Elsevier

Applied Catalysis A: General

Volume 452, 15 February 2013, Pages 214-221
Applied Catalysis A: General

Exploring anatase-TiO2 doped dilutely with transition metal ions as nano-catalyst for H2O2 decomposition: Spectroscopic and kinetic studies

https://doi.org/10.1016/j.apcata.2012.12.003Get rights and content

Abstract

Pure and dilutely doped nano anatase-TiO2 with 3 at% Nb, Cr, Fe, Ni, Mn, Co or Cu ions were prepared by sol–gel processing. The doping was found by X-ray diffractometry and laser Raman spectroscopy to reduce the average crystallite size of the titania (8 nm) down to <8–3 nm without formation of separate or mixed oxide phases and without deviation from the anatase structure of the titania. Ion scattering spectroscopy observed surface exposure of the dopant ions, but only Mn, Co or Cu dopant ions were found by X-ray photoelectron spectroscopy to form oxygen-mediated bonds with titanium ions. The likely facilitation of d–d electron exchange interactions via such dopant–O–Ti linkages is suggested to provide the necessary bulk and surface attributes for the observed rendering of the host anatase-TiO2 a visible light absorber (as revealed by UV-visible diffuse reflectance spectroscopy) and an active heterogeneous catalyst for H2O2 decomposition (as revealed by gravimetric gasometry of the decomposition reaction). These two dopant-mediated behaviors were exhibited uniquely by the anatase-TiO2 nanoparticles after doping with Mn, Co or Cu ions.

Highlights

► 3 at% doping of anatase-TiO2 was affected with Nb, Cr, Fe, Ni, Mn, Co or Cu ions. ► The anatase structure is maintained but the crystallite size decreases down 8–3 nm. ► Only the Mn, Co or Cu ions form dopant–O–Ti linkages. ► Consequently, anatase-TiO2 is rendered a visible light absorber. ► Consequently, anatase-TiO2 is rendered a H2O2 decomposition heterogeneous catalyst.

Introduction

Anatase-TiO2 (semiconductive band gap >3 eV [1]) is a widely used catalytic material, i.e. a catalyst or an SMSI support material for active metal and metal oxide catalysts [2]. However, its fame in the field of catalysis is due, foremost, to its successful application in the chemical makeup of the DeNOx environmental catalyst [3] and as a heterogeneous UV-sensitive photocatalyst [4]. Consequently, anatase-TiO2 has attracted considerable attention of nano-sized and doped materials developers. This is due to two considerations: (i) when the size of the material particle becomes smaller down to the nanometer scale, the surface-to-volume ratio and free surface energy increase dramatically [5], and (ii) when doping is affected, a number of catalysis-worthy material properties are developed favorably [6], [7], [8]. Impacts of doping with transition metal ions on physical and chemical properties of a host metal oxide have been found to be critically dependent on the size, charge and electronic configuration of the dopant relative to the host metal ions [9], [10], [11]. Accordingly, some rules have evolved [12] and have been found to be helpful in designing doped metal oxides for specific industrial and technological applications [6].

Recent research works performed in these laboratories [13], [14] have revealed that decreasing the particle size of anatase-TiO2 down the range 19–8 nm, stabilizes the anatase structure, blueshifts the UV absorption edge, and weakens the surface acidity. On the other hand, doping of anatase-TiO2 with cationic and anionic dopants has been found to further the particle size decrease to <8 nm [15] and to render it a visible-light photocatalyst [16].

The decomposition of hydrogen peroxide (H2O2) into water and oxygen is a straightforward, simple reaction. It is initiated by the generation of HOradical dot and HOOradical dot radicals, which, then, trigger a chain reaction propagating till the decomposition is ceased [17]. The radical generation may occur via (i) homolytic fission of HOsingle bondOH bonds prompted pyrolytically or photolytically [17], [18], or (ii) heterolytic fission of HOsingle bondOH and/or HOOsingle bondH bonds prompted catalytically over redox active sites (Mn+/M(n+1)+) facilitating the necessary electron mobility, as depicted in the following reaction equations [19]:H2O2 + Mn+  OH + M(n+1)+ + HOradical dot ….(H2O2 reduction)H2O2 + M(n+1)+  H+ + Mn+ + HOOradical dot ….(H2O2 oxidation)

Therefore, the decomposition of H2O2 has for long been used as a probe reaction for redox catalytic activity of metals and metal oxides [20], [21], [22]. In the past decade, the reaction has been under scrutiny of space agencies as a prospective main component in some types of propulsion systems for space technologies [23], [24], [25]. Active metal catalysts examined thus far have been found to be unable to withstand the build-up of high temperature regimes in the adiabatic thrusters frequently used [23]. Therefore, thermally more stable, though less active, alternative metal oxide based catalysts have been sought [19].

In the stated context, it is plausible to think of anatase-TiO2 as heterogeneous photocatalyst for the H2O2 decomposition reaction, provided that it is modified to function under visible light, or as a heterogeneous catalyst capable of functioning in the absence or presence of visible light. This is to avoid the explosive UV-photolytic decomposition of the reactant on one hand [17], and to also warrant technological and economic feasibilities. Compatibly, the present investigation was designed to explore the possibility of turning anatase-TiO2 into either a photocatalyst or a heterogeneous catalyst functioning under natural visible light. To accomplish this objective, (i) pure and dilutely doped (to 3 at% with Nb, Cr, Fe, Mn, Ni, Cu and Co ions) anatase-TiO2 nanoparticles (8–3 nm) were prepared by sol–gel processing, (ii) influence of the dopants on the anatase structure and optical/vibrational characteristics were examined by X-ray diffractometry, laser Raman spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy, (iii) surface chemical composition was determined by ion scattering spectroscopy, and X-ray and ultraviolet photoelectron spectroscopies, and (iv) kinetics of the catalysis of the decomposition of dilute solutions (1.8 wt%) of H2O2 was followed by gravimetric gasometry.

Section snippets

Pure and doped titanias

Sol–gel processing was used to prepare pure (TiO2) and doped (3 at% M-TiO2) titanias, where M stands for Nb, Cr, Fe, Ni, Cu, Mn or Co cationic dopant), following the recipe detailed previously [13]. Accordingly, a 5-mL portion of titanium isopropoxide (Ti(OC3H7)4, 98% pure product of Spectrochem Pvt. Ltd.) and ethanol (C2H5OH, 99% pure product of Cymprangludt B.V.) in 1:8 volume ratio were mixed, and the dopant precursor nitrate or chloride compound (whose molecular formula, purity and producer

X-ray powder diffractograms

Full X-ray diffractograms obtained for pure and doped titania samples are displayed in Fig. 1A, whereas the adjacent panel (Fig. 1B) focuses only on the strongest peak monitored. With reference to the standard diffraction data filed in ICDD PDF 84-1286 [27], all of the peaks monitored (at 2θ = 25.3, 37.8, 48.1, 53.9, 55.1, 62.7, 68.8, 70.3 and 75.1°) in the diffractogram of the pure titania (Fig. 1A) are assignable solely to anatase-TiO2. The comparison also reveals the emergence of a tiny peak

Conclusions

The above presented and discussed results may help drawing the following conclusions:

  • 1.

    Dilute doping of anatase-TiO2 nanoparticles (8 nm) with Nb, Cr, Fe, Ni, Mn, Co or Cu ions at 3 at% reduces the average crystallite size (<8–3 nm) without XRD-detectable formation of separate or mixed oxide phases and without significant deviation from the anatase structure of the titania.

  • 2.

    Only when dopants form XPS-detectable dopant–oxygen–titanium bonds (i.e. the case of Mn, Co and Cu) the titania anatase lattice

Acknowledgments

M.I.Z. is indebted to the TWAS(Italy)/CSIR(India) for a joint grant, and Minia University for a leave of absence that facilitated a 3-month visit to Pune/India and, hence, a fruitful collaboration between Minia University and the CSIR-NCL. S.B.O. likes to acknowledge support under CSIR's TAPSUN program.

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