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J. Phys. Chem. B 2001, 105, 5950-5956
Study of the Partial Oxidation of Methanol to Formaldehyde on a Polycrystalline Ag Foil R. J. Beuhler, R. M. Rao, J. Hrbek, and M. G. White* Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: January 11, 2001; In Final Form: April 11, 2001
A study of the partial oxidation reaction of methanol to formaldehyde has been carried out on the surface of an oxygen permeable, Ag-membrane. Surface oxygen species are formed on the high vacuum side of the Ag-foil by diffusion of O atoms generated by dissociative adsorption on the high pressure side at temperatures >500 K. Methanol pressures were kept sufficiently low (e10-5 Torr) to ensure collision free sampling of the products using electron-impact mass spectrometry. Starting with a new Ag-foil, reaction rates were found to increase over time, with maximum rates achieved after activation times of several hours. Activation is accompanied by significant changes in membrane morphology. For a methanol pressure of 1 × 10-6 Torr, the maximum rate of formaldehyde formation is 7 × 1014 mol/s-cm2 at 900 K and only decreases by a factor of 2 as the foil temperature is reduced to 300 K. Reaction at room temperature is attributed to the high diffusion rate of O atoms dissolved in the bulk which maintains the surface oxygen concentration sufficiently high to achieve reaction rates of 5 × 1014 mol/s-cm2 for hours when the methanol pressure is held at 2 × 10-6 Torr. Attempts to titrate all the membrane O atoms through reaction with methanol were unsuccessful, and suggest that the loading must be greater than 1019 O atoms/gram of Ag.
I. Introduction The reaction of methanol to form formaldehyde is a largescale industrial process carried out over silver or copper catalysts with worldwide production in excess of 1.5 × 107 tons in 1997.1 Due to its commercial importance, this reaction has been studied extensively in efforts to improve the efficiency of the process and to understand the reaction on a molecular basis.2,3 Under oxidizing conditions, the reaction is thought to proceed by the dissociative adsorption of methanol to form the methoxy radical, followed by the loss of a methyl hydrogen to form formaldehyde and water which desorb from the surface.4 The overall reaction is highly exothermic (∆H ) -156 kJ/mol)1 and can be written as
CH3OH + O(a) f CH2O + H2O
(1)
On Ag surfaces, the methanol-to-formaldehyde reaction is optimally run at temperatures between 800 K to 900 K, where O2 molecules undergo dissociative adsorption and O atoms have high mobility and can readily dissolve into the Ag bulk.3,4,5 We exploit this unique property of Ag metal to prepare reactive surfaces with high coverages of O atoms through the use of a permeable Ag membrane. Surface oxygen species are formed on the high vacuum side of a Ag surface by bulk diffusion of O atoms produced by dissociative adsorption on the highpressure side of a heated foil (∼400 Torr, >400 K).6 Because the sticking coefficient for dissociative adsorption of O2 on Ag surfaces is relatively small (500 K undergoes crystallization of the bulk with facet formation at the surface with low index faces ((111) and (110)) [5,10,11]. Reaction with excess methanol is also known to lead to hole formation that disrupts the surface order as a result of the reaction of hydrogen with bulk dissolved oxygen to form water.10 In the present work, the reactivity toward formaldehyde
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5953
Figure 5. Plot of the fraction of methanol reacting as a function of the activation time for four new silver foils at two temperatures and with different exposures. For the data labeled “external O2”, the oxygen exposures were 8640 L at 0.6 h, 6000 L at 1.5 h, and 3360 L at 1.9 h. For the “external methanol” activation at 925 K, the methanol exposure was continuous at a pressure of 1.1 × 10-5 Torr.
formation varied with high-temperature oxygen and methanol pretreatment and it became necessary to investigate the activation of the Ag surface in a series of controlled experiments. The activation histories of several Ag-membranes are shown in Figure 5. Following an overnight bake under high vacuum (∼10-9 Torr), each new foil was heated to temperatures greater than 800 K with 400 Torr of oxygen on the high-pressure side for several hours at a time. The reactivity of the Ag-membrane at high temperatures was periodically checked by introducing methanol to a pressure of about 5 × 10-6 Torr and extracting the reacting fraction of methanol. The total exposure of methanol for each measurement in Figure 5 was about 3000 L, with the exception of the bottom curve for which the methanol was continually introduced at a pressure of 1.1 × 10-5 Torr. Interestingly, the Ag-surface was not found to activate under these latter conditions, with the methanol reaction fraction reaching only 7%. On the other hand, the foils treated in the absence of methanol reached maximum activation over the same time period, with reacting fractions approaching 40%. The initial rate of activation at 850 K appears to be faster than at 925 K, and exposure to gas-phase oxygen on the highvacuum side of the foil further accelerates activation in the first few hours. For the data labeled “External O2” in Figure 5, the oxygen exposures were 8640 L at 0.6 h, 6000 L at 1.5 h, and 3360 L at 1.9 h. In all cases, however, the limiting activity level was reached only after a period of 5 or 6 h. Furthermore, the amount of oxygen diffusing through the foils, as measured by the partial pressure of desorbing molecular oxygen, was comparable. The decreased activation rates at 925 K are qualitatively consistent with the discussion by Nagy, et al, who noted that a polycrystalline Ag-foil pretreated in oxygen can undergo local restructuring to close packed surfaces at temperatures above 900 K.5 This “healing” evidently hinders the formation of reactive O atom sites for the Ag-membranes studied here. Furthermore, Nagy, et al, found that the optimum temperature for morphological change was ∼775 K [5], which is also consistent with the higher rate of activation observed at 850 K. The data in Figure 5 also suggest that exposure to methanol, even at the low pressures used here (1.1 × 10-5 Torr), has an even more dramatic influence on the ability of the Agsurface to restructure and strongly inhibits the formation of a reactive surface. This inhibitory effect of methanol is similar
5954 J. Phys. Chem. B, Vol. 105, No. 25, 2001
Figure 6. (a) Formaldehyde formation rates (molecules/s-cm2) as a function of temperature on activated Ag-membrane. The methanol reactant pressure is 1 × 10-6 Torr. (b) Fraction of methanol reacting as derived from the reaction rates in (a).
to that found by Millar, et al., who showed that methanol inhibits grain growth on polycrystalline Ag [16]. D. Temperature Dependence of Reaction Rate. Figure 6(a) shows the temperature dependence of the methanol-to-formaldehyde reaction rate, evaluated at a methanol pressure of 1 × 10-6 Torr. The data were obtained by rapidly heating the already activated foil to about 900 K and measuring the maximum reaction rate at each temperature as the foil cooled. The oxygen pressure behind the foil was approximately 400 Torr for these measurements, but as discussed previously, this has only a very small effect on the formaldehyde formation rate. The error bars represent the reproducibility of successive measurements and are typically less than 20%. A much wider range of rates was found at room temperature and is addressed below. No attempt was made to look for finer details of structure in the reaction rate temperature dependence. Figure 6b is a plot of the fraction of methanol reacting vs the foil temperature. The highest reacting fractions are ∼50% at 875 K. Surprisingly, the rate of formaldehyde formation decreases by less than a factor of 2 as the temperature is decreased from 900 K to room temperature. This result differs significantly from the results of Nagy, et al., who studied the methanol reaction over an aged electrolytic silver catalyst.10 In that work, the yield of formaldehyde has a sharp onset at ∼475 K, beyond which the yield rises to 55% at 900 K. In addition, the ratio of carbon dioxide to formaldehyde formation is 2.8 at 500 K and drops to 0.18 at 900 K, which indicates a marked increase in the selectivity of the reaction as the temperature is increased. For the Ag-membrane used in this work, the observed ratio of carbon dioxide to formaldehyde products is significantly smaller and also varies by less than a factor of 2 over the entire temperature range studied (300-900 K). Specifically, the CO2/H2CO ratio is 1.9 × 10-2 at 300 K and increases to 2.7 × 10-2 at 880 K. These differences in temperature dependence and selectivity reflect the very different processes that provide the active surface oxygen species in the Ag-membrane and dispersed Ag catalysts. Specifically, active oxygen species must be continuously created on the electrolytic Ag substrate via dissociative adsorption of molecular oxygen in the feed gas. Furthermore, high pressure exposure leads to at least three bound O atom species with desorption temperatures of 480 K, 600 K and >900 K. The latter two are thought to be bulk dissolved O atoms and strongly bound near-surface O atoms, respectively, whereas the lowtemperature peak at 480 K has been assigned to both chemisorbed O atoms and molecular oxygen.5,7,10,11 The reaction onset
Beuhler et al.
Figure 7. Room-temperature reaction rates for formaldehyde formation as a function of methanol exposure for two different, initial silver foil conditions. Reaction rates are normalized to the rate of formaldehyde formation at a methanol pressure of 1 × 10-6 Torr. The continuous data were obtained for a foil activated at high temperatures, cooled overnight, and then exposed to methanol over a period of several hours. The three data points were obtained from the same foil on a subsequent day after room-temperature exposure to 2570 L of oxygen, prior to exposure to methanol.
at ∼475 K reported by Nagy, et al., indicates that chemisorbed O atoms (desorption at 480 K) do not play a role in the reaction. More likely, the onset corresponds to thermally driven surface reconstruction and/or diffusion of O atoms into near-surface or subsurface sites. In the present experiment, the active O atom species in near-surface or subsurface sites are provided by diffusion of bulk dissolved oxygen, which is loaded into the foil at high temperatures and remains as a reservoir at low temperatures. As a result, the reactivity of the Ag-membrane can extend to lower temperatures, providing there are no significant barriers to reaction or chemisorption of methanol. As noted above, the reaction rates measured at room temperature (see Figure 6) exhibited a greater range of values than at higher temperatures and, furthermore, appeared to be correlated with prior treatment of the Ag-foil. Room temperature measurements were usually performed the day after high temperature runs due to the exponentially long cooling time of the stainless steel mounting hardware for the foil. The cooled Ag-surface was found to be relatively inactive, but the reaction rate could be increased by exposure to either oxygen or methanol. Results of two types of room-temperature activation experiments are shown in Figure 7, where the rate of formaldehyde formation is plotted vs the integrated exposure to methanol. The filled circles show the reactivity when the roomtemperature foil was first exposed to 2570 L of molecular oxygen, followed by exposure to methanol. Two subsequent methanol exposures at integrated doses of 1070 and 5700 L resulted in lower rates of formaldehyde formation and appear to level off at about 3.0 × 1014 molecules/s-cm2. Activation of the surface with methanol alone is shown in the lower curve in Figure 7 which was obtained by exposing the foil to a constant pressure of methanol (5 × 10-6 Torr) for approximately 70 min. Reactivation with methanol requires significantly more exposure than activation with oxygen, but both treatments reach the same steady-state rate of 3 × 1014 molecules/s-cm2 (within experimental errors). E. Oxygen Loading and Diffusivity. At room temperature, the rate of dissociative adsorption of oxygen and hence the rate of dissolution of new O atoms into the Ag-membrane is very small. This raises the question, how long would the formaldehyde reaction proceed at room temperature before a decrease
Partial Oxidation of Methanol to Formaldehyde
J. Phys. Chem. B, Vol. 105, No. 25, 2001 5955
Figure 8. (a) Rate of formation of formaldehyde from a roomtemperature silver foil as a function of exposure time to methanol. The methanol pressure was 1.5 × 10-6 Torr. The total pressure (primarily methanol) is also shown.
in rate is observed as a result of consumption of the active O atom species? This question is addressed in Figure 8, where the room temperature rate of formation for both formaldehyde and water were monitored for approximately 1.8 h at a constant methanol pressure of 1.5 × 10-6 Torr. This particular Ag-foil had been activated by heating, permeation with oxygen, and methanol exposure. Remarkably, the formation rates exhibit very little change over the entire period, with the formaldehyde rate actually showing a slight increase with time. The total number of O atoms consumed in this measurement can be roughly estimated by multiplying the apparent formation rate of water by the area of the foil and the exposure time, i.e., 1.5 × 1014 mol/s-cm2 × 3.2 cm2 × 7000 sec ) 3.4 × 1018 atoms. This number is much larger than the number of Ag-atoms on the surface of a close packed lattice of the same area, e.g. 1.2 × 1015 atoms/cm2 on a Ag (111) surface.17 The consumption of such a large number of O atoms at the membrane surface, therefore, suggests that O atoms dissolved in the bulk can rapidly diffuse to active surface sites, even at room temperature. The question then becomes whether the observed reaction rates at room temperature are consistent with the solubility and the diffusion of oxygen in silver. An upper limit to the amount of oxygen that can be dissolved into silver can be crudely estimated assuming that each octahedral hole of the Ag-lattice is filled with an O atom.18 The number of such octahedral holes is 5.58 × 1021 per gram of Ag and leads to 5.3 × 1021 such O atom sites in a typical Ag-foil used in this work (area, 3.24 cm2, and thickness, 0.0279 cm). Using this estimate and an O atom consumption rate (upper limit) of 1 × 1015 atoms/s (see Figure 8), it would take 61 days to exhaust the dissolved oxygen supply! The 3.4 × 1018 O atoms consumed in the experiment shown in Figure 8 represent less than 1% of the total oxygen available if all the octahedral holes were occupied by an O atom. The remaining issue is whether the diffusivity of O atoms in silver at room temperature is high enough to support the observed reaction rate. A consumption rate of 1 × 1015 O atoms/ s-cm2 suggests that approximately a monolayer of O atoms is removed from the near-surface region per second and needs to be replenished by diffusion from the bulk. If one assumes that the diffusion coefficient does not depend on the oxygen concentration, then the diffusion coefficient can be estimated from the expression, ∆x2 ) 2Dt, which relates the displacement (∆x) of an O atom to the time (t) and diffusion coefficient (D) in units of cm2/s.19 Assuming ∆x is the distance between Ag layers, 2.2 Å, a diffusion coefficient of 2 × 10-16 cm2/s would be required. To check the validity of this estimate, it was necessary to extrapolate the experimental diffusivity data of
Figure 9. Extrapolation of the diffusion coefficient (D) of oxygen in silver from the high-temperature data (open circles, ref 6) to 300 K. Solid line: Linear extrapolation from ref 20; Dashed line: Linear extrapolation of the data from ref 6 between 900 K to 650 K. The single data point at 300 K is derived from the observed reaction rate for formaldehyde formation.
Outlaw and co-workers6 (650 K to 1050 K) to lower temperatures. This is shown in Figure 9, where the previous experimental data are given as open circles and the dashed line is a linear fit for data below 903 K.6 Extrapolation of this line to 300 K yields a D value of about 2 × 10-13 cm2/s. An alternative linear fit was carried out by Baird, King, and Stein20 and yields a value of 1 × 10-12 cm2/s at room temperature (solid line in Figure 9). Both extrapolations yield diffusion coefficients that are more than 3 orders of magnitude higher than the value estimated to support the observed reaction rates. Although such extrapolations well away from the experimental points must be viewed with caution, they provide some support for a diffusion rate high enough to support the observed room-temperature reactivity for extended periods of time. Finally, we note that the total O atoms estimated to be consumed in Figure 8 (about 3.4 × 1018 atoms) is larger than the total oxygen loading on the electrolytic Ag substrate used by Rehren et al.19 In those experiments, the uptake of oxygen was measured as a function of temperature for an exposure of 1 atm of O2 for 5 min. A maximum uptake of 2.5 × 10-6 mole of O atoms was adsorbed by a 3.147 gm foil of Ag at 820 K. This leads to an oxygen loading of approximately 4.8 × 1017 per gram, a factor of 30 less than that estimated from the room temperature titration experiment discussed above. The increased loading in our experiment clearly reflects the longer exposures of our foil to high-pressure oxygen (400 Torr for ∼6 h). In addition, the oxygen-induced morphology changes in the Agmembrane foil may also facilitate oxygen uptake. As a result, the oxygen loading of the Ag membrane estimated here may represent a more realistic upper bound to the oxygen loading capabilites of Ag metal. IV. Summary The reactivity studies presented here for the partial oxidation of methanol have demonstrated the utility of the Ag-membrane reactor for obtaining product branching ratios and formation rates under steady-state and collision-free conditions. In particular, the ability to separate oxygen loading of the Ag-foil from the reaction with methanol, allowed kinetic measurements to be performed at very low pressures (10-7-10-5 Torr) and
5956 J. Phys. Chem. B, Vol. 105, No. 25, 2001 over a wide range of surface temperatures. At these low pressures, the rate of formaldehyde and water formation were found to be first order in methanol pressure with absolute rates corresponding to methanol reaction fractions of ∼50%. The highest reaction rates were obtained after several hours of high temperature and high oxygen exposure, which induces extensive changes in surface morphology that are key for reactivity. Previous work on polycrystalline foils5 suggests that the only O atom species present on the activated Ag-membrane are bulk dissolved (Oβ) and strongly bound near-surface (Oγ) species. The low temperature capability led to the unexpected observation that the formation rates for both formaldehyde and water decrease by only a factor of 2 from 900 K to room temperature, even though previous work with other Ag surfaces exhibit no activity below 475-500K.3,5,10 More surprisingly, the roomtemperature activity could be maintained for long periods of time without a significant drop in the formation rates. This observation suggests that reactive oxygen species (Oβ) removed by water formation can be replaced by rapid diffusion from the bulk which acts as an O atom reservoir. An extrapolation of diffusion data at high temperatures indicates that the oxygen diffusion coefficient at room temperature (10-12-10-13 cm2/s) is at least 3 orders of magnitude faster than that required to support the observed reaction rates. Another consequence of using the oxygenated Ag membrane is the very low yield (e2.5%) of CO2 over the entire temperature range investigated (300 K to 900 K). This is significantly smaller than that observed in reactivity studies where oxygen is part of the feed gas,10 indicating that other O atom species not present on the activated Ag-membrane, e.g., chemisorbed (OR), are responsible for total combustion. Acknowledgment. This work was performed at Brookhaven National Laboratory and was supported by the U.S. Department
Beuhler et al. of Energy, Office of Basic Energy Sciences under Contract No. DE-AC02-98CH10886. References and Notes (1) See for example: Othmer, K. Encyclopedia of Chemical Technology, 4th ed.; Wiley Interscience: New York, 1994, Vol. 11, 929. (2) Wachs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208. (3) Nagy, A.; Mestl, G. Appl. Catal. 1999, 188, 337. (4) Barteau, M. A.; Madix, R. J. In Chemical Physics of Solid Surfaces and Heterogeneous Catalysis: Fundamental Studies of Heterogeneous Catalysis; Elsevier: Amsterdam, 1982; Vol. 4, p 95. (5) Nagy, A. J.; Mestl, G.; Herein, D.; Weinberg, G.; Kitzelmann, E.; Schlogl, R. J. Catal. 1999, 182, 417. (6) Outlaw, R. A.; Sankaran, S. N.; Hoflund, G. B.; Davidson, M. R. J. Mater. Res. 1988, 3, 1378. (7) Besenbacher, F.; Norskov, J. K. Prog. Surf. Sci. 1993, 44, 1, and references therein. (8) Rao, R. M.; Beuhler R. J.; White, M. G., to be published. (9) van Santen, R. A.; Kuipers, H. P. C. E. AdV. Catal. 1987, 35, 265. (10) Nagy, A.; Mestl, G.; Ruhle, G.; Weinberg, G.; Schlogl, R. J. Catal. 1998, 179, 548. (11) Herein, D.; Nagy, A.; Schubert, H.; Weinberg, G.; Kitzelmann, E.; Schlogl, R. Z. Phys. Chem. 1996, 197, 67. (12) Gryaznov, V. Catal. Today 1999, 51, 391. (13) Schwartz, S. B.; Schmidt, L. D.; Fisher, G. B. J. Phys. Chem. 1986, 90, 6194. (14) The uncorrected formaldehyde to water intensities would be in the ratio of the numbers of electrons in the two molecules, i.e., 18 to 10. See: Dushman, J. In Scientific Foundations of Vacuum Technique, 2nd ed.; Lafferty, J. M., Ed., John Wiley and Sons: New York, 1965; p 325. (15) See, for example: Beuhler, R. J.; Friedman, L. J. App. Phys. 1977, 48, 3928. (16) Millar, G. J.; Nelson, M. L.; Uwins, P. J. R. Catal. Lett. 1997, 43, 97. (17) Joyner, R. W.; Roberts, M. W. Chem. Phys. Lett. 1979, 60, 459. (18) Rehren, C.; Isaac, G.; Schlogl, R.; Ertl, G. Catal. Lett. 1991, 11, 253. (19) Moore, W. J. Physical Chemistry; 3rd ed.; Prentice-Hall: New York, 1964; p 343. (20) Baird, J. K.; King T. R.; Stein, C. J. Phys. Chem. Solids 1999, 60, 891.
