Chemically Induced Formation of Monovalent Cd
+Ions and Reversible O
2Activation in Cadmium Loaded ZSM-5 Zeolite
Elena Morra and Mario Chiesa*
Department of Chemistry, NIS Centre, University of Turin, Turin, Via Giuria 7, 10125 Italy.
Abstract
We have studied the nature of monovalent Cd+ (4d10 5s1) species generated in a Cd loaded HZSM-5
zeolite by reaction with molecular O2. The openshell species formed in the different steps of the
-reversible - reaction are characterized by Electron Paramagnetic Resonance (EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopies at X- and Q- band frequencies. The same Cd+
species are obtained via a photochemical pathway, through UV irradiation of the Cd loaded zeolite.
Unambiguous evidence for the formation of mononuclear Cd+ species is obtained by detection of
the hyperfine interaction associated to naturally abundant 111Cd and 113Cd magnetic isotopes (I = ½
natural abundance 12.8 % and 12.22 % respectively). The full 111Cd A tensor (A
x=10620 Ay=10639,
Az=10790) MHz is resolved, indicating that 84% of the unpaired electron spin density is localized on
the Cd ion. The small spin density delocalization on the zeolite framework is observed through detection of 27Al hyperfine interactions by means of 6 pulse HYSCORE experiments, allowing for a
detailed description of the geometric and electronic structure of the monovalent cadmium species and the zeolite stabilizing sites.
1. Introduction
Metal-loaded zeolites are still attracting persistent interest because of the unusual ions, clusters, and filamentary structures, which can be stabilized and the unique physical and chemical properties associated to these exotic chemical species. Spatially and chemically stabilized metal species ranging from single ions to small clusters of a few metal atoms (Mnδ+) are known, which are
of considerable interest in advancing our understanding of the size dependency of metal properties between the atomic and nanoparticle regime. In this size regime the electronic configuration and discrete energy levels of metals manifest some remarkable electronic and optical properties, such as strong photoluminescence,1 and high catalytic activity.2 In the field of catalysis,
it is well known that the Al distribution inside zeolites determines the chemical properties of countercations 3-45, priming single cations or small ionic clusters for high activity towards many
relevant reactions.6
Many examples of molecular metal clusters involving alkali metals 7 and silver 8 have been
reported, while only few reports are available for heavy metals of group 12 (Zn, Cd and Hg).9-10111213
Cadmium cluster formation within zeolites by Cd metal vapor sorption has been reported by Seff and co-workers,11,14 which were able to identify different sorption complexes including Cd+ ,Cd
22+
and Cd34+ species in Cd(II) exchanged zeolites. Recently, a weak and poorly resolved EPR signal
resonating at g=2.003, observed on a UV irradiated Cd-ZSM-5 system, was attributed to Cd+.13
Cd+ is a paramagnetic ion with 4d10 5s1 electron configuration with a very high chemical
the past as the result of high energy radiolysis experiments involving the use of gamma rays15,16 or
atom beams at cryogenic temperatures 17 and detailed X-band EPR studies are available for CdH,
CdOH and CdCN radical molecules isolated in rare gas matrices.18-
Given its paramagnetic nature, the most direct evidence for the formation of this unusual oxidation state is Electron Paramagnetic resonance (EPR) spectroscopy. However, unambiguous identification of this species by EPR can be obtained only by the detection of the hyperfine interaction associated to 111Cd and 113Cd magnetic isotopes (I = ½ natural abundance 12.8 % and
12.22 % respectively). This is non trivial due to the low natural abundance of the magnetically active isotopes and very large hyperfine splitting, which make the low field component undetectable at standard operating frequency (9.5 GHz).
As part of a general study of the chemistry of Group 12 metals in zeolites, we report in the following on the reversible chemical formation of monovalent Cd ions induced by the reaction of molecular O2 with a Cd loaded HZSM-5 zeolite. For the first time the full set of hyperfine
interactions relative to 111Cd and 113Cd cadmium isotopes and the magnetic nuclei of the zeolite
framework (27Al, 29Si and 1H) are resolved, providing direct evidence for the formation and
localization of monovalent cadmium ions, the nature of the zeolite framework trapping site and the reversible chemical reactivity of Cd+ towards molecular oxygen.
2 Experimental Section
2.1 Sample Preparation
Cd-loaded ZSM-5 was prepared by in situ sublimation of metallic cadmium on ZSM-5. The H-ZSM-5 zeolite (Si/Al = 15) was dehydrated by thermal treatment at 673 K under dynamic vacuum (residual pressure <10-4 mbar) for two hours and subsequently calcined at 773 K in O
2 atmosphere
to remove spurious organic residues. The activated zeolite was exposed for 5 minutes to metallic cadmium vapours generated in situ by heating a Cd metal bead at 583 K. Photochemical experiments were performed using a 1500W xenon lamp (New Port Instruments) irradiating the sample with UV/Vis light for 80 minutes. The sample obtained with this procedure is abbreviated as Cd-ZSM-5/UV.
2.2 EPR Characterization
X-band (microwave frequency 9.46 GHz) CW EPR spectra were detected at T = 77 K and T = 298 K on a Bruker EMX spectrometer equipped with a cylindrical cavity. A modulation frequency of 100 kHz, a modulation amplitude of 0.2 mT, and a microwave power of 0.02 mW were used. X-band (microwave frequency 9.76 GHz) and Q-band (microwave frequency 33.7 GHz) pulse EPR experiments were carried out at T = 50 K and T = 298 K on a Bruker ELEXYS 580 EPR spectrometer, equipped with helium gas-flow cryostat from Oxford Inc. The magnetic field was measured with a Bruker ER035M NMR gaussmeter.
Electron-spin-echo (ESE) detected EPR spectra were recorded with the pulse sequence
π/2−τ−π−τ−echo. Pulse lengths tπ/2 = 16 ns and tπ = 32 ns, a τ value of 200 ns and a 0.5 kHz shot
Six-pulse Hyperfine Sublevel Correlation experiments23,24 were carried out with the extended pulse
sequence (π/2)x−τ1−(π)x−τ1−(π/2)y−t1−(π)y−t2−(π/2)y−τ2−(π)y−τ2−echo, applying a eight-step phase
cycle in order to eliminate unwanted echoes. The t1 and t2 time intervals were incremented in
steps of 16 ns, starting from 100 ns to 4900 ns. Pulse lengths tπ/2 = 16 ns and tπ = 32 ns, and a 0.5
kHz shot repetition rate, and equal τ1 andτ2 values were used. The time traces of the HYSCORE
spectra were baseline corrected with a third-order polynomial, apodized with a Hamming window, and zero-filled. After two-dimensional Fourier transformation, the absolute-value spectra were calculated. Spectra recorded with different τ1 = τ2 values (specified in the figure captions) were
added to average out blind-spots. EPR and HYSCORE spectra were simulated using the Easyspin package.25
Phase-memory times (Tm) were measured by the Hahn Echo sequence upon increasing the
interpulse delay starting from = 98 ns. Spin–lattice relaxation times (T1) were measured using
the standard inversion recovery sequence π − td − π/2 − − π − − echo. All relaxation times were
measured at room temperature.
3. Results and Discussion
3.1 Generation of Cd+ Species in HZSM-5
Upon exposure of the dehydrated H-ZSM-5 zeolite to Cd vapours, diamagnetic species are formed and the sample is EPR silent (Figure 1a). Reaction with molecular oxygen leads to the formation of an intense EPR signal amenable to the formation of superoxide (O2-) molecular anions as described
by the first part of Equation 1. (Cd+)
n + nO2 n[Cd2+ -O2-]+ nO2 + nCd+ (1)
Figure 1. X-band CW-EPR spectra of a) Cd-ZSM-5, b) contacted with O2, c) outgassed at RT. The spectra were recorded
at 77 K, with microwave power of 0.02 mW. A schematic description of the reaction cycle is presented on the right hand side.
This behavior is in accord with the presence of strongly reducing monovalent cadmium species in the form of either Cd22+ dimers or Cdnn+ diamagnetic clusters, as reported by Seff and co-workers.11, 14
Oxygen removal at room temperature by out-gassing the sample down to a residual pressure of 10-4 mbar, leads to a new spectrum characterized by a pseudo-axial powder pattern resonating at g
values close to the free electron value (ge=2.0023). Anticipating the result of this study, this signal
is assigned to monomeric Cd+ (4d10 5s1) ions based on the detection of a large hyperfine interaction
to 111Cd and 113Cd isotopes (I = ½), dominated by the isotropic “contact” term (vide infra). Upon
annealing of the sample at 473 K the Cd+ signal disappears, suggesting that the initial diamagnetic
Cd+ polynuclear species are restored. All these steps are fully reversible and the reaction can be
cycled interconverting O2- radical ions into mononuclear Cd+ species in a quantitative way, as
shown in Figure S1 of Supporting Informations.
Interestingly, the same Cd+ species, responsible for the EPR spectrum reported in Figure 1c, are
formed upon UV irradiation of the diamagnetic Cd-ZSM-5 sample (Figure 2 and Figure S2 in Supporting Informations). The formation of monovalent Cd+ ions in this case can be explained
considering the homolytic splitting of the diamagnetic precursors to mononuclear Cd+ ions as
already proposed for the formation of Zn+ species in the case of zinc-doped ZSM-5.24,26
This reversible redox cycle (Figure 1 and Equation 1), which involves the activation of the oxygen
molecule and the formation of stable Cd+ species generated by a chemical reaction under mild
conditions, is of great interest. In the following, we report the detailed EPR characterization of the Cd+ species in H-ZSM5, while details on the reversible reactivity towards O
2 will be dealt with
elsewhere.
3.2 EPR Spectra of Cd+
The unambiguous identification of Cd+ species is possible thanks to the detection of the
hyperfine interactions of 111Cd and 113Cd isotopes with 12.8 % and 12.22 % natural abundance
respectively and both with nuclear spin I = ½. Detection of the hyperfine transitions is not trivial because the hyperfine coupling is so large that at standard X-band frequency (9.5 GHz) the low field components are usually undetectable (Figure 2a). This problem can be overcome by working at higher operational frequencies as demonstrated by the Q-band (35 GHz) spectra reported in Figure 2b, where the expected doublets are fully resolved. This observation firmly proves that the spectra observed upon superoxide decomposition (Figure 1c) and after UV irradiation of the Cd-ZSM-5 diamagnetic sample (Figure 2) are due to isolated Cd+ ions.
The computer simulation of the X- and Q-band CW-EPR spectra was carried out assuming the following spin Hamiltonian:
H = eBTgS + i STAiIi (2)
with slightly rhombic g and A tensors. Two different species, with slightly different spin-Hamiltonian parameters – reported in Table 1 – and with nearly equal abundance were needed in order to fully simulate the experimental spectrum. This suggests the presence of two distinct stabilizing sites, with slightly different topological characteristics.
Figure 2. a) X-band and b) Q-band experimental (black) and simulated (red) CW-EPR spectra of Cd-ZSM-5/UV recorded at room temperature. The asterisk indicates a spurious cavity signal. The spectra are shown with normalized intensities, except the X-band low-field signal. The parameters employed for the simulations are listed in Table 1. The deconvoluted spectra showing the individual contributions of the two components are shown in supporting information, Figure S3.
To fully understand the spectra reported in Figure 2, it is useful to analyse the splitting of the energy levels as a function of the magnetic field at the two frequencies that are shown in Figure 3. Given the very large hyperfine coupling A (10.6 GHz), the low field transition at X-band frequency (9.5 GHz) falls at a magnetic field where the usual uncoupled representation is not valid and the electron and nuclear spin angular momenta (S and I) couple to form a resultant F = I + S total angular momentum. In this way a triplet (F = 1) and a singlet (F = 0) level are present at zero field, separated by the hyperfine coupling constant A. Under these circumstances the allowed transitions correspond to mF=±1 for F=1 or 0. The allowed low field transition indicated in Figure 3a
corresponds therefore to a NMR transition. In the experimental spectrum, such NMR transition is actually barely observed due to the combination of a slightly lower transition probability and strain effects, which specifically affect this low field component. This can be appreciated by examining a plot of the hyperfine coupling constant versus the resonant magnetic field for the two 2I+1 transitions (Figure 3b). The mI linewidth dependency originates in combined g and A strains, which
reflect the slightly different local topological details of each monomeric species. The observed hyperfine coupling constant to the metal nucleus is thus an “average” value, with a distribution of values between the limits A±A. It is such a distribution, which is at the origin of the mI-dependent
line width of the X-band spectrum. The mI dependent contribution is sensitive to the gradient
(A/B) of the plot of A against Bres at a given A value.
Figure 3. (a) Energy level diagram, the full set of quantum numbers mF (coupled representation, low field) and mI and ms (uncoupled representation, high field) are indicated. The allowed transitions at X band (9.5 GHz, purple) and at Q
band (34 GHz, green) frequencies are indicated. (b, c) Calculated diagrams of the resonant magnetic field for the two spin manifolds (mI=1/2) as a function of the hyperfine coupling constants at the two frequencies. The dotted lines in
the A vs Bres plot indicate the experimental hyperfine coupling of 111Cd. A negative sign for the hyperfine coupling
constant is assumed in accordance with the negative nuclear g factor of 111Cd.
The result is visualized in the plot of Figure 3b, where the effect of a distribution around the experimentally determined A value is shown. Given the different slope of the two curves, the broadening effect is particularly large for the low-field NMR transition. At Q-band frequency (34 GHz), the high-field approximation holds, and the two EPR transitions are observed, without appreciable mI-dependent line widths.
The g values of the two species are close to the free-atom value ge = 2.0023, as it may be
expected for a Cd+ ion (4d10 5s1), in a nominally perturbed 2S
1/2 ground state.
The shift of the g values from the free electron value ge can be calculated in second-order of
perturbation as: ∆ gij=−2 λCd
∑
n⟨
n|
Li|
0⟩ ⟨
0|
Lj|
n⟩
E0−En (3)where λCd = 1142 cm-1 is the Cd spin-orbit coupling constant, Li are components of the angular
momentum operator, 0 is the wave function of the ground-state (SOMO) and n is the wave function of the excited states. The slight negative shift Δgx,y indicates spin-orbit induced couplings
with empty states. In the ionic approximation and assuming only coupling to one excited state represented by a Cd 5p orbital, the deviation of g from ge can be expressed by the relation
Δ g⊥=g⊥−ge=−2 λCd c2
2
Δ E (4)
where c22 is the 5p character in the hybridized orbital, and ΔE is the energy separation between the
ground state and the excited state. Considering the experimentally observed Δg -0.01 and ΔE =
32000 cm-1,17 estimated for Cd monohalides, a c
22 value of approximately 0.14 is obtained from
Equation 4.
Compared to the g tensor of Zn+ ions in ZSM-5, the g tensor of Cd+ ions shows a higher
deviation from the ge value (Table 1), consistently with the higher spin-orbit coupling constant of
cadmium compared to the one of zinc (λZn = 342 cm-1). Long relaxation times were measured at RT,
with a phase-memory time (Tm) of the order of 1 s and a spin-lattice relaxation time (T1) of 3.8 s
at X band frequency. These values are slightly shorter compared to those observed for Zn+ ions on
ZSM-5 27 again in agreement with the larger spin-orbit coupling (and larger g anisotropy) of Cd with
respect to Zn.
The extremely large hyperfine coupling of 111,113Cd suggests a substantial ionic character of the
system with little delocalization of the spin density to the zeolite framework. The hyperfine coupling tensor can be decomposed in the isotropic Fermi contact term aiso = -10683 MHz and the
anisotropic dipolar tensor T = [+63 +44 -107] MHz, where the signs have been chosen considering that gCd < 0 for both Cd magnetic isotopes. The small anisotropy of the hyperfine tensor, with the
larger departure along the z direction, indicates a SOMO with a1 symmetry, with mainly |5s
character and small admixture of |4pz orbital, in line with the g tensor analysis. The magnitude of
aiso and T reflects the amount of unpaired s and p character respectively. Assuming a simple ionic
model, the wave function for the unpaired electron can be represented by a normalized LCAO wave function
ψ=c1χ5 s+c2χ5 p (5)
where χ5s and χ5p represent the valence atomic metal orbitals, and the values c1 2
=ρ5 s and
c22=ρ5 p express the amount of 5s and 5p metal orbital character respectively.
The unpaired electron spin population in the Cd 5s orbital can be estimated as Cd(5s) = aiso/a0Cd =
0.74, taking the value a0
Cd= -14385 MHz experimentally measured for 111Cd+ isolated in argon
matrix.18 Assuming an atomic value for the 111Cd dipolar hyperfine constant T0
Cd = 1032 MHz 28, the
Cd 5p character is estimated to be Cd(5p) = T/T0Cd 0.1, in agreement with the estimate obtained
from the analysis of the g tensor (Equation 4) The total spin density on the Cd atom is thus in the order of Cd(total) = Cd(5s) + Cd(5p) = 0.84.
The unpaired-electron spin density, which is directly monitored via the EPR hyperfine coupling constant aiso, is a sensitive probe of the degree and of the nature of the metal-to-matrix
interaction. The large spin density observed for the Cd+ species points to large ionic character with
little spin delocalization over the matrix and small matrix perturbing effects.
An interesting comparison can be set to the case of alkali metals adsorbed on metal oxides, where can be interpreted as a quantitative measure of the perturbation of the alkali atom gas phase ns wave function by the local environment. We reported values in the range of 0.45 – 0.52 for alkali-metal atoms adsorbed on alkaline-earth oxides.29 In particular a value of = 0.42 was
observed for Rb atoms – isoelectronic with Cd+ – on MgO. Due to atom-matrix interactions,
alkali-metal atoms adsorbed on alkali-metal oxides show a large decrease of the alkali-metal hyperfine coupling constant, compared to the gas-phase atoms. This reduction was found to correlate with the basicity of the substrate and interpreted in term of a nephelauxetic effect where the ns orbital of the bound atom is strongly destabilized in energy by the interaction with the matrix oxygens and almost no spin delocalization over the oxide matrix.30 In the case of Cd+ on ZSM-5, a remarkably
higher amount of spin-density is found to be localized on the Cd atom in comparison with the case of the isoelectronic Rb on MgO, which is consistent with the acidic nature of the ZSM-5 substrate and the low electron charge donaticity of the matrix oxide ions.
Table 1. Spin-Hamiltonian parameters extracted from simulation of X- and Q-band CW-EPR spectra of Cd-ZSM-5/UV and Zn-ZSM-5/UV.
species % gx gy gz |Ax| |Ay| |Az| |Q| Ref.
Cd-ZSM-5 /UV Cd+(1) 50 1.9891 0.0002 1.9932 0.0001 2.0007 0.0001 111Cd 10620 40 10639 40 10790 40 - - This work Cd+(2) 50 1.9875 0.0005 1.9918 0.0005 2.0007 0.0005 27Al 1.2 0.2 1.1 0.2 3.1 0.2 8 2 0.8 0.2 Zn-ZSM-5 /UV Zn+(1) 55 1.9951 0.0002 1.9984 0.0001 2.0015 0.0001 27Al 2.8 0.2 -2.7 0.2 -4.6 0.2 -10 2 -0.8 0.2 -27 Zn+ (2) 45 0.00051.9953 0.00051.9979 0.00052.0015 3.3 Superhyperfine interactions with the zeolite matrix
Despite the dominating ionic bond character, the overlap between the Cd and the matrix orbitals leads to a certain degree of spin delocalization, which gives rise to superhyperfine interactions between the unpaired electron and matrix species bearing a nuclear magnetic moment.
Figure 4. a,b) X-band 6p-HYSCORE spectrum of Cd-ZSM-5/UV, recorded with 1 = 2 = 100 + 130 + 154 ns, T = 50 K, at
the field position corresponding at the top of the echo; the frequency regions of a) 29Si, 27Al and b) 1H are shown. c)
Schematic representation of the formation of Cd+ species substituting the proton Brønsted sites of ZSM-5.
The small superhyperfine interactions of Cd+ with surrounding magnetic nuclei of the zeolite
framework (27Al, I = 5/2) can be detected and studied by exploiting advanced pulse EPR methods.
These interactions are particularly important to understand the nature of the Cd+ trapping site. In
particular, X- and Q-band two-dimensional hyperfine-sublevel-correlation experiments carried out with the extended six-pulse sequence (6p-HYSCORE)23,24 were employed in this study. Compared to
the standard four-pulse HYSCORE,31 the six-pulse sequence suffers less strongly from
cross-suppression effects which can occur when several kind of nuclei are coupled to the same electron spin. Another advantage is the suppression of multi-quantum transitions in the case of nuclei with large spins, which results in the simplification of the spectra. Finally, a general enhancement of sensitivity, providing a good signal-to-noise ratio in shorter acquisition times is usually observed. These features make this experiment particularly useful in the detection of the weak superhyperfine interactions of Cd+ with surrounding 29Si (I = 1/2), 27Al (I = 5/2) and 1H (I = 1/2)
matrix nuclei as described in the following.
27Al interaction. The X-band 6p-HYSCORE spectrum (Figure 4a) is dominated by a signal centred
at the 27Al Larmor frequency. A better resolution is obtained at Q-band frequency, where two cross
peaks with frequencies approximately = I ± A/2, where I is the 27Al Zeeman frequency and A is
the hyperfine coupling are observed. The observation of two well resolved off-diagonal cross peaks indicates that the hyperfine interaction with aluminum nuclear spins is dominated by the Fermi contact term. The full hyperfine coupling tensor, can thus be obtained by orientation selective Q-band 6p-HYSCORE experiments. 6p-HYSCORE spectra were recorded and simulated at magnetic fields corresponding to the principal values of the g tensor as shown in Figure 5. The 27Al hyperfine
tensor, extracted from the simulation of the spectra (Table 1) can be decomposed in the usual way into the isotropic constant aiso = 1.8 MHz and the dipolar tensor T = [-0.6 -0.7 +1.3], where the signs
have been chosen accordingly to the positive gAl value. These values translate into a total spin
density transfer on the Al nucleus of approximately Al = 0.0085 (using a0Al = 3911 MHz and T0Al =83
MHz). 28 Similar values were observed for Zn+-ZSM-5,27 for which however a larger isotropic contact
Figure 5. Experimental (left) and simulated (right) Q-band 6p-HYSCORE spectra of Cd-ZSM-5/UV. The spectra were recorded at T = 50 K with 1 = 2 = 112 ns at the field positions indicated in the insets.
29Si interaction. The X-band 6p-HYSCORE spectrum shows a strong diagonal signal centred at
the 29Si Larmor frequency (Figure 4a), with maximum extension of approximately 1.4 MHz, which is
associated with remote Si framework nuclei. Statistically, only remote silicon nuclei are detected due to the low natural abundance (4.68 %) of the 29Si isotopes, which hampers the detection of
directly coupled silicon nuclei. This is also true at Q band frequency, therefore no 6 pulse HYSCORE experiments optimized for the detection of 29Si were performed.
1H interaction. A weak diagonal 1H signal is observed in the X-band 6p-HYSCORE spectrum
(Figure 4b), with a maximum extension of approximately 1.0 MHz. In the point-dipole approximation and assuming a pure dipolar interaction, such a coupling corresponds to a Cd+–1H
distance of approximately 0.54 nm corresponding to remote matrix protons, indicating the presence of residual protons in the matrix but absence of H+ within the first coordination sphere of
Cd+. This is consistent with previous investigation of Zn-ZSM-5 27 and with NMR reports showing
that proton Brønsted sites are consumed upon reaction of zeolites with the metal.32
The observation of a substantial hyperfine interaction of the unpaired electron of Cd+ with 27Al
framework nuclei, together with the lack of evidence for protons within the first coordination sphere, is the experimental proof that mononuclear Cd+ ions are stabilized at the Brønsted sites of
4. CONCLUSIONS
The local environment of unusual Cd+ ions stabilized at ZSM-5 zeolite was investigated
exploiting a combination of X- and Q-band CW and HYSCORE EPR techniques. No paramagnetic species are directly formed by sublimation of metallic cadmium on H-ZSM-5. Two possible routes for generating Cd+ ions upon the sublimation process were tested: i) the first procedure, which has
been already reported in literature, involves the irradiation with UV light; ii) a second procedure, which to the best of our knowledge has been reported here for the first time, exploits the reaction
with O2 followed by evacuation at room temperature. Both preparations lead to the formation of
stable mononuclear Cd+ paramagnetic species with the same properties. CW-EPR spectra testified
the formation of Cd+ species. By means of Q-band CW EPR the full hyperfine tensor associated to
the interaction with natural abundant 111Cd and 113Cd nuclei was observed, allowing to characterize
the hybridization of the Cd+ ions. Moreover, the analysis of the 111,113Cd hyperfine tensor ascertains
that the observed Cd+ paramagnetic species are mononuclear. To gain insight into the coordination
spheres of Cd+, advanced X- and Q-band HYSCORE experiments were exploited, which allowed
investigating the nature of the adsorption site. Superhyperfine interactions to 27Al, 29Si and 1H
nuclei belonging to the ZSM-5 framework were detected; the observation of a consistent interaction of Cd+ with 27Al nuclei and the lack of evidence of nearby protons strongly indicate that
mononuclear Cd+ ions are formed and stabilized at the acidic Brønsted sites of H-ZSM-5 replacing
for the protons. The nature of the diamagnetic Cd precursors formed upon sublimation of metallic cadmium on H-ZSM-5 remains elusive; however, the peculiar reactivity with O2 and the reversible
conversion between diamagnetic and paramagnetic form observed by EPR experiments suggests that these precursors are Cd22+ dimers.
The peculiar reactivity of Cd-ZSM-5, featuring a reversible charge transfer towards O2 at room
temperature may reveal important in the context of catalytic processes for the oxidation of hydrocarbons by air as well as in the development of materials for the selective separation of O2,
although further studies on the selectivity with respect to other gases are needed in this context.
ACKNOWLEDGEMENTS
This work was supported by the Italian MIUR (through Project No. PRIN 2015-HYFSRT).
AUTHOR INFORMATION Corresponding author
E-mail: [email protected]
SUPPORTING INFORMATION
EPR signal intensity of Cd+ and O
2- for subsequent treatments of Cd-ZSM-5. Comparison of the EPR
spectra of Cd+ ions generated by UV irradiation of Cd-ZSM-5 and by reaction of Cd-ZSM-5 with O 2
followed by evacuation. Deconvolution of the individual contributions of the two Cd+ species to the
X- and Q-band CW EPR spectra.
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