2906
J . Phys. Chem. 1986,90, 2906-2910
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Adsorption and Decomposition of Ammonia on Metal Films of Nickel, Palladium, Tungsten, and Aluminum Kamal K. AI-Shammeri and Jalal Mohammed Saleh* Department of Chemistry, College of Science, University of Baghdad, Baghdad, Jadiriya, Republic of Iraq (Received: March 29, 1985)
The interaction of ammonia with evaporated metal films of Ni, Pd, W, and AI has been studied in the temperature range from 193 to 508 K. Both molecular and dissociative adsorption of NH3 took place on all films at 193 K. The adsorbed NH3 at such temperature did not displace presorted hydrogen at 300 K on Ni and Pd films. The extent (8) of NH3 adsorption on clean Ni and Pd films at 193 K was greater by 20 to 30% than that attained on films which had been covered with hydrogen at 300 K. Extensive NH3 adsorption also occurred at 193 K on surfaces of Ni, Pd, and AI which had been saturated with oxygen at 300 K. The rate of NH3 adsorption increased with temperature on clean and oxidized films subsequent to the maximum uptake at 193 K. The activation energies (E,) of NH3 adsorption increased rapidly over a small range of 8 values ultimately attaining a constant value at saturation. From the values of E, and by use of the appropriate rates and temperatures of adsorption, the values of the preexponential factor ( A ) of the rate equation were determined. A linear relationship existed between log A and E,, suggesting the operation of a compensation effect in the interaction of NH3 with different surfaces. Hydrogen was found to be the only gas resulting from NH, dissociation on the films.
Introduction The adsorption, decomposition, and synthesis of ammonia have been extensively studied on various types of surfaces. Most of the early work1-" have been performed under poor experimental conditions at relatively high temperatures and pressures. Recent on the other hand, have mainly been dealing with single crystals with the aim of understanding the nature of the adsorbed species resulting from NH3 adsorption and dissociation on such surfaces. Despite such variety of studies, the mechanism of adsorption and decomposition is not understood clearly yet. There is still a need for further fundamental studies involving adsorption and kinetics, particularly on polycrystalline surfaces, under carefully controlled conditions of temperature and pressure. The object of the present work was to follow the adsorption and decomposition of N H 3 on metal films of Ni, Pd, W, and A1 over the temperature range from 193 to 508 K. The catalytic characteristics of these metals have been examined under clean surface conditions and the extent of adsorption and decomposition was followed throughout the interaction of N H 3 with the films. Metal oxides"2 may catalyze quite a number of surface reactions such as dehydrogenation, dehydration, and oxidation. The oxidation of ammonia is the only economical methodl3,I4for nitric ( I ) M. Wahba and C. Kemball, Trans. Faraday Soc., 49, 1351 (1953). (2) 0.Ozaki, H.S. Tayler, and M. Beudart, Proc. R . SOC.London, Ser. A, 47,258 (1960). ( 3 ) M. I. Temkin and V . Pyzechen, Acta Physicochim. URSS, 12, 327
(1940). (4) P. H.Emmet and S. Brunauer, J . Am. Chem. SOC.,56, 35 (1934). (5) S.K. Roy, N. Roy, D. K. Mukherjee, and S . P. Sen, Proc. Indian Natl. Sci. Acad., 41,b85 (1975). (6) K. Kunimeri and T. Kawai, Surf. Sci., 59, 302 (1976). (7) H.Shinde. C.Egawa, - T. Onishi. and K. Tamara. Z . Nuturforsch., 96, 344 (1979). (8) I. D. Gray, M. Texter, R. Nasen, and Y. Iwashwa, Proc. R. SOC. London, Ser. A, 25, 356 (1977). (9) H. I. Kemball and D. A. Whan, Proc. R . SOC.London, Ser. A , 30,385 (1968). (10) J. L. Falcener and H. Wise, J . Catal., 43, 220 (1976). (11) F. Steinbach and D. Heffer, Surf. Sci., 79, 311 (1979). (12) S . L.Parrett, J. W. Rogers, and J. M. White, Appl. Surf. Sci., 1, 443 (1978).
0022-3654/86/2090-2906$01.50/0
acid manufacture for which the industrial demand is tremendous and progressively increasing. An attempt was therefore made to find the influence of presorbed oxygen oqsubsequent adsorption and decomposition of ammonia on the metal films. No such work has been reported previously to our knowledge.
Experimental Section The apparatus consisted of two diffusion-pumped sections. The first was for gas handling and was capable of ultimate pressures of N m-*. The second was a bakeable ultrahigh vacuum system capable of ultimate pressures of N m-*. Films of Ni, Pd, and A1 were prepared from spectroscopically standardized metal wires, 0.5 mm diameter, obtained from Johnson Matthey and Co. Nickel and palladium films were prepared from filaments by evaporation onto the glass walls of the adsorption vessel, 240 cm3 capacity, maintained at 78 K. With Al, a known length ( 5 cm) of metal wire was supported on a tungsten coil, prepared from 0.2-mm tungsten wire, and the latter was heated electrically. The tungsten coil was first degassed in the absence of A1 wire, and the same wire was used for up to five evaporations, thereby reducing contamination to a minimum. Tungsten films were prepared from 0.1-mm-diameter tungsten filaments. Tungsten wires were pure grade samples supplied by the British Tungsten Manufacturing Co. Ni and Pd wires were reduced in pure hydrogen at about 1300 K before they were degassed. During the deposition of the film the reaction vessel was maintained open to the pumps and the pressure was always less than N m-2. The experimental technique for degassing of the apparatus and the metal wires, and for preparing and sintering of the films have been described p r e v i ~ u s l y . ' ~ -The ~ ~ cleanliness of the metal surfaces that have been prepared under our experimental conditions was (13) A.W.Holmes, Plat. Mer. Rev., 3, 2 (1959). (14) J. Zawadzki, Discuss.Faraday Soc.. 8, 140 (1950). (15) D.A. Mawlawi and J. M. Saleh, J . Chem. SOC.,Faraday Trans. I , 77,2965 (1981). (16) J. M. Saleh and F. A. K. Nasser. J . Phvs. Chem.. 89. 3392 (1985). (17j Y.K.AI-Hayderi, J. M. Saleh, and M. H. Matloob, J . Phys. Chek., 89, 3286 (1985).
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2907
Decomposition of N H 3 on Metal Films TABLE I: Adsorption of NH3 on Ni, Pd, W, and AI Films at 193 K" film Vk,, mm3 (STP) V,, mm3 (STP) e
Ni Pd W AI
106.2 110.0 300.0 298.0
127.3 134.0 370.0 3 12.0
1.20 f 1.21 f 1.23 1.31 f
*
0.01 0.01 0.01 0.01
checked by the reproducibility of the adsorption and kinetic results as well as by the maximum capacities of the films for hydrogen adsorption at 300 K. The maximum volume of hydrogen that could be adsorbed on a film at 300 K was equivalent or greater than the volume of krypton monolayer on the film at 78 K; the maximum hydrogen uptake on a Ni film at 300 K and hydrogen pressure of lo-' N rn-, corresponded in this work to 1.46 X 1015 atoms cm-, as compared with 1.5 X lOI5 atoms cm-, reported'* for a similar system. Anhydrous ammonia was supplied by the British Oxygen Co. with a purity greater than 99% as checked mass spectrometrically, the main impurity being water. It was conveniently dried over solid caustic potash, freshly prepared calcium oxide, and sodium-sputtered glass wool and finally passed over phosphoric pentoxide. Liquefaction in a liquid nitrogen cooled trap allowed the removal of permanent gases, and distillation yielded a product containing less than 0.0001% of mainly water contaminants. Krypton was also obtained from British Oxygen Co. and was further purified before surface area determinations. Calibration with ammonia showed that negligible adsorption and decomposition occurred on the glass walls of the reaction vessel. At the end of each run the apparatus was usually brought back to room temperature, the cold trap being removed first and, thereafter, a final analysis of the gas phase was performed. It was demonstrated that no condensable gases (N2H4,H 2 0 , ",OH, etc.) were present in the cold traps. The composition of the gas phase at each temperature was analyzed mass spectrometrically as well as by using a precalibrated Pirani gauge as described in previous papers.I6J7
Results The extent (6) of NH3 adsorption on each film is expressed as =
Vg/Vkr
(1)
where V, is the volume of N H 3 adsorbed and Vk, is the volume of the krypton monolayer on the film at 78 K prior to ammonia adsorption; the volumes are expressed in mm3 at STP. The rate of N H 3 uptake was calculated from the volume of gas uptake (mm3) per unit time (SI) per unit area (ern-,) of the film surface. Adsorption and Decompoyition of NH3. The initial ammonia uptake on all films at 193 K occurred rapidly (< 1 min) until the value of 6 approached 1.2 to 1.3. Adsorption became slow then and preceeded at a rate of mm3 cm-, SI. The adsorption of N H 3 on Ni, Pd, and AI films at 193 K was irreversible, as the adsorbed gas could not be removed either by pumping of the reaction vessel down to N rn-, or by heating the film to temperatures as high as 450 K. On the other hand, about 25% of N H 3 adsorption on W film was reversible. Table I summarizes the surface area ( Vh), the volume of N H 3 adsorbed (V,) and the corresponding values of 6 for adsorption on the films at 193 K. The volumes of N H 3 adsorbed (V,) and the values of 6 corresponded to pseudo-equilibrium under N H 3 pressure of 3.0 N m-2. No gases were liberated subsequent to N H 3 adsorption on Ni, Pd, and W films at 193 K while some H,, amounting to 15% of the hydrogen content of the adsorbed ammonia, desorbed throughout N H 3 adsorption on A1 at such temperature. Further ammonia adsorption occurred on all films at >193 K and the rate increased with temperature. The extent was appreciable only on N i and A1 films. The reaction on Ni and A1 films above 193 K was accompanied by H2 evolution. The total gas pressure over both metals decreased with time despite the H, evolution. The liberated H2gas represented less than half of the ( 1 8) G . Wedler, Chemisorption; An Experimental Approach, Translated by D. F. Klemperer, Butterworths, London, 1976, p 39.
TABLE II: Adsorption of Ammonia at 193 K on Ni and Pd Films Which Had Been Covered with Hydrogen at 300 K"
film Ni Pd
Vk, V,. 157 182
126 458
Z
V.
8
8+Z
0.8 f 0.01 2.51 f 0.03
160 140
1.02 f 0.01 0.77 f 0.01
1.8 3.28
TABLE 111: Adsorption of NH3 at 193 K on Oxidized Ni, Pd, and AI"
film Ni Pd AI
Vk, 155.0 127.0 106.0
Vo2 151.9 54.6 178.1
X 0.98 f 0.01 0.43 f 0.01 1.68 f 0.01
V8 113.3 85.0 140.2
0
vkkr
0.82 f 0.01 0.74 f 0.01 1.40 f 0.01
138.5 115.0 100.2
"Volumes are in mm3 at STP.
hydrogen content of the adsorbed ammonia. When a Pd film which had adsorbed N H 3 at 193 K to the extent 6 = 1.21 was heated to temperatures around 508 K, a very slow desorption of N H 3 occurred and consequently the value of 6 dropped to 1.O. A slow N H 3 uptake on Pd was shown only to occur at temperatures exceeding 508 K. A slow desorption of N H 3 was detected on heating W films in the temperature range 193-400 K, but above 400 K the desorption ceased and NH3 adsorption began to occur at a very slow rate. The slow N H 3 adsorption on W at temperatures > 400 K took place with H2 evolution. No gases other than N H 3 together with some H2were detected on careful analysis of the gas phase in the temperature range from 193 to 500 K. Preadsorbed Hydrogen. In a series of experiments, the Ni and Pd films were first covered at 300 K at a pressure of lo-, N rn-, with pure hydrogen. The extent (2)of hydrogen adsorption on each film at 300 K is represented as =
VH2/Vkr
(2)
where VH2is the volume of hydrogen adsorbed on the film at 300 K under a pressure of lo-, N m-2 and V, is the volume of krypton monolayer on the film at 78 K before hydrogen adsorption. The excess hydrogen gas was then pumped out to a pressure of N m-2 and the film was cooled to 193 K at which point N H 3 gas was admitted to the film in small doses until a pressure of 10-1 N rn-, of N H 3 remained in the gas phase. Fast irreversible adsorption of N H 3 took place on hydrogen-covered Ni and Pd films until the rate of adsorption slowed to mm3 cm-, s-I. The results are given in Table 11. Above 193 K, the films behaved with respect to N H 3 adsorption in a very similar manner to N H 3 adsorption on the clean films. The extent of H, evolution subsequent to N H 3 adsorption on Ni film did not differ significantly from that on clean Ni film at temperatures below 508 K. Preadsorbed Oxygen. In these experiments the metal film was first oxidized with pure oxygen gas at 300 K until some 02, amounting to lo-, N m-,, remained in the gas phase. The volume of oxygen adsorbed at this stage was denoted VO2. The extent (X)of oxygen adsorption on the film is =
V02/Vkr
(3)
where Vk, is the krypton monolayer on the clean metal prior to oxidation. The excess oxygen was then pumped out and the surface area of the oxidized film was determined at 78 K; the volume of the krypton monolayer on such a surface was represented as r k r . The krypton was then pumped out to a pressure of N rn-, and N H 3 was admitted to the film at 193 K until the rate of uptake decreased to mm3 cm-2 s-l. The extent (0) of N H 3 adsorption is represented as 0 = Vg/Vkr
(4)
The difference between the values of Vk, and Vk was significant in the case of preadsorbed oxygen and this accounts for the use of eq 4 for the estimation of 6. Adsorption of N H 3 on the oxidized films at 193 K occurred rapidly and the results of adsorption are indicated in Table 111. Further N H 3 adsorption on the films at temperatures >193 K proceeded at a slower rate compared with the corresponding clean films. Analysis of the gas phase did not
2908
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986
Al-Shammeri and Saleh TABLE I V Energies of Activation ( E , ) and the Preexponential Factor ( A ) in the Rate Equation for the Adsorption of NH3 on the Clean and Oxidized Films temp range, K 295-508 Ni Pd 508-603 A1 301-553 W 400-453 oxid Ni 323-453 oxid Pd 353-623 oxid AI 333-503
film
0.6
{
10
20
30
40
tia/mm
Figure 1. First-order plots for the interaction of NH3 with (a) a clean Ni film at 300 K (0)and (b) oxidized Ni film at 360 K ( 0 ) .
8
3.75
3.8
0.85 e 0.9
I
E,,
",(a)
13
I35
Figure 2. Activation energy (E,) for the adsorption of NH3on clean Pd (0) and on oxidized Ni ( 0 )films as a function of surface coverage (e).
show gases ether than the unreacted NH3 and some hydrogen. The adsorption of NH, on the oxidized films was irreversible. The adsorption of NH, on the oxidized AI films above 300 K and under similar ammonia pressure proceeded at an appreciably higher rate than on oxidized Ni and Pd films. Kinetics of Adsorption. The dependence of the adsorption rate on NH3 pressure was found from a large number of measurements against to be unity. A check of this result was to plot log PNH, time as shown in Figure 1. No evidence was obtained for the adsorption rate depending on hydrogen pressure. The interaction of NH, with the films, subsequent to the adsorption at 193 K (Tables 1-111) proceeded at a rate which increased with temperature. The activation energy (E,) of adsorption was deterrninedI6-l7from the rates of NH3 uptake at two successive temperatures but essentially the same value of % and under the same NH, pressure. Over a small range of 0 values (about 0.15), the values of E, increased rapidly with increasing % as indicated in Figure 2. The activation energy, after such rapid increase with 8, attained a constant value which did not change throughout the subsequnt NH3 interaction with the films at higher temperatures. From such constant values of E,, and by use of the appropriate rates of NH3 adsorption and temperatures, the values of the preexponential factor ( A ) in the rate equation were estimated by using rate = A = exp(-E,/RT) (5) The results obtained are given in Table IV. The data of the table indicate also the range of 8 values and temperatures for which the values of A and E , were derived. Discussion Decomposition on Clean Films. The results indicate that both molecular and dissociative adsorption of NH3 occurred on the films
(1.4 f 0.04) (8.2 f 0.12) (8.2 f 0.12) (6.4 f 0.10) (1.0 f 0.04) (3.1 f 0.16) (1.0 f 0.04)
X X X X X X X
lo1* lOI4 lOI5 lOI4 10l6 10l6 lOI4
-
-",(a)
(6)
NH2(a)
+ H(a)
(7)
where (g) and (a) refer to the gaseous and adsorbed species, respectively. Grabke24 concluded from kinetic data for NH, formation on iron foils that with pressures CO.1 atm the reaction H(a) ",(a) played a dominant role and proceeded ",(a) with an activation energy of 48 kJ mol-'. Moreover, the thermal desorption spectra25for N H 3 showed the appearance of a p3-state which was ascribed to a reaction between surface amine groups and hydrogen adatoms. The existence of NH2(a) as the predominant surface species was also confirmed by Nakata and Matsushita26 on the basis of the infrared spectroscopic measurements with N H 3 as well as with N 2 H2 mixtures interacting with Fe on silica. Further decomposition of NH2(a) may occur as293321-23
+
e
60.9 f 0.3 73.6 f 0.35 76.4 f 0.35 69.4 f 0.3 83.6 f 0.4 96.4 f 0.4 42.9 f 0.3
at 193 K despite the fact that only a small amount of hydrogen appeared in the gas phase subsequent to NH3 adsorption on the films at this temperature. Field emission studies19 indicate that N H 3 adsorbed on Mo surfaces is partially dissociated at temperatures 5300 K. The chemisorption of N H 3 on W( 100) has also been reported20 to occur with partial decomposition of the adsorbate. Only hydrogen evolution is seen up to 600 K. The adsorption and the initial dissociation of NH3 on the metal surface may result in the formation of adsorbed amine radicals and hydrogen atoms as21-23 ",(d
125
A , molecule cm-* s-l
kJ rno1-l
0 1.24-2.5 1.0-1.63 1.28-2.51 1.02-1.14 0.88-1.20 0.95-1.34 1.27-1.7
-
+
",(a)
-
"(a)
+ H(a)
(8)
in which a surface imide group and an adsorbed hydrogen atom are formed. The direct decomposition of NH2(a) into adsorbed nitrogen and hydrogen atoms in a single step is expected to be energetically highly i m ~ r o b a b l e . ~ ~Further - ~ ~ dissociation of "(a) may continue according to "(a)
-
N(a)
+ H(a)
(9)
The energies of the metal-" and for the dissociation of N-H bonds may be overcompensated by the formation of relatively strong metal-nitrogen and metal-hydrogen bonds.27 It has been found that NH, adsorption on Ni( 100) takes place in steps similar to these of the eq 6-9 and that the activation energy for the initial dissociation of ",(a) to NH2(a) and H(a) was 198 kJ mol-'. The dissociation of ",(a) occurred only above 150 K. Similar dissociation steps are also revealed on Pd by Auger spectroscopy.6 If we ignore the small entropy contribution to the value of the standard free energy change for NH, decomposition on metal surfaces, the thermodynamic feasibility of the decomposition (19) G. Bergert, M. Aben, E. Terdy, and S. J. Teichner, Colloq. Int. Phys. Chim. Surf.Znd, 1975, 30A, 104 (1975). (20) A. P. C. Reed and R.M. Lambert, J . Phys. Chem., 88, 1954 (1984). (21) G. Ertl and M. Huber, J. Card., 61, 537 (1980). (22) J. W. Rogers, Jr., C. T. Campbell, R. L. Hance, and J. W. White, Surf. Sci., 97,425 (1980). (23) V. D. Yagederskii, Zh. Fiz. Kim., 54, 257 (1980). (24) M. J. Grabke, Ber. Bumenges. Phys. Chem., 72, 533 (1968). (25) F. Bazse, G. Ertl, and M. Weiss, Appl. Surf. Sci., 1, 341 (1978). (26) T. Nakata and S . Natsushita, J . Phys. Chem., 72, 458 (1968). (27) M. Grunze, Surf. Sci., 81, 603 (1979).
The Journal of Physical Chemistry, Vol. 90, No. 13, 1986 2909
Decomposition of NH, on Metal Films TABLE V Enthalpy Changes Associated with the Decomposition of NH3 on Metal Films of AI, W, Ni, and Pd Resulting in the Formation of (a) Adsorbed Nitrogen and Hydrogen Atoms (AH,) and (b) Gaseous Nitrogen and Hydrogen (AH2) metal AH,,kJ mol-' AH?,kJ mol-' AI -62 27 W -98 20 Ni -40 13 Pd -29 16
process may be judged on the basis of the standard enthalpy change involved in the reactions 6-9 in a procedure similar to that discussed by G r ~ n z e . ~The ' energy involved in each reaction was estimated from the bond dissociation energies of the metal-adsorbates and the gas-phase enthalpies of the formation of the for the various species.27 The overall change of enthalpy (AH,) steps 6-9 was found from such estimation to be appreciably negative on all films as indicated in Table V. These energetic considerations demonstrate the feasibility of the whole sequence of the reaction steps: NH3(g) ",(a) NHz(a) H(a) "(a) 2H(a) N(a) 3H(a). The decomposition mechanism of N H 3 on various films should therefore depend on the (x = 0 3) energy gained by the formation of the metal-", and metal-hydrogen bonds. The formation of such bonds makes the decomposition mechanism via these surface species thermodynamically feasible. When the evolution of nitrogen and hydrogen gases is considered as the final decomposition steps
+
-
+
-
-
+
-
the overall enthalpy change (AH2)for N H 3 decomposition leading to N 2 and H2 evolution was found from our estimation to be endothermic. This suggests that the breakage of the metal-nitrogen and metal-hydrogen bonds and the subsequent evolution of nitrogen and hydrogen gases should be energetically the most unfavorable reaction steps. The fact that only a small amount of hydrogen, and no nitrogen, evolution was detected in the present work agrees with this conclusion. It has been foundz0 that N 2 evolution resulting from NH, adsorption on W( 100) occurs only above 750 K. For the reverse reaction, namely the synthesis of ammonia from nitrogen and hydrogen gases on the metal surfaces, the overall change of enthalpy should therefore be negative and the synthesis process should be thermodynamically feasible at ordinary and higher temperatures. The alternative (molecular) mechanismz7 for the decomposition of N H 3 through the steps 2NH3(g) 2NH3(a) N2H4(a) 2H(a) N2H,(a) 4H(a) Nz(a) 6H(a) N2(g) 3H2(g) was disregarded because of (i) the strong evidence that are available in the literature, as discussed earlier supporting our experimental results, for the presence of NH, intermediates on the surface, (ii) no N 2 gas was observed throughout the decomposition steps despite the weak metal-Nz bond involved in this mechanism as compared with the dissociation of metal-N (metal nitride) in the previous mechanism, and (iii) no NzH4to any extent was detected in the dissociation of NH, on the metals, as the formation of this compound in the gas phase according to the molecular mechanism through the desorption of NzH4(a) species is probable. Preadsorbed Hydrogen. The adsorbed N H 3 at 193 K did not displace preadsorbed hydrogen on Ni and Pd films, suggesting that the heat of NH, adsomtion on these metals is not significantlv greater than the heat of hidrogen adsorption on the same films. Wahba and Kemball' have reported that when a chemisorbed ammonia laver was exmsed to excess NH,. there was a tendencv for the chemisorbed iydrogen resulting from N H 3 dissociatioi to be displaced by amine radicals on W and Fe films. However, no such behavior was detected on a Ni film.' The heat of chemisorption of hydrogen on a Ni filmz8 at surface coverages
-
+
- -+ - - +
+
(28) F. J. Brecker and G. Wedler, Discuss. Faraday Soc., 41,87 (1966).
of 0.8-1.0 is about 84 kJ mol-'. The initial heat of NH, adsorption on a "clean" N i film' is about 155 kJ mol-' and this value is expected to decrease considerably on surfaces which have been covered with hydrogen to the extent Z = 0.8. The values of 8 on clean Ni and Pd films at 193 K (Table I) did not differ significantly from those attained on films that had been covered with hydrogen at 300 K to the extent 2 = 0.8 (Table 11). This suggests that the maximum values of 0 on clean Ni and Pd films at 193 K should correspond to an incomplete surface coverage with the adsorbed NH3. An alternative i n t e r p r e t a t i ~ n ~ ~ of the extensive N H 3 and Hz adsorption on the same film is based on the formation of N H 3 clusters. Clustering of N H 3 at small surface coverages and low temperatures is thought to be a general phenomenon on Ni surfaces. The clustering is assumed to involve hydrogen bridge bonding between the chemisorbed species and the second layer N H 3 molecules. The comparatively higher value of Z (Table 11) on Pd than on Ni refers probably to some hydrogen penetration into the bulk of the metal at such t e r n p e r a t ~ r e . ' ~ . ~ ~ Oxidized Films. Adsorption of NH, on the oxidized surfaces at 193 K is likely to be both molecular and dissociative. The maximum values of 8 on the oxidized Ni and Pd films (Table 111) were lower, and on oxidized A1 higher, than the corresponding values on the clean metals (Table I). The presence of a chemisorbed oxygen on a metal alters the geometry and chemical composition of its surface and hence, affects the chemisorption and catalytic properties of the resulting surface.,' The greater N H 3 adsorption which occurred on the oxidized A1 than on the clean film, and as compared also with the oxidized Ni and Pd films, may be ascribed to the geometrical suitability of the former oxide than the latter surface. Extensive irreversible adsorption of water vapor and of alcohols is known to take place on both anhydrous alumina (7-Al,O,) and on bachmite (7-AlO, O H ) through the conversion of the oxide to hydroxide group. The surface then takes on the character of gibbsite (yAl(OH),) as has been confirmed from X-ray studies.3z Further adsorption of NH3 on the oxidized films continued above 193 K and only a very small amount of hydrogen was observed in the gas phase. The initial dissociation of the adsorbed N H 3 on the oxidized surfaces is assumed to i n ~ o l v e ' ~ ~ ~ ~ ~ ~ ~ NH3(g) + O(a) -",(a)
+ OH(a)
(12)
The N H 2 radicals are assumed to be attached to the metallic sites of the surface. Further interaction among the adsorbed species may c ~ n t i n u e ' ~as* ~ ~ , ~ ~ ",(a)
+ OH(a)
-+
"(a)
+ H20(a)
(13)
The transition state in reaction 13 is r e p ~ r t e dto~ be ~ . also ~ ~ capable of giving hydroxylamine (",OH) according to NHz(a)
+ OH(a)
-
NH20H(a)
(14)
The overall process of N H 3 interaction with surface oxygen according to the eq 12-14 is s h o ~ nto~be~ exothermic * ~ ~ provided the imide radical is chemisorbed with a heat of more than 34 kJ mol-'; our estimation of the heat of NH3 adsorption on the metals is appreciably higher than this value and the thermodynamic feasibility of the processes in the present work may thus be justified. The evolution of some hydrogen subsequent to the adsorption of NH, on the oxidized surfaces above 193 K may be attributed to (i) combination of hydrogen atoms that are attached to the oxide (or to the metallic) sites of the surface, and (ii) dissociation of the surface HzO and N H z O H species producing hydrogen gas (29) M.Grunze, P. A. Derben, and C. H. Brundle, Surf. Sci., 128, 311 (19831. . (30) G. Wedler, Chemisorption; An Experimental Approach, Butterworths, London, 1976,p 196. (31) A. V. Kiselev, Discuss. Faraday Soc., 52, 14 (1971). (32) J. J. Kipling and D. S. Peakall, In Chemisorption, W. E. Garner, Ed., Butterworths, London, 1957,pp 59-67. (33) J. K. Dixon and J. E. Lengfield, In Catalysis, P. H. Ernrnet, Ed., Reinhold, New York, 1960,Vol. 2, p 281. (34) S.R.Logan and C. Kernball, Trans. Faraday SOC.,56, 144 (1960).
2910
The Journal of Physical Chemistry, Vol. 90, No. 13. 1986
20
40
E,/
60
80
IOD
kJmoi'
Figure 3. log A vs. E, plots for the interaction of N H 3 with clean and oxidized metal films: Ni ( 0 ) ,W (A), Pd ( O ) , AI (X), oxid Pd (m), oxid Ni (O), oxid AI (A).
and forming surface oxygen or surface N H O species respectively. N o H 2 0 , ",OH, or any other product other than a small amount of hydrogen was detected at any temperature below 508 K after careful analysis of the gas phase. A higher temperature is probably needed for the desorption of the reaction products. Kinetics of Adsorption. The kinetic data revealed the following: 1. The rate of adsorption directly depended on N H 3 pressure. The pressure dependency of unity together with the linearity of ~ (Figure 1) supports the relationship between log P N vs.~ time this conclusion. This agrees with the available data for the pressure dependence of ammonia a d s ~ r p t i o non . ~ Ni ~ of about 0.96. The rate of N H 3 adsorption did not depend on H2 gas pressure. 2. The values of E, on each film increased with the extent (0) of adsorption (Figure 2) over a small range of 0 values approaching constancy at relatively high values of 0 when the surface is rather highly covered with the adsorbate. The variation of E, with 0 may be due to sites possessing different adsorption properties and the constant values of E, (Table IV) probably correspond to the stage of steady dissociative of ammonia through the eq 6-9. 3. A linear relationship existed between log A and E, for the interaction of NH3 with different clean and oxidized films (Figure 3). This suggests the operation of a compensation effect in the process of adsorption, which may be represented log A = mE, + C (15) where m and C are constants representing the slope and the intercept of the relationships. Such a behavior suggests that adsorption of N H 3 should take place first on surfaces with low energy of activation and that adsorption, thereafter, continues on surfaces with higher energy. The activation energies of 181, 117-135, and 160 kJ mol-' have been r e p ~ r t e d ~for ~ >N~H*3 adsorption, at pressures between 10 and 53 Torr, on Ni film, Pd deposited on A1203, and W films, respectively, over the temperature range from 380 to 570 OC. Under such condition, the value of log A on Ni film was found t o be 25.7. These values are generally higher than those obtained in the present work (Table IV). The difference may be due to (35) F. C. Constable, Proc. R. SOC.London, Ser. A, 108, 355 (1925). (36) E. Cremer and G. M . Schwab, Z. Phys. Chem., Abt A , 144, 243 (1929). (37) G. M. Schwab, Z. Phys. Chem., Abt B, 5, 355 (1929). (38) A. Amene and H. S . Taylor, J . Am. Chem. SOC.,76, 4201 (1954).
AI-Shammeri and Saleh the different experimental conditions for the work referred to above34*38 than these of the present study. Figure 3 indicates that the reactivity of the metal films for NH3 adsorption follows the sequence Ni > Pd, W > Al. Thus Pd and W films acquire a similar reactivity. The higher value of E, and log A on A1 makes this metal the least active of the four metals for N H 3 adsorption. Ni film, on the other hand, showed the greatest activity for N H 3 adsorption. In the same manner, the oxidized surfaces may also be arranged in the sequence oxid A1 > oxid Ni > oxid Pd. The oxidized AI is thus the most active of the three oxidized films, and oxidized Pd the least, for N H 3 adsorption. The surface of metal oxides is known39,40to contain both acidic (electrophilic) and basic (nucleophilic) sites, and the degree of the acidity was found to depend on the nature of the metallic sites. Cracking activity has been reported4I to increase with the acidity of the oxide surface. The activation energy for the decomposition of formic acid was shown42to decrease with increasing acidity of the oxide catalyst. Nitrogen bases have been shown43to undergo strong adsorption on alumina catalyst thereby reducing the activity of the acidic sites for subsequent olefin isomerization. On this basis, the greater activity of oxidized AI than oxidized Ni and Pd for ammonia adsorption and decomposition may thus be ascribed to higher acidic surface sites on the former as compared with the latter two oxidized films. The value of m in eq 15 as derived from the slope of the lines in Figure 3 is found to be greater for clean metals than for oxidized surfaces. Thus, the presence of preadsorbed oxygen reduces the dependence of log A values on E,. Moreover, the preadsorbed oxygen also reduces the activity of Ni and Pd films, but enhances the activity of AI, for N H 3 adsorption and subsequent decomposition.
Summary and Conclusion The experiments performed in this study on the interaction of ammonia with metal films of Ni, Pd, W, and A1 revealed that, depending on the temperature, molecular adsorption, desorption, and dissociation could be observed. It was concluded from the resulting data that N H 3 decomposes according to NH&) NH3(a)- NH2(a)+ H(a)-"(a)+ 2H(a)- N(a) + 3H(a), and simple thermochemical considerations support the suggested model of decomposition on the films. Some H2desorption occurred '/2H2(g) but no N 2 was detected in due to the reaction H(a) the gas phase at any temperature in the range from 193 to 508 K. The formation of nitrogen gas subsequent to the breakage of the metal-nitrogen bonds was shown to be the most unfavorable reaction step. On this basis, the synthesis of ammonia from nitrogen and hydrogen gases on the same metals should therefore be thermodynamically feasible. The sequence of the decomposition steps on the oxidized surfaces is assumed to be NH3(g) O(a) "*(a) + OH(a) and this + H,O(a) or NH20H(a). The kinetic may result either "(a) data showed that Ni was the most, and AI was the least, active of the four metals for NH, adsorption and decomposition. With oxidized surfaces, oxidized AI was the most, and oxidized Pd was the least, active of the three oxidized films for ammonia adsorption and dissociation.
-
+
-
Registry No. NH3, 7664-41-7; Ni, 7440-02-0; Pd, 7440-05-3; W, 7440-33-7; AI, 7429-90-5; Hz, 1333-74-0. (39) J. R. Jain and C. N. Pillai, J. Coral. 9, 322 (1967). (40) N. Takezawa, C. Hanamaki, and H. Kobayoshi, J . Catal. 38, 101 (1978). (41) G. C. Bond, Catalysis by Metals, Academic, New York, 1962, pp 440-44 1. (42) J. B. Fisher and F. Sebba, Proc. 2nd. Int. Congr. Catal. 1, 1961, 71 1. (43) H. Pines and C. N. Pillai, J . Am. Chem. SOC.,83, 3274 (1961).
