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zinc oxide catalysts
336
Surface Science 192 (1987) 336-343 North-Holland, Amsterdam
a-BRASS FORMATION IN COPPER/ZINC OXIDE CATALYSTS III. Surface segregation of zinc in a-brass M.S. SPENCER ICI Chemicals and Polymers Group, Research and Technology Department, Billingham Catalysis Centre, P.O. Box No.1, Billingham, Cleveland TS23 1LB, UK
Received 19 June 1987; accepted for publication 12 August 1987
Conventional simple criteria for surface segregation indicate that marked enrichment of zinc should occur in the surface of a-brasses. These conclusions are confirmed by calculations with a broken-bond model and thermodynamic data for a-brass as well as for the pure metals. Partial oxidation of the surface, e.g. under methanol synthesis from carbon dioxide, probably lessens surface segregation of zinc. Experimental results for methanol synthesis and the water-gas shift reaction over copper/zinc oxide and other supported copper catalysts are in qualitative agreement with the calculations.
1. Introduction In the previous papers [1,2] the bulk concentrations of zinc in supported a-brass crystallites were calculated for various practical reaction conditions. However, the consequences of a-brass formation for catalysis depend on the surface zinc concentrations, which may, if surface segregation occurs, differ from bulk concentrations. In this paper a broken-bond model, used successfully before to predict surface segregation in non-ideal platinum alloys [3,4], is applied to a-brass.
2. Simple criteria for surface segregation Various simple criteria are frequently used qualitatively to assess surface segregation and these all indicate that surface segregation of zinc should occur on a-brass in vacuo. Zinc has the lower melting point (Zn, 692.7 K; Cu, 1357 K) and the larger metallic radius (Zn, 0.137 nm; Cu, 0.128 nm). The heats of sublimation [5] at 500 K are zinc, 30.98 kcal mol-I, and copper, 80.65 kcal mol-I, Surface free energies at 500 K can be calculated from published data [6] to be, for zinc, 0.22 cal m - 2; for copper, 0.42 cal m- 2. Although the large differences in each pair of values indicate considerable surface enrichment of 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M.S. Spencer / a-brass formation in Cu/ 2nO catalysis. III
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zinc is to be expected, calculations with the properties of a-brass, as well as of copper and zinc, are desirable because of non-ideality of the system. Simple criteria were found before [3] to be inadequate in other non-ideal systems.
3. Broken-bond model for dilute a-brasses The treatment used here follows that described before [3,4] for non-ideal platinum alloys. The basic equation for surface concentrations of metals 1 and 2 in very dilute alloys (see, e.g. ref. [7]) is:
(aVaf) = (a~/an exp( -!:J.Gs/RT),
(3.1)
a:
where is the surface activity of the ith component, af is the bulk activity of the ith component and D.Gs is the free energy of segregation, defined by:
(3.2) The quantities G b and G, denote the free energy for a system with solute in the bulk and in the surface, respectively, and Gg and GsD denote the respective free energies for a pure solvent. For this application the subscript 2 refers to the solute (Zn) and 1 to the solvent (Cu). The free energy of segregation is frequently separated into two components, a strain energy arising from a mismatch of solute and solvent atom sizes and a difference in surface free energy due to differences in the various atom-atom bond energies. Strain energy is examined first. The incorporation of zinc in a-brass causes an increase in lattice dimensions, so the zinc atom in a copper lattice has a larger atom radius than that of the copper atoms (this does not necessarily follow from the atomic radii of the pure metals [3]). In this case the contribution to free energy of segregation from strain energy would be expected [7] to be significant. However, it has been argued [8-11) that when the bulk properties of the appropriate alloy are used as well as the properties of the pure metals, the strain energy is implicitly included. Further, the differences in bond energies are so large (see below) that any strain energy contribution is likely to be relatively small. For these reasons strain energy will be neglected in this treatment. The a-brasses under consideration here are dilute [1], so that each zinc atom is coordinated only by copper atoms and any Zn-Zn bonding can be ignored. The relevant reaction Cu S + Znb = CUb
+ Zn",
(3.3)
then gives for the general case for the free energy of segregation
!:J.Gs = !:J.Z(E 12 - Ell)' (3.4) where Ell and E 12 are the nearest-neighbour bond strengths (as free energies,
M.S. Spencer / a-brass formation in Cu / 2nO catalysts. III
338
Table 1 Atom-atom bond free energies in copper, zinc and dilute a-brass Temperature (K)
Ell (kcal mol-I)
400 500 600
5.680 5.419 5.159
E12 (kcal mol-I) 1.655 1.422
1.192
4.210 3.975 3.742
not enthalpies) between solvent (1) atoms, and between solvent (1) and solute (2) atoms respectively; I::. Z is the difference in effective coordination number of a bulk site and a surface site. The free energy of the Cu-Cu bond, E u , can be calculated from the free energy of vapourisation of copper [5] and the coordination number of copper, 12, in the fcc structure of a-brass. Values of E u for different temperatures are given in table 1, together with those of E 22 , calculated in the same way. For E 12 the activity coefficient for zinc in dilute brasses determined by Rapperport and Pemsler [12] from lnYZn= -O.7360-2977I T
(3.5)
was used as in earlier work. The extrapolation involved in the use of eq. (3.5) for these copper catalysts has been discussed before [1]. Let the surface segregation, cp, be defined by:
ep =
(xVxD bl b) , X2 Xl
(3.6)
(
and then from eqs. (3.1), (3.4) and (3.6), with the assumption that the activity coefficient of zinc in the surface is the same as in the bulk a-brass, cj> is given by
ep = exp[( -I::.Z(E12 - El1)/RT)].
(3.7)
Values of cj> for different temperatures and different crystal faces are given in table 2. It is clear that, as indicated by the simple criteria, that extensive surface segregation of zinc should be expected in vacuo. Table 2 Calculated surface segregation ratio for zinc in dilute a-brass Temperature
Surface segregation ratio,
(K)
(100) face A2=4
(110) face AZ=5
(111) face AZ=3
400 500 600
1630 335 116
10380 1430 381
257 78 35
M.S. Spencer / a-brassformation in Cu/ZnO catalysts. III
339
4. Effects of surface oxygen Oxygen adsorbed on the surface of bimetallic solids can modify surface segregation on one of the metal constituents. For example, surface segregation of the minor component in some platinum alloys (Pty'Ti , PtjFe [13] and Pt 3Sn [14]) is greatly enhanced by oxygen adsorption. As the surface of copper catalysts is partially covered by adsorbed oxygen under methanol synthesis and water-gas shift reaction conditions [17], it is necessary to assess the effect of this oxygen on the surface segregation of zinc in dilute a-brasses. The broken-bond model can be readily modified [13] to allow for the adsorption of other atoms on the metal surface, but a problem arises from the lack of an experimental value for the strength of the O-Zn(Cu) bond. A semi-quantitative approach can give an indication of probable changes in the surface segregation of zinc. The bond free energies of various copper-oxygen and zinc-oxygen bonds are given in table 3. Those for the bulk oxides were calculated from standard data for oxygen [15], the metals [5] and the oxides [16]. A mean value of the free energy of formation, - 57 kcal mol-) at 513 K, of adsorbed oxygen on copper metal in copper/zinc oxide/alumina catalysts was obtained [17] from the variation in oxygen coverage with PcoJpco ratio. This can be used with standard data to derive approximate values of the Cu-O bond free energy for different assumed coordinations of the surface oxygen (as the crystal planes exposed by copper crystallites in the catalyst are not known, coordinations cannot be assigned from single crystal work). It should be noted that although the free energy of formation of 0 ca/Cu is much greater than that of Cu 20 [17], thus allowing the water-gas shift reaction to proceed, the Cu-O bond free energies do not show this difference. The Zn-O bond energy in ZnO(C) is less than the Cu-O bond energy in either Cu 20(C) or CuOCC)' so it is plausible that the bond energy of O(a)-Zn(Cu) Table 3 Metal atom-oxygen atom bond free energies for various copper and zinc oxides at 500 K Compound
Atom coordination Cu :2
ZnO(c) a)
Values at 513 K.
0 :4 Cu :4 0 :4 0 :2 0 :2 Zn :4 0 :4
Bond free energy (kcal mol - I) 53.51 36.03
- 55 a) - 27 a) 35.35
340
M,S. Spencer
I a-brass formation in Cui 2nD catalysts. III
is less than that of Ora)-Cu, It then follows that the stability of the copper surface is probably enhanced by adsorbed oxygen more than that of the surface zinc atoms, Thus surface segregation of zinc probably decreases on oxygen adsorption, in contrast to the pattern found with platinum alloys [13,14]. Other adsorbates present in the metal surface under reaction conditions, e.g. hydrogen atoms, methoxy and formate radicals, may also modify zinc segregation, However, all these intermediates are adsorbed more weakly than oxygen, so their effect should be less.
5. Comparison with experimental results
From the calculations in this and the two previous papers [1,2] it is possible to estimate the surface zinc concentration in the copper crystallites of the catalysts under various reaction conditions. 5,], Methanol synthesis from C021CO I H 2 mixtures
In these mixtures, as in C02/H2 mixtures, carbon dioxide is the source of carbon for methanol [18] and the reaction takes place on the surface of copper crystallites [17,19], The calculated equilibrium zinc concentrations, both bulk and surface, are given in table 4 for typical reaction conditions. The rate of zinc diffusion at 500-550 K is fast enough [2] to ensure surface equilibration, The surface mobility of the copper crystallites under reaction conditions is significant [24], so the choice of the (111) face for the fcc crystals is reasonable, As the coverage of adsorbed oxygen under these conditions is about 0.3 Table 4 Surface concentrations of zinc in (111) faces of copper crystallites in Cu/ZnO catalysts under various reaction conditions Reaction
Temperature (K)
Gas ratio
Methanol synthesis from CO 2/COIH 2
500 550
CO/C02 =1 CO/C0 2 =1
Methanol synthesis from CO/H 2
500 500
CO/C0 2 =10 3 CO/C02 =10 2
Water-gas shift
500
H2IH 20 =1
oj b)
c)
Equilibrium zinc concentration Bulk (atom fraction) 0.00065 0.0010 (0.65) b) 0.065
8XlO- 5
Surface a) (atom fraction) 0.051 c) 0,052 c) (> 0.9) (> 0.9)
b)
b)
0.0062
c)
Values for surface in vacuo, in equilibrium with bulk a-brass. Values uncertain because calculation bases on assumption of dilute a-brass. Values under reaction conditions probably lower because of adsorbed oxygen (see text).
M.S. Spencer I a-brass formation in Cu I ZnO catalysts. III
341
monolayer [17,19], the surface zinc concentration may well be lower, e.g. about 0.001 atom fraction (section 4). This low level of surface brass formation seems unlikely to have much influence on the methanol synthesis reaction and this is borne out by several experimental observations: (a) In contrast to earlier claims, there now appears to be no special synergy between copper and zinc oxide [17,19] in the reaction, i.e. the specific activity of methanol synthesis from carbon dioxide on copper is almost independent of the support phase. Zinc oxide is important in commercial catalysts for its role in the formation and maintenance of small copper crystallites [17,20,21]. (b) Chadwick et al. [22] have pre-reduced standard copper/zinc oxide/ alumina catalysts at high temperatures to generate a-brass crystallites, but on subsequent tests the specific activity was almost unaffected by brass formation. Zinc diffusion and re-formation of zinc oxide under reaction conditions presumably gave an essentially copper surface to the a-brass crystallites. The only experimental result which may be linked to brass formation is the higher surface concentrations of adsorbed oxygen found [23] on ZnO-supported copper catalyts. 5.2. Methanol systhesis from
co / H 2
mixtures
Methanol can be synthesised over supported copper catalysts from nominally CO2-free CO /H 2 mixtures, but in practice some carbon dioxide is present, even when the reactants are purified rigorously, because the traces of byproducts have associated H 20/C02 formation [17]. The possible mechanisms of methanol synthesis under these conditions are detailed elsewhere [17,18,25), but here some of the consequences of a-brass formation are assessed. The conditions are the same as in section 5.1, but with the CO/C02 pressure ratio set at 10 3 and 10 2 at 500 K only. The calculated values of equilibrium concentrations are given in table 4. Although some of these values are much larger than the ranges for which the calculations are valid (e.g., beyond the a-brass phase boundary), it is clear that the surfaces of the brass crystallites in Cu/ZnO catalysts will contain significant amounts of zinc. There is little adsorbed oxygen at these high CO/C02 ratios [17], so qafmodified segregation can be ignored. Different groups of workers have obtained inconsistent results in methanol synthesis from CO/H 2 mixtures, even when using nominally identical Cu/ZnO catalyts [26]. It seems possible that some of the variability is due to different trace levels of carbon dioxide and hence different surface concentrations of zinc. Burch [27] has found that methanol synthesis from CO/H 2 mixtures on supported copper catalysts is markedly support-specific, unlike the synthesis from CO2/CO/H 2 mixtures [19]. Of relevance here is the high specific activity of eu/ZnO catalysts, compared with e.g. Cu/Si02 catalysts. As specific activity was calculated from measured copper metal areas, encapsulation
M.S. Spencer / a-brass formation in Cui 2nO catalysts. III
342
cannot be the explanation. The difference in activity could be a consequence of brass formation. The relation between these results, the support-specificity of palladium catalysts for methanol synthesis and the activation of related catalysts for CO hydrogenation is discussed elsewhere [25,26].
5.3. The water-gas shift reaction The water-gas shift reaction CO
+ H 20 = CO2 + H 2
(5.1)
can be catalysed by the same catalysts as those used for methanol synthesis [17], as well as by many others. Copperlzinc oxide-based catalysts are used at about 500 K with, typically, the H 2/H 2 0 pressure ratio of about unity. Under these conditions (table 4) the equilibrium zinc concentrations, both bulk and surface, are negligibly small and a-brass formation is not relevant for the water-gas shift reaction. On copper catalysts the reaction proceeds via a dissociative mechanism, with 0(3) as reaction intermediate, and although no rigorous test of support-specificity has been reported there is no evidence of synergy between copper and zinc oxide in these catalysts [17]. In earlier work, van Herwijnen and de Jong [28] found that the prereduction of a copper/zinc oxide catalyst gave a-brass crystallites containing several percent of zinc. These catalysts had lower total surface areas and decreased catalytic activity, but as no copper metal areas were measured it is uncertain whether the specific activity was lower. It is plausible that the available copper surface was decreased either by sintering (decrease of total surface area) or by blockage with zinc oxide (surface equilibration under reaction conditions would have given a copper metal surface, with almost no zinc metal, and zinc oxide).
References [1] [2] [3] [4] [5] [6] (7]
[8] [9] [10] [11] [12]
M.S. Spencer, Surface Sci. 192 (1987) 323. M.S. Spencer, Surface Sci. 192 (1987) 329. M.S. Spencer, Surface Sci. 145 (1984) 145. M.S. Spencer, Surface Sci. 145 (1984) 153. R. Hultgren, R.L. Orr, P.D. Anderson and K.K. Kelly, Selected Values of Thermodynamic Properties of Metals and Alloys (Wiley, New York, 1963). W.R. Tyson and W.A. Miller, Surface Sci. 62 (1977) 267. F.F. Abraham, N.H. Tsai and G.M. Pound, Surface Sci. 83 (1979) 406. A.D. van Langeveld and V. Ponec, App!. Surface Sci. 16 (1983) 405. W.M.H. Sachtler, App!. Surface Sci. 19 (1984) 167. P. Wynblatt and R.C. Ku, Surface Sci. 65 (1977) 511. M.J. Kelley and V. Ponee, Progr, Surface Sci. 11 (1981) 139. E.J. Rapperport and J.P. Pemsler, Met. Trans. 3 (1972) 827.
M.S. Spencer / a-brass/ormation in Cu r Zno catalysts.LlI
343
[13J M.S. Spencer, J. Catalysis 93 (1985) 216. [14J G.B. Hoflund and D.A. Asbury, Langmuir 2 (1986) 695. [15J D.R. Stull and H. Prophet, JANAF Thermochemical Tables, 2nd ed. (Natl, Bur. Std., Washington, DC, 1971). [16J J.P. Coughlin, US Bureau of Mines Bulletin 542 (1954). [17J G.c. Chinchen, M.S. Spencer, K.C. Waugh and D.A. Whan, J. Chern. Soc. Faraday Trans. I, 83 (1987) 2193. [18] G.c. Chinchen, P.J. Denny, D.G. Parker, M.S. Spencer and D.A. Whan, Appl. Catalysis 30 (1987) 333. (19J G.C. Chinch en, K.c. Waugh and D.A. Whan, Appl. Catalysis 25 (1986) 10l. [20] K. Klier, Advan. Catalysis 31 (1982) 243. [21] J.J.F. Scholten, J. Chern. Soc. Faraday Trans. I, 83 (1987) 2246. [22] D. Chadwick, C. Cawthorne and P.J. Denny, to be published. [23] G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, Preprints, Am. Chern. Soc. Div. Fuel Chern. 29 (1984) 178. [24] M.S. Spencer, Nature 323 (1986) 625. [25] M.E. Fakley, J.R. Jennings and M.S. Spencer, to be published. [26] G.c. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, AppL Catalysis, in press. [27] R. Burch, 1. Chern. Soc. Faraday Trans. I, 83 (1987) 2250. [28] T. van Herwijnen and W.A. de Jong, J. Catalysis 34 (1974) 209.
Surface Science 192 (1987) 336-343 North-Holland, Amsterdam
a-BRASS FORMATION IN COPPER/ZINC OXIDE CATALYSTS III. Surface segregation of zinc in a-brass M.S. SPENCER ICI Chemicals and Polymers Group, Research and Technology Department, Billingham Catalysis Centre, P.O. Box No.1, Billingham, Cleveland TS23 1LB, UK
Received 19 June 1987; accepted for publication 12 August 1987
Conventional simple criteria for surface segregation indicate that marked enrichment of zinc should occur in the surface of a-brasses. These conclusions are confirmed by calculations with a broken-bond model and thermodynamic data for a-brass as well as for the pure metals. Partial oxidation of the surface, e.g. under methanol synthesis from carbon dioxide, probably lessens surface segregation of zinc. Experimental results for methanol synthesis and the water-gas shift reaction over copper/zinc oxide and other supported copper catalysts are in qualitative agreement with the calculations.
1. Introduction In the previous papers [1,2] the bulk concentrations of zinc in supported a-brass crystallites were calculated for various practical reaction conditions. However, the consequences of a-brass formation for catalysis depend on the surface zinc concentrations, which may, if surface segregation occurs, differ from bulk concentrations. In this paper a broken-bond model, used successfully before to predict surface segregation in non-ideal platinum alloys [3,4], is applied to a-brass.
2. Simple criteria for surface segregation Various simple criteria are frequently used qualitatively to assess surface segregation and these all indicate that surface segregation of zinc should occur on a-brass in vacuo. Zinc has the lower melting point (Zn, 692.7 K; Cu, 1357 K) and the larger metallic radius (Zn, 0.137 nm; Cu, 0.128 nm). The heats of sublimation [5] at 500 K are zinc, 30.98 kcal mol-I, and copper, 80.65 kcal mol-I, Surface free energies at 500 K can be calculated from published data [6] to be, for zinc, 0.22 cal m - 2; for copper, 0.42 cal m- 2. Although the large differences in each pair of values indicate considerable surface enrichment of 0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M.S. Spencer / a-brass formation in Cu/ 2nO catalysis. III
337
zinc is to be expected, calculations with the properties of a-brass, as well as of copper and zinc, are desirable because of non-ideality of the system. Simple criteria were found before [3] to be inadequate in other non-ideal systems.
3. Broken-bond model for dilute a-brasses The treatment used here follows that described before [3,4] for non-ideal platinum alloys. The basic equation for surface concentrations of metals 1 and 2 in very dilute alloys (see, e.g. ref. [7]) is:
(aVaf) = (a~/an exp( -!:J.Gs/RT),
(3.1)
a:
where is the surface activity of the ith component, af is the bulk activity of the ith component and D.Gs is the free energy of segregation, defined by:
(3.2) The quantities G b and G, denote the free energy for a system with solute in the bulk and in the surface, respectively, and Gg and GsD denote the respective free energies for a pure solvent. For this application the subscript 2 refers to the solute (Zn) and 1 to the solvent (Cu). The free energy of segregation is frequently separated into two components, a strain energy arising from a mismatch of solute and solvent atom sizes and a difference in surface free energy due to differences in the various atom-atom bond energies. Strain energy is examined first. The incorporation of zinc in a-brass causes an increase in lattice dimensions, so the zinc atom in a copper lattice has a larger atom radius than that of the copper atoms (this does not necessarily follow from the atomic radii of the pure metals [3]). In this case the contribution to free energy of segregation from strain energy would be expected [7] to be significant. However, it has been argued [8-11) that when the bulk properties of the appropriate alloy are used as well as the properties of the pure metals, the strain energy is implicitly included. Further, the differences in bond energies are so large (see below) that any strain energy contribution is likely to be relatively small. For these reasons strain energy will be neglected in this treatment. The a-brasses under consideration here are dilute [1], so that each zinc atom is coordinated only by copper atoms and any Zn-Zn bonding can be ignored. The relevant reaction Cu S + Znb = CUb
+ Zn",
(3.3)
then gives for the general case for the free energy of segregation
!:J.Gs = !:J.Z(E 12 - Ell)' (3.4) where Ell and E 12 are the nearest-neighbour bond strengths (as free energies,
M.S. Spencer / a-brass formation in Cu / 2nO catalysts. III
338
Table 1 Atom-atom bond free energies in copper, zinc and dilute a-brass Temperature (K)
Ell (kcal mol-I)
400 500 600
5.680 5.419 5.159
E12 (kcal mol-I) 1.655 1.422
1.192
4.210 3.975 3.742
not enthalpies) between solvent (1) atoms, and between solvent (1) and solute (2) atoms respectively; I::. Z is the difference in effective coordination number of a bulk site and a surface site. The free energy of the Cu-Cu bond, E u , can be calculated from the free energy of vapourisation of copper [5] and the coordination number of copper, 12, in the fcc structure of a-brass. Values of E u for different temperatures are given in table 1, together with those of E 22 , calculated in the same way. For E 12 the activity coefficient for zinc in dilute brasses determined by Rapperport and Pemsler [12] from lnYZn= -O.7360-2977I T
(3.5)
was used as in earlier work. The extrapolation involved in the use of eq. (3.5) for these copper catalysts has been discussed before [1]. Let the surface segregation, cp, be defined by:
ep =
(xVxD bl b) , X2 Xl
(3.6)
(
and then from eqs. (3.1), (3.4) and (3.6), with the assumption that the activity coefficient of zinc in the surface is the same as in the bulk a-brass, cj> is given by
ep = exp[( -I::.Z(E12 - El1)/RT)].
(3.7)
Values of cj> for different temperatures and different crystal faces are given in table 2. It is clear that, as indicated by the simple criteria, that extensive surface segregation of zinc should be expected in vacuo. Table 2 Calculated surface segregation ratio for zinc in dilute a-brass Temperature
Surface segregation ratio,
(K)
(100) face A2=4
(110) face AZ=5
(111) face AZ=3
400 500 600
1630 335 116
10380 1430 381
257 78 35
M.S. Spencer / a-brassformation in Cu/ZnO catalysts. III
339
4. Effects of surface oxygen Oxygen adsorbed on the surface of bimetallic solids can modify surface segregation on one of the metal constituents. For example, surface segregation of the minor component in some platinum alloys (Pty'Ti , PtjFe [13] and Pt 3Sn [14]) is greatly enhanced by oxygen adsorption. As the surface of copper catalysts is partially covered by adsorbed oxygen under methanol synthesis and water-gas shift reaction conditions [17], it is necessary to assess the effect of this oxygen on the surface segregation of zinc in dilute a-brasses. The broken-bond model can be readily modified [13] to allow for the adsorption of other atoms on the metal surface, but a problem arises from the lack of an experimental value for the strength of the O-Zn(Cu) bond. A semi-quantitative approach can give an indication of probable changes in the surface segregation of zinc. The bond free energies of various copper-oxygen and zinc-oxygen bonds are given in table 3. Those for the bulk oxides were calculated from standard data for oxygen [15], the metals [5] and the oxides [16]. A mean value of the free energy of formation, - 57 kcal mol-) at 513 K, of adsorbed oxygen on copper metal in copper/zinc oxide/alumina catalysts was obtained [17] from the variation in oxygen coverage with PcoJpco ratio. This can be used with standard data to derive approximate values of the Cu-O bond free energy for different assumed coordinations of the surface oxygen (as the crystal planes exposed by copper crystallites in the catalyst are not known, coordinations cannot be assigned from single crystal work). It should be noted that although the free energy of formation of 0 ca/Cu is much greater than that of Cu 20 [17], thus allowing the water-gas shift reaction to proceed, the Cu-O bond free energies do not show this difference. The Zn-O bond energy in ZnO(C) is less than the Cu-O bond energy in either Cu 20(C) or CuOCC)' so it is plausible that the bond energy of O(a)-Zn(Cu) Table 3 Metal atom-oxygen atom bond free energies for various copper and zinc oxides at 500 K Compound
Atom coordination Cu :2
ZnO(c) a)
Values at 513 K.
0 :4 Cu :4 0 :4 0 :2 0 :2 Zn :4 0 :4
Bond free energy (kcal mol - I) 53.51 36.03
- 55 a) - 27 a) 35.35
340
M,S. Spencer
I a-brass formation in Cui 2nD catalysts. III
is less than that of Ora)-Cu, It then follows that the stability of the copper surface is probably enhanced by adsorbed oxygen more than that of the surface zinc atoms, Thus surface segregation of zinc probably decreases on oxygen adsorption, in contrast to the pattern found with platinum alloys [13,14]. Other adsorbates present in the metal surface under reaction conditions, e.g. hydrogen atoms, methoxy and formate radicals, may also modify zinc segregation, However, all these intermediates are adsorbed more weakly than oxygen, so their effect should be less.
5. Comparison with experimental results
From the calculations in this and the two previous papers [1,2] it is possible to estimate the surface zinc concentration in the copper crystallites of the catalysts under various reaction conditions. 5,], Methanol synthesis from C021CO I H 2 mixtures
In these mixtures, as in C02/H2 mixtures, carbon dioxide is the source of carbon for methanol [18] and the reaction takes place on the surface of copper crystallites [17,19], The calculated equilibrium zinc concentrations, both bulk and surface, are given in table 4 for typical reaction conditions. The rate of zinc diffusion at 500-550 K is fast enough [2] to ensure surface equilibration, The surface mobility of the copper crystallites under reaction conditions is significant [24], so the choice of the (111) face for the fcc crystals is reasonable, As the coverage of adsorbed oxygen under these conditions is about 0.3 Table 4 Surface concentrations of zinc in (111) faces of copper crystallites in Cu/ZnO catalysts under various reaction conditions Reaction
Temperature (K)
Gas ratio
Methanol synthesis from CO 2/COIH 2
500 550
CO/C02 =1 CO/C0 2 =1
Methanol synthesis from CO/H 2
500 500
CO/C0 2 =10 3 CO/C02 =10 2
Water-gas shift
500
H2IH 20 =1
oj b)
c)
Equilibrium zinc concentration Bulk (atom fraction) 0.00065 0.0010 (0.65) b) 0.065
8XlO- 5
Surface a) (atom fraction) 0.051 c) 0,052 c) (> 0.9) (> 0.9)
b)
b)
0.0062
c)
Values for surface in vacuo, in equilibrium with bulk a-brass. Values uncertain because calculation bases on assumption of dilute a-brass. Values under reaction conditions probably lower because of adsorbed oxygen (see text).
M.S. Spencer I a-brass formation in Cu I ZnO catalysts. III
341
monolayer [17,19], the surface zinc concentration may well be lower, e.g. about 0.001 atom fraction (section 4). This low level of surface brass formation seems unlikely to have much influence on the methanol synthesis reaction and this is borne out by several experimental observations: (a) In contrast to earlier claims, there now appears to be no special synergy between copper and zinc oxide [17,19] in the reaction, i.e. the specific activity of methanol synthesis from carbon dioxide on copper is almost independent of the support phase. Zinc oxide is important in commercial catalysts for its role in the formation and maintenance of small copper crystallites [17,20,21]. (b) Chadwick et al. [22] have pre-reduced standard copper/zinc oxide/ alumina catalysts at high temperatures to generate a-brass crystallites, but on subsequent tests the specific activity was almost unaffected by brass formation. Zinc diffusion and re-formation of zinc oxide under reaction conditions presumably gave an essentially copper surface to the a-brass crystallites. The only experimental result which may be linked to brass formation is the higher surface concentrations of adsorbed oxygen found [23] on ZnO-supported copper catalyts. 5.2. Methanol systhesis from
co / H 2
mixtures
Methanol can be synthesised over supported copper catalysts from nominally CO2-free CO /H 2 mixtures, but in practice some carbon dioxide is present, even when the reactants are purified rigorously, because the traces of byproducts have associated H 20/C02 formation [17]. The possible mechanisms of methanol synthesis under these conditions are detailed elsewhere [17,18,25), but here some of the consequences of a-brass formation are assessed. The conditions are the same as in section 5.1, but with the CO/C02 pressure ratio set at 10 3 and 10 2 at 500 K only. The calculated values of equilibrium concentrations are given in table 4. Although some of these values are much larger than the ranges for which the calculations are valid (e.g., beyond the a-brass phase boundary), it is clear that the surfaces of the brass crystallites in Cu/ZnO catalysts will contain significant amounts of zinc. There is little adsorbed oxygen at these high CO/C02 ratios [17], so qafmodified segregation can be ignored. Different groups of workers have obtained inconsistent results in methanol synthesis from CO/H 2 mixtures, even when using nominally identical Cu/ZnO catalyts [26]. It seems possible that some of the variability is due to different trace levels of carbon dioxide and hence different surface concentrations of zinc. Burch [27] has found that methanol synthesis from CO/H 2 mixtures on supported copper catalysts is markedly support-specific, unlike the synthesis from CO2/CO/H 2 mixtures [19]. Of relevance here is the high specific activity of eu/ZnO catalysts, compared with e.g. Cu/Si02 catalysts. As specific activity was calculated from measured copper metal areas, encapsulation
M.S. Spencer / a-brass formation in Cui 2nO catalysts. III
342
cannot be the explanation. The difference in activity could be a consequence of brass formation. The relation between these results, the support-specificity of palladium catalysts for methanol synthesis and the activation of related catalysts for CO hydrogenation is discussed elsewhere [25,26].
5.3. The water-gas shift reaction The water-gas shift reaction CO
+ H 20 = CO2 + H 2
(5.1)
can be catalysed by the same catalysts as those used for methanol synthesis [17], as well as by many others. Copperlzinc oxide-based catalysts are used at about 500 K with, typically, the H 2/H 2 0 pressure ratio of about unity. Under these conditions (table 4) the equilibrium zinc concentrations, both bulk and surface, are negligibly small and a-brass formation is not relevant for the water-gas shift reaction. On copper catalysts the reaction proceeds via a dissociative mechanism, with 0(3) as reaction intermediate, and although no rigorous test of support-specificity has been reported there is no evidence of synergy between copper and zinc oxide in these catalysts [17]. In earlier work, van Herwijnen and de Jong [28] found that the prereduction of a copper/zinc oxide catalyst gave a-brass crystallites containing several percent of zinc. These catalysts had lower total surface areas and decreased catalytic activity, but as no copper metal areas were measured it is uncertain whether the specific activity was lower. It is plausible that the available copper surface was decreased either by sintering (decrease of total surface area) or by blockage with zinc oxide (surface equilibration under reaction conditions would have given a copper metal surface, with almost no zinc metal, and zinc oxide).
References [1] [2] [3] [4] [5] [6] (7]
[8] [9] [10] [11] [12]
M.S. Spencer, Surface Sci. 192 (1987) 323. M.S. Spencer, Surface Sci. 192 (1987) 329. M.S. Spencer, Surface Sci. 145 (1984) 145. M.S. Spencer, Surface Sci. 145 (1984) 153. R. Hultgren, R.L. Orr, P.D. Anderson and K.K. Kelly, Selected Values of Thermodynamic Properties of Metals and Alloys (Wiley, New York, 1963). W.R. Tyson and W.A. Miller, Surface Sci. 62 (1977) 267. F.F. Abraham, N.H. Tsai and G.M. Pound, Surface Sci. 83 (1979) 406. A.D. van Langeveld and V. Ponec, App!. Surface Sci. 16 (1983) 405. W.M.H. Sachtler, App!. Surface Sci. 19 (1984) 167. P. Wynblatt and R.C. Ku, Surface Sci. 65 (1977) 511. M.J. Kelley and V. Ponee, Progr, Surface Sci. 11 (1981) 139. E.J. Rapperport and J.P. Pemsler, Met. Trans. 3 (1972) 827.
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[13J M.S. Spencer, J. Catalysis 93 (1985) 216. [14J G.B. Hoflund and D.A. Asbury, Langmuir 2 (1986) 695. [15J D.R. Stull and H. Prophet, JANAF Thermochemical Tables, 2nd ed. (Natl, Bur. Std., Washington, DC, 1971). [16J J.P. Coughlin, US Bureau of Mines Bulletin 542 (1954). [17J G.c. Chinchen, M.S. Spencer, K.C. Waugh and D.A. Whan, J. Chern. Soc. Faraday Trans. I, 83 (1987) 2193. [18] G.c. Chinchen, P.J. Denny, D.G. Parker, M.S. Spencer and D.A. Whan, Appl. Catalysis 30 (1987) 333. (19J G.C. Chinch en, K.c. Waugh and D.A. Whan, Appl. Catalysis 25 (1986) 10l. [20] K. Klier, Advan. Catalysis 31 (1982) 243. [21] J.J.F. Scholten, J. Chern. Soc. Faraday Trans. I, 83 (1987) 2246. [22] D. Chadwick, C. Cawthorne and P.J. Denny, to be published. [23] G.C. Chinchen, P.J. Denny, D.G. Parker, G.D. Short, M.S. Spencer, K.C. Waugh and D.A. Whan, Preprints, Am. Chern. Soc. Div. Fuel Chern. 29 (1984) 178. [24] M.S. Spencer, Nature 323 (1986) 625. [25] M.E. Fakley, J.R. Jennings and M.S. Spencer, to be published. [26] G.c. Chinchen, P.J. Denny, J.R. Jennings, M.S. Spencer and K.C. Waugh, AppL Catalysis, in press. [27] R. Burch, 1. Chern. Soc. Faraday Trans. I, 83 (1987) 2250. [28] T. van Herwijnen and W.A. de Jong, J. Catalysis 34 (1974) 209.