040N Z020N
2.0 I.7 1.o 0.6
-
ZIOOCI ZO8OCl ZO4OCl ZO2OCI
2.0 1.7 1.0 0.6
-
ZOO0
and CO uptakes of Ru-Cu/ZSMS
Ru Cu (wt%z) (w&b)
-
Ru/(Ru (at%)
+ Cu)
CO uptake (cc6TP) g cat-’ )
samples CO/Ru
0.3 1.0 1.4
100 80 40 20
2.44 I .47 2.23 0.92
0.55 0.69 1.00 0.72
0.3 I.0 1.4
100 80 40 20
0.44 0.65 0.31 0.12
0.10 0.17 0.09 0.14
2.0
0
n.d.
-
n.d.: not detectable.
2. Experimental Ru-Cu samples were prepared by incipient wetness impregnation of the support with aqueous solutions of Ru(NOXNO,), or RuCl, (Johnson Matthey) and Cu(NO,), (Carlo Erba), having an appropriate concentration of metals. The salt(s) concentration in the solution was adjusted to yield a total (Cu + Ru) metal content of about 2 wt%. The support used (supplied as powder by Conteka) was a ZSMS zeolite with a BET surface area of 410 m’ g-’ and a SiO,/Al,O, ratio of 30. After impregnation catalyst samples were dried at 393 K for about 24 h, reduced in a flow reactor at 673 K for 1 h with a pure H, stream and then cooled at room temperature CRT) in flowing H,. Chemisorption of CO was measured in a conventional pulse system operating at RT. Pulses of a mixture of 10 ~01% CO in He were used. In order to avoid CO chemisorption on Cu/ZSMS all samples have been reduced in H, for 1 h at 673 K, cooled to 443 K in flowing H,, held in He at this temperature for 1 h to permit H, desorption, and then cooled in flowing He down to RT. Under these conditions negligible amounts of CO were found to chemisorb on the support and on Cu/ZSMS. Chemical composition and CO/Ru ratio of the Ru-Cu/ZSMS samples are reported in Table 1. Chemisorption of CO, instead of H,, was used to avoid spillover phenomena. Spillover of hydrogen from Ru to Cu has been previously reported [5]. The sample code used has the following meaning:
the first letter indicates the support used (Z = ZSMS), while the three digits indicate the atomic percentage of ruthenium in the metallic phase. The last letters indicate the precursor used (Cl = RuCl,, N = Ru(NOXNO,),). For IR studies the powdered samples were pressed into thin self-supporting discs of about 25 mg cm-* and 0.1 mm thick using a pressure of 18 X 1O3 bar. Pellets were evacuated at RT and reduced in pure H,, raising slowly the temperature to 673 K over a period of 2 h and held there for another 1 h. The sample was then cooled in H, at 443 K and evacuated for 1 h at 10m6 mbar and finally cooled at RT. On this reduced sample, CO was admitted at a pressure of 30 mbar, unless otherwise specified. Subsequent evacuations were performed at room temperature or at higher temperatures. The IR spectra were collected on a IT-IR spectrophotometer Perkin-Elmer System 2000 with a resolution of 2 cm-’ and equipped with a MCT detector. Data are reported as difference spectra obtained subtracting the spectrum on the sample recorded before the interaction with CO and are normalized to the same amount of catalyst per cm’ (25 mg/cm*).
3. Results 3. I. Monometallic
Ru and Cu catalysts
Fig. 1A shows the infrared spectra of CO adsorbed at room temperature on the monometallic
C. Crisafulli
22al
et al. /Applied
2m wavsnumbar(cm-l)
Fig. I. (A) Fl-IR spectra of CO adsorbed on the ‘reduced’ ZlOON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO: curve c. after 15 min outgassing at RT. (B) FT-IR spectra of CO adsorbed on the ‘oxidized’ ZIOON sample: curve a. 30 mbar of CO; curve b, 2 mbar of CO: curve c. after IS min outgassing at RT.
Ru/ZSM.5 sample prepared from Ru(NOXNO,), (sample ZIOON). After admission of CO the FT-IR spectrum (spectrum a> shows an asymmetric band of large intensity centred at 2046 cm- ’, attributed to CO linearly adsorbed on metallic Ru [21], and a band of smaller intensity at 2138 cm-‘, assigned to a tricarbonyl species on RuS+ atoms located inside the pores [21,23]. A not well-resolved peak at 2081 cm-’ and several shoulders (region 2 100-2 120 cm _’ ) are also present. The peak at 2081 cm- ’ has been reported to be related to CO linearly adsorbed on Ru’+ and/or to be partially associated to Ru tricarbonyl species [23]. The bands in the spectral region 2100-2120 cm-’ can be attributed to multicarbonyl CO species on RuS+ [24]. A band at 2 166 cm- ’ has been also observed in the spectrum of Fig. 1A. This band could be probably attributed to Ru’+(CO),, species [24] which were formed as a results of an ionic exchange with zeolite protons. The attribution of this peak to CO adsorbed
Sueace Science 94, C19%)40-409
JO3
on the zeolitic support can be excluded due to the high stability to outgassing of this peak (Fig. IA). Upon evacuation at 298 K the band at 2046 cm- ’ (Fig. IA) is quite stable and shifts slightly to lower frequency (2043 cm _ ’). The very low shift observed as the coverage decreases can be related to the very high Ru dispersion of this sample as confirmed by chemisorption results. The intensities of all other bands decrease very slightly upon evacuation. Fig. 1B shows the IR spectra of CO adsorbed on the ZlOON sample after interaction of the sample with O2 at room temperature (‘oxidized’ sample). After evacuation of the sample previously used to record the spectra of CO adsorbed on the ‘reduced’ catalyst, oxygen (20 mbar) was contacted with the sample at RT. After equilibration, CO was admitted. in the presence of Oz. into the cell. The same procedure was used for all the ‘oxidized’ samples. The spectra show a very intense band at 2081 cm _’ and another relevant one at 2 138 cm ’ Minor features are the shoulders at 2046 cm- ’ and 2020 cm-‘. The 2138, 2081 and 2046 cm _ ’ bands have also been observed on the ‘reduced’ sample and have already been discussed. The 2020 cm-’ band can be attributed to CO adsorbed on isolated Ru” atoms. This band is also present (as a shoulder) on the ‘reduced’ sample but it is covered bq the broad 2046 cm- ’ band. The comparison of these spectra with those of the ‘reduced’ sample shows clearly that in the presence of oxygen the intensity of Ru’+ bands (208 I and 2 138 cm-’ 1 strongly increases. with a corresponding decrease of the Ru” band (2046 cm-‘). showing a high affinity of Ru” towards oxygen. The spectra of CO adsorbed on the Ru/ZSMS sample prepared from RuCI, (ZlOOCI) are reported in Fig. 2A. The main feature of the spectrum is the peak at 2046 cm- ’ with a shoulder at 2088 cm- ’. These two bands have also been observed on the ZlOON sample of Fig. 1 and attributed to CO linearly adsorbed respectively on Ru” and Ru”. It is important to note that in this case the 2088 cm--’ /2046 cm-’ intensity ratio. i.e. the Ru”’ fraction. is much lower than that observed on ZIOON. This behaviour could be related to the bigger particle size of ZIOOCI (see chemisorption data of Table 1). which leads to a lower ruthenium-ZSM5 interaction. The larger particle size is confirmed by the stronger shift (IO cm ’)
C. Crisajdli et al. /Applied Surface Science 99 (1996) 401-409
404
2200
2uoo wubwlumba(cm-l)
Fig. 2. (A) FI-IR spectra of CO adsorbed on the ‘reduced’ ZlOOCl sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT. (B) FT-IR spectra of CO adsorbed on the ‘oxidized’ ZlOOCl sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT.
this is in contrast with the absence of this band on the samples prepared from Ru(NOXNO,), suggesting that the band at 2186 cm-’ is related to the higher acidity of the samples containing chlorine. The spectra of the ‘oxidized’ samples (Fig. 2B) show that also on ZlOOCl in the presence of oxygen the intensity of the Rus+ bands (2088, 2138 and 2152 cm-’ bands) strongly increases with a corresponding decrease of the Rue band (2046 cm-’ >. Fig. 3A shows the IR spectra of CO chemisorbed at room temperature on Cu/ZSMS (ZOO0 sample). After admission of CO two main bands (sharp and near) at 2158 and 2151 cm-’ are observed. The 2 158 cm -’ band, attributed to CO adsorbed on Cur+, is relatively stable to outgassing, whereas the 2151 cm-’ band disappears easily upon evacuation at RT. According to Sarkany et al. 1181 this latter band can be attributed to CO adsorbed on Cu’+ sites slightly perturbed by the CO present in the gas phase. A small band at 2180 cm-’ is also present and it has been assigned to CO on aluminium sites of
to lower frequency of the 2046 cm-’ band by evacuation treatments (Fig. 2A). Moreover, on ZlOOCl the intensity of Ru peaks is quite somewhat lower than on ZlOON. Formation of smaller metal particles on samples prepared from Ru(NOXNO,), with respect to those prepared from RuCl, has been previously reported on silica supported samples 14,251. Fig. 2A also shows the presence of a well defined band at 2152 cm-‘, just present as a shoulder in the spectrum of ZlOON sample. This band has been attributed to a Rus+ tricarbonyl species located outside the zeolite pores [21]. As reported in the literature [26], the presence of chlorine strongly reduces the mobility of Ru atoms, so hindering their migration inside the zeolite pores. A small peak at 2186 cm-’ is also present in the spectrum of Fig. 2A. This band easily disappears by RT evacuation and can be attributed to CO adsorbed on aluminium species of the zeolite [28]. However,
Fig. 3. (A) FI-IR spectra of CO adsorbed on the ZOO0 sample: curve a, 30 mbar of CO: curve b, 2 mbar of CO; curve c, after 15 mitt outgassing at RT. (B) FI-IR spectra of CO adsorbed on the ‘oxidized’ ZOO0 sample: curve a, 30 mbar of CO: curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT.
C. Crisafidli
et al. /Applied
305
Surface Science 99 (1996) 401-409
In the presence of oxygen (Fig. 3B) no important changes in the spectrum of CO on Cu/ZSMS are observed. Anyway, the appearance of a small band (shoulder) at 2117 cm-‘, attributed to CO adsorbed on Cu’+ [ 181, can be noted. 3.2. Bimetallic
Fig. 4. Influence of pretreatment conditions on IT-IR spectra of CO adsorbed on the ZOO0 sample after admission of 30 mbar di CO. Pretreatment: reduction in Hz at 673 K and curve a, evacuation at 773 K; curve b. evacuation at 673 K; curve c, cooling in Hz at 443 K and evacuation at 443 K.
the zeolite [28] and/or [311. It is important
associated
to Cu’+
species
to note that the intensity of the bands of CO on Cu is relatively small compared to Ru peaks. However outgassing the copper monometallic sample at 673 K (instead of cooling in H ? at 443 K and then outgassing) the 2 1.51 and 2 158 cm-’ bands (related to Cu” ) become far more intense (Fig. 4). As reported in the literature [ 18,201 this behaviour can be explained with the strong tendency of the Cu atoms (faster at higher temperature) to be oxidized to Cu’+ when supported on H-ZSM5 zeolite through a reaction with the OH groups of the zeolite. On the contrary on Cu/SiO,, Cu” is quite stable to oxidation (at least at 10d6 mbar and 673 K) as confirmed by the presence of just a small band at 2 117 cm-‘, attributed to CO adsorbed on Cu”, which disappears upon evacuation anyway [ 15,291. It has been widely accepted, in fact, that only Cu’+ can irreversibly absorb CO at RT while Cu” and Cu” cannot [19,30]. On the Cu/ZSMS spectrum it is possible to observe also a very small and relatively weak band at 2108 cm- ’ attributed to CO on Cu’. The very small intensity of this peak must be attributed both to the low relative response of the CO on the Cue band and/or to a large Cu metal particle size. A relatively intense peak attributed to CO on Cue has been in fact reported for highly dispersed silica supported copper [3 I I.
Ru-Cu
catalysts
The spectra of bimetallic Ru-Cu samples prepared both from Ru(NOXNO,), and from RuCI, are quite complicated. They present practically all characteristic bands of Ru and Cu monometallic samples with some important differences which will be discussed below. Fig. 5 shows the spectra of bimetallic Ru-Cu samples, prepared from Ru(NOXNO,),, at different Cu/Ru ratios and recorded at 2 mbar of CO. It can be noted that on increasing the Cu/Ru ratio there is a progressive decrease of the broad band at 2046 cm-‘, due to CO linearly adsorbed on Ru”. Also the band at 2081 and the shoulders at higher frequency, related to Ru’+. decrease in intensity on addition of Cu, even if at a less extent. Therefore on increasing the amount of copper the Ru”/Ru’ ratio increases continuously. A different behaviour is instead observed for the
H
1
2000
2200 wovrnumbrr
1600
(cm-l)
Fig. 5. ITIR spectra of CO adsorbed on Ru-Cu/ZSMS prepared from Ru(NOXNO,),. 2 mbar of CO.
samples
406
C. Crisqfulli et al. /Applied
Surface Science 99 (IYYri) 40/-409
2138 cm- ’ band. The intensity of this band at first increases with the Cu/Ru ratio, reaching a maximum on Z080N, and then decreases at higher Cu contents. This band is in the same spectral region of the band of CO adsorbed on Ru6+. However it cannot be assigned to Rus+-CO species in so far it shows a quite low stability to outgassing as shown in Fig. 6, where the behaviour of the CO bands to evacuation on the monometallic ZlOON (Fig. 6A) and on the bimetallic Z080N samples (Fig. 6B) is reported. In agreement with previous results of Knozinger et al. [16] this band can be attributed to CO adsorbed on Cu surface atoms bound to Ru atoms. A frequency of 2 138 cm-’ also has been reported on single Cu atoms adsorbed on Ru(OOl> [32]. A similar behaviour also has been observed on Ru-Cu silica supported catalysts and correlated with bimetallic formation [ 151. It can be therefore concluded that the band at 2138 cm-’ is made of two components. One is related to CO adsorbed on RuGi which is stable upon evacuation. The other component is assigned to CO adsorbed on Cu atoms bound to Ru and it is easily eliminated by evacuation at room temperature. The relative amount of the two species can be calculated by recording the spectra before and after evacuation. The attribution of the 2138 cm-’ band is further confirmed by the spectra of the Ru-Cu samples
2200 wav.numbrr
2000 (om-
2200 1)
wavmumbrr
2000 2200 wovwumbor
2000 (cm-
1 BOO 1)
Fig. 7. FT-IR spectra of CO adsorbed on Ru-Cu/ZSM5 prepared from R&I,. 2 mbar of CO.
samples
prepared from RuCl, (Fig. 7) and recorded at 2 mbar of co. On increasing the Cu/Ru ratio, in fact, a behaviour similar to the ex-Ru(NOXNO,), samples is observed. In this case the attribution of the 2138 cm -’ band to CO adsorbed on Cu bound to Ru is more simple due to the fact that on the ex-RuCl, monometallic Ru sample the 2138 cm-’ band is very small. The intensity of the band at 2 158 cm-’ (CO on Cu’+ > measured on the Ru-Cu bimetallic samples is higher than that observed on ZOO0 (monometallic Cu/ZSMS sample), notwithstanding the lower amount of Cu present on the bimetallic samples. Even though this result needs further investigations it is possible that on the Ru-Cu samples ruthenium catalyses the oxidation of copper by the OH groups of the zeolite at low temperature, thus increasing the fraction of Cu’ + present on the sample.
Ia00 (cm-
1)
Fig. 6. (A) Influence of outgassing at RT on the ZlOON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO; curve c, after 15 min outgassing at RT. (B) Influence of outgassing at RT on the ZO8ON sample: curve a, 30 mbar of CO; curve b, 2 mbar of CO: curve c, after I5 min outgassing at RT.
4. Discussion To correlate the ET-IR spectra of the bimetallic Ru-Cu/ZSMS samples and their catalytic performance, let us summarize briefly the results of cat-
.u~ouyun
II!IS
s!
saw3aJ33v
3g
-1elauuq ns-na 40 uo~lw_i~o~ aql saxanuu! au!lolq3 qxq~ qBnolq1uyueqsaw aqL ‘[EE’ 1[ ‘11 pasn lJod -dns aql JO ssalpla3al uogcw~alu! ns-nx 01 pauodal
.saldwes asaql uo UO~ELLI~OJ
laq3!q G 3uy.1yuo~
-aId slsiCpwz aql uo doe
"‘Iya
uroy
Lq
aql 3ugzwqns
paled
u! aseal3ap laq%!q q3t-w
l2lCp &!h!lX
u! air? silnsal asaql
3!1/ClElt?:, aql ql!M
'pueq
aqi JO /(i!suaiu!leioi aqi u101~ (+yna lti IL' uo!ianDeAa
luauodwos
aql 1noAEJ
i(lsno!Aald uaaq seq au!lolqD 3u~uy~uo~
slos.uwald JO asn aqL 3qlwaw!q
t?MOqS L/3!ljM ‘(8 %J) iuakuaal3v
03)
paAolua1
,_um 8EIz uo paqlospr!
1OU S!
Lq pauyqo
qD!qM
uaaq wq
nx
01 punoq n3 uo 03 JO pueq aqi JO Qsuaiu! aqi ieqi palou aq isnlu 11 ‘(6 .3y> (KON)(()N)n’d-Xa aqi UO usql saldues 's! (,+v
uo
'13nx-xa
aql uo laq3!q qzcnu 'IDVJ u!
03) , -m3 5x70~.1~ pucq w JO ieqi pue
( , ~3 8~ I z) na ol punoq n3 uo 03 ~0 puw aql ~0 iCl!sualu!aql uaawaq ns-na
oyl
aql
'UO~RLLI.IOJsaisgled
~!1lvIau_uqJO iuaixa aqi sawanuu!
pasn loslnaa.td aql leql aDuap!Aa aA! s! y saldlues na-na
JO ewads
43~011s
01 aIq!ssod asp
81-u
aql LLIOLJ
'[go] yr?aM /(laAgeIal s! uo!i3waiu! D!uoliJala aqi ieqi papnlwo3 na/03
uaaq seq i! ‘pahlasqo IOU s! pueq
aqi JO slaqwnuaAvm
'OJ'4 3wpuodsauo3
aqi wqi
lam01
sple~oi
aql SE '1aAaMOH
-DeJalu! aql i(q pazylod n3
sno!,taJd
iyqs
'IQ ql!M UO!l
i([aAysod air!surole azzpns
saldum
pavoddns
S~SZ-H
uo
oslr! ieqi su~lyuo~
laded s!ql u! paluasald uope3psaAu! '[Lz] ysLloua3olpLq
auedold
x1-u
aqL
104 alq!suodsal
‘salqmasua na aApze aqiJ0 laqwnu aqi aloJalaqi puv stole na luaze[ppe aql JO Jaqumu aql aseaJ3ap p[noM um!uaqlnJ JO aDepns aql uo Jaddo3 tlau! JO ama -saJd aqL ‘[I] ylnq aql u! sp.?laLu 0~1 aqi ~0 Ll!l!q!D -syu~ aql alldsap pauuoJ aJtz sale3aJ33e zqlclauuq *saldu_m na-na aql ug .saIqluasua qms SKI! -pug JO AlyqeqoJd aql 01 paleIa1 s! h!l\p2 3!1@?1~?3 aql pue surole na lua3rz[ppI?I( JO salqwasua ~0 apwu salis aym uo sJnmo auedoJd ~0 s!sL[oua%oJpAq leql palsa%%ns uaaq svq l! Jno!mqaq s!ql u!eIdxa OJ. ‘[ 1I] ~001~ ueql ai\!m ssal Japio au0 KIUO SBM NOZOZ seaiaqm [3()()[Z 3gpwuouom aqi ueq1 an!lX SsaI apnl!U%k?lu JO slap10 aalql SBM aldures
130202 aqL “(‘ON)(ON)nX uoJ.4 ueql ‘13nIl u’0J.I paJedaJd sa1du.m aql uo a+lDaga aJow qmw *IXJ u! ‘SBM n3 ~0 uop!ppe aql .pasn JosJnsald aql uo luapuadap ~~3uoJls SBM aseaJDap s!qi JO luaixa aqL .slsiCp2le;, ~0 sa!Jas qloq JOJ aseaJ3ap 01 pun03 SBM ‘aDt+ns aql uo ruyuaqlru 40 mole pm puoDas Jad palJam auedoJd JO sa[otu se passaidxa pm &OOZ le pamwarn ‘(8 ‘3g) ysbloua3oJpLq auedoJd JO (SOL) huanbay Jar\ouJnl aql n3 JO uoyppa uo ‘[I I] auedold JO s!slc -1oua3oJpdq aql u! pauyqo
@nO!Aald
di!lzp3e 3pL1v
io!s/n3-n~ uo uoy23!lsahu! luawaaJ3e u! ‘aleJ!pu! plnos ‘( , ~13
'[g1'g1]
qly
8())1 1”) ,$Q/o3 Jo Wql ql!M paledmo:, ‘pueq s!ql JO Lmanbaq Jaqd!q aql wuolr! na JO mnomr! aql uo luapuadap oslr! s! I! lnq su101e n3 01 paleIaJ s! puzq s!ql laql salmpu! i(jJeaj3 (saseaiaap uaqi pue aldmes 080~ aql uo uInLu!xelu t! 3wqXaJ SawaiD -u! lsig /Ci!sualu! aqi) oyi na/ns aql Bu!svaJm! uo pai\Jasqo / .W3 8EIZ le puCq aql Jo h!Wlu! uoy~l~~ aql ‘JaAoaJoI/\I .3u!sse%no 01 icl!l!qw
‘>I EL@= 1 ,mddoD JO uoyppo JO uopmJ e se SNSZ-H uo pawd -dns saldues (NXXXz) (( LON)(ON)ll~-Xa puk!(131-I'xz) '[311x -xa uo s!s/(loua%o~pLq auedoJd JO sa!suanba.g DAOUXIJ ‘8 %J (x ruo)l, ) [(w + n3) I wl
oa
09
02
w
0
$0
~01 SI!JO asnwaq aq iouuw
+,na
uo paqlospc 03 ‘iy na
i! 1aAaMoq
aql sr! uo!daJ
JO puvq
jellDads awes
oipau3!sse
ILuoqlezypu_u
aql u! y
pueq
s!qL
'[g1’s I] saledaJ33e s!lIelauuq n3-nx Jo uo!lw~~o~ aql 01 palelaJJo3 ApzyJis s! l! aiojaiaql pur! ““8 01 punoq SLLIO~I! Jaddoa uo paq.Iospe 03 01 pawq!Jlll? s! saldum yp?iaw!q [[e uo , _u13 8~ IZ $13pueq aqL ma pue na JO spueq aql JO uopeu!qluo3 aIdLu!s B IOU aie qJ!qM eJlDads 81 ‘13ej u! ‘WA!8 sls/Cpm?~ gysz/n3-n~ uo 03 Jo uoylJospe aqL .aDkyns 1sLl -8183 aql uo paluloj am sale3aJ33v y-n8 ~qpzlauuq
C. Crisafulli et al. /Applied
408
Q g
0.4
2
f0.* 0
0 0
20
[cl’ 1(CU
60
+Iu), atom (
60
X)
Fig. 9. Ratio between the intensity of the evacuable 2138 cm-’ band and the 2046 cm-’ band (Cq,,, ,/Ru) as a function of addition of copper on ex-R&l, (ZxxxCl) and ex-Ru(NOXNO1), (Z.vcxN) samples supported on H-ZSMS. 30 mbar of CO.
However, it is noteworthy that the higher bimetallic formation on the ex-RuC1, samples cannot be attributed to their lower metal dispersion (Table 1). In fact, previous investigations carried out on Ru-Cu samples prepared by using the same ruthenium precursor have shown that the highest degree of interaction between the two metals and the highest Cu surface coverage are obtained on the samples with higher dispersion [25]. The FT-IR data reported in this paper do not allow to draw any conclusion on the nature of the interaction occurring between the two metals. However taking into account that no decrease in the CO/Ru ratio has been observed in addition of Cu both on ex-RuCl, and ex-Ru(NOXNO,), samples (Table l), it seems likely that the amount of copper interacting with ruthenium is relatively small. However, due to the large number of Ru atoms required to form an active ensemble for the hydrogenolysis of propane [1,9,27], this small amount of copper diluting the Ru aggregates is enough to strongly decrease the catalytic activity. A comparison between the FT-IR data presented in this paper and those reported previously on RuCu/SiO, samples [15] shows that the Ru-Cu interaction on H-ZSMS is lower than on SiO,. In fact, whereas on Ru-Cu/ZSMS samples pre-
Surface Science 99 (1996) 401-409
pared from RuCl, at the highest Cu/Ru ratio (sample ZO2OCl) the bands of CO on Ru are still quite evident (Fig. 7), on the contrary on silica, at the same Cu/Ru ratio (ex-RuCl, samples), only the band at 2 138 cm-’ (CO on Cu bound to Rul has been detected. In the spectral region where the band of CO adsorbed on Ru is expected, only a very weak and broad peak has been observed [ 151. On the basis of this observation it can be concluded that on ZSMS bimetallic supported samples the extent of Ru-Cu interaction is lower than on silica, where a complete coverage of the Ru surface atoms by Cu occurs [15,16]. In order to explain the different behaviour observed on silica and zeolite it can be suggested that the degree of interaction between Ru and Cu (i.e. the extent of bimetallic formation) is strongly influenced by the strength of the metal-support interaction: the higher the interaction with the support, the lower is the formation of bimetallic particles [ 10,341. A higher metal-support interaction of the samples supported on ZSMS, compared to the silica ones, has been confirmed by TPR experiments which have shown that the reduction temperature of RuCl,/ZSMS is considerably higher than RuCl JSiO,. On ZSMS the reduction of the Ru precursor starts at 165°C with several peaks at higher temperature, whereas on SiO, the reduction peak is sharp with a maximum at 130°C. A direct correlation between the reduction temperature of the precursor salts used and the strength of the metal-support interaction has been previously reported [ 10,341. The strong metal-support interaction on ZSMS is further confirmed by the FT-IR spectra which show that Ru and Cu metal particles undergo a positive polarization (high extent of Ru6+ or formation of positive ions of copper) through the participation of the OH groups of the zeolite [l&21].
5. Conclusions On the basis of the FT-IR results discussed in this paper the following conclusions can be drawn: 1. On H-ZSMS supported samples, bimetallic RuCu aggregates are formed on the catalyst surface, as confirmed by the 2138 cm-’ band detected on
C. Crisufulli
et al. /Applied
Sutface
the bimetallic samples and attributed to CO adsorbed on Cu bound to Ru’. The degree of Ru-Cu bimetallic formation strongly depends on the precursor used for catalyst preparation. On ex-RuCl, samples the extent of bimetallic Ru-Cu particles is higher than on ex-Ru(NOXNO,),, according to that previously reported on other supports [1.5,33]. These results are in good agreement with the catalytic performance of these systems in the hydrogenolysis of propane. A strong metal-support interaction has been observed on ZSM5 supported samples, leading to a positive polarization of Ru and Cu metal particles. This behaviour can account for the lower degree of Ru-Cu interaction observed on these samples compared to silica supported ones.
Acknowledgements This work was partially supported by a financial contribution from MURST and CNR.
References [II J.H. Sinfelt, J. Catal. 29 (1973) 308. 01 S.Y. Lai and J.C. Vickerman, J. Catal. 90 (1984) 337. i.31D.E. Damiani, E.D.P. Millan and A.J. Rouco, J. Catal. IO1 (1986) 162.
II A.J. Hong. A.J. Rouco. D.E. Resasco and G.L. Haller. J. Phys. Chem. 91 (1987) 2665.
El X. Wu. B.C. Gerstein and T.S. King, J. Catal. 121 (1990) 271.
161G. Ghtotti, F. Boccuzzi, A. Chiorino, S. Galvagno
and C. Crisafulli, J. Catal. 142 (1993) 437. [71 J.E. Houston, C.H.F. Peden, D.S. Blair and D.W. Goodman, Surf. Sci. 167 (1986) 427. 181 B. Sakakini. A.J. Swift, J.C. Vickerman, C. Harendt and K. Christman. J. Chem. Sot. Faraday Trans. I, 83 (1987) 1975.
Science
99 (19961
401-409
409
[9] C. Crisafulli. R. Maggiore, G. Schembari. S. Scire and S. Galvagno. J. Mol. Catal. 50 (1989) 67. [IO] C. Crisafulli. S. Galvagno. R. Ma&ore. S. Scire and A. Saeli. Catal. Lett. 6 (1990) 77. [Ii] R. Ma&ore, C. Criaafulli. S. Scire, L. Solarino and S. Galvagno, React. Kinet. Catal. Len. 48(2) (1992) 367. [12] N. Sheppard and T.T. Nguyen. in: Advances in Infrared and Raman Spectroscopy. Vol. 5 (Heyden and Son. London. 1978). [I31 M. Primet, J. Catal. 88 (1984) 273. [l4] F.J.C.M. Toolenar. F. Stoop and V. Ponec. J. Catal. 82 (1983) I. [l5] C. Crisafulli. R. Maggiore. S. Sciri- and S. Galvagno. J. Chem. Sot. Faraday Trans. 90 (I 994) 2809. 1161R. Liu. B. Tesche and H. Knotinger. J. Catal. 129 (1991) 402. 1171 K. Balakrishnan and J. Schwank, J. Catal. 138 (1992) 491. [I81 J. Sarkany. J.L. d’Itri and W.M.H. Sachtler. Catal. Lett. I6 (1992) 241. 1191 M. Iwamoto, H. Yahiro. K. Tanda. N. Mizuno, Y. Mine and S. Kagawa. J. Phys. Chem. 95 (1991) 3727. G.S. McDougall. A. Guerrero-Ruiz and DO1 C. Marquez-Alvarez. I. Rodriguez-Ramoa. Appl. Surf. Sci. 78 t 1994) 477. 1211 J.Y. Shen. A. Sayari and S. Kaliaguine. Appl. Spectrosc. 46 (1992) 1279. [22] J.Y. Shen. A. Sayari and S. Kaliaguine. Appl. Spectrosc. 16 (1992) 1288. I231 G.H. Yokomizo. C. Louis and A.T. Bell, J. Catal. I20 (1989) I. [241 J.G. Goodwin and C. Naccache, J. Catal. 64 (1980) 482. I251 C. Criaafulli. R. Maggiore. S. Scire. C. Milone and S. Galv*agno, J. Mol. Catal. 83 (1993) 237. [261 J.A. Mieth and A. Schwarz. J. Catal. I I8 (1989) 203. I271 J.A. Dalmon and G.A. Martin, J. Catal. 66 (1980) 214. t281 L. Kustov. V.B. Kazansky, S. Berdn, L. Kuhelkova and P. Jiru, J. Phys. Chem. 91 (1987) 5247. 1291G.J. Millar. C.H. Rochester and K.C. Waugh. J. Chem. Sot. Faraday Trans. 1. 87 (1991) 1367. [301 Y.Y. Huang. J. Catal. 30 (1973) 187. [3ll M.A. Kohler. N.W. Cant, MS. Wainwrtght and D.L. Trimm, J. Catal. 1 I7 (1989) 188. ]32] F.M. Hoffmann and J. Paul. J. Chem. Phys. 87 (1987) 1857. S. Scire. L. Solarino and S. [331 C. Crisafulli. R. Maggiorr. Galvagno. J. Mol. Catal. 63 (1990) 55. ]341 S. Galvagno. C. Crisafulli, R. Maggiore. G.R. Tauszik and A. Giannetto. J. Therm. Anal. 30 (1985) 61 I.