TiO2 Studied by

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Photothermal Catalysis over Non-plasmonic Pt/TiO Studied by Operando HERFD-XANES, Resonant XES and DRIFTS Ying Zhou, Dmitry E. Doronkin, Ziyan Zhao, Philipp N. Plessow, Jelena Jelic, Blanka Detlefs, Tim Pruessmann, Felix Studt, and Jan-Dierk Grunwaldt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03724 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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ACS Catalysis

Photothermal Catalysis over Non-plasmonic Pt/TiO2 Studied by Operando HERFD-XANES, Resonant XES and DRIFTS Ying Zhou,*,†,‡ Dmitry E. Doronkin,†,§Ziyan Zhao,†,‡ Philipp N. Plessow,§ Jelena Jelic,§ Blanka Detlefs,∥ Tim Pruessmann,†,§Felix Studt,†,§ and Jan-Dierk Grunwaldt*,†,§. † Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology, Karlsruhe (KIT), 76131 Karlsruhe, Germany ‡ School of Materials Science and Engineering, Southwest Petroleum University, Chengdu, 610500, China § Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany ∥ European Synchrotron Radiation Facility (ESRF), 38043 Grenoble, France * To whom correspondence should be addressed: [email protected] (Y. Z.); [email protected] (J.-D. G.)

ABSTRACT: There is strong interest in developing photothermal catalysts which can utilize both thermal energy and low-intensity photon flux. The complementary use of operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), High-Energy-Resolution Fluorescence Detected X-ray Absorption Near Edge Structure (HERFD-XANES) and X-ray Emission Spectroscopy (XES) provided mechanistic insight into photothermal catalytic oxidation of CO over Pt/TiO2. The methods turned out to be sensitive enough to uncover a change in the electronic structure of the Pt sites upon light illumination. The Pt-sites on TiO2 were found more oxidized upon light illumination. The CO-coverage is reduced which results in a ~20 fold rate enhancement of CO oxidation at 45 °C. Finally, a promising operando spectroscopic route to understand photothermal catalytic reactions is proposed.

Keywords: Photothermal catalysis • CO oxidation • platinum • XANES • XES

■ INTRODUCTION Catalysis plays a key role in chemical conversion, energy production and environmental protection to address the challenge of our modern society.[1,2] Lower operating temperatures could not only reduce energy consumption but also improve the long-term stability of catalysts.[3] As a fascinating material, plasmonic metal nanoparticles (NPs) have recently been proposed as a new type of photocatalysts which can utilize concurrently thermal energy and photon flux to drive several catalytic reactions such as ethylene epoxidation, CO oxidation and reverse water-gas shift reaction at remarkably lower temperature compared to conventional thermal processes due to the strong interaction with resonant photons through the

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excitation of localized surface plasmon resonance (SPR).[4-11] Moreover, such plasmonic nanostructures can improve catalytic selectivity.[12,13] Unlike the intensively studied Ag and Au, non-plasmonic catalytic metals (Pt, Pd) generally reveal broad extinction from ultraviolet to visible light without SPR absorption peaks.[14] Interestingly, it was found that non-plasmonic metals can also enhance the intrinsic catalytic performance under visible light irradiation for various reactions,[15-18] even at high temperature (up to 700 °C).[3] This brings evidence that non-plasmonic metals have the potential to co-utilize thermal energy and photon flux as well. However, the underlying origin of the enhanced photocatalytic activity over non-plasmonic metals remains far from understood.[19,20] This calls for studies under real operating conditions. Although in situ and operando characterization techniques for thermally driven catalytic reactions develop rapidly,[21-24] very little work is reported on the application of operando techniques to study photocatalysts.[25-27] This deficiency is mainly due to the following challenges: 1) Typical window materials for X-rays such as Kapton® or aluminium foils or the use of quartz capillaries for operando techniques[28] are not suitable for photocatalysis due to the very limited irradiation area; 2) the design of the experiment is challenging to combine excitation by light, temperature, X-ray probe, gases, catalyst, gas analysis in one cell; 3) the resolution of X-ray absorption near edge structure (XANES) is not high enough to discriminate tiny changes in the spectra. For this purpose, High-Energy-Resolution Fluorescence Detected X-ray Absorption Near Edge Structure (HERFD-XANES) provides much sharper spectral features and enables better discrimination of small spectral changes than conventional XANES.[29,30] While XANES probes the electronic structure of unfilled states, X-ray emission spectroscopy (XES) monitoring Pt Lβ5 emission line is a complementary technique probing the electronic density of filled valence orbitals.[31,32] Hence, the combination of HERFD-XANES and XES opens up a way to probe the electronic structure of catalytically active NPs. Nevertheless, to date, no operando HERFD-XANES/XES study on photothermal catalysis has been reported. Herein, we report such a first operando HERFD-XANES and XES investigation on photothermal catalysis over non-plasmonic metal using CO oxidation as a probe reaction. Pt NPs were selected as they are highly active oxidation catalysts and their behavior in thermal catalysis is relatively well understood.[33]

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■ EXPERIMENTAL METHODS Pt/TiO2 was prepared by wet impregnation of hydrothermally-made anatase TiO2 dominated with {101} facet (a detailed description is given in our previous work[34] and in the supporting information (SI)). Reference Pt/Al2O3 catalyst was prepared by incipient wetness impregnation of commercial γ-Al2O3 with aqueous H2PtCl6 solution as described elsewhere.[35] X-ray diffraction (XRD) patterns were recorded on a PANalytical X'pert diffractometer operating at 40 mA and 40 kV using Cu Kα radiation. Transmission electron microscopy (TEM) was performed using a FEI Tecnai G2 20 microscope operating at 200 kV. Elemental compositions were measured by ICP-AES on a Vista MPX ICP system (Varian). Surface area was determined by the BET method on a nitrogen adsorption apparatus. UV−vis absorption spectra were recorded on a Shimadzu 2600 UV-vis spectrometer. Photothermal catalytic performance evaluation, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and operando HERFD-XANES/XES measurements were performed in the Praying MantisTM high temperature reaction chamber (Harrick, Figure. 1). To our knowledge, this design is used for the first time to obtain HERFD-XANES and XES spectra of working photocatalysts. More details on the experiment as well as on the theoretical calculations are described in the SI. ■ RESULTS AND DISCUSSION A Pt/TiO2 catalyst was prepared by wet impregnation of hydrothermally-made anatase TiO2 (cf. Figures S1 and S2).[34] TiO2 with predominant {101} facets was selected because it was found earlier that it can stabilize dispersed Pt species and avoid sintering Pt NPs.[34] The surface area of the obtained Pt/TiO2 catalyst was SBET = 91 m2/g, Pt loading estimated by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was 1.1 wt.%, and the average particle size determined by Transmission Electron Microscopy (TEM) was ca. 1.7 nm.[34] UV-vis absorption spectra of TiO2 and Pt/TiO2 showed that no new distinct absorption peaks in either the UV or visible light regions were observed after the introduction of Pt NPs, confirming the non-plasmonic character of Pt NPs (Figure S3). A modified Harrick cell equipped with a 1 mm thick CaF2 (for DRIFTS) and 60 μm thick mica (for XAS/XES studies) window was used to carry out catalytic reaction which allowed both temperature control and continuous wave


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ACS Catalysis

(CW) illumination (on the order of solar intensity) (Figure S4).[36] The catalyst showed hardly any CO conversion at room temperature (25 °C) in the absence of light irradiation, while more than 200 ppm CO in a mixture of 1000 ppm CO and 10% O2 in N2 (SI) can be oxidized to CO2 under light illumination with an intensity of ~680 mW cm-2 (cf. Figure 2a). This effect is reversible and the slow recovery (compared to the temperature effect, Figure 2c) of CO and CO2 concentrations after switching off light is attributed to the hysteresis of adsorption-desorption equilibrium between Pt/TiO2 and CO in the gas phase.[34] The same conversion level would require up to ~ 60 °C higher temperature if no light irradiation is used. It should be noted that Pt NPs supported on chemically and optically inert γ-Al2O3 exhibit a comparable photocatalytic activity (Figure S5) with that over Pt/TiO2. At the same time, pristine TiO2 support does not exhibit any activity in CO oxidation at any of the used light intensities (Figure S6). Therefore, the origin of the photocatalytic activity should be ascribed to Pt NPs instead of TiO2.

irradiation can heat the catalyst bed significantly. Temperature increase up to 25 °C was recorded during irradiation with ~ 680 mW/cm2 and even for the lowest used intensity (114 mW/cm2) temperature of the catalyst bed increased by approx. 2 ~ 3 °C. Therefore, temperature effect cannot be ignored. On the other hand, for purely thermal process the reactor temperature increase from 45 to 70 °C (Figure 2b) resulted in the CO oxidation rate increase from 7 μmol gcat-1 to ~ 25 μmol gcat-1 min-1, which is much lower than the photothermal conversion rate at 45 °C (with ~ 680 mW/cm2 light irradiation, the temperature of catalyst bed also increased to ~ 70 °C, and the conversion rate is 151 μmol gcat-1 min-1). At the same time, photothermal CO conversion 45 °C and ~ 970 mW/cm2 irradiation is similar to the rate achieved at ~ 680 mW/cm2 although catalyst bed temperature is 35 °C higher in the first case and the reaction rate can still be increased by the corresponding heating (Figure 2b). Hence, the light effect is more complex than a temperature effect measurable with conventional techniques.

Figure 2b shows the CO conversion through thermal (no light) and photothermal (light on) activation over Pt/TiO2 as a function of temperature. With increasing temperature to 45 °C, a conversion of 7 μmol gcat-1 min-1 was observed, which can be enhanced to 151 μmol gcat-1 min-1 once the light was turned on. Thus, the reaction conversion increased by a factor of ca. 22 compared to the thermal process (no light). Further heating leads to very high reaction rate and the reaction switches from kinetics-limited to mass-transfer limited regime, which further limits light-induced increase in the CO oxidation conversion and does not allow separate evaluation of the light effect. The maximum CO reaction rate of 219 μmol gcat-1 min-1 was reached at 133 °C and it did not increase at higher temperatures, most probably due to mass transfer limitations. Also light illumination does not enhance CO oxidation at this temperature in the long term run.

In situ DRIFTS of adsorbed CO as a probe molecule provides relevant information on both the nature of Pt sites and the state of adsorbed CO. DRIFTS spectra in the region of CO vibrations recorded at room temperature without light irradiation and after two light on cycles are shown in Figure 2d. Without light, only a peak at 2064 cm-1 is assigned to linearly-bonded CO adsorbed on low-coordinated metallic Pt sites[38, 39] was observed. No peaks in the studied region were found in the control experiment with pristine TiO2 (Figure S7). Further attribution was supported by DFT calculations. In the case of CO adsorbed on a reduced Pt3/TiO2 model cluster of metallic nature (all neighboring O atoms are part of TiO2 framework, Figure 3, left model) CO vibrational frequency was calculated as 2054 cm-1. A similar feature (double peak with maxima at 2055 and 2075 cm-1) was also observed in the spectra of Pt/Al2O3 reference sample (Figure S8). After adsorption of CO on Pt/TiO2 and even after flushing with He for 20 min at 45 °C the peak at 2064 cm-1 was still very intense (Figure S9a) confirming strong chemisorption of CO on Pt sites.

Generally, the light irradiation may increase catalyst temperature and, thus, result in the enhancement of the reaction rate. For example, under intense laser illumination (~1 kW·cm-2) localized plasmon-mediated heating of metals leads to a rate enhancement of the thermocatalytic reaction.[37] However, this may be only partially responsible for the observations in the current work. To evaluate the temperature increase in the catalyst bed induced by light illumination, a thin thermocouple (on the order of catalyst grain size) was placed in the Harrick cell (at 0.5 mm below the top of the catalyst bed) and temperature increase was recorded at a time resolution of 0.1 s (Figure 2c). Even moderate intensity

After light irradiation (~ 680 mW/cm2), the peak at 2064 cm-1 shifted to 2076 cm-1 and a new intense peak at 2120 cm-1 emerged (Figure 2d). The peak shift from 2064 cm-1 to 2076 cm-1 was attributed to higher amount of O in the vicinity of Pt (but not in direct contact, modeled by Pt1/TiO2 with a higher ratio between Pt and surface O from TiO2, i.e. still metallic nature of Pt, Figure 3). The DFT modeling also predicted blue shift of CO vibration by 13 cm-1. In this case due to lower electronic density on


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Pt back donation from the Pt site to the anti-bonding molecular orbital of CO is lowered. This weakens the Pt-CO bond and leads to CO desorption.[17] Pt sites without strongly adsorbed CO facilitate dissociative adsorption of oxygen in addition to the oxygen supplied by reducible support such as TiO2,[40] thus, increasing CO oxidation rate.[33] Full oxidation of the Pt site to PtO2 site further blue shifts the CO vibration by 58 cm-1 to 2125 cm-1. Hence, the calculations confirm the trend observed by DRIFTS: turning on light resulted in approx. 12 cm-1 blue shift of CO vibrational frequency due to more O atoms in the

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vicinity of Pt sites, possibly at the Pt-TiO2 interface, and further exposure to light led to oxidation of Pt sites with the further blue shift to 2120 cm-1. The spectra of the reference Pt/Al2O3 sample showed similar peak shifts as the Pt/TiO2 (Figure S8) confirming that the signal at 2120 cm-1 originates from the interaction between CO and oxidized Pt, instead of the TiO2 support. The spectral changes were only partly reversible with the 2120 cm-1 peak diminished but not disappearing also after 15 minutes without light. Such a lack of full reversibility of the spectra results from the strong interaction between CO and oxidized Pt sites, which was shown recently.[40]

Figure 1. Scheme of the experiment and photos of the experimental setup for photo- and thermal catalysis at ID20 beamline (ESRF) with a modified Harrick in situ cell.


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Figure 2. (a)The CO concentration and produced CO2 at room temperature (~ 25 °C) in the dark and under light irradiation 1000 ppm of CO, 10 vol % O2/N2, CW light illumination with a intensity of ~680 mW/cm2,time resolution 2 s/point; (b) the CO conversion rate through thermal and photothermal activation over Pt/TiO2 as a function of temperature; (c) dependence of catalyst bed temperature on the light irradiation of different intensity; (d) in situ DRIFTS of CO adsorbed on Pt/TiO2 during photocatalytic reaction process recorded at room temperature under 1000 ppm of CO, 10 vol % O2/N2 (light intensity ~680 mW/cm2); (e) the CO concentration and produced CO2 at 45 °C in the dark and under light irradiation with different intensities. For the purely thermal CO oxidation (Figure S10), CO molecules cover the surface of Pt0 sites below 100 °C. At 133 °C the 2064 cm-1 peak blue shifts to 2076 cm-1 and the peak at 2120 cm-1 emerges corresponding to CO adsorbed on fully oxidized Pt atoms. The latter peak becomes dominating only above 205 °C. Light irradiation could induce the same spectral changes already at room temperature (Figure 2d). Hence, changes in the IR spectra during photocatalytic and thermal catalytic processes below 133 °C are significantly different. If the light-induced changes are to be attributed to a heating effect only, an increase by ~180 °C in local temperature around Pt NPs would be required. It was previously suggested that solar light irradiation may indeed induce local heating on the order of hundreds of degrees,[41] however, available techniques could not confirm such temperature increase.

Figure 2e shows the CO concentration and produced CO2 at 45 °C in the dark and under light illumination with different light intensities (114-972 mW cm-2), all of which can promote the CO oxidation. When increasing light intensity from ~ 680 to ~ 970 mW/cm2, the photocatalytic rate decreased slightly instead of increasing. Therefore, the behavior of non-plasmonic Pt NPs is significantly different from the plasmonic metals as intensity dependent transition from the linear to super-linear regime has been observed for Ag and Rh.[5,10] Moreover, at higher intensity (461-972 mW cm-2), a higher amount of CO2 was observed when just switching on the light following a fast decay process (Figure 2e). The influence of the light intensity variation was investigated by in situ DRIFTS (Figure S11a-c). At an intensity of ~ 970 mW/cm2 (Figure S11a), the shift of the CO peak from 2064 to 2076 cm-1 as well as the decrease in the intensity was observed. At the same time, the intensity of the peak at 2120 cm-1 significantly increased. Therefore, light illumination can


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reduce the electronic density on Pt sites during CO oxidation which was evidenced as a shift of the adsorbed CO peak to 2076 cm-1. It leads to desorption of CO and chemisorption of O, in turn, increasing the CO oxidation rate.[42] Moreover, chemisorbed O also oxidizes Pt and this results in a peak at 2120 cm-1. CO seems to be rearranged between Pt0 and oxidized Pt, which may form as a result of CO desorption, i.e. freeing up the Pt surface.[43-47] When decreasing the light intensity to ~460 mW cm-2, no peak shift was observed and the intensity of the peak at 2064 cm-1 decreased slightly with a simultaneous increase of the 2120 cm-1 peak (Figure S11b). Under illumination with ~110 mW/cm2, no changes in CO peaks were observed (Figure S11c). Operando HERFD-XANES and XES measurements were performed using the same in situ cell with a thin mica window instead of CaF2 to minimize X-ray absorption by the window (Figure 1). Due to the different gas flow rate at a synchrotron, the reaction rate measured during operando measurements was different from the laboratory results, but the trend is the same (Figures 2b and S12), which allows determining structure-performance relationships under working conditions. The HERFD-XANES spectra of the Pt/TiO2 catalyst measured after reductive pretreatment are reported in Figure 4a and the calculated model spectra for TiO2-supported Pt3 clusters with chemisorbed CO and O are given in Figure 4b. Figures 4c and 4d show model Pt3(CO)3 and Pt3O3 clusters supported on anatase TiO2 {101} (the model site structure is chosen on the basis of EXAFS analysis of the

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respective catalyst under thermally-driven CO oxidation[34]). The spectrum of the as-received sample (ex situ, not shown) lies in between the reduced sample and the sample exposed to O2/He which suggests some interaction with O2 from the air. To achieve a defined starting state, a reductive pretreatment was conducted. The spectrum of Pt/TiO2 after reduction showed the lowest intensity of the “white line” (the first maximum after the absorption edge). The intensity of the white line is often used as an indicator of Pt oxidation state since it is proportional to the amount of unoccupied 5d states[29,30] and in our case the H2-pretreated sample revealed the lowest white line intensity confirming the reduction. The spectrum measured in O2/He atmosphere shows some increase of the white line which suggests partial oxidation. However, the white line intensity still stays very low, lower than the one reported for Pt/Al2O3 catalysts earlier[29] and much lower than in the corresponding spectra of our reference Pt/Al2O3 material (Figure S14). This leads to a suggestion that upon exposure to O2 at room temperature no prominent reconstruction of Pt sites on titania occurs, the calculated spectrum of the Pt3O3/TiO2 model cluster qualitatively supports the O-covered Pt surface. With addition of CO to the O2/He feed the white line becomes less intense than in O2 and significantly broader. The broad white line was previously attributed to CO-covered Pt species,[29,30] which is also confirmed by the calculations for the Pt3(CO)3 model cluster (cf. Figure 4) and the reference experiment with Pt/Al2O3 (Figure S14).

Pt3(CO)3/TiO2

Pt(CO)/TiO2

PtO2(CO)/TiO2

2053.7 cm-1

2066.8 cm-1

2125.3 cm-1

Figure 3. Model Pt sites used to calculate the vibrational frequencies of CO on different Pt-sites.

Next, the HERFD-XANES and XES spectra upon CO oxidation without light at different temperatures are compared in Figure 5. The aim of the comparison is to

set the observed spectral features in relation to the generally accepted mechanism of CO oxidation over Pt catalysts.[33] According to this mechanism, active metal


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ACS Catalysis

sites are fully covered at low temperatures with CO

leads an even higher fraction of free sites. This is a chain

which blocks or poisons the sites and inhibits CO

reaction which is described as light-off and leads to a Pt

oxidation.[33, 39]

A CO-covered Pt surface is indeed found

surface free from CO, only with chemisorbed oxygen at

by HERFD-XANES (Figures 4a and 5a) at room

high temperature. Indeed, at the temperature of

temperature. CO oxidation requires some free surface

maximum CO conversion a much higher white line is

sites for chemisorption of oxygen or oxygen supplied

observed indicating the presence of oxidic Pt.[30] The

from the reducible oxide support.[40] A temperature

white line becomes significantly higher than in the case

increase leads to partial desorption (consumption) of CO

of O-covered Pt NPs at 20 °C (Figure 4b) suggesting bulk

and thus to a higher fraction of free Pt sites. This is

oxidation and / or reconstruction of Pt NPs instead of

supported

the

simply O-covered surface.[46] The same qualitative trend

HERFD-XANES spectrum measured at 80 °C which

is observed for the reference Pt/Al2O3, although without

indicates less CO on the surface (Figures 5a and S14a for

stabilizing effect of titania, which leads to significantly

the reference Pt/Al2O3). Oxygen adsorbs on the free Pt

higher average oxidation state of Pt (Figure S14a).

by

a

narrower

white

line

in

sites, reacts with CO with fast desorption of CO2 and

Figure 4. (a) HERFD-XANES spectra of Pt/TiO2 measured at RT (20 °C) after reductive pretreatment under 5%H2/He, after exposure to 10% O2/He, and 1000 ppm CO/10% O2/He. (b) Calculated HERFD-XANES spectra of model Pt3TiO2 clusters with chemisorbed O or CO. (c) Model Pt3(CO)3 and (d) Pt3O3 clusters supported on anatase TiO2 {101} used for HERFD-XANES and XES calculations.

In the XES spectrum measured at room temperature, two broad features can be identified centered around 11560 and 11562 eV stemming from the highest occupied molecular orbitals of the Pt-CO complex (Figure 5b). No structural information can be extracted from the region

above 11564 eV because of elastically scattered incident beam dominating the spectrum (energy of incoming X-ray photons set as 11566 eV). A drastic change occurred in the XES spectrum at the temperature of maximum CO conversion, the feature at 11562 eV slightly shifted to


ACS Catalysis

~11563 eV, the feature at 11560 eV disappeared and a new

3

a

catalyst (Figure S14b). There is only scarce information available

about

valence-to-core

XES (vtc)

of

5d

elements

region.[31,32]

The

in

biggest

(density of states, DOS) on Pt and s- and p-DOS on [32]

In our case contribution of d-DOS on Ti

atoms of the TiO2 support also could not be neglected. Therefore, to be able to attribute the obtained Lβ5 XES spectra to Pt sites with different structure the local DOS and Lβ5 XES the spectra were calculated using FEFF code[48] (cf. SI for details, Figure S15). The most prominent difference is related to the relatively narrow band in the d-DOS on Pt in the spectrum of the Pt3(CO)3 model cluster. The d-DOS on Pt atoms in this case has two maxima with a similar intensity at 11559 and 11562 eV (Figure S15). This corresponds well to the broad experimental spectrum of the Pt/TiO2 catalyst at room temperature which features Pt sites covered with CO[29,30] and shows maxima at 11560 and 11562 eV (Figure 5b, RT). On the contrary, the Pt3(CO)3 model cluster shows significantly broader d-DOS on Pt with maxima at 11557 eV (lower intensity) and 11561 eV (higher intensity) which corresponds to the experimentally observed 11558 and 11562 eV (Figure 5b, 133 °C) Thus, comparison of the calculated XES spectra of model Pt3O3/TiO2 and Pt3(CO)3/TiO2 clusters (Figure S13a) confirms that the shoulder at 11560 eV can be traced back to structures involving Pt-CO interaction. This agrees well with the fact that during heating chemisorbed CO desorbs from Pt (feature at 11560 eV disappears) and Pt sites become covered with oxygen (feature at 11558 eV appears). In addition, the higher overall intensity of the XES spectrum at high temperature stems probably from X-ray absorption (higher white line in XAS) and subsequently more holes in 2p (core) level of Pt. A shift of the peak at 11562.2 to 11562.6 eV seems to be related to the increase of Pt oxidation state.

RT 80 °C 133 °C

11560 eV 11558 eV

1

the

contributions to the Pt Lβ5 line occur from d-states ligands.

I / a.u.

evidenced in the spectra of the Pt/Al2O3 reference

2

11562.5 eV

b

RT 80 °C 133 °C

feature at 11558 eV appeared. Similar shifts were I / a.u.

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0

11560

11570

Energy / eV

11580

11550

11555

11560

11565

Energy / eV

Figure 5. (a) HERFD-XANES and (b) XES spectra of Pt/TiO2 measured under CO oxidation feed at different temperatures. Conditions: 1000 ppm CO, 10% O2 in He, flow rate 135 ml/min. The Pt/TiO2 catalyst measured under CO oxidation feed at room temperature (25 °C) exhibited no CO conversion in the absence of light while conversion increased to 7.4% (74 ppm of CO converted) after the light was turned on (Figure S12). Turning on the light resulted in a small decrease of the width of the white line (Figure S16a) which suggests removal of a small fraction of CO from Pt surface. Such small changes would not be visible in conventional XAS spectra and require HERFD measurements as was shown in the case of Pt:SnO2 sensing materials.[49] Notably, while DRIFTS after light irradiation revealed features characteristic for the catalyst heated to 205 °C (e.g. peak at 2120 cm-1), HERFD-XANES and XES do not point at strong heating (compare with the thermal catalytic data at 133 °C or even 80 °C, Figure 5a). The same applies to the reference Pt/Al2O3 experiment (SI). In theory, the difference may be attributed to different probing depth of the techniques with DRIFTS being surface-sensitive and XAS/XES probing the bulk. Hence, the photoactivation is limited to the catalyst surface only, which is different from conventional heating. Thus, a new pathway for CO-oxidation seems to be opened up. This points to similar outcomes of photo- and thermal activation of Pt/TiO2, i.e. desorption/consumption of CO freeing sites for chemisorption of oxygen, however, HERFD-XANES and XES data does not point at significant thermally induced CO desorption. No line shifts are observed in the XES spectra, decrease in total intensity of a spectrum


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measured with light may probably be associated with the

conversion

decrease in absorption of incident X-ray photons (lower

HERFD-XANES (Figure 7a) and XES spectra (Figure 7b)

XANES white line).

are markedly different from those measured at lower

At 80 °C activation by light brings the enhancement in CO conversion (Figure S12). However, it again does not result in dramatic changes in HERFD-XANES spectra (Figure 6a). When the light is turned on the white line becomes narrower (as at room temperature) indicating further removal of CO. Meanwhile, the intensity of the white line increases which may be due to now significant oxygen coverage at Pt sites. More interesting changes are indicated in the XES spectra (Figure 6b): high-energy feature at 11562 eV shifts slightly to higher energies as we have observed for the oxidized catalyst in Figure 5b (this is probably a stronger indication of higher oxidation state of Pt than seen in HERFD-XANES), and the feature at 11560 eV preliminary ascribed to Pt carbonyls gains intensity.

According

to

XANES

and

mechanistic

considerations, less CO should be adsorbed on Pt at higher conversions, therefore, increased intensity does not stem from the increased CO coverage. The higher intensity of the C-feature at 11560 eV can stem from higher electronic density at the corresponding molecular orbital, electronic density decreases on Pt as a result of light illumination, leading to higher probability of transition from the ligand. a

2b

and

S12).

Moreover,

temperatures. First, the width (FWHM) of the white line in the HERFD-XANES spectra suggest CO-free surface of Pt, second, its height suggests the presence of oxidic Pt which occurs in CO oxidation at high conversions of CO [29,30]

when Pt surface is not covered with CO and

available for interaction with oxygen. Hence, the oxidation power of oxygen is higher compared to the situation without light, which may be due to activation of oxygen through electron-assisted O2-dissociation process.[4, 5, 9, 10, 12] A shoulder at 11558 eV and absence of the shoulder at 11560 eV in the XES spectra also suggests a direct Pt-O bond instead of a Pt-CO interaction. Under light illumination, the average oxidation state of Pt increases and the conversion of CO slightly decreases as seen by the higher white line in XANES and a small shift of the peak at 11562 eV to higher energies. This is in line with the results by Gänzler et al.[46] who found, that oxidized Pt-particles are less active. The difference between low and high temperature cases is then due to the different limiting steps. At low temperature, the Pt surface is blocked by CO and activation of oxygen is rate-limiting. At higher temperature more CO is desorbed/consumed and more O can dissociatively Alternatively, more reactive oxygen can be produced as

b I / a.u.

1

(Figure

adsorb which, in turn, results in higher CO conversion.

80 °C no light with light

2

I / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

11562.2 eV


11560 eV

demonstrated in this study using light irradiation and this in turn also removes some of the CO. At high temperatures, on the contrary, the Pt surface is covered with O (because the partial pressure of O2 is 100 times

0

higher than of CO) and now adsorption of CO limits CO 11560

11570

11580

Energy / eV

11550

11555

11560

11565

Energy / eV

oxidation. In this case, if irradiation by light further decreases the CO coverage (by activating oxygen or

Figure 6. (a) HERFD-XANES and (b) XES spectra of

simple heating), O coverage will increase and the rate of

Pt/TiO2 measured under CO oxidation feed at 80-85 °C

CO oxidation will decrease due to now oxidized nature

with and without light. Conditions: 1000 ppm CO, 10%

of Pt.[46] Hence, a moderate oxidation state of Pt results

O2 in He, flow rate 135 ml/min.

in the highest reaction rates on Pt/TiO2.[40] This explains

At the temperature of maximum CO conversion turning on light does not increase but even slightly decreases CO

the observed lower CO conversion and the higher white line in XANES.


ACS Catalysis

The reference Pt/Al2O3 catalyst data shows a different

definitely plays a role in the photothermocatalytic

behavior under light irradiation (Figure S17). In this case

oxidation of CO, however, temperature increase by ~180

turning on light at low temperatures decreases the

°C would be required to account for the changes in

"white line" pointing at reduction of oxidic Pt. This is in

DRIFTS spectra while HERFD-XANES / XES and direct

line with reduction of PtOx by CO at higher

temperature measurements point at values on the order

temperatures[29,34,47],

hence proves, that light irradiation

of several tens of degrees. The difference may be

does lead to certain heating of the catalyst bed and

attributed to different probing depth of the techniques

HERFD-XANES is sensitive enough to observe this effect.

with DRIFTS being surface-sensitive and XAS/XES

However, once again, this heating effect is much smaller

probing the bulk. Hence, the photoactivation is limited

than ~180 °C required for the appearance of the CO band

to the catalyst surface only, even such very localized

at 2120

cm-1

seen in DRIFTS of Pt/TiO2.

heating effect is different from the conventional heating. Her et al.[50] have demonstrated that irradiation of

a


2

CO-covered Pt surface with high intensity (laser) light

11562.6 eV

b 133 °C no light with light

I / a.u.

3

I / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 15

causes desorption of CO which, in turn, can enhance CO oxidation activity. In our case of low intensity light, we

11558 eV

relate the photocatalytic rate enhancement, apart from

1

the heating, mostly to activation of O2, similar to the 0

11560

11570

11580

11550

Energy / eV

11555

11560

11565

Energy / eV

O2 through photo-induced electronic transitions leading

Figure 7. (a) HERFD-XANES and (b) XES spectra of Pt/TiO2

measured

under

CO

oxidation

results of Christopher et al.[4] Light irradiation can excite

feed

at

temperature of maximum CO conversion with and without light. Conditions: 1000 ppm CO, 10% O2 in He, flow rate 135 ml/min.

to the lower CO-coverage and the higher oxidation state of Pt (which is visible in HERFD-XAS and vtc-XES) (cf. Figure 8). This was observed over several supports but can be especially promoted on the {101} anatase TiO2 surface

[51]

such as used in this work due to activated

oxygen promoting CO oxidation according to the

Based on these results, we can propose several ways how

Mars-van Krevelen mechanism.[40]

light can enhance CO oxidation on Pt catalysts as shown in Figure 8. The first possible mechanism can be based on light irradiation creating electron-holes in TiO2, which

alters

the

Pt-TiO2

interface

via

strong

metal-support interaction (SMSI). SMSI in the Pt/TiO2 system can change the morphology, size and, thus, the reactivity of Pt species enhancing catalytic oxidation of CO according to our previous work.[34] Besides, according to the Mars-van Krevelen mechanism, oxygen atoms in TiO2 could also take part in the CO oxidation as active species, and the reducibility of TiO2 may be influenced by light.[40] However, these two hypotheses do not agree with the fact that photocatalytic oxidation of CO was also observed over Pt/Al2O3 (Figure S5), in that case, at

Figure 8. Possibilities to circumvent CO poisoning at low temperatures. ■ CONCLUSIONS

least in part, induced by heating effect. Heating by light


Page 11 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

In conclusion, small Pt NPs (< 2 nm) can utilize both

China (U1232119, 51102245) and the Innovative Research

thermal energy and photons for CO oxidation which is

Team of Sichuan Province (2016TD0011). We thank ESRF

similar to plasmonic metal NPs. The combination of

(Grenoble) for provision of time at the synchrotron

operando DRIFTS, HERFD-XANES and XES provides

radiation facility and Christian Henriquet for the

direct evidence for the changes in the coverages of the

technical support during the beamtime at ID20. Y. Z.

CO and O adsorbates on the Pt surface and the

acknowledges financial support by the Alexander von

electronic structure of Pt sites. Not only DRIFTS but also

Humboldt Foundation. P. N. P. and J. J. acknowledge

HERFD-XANES and XES proved to be sensitive enough

support by the state of Baden-Württemberg through

using a high dispersion of Pt to observe changes in the

bwHPC (bwunicluster and JUSTUS, RV bw16G001 and

electronic structure of Pt sites upon light irradiation.

RV bw17D011).

Apart from the temperature increase due to light illumination, it is proposed that oxygen is activated upon light irradiation and reacts with adsorbed CO either directly or via TiO2 support according to the Mars-van

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ACS Catalysis

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