X. Zhang, et al.
CatalysisTodayxxx(xxxx)xxx–xxx
DCE. Yang et al. [11] prepared cerium-transition metal mixed oxides
(4Ce1M, M = V, Cr, Mn, Fe, Co, Ni, and Cu) by the coprecipitation
method, investigated catalytic properties of these materials for deep
oxidation of four typical CVOCs, and found that the 4Ce1Cr catalyst
possessed the optimal activity (T90% = 232 °C at SV = 15,000 h−1) and
selectivity, which was mainly associated with strong oxidizing ability of
the Cr6+ species. After investigating the (Ce,Cr)xO2/MOy (M = Ti, V,
Nb, Mo, W, and La) catalysts for total oxidation of 1,2-DCE, Zhou and
coworkers [12] concluded that the improved redox property and in-
creased surface Ce3+ and Cr6+ species contents enhanced oxidation
performance of the catalyst, in which (Ce,Cr)xO2/Nb2O5 performed the
best (T90% = 225 °C at SV = 9000 mL/(g h)).
During the oxidation process of CVOCs, the Cl-containing inorganic
species (HCl and/or Cl2) are easily adsorbed on the surface of a catalyst
or react with the active species to form volatile metal chlorides, re-
sulting in partial or even complete deactivation of the catalyst. The
mixed transition metal oxide catalysts are expected to solve this pro-
blem. Introduction of a certain amount of transition metal (M = Cr,
Mn, Fe, Ni or Cu) oxide is a useful strategy in modifying physico-
chemical properties and improving thermal stability of the TiO2-based
materials. The synergistic action between MOx and TiO2 and formation
of oxygen vacancies in TiO2 due to partial replacement of Ti4+ by Mn+
could enhance activity and Cl-resistant stability of the MOx–TiO2 cat-
alysts for the oxidative removal of CVOCs. Owing to the advantages
(e.g., nontoxicity, cheapness, corrosion resistance, and long-term sta-
bility) of TiO2, the TiO2-based materials possessed highly selective and
anti-poisoning properties in CVOCs deep oxidation [13,14].
Fig. 1. XRD patterns of (a) TiO2, (b) 10CrOx–TiO2, (c) 10MnOx–TiO2, (d)
10FeOx–TiO2, (e) 10NiOx–TiO2, and (f) 10CuOx–TiO2.
In this work, we prepared the MOx–TiO2 (M = Cr, Mn, Fe, Ni, and
Cu) mixed oxide catalysts using the coprecipitation method, char-
acterized their physicochemical properties, measured their catalytic
activities for deep oxidation of 1,2-DCE, examined impact of water
vapor or hydrochloric acid on catalytic performance of the typical
samples, and probed the involved oxidation mechanisms.
reaction products were analyzed online by an Agilent GC–MS equip-
ment. 1,2-DCE conversion = (cinlet − coutlet)/cinlet × 100 %, where the
cinlet and coutlet are the inlet and outlet 1,2-DCE concentrations in the
feed stream. HCl and Cl2 selectivities were determined by bubbling the
outlet gas mixture in a sodium hydroxide aqueous solution (12.5 mmol/
L) for 0.5 h. The Cl2 concentration in the bubbled solution was mea-
sured via a chemical titration route [15]. The detailed chemical analysis
procedures are described in the Supplementary material. 5.0 vol% H2O
was added to the feed stream through a water saturator at 34 °C.
100 ppm HCl provided by a gas cylinder with N2 as balance was in-
troduced to the reaction system. The balance of carbon throughout the
2. Experimental
2.1. Catalyst preparation
The 10 wt% MOx–TiO2 (M = Cr, Mn, Fe, Ni, and Cu) mixed oxide
catalysts were prepared using the coprecipitation method. 1.0 g of tet-
rabutyl titanate and a desired amount of Cr(NO3)3, Mn(NO3)2, Fe
(NO3)3, Ni(NO3)2 or Cu(NO3)2 were dissolved in 20 ml of ethanol
aqueous solution (deionized water/ethanol volumetric ration = 1 : 9).
A certain amount of NH4OH (1.0 mol/L)-ethanol aqueous solution was
added dropwise into the above mixed solution at 50 °C under vigorous
stirring for 8 h, in which the pH was adjusted to 10.0. The precipitated
solids were filtered, washed with deionized water and ethanol three
times, dried at 80 °C overnight, and calcined in air at 500 °C for 5 h, thus
obtaining the 10 wt% MOx–TiO2 (denoted as 10MOx–TiO2) samples.
catalytic system was estimated to be 98.5
1.5 %.
3. Results and discussion
3.1. Crystal structure, surface area, and morphology
XRD patterns of the 10MOx–TiO2 samples are shown in Fig. 1. The
diffraction signals at 2θ = 25.3°, 36.5°, 37.8°, 38.6°, 48.0°, 53.9 °, 55.1°,
62.7°, 68.6°, 70.1°, and 75.0° could be attributed to the (101), (103),
(004), (112), (200), (105), (211), (204), (116), (220), and (215) crystal
planes of the anatase-phase TiO2 (JCPDS PDF# 21-1272) [5]. For the
10MOx–TiO2 samples, the feature diffraction peak(s) assignable to the
Cr2O3 (104), (110), and (300) [16], Mn2O3 (104) [17], Fe2O3 (104) and
The ionic radii of the Mn+ are smaller than that of Ti4+, doping of the
Mn+ into the TiO2 lattice would give rise to a shrinking in crystal cell of
TiO2. That is to say, the Mn+ might be incorporated into the lattice of
TiO2, which contributed to enhancement in interaction between TiO2
and MOx and hence improvement in catalytic performance of the
10MOx–TiO2 samples for deep oxidation of CVOCs. It should be noted
that chromium is not environmentally friendly, and the Cr-based cat-
alysts sometimes suffer from formation of the toxic residues. However,
there was no formation of toxic Cr-containing compounds in our
Cr2O3–TiO2 samples during the TCE combustion process below 300 °C,
since Cr2O3 strongly interacted with TiO2 to generate a Cr2O3–TiO2
solid solution and partial Cr3+ ions were incorporated into the lattice of
TiO2.
2.2. Catalyst characterization
The XRD, TEM, HAADF-STEM, elemental mapping, BET, XPS, H2-
TPR, O2-TPD, NH3-TPD, and CO2-TPD, TPSR, and in situ DRIFTS
techniques were used to characterize physicochemical properties of the
samples. The detailed characterization procedures are described in the
Supplementary material.
2.3. Catalytic activity evaluation
A continuous flow fixed-bed quartz microreactor (i.d. = 6 mm) was
employed to evaluate catalytic activities of the samples for the oxida-
tion of 1,2-DCE. To avoid the presence of hot spots, the sample (50 mg,
40–60 mesh) was mixed with quartz sand (0.25 g, 40–60 mesh). The
total flow of the reactant feed of 1000 ppm 1,2-DCE +20 vol% O2 + N2
(balance) was 33.4 mL/min, and the SV was ca. 40,000 mL/(g h). The
2