Ammonia Oxidation Process Catalyzed by Pt@XO2 (X = Ti, Zr, Ce, and

Subscriber access provided by University of Winnipeg Library

Kinetics, Catalysis, and Reaction Engineering

Ammonia Oxidation Process Catalyzed by Pt@XO2 (X = Ti, Zr, Ce, and Ce/Zr) prepared by Photoreduction process Qiang Liu, Xueling Pu, Hairong Yue, Wei Jiang, and Bin Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00391 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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

Industrial & Engineering Chemistry Research

Ammonia Oxidation Process Catalyzed by Pt@XO2 (X = Ti, Zr, Ce, and Ce/Zr) prepared by Photoreduction process Qiang Liu, Xueling Pu, Hairong Yue, Wei Jiang*, Bin Liang Multi-Phase Mass Transfer and Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu, 610065, China E-mail: [email protected]

Abstract

In the nitric acid industry, platinum loss is the second largest cost due to vaporization of the Pt at high temperature. In this research, Pt is loaded on TiO2, ZrO2, CeO2 and ceria-zirconia solid solution by photoreduction and used as catalysts for ammonia oxidation. Results show Pt content of the obtained catalysts is effectively reduced to 0.08 wt %, but their catalytic performance is better than that of the traditional Pt gauze between 300 and 800 °C. The best catalyst, 3Pt@CeO2, enabled complete ammonia transformation at 300 °C and 80 % NOx yield at 800 °C with the space velocity of 235000 h-1 and the oxygen to ammonia ratio of 15. After 50 h of continuous operation, no observable decay was detected. Such performance is ascribed to lattice oxygen provided by CeO2 and the formation of Pt-Ce-O. These results provide a promising method of improving the current industrial Pt catalysts. KEY WORDS: Ammonia selective oxidation, Supported Pt catalysts, Photo reduced 1 ACS Paragon Plus Environment

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

Page 2 of 35

1. Introduction Generation of NOx from ammonia by high-temperature catalytic oxidation is the crucial step for the industrial production of nitric acid1. Platinum and platinum alloy gauzes are usually employed as the catalysts for this process, and the ammonia conversion can reach 94-98 % depending on the reaction conditions2-6. However, the platinum loss caused by volatile PtO2 generated under high-temperature conditions in an oxygen atmosphere results in a significant catalytic performance decay and cost increase. Such Pt loss is responsible for the second largest cost except for the raw material, ammonia, in nitric acid production4, 7. Therefore, lowering Pt loss during the ammonia oxidation process is the key to increasing nitric acid production yield.

At present, three strategies are used to lower Pt loss8: improving Pt gauze woven technology, entrapping the violated Pt and replacing Pt with a cheaper metal. However, the first two methods only cause a negligible change in the operating temperature, resulting in Pt loss. The third method is not yet commercialized due to the relatively poor catalytic performance of non-precious metals such as Fe, Co, and Cr. Thus, obtaining a Pt catalyst for ammonia oxidation with high activity at low reaction temperature and reducing Pt consumption are still attractive research areas.

Preparing a highly dispersed Pt supported catalyst is a possible approach to achieve this goal. The high-dispersion state of the active component can effectively reduce the Pt content and improve the Pt activity. However, only a few studies have been

2 ACS Paragon Plus Environment

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


conducted in this field. Schäffer et al.5 prepared a catalyst with 0.005 wt % Pt content deposited on α-Al2O3, γ-Al2O3 and CeO2-TiO2. However, the selectivity of NO was nearly 90 % at 800 °C

and the NH3 conversion was only about 60 %. Jiang et al9.

prepared a wall-loaded Pt/TiO2/Ti catalyst by anodizing a Ti foil and photo-depositing Pt on the anodic surface layer in a microchannel reactor, obtaining 100 % ammonia conversion at 300 °C. However, its industrialization has been difficult. Hung et al10 elucidated selective catalytic oxidation (SCO) of ammonia over a Pt-Pd-Rh ternary composite cordierite substrate catalyst at 150-350 °C. The method resulted in the total conversion of ammonia at 327 °C with only 7 % NO yield. The performance of the loaded Pt catalyst should be promoted further for practical applications.

Photoreduction is a promising technology for noble metal deposition on the surface of photocatalysts, thus enhancing their photocatalytic performance. This method is also used to obtain a high-efficiency catalyst with highly dispersed active noble metal sites. Electron-hole pairs are generated in the photocatalysts by illumination, and the photo-generated electrons can reduce noble metal ions and immobilize them on the photocatalyst surface. Theoretically, noble metals loaded in this way are deposited in single-atom layer dispersions on the photocatalyst support, which improves their catalytic performance and decreases the loading amount. It is expected that the performance of ammonia oxidation catalyst can be improved since the common active components involved are noble metals.



Page 4 of 35

TiO2 is a popular photocatalyst and has been widely used to photoreduce metals. ZrO2, a wide band-gap photocatalyst, also has been widely used as a catalyst or catalyst support due to its acidic and basic sites11. In addition, CeO2 and ceria-zirconia solids are widely studied in the field of catalytic oxidation and reduction, especially denitrification, owing to their excellent oxygen storage capacity12-14. Thus, the four photocatalysts have potential applications as ammonia oxidation catalysts and supports.

In this study, Pt-supported catalysts are prepared by photoreduction to decrease Pt usage, thus improving activity and stability. Photocatalysts, TiO2, ZrO2, CeO2, and Ce0.5Zr0.5O2, are employed as the carriers in this process. The catalytic performance of four composites are evaluated to screen out the best one. The influence of carriers on Pt-supported catalyst performance is determined and discussed.


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


2. Experimental 2.1 Chemicals Potassium chloroplatinate (99 %) is provided by Beijing Hwrk Chemical Co. Ltd. Analytical-grade reagents, including butyl titanate, butyl acrylate, nitric acid, ethanol, cerium nitrate, zirconium nitrate, formic acid, urea, polyvinylpyrrolidone, ammonia, diamine, and zirconium acid butyl ester, were purchased from KeLong Chemical Reagent Co., Ltd., and were used without further purification.

2.2 Preparation of photocatalyst carriers Three photocatalyst samples, TiO2, ZrO2, and CeO2, were prepared by sol-gel method, and Ce0.5Zr0.5O2 was prepared by co-precipitation.

For preparation of TiO2, 20 mL butyl titanate was added to 50 mL ethanol and stirred to form solution A. Another 20 mL deionized water, 30 mL ethanol, and a small amount of nitric acid were mixed to obtain solution B. Then, solution B was slowly added into solution A with continuous stir. The mixed solution was aged for 24 h to obtain the sol and then dried for 24 h at 70 °C. When the anhydrous ethanol solvent completely evaporated, the sample was dried at 100 °C for 24 h and the sample particles were obtained. The sample particles were annealed for 3 h at 800 °C in a muffle furnace. After removal, cooling, grinding and screening, the sample with 0.15-0.3 mm particles was used as the photoreduction carrier.



Page 6 of 35

The preparation methods of ZrO2 and TiO2 were similar. Solution A was replaced by adding 10 mL zirconium acid butyl ester into 34 mL ethanol, and solution B was replaced by a mixture of 2 mL deionized water, 17 mL ethanol, and 0.58 mL nitric acid. For the preparation of CeO2, only one solution was employed by mixing 1.5 g cerium nitrate, 1.2 g urea, and 0.6 g polyvinylpyrrolidone in 40 mL beaker containing deionized water, and adding certain ammonia after stirring for 1 h.

For the preparation of Ce0.5Zr0.5O2, 0.01 mol cerium nitrate and 0.01 mol zirconium nitrate were added into a 100 mL beaker containing 100 mL deionized water. After stirring for 1 h, the solution was mixed with 5 mL diamine solution, and the solution was mixed with a magnetic stir bar for 24 h. After placing the mix in a centrifuge, the obtained powders were dried at 100 °C for 24 h and annealed at 800 °C for 3 h in a muffle furnace. After cooling, grinding, and dividing, the sample with 0.15-0.3 mm particles was obtained.

2.3 Preparation of Pt-loaded catalysts for ammonia oxidation The typical preparation method was as follows: 0.1 g of carrier (TiO2, ZrO2, CeO2, or Ce0.5Zr0.5O2) was mixed with 2 mL potassium chloroplatinate solution with 0.0047 mol/L concentration in a culture vessel, and 20 mL deionized water was added. Formic acid was used as a sacrificial agent. The culture vessel was placed in a photoreactor at room temperature. The sample was irradiated by a 500 W UV lamp for an arbitrary amount of time. The sample was dried at 80 °C for 24 h after it was


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


separated from the photoreactor. The dried sample was annealed at 800 °C for 3 h in the muffle furnace. The obtained product was the catalyst xPt@carriers, of which the term, x, referred to the irradiation time in hours.

2.4 Characterization The crystal structure of each sample was identified by powder X-ray diffraction (XRD) measurements (X 'Pert pro MPD, Cu target Ka ray). Scanning electron microscopy (SEM) (S-4800) was used to analyze the morphology of the samples. X ray spectra (EDS) (IE250) was used to analyze the catalyst elements present in the samples. The microstructure of the catalysts was characterized by transmission electron microscopy (JEM-2100F, TEM). The Brunauer-Emmett-Teller (BET) surface area and the samples’ pore volume were obtained with a Micro ASAP2460 catalyst area analyzer. Electric high frequency plasma emission spectroscopy (ICP) (Vista, Axial) was used to obtain the platinum contents in catalysts. An X-ray photoelectron spectroscopy analyzer (XPS VG, ESCALAB MK II) was used to analyze the chemical structures of the prepared catalysts. Temperature programmed desorption (TPD) was characterized with a Micro2920 analyzer. Raman spectra (Thermo Fisher DXR Raman Microscope) operating at 455 nm line for excitation was used to identify the interaction between Pt and carriers.



Page 8 of 35

2.5 Ammonia oxidation reaction The catalytic performance evaluation of the different xPt@carrier catalysts was carried out in a fixed bed reactor. The influence of internal and external diffusion was eliminated when the catalyst particle size was less than 0.15-0.3 mm and the space velocity was greater than 235000 h-1 15. About 50 mg of the catalyst was packed in a 4 mm×2 mm quartz reaction tube. The raw material ratio of oxygen to ammonia was adjusted from 1:1.94 to 1:15. The reaction products were analyzed by gas chromatography (GC-9790 II) with a GDX column for ammonia and NOx detection and a 5A molecular sieve column for nitrogen detection.

In this process, the main products of ammonia oxidation were NO2 and NO16,17, which are collectively known as NOx, and main by-product was N2. Here, the catalyst activity was evaluated by two index, ammonia conversion and NOx yield, which can be calculated using the following equation：

𝑋(NH3 )(%) =

NH3 𝑖𝑛 −NH3 𝑜𝑢𝑡 NH3 𝑖𝑛

× 100%

2N

2 𝑆(NOx )(%) = [1 − NHi𝑛−NH 𝑜𝑢𝑡 ] × 100% 3

(1) (2)

3


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


3. Results and discussion 3.1 Characterization of the four Pt-loaded catalysts

Figure 1. The XRD patterns of: (a) xPt@TiO2 (b) xPt@ZrO2 (c) xPt@CeO2 (d) [email protected]

The XRD characterization results of TiO2, ZrO2, CeO2, and Ce0.5Zr0.5O2 photocatalysts and the corresponding Pt-loaded composites after different photodegradation time are shown in Fig. 1. In Fig. 1-A, it can be observed that the crystal structure of TiO2 and three Pt@TiO2 samples exhibit the same pattern as rutile TiO2 without any detectable changes in photoreduction. The characteristic peaks at 27.446°, 36.085°, 41.225°, and 54.322° were assigned to the crystal face of (110),



Page 10 of 35

(101), (111), and (211) of rutile TiO2 (PDF#21-1276), respectively. The calculated TiO2 grain size of four samples is 31.9 nm. No peaks ascribed to Pt can be detected, thus confirming the amorphous state of Pt.

Similarly, the peaks belonging to Pt are unobservable, and only the peaks of carriers can be detected for Pt@ZrO2, Pt@CeO2, and Ce0.5Zr0.5O2. The crystal pattern of serial ZrO2 samples shown in Fig. 1-B is a mixture of monoclinic and tetragonal. The peaks observed at 24.047°, 28.174°, and 34.159° are assigned to monoclinic crystal structure (PDF#37-148), and the peak at 30.167° is assigned to tetragonal crystal structure (PDF#42-1164). The grain size determined in different samples is about 28.6 nm. For CeO2, the main peaks of serial samples shown in Fig. 1-C are 28.554°, 33.081°, 47.478°, and 56.334° correspond to (111), (200), (220), and (311) facets of fluorite CeO2 (PDF#43-1002), respectively. The calculated grain size is 31.9 nm. As shown in Fig. 1-D, all the reflection data from the serial Ce0.5Zr0.5O2 samples provide typical patterns (PDF#38-1436) without any characteristic peak due to Pt. The calculated grain size is about 5.8 nm.

Since the existence of Pt on the carriers cannot be directly detected by XRD, EDS and ICP analyses were conducted to determine the Pt content on the selected surface and in bulk, and BET analysis was used to obtain the surface area. As shown in Table 1, the existence of loaded Pt on the four carriers via photoreduction can be confirmed. The Pt content increased as the UV light irradiation time increased, but the surface area decreases significantly due to the increasing amount of Pt. This fact confirms the 10 ACS Paragon Plus Environment

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


reduction and capture of Pt on all four photocatalyst carriers. A noticeable fact is the great deviation of Pt content determined by ICP and EDS. EDS only analyzed the selected area of surface while ICP measured the total body. Furthermore, one can notice that the Pt content on CeO2 is the largest with the same photoreduction time. However, the surface area of CeO2 is smaller than ZrO2 and Ce0.5Zr0.5O2, which suggests its higher activity for ammonia oxidation. The Pt loaded amount on Ce0.5Zr0.5O2 is lowest due to its poorer photoactivity despite the fact that its surface area is the largest.

Compared with the industrial pure platinum gauze, it can be found that even the largest Pt content of 5Pt@CeO2 is only 0.1212 wt %, far less than the industrial Pt catalyst. While the smallest surface area of 5Pt@TiO2 reaches 0.4620 m2/g, far higher than the value of 46.50 cm2/g in platinum gauze. This feature is beneficial to the performance improvement of Pt catalysts.

Table 1. Surface area and Pt content of photocatalyst carriers and obtained xPt@carrier. Surface area Sample m2/g

wt %Pt by ICP

--

wt %Pt by EDS

TiO2

1.592

--

1Pt@TiO2

1.360

0.06130

0.210

3Pt@TiO2

1.346

0.07110

0.250



Page 12 of 35

5Pt@TiO2

0.4620

0.1047

0.620

ZrO2

16.80

--

--

1Pt@ZrO2

16.30

0.05190

0.170

3Pt@ZrO2

15.90

0.05960

1.24

5Pt@ZrO2

14.60

0.1212

2.58

CeO2

10.28

--

--

1Pt@CeO2

8.500

0.06490

0.720

3Pt@CeO2

7.850

0.06590

2.87

5Pt@CeO2

7.670

0.1365

5.34

Ce0.5Zr0.5O2

35.91

--

--

[email protected]

31.41

0.04290

1.20

[email protected]

31.07

0.06730

2.36

[email protected]

30.18

0.07150

4.23


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


Figure 2.The SEM images of: (a) 5Pt@TiO2 (b) 5Pt@ZrO2 (c) 3Pt@CeO2 (d)[email protected], the size of SEM images was magnified 20000 times.

The morphology of the 5Pt@TiO2, 5Pt@ZrO2, 3Pt@CeO2, and [email protected] catalysts were selected and characterized by SEM. The dispersion of nanoparticles can be observed on the surface of the four carriers as shown in Fig. 2. The relatively flat and dense TiO2 substrate in Fig. 2-A explicates its low surface area, and lots of nanoparticles with ~50 nm diameter are dispersed on the substrate surface. However, for ZrO2 in Fig. 2-B and CeO2 in Fig. 2-C, a loose porous film is observed. This is in agreement with their relatively high surface area. The diameter of dispersed nanoparticles has a wide size distribution ranging from 20 nm to 200 nm. For Ce0.5ZrO0.5O2, a flat but porous structure corresponding to its highest surface area is



Page 14 of 35

obtained in Fig. 2-D. The sizes of the nanoparticles is inhomogeneous with diameters ranging from about 10 nm to 300 nm. This range is larger than the substrate pore diameter and suggests the occurrence of surface deposition rather than internal deposition.

Figure 3.The HRTEM images of: (a) 5Pt@TiO2 (b) 5Pt@ZrO2 (c) 3Pt@CeO2 (d) [email protected].

The HRTEM results confirmed the nanoparticles were indeed Pt. In Fig. 3-A, two crystal lattice peaks at 0.325 nm and 0.227 nm are assigned to TiO2(110) and Pt(111), respectively. Similarly, Pt(111) is detected in Fig. 3-B and Fig. 3-C. Peaks at 0.369 nm and 0.281 nm are assigned to ZrO2(110) and ZrO2(111), respectively, while a peak at 0.31 nm is assigned to CeO2(111). Finally, in Fig. 3-D, lattice spacing are 14 ACS Paragon Plus Environment

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


measured; a peak at 0.312 nm is ascribed to CeO2(111), 0.262 nm is ascribed to ZrO2(211), and 0.227 nm is ascribed to Pt(111).

Figure 4. The XPS results of: (a) 5Pt@TiO2, (b) 5Pt@ZrO2, (c) 3Pt@CeO2, (d) [email protected].

The XPS results of four typical samples are shown in Fig. 4. Unionized Pt(0) emission peaks are observed at 74.50 eV from Pt 4f(5/2) and 71.20 eV from Pt 4f(7/2) with peak separation of 3.30 eV18, 19. Ionized Pt(II) emission peaks are observed at 75.80 eV of Pt 4f(5/2) and 72.50 eV of Pt 4f(7/2) with peak separation of 3.30 eV can be detected20. Ionized Pt(IV) emission peaks are also observed at 74.40 eV of Pt 4f(5/2) and 77.80 eV of Pt 4f(7/2) with peak separation of 3.40 eV can be detected



Page 16 of 35

after deconvolution20. These observations confirm the co-existence of pure Pt, Pt(II) and Pt(IV) ion in all four samples. The former two are the photoreduction product, while the latter should originate from unreacted K2PtCl6 that was adsorbed by the carriers. As shown in Fig. 4-A to Fig. 4-D, the calculated Pt(0), Pt(II) to Pt(IV) ratios in 5Pt@TiO2, 5Pt@ZrO2, 3Pt@CeO2, and [email protected] are 3.84:1.69:1, 1.13:0.56:1, 3.69:2.28:1 and 2.24:1.27:1, and the ratio of Pt(0) is 0.59, 0.42, 0.53, 0.50, respectively. This ratio confirms that the photocatalytic performance of TiO2 should be highest since most of the Pt has been reduced in the same time interval.

3.2 The effect of photocatalysts as support

Figure 5. The Raman spectrum of: (a) TiO2 and 5Pt@TiO2 (b) ZrO2 and 5Pt@ZrO2 (c) CeO2 and 3Pt@CeO2

(d) Ce0.5Zr0.5O2 and 1Pt@ Ce0.5Zr0.5O2


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


Here, Raman spectra from four samples were analyzed to determine the possible interaction between the carriers and loaded Pt, and the results are shown in Fig. 5. For TiO2 and 5Pt@TiO2 in Fig. 5-A, it can be seen that the two curves nearly overlapped without significant distortion and shift. It can be deduced that the interaction between the TiO2 support and Pt can be ignored, suggesting that the TiO2 support do not affect the catalytic performance of Pt.

However, the Raman results of the other three samples are different. Comparing the spectra of pure ZrO2 and 5Pt@ZrO2 in Fig. 5-B, two peaks at 147 cm-1 and 264 cm-1 are ascribed to a tetragonal ZrO2 phase that disappeared after loading Pt21, 22. The disappearance of two peaks could result from photoreduction and deposition of Pt on tetragonal ZrO2 since its direct band gap (5.0 eV) is smaller than that in monoclinic ZrO2 (5.3 eV). Pt could deposit on the active site of tetragonal ZrO2 preferentially, thus causing the transformation of the Raman peaks23-25.

Comparing pure CeO2 and 5Pt@CeO2 in Fig. 5-C, the fluorite structure of CeO2 stays constant and no large changes occurred in the 462 cm-1 peak. However, a remarkable enhancement in the 600-700 cm-1 spectral range can be found after loading Pt. The band is related to the presence of oxygen vacancies. This enhancement suggests the generation of extra oxygen vacancies by the incorporation of Pt(VI), Pt(II) into the CeO2 fluorite lattice, which compensates the valence mismatch between the Pt(II) and Ce(VI)26-28. Such enhancement in oxygen vacancy should be advantageous to the oxidation of ammonia. This band is thought to 17 ACS Paragon Plus Environment


Page 18 of 35

originate from a Pt-CeO surface species containing a Pt-O-Ce link. Another observable change is a weakening of the peak at 830 cm-1 after loading Pt29-31. This could be due to the interaction between deposited Pt and a CeO2 active site. In Fig. 5D, no remarkable changes in the Raman spectra of Ce0.5Zr0.5O2 are observed before and after loading Pt. A peak at 473 cm-1 attributed to the T2g vibration absorption of fluorite CeO2 remained unchanged, suggesting the interaction between Pt and Ce0.5Zr0.5O2 is negligible32.

Figure 6. The UV-Vis DRS spectra of; (a) TiO2 and 5Pt@TiO2 (b) ZrO2 and 5Pt@ZrO2 (c) CeO2 and 3Pt@CeO2 (d) Ce0.5Zr0.5O2 and 1Pt@ Ce0.5Zr0.5O2.


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


In order to further understand the interaction between Pt and the four carriers, UV-vis diffuse reflectance was measured, and the results are shown in Fig. 6. In Fig. 6-A, the absorption edge at 420 nm corresponding to a band gap of 2.95 eV33-35 in TiO2 can be observed. After Pt loading, the absorption edge redshifted to about 440 nm with a band gap value of 2.82 eV. This redshift is ascribed to the TiO2 nanograin growth after loading36, and also possibly ascribed to Schottky barrier between the loaded metal and semiconductor oxide support, which reduce the oxygen vacancy formation enthalpy benefitting from electronic shift from oxides to metals, thus benefitting the oxidization process37-39. A small new broad peak at 490 nm is observed, which ascribed to Pt molecular electron transition in π-π band40.

For ZrO2 and 5Pt@ZrO2 shown in Fig. 6-B, a redshift of the ZrO2 absorbance edge from 253 nm to 260 nm is observed, which corresponds to a band gap change from 4.91 eV to 4.76 eV. The small wide peak at 490 nm is also observed. This peak is slightly larger than 5Pt@TiO2, suggesting stronger catalytic performance.

However, the observations in CeO2 after loading Pt shown in Fig. 6-C is totally different. The adsorption edge of CeO2 transfers from 365 nm to 376 nm after loading Pt, corresponding to a band gap change from 3.4 eV to 3.3 eV. However, a wide but strong absorption range between 400 and 650 nm was observed after Pt loading. This change is ascribed to the Schottky barrier constructed between Pt and CeO237. The presence of such Pt-CeO2 is advantageous to the transfer of electrons from O2p to Ce4e41, 42, significantly increasing the number of oxygen vacancies and improving the 19 ACS Paragon Plus Environment


Page 20 of 35

catalytic performance. The results from Ce0.5Zr0.5O2 and [email protected] shown in Fig. 6-D are similar to CeO2 with a band gap change from 3.61 eV to 3.41 eV. However, the interaction between Pt and the carrier is not as strong as CeO2.

3.3 Performance evaluation of four Pt-loaded catalysts 3.3.1 Catalyst performance

Figure 7. The NH3 conversion and NOx yield of ammonia oxidation catalyzed by: (a) xPt@TiO2, (b) xPt@ZrO2 , (c) xPt@CeO2 , (d) [email protected], and the content of NH3 is 5.71 vol %, VO2/VNH3=15 , GHSV=235000 h-1 in feed, then the conversion values are of blank test and unloaded supports, while the NH3 conversion of Pt@XO2 are 100 %.


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


Now, the obtained four serial catalysts by loading Pt on different carriers were evaluated under the same conditions in order to screen out the best one. Comparative experiments with four blank carriers also were conducted to determine their possible effects on catalytic performance. The ammonia conversion and NOx selectivity were selected to assess the performance since ammonia cost is the main factor in nitric acid production, and NOx is the primary product required for ammonia oxidation. The results were shown in Fig. 7, and the value of ammonia conversion without any catalyst also was plotted for comparison.

As shown in Fig. 7-A, it can be found that the ammonia conversion without any catalyst only increases from 0 % at 300 °C to 21.6 % at 800 °C, and the NOx yield increases from 0 % to 23 %. When introducing pure TiO2 as the catalyst, the ammonia conversion slightly increases from 0 % at 300 °C to 44.3 % at 800 °C, but no more NOx can be detected. This conversion increase can be ignored, and TiO2 can be regarded as an inert catalyst in the ammonia oxidation process. However, when Pt was photoreduced and deposited on the TiO2 surface, the catalytic performance is significantly improved. Ammonia can be completely converted with the Pt@TiO2 catalyst at 300 °C with an ignition temperature of about 237 °C regardless of the Pt content. Thus, the ammonia conversion does not need to be plotted in this figure. However, the product distribution of three xPt@TiO2 samples are different. As shown in Fig. 7-A, the NOx yield from 1Pt@TiO2, 3Pt@TiO2, and 5Pt@TiO2 catalysts



Page 22 of 35

increase from 56.07 %, 64.38 %, and 70.12 % to 69 %, 70.81 %, and 71.13 %, respectively, with temperature rising form 600 °C to 800 °C.

However, for the other three samples, the observations are different after introducing blank carriers. Ammonia conversion with ZrO2 (Fig. 7-B), CeO2 (Fig. 7C), and Ce0.5Zr0.5O2 (Fig. 7-D) are significantly improved from 5.00 % at 300 °C to 100 % at 600 °C, from 4.50 % at 300 °C to 100 % at 625 °C, and from 4.1 % at 300 °C to 100 % at 525 °C, respectively. Their NOx selectivity is unsatisfactory since the highest determined NOx yield value was only 33.81 % of pure ZrO2, 39.76 % for CeO2, and 39.50 % for Ce0.5Zr0.5O2 at 800° C, respectively. This result confirms that the ZrO2, CeO2, and Ce0.5Zr0.5O2 carriers can effectively promote the conversion of NH3. However, the generation of nitrogen is the prior, although the NOx yield gradually increased with rising temperature.

This awkward situation of the three carriers improved when Pt was introduced as the active substance after the photoreduction process. For the three serial catalysts xPt@ZrO2, xPt@CeO2, and [email protected], complete ammonia conversion can be achieved at 300°C, regardless of the loaded Pt content. However, their product distributions are different. As shown in Fig. 7-B, when the temperature increases from 600 °C to 800 °C, the NOx yield of 1Pt@ZrO2, 3Pt@ZrO2, and 5Pt@ZrO2 increase from 49.36 %, 48.68 %, and 54.59 % to 60.14 %, 61.65 %, and 62.93 %, respectively. The addition of Pt content against the photoreduction time effectively promotes the production of NOx. Similarly, for xPt@CeO2 shown in Fig. 7-C, the increased Pt 22 ACS Paragon Plus Environment

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


content also positively boosts the NOx yield. The determined NOx yield increases from 66.32 % to 76.6 % in 1Pt@CeO2 and from 68.5 % to 80 % in 3Pt@CeO2 in the testing temperature zone. However, for 5Pt@CeO2, the NOx yield only slightly increases from 61 % to 71 % with increased temperature. This occurred despite its maximum Pt content of 0.1626 % compared with that of the other two samples. This performance decay could be ascribed to the overgrowth of nanoparticles, resulting in a decrease in the number of active sites. The occurrence of [email protected] shown in Fig. 7-D is similar since the determined NOx yield values in [email protected] and [email protected] increase from 60.29 % and 60.79 % to 72.46 % and 70.57 % when temperature rose from 600 to 800°C, respectively. However, that of [email protected] only increase from 58.34 % to 71.09 % in the same temperature zone despite the significantly increased Pt content.

Based on the catalytic performance evaluation of the four serial Pt@XO2 catalysts, it can be concluded that 5Pt@TiO2, 5Pt@ZrO2, 3Pt@CeO2, and [email protected] were screened out as the catalysts. The photo-deposited Pt on photocatalyst carriers can effectively catalyze ammonia oxidation. With these catalysts, complete conversion of ammonia is achieved beyond 300 °C, and the NOx selectivity improved with increasing reaction temperature.



Page 24 of 35

3.3.2 Optimal catalysts and their performance comparison

Figure 8. Catalyst performance evaluation: (a) The effect of V(O2)/V(NH3) on NOx yield, the content of NH3 is 5.71 vol % and GHSV=235000 h-1 in feed, 800 °C (b) Catalyst stability, the content of NH3 is 5.71 vol %,, VO2/VNH3=15 and GHSV=235000 h-1 in feed, 800 °C.

The four best supported Pt catalysts, 5Pt@TiO2, 5Pt@ZrO2, 3Pt@CeO2, and [email protected], were contrasted to determine the best candidate for the ammonia oxidation process. After plotting the temperature-dependent tendencies of the four samples as shown in Fig. 7, the sequence of the NOx yield amount is 3Pt@CeO2 (80 %) ＞ [email protected] (72.46 %) ＞ 5Pt@TiO2 (71.13 %) ＞ 5Pt@ZrO2 (62.93 %) at 800 °C. Actually, the 3Pt@CeO2 sample exhibited the best NOx yield throughout the temperature testing zone. Then the raw material ratio of oxygen to ammonia was adjusted from 1:1.94 to 1:15 at 800 °C to determine its effect. As shown in Fig. 8-A, it can be observed that the NOx yield can be seriously affected regardless of the oxygen to ammonia ratio, despite the complete conversion of ammonia at 300 °C. The determined NOx yield ranges of 5Pt@TiO2, 5Pt@ZrO2,


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


3Pt@CeO2, and [email protected] are 64.14-71.13 %, 53.47-62.93 %, 60.37-78.36 %, and 57.05-72.46 % when V(O2)/V(NH3) increases from 1.94 to 15, respectively. The increase of oxygen in feed is conducive to the formation of NOx. However, it is noteworthy that the selectivity increase of 3Pt@CeO2 is sharpest when the oxygen to ammonia ratio increased to 15. This significant increase of NOx yield with 3Pt@CeO2 could be ascribed to the special oxygen storage and release property of CeO2 carrier.

The service life of the four selected catalysts were determined with a volume ratio of oxygen to ammonia of 15, space velocity of 235000 h-1, and temperature of 800 °C. After running continuously for 50 h, ammonia could not be detected in the gas product, confirming the complete conversion of ammonia. The determined values of the NOx yield in Fig. 8-B stay constant for all the four catalysts. No observable fluctuations are detected, which confirms the excellent stability of the 5Pt@TiO2, 5Pt@ZrO2, 3Pt@CeO2, and [email protected] catalysts.

3.4 Investigation of catalytic mechanism of four Pt-loaded catalysts Since the obtained catalysts exhibited attractive performance with low Pt content and high stability, it is necessary to determine the mechanism for obtaining catalysts by the photoreduction process. Here, TPD analyses of four serial samples were conducted. NH3 and O2 were included as the reactants and NO as the product.



Page 26 of 35

Figure 9.The TPD analysis of: (a) NH3-TPD of xPt@TiO2, (b) NH3-TPD of xPt@ZrO2, (c) NH3TPD of xPt@CeO2, (d) NH3-TPD of [email protected], (e) O2-TPD of 5Pt@TiO2, 5Pt-ZrO2,3PtCeO2,1Pt-Ce0.5Zr0.5O2, (f) NO-TPD of 5Pt@TiO2,5Pt-ZrO2,3Pt-CeO2,1Pt-Ce0.5Zr0.5O2.

NH3-TPD measurements on four serial catalysts were conducted one-by-one and are shown in Fig. 9-A to Fig. 9-D. As shown in Fig. 9-A, pure TiO2 exhibits a 26 ACS Paragon Plus Environment

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


significant strong acidic site at 600 °C. However, after loading Pt, the acidic sites weakened and gradually shifted to about 580 °C with increasing Pt content. This strong NH3 chemisorption is beneficial to the dissociation of N-H bonds, but the low adsorption amount and relatively low desorption temperature peak is not good for further reaction. The loading of Pt shows a limited improvement on NH3 adsorption by TiO2, but this improvement is unsatisfactory.

The NH3-TPD results from ZrO2 shown in Fig. 9-B show that ZrO2 had stronger surface acidity in a temperature range between 300 °C and 600 °C, except for the remarkable NH3 physical adsorption at 110 °C. This strong ammonia adsorption is not only beneficial to the formation of NOx, but also to N2 generation43. When Pt is deposited on ZrO2, the surface acidic site density of 1Pt@ZrO2, 3Pt@ZrO2, and 5Pt@ZrO2 decreased from 13.93 to 11.53, 11.57, and 12.26 μmol/m3, respectively. In addition, the desorption temperature is mainly at 320-660 °C. As the deposition time increases, 1Pt@ZrO2 possesses weaker desorption peaks at 260 °C, 400 °C, and 520 °C, beneficial to desorption of NOx. Similarly, 3Pt@ZrO2 desorption occurs at 280 °C, 440 °C, and 560 °C, while desorption in 5Pt@ZrO2 occurs at 280 °C and 480 °C. As a whole, the desorption temperature decreases with increasing deposition time, suggesting a performance improvement in xPt@ZrO2 with increasing Pt.

The amount of adsorbed NH3 on pure CeO2 is relatively low after excluding physical adsorption. As shown in Fig. 9-C, the major NH3 desorption peak of CeO2 is at 375 °C, and the acid site density is determined to be 20.04 μmol/m3. However, after 27 ACS Paragon Plus Environment


Page 28 of 35

photo-depositing Pt on the CeO2 surface, the acid density value is changed to 19.41 μmol/m3, 20.51 μmol/m3, and 22.43 μmol/m3 for 1Pt@CeO2, 3Pt@CeO2, and 5Pt@CeO2, respectively. The corresponding desorption temperature changes from 370 °C and 600 °C in CeO2 to 440 °C in 1Pt@CeO2, 340-610 °C in 3Pt@CeO2, and 440-610 °C in 5Pt@CeO2, respectively. For [email protected] in Fig. 9-D, the calculated acid site density values of pure solid solution and other composites are estimated as 6.043 μmol/m3, 5.317 μmol/m3, 5.665 μmol/m3, and 5.898 μmol/m3, respectively. Their determined NH3 desorption temperature zones are relatively wide between 300 and 600 °C (Ce0.5Zr0.5O2), 200 and 600 °C ([email protected]), 340 and 430 °C ([email protected]), and 350 °C ([email protected]).

The NH3-TPD results from the four serial samples confirms that ZrO2, CeO2, and the solid solutions possess strongly acidic sites for NH3 adsorption, which are advantageous for the dissociation and activation of N-H bonds. Pt loading widens the NH3 adsorption zone to higher temperatures, thus intensifying NH3 conversion. However, this difference between the four samples do not change the different NOx selectivity since they can all adsorb NH3 strongly. This is excluded as the controlling step in ammonia oxidation.

Therefore, O2-TPD analysis of the four optimal catalysts was conducted, and the results are shown in Fig. 9-E. It can be observed that the main desorption peaks in 5Pt@TiO2 are at 281 °C and 694.5 °C. For 5Pt@ZrO2, there are three desorption peaks at 108 °C, 247 °C, and 694.5 °C, but the peak at 108 °C can be attributed to 28 ACS Paragon Plus Environment

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


physical absorption. For 3Pt@CeO2, only the peak at 61 °C attributed to physical absorption can be observed. It can be inferred that in the total temperature testing zone, oxygen can be strongly adsorbed. For [email protected], the O2 released at 88 °C and 417 °C are ascribed to the physical adsorption and chemisorption resulting from ZrO2 components. Thus, this result suggests the best O2 dissociation capability of the CeO2 carrier.

NO-TPD analysis of four samples was conducted and shown in Fig. 9-F. It can be determined from the desorption peak that NO easily desorbed from 3Pt@CeO2 at 220 °C. For [email protected], the desorption peaks are at 227 °C and 350 °C after excluding physical desorption at 144 °C, but its total NO adsorption amount is the largest. The determined desorption peaks in 5Pt@TiO2 are 258 °C and 625 °C, and the desorption peaks in 5Pt@ZrO2 are 228 °C and 660 °C, respectively. This strong NO adsorption is disadvantageous to the fast desorption of the NOx product, causing low NOx selectivity.

In summary, although the ammonia conversion can be effectively improved since highly dispersed Pt was deposited by the photoreduction process. The improved NOx selectivity of these composites with photocatalyst carriers was ascribed to abundant oxygen vacancies supplied by special carriers such as CeO244-47. NH3 conversion had been completely transformed temperatures as low as 300°C. Herein, the NOx selectivity was key for evaluating the catalyst performance in this process within a



Page 30 of 35

relatively high temperature zone. Thus, the obtained Pt@CeO2 exhibited the best catalytic activity for ammonia oxidation.

4. Conclusions In this research, four supported Pt catalysts for ammonia oxidation were successfully prepared with selected photocatalysts as carriers by the photoreduction deposition process. The catalytic performance of the obtained Pt-loaded composites was evaluated, and the best one was screened out. The results showed that Pt species can be effectively deposited on the surface of photocatalyst carriers with low Pt content, high surface area, and high dispersion. The maximum Pt content of the obtained sample was only 0.1365 wt % in 5Pt@CeO2. The minimum specific surface area was 0.462 m2/g of 5Pt@TiO2, which was still far greater than the value of 46.5 cm2/g for the industrial platinum gauze catalyst. Although the Pt amount of the four composite catalysts sharply decreased, their catalytic performance improved. Ammonia can be completely transformed at about 300 °C with all the photodeposited Pt-loaded composites. However, the highest NOx yield (80.00 %) was achieved with 3Pt@CeO2 as the catalyst under 1 atm at 800°C, with an oxygen to ammonia molar ratio of 15 and space velocity of 235000 h-1. This excellent and stable catalytic performance of 3Pt@CeO2 is ascribed to the loaded high-dispersion Pt species as Pt-O-Ce bonds and the abundant oxygen vacancies provided by CeO2. These results demonstrate a cheap but promising prospect to improve the current industrial Pt catalyst for industrial nitric acid production. 30 ACS Paragon Plus Environment

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


Acknowledgements We appreciated the financial support from the National Natural Science Foundation of China [grant number 21176157 & 21676168].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Qiang Liu and Xueling Pu conducted the experiments and characterizations; Hairong Yue, Wei Jiang, Bin Liang gave supports on experiment guides and facilities.

Reference (1) Hunt, L. B., The Ammonia Oxidation Process for Nitric Acid Manufacture. Platinum Met. Rev. 1958, 2. (2) Benner, L. S., Precious Metals Science and Technology. Platinum Met. Rev.1991. (3) Black Pig Ltd, T. S., Upper King Street, Royston, Hertfordshire. SG AZ.; TechRevMatthey, Recovery of Palladium from Spent Activated Carbon-Supported Palladium Catalysts - Johnson Matthey Technol. Rev.. (4) Ning, Y., The Centennial Development of Platinum Alloy Catalytic Gauze for Ammonia Oxidation in Nitric Acid Plants:Development of Platinum Alloy Catalytic Gauze and Some Technological Improvements. Precious Met. 2008. (5) Schäffer, J.; Kondratenko, V. A.; Steinfeldt, N.; Sebek, M.; Kondratenko, E. V., Highly selective ammonia oxidation to nitric oxide over supported Pt nanoparticles. J. Catal. 2013, 301, 210-216. (6) Hickman, D. A.; Huff, M.; Schmidt, L. D., Alternative catalyst supports for HCN synthesis and NH [sub 3] oxidation. Ind. Eng. Chem. Res. 1993, 32, 809-817. (7) Dai, H.; Ning, Y., Oxidation and Surface Chemical Species of Palladium. Rare Met. Mater. Eng. 2011, 40, 1256-1259. (8) Sadykov, V. A.; Isupova, L. A.; Zolotarskii, I. A.; Bobrova, L. N.; Noskov, A. S.; Parmon, V. N.; Brushtein, E. A.; Telyatnikova, T. V.; Chernyshev, V. I.; Lunin, V. V., ChemInform Abstract: Oxide Catalysts for Ammonia Oxidation in Nitric Acid Production: Properties and Perspectives. Appl. Catal., A 2000, 204, 59-87. 31 ACS Paragon Plus Environment


Page 32 of 35

(9) Jiang, W.; Qiu, J.; Qiu, W.; Yuan, S.; Lu, H.; Liang, B., Wall-loaded Pt/TiO2/Ti catalyst and its application in ammonia oxidation reaction in microchannel reactor. RSC Adv. 2016, 6, 26637-26649. (10) Hung, C. M., Characterization and performance of Pt-Pd-Rh cordierite monolith catalyst for selectivity catalytic oxidation of ammonia. J. Hazard. Mater. 2010, 180, 561. (11) Luo, X.; Rong, W.; Ni, J.; Lin, J.; Lin, B.; Xu, X.; Wei, K., Effect of La2O3 on Ru/CeO2-La2O3 Catalyst for Ammonia Synthesis. Catal. Lett. 2009, 133, 382. (12) Gao, X.; Jiang, Y.; Zhong, Y.; Luo, Z.; Cen, K., The activity and characterization of CeO2-TiO2 catalysts prepared by the sol-gel method for selective catalytic reduction of NO with NH3. J. Hazard. Mater. 2010, 174, 734. (13) Li, Y.; Wang, H.; Arowo, M.; Sun, B.; Chen, J.; Shao, L., Synthesis of nano-Ce 0.5Zr0.5O2 by absorption of ammonia into water-in-oil microemulsion in a rotor-stator reactor. J. Nanopart. Res. 2015, 17, 1-10. (14) Pérez-Ramírez, J.; Kondratenko, E. V., Mechanism of ammonia oxidation over oxides studied by temporal analysis of products. Journal of Catalysis 2007, 250, (2), 240-246. (15) Xiao-Bo, X. U.; Xue-Ling, P. U.; Yuan, S. J.; Wei, J.; Liang, B., Preparation and Application of Core-shell Pt/TiO2 Catalyst for Ammonia Oxidation Process. J. Mol. Catal. 2014, 28, 75-81. (16) Chmielarz, L.; Jabłońska, M.; Strumiński, A.; Piwowarska, Z.; Węgrzyn, A.; Witkowski, S.; Michalik, M., Selective catalytic oxidation of ammonia to nitrogen over Mg-Al, Cu-Mg-Al and Fe-Mg-Al mixed metal oxides doped with noble metals. Applied Catalysis B Environmental 2013, s 130–131, (3), 152-162. (17) Biausque, G.; Schuurman, Y., The reaction mechanism of the high temperature ammonia oxidation to nitric oxide over LaCoO3. Journal of Catalysis 2010, 276, (2), 306-313. (18) Contour, J. P.; Mouvier, G.; Hoogewys, M.; Leclere, C., X-ray photoelectron spectroscopy and electron microscopy of PtRh gauzes used for catalytic oxidation of ammonia. J. Catal. 1977, 48, 217-228. (19) Schneider, W. D.; Laubschat, C., Actinide-noble-metal systems: An x-rayphotoelectron-spectroscopy study of thorium-platinum, uranium-platinum, and uranium-gold intermetallics. Phys. Rev. B 1981, 23, 997-1005. (20) Liang, Y.; Zhang, H.; Zhong, H.; Zhu, X.; Tian, Z.; Xu, D.; Yi, B., Preparation and characterization of carbon-supported PtRuIr catalyst with excellent CO-tolerant performance for proton-exchange membrane fuel cells. J. Catal. 2006, 238, 468-476. (21) Zhang, W.; Gan, J.; Hu, Z.; Yu, W.; Li, Q.; Sun, J.; Xu, N.; Wu, J.; Ying, Z., Infrared and raman spectroscopic studies of optically transparent zirconia (ZrO2) films deposited by plasma-assisted reactive pulsed laser deposition. Appl. Spectrosc. 2011, 65, 522. (22) Zhao, X.; Vanderbilt, D., Phonons and Lattice Dielectric Properties of Zirconia. Phys. Rev. B 2001, 65.


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


(23) Štefanić, G.; Musić, S.; Gajović, A., A comparative study of the influence of milling media on the structural and microstructural changes in monoclinic ZrO2. J. Eur. Ceram. Soc. 2007, 27, 1001-1016. (24) Keogh, R. A.; Srinivasan, R.; Sparks, D. E.; Khorfan, S.; Davis, B. H., The regeneration of Pt promoted SO42−-ZrO2 catalysts. Fuel 1999, 78, 721-727. (25) Chandra, P.; Doke, D.; Umbarkar, S.; Biradar, A., One-pot synthesis of ultrasmall MoO3 nanoparticles supported on SiO2, TiO2, and ZrO2 nanospheres: an efficient epoxidation catalyst. J. Mater. Chem. A 2014, 2, 19060-19066. (26) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S., Surface characterization of aluminasupported ceria. J.phys.chem 1988, 92, 4964-4970. (27) Keramidas, V. G.; White, W. B., Raman spectra of oxides with the fluorite structure. J. Chem. Phys. 1973, 59, 1561-1562. (28) Mochizuki, S.; Tateyama, S., Reststrahlen Spectra of Cerium Dioxide. Phys. Status Solidi 1982, 110, K1-K4. (29) Avgouropoulos, G.; Ioannides, T.; Watralis, H. K.; Batista, J.; Hocevar, S., CuOCeO2 mixed oxide catalysts for the selective oxidation of carbon monoxide in excess hydrogen. Catal. Lett. 2001, 73, 33-40. (30) Yang, F.; Wei, J.; Liu, W.; Guo, J.; Yang, Y., Copper doped ceria nanospheres: surface defects promoted catalytic activity and a versatile approach. J. Mater. Chem. A 2014, 2, 5662-5667. (31) Brogan, M. S.; Dines, T. J.; Cairns, J. A., Raman spectroscopic study of the PtCeO2 interaction in the Pt/Al2O3-CeO2 catalyst. J. Chem. Soc., Faraday Trans. 1994, 90, 1461-1466. (32) Miao, J., In Situ Raman Spectroscopic Study of CeO2 and Ce0.5Zr0.5O2 Solid Solution. J. Xiamen Univ. 1999. (33) Tandon, S. P.; Gupta, J. P., Measurement of Forbidden Energy Gap of Semiconductors by Diffuse Reflectance Technique. Phys. Status Solidi 1970, 38, 363367. (34) †, D. G. B.; Max Shtein; †, R. D. W.; And, S. L. S., ‡; Enrique Iglesia, Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures. J. Phys. Chem. B 1999, 103. (35) Weber, R. S., Effect of Local Structure on the UV-Visible Absorption Edges of Molybdenum Oxide Clusters and Supported Molybdenum Oxides. J. Catal. 1995, 151, 470-474. (36) Centeno, M. Á.; Portales, C.; Carrizosa, I.; Odriozola, J. A., Gold supported CeO 2/Al2O3 catalysts for CO oxidation: influence of the ceria phase. Catal. Lett. 2005, 102, 289-297. (37) Frost, J. C., Junction effect interactions in methanol synthesis catalysts. Nature 1988, 334, 577-580. (38) Bi, Y. D.; Zhang, W.; Xu, H. Y.; Li, W. Z., Interaction at Pt-CeO2 contact interface over Pt/CeO2/SBA-15 catalyst. Chem. Res. Chin. Univ. 2009, 30, 2032-2035. (39) And, M. J.; Levanon, H.; Kamat, P. V., Charge Distribution between UV-Irradiated TiO2 and Gold Nanoparticles: Determination of Shift in the Fermi Level. Nano Letters 2003, 3(3), 353-358. 33 ACS Paragon Plus Environment


Page 34 of 35

(40) Zhu, Y. F.; Pan, K. L.; Dan, Y., Performance of photocatalytic degrading organic pollutants of Pt/TiO2 composites. Polym. Mater.: Sci. Eng. 2008, 24, 102-105. (41) A. H., Ceria on silica and alumina catalysts: dispersion and surface acid-base properties as probed by X-ray diffractometry, UV-Vis diffuse reflectance and in situ IR absorption studies. Colloids Surf., A 1997, 127, 47-56. (42) Bensalem, A.; Muller, J. C.; Bozon-Verduraz, F., Faraday communications. From bulk CeO2 to supported cerium-oxygen clusters: A diffuse reflectance approach. J. Chem. Soc., Faraday Trans. 1992, 88, 153-154. (43) Amblard, M.; Burch, R.; Southward, B., The selective conversion of ammonia to nitrogen on metal oxide catalysts under strongly oxidising conditions. Appl. Catal. B 1999, 22, L159-L166. (44) Fang, C.; Zhang, D.; Shi, L.; Gao, R.; Li, H.; Ye, L.; Zhang, J., Highly dispersed CeO2 on carbon nanotubes for selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 2013, 3, 803-811. (45) Gao, X.; Jiang, Y.; Fu, Y.; Zhong, Y.; Luo, Z.; Cen, K., Preparation and characterization of CeO2 /TiO2 catalysts for selective catalytic reduction of NO with NH3. Catal. Commun. 2010, 11, 465-469. (46) Si, Z.; Weng, D.; Wu, X.; Ma, Z.; Ma, J.; Ran, R., Lattice oxygen mobility and acidity improvements of NiO-CeO2-ZrO2 catalyst by sulfation for NOx reduction by ammonia. Catal. Today 2013, 201, 122-130. (47) Shen, Y.; Zhu, S.; Qiu, T.; Shen, S., A novel catalyst of CeO2/Al2O3 for selective catalytic reduction of NO by NH3. Catal. Commun. 2009, 11, 20-23.


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


Table Of Content


Ammonia Oxidation Process Catalyzed by Pt@XO2 (X = Ti, Zr, Ce, and

Recommend Documents