Al ratio and

Subscriber access provided by IDAHO STATE UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Autoreduction of Copper in Zeolites: Role of Topology, Si/Al ratio and Copper Loading Vitaly L. Sushkevich, Andrey Valentinovich Smirnov, and Jeroen A. van Bokhoven J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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 30 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

The Journal of Physical Chemistry

Autoreduction of Copper in Zeolites: Role of Topology, Si/Al ratio and Copper Loading Vitaly L. Sushkevicha,*, Andrei V. Smirnovb, Jeroen A. van Bokhovena,c,* a Laboratory

for Catalysis and Sustainable Chemistry, Paul Scherrer Institut, 5232 Villigen PSI,

b Lomonosov

Moscow State University, Department of Chemistry, 119991, Moscow, Russia

Switzerland

c

Institute for Chemistry and Bioengineering, ETH Zurich, Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland

Abstract The autoreduction of CuII-oxo species in the copper-exchanged zeolites MOR, MFI, BEA and FAU was studied using temperature-programmed desorption of oxygen and in situ Xray absorption spectroscopy. The zeolite topology and the Si/Al ratio have a significant effect on the onset temperature and rate of copper reduction, which originates from a different structure of the copper-oxo species, as dictated by the nature of the zeolite and the Si/Al ratio. In contrast, the autoreduction of copper hardly reacted to the variation in the copper loading when the Si/Al ratio was constant, as shown for mordenite. Thus, there is a disparate effect of copper concentration on copper speciation and, consequently, on autoreduction, due either to the variation in the Si/Al ratio or to incomplete exchange. Quantitative analysis revealed that the release of one oxygen molecule is accompanied by the reduction of four CuII ions to CuI.

__________________________________________ *Corresponding authors: Tel.: +41563103518; E-mail address: [email protected], [email protected].

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 30

1. Introduction Transition metals hosted in zeolites represent an important class of catalysts for various chemical transformations. The best examples of such materials are iron- and copper-exchanged zeolites, which are important in the decomposition of N2O1,2, selective reduction of NOx3-6, oxidation of benzene into phenol7-9, and, more recently, the stepwise selective oxidation of methane to methanol10-14. Their high activity and selectivity originates from the unique structure of the zeolite matrix, which enables the stabilization of polyvalent species inside the pores of the zeolite. By varying the zeolite topology, the Si/Al ratio, and the transition metal loading, it is possible to obtain different iron and copper species, ranging from monomeric and dimeric oxo species to oligomers and metal oxide clusters of different sizes10-12,

14-18.

The

structure of these species can also be affected by the method of synthesis and thermal activation, hence leading to changes in the redox properties19, 20. One of the most common but poorly understood processes is the so-called “autoreduction” or “self-reduction”, which takes place during the activation of copper-exchanged zeolites. It implies the spontaneous transformation of some of the CuII species to CuI at elevated temperature, typically above 673 K, in an inert environment in the absence of a reducing agent21-29. According to Iwamoto25, 26 and Hall22-24, autoreduction of CuII species to CuI leads to a decrease in the weight of copper-exchanged zeolite, which is associated with the release of molecular oxygen, as is also detected by mass spectrometry31. Scheme 1 is generally agreed to describe this process.

Scheme 1. Generally accepted pathway for copper autoreduction in zeolites

2


Page 3 of 30 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


First, elimination of water from two neighboring hydroxyl-bridged species yields [Cu-OCu]2+ species. Many researchers have reported that these oxygen-bridged species in the copper ion-exchanged MFI, MOR and FAU type zeolites are considered to be ‘‘extra-lattice oxygen’’10-12, 15. At the high temperature, copper-oxo sites decompose to CuI and molecular oxygen, which desorbs from the material. In this respect, copper autoreduction should not be confused with the reduction of copper by reacting with the residual carbonaceous deposits, which remain in the catalysts after synthesis31-33. The latter process leads to almost complete conversion of CuII to CuI sites at low temperature and yields carbon dioxide rather than oxygen31. Additionally, the formation of Cu-O-· anion-radicals, which might undergo autoreduction with stoichiometry different to Scheme 1, was suggested34,

35,

however, their presence in copper-

exchanged zeolites still did not find experimental confirmation.

Copper-exchanged MFI zeolite is the most-studied system. Li and Hall showed that, by replacing pure oxygen with oxygen-lean mixtures, CuMFI can lose up to 0.046 oxygen atoms per copper at 773 K22. Further experiments revealed that the switch from pure oxygen to pure helium leads to desorption of 0.1 O/Cu, while the total redox capacity was determined to be 0.5 O/Cu22. Thus, only 20% of CuII underwent autoreduction to CuI. 23 Simultaneously, Iwamoto et al. and Robota et al. used Cu K-edge X-ray absorption spectroscopy (XAS) to determine the copper oxidation state in CuMFI, depending on the evacuation temperature, and showed that CuI species appear first, starting at 473 K and develop progressively until 873 K26, 28. Later on, Giamello et al. showed by XAS that the CuI fraction after heating at 873 K accounts for 70% of the total amount of copper introduced into CuMFI30. The presence of water during the heating of CuMFI without oxygen promotes autoreduction, leading to a higher fraction of CuI; however, the mechanism of this promotional effect is unclear33. As well as in zeolites, copper supported on oxides can undergo autoreduction at elevated temperature. Tanaka et al. found that, over Cu/Al2O3, isolated CuII species reduce easily to CuI, 3



Page 4 of 30

while copper-oxo aggregates are stable with regard to autoreduction up to 973 K36. Analogously, CuII-oxo species supported over SBA-15 showed 79% conversion to CuI at 873 K37. In more recent work devoted to copper-exchanged MOR, MFI, BEA, CHA, and FAU, the autoreduction of copper occurred during the activation of the samples in an inert gas21, 39-42. Although the extent and the rate of autoreduction are influenced by the zeolite structure, the Si/Al ratio, and the copper loading, the underlying causes remain unclear. These parameters, however, are crucial for the activity of copper materials in the reduction of NOx and the selective oxidation of methane. Therefore, a systematic study of autoreduction in copperexchanged zeolites is highly desirable. In this respect, we report the autoreduction of copper over a series of copper-exchanged zeolites with a MOR, MFI, BEA, or FAU topology and different Si/Al ratios and copper loadings. Correlating in situ Cu K-edge X-ray absorption spectroscopy with temperatureprogrammed desorption of oxygen, we identified the quantitative ratio of four copper (II) atoms required for the release of one oxygen molecule. While copper loading has no effect on the CuII to CuI transformation temperature and the amount of released oxygen per copper basis, the zeolite topology and Si/Al ratio are key factors that affect autoreduction. These results illustrate the copper redox chemistry of zeolites, which is of practical importance for the design and optimization of copper-based materials in environmental applications. 2. Experimental 2.1 Materials All samples were prepared by conventional ion exchange with an aqueous solution of copper nitrate. The commercial zeolites, MOR (CBV10A, CBV21A, CBV90A, Zeolyst), MFI

4


Page 5 of 30 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


(CBV2314, Zeolyst), BEA (CP814E, Zeolyst), and FAU (CBV720, Zeolyst) were the starting materials. Table 1 gives the Si/Al ratios of the parent zeolites. In a first step, all zeolites, with the exception of CBV10A, were converted to the ammonium form. For ion exchange, 30 g of commercial zeolite were added to 2500 ml of 0.2 M NH4NO3 (99.8%, Sigma Aldrich) solution and stirred at 323 K overnight. The resulting sample was filtered, washed with deionized water, and dried at 393 K for 1 h. The ion exchange was repeated twice. The final material was dried at 393 K for 12 h. For the ion exchange with copper, 5 g of each zeolite in the ammonium form were stirred in 500 ml of a 0.05 M aqueous solution of copper nitrate (99%, Sigma Aldrich) at 323 K overnight. The ion exchange was repeated twice. The resulting sample was filtered, washed with deionized water, dried at 393 K for 1 h, and calcined at 773 K for 4 h in a flow of dry synthetic air. In the event of incomplete ion exchange of zeolite mordenite (CBV10A, Si/Al = 6.5), the aqueous solutions of copper nitrate with concentrations ranging from 0.005 to 0.05 M were used. The samples were designated as Cu(x)ZEO(y), where “ZEO” represents the zeolite framework type of the material, and “x” and ”y” correspond to the copper loading in wt% and the Si/Al ratio, respectively. 2.2 Characterization of the materials The copper loading as well as the Si:Al:Na ratio of the parent zeolite and the final material were determined by inductively coupled plasma mass spectrometry (ICP–MS) with an Agilent 77009 ICPMS instrument after complete digestion of the samples in the HF (2 M) in a 3000 Anton Paar microwave digestion unit. Nitrogen sorption-desorption isotherms were measured at 77 K using a Micromeritics 3D Flex automatic surface area and pore size analyzer. Prior to the measurements, the samples

5



Page 6 of 30

were evacuated at 623 K. Powder X-ray diffraction patterns were recorded on a Bruker D8 diffractometer; applying Cu Kα radiation at a wavelength of 1.5456 Å. Infrared spectroscopy (FTIR) measurements were carried out on a Thermo iS50 spectrometer equipped with a DTGS detector. A quartz low-temperature cell, which enables high-temperature pretreatment, was used for adsorption of nitrogen monoxide. In a typical experiment, 20 mg of sample were pressed into a self-supported wafer (2 cm2). A sample was placed in a quartz FTIR cell and activated as follows: ramping at 8 K/min to 673 K and a dwell time of 3 h. Then, 300 torr of oxygen were introduced into the cell and the activation continued for 1 h at 673 K. Next, the oxygen and traces of water were evacuated at 623 K for 1 h. The sample was cooled to liquid nitrogen temperature and the reference spectrum was recorded (128 scans, 4 cm-1 resolution). Nitrogen monoxide (Messer, 4.8) aliquots were gradually introduced into the cell and IR spectra were subsequently recorded until full saturation of the sample. Spectra of surface species were obtained by subtracting the reference spectra from the spectra of the samples with the adsorbate. The processing of the spectra was carried out by means of the OMNIC 9.1 package. Temperature-programmed desorption of oxygen (TPD-O2) was measured using an automatic USGA-101 instrument (UNISIT) equipped with a TCD detector. The samples (~300 mg, sieved to 0.25-0.5 mm fractions) were placed in a quartz reactor and treated as follows: ramping at 8 K/min to 773 K in a flow of oxygen (30 ml/min), dwell for 1 h at 773K, cooling to 373 K. The gas flow was then switched to helium (30 ml/min, 6.0 grade) and, after reaching equilibration of the detector baseline, heating to 1223 K was started at 8 K/min. For TCD calibration, silver (I) oxide that releases a stoichiometric amount of oxygen upon heating to 673 K was used as the standard. TPD-O2 of parent zeolites showed very low oxygen (1940 cm-1) are associated with aggregated copper sites, such as copper-oxo dimers and trimers16,

18, 29-31.

This indicates the preferential formation of copper

monomeric sites in Cu(2.8)BEA(12) and Cu(2.7)FAU(15) and a mixture of monomers and copper oligomers in the case of Cu(3.4)MOR(10) and Cu(4.0)MFI(12). Similarly, the significant variation in the intensity of the bands in the spectra of the different zeolites, associated with the changes in the dipole moment of copper nitrozyls (Fig. 1a), indicates the dissimilar structure of CuII(NO) species in different copper-exchanged zeolites. Fig. 1b shows the infrared spectra of CuMOR with different Si/Al ratios. The spectra of adsorbed nitrogen monoxide of Cu(3.4)MOR(10) and Cu(4.4)MOR(6) strongly resemble each other: the intensity of the three most intense bands, centered at 1908, 1950 and 1995 cm-1, is very similar. The spectrum of Cu(1.2)MOR(46) demonstrates the presence of a single band at 1908 cm-1, which was previously assigned to a copper monomeric species, in contrast to the spectra of Cu(4.4)MOR(6) and Cu(3.4)MOR(10), which reveal the formation of a mixture of copper-oxo sites. The intensity of the band centered at 1908 cm-1 decreases in the following order: Cu(4.4)MOR(6) > Cu(3.4)MOR(10) > Cu(1.2)MOR(46), indicating a decrease in the amount of copper monomers in the samples (Table S1). This is in line with the total copper content, as measured by elemental analysis. The areas of the bands at 1830, 1800 and 1730 cm-

9



1

Page 10 of 30

due to CuI sites decrease with the increasing Si/Al ratio, showing greater stability of CuII

species during autoreduction in copper-exchanged mordenite with high Si/Al. It is notable that the variation in the copper loading in the CuMOR(6) zeolite does not result in a change in the position of the bands in the spectra of adsorbed nitrogen monoxide (Fig. 1c). The decrease in the intensity of the bands attributed to the interaction of both CuI and CuII species with nitrogen monoxide is in line with the copper content of the samples, decreasing from 4.4 to 1.7 wt% (Table 1). This suggests the persistence of the structure of copper-oxo species in the mordenite with a fixed Si/Al ratio and different copper loadings (Table S1). 3.3 Cu K-edge XANES spectroscopy Cu K-edge XANES spectra of oxygen-activated samples show spectroscopic features typical of CuII species (Figs. S4, S5). The pre-edge peak at 8977 eV is associated with dipoleforbidden 1s→3d electronic transition in CuII, whereas the shoulder at 8986 eV and the peak at 9000 eV are due to the 1s →4p transition.46, 47 After introducing a flow of helium, there is no change in the spectrum at 373K. Fig. 2 represents XANES spectra of copper-exchanged zeolite of different topology, acquired upon heating from 373 to 950 K. Starting at 550 K, a new shoulder appears at 8983 eV and continues to develop as temperature increases. This feature is a typical of the CuI species, corresponding to the 1s → 4p transition. Simultaneously, the intensity of the peak at ~9000 eV and the shoulder at 8986 eV decrease, suggesting a gradual transformation of CuII sites to CuI sites. Heating in a flow of helium at 950 K does not lead to the complete reduction of copper. The reference CuI spectrum, obtained by high-temperature reduction of the same sample in methane, shows a considerably higher intensity of the 8983 eV peak (Fig. S6). There is a small detectable peak at 8981 eV, assigned to Cu0 at temperatures above 850K, indicating the complete loss of oxygen by a small fraction (

Al ratio and

Recommend Documents