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TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions
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Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions Jerry Pui Ho Li, Zebang Liu, Hao Wu, Yong Yang ∗ School of Physical Science and Technology, ShanghaiTech University,100 Haike Road, Shanghai, China
a r t i c l e
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Article history: Received 13 October 2016 Received in revised form 17 February 2017 Accepted 24 February 2017 Available online xxx Keywords: Au/TiO2 CO catalytic oxidation MS Perimeter sites Temperature programmed reaction
a b s t r a c t Using a temperature-programmed online MS system, gas products are directly analyzed at low temperature to understand the reactivity of the surface intermediates on an Au/TiO2 catalyst that exhibits catalytic activity for CO oxidation. CO oxidation was performed over Au/TiO2 catalysts with Au particle size of 3 nm, and a 7.3 wt% loading, focusing on CO-TPD, steady state isothermal titration, high temperature TPR, and low temperature selective TPO experiments. With CO-TPD, we were able to observe directly, the presence of two different CO species adsorbed on the catalyst surface, as well as the respective desorption energies. Steady state isothermal titrations showed two transient processes with different redox sequences, both forming catalytic layers for intermediate storage and subsequent CO2 product formation. High temperature CO titration transients with 200 ◦ C pre-oxidation following either isothermal CO titration or high temperature TPR highlights the presence of two oxidative surface intermediate species with different kinetics, with the intermediates purposed as atomic oxygen at the Au NP perimeter and neighboring lattice sites. Finally, low temperature selective TPO beginning at −165 ◦ C showed that prereduction following oxidation up to −100 ◦ C is able to partially convert surface CO species into stable CO2 intermediates, as a higher activation energy was required to fully desorb the CO2 product from the catalyst surface. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Over the past two decades, there has been increasing interest in the use of oxide-supported nanoparticle Au catalysts [1–4]. Catalysis by Au continues to be one of the fastest growing areas in research for chemical science today [5]. In particular, the use of metal oxide supports such as Au/TiO2 , Au/CeO2 and Au/Fe2 O3 due to their high catalytic activity for a variety of oxidation and reduction reactions at low temperature (LT) [6–8]. The catalytic oxidation of CO is of particular importance for fuel gas processing. This is important for the selective removal of CO from H2 rich syngas can then be applied to fuel cells, and has been adapted for use in industry [9,10]. Within the literature, there have been extensive efforts to clarify the reaction mechanism and the physical origin of the high catalytic activity observed for Au catalysts. It has been found that Au nanoparticles (NP) have significantly enhanced catalytic activity
∗ Corresponding author. E-mail addresses: [email protected], jerryli @hotmail.com (Y. Yang).
compared to bulk Au [11,12]. Au NP catalysts over TiO2 surfaces are of particular interest [13,14], where not only do Au cluster formation occur at much higher density than other surfaces which allows for a higher loading of Au [15], but an improved resistance to Au cluster sintering due to defect sites on the support [16]. It has also been well documented from experiments that at LT O2 does not readily chemisorb on Au clusters larger than ∼1 nm [17–19], or Au supported on TiO2 [20] either dissociatively or molecularly. Though these studies do provide some useful knowledge, it is important to consider that these studies are operated using single crystal support surfaces with experiments performed often under vacuum conditions. Thus on a powder surface and under more ambient conditions, the mechanisms proposed may vary in comparison as an effect of pressure and materials gap. More recent studies on powder samples in the field have focused on two key parts: CO adsorption, and O2 activation. There is general agreement that at LT CO is largely adsorbed on the Au nanoparticles, as highlighted by some key investigations in the literature [21,22]. Investigations from Green et al. extend upon this, observing adsorption at not only on the metallic Au NP, but also adsorption over Ti sites. [23,24], observing that in agreement with Haruta et al. [1],
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Please cite this article in press as: J.P.H. Li, et al., Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.031
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CO oxidation already occurs at −70 ◦ C. Green et al. using IR studies, observed dual perimeter sites, whereby under LT, CO is adsorbed on both the metallic Au and the Ti interface, as IR bands assigned to COAu and CO-TiO2 were observed at −153 ◦ C. Green et al. goes further to suggest that the activity on the perimeter sites occurs through O-O scission, activated by formation of CO-O2 complexes at Ti-Au sites found on the Au/TiO2 interface, induced by pre-reduction with CO [23]. O2 activation is another key aspect to CO oxidation that requires clear resolution. This includes the nature of the active adsorbed oxygen species, whether they are in dissociated form or in molecular form, and its location on the catalyst, i.e. on the Au particle, on the TiO2 support surface, or at the perimeter of the interface between Au and Ti. Koga et al. proposes that the active oxygen can exist as both dissociated form and molecular form, depending on the concentration of oxygen relative to the contact area which they believe to be located at the Au-Ti perimeter, with coordination primarily at the Ti sites [25]. Widmann et al. go further in proposing that the catalytic activity is dependent on oxygen vacancies on the TiO2 lattice; particularly along the Au-Ti perimeter, which are the key reason for the high activity of CO oxidation [26]. These studies would strongly suggest that oxygen activation is more crucial than the CO adsorption itself. However, findings by Green et al. show that pre-treatment with O2 prior to CO introduction inhibits activity [24]. It is clear that there is agreement that the key active sites are found along the Au-Ti perimeter, but whether the key steps involve CO2 intermediate formation, or formation of active oxygen species, is yet to be fully conclusive. Though the catalytic activity of Au nanoparticles have been well documented, it is crucial that one is able to quantify the active sites on the catalyst surface. Studies in this field have focused on characterizing the catalyst active sites and relating them to reaction data. In addition, direct observations of CO desorption at subzero temperatures as well as CO2 desorption at the same conditions, has to our knowledge, not been investigated; particularly over powder catalysts instead of a model single crystal surface. Direct observation of the desorption of these species will provide key data for the energy barrier required to facilitate desorption. In the work reported here, we characterize the catalytic sites activity of the Au/TiO2 NP involved in the CO oxidation reaction, and demonstrate that under LT conditions, the CO oxidation process significantly differs from that at room temperature to high temperature (HT) region. Titration experiments were performed at a temperature region of in −165- 200 ◦ C, with various programmed experiments at ambient pressure with flow of CO or O2 . We aim to show that under different temperature conditions, both the CO oxidation mechanisms previously presented by Green et al. [23,24,27] and Widmann et al. [26,28] are valid for transient CO2 products. Detailed kinetic parameter fitting indicates two different surface active sites with new online quantification measurements.
1.1. Catalyst preparation Au/TiO2 was prepared using the deposition-precipitation (DP) method as outlined by Zanella et. al [29] which was the same catalyst used in studies by Green et al. [23]. Briefly, 1 g of TiO2 was added to 100 mL of an aqueous solution of HAuCl4 (4.2 × 10−3 M) and of urea (0.42 M). The initial pH was ∼2. The suspension was held at 80 ◦ C and was vigorously stirred for 4 h, where by the pH increased. The suspension was then centrifuged, washed, dried, and calcined at 300 ◦ C.
1.2. Catalyst characterization The total surface area of the catalysts was measured by nitrogen adsorption at −196 ◦ C using a Quantachrome Instruments Autosorb iQ2. Analysis showed a BET surface area of 59 m2 /g with a type I isotherm. Powder X-ray diffraction (XRD) of the catalysts was obtained using Cu K␣ radiation using a Bruker D2 Phaser diffractometer. Diffractograms were collected in the 2 angle range from 10◦ to 80◦ with the 0.008◦ 2 step resolution. The catalyst sample was composed of 7.3% Au, 82.1% TiO2 in the anatase phase, and 10.6% TiO2 in the rutile phase. Average size of the Au particles was found to be in the range of 3 nm. This was calculated using the Scherrer equation. The results are shown in Fig. S1 from the supporting information. Transmission electron microscopy measurements, conducted using a Fei TF20 Tecnai G20, showed an average particle size in the range of 2.8 nm, which is in agreement with the XRD analysis. The results are shown in Fig. S2 from the supporting information. Combining the XRD and TEM results and assuming the surface lattice density as 1.0–1.3 × 1015 /cm2 , this yields surface gold sites of about 120–150 mol/g. 1.3. Reactions The full detailed setup was introduced in a previous publication [30]. In each of these experiments, reactant gas mixtures were prepared in flow-controlled manifolds. In all experiments reported, 100 sccm of total gas flow was used with total pressures of 1.4 bar. In all processes of the MS measurements results, the small signal flow rates are calibrated with 36 Ar signal as internal standard with fragmentation correction. The activity of the catalyst was measured in a high pressure microreactor using a quartz tube reactor vessel. Gas analysis was achieved with a Pfeiffer Prisma quadrupole mass spectrometer, observing the key reactants using the following mass spectrum signals: CO (m/z −28), O2 (m/z −32), and CO2 (m/z −44). 1.3.1. Low temperature (LT) CO oxidation reaction The sample was first tested for its activity in LT CO oxidation reaction. 100 mg sample was loaded in the middle of reactor 6 mm quartz tube with an 100 sccm gas mixture (CO:O2 :Ar = 20:10:70) flowed though. The reactor temperature (Fig. 1, thick dashed line) was first cooled down to −150 ◦ C and then heated up with ramping rate of 10 ◦ C/min. 1.3.2. Isothermal CO titration The CO titration procedure is detailed elsewhere in previous work [31]. Briefly, 50 mg of the catalyst was heated to 200 ◦ C under Ar flow, and held for 30 min. During this time, the catalyst was oxidized in a flow of 3% O2 . This was followed by flushing in an Ar stream, followed by cooling to the desired reaction temperature. An injection of 3% CO was then put into the system. 1.3.3. CO-TPD and Temperature Programmed Oxidation – Temperature Programmed Desorption (TPO-TPD) The CO-TPD procedure involved heating the catalyst (100 mg) was first heated to 200 ◦ C under Ar flow. It is well documented that above certain temperatures, the Au/TiO2 begins to sinter [32]. After heating, the catalyst was cooled to −170 ◦ C and held for 1 h, where during this time 10% CO was introduced. This is followed by purging with Ar flow. The catalyst was then heated at a ramp rate of 10 ◦ C/min heating up to 100 ◦ C. The TPO procedures are divided into a full TPO and a selective oxidation experiment. The catalyst (100 mg) is first heated to 200 ◦ C under Ar flow and held for 30 min. After heating, the catalyst was cooled to −170 ◦ C and held for 10 min, where during this time 3%
Please cite this article in press as: J.P.H. Li, et al., Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.031
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Fig. 1. Activation and deactivation vs. temperature of CO oxidation on AuTiO2 by Co-feeding 20% CO and 10% O2 . Showing the temperature profile (dashed line) used during the course of the reaction, and the normalized flowrate of a) CO (m/z −28) and b) CO2 (m/z −44) vs time.
CO is introduced for surface reduction. After, CO is switched with Ar flow. For the full TPO procedure, a flow of 3% O2 was introduced. During this procedure, the catalyst was heated at a ramp rate of 10 ◦ C/min, heating up to 100 ◦ C. The selective oxidation procedure, uses the same catalyst CO reduction procedure as the full TPO procedure. After the pre-treatment (heating to 200 ◦ C and CO reduction/Ar flushing procedure at −150 ◦ C), a flow of 3% O2 was introduced. During this procedure, the catalyst was heated at a ramp rate of 10 ◦ C/min heating up to −100 ◦ C and held for 10 min. O2 flow was switched to an Ar flow, followed by cooling back to −150 ◦ C. Under Ar flow, the catalyst was heated at a ramp rate of 10 ◦ C/min heating up to 100 ◦ C. 1.3.4. Temperature Programmed Reduction (TPR) – High Temperature (HT) The HT TPR experiment utilized 100 mg of the catalyst and was first heated to 200 ◦ C under Ar flow, and held for 30 min. During this time, the catalyst was reduced in a flow of 3% CO. This was followed by flushing in an Ar stream, followed by cooling. After the catalyst reduction at 200 ◦ C, the catalyst was treated with a flow of 3% O2 for 1 h. This is followed by purging with Ar flow. The catalyst was cooled to 10 ◦ C, followed immediately by an introduction of 3% CO stream. The TPR procedure was performed in the temperature range from 10 to 280 ◦ C at a ramp rate of 10 ◦ C/min. 2. Results and discussion 2.1. LT CO oxidation reaction The sample was first tested for its activity in LT CO oxidation reaction. 100 mg sample was loaded in the middle of reactor 6 mm quartz tube with an 100 sccm gas mixture (CO:O2 :Ar = 20:10:70) flowed though. The reactor temperature (Fig. 1, thick dashed line) was first cooled down to −150 ◦ C and then heated up with ramping rate of 10 ◦ C/min. Both CO (Fig. 1a) and CO2 (Fig. 1b) MS signals are plotted, with all the fragmentation factors corrected. At −67 ◦ C the oxidation reaction started and CO flow rate output immediately dropped to zero level and 100% CO2 conversion was observed afterward. Heating was continued to 100 ◦ C and held at that temperature for additional 10 min. Afterwards the sample was cooled
back to −150 ◦ C using the same rate of 10 ◦ C/min. At approximately −100 ◦ C the oxidation reaction stops with CO2 conversion dropped sharply to zero and while CO output signal recovered. This is a typical LT performance of CO oxidation on Au/TiO2 sample with NP loaded [1].
2.2. CO-TPD To observe the CO adsorbates desorption behavior, CO exposure was performed at −165 ◦ C by flowing 10% CO diluted in Ar with total flow rate of 100 sccm for 10 min. The sample was thus flushed in pure Ar flow for 20 min and heated up in the same flow with ramping rate of 10 ◦ C/min. The results obtained at LT conditions showed clearly that the desorption profile (Fig. 2, main panel) of CO desorption. CO (m/z − 28) desorption signal appears at ∼−135 ◦ C and formed a shoulder, found at −113 ◦ C. The slope of desorption curve became steeper again after temperature surpasses −90 ◦ C, and formed a main peak at −55 ◦ C. The total desorption yields approximately 25 mol of CO. This equates to all the CO adsorption sites on the catalyst surface which incorporates the sum of chemisorbed surfaces of the Au NP and TiO2 surface. Comparing previous temperature desorption studies observed using IR by Green et al. [10,12], these desorption features may be connected with specific IR assignment in that study. The inset of Fig. 2 is from Fig. 2C inset of that report. In that report, two CO adsorbates features were observed on CO exposure saturated Au/TiO2 surface, assigned as CO on TiO2 sites and CO on Au sites. As temperature increases, the CO/TiO2 feature coverage decreases sharply at ∼−113 ◦ C and the CO/Au feature coverage decreases to ∼50% at −58 ◦ C. Considering the derivative relation between the CO total coverage and desorption rate, our TPD results are in agreement with this previous IR intensity behavior obtain during similar thermal desorption process. Thus the shoulder that centers at −113 ◦ C in our results is assigned to the CO/TiO2 IR feature observed by Green et al. while the main peak at −55 ◦ C is assigned to be the CO/Au chemisorbed species in that study [10,12]. With a rough estimation of the ratio between the shoulder and the main peak picked between 1:6 and 1:3, the main peak area falls between 18 and 21 mol. This yields the main peak related surface CO chemisorption sites to be 180–210 mol/g. This is about
Please cite this article in press as: J.P.H. Li, et al., Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.031
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Fig. 2. TPD plot of CO desorbed versus temperature (CO signal taken at m/z −28) after −165 ◦ C CO pre-exposure; inset: The normalized integrated IR absorbance of CO/Au and CO/TiO2 against temperature from Yates et al. [23].
the same scale as the earlier Au metallic surface area estimation based on our TEM and XRD diameter measurements. 2.3. Isothermal CO titration CO titration experiments were performed on oxidized sample to quantify the surface oxygen storage after pre-oxidation at a certain temperature. Two types of temperature controlled titration process were conducted as isothermal reduction and temperature programmed reduction (TPR). The sample was first exposed to 3% O2 at 200 ◦ C, as previously described. CO isothermal titrations were performed at a lower fixed temperature by flow 3% CO diluted in Ar carrier gas. Fig. 3a shows four calibrated CO2 product flow rate measured by online MS at 40, 80, 100 and 130 ◦ C (Fig. 3, panel a, curve a–d). At 40 ◦ C, the CO titration yields almost no CO2 products (Fig. 3, panel a, curve d). At 80 and 100 ◦ C titration product are significant, with the respective curves (Fig. 3, panel a, curves b and c) being very close to each other. The 130 ◦ C (Fig. 3, panel b, curve a) titration yields CO2 product that is around two times the amount of the previous two curves. The full plots of the tabulated CO2 total products from each curve obtained at all temperatures is showed in Fig. 3, panel b. Overall the isothermal temperatures covered the temperature range from 40 ◦ C to 200 ◦ C, thus 17 temperatures in total. For some of the isothermal temperatures, repeat measurements was performed for consistency. At the same isothermal temperature, the difference in the pre-oxidation temperature does not affect the CO2 products significantly, with repeat measurements showing good consistency, being within ±10%. At lower temperatures from 30 to 60 ◦ C the titrations yield almost no CO2 , indicating there is little CO oxidation activated on the surface. Between 70 and 100 ◦ C, the CO2 products kept at a constant level of 4.0 ± 0.2 mol/g which suggests there is only a uniform oxygen species activated for surface CO oxidation in this temperature region. This is a very small amount compared to either the surface Au metallic sites (<3%) or oxygen sites on the oxide («1%). Thus such storage of surface oxygen may not be directly related with either of these two sites. At higher temperatures as titration increase from 110 to 200 ◦ C, the total CO2 products increase up to more than 10 times and the highest CO2 products are at around 50 mol/g. This indicates that at tempera-
tures higher than 100 ◦ C, there is secondary surface oxygen species which is activated during in CO oxidation. This assumption is also supported by the shape of the curves in Fig. 3 panel a. At both 80 and 100 ◦ C, the descending sides of CO2 product peaks decays smoothly (curves b and c), but the 130 ◦ C peak appears with 2 decay rates. This trend is more obviously in the inset of panel a, by directly plotting the linear dependence between adsorbates coverage in titration decay curve and time (Fig. 3, panel a, inset). Here after several fitting attempts, the 2nd order desorption model is found to be the best to fit the decay curves (see supplementary materials) The reciprocal of the 80 ◦ C titration curve integration (inset, c), of CO2 * coverage, shows good linear dependence on time, as predicted by the 2nd desorption order model. Meanwhile, the 130 ◦ C curve (inset, a) shows an obvious slope change at the same time. 2.4. HT TPR The results of the HT TPR experiment is illustrated in Fig. 4. The temperature ramping starts at −15 ◦ C. A very small yet repeatable initial CO2 peak was found at 29 ◦ C, with the integration yields only about 2 mol/g active surface sites related. The full curve of this peak is amplified in the inset of Fig. 4. A much larger and broad peak with CO2 product formed was observed at 140 ◦ C. The total integration of this peak shows there is 150 mol/g of intermediate involved in this temperature region. In this TPR process, at 40 ◦ C the desorption rate is zero, while the two observed desorption peaks do not overlap each other. This implies that there are two different desorption species which contribute to the CO2 products. Applying first order reaction model to the Redhead equation [33], the two peak temperatures convert to 0.89 eV and 1.23 eV CO2 formation barrier respectively. The results of TPR along with the isothermal CO titration both prove that there are two different CO2 intermediate species formed on the pre-oxidized sample surface with CO flow exposure. One species is of very small amount, less than 3% of the total gold metallic sites, and with a smaller surface formation barrier for CO2 production. As reaction temperature increases, the second surface intermediates show up with a higher and broadly distributed energy formation barrier for additional CO oxidation. This result
Please cite this article in press as: J.P.H. Li, et al., Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.02.031
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Fig. 3. Panel a): CO2 product from 200 ◦ C pre-oxidized Au/TiO2 in isothermal CO titration at: a) 130 ◦ C, b) 100 ◦ C, c) 80 ◦ C, and d) 40 ◦ C (Inset: Comparison of 80 ◦ C and 130 ◦ C decay curves; 1/CO2 * coverage vs time, Panel b): Tabulated results of total CO2 desorbed versus experiment temperature (Inset: expanded view of plateau region).
is similar with the earlier TAP studies [26,28] with similar reaction condition mentioned in the introduction part. After Au/TiO2 surface pre-oxidized in O2 ambient at 400 ◦ C, cyclical CO and O2 pulses were applied on the surface yield reversible and stoichiometric pulse losses to surface CO2 products, which is quantified as surface oxygen storage capacity (OSC). Measurements from 80 to 400 ◦ C with interval of every 80 ◦ C, the OSC was also found strongly depends on the temperature of the TAP measurement. At 80 ◦ C, the oxygen storage sites were assigned to lattice oxygen close around the Au NP perimeter. At higher temperatures, the additional oxygen species are assigned to additional accessible neighboring lattice oxygen sites. Comparing the similarity with our results, the Au NP
perimeter lattice oxygen sites are assigned to the CO2 intermediates in 70–100 ◦ C isothermal CO titration, i.e. the plateau in Fig. 3, panel b and the lower peak of TPR results (Fig. 4). And the neighboring lattice oxygen sites are assigned to the intermediates of the additional CO2 in higher temperature CO isothermal titration and the TPR large peak. 2.5. LT selective TPO As CO TPD results shows, stable CO storage on Au/TiO2 system may only be available at lower subzero temperature. To study the CO oxidation activity of the LT adsorbed CO species, LT selective TPO
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Fig. 4. Au/TiO2 TPR results under CO flow after 200 ◦ C pre-oxidized, showing calibrated CO2 flowrate versus temperature (Inset: zoomed in view of 30 ◦ C TPR curve region in main panel).
is performed as detailed as follows: The catalyst was first exposed to CO flow at −165 ◦ C and purged by Ar flow, The system was then exposed to 3% O2 flow and heated to −100 ◦ C at 10 ◦ C/min, held for a period of time, and followed with purging by Ar flow while cooling back to −165 ◦ C. Finally the sample in pure Ar was ramped from −165 ◦ C to 100 ◦ C to desorb all surface carbon species formed after the previous oxidation treatment. Both CO and CO2 species were found in the whole process and the MS signal are plotted in Fig. 5, panel a (a, b, c) along with the temperature vs. time. The total carbon balance is compared with the CO TPD total yields (Fig. 2) and the relative products ratio from each temperature interval is plotted in Fig. 5, panel b. During the oxidation section below than −100 ◦ C, only CO desorption was observed, accounting for 10% of total carbon yields. No CO2 products was found in this section. The CO desorption rate reached the maximum as the ramping temperature stopped at −100 ◦ C. In the second temperature ramping session in pure Ar, CO desorption resumed as the temperature reached −100 ◦ C and reached its maximum at −52 ◦ C; a similar desorption temperature as the CO TPD main peak shown in Fig. 2. CO2 desorption from the surface also observed in this section which started at higher temperature of −75 and peaked at −12 ◦ C. The CO and CO2 desorption in the second temperature ramp each counted for 63% and 27% of total carbon yields, respectively. The selective oxidation and the following ramping indicated that during the first ramp to −100 ◦ C, simultaneous oxidation on CO adsorbed catalyst was able to produce stable CO2 surface intermediates at this temperature. This process provides another CO2 product procedure from elementary steps operation on an initially different prepared surface of AuTiO2 which was LT CO reduced. Meanwhile the majority of the original CO adsorbates (73% by combining CO desorption in the two ramping process) during the oxidation remained unchanged and desorbed as CO product. Comparing the desorption of CO in the both ramping process with the CO TPD, the behaviors are almost identical as following: 1) In the initial ramping from around −165 ◦ C, CO desorption started at around −130 ◦ C, whether or not oxygen was present. 2) In the second ramp the desorption peak of CO resembles the TPD main peak with the same peak temperature. Combining all these observations, it is concluded that the oxidation of the surface CO is not complete The upper limit of oxidized species at −100 ◦ C is 27% while the remaining species remain on the surface bonding status similar to the original. The oxidation process may either generated a surface 1) with co-adsorbed oxygen and CO species, which is similar to the
LT Pt CO pre-adsorbed surface [34,35] or 2) with the surface CO partially oxidized into CO2 * adsorbates or CO3 −2 group on the surface at this temperature region. In previous CO LT oxidation studies based on IR methods, Green et al. reported that at −153 ◦ C, on a similar Au/TiO2 catalyst, the surface pre-adsorbed CO species, assigned to TiO2 sites, oxidized to CO2 species bonded to titania support upon O2 exposure, while another CO adsorbates assigned to Au sites remains the same absorbance intensity. No gaseous CO2 at this temperature was mentioned. This result provides a CO2 intermediate explanation compatible to the second assumption above for the selective oxidation reported. However, in order to directly confirm the IR CO2 /TiO2 species with the CO2 gas product after the selective oxidation, further online experiments such as online IR-MS characterization of the same reaction process is required.
2.6. Discussion Different models have been purposed for CO oxidation reaction over AuTiO2 catalysts. Generally, most viewpoints are in agreement where both Au metallic and Au/TiO2 perimeter sites play important roles [23,26,36,37]. With the above experiments performed, we scanned the temperature region from liquid nitrogen temperatures to 200 ◦ C for several steady states and transient reactions that produce CO2 gas products, including co-feeding CO and O2 , CO reduction-oxidation and oxidation-CO reduction sequences with precise temperature controlling. The Table 1 summarizes the main experimental results associated with the reaction products. The last column also provides the purposed catalytic active sites based on earlier discussion for the involved key intermediates by comparing the consistency of product amount, characteristic temperature combining our kinetics measurements results, previous characterization from IR and TAP measurements and the TEM, XRD results. Both transient reaction with different sequence of reduction and oxidation were able to produce CO2 , however, under very different condition and with very different product behaviors. Both HT TPR/isothermal CO titration and LT TPO generates catalytic active AuTiO2 surface after initial oxidation/reduction with surface storage of O*/CO* species as intermediates of final CO2 products. However, the reaction conditions and CO2 product characteristics are dissimilar to each other. In summary:
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Fig. 5. Panel a),TPO desorption profile of the selective oxidation experiment. The procedure was as follows; Zone I: Introduction of O2 stream over pre-reduced catalyst, Zone II: Temperature ramping from −150 ◦ C to −100 ◦ C under O2 stream, Zone III: Temperature held at −100 ◦ C, Zone IV: Cooling to −150 ◦ C and removing O2 stream. Zone V: Temperature ramping from −150 ◦ C to 100 ◦ C in pure Ar stream. Showing the MS desorption signal calibrated to flowrates of a) – CO (1st temperature ramp), b) – CO (2nd temperature ramp) and c) – CO2 , Panel b), Distribution of the different desorbed carbon species from the selective oxidation experiment.
Table 1 Summary of the main experimental results associated with the reaction products. Desorption/products
Method
Uptake per gram of catalyst (mol/g)
Tp (◦ C)
Reaction barrier (eV)
Purposed intermediate site
CO2
Isothermal CO titration
4.0 for 70–100 ◦ C plateau 4.0–50 for higher T 27% of total C uptake 2.0 150
n/a
n/a
Au perimeter oxygen
−12 29 140
0.77 0.89 1.23
n/a Au perimeter oxygen Au perimeter neighboring lattice oxygen
250
40–70 180–210 10% of total C uptake1st ramp 63% of total C uptake2nd ramp
−113 −55 n/a −52
0.47 0.64 n/a 0.64
TiO2 surface CO Au metallic surface CO TiO2 surface CO (mostly) Au metallic surface CO (mostly)
135–170
n/a
n/a
n/a
Selective oxidation LT TPR HT CO
TPD Selective oxidation LT
Total Au NP physical surface sites
TEM, XRD
Au perimeter neighboring lattice oxygen
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1) Catalytic surface activation temperature: In the isothermal process, CO exposed on pre-oxidized AuTiO2 surface with oxygen storage at perimeter sites yield no reaction products below 60 ◦ C. The activation of perimeter oxygen with CO titration only starts after 70 ◦ C. The HT TPR also indicates a similar species only active at room temperature or higher. On the side of LT TPO, on CO preexposed surface, even at temperature as low as −100 ◦ C, oxygen already reacts with surface CO species and forms stable intermediates layer which itself directly produce CO2 without additional CO or oxygen refill at higher temperature. Compared to the initial LT CO oxidation reaction (with CO/O2 co-feeding) in Fig. 1, the CO2 does not appear until −12 ◦ C. The appearance of the intermediate though, will require further spectroscopic analysis, which will also provide clarity to the reaction mechanism during this particular reaction pathway. 2) Amount of active sites: In the TPR process, CO2 desorbs from the catalytic surface upon CO exposure over 70 ◦ C, and the amount of products is limited by the purposed perimeter sites, i.e., surface oxygen storage capacity, which is less than 3% of the Au NP surface sites. On the selective oxidation side, the surface provides stable CO2 intermediates storage after −100 ◦ C oxygen exposure. This storage, which counts 27% of the carbon adsorbates at LT CO adsorption, is therefore more than the purposed perimeter sites. Applying the TEM results of 3 nm diameter average for Au NP and assuming 0.3 nm lattice constant, the maximum percentage of perimeter sites is no larger than 4%. Thus the oxidation up to −100 ◦ C must activate additional surface sites. 3) The temperature that surface produce CO2 : Comparing the CO2 desorption initial temperature, the selective oxidation process also starts produce gas CO2 at least 80 ◦ C lower than either the isothermal titration or the TPR processes. It is also noted here that these surface CO2 precursors are more kinetically stable than any of the CO surface species, when comparing to all CO desorption in this study.
sequences were both found forming catalytic layer for intermediates and produce CO2 products. The HT CO titration transients, with 200 ◦ C pre-oxidation following either isothermal CO titration or TPR above room temperature, indicates there are two oxidative surface intermediates species with different kinetics. The two intermediates are purposed as atomic oxygen at Au NP perimeter site and neighboring lattice sites. The LT selective oxidation, with −165 ◦ C CO pre reduction following oxidation up to −100 ◦ C, is able to partially convert surface CO species into a more stable CO2 surface intermediates. Detailed measurement and calibration are performed to obtain the kinetics and product quantification for above reactions. The results support previous findings in the literatures which show activity at the same temperature conditions. The experiments also provides methods for direct surface active sites quantification with one step transient online MS measurements for both Au NP surface and perimeter sites. Acknowledgements The authors would like to thank the startup funding provided by ShanghaiTech University, the National Natural Science Foundation of China (No. 21573148) and Shanghai Pujiang Program (No. 15PJ1405800) for funding their participation in this work. This work was conducted in the facility of School of Physical Science and technology in ShanghaiTech University. The authors would like to thank Mimi Dai and Minghui Fan for their assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.02. 031. References
Previous studies in the literature have focused primarily at either liquid nitrogen temperature regions or from room temperature and above. Here we have performed both procedures by using real-time MS sampling from simulated reaction conditions. Whether these two CO2 products share the same reaction pathway or not, this will involve further detailed and thorough kinetics investigation and theoretical modeling. The work reported here for both CO oxidation results, further supports results from other results, indicating that the perimeter site certainly plays a key role in oxygen dissociation and further CO2 intermediates formation. By selecting proper temperature controlled titration method, quantification of the two characteristic active sites on AuTiO2 surface, Au cluster perimeter sites and Au metallic surface sites, may be directly measured. The LT CO TPD may be adapted for Au metallic sites quantification. And the isothermal titration plateau region appears to be a good probe for the Au cluster perimeter sites. It should be taken into consideration that Au cluster activity varies significantly according to the average size and the interaction with support materials, thus the measurement procedure may need proper adjustment depends on the catalyst conditions. 3. Conclusions CO oxidation experiments was performed over Au/TiO2 catalysts with Au particle size of 3 nm, and a 7.3 wt% loading in a broad temperature range from liquid nitrogen to 200 ◦ C. The catalyst low temperature (LT) CO oxidation reactivity was measured with online MS. LT CO TPD yielded two CO species at different temperature ranges and bonding status was assigned to surface TiO2 and Au island surface sites. Two transient processes with different redox
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Investigation of CO oxidation over Au/TiO2 catalyst through detailed temperature programmed desorption study under low temperature and Operando conditions Jerry Pui Ho Li, Zebang Liu, Hao Wu, Yong Yang ∗ School of Physical Science and Technology, ShanghaiTech University,100 Haike Road, Shanghai, China
a r t i c l e
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Article history: Received 13 October 2016 Received in revised form 17 February 2017 Accepted 24 February 2017 Available online xxx Keywords: Au/TiO2 CO catalytic oxidation MS Perimeter sites Temperature programmed reaction
a b s t r a c t Using a temperature-programmed online MS system, gas products are directly analyzed at low temperature to understand the reactivity of the surface intermediates on an Au/TiO2 catalyst that exhibits catalytic activity for CO oxidation. CO oxidation was performed over Au/TiO2 catalysts with Au particle size of 3 nm, and a 7.3 wt% loading, focusing on CO-TPD, steady state isothermal titration, high temperature TPR, and low temperature selective TPO experiments. With CO-TPD, we were able to observe directly, the presence of two different CO species adsorbed on the catalyst surface, as well as the respective desorption energies. Steady state isothermal titrations showed two transient processes with different redox sequences, both forming catalytic layers for intermediate storage and subsequent CO2 product formation. High temperature CO titration transients with 200 ◦ C pre-oxidation following either isothermal CO titration or high temperature TPR highlights the presence of two oxidative surface intermediate species with different kinetics, with the intermediates purposed as atomic oxygen at the Au NP perimeter and neighboring lattice sites. Finally, low temperature selective TPO beginning at −165 ◦ C showed that prereduction following oxidation up to −100 ◦ C is able to partially convert surface CO species into stable CO2 intermediates, as a higher activation energy was required to fully desorb the CO2 product from the catalyst surface. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Over the past two decades, there has been increasing interest in the use of oxide-supported nanoparticle Au catalysts [1–4]. Catalysis by Au continues to be one of the fastest growing areas in research for chemical science today [5]. In particular, the use of metal oxide supports such as Au/TiO2 , Au/CeO2 and Au/Fe2 O3 due to their high catalytic activity for a variety of oxidation and reduction reactions at low temperature (LT) [6–8]. The catalytic oxidation of CO is of particular importance for fuel gas processing. This is important for the selective removal of CO from H2 rich syngas can then be applied to fuel cells, and has been adapted for use in industry [9,10]. Within the literature, there have been extensive efforts to clarify the reaction mechanism and the physical origin of the high catalytic activity observed for Au catalysts. It has been found that Au nanoparticles (NP) have significantly enhanced catalytic activity
∗ Corresponding author. E-mail addresses: [email protected], jerryli @hotmail.com (Y. Yang).
compared to bulk Au [11,12]. Au NP catalysts over TiO2 surfaces are of particular interest [13,14], where not only do Au cluster formation occur at much higher density than other surfaces which allows for a higher loading of Au [15], but an improved resistance to Au cluster sintering due to defect sites on the support [16]. It has also been well documented from experiments that at LT O2 does not readily chemisorb on Au clusters larger than ∼1 nm [17–19], or Au supported on TiO2 [20] either dissociatively or molecularly. Though these studies do provide some useful knowledge, it is important to consider that these studies are operated using single crystal support surfaces with experiments performed often under vacuum conditions. Thus on a powder surface and under more ambient conditions, the mechanisms proposed may vary in comparison as an effect of pressure and materials gap. More recent studies on powder samples in the field have focused on two key parts: CO adsorption, and O2 activation. There is general agreement that at LT CO is largely adsorbed on the Au nanoparticles, as highlighted by some key investigations in the literature [21,22]. Investigations from Green et al. extend upon this, observing adsorption at not only on the metallic Au NP, but also adsorption over Ti sites. [23,24], observing that in agreement with Haruta et al. [1],
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CO oxidation already occurs at −70 ◦ C. Green et al. using IR studies, observed dual perimeter sites, whereby under LT, CO is adsorbed on both the metallic Au and the Ti interface, as IR bands assigned to COAu and CO-TiO2 were observed at −153 ◦ C. Green et al. goes further to suggest that the activity on the perimeter sites occurs through O-O scission, activated by formation of CO-O2 complexes at Ti-Au sites found on the Au/TiO2 interface, induced by pre-reduction with CO [23]. O2 activation is another key aspect to CO oxidation that requires clear resolution. This includes the nature of the active adsorbed oxygen species, whether they are in dissociated form or in molecular form, and its location on the catalyst, i.e. on the Au particle, on the TiO2 support surface, or at the perimeter of the interface between Au and Ti. Koga et al. proposes that the active oxygen can exist as both dissociated form and molecular form, depending on the concentration of oxygen relative to the contact area which they believe to be located at the Au-Ti perimeter, with coordination primarily at the Ti sites [25]. Widmann et al. go further in proposing that the catalytic activity is dependent on oxygen vacancies on the TiO2 lattice; particularly along the Au-Ti perimeter, which are the key reason for the high activity of CO oxidation [26]. These studies would strongly suggest that oxygen activation is more crucial than the CO adsorption itself. However, findings by Green et al. show that pre-treatment with O2 prior to CO introduction inhibits activity [24]. It is clear that there is agreement that the key active sites are found along the Au-Ti perimeter, but whether the key steps involve CO2 intermediate formation, or formation of active oxygen species, is yet to be fully conclusive. Though the catalytic activity of Au nanoparticles have been well documented, it is crucial that one is able to quantify the active sites on the catalyst surface. Studies in this field have focused on characterizing the catalyst active sites and relating them to reaction data. In addition, direct observations of CO desorption at subzero temperatures as well as CO2 desorption at the same conditions, has to our knowledge, not been investigated; particularly over powder catalysts instead of a model single crystal surface. Direct observation of the desorption of these species will provide key data for the energy barrier required to facilitate desorption. In the work reported here, we characterize the catalytic sites activity of the Au/TiO2 NP involved in the CO oxidation reaction, and demonstrate that under LT conditions, the CO oxidation process significantly differs from that at room temperature to high temperature (HT) region. Titration experiments were performed at a temperature region of in −165- 200 ◦ C, with various programmed experiments at ambient pressure with flow of CO or O2 . We aim to show that under different temperature conditions, both the CO oxidation mechanisms previously presented by Green et al. [23,24,27] and Widmann et al. [26,28] are valid for transient CO2 products. Detailed kinetic parameter fitting indicates two different surface active sites with new online quantification measurements.
1.1. Catalyst preparation Au/TiO2 was prepared using the deposition-precipitation (DP) method as outlined by Zanella et. al [29] which was the same catalyst used in studies by Green et al. [23]. Briefly, 1 g of TiO2 was added to 100 mL of an aqueous solution of HAuCl4 (4.2 × 10−3 M) and of urea (0.42 M). The initial pH was ∼2. The suspension was held at 80 ◦ C and was vigorously stirred for 4 h, where by the pH increased. The suspension was then centrifuged, washed, dried, and calcined at 300 ◦ C.
1.2. Catalyst characterization The total surface area of the catalysts was measured by nitrogen adsorption at −196 ◦ C using a Quantachrome Instruments Autosorb iQ2. Analysis showed a BET surface area of 59 m2 /g with a type I isotherm. Powder X-ray diffraction (XRD) of the catalysts was obtained using Cu K␣ radiation using a Bruker D2 Phaser diffractometer. Diffractograms were collected in the 2 angle range from 10◦ to 80◦ with the 0.008◦ 2 step resolution. The catalyst sample was composed of 7.3% Au, 82.1% TiO2 in the anatase phase, and 10.6% TiO2 in the rutile phase. Average size of the Au particles was found to be in the range of 3 nm. This was calculated using the Scherrer equation. The results are shown in Fig. S1 from the supporting information. Transmission electron microscopy measurements, conducted using a Fei TF20 Tecnai G20, showed an average particle size in the range of 2.8 nm, which is in agreement with the XRD analysis. The results are shown in Fig. S2 from the supporting information. Combining the XRD and TEM results and assuming the surface lattice density as 1.0–1.3 × 1015 /cm2 , this yields surface gold sites of about 120–150 mol/g. 1.3. Reactions The full detailed setup was introduced in a previous publication [30]. In each of these experiments, reactant gas mixtures were prepared in flow-controlled manifolds. In all experiments reported, 100 sccm of total gas flow was used with total pressures of 1.4 bar. In all processes of the MS measurements results, the small signal flow rates are calibrated with 36 Ar signal as internal standard with fragmentation correction. The activity of the catalyst was measured in a high pressure microreactor using a quartz tube reactor vessel. Gas analysis was achieved with a Pfeiffer Prisma quadrupole mass spectrometer, observing the key reactants using the following mass spectrum signals: CO (m/z −28), O2 (m/z −32), and CO2 (m/z −44). 1.3.1. Low temperature (LT) CO oxidation reaction The sample was first tested for its activity in LT CO oxidation reaction. 100 mg sample was loaded in the middle of reactor 6 mm quartz tube with an 100 sccm gas mixture (CO:O2 :Ar = 20:10:70) flowed though. The reactor temperature (Fig. 1, thick dashed line) was first cooled down to −150 ◦ C and then heated up with ramping rate of 10 ◦ C/min. 1.3.2. Isothermal CO titration The CO titration procedure is detailed elsewhere in previous work [31]. Briefly, 50 mg of the catalyst was heated to 200 ◦ C under Ar flow, and held for 30 min. During this time, the catalyst was oxidized in a flow of 3% O2 . This was followed by flushing in an Ar stream, followed by cooling to the desired reaction temperature. An injection of 3% CO was then put into the system. 1.3.3. CO-TPD and Temperature Programmed Oxidation – Temperature Programmed Desorption (TPO-TPD) The CO-TPD procedure involved heating the catalyst (100 mg) was first heated to 200 ◦ C under Ar flow. It is well documented that above certain temperatures, the Au/TiO2 begins to sinter [32]. After heating, the catalyst was cooled to −170 ◦ C and held for 1 h, where during this time 10% CO was introduced. This is followed by purging with Ar flow. The catalyst was then heated at a ramp rate of 10 ◦ C/min heating up to 100 ◦ C. The TPO procedures are divided into a full TPO and a selective oxidation experiment. The catalyst (100 mg) is first heated to 200 ◦ C under Ar flow and held for 30 min. After heating, the catalyst was cooled to −170 ◦ C and held for 10 min, where during this time 3%
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Fig. 1. Activation and deactivation vs. temperature of CO oxidation on AuTiO2 by Co-feeding 20% CO and 10% O2 . Showing the temperature profile (dashed line) used during the course of the reaction, and the normalized flowrate of a) CO (m/z −28) and b) CO2 (m/z −44) vs time.
CO is introduced for surface reduction. After, CO is switched with Ar flow. For the full TPO procedure, a flow of 3% O2 was introduced. During this procedure, the catalyst was heated at a ramp rate of 10 ◦ C/min, heating up to 100 ◦ C. The selective oxidation procedure, uses the same catalyst CO reduction procedure as the full TPO procedure. After the pre-treatment (heating to 200 ◦ C and CO reduction/Ar flushing procedure at −150 ◦ C), a flow of 3% O2 was introduced. During this procedure, the catalyst was heated at a ramp rate of 10 ◦ C/min heating up to −100 ◦ C and held for 10 min. O2 flow was switched to an Ar flow, followed by cooling back to −150 ◦ C. Under Ar flow, the catalyst was heated at a ramp rate of 10 ◦ C/min heating up to 100 ◦ C. 1.3.4. Temperature Programmed Reduction (TPR) – High Temperature (HT) The HT TPR experiment utilized 100 mg of the catalyst and was first heated to 200 ◦ C under Ar flow, and held for 30 min. During this time, the catalyst was reduced in a flow of 3% CO. This was followed by flushing in an Ar stream, followed by cooling. After the catalyst reduction at 200 ◦ C, the catalyst was treated with a flow of 3% O2 for 1 h. This is followed by purging with Ar flow. The catalyst was cooled to 10 ◦ C, followed immediately by an introduction of 3% CO stream. The TPR procedure was performed in the temperature range from 10 to 280 ◦ C at a ramp rate of 10 ◦ C/min. 2. Results and discussion 2.1. LT CO oxidation reaction The sample was first tested for its activity in LT CO oxidation reaction. 100 mg sample was loaded in the middle of reactor 6 mm quartz tube with an 100 sccm gas mixture (CO:O2 :Ar = 20:10:70) flowed though. The reactor temperature (Fig. 1, thick dashed line) was first cooled down to −150 ◦ C and then heated up with ramping rate of 10 ◦ C/min. Both CO (Fig. 1a) and CO2 (Fig. 1b) MS signals are plotted, with all the fragmentation factors corrected. At −67 ◦ C the oxidation reaction started and CO flow rate output immediately dropped to zero level and 100% CO2 conversion was observed afterward. Heating was continued to 100 ◦ C and held at that temperature for additional 10 min. Afterwards the sample was cooled
back to −150 ◦ C using the same rate of 10 ◦ C/min. At approximately −100 ◦ C the oxidation reaction stops with CO2 conversion dropped sharply to zero and while CO output signal recovered. This is a typical LT performance of CO oxidation on Au/TiO2 sample with NP loaded [1].
2.2. CO-TPD To observe the CO adsorbates desorption behavior, CO exposure was performed at −165 ◦ C by flowing 10% CO diluted in Ar with total flow rate of 100 sccm for 10 min. The sample was thus flushed in pure Ar flow for 20 min and heated up in the same flow with ramping rate of 10 ◦ C/min. The results obtained at LT conditions showed clearly that the desorption profile (Fig. 2, main panel) of CO desorption. CO (m/z − 28) desorption signal appears at ∼−135 ◦ C and formed a shoulder, found at −113 ◦ C. The slope of desorption curve became steeper again after temperature surpasses −90 ◦ C, and formed a main peak at −55 ◦ C. The total desorption yields approximately 25 mol of CO. This equates to all the CO adsorption sites on the catalyst surface which incorporates the sum of chemisorbed surfaces of the Au NP and TiO2 surface. Comparing previous temperature desorption studies observed using IR by Green et al. [10,12], these desorption features may be connected with specific IR assignment in that study. The inset of Fig. 2 is from Fig. 2C inset of that report. In that report, two CO adsorbates features were observed on CO exposure saturated Au/TiO2 surface, assigned as CO on TiO2 sites and CO on Au sites. As temperature increases, the CO/TiO2 feature coverage decreases sharply at ∼−113 ◦ C and the CO/Au feature coverage decreases to ∼50% at −58 ◦ C. Considering the derivative relation between the CO total coverage and desorption rate, our TPD results are in agreement with this previous IR intensity behavior obtain during similar thermal desorption process. Thus the shoulder that centers at −113 ◦ C in our results is assigned to the CO/TiO2 IR feature observed by Green et al. while the main peak at −55 ◦ C is assigned to be the CO/Au chemisorbed species in that study [10,12]. With a rough estimation of the ratio between the shoulder and the main peak picked between 1:6 and 1:3, the main peak area falls between 18 and 21 mol. This yields the main peak related surface CO chemisorption sites to be 180–210 mol/g. This is about
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Fig. 2. TPD plot of CO desorbed versus temperature (CO signal taken at m/z −28) after −165 ◦ C CO pre-exposure; inset: The normalized integrated IR absorbance of CO/Au and CO/TiO2 against temperature from Yates et al. [23].
the same scale as the earlier Au metallic surface area estimation based on our TEM and XRD diameter measurements. 2.3. Isothermal CO titration CO titration experiments were performed on oxidized sample to quantify the surface oxygen storage after pre-oxidation at a certain temperature. Two types of temperature controlled titration process were conducted as isothermal reduction and temperature programmed reduction (TPR). The sample was first exposed to 3% O2 at 200 ◦ C, as previously described. CO isothermal titrations were performed at a lower fixed temperature by flow 3% CO diluted in Ar carrier gas. Fig. 3a shows four calibrated CO2 product flow rate measured by online MS at 40, 80, 100 and 130 ◦ C (Fig. 3, panel a, curve a–d). At 40 ◦ C, the CO titration yields almost no CO2 products (Fig. 3, panel a, curve d). At 80 and 100 ◦ C titration product are significant, with the respective curves (Fig. 3, panel a, curves b and c) being very close to each other. The 130 ◦ C (Fig. 3, panel b, curve a) titration yields CO2 product that is around two times the amount of the previous two curves. The full plots of the tabulated CO2 total products from each curve obtained at all temperatures is showed in Fig. 3, panel b. Overall the isothermal temperatures covered the temperature range from 40 ◦ C to 200 ◦ C, thus 17 temperatures in total. For some of the isothermal temperatures, repeat measurements was performed for consistency. At the same isothermal temperature, the difference in the pre-oxidation temperature does not affect the CO2 products significantly, with repeat measurements showing good consistency, being within ±10%. At lower temperatures from 30 to 60 ◦ C the titrations yield almost no CO2 , indicating there is little CO oxidation activated on the surface. Between 70 and 100 ◦ C, the CO2 products kept at a constant level of 4.0 ± 0.2 mol/g which suggests there is only a uniform oxygen species activated for surface CO oxidation in this temperature region. This is a very small amount compared to either the surface Au metallic sites (<3%) or oxygen sites on the oxide («1%). Thus such storage of surface oxygen may not be directly related with either of these two sites. At higher temperatures as titration increase from 110 to 200 ◦ C, the total CO2 products increase up to more than 10 times and the highest CO2 products are at around 50 mol/g. This indicates that at tempera-
tures higher than 100 ◦ C, there is secondary surface oxygen species which is activated during in CO oxidation. This assumption is also supported by the shape of the curves in Fig. 3 panel a. At both 80 and 100 ◦ C, the descending sides of CO2 product peaks decays smoothly (curves b and c), but the 130 ◦ C peak appears with 2 decay rates. This trend is more obviously in the inset of panel a, by directly plotting the linear dependence between adsorbates coverage in titration decay curve and time (Fig. 3, panel a, inset). Here after several fitting attempts, the 2nd order desorption model is found to be the best to fit the decay curves (see supplementary materials) The reciprocal of the 80 ◦ C titration curve integration (inset, c), of CO2 * coverage, shows good linear dependence on time, as predicted by the 2nd desorption order model. Meanwhile, the 130 ◦ C curve (inset, a) shows an obvious slope change at the same time. 2.4. HT TPR The results of the HT TPR experiment is illustrated in Fig. 4. The temperature ramping starts at −15 ◦ C. A very small yet repeatable initial CO2 peak was found at 29 ◦ C, with the integration yields only about 2 mol/g active surface sites related. The full curve of this peak is amplified in the inset of Fig. 4. A much larger and broad peak with CO2 product formed was observed at 140 ◦ C. The total integration of this peak shows there is 150 mol/g of intermediate involved in this temperature region. In this TPR process, at 40 ◦ C the desorption rate is zero, while the two observed desorption peaks do not overlap each other. This implies that there are two different desorption species which contribute to the CO2 products. Applying first order reaction model to the Redhead equation [33], the two peak temperatures convert to 0.89 eV and 1.23 eV CO2 formation barrier respectively. The results of TPR along with the isothermal CO titration both prove that there are two different CO2 intermediate species formed on the pre-oxidized sample surface with CO flow exposure. One species is of very small amount, less than 3% of the total gold metallic sites, and with a smaller surface formation barrier for CO2 production. As reaction temperature increases, the second surface intermediates show up with a higher and broadly distributed energy formation barrier for additional CO oxidation. This result
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Fig. 3. Panel a): CO2 product from 200 ◦ C pre-oxidized Au/TiO2 in isothermal CO titration at: a) 130 ◦ C, b) 100 ◦ C, c) 80 ◦ C, and d) 40 ◦ C (Inset: Comparison of 80 ◦ C and 130 ◦ C decay curves; 1/CO2 * coverage vs time, Panel b): Tabulated results of total CO2 desorbed versus experiment temperature (Inset: expanded view of plateau region).
is similar with the earlier TAP studies [26,28] with similar reaction condition mentioned in the introduction part. After Au/TiO2 surface pre-oxidized in O2 ambient at 400 ◦ C, cyclical CO and O2 pulses were applied on the surface yield reversible and stoichiometric pulse losses to surface CO2 products, which is quantified as surface oxygen storage capacity (OSC). Measurements from 80 to 400 ◦ C with interval of every 80 ◦ C, the OSC was also found strongly depends on the temperature of the TAP measurement. At 80 ◦ C, the oxygen storage sites were assigned to lattice oxygen close around the Au NP perimeter. At higher temperatures, the additional oxygen species are assigned to additional accessible neighboring lattice oxygen sites. Comparing the similarity with our results, the Au NP
perimeter lattice oxygen sites are assigned to the CO2 intermediates in 70–100 ◦ C isothermal CO titration, i.e. the plateau in Fig. 3, panel b and the lower peak of TPR results (Fig. 4). And the neighboring lattice oxygen sites are assigned to the intermediates of the additional CO2 in higher temperature CO isothermal titration and the TPR large peak. 2.5. LT selective TPO As CO TPD results shows, stable CO storage on Au/TiO2 system may only be available at lower subzero temperature. To study the CO oxidation activity of the LT adsorbed CO species, LT selective TPO
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Fig. 4. Au/TiO2 TPR results under CO flow after 200 ◦ C pre-oxidized, showing calibrated CO2 flowrate versus temperature (Inset: zoomed in view of 30 ◦ C TPR curve region in main panel).
is performed as detailed as follows: The catalyst was first exposed to CO flow at −165 ◦ C and purged by Ar flow, The system was then exposed to 3% O2 flow and heated to −100 ◦ C at 10 ◦ C/min, held for a period of time, and followed with purging by Ar flow while cooling back to −165 ◦ C. Finally the sample in pure Ar was ramped from −165 ◦ C to 100 ◦ C to desorb all surface carbon species formed after the previous oxidation treatment. Both CO and CO2 species were found in the whole process and the MS signal are plotted in Fig. 5, panel a (a, b, c) along with the temperature vs. time. The total carbon balance is compared with the CO TPD total yields (Fig. 2) and the relative products ratio from each temperature interval is plotted in Fig. 5, panel b. During the oxidation section below than −100 ◦ C, only CO desorption was observed, accounting for 10% of total carbon yields. No CO2 products was found in this section. The CO desorption rate reached the maximum as the ramping temperature stopped at −100 ◦ C. In the second temperature ramping session in pure Ar, CO desorption resumed as the temperature reached −100 ◦ C and reached its maximum at −52 ◦ C; a similar desorption temperature as the CO TPD main peak shown in Fig. 2. CO2 desorption from the surface also observed in this section which started at higher temperature of −75 and peaked at −12 ◦ C. The CO and CO2 desorption in the second temperature ramp each counted for 63% and 27% of total carbon yields, respectively. The selective oxidation and the following ramping indicated that during the first ramp to −100 ◦ C, simultaneous oxidation on CO adsorbed catalyst was able to produce stable CO2 surface intermediates at this temperature. This process provides another CO2 product procedure from elementary steps operation on an initially different prepared surface of AuTiO2 which was LT CO reduced. Meanwhile the majority of the original CO adsorbates (73% by combining CO desorption in the two ramping process) during the oxidation remained unchanged and desorbed as CO product. Comparing the desorption of CO in the both ramping process with the CO TPD, the behaviors are almost identical as following: 1) In the initial ramping from around −165 ◦ C, CO desorption started at around −130 ◦ C, whether or not oxygen was present. 2) In the second ramp the desorption peak of CO resembles the TPD main peak with the same peak temperature. Combining all these observations, it is concluded that the oxidation of the surface CO is not complete The upper limit of oxidized species at −100 ◦ C is 27% while the remaining species remain on the surface bonding status similar to the original. The oxidation process may either generated a surface 1) with co-adsorbed oxygen and CO species, which is similar to the
LT Pt CO pre-adsorbed surface [34,35] or 2) with the surface CO partially oxidized into CO2 * adsorbates or CO3 −2 group on the surface at this temperature region. In previous CO LT oxidation studies based on IR methods, Green et al. reported that at −153 ◦ C, on a similar Au/TiO2 catalyst, the surface pre-adsorbed CO species, assigned to TiO2 sites, oxidized to CO2 species bonded to titania support upon O2 exposure, while another CO adsorbates assigned to Au sites remains the same absorbance intensity. No gaseous CO2 at this temperature was mentioned. This result provides a CO2 intermediate explanation compatible to the second assumption above for the selective oxidation reported. However, in order to directly confirm the IR CO2 /TiO2 species with the CO2 gas product after the selective oxidation, further online experiments such as online IR-MS characterization of the same reaction process is required.
2.6. Discussion Different models have been purposed for CO oxidation reaction over AuTiO2 catalysts. Generally, most viewpoints are in agreement where both Au metallic and Au/TiO2 perimeter sites play important roles [23,26,36,37]. With the above experiments performed, we scanned the temperature region from liquid nitrogen temperatures to 200 ◦ C for several steady states and transient reactions that produce CO2 gas products, including co-feeding CO and O2 , CO reduction-oxidation and oxidation-CO reduction sequences with precise temperature controlling. The Table 1 summarizes the main experimental results associated with the reaction products. The last column also provides the purposed catalytic active sites based on earlier discussion for the involved key intermediates by comparing the consistency of product amount, characteristic temperature combining our kinetics measurements results, previous characterization from IR and TAP measurements and the TEM, XRD results. Both transient reaction with different sequence of reduction and oxidation were able to produce CO2 , however, under very different condition and with very different product behaviors. Both HT TPR/isothermal CO titration and LT TPO generates catalytic active AuTiO2 surface after initial oxidation/reduction with surface storage of O*/CO* species as intermediates of final CO2 products. However, the reaction conditions and CO2 product characteristics are dissimilar to each other. In summary:
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Fig. 5. Panel a),TPO desorption profile of the selective oxidation experiment. The procedure was as follows; Zone I: Introduction of O2 stream over pre-reduced catalyst, Zone II: Temperature ramping from −150 ◦ C to −100 ◦ C under O2 stream, Zone III: Temperature held at −100 ◦ C, Zone IV: Cooling to −150 ◦ C and removing O2 stream. Zone V: Temperature ramping from −150 ◦ C to 100 ◦ C in pure Ar stream. Showing the MS desorption signal calibrated to flowrates of a) – CO (1st temperature ramp), b) – CO (2nd temperature ramp) and c) – CO2 , Panel b), Distribution of the different desorbed carbon species from the selective oxidation experiment.
Table 1 Summary of the main experimental results associated with the reaction products. Desorption/products
Method
Uptake per gram of catalyst (mol/g)
Tp (◦ C)
Reaction barrier (eV)
Purposed intermediate site
CO2
Isothermal CO titration
4.0 for 70–100 ◦ C plateau 4.0–50 for higher T 27% of total C uptake 2.0 150
n/a
n/a
Au perimeter oxygen
−12 29 140
0.77 0.89 1.23
n/a Au perimeter oxygen Au perimeter neighboring lattice oxygen
250
40–70 180–210 10% of total C uptake1st ramp 63% of total C uptake2nd ramp
−113 −55 n/a −52
0.47 0.64 n/a 0.64
TiO2 surface CO Au metallic surface CO TiO2 surface CO (mostly) Au metallic surface CO (mostly)
135–170
n/a
n/a
n/a
Selective oxidation LT TPR HT CO
TPD Selective oxidation LT
Total Au NP physical surface sites
TEM, XRD
Au perimeter neighboring lattice oxygen
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1) Catalytic surface activation temperature: In the isothermal process, CO exposed on pre-oxidized AuTiO2 surface with oxygen storage at perimeter sites yield no reaction products below 60 ◦ C. The activation of perimeter oxygen with CO titration only starts after 70 ◦ C. The HT TPR also indicates a similar species only active at room temperature or higher. On the side of LT TPO, on CO preexposed surface, even at temperature as low as −100 ◦ C, oxygen already reacts with surface CO species and forms stable intermediates layer which itself directly produce CO2 without additional CO or oxygen refill at higher temperature. Compared to the initial LT CO oxidation reaction (with CO/O2 co-feeding) in Fig. 1, the CO2 does not appear until −12 ◦ C. The appearance of the intermediate though, will require further spectroscopic analysis, which will also provide clarity to the reaction mechanism during this particular reaction pathway. 2) Amount of active sites: In the TPR process, CO2 desorbs from the catalytic surface upon CO exposure over 70 ◦ C, and the amount of products is limited by the purposed perimeter sites, i.e., surface oxygen storage capacity, which is less than 3% of the Au NP surface sites. On the selective oxidation side, the surface provides stable CO2 intermediates storage after −100 ◦ C oxygen exposure. This storage, which counts 27% of the carbon adsorbates at LT CO adsorption, is therefore more than the purposed perimeter sites. Applying the TEM results of 3 nm diameter average for Au NP and assuming 0.3 nm lattice constant, the maximum percentage of perimeter sites is no larger than 4%. Thus the oxidation up to −100 ◦ C must activate additional surface sites. 3) The temperature that surface produce CO2 : Comparing the CO2 desorption initial temperature, the selective oxidation process also starts produce gas CO2 at least 80 ◦ C lower than either the isothermal titration or the TPR processes. It is also noted here that these surface CO2 precursors are more kinetically stable than any of the CO surface species, when comparing to all CO desorption in this study.
sequences were both found forming catalytic layer for intermediates and produce CO2 products. The HT CO titration transients, with 200 ◦ C pre-oxidation following either isothermal CO titration or TPR above room temperature, indicates there are two oxidative surface intermediates species with different kinetics. The two intermediates are purposed as atomic oxygen at Au NP perimeter site and neighboring lattice sites. The LT selective oxidation, with −165 ◦ C CO pre reduction following oxidation up to −100 ◦ C, is able to partially convert surface CO species into a more stable CO2 surface intermediates. Detailed measurement and calibration are performed to obtain the kinetics and product quantification for above reactions. The results support previous findings in the literatures which show activity at the same temperature conditions. The experiments also provides methods for direct surface active sites quantification with one step transient online MS measurements for both Au NP surface and perimeter sites. Acknowledgements The authors would like to thank the startup funding provided by ShanghaiTech University, the National Natural Science Foundation of China (No. 21573148) and Shanghai Pujiang Program (No. 15PJ1405800) for funding their participation in this work. This work was conducted in the facility of School of Physical Science and technology in ShanghaiTech University. The authors would like to thank Mimi Dai and Minghui Fan for their assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2017.02. 031. References
Previous studies in the literature have focused primarily at either liquid nitrogen temperature regions or from room temperature and above. Here we have performed both procedures by using real-time MS sampling from simulated reaction conditions. Whether these two CO2 products share the same reaction pathway or not, this will involve further detailed and thorough kinetics investigation and theoretical modeling. The work reported here for both CO oxidation results, further supports results from other results, indicating that the perimeter site certainly plays a key role in oxygen dissociation and further CO2 intermediates formation. By selecting proper temperature controlled titration method, quantification of the two characteristic active sites on AuTiO2 surface, Au cluster perimeter sites and Au metallic surface sites, may be directly measured. The LT CO TPD may be adapted for Au metallic sites quantification. And the isothermal titration plateau region appears to be a good probe for the Au cluster perimeter sites. It should be taken into consideration that Au cluster activity varies significantly according to the average size and the interaction with support materials, thus the measurement procedure may need proper adjustment depends on the catalyst conditions. 3. Conclusions CO oxidation experiments was performed over Au/TiO2 catalysts with Au particle size of 3 nm, and a 7.3 wt% loading in a broad temperature range from liquid nitrogen to 200 ◦ C. The catalyst low temperature (LT) CO oxidation reactivity was measured with online MS. LT CO TPD yielded two CO species at different temperature ranges and bonding status was assigned to surface TiO2 and Au island surface sites. Two transient processes with different redox
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