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Al2O3
Applied Catalysis B: Environmental 70 (2007) 294–304 www.elsevier.com/locate/apcatb
Millisecond step-scan FT-IR transmission spectroscopy under transient reaction conditions: CO oxidation over Pt/Al2O3 Ansgar Wille 1, Erik Fridell * Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Available online 30 June 2006
Abstract Time-resolved FT-IR spectra of CO oxidation over Pt/Al2O3 catalysts were collected in situ in transmission mode under transient reaction conditions. Using the step-scan acquisition mode of a commercial FT-IR spectrometer, time-resolution of a few milliseconds, compared to several hundred milliseconds in the normal rapid-scan mode, could be achieved. The experiments were triggered by reproducible oxygen gas pulses at constant temperature. Infrared light transmission, uniform heating and fast gas exchange were realized by pressing the pure catalyst powder onto a thin stainless steel wire mesh using a low volume reaction cell. By comparing the results of both acquisition modes, rapid-scan and step-scan, we will demonstrate, that it is possible, in a rather simple way, to investigate heterogeneously catalysed reactions under reaction conditions by means of FT-IR spectroscopy with time-resolutions down to a few milliseconds, and, in principle, lower. CO oxidation between 443 and 573 K over 1%Pt/ g-Al2O3 and 4%Pt/g-Al2O3 catalysts has been investigated as a test reaction. By using the high time-resolution of the step-scan acquisition mode we could obtain new details of the dynamics of the CO oxidation reaction over a supported Pt catalyst at high temperatures and 1 atm pressure. # 2006 Elsevier B.V. All rights reserved. Keywords: Time-resolved FTIR; Step-scan; CO oxidation; Platinum; Transient reaction
1. Introduction The development of new heterogeneous catalysts for the use in industrial applications is still based on empirical methods. Only in a few cases it succeeded to understand the complex mechanism of the chemical reaction by elucidating the different steps of the heterogeneously catalysed reaction [1–3]. It is crucial to get a detailed insight into the elementary steps proceeding at the solid surface of the catalyst under as real conditions as possible (temperature, partial pressure of reactants, flow dynamics) to create the ability of improving the conversion or the selectivity. The oxidation of CO on supported noble metal catalysts is one of the most widely studied catalytic reactions. However, the involved elementary steps, especially the rate of CO2 production, seem to differ if one is comparing results obtained in UHV studies at fairly low
* Corresponding author. Present address: IVL Swedish Environmental Research Institute, PO Box 5302, 400 14 Go¨teborg, Sweden. Tel.: +46 31 772 33 72; fax: +46 31 772 31 34. E-mail address: [email protected] (E. Fridell). 1 Present address: W.C. Heraeus GmbH, Heraeusstr. 12–14, D-63450 Hanau, Germany. 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.10.031
CO partial pressures (<104 Pa) with experiments under real conditions (>102 Pa) and they are still under debate [3–6]. This pressure gap is accompanied by the materials gap revealing morphological differences as defect sites, irregular edges and kinks at supported particles and flat and almost perfectly oriented surfaces on single crystals, directly influencing elementary reaction steps. Hence, investigations of these basic steps should be accomplished at supported catalysts under real reaction conditions, preferably. Infrared spectroscopy is one of the most important techniques used for conducting in situ studies of catalytic solids under reaction conditions [7]. The interaction of the infrared light with the catalyst can be arranged in a transmission experiment (pressed self-supported catalyst pellet), a reflectance experiment (diffuse reflectance infrared Fourier Transform (DRIFT), using catalyst powder) or by applying an attenuated total reflection (ATR) of the infrared light with catalyst particles in contact with an ATR crystal. A simple quantitative evaluation of the absorbance data is only possible for a transmission experiment, whereas for a reflected infrared beam the knowledge of frequency dependent extinction coefficients is crucial [8]. Within the normal operating mode of an FT-IR spectrometer the moving mirror in the interferometer is driven at a constant
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velocity acting this mirror continuously forward and backward. This is the so-called rapid-scan mode. During one single-sided forward or backward move a complete interferogram, and after Fourier Transformation a spectrum, is collected. All processes which are proceeding at the catalyst surface are averaged resulting in one spectrum related to the time slice of the mirror moving period (e.g. a complete forward move). In order to produce shorter time slices the velocity of the moving mirror has to be increased. However, a finite mass of the mirror and, hence, a certain mechanical stability of the interferometer constrains the mirror velocity and thus a maximum timeresolution. Additionally, a high optical resolution requires a long distance of the moving mirror, which constricts a high time-resolution at high optical resolutions. Nowadays, commercial FT-IR spectrometer are able to produce spectra at a time-resolution of about 50 ms at an optical resolution of 4 cm1. Higher time-resolution can be obtained only at the cost of optical resolution. A different way obtaining an interferogram by an interferometer is the so-called step-scan mode in which the mirror movement is decoupled from time [9,10]. Within this mode the moving mirror is acting in a step-wise mode holding the mirror fixed at a specific position. Now an event is triggered (e.g. by a laser pulse, gas pulse, temperature ramp) and an altering of the sample is followed in time by triggering the infrared detector very fast. Then the mirror is moving to the next step position and the event is triggered again. Repeating this procedure several hundred or thousand times, a set of data points is collected resulting in complete interferograms with time segments depending on the triggering rate of the detector. This means that the time-resolution of a step-scan experiment is limited in principle simply by the triggering rate of the spectrometer electronics and the response time of the IR detector. Thus, time-resolutions of a few nanoseconds are possible [11]. However, this technique requires a reproducible event and a very good long-term stability of the complete experimental setup. This means for an application on heterogeneous catalysts, that the reaction trigger pulses (gas pulse) and the sample temperature have to be constant and, that the catalyst should not age during the entire experiment. We will show that this technique can be used for studying the effects of fast gas transients in heterogeneous catalysis by means of the CO oxidation reaction over supported platinum catalysts with a time-resolution of 5 ms, which can be chosen, in principle, much lower (see Section 2.3). The elementary steps of the oxidation of CO valid for UHV conditions have been summarized by Engel and Ertl in 1979 into the so-called conventional three-step Langmuir–Hinshelwood (L–H) reaction scheme [3]: COgas $ COads
(1.1)
ðO2 Þgas ! 2Oads
(1.2)
COads þ Oads ! ðCO2 Þgas
(1.3)
The indices ‘‘gas’’ and ‘‘ads’’ refer to gas-phase and adsorbed species, respectively. CO adsorbs associatively whereas O2
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adsorbs dissociatively already at temperatures about 100 K [12]. The formed product CO2 of the reaction between adsorbed COads and Oads species is desorbing immediately after reaction. However, this general picture cannot be translated directly to supported platinum particles working under real conditions and some aspects have to be added to explain experimental results. In a series of papers Bourane and Bianchi [13–17] have elucidated parameters playing significant roles in the elementary steps of this ‘‘simple’’ surface reaction over Pt/Al2O3. They performed kinetic studies under isothermal conditions, mainly at low temperatures, determining the effect of transient changes of partial pressures of the reactants and products on the rate of reaction. It could be shown, that different CO species, like linear and bridged adsorbed molecules, as well as different types of adsorbed oxygen atoms are involved in the elementary steps. At elevated temperatures, however, it is difficult to follow surface species, especially at the ignition state of the CO oxidation reaction, which proceeds very fast [17]. At this stage it would be interesting to have the ability investigating the behaviour of surface species like linear adsorbed CO molecules at platinum sites at these elevated temperatures in time. We have studied the oxidation of CO over Pt/g-Al2O3 catalysts at constant temperatures under transient conditions applying oxygen gas pulses. Utilizing the step-scan acquisition mode of a commercial FT-IR spectrometer we have investigated the reactivity of linear adsorbed CO species as well as the production of gas-phase CO2 molecules with a time-resolution of 5 ms. All experiments have been performed at elevated temperatures between 443 and 573 K at 0.5% CO constant feed and O2 pulses of 1–3% O2 concentration at 1 or 2 s duration. 2. Experimental 2.1. Preparation of catalyst In this study two different Pt/g-Al2O3 catalysts with 1 and 4% Pt have been used. Both catalysts have been prepared by the wet impregnation method. g-Al2O3 (SCC a-150/200, Sasol) was dispersed in distilled water and the pH was adjusted to 2.5 by adding diluted HNO3 solution (Merck). An aqueous platinum nitrate solution (Pt(NO3)2, Johnson Matthey) was added dropwise to the alumina slurry under continuous stirring to yield the desired platinum loading. The slurry was kept under continuous stirring for 20 min, frozen with liquid nitrogen and freeze-dried. In order to preserve a high dispersion the catalysts were calcined carefully at 723 K for 4 h in air starting this procedure at room temperature and ramping the temperature with a rate of 5 K/min up to 723 K. The Pt dispersions have been determined by CO chemisorption (Micromeritics, ASAP2010C) at 300 K giving 98% dispersion (40.4 mmol CO/g catalyst) for the 1%Pt/Al2O3 catalyst and 65% dispersion (106.2 mmol CO/g catalyst) for the 4%Pt/Al2O3 catalyst assuming a maximum CO/Pt ratio of 0.8 which is reasonable for highly dispersed Pt particles [18]. However, the very high dispersion for the 1%Pt/Al2O3 catalyst shows that the value of 0.8 is probably underestimated. The BET surface areas (Micromeritics Tristar) have been measured as 187 and
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182 m2/g for the 1%Pt/Al2O3 and the 4%Pt/Al2O3 catalyst, respectively. 2.2. Reaction cell and sample All experiments have been performed in a home-made stainless steel IR reaction cell at atmospheric pressure. The cell design facilitates in situ FTIR transmission experiments under reaction conditions at atmospheric pressure as well as continuous following of the product stream by a mass spectrometer. The cell consists of a main stainless steel body with feed throughs for gas inlet and outlet and thermocouples to measure the sample and the cell temperature, as well as high power feed throughs for resistive heating of the sample wire mesh. Two CaF2 windows (50 mm diameter 10 mm) sealed by Kalrez o-rings are attached to both sides of the main body by stainless steel holders. To reduce the internal volume of the reaction cell two macor inlays with cut-outs for the sample holder and a 12 mm hole for infrared light pass through are placed between sample holder and the CaF2 windows. The residual cell volume is about 4 cm3. When investigating fast transients applied on a high surface area solid by transmission infrared spectroscopy, some requirements like fast gas-surface response, uniform heat distribution and an IR-transparent sample form have to be fulfilled. Hence, following a procedure of Ballinger et al. [19], we have chosen a sample preparation by pressing the pure catalyst onto a stainless steel plain woven wire mesh. The wire mesh is made out of highly oxidation resistive stainless steel wire (0.066 mm diameter, 1.4301 steel: Fe/Cr18/Ni10/Mo3) with a nominal aperture of 0.103 mm and 37% open area. The ground catalyst powder is distributed over an area of 1.2 cm 1.4 cm of the wire mesh and scratched with a spattle into the openings of the mesh. This procedure guarantees a uniform distribution of the catalyst over the area probed by the infrared beam. Subsequently the catalyst supported wire mesh is pressed between two stainless steel plates by means of a press. After this pressing the catalyst is fixed to the wire mesh firmly with a maximum sample thickness of 150 mm. An average amount of 10 mg of catalyst cm2 are pressed onto the wire mesh. The thermocouple for measuring the sample temperature is placed 3 mm off the mesh border in close contact to the supported catalyst. Finally, the sample wire mesh is placed in the middle of the main reaction cell body by two stainless steel clamps which are attached to power feed throughs. 2.3. Transient FTIR experiments All gases used in this study were of high purity (>99.995%) and were introduced to the reaction cell without further purification by individual mass flow controllers (MFC, Bronckhorst). The reactor was fed by two individual gas streams, first the main constant reactant feed and secondly the pulsed reactant feed (see Fig. 1). If no reactant was pulsed argon was used as balance for this gas stream to obtain a constant flow. The switching system consists of two fast switching three-way valves (Parker, Skinner solenoid valves, <16 ms response) arranged in a four-port acting mode. For all experiments a
Fig. 1. Experimental setup displaying gas flows and trigger signals.
constant flow of 0.5% CO and O2 pulses with varying concentrations (1–3%) and durations (1 and 2 s) were introduced to the reaction cell. In all FT-IR experiments a flow of 600 ml/min has been chosen. The fresh sample was dried in a 300 ml/min Ar flow at 573 K for 2 h before being exposed to reactant gas compositions. The sample was then exposed first to a 4% O2 in Ar mixture at 623 K at 300 ml/min for 30 min and second to 4% H2 in Ar at 623 K at 150 ml/min for 30 min for oxidation and reduction of the sample, respectively. Before each experiment the sample was oxidized and reduced under the same conditions but only for a period of 5 min. This procedure has been followed by a net-oxidation with 0.5% CO/2% O2 in Ar at 600 ml/min for 5 min at the actual temperature to be studied and an FT-IR background was collected. By using this routine, the appearance of gas-phaserelated CO and CO2 bands and of alumina-related carbonates, acting as spectators, in the spectra was minimized. Additionally, the background spectra will not contain any IR bands of CO bonded to Pt, since the CO coverage is low due to the CO + O reaction. The time-resolved in situ FTIR spectroscopy experiments were performed in transmission mode with a Bio-Rad FTS 6000 spectrometer. To obtain time-resolutions of 5 ms the internal step-scan option has been used. By means of utilizing gas pulses an event period of 10 s have been chosen. The release of a gas pulse or a temperature ramp (not applied in this study) is triggered by the spectrometers step pulse output (Fig. 1). This trigger pulse can be altered by a pulse generator (HP 8013B) to TTL level which is used to trigger the switching of the threeway valves. The gas pulse duration can be controlled by a computer program. The FT-IR spectra in all step-scan experiments were recorded with an optical resolution of 8 cm1. To obtain a background spectrum a complete TRS run under oxidation conditions (continuous feed of 0.5% CO + 2% O2) at the temperature to be studied was carried out. Using a 10 s step cycle at 8 cm1 optical resolution a complete TRS run requires about 6 h for collecting the data. In all illustrations showing step-scan data the graphs of integrated peak areas are smoothed by a 10 point FFT filter procedure for clear presentation.
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Fig. 2. Pulse profiles of CO2 pulses introduced to the empty reaction cell and characterized by the CO2 infrared absorption at 2348 cm1 by means of a step-scan run: (a) 1 s pulse 1–3% CO2 and (b) 2 s pulse 1–3% CO2 (600 ml/min flow in Ar, 293 K).
For rapid-scan experiments the data collection procedure is similar. A background spectrum has been collected under continuous CO oxidation reaction conditions (0.5% CO + 2% O2) at the temperature to be studied. Under transient reaction conditions the gas pulses were released automatically under software control and data collection was started manually. All rapid-scan spectra have been collected with an optical resolution of 4 cm1 at a maximum time-resolution of about 190 ms. The product stream leaving the IR reaction cell was continuously analysed using a Balzers Quadstar 422 quadrupole mass spectrometer. The signals of H2 (m/e 2), H2O (m/e 18), CO/N2 (m/e 28), O2 (m/e 32), Ar (m/e 40) and CO2 (m/e 44) have been detected with a time-resolution of about 0.3 s. In order to correlate the O2 pulse profile with the characteristics of the absorption peaks of linear adsorbed CO and produced gas-phase CO2 a characterization of the O2 pulse has to be performed. We used CO2 as a probe molecule having an infrared absorption feature at 2348 cm1. We assume that the pulsed CO2 gas behaves approximately in the same way as O2 gas. The slope of pulsed CO2 gas, as integrated peak area of the CO2 absorption against time, for 1, 2 and 3% CO2 concentration of 1 and 2 s pulses is shown in Fig. 2a and b, respectively. The infrared spectra have been recorded in the step-scan mode with a step period of 10 s at a time-resolution of 15 ms. For both pulse durations the concentration dependence shows an almost linear relationship with a peak half width matching the forced pulse duration. However, the late part of the pulses reveal a broad tailing increasing the mean residence time of the gas in the reaction cell. On the other hand the entrance time of the pulse is very reproducibly detected at about 1070 ms. 3. Results and discussion 3.1. CO oxidation at different O2 concentrations: stepscan versus rapid-scan In Fig. 3 infrared absorbance spectra of linear adsorbed CO species on platinum particles in a 4%Pt/g-Al2O3 catalyst and of
produced gas-phase CO2 are presented showing the evolution of the ignition of the CO oxidation reaction after a 2% O2 pulse for 2 s at 523 K catalyst temperature comparing a rapid-scan (Fig. 3a) and a step-scan FT-IR experiment (Fig. 3b) in the period of 1–3.4 s. The experiment is characterized by steps of 10 s duration with one O2 pulse within one step. In both sets of spectra the absorption features of gas-phase CO2 at 2348 cm1 and of linear adsorbed CO species at 2056 cm1 together with a shoulder at 2064 cm1 are observable. The low frequency part of this absorption can be assigned to CO adsorbed on reduced platinum atoms (denoted as Pt0) and the high frequency shoulder to CO molecules adsorbed on partially oxidized platinum (denoted as Ptd+) in agreement with several studies [13,20–22]. Additionally, at about 1840 cm1 a very weak absorption of bridged adsorbed CO species can be observed. Thus, more than 90% of the platinum surface is covered by linear adsorbed CO as observed by other authors [22,23]. The spectra collected in both modes indicate a constant decrease in intensity of the adsorbed CO species for the ignition of the reaction, while the absorbance of CO2 is increasing. The absorption feature of linear CO is red-shifting to 2050 cm1 which can be explained by reduced CO–CO dipole coupling by the decrease of CO coverage at the platinum particles [24]. Fig. 3a is presenting all spectra collected in the time period between 1 and 3.4 s, whereas Fig. 3b shows some selected spectra of a step-scan run under identical conditions representing similar time stages as for the rapid-scan experiment but obtained with a time-resolution of 5 ms. By comparing the two sets of FT-IR spectra both acquisition modes present the same evolution of the spectra for the ignition process, but with an obvious difference in the signal-noise ratio. The fast triggering of the infrared detector at a step position, creating the high time-resolution, is definitely accompanied by a lower spectrum quality. Increasing the signal-noise ratio is possible only by repeating the complete step-scan run and coadding different scans. However, this would increase the measuring time enormously. In principle, both methods are comparable with a lower signal-noise ratio for the step-scan mode and with a much higher time-resolution.
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Fig. 3. Evolution of infrared absorptions of linear adsorbed CO molecules and produced gas-phase CO2 after an O2 pulse (2% 2 s) with a 0.5% CO continuous feed in Ar at 523 K: (a) rapid-scan mode (1–3.4 s, 190 ms resolution and (b) step-scan mode; (a–h) 1000, 1800, 2000, 2250, 2500, 2800, 3000, 3400 ms).
Fig. 4 shows the evolution of the linear adsorbed CO species and the production of CO2 at 523 K as integrated infrared absorption peak areas over a 1%Pt/g-Al2O3 (a–c) and a 4%Pt/gAl2O3 (d–f) catalyst after introduction of a continuous feed of 0.5% CO in Ar and different O2 concentrations by a 2 s pulse comparing rapid-scan (c and f) and step-scan data acquisition mode (a, b, d and e). For the sake of clarity in the diagrams concerning step-scan mode only every fourth data point of the smoothed original data set is plotted, whereas for the rapid-scan mode all collected data are presented. In the first 1000 ms the coverage of linear adsorbed CO (a and d) is close to saturation for this temperature at the beginning of the reaction cycle. Different levels of adsorbed CO are created due to different samples with slightly different amounts of probed catalyst. In addition, the 10 s cycles are repeated several hundred times, thus the surface is probably not completely reduced by CO. Carlsson et al. [22] have shown, that a reduction period by 1% CO at 523 K over a 2%Pt/Al2O3 catalyst of a completely O covered surface can be as long as several tens of seconds. Therefore, low production rate of CO2, which is not observable by infrared absorption, seems to be reasonable at this elevated temperature and the surface is not in a completely unreactive, CO covered state. The additional displayed pulse profiles indicate, that immediately after introduction of an oxygen pulse the CO coverage decreases for all three O2 concentrations. Here the 2 and 3% O2 pulses lead to a complete removal of the CO coverage, the 1% O2 pulse decreases the coverage only partially. Whereas the data plots obtained by the rapid-scan mode (Fig. 4c and f) indicate a more or less simultaneous point of decrease in CO coverage (1 s), the step-scan plots reveal in more detail the CO coverage evolution. The rate of decreasing the CO coverage depends on the O2 concentration in the pulse, as well. The higher the concentration the faster the surface is freed from CO molecules, which is a well known feature [13,14,22,25]. One has to have in mind, that the reaction cell is fed by a constant amount of CO during the complete cycle. Approximation of mass transfer limitations (50% maximum conversion, Deff = 3.7 exp–7 m2/s) indicate a diffusion limitation for CO at the high reaction state of the O2 pulse at maximum CO conversion (Weisz–Prater criterion: F > 1).
However, the early stages of the ignition process should not be influenced by mass transfer limitations, hence, obtained differences originate from intrinsic parameters, like e.g. platinum particle size dependent reactivities. When the O2 concentration at the end of the pulse is decreasing readsorption of CO onto the platinum particles initiates. This process is definitely influenced by mass transfer limitations at a high reaction rate, as well. However, the data of the linear CO species concerning the different O2 concentrations indicate a slower readsorption to saturation level for higher O2 concentrations and bigger platinum particles (Fig. 4a and d). Even if the CO level is close to saturation a CO2 production can be observed, especially for the 1% O2 pulse data (Fig. 4b and e). The non-symmetric behaviour of the COads profile could be generated by several parameters. First, the pulse profiles are not symmetrical but being characterized by a tailing. This causes higher O2 concentrations at times > 5000 ms being high enough to sustain a high reaction rate, possibly. However, many authors have described a situation for transient CO oxidation over supported platinum surfaces, in which an oxidation process of the platinum surface creating strongly adsorbed O atoms has to be considered [14,16,22,23]. Bourane and Bianchi [16,17] have shown, that these strongly adsorbed O atoms are removed from the platinum surface by CO only at elevated temperatures at a much lower reaction rate for CO2 production compared to the ignition reaction rate of a CO covered surface. They distinguish between a weakly adsorbed O atom responsible for the first stage of the oxidation ignition inducing a phase transition Pt–CO ! Pt–O and a strongly adsorbed O atom produced at the sites of removed CO molecules reacting in the phase transition Pt–O ! Pt–CO [17]. This could be an additional explanation for a non-symmetrical coverage profile of linear adsorbed CO molecules. Especially the differences in the slope between 1% O2 and a 2 or 3% O2 pulse are rather obvious. If the particles are freed from CO molecules and a high amount of strongly adsorbed O atoms is created, following the interpretation of Bourane and Bianchi, readsorption of CO seems to be hindered. Furthermore, the coverage profile after a 1% O2 pulse is much more symmetric, indicating that a less amount of O2 has a less oxidative impact on the platinum surface. A high amount of strongly adsorbed O
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Fig. 4. CO coverage of linear adsorbed CO species and CO2 production over a 1%Pt/Al2O3 (a–c) and a 4%Pt/Al2O3 (d–f) catalyst at 523 K after introduction of 1, 2 and 3% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar comparing step-scan (a, b, d and e) and rapid-scan (c and f; filled symbols: CO2) data. All obtained data are plotted for rapid-scan (190 ms resolution), every fourth data point is plotted for step-scan data (5 ms resolution).
atoms is created, if CO is removed almost completely from the platinum surface [16]. This picture is sustained by the CO2 production profiles in Fig. 4b and e. The two high concentration pulses produce a much higher level of CO2 in the readsorption phase of CO at t > 5000 ms than the 1% O2 pulse. However, the peak CO2 productions at times t < 4000 ms do not reveal huge differences between the three O2 concentrations as is observable for the CO coverage.
Focussing on the start of CO2 production almost no difference between the three O2 concentrations can be observed. After entrance of the pulse into the reaction cell almost no delay (<100 ms) in CO2 production can be measured. Comparing both catalysts the difference between the 1% O2 and the 2 and 3% O2 pulse is more pronounced for the larger particle containing 4%Pt/Al2O3 catalyst. Additionally the amount of produced CO2 per pulse is slightly higher
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(20% obtained by mass spectrometer), indicating a higher reactivity of this catalyst. The data obtained by the rapid-scan mode are in good agreement with the data collected in step-scan mode. The higher sensitivity of the rapid-scan experiments, however, shows that the readsorption phase is divided into two parts: first, a rapid increase in CO coverage when the O2 pulse is leaving the reaction cell, second, a slow increase in absorption intensity close to saturation level. The step-scan data do not exhibit the second phase of the slow increase in CO absorption intensity as clear, especially for the 1%Pt/Al2O3 catalyst, probably due to the lower signal-noise ratio of the spectra. The first phase can be characterized by the phase transition Pt–O ! Pt–CO with a rather fast reaction rate. The slow increase in CO coverage in the second phase could be ascribed to a slow oxidation process of strongly adsorbed O atoms at defect sites giving these platinum sites free for CO adsorption very slowly. It is known, that oxygen prefers the adsorption at defect sites, e.g. the upper site of a step edge, whereas CO adsorbs preferentially at terrace sites [26–28]. Furthermore, O atoms can diffuse at elevated temperatures into the first atom layers of precious metal particles creating so-called sub-surface oxygen [26]. By increasing the defect density the ability of diffusion into the particle is generally enhanced [29]. Hence, the reaction of these O atoms could lead to a slow oxidation reaction influencing the CO readsorption. Another parameter effecting the reactivity of these O atoms is the coverage dependent adsorption energy. Bourane and Bianchi could show, that the activation energy of the reaction between COads and strongly adsorbed O atoms increases with decreasing O coverage [16]. However, the abrupt change between the different readsorption phases cannot be explained by these considerations. Finally, the reaction of residual O2, arising from the tailing of the O2 pulse and dissociating at free surface sites, with adsorbed CO molecules cannot be excluded. This step would involve the reaction of weakly adsorbed O atoms [17]. A comparison of the reactivity over a 1%Pt/Al2O3 catalyst at 523 K of a continuous feed of 0.5% CO in Ar plus two different O2 pulses, 2% O2 for 1 s and 1% O2 for 2 s, having an almost identical amount of pulsed O2 (D < 5%), is presented in Fig. 5. Data plots of the integrated peak areas of linear adsorbed CO molecules and produced gas-phase CO2 obtained by step-scan FT-IR spectroscopy (Fig. 5a and b, respectively) and the corresponding data obtained by the rapid-scan mode (Fig. 5c) are depicted. In addition, in Fig. 5a the profiles of the used pulses are shown. Applying a 2% O2 pulse for a duration of 1 s the coverage of the linear adsorbed CO molecules decreases more rapidly than for 1% O2 pulse, not reaching a zero CO coverage in both cases. The minima of the coverages are following nicely the maxima of the O2 pulses. The readsorption process of CO at the end of the pulses is almost comparable, however, the entire evolution of the CO coverage within this 10 s step cycle indicates a stronger non-symmetry for the 2% O2 pulse. This is in agreement with the data presented and discussed in Fig. 4. A comparable behaviour is found for the production of CO2 in Fig. 5b, where the maxima of CO2 production follow the O2 pulse maxima with a stronger
Fig. 5. Comparison between the reactivity of 2% O2 pulses for 1 s and 1% O2 pulses for 2 s with continuously fed 0.5% CO in Ar over a 1%Pt/Al2O3 catalyst at 523 K: CO coverage of linear adsorbed species and CO2 production obtained by step-scan mode (a and b, respectively); rapid-scan data of the evolution of CO coverage and CO2 production (c; filled symbols: CO2). All obtained data are plotted for rapid-scan (190 ms resolution), every fourth data point is plotted for step-scan data (5 ms resolution).
non-symmetric slope for the 2% O2 1 s pulse. Comparing the data obtained by the step-scan mode with the rapid-scan mode in Fig. 5c, the graphs are rather similar. Again, the second phase
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of the CO readsorption for times t > 5000 ms is revealed more clearly for the rapid-scan data. The most interesting results are gained, if we compare the amount of produced CO2 and the evolution of the coverage for linear adsorbed species for both O2 pulses. While the reduction of the CO adlayer during a 1% O2 2 s pulse is only 79% of that of a 2% O2 1 s pulse, the produced amount of CO2 per pulse detected by mass spectrometry is about 15% higher for the lower concentrated pulse, though the overall amount of pulsed O2 is similar. Apparently, the distribution of the O2 in a pulse has an effect on the amount of CO2 production. Applying a 2% O2 pulse this high concentration leads to a rapid decrease in CO coverage. Following the interpretation of Bourane and Bianchi, this ignition is characterized by weakly adsorbed O atoms at a high coverage of CO molecules [14,16]. The reaction of these weakly adsorbed O atoms and the linear adsorbed CO molecules producing CO2 are creating free adsorption sites at the platinum particle surface. Due to a high O2 concentration these free Pt0 sites will be occupied preferably by O species resulting in Ptd+–O sites. As discussed above the reduction of these sites is processed at a significantly lower reaction rate [14,16]. If 1% O2 pulses for 2 s are released, the CO coverage is decreasing slower and its minimum level is higher compared to 2% O2 pulses. Though, averaged less CO is removed from the surface as depicted in Fig. 5a more CO2 is produced. The faster readsorption period of CO of this lower concentrated O2 pulse indicates, that the platinum surface is probably covered by a less amount of strongly adsorbed O species. Therefore, the CO2 production by this peak is more strongly influenced by weakly adsorbed O atoms, whose reaction with COads to CO2 has a lower activation energy and a higher reaction rate at elevated temperatures [16]. Due to the fact, that in the case of a 1% O2 pulse less free Pt sites produce more CO2 the turnover frequency (TOF) of such a surface state must be higher compared to the situation produced by a 2% O2 pulse providing the same amount of oxygen. This behaviour is in agreement with experiments performed at a high and low coverage of strongly adsorbed O atoms [16,17]. Regarding to the literature [15–17] the following transient reaction scheme involving L–H type elementary steps can be proposed: (a) Linear adsorbed CO molecules are reacting immediately with adsorbed O atoms. These O atoms are weakly adsorbed at the platinum surface without competition to linear adsorbed CO molecules due to occupation of different adsorption sites [16]: COads þ Oads weak ! CO2;gas
(3.1)
The reaction proceeds with a high TOF and produced CO2 is desorbing immediately at these high temperatures used in this study. (b) The CO coverage is decreasing very fast, dependent on the concentration of the applied O2 pulse. The higher the O2 concentration, the faster the decrease in CO coverage. This step is characterized by a Pt–CO ! Pt–O transition, where the O atoms are described as strongly adsorbed. Here a
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higher amount of available gas-phase O2 creates a higher coverage of strongly adsorbed O atoms. (c) CO readsorption is affected by the reaction of adsorbed CO molecules with strongly adsorbed O atoms at the platinum surface: COads þ Oads strong ! CO2;gas
(3.2)
Even if the platinum surface is saturated by O atoms Lynch and Hu [30] calculated, that the heat of adsorption of linear CO molecules is very close to that on a completely reduced surface indicating that the readsorption should not be hindered drastically. However the reaction rate of (3.2) has to be significantly lower than that of (3.1). The surface state is represented by a phase transition Pt–O ! Pt–CO. (d) Close to CO saturation a slow increase in the CO coverage can be observed. This step cannot be explicitly explained. On the one hand the slow decrease of gas-phase O2 in the reaction cell regarding to the tailing of the O2 pulse could cause a residual CO oxidation. By the interpretation of Bourane and Bianchi this step would be explained by reaction (3.1). On the other hand strongly adsorbed O atoms at defect sites could be involved following a reaction step regarding to (3.2) sustained by generally higher adsorption energies at low coverages. 3.2. Induction periods: influence of temperature and platinum particle size In this second part we will demonstrate the performance of a step-scan experiment with a high time-resolution concerning the induction period of a CO oxidation reaction over Pt/Al2O3 catalysts at elevated temperatures. In Fig. 6a and b the evolution of the CO coverage and the CO2 production, respectively, after introduction of 2% O2 pulses for 2 s and continuously 0.5% CO in Ar are depicted using a 4%Pt/Al2O3 catalyst at 443, 523 and 573 K. The data for CO coverage and CO2 production are plotted as integrated peak areas of the specific step-scan FT-IR absorption spectra of linear adsorbed CO species and the gasphase vibration of CO2 at 2348 cm1. The profile of the applied O2 pulse is also given indicating the entrance of O2 into the reaction cell at about 1070 ms. The overall absorption intensity of linear adsorbed CO species is varying with temperature. At 523 K a higher absorption intensity is observed compared to 443 K (spectra not shown for sake of brevity) and this has been detected by other authors, as well [31,32], interpreted as an adsorbate induced reconstruction of the platinum particle surface. At the highest studied temperature of 573 K absorption intensity is decreasing due to enhanced desorption of CO molecules. In addition, the usage of samples with slightly different amounts of catalyst probed by the infrared beam cause variations in the overall absorption intensity. Focussing onto the point when the CO coverage of linear adsorbed species is decreasing, it is obvious, that with increasing temperature this point is shifted to earlier times, e.g. 1850 ms at 443 K to 1150 ms at 523 K. The start of CO coverage decrease at 573 K is found at an intermediate time at 1250 ms, which is shown more clearly in Fig. 6b concerning the evolution of CO2
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Fig. 6. CO coverage of linear adsorbed CO species (a) and CO2 production (b) over a 4%Pt/Al2O3 catalyst at 443, 523 and 573 K after introduction of 2% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar obtained by step-scan mode; every fourth data point is plotted for step-scan data (5 ms resolution).
production. Here the pulse starting at 1070 ms is followed by the first pronounced CO2 production for pulses applied at 523 K, and then a CO2 production for pulses applied at 573 K can be observed at 1250 ms, indicating a delay of about 100 ms between both temperatures and a delay of about 80 and 180 ms regarding to the entrance of the pulse into the reaction cell at 1070 ms. For O2 pulses introduced at a catalyst temperature of 443 K the first CO2 production is detectable at about 1500 ms. Whereas the production of CO2 and the decrease in CO coverage for both higher temperatures are proceeding almost simultaneously, a delay of more than 400 ms can be found between the coverage decrease and the CO2 production at 443 K. Furthermore, the CO oxidation reaction is igniting about 100 ms earlier for the intermediate temperature at 523 K compared to 573 K. In this context it is interesting to investigate not only the temperature dependence of a transient CO oxidation but the particle size dependence, as well. In Fig. 7 the evolutions of the coverage of linear adsorbed CO species and of produced gasphase CO2 applying 2% O2 pulses for 2 s together with continuously fed 0.5% CO in Ar at 443 K are presented comparing a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst. Both catalysts have been characterized showing a high dispersion of 98 and 65%, respectively (see Section 2.1), assuming the particles of the 4%Pt/Al2O3 catalyst being slightly larger. The obtained step-scan data indicate a higher reactivity of the larger platinum particles concerning the decrease in linear adsorbed CO coverage and the CO2 production. The profile of the applied O2 pulse is depicted as well, revealing a pulse entrance into the reaction cell at about 1070 ms. A first CO2 production is detected at about 1450 ms for both catalysts whereas a decrease in the linear adsorbed CO coverage is found at about 2400 and 1850 ms for the 1%Pt/Al2O3 and 4%Pt/Al2O3 catalyst, respectively. This indicates a clear difference between both catalysts. Four basic questions concerning the main phenomena of these experiments presented in Figs. 6 and 7 have to be answered. (a) Why is the ignition of the start of the oxidation reaction shifting with different temperatures for a constant O2
pulse? (b) Why is the CO2 production starting prior to the reduction of linear adsorbed species, which is observable at lower temperatures? (c) Why is the reaction rate higher at larger particles? (d) Why is the ignition of the reaction not proceeding fastest for the highest temperature? Concerning the first question several authors have observed, that the duration from a step or a pulse release of O2 in an isothermal oxidation study to the start of reaction ignition is dependent on the oxidation temperature [33–35]. The main CO2 production arising after this delay originates from the oxidation of linear adsorbed CO species [23] and is called induction period. In the literature this induction period is coupled to an increase in the number of free Pt0 sites available for adsorption of oxygen species [33–35]. This increase is ascribed to the desorption of an adsorbed CO species. Because of a variation of the heat of adsorption of CO on platinum with temperature the induction period should increase with
Fig. 7. CO coverage of linear adsorbed CO species and CO2 production over a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst at 443 K after introduction of 2% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar obtained by stepscan mode; every fourth data point is plotted for step-scan data (5 ms resolution).
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increasing heat of adsorption and, hence, with decreasing temperature [36]. The discussion of the influence of heat of adsorption of linear CO species on the induction period guides directly to question (b). At supported platinum particles not only linear adsorbed CO molecules are present at the surface but bridged and hollow site adsorbed molecules, as well. The number of these multiple bonded CO species is, however, rather small. Derrouiche and Bianchi [23] suggested, that the free Pt0 sites are formed by the removal of bridged adsorbed CO molecules during the induction period. The heat of adsorption of these bridged adsorbed species is lower than for linear adsorbed CO molecules and desorption at elevated temperatures should be preferred. They observed over a 1.2%Pt/Al2O3 catalyst at 300 K a partial desorption of these CO species under pure He flow, and at a switch to 2% O2 in He first the bridged adsorbed species were removed by reaction followed by the reaction of the linear adsorbed CO molecules [23]. However, they did not perform the experiments under a constant CO flow. For a constant 0.5% CO atmosphere it could be assumed that even at higher temperatures (443 K) the coverage of bridged adsorbed CO molecules is still high. Therefore, the removal of these CO species could be the origin of a CO2 production prior to a decrease in the coverage of linear adsorbed CO molecules. The free adsorption sites of the removed bridged species are then available for O2 species, if the concentration of O2 is high enough, and a high number of weakly adsorbed O atoms is generated inducing the ignition of the reaction with linear adsorbed CO molecules. If the bridged species desorb due to a competitive adsorption or react directly with O2 to CO2, as suggested in a modified Eley–Rideal mechanism by Dwyer and Bennett [33], is not clear. At higher temperatures the coverage of the bridged CO species is apparently so low, that the ignition process can be started due to a high number of free Pt0 sites for O2 dissociation. However, the lower sensitivity of the step-scan mode compared to the rapid-scan mode has to be taken into account. Hence, a very weak decrease in coverage of linear adsorbed species during the induction period is hardly detectable. The same problem does not allows us to investigate the evolution of the low number of bridged adsorbed CO species, which would sustain our suggestions. As presented in Fig. 7 the platinum particle dispersion has a huge effect on the reactivity concerning the CO oxidation reaction. It has been shown, that an increase of the dispersion of platinum particles decreases the rate constant of oxidation of adsorbed linear CO species at low temperatures and increases the heat of adsorption of bridged CO species [23]. Furthermore the heat of adsorption of these species is increasing with decreasing coverage [36]. In contrast, the heat of adsorption of linear adsorbed species does not depend strongly on the dispersion [23]. Derrouiche and Bianchi [23] did not observe desorption of CO during induction periods of transient isothermal CO oxidation experiments, indicating, that probably not a competitive chemisorption between oxygen and bridged CO species contributes to their removal but an oxidation of the bridged adsorbed CO molecules is preferred, which is sustained by the detection of a fraction of the CO2 production during the
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induction period. Finally, this means, that at larger particles the removal of bridged adsorbed CO molecules is favoured compared to smaller particles due to a lower heat of adsorption and as a consequence the number of O atoms, after O2 dissociation at these sites, is higher and ignition regarding to reaction (3.1) proceeds faster with a higher CO2 production rate. The last question (d) points onto another parameter of the ignition of the oxidation of linear adsorbed CO species. Referring to Section 3.1 and reaction (3.1) the transition of the CO covered platinum surface to an oxygen covered surface (Pt– CO ! Pt–O) is characterized by weakly adsorbed O atoms. These species are generated by dissociation of O2 molecules at free platinum sites, which are created due to the oxidation of bridged adsorbed CO molecules, probably [23]. As shown above increasing of the reaction temperature favours this last reaction. However, an increase of reaction temperature means a decrease in the heat of adsorption of the weakly adsorbed O atoms and finally a lower coverage of these species [14]. Because a higher level of gas-phase O2 is required to obtain a sufficient coverage of O species at 573 K compared to 523 K, a delay in the ignition of the linear CO oxidation is produced, as depicted in Fig. 6b. 4. Conclusions A time-resolved FT-IR study of the transient CO oxidation reaction over a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst triggered by O2 pulses comparing the step-scan and rapidscan acquisition modes of a commercial FT-IR spectrometer has been performed. Time-resolutions of 5 ms in the step-scan mode compared to several hundred milliseconds in the normal rapid-scan mode could be achieved. O2 pulses with concentrations of 1, 2 and 3% for 1 and 2 s duration at reaction temperatures of 443, 523 and 573 K and a constant flow of 0.5% CO in Ar have been applied. The data obtained by step-scan and rapid-scan mode are in good agreement, showing a lower signal-noise ratio and therefore a lower sensitivity for small IR absorption intensity changes for the step-scan mode. However, a high time-resolution, in principle much higher than the chosen 5 ms, can be used for studying fast transients in heterogeneously catalysed reactions. By utilizing the high time-resolution of the step-scan acquisition mode we could obtain new details of the dynamics of the CO oxidation reaction over a supported Pt catalysts at high temperatures in the range of 443–573 K and at 1 atm pressure. The obtained data reveal different elementary steps involving weakly adsorbed O atoms for the surface transition from Pt–CO ! Pt–O and strongly adsorbed O atoms for the surface transition from Pt–O ! Pt–CO. We could show, that not only the O2 concentration is influencing the reaction rate, but the distribution of O2 within a pulse has an effect on the surface transition and on the reactivity, as well. By means of the step-scan mode induction periods of the CO oxidation reaction could be investigated in detail at elevated temperature. Here the oxidation of bridged adsorbed CO species probably plays a key role in the evolution of the induction period. We could show
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that an intermediate temperature of 523 K generates the shortest induction period compared to reaction temperatures of 443 and 573 K. Finally, the comparison of the transient CO oxidation reaction between a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst reveals the importance of controlling particle dispersions during catalyst preparation indicated by a higher reactivity due to a shorter induction period for the catalyst containing larger particles. Within this study we could demonstrate the performance of the high-time resolution step-scan FT-IR spectroscopy, which will, hopefully, be an additional tool gaining a better understanding of heterogeneously catalysed reaction mechanisms in the future. Acknowledgements This work has been performed within the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Agency and the member companies: AB Volvo, Johnson Matthey-CSD, Saab Automobile Powertrain AB, Perstorp AB, AVL-MTC AB, Albemarle Catalysts and The Swedish Space Corporation. The Swedish Research Council is greatly acknowledged for financial support. References [1] G. Ertl, in: J.R. Anderson, M. Boudart (Eds.), Catalysis, Science and Technology, vol. 4, Springer, Berlin, 1983, p. 209. [2] T.S. Askgaard, J.K. Nørskov, C.V. Ovesen, P.J. Stoltze, J. Catal. 156 (1995) 229. [3] T. Engel, G. Ertl, Adv. Catal. 28 (1979) 1. [4] L.F. Razon, R.A. Schmitz, Catal. Rev. Sci. Eng. 28 (1986) 89. [5] R.H. Nibbelke, M.A.J. Campman, J.H.B.J. Hoebink, G.B. Marin, J. Catal. 171 (1997) 358.
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Millisecond step-scan FT-IR transmission spectroscopy under transient reaction conditions: CO oxidation over Pt/Al2O3 Ansgar Wille 1, Erik Fridell * Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden Available online 30 June 2006
Abstract Time-resolved FT-IR spectra of CO oxidation over Pt/Al2O3 catalysts were collected in situ in transmission mode under transient reaction conditions. Using the step-scan acquisition mode of a commercial FT-IR spectrometer, time-resolution of a few milliseconds, compared to several hundred milliseconds in the normal rapid-scan mode, could be achieved. The experiments were triggered by reproducible oxygen gas pulses at constant temperature. Infrared light transmission, uniform heating and fast gas exchange were realized by pressing the pure catalyst powder onto a thin stainless steel wire mesh using a low volume reaction cell. By comparing the results of both acquisition modes, rapid-scan and step-scan, we will demonstrate, that it is possible, in a rather simple way, to investigate heterogeneously catalysed reactions under reaction conditions by means of FT-IR spectroscopy with time-resolutions down to a few milliseconds, and, in principle, lower. CO oxidation between 443 and 573 K over 1%Pt/ g-Al2O3 and 4%Pt/g-Al2O3 catalysts has been investigated as a test reaction. By using the high time-resolution of the step-scan acquisition mode we could obtain new details of the dynamics of the CO oxidation reaction over a supported Pt catalyst at high temperatures and 1 atm pressure. # 2006 Elsevier B.V. All rights reserved. Keywords: Time-resolved FTIR; Step-scan; CO oxidation; Platinum; Transient reaction
1. Introduction The development of new heterogeneous catalysts for the use in industrial applications is still based on empirical methods. Only in a few cases it succeeded to understand the complex mechanism of the chemical reaction by elucidating the different steps of the heterogeneously catalysed reaction [1–3]. It is crucial to get a detailed insight into the elementary steps proceeding at the solid surface of the catalyst under as real conditions as possible (temperature, partial pressure of reactants, flow dynamics) to create the ability of improving the conversion or the selectivity. The oxidation of CO on supported noble metal catalysts is one of the most widely studied catalytic reactions. However, the involved elementary steps, especially the rate of CO2 production, seem to differ if one is comparing results obtained in UHV studies at fairly low
* Corresponding author. Present address: IVL Swedish Environmental Research Institute, PO Box 5302, 400 14 Go¨teborg, Sweden. Tel.: +46 31 772 33 72; fax: +46 31 772 31 34. E-mail address: [email protected] (E. Fridell). 1 Present address: W.C. Heraeus GmbH, Heraeusstr. 12–14, D-63450 Hanau, Germany. 0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.10.031
CO partial pressures (<104 Pa) with experiments under real conditions (>102 Pa) and they are still under debate [3–6]. This pressure gap is accompanied by the materials gap revealing morphological differences as defect sites, irregular edges and kinks at supported particles and flat and almost perfectly oriented surfaces on single crystals, directly influencing elementary reaction steps. Hence, investigations of these basic steps should be accomplished at supported catalysts under real reaction conditions, preferably. Infrared spectroscopy is one of the most important techniques used for conducting in situ studies of catalytic solids under reaction conditions [7]. The interaction of the infrared light with the catalyst can be arranged in a transmission experiment (pressed self-supported catalyst pellet), a reflectance experiment (diffuse reflectance infrared Fourier Transform (DRIFT), using catalyst powder) or by applying an attenuated total reflection (ATR) of the infrared light with catalyst particles in contact with an ATR crystal. A simple quantitative evaluation of the absorbance data is only possible for a transmission experiment, whereas for a reflected infrared beam the knowledge of frequency dependent extinction coefficients is crucial [8]. Within the normal operating mode of an FT-IR spectrometer the moving mirror in the interferometer is driven at a constant
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velocity acting this mirror continuously forward and backward. This is the so-called rapid-scan mode. During one single-sided forward or backward move a complete interferogram, and after Fourier Transformation a spectrum, is collected. All processes which are proceeding at the catalyst surface are averaged resulting in one spectrum related to the time slice of the mirror moving period (e.g. a complete forward move). In order to produce shorter time slices the velocity of the moving mirror has to be increased. However, a finite mass of the mirror and, hence, a certain mechanical stability of the interferometer constrains the mirror velocity and thus a maximum timeresolution. Additionally, a high optical resolution requires a long distance of the moving mirror, which constricts a high time-resolution at high optical resolutions. Nowadays, commercial FT-IR spectrometer are able to produce spectra at a time-resolution of about 50 ms at an optical resolution of 4 cm1. Higher time-resolution can be obtained only at the cost of optical resolution. A different way obtaining an interferogram by an interferometer is the so-called step-scan mode in which the mirror movement is decoupled from time [9,10]. Within this mode the moving mirror is acting in a step-wise mode holding the mirror fixed at a specific position. Now an event is triggered (e.g. by a laser pulse, gas pulse, temperature ramp) and an altering of the sample is followed in time by triggering the infrared detector very fast. Then the mirror is moving to the next step position and the event is triggered again. Repeating this procedure several hundred or thousand times, a set of data points is collected resulting in complete interferograms with time segments depending on the triggering rate of the detector. This means that the time-resolution of a step-scan experiment is limited in principle simply by the triggering rate of the spectrometer electronics and the response time of the IR detector. Thus, time-resolutions of a few nanoseconds are possible [11]. However, this technique requires a reproducible event and a very good long-term stability of the complete experimental setup. This means for an application on heterogeneous catalysts, that the reaction trigger pulses (gas pulse) and the sample temperature have to be constant and, that the catalyst should not age during the entire experiment. We will show that this technique can be used for studying the effects of fast gas transients in heterogeneous catalysis by means of the CO oxidation reaction over supported platinum catalysts with a time-resolution of 5 ms, which can be chosen, in principle, much lower (see Section 2.3). The elementary steps of the oxidation of CO valid for UHV conditions have been summarized by Engel and Ertl in 1979 into the so-called conventional three-step Langmuir–Hinshelwood (L–H) reaction scheme [3]: COgas $ COads
(1.1)
ðO2 Þgas ! 2Oads
(1.2)
COads þ Oads ! ðCO2 Þgas
(1.3)
The indices ‘‘gas’’ and ‘‘ads’’ refer to gas-phase and adsorbed species, respectively. CO adsorbs associatively whereas O2
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adsorbs dissociatively already at temperatures about 100 K [12]. The formed product CO2 of the reaction between adsorbed COads and Oads species is desorbing immediately after reaction. However, this general picture cannot be translated directly to supported platinum particles working under real conditions and some aspects have to be added to explain experimental results. In a series of papers Bourane and Bianchi [13–17] have elucidated parameters playing significant roles in the elementary steps of this ‘‘simple’’ surface reaction over Pt/Al2O3. They performed kinetic studies under isothermal conditions, mainly at low temperatures, determining the effect of transient changes of partial pressures of the reactants and products on the rate of reaction. It could be shown, that different CO species, like linear and bridged adsorbed molecules, as well as different types of adsorbed oxygen atoms are involved in the elementary steps. At elevated temperatures, however, it is difficult to follow surface species, especially at the ignition state of the CO oxidation reaction, which proceeds very fast [17]. At this stage it would be interesting to have the ability investigating the behaviour of surface species like linear adsorbed CO molecules at platinum sites at these elevated temperatures in time. We have studied the oxidation of CO over Pt/g-Al2O3 catalysts at constant temperatures under transient conditions applying oxygen gas pulses. Utilizing the step-scan acquisition mode of a commercial FT-IR spectrometer we have investigated the reactivity of linear adsorbed CO species as well as the production of gas-phase CO2 molecules with a time-resolution of 5 ms. All experiments have been performed at elevated temperatures between 443 and 573 K at 0.5% CO constant feed and O2 pulses of 1–3% O2 concentration at 1 or 2 s duration. 2. Experimental 2.1. Preparation of catalyst In this study two different Pt/g-Al2O3 catalysts with 1 and 4% Pt have been used. Both catalysts have been prepared by the wet impregnation method. g-Al2O3 (SCC a-150/200, Sasol) was dispersed in distilled water and the pH was adjusted to 2.5 by adding diluted HNO3 solution (Merck). An aqueous platinum nitrate solution (Pt(NO3)2, Johnson Matthey) was added dropwise to the alumina slurry under continuous stirring to yield the desired platinum loading. The slurry was kept under continuous stirring for 20 min, frozen with liquid nitrogen and freeze-dried. In order to preserve a high dispersion the catalysts were calcined carefully at 723 K for 4 h in air starting this procedure at room temperature and ramping the temperature with a rate of 5 K/min up to 723 K. The Pt dispersions have been determined by CO chemisorption (Micromeritics, ASAP2010C) at 300 K giving 98% dispersion (40.4 mmol CO/g catalyst) for the 1%Pt/Al2O3 catalyst and 65% dispersion (106.2 mmol CO/g catalyst) for the 4%Pt/Al2O3 catalyst assuming a maximum CO/Pt ratio of 0.8 which is reasonable for highly dispersed Pt particles [18]. However, the very high dispersion for the 1%Pt/Al2O3 catalyst shows that the value of 0.8 is probably underestimated. The BET surface areas (Micromeritics Tristar) have been measured as 187 and
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182 m2/g for the 1%Pt/Al2O3 and the 4%Pt/Al2O3 catalyst, respectively. 2.2. Reaction cell and sample All experiments have been performed in a home-made stainless steel IR reaction cell at atmospheric pressure. The cell design facilitates in situ FTIR transmission experiments under reaction conditions at atmospheric pressure as well as continuous following of the product stream by a mass spectrometer. The cell consists of a main stainless steel body with feed throughs for gas inlet and outlet and thermocouples to measure the sample and the cell temperature, as well as high power feed throughs for resistive heating of the sample wire mesh. Two CaF2 windows (50 mm diameter 10 mm) sealed by Kalrez o-rings are attached to both sides of the main body by stainless steel holders. To reduce the internal volume of the reaction cell two macor inlays with cut-outs for the sample holder and a 12 mm hole for infrared light pass through are placed between sample holder and the CaF2 windows. The residual cell volume is about 4 cm3. When investigating fast transients applied on a high surface area solid by transmission infrared spectroscopy, some requirements like fast gas-surface response, uniform heat distribution and an IR-transparent sample form have to be fulfilled. Hence, following a procedure of Ballinger et al. [19], we have chosen a sample preparation by pressing the pure catalyst onto a stainless steel plain woven wire mesh. The wire mesh is made out of highly oxidation resistive stainless steel wire (0.066 mm diameter, 1.4301 steel: Fe/Cr18/Ni10/Mo3) with a nominal aperture of 0.103 mm and 37% open area. The ground catalyst powder is distributed over an area of 1.2 cm 1.4 cm of the wire mesh and scratched with a spattle into the openings of the mesh. This procedure guarantees a uniform distribution of the catalyst over the area probed by the infrared beam. Subsequently the catalyst supported wire mesh is pressed between two stainless steel plates by means of a press. After this pressing the catalyst is fixed to the wire mesh firmly with a maximum sample thickness of 150 mm. An average amount of 10 mg of catalyst cm2 are pressed onto the wire mesh. The thermocouple for measuring the sample temperature is placed 3 mm off the mesh border in close contact to the supported catalyst. Finally, the sample wire mesh is placed in the middle of the main reaction cell body by two stainless steel clamps which are attached to power feed throughs. 2.3. Transient FTIR experiments All gases used in this study were of high purity (>99.995%) and were introduced to the reaction cell without further purification by individual mass flow controllers (MFC, Bronckhorst). The reactor was fed by two individual gas streams, first the main constant reactant feed and secondly the pulsed reactant feed (see Fig. 1). If no reactant was pulsed argon was used as balance for this gas stream to obtain a constant flow. The switching system consists of two fast switching three-way valves (Parker, Skinner solenoid valves, <16 ms response) arranged in a four-port acting mode. For all experiments a
Fig. 1. Experimental setup displaying gas flows and trigger signals.
constant flow of 0.5% CO and O2 pulses with varying concentrations (1–3%) and durations (1 and 2 s) were introduced to the reaction cell. In all FT-IR experiments a flow of 600 ml/min has been chosen. The fresh sample was dried in a 300 ml/min Ar flow at 573 K for 2 h before being exposed to reactant gas compositions. The sample was then exposed first to a 4% O2 in Ar mixture at 623 K at 300 ml/min for 30 min and second to 4% H2 in Ar at 623 K at 150 ml/min for 30 min for oxidation and reduction of the sample, respectively. Before each experiment the sample was oxidized and reduced under the same conditions but only for a period of 5 min. This procedure has been followed by a net-oxidation with 0.5% CO/2% O2 in Ar at 600 ml/min for 5 min at the actual temperature to be studied and an FT-IR background was collected. By using this routine, the appearance of gas-phaserelated CO and CO2 bands and of alumina-related carbonates, acting as spectators, in the spectra was minimized. Additionally, the background spectra will not contain any IR bands of CO bonded to Pt, since the CO coverage is low due to the CO + O reaction. The time-resolved in situ FTIR spectroscopy experiments were performed in transmission mode with a Bio-Rad FTS 6000 spectrometer. To obtain time-resolutions of 5 ms the internal step-scan option has been used. By means of utilizing gas pulses an event period of 10 s have been chosen. The release of a gas pulse or a temperature ramp (not applied in this study) is triggered by the spectrometers step pulse output (Fig. 1). This trigger pulse can be altered by a pulse generator (HP 8013B) to TTL level which is used to trigger the switching of the threeway valves. The gas pulse duration can be controlled by a computer program. The FT-IR spectra in all step-scan experiments were recorded with an optical resolution of 8 cm1. To obtain a background spectrum a complete TRS run under oxidation conditions (continuous feed of 0.5% CO + 2% O2) at the temperature to be studied was carried out. Using a 10 s step cycle at 8 cm1 optical resolution a complete TRS run requires about 6 h for collecting the data. In all illustrations showing step-scan data the graphs of integrated peak areas are smoothed by a 10 point FFT filter procedure for clear presentation.
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Fig. 2. Pulse profiles of CO2 pulses introduced to the empty reaction cell and characterized by the CO2 infrared absorption at 2348 cm1 by means of a step-scan run: (a) 1 s pulse 1–3% CO2 and (b) 2 s pulse 1–3% CO2 (600 ml/min flow in Ar, 293 K).
For rapid-scan experiments the data collection procedure is similar. A background spectrum has been collected under continuous CO oxidation reaction conditions (0.5% CO + 2% O2) at the temperature to be studied. Under transient reaction conditions the gas pulses were released automatically under software control and data collection was started manually. All rapid-scan spectra have been collected with an optical resolution of 4 cm1 at a maximum time-resolution of about 190 ms. The product stream leaving the IR reaction cell was continuously analysed using a Balzers Quadstar 422 quadrupole mass spectrometer. The signals of H2 (m/e 2), H2O (m/e 18), CO/N2 (m/e 28), O2 (m/e 32), Ar (m/e 40) and CO2 (m/e 44) have been detected with a time-resolution of about 0.3 s. In order to correlate the O2 pulse profile with the characteristics of the absorption peaks of linear adsorbed CO and produced gas-phase CO2 a characterization of the O2 pulse has to be performed. We used CO2 as a probe molecule having an infrared absorption feature at 2348 cm1. We assume that the pulsed CO2 gas behaves approximately in the same way as O2 gas. The slope of pulsed CO2 gas, as integrated peak area of the CO2 absorption against time, for 1, 2 and 3% CO2 concentration of 1 and 2 s pulses is shown in Fig. 2a and b, respectively. The infrared spectra have been recorded in the step-scan mode with a step period of 10 s at a time-resolution of 15 ms. For both pulse durations the concentration dependence shows an almost linear relationship with a peak half width matching the forced pulse duration. However, the late part of the pulses reveal a broad tailing increasing the mean residence time of the gas in the reaction cell. On the other hand the entrance time of the pulse is very reproducibly detected at about 1070 ms. 3. Results and discussion 3.1. CO oxidation at different O2 concentrations: stepscan versus rapid-scan In Fig. 3 infrared absorbance spectra of linear adsorbed CO species on platinum particles in a 4%Pt/g-Al2O3 catalyst and of
produced gas-phase CO2 are presented showing the evolution of the ignition of the CO oxidation reaction after a 2% O2 pulse for 2 s at 523 K catalyst temperature comparing a rapid-scan (Fig. 3a) and a step-scan FT-IR experiment (Fig. 3b) in the period of 1–3.4 s. The experiment is characterized by steps of 10 s duration with one O2 pulse within one step. In both sets of spectra the absorption features of gas-phase CO2 at 2348 cm1 and of linear adsorbed CO species at 2056 cm1 together with a shoulder at 2064 cm1 are observable. The low frequency part of this absorption can be assigned to CO adsorbed on reduced platinum atoms (denoted as Pt0) and the high frequency shoulder to CO molecules adsorbed on partially oxidized platinum (denoted as Ptd+) in agreement with several studies [13,20–22]. Additionally, at about 1840 cm1 a very weak absorption of bridged adsorbed CO species can be observed. Thus, more than 90% of the platinum surface is covered by linear adsorbed CO as observed by other authors [22,23]. The spectra collected in both modes indicate a constant decrease in intensity of the adsorbed CO species for the ignition of the reaction, while the absorbance of CO2 is increasing. The absorption feature of linear CO is red-shifting to 2050 cm1 which can be explained by reduced CO–CO dipole coupling by the decrease of CO coverage at the platinum particles [24]. Fig. 3a is presenting all spectra collected in the time period between 1 and 3.4 s, whereas Fig. 3b shows some selected spectra of a step-scan run under identical conditions representing similar time stages as for the rapid-scan experiment but obtained with a time-resolution of 5 ms. By comparing the two sets of FT-IR spectra both acquisition modes present the same evolution of the spectra for the ignition process, but with an obvious difference in the signal-noise ratio. The fast triggering of the infrared detector at a step position, creating the high time-resolution, is definitely accompanied by a lower spectrum quality. Increasing the signal-noise ratio is possible only by repeating the complete step-scan run and coadding different scans. However, this would increase the measuring time enormously. In principle, both methods are comparable with a lower signal-noise ratio for the step-scan mode and with a much higher time-resolution.
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Fig. 3. Evolution of infrared absorptions of linear adsorbed CO molecules and produced gas-phase CO2 after an O2 pulse (2% 2 s) with a 0.5% CO continuous feed in Ar at 523 K: (a) rapid-scan mode (1–3.4 s, 190 ms resolution and (b) step-scan mode; (a–h) 1000, 1800, 2000, 2250, 2500, 2800, 3000, 3400 ms).
Fig. 4 shows the evolution of the linear adsorbed CO species and the production of CO2 at 523 K as integrated infrared absorption peak areas over a 1%Pt/g-Al2O3 (a–c) and a 4%Pt/gAl2O3 (d–f) catalyst after introduction of a continuous feed of 0.5% CO in Ar and different O2 concentrations by a 2 s pulse comparing rapid-scan (c and f) and step-scan data acquisition mode (a, b, d and e). For the sake of clarity in the diagrams concerning step-scan mode only every fourth data point of the smoothed original data set is plotted, whereas for the rapid-scan mode all collected data are presented. In the first 1000 ms the coverage of linear adsorbed CO (a and d) is close to saturation for this temperature at the beginning of the reaction cycle. Different levels of adsorbed CO are created due to different samples with slightly different amounts of probed catalyst. In addition, the 10 s cycles are repeated several hundred times, thus the surface is probably not completely reduced by CO. Carlsson et al. [22] have shown, that a reduction period by 1% CO at 523 K over a 2%Pt/Al2O3 catalyst of a completely O covered surface can be as long as several tens of seconds. Therefore, low production rate of CO2, which is not observable by infrared absorption, seems to be reasonable at this elevated temperature and the surface is not in a completely unreactive, CO covered state. The additional displayed pulse profiles indicate, that immediately after introduction of an oxygen pulse the CO coverage decreases for all three O2 concentrations. Here the 2 and 3% O2 pulses lead to a complete removal of the CO coverage, the 1% O2 pulse decreases the coverage only partially. Whereas the data plots obtained by the rapid-scan mode (Fig. 4c and f) indicate a more or less simultaneous point of decrease in CO coverage (1 s), the step-scan plots reveal in more detail the CO coverage evolution. The rate of decreasing the CO coverage depends on the O2 concentration in the pulse, as well. The higher the concentration the faster the surface is freed from CO molecules, which is a well known feature [13,14,22,25]. One has to have in mind, that the reaction cell is fed by a constant amount of CO during the complete cycle. Approximation of mass transfer limitations (50% maximum conversion, Deff = 3.7 exp–7 m2/s) indicate a diffusion limitation for CO at the high reaction state of the O2 pulse at maximum CO conversion (Weisz–Prater criterion: F > 1).
However, the early stages of the ignition process should not be influenced by mass transfer limitations, hence, obtained differences originate from intrinsic parameters, like e.g. platinum particle size dependent reactivities. When the O2 concentration at the end of the pulse is decreasing readsorption of CO onto the platinum particles initiates. This process is definitely influenced by mass transfer limitations at a high reaction rate, as well. However, the data of the linear CO species concerning the different O2 concentrations indicate a slower readsorption to saturation level for higher O2 concentrations and bigger platinum particles (Fig. 4a and d). Even if the CO level is close to saturation a CO2 production can be observed, especially for the 1% O2 pulse data (Fig. 4b and e). The non-symmetric behaviour of the COads profile could be generated by several parameters. First, the pulse profiles are not symmetrical but being characterized by a tailing. This causes higher O2 concentrations at times > 5000 ms being high enough to sustain a high reaction rate, possibly. However, many authors have described a situation for transient CO oxidation over supported platinum surfaces, in which an oxidation process of the platinum surface creating strongly adsorbed O atoms has to be considered [14,16,22,23]. Bourane and Bianchi [16,17] have shown, that these strongly adsorbed O atoms are removed from the platinum surface by CO only at elevated temperatures at a much lower reaction rate for CO2 production compared to the ignition reaction rate of a CO covered surface. They distinguish between a weakly adsorbed O atom responsible for the first stage of the oxidation ignition inducing a phase transition Pt–CO ! Pt–O and a strongly adsorbed O atom produced at the sites of removed CO molecules reacting in the phase transition Pt–O ! Pt–CO [17]. This could be an additional explanation for a non-symmetrical coverage profile of linear adsorbed CO molecules. Especially the differences in the slope between 1% O2 and a 2 or 3% O2 pulse are rather obvious. If the particles are freed from CO molecules and a high amount of strongly adsorbed O atoms is created, following the interpretation of Bourane and Bianchi, readsorption of CO seems to be hindered. Furthermore, the coverage profile after a 1% O2 pulse is much more symmetric, indicating that a less amount of O2 has a less oxidative impact on the platinum surface. A high amount of strongly adsorbed O
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Fig. 4. CO coverage of linear adsorbed CO species and CO2 production over a 1%Pt/Al2O3 (a–c) and a 4%Pt/Al2O3 (d–f) catalyst at 523 K after introduction of 1, 2 and 3% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar comparing step-scan (a, b, d and e) and rapid-scan (c and f; filled symbols: CO2) data. All obtained data are plotted for rapid-scan (190 ms resolution), every fourth data point is plotted for step-scan data (5 ms resolution).
atoms is created, if CO is removed almost completely from the platinum surface [16]. This picture is sustained by the CO2 production profiles in Fig. 4b and e. The two high concentration pulses produce a much higher level of CO2 in the readsorption phase of CO at t > 5000 ms than the 1% O2 pulse. However, the peak CO2 productions at times t < 4000 ms do not reveal huge differences between the three O2 concentrations as is observable for the CO coverage.
Focussing on the start of CO2 production almost no difference between the three O2 concentrations can be observed. After entrance of the pulse into the reaction cell almost no delay (<100 ms) in CO2 production can be measured. Comparing both catalysts the difference between the 1% O2 and the 2 and 3% O2 pulse is more pronounced for the larger particle containing 4%Pt/Al2O3 catalyst. Additionally the amount of produced CO2 per pulse is slightly higher
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(20% obtained by mass spectrometer), indicating a higher reactivity of this catalyst. The data obtained by the rapid-scan mode are in good agreement with the data collected in step-scan mode. The higher sensitivity of the rapid-scan experiments, however, shows that the readsorption phase is divided into two parts: first, a rapid increase in CO coverage when the O2 pulse is leaving the reaction cell, second, a slow increase in absorption intensity close to saturation level. The step-scan data do not exhibit the second phase of the slow increase in CO absorption intensity as clear, especially for the 1%Pt/Al2O3 catalyst, probably due to the lower signal-noise ratio of the spectra. The first phase can be characterized by the phase transition Pt–O ! Pt–CO with a rather fast reaction rate. The slow increase in CO coverage in the second phase could be ascribed to a slow oxidation process of strongly adsorbed O atoms at defect sites giving these platinum sites free for CO adsorption very slowly. It is known, that oxygen prefers the adsorption at defect sites, e.g. the upper site of a step edge, whereas CO adsorbs preferentially at terrace sites [26–28]. Furthermore, O atoms can diffuse at elevated temperatures into the first atom layers of precious metal particles creating so-called sub-surface oxygen [26]. By increasing the defect density the ability of diffusion into the particle is generally enhanced [29]. Hence, the reaction of these O atoms could lead to a slow oxidation reaction influencing the CO readsorption. Another parameter effecting the reactivity of these O atoms is the coverage dependent adsorption energy. Bourane and Bianchi could show, that the activation energy of the reaction between COads and strongly adsorbed O atoms increases with decreasing O coverage [16]. However, the abrupt change between the different readsorption phases cannot be explained by these considerations. Finally, the reaction of residual O2, arising from the tailing of the O2 pulse and dissociating at free surface sites, with adsorbed CO molecules cannot be excluded. This step would involve the reaction of weakly adsorbed O atoms [17]. A comparison of the reactivity over a 1%Pt/Al2O3 catalyst at 523 K of a continuous feed of 0.5% CO in Ar plus two different O2 pulses, 2% O2 for 1 s and 1% O2 for 2 s, having an almost identical amount of pulsed O2 (D < 5%), is presented in Fig. 5. Data plots of the integrated peak areas of linear adsorbed CO molecules and produced gas-phase CO2 obtained by step-scan FT-IR spectroscopy (Fig. 5a and b, respectively) and the corresponding data obtained by the rapid-scan mode (Fig. 5c) are depicted. In addition, in Fig. 5a the profiles of the used pulses are shown. Applying a 2% O2 pulse for a duration of 1 s the coverage of the linear adsorbed CO molecules decreases more rapidly than for 1% O2 pulse, not reaching a zero CO coverage in both cases. The minima of the coverages are following nicely the maxima of the O2 pulses. The readsorption process of CO at the end of the pulses is almost comparable, however, the entire evolution of the CO coverage within this 10 s step cycle indicates a stronger non-symmetry for the 2% O2 pulse. This is in agreement with the data presented and discussed in Fig. 4. A comparable behaviour is found for the production of CO2 in Fig. 5b, where the maxima of CO2 production follow the O2 pulse maxima with a stronger
Fig. 5. Comparison between the reactivity of 2% O2 pulses for 1 s and 1% O2 pulses for 2 s with continuously fed 0.5% CO in Ar over a 1%Pt/Al2O3 catalyst at 523 K: CO coverage of linear adsorbed species and CO2 production obtained by step-scan mode (a and b, respectively); rapid-scan data of the evolution of CO coverage and CO2 production (c; filled symbols: CO2). All obtained data are plotted for rapid-scan (190 ms resolution), every fourth data point is plotted for step-scan data (5 ms resolution).
non-symmetric slope for the 2% O2 1 s pulse. Comparing the data obtained by the step-scan mode with the rapid-scan mode in Fig. 5c, the graphs are rather similar. Again, the second phase
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of the CO readsorption for times t > 5000 ms is revealed more clearly for the rapid-scan data. The most interesting results are gained, if we compare the amount of produced CO2 and the evolution of the coverage for linear adsorbed species for both O2 pulses. While the reduction of the CO adlayer during a 1% O2 2 s pulse is only 79% of that of a 2% O2 1 s pulse, the produced amount of CO2 per pulse detected by mass spectrometry is about 15% higher for the lower concentrated pulse, though the overall amount of pulsed O2 is similar. Apparently, the distribution of the O2 in a pulse has an effect on the amount of CO2 production. Applying a 2% O2 pulse this high concentration leads to a rapid decrease in CO coverage. Following the interpretation of Bourane and Bianchi, this ignition is characterized by weakly adsorbed O atoms at a high coverage of CO molecules [14,16]. The reaction of these weakly adsorbed O atoms and the linear adsorbed CO molecules producing CO2 are creating free adsorption sites at the platinum particle surface. Due to a high O2 concentration these free Pt0 sites will be occupied preferably by O species resulting in Ptd+–O sites. As discussed above the reduction of these sites is processed at a significantly lower reaction rate [14,16]. If 1% O2 pulses for 2 s are released, the CO coverage is decreasing slower and its minimum level is higher compared to 2% O2 pulses. Though, averaged less CO is removed from the surface as depicted in Fig. 5a more CO2 is produced. The faster readsorption period of CO of this lower concentrated O2 pulse indicates, that the platinum surface is probably covered by a less amount of strongly adsorbed O species. Therefore, the CO2 production by this peak is more strongly influenced by weakly adsorbed O atoms, whose reaction with COads to CO2 has a lower activation energy and a higher reaction rate at elevated temperatures [16]. Due to the fact, that in the case of a 1% O2 pulse less free Pt sites produce more CO2 the turnover frequency (TOF) of such a surface state must be higher compared to the situation produced by a 2% O2 pulse providing the same amount of oxygen. This behaviour is in agreement with experiments performed at a high and low coverage of strongly adsorbed O atoms [16,17]. Regarding to the literature [15–17] the following transient reaction scheme involving L–H type elementary steps can be proposed: (a) Linear adsorbed CO molecules are reacting immediately with adsorbed O atoms. These O atoms are weakly adsorbed at the platinum surface without competition to linear adsorbed CO molecules due to occupation of different adsorption sites [16]: COads þ Oads weak ! CO2;gas
(3.1)
The reaction proceeds with a high TOF and produced CO2 is desorbing immediately at these high temperatures used in this study. (b) The CO coverage is decreasing very fast, dependent on the concentration of the applied O2 pulse. The higher the O2 concentration, the faster the decrease in CO coverage. This step is characterized by a Pt–CO ! Pt–O transition, where the O atoms are described as strongly adsorbed. Here a
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higher amount of available gas-phase O2 creates a higher coverage of strongly adsorbed O atoms. (c) CO readsorption is affected by the reaction of adsorbed CO molecules with strongly adsorbed O atoms at the platinum surface: COads þ Oads strong ! CO2;gas
(3.2)
Even if the platinum surface is saturated by O atoms Lynch and Hu [30] calculated, that the heat of adsorption of linear CO molecules is very close to that on a completely reduced surface indicating that the readsorption should not be hindered drastically. However the reaction rate of (3.2) has to be significantly lower than that of (3.1). The surface state is represented by a phase transition Pt–O ! Pt–CO. (d) Close to CO saturation a slow increase in the CO coverage can be observed. This step cannot be explicitly explained. On the one hand the slow decrease of gas-phase O2 in the reaction cell regarding to the tailing of the O2 pulse could cause a residual CO oxidation. By the interpretation of Bourane and Bianchi this step would be explained by reaction (3.1). On the other hand strongly adsorbed O atoms at defect sites could be involved following a reaction step regarding to (3.2) sustained by generally higher adsorption energies at low coverages. 3.2. Induction periods: influence of temperature and platinum particle size In this second part we will demonstrate the performance of a step-scan experiment with a high time-resolution concerning the induction period of a CO oxidation reaction over Pt/Al2O3 catalysts at elevated temperatures. In Fig. 6a and b the evolution of the CO coverage and the CO2 production, respectively, after introduction of 2% O2 pulses for 2 s and continuously 0.5% CO in Ar are depicted using a 4%Pt/Al2O3 catalyst at 443, 523 and 573 K. The data for CO coverage and CO2 production are plotted as integrated peak areas of the specific step-scan FT-IR absorption spectra of linear adsorbed CO species and the gasphase vibration of CO2 at 2348 cm1. The profile of the applied O2 pulse is also given indicating the entrance of O2 into the reaction cell at about 1070 ms. The overall absorption intensity of linear adsorbed CO species is varying with temperature. At 523 K a higher absorption intensity is observed compared to 443 K (spectra not shown for sake of brevity) and this has been detected by other authors, as well [31,32], interpreted as an adsorbate induced reconstruction of the platinum particle surface. At the highest studied temperature of 573 K absorption intensity is decreasing due to enhanced desorption of CO molecules. In addition, the usage of samples with slightly different amounts of catalyst probed by the infrared beam cause variations in the overall absorption intensity. Focussing onto the point when the CO coverage of linear adsorbed species is decreasing, it is obvious, that with increasing temperature this point is shifted to earlier times, e.g. 1850 ms at 443 K to 1150 ms at 523 K. The start of CO coverage decrease at 573 K is found at an intermediate time at 1250 ms, which is shown more clearly in Fig. 6b concerning the evolution of CO2
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Fig. 6. CO coverage of linear adsorbed CO species (a) and CO2 production (b) over a 4%Pt/Al2O3 catalyst at 443, 523 and 573 K after introduction of 2% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar obtained by step-scan mode; every fourth data point is plotted for step-scan data (5 ms resolution).
production. Here the pulse starting at 1070 ms is followed by the first pronounced CO2 production for pulses applied at 523 K, and then a CO2 production for pulses applied at 573 K can be observed at 1250 ms, indicating a delay of about 100 ms between both temperatures and a delay of about 80 and 180 ms regarding to the entrance of the pulse into the reaction cell at 1070 ms. For O2 pulses introduced at a catalyst temperature of 443 K the first CO2 production is detectable at about 1500 ms. Whereas the production of CO2 and the decrease in CO coverage for both higher temperatures are proceeding almost simultaneously, a delay of more than 400 ms can be found between the coverage decrease and the CO2 production at 443 K. Furthermore, the CO oxidation reaction is igniting about 100 ms earlier for the intermediate temperature at 523 K compared to 573 K. In this context it is interesting to investigate not only the temperature dependence of a transient CO oxidation but the particle size dependence, as well. In Fig. 7 the evolutions of the coverage of linear adsorbed CO species and of produced gasphase CO2 applying 2% O2 pulses for 2 s together with continuously fed 0.5% CO in Ar at 443 K are presented comparing a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst. Both catalysts have been characterized showing a high dispersion of 98 and 65%, respectively (see Section 2.1), assuming the particles of the 4%Pt/Al2O3 catalyst being slightly larger. The obtained step-scan data indicate a higher reactivity of the larger platinum particles concerning the decrease in linear adsorbed CO coverage and the CO2 production. The profile of the applied O2 pulse is depicted as well, revealing a pulse entrance into the reaction cell at about 1070 ms. A first CO2 production is detected at about 1450 ms for both catalysts whereas a decrease in the linear adsorbed CO coverage is found at about 2400 and 1850 ms for the 1%Pt/Al2O3 and 4%Pt/Al2O3 catalyst, respectively. This indicates a clear difference between both catalysts. Four basic questions concerning the main phenomena of these experiments presented in Figs. 6 and 7 have to be answered. (a) Why is the ignition of the start of the oxidation reaction shifting with different temperatures for a constant O2
pulse? (b) Why is the CO2 production starting prior to the reduction of linear adsorbed species, which is observable at lower temperatures? (c) Why is the reaction rate higher at larger particles? (d) Why is the ignition of the reaction not proceeding fastest for the highest temperature? Concerning the first question several authors have observed, that the duration from a step or a pulse release of O2 in an isothermal oxidation study to the start of reaction ignition is dependent on the oxidation temperature [33–35]. The main CO2 production arising after this delay originates from the oxidation of linear adsorbed CO species [23] and is called induction period. In the literature this induction period is coupled to an increase in the number of free Pt0 sites available for adsorption of oxygen species [33–35]. This increase is ascribed to the desorption of an adsorbed CO species. Because of a variation of the heat of adsorption of CO on platinum with temperature the induction period should increase with
Fig. 7. CO coverage of linear adsorbed CO species and CO2 production over a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst at 443 K after introduction of 2% O2 pulses of 2 s duration with a 0.5% CO continuous feed in Ar obtained by stepscan mode; every fourth data point is plotted for step-scan data (5 ms resolution).
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increasing heat of adsorption and, hence, with decreasing temperature [36]. The discussion of the influence of heat of adsorption of linear CO species on the induction period guides directly to question (b). At supported platinum particles not only linear adsorbed CO molecules are present at the surface but bridged and hollow site adsorbed molecules, as well. The number of these multiple bonded CO species is, however, rather small. Derrouiche and Bianchi [23] suggested, that the free Pt0 sites are formed by the removal of bridged adsorbed CO molecules during the induction period. The heat of adsorption of these bridged adsorbed species is lower than for linear adsorbed CO molecules and desorption at elevated temperatures should be preferred. They observed over a 1.2%Pt/Al2O3 catalyst at 300 K a partial desorption of these CO species under pure He flow, and at a switch to 2% O2 in He first the bridged adsorbed species were removed by reaction followed by the reaction of the linear adsorbed CO molecules [23]. However, they did not perform the experiments under a constant CO flow. For a constant 0.5% CO atmosphere it could be assumed that even at higher temperatures (443 K) the coverage of bridged adsorbed CO molecules is still high. Therefore, the removal of these CO species could be the origin of a CO2 production prior to a decrease in the coverage of linear adsorbed CO molecules. The free adsorption sites of the removed bridged species are then available for O2 species, if the concentration of O2 is high enough, and a high number of weakly adsorbed O atoms is generated inducing the ignition of the reaction with linear adsorbed CO molecules. If the bridged species desorb due to a competitive adsorption or react directly with O2 to CO2, as suggested in a modified Eley–Rideal mechanism by Dwyer and Bennett [33], is not clear. At higher temperatures the coverage of the bridged CO species is apparently so low, that the ignition process can be started due to a high number of free Pt0 sites for O2 dissociation. However, the lower sensitivity of the step-scan mode compared to the rapid-scan mode has to be taken into account. Hence, a very weak decrease in coverage of linear adsorbed species during the induction period is hardly detectable. The same problem does not allows us to investigate the evolution of the low number of bridged adsorbed CO species, which would sustain our suggestions. As presented in Fig. 7 the platinum particle dispersion has a huge effect on the reactivity concerning the CO oxidation reaction. It has been shown, that an increase of the dispersion of platinum particles decreases the rate constant of oxidation of adsorbed linear CO species at low temperatures and increases the heat of adsorption of bridged CO species [23]. Furthermore the heat of adsorption of these species is increasing with decreasing coverage [36]. In contrast, the heat of adsorption of linear adsorbed species does not depend strongly on the dispersion [23]. Derrouiche and Bianchi [23] did not observe desorption of CO during induction periods of transient isothermal CO oxidation experiments, indicating, that probably not a competitive chemisorption between oxygen and bridged CO species contributes to their removal but an oxidation of the bridged adsorbed CO molecules is preferred, which is sustained by the detection of a fraction of the CO2 production during the
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induction period. Finally, this means, that at larger particles the removal of bridged adsorbed CO molecules is favoured compared to smaller particles due to a lower heat of adsorption and as a consequence the number of O atoms, after O2 dissociation at these sites, is higher and ignition regarding to reaction (3.1) proceeds faster with a higher CO2 production rate. The last question (d) points onto another parameter of the ignition of the oxidation of linear adsorbed CO species. Referring to Section 3.1 and reaction (3.1) the transition of the CO covered platinum surface to an oxygen covered surface (Pt– CO ! Pt–O) is characterized by weakly adsorbed O atoms. These species are generated by dissociation of O2 molecules at free platinum sites, which are created due to the oxidation of bridged adsorbed CO molecules, probably [23]. As shown above increasing of the reaction temperature favours this last reaction. However, an increase of reaction temperature means a decrease in the heat of adsorption of the weakly adsorbed O atoms and finally a lower coverage of these species [14]. Because a higher level of gas-phase O2 is required to obtain a sufficient coverage of O species at 573 K compared to 523 K, a delay in the ignition of the linear CO oxidation is produced, as depicted in Fig. 6b. 4. Conclusions A time-resolved FT-IR study of the transient CO oxidation reaction over a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst triggered by O2 pulses comparing the step-scan and rapidscan acquisition modes of a commercial FT-IR spectrometer has been performed. Time-resolutions of 5 ms in the step-scan mode compared to several hundred milliseconds in the normal rapid-scan mode could be achieved. O2 pulses with concentrations of 1, 2 and 3% for 1 and 2 s duration at reaction temperatures of 443, 523 and 573 K and a constant flow of 0.5% CO in Ar have been applied. The data obtained by step-scan and rapid-scan mode are in good agreement, showing a lower signal-noise ratio and therefore a lower sensitivity for small IR absorption intensity changes for the step-scan mode. However, a high time-resolution, in principle much higher than the chosen 5 ms, can be used for studying fast transients in heterogeneously catalysed reactions. By utilizing the high time-resolution of the step-scan acquisition mode we could obtain new details of the dynamics of the CO oxidation reaction over a supported Pt catalysts at high temperatures in the range of 443–573 K and at 1 atm pressure. The obtained data reveal different elementary steps involving weakly adsorbed O atoms for the surface transition from Pt–CO ! Pt–O and strongly adsorbed O atoms for the surface transition from Pt–O ! Pt–CO. We could show, that not only the O2 concentration is influencing the reaction rate, but the distribution of O2 within a pulse has an effect on the surface transition and on the reactivity, as well. By means of the step-scan mode induction periods of the CO oxidation reaction could be investigated in detail at elevated temperature. Here the oxidation of bridged adsorbed CO species probably plays a key role in the evolution of the induction period. We could show
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that an intermediate temperature of 523 K generates the shortest induction period compared to reaction temperatures of 443 and 573 K. Finally, the comparison of the transient CO oxidation reaction between a 1%Pt/Al2O3 and a 4%Pt/Al2O3 catalyst reveals the importance of controlling particle dispersions during catalyst preparation indicated by a higher reactivity due to a shorter induction period for the catalyst containing larger particles. Within this study we could demonstrate the performance of the high-time resolution step-scan FT-IR spectroscopy, which will, hopefully, be an additional tool gaining a better understanding of heterogeneously catalysed reaction mechanisms in the future. Acknowledgements This work has been performed within the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Agency and the member companies: AB Volvo, Johnson Matthey-CSD, Saab Automobile Powertrain AB, Perstorp AB, AVL-MTC AB, Albemarle Catalysts and The Swedish Space Corporation. The Swedish Research Council is greatly acknowledged for financial support. References [1] G. Ertl, in: J.R. Anderson, M. Boudart (Eds.), Catalysis, Science and Technology, vol. 4, Springer, Berlin, 1983, p. 209. [2] T.S. Askgaard, J.K. Nørskov, C.V. Ovesen, P.J. Stoltze, J. Catal. 156 (1995) 229. [3] T. Engel, G. Ertl, Adv. Catal. 28 (1979) 1. [4] L.F. Razon, R.A. Schmitz, Catal. Rev. Sci. Eng. 28 (1986) 89. [5] R.H. Nibbelke, M.A.J. Campman, J.H.B.J. Hoebink, G.B. Marin, J. Catal. 171 (1997) 358.
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