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Base metal oxides as promotors for the electrochemical oxidation of methanol
349
J. Electroanal. Chem., 240 (1988) 349-353 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note BASE METAL OXIDES AS PROMOTORS OXIDATION OF METHANOL
ANDREW
HAMNETT,
BRENDAN
J. KENNEDY
l
FOR THE ELECTROCHEMICAL
and SIMON A. WEEKS
l
*
Inorganrc Chemrstry Luborafory, South Parks Roaci, Oxford OXI 3QR (Great Britain) (Received 13th November 1987)
The use of base-metal oxides, especially the Group IV-VI transition metal oxides, as gas-phase catalysts for a wide variety of oxidation reactions including the partial oxidation of methanol to formaldehyde [l] and the oxidative dimerisation of methane [2] is now well known. As a consequence of the fact that most of these oxides are poor electrical conductors very little is known about their ability to catalyse electrochemical reactions, although there is evidence to suggest that some conducting metal oxides are electrocatalytically active [3,4]. Deposition of insulating transition metal oxides onto a conductive support, such as carbon black, does offer, potentially, a method of utilising their catalytic ability in electrochemical reactions. However, for the oxidation of methanol, both adsorption of the fuel from solution and oxygen atom transfer must take place. Whilst the oxides could, in principle, supply the latter, it is unlikely that methanol will adsorb preferentially on oxide surfaces, and we must also incorporate platinum as a co-catalyst. Although the short term activity reported below for the oxidation of methanol by Pt/MO, : C anodes is less than that found in the well studied, and carefully optimised Pt/Ru anodes [5,6], the mechanism by which a second element can promote the activity of Pt is of considerable interest and a full understanding of this mechanism is of great importance. The binary metal-oxide Pt catalysts were prepared as follows. High purity carbon was suspended in water and heated to boiling. A dilute solution of the metal chloride was then added and the pH adjusted to ensure complete hydrolysis. Chloroplatinic acid was then added and reduced with an excess of HCHO [6]. The slurry was then filtered and washed with copious amounts of water. Teflon bonded
Present address: Research School of Chemistry, Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia. l * Present address: BP Research Centre, Sunbury-on-Thames, Middlesex TW16 7LN, Great Britain. l
0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
350
electrodes were prepared as described elsewhere [6,7]. Polarisation curves were measured in an argon saturated 2.5 M H,SO, + 1 M CH,OH solution at 60 o C, and potentials are referred to the reversible mercury-mercury sulphate electrode in 1 M H,SO, (RMSE) which has a potential of ca. 640 mV vs. SHE. The catalysts were characterised by X-ray diffraction and electron microscopy. The former showed very broad structure under the main platinum diffraction peaks, suggesting that the platinum particles were extremely small, and this was borne out by electron microscopy which revealed a mean particle size of ca. 2 nm. On a platinum electrode the direct electro-oxidation of methanol involves two distinct processes [5]: (A) Methanol adsorption and dehydrogenation: CH,OH
+
Pt J-OH +3H++3e-
--* Pt-CHO
(1)
(2)
+
Pt ,=CO +H++e(3)
+ Pt-CO (4)
where either (1) or (2) are thought to be active intermediates and (3) and (4) act as poisons. (B) provision of “active” oxy-species to complete methanol oxidation: H,O -+ Pt-OH,,,,,
+ H+ + e-
where (5) can complete the oxidation of (1) or (2) to CO,. Whilst the dehydrogenation of methanol is believed to occur at a fairly low overpotential, the formation of (5) on bulk platinum is not expected to occur to any great extent below ca. 0 V vs. RMSE (i.e. 0.6 V overpotential) and the overall reaction will not, therefore, occur to any great extent [5-71. We note, however, that small platinum particles ca. 2 nm dia. can promote the formation of platinum oxides at much lower potentials [6]. The short term polarisation behaviour of carbon supported binary catalysts for the electro-oxidation of methanol in acid solutions is shown in Fig. 1. From this figure it is seen that the co-precipitation of a base metal oxide with platinum has a dramatic influence on the catalytic behaviour. The major points to be noted from this figure are: (1) The Group IV oxides all behave rather similarly, with only TiO, shown; all act as promotors at low current densities (and potentials) only. Above a threshold region they act as inhibitors. (2) The Group V oxides Nb,OS and Ta,O, behave similarly, acting as promotors at all current densities and (3) the Group VI oxide WO, acts as an inhibitor. X-ray photoelectron spectroscopy of the materials show the presence of at least two platinum species in all the catalysts. The majority species has binding energies similar to those found for the 4f,,* and 4f,,, ionisations of Pt(O), whilst a second species, with an intensity 30-50% of the total, has binding energies characteristic of Pt(I1). In all cases where a metal oxide was present, the coverage by Pt(I1) was substantially higher than for platinum particles dispersed on carbon alone. Save for the Pt/TiO, system, in which both Ti3+ and Ti4+ species were found to be present, the XPS indicated that the metal oxides all possess their group valence. Thus it
351
-0
3
0
50
100
150 CurredmA
Fig. 1. Current (mA per mg of Pt)-potential Pt/Nb,O, : Ct, ( X) Pt : Ct and (*) Pt/WO,
200 mg-’
curves
for methanol
: Ct in 2.5 A4 H,SO,
oxidation on: ( +) Pt/TiO, + 1 M CH,OH at 60 o C.
: Ct. (0)
appears that the provision of an active oxygen species to complete the oxidation of methanol does not take place by a redox couple based on the metal oxide, but rather the oxide acts by promoting the formation of active platinum oxides. It has recently been shown that when these metal oxides are dispersed on an oxide support, such as A1203, they exist in an approximately tetrahedral arrangement of oxy and hydroxy groups [S]. In aqueous electrolytes, it is likely that this basic bonding scheme is retained using Pt-0 linkages, though it is, of course, probable that additional coordination of H,O molecules will take place. For TiO, and ZrO, this means that only M-OH and M-OH, groups are present whilst for Nb there is evidence for a single M=O group. This, taken with the above observations on the catalytic activity, suggests the following mechanistic scheme shown in Scheme 1. For M=Ti and Zr the initial step in the reaction is a rearrangement of (6) to (7) in which a M=O group is formed; this gives a Pt-OH group which can then oxidise a nearby Pt-CHO or Pt,H=COH residue to CO,, liberating two protons and two electrons. A water molecule, possibly one already present in the inner coordination sphere of the metal, then regenerates the starting material. It is furiher plausible that an increase in potential may favour the formation of a M=O group without generating a Pt-OH moiety, such as that illustrated as species (9). This cannot then rearrange to give a Pt-OH group without breaking both M-0-Pt linkages and hence an upper potential limit for promotion by M(IV) oxy species is expected. For M=Nb and Ta the reaction pathway is essentially the same, but now increasing the potential results in the formation of a Pt-OH group as well as the M=O group and so no potential dependence is expected. For M=W it is not possible to form a Pt-OH group without breaking both Pt-O-M linkages. The inhibiting
352 For
I=Ti or Zr
“\ 7\
-
j”
Pt-CHO
7
~
Ho\ 7
04 o\*t~oH
O\P,/O
O\ Pt + CO2 + 2H+ + 2e-
(6)
'Pi! t H+ t e-
(8)
(7)
I
Increased Potential
c
19 /*\ Olpt/O (9)
For lI=Hbor Ta the initial species is Ho\
//"
/\ O\
/O Pt (10)
and
for
I=W it is
0
Pt
Scheme 1. Proposed mechanism oxide promotors.
for the promoting/inhibiting
effect displayed
by the transition
metal
353
effect observed for WO, is presumably surface area of the platinum catalyst.
a consequence
of a lowering of the effective
ACKNOWLEDGEMENTS
We thank the SERC and EEC for support and Johnson Matthey Ltd. for the loan of platinum and a CASE award (S.A.W.). REFERENCES 1 2 3 4 5 6
T.J. Yang and J.H. Lunsford, J. Catal., 103 (1987) 55. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. M. Koudelka, A. Momtier and J. Augustynski, J. Electrochem. Sot., 131 (1984) 745. D.E. Aspnes and A. Heller, J. Phys. Chem., 87 (1983) 4919. B.D. McNicol, J. Electroanal. Chem., 118 (1981) 71. J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan and S.A. Weeks, J. Electroanak Chem., 240 (1988) 133. 7 J.B. Goodenough, A. Hamnett, B.J. Kennedy and S.A. Weeks, Electrochim. Acta, 32 (1987) 1233. 8 J. Bemholc, J.A. Horsley, L.L. Murrefl, L.G. Sherman and S. Soled, J. Phys. Chem., 91 (1987) 1526.
J. Electroanal. Chem., 240 (1988) 349-353 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note BASE METAL OXIDES AS PROMOTORS OXIDATION OF METHANOL
ANDREW
HAMNETT,
BRENDAN
J. KENNEDY
l
FOR THE ELECTROCHEMICAL
and SIMON A. WEEKS
l
*
Inorganrc Chemrstry Luborafory, South Parks Roaci, Oxford OXI 3QR (Great Britain) (Received 13th November 1987)
The use of base-metal oxides, especially the Group IV-VI transition metal oxides, as gas-phase catalysts for a wide variety of oxidation reactions including the partial oxidation of methanol to formaldehyde [l] and the oxidative dimerisation of methane [2] is now well known. As a consequence of the fact that most of these oxides are poor electrical conductors very little is known about their ability to catalyse electrochemical reactions, although there is evidence to suggest that some conducting metal oxides are electrocatalytically active [3,4]. Deposition of insulating transition metal oxides onto a conductive support, such as carbon black, does offer, potentially, a method of utilising their catalytic ability in electrochemical reactions. However, for the oxidation of methanol, both adsorption of the fuel from solution and oxygen atom transfer must take place. Whilst the oxides could, in principle, supply the latter, it is unlikely that methanol will adsorb preferentially on oxide surfaces, and we must also incorporate platinum as a co-catalyst. Although the short term activity reported below for the oxidation of methanol by Pt/MO, : C anodes is less than that found in the well studied, and carefully optimised Pt/Ru anodes [5,6], the mechanism by which a second element can promote the activity of Pt is of considerable interest and a full understanding of this mechanism is of great importance. The binary metal-oxide Pt catalysts were prepared as follows. High purity carbon was suspended in water and heated to boiling. A dilute solution of the metal chloride was then added and the pH adjusted to ensure complete hydrolysis. Chloroplatinic acid was then added and reduced with an excess of HCHO [6]. The slurry was then filtered and washed with copious amounts of water. Teflon bonded
Present address: Research School of Chemistry, Australian National University, G.P.O. Box 4, Canberra, A.C.T. 2601, Australia. l * Present address: BP Research Centre, Sunbury-on-Thames, Middlesex TW16 7LN, Great Britain. l
0022-0728/88/$03.50
0 1988 Elsevier Sequoia S.A.
350
electrodes were prepared as described elsewhere [6,7]. Polarisation curves were measured in an argon saturated 2.5 M H,SO, + 1 M CH,OH solution at 60 o C, and potentials are referred to the reversible mercury-mercury sulphate electrode in 1 M H,SO, (RMSE) which has a potential of ca. 640 mV vs. SHE. The catalysts were characterised by X-ray diffraction and electron microscopy. The former showed very broad structure under the main platinum diffraction peaks, suggesting that the platinum particles were extremely small, and this was borne out by electron microscopy which revealed a mean particle size of ca. 2 nm. On a platinum electrode the direct electro-oxidation of methanol involves two distinct processes [5]: (A) Methanol adsorption and dehydrogenation: CH,OH
+
Pt J-OH +3H++3e-
--* Pt-CHO
(1)
(2)
+
Pt ,=CO +H++e(3)
+ Pt-CO (4)
where either (1) or (2) are thought to be active intermediates and (3) and (4) act as poisons. (B) provision of “active” oxy-species to complete methanol oxidation: H,O -+ Pt-OH,,,,,
+ H+ + e-
where (5) can complete the oxidation of (1) or (2) to CO,. Whilst the dehydrogenation of methanol is believed to occur at a fairly low overpotential, the formation of (5) on bulk platinum is not expected to occur to any great extent below ca. 0 V vs. RMSE (i.e. 0.6 V overpotential) and the overall reaction will not, therefore, occur to any great extent [5-71. We note, however, that small platinum particles ca. 2 nm dia. can promote the formation of platinum oxides at much lower potentials [6]. The short term polarisation behaviour of carbon supported binary catalysts for the electro-oxidation of methanol in acid solutions is shown in Fig. 1. From this figure it is seen that the co-precipitation of a base metal oxide with platinum has a dramatic influence on the catalytic behaviour. The major points to be noted from this figure are: (1) The Group IV oxides all behave rather similarly, with only TiO, shown; all act as promotors at low current densities (and potentials) only. Above a threshold region they act as inhibitors. (2) The Group V oxides Nb,OS and Ta,O, behave similarly, acting as promotors at all current densities and (3) the Group VI oxide WO, acts as an inhibitor. X-ray photoelectron spectroscopy of the materials show the presence of at least two platinum species in all the catalysts. The majority species has binding energies similar to those found for the 4f,,* and 4f,,, ionisations of Pt(O), whilst a second species, with an intensity 30-50% of the total, has binding energies characteristic of Pt(I1). In all cases where a metal oxide was present, the coverage by Pt(I1) was substantially higher than for platinum particles dispersed on carbon alone. Save for the Pt/TiO, system, in which both Ti3+ and Ti4+ species were found to be present, the XPS indicated that the metal oxides all possess their group valence. Thus it
351
-0
3
0
50
100
150 CurredmA
Fig. 1. Current (mA per mg of Pt)-potential Pt/Nb,O, : Ct, ( X) Pt : Ct and (*) Pt/WO,
200 mg-’
curves
for methanol
: Ct in 2.5 A4 H,SO,
oxidation on: ( +) Pt/TiO, + 1 M CH,OH at 60 o C.
: Ct. (0)
appears that the provision of an active oxygen species to complete the oxidation of methanol does not take place by a redox couple based on the metal oxide, but rather the oxide acts by promoting the formation of active platinum oxides. It has recently been shown that when these metal oxides are dispersed on an oxide support, such as A1203, they exist in an approximately tetrahedral arrangement of oxy and hydroxy groups [S]. In aqueous electrolytes, it is likely that this basic bonding scheme is retained using Pt-0 linkages, though it is, of course, probable that additional coordination of H,O molecules will take place. For TiO, and ZrO, this means that only M-OH and M-OH, groups are present whilst for Nb there is evidence for a single M=O group. This, taken with the above observations on the catalytic activity, suggests the following mechanistic scheme shown in Scheme 1. For M=Ti and Zr the initial step in the reaction is a rearrangement of (6) to (7) in which a M=O group is formed; this gives a Pt-OH group which can then oxidise a nearby Pt-CHO or Pt,H=COH residue to CO,, liberating two protons and two electrons. A water molecule, possibly one already present in the inner coordination sphere of the metal, then regenerates the starting material. It is furiher plausible that an increase in potential may favour the formation of a M=O group without generating a Pt-OH moiety, such as that illustrated as species (9). This cannot then rearrange to give a Pt-OH group without breaking both M-0-Pt linkages and hence an upper potential limit for promotion by M(IV) oxy species is expected. For M=Nb and Ta the reaction pathway is essentially the same, but now increasing the potential results in the formation of a Pt-OH group as well as the M=O group and so no potential dependence is expected. For M=W it is not possible to form a Pt-OH group without breaking both Pt-O-M linkages. The inhibiting
352 For
I=Ti or Zr
“\ 7\
-
j”
Pt-CHO
7
~
Ho\ 7
04 o\*t~oH
O\P,/O
O\ Pt + CO2 + 2H+ + 2e-
(6)
'Pi! t H+ t e-
(8)
(7)
I
Increased Potential
c
19 /*\ Olpt/O (9)
For lI=Hbor Ta the initial species is Ho\
//"
/\ O\
/O Pt (10)
and
for
I=W it is
0
Pt
Scheme 1. Proposed mechanism oxide promotors.
for the promoting/inhibiting
effect displayed
by the transition
metal
353
effect observed for WO, is presumably surface area of the platinum catalyst.
a consequence
of a lowering of the effective
ACKNOWLEDGEMENTS
We thank the SERC and EEC for support and Johnson Matthey Ltd. for the loan of platinum and a CASE award (S.A.W.). REFERENCES 1 2 3 4 5 6
T.J. Yang and J.H. Lunsford, J. Catal., 103 (1987) 55. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. M. Koudelka, A. Momtier and J. Augustynski, J. Electrochem. Sot., 131 (1984) 745. D.E. Aspnes and A. Heller, J. Phys. Chem., 87 (1983) 4919. B.D. McNicol, J. Electroanal. Chem., 118 (1981) 71. J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan and S.A. Weeks, J. Electroanak Chem., 240 (1988) 133. 7 J.B. Goodenough, A. Hamnett, B.J. Kennedy and S.A. Weeks, Electrochim. Acta, 32 (1987) 1233. 8 J. Bemholc, J.A. Horsley, L.L. Murrefl, L.G. Sherman and S. Soled, J. Phys. Chem., 91 (1987) 1526.