2480
The Journal of Physical Chemistry, Vol. 83,No. 19, 1979
R. T. Rewick, B. J. Wood,
and H. Wise
Mixed Metal Catalysis. 1. Properties of AI2O3 Supported Ir/Ni for Hydrazine Decomposition Robert T. Rewick, Bernard J. Wood, and Henry Wise" Solid State Catalysis Laboratory, SRI International, Menlo Park, California 94025 (Received March 26, 1979) Publication costs assisted by SRl li?lernationai
The kinetics of hydrazine decomposition catalyzed by alumina-supported Ir/Ni have been examined over a range of mixed-metal compositions. The results demonstrate that with Ni additions up to 40 atom 70the level of activity of the Ir catalyst is maintained, and the degree of catalyst deactivation,caused by prolonged hydrazine exposure, greatly reduced. In the bulk phase the Ir/Ni system forms a homogeneous fccub random alloy. However, on the surface, enrichment with Ir is observed by Auger electron spectroscopy. The kinetics of hydrazine decomposition and catalyst deactivation are interpreted in terms of the surface compositions of the two-component metal catalyst system.
Introduction In previous studies,'V2 we noted the formation of strongly bound nitrogen adspecies on alumina-supported iridium catalysts during prolonged exposure to hydrazine. Accumulation of this irreversibly sorbed adsorbate was accompanied by a decay in the catalytic activity of the surface for hydrazine decomposition. In the present study we explore the possibility of modifying the surface properties of Ir by the addition of Ni, with which Ir forms a homogeneous, random, binary alloy over the entire composition range.3 Changes in the properties of group VI11 metals by alloy formation with another metal component are known to occur for a number of catalytic rea c t i o n ~ .Also, ~ recent photoelectron emission studies with two-component systems have provided some information on the changes in electronic properties brought about by alloy f ~ r m a t i o n .Combined ~ with thermal desorption data, these results have given some insight into the local nature of chemical surface bonding and its modification by substrate alloying. For reactions requiring clusters of identical surface atoms, a geometric effect has been demonstrable6 in which the added metal alters the surface distribution of neighboring atoms. In some cases the experimental results have suggested a change in the electronic properties of the solid by alloy f ~ r m a t i o n . ~ Experimental Section Catalyst Preparation and Characterization. The Ir/ Alz03 catalyst, containing 10 wt % Ir, was supplied by Shell Development Co. It was used in the preparation of the mixed metal catalyst samples. The nickel-promoted catalyst was prepared by treating the Ir/A1203 catalyst with an aqueous solution of Ni(NO3),.6H20. We adjusted the concentration of the nickel salt in the aqueous solution to the desired weight loading of metallic Ni and allowed a measured aliquot of the solution to wet the A1203-supported iridium catalyst (10 wt % Ir). The mixture was evaporated to dryness on a steam bath, air dried at 373 K, and calcined for 1 h at 573 K. The samples were reduced in H, (1 atm) for several hours at 723 K followed by heat treatment in He at 723 K for 15 h. The nickel weight loadings prepared ranged from 1to 10 w t % on a dried and reduced basis. The bulk compositions of the mixed metal catalysts were evaluated by X-ray diffraction. The elemental surface compositions of the catalysts were determined by Auger electron spectroscopy (AES). 0022-3654/79/2083-2480$0 1.OO/O
For the infrared absorption measurements of surface adsorbates, a catalyst was required with relatively higher optical transparency than could be obtained with the wet impregnation technique. For this purpose the catalyst was prepared by deposition of five 0.05-cm3 aliquots of an aqueous dispersion of IrC14 in colloidal A1203 (Baymal) on each side of a BaFz crystal disk (8 X 8 X 25 mm). Each aliquot was spread to wet the entire optical surfaces and evaporated to dryness with an IR heat lamp before addition of the next aliquot. By this technique, 3 mg of 10 wt % Ir on A1203were dispersed on the BaF2 disk. This technique was also used to prepare a blank containing only Al,03. Before study, the catalyst was reduced in H, at 723 K for 30 min, flushed in He at 723 K for 10 min, then cooled to room temperature in He. Apparatus and Procedure. A powdered sample of catalyst (1.5 f 0.3 mg) was placed on the glass frit of a differential flow microreactor made of Pyrex glass. By use of switching valves, the reactor could be operated in either of two modes. In the reduction mode, the catalyst was exposed in situ to a stream of H2 (723 K, 1 h) and then flushed in He (723 K, 0.5 h). In the reaction mode, the catalyst was exposed at a specified temperature to a N2H4 reactant stream (about 2 vol % N2H4, 98 vol % He) generated by bubbling He at a slow rate through liquid N2H4( O h technical grade) maintained at 298 K. After a specified time of exposure to gaseous N2H4,the catalyst activity was measured a t 300 K by injecting from a glass syringe an aliquot of liquid N2H4(6.3 X lo4 mol) through a rubber septum above the reactor into the He carrier stream. The reactant was vaporized during passage through the heated upper inlet section of the reactor before coming in contact with the catalyst. The degree of N2H4 conversion was determined by gas chromatographic analysis (GC) of the mass of Nzformed after removal of unreacted N2H4and NHB from the product stream by condensation in a cold trap (77 K). This analysis provides a quantitative measure of catalyst activity, because "3, Nz, and Hz were found to be the only products of N2H4 decomposition at 300 K in accordance with the stoichiometry:2 2N2H4= 2NH3 + N2 + H2. To obtain estimates of the metal surface site density, we exposed the reduced catalyst samples to pulses of a 10 vol % CO-in-He mixture at 300 K and a total pressure of 1 atm until GC analysis indicated surface saturation. All gases used in this study were purified. Hydrogen was diffused through a Pd thimble at 573 K; helium was passed
0 1979 American Chemical Society
Mixed Metal Catalysis
The JOUt'flal
Qf
Physical Chemistry, V d . 83, NO. 19, 1979 2481
tained at 300 K. Before entering the cell the NzH,-Me gas stream was further diluted with Me to yield -1 vol % NZH.4.
Results Bulk Composition of Catalyst. The X-ray diffraction results indicated that the 15-h heat treatment in Hzat 723 K led to the formation of Tr/Ni alloy crystallites. The
I
i
change in interatomic spacing of the fccub two-component lattice, as given by the X-ray diffraction maxima, followed Vegard's law. Surface Composition of Catalyst, Auger electron spectra of the catalysts were obtained with a Varian cylindrical mirror analyzer that displayed the Auger electron lines in the usual derivative mode. For our analysis, we used the MjN704(230 eV) transition for Ir, and the L,M4,jN14,6(850 eV) transition for Ni. In the absence of chemical line shifts, as was the case in our measurements, the amplitudes of these Auger peaks are proportional t o the concentrations of the respective components6 and the peak height ratio, R = INi/IIr, is a measure of the relative surface abundance of the respective metals. However, to compensate for differences in Auger electron yield between the two lines, the peak height ratio for samples of the pure metals, Ro = IF:/II:, obtained under ident,ical instrumental conditions, is used to obtain the normalized relative abundance
A
N (a1
Gas Outlet
Ibl
Pyrex String
IC1
Gas Inlet
id)
S S Sample Holder
(e)
Sample 1 2 7 x 0 36 mm
If) Igl
Teflon Sleeve
(hl
Horizontal Section 70 x 2 5 4 mm
(11
NaCl Window
(11
Nichrome qeating Wire
Sample Positioning Tube
(kl
Thermowell
(I1
Vertical Section 130 x 21
= R/RO
(1)
An additional correction in N is necessary for this alloy system to account for the significant difference in the escape depths of the Auger electrons for the two metals. Auger electrons from Ir with an energy of 230 eV have a mean free path, XIr, in the solid of about 68 nm, whereas those from Ni with an energy of 850 eV possess a mean escape depth, .ANi, of about 110 n n 7 To a good approximation, the attenuation of Auger electrons in the solid foIlows an exponential decay law: m
Ii = kici C X,'exp(-zni/Xi cos 6 )
mm
in sequence over copper turnings at 523 K an, a molecular sieve (Union Carbide 5A) at 77 K; the CO/He mixture was passed through a glass bead trap at 77 K (to remove metal carbonyls). The infrared (IR) absorption studies were carried out with a T-shaped cell made of Pyrex glass (Figure 1). The outside of the vertical section of the cell was wrapped with resistance wire for external heating, and provided with a thermowell for temperature measurement. The end plates of the horizontal section of the cell were NaCl windows. The catalyst, deposited on the BaF, crystal, suspended from a Pyrex fiber string, could be raised and lowered inside the vertical section of the cell by means of a specially designed Teflon stopcock. In the vertical section, the catalyst could be pretreated by exposure to gases at elevated temperatures. For IR measurements, the crystal was lowered into the horizontal window section where it could be exposed to gaseous reactants or to inert carrier gas (He or Ar) at room temperature. Infrared spectra were obtained under flow conditions at 1 atm total pressure with a Perkin-Elmer Model 457 spectrometer. With the catalyst samples under study, good spectral transmission was achieved in the frequency region from 4000 to 1200 cm-l. A scan over this spectral region is sufficient to detect most of the reported OH and NH vibrational frequencies. Hydrazine was introduced into the feed stream hy bubbling the He carrier gas through liquid hydrazine main-
(2)
n=l
Figure 1. The infrared cell.
where hi is a constant including instrumental parameters, sample geometry, and the backscattering factor; ci is the number of atoms per unit surface area; X,' is the atom fraction of component i in the ntb atomic layer; zEiis the distance of the nth layer from the surface; and 0 is the acceptance angle of the cylindrical mirror analyzer. Combining eq 1 and 2, we obtains m
N
m
= (ICX," eXp(-znd*Y/XWi cos S ) j / [ f Cexp(-ZEN'/ n=l
?Z-1 m
XNi cos 6)])/( [
c Xnl' exp(-znauOY/Xlycos a)]/
.i=l
m
[dC exp(-znIr/.A1, n=l
cos
@I] (3)
Values of z are computed from the lattice constants, a, of the pure metals and the various alloy compositions, and f = (ad0Y/ai)*.If we assume that the chemical composition changes are restricted to the surface and that the submonolayer strata are identical in composition with the bulk, the atom fraction, x:, of each component for the surface layer ( n = 11,can be computed from the measured value of N by using eq 3. Subsequently, different values of Xzic a n be evaluated by using the previously calculated values of xli and assuming that Xn,2i = Xbulki. We have analyzed our experimental data by this procedure for values of n = 1-10, The results (Figure 2) indicate that
R. T. Rewick, B. J. Wood, and H. Wise
TABLE 11: @8 Adsorption Studies with [Ni-Ir]/Al,O, Catalysts at 300 K CO adsorbed catalyst compn (bulk), -+
ab.
07
/o
l l _ _ l _
I _
wt (mol'g
loading (Xr t NP),
Ir
Ni
wt%
100 77 52 35 21
0 23 48 65 79 100
10.0 10.8 12.0 14.8 19.1 10.1
0
catalyst) X
lo4
1.55 1.53 1.79 2.16 2.51 0.50
imol/g mol CO/ ketal7 mol x I O 3 ( l r -t Ni) 1.85 1.42 1.49 1.46 1.31 0.50
0.36 0.18 0.14 0.11 0.09 0.03
TABLE HI: Activity of h / A 1 2 0 3and [NI-Ir]/AI,O, Catalysts for N2H4Decomposition at 300 K catalyst compn Ir, at. % 0
0.2
04
o.a
05
bulk
10
surface'
Figure 2. Surface composition of Ni/Ir/AIPO, catalysts Auger electron specirascopy.
determined by
TABLE I : Surface ~ o ~ Catalysts Determined b y AES
~
~ of oMixed s ~
~
~
n
l_lll"--_-~I1_--
Ni atomic fraction l l l _ l l -
catalyst prepna pretreatment x b U , k N i I _
Ni added t o Ir!A1, 0
Ir added to Ni/Al,O, Ni added to IriA1,O
__--_l___l_
reduced and annealedb
redtieedb
Art sputter etchC
xLNi xZWi __
100 100 100 87 64 0
100 77 52 35 21 0
P
re1 log conversna
I_________x
BULL< ATOM FRAC.I-IOV NI, X I '
a 10 wt % Ir/Ai,O, = 1.00. Cluster probability (see text). Based on smooth curve of Figure 2.
TABLE IV: Catalyst Deactivation b y Hydrazine ExDosurea
0
0.11
0.65 0.79
0.23
0.15
0.65 0.79
0.89
0.78
0.89
bulk
surface'
loss,b %
100 77 52 35 21
100 100 100 87 64
52 53 29 27 32
0.79
0.97
0.48 0.65 0.79
0.41 0.67
0.48 0.65 0.79
0.7')
See text. ADdescribed in text. One minute at approximately 40 X 1 0 ' A C ~ I - ~At . unit sputtering efficiency this ion current ~ o t n l dremove about, 10 atomic layers. a
significant Ir enrichment OCCUR near the surface. These residt,s are supported hy Rubsequent AES examination of subsurface atomic layers exposed by removal of a few outer layers of the metals by argon-ion hoaiibardment. In the case of N1--Ir alloys, selective sputtering would not be expected to occur, as the low-energy Ar" sputter yields for the two individual metals are comparable within 2 5 % m9 The observed composition of the deeper layers corresponds closely to that, of the bulk (Table I). AES measurement were made on two additioxial catalysts prepared by impregnation of a 26 wt 910 Ni/A120, ca.talyst with i?q~ieous KzIrCls to give respective hulk compositions of 89 and 97 atom % Ni. They were subsequently reduced in H2 for 18 h at 7 2 3 K . The AES analysis indicates that tho surfaces of these prepmations also are enriched in Br (Table I and Figure 2 ) . Surface Site Density. In comparing the activities of the e ~ t composi.tions,we found it useful cat,alysLs of ~ ~ f f e rmetal to relate the fractional conversion of N2M,to the total number of metal adsorption sites available. Determinations of CO adsorption to saturation coverage (Table 11) provide a measure of the total number of Ir and Ni surface sites capable of bonding C 8 . Since the metal-to-carbon monoxide bonding stoichiometry is not !mourn for the pure metal or the mixed metal crystallites, quantitative evaluatioa of the respective surface site densities cannot be
,
1.0 1.0 1.0 0.7 0.5 0
0.48
0,97
~
catalyst compn Ir, at. %
fractional activity
Each catalyst sample exposed continuously to 8.4 x mol of N,H, at 448 K before testing for activity with a pulse of N,H, (6.3 X mol) a t 300 K Evaluated from logarithmic fractional conversion per gram of total metal of fresh and used catalyst semples. ' Based o n smooth curve of Figure 2. a
made. However, in terms of the number of CO molecules adsorbed per total number of metal atoms in the sample (CO/M, where M = Ir Ni) we note a gradual decrease as Ni is incorporated into the samples. Most likely this decrease is associated with growth in mixed metal crystallite size, since both Ni and Ir strongly bond CQ. Temperature-programmed desorption experiments with the catalysts under study demonstrate the formation of a new binding energy level located in between that of Ir and Ni.'O Catulytic Activity for Hydrazine Decomposition. For a first-order surface reaction, such as N2H4decomposition on Ir/A1203,1,2the pulse microreactor technique allows quantitative evaluation of kinetic rate data. Also for a first-order reaction in which the surface reaction rather than the attainment of adsorption equilibrium is rate controllling,ll the logarithmic fractional conversion of reactant in the pulse is proportional to the conversion coefficient. This coefficient is the product of the rate constant, the adsorption equilibrium constant, and the retention time. To compare the activities of the various catalyst samples, we have expressed our results (Table 111) in terms of logarithmic conversion relative to that of the Ir/A1203catalyst.
+
~
~
_ l _ _ _ l
1.0 i 0.1 1.1I 0.1 1.9 i 0.1 0.60 f 0.06 0.44 I 0.04 0.01i 0.005
The Journal of Physical Ch:hemistry, Vol. 89,No. 19, 1979 2483
Mixed Metal Catalysis
0.0
I
I
1
I
0.11
W
z m
a
v1
0.3
:
o'2
1
1603 IN2HII
l 4000
3500
I
_
I
I 2500
3000
_
i
_
2000
_
i
d
I 14W
1600
1800
I
_ 1200
WAVENUMBER (ern-'/
Figure 3. Infrared spectrum of Ir/AI20, exposed to N,H,
(6.0
band cm-'
TABLE VI: Specific Surface Assignments for Infrared Bands Found on N,H,-Exposed Ir/Al,O, band, em-'
A1203
Catalyst Deactivation. An important aspect of our study was the problem of catalyst deactivation1 resulting from prolonged exposure of the Ir/A1203and Ir/Ni/A1203 catalysts to hydrazine. The degree of deactivation diminishes significantly with Ni addition (Table IV). The bimetallic catalyst composed of 52 Ir/48 Ni (bulk atom %) is of special interest. Its activity for hydrazine decomposition has been maintained relative to 100 Ir (Table 111), yet its sensitivity to deactivation has been greatly reduced (Table IV). Infrared Studies of Adsorbates. A typical infrared (IR) spectrum obtained for the Ir/A1203catalyst and the A1203 mol) is blank after 16-h exposure to N2H4 (-6 X shown in Figure 3. The bands observed can be assigned to OH, NH, and NH2 absorption frequencies in addition to molecularly adsorbed N2H4 and NH, (Table V), although the magnitude of absorption-band intensities can only be evaluated semiquantitatively because small peak-overlap corrections have not been made. While many of the absorption bands are common to both the A1203 support ("blank") and the Ir/A1203catalyst, the absorption bands at 1499 and 1469 cm-l are found only on Ir/A1203. These absorption bands are situated a t frequencies characteristic of NH2 and NH4* species, respectively. Comparison of the spectra observed for Ir/A1203exposed to N&14 and to NH3 indicates that the band at 1499 cm-l is formed only during N2H4 exposure, whereas the 1469-cm-l band appears with both NH3 and NzH4exposure. The band at 1623 cm-' appears to be characteristic of OH or adsorbed molecular N2H4. By comparison with
assignrnent/surface
- 3474
N,H, NH, OM 3474 VS,B m , B M , B M,B N2H,,NH3 V S , B S , B S , B S , B -3335 -3177 NH VS,B S,B M,B M,B 1688 "3 A W A A A M A 1623 OH, N,H, M 1603 N2 H4 S A M M 1588 "3 A M A A 1549 NH A A W A M A A A 1499 2" 1469 NH,' W M A A 1360 A R A W 1216-1271 NH, w w w w a N,H, and NH, exposure = 6.0 x mol at 295 K . VS, very strong; S, strong; M,medium; W, weak; A , absent; B, broad.
-
assignment
lo00
x I O 3 mol).
TABLE V : Summary of IR Spectra Observed on Ir/A1203and o n A120, Exposed t o N,H, or Ir/A1203 N,H, NH,
\I
1
0H/Al2O, OH/AI20
1623 1603 1499 1469
I
1
N,H,/Al, 0 NH,/Ir NH,"/Ir 1
I
I
I
I
6
I
18
4
1.6
3
-9
B
I
r-e
1U
2
0.5
I
0.0
O
1
2
3
4
6
0
7
8
0
10
N2H4 E X P O S W E TIME imlnuteil
Figure 4. Kinetic analysis of infrared absorption bands on Ir/AI2O3as a function of N,H, exposure. 8 is the fractional surface coverage.
I
the data given in Table V, we interpreted the IR spectrum of N2H4-exposed Ir/A1203 in terms of specific surface assignmenki. The IR bands at -3473,1623, and 1603 cm-' are associated with the A1203 surface, and those at 1499 and 1469 cm-l, with the Br surface (Table VI). Although all bands, except those at -3474 cm-' (OH) and 1469 cm-l (NH4*),were observed to grow in peak intensity with N2H4 exposure, the bands showing the largest intensity increase were located at 3335,1306, and at 1499 cm-l. We examined the initial rates of change of these band intensities in terms of typical Eangmuir-type first- and second-order rate laws (Figure 4). For this purpose, the maximum observed band
2484
R. T. Rewick, B. J. Wood, and H. Wise
The JournaE of Physical Chemistry, Vol. 83, No. 19, 7979
TABLE VII: Effect of Heating in Hen on Absorption Spectrum of Ir/A1203Exposed to Hydrazineb
a
50
I
I
6
absorbance (log I J I ) at temp, K
band, cm-'
__ 295
-3474 1623 1603 1499 1469
0.32 0.12 0.21 0.15
-0 0.04 0.10 0.12
0.08
0.03 0.05
0.05
0.04
0.03
0.03
373
473
-0 -0 0.03
573
-0 0
For 1 5 min at a He flow rate of 30 cm3/min. mol at 295 K.
725 40
-0 -0
5
n .-
4-
0.03 0.04 0.02 6.0 X
intensity was assumed to correspond to surface saturation (0 = 1). The band at 1499 cm-I (NH2/Ir) is found to be of second order in the unoccupied site density, s, on the metal surface, i.e., NZH4 f 2s 2(NHz-s). The other two bands follow first-order kinetics. A measure of the relative binding strengths of the surface adsorbates seen by IR spectroscopy can be obtained from the change in absorbance as a function of temperature in a He carrier stream (Table VII). The results indicate that, at 373 K, all the band intensities are reduced to near background levels, except those at 1603, 1499, and 1469 cm-l. The last two band intensities show the smallest relative changes over the entire temperature range studied.
-
Discussion At relatively high bulk atom fractions of Ni, the Alz03-supportedmixed metal system Ir/Ni exhibits significantly higher resistance over Ir/A1203 to surface deactivation without loss in relative conversion activity. A priori, the addition of Ni, which by itself exhibits poor catalytic activity, would be expected to dilute the surface concentration of Ir and thereby cause a reduction in activity. In fact, there is an observed lack of sensitivity of catalytic activity to Ni addition (up to 40 atom %), which appears to be due to the high degree of surface enrichment with Ir as observed by AES. If we make the assumption that at least two neighboring surface Ir sites are needed for the initial process of N2H4adsorption, we can compute the probability of finding such an ensemble in a cluster of surface atoms. For the fccub (111) surface we have computed the ensemble probability P5,2of finding two or more Ir atoms in a cluster of five surface atoms. As shown by the data in Table 111the activity pattern correlates with the Ir-atom distribution on the surface in terms of the ensemble theory.12 Thus the activity of the Ir catalyst with Ni added is interpretable in terms of such a geometric effect controlled by the surface composition of the mixed-metal system. By way of contrast, the observed enhanced resistance to deactivation by the nickel-containing catalysts bears no simple relationship to the relative abundances of the respective metals a t the catalyst surface (Table IV). Rather, the results suggest a decrease in the binding energy of the reaction intermediate responsible for surface poisoning by the addition of Ni to Ir. We conclude that activity and deactivation are governed by different factors. The kinetics of N2H4decomposition depend on the capability of the catalyst to adsorb the reactant molecule. This capability is determined largely by the surface density of neighboring Ir atoms. Dilution of the Ir atoms in the surface layer with Ni atoms reduces the abundances of Ir with a resulting proportionate loss in activity. However, this effect is not proportional to the bulk Ni concentration because of the pronounced degree of surface enrichment with Ir. Deactivation, however, is related to the NzH4 decomposition only to the extent that the strongly bound
E
E
i
A
4
g
m
VI
a
I
IT
2 n
>
.
q
30
3r
a
z
20
0
j
t 51
2 e O
N
I
10 1
0 2 73
0 473
3 73
573
TEMPERATURE (KI
Flgure 5. Catalyst activity decrease after exposure to 8.0 X of NH , , (5.5 X g of 10 wt % Ir/Alz03).
mol
reaction intermediate responsible for deactivation is produced by the surface-catalyzed reaction. An identification of the adspecies responsible for Ir catalyst poisoning is obtained from the observed sensitivity of the catalyst to deactivation a t elevated temperatures (Figure 5). The temperature pattern of catalyst poisoning exhibits a maximum near 450 K. This temperature pattern is similar to the Hzdesorption rate observed in a temperature-programmed desorption experiment2 involving N2H4preadsorbed on the catalyst at 300 K. Because of the rapid rate of depletion of H adatoms by recombination and Hzdesorption a t 450 K, the NHz(s) surface intermediate formed in NzH4 decomposition1 NzH4 -.+ 2NHz(s)
(4)
can undergo further dehydrogenation to give strongly bound nitrogen adatoms (N(s)) NH,(s) + NH(s) -+ H(s) (5) NH(s) e N(s) + H(s) (6) 2H(s)
~ 1Hz .
rather than
+ H(s) 2NH(s)
+
-
Nzk)
(7)
NH,(g)
(8)
+ Hzk)
(9)
On the basis of such a mechanism, one would expect less deactivation in the presence of H,. Indeed, we have observed such an effect. As shown in Figure 6, the activity decline of Ir/A1203 on exposure to NzH4 is substantially reduced when H2 is admixed. In agreement with the IR data, these kinetic results suggest that NH2 adspecies are the precursors of the N adatoms that cause catalyst poisoning. The formation of strongly bound N adatoms leads to irreversible deactivation of the Ir surface possibly by the formation of a two-dimensional iridium nitride. Such a deactivation mechanism is supported by the results of our earlier AES observations1 of a nitrogen adspecies that remained adsorbed at temperatures up to 1000 K.
IR Study of the Silica Gel Surface
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
2485
average potential, indicative of the inadequacy of the rigid-band model. However, the state density and the bonding orbitals are modified by the introduction of the foreign metal. As a result, the strength of the chemisorption bond can change. In the case of Ir/Ni, such a change is manifested as a weakening of the bond between surface iridium and nitrogen adspecies. Acknowledgment. Support of this research by the Air Force Office of Scientific Research (Contract No. F44620-73-C-0069) is gratefully acknowledged.
V
1
1
I
I
I
I
B
0 N*i/< E X P O S U R E ,.IOLES
x 1011
Figure 6. Effect of H, on catalyst deactivation during N2H, exposure at 373 K.
We conclude from our studies that the surface bonding of N H and N adspecies to Ir is weakened by the addition of Ni atoms. Their presence in subsurface atomic layers affects the nature of the chemisorption bond between the adspecies and the Ir surface atoms. Examination of valence-band electron populations by means of photoelectron spectroscopy have demonstrated that for bimetallic alloys the d states undergo little movement in energy position upon alloy f ~ r m a t i o n . ' ~ Thus J ~ the energy states in the alloy are more affected by the local potential than the
References and Notes (1) B. J. Wood and H. Wise, J . Catal., 39, 471 (1975). 121 J. L. Falconer and H. Wise. J. Cafal.. 43. 220 (1976). (3) A. E. Bucher, W. F. Brinkman, J. P. Maitu, and A: J. Cooper, Phys. Rev. B , 1, 274 (1970). (4) V. Ponec, Cafal. Rev. Sci. Eng., 11, 1-40 (1975). ( 5 ) G. M. Stocks, R. W. Williams, and J. S.Faulkner, Phys. Rev. B , 4, 4390 (19711. (6) R. E. Weber and A. L. Johnson, J . Appl. Phys., 40, 314 (1969). (7) P. W. Palmberg, Anal. Chem., 45, 549A (1973). (8) J. M. McDavid and S. C. Fain, Jr., Surf. Sci., 52, 161 (1975). (9) N. Laegried and G. K. Wehner, J. Appl. Phys., 32, 365 (1961). (10) P. Hou, J. McCarty, and H. Wise, to be published. (11) D. W. Bassett and H. W. Habgood, J. Phys. Chem., 64, 769 (1960). (12) D. A. Dowden, "Proceedings of the Fifth International Congress on Catalysis", North Holland Publishing Co., Amsterdam, 1973, Paper No. 41. (13) D. H. Seib and W. E. Splcer, Phys. Rev. B , 2, 1676 (1970). (14) C. Norris and H. P. Myers, J . Phys. F , 1, 62 (1971).
An Infrared Study of the Silica Gel Surface. 2, Hydration and Dehydration A. J. van Roosmalen" and J. C. Mol University of Amsterdam, Institute of Chemical Technology, Amsterdam, The Netherlands (Received September 29, 1978; Revised Manuscript Received February 28, 1979) Publication costs assisted by the University of Amsterdam
The hydration and dehydration of transparent silica aerogel plates was studied by infrared spectroscopy and thermogravimetry. The following mechanism is proposed for the hydration of silica gel degassed at 875 K: (1)physical adsorption of water on residual hydrogen-bonded vicinal silanol pairs; (2) formation of more vicinal pairs by hydrolysis of surface siloxanes reached by the growing water aggregates; (3) additional water adsorption on these new hydroxyls. Isolated surface silanols appeared to be hardly involved in water vapor adsorption.
Introduction In part 1 of this series we presented a study on the surface structure of dry silica gel plates prepared according to a method described by Peri.1,2 Silica gel was found to have four infrared absorptions in the O-H stretching region after vacuum treatment at 875 K. We assigned these bands to four different types of surface hydroxyls, viz. isolated single (3749 cnn-l) and geminal (3742 cm-') silanols, and asymmetric hydrogen-bonded vicinal silanol pairs (3720 and 3500-3700 cm-l). Silica powders, such as Aerosil and Cabosil, show only one absorption (3748 cm-l) in this spectral region after degassing above 700 Ka3From this we concluded that the surface structures of silica gel and silica powder are not identical. Relatively little is known about the hydration mechanism of dry silica geL4l6 In part, this is caused by the bad transparancy of samples obtained by milling and pressing commercial silica gels. The interaction of water vapor with dehydrated silica powders has been extensively studied. Physical adsorption of molecular water was reported to take place on isolated single surface silanols.6%7 However, 0022-3654/79/2083-2485$0 1.OO/O
this result does not necessarily apply to silica gel, because of the observed dissimilarity between silica gel and silica p0wder.l The silica aerogel plates we used in part 1 of this series lack scattering. Therefore, they will be an ideal material for measurements at high degrees of hydroxylation. In the work presented here, infrared spectroscopy and thermogravimetry were combined in order to obtain information about the hydration mechanism and the nature of the hydrogen-bonded hydroxyl groups on silica gel.
Experimental Section The infrared spectra were recorded in the absorbance mode (log Io/l). This allowed direct substraction of spectra obtained with the same sample under different conditions. Difference curves often give more detailed information about (dis)appearing vibrations than the original spectra do. Sample heating by the infrared beam was suppressed by admitting lo3 N m-2 of He to the measuring cell if no other gases were present. In this way, a temperature of 335 f 5 K could be maintained. Details about the @ 1979 American Chemical Society
