Monitoring Structural and Electronic Changes of Supported Metal Catalysts Using Combined X‐Ray Techniques

Supported metal nanoparticle catalysts have become increasingly crucial for many catalytic applications. However, long‐term catalyst stability remains a problem due to catalyst deactivation caused by coke formation and sintering. The deposition of a thin overcoating via atomic layer deposition (ALD) onto metal‐supported nanoparticles has shown to greatly inhibit catalyst deactivation. This work utilizes a model catalyst system comprised of Pt nanoparticles supported on Al2O3 to demonstrate the effect of an atomically thin overcoating on supported metal nanoparticles. Continuous operando small‐angle X‐ray scattering (SAXS) and X‐ray absorption near edge spectroscopy (XANES) monitor structural and electronic changes to the catalyst and overcoating during calcination. SAXS data fitting reveals the formation of nanopores in the overcoating at high temperatures, while XANES monitors the oxidation state of the Pt catalyst. Herein, the usefulness of combined X‐ray techniques is demonstrated to characterize supported metal catalysts to further understanding of the synergistic effects of the ALD overcoating to aid in the design of new catalyst materials.


Introduction
Supported metal nanoparticle (NP) catalysts are used in many industrial and commercial applications due to their high activity and selectivity.However, these catalysts are highly susceptible to catalyst deactivation via sintering and coking.The major tradeoff for the high-specific activity of metal catalyst NPs is their poor thermal stability.[3] Carbon deposition (coke formation) can also occur on the surface of metal catalysts during hydrocarbon reactions.6] The stability and long-term effectiveness of a supported metal NP catalyst play a crucial role in assessing their feasibility.
8][9][10][11][12][13][14][15][16][17][18][19] The thickness of the ALD overcoat can range from submonolayer to tens of nanometers thick.During the initial stages of deposition, overcoats have been shown to preferentially nucleate at corners, edges, and steps of metal NPs.This results in the formation of overcoat "islands" on the surface of the metal catalyst, creating an energetic barrier that helps prevent sintering. [20,21]The blocking of the undercoordinated metal sites has also been shown to alter catalyst selectivity and prevent coke formation. [11,22,23]In the case of thicker, continuous overcoatings, the catalyst is calcined prior to use resulting in the formation of nanosized pores in the overcoat which allow access to catalytic sites while inhibiting sintering and coke formation.[26][27] It is vital to understand the effect of overcoat transitions and pore formation in catalytic systems to aid in the design of new, efficient catalysts.
Small-angle X-ray scattering (SAXS) is a useful technique for acquiring structural information of a sample such as shape, size, distribution, and surface structure for features ranging from 1 to 100 nm.[30] Synchrotron X-ray sources offer unique benefits compared to lab source SAXS instruments such as high X-ray flux and a tunable energy range.This allows for weakly scattering features, such as pores within an overcoat, to be measured.The versatility of synchrotron beamlines also allows for design of unconventional experiments utilizing in situ/in operando conditions.X-ray absorption spectroscopy (XAS) is a technique that well complements SAXS when making time-resolved measurements of a catalytic system.[33] This allows for element-specific information related to coordination number, oxidation state, and identifying neighboring atoms within a sample.
This work utilizes a model system to demonstrate the applicability and usefulness of combined X-ray techniques for material characterization.Combined in situ SAXS and XAS are used to monitor changes to the ALD Al 2 O 3 overcoat and Pt nanoparticles on an Al 2 O 3 support.Pore formation was monitored by time-resolved SAXS along with thermogravimetric analysis (TGA) to relate mass loss at high temperatures associated with the overcoat transition and pore formation.Structural and electronic changes to Pt were monitored by XAS.Transmission electron microscopy (TEM) was used to confirm Pt particle size and identify any particle sintering caused by heating.

Results and Discussion
Samples were prepared with 5, 15, and 25 cycles of Al 2 O 3 ALD.For clarification purposes, samples will be referred to as 15c, where "c" represents the number of ALD cycles.The ALD-based method of overcoating preparation allows for precise control of the overlayer thickness.In situ, SAXS was used to investigate pore formation in overcoated supported Pt NPs.During each SAXS experiment, samples were heated from 30 °C to a max temperature of 650 °C.Samples were held at 650 °C for 1 h before being cooled back to room temperature.Heating rates of 2 and 20 °C min À1 were tested.Figure 1a shows SAXS data for the 15c sample with a heating rate of 2 °C min À1 .As temperature is increased, the growth of a feature can be seen between q range 0.09 and 0.2 Å À1 .Past work has shown this is related to the formation of pores in the ALD overcoat. [25]To analyze the size distribution of pores, background subtracted SAXS curves were analyzed in which a room temperature SAXS measurement was used as the background.Figure 1b shows an example of two background subtracted, fitted SAXS curves.The plotted curve for the 300 °C data shows the first measurement in the series in which the 0.09-0.2Å À1 q region could be fit.Figure 1c shows lognormal size distribution plots of measurements taken near 300, 400, 500, and 650 °C.The plot shows a gradual increase in the pore size as a function of temperature with the largest increase taking place after holding at 650 °C for 1 h with a lognormal mean radius of 2.23 nm.The Porod invariant of the SAXS curves was also calculated to visualize changes in the curves as a function of temperature.The Porod invariant calculates the total scattering cross section by integrating over a q space region according to Q ¼ ∫ IðqÞ4πq 2 dq. [34,35]This allows another method of visualization of pore growth from the SAXS data, as shown in Figure 1d.The figure shows a gradual increase in the invariant, which is consistent with the size distribution data from the data fitting.The sharp increase in invariant intensity seen at 650 °C is also in good agreement with the intensity increase in the final size distribution data in Figure 1c.
Samples were also measured with a heating rate of 20 °C min À1 to investigate if the heating rate influences pore formation.SAXS curves measured for the 15c ALD samples showed visually similar data to the 2 °C min À1 data seen in Figure 1a.As a reference, a sample without any ALD overcoating was also measured with a heating rate 20 °C min À1 , as shown in Figure 2a.The uncoated sample shows no change in the SAXS at temperature increases.This helps confirm the changes in the SAXS pattern are due to pore formation in the overcoated samples.Data fitting was then performed on the measured samples to determine pore size.Size distribution models can be seen in Figure 2c for the 15c sample.At 390 °C, the size distribution data show the emergence of features roughly 2 nm in diameter.Since SAXS measures the difference in electron density, nanopores can be thought of as an "inverse" nanoparticle when being observed by SAXS. [30]This allows for the same theory used to characterize colloids to be used for nanopores.TEM was also used to characterize the samples before and after the calcination process.After heating, no observable difference was seen in the 5c, 15c, and 25c samples (Figure S1-S3, Supporting Information).However, Pt particle sintering was observed throughout the uncoated Pt sample (Figure S4, Supporting Information).
XAS was used to monitor the local electronic structure of the Pt nanoparticles during the in situ experiments.The X-ray absorption spectroscopy near edge structure (XANES) data provides insight to the density of empty electron states in the absorbing atom.For Pt, features in the L III absorption edge correspond to the 2p !5d photoelectron transition and the number of unoccupied states in the 5d shell. [36,37][39] XANES spectra for each of the ALD overcoated samples along with the sample containing no overcoat can be seen in Figure 3. XANES curve for the uncoated and 5c sample show similar results in that there is no observable change to the white line intensity suggesting there is no change in oxidation state to the Pt.The dashed line in each spectrum in the Pt foil represents the Pt 0 state.Interestingly, both the 15 and 25c samples show a slight increase in oxidation state as temperature increases.As the sample reaches higher temperatures and the pores in the ALD overcoat are formed, the Pt becomes exposed to atmospheric conditions.Since the Al 2 O 3 growth per cycle is around 1.1 Å, [40,41] the 15 and 25c overcoatings likely provide greater resistance to particle sintering since the 5c ALD results in sub-nanometer thickness.Since the XANES spectrum is the average of all the absorbing Pt atoms, particle sintering would result in a lower surface area and a greater amount of Pt 0 .This could partially explain why no white line intensity increase is seen in the uncoated and 5c samples.Al 2 O 3 overcoating has also been shown to enhance activity and alter the selectivity in Pt-based catalytic systems. [42,43]his, along with the greater protection against particle sintering provided by the thicker 15 and 25c overcoating could help explain the slight white line intensity increase seen in Figure 3c,d.

Conclusion
The in situ sequential SAXS/XANES experimental setup used in this work outlines an extremely useful technique for monitoring changes in the catalytic system.The results shown in this work will allow for better design and fabrication of ALD-supported metal catalysts.Understanding pore formation and how it affects catalyst stability and activity will greatly benefit the catalyst scientific community which has a variety of both industrial and commercial applications.

Experimental Section
Material Synthesis: Al 2 O 3 overcoating was performed on 1%Pt/Al 2 O 3 catalyst (Sigma-Aldrich) in a low vacuum (%0.5 Torr) atomic layer deposition (ALD) reactor (Gemstar-6, Arradiance) at 200 °C.Prior to the overcoating, this catalyst was calcined at 700 °C in 30 sccm flowing air for 2 h.Ultrahigh purity N 2 (Airgas, 99.999%) was used as carrier gas and further purified using a Supelco gas purifier (Sigma-Aldrich) before entering the reactor.In a typical preparation, 500 mg of 1%Pt/Al 2 O 3 was uniformly spread onto a stainless-steel tray with a mesh cover on top of it.The mesh can prevent the spill of the samples during vacuuming but still provides sufficient diffusion of the ALD precursors and product gases in and out of the tray.The time sequence for one Al 2 O 3 ALD cycle can be expressed as t 1 :5sÀt 2 :25sÀt 3 :5sÀt :25s, where t 1 and t 3 correspond to the exposure times of the precursor trimethylaluminum (TMA) and reactant deionized water, respectively, and t 2 and t 4 are the nitrogen purge times in between.Both TMA and deionized water were stored in sealed stainless-steel bottles at room temperature.Samples were prepared 15 cycles of Al 2 O 3 ALD.
Small-Angle X-ray Scattering (SAXS)/X-ray Absorption Spectroscopy (XAS): In situ SAXS/XAS experiments were performed at the 12-ID-C beamline at the Advanced Photon Source at Argonne National Laboratory.SAXS images were taken with an X-ray energy of 11.5 keV on a 2D CCD detector with a typical exposure time of 1.0 s.The sampleto-detector distance was adjusted to provide a detecting range for scattering vector q = 4π(sinθ)/λ between 0.005 and 0.7 Å À1 .2D images were radially averaged before undergoing data processing and fitting using the Irena software package. [44]SAXS and XAS were measured sequentially with a 1.0 s delay between measurements.After a SAXS image was taken at 11.5 keV, the energy was scanned from 11.5 to 11.7 keV.A Vortex 4-element (ME4) silicon drift detector was positioned at a 45°angle from the beam path to measure the fluorescence XAS data.A Pt foil was also measured and was used to align the sample data.Samples were prepared as pellets using a 7.0 mm die kit and pressed via hydraulic press.Samples were heated and measured inside of a Linkam TS1500 temperature control stage.
Thermogravimetric Analysis (TGA): TGA measurements were made using a Mettler Toledo TGA/SDTA851e.Samples were heated from 40 to 700 °C at a rate of 20 °C min À1 under a nitrogen atmosphere.For a standard measurement, roughly 5 mg of powdered sample was placed in a 70 μL aluminum oxide crucible.Prior to each sample measurement, each crucible underwent the heating program to remove residual moisture and to provide a "blank" measurement to account for the buoyancy effect, if necessary.
Field-Emission Transmission Electron Microscopy (FE-TEM): FE-TEM images were collected using a JEOL JEM2100F microscope operating at 200 kV.Samples were prepared on 400-mesh carbon-coated copper grids (Ted Pella Inc.).For each sample, %5 mg was added to a 3.7 mL glass vial with 100 μL ethanol (99.9%,Fisher Sci.).The vial then underwent sonication for thirty minutes.A droplet was then placed directly onto a TEM grid and allowed to dry.Images were collected at both low and high magnification modes.

Figure 1 .
Figure 1.a) SAXS curves during in situ heating for the 15c sample.b) Background subtracted SAXS curves and fitting (red line) for data at 300 and 650 °C.c) Lognormal size distribution from data fitting for measurements at different heating increments.d) Porod invariant of the SAXS curves as a function of temperature.

Figure 2 .
Figure 2. a,b) In situ SAXS curves of the uncoated and 15c samples, respectively.c) Lognormal size distributions from SAXS data fitting of the 15c sample.

Figure 3 .
Figure 3.In situ XANES curves with a heating rate of 20 °C min À1 for a) the sample with no ALD overcoating and b-d) 5, 15, and 25c ALD overcoated samples, respectively.Pt foil was plotted with each data set for reference.