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Rate Expression for WaterGas Shift over a Gold/Ferrochrome Catalyst Gaurav N. Vajani, Shea Lerk Ng, and Carl R. F. Lund* Department of Chemical and Biological Engineering, University at Buffalo, SUNY, Buffalo, New York 14260, United States ABSTRACT: The kinetics of low-temperature watergas shift over gold/ferrochrome catalysts were studied at atmospheric pressure and temperatures between 160 and 180 °C. During two extended kinetic experiments lasting 903 and 1313 h, the activity of the catalyst decreased by 52% and 78%, respectively. The corresponding changes in the Brunauer-Emmett-Teller surface areas were 16% and 12%, respectively, suggesting that changes in the support surface area are not the primary cause for deactivation. The use of a bracketing technique, wherein the system was regularly returned to a standard set of reaction conditions, allowed the rate expression to be written as the product of an activity factor, which accounted for catalyst deactivation, and a power-law rate expression, which accounted for the dependence of the reaction rate upon temperature and composition. The power-law kinetic expression, including a term to account for the approach to equilibrium, was found to describe the kinetics of the reaction reasonably well. The resulting reaction orders were 0.39 for CO, 0.99 for H2O, 0.28 for CO2, and 0.25 for H2, and the apparent activation energy was equal to 88.2 kJ/mol.
’ INTRODUCTION Watergas shift (WGS), eq 1, can be used to reduce the concentration of CO in hydrogen that is generated via steam reforming or gasification. This is necessary when CO poisons subsequent processes, such as in ammonia synthesis or the operation of a proton exchange membrane fuel cell. WGS is exothermic and reversible, and consequently the thermodynamically allowed conversion increases as the temperature decreases. For this reason, the development of catalysts that are active at low temperatures has been a long-standing objective of WGS research. COðgÞ þ H2 OðgÞ f CO2 ðgÞ þ H2 ðgÞ
ð1Þ 1
Following the seminal work of Haruta et al. that showed gold nanoparticles to be active as CO oxidation catalysts at very low temperatures, it was demonstrated that supported gold nanoparticles are also highly active for reverse2 and forward WGS.3,4 Catalysts containing gold nanoparticles have been reported to be active at temperatures as low as 110 °C,5,6 well below the operating range of commercial low-temperature shift catalysts. Since that time, there have been numerous studies on the preparation of highly dispersed gold/metal oxide catalysts, the influence of preprocessing conditions like calcination and reduction, the effect of goldsupport interactions, the active state of gold, and the role of the gold particle size and electronic structure of these catalysts.5,733 Mechanistic aspects of catalysis have also been considered, including active sites for the WGS reaction, adsorption of various species on gold-based catalysts, catalyst deactivation, and possible reaction pathways for low-temperature WGS.8,15,16,22,30,3439 Somewhat surprisingly, it does not appear that a complete rate expression for WGS over a gold nanoparticle catalyst has been published, but there have been reports5,8,26,34,4048 of apparent activation energies and selected kinetic reaction orders. Gold-based catalysts for WGS are prone to rapid deactivation, and this is probably one reason why rate expressions have not been reported. r 2011 American Chemical Society
Gold nanoparticles that are active for WGS can be supported on a wide variety of materials. Iron oxide was popular as a support initially, but more reducible oxides like doped ceria and titania have predominated more recent investigations. Deactivation is reported for most, if not all, of the supports that have been used, but it is quite possible that the cause for deactivation may vary depending upon the particular support in question. For example, the formation of very strongly adsorbed carbon oxide species is proposed to cause deactivation with ceria and lanthana supports but not with titania and mixed ceria/titania, where other reasons are invoked.49 For gold supported on iron oxide, deactivation has been attributed primarily to sintering of iron oxide, leading to a significant decrease in the support surface area.30,38 The study reported here was undertaken with the objective of finding an accurate rate expression for WGS over gold nanoparticles supported on iron oxide. It is known from the hightemperature WGS literature that the addition of chromium oxide to iron oxide, up to ca. 7 wt %, results in the substitution of chromium cations in the iron oxide lattice and that this stabilizes the surface area of the resulting ferrochrome without affecting the specific catalytic activity.5055 Because support sintering has been identified as a primary cause for deactivation of gold nanoparticles supported on iron oxide, the present study used a ferrochrome support for which sintering is not expected to be as significant. The objectives of the work were to determine whether gold supported on ferrochrome would deactivate like gold supported on iron oxide and to develop an accurate rate expression for WGS over gold supported on ferrochrome.
Received: May 12, 2011 Accepted: August 8, 2011 Revised: August 3, 2011 Published: August 08, 2011 10493
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’ EXPERIMENTAL SECTION The gold/ferrochrome catalyst was prepared in two steps. All chemicals used were analytical grade and obtained from Sigma Aldrich. In the first step, the ferrochrome (iron/chromium oxide) support was prepared by coprecipitation using appropriate quantities of the precursors Fe(NO3)3 3 9H2O and Cr(NO3)3 3 9H2O in an aqueous medium, as described elsewhere56 in detail. Briefly, an aqueous solution of NH4OH was added until a pH of 8.0 was obtained under rapid stirring at 70 °C. The resulting precipitate was allowed to age for 1 h at 70 °C, followed by filtration and washing until all NO3 was removed. The solid then was dried in static air for approximately 16 h using a muffle furnace at 110 °C. Once dried, the oxide was ground into a powder and calcined in static air at 400 °C for 3 h. After cooling, the fraction passing through a 60-mesh sieve but retained on an 80-mesh sieve was used for gold deposition. The second step involved the addition of gold to the oxide support. In the depositionprecipitation step, an aqueous solution of HAuCl4 3 3H2O was prepared at a concentration of 1.8 mmol/L. This solution was alkalized by the addition of a solution of K2CO3 dropwise to obtain a pH of 8.0 under strong stirring at 60 °C. A measured quantity of the previously prepared and calcined ferrochrome support was added to the Au(OH)3 suspension, and the catalyst was allowed to age for 1 h at 60 °C under rapid stirring. The resulting catalyst was thoroughly washed multiple times with deionized water until chloride could no longer be detected. The catalyst was then dried in static air at 110 °C for 16 h, followed by a second calcination at 300 °C for 2 h. The catalyst used for kinetic studies consisted of particles in the size range of 180250 μm corresponding again to the fraction passing through a 60-mesh sieve but retained on an 80-mesh sieve. The weight percentages of gold and chromium in the catalyst were determined by Galbraith Laboratories. The Brunauer EmmettTeller (BET) surface areas of the catalyst samples were measured using a Micromeritics ASAP 2010 instrument. A JEOL JEM 2010 transmission electron microscope was used to examine the size of the supported gold nanoparticles. Guidelines for avoiding experimental artifacts, as suggested by Satterfield,57 were employed when operating the laboratory reactor. Isothermal operation was verified by measuring the temperature at the entrance to the catalyst bed and at the exit from the bed and ensuring they were equal to the measured temperature of the oil bath in which the reactor was immersed. The observed conversions did not change when the catalyst particle size was decreased by 50%, suggesting that internal transport processes were not affecting the measured rates. At a constant space velocity, it was observed that the conversion remained constant as the flow rate was changed, indicating that the influence of external mass transfer is negligible. The gases used for the catalytic activity and stability measurements were high-purity grade. Omega FMA-764 mass flow controllers were used to meter the individual gases into a mixing manifold. The combined gas flow leaving that manifold flowed into a high-temperature vaporizer filled with glass beads. An ISCO Series D syringe pump metered liquid water into the vaporizer. Provision was made for measuring the composition of the reactor inlet and exit streams by using a heated switching valve in conjunction with an SRI 8610C gas chromatograph. All gas flow lines downstream of the vaporizer were maintained at sufficiently high temperature to ensure no condensation of water
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vapor. The gas chromatograph was equipped with a 10-port sampling valve with 0.25 μL sampling loops for online analysis of the gas composition. The combination of a molecular sieve 5A column and a silica gel column was used for separation of the sample with helium as the carrier gas flowing at 25 mL/min. Detection was performed using a thermal conductivity detector, and PeakSimple II software was used for data collection. The reactor consisted of a U-shaped section of 0.25-o.d. stainless steel tubing with a 0.4 g catalyst bed packed with quartz wool at both ends. The reactor was operated isothermally and at atmospheric pressure. K-type thermocouples were introduced on both the entrance and exit of the catalyst bed. The U tube was placed within a hot oil bath held at a constant temperature through the use of an OptiChem temperature controller connected to a Romalox BA-341420 band heater placed around a stainless steel beaker upon a magnetic stir plate. Prior to activity measurements, the catalyst sample was reduced in a flow of 10% hydrogen in argon for 3 h at 230 °C. After reduction, the catalyst was exposed to WGS conditions for kinetic and deactivation measurements. A volume hourly space velocity of 7800 h1 and a total flow rate of 48.5 mL/min were used for the kinetic study. Measurements were taken only after allowing sufficient time for steady state to be reached, and multiple readings were taken at the same conditions to ensure consistency and reproducibility of the results. The full compositions of the inlet and outlet streams were measured chromatographically. The experimentally determined fractional conversion of carbon monoxide, (xCO)expt, then was calculated from the measured inlet and exit molar ratios of CO2 to CO using eq 2. To check the data for consistency, the conversion was next used to calculate the outlet H2O and H2 partial pressures, and the results were compared to the measured outlet partial pressures of these species. CO2 CO2 CO out CO in ð2Þ ðxCO Þexpt ¼ CO2 1 þ CO out The kinetic studies utilized feed gas containing argon, CO, CO2, H2, and water vapor. Argon acted as a makeup gas to maintain the total flow rate constant, while the partial pressure of one species was varied. The kinetics were measured over a nominal temperature range of 160180 °C. A range of feed mixtures was used, varying the composition of each reacting species, as well as the steam/CO ratio. The ferrochrome support without added gold did not show any WGS activity under the reaction conditions reported in this study. Kinetic experiments were performed using a bracketing scheme wherein the reactor was reset to arbitrarily selected standard conditions at regular intervals. The standard conditions for this study were 183 °C, 1 atm, and a total feed flow rate of 48.5 cm3/min containing 40% H2O, 20% CO, and 40% argon. (An excess of steam was used for the standard state in order to avoid the possibility of carbon deposition and methanation, which can occur with feed ratios near stoichiometric.) In this way, the conversion of CO at these standard conditions was measured as a function of time on stream during the course of the kinetic experiments. An activity factor could be computed for any experimental data point measured at some time t, with that activity factor being equal to the ratio of the conversion at standard conditions at time zero to the conversion at standard conditions at time t. 10494
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Figure 1. Deactivation of the ferrochrome-supported gold catalyst over time as described in the text.
The reactor was modeled as a steady-state, isothermal, isobaric plug-flow reactor. The corresponding CO mole balance for the reactor is given in eq 3, where mcat represents the mass of catalyst used in the experiment, n_ 0CO is the inlet molar flow rate of CO, η is the experimental activity factor, fCO is the integration variable representing the fractional conversion of CO within the integrand, rCO is the rate of generation of CO, and (xCO)model is the conversion of CO predicted by the kinetic model. Equation 4 was used as the rate expression after rewriting the partial pressures of the species in terms of the feed composition and the fractional conversion of CO. The equilibrium constant appearing in eq 4 was evaluated for each experimental data point using thermochemical data from the NIST Chemistry Web Book.58 Z ðxCO Þmodel η dfCO mcat ¼ n_ 0CO ð3Þ rCO 0
rCO
E R β PCO2 PH2 γ ε P P P P 1 ¼ k0 exp RT CO H2 O CO2 H2 KWGS PCO PH2 O
ð4Þ The conversion predicted by the kinetic model was found by integrating eq 3 using the trapezoid rule with increments of the fractional conversion equal to 0.000 05 and continuing the integration until the value of the integral equaled the mass of the catalyst used in the experiment. (If the integrand became negative during the integration, this signified that the reaction had reached thermodynamic equilibrium, and the conversion for that experiment was taken to equal the equilibrium conversion.) In this way, the kinetic model, eq 4, was fit to the experimental data using the MatLab functions nlinfit and nlparci to minimize the sum of squares of errors (predicted vs measured conversion) and to estimate the 95% confidence limits for the resulting parameters: E, R, β, γ, and ε. The calculations were repeated using a smaller integration increment in order to ensure that the numerical integrations were converged.
’ RESULTS AND DISCUSSION The ferrochrome-supported gold catalyst does deactivate during use, as can be seen in Figure 1. The rate of deactivation is appreciable, and it appears to depend upon the reaction conditions. The data plotted as circles and crosses in the figure are measurements made while the system was operating at steady
Figure 2. CO conversion at standard conditions during the first extended kinetic run.
state at the standard conditions described in the Experimental Section, and the data plotted as diamonds are measurements at a higher steam-to-CO ratio (6.9) and volume hourly space velocity (14 000 h1). It is clear from the slopes of the curves that the catalyst at the latter conditions is deactivating faster than the ones at standard conditions. Looking more closely at the two sets of measurements taken at standard conditions, one observes that the apparent rates of deactivation also differ. The reason is that, during the time between the measurements plotted in Figure 1, kinetic measurements were being made at nonstandard conditions. Furthermore, as discussed presently, the conditions being used for the kinetic experiments in the two experiments were different. It is believed that a different apparent rate of deactivation is observed in the figure because (a) the rate of deactivation varies with reaction conditions and (b) during the time between the points plotted in the figure, the two systems were operating at different conditions. The results shown in Figure 1 show that deactivation must be accounted for in the development of a rate expression for WGS over gold supported on ferrochrome. To do so, two extended kinetic runs were performed wherein the system alternated between steady state at standard conditions, as defined previously, and steady state at conditions of interest for the development of the rate expression. The first extended run of this kind lasted slightly over 900 h. The CO conversions measured at steady state at standard conditions during the first 50 h of this run are shown as the crosses in Figure 1; the corresponding standard state CO conversions for the full run are shown in Figure 2. The starting catalyst sample used in this run contained 2.08 wt % Au and 2.59 wt % Cr. The BET surface area was 35.8 m2/g, and examination of the catalyst using transmission electron microscopy (TEM) indicated that ∼85% of the gold particles had diameters below 5 nm. The kinetic measurements (i.e., measurements not at standard conditions) made during the extended kinetic run of Figure 2 involved the perturbation of one partial pressure (and perhaps the temperature) from its value at standard conditions while the others remained at their standard condition values. (The partial pressure of argon was used to compensate for the partial pressure that was changed.) The resulting data set yielded a good rate expression, but we were concerned that our operating conditions were not particularly close to the conditions that most likely 10495
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Table 1. Changes in the Catalyst Properties during Extended Kinetic Runs
Table 2. WGS Kinetic Parameters E (kJ/mol)
88.2 ( 11.8
extended
run duration
fresh sample
BET area
activity
R
0.39 ( 0.09
run
(h)
BET (m2/g)
decrease (%)
decrease (%)
β
0.99 ( 0.13
1 2
903 1313
35.8 38.7
15.8 12.4
51.8 77.6
would be encountered in commercial situations. Indeed, the partial pressures of CO2 and H2 were zero at standard conditions. Therefore, in order to ensure that the resulting rate expression was valid at compositions more representative of a feed coming from a steam reformer or coal gasifier, a second extended kinetic run was performed. The second extended kinetic run lasted slightly over 1300 h. The same standard conditions were used for the purpose of calculating the activity factor. However, the intervening kinetic measurements involved perturbations of the reagent partial pressures and the temperature from a base case more representative of the effluent from a reformer or gasifier (50% H2, 20% H2O, 15% CO2, and 6% CO, with the balance being argon). The experiments consistently revealed that the gold/ferrochrome catalyst deactivated under reaction conditions. Moulijn et al.38 suggested that deactivation of gold supported on iron oxide (not ferrochrome) was primarily caused by decreases in the support surface area. They noted that the relationship between the catalyst activity and support surface area was nonlinear. This is one of the reasons why a mixed iron/chromium oxide was used as the support in the present work; it was anticipated that the ferrochrome support surface area would be more stable under reaction conditions. While the gold/ferrochrome catalysts did deactivate, we do not believe that it can be attributed to decreases in the surface area of the support. Table 1 compares the change in the BET surface area of the gold/ferrochrome catalyst to the change in its catalytic activity for the two extended kinetic runs of this study. The BET surface area represents the total surface area of the catalyst, support plus metal, but the predominant contribution is from the support. Table 1 shows that the extended kinetic run with the greater decrease in the total surface area had the lesser decrease in the activity. Clearly, then the activity is not proportional to the total surface area, and it strongly suggests that the activity also is not proportional to the support surface area. Even if the catalytic activity depended nonlinearly upon either of these surface areas, the activity would be expected to decrease monotonically as the surface area decreased. That is not the case here, which strongly suggests that other factors are primarily responsible for deactivation. One might suspect that changes in the gold metal surface area are responsible, but changes in the gold particle size as estimated from TEM images suggest that the gold surface area decreases by ca. 35%, which is not enough to explain the deactivation. The causes for the deactivation of these catalysts continue to be studied and will be reported separately. The relevant point here is that no simple titration can be used to account for deactivation, necessitating the use of the activity factor as implemented here. In essence, the activity factor uses the rate of reaction at a single set of conditions to titrate the number of active sites. In fact, the observed decreases in the BET surface area, as reported in Table 1, are a bit surprising. The reaction studies took place at temperatures no greater than 185 °C, whereas the
γ
0.28 ( 0.08
ε
0.25 ( 0.06
Figure 3. Rectifying plot comparing the measured conversion of CO to that predicted by the final rate expression.
support had initially been calcined at 400 °C; after gold was introduced, it was again calcined at 300 °C and the reductive pretreatment of the catalyst was performed at 230 °C. Hence, it was anticipated that the support would have attained a stable surface area during these higher temperature treatments prior to the kinetic studies. In addition, we have previously used ferrochrome prepared in the same way, but without added gold, as a high-temperature WGS catalyst. In those studies,56 the initial ferrochrome BET surface area of 39.6 m2/g decreased by only 4% during extended kinetic experiments at temperatures between 400 and 550 °C. Given the milder conditions of the present studies, a smaller decrease in the support surface area was expected. We can speculate that the presence of gold accelerates the sintering of the support or that gold particles grow under reaction conditions in a way that blocks access to some fraction of the support pore structure, but no definitive explanation can be given. A total of 136 data points were generated during the two extended kinetic runs. The experimental conversions of CO in this data set ranged from 4% to 98%. They were used to develop a power-law rate expression as shown in eq 4. The resulting kinetic parameters are presented in Table 2, where the given uncertainties represent the 95% confidence limits. The rate expression shows that both CO2 and H2 inhibit the forward rate of WGS. The activation energy, 88 kJ/mol, is within the range of values (20112 kJ/mol) reported for various low-temperature shift catalysts.5,8,26,34,4048 The only other values we are aware of for iron oxide supported catalysts (but not ferrochrome-supported) are those of Sakurai et al.5 and Venugopal et al.,32 who reported values of 52 and 21 kJ/mol, respectively. We do not know whether the approach to equilibrium was included when these activation energies were determined. In Figure 3, the experimentally measured CO conversion is plotted against the CO conversion predicted by the rate expression. 10496
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Figure 4. Measured and predicted variations in the apparent WGS reaction rate as a function of (a) the inlet CO partial pressure, (b) the inlet H2O partial pressure, (c) the inlet CO2 partial pressure, and (d) the inlet H2 partial pressure. The squares correspond to experiments at 183 °C; the triangles, 170 °C and the circles, 160 °C.
If both the data and model were perfect, every point would fall on the diagonal of the plot. There is scatter about the diagonal, but the fit is reasonably good. The correlation coefficient, r2, is equal to 0.945. The residuals (difference between the measured and predicted conversion) were computed for all of the data. They did not display any systematic variation with the temperature, conversion, time on stream, activity factor, or any of the reagent’s inlet partial pressures. Figure 4 uses data from the first extended kinetic experiment to show that the model does capture trends in conversion as a function of the composition and temperature. Upon examination of these figures, it should be noted that the lines plotted in the figure represent the conversions predicted by substitution of the rate expression into the reactor design equation for each of the experimental data plotted as symbols. That is, they compare experiments where the inlet composition was varied systematically. Because an integral reactor is being used, the rate of reaction will vary continuously along the length of that reactor, and the plotted ordinate is an average over the length of the reactor. Furthermore, the variation along the length of the reactor will not be the same, even for two points on the same curve in Figure 4. (We have not normalized the ordinate, e.g., per unit mass, in Figure 4 to avoid giving the impression that the ordinate represents the actual rate at the corresponding abscissa value. In fact it is the integral of the instantaneous rate over a range of inlet partial pressures beginning at the plotted value of the abscissa.)
As a consequence, the plots in Figure 4 should not be taken to indicate how the rate varies with the reactant partial pressures; the reaction orders provide that information. Instead, they can be used to assess how well the rate expression predicts trends in conversion (or the average rate) as the inlet composition is systematically varied. Similarly, because only a fraction of the data are used in Figure 4, it should not be used to judge the overall fit of the rate expression to the data. For that purpose, Figure 3 and the correlation coefficient allow for a better assessment. The important point is that the rate expression captures all of the trends that are apparent in the data, and it is valid across a wide range of compositions. (The data points from experiments closer to reformer/gasifier conditions do not lend themselves to plots against a single inlet partial pressure, but Figure 3 and the fitting statistics show that the rate expression fits them equally well.) The results demonstrate that deactivation of the catalyst can be accounted for simply by multiplying the rate expression by an activity factor. As was already noted, the residuals were not observed to vary systematically with the activity factor or with time on stream. This suggests that deactivation is primarily a result of decreasing accessibility of the active sites, either because the number of such sites is decreasing or because sites are being rendered inaccessible. If deactivation were primarily the result of an electronic effect (perhaps due to a change in the metal support interaction or a change in the metal particle size), one might expect the form of the rate expression to change. In a power-law 10497
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Industrial & Engineering Chemistry Research type of kinetic model, this would result in changes in the apparent reaction orders. In the present case, it was not necessary to vary the reaction orders to account for deactivation. This does not mean that such electronic effects are absent, but that if they are present, either they do not strongly affect the kinetics of the reaction or they uniformly affect all of the important steps in the reaction mechanism.
’ CONCLUSIONS Gold nanoparticles supported on a mixed iron/chromium oxide are active as WGS catalysts at temperatures in the range of 160185 °C. These catalysts deactivate severely under reaction conditions. The catalytic activity does not decrease monotonically with the total BET surface area of the catalyst, suggesting that loss of the support surface area is not the primary cause of deactivation. The kinetics can be well-described using a rate expression that is independent of the degree of deactivation and multiplying that rate expression by an activity factor. The activity factor at any arbitrary time can be taken to equal the ratio of the initial CO conversion to the CO conversion at that time, both being measured at the same standard conditions. The finding that a term separate from the rest of the rate expression can fully account for the observed deactivation suggests that deactivation results from a decrease in the number of accessible active sites. A power-law rate expression (including a term accounting for the approach to equilibrium) was found to fit the experimental data well. According to that expression, the reaction orders were 0.39, 0.99, 0.28, and 0.25 for CO, H2O, CO2, and H2, respectively, and the activation energy was 88.2 kJ/mol. This expression was found to apply over a range of compositions that included very little product as well as at conditions more typical of a commercial process. After accounting for deactivation via the activity factor, it also applied equally to fresh catalyst and to catalyst deactivated by 5075%. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:lund@buffalo.edu. Tel.: (716) 645-1180. Fax: (716) 645-3822.
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