This work was supported by the School of Chemical Engineering, Purdue University. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. MRCAT operations, beamline 10-BM are supported by the Department of Energy and the MRCAT member institutions. The authors also acknowledge the use of beamline 11-ID-C. We thank Evan Wegener for experimental assistance with the XAS.

1. Synthesis of Supported 2 nm Pt-Cu Bimetallic Nanoparticle Catalysts
<ol>
	<li>Preparation of metal precursor solution
	<ol>
		<li>Dissolve 0.125 g of copper nitrate trihydrate (Cu(NO3)2&#183;3H2O) in 1 mL of water to achieve a sky blue solution. 
		Caution: Use protective gloves when handling chemicals.</li>
		<li>Add ammonia dropwise to the copper nitrate solution, forming dark blue precipitates of copper hydroxide. 
		Caution: Use a fume hood for handling bases and volatile chemicals.</li>
		<li>Keep adding ammonia until the dark blue precipitates dissolve to form a dark blue solution and the pH&#160;&#62;10.</li>
		<li>Add 0.198 g of tetraammineplatinum nitrate ((NH3)4Pt(NO3)2) to the solution and additional water so that the total volume of the solution is 3.5 mL. Add additional ammonia if necessary to keep the pH of the solution greater than 10.</li>
		<li>Heat the solution to 70 &#176;C until all the tetraammineplatinum nitrate salts are dissolved in the solution. Allow the solution cool to RT.</li>
	</ol>
	</li>
	<li>Co-impregnation of metal precursor solution
	<ol>
		<li>Prior to catalyst preparation, determine the impregnating pore volume of the silica support. Carefully weigh approximately 5 g of dry silica into a weighting dish. While mixing, add H2O dropwise until the silica is completely wet, but with no excess solution. Reweigh the wet silica. Divide the grams of added water by the grams of sample to calculate the pore volume.</li>
		<li>Add the dissolved metal precursor solution a few drops at a time to 5 g of high-porous silica (SiO2) in a ceramic evaporating dish and stir gently to break up the particles that stick together to achieve a homogeneous distribution of the solution. 
		NOTE: The white silica will turn dark blue once it adsorbs all 3.5 mL of metal precursor solution.
		<ol>
			<li>Make sure that the texture of the silica particles remains like that of dry sand. Avoid the accumulation of excess solution during impregnation.</li>
		</ol>
		</li>
		<li>Place the impregnated Pt-0.7Cu/SiO2 catalyst precursor into a drying oven and dry it at 125 &#176;C O/N.</li>
	</ol>
	</li>
	<li>Calcination and reduction
	<ol>
		<li>Calcine the catalyst in a furnace at 250 &#176;C with a 5 &#176;C/min ramp rate in air for 3 h. 
		NOTE: Calcination at higher temperatures generally leads to larger Pt nanoparticles.</li>
		<li>Place a 2 cm layer of quartz wool in the middle of a 1&#34; quartz tube reactor and load the calcined Pt-0.7Cu/SiO2 catalyst into the tube through a plastic funnel. Place the tube in a clamshell temperature-programmed furnace.</li>
		<li>After purging the tube with N2 for 5 min at RT, start to flow H2 (at RT) at the same flow rate as N2 (100 ccm) to reduce the Pt-0.7Cu/SiO2 catalyst.</li>
		<li>Increase the temperature to 150 &#176;C with a 5 &#176;C/min ramp rate and hold for 5 min.</li>
		<li>Start slow ramping the temperature at a rate of 2.5 &#176;C/min to 250 &#176;C. Hold the temperature for 15 min at every 25 &#176;C. 
		NOTE: Other metals may require lower or higher temperatures of reduction. The exact temperature can generally be determined by examining color changes of the catalyst (e.g., from blue to black) for Pt-Cu.</li>
		<li>Ramp to 550 &#176;C (or the reaction temperature, if higher) at 10 &#176;C/min and stay for 30 min to complete the reduction. Purge with N2 and cool to room temperature.</li>
		<li>Unload the Pt-0.7Cu/SiO2 catalysts and store in a vial for future use. 
		NOTE: Repeat similar synthesis procedures using different amount of Cu(NO3)2&#183;3H2O and (NH3)4Pt(NO3)2 to prepare the other Pt-X Cu/SiO2 catalysts (X = 0.7, 2.3, and 7.3 and stands for Cu:Pt atomic ratios) and Pt/SiO2 catalysts.</li>
	</ol>
	</li>
</ol>
2. Propane-dehydrogenation Performance Test
<ol>
	<li>Catalyst loading
	<ol>
		<li>Take a 3/8&#34; quartz tube reactor and place a 1 cm layer of quartz wool against the dimple in the middle. 
		Caution: Use protective gloves when handling quartz wool, since the fine needles can get imbedded in the skin.</li>
		<li>Weigh 40 mg of Pt-0.7Cu/SiO2 catalyst and 960 mg of the silica for dilution. Mix the particles (1 g total weight) in an empty vial.</li>
		<li>Use a plastic funnel to load all the catalyst mixture into the reaction tube. Wipe the outer wall of both tube ends with lint-free wipes to remove any dirt to get a good seal with the O-ring.</li>
		<li>Put the tube fittings onto both ends of the quartz reactor tube and attach them to the reactor system equipped with a clamshell furnace.</li>
	</ol>
	</li>
	<li>Leak test and catalyst pretreatment
	<ol>
		<li>Turn on 50 cm3/min N2 flow through the tube reactor. After 1 min, close the ball valve on the reactor outlet. Wait for the system pressure to increase to 5 psig. Close the ball valve on the inlet N2 line to stop N2 flow and seal the reactor system.</li>
		<li>Wait for 1 min and record the pressure read from the gauge. If the pressure drops, open the ball valve on the reactor outlet to release the pressure and re-attach the fittings. If not, first open the ball valve on the reactor outlet to release the pressure before restarting the N2 flow by turning on the ball valve on the inlet N2 line to purge the system for 1 min.</li>
		<li>Start flowing 50 cm3/min of 5% H2/N2 for catalyst reduction before running a reaction and stop the N2 flow. Start heating the tube to the reaction temperature of 550 &#176;C, with a rate of 10 &#176;C/min. Wait for 30 min after the furnace reaches the set point and allow the system temperature to stabilize at the target temperature.</li>
	</ol>
	</li>
	<li>Propane dehydrogenation reaction testing
	<ol>
		<li>Start the gas chromatograph (GC) in the reactor system and select the proper method for gas component analysis.
		<ol>
			<li>Switch the reactor gas flow to a bypass line. Flow 100 cm3/min of 5% propane/N2 and 100 cm3/min of 5% H2/N2. Wait for 1 min so that the propane flow stabilizes and inject the bypass flow into GC as a reference sample.</li>
		</ol>
		</li>
		<li>Switch the gas flow back to the reactor tube line to start the reaction and record the time.</li>
		<li>After the reaction runs for 4 min, inject the reactor outlet gas flow (a GC sample) into the GC to get the outlet gas component information. Inject samples every 4 min and run the test until the conversion reaches steady state or the conversion is very low.</li>
		<li>Use the corresponding peak analysis software to analyze each peak.
		<ol>
			<li>Click to select the start and end points of the peak. Use the integrate function to get the peak area. Write down the peak area for the propane (C3H8) reactant; the propylene (C3H6) product; and the side products, methane (CH4), ethane (C2H4), and ethylene (C2H6). 
			NOTE: For each injection, a GC pattern with multiple peaks is obtained whose area relates to the number of moles of different gas species.</li>
		</ol>
		</li>
		<li>Convert the peak area to the number of moles for each species using the response factor. Determine the propane conversion and propylene selectivity at the time for each sample according to the following formulas: 
		<img alt="Equation" src="/files/ftp_upload/56040/56040eq1.jpg" /> 
		<img alt="Equation" src="/files/ftp_upload/56040/56040eq2.jpg" /> 
		where <img alt="Equation" src="/files/ftp_upload/56040/56040eq3.jpg" /> is the conversion of propane, <img alt="Equation" src="/files/ftp_upload/56040/56040eq4.jpg" /> is the propylene selectivity, <img alt="Equation" src="/files/ftp_upload/56040/56040eq5.jpg" /> is the number of moles of propane, and <img alt="Equation" src="/files/ftp_upload/56040/56040eq6.jpg" /> is the number of moles of propylene.</li>
		<li>Obtain the initial conversion and selectivity value at t = 0 by extrapolating the measured conversion and selectivity versus time on stream using an exponential fit.</li>
	</ol>
	</li>
	<li>Post-reaction
	<ol>
		<li>Stop heating the reactor by turning off the temperature program. Switch the gas flow to 10 cm3/min N2.</li>
		<li>Switch the gas chromatograph back to standby method to reduce the flow rate of the carrier gas.</li>
		<li>Unload the used catalyst from the quartz fix-bed reactor after cooling to room temperature. Collect the catalyst weight in a designated waste disposal area.</li>
	</ol>
	</li>
</ol>
3. Characterization of Catalyst Samples
<ol>
	<li>Scanning Transmission Electron Microscopy4,20
	<ol>
		<li>Load the catalyst in a mortar and grind it into less than 100 mesh powder using a pestle.</li>
		<li>Disperse about 30 mg of catalyst powder into about 5 mL of isopropyl alcohol in a small vial. Shake the vial for full mixing and then let the vial sit for 5 min to allow for the deposition of the relatively large particles. 
		NOTE: The obtained suspension should contain very small particles of supported catalysts.</li>
		<li>Place a Au TEM ready grid on an evaporating dish. Heat the dish to 80 &#176;C on a hot plate. Add three drops of the catalyst suspension to the grid. 
		NOTE: The isopropyl alcohol will evaporate quickly and leave the catalyst particles on the grid.</li>
		<li>Load the grid onto the sample holder for electron microscopy sample imaging.</li>
	</ol>
	</li>
	<li>In situ X-ray absorption spectroscopy3,4,19,20
	<ol>
		<li>Load the catalyst into a mortar and grind it into less than 100 mesh powder using a pestle. Load the fine powders into a die set and press it with the fingers to form a self-supporting wafer.</li>
		<li>Load a ~100 mg sample into the sample holder.</li>
		<li>Place the sample holder into a quartz tube reactor and pretreat the sample by reducing it in 50 cm3/min 3% H2/He.</li>
		<li>After cooling to RT, seal the tube and transfer it to the synchrotron beamline to collect the XAS data.</li>
	</ol>
	</li>
	<li>In situ X-ray diffraction 19,20
	<ol>
		<li>Load the catalyst in a mortar and grind it into less than 100 mesh powder using a pestle.</li>
		<li>Press a thin wafer using a standard 7 mm diameter die set. 
		NOTE: The die set contains a female piece and top and bottom male pieces.
		<ol>
			<li>Attach the bottom male piece to the female part. Load the sample onto the polished surface of the bottom part. Attach the top male piece and transfer the die set to the sample stage of the press. Press with appropriate strength.</li>
		</ol>
		</li>
		<li>Unload the wafer and transfer it to the ceramic cup of the specialized sample stage (see the Table of Materials). Seal the stage and fix it on the sample table on the beamline.</li>
		<li>Reduce the sample by flowing and ramping the temperature to 550 &#176;C. Collect the in situ X-ray diffraction data under 3% H2/He gas flow at 550 &#176;C and after cooling down to RT20.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

The Pt-Cu catalysts prepared in this work contain uniform nanoparticles around 2 nm in size, similar to heterogeneous catalysts qualified for industrial application. All the Pt and Cu precursors form bimetallic structures, as opposed to separate monometallic particles. This bimetallic interaction and small particle size are realized by careful control over the synthesis procedures. The impregnation process makes use of the Strong Electrostatic Adsorption (SEA) between metal ions and the surface of certain oxide supports21. Oxide materials such as silica have hydroxyl groups on the surface, which can be protonated or deprotonated in a solution depending on the pH. Silica has a characteristic Point of Zero Charge (PZC) around pH = 422, which means that the surface is electronically neutral at this pH. The oxide surface would be deprotonated and adsorb cations if the pH of the solution goes above its PZC, whereas at Ph values below PZC, it would protonate and adsorb anions. By adjusting the pH of the precursor solution to greater than 10 by adding ammonia, as shown in step 1.1.3, the solution is under basic conditions, so the silica hydroxyl groups are sufficiently deprotonated and are able to strongly adsorb cations such as platinum tetraammine [(NH3)4Pt]2+ and copper tetraammine complexes [(NH3)4Cu]2+. The electronic attraction between the cations and the deprotonated hydroxyl groups anchors the precursor on the catalyst, and the repulsion between the cations aids in their dispersion, both of which help to prevent the agglomeration of the metal species upon calcination and reduction. The impregnation processes of the two different metal precursors are conducted in parallel. Compared with sequential impregnation, the co-impregnation method ensures the uniform distribution of the two metal precursors in the solution to maximize their mixing. The obtained supported Pt-Cu catalysts, therefore, show strong bimetallic interaction, and no monometallic particles are formed. The impregnation is also carried out in incipient wetness, which optimizes the dispersion of the precursor solution in the pores of the support oxide by capillary effect. The impregnated Pt-Cu catalysts are calcined at 250 &#176;C to remove the ligand attached to the metals while minimizing the growth of the oxide clusters. The calcination temperature of the impregnated metal precursors has been shown to affect the size of the reduced metal nanoparticles23,24.
For Pt on silica, the particle size increases with increasing temperature, and temperatures near 250 &#176;C are necessary to give small nanoparticles. The optimum calcination temperature is different for other metals and supports. When reducing the Pt-Cu catalysts, a high H2 flow rate and a slow ramping rate are used at around the reduction temperature of Pt to quickly remove the water formed during reduction, thus suppressing the growth of the metal nucleus. The goal is to raise the temperature to just below the point where rapid reduction occurs to slowly reduce the metal oxide while quickly removing the water formed during reduction. Thus, it is important to approach the reduction slowly and to hold at that temperature for sufficient time while removing the water under high flow. Once reduced, the temperature can be raised quickly to the reaction temperature. It is important to reduce the entirety of the catalysts to the highest temperature they will be exposed to during the reaction condition so that they do not go through additional sintering when the reaction is running. Once reduced, exposure to air at room temperature will oxidize the particle surface but not the entire particle. Further rereduction will give almost the same particle size. Thus, if the entire particle is reduced, the size will be fixed for every analysis (i.e.&#160;the particle size for kinetics, infrared (IR) spectroscopy, XAS, XRD, etc. will be identical). The whole catalyst synthesis protocol is also applicable to the preparation of other metal catalysts used in various applications25 to obtain strong bimetallic interactions, and smaller or larger particle sizes can be obtained via minor modification of the protocol.
In situ synchrotron XRD was performed to investigate the crystal structure of the 2-nm Pt-Cu nanoparticles. For supported nanoparticles below 3 nm in size, diffraction is very challenging to measure using laboratory instrument19. The diffraction peaks are strongly broadened and very low in intensity due to the limited repetition of unit cells. In addition, the metal loading of supported metal catalysts is typically low (&#8804;5%), which further lowers the diffraction signal. Moreover, a large fraction of the atoms is at the particle surface (close to 50% for 2 nm nanoparticles) and is oxidized when measured in air. Therefore, to obtain meaningful structural information, the diffraction must be collected in situ using high-flux X-ray, typically only available with a synchrotron. In this case, the X-ray diffraction patterns are first measured under 3% H2/He at 550 &#176;C after reduction and then at room temperature in the same atmosphere after cooling. The diffraction signals from the metals in the catalysts are isolated from the raw data by subtracting the SiO2 support and sample cell diffraction using the patterns taken at the same condition. The two patterns show the same phase, indicating the unchanged crystal structure of the catalysts at the two different temperatures.
In situ XRD and XAS suggest that, for the Pt-Cu catalysts with increasing Cu:Pt atomic ratio, a series of substitutional solid-solution structure with increasing Cu content percentage are formed. In a solid-solution structure, Pt and Cu atoms are randomly distributed; therefore, the Pt atoms are not necessarily bonded each other. However, the isolation of Pt atoms by Cu is realized at a high Cu:Pt ratio, and the extent of the Pt site isolation improves as the atomic percentage of Cu increases. This change in the structure of active sites translates into improvements in the PDH selectivity of the Pt-Cu catalysts (Figure 1C), confirming the relationship between site isolation and high dehydrogenation selectivity. For the Pt-7.3Cu catalyst with the highest Cu content, almost all the Pt atoms are isolated, as shown by EXAFS. As a result, the selectivity of this catalyst remains high (close to 100%) at varied initial conversion. In addition to changes in the selectivity, improved site isolation also introduces an increasing amount of Cu neighbors bonded to the Pt atoms and electronically modifies the Pt active sites. As a result, the turnover rate of the catalyst for PDH also continues to increase with the atomic ratio of Cu:Pt (Figure 1D). A TOR of 0.98 s-1 for Pt-7.3Cu is 16 times higher than the TOR of 0.06 s-1 for monometallic Pt and is also higher than the typical TOR values (0.1-0.5 s-1) of Pt-Sn catalysts under similar reaction conditions1.
In the manuscript, we have demonstrated a convenient method for the synthesis of bimetallic Pt-Cu catalysts, as well as performance tests for propane dehydrogenation and characterization. The catalyst forms a substitutional solid-solution structure, with a small and uniform particle size around 2 nm, which is realized by careful control over the impregnation, calcination, and reduction steps during catalyst preparation and is identified by advanced in situ synchrotron techniques. The catalyst performance continuously improves with increasing Cu:Pt atomic ratio.

Propane dehydrogenation (PDH) is a key processing step in the production of propylene, taking advantage of shale gas, the fastest growing source of gas in the country1. This reaction breaks two C-H bonds in a propane molecule to form one propylene and molecular hydrogen. Noble metal catalysts, including Pd nanoparticles, exhibit poor selectivity for PDH, breaking the C-C bond to produce methane with a high yield, with the concomitant production of coke, leading to catalyst deactivation. Recent reports showed that selective PDH catalysts could be obtained by the addition of promoters like Zn or In to Pd2,3,4. The promoted catalysts are near 100% selective to PDH, as opposed to less than 50% for monometallic Pd nanoparticles of the same size. The great improvement in selectivity was attributed to the formation of PdZn or PdIn intermetallic compound (IMC) structures on the catalyst surface. The ordered array of two different types of atoms in the IMCs geometrically isolated the Pd active sites with non-catalytic Zn or In atoms, which turned off the side reactions catalyzed by an ensemble (group) of neighboring Pd active sites.
Platinum has the highest intrinsic selectivity among noble metals for propane dehydrogenation, but it is still not satisfactory for commercial use1. Typically, Sn, Zn, In, or Ga is added as promoter for Pt5,6,7,8,9,10,11,12,13. Based on the idea that geometric active site isolation contributes to high selectivity, any non-catalytic element forming an alloy structure with Pt, such as Cu, should also potentially promote catalyst performance14. Several previous studies suggested that the addition of Cu indeed improved the PDH selectivity of Pt catalysts15,16,17,18. Nevertheless, no direct evidence has been reported to determine whether Pt and Cu form bimetallic nanoparticles or ordered structures, which is crucial to understanding the promotional effect of Cu. In the binary phase diagram of Pt-Cu, two different structure types are possible over a wide composition range16,18: intermetallic compound, in which Pt and Cu each occupy specific crystal sites, and solid solution, in which Cu randomly substitutes in the Pt lattice. IMCs form at low temperature and transform into solid solution at around 600 - 800 &#176;C for bulk materials14. This transformation temperature may be lower for nanoparticles, near the reaction temperature of PDH (i.e.&#160;550 &#176;C). Therefore, it is essential to investigate the atomic order of Pt-Cu under reaction conditions. For supported nanoparticles with small particle sizes, it is very challenging to obtain meaningful structural information using laboratory instruments19. The limited repetition of unit cells leads to very broad diffraction peaks with very low intensities. Because of the high fraction of surface atoms in nanoparticles 1 - 3 nm in size, which are oxidized in air, diffraction must be collected in situ using high-flux X-ray, typically available with synchrotron techniques.
The previously reported Pt-Cu PDH catalysts were all larger than 5 nm in size15,16,17,18. However, for noble metal nanoparticle catalysts, there is always a strong desire to maximize catalytic activity per unit cost by synthesizing catalysts with high dispersions (typically around or less than 2 nm in size)19. Though the preparation of bimetallic nanoparticles of this size is possible by standard impregnation methods, rational control over the procedures is necessary. The metal precursors, pH of the impregnating solution, and support type need to be controlled to optimize the anchoring of the metal species onto high-surface area supports. The subsequent calcination and reduction heat treatments should also be carefully controlled to suppress the growth of the metallic nanoparticles.
This article covers the protocol for the synthesis of supported 2 nm Pt-Cu bimetallic nanoparticle catalysts and for the testing of their propane dehydrogenation performance. The structure of the catalysts is investigated by Scanning Transmission Electron Microscopy (STEM), in situ synchrotron X-ray Absorption Spectroscopy (XAS), and in situ synchrotron X-ray diffraction (XRD), which help elucidate the improved catalyst performance upon the introduction of Cu.

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synthesis and testing of supported pt-cu solid solution nanoparticle catalysts for propane dehydrogenation

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are demonstrated here. The catalyst forms a substitutional solid solution structure, with a small and uniform particle size around 2 nm. This is realized by careful control over the impregnation, calcination, and reduction steps during catalyst preparation and is identified by advanced in situ synchrotron techniques. The catalyst propane dehydrogenation performance continuously improves with increasing Cu:Pt atomic ratio.

The propylene selectivity versus time for Pt and Pt-Cu catalysts measured at an initial propane conversion of about 20% is presented in Figure 1A. Pt catalyst has an initial selectivity of 61%, which increases to about 82% with time on-stream as the catalyst deactivates for 1h. The Pt-0.7Cu catalyst shows a better initial propylene selectivity of 72%. For Pt-2.3Cu and Pt-7.3Cu catalysts, their initial selectivity reach 90% and 96%, respectively, and are maintained with reaction time on-stream. Figure 1B shows the change in initial propylene selectivity versus initial propane conversion for the Pt and Pt-7.3Cu catalysts. While the selectivity of the Pt catalysts decreases at higher conversion, the Pt-7.3Cu catalyst retains high propylene selectivity, over 95%, at different propane conversions. When comparing different catalysts, it seems that the catalyst selectivity increases almost linearly with the content of Cu in Pt-Cu catalysts, expressed in atomic percentage, as shown in Figure 1C. Increase in the Cu content also improves the turnover rates (TOR) for propane dehydrogenation. Figure 1D displace the close linear relationship between the TOR and the catalyst atomic ratio of Cu to Pt. The carbon balance is close to 100% during all reaction tests, although minor coke formation occurs throughout the reaction.
STEM images were collected for monometallic Pt and three Pt-Cu catalysts with different metal loadings and Cu:Pt ratios, as confirmed by atomic absorption spectroscopy. The particle sizes of all catalysts determined by STEM imaging are between 2 and 3 nm. Figure 2A shows an STEM image of a Pt-2.3Cu catalyst, which is typical of the other samples. The average particle size of the sample is determined to be 2.2 nm, with a standard deviation of 0.4 nm. For Pt-0.7Cu and Pt-7.3Cu, the obtained STEM particle sizes are 2.5 &#177;0.4 nm and 2.1 &#177;0.4 nm. In situ X-ray absorption near edge spectroscopy at both the Pt and Cu edges have been done to determine the valence states of Pt and Cu under conditions similar to the reaction environment. Both Pt and Cu are found to be fully reduced to the metallic state. The magnitude of the k2 weighted Fourier transform of extended X-ray absorption fine structure (EXAFS) spectra for Pt and Pt-Cu catalysts at the Pt LIII edge are shown in Figure 2B. At the Pt LIII edge, the Pt catalyst shows a three-peak pattern typical of Pt-Pt scattering. For Pt-0.7Cu, the peak position of three-peak pattern is shifted to lower R, suggesting the interference of the Pt-Pt scattering by a second Pt-Cu scattering with a shorter bond distance. The EXAFS spectra of Pt-2.3Cu and Pt-7.3Cu, which contain a relative high amount of Cu, show only one peak, typical of Pt-Cu metal scattering. The change of the scattering pattern with increasing Cu:Pt atomic ratio suggests the formation of bimetallic nanoparticles with increasing Cu content percentage. Figure 2C displays a background-subtracted in situ synchrotron X-ray diffraction (XRD) pattern of Pt and Pt-Cu catalysts and the corresponding simulated XRD pattern of the identified Pt-Cu phases. No superlattice diffraction is found, and the composition of the Pt-Cu catalysts is different from the ideal composition of ordered alloys, indicating that Pt and Cu form a solid solution structure in the Pt-Cu catalysts. The diffraction peaks of the catalysts with increasing Cu:Pt atomic ratio shift to higher angles, and their normalized intensities decrease, all confirming that the solid solution becomes richer in Cu. The composition of the solid solution is calculated from the diffraction pattern using Bragg&#39;s Law and Vegard&#39;s Law.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56040/56040fig1.jpg" /> 
Figure 1: Propane-dehydrogenation Performance of Pt and Pt-Cu Catalysts. (A) Propylene selectivity versus the time measured at 550 &#176;C for the Pt (black squares), Pt-0.7Cu (red circles), Pt-2.3Cu (blue triangles), and Pt-7.3Cu (magenta down triangles) catalysts. (B) Propylene selectivity versus propane conversion measured at 550 &#176;C for the Pt (black squares) and Pt-7.3Cu (magenta down triangles) catalysts. (C) Dehydrogenation selectivity versus Cu content in atomic percentage. (D) Turnover rate versus the Cu:Pt atomic ratio of each catalyst <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56040/56040fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56040/56040fig2.jpg" /> 
Figure 2: Structures of Pt and Pt-Cu Catalysts. (A) STEM high-angle annular dark field (HAADF) image of the Pt-2.3Cu catalyst. (B) Pt LIII edge magnitude of the Fourier transform of the EXAFS of Pt (black), Pt-0.7Cu (red), Pt-2.3Cu (blue), and Pt-7.3Cu (magenta). (C) Background-subtracted in situ XRD pattern of: Pt (black, solid), Pt-0.7Cu (red, solid), Pt-2.3Cu (blue, solid), and Pt-7.3Cu (magenta, solid) compared with the simulated XRD pattern of: bulk FCC Pt (black, dotted), Pt0.70Cu0.30 (red, dotted), Pt0.32Cu0.68 (blue, dotted), and Pt0.13Cu0.87 (magenta, dotted), respectively. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56040/56040fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation at JoVE.com

Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation

A convenient method for the synthesis of 2 nm supported bimetallic nanoparticle Pt-Cu catalysts for propane dehydrogenation is reported here. In situ synchrotron X-ray techniques allow for the determination of the catalyst structure, which is typically unobtainable using laboratory instruments.

A convenient method for the synthesis of bimetallic Pt-Cu catalysts and performance tests for propane dehydrogenation and characterization are ...

synthesis-testing-supported-pt-cu-solid-solution-nanoparticle

Nanyang Technological University

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JoVE Journal

Chemistry

11.8K Views.  Purdue University. A convenient method for the synthesis of 2 nm supported bimetallic nanoparticle Pt-Cu catalysts for propane dehydrogenation is reported here. In situ synchrotron X-ray techniques allow for the determination of the catalyst structure, which is typically unobtainable using laboratory instruments.

Video: Synthesis and Testing of Supported Pt-Cu Solid Solution Nanoparticle Catalysts for Propane Dehydrogenation

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of Chalcogenidoplumbates(II or IV)

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica glass ampules). Reduction of such phases by elemental alkaline metals in amines affords crystalline chalcogenidoplumbate(II) salts comprised of [PbTe3]2- or [Pb2Ch3]2- anions, depending upon which sequestering agent for the cations is present: crown ethers, like 18-crown-6, or cryptands, like [2.2.2]crypt. Reactions of solutions of such anions with transition-metal compounds yield (poly-)chalcogenide anions or transition-metal chalcogenide clusters, including one with a &#181;-PbSe ligand (i.e., the heaviest-known CO homolog).
In contrast, the solid-state synthesis of a phase of the nominal composition &#34;K2PbSe2&#34; by successive reactions of the elements and by the subsequent solvothermal treatment in amines yields the first non-oxide/halide inorganic lead(IV) compound: a salt of the ortho-selenidoplumbate(IV) anion [PbSe4]4-. This was unexpected due to the redox potentials of Pb(IV) and Se(-II). Such methods can further be applied to other elemental combinations, leading to the formation of solutions with binary [HgTe2]2- or [BiSe3]3- anions, or to large-scale syntheses of K2Hg2Se3 or K3BiSe3 via the solid-state route.
All compounds are characterized by single-crystal X-ray diffraction and elemental analysis; solutions of plumbate salts can be investigated by 205Pb and 77Se or 127Te NMR techniques. Quantum chemical calculations using density functional theory methods enable energy comparisons. They further allow for insights into the electronic configuration and thus, the bonding situation. Molecular Rh-containing Chevrel-type compounds were found to exhibit delocalized mixed valence, whereas similar telluridopalladate anions are electron-precise; the cluster with the &#181;-PbSe ligand is energetically favored over a hypothetical CO analog, in line with the unsuccessful attempt at its synthesis. The stability of formal Pb(IV) within the [PbSe4]4- anion is mainly due to a suitable stabilization within the crystal lattice.

Combining Solid-state and Solution-based Techniques: Synthesis and Reactivity of ChalcogenidoplumbatesII or IV

The synthesis of chalcogenidoplumbates(II,IV) via the in situ reduction of nominal "PbCh2" (Ch = Chalcogen) and via a solid-state reaction and subsequent solvothermal reactions is presented. Additionally, reactivities of plumbate(II) solutions are portrayed, which yield the heaviest-known CO homolog known to date: the &#181;-PbSe ligand.

The phases of &#34;PbCh2&#34; (Ch = Se, Te) are obtained from solid-state syntheses (i.e., by the fusion of the elements under inert conditions in silica ...

Measurement of Particle Size Distribution in Turbid Solutions by Dynamic Light Scattering Microscopy

A protocol for measuring polydispersity of concentrated polymer solutions using dynamic light scattering is described. Dynamic light scattering is a technique used to measure the size distribution of polymer solutions or colloidal particles. Although this technique is widely used for the assessment of polymer solutions, it is difficult to measure the particle size in concentrated solutions due to the multiple scattering effect or strong light absorption. Therefore, the concentrated solutions should be diluted before measurement. Implementation of the confocal optical component in a dynamic light scattering microscope1 helps to overcome this barrier. Using such a microscopic system, both transparent and turbid systems can be analyzed under the same experimental setup without a dilution. As a representative example, a size distribution measurement of a temperature-responsive polymer solution was performed. The sizes of the polymer chains in an aqueous solution were several tens of nanometers at a temperature below the lower critical solution temperature (LCST). In contrast, the sizes increased to more than 1.0 &#181;m when above the LCST. This result is consistent with the observation that the solution turned turbid above the LCST.

A protocol for the direct measurement of particle size distribution in concentrated solutions using dynamic light scattering microscopy is presented.

A protocol for measuring polydispersity of concentrated polymer solutions using dynamic light scattering is described. Dynamic light scattering is a ...

Protocol for the Solid-phase Synthesis of Oligomers of RNA Containing a 2'-O-thiophenylmethyl Modification and Characterization via Circular Dichroism

Solid-phase synthesis has been used to obtain canonical and modified polymers of nucleic acids, specifically of DNA or RNA, which has made it a popular methodology for applications in various fields and for different research purposes. The procedure described herein focuses on the synthesis, purification, and characterization of dodecamers of RNA 5&#39;-[CUA CGG AAU CAU]-3&#39; containing zero, one, or two modifications located at the C2&#39;-O-position. The probes are based on 2-thiophenylmethyl groups, incorporated into RNA nucleotides via standard organic synthesis and introduced into the corresponding oligonucleotides via their respective phosphoramidites. This report makes use of phosphoramidite chemistry via the four canonical nucleobases (Uridine (U), Cytosine (C), Guanosine (G), Adenosine (A)), as well as 2-thiophenylmethyl functionalized nucleotides modified at the 2&#39;-O-position; however, the methodology is amenable for a large variety of modifications that have been developed over the years. The oligonucleotides were synthesized on a controlled-pore glass (CPG) support followed by cleavage from the resin and deprotection under standard conditions, i.e., a mixture of ammonia and methylamine (AMA) followed by hydrogen fluoride/triethylamine/N-methylpyrrolidinone. The corresponding oligonucleotides were purified via polyacrylamide electrophoresis (20% denaturing) followed by elution, desalting, and isolation via reversed-phase chromatography (Sep-pak, C18-column). Quantification and structural parameters were assessed via ultraviolet-visible (UV-vis) and circular dichroism (CD) photometric analysis, respectively. This report aims to serve as a resource and guide for beginner and expert researchers interested in embarking in this field. It is expected to serve as a work-in-progress as new technologies and methodologies are developed. The description of the methodologies and techniques within this document correspond to a DNA/RNA synthesizer (refurbished and purchased in 2013) that uses phosphoramidite chemistry.

Protocol for the Solid-phase Synthesis of Oligomers of RNA Containing a 2'-O-thiophenylmethyl Modification and Characterization via Circular Dichroism

This article provides a detailed procedure on the solid-phase synthesis, purification, and characterization of dodecamers of RNA modified at the C2&#39;-O-position. UV-vis and circular dichroism photometric analyses are used to quantify and characterize structural aspects, i.e., single-strands or double-strands.

Solid-phase synthesis has been used to obtain canonical and modified polymers of nucleic acids, specifically of DNA or RNA, which has made it a popular ...

Liquid-cell Transmission Electron Microscopy for Tracking Self-assembly of Nanoparticles

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully understood. We have traced the trajectories of individual nanoparticles using liquid-cell transmission electron microscopy (TEM) to investigate the mechanism of the assembly process. Herein, we present the protocols used for liquid-cell TEM studies of the self-assembly mechanism. First, we introduce the detailed synthetic protocols used to produce uniformly sized platinum and lead selenide nanoparticles. Next, we present the microfabrication processes used to produce liquid cells with silicon nitride or silicon windows and then describe the loading and imaging procedures of the liquid-cell TEM technique. Several notes are included to provide helpful tips for the entire process, including how to manage the fragile cell windows. The individual motions of nanoparticles tracked by liquid-cell TEM revealed that changes in the solvent boundaries caused by evaporation affected the self-assembly process of nanoparticles. The solvent boundaries drove nanoparticles to primarily form amorphous aggregates, followed by flattening of the aggregates to produce a 2-dimensional (2D) self-assembled structure. These behaviors are also observed for different nanoparticle types and different liquid-cell compositions.

Here we introduce experimental protocols for the real-time observation of a self-assembly process using liquid-cell transmission electron microscopy.

Drying a nanoparticle dispersion is a versatile way to create self-assembled structures of nanoparticles, but the mechanism of this process is not fully ...

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of NixNb1-xO nanoparticles with different atomic compositions (x = 0.03, 0.08, 0.15, and 0.20) have been prepared by chemical precipitation. These NixNb1-xO catalysts are characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. The study revealed the sponge-like and fold-like appearance of Ni0.97Nb0.03O and Ni0.92Nb0.08O on the NiO surface, and the larger surface area of these NixNb1-xO catalysts, compared with the bulk NiO. Maximum surface area of 173 m2/g can be obtained for Ni0.92Nb0.08O catalysts. In addition, the catalytic hydroconversion of lignin-derived compounds using the synthesized Ni0.92Nb0.08O catalysts have been investigated.

Chemical Precipitation Method for the Synthesis of Nb2O5 Modified Bulk Nickel Catalysts with High Specific Surface Area

A protocol for the synthesis of sponge-like and fold-like Ni1-xNbxO nanoparticles by chemical precipitation is presented.

We demonstrate a method for the synthesis of NixNb1-xO catalysts with sponge-like and fold-like nanostructures. By varying the Nb:Ni ratio, a series of ...

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

The use of nanoprobes such as gold, silver, silica or iron-oxide nanoparticles as detection reagents in bioanalytical assays can enable high sensitivity and convenient colorimetric readout. However, high densities of nanoparticles are typically needed for detection. The available synthesis-based enhancement protocols are either limited to gold and silver nanoparticles or rely on precise enzymatic control and optimization. Here, we present a protocol to enhance the colorimetric readout of gold, silver, silica, and iron oxide nanoprobes. It was observed that the colorimetric signal can be improved by up to a 10000-fold factor. The basis for such signal enhancement strategies is the chemical reduction of Au3+ to Au0. There are several chemical reactions that enable the reduction of Au3+ to Au0. In the protocol, Good&#39;s buffers and H2O2 are used and it is possible to favor the deposition of Au0 onto the surface of existing nanoprobes, in detriment of the formation of new gold nanoparticles. The protocol consists of the incubation of the microarray with a solution consisting of chloroauric acid and H2O2 in 2-(N-morpholino)ethanesulfonic acid pH 6 buffer following the nanoprobe-based detection assay. The enhancement solution can be applied to paper and glass-based sensors. Moreover, it can be used in commercially available immunoassays as demonstrated by the application of the method to a commercial allergen microarray. The signal development requires less than 5 min of incubation with the enhancement solution and the readout can be assessed by naked eye or low-end image acquisition devices such as a table-top scanner or a digital camera.

Rapid Nanoprobe Signal Enhancement by In Situ Gold Nanoparticle Synthesis

In this work, a protocol for signal enhancement of nanoprobe-based biosensing is presented. The protocol is based on the reduction of chloroauric acid onto the surface of existing nanoprobes that consist of gold, silver, silica or iron-oxide nanoparticles.

The use of nanoprobes such as gold, silver, silica or iron-oxide nanoparticles as detection reagents in bioanalytical assays can enable high sensitivity ...

A Rapid Synthesis Method for Au, Pd, and Pt Aerogels Via Direct Solution-Based Reduction

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various precursor noble metal ions with reducing agents in a 1:1 (v/v) ratio results in the formation of metal gels within seconds to minutes compared to much longer synthesis times for other techniques such as sol-gel. Conducting the reduction step in a microcentrifuge tube or small volume conical tube facilitates a proposed nucleation, growth, densification, fusion, equilibration model for gel formation, with final gel geometry smaller than the initial reaction volume. This method takes advantage of the vigorous hydrogen gas evolution as a by-product of the reduction step, and as a consequence of reagent concentrations. The solvent accessible specific surface area is determined with both electrochemical impedance spectroscopy and cyclic voltammetry. After rinsing and freeze drying, the resulting aerogel structure is examined with scanning electron microscopy, X-ray diffractometry, and nitrogen gas adsorption. The synthesis method and characterization techniques result in a close correspondence of aerogel ligament sizes. This synthesis method for noble metal aerogels demonstrates that high specific surface area monoliths may be achieved with a rapid and direct reduction approach.

A rapid, direct solution-based reduction synthesis method to obtain Au, Pd, and Pt aerogels is presented.

Here, a method to synthesize gold, palladium, and platinum aerogels via a rapid, direct solution-based reduction is presented. The combination of various ...

Solid-phase Synthesis of [4.4] Spirocyclic Oximes

A convenient synthetic route for spirocyclic heterocycles is well sought after due to the molecule&#39;s potential use in biological systems. By means of solid-phase synthesis, regenerating Michael (REM) linker strategies, and 1,3-dipolar cycloaddition, a library of structurally similar heterocycles, both with and without a spirocyclic center, can be constructed. The main advantages of the solid-support synthesis are as follows: first, each reaction step can be driven to completion using a large excess of reagents resulting in high yields; next, the use of commercially available starting materials and reagents keep the costs low; finally, the reaction steps are easy to purify via simple filtration. The REM linker strategy is attractive because of its recyclability and traceless nature. Once a reaction scheme is completed, the linker can be reused multiple times. In a typical solid-phase synthesis, the product contains either a part of or the whole linker, which can prove undesirable. The REM linker is &#34;traceless&#34; and the point of attachment between the product and the polymer is indistinguishable. The high diastereoselectivity of the intramolecular 1,3-dipolar cycloaddition is well documented. Limited by the insolubility of the solid support, the reaction progression can only be monitored by a change in the functional groups (if any) via infrared (IR) spectroscopy. Thus, the structural identification of intermediates cannot be characterized by conventional nuclear magnetic resonance (NMR) spectroscopy. Other limitations to this method stem from the compatibilities of the polymer/linker to the desired chemical reaction scheme. Herein we report a protocol that allows for the convenient production of spirocyclic heterocycles that, with simple modifications, can be automated with high-throughput techniques.

Here we present a protocol to demonstrate an efficient method for the synthesis of spirocyclic heterocycles. The five-step process utilizes solid-phase synthesis and regenerating Michael linker strategies. Generally difficult to synthesize, we present a customizable method for the synthesis of spirocyclic molecules otherwise inaccessible to other modern approaches.

A convenient synthetic route for spirocyclic heterocycles is well sought after due to the molecule&#39;s potential use in biological systems. By means of ...

Imine Metathesis by Silica-Supported Catalysts Using the Methodology of Surface Organometallic Chemistry

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is designed and prepared according to the methodology developed by surface organometallic chemistry (SOMC). In this framework, catalytic cycles can be determined by isolating crucial intermediates. All air-sensitive materials are handled under inert atmosphere (using gloveboxes or a Schlenk line) or high vacuum lines (HVLs, &#60;10-5 mbar). The preparation of SiO2-700 (silica dehydroxylated at 700 &#176;C) and subsequent applications (the grafting of complexes and catalytic runs) requires the use of HVLs and double-Schlenk techniques. Several well-known characterization methods are used, such as Fourier-transform infrared spectroscopy (FTIR), elemental microanalysis, solid-state nuclear magnetic resonance spectroscopy (SSNMR), and state-of-the-art dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP-SENS). FTIR and elemental microanalysis permit scientists to establish the grafting and its stoichiometry. 1H and 13C SSNMR allows the structural determination of the hydrocarbon ligands coordination sphere. DNP SENS is an emerging powerful technique in solid characterization for the detection of poorly sensitive nuclei (15N, in our case). SiO2-700 is treated with about one equivalent of the metal precursor compared to the amount of surface silanol (0.30 mmol&#183;g-1) in pentane at room temperature. Then, volatiles are removed, and the powder samples are dried under dynamic high vacuum to afford the desired materials [(&#8801;Si-O-)Hf(&#951;2&#960;-MeNCH2)(&#951;1-NMe2)(&#951;1-HNMe2)]. After a thermal treatment under high vacuum, the grafted complex is converted into metal imido silica complex [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)]. [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] effectively promotes the metathesis of imines, using the combination of two imine substrates, N-(4-phenylbenzylidene)benzylamine, or N-(4-fluorobenzylidene)-4-fluoroaniline, with N-benzylidenetert-butylamine as substrates. A significantly lower conversion is observed with the blank runs; thus, the presence of the imido group in [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is correlated to the catalytic performance.

A new group IV metal catalyst for imine metathesis is prepared by grafting amine metal complex onto dehydroxylated silica. Surface metal fragments are characterized using FT-IR, elemental microanalysis, and solid-state NMR spectroscopy. Further dynamic nuclear polarization surface enhanced NMR spectroscopy experiments complement the determination of the coordination sphere.

With this protocol, a well-defined singlesite silica-supported heterogeneous catalyst [(&#8801;Si-O-)Hf(=NMe)(&#951;1-NMe2)] is designed and prepared ...

Tuning the Acidity of Pt/ CNTs Catalysts for Hydrodeoxygenation of Diphenyl Ether

We herein present a method for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs. By varying the weight of various solid acid nanosheets, a series of Pt/xHMNO6/CNTs with different solid acid compositions (x = 5, 20 wt%; M = Nb, Ta; N = Mo, W) have been prepared by carbon nanotube pretreatment, protonic exchange, solid acid exfoliation, aggregation and finally Pt particles impregnation. The Pt/xHMNO6/CNTs are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and NH3-temperature programmed desorption. The study revealed that HNbWO6 nanosheets were attached on CNTs, with some edges of the nanosheets being bent in shape. The acid strength of the supported Pt catalysts increases in the following order: Pt/CNTs &#60; Pt/5HNbWO6/CNTs &#60; Pt/20HNbMoO6/CNTs &#60; Pt/20HNbWO6/CNTs &#60; Pt/20HTaWO6/CNTs. In addition, the catalytic hydroconversion of lignin-derived model compound: diphenyl ether using the synthesized Pt/20HNbWO6 catalyst has been investigated.

A protocol for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs is presented.

We herein present a method for the synthesis of HNbWO6, HNbMoO6, HTaWO6 solid acid nanosheet modified Pt/CNTs. By varying the weight of various solid acid ...