Experimental and Theoretical Understanding of Nitrogen-Doping

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Experimental and Theoretical Understanding of Nitrogen Doping-Induced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation Peirong Chen, Abhishek Khetan, Fengkai Yang, Vadim Migunov, Philipp Weide, Sascha P Stürmer, Penghu Guo, Kevin Kähler, Wei Xia, Joachim Mayer, Heinz Pitsch, Ulrich Simon, and Martin Muhler ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02963 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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Experimental and Theoretical Understanding of Nitrogen DopingInduced Strong Metal-Support Interactions in Pd/TiO2 Catalysts for Nitrobenzene Hydrogenation Peirong Chen,*†‡§ Abhishek Khetan,*§# Fengkai Yang,‡ Vadim Migunov,¶ Philipp Weide,‡ Sascha P. Stürmer,‡ Penghu Guo,‡ Kevin Kähler,‡ Wei Xia,‡ Joachim Mayer,§¶ Heinz Pitsch,§# Ulrich Simon,**†§ and Martin Muhler**‡ †

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen,

Germany ‡

Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany

§

Center for Automotive Catalytic Systems Aachen, RWTH Aachen University, Aachen, Germany

#

Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, 52056

Aachen, Germany ¶

Ernst Ruska Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich

GmbH, 52428 Jülich, Germany * These authors contribute equally to this work. ** Corresponding authors. Emails: [email protected]; [email protected]

ABSTRACT: By doping the TiO2 support with nitrogen, strong metal-support interactions (SMSI) in Pd/TiO2 catalysts can be tailored to obtain high-performance supported Pd nanoparticles (NPs) in nitrobenzene (NB) hydrogenation catalysis. According to the comparative studies by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance CO FTIR (CO-DRIFTS), N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. High-angle annular dark-field scanning transmission electron microscopy study of Pd on N-doped TiO2 confirmed a predominant presence of sub-2 nm Pd NPs, which are stable under the applied hydrogenation conditions. XPS and CO-DRIFTS revealed the formation of strongly coupled Pd-N species in Pd/TiO2 1 ACS Paragon Plus Environment

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with N-doped TiO2 as support. Density functional theory (DFT) calculations over model systems with Pdn (n = 1, 5 or 10) clusters deposited on TiO2(101) surface were performed to verify and supplement the experimental observations. In hydrogenation catalysis using NB as a model molecule, Pd NPs on N-doped TiO2 outperformed those on N-free TiO2 in terms of both catalytic activity and stability, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and the easier desorption of the product aniline. KEYWORDS: hydrogenation, TiO2, palladium nanoparticles, nitrogen doping, strong metalsupport interactions, DFT calculations

1. INTRODUCTION Strong metal-support interactions (SMSI), which were initially used to describe the unusual change of chemisorption and catalytic properties of TiO2-supported platinum-group metal nanoparticles (NPs),1,2 have been intensively investigated for various catalytic applications.1-6 In the last years, the widespread application of in situ or operando spectroscopy, aberrationcorrected electron microscopy and density-functional theory (DFT) calculation led to the understanding of the SMSI effect at the atomic level.7-10 In the regime of SMSI, both structural interactions in the form of mass redistribution and electronic interactions in the form of charge redistribution can occur between the oxides and the supported metal NPs.7-12 Typical phenomena induced by SMSI include the encapsulation of metal NPs by oxide layers at high temperatures, enhanced bonding between the supported metal and the oxide support, formation of metal nanostructures in close contact with the oxide support, and electron transfer between metal NPs and the oxide support.7-12 These interactions, as revealed by mechanistic investigations of model systems consisting of metal NPs supported on oxide films deposited on single crystals, consequently influence and even dominate the catalytic behavior of the supported metal NPs.4,10,13,14 For example, in the water gas shift reaction, 2 ACS Paragon Plus Environment

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Pt/CeO2 showed very high activity due to the electron transfer from the Pt NPs to the CeO2 support and the reverse spillover of activated oxygen from the CeO2 support to the Pt NPs.10 In furfuraldehyde hydrogenation using TiO2-supported Pt NPs as catalyst, the electron transfer from oxygen vacancies at the TiO2 surface to furfuraldehyde led to the formation of a furfuryl-oxy intermediate, which is a highly active and selective precursor to furfuryl alcohol.13 Although a model description of SMSI phenomena may differ from the actual reaction conditions, it does provide important design criteria for the development of metal/oxide catalysts with improved performance. Several approaches, such as tailoring the size or morphology of the oxide supports,15-17 doping the oxide supports with metallic or nonmetallic elements,18 creating surface defects on the oxide supports,9 controlling the size and dispersion of the supported metal particles16 and forming bimetallic or intermetallic compounds19 were developed in order to take advantage of the SMSI effect in catalysis. In the liquid-phase hydrogenation of nitrobenzene (NB), by using CeO2 NPs rather than low surface area CeO2 as the support of Au NPs, the selectivity to azobenzene can be significantly improved from 15% to >95% over Au/ CeO2 catalyst.15 Compounding Pd or Rh with more electronegative metals (e.g. Pb) on SiO2 formed polar sites in the catalyst favoring the chemoselective hydrogenation of p-nitrostyrene to p-aminostyrene.19 A recent study showed that atomically dispersed Pd on TiO2 nanosheets exhibited high activity for hydrogenating alkenes and aldehydes, and outperformed significantly Pd NPs supported on carbon.20 In our previous studies, by rationally introducing N-containing functional groups onto carbon surfaces, we were able to achieve favorable structural conditions (e.g. improved metal dispersion) and electronic interactions (e.g. modified oxidation state of the supported metals) between the carbon substrate and the supported metal NPs (Pd, Pt, Co).21-23 The structural and electronic promoting effects induced by N-doping were found to improve the activity and/or 3 ACS Paragon Plus Environment

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selectivity in gas-phase hydrogenation of olefins or nitroaromatics, and liquid-phase alcohol oxidation.21-24 For catalysts consisting of metal NPs on oxide supports such as TiO2 or ZnO, the N-doping strategy was shown to be highly beneficial in photocatalytic reactions.25-27 However, it is still not sufficiently understood whether N-doping is able to induce SMSI effects promoting the catalytic performance of supported metal catalysts in hydrocarbon conversion reactions such as selective hydrogenation. In this work, a comparative study by supporting Pd NPs on TiO2 after post-treatment in different atmospheres (NH3, H2, He) is performed in order to clarify the effect of the created N-dopants and defects on the structural, electronic and catalytic properties of supported Pd NPs. Combined experimental and theoretical investigations by CO chemisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution electron microscopy, and first-principles DFT calculations were conducted to understand the structural and electronic interactions in the synthesized Pd/TiO2 catalysts at the atomic level. Catalytic activities of the Pd/TiO2 catalysts were evaluated in the gas-phase hydrogenation of NB to aniline (AN) as a model reaction. Mechanistic understanding was achieved by diffuse reflection infrared Fourier transform spectroscopy using CO as a probe molecule (CODRIFTS), and DFT modeling of NB and AN adsorption on Pd/TiO2 supercells.

2. EXPERIMENTAL SECTION 2.1. Catalyst preparation As-received anatase TiO2 (Sachtleben Chemie, Germany) was treated at 550 °C for 48 h in He, H2 or NH3 resulting in three modified TiO2 samples, i.e., TiO2-He, TiO2-H2 and TiO2-NH3, respectively. Pd NPs pre-formed via a modified colloidal method21,28 were deposited on the as-received and modified TiO2 samples. The obtained powders were subsequently treated at 223 °C in He and reduced at 200 °C in H2, resulting in four catalysts designated as Pd/TiO2-A, Pd/TiO2-He, Pd/TiO2-H2 and Pd/TiO2-NH3, respectively. 4 ACS Paragon Plus Environment

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2.2. Physicochemical characterization The as-received and modified TiO2 samples were characterized by N2-physisorption, temperature-programmed desorption using NH3 as a probe molecule (NH3-TPD), and scanning electron microscopy (SEM). Static N2-physisorption measurements were carried out at 77 K using an Autosorb-1 MP Quantachrome system. The TiO2 samples were degassed at 200 °C for 2 h before the measurements. NH3-TPD was performed following a procedure reported elsewhere.22 Briefly, 100 mg of TiO2 samples were first saturated with diluted NH3 (4000 ppm in He, 100 ml min-1), then heated to 650 °C in pure He at a ramp of 5 K min-1. The desorbed NH3 was recorded using a pre-calibrated, non-dispersive infrared detector (Emerson Process Management, Rosemount NGA 2000 MLT 4). SEM analysis was conducted using a Leo supra 35 VP microscope (Zeiss). The specimens for SEM were prepared by ultrasonically dispersing the powder samples in ethanol and dropping the suspension on Si wafer. The Pd/TiO2 catalysts were examined using inductively coupled plasma-optical emission spectroscopy (ICP-OES), XRD, CO chemisorption, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron microscopy (HRTEM), XPS, and CO-DRIFTS. ICP-OES was performed with an UNICAM PU701 instrument to determine the Pd loadings on TiO2 supports. XRD patterns were recorded in a Philips X’Pert MPD system with Cu Kα radiation. CO-chemisorption was performed in a stainless-steel microreactor setup equipped with He and a CO/Ar/He mixture (1.0 vol% CO and 1.0 vol% Ar in He). For each experiment, 70 mg of catalyst powders were used and pretreated at 200 °C before CO adsorption. Gas analysis was conducted by a calibrated online mass spectrometer (Balzers GAM 400). Pd dispersion was calculated based on the adsorbed amounts of CO and the Pd loadings obtained by ICP-OES, assuming a Pd/CO adsorption stoichiometry of 1.5.29 HAADF-STEM and HRTEM measurements were carried out with an aberration-corrected FEI Titan 80-300 STEM microscope and an aberration5 ACS Paragon Plus Environment

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corrected FEI Titan 80-300 TEM microscope (both were operated at 300 kV), respectively. The specimens for HAADF-STEM and HRTEM were prepared by ultrasonically dispersing the powder samples in ethanol and dropping the suspension on a carbon-coated Cu grid. XPS measurements were performed in an ultra-high vacuum setup equipped with a high-resolution Gammadata Scienta SES 2002 analyzer and a monochromatic Al Kα X-ray source (1486.6 eV, operated at 14.5 kV and 30 mA). The pressure inside the measuring chamber was kept in the range of 3.5 to 6 × 10−10 mbar during each measurement. The spectra were taken at a pass energy of 200 eV (high pass energy mode). Charging effects due to the insufficient conductivity of the samples were mediated by applying a flood gun (SPECS). CasaXPS program was employed in the analysis of XPS data. Calibration of the measured spectra was performed by positioning the Ti 2p3/2 peak at 459.2 eV for TiO2.30 CO-DRIFTS studies over the TiO2 and Pd/TiO2 samples were performed in a Nicolet Nexus spectrometer equipped with a MCT (mercury cadmium telluride) detector and a Nicolet environmental cell with ZnSe windows. About 10 mg of fine powders were loaded for each CO-DRIFTS experiment. The samples were first treated at 110 °C in flowing He for 2 h, then cooled down to 50 °C, and finally saturated with diluted CO (1 vol% CO in He) at 50 °C for 1 h. The weakly adsorbed CO molecules were removed by purging the samples with flowing He before taking spectra. A spectral resolution of 4 cm−1 was chosen for all the measurements. 2.3. Catalytic tests The hydrogenation of NB is one of the most widely used approaches for the production of AN, which is an indispensable raw material in the production of methylene diphenyl diisocyanate. The proposed reaction pathway of NB hydrogenation is shown in Scheme 1. NB can be partially hydrogenated to nitrosobenzene (NSB) and fully hydrogenated to AN.

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Nitrobenzene (NB)

Nitrosobenzene (NSB)

O

Aniline (AN)

O

N O

N

H2

H N

H2

H

Scheme 1. Reaction pathway of nitrobenzene hydrogenation. The NB hydrogenation reactions in this study were performed in the gas phase using a fixed-bed reactor under plug-flow conditions. A constant NB flow was obtained by passing He through liquid NB at 70 °C. For each test, 10 mg of freshly reduced Pd/TiO2 catalysts diluted in 1 g quartz sand were loaded. For the NB hydrogenation at different temperatures (100-200 °C; 20 °C per step; ramping up), a gas feed containing 0.25 vol% NB, 2.5 vol% H2 and 97.25 vol% He was applied; the hydrogenation reaction was stabilized for 30 min at each temperature. For the NB hydrogenation at a constant temperature of 160 °C, a gas feed containing 0.15 vol% NB, 6 vol% H2 and 93.85 vol% He was applied. The reactants and products were monitored by an online gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector, a HP-5 capillary column and a Valco sample loop. Control experiment without any catalyst was performed prior to the catalytic tests, and conversion of NB was not detected under applied conditions. The conversion of NB (XNB) and the selectivity to AN (SAN) and NSB (SNSB) were calculated according to Eqs. 1, 2 and 3, respectively. XNB =

CNB , 0 − CNB ×100% CNB , 0

(1)

SAN =

CAN × 100% CNB , 0 − CNB

(2)

SNSB =

CNSB × 100% CNB , 0 − CNB

(3)


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In the equations, CNB,0 is the initial NB concentration in the gas feed, CNB, CAN and CNSB are the concentrations of NB, AN and NSB monitored by the online GC during the hydrogenation reaction. A carbon balance of 100±6 % was achieved in all the tests. Turnover frequencies (TOFs) were calculated based on the conversion rates of NB (in mol min-1, determined according to the measured NB conversion) and the amount of active Pd sites (in mol, based on the Pd loading by ICP-OES and the Pd dispersion by CO chemisorption) in the freshly reduced Pd/TiO2 catalysts. 2.4. Computational details The first-principles DFT calculations were performed using the VASP code.31 All the calculations were carried out using spin polarized GGA+U with Perdew-Burke-Ernzerhof exchange-correlation functional (PBE).32,33 The spin-polarized Kohn-Sham equations were solved in the plane-wave and pseudopotential framework with a cutoff of 500 eV, on a 6x5x1 Monkhorst –Pack kpoint grid. We used a U=4.5 eV, similar to the value of U=4.2 eV which is recommended by de Gironcoli and Cococcioni in the LDA+U implementation using a selfconsistent linear response approach.34,35 Pd/TiO2 systems consisting of Pdn (n = 1, 5, or 10) cluster on anatase TiO2 (101) phase were considered. The computational details for Pd1/TiO2 systems can be found in the Supporting Information. For Pd5 and Pd10 clusters, the TiO2(101) surface was modelled using three tri-layers of TiO2 supercell slab containing 36 Ti and 72 O atoms separated by more than 16 Å. The atoms in the bottom tri-layer were constrained to their equilibrium positions to simulate bulk behavior, while all the other atoms were free to move. The differential changes in electronic charge, ∆σPd, on Pd nucleus (nuclei) in a Pdn/TiO2 system, with or without N-dopant, can be given by ∆σPd = σPd/TiO2 - σPd - σTiO2

(4) 8

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where σPd/TiO2 represents the electronic charge in the total Pdn/TiO2 system evaluated by Bader charge analysis using the code from the Henkelmann group.36 Similarly, σPd represents the charge on the Pdn cluster alone, with atoms fixed at their converged positions from the total system. σTiO2 represents the charge on the TiO2 system alone with the positions fixed at the converged positions from the total system. The differential charge on the Pd nucleus (nuclei) upon doping, §Pd, can be evaluated by taking the difference between the ∆σPd of the doped and the non-doped systems as §Pd = ∆σPd (with N doping) - ∆σPd (no doping)

(5)

3. RESULTS AND DISCUSSION 3.1. Catalyst synthesis and characterization Thermal treatment in dynamic gas atmospheres has been shown to be efficient for creating defects (e.g. oxygen vacancies) or introducing dopants on the surface of TiO2.26,37,38 Several properties of TiO2 including color, morphology, specific surface area, porosity and surface acidity were changed due to the treatments (Figure S1 and Table S1). The color change from white (TiO2-A) to bluish (TiO2-He and TiO2-H2) or yellowish (TiO2-NH3) is due to the presence of defects (e.g. oxygen vacancies created by thermal treatment in He or reduction in H2) or N-dopants.26,37,38 The morphology change due to thermal treatments (see SEM images in Figure S1) may result from the agglomeration of small TiO2 particles under dynamic gas conditions.7 The change of surface hydrophilicity/hydrophobicity, resulting from the removal of surface hydroxyl groups, is likely to play an important role in the agglomeration process.39 In this regard, TiO2-NH3 with the lowest surface acidity (as indicated by the NH3 uptake in NH3 TPD studies; Table S1) displays a very different morphology (Figure S1). Depending on the applied gas atmosphere, the specific surface area and pore volume of TiO2 were decreased to different degrees (Table S1). The crystal structure of TiO2 was largely retained according to XRD analysis (not shown), indicating that the introduced defects and N-dopants did not affect 9 ACS Paragon Plus Environment

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the long-range order of TiO2. Four Pd/TiO2 catalysts, namely Pd/TiO2-A, Pd/TiO2-He, Pd/TiO2-H2 and Pd/TiO2-NH3, were prepared by depositing Pd NPs no larger than 2 nm on the as-received and differently treated TiO2 powders.21 The physicochemical properties of the Pd/TiO2 catalysts after reduction were thoroughly characterized using different methods including ICP-OES, CO chemisorption, N2 physisorption, HAADF-STEM, HRTEM, XPS, and DRIFTS using CO. Table 1. Physicochemical properties and catalytic activities of the Pd/TiO2 catalysts (after reduction at 200 °C). Pd wt% a

Pd at% b

Pd/TiO2-A

1.33

Pd/TiO2-He

Catalyst

a

SBET

c

Pore volume c

Dd

dp d

TOF e

(nm)

(min-1)

(m2 g-1)

(cm3 g-1)

0.19

103

0.45

0.32

3.4

4.1

1.38

0.32

67

0.33

0.39

2.8

3.7

Pd/TiO2-H2

1.33

0.45

57

0.29

0.44

2.5

2.7

Pd/TiO2-NH3

1.28

0.46

67

0.32

0.35

3.1

14.6

Pd weight loading (wt%) determined by ICP-OES. b Surface atomic concentration (at%) derived from

the quantitative XPS analysis. physisorption measurements.

d

c

Specific surface area (SBET) and pore volume derived from N2

Dispersion (D) and average particle size (dP) of the supported Pd

particles determined by CO chemisorption. e Calculated turnover frequencies (TOF) after 240 min of time on stream in NB hydrogenation (XNB < 100% for all the catalysts).

The achieved Pd loadings were found to be ca. 1.3 wt% in all the four Pd/TiO2 catalysts without significant variation (Table 1). The Pd dispersion of the freshly reduced Pd/TiO2 catalysts was determined by CO chemisorption to be 0.38±0.06, corresponding to an average Pd particle size of 3.0±0.5 nm (Table 1). As compared to TiO2-A, the post-treated TiO2 supports allow a higher Pd dispersion and, in turn, a smaller average Pd particle size, pointing to the beneficial effects from the created surface defects or nitrogen dopants.21-23 It has to be noted that the determination of Pd dispersion by CO chemisorption depends on the chosen Pd/CO stoichiometry, which remains as a strong debate because of the co-existence of both linear and bridged adsorption of CO on Pd (see the CO-DRIFTS results in section 3.4). Besides, SMSI is known to influence strongly the CO adsorption on Pd sites leading to 10 ACS Paragon Plus Environment

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additional uncertainties in CO chemisorption.29 Therefore, further evidence from other techniques is required for a better comparison of the Pd dispersion and particle size on different TiO2 supports.

Figure 1. XRD patterns of Pd/TiO2 catalysts after reduction at 200 °C. The inset shows an enlarged view of the Pd(111) reflections after background subtraction and the estimated full width at half maximum (FWHM; indicated by the horizontal lines with arrowheads and the corresponding values).

In the XRD patterns shown in Figure 1, intense peaks were detected at 38°, 49°, 54° and 55° in all the four catalysts due to the characteristic (004), (200), (105) and (211) reflections of anatase TiO2, respectively.40 Interestingly, while the Pd(111) reflection at ca. 40.2° was clearly observed in Pd/TiO2-A, Pd/TiO2-He or even Pd/TiO2-H2, that of Pd/TiO2-NH3 is significantly less intense and hardly visible in the XRD pattern. Considering that the four catalysts have similar Pd loadings (Table 1), the clearly weaker Pd(111) reflection in Pd/TiO2NH3 can be attributed to a significantly smaller number of relatively larger Pd particles (above 11 ACS Paragon Plus Environment

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5 nm) that are detectable by XRD.41,42 A comparison of the peak width (in terms of full width at half maximum, i.e. FWHM) demonstrated that the Pd(111) reflection peak of Pd/TiO2-NH3 is clearly broader than those of the other catalysts (inset in Figure 1), suggesting a smaller crystalline size of the detected Pd NPs on TiO2-NH3.41,42 A similar smaller particle size effect was observed by XRD also in Pd/TiO2-NH3 after reduction at a higher temperature of 400 °C (Figure S2a) or with a higher Pd loading of ca. 3 wt% (Figure S2b), demonstrating the outstanding ability of N-dopants for the efficient anchoring of small Pd NPs against sintering.21-23,43 This observation is in good agreement with a previous report showing that Ndoped TiO2 can effectively anchor and stabilize supported metal NPs due to the strong metalN interaction.25 The loading and reduction of Pd NPs did not change noticeably the surface areas of the respective TiO2 support (Table S1).

Figure 2. HAADF-STEM images for freshly reduced Pd/TiO2 -He (a) and Pd/TiO2 -NH3 (b). The readers are referred to the electronic version of this work for a better visualization of the sub-2 nm Pd NPs.

Both relatively large Pd NPs (above 3 nm) and highly dispersed Pd NPs or even clusters (below 2 nm) were observed on Pd/TiO2-He and Pd/TiO2-NH3 by HAADF-STEM using an aberration-corrected microscope. The HAADF-STEM images (also known as Z-contrast images) revealed a predominant presence of sub-2 nm Pd NPs or clusters together with seldom presence of Pd NPs larger than 3 nm in both catalysts (Figures 2, S3 and S4). Such 12 ACS Paragon Plus Environment

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predominant presence of sub-2 nm Pd NPs or clusters was also observed in Pd/TiO2-NH3 after NB hydrogenation reaction (Figure S5). In HRTEM, while the relatively large Pd NPs (3-3.5 nm) in Pd/TiO2-NH3 can be clearly observed (Figure 3), a clear visualization of the sub-2 nm Pd NPs or clusters is practically challenging due to the very low Z contrast between Pd and TiO2.

Figure 3 HRTEM images for Pd NPs in the freshly reduced Pd/TiO2-NH3. The red arrows in images a and b indicate the locations of the Pd NPs. The exposed facets of a Pd NP (ca. 3.4 nm) in Pd/TiO2-NH3 are indicated in (c), along with the fast Fourier transform (FFT) pattern of the NP (inset in c).

The chemical nature of Pd on different TiO2 supports was investigated by XPS. In addition to Pd, Ti, O and N (only for Pd/TiO2-NH3), carbonaceous species were observed for all the catalysts due to carbon contamination during the storage and transfer of the Pd/TiO2 catalysts under ambient conditions. The collected XP Pd 3d, Ti 2p, and O 1s spectra of the freshly reduced Pd/TiO2 catalysts are displayed in Figures 4, S4a and S4b, respectively. In the 3d5/2 portion of the XP Pd 3d spectra, metallic Pd (Pd0 ) with a binding energy (BE) of 335.0±0.1 eV (for the Pd 3d5/2 peak) is dominating in catalysts Pd/TiO2-A, Pd/TiO2 -He and Pd/TiO2 -H2. As compared to that of Pd/TiO2-A, the Pd 3d5/2 peak for Pd/TiO2 -He and Pd/TiO2-H2 shifted slightly to a higher BE by ca. 13 ACS Paragon Plus Environment

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0.1 eV (Table S2), which can be assigned to more pronounced final-state effects in small Pd NPs (2-5 nm). 44,45 This observation of final-state effects suggests a smaller particle size of Pd in Pd/TiO2-He and Pd/TiO2 -H2 than in Pd/TiO2-A, corresponding to the results by CO chemisorption (Table 1). Surprisingly, a significant shift of ca. 0.5 eV to higher BE positions was observed for Pd 3d5/2 in the catalyst Pd/TiO2 -NH3 (Figure 4 and Table S2). Such shift, which is notably more pronounced than that induced by the finalstate effects (as in the case of Pd/TiO2 -He or Pd/TiO2-H2 ) but less pronounced than the BE difference between Pd0 and Pd2+ in PdO (ca. 1.1 eV), suggests the presence of electron-deficient Pd species (Pdn+ ) exclusively on the TiO2-NH3 support. A significant electronic modification by the N-dopants is likely responsible for this change of Pd oxidation state.25 In the 3d3/2 part of the Pd 3d spectra, the corresponding shift can be clearly seen (Figure 4). The electron-deficient Pd NPs in supported Pd catalysts were found to be highly active in the activation of –NO2 groups in nitroarene molecules and are crucial for superior catalytic performance in nitroarene hydrogenation reactions.46 No traceable difference was observed in the Ti 2p and O 1s spectra for the 4 Pd/TiO2 catalysts (Figure S6) due to the low amount of created defects or nitrogen species in the post-treated TiO2.47 Quantitative XPS studies revealed significantly different atomic concentrations of Pd (Pd at%) on the surface of Pd/TiO2 catalysts despite similar Pd loadings (Table S2). While a Pd content (at%) of only 0.19% was achieved on TiO2 -A, the Pd content was significantly increased to 0.32% on TiO2-He, 0.45% on TiO2 -H2, and 0.46% on TiO2 NH3, respectively (Table S2). This increase implies that higher degrees of Pd dispersion were achieved on the post-treated TiO2 samples. The higher Pd at% values on TiO2 -H2 and TiO2 -NH3, resulting from a better dispersion of the Pd species, indicate that both surface defects and N-dopants could serve as anchoring sites to retain small Pd NPs 14 ACS Paragon Plus Environment

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under a reductive atmosphere.21,25,42 Nevertheless, Pd/TiO2-NH3 possesses the highest Pd at% value among the four catalysts with similar Pd loadings, confirming that TiO2NH3 is the most advantageous support for the highly dispersed Pd NPs.

Figure 4. XP Pd 3d spectra of freshly reduced Pd/TiO2 catalysts. Before the XPS measurements, the samples were stored and transferred under ambient conditions (in air).

The XP N 1s spectra of TiO2 -NH3 and Pd/TiO2-NH3 were examined and compared to understand the possible Pd-N interaction. The deconvoluted N 1s spectra are shown in Figure 5. For both TiO2 -NH3 and Pd/TiO2-NH3, three different nitrogen species can be distinguished from the N 1s spectra at 396.7 eV (N1), 397.8 eV (N2), and 400.3 eV (N3), which can be assigned to substitutional N at the O sites (i.e. nitride N in Ti–N bonds), interstitial N bonding with the lattice O, and surface NOx or NHx species, respectively.47 15 ACS Paragon Plus Environment

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Table 2 summarizes the quantitative analysis results from the deconvolution of the N 1s spectra. It can be seen that the loading of Pd led to a decrease of all the nitrogen species. While the N3 species seems to be largely intact, the amounts of N1 and N2 species decreased considerably due to Pd loading suggesting that these nitrogen species may be the preferential anchoring sites for the Pd species. It has to be noted that the low overall N 1s intensity for both samples may lead to a relatively high uncertainty in the quantitative analysis.

Figure 5. Deconvoluted XP N 1s spectra for TiO2-NH3 and Pd/TiO2-NH3. Empty circles refer to the measured data points. Table 2 Atomic concentrations (at%) of the surface nitrogen species derived from the deconvoluted XP N 1s spectra. Total

N1

N2

N3

(396.7 eV)

(397.8 eV)

(400.3 ev)

TiO2-NH3

0.76

0.36

0.14

0.26

Pd/TiO2-NH3

0.53

0.24

0.07

0.22


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3.2. DFT calculations In order to validate the electronic modification of Pd by the TiO2-NH3 support, we performed DFT calculations with model systems consisting of a Pdn (n = 1, 5, or 10) cluster deposited on the TiO2(101) surface.25 For the N-doping of TiO2, three different situations, namely Nadsorption on the surface, N-substitution of O on the surface and N-substitution of O in the subsurface layer, were first examined. The optimized geometries of N-doped TiO2 and Pd1/TiO2 systems are displayed in Figures S7 and S8, respectively. The electron charge density distribution in the Pd1/TiO2 systems are shown in Figure S9. In all the three N-doping cases, charge depletion was observed around the Pd nucleus and was found to increase as the proximity of the N atom to the Pd atom increased. These observations indicate that the effect of N-doping is more pronounced when Pd and N atoms can couple electronically on the surface. Electron accumulation was observed around the N nucleus leading to a strongly interacting Pd-N couple, which is in agreement with literature.25 The energetically most stable N-doping case, i.e. N-substitution of O atom on the surface, was then considered for the Pd5/TiO2 and Pd10/TiO2 systems to investigate the interactions between Pdn clusters and N-doped TiO2. The converged geometries for the Ndoped TiO2, Pd5/TiO2 and Pd10/TiO2 systems are presented in Figure 6 and are found to be in agreement with existing reports.48 Electronic charge reorganization because of support, ∆σPd, for the case of Pd5/TiO2 systems without and with N-dopant is shown as an isosurface in Figure 7, where yellow regions indicate excess electronic charge and blue regions indicate charge depletion. As can be clearly seen, there is a significant amount of charge reorganization upon N-doping. The N atom has excess electrons, whereas the Pd5 cluster has clear charge depletion. The charge reorganization by N-doping leads to a strongly interacting Pd-N couple similar as that observed in Pd1/TiO2 systems (Figure S9). These findings by first-


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principles calculations confirm our hypothesis that the electron-deficient Pd species in Pd/TiO2-NH3 by XPS originate from the formation of Pd-N couples .25

Figure 6. Top (a-c) and side (d-f) views of the optimized geometries of surface N-doped TiO2 (101) termination without Pd (a, d), with Pd5 cluster (b, e) and with Pd10 cluster (c, f). Spheres in red, blue, gray and light purple denote oxygen, titanium, palladium and nitrogen, respectively.

Figure 7. Electron charge density distribution for a Pd5 cluster on (a) non-doped TiO2 and (b) TiO2 with N-substitution on the surface. The figures show the isosurfaces of the change in electron charge density between the non-doped and N-doped systems. Spheres in red, blue, gray and light purple


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denote oxygen, titanium, palladium and nitrogen, respectively. The yellow and blue regions represent electronic charge accumulation and depletion, respectively. Table 3. Charge depletion on the Pdn (n = 5 or 10) clusters on non-doped and N-doped TiO2 using Bader charge analysis. Pdn/TiO2 system

∆σPd

∆σPd

(number of N atoms)

(Non-doped)

(N-doped)

Pd5/TiO2 (1)

0.038

0.801

0.763

Pd10/TiO2 (1)

1.178

1.478

0.300

Pd10/TiO2 (2)

1.178

1.663

0.485

§Pd

Quantitative estimates of charge depletion in terms of ∆σPd and §Pd are presented in Table 3. The difference in charge depletion upon N-doping was found to be 0.76 electrons for the Pd5 cluster and 0.30 electrons for the Pd10 cluster. A TiO2 supercell with two N-dopants on the surface was also considered as the support for the Pd10 cluster. Increasing the N-doping level was found to result in a more pronounced charge depletion on the Pd10 cluster (Table 3), suggesting that increasing the amount of N-dopants on the TiO2 surface can be an efficient way to tune the electronic structures and eventually the catalytic properties of the supported Pd. 3.3. Catalytic performance in NB hydrogenation The Pd/TiO2 catalysts after reduction were tested in NB hydrogenation in order to clarify the effect of defects or nitrogen dopants on the catalytic performance of support Pd NPs. Figure 8 compares the NB conversion of freshly reduced (at 200 °C or 400 °C) Pd/TiO2-A and Pd/TiO2-NH3 at different reaction temperatures (100-200 °C). After reduction at 200 °C, Pd/TiO2-NH3 achieved full conversion of NB at all the selected temperatures. The 200 °C reduced Pd/TiO2-A showed a significantly lower NB conversion than Pd/TiO2-NH3 at the all reaction temperatures and achieved a NB conversion of only 66% even at an elevated reaction temperature of 200 °C. Although both catalysts were considerably deactivated by reduction at 19 ACS Paragon Plus Environment

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the higher temperature of 400 °C, Pd/TiO2-NH3 still outperformed significantly Pd/TiO2-A in terms of NB conversion. The deactivation of Pd/TiO2 catalysts by high-temperature reduction has been frequently reported in literature and can be attributed to the encapsulation of Pd NPs by a reduced TiOx over-layer.2,7,11 As demonstrated by SEM studies (Figure S1), a clear more pronounced agglomeration of small TiO2 particles occurred during the treatment in NH3. During reduction at high temperature, the encapsulation of Pd NPs by a TiO2 over-layer may be prohibited to a greater extent because of a lower number of small and mobile TiO2 particles in TiO2-NH3.7 On the other hand, the strong anchoring ability of N-sites allows to maintain the small Pd NPs against severe agglomeration during reduction at high temperature, as suggested by the hardly visible Pd(111) reflection in the XRD pattern for Pd/TiO2-NH3 reduced at 400 °C (Figure S2a) and previous studies.22,43 Thus, the catalytic activity of Pd/TiO2-NH3 was retained to a large extent even after reduction at 400 °C.

Figure 8. The conversion of NB over freshly reduced Pd/TiO2-A (black squares) and Pd/TiO2-NH3 (red circles) at temperatures from 100 °C to 200 °C (ramping up). The Pd/TiO2 catalysts were reduced off-site at 200 °C (solid symbols) or 400 °C (empty symbols). Gas feed: 0.25 vol% NB, 2.5 vol%, He balance.


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Figure 9a demonstrates the temporal NB conversion of freshly reduced (at 200 °C) Pd/TiO2-A, Pd/TiO2-He, Pd/TiO2-H2 and Pd/TiO2-NH3 in hydrogenation reactions at 160 °C. Pd/TiO2-A showed a NB conversion of ca. 52% after 30 min and deactivated continuously with increasing reaction time. Although the initial degrees of NB conversion were high over Pd/TiO2-He (100%) and Pd/TiO2-H2 (85%), both catalysts deactivated rapidly reaching similar NB conversion as Pd/TiO2-A after 240 min (ca. 30%). Pd/TiO2-NH3 displayed a high activity as indicated by full conversion after 30 min and, more importantly, a high stability with only negligible deactivation even after 240 min. A comparison of the TOF numbers after 240 min of time on stream (XNB < 100% for all the catalysts) emphasizes the significantly higher performance of Pd on TiO2-NH3 compared with N-free TiO2 supports (Table 1). To examine the resistance to NB poisoning,49 the freshly reduced Pd/TiO2-He and Pd/TiO2-NH3 catalysts, which both achieved full conversion of NB at 30 min, were intentionally exposed to flowing NB without H2 prior to the hydrogenation tests. As compared with the freshly reduced catalysts, both Pd/TiO2-He and Pd/TiO2-NH3 were slightly deactivated by the exposure to the NB stream for 5 min (Figure 9b). After 30 min of exposure, however, Pd/TiO2-He was more significantly deactivated than Pd/TiO2-NH3, revealing that the resistance to NB poisoning was remarkably higher for Pd NPs on TiO2-NH3 than on TiO2-He. Further comparative studies revealed that reduction in H2 reactivated considerably the partially deactivated Pd/TiO2-NH3, but hardly the partially deactivated Pd/TiO2-He (Figure S10). NB hydrogenation over Pd/TiO2 catalysts with a higher Pd loading of ca. 3 wt% (Figure S11) shows that increasing the Pd loading did not essentially alter the deactivation behavior of the freshly reduced Pd/TiO2-He and Pd/TiO2-NH3 catalysts. Therefore, the properties, rather than the amount of the Pd surface atoms, are decisive for the catalytic performance in NB hydrogenation under the applied reaction conditions.


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Figure 9. Temporal conversion of NB at 160 °C over Pd/TiO2 catalysts (a) in the freshly reduced state and (b) after exposure to NB stream (the same gas feed for hydrogenation reaction but without H2 ) for different durations. The catalysts were reduced off-site at 200 °C before the tests. Gas feed: 0.15 vol% NB, 6.0 vol% H2 , He balance.

The performance of the Pd/TiO2 catalysts was also compared to commercial Pd/C and Pd/Al2O3 hydrogenation catalysts under different testing conditions. At a fixed reaction temperature of 160 °C (similar as Figure 9), both commercial catalysts displayed full NB conversion throughout a test period of 240 min. In temperature-varied catalytic tests (100200 °C; similar as Figure 8), while the Pd/C required a reaction temperature higher than 140 °C to achieve a full NB conversion, the Pd/Al2O3 catalyst deactivated at temperatures above 140 °C (Figure S12). On the contrary, the Pd/TiO2-NH3 catalyst showed full NB conversion at all the selected temperatures (Figure 8), demonstrating that Pd/TiO2-NH3 possesses superior activity and stability in NB hydrogenation over a relatively wide temperature range. The selectivity to AN was observed to be 100% in all the above-mentioned


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hydrogenation tests, except that Pd/C showed a drop of AN selectivity from 100% to ca. 96.5% after 180 min at 160 °C. 3.4. Mechanistic studies CO-DRIFTS was performed over the Pd/TiO2-He and Pd/TiO2-NH3 catalysts in order to further investigate the differences in particle size, oxidation state, and in particular chemical reactivity of the supported Pd NPs. The spectra obtained with the two Pd/TiO2 catalysts and the corresponding TiO2 supports are shown in Figure 10. Chemisorption of CO was not observed on the TiO2 support regardless of treatments in NH3 or He, implying a low chemical reactivity of the TiO2 supports. In case of Pd/TiO2-NH3, a strong band appeared at 2082 cm-1 due to the linear CO adsorption on Pd0 sites in very small Pd NPs, whereas the relatively weak band at ca. 1945 cm-1 is attributed to bridged CO adsorbed on Pd0 sites.50,51 For Pd/TiO2-He, the band at about 1945 cm-1 is stronger than that at 2082 cm-1 as a consequence of a larger particle size of Pd.50 Minor contributions resulting from CO adsorption on Pd2+ (2173 cm-1) and Pd+ (2121 cm-1) were observed as well in both catalysts.50,51 Surprisingly, a strong band at 2210 cm-1, which has been attributed to -NCO species in literature,5,52 was observed exclusively in the DRIFT spectrum for Pd/TiO2-NH3. Considering that no additional N-containing source was fed to the system, such -NCO species can only form by association of dopant N atoms on the support with CO adsorbed nearby on Pd NPs, thus confirming the formation of Pd-N moieties in the catalyst.5,52 This unexpected observation of -NCO species corroborates further that the electron deficiency of Pd detected by XPS is due to the strong Pd-N interactions, rather than Pdn+ in oxidic form as also indicated by the weak bands at 2173 cm-1 and 2121 cm-1.


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Figure 10. DRIFT spectra of freshly reduced Pd/TiO2 catalysts and the corresponding TiO2 supports after CO adsorption at 323 K.

Comparing the two Pd/TiO2 catalysts, we clearly see that the overall intensities of the characteristic bands, in particular the one at 2082 cm-1 for linear CO adsorption on highly dispersed Pd0 sites, are much stronger on Pd/TiO2-NH3 than on Pd/TiO2-He, indicating a larger amount of active Pd sites available for CO chemisorption. The CO-DRIFTS studies also provide additional evidence for the smaller Pd particle sizes on TiO2-NH3 as indicated by XRD and XPS. The adsorption of the reactant NB and the product AN on Pd1/TiO2 (with or without N doping) was calculated using DFT for gaining a mechanistic understanding of the catalytic reaction. N-substitution of O on the surface (Figure S8c) was taken as an example for Ndoped TiO2. As illustrated in Figure 11, both NB and AN are perpendicularly adsorbed on the 24 ACS Paragon Plus Environment

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Pd atom in accordance with literature results.53 As seen from the adsorption geometry of NB (Figure 11b) on Pd/TiO2 with N-dopant, while the N atom in the -NO2 group of NB couples intimately with the Pd atom, one of the two O atoms in the -NO2 group intimately couples with the N-dopant on the TiO2 surface. As a result, the length of the N-O bond was extended from 1.2 Å (Figure 11a) to 3.0 Å (Figure 11b) suggesting the dissociative adsorption of NB. The calculation of the adsorption energies (Eads) of NB on Pd/TiO2 revealed that N doping shifted Eads negatively by ca. 1.0 eV, indicating a stronger adsorption of NB. Although the adsorption geometry of AN was not significantly changed by N-doping (Figures 11c and d), Eads was calculated to be shifted positively by ca. 0.2 eV, pointing to a weaker adsorption of AN on Pd/TiO2 with N-dopant. While it is clear that a better dispersion of Pd species on TiO2-NH3, as evidenced by XRD, XPS and CO-DRIFTS, results in more active Pd sites for the adsorption and activation of NB,20 the strong electronic interaction between Pd and N altered the adsorption geometry of NB, leading to the activation of the NB molecule by weakening the N-O bond (Figure 11b).54,55 As a result, Pd is significantly more active and stable on TiO2-NH3 than on N-free TiO2 in a wide reaction temperature range from 100 °C to 200 °C (Figures 8 and S12). On the other hand, the positive shift of Eads for AN leads to an easier desorption from the catalyst surface, which was confirmed by the temperature-programmed desorption of AN from the Pd/TiO2 catalysts (Figure S13). In addition, the elimination of acidic sites on the TiO2 support by the NH3 treatment, as indicated by the significantly lower NH3 uptake of TiO2-NH3 than N-free TiO2 in NH3-TPD (Table S1), may favor the desorption of AN from the catalyst surface as well.56 Consequently, the formation of organic compounds resulting from the buildup of AN on the catalyst surface, which is believed to be the main reason for the deactivation of Pd catalysts in NB hydrogenation,49,57 is hindered. As a whole, the changes in the binding of NB and AN enhanced the activity and stability of Pd/TiO2-NH3 in NB hydrogenation. 25 ACS Paragon Plus Environment

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Figure 11. A side view of the optimized structures for the adsorption of NB on Pd/TiO2 (a) without N doping and (b) with N-substitution on the TiO2 surface, and for the adsorption of AN on Pd/TiO2 (c) without N doping and (d) with N-substitution on the TiO2 surface. Spheres in red, light blue, gray, light purple, brown and light pink denote oxygen, titanium, palladium, nitrogen, carbon and hydrogen, respectively.

4. CONCLUSIONS In summary, by doping the TiO2 support with nitrogen, the metal-support interactions can be tailored favoring a high catalytic performance of supported Pd NPs in nitrobenzene hydrogenation catalysis. Comparative studies by XRD, XPS and CO-DRIFTS revealed that N-doping induced a structural promoting effect, which is beneficial for the dispersion of Pd species on TiO2. HAADF-STEM investigations confirmed that the Pd NPs on N-doped TiO2 are mainly below 2 nm, and that they are stable under the applied hydrogenation conditions. The electronic promoting effect of nitrogen doping was corroborated by the formation of strongly coupled Pd-N species, which were observed by XPS and CO-DRIFTS and further verified by DFT calculations in model Pd/TiO2 systems consisting of Pdn (n = 1, 5, or 10) clusters on N-doped TiO2. In NB hydrogenation, Pd NPs on N-doped TiO2 were highly active 26 ACS Paragon Plus Environment

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and stable, which can be attributed to the presence of highly dispersed Pd NPs providing more active sites, and to the formation of Pd-N species favoring the dissociative adsorption of the reactant NB and an easier desorption of the product AN. Nitrogen doping thus paves the way for efficiently tailoring the properties of TiO2-supported catalysts toward improved performance.

SUPPORTING INFORMATION Physicochemical properties of the TiO2 supports, further XRD results, HAADF-STEM images for Pd/TiO2-NH3, XP Ti 2p and O 1s spectra, quantitative XPS results, optimized geometries of TiO2 and Pd1/TiO2 systems (without and with N-doping), charge analysis of Pd1/TiO2 systems (without and with N-doping), deactivation and regeneration of Pd/TiO2 catalysts in NB hydrogenation, NB hydrogenation tests over Pd/TiO2 with 3 wt% Pd, temperature-varied NB hydrogenation tests over Pd/TiO2 and commercial Pd catalysts, and TPD of aniline. This information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGEMENT Dr. Aleksander Kostka (MPIE, Düsseldorf) is acknowledged for experimental support and scientific discussions.

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GRAPHIC ABSTRACT


Experimental and Theoretical Understanding of Nitrogen-Doping

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