CoreâShell Al-MCM-41 as a Catalyst for

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Characterization of Cu-Zn/core-shell Al-MCM-41 as a Catalyst for Reduction of NO: Effect of Zn Promoter Thidarat Imyen, Nevzat Yigit, Peerapan Dittanet, Noelia Barrabes, Karin Föttinger, Gunther Rupprechter, and Paisan Kongkachuichay Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03990 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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Industrial & Engineering Chemistry Research

Characterization of Cu-Zn/core-shell Al-MCM-41 as a Catalyst for Reduction of NO: Effect of Zn Promoter Thidarat Imyena, Nevzat Yigitb, Peerapan Dittanetc, Noelia Barrabésb, Karin Föttingerb, Günther Rupprechterb, Paisan Kongkachuichaya a

1

Department of Chemical Engineering, Faculty of Engineering, NANOTEC Center for

Nanoscale Materials Design for Green Nanotechnology and Center for Advanced Studies in Nanotechnology and its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand b

Institute of Materials Chemistry/Physical Chemistry Division, Vienna University of Technology, Getreidemarkt 9, A-1060, Vienna, Austria

c

Department of Chemical Engineering, Faculty of Engineering, Center for Advanced Studies in Industrial Technology, Kasetsart University, Bangkok 10900 Thailand

Keywords: Cu; Zn; Al-MCM-41; NH3-SCR; Nitrogen oxide (NOx) 1

Corresponding author. Tel.: +66 2797 0999 ext. 1207; Fax: +66 2561 4621.

E-mail address: [email protected] (P. Kongkachuichay)

ACS Paragon Plus Environment

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Abstract

A combination of three methods—substitution, ion-exchange, and impregnation method—was used to prepare the Cu-Zn/core-shell Al-MCM-41 catalyst with various copper species. The roles of each preparation method and of Zn promoter in the nature of copper were studied by means of in situ FTIR of CO and NO adsorption, DR UV-vis-NIR, XPS, and H2-TPR. The results suggested that the different preparation methods strongly affected the nature of copper species. The amount of Cu(I) species in reduced catalysts was promoted by the stabilization effect of Zn. In addition, Zn species also provide additional sites for the formation of nitrate and enhance the acidity of the catalysts.

1. Introduction Transition-metal-based zeolite catalysts have received a great deal of attention from many researchers as potential catalysts for NOx removal.1,2 Copper has been utilized as an active metal for these catalysts because of its low cost and unique catalytic properties.3 Since Iwamoto et al.4 discovered the exceptional performance of Cu/ZSM-5 as active and selective catalyst for NO decomposition and selective catalytic reduction (SCR) with hydrocarbons, many coppercontaining zeolite catalysts have been extensively studied for direct decomposition of NO4-6 and N2O,6,7 as well as selective catalytic reduction by hydrocarbons,8–10 ammonia,11–14 or hydrogen.15 Among the NOx removal processes, the selective catalytic reduction by ammonia (NH3-SCR) has been considered as the most effective method for reducing NOx emission under lean condition16−18 due to the high selectivity of ammonia reaction with NOx under the presence of excess O2.11


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The reduction of NOx by NH3-SCR depends on dual functions of the catalysts: the acidic properties of the support and the redox properties of active metals. The acidic support is critical for activating NH3 by providing sites for NH3 adsorption, while the transition metal plays an important role for activating NOx by catalyzing the NO oxidation and the subsequent formation of surface NOx adsorption complexes.11,19 Many previous works have suggested that nitrites and nitrates are likely to be the reaction intermediates for NH3-SCR.13,18,20 Cu/Al-MCM-41 has been found to have an activity close to that of Cu/ZSM-5 but possesses higher thermal stability as its mesoporous structure is still retained after reaction.9 As reported in our previous works,15,21 the core-shell structure of Al-MCM-41 can improve the catalytic activity for NOx reduction because the aluminosilicate-shell creates the specific active sites of copper, leading to a better accessibility of active sites. There is still considerable interest in further improvement of the catalytic performance of copper-based catalysts. For instance, the nature of copper species could be modified by changes to the preparation method and addition of a promoter. The nature of copper species has been found to be strongly affected by the preparation methods.3,12,15,21 Current preparation methods for SCR catalysts include the direct incorporation of copper into the framework of the catalyst support,15,21−23 the ion-exchange method,11,12,14,21,24 and the impregnation method.14,15,21 The Cu(I) species has been suggested as the main active sites for NOx reduction15,21,25 and has also been assigned as the sites for N2 generation from two adsorbed NO molecules.26 Even up to today, much research has been devoted to resolve the nature of the copper sites in this process.12,14 The use of Zn as a promoter for Cu-containing catalysts has been studied for many reactions— partial oxidation of methanol,27,28 methanol synthesis,29,30 low-temperature water-gas shift,29


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synthesis of dimethyl ether from syngas,31 and methanol steam reforming.32,33 Indeed, ZnO is well known that it can improve the dispersion of copper.22,34 The interaction between Cu and Zn plays a critical role in the improvement of the catalytic activity of the catalyst. It has also been found that the concentration of reduced Cu species was related to the addition of Zn.34 The introduction of Zn is helpful to create active sites at the Cu-Zn interface and stabilize the Cu(I) species by a strong interaction.35-39 Despite all works focusing on Zn, there are few works focusing on the use of Zn as the promoter for NO reduction.34 Recently, Zn has been used as an active metal for NO reduction, with Wang and coworkers40 suggesting that Zn/ZSM-5 was active for SCR of NO with H2, as it showed low temperature activity and high N2 selectivity, these two results being attributed to the presence of Zn cations in the framework. The Zn/ZSM-5 was also reported to exhibit outstanding performance for SCR of NO with NH3 over a wide temperature range.41 In order to understand the nature of metal species existing in the catalyst, the fundamental physical and chemical properties should be the focus of investigation. In this work, Cu-Zn/coreshell Al-MCM-41 was investigated as a promising catalyst for NH3-SCR for the first time. The higher amount of Cu(I) species caused by the stabilization effect of Zn can be beneficial for NO reduction, as the Cu(I) species has been considered as the active species for this reaction. Previous work21 showed that different techniques of copper loading on the core-shell Al-MCM41 support—substitution, ion-exchange, and impregnation methods—give the existence of different copper species in the structure of the catalyst, and these copper species are expected to exhibit different values of activity for NO reduction. In this work, a combination of those three methods (substitution, ion-exchange, and impregnation) was used to prepare a Cu-Zn/core-shell Al-MCM-41 catalyst with occurring of


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various copper species. In order to investigate the effect of Zn promoter on each copper species in the catalyst, the Cu-Zn/core-shell Al-MCM-41 catalysts were also prepared separately using each of those three methods. The roles of each method and of Zn promoter in the nature of copper species were studied by means of several characterization techniques. 2. Experimental 2.1. Preparation of catalysts A core-shell-structured Al-MCM-41 was synthesized according to the procedure reported by Chamnankid et al.,15 in which tetraethyl orthosilicate (TEOS: 98%, Sigma-Aldrich) and aluminum nitrate (Al(NO3)3·9H2O: 98%, QREC) were used as silica and alumina sources, respectively. Cetyltrimethylammonium bromide (CTAB: 98%, APS Ajax Finechem) was used as a structural directing agent. The molar gel composition was fixed at 1SiO2:0.2CTAB:100H2O, while the Al2O3/SiO2 ratio was fixed at 0.1. Firstly, TEOS was added into the mixed solution of Al(NO3)3 and CTAB that was being stirred at 40°C. After stirring for 1 h, the pH of the mixture was adjusted to 6.5 by using 1 M sodium hydroxide (NaOH 98%, Carlo Erba) solution, and the mixture was kept stirring for another 5 h. The obtained mixture was sealed in a Teflon-lined autoclave for hydrothermal treatment at 100°C for 24 h. Then, the solid product was obtained through filtering, washing with distilled water, and drying at 80°C overnight. Finally, the samples were calcined in air at 600°C for 5 h. Copper- and/or zinc-containing Al-MCM-41 catalysts were prepared by different methods, including substitution (Sub), ion-exchange (Ex), incipient wetness impregnation (Im), and a combination of these three methods (SubExIm). The ratio of copper to zinc used in this work was 1:1. The procedure of the substitution method was same as the synthesis of core-shell AlMCM-41 except that copper nitrate (Cu(NO3)2·3H2O: 99.5 %, LobaChemie) and/or zinc nitrate


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(Zn(NO3)2·6H2O: 98%, LobaChemie) solution was added before adding TEOS. For the ionexchange process, Al-MCM-41 powder was suspended in 0.025 M of copper and/or zinc nitrate solution at room temperature for 24 h. Then, the solid was filtered, washed with deionized water, dried at 100°C overnight, and calcined at 400°C for 5 h. For the impregnation method, Al-MCM41 was impregnated with copper and/or zinc nitrate solution. After stirring for 1 h, the slurry was evaporated while being stirred at 100°C and then calcined in air at 400°C for 5 h. For the combined method, copper and/or zinc were sequentially loaded onto Al-MCM-41 by the three methods as mentioned earlier, starting with substitution, followed by ion-exchange and incipient wetness impregnation. The nomenclature and the metal content are shown in Table 1. The core-shell structure of AlMCM-41 and Cu-Zn/Al-MCM-41 is observed by TEM images (Fig. S1). It was revealed that the core-shell structure remains intact even after the introduction of Cu and Zn into Al-MCM-41 framework by the combined method. Table 1. The metal contents of Cu-Zn/Al-MCM-41, Cu/Al-MCM-41, and Zn/Al-MCM-41 catalysts (determined by an ICP-OES). Cu content

Zn content

(wt.%)

(wt.%)

Cu-Zn_Sub

0.36

0.41

Cu-Zn_Ex

0.21

0.18

Cu-Zn_Im

0.69

0.66

Cu-Zn_SubExIm

0.96

0.93

Cu_SubExIm

1.06

-

Sample


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Zn_SubExIm

-

0.97

2.2. Catalyst characterization The core-shell structure of the sample was observed by a Transmission Electron Microscope (TEM, JEOL JEM-2100 LaB6) operated with an acceleration of 200 kV. Before the examination, the sample was prepared by suspension in ethanol, followed by the thermal evaporation of ethanol on a copper grid coated with a carbon film. The amounts of loaded metals in the prepared catalysts as shown in Table 1 were determined by an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES, Agilent 700-ES series). Prior the ICP-OES measurement, the sample was fused with NaOH in order to convert the sample powder into the solution form. Diffuse reflectance UV-vis-near-infrared (DR UV-vis-NIR) spectra were recorded in the range of 150–900 nm by a Lambda 750 PerkinElmer UV/vis spectrometer equipped with an integrator and a double monochromator. X-ray Adsorption Near Edge Structure (XANES) spectra were collected in a florescence mode for the Cu K-edge (8979 eV). The spectra of Cu foil, Cu2O, and CuO were also collected as the standard references. Typical operation was with a generation electron storage ring (1.2 GeV; beam current: 80-150 mA) at the Synchrotron Light Research Institute (SLRI), Thailand. X-ray Photoelectron Spectroscopy (XPS) was carried out with an AXIS Ultra DLD spectrometer. The XPS measurement was performed in the electron binding energy ranges corresponding to copper 2p, zinc 2p, oxygen 1s, silicon 2p, aluminum 2p, and carbon 1s core excitations.14 The spectra were deconvoluted by using XPSPEAK 4.1 software after applying a Shirley background. The type of peak was set as p-type, which an area ratio of 2p3/2 to 2p1/2 was


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fixed at 2 to 1. The peak shape was fitted with a mixture of Lorentzian-Gaussian with an asymmetric parameter of zero. 2.3. Temperature-programmed reduction and desorption Temperature Programmed Desorption of Ammonia (NH3-TPD) was used to evaluate the acidity of the prepared catalysts. The catalyst (0.1 g) was packed in a tubular reactor. Prior to measurement, the catalyst was evacuated at 400°C for 1 h and then cooled down to 100°C.21 Next, NH3 was fed into the reactor to perform NH3 adsorption at 100°C for 1 h. Subsequently, NH3 adsorbed on the catalysts was desorbed, and the amount of the desorbed NH3 was monitored by a mass spectrometer (BalzersPrisma 260). Temperature Programmed Reduction (H2-TPR) technique was applied to characterize the reducibility of copper species in the catalysts. The catalyst (0.25 g) was blended with quartz sand (1 g), and then packed in a tubular reactor (Inconel-600, O.D. 3/8 in). The gas mixture of H2 and Ar (9.6% H2 balanced with Ar) was continuously fed at flow rate of 15 ml/min through the reactor. The H2-TPR was performed by heating the sample from room temperature to 900°C at a heating rate of 5°C/min. The H2 consumption was detected by a Shimadzu gas chromatograph (GC-2014) equipped with a thermal conductivity detector (TCD). All H2-TPR profiles were deconvoluted by OriginPro 8.5 software with a Gaussian peak function. 2.4. FTIR of CO, NO, and NH3 adsorption The Fourier Transform Infrared (FTIR) spectra were recorded using a Bruker IFS 28 spectrometer equipped with a MCT detector at a resolution of 4 cm−1. Firstly, the sample was added into a mold and pressed with a pressure of 3 MPa by a hydraulic press in order to obtain a circular tablet having a diameter of 1 cm. Then, the pressed sample was put inside a sample holder. Prior to the gas adsorption, the sample was pretreated by oxidation with 100 mbar O2 at


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400°C for 1 h in order to remove any impurities from the surface of the sample, followed by reduction with 100 mbar H2 and 400 mbar N2 at 350°C for 1 h. For CO adsorption, 5 mbar of CO was fed into the cell, and the FTIR spectra were recorded by using vacuum condition as a background. In the case of NO and NH3 adsorption, 1 mbar of NO and 0.1 mbar of NH3 was injected into the cell instead of CO, respectively. 3. Results and discussion 3.1. Acid properties of the catalysts The surface acidity of the catalyst is crucial for NH3-SCR since the acid sites are needed for NH3 activation.11,13 As shown in Fig. 1, NH3-TPD profiles of all samples show two NH3 desorption peaks, indicating ammonia bound to the acid sites with different acid strength. The NH3 desorption peak at the lower temperature corresponds to ammonia weakly adsorbed on weak acid sites,42−44 whereas the NH3 desorption peak at the higher temperature corresponds to strongly bound ammonia, which could arise from ammonia bound to protons of the Al-MCM-41 support.43,45 Since NH3 is still adsorbed on the acid sites at high temperatures, this profile indicates the presence of the high acid strength of the catalysts in this work, a fact which is expected to show high de-NOx activity for the NH3-SCR.43


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Figure 1. NH3-TPD profiles of Cu-Zn/Al-MCM-41, Cu/Al-MCM-41, and Zn/Al-MCM-41 catalysts. The total amount of adsorbed ammonia species, represented by the area of NH3-TPD peaks, is considerably increased with the introduction of transition metals (i.e. Cu and Zn) into Al-MCM41 material, while the acid strength is affected to a lesser degree. By introducing bivalent Cu and Zn cations into the Al-MCM-41 structure, protons of Brønsted acid sites are replaced by Cu(II) and Zn(II) cations, enhancing numbers of Lewis acid sites. However, our previous works21 stated that the exchanged Cu(II) loosely bonded with Al–O–Si framework could interact with OH groups and create the Brønsted acid sites. The promoted acidity of Zn containing catalyst suggests that exchanged Zn(II) ions can also create Brønsted acid sites in the same way as exchanged Cu(II) ions. In addition, Cu-Zn_SubExIm shows slightly higher total amount of acid than that of Cu_SubExIm as shown by a larger area of NH3-TPD peaks. This increase suggests that the introduction of Zn could improve the total amount of acid of the catalyst and makes this catalyst to be a promising one for NH3-SCR. Such an improvement can be explained by the fact that the Cu-Zn_SubExIm catalyst consists of two cations, which both can promote the acidity of the catalyst in the same manner. Moreover, after the catalyst was pretreated by heating under vacuum at 400°C, some part of exchanged Cu(II) will be automatically reduced to Cu(I),46 while Zn(II) will not be reduced under this condition due to its high stability.47 The Zn(II) species in treated catalyst can still interact with OH groups, benefiting Brønsted acid sites creation as mentioned above. The promoted acidity caused by Zn(II) species in treated catalyst can be confirmed by the higher total acid amount of Zn_SubExIm compared with that of Cu_SubExIm.


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In order to gain more information regarding to the surface acid sites and the NH3 adsorption properties of the catalysts, in situ FTIR of NH3 adsorption was also conducted. Fig. 2 shows the FTIR spectra of all catalysts after NH3 adsorption at room temperature. All samples show the IR bands at the same frequency, indicating to the similarity of ammonia adsorbed species over these catalysts. The negative band at 3743 cm-1 in the O-H stretching mode region relates to O-H vibrations at Brønsted acid sites, which is indicative of NH4+ adsorbed on Brønsted acid sites of the catalyst.12 For the N-H stretching mode region, the IR bands at 3350 and 3281 cm-1 are assigned to NH4+, while the IR bands at 3192 cm-1 is attributed to coordinated ammonia on Lewis acid sites.12,13 In the N-H deformation region, the band at 1622 cm-1 corresponds to asymmetric bending vibration of ammonia coordinated to the Lewis acid sites, whereas the band at 1458 cm-1 relates to asymmetric bending vibration of NH4+ adsorbed on the Brønsted acid sites.12,13,48 However, the spectrum of Cu-Zn_SubExIm catalyst in the N-H deformation region is a little bit different from that of other samples—that is the ratio of the band corresponding to ammonia coordinated to the Lewis acid sites (1622 cm-1) to the band corresponding to NH4+ adsorbed on the Brønsted acid sites is higher. This can be explained by the highest metal content (Cu and Zn) in CuZn_SubExIm catalyst (see Table 1). As discussed earlier, the enhanced number of Lewis acid sites is a result of the replacement of proton of Brønsted acid sites by the Cu(II) and Zn(II) ions


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Figure 2. FTIR of NH3 adsorption on (a) reduced Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, (d) Cu-Zn_SubExIm, (e) Cu_SubExIm, and (f) Zn_SubExIm. 3.2. In situ FTIR spectroscopy 3.2.1. CO adsorption Molecules of CO fed in adsorption were able to strongly adsorb onto Cu(I) ions at room temperature, forming highly stable carbonyl. Hence, CO adsorption has been considered as an effective technique to probe the existence of Cu(I) sites in the catalyst.49−51 As shown in Fig. 3, at low CO pressure (1 mbar), the IR band at 2155−2156 cm−1 is observed over Cu-Zn/Al-MCM-41 and Cu/Al-MCM-41 catalysts, while no IR band is observed over Zn/Al-MCM-41 catalyst. The band at 2155–2156 cm−1 has been assigned to the stretching vibration of monocarbonyls (Cu(I)– CO), resulting from CO adsorbed on Cu(I) ions that are located in cavities and pores of the support.24,46,52,53 This Cu(I)–CO species has been reported as the most favorable adsorption form and as rather stable.51,53


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Figure 3. FTIR of CO adsorption on (a) reduced Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, (d) Cu-Zn_SubExIm, (e) Cu_SubExIm, and (f) Zn_SubExIm for pressures of 1 mbar (black), 3 mbar (red), and 5 mbar (blue). With further increase of CO pressure, the intensity of the band at 2155 cm−1 increases, coinciding with a red shift, and an additional band at 2182 cm−1, which is attributed to the symmetric stretching of dicarbonyls (Cu(I)–(CO)2), develops.51–54 Both the features of the IR band at 2155 cm−1 and 2182 cm−1 confirm the existence of Cu(I) ions resulted from the partial reduction of Cu(II) to Cu(I) under H2 at 350°C. In addition, the appearance of the band at 2182 cm−1 and the red shift of the band at 2155 cm–1 indicate the formation of dicarbonyl species bound to Cu(I) cations.24,53 It has been reported that Cu(I)–CO species are formed upon CO


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adsorption and converted to Cu(I)–(CO)2 species upon increasing CO pressure.53 The results obtained from IR demonstrate that the Cu(I) sites in this work are able to coordinate up to two CO molecules, suggesting the coordinative unsaturation of Cu(I) ions, which is generally observed in many zeolites.50−52 Interestingly, Cu-Zn_SubExIm catalyst possesses a higher number of Cu(I) sites than that in Cu_SubExIm as shown by a higher intensity of the band involving Cu(I)–CO species. This higher number of Cu(I) species in Zn-promoted catalyst can be explained by the promotional effects of Zn regarding the creation and stabilization of Cu(I) ions.35,36,55,56 The introduction of Zn in a form of ZnO is found to be helpful in stabilizing Cu(I) species in reduced Cu-Zn/AlMCM-41 catalyst. Meanwhile, the stabilization effect of ZnO on Cu(I) species has been reported in many previous works.35,36,55 ZnO can promote the creation of active Cu(I) sites at the Cu-Zn interface, and the Cu(I) species is stabilized by the strong interaction between Cu and Zn.35−39,55 Upon the catalyst reduction by H2, Zn species can migrate to the Cu surface, forming Cu(I)–O – Zn species.29,57 It was reported that ZnO could modify the electronic properties of Cu sites by an electron exchange and interaction with copper.57 This interaction between ZnO and Cu(I) is beneficial to stabilize Cu(I) and impede the reduction of Cu(I) to Cu(0).29,57 Therefore, the high stability of Cu(I) species caused by Zn promoter is responsible for the higher amount of Cu(I) species in the catalyst, a fact which benefits de-NOx catalytic reaction as Cu(I) has been considered as the active site for NOx catalytic reduction,15,21,25 leading Cu-Zn/AlMCM-41 being a promising catalyst for the SCR process. The stabilization effect of Zn in CuZn/Al-MCM-41 is further confirmed by H2-TPR results (see section 3.3.3). 3.2.2. NO adsorption


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In contrast to CO molecules and their behavior, NO molecules are preferentially bonded to Cu(II) ions. Consequently, NO adsorption is a particularly useful technique for probing Cu(II) sites and complementing the information gained about the state of copper in the catalyst from CO adsorption.49 In addition, it is important to obtain information regarding different NO adsorbed species as it is the reactant for NH3-SCR reaction.

Figure 4. FTIR of NO adsorption on (a) reduced Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, (d) Cu-Zn_SubExIm, (e) Cu_SubExIm, and (f) Zn_SubExIm. The FTIR spectra of NO adsorption on the reduced catalysts can be divided into three ranges: 2150–2250 cm−1, 1800–1910 cm−1, and 1600–1630 cm−1, as shown in Fig. 4. For Cu-containing catalysts, the IR band at 1810 cm−1 arises from the stretching vibration of mononytrosyls on Cu(I) species (Cu(I)–NO), while the IR band at 1905 cm−1 represents to mononytrosyls associated with isolated Cu(II) ions (Cu(II)–NO).21,24,49,53,58 The presence of these two IR bands confirms the co-existence of Cu(II) and Cu(I) sites on the reduced catalyst. The Cu(II) sites on all Cu-containing catalysts in this work are found to have similar coordination geometry, showing the IR band at the same frequency (1905 cm−1) upon NO adsorption. This similarity suggests that the bond length of Cu(II)−NO vibration does not shift with the presence of Zn. In


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addition to Cu sites, NO molecules were able to adsorb on Zn species, as confirmed by the observed IR band at about 1900 cm−1, which is assigned to Zn(II)–NO species.59 The IR band at 2250 cm−1 is observed for all catalysts and has been assigned to N2O resulting from NO disproportionation after contacting with the sample.24,51,58,60 Meanwhile, the intense band at 2157 cm−1, observed only for Cu-containing catalysts has been previously reported and suggested to be attributable to Cu(I)–NO+ species.20,24,61,62 This band is absent over Zn/AlMCM-41 catalyst, confirming the assignment of this feature to Cu(I)–NO+ species. Previous works have suggested that Cu(I)–NO+ complexes can be formed either by the redox reaction between Cu(II) and NO or the disproportionation of NO2.24,61 For the redox reaction between Cu(II) and NO, the amphoteric NO molecule can reduce Cu(II) ion by donating its electron, and form Cu(I)–NO+ adduct.24,46,61 The formation of NO+ originating from the disproportionation of NO2 can take place even though only NO is used for adsorption study, since it was shown that NO is able to decompose to N2O over the catalysts as evidenced by the IR feature at 2250 cm−1. The NO decomposition can produce N2O and also O atom by the following reaction:24 2NO → N2O + O

(1)

This formed O atom is picked up by another NO molecule to produce NO2: NO + O → NO2

(2)

The disproportionation of NO2 simultaneously produces both NO+ and NO3−: NO2→ NO+ + NO3−

(3)

It was found that nitrosyl species are easily bound with copper, and surface NO+ has been suggested to be the key intermediate for selective catalytic reduction of NO to N2.24,61,63


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Moreover, this NO+ species is a stable NOx species that was not able to be easily desorbed by evacuation at room temperature (not shown).46,64 Additionally, the bands in the range of 1600–1630 cm−1 observed for all catalysts have been assigned to different surface nitrite and nitrate species adsorbed on the transition metal sites.13,19,52,53,58,60,65 However, the exact assignment of these bands is difficult due to the similar vibration patterns of different nitrite and nitrate species. As for NO+ formation, nitrate species are formed from NO2 disproportionation, even though only NO is introduced into the system. Interestingly, although NO+ species is absent in Zn/Al-MCM-41 catalyst, surface nitrate species are able to form on Zn sites in Zn/Al-MCM-41 catalyst. This result indicates that Zn promoter does not only affect the nature of Cu in the catalyst, but also provides additional sites for NO to adsorb in the form of nitrate species, which play critical role in selective catalytic reduction of NO. It is well known that nitrate species are the intermediates for NH3-SCR of NO and that they are active with ammonia.65 The reaction between surface nitrate species results in the formation of NH4NO3 species, which can be further reduced by NO to the final products of N2 and H2O.13,62,63 3.3. The nature of copper and zinc species in the catalysts In order to acquire more understanding of the nature of copper species and elucidate the effect of Zn promoter on each copper species obtained from different preparation methods, another set of Cu/Al-MCM-41 catalysts was prepared by the same method as that of Cu-Zn/Al-MCM-41 catalysts; the properties of Cu-Zn/Al-MCM-41 are compared with those of Cu/Al-MCM-41. 3.3.1. DR UV-vis-NIR spectroscopy Both Cu-Zn/Al-MCM-41 and Cu/Al-MCM-41 catalysts prepared by same method show similar spectra (see Fig. 5 and 6), suggesting that the introduction of Zn does not alter the


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environment of Cu(II) ions. All samples exhibit the adsorption band at 275 nm, which has been assigned to charge transfer (CT) of O2−→Cu2+ transitions between isolated Cu(II) ions and lattice oxygen.23,66,67 The second band at 330 nm corresponds to small crystalline CuO species, well dispersed on the surface of the catalyst.67 This band is most distinctly observed in the spectra of samples prepared by impregnation method compared with others, due to the nature of this preparation method. By using impregnation method, copper species predominately exist as CuO species.21 Some part of copper precursor (i.e., Cu(NO3)2) was confined inside the channels of the catalyst by the capillary effect, leading to the formation of small crystalline CuO species. The broad band at 750 nm has been assigned to d-d transition of Cu(II) in octahedral coordination with 4–6 oxygen in extra-framework or bulk CuO.67 This feature appears only in the samples prepared by impregnation (Im) and the combined method (SubExIm), confirming the presence of CuO species as the dominant phase resulting from the impregnation method.

Figure 5. DR UV-vis-NIR spectra of (a) Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, and (d) CuZn_SubExIm.


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Figure 6. DR UV-vis-NIR spectra of (a) Cu_Sub, (b) Cu_Ex, (c) Cu_Im, and (d) Cu_SubExIm. Since Zn species could also show the absorption bands in the wavelength close to those of Cu at around 275 nm,68 there might be some overlapping between the absorption bands of these two metals and thus difficulty in differentiating the nature of both copper and zinc species in CuZn/Al-MCM-41 catalysts by using the spectra of Cu-Zn/Al-MCM-41 alone. Therefore, DR UVvis NIR spectroscopy was applied to Zn/Al-MCM-41 catalysts in order to study the nature of Zn species in the catalysts prepared by different methods. It has been reported that the appearance of the absorption band below 230 nm is generally considered as the proof of metal atoms being in the framework of zeolite by isomorphous substitution.69 Since the band at 228 nm (Fig. 7) is observed only in Zn_Sub and Zn_SubExIm, it is reasonable to assign this band to the framework zinc species.69 The presence of this feature confirms that Zn was able to replace Al atom in the framework position by using substitution method. The band at 284 and 391 nm appear in the spectra of Zn_Ex, Zn_Im, and Zn_SubExIm catalysts. The former of these two bands is attributed to ZnO cluster with a diameter around 1 nm, while the latter represents the charge transfer (CT) O2−→Zn2+ transitions in bulk ZnO.68,69 This latter result indicates that both ionexchange and impregnation methods could lead to the formation of ZnO species.


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Figure 7. DR UV-vis-NIR spectra of (a) Zn_Sub, (b) Zn_Ex, (c) Zn_Im, and (d) Zn_SubExIm. 3.3.2. XPS XPS measurement was further carried out in the electron binding energy regions corresponding to Cu 2p and Zn 2p core excitations, in order to clarify both the nature of Cu species and the chemical state of Zn in the catalyst. All Cu 2p regions (Fig. S2 and S3) exhibit two main peaks centered at around 933 and 953 eV, assigned to 2p3/2 and 2p1/2 transition of Cu(II), respectively, along with the presence of a weak satellite peak at around 943 eV, which is the typical characteristic of the Cu(II) oxidation state.14,23,70 The weak intensity of the satellite peak of all samples might be due to low content of highly dispersed copper.23 The presence of only Cu(II) species in these fresh Cu-containing catalysts is evidenced by Cu K-edge XANES spectra of different fresh Cu-Zn/Al-MCM-41 catalysts (see Fig. S4). These spectra illustrate three features at 8977, 8987, and 8998 eV, all of which are the characteristic of Cu(II).71,72 The pre-edge feature with low intensity at 8977 eV (see the inset of Fig. S4) has been assigned to a dipole forbidden 1s–to–3d transition found only in Cu(II) complexes, while the shoulder at 8987 eV and the intense feature at 8998 eV arise from 1s–to–4p transition in


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Cu(II).71-73 Apart from the feature at 8977 eV, the absence of other pre-edge features below 8985 eV confirms the presence of copper only in a completely oxidation state (Cu(II)).73 As shown in Fig. 8 and 9, the Cu 2p3/2 transition was deconvoluted into three peaks (A−C) by XPSPEAK 4.1 software, which are attributed to different Cu(II) species. The binding energy value and the percentage contribution of each individual peak are summarized in Table 2.

Figure 8. XPS spectra for Cu 2p3/2 transition of (a) Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, and (d) Cu-Zn_SubExIm.


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Figure 9. XPS spectra for Cu 2p3/2 transition of (a) Cu_Sub, (b) Cu_Ex, (c) Cu_Im, and (d) Cu_SubExIm. Table 2. Binding energy (BE) and percentage of the different copper species identified in the Cu 2p3/2 transition. Component A Sample

BE (eV)

%

Component B BE (eV)

%

Component C BE (eV)

%

Cu-Zn_Sub

932.4

21.66

933.3

46.74

934.8

31.6

Cu-Zn_Ex

932.9

39.77

934.0

41.70

935.01

18.53

Cu-Zn_Im

933.3

63.09

934.1

32.22

935.1

4.69

Cu-Zn_SubExIm

932.7

29.7

933.3

39.15

933.9

31.15

Cu_Sub

932.1

19.68

933.3

59.50

934.3

20.82

Cu_Ex

932.7

38.75

933.7

42.36

935.5

18.89

Cu_Im

933.0

50.86

933.8

36.7

935.1

12.44

Cu_SubExIm

932.8

21.84

933.5

56.37

934.9

21.79

The first component (A) at the lowest binding energy is ascribed to CuO species,14,23 while the second (B) and third (C) components are likely due to isolated Cu(II) ions in tetrahedral and octahedral coordination, respectively.14,23,74 The CuO appears at the lowest binding energy due to its weakest interaction with Al-MCM-41 support compared with other copper species. Subsequently, it was easily reduced, as evidenced by the H2-TPR results (see section 3.3.3). In the hydrated sample, the isolated Cu(II) ions can link with up to six water molecules as a hexaaqua copper(II) complex ([Cu(H2O)6]2+).74−76 However, some of these coordinated water molecules could be lost upon dehydration, leaving Cu(II) coordinated with oxygen as an


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octahedral or tetrahedral coordination. The co-existence of Cu(II) ions in tetrahedral and octahedral coordination suggests that there are two kinds of exchange sites available in the framework of the prepared catalyst.14 As shown in Table 2, the contribution of each individual peak in the Cu 2p3/2 transition depends both on the Zn promoter and preparation method. It is clearly observed that the contribution of CuO increases with the introduction of Zn regardless of the preparation method, which is consistent with the H2-TPR results. It has been suggested that copper preferentially occupies the exchange sites first, and when those sites are saturated, CuO accumulates on the surface of the catalyst.14 Due to their similar ionic radii, both copper and zinc cations compete for the available sites. Therefore, some part of copper was not able to occupy the sites and agglomerated as CuO species on the surface of the catalyst. The nature of copper species is strongly affected by the methods used for metal loading, as seen by the different contribution of each copper species obtained from different catalysts (see Table 2). For substitution and ion-exchange methods, most Cu(II) species are in the form of isolated Cu(II) ions. For the impregnation method, the contribution is somewhat different; most Cu(II) species are in the form of CuO, which is in line with DR UV-vis-NIR results. The contribution also suggests that the isolated Cu(II) ions preferentially exist in tetrahedral rather than octahedral coordination. However, when using combined preparation method, the contribution of the isolated Cu(II) in octahedral coordination becomes greater, and the contribution of copper species becomes closer to each other, indicating that the combination of different copper species is a result of each method. The XPS spectra corresponding to Zn 2p3/2 transition of Cu-Zn/Al-MCM-41 catalysts are shown in Fig. 10, while Table 3 shows the binding energy with individual contributions. The Zn


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2p spectra (not shown here) exhibit the spin-orbit splitting in the vicinity of 23 eV for Zn 2p3/2 and Zn 2p1/2, confirming that Zn was in a completely oxidized state (i.e., ZnO) in all samples.77 By fitting XPS spectra by XPSPEAK 4.1 software, it can be seen that all Cu-Zn/Al-MCM-41 samples show two peaks located around 1023 and 1024 eV, indicating the presence of different types of Zn species. As pure ZnO exhibits a peak at 1021.8 eV,68 the component at the lower binding energy (A) is reasonably assigned to ZnO species, while the component at the higher binding energy (B) is attributed to Zn(II) ions at exchange sites.68,78 The fitting results vary depending on the preparation method, demonstrating that Zn species are directly influenced by the method of zinc loading as in the case of copper. For substitution method (Cu-Zn_Sub), fewer ZnO species were formed compared to those obtained with other methods because Zn could be incorporated to Al-MCM-41 in many forms, including framework Zn species, isolated Zn(II) ions, and ZnO species. On the other hand, when the ion-exchange technique is used, the contribution of ZnO species becomes slightly greater than that of isolated Zn(II) ions. However, for impregnation (Cu-Zn_Im) and for the combined method (Cu-Zn_SubExIm), the majority of Zn species is presented as ZnO, as evidenced by DR UV-vis-NIR spectra.

Figure 10. XPS spectra for Zn 2p3/2 transition of (a) Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, and (d) Cu-Zn_SubExIm.


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Table 3. Binding energy (BE) and percentage of the different zinc species identified in the Zn 2p3/2 transition. Component A Sample

BE (eV)

Component B %

BE (eV)

%

Cu-Zn_Sub

1022.07

29.72

1023.19

70.28

Cu-Zn_Ex

1022.34

59.05

1023.79

40.95

Cu-Zn_Im

1022.89

81.60

1023.91

18.40

Cu_Zn_SubExIm

1022.79

86.69

1024.00

13.31

3.3.3. H2-TPR The reducibility of the catalysts was studied by H2-TPR technique. Each H2-TPR profile (Fig. 11 and 12) was deconvoluted into sub-peaks to dissect the TPR profile from the catalyst. Although the reduction of zinc species can take place at high temperatures, no distinct reduction peak of zinc species is observed in this study, a fact which is in agreement with previous works.33,47 This stable zinc species can be explained by the low reducibility of zinc species compared with copper species, and the low content of Zn in the catalysts, which might make the identification of zinc reducibility to be even more difficult. Therefore, these deconvoluted subpeaks are ascribed to the reduction of different types of copper species.


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Figure 11. H2 TPR profiles of (a) Cu-Zn_Sub, (b) Cu-Zn_Ex, (c) Cu-Zn_Im, and (d) CuZn_SubExIm.

Figure 12. H2 TPR profiles of (a) Cu_Sub, (b) Cu_Ex, (c) Cu_Im, and (d) Cu_SubExIm. The first peak at around 220–260°C, which exists for all catalysts, likely corresponds to a onestep reduction of CuO to Cu.21,62 It is clearly seen that the area under the peak corresponding to surface CuO species in each copper-zinc catalyst increases with the introduction of Zn, a fact which is consistent with the XPS results. It should be noted that the copper content in both CuZn/Al-MCM-41 and Cu/Al-MCM-41 catalysts was equal. The incorporation of zinc together


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with copper on Al-MCM-41 support as the promoter results in competition between copper and zinc for available sites since divalent copper and zinc cations have similar ionic radii. It can be observed that the substitution catalyst shows TPR profile that is quite similar to that of the ion-exchange catalyst, but the intensity of H2 consumption is lower. By using the substitution technique, copper is able to be incorporated into the tetrahedral-coordinated position as the stable framework copper, which is a situation in which copper is difficultly reduced.15,21 However, since it is generally difficult to introduce a bivalent cation in the tetravalent silicon framework,22 some parts of copper could not be incorporated into the tetrahedral position and existed as isolated Cu2+ ions at the exchange sites. For substitution method, the reduction peaks at 372 and 602°C for Cu_Sub (Fig. 12(a)) and 378 and 514°C for Cu-Zn_Sub (Fig. 11(a)) correspond to the two-step reduction of isolated Cu(II) ions to Cu(I) and Cu(I) to Cu(0), respectively. For ion-exchange method, the two peaks at 373 and 626°C for Cu_Ex (Fig. 12(b)), and 398 and 572°C for Cu-Zn_Ex (Fig. 11(b)) are also assigned to a two-step reduction of isolated Cu(II) ions. The higher reduction temperature of Cu(II) ions in substitution and ion-exchange Cu-Zn/AlMCM-41 catalysts indicates that the incorporation of Zn in these catalysts impedes the reduction of copper. A similar effect has also been found in Cu-Zn/MCM-41, Cu-Zn/MCM-48, and CuZn/Y with low concentration.22,79,80 This phenomenon has been explained by the promotion of Cu dispersion by Zn. It was reported that the presence of zinc in the copper-based catalyst promotes the dispersion of copper and the dispersed copper then strongly interacts with the support, resulting in lower reducibility of copper.22 In addition, the interaction between Cu and Zn could be conducive to lower reducibility of copper since zinc is difficultly reducible and might restrain copper from being reduced. Moreover, the increased electron cloud density of Cu


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ions could also contribute to the hindered copper reducibility as reported by Tseng and coworkers:34 for Cu-Zn catalysts with Zn/Cu ratio higher than 0.75, the electrons of Cu may be hidden by the electron transfer from the conduction band of Zn to Cu due to the lower Fermi level of Cu, resulting in the loss of reducibility of Cu(II). For the impregnation method, the reduction peak at 258°C for Cu_Im is undoubtedly attributable to the reduction of CuO species located at the external surface of the catalyst. However, the reduction of CuO species in Cu-Zn/Al-MCM-41 was split into two peaks at 184 and 270°C, which correspond to the reduction of CuO species at different levels in the catalyst structure. The peak at 184°C is due to the reduction of CuO species formed from the copper precursors that could not penetrate deeper into the catalyst because of the hindering effect of ZnO particles. Meanwhile, the peak at 346°C for Cu_Im and at 324°C for Cu-Zn_Im are quite difficult to definitively assign to a specific type of copper species due to the overlapping range of the reduction of CuO species located deeper within the pores and the isolated Cu(II) ions at the exchange sites. Since the copper loading in this work is low and since, in the case of low copper loading for impregnated catalyst, the copper could be presented as highly dispersed CuO and/or as isolated copper ions,81 the peaks at 346°C for Cu_Im and 324°C for Cu-Zn_Im are ascribed to the reduction of smaller CuO clusters within the pores and/or isolated Cu(II) ions with similar reducibility. Finally, the peaks at 603°C for Cu_Im and 569°C for Cu-Zn_Im are attributed to the reduction of Cu(I) to Cu(0). However, the presence of Zn in the impregnation catalysts gives a different effect on the reducibility of Cu from that of the substitution and the ion-exchange catalysts. The reduction temperatures of Cu-Zn_Im are lower than those of Cu_Im due to the hindering effect of ZnO, which blocks the Cu species in Cu-Zn/Al-MCM-41 from penetrating


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deeply into the pores, as in the case of Cu/Al-MCM-41, making them more accessible to H2 molecules and thus easily reduced. For the catalysts prepared by the combined method (Cu-Zn_SubExIm and Cu_SubExIm), the reduction temperatures decrease compared with those of catalysts prepared by any single method. The lowered reduction temperature was likely caused by the higher copper loading than that in the catalysts prepared by single method. A higher concentration of copper (i.e., higher Cu/Al ratio) can weaken the interaction between copper species and the catalyst support, resulting in easier reduction of copper species.82,83 Since both Cu-Zn_SubExIm and Cu_SubExIm catalysts were prepared by the combination of three methods, their TPR profiles combine the reduction behavior resulting from each preparation method. The higher reduction temperatures of Cu-Zn_SubExIm compared with those of Cu_SubExIm are caused by the effect of Zn as previously discussed. The information obtained from H2 TPR study can be used to estimate the ratio of Cu(I)/Cu(II) in the catalysts pretreated by the reduction with H2. The Cu(I)/Cu(II) ratios shown in Table S1 were determined based on the peak corresponding to the reduction of Cu(II) to Cu(I) in H2-TPR profile. The catalysts in this work are pre-reduced by H2 at 350˚C, therefore the reduction temperature of 350˚C in the H2-TPR profiles was used as the cut-off point. The area under the curve below 350˚C represents the number of Cu(I) formed from the reduction of Cu(II), while the area under the curve above 350 ˚C represents the number of unreduced Cu(II). These Cu(I)/Cu(II) ratios indicate the Cu(II) reducibility to Cu(I). Higher reducibility of Cu(II) is represented by higher Cu(I)/Cu(II) ratio. As discussed previously, the introduction of Zn into Cu-Zn/Al-MCM-41 prepared by substitution and ion-exchange methods increases the reduction temperature of Cu(II) species, resulting in lower Cu(II) reducibility than that of Cu/Al-MCM-41


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catalysts (see Table S1). On the other hand, the effect of Zn on the reduction temperature is different for impregnation method as the reduction temperature of Cu(II) decreases with the Zn introduction by hindering effect of ZnO as reflected by higher Cu(I)/Cu(II) ratio of Cu-Zn_Im than that of Cu_Im (see Table S1). However, it is worth to note that these Cu(I)/Cu(II) ratios are only the estimation, because H2-TPR is a transient process, whereas the catalyst pretreatment is an isothermal process as the reduction temperature is hold at 350˚C for 1 hour. Notably, the amount of reduced copper species was also affected by Zn. For all Cu-Zn/AlMCM-41 catalysts, the area of the peak corresponding to the reduction of Cu(I) to Cu(0) is lower than that of Cu/Al-MCM-41 catalysts, indicating that some part of Cu(I) in Cu-Zn/Al-MCM-41 is impeded by Zn from being completely reduced to Cu(0). This result suggests that the Cu(I) species in Cu-Zn/Al-MCM-41 catalysts was highly stabilized and not easily reduced to Cu(0), resulting in a higher amount of Cu(I) in the system, as confirmed by FTIR of CO adsorption. This result was caused by the stabilization effect of Zn on Cu(I) species as discussed earlier. 4. Conclusions Combining the characterization results shown above, the Cu-Zn/Al-MCM-41 catalysts, in particular the Cu-Zn/Al-MCM-41 catalyst prepared by combined methods (substitution, ionexchange, and incipient wetness impregnation), are very promising for NH3-SCR of NO. By using the combined method, various copper species are able to coexist in the catalyst as a result of the combination of different copper species obtained from each step of metal loading. The introduction of Zn to Cu/Al-MCM-41 catalysts directly affects the properties of Cu-Zn/AlMCM-41 catalysts, including the acidic properties and the nature of the copper, enhances the total amount of acid, which is crucial property for NH3-SCR. Apart from Lewis acid sites


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generation, Brønsted acid sites are able to be created by the high stability of Zn(II) ions, which can interact with OH groups. It is revealed that the results from both FTIR of CO adsorption and H2-TPR indicate that the introduction of Zn promotes the amount of Cu(I) species in reduced Cu-Zn/Al-MCM-41 by stabilizing Cu(I) species, hindering these species from being completely reduced to Cu(0). Moreover, Zn also provides additional sites for NO adsorption in the form of nitrate species, which has been considered as the key intermediate for NH3-SCR of NO. The higher amount of Cu(I) caused by the stabilization effect of Zn, as well as the promoted acidity due to zinc addition, are expected to enhance the NO conversion over Cu-Zn/Al-MCM41 catalysts. However, further investigation will be conducted in order to test the catalytic performance and relate the properties of these catalysts to NH3-SCR performance. Acknowledgements This work was supported by the Thailand Research Fund (TRF) through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0139/2556 for Thidarat Imyen), the Synchrotron Light Research Institute (Public Organization), and the Kasetsart University Research and Development Institute (KURDI). References (1)

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Abstract Graphic


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CoreâShell Al-MCM-41 as a Catalyst for

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