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Selective Methanol Oxidation to Hydrogen over Ag/ZnO Catalysts Doped with Mono- and Bi-Rare Earth Oxides Hany Mohamed Abdeldayem, Shar Saad Al-shihry, and Salah Abdou HASSAN Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 26 Nov 2014 Downloaded from http://pubs.acs.org on November 27, 2014
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Selective Methanol Oxidation to Hydrogen over Ag/ZnO Catalysts
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Doped with Mono- and Bi-Rare Earth Oxides
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Hany M. AbdelDayem,†, ‡* Shar S. Al-Shihry,† and Salah A. Hassan‡
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†
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‡
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ABSTRACT: The hydrogen production by the catalytic partial oxidation of methanol (POM)
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over rare earth oxide (RE = La, Dy, Gd and Ce)-modified ZnO-supported silver catalysts, as well
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as silver supported on Ce1-XGdXOy-modified ZnO catalysts, was investigated. The effect of the
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temperature on the activity was studied in the range between 150°C and 400°C, and the catalyst
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stability was monitored with time-on-stream (TOS). The addition of rare earth metal oxide
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promoters resulted in a significant improvement in the catalytic performance. The optimal
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performance in methanol oxidation was achieved using AgCe20Zn, which exhibited a hydrogen
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selectivity of 90.8% with 95.2% methanol conversion at 350°C; however, the catalyst suffered
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from marked deactivation with TOS. The good stability of the Ag/ZnO catalyst was verified
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using a gadolinium promoter. Doping of the AgCe20Zn catalyst by Gd greatly enhanced its life
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span over at least 24 h on-stream and markedly reduced the CO content (down to 97% at methanol conversion ca.
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92%) than Ag/ZnO.[9,10] The physical properties of the promoted catalyst have been subsequently
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characterized, and it was suggested that the Ag+ ions were reduced by Ce3+. In the same catalytic
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reaction, La and Sm have been introduced to promote the activity and stability of Au and Cu
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based catalysts, respectively.[14,15]
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The use of CeO2-Gd2O3 in the composition of solid oxide fuel-cell (SOFC) anodes has been
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proposed as an approach for suppressing the carbon deposition of SOFC running on ethanol.[16]
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Recently, Huang and Chen have demonstrated that the methanol steam reforming activity of
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Cu/CeO2 can be enhanced by the addition of Gd2O3 to ceria and that the CO content in reformate
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is approximately 1% at 240°C.[17]
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The objective of this work focused on the study of the influence of trivalent rare earth oxide
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(RE = La, Gd, and Dy) doping on the activity and stability of an Ag/ZnO catalyst for the partial
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oxidation of methanol to hydrogen, compared with Ag/CeO2-ZnO. Ag supported on CexGd1-xOy-
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ZnO (x = 0.1, 0.2, 0.3, and 0.5) catalysts were also studied for methanol oxidation to explore the
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effect of promotion by both Ce and Gd on the catalyst stability and the reduction of CO
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formation.
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2. EXPRIMENTAL SECTION
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2.1. Catalysts preparation, ZnO particles were synthesized by the direct-precipitation
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method.[18] Analytical grade Zn(NO3)2 and (NH4)2CO3 (Sigma-Aldrich, 99.5%) were first
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dissolved in high-purity water to form solutions with 1.5 and 2.25 mol/L, respectively. The
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Zn(NO3)2 solution was slowly poured into the (NH4)2CO3 solution with vigorous stirring, and
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then the precipitate derived from the reaction was collected by filtration and rinsed three times
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each with high-purity water and ethanol. The product was dried at 80°C to form the ZnO
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precursor. Finally, the precursor was calcined at 550°C for 2 h in a muffle furnace to obtain
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nanoscale ZnO particles.
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Rare earth metal (20 wt %) (RE = Ce, La, Dy, and Gd) doped-ZnO supports were prepared
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by the dry-impregnation technique. The ZnO particles were impregnated with a salt solution
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containing the appropriate amounts of the rare earth metal nitrates. The resulting slurry was
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dried by continuous stirring and heating at 70°C. The solid obtained was then kept in an oven
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overnight at 110°C, crushed in an agate mortar and calcined for 4 h at 550°C. The products
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were denoted as RE20Zn, namely, Ce20Zn, La20Zn, Gd20Zn, and Dy20Zn.
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Ce1-XGdXOy (x= 0.1, 0.2. 0.3 and 0.5, where x is the atom fraction of gadolinium)-doped
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ZnO with (20 wt % Ce1-XGdXO(4-X)/2) supports were prepared using the carbonate coprecipitation
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method.[19] Each Ce1-XGdXOy solid was prepared following the same procedure below in the
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absence of ZnO to identify its yield (wt). The high-purity (>99.5%) reagents Ce(NO3)3·6H2O and
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Gd(NO3)3·6H2O were used as the starting materials, and ammonium carbonate as the
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precipitation medium. The typical procedure involved the following steps: (1) Ce(NO3)3·6H2O
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and Gd(NO3)3·6H2O were dissolved separate in high-purity water. A 150-mL mixture of Gd3+
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and Ce3+ ions was prepared to be 0.3 M by blending the two solutions. (2) The mixture was 5 ACS Paragon Plus Environment
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slowly added to 150 mL of an ammonium carbonate solution (0.75 M) containing the appropriate
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amount of ZnO, with constant stirring at room temperature. (3) After the precipitation was
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completed, the stirring was continued for 1 h at ca. 70°C. (4) The precipitate was filtered and
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then washed several times with high-purity water and ethanol. Finally, the precursor produced
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was dried at room temperature with flowing nitrogen gas for 20 h and calcined at 800°C for 2 h.
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The solids produced were denoted as Ce0.9Gd0.1Zn, Ce0.8Gd0.2Zn, Ce0.7Gd0.3Zn and
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Ce0.5Gd0.5Zn.
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Two catalyst series were prepared via the dry-impregnation technique as follows: the first
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series was silver supported on REZnO (RE= Ce, La, Dy, Gd) with 5 wt% loading, and the
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second series was silver supported on CexGd1-xZn supports with 5 wt % loading. Each support
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was impregnated with an aqueous solution containing the appropriate amount of silver nitrate
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under continuous stirring for 24 h at 25°C. The silver-loaded samples were washed with
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methanol several times, filtered, dried at 110°C for 12 h and then calcined at 350°C for 4 h. The
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catalysts in the first series were labeled as AgCe20Zn, AgLa20Zn, AgDy20Zn and AgGd20Zn
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and in the second series as Ag/ Ce1-xGdxZn; where x= 0.1, 0.2, 0.3, and 0.5. Silver supported on
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undoped ZnO was denoted as AgZn.
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2.2. Characterization, The silver content in these catalysts was determined by atomic
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absorption spectroscopy (AAS) on a Perkin Elmer model 3100. X-ray diffraction (XRD)
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measurements were performed employing a Philips X’Pert MPD (multipurpose X-ray
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diffractometer) employing Cu Kα1,2 radiation (λ = 1.5405 Å) for 2θ angles varying from 10° to
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80°. H2-TGA analysis was performed using a Shimadzu - Japan Thermal Analyzer model 50
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using a heating rate of 10.0 °C min-1 from 50 to 1000°C with a hydrogen flow rate of 20 mL min-
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. Temperature-programmed reduction (H2-TPR) was performed using a ChemBET 300 6 ACS Paragon Plus Environment
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Quantachrome. A 100-mg sample of the freshly calcined catalyst was subjected to a heat
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treatment (20°C min-1 up to 1000°C) in a gas flow (85 mL min-1) composed of a mixture of
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hydrogen/nitrogen (5/95 vol.%). Prior to the TPR experiments, the samples were heated for 3 h
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under an inert atmosphere (nitrogen) at 200°C. The surface areas (SBET) of the various samples
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were determined from the adsorption of nitrogen gas at liquid-nitrogen temperature (-195.8 °C)
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using a NOVA3200e (Quantachrome-USA). Prior to the measurements, all samples were
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perfectly degassed at 150°C and 10–4 Torr overnight. N2O pulse chemisorption was applied to
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determine the Ag degree of dispersion using ChemBET 3000 and the TPR-Win V. 1.50 software.
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Before chemisorption a pretreatment procedure has been applied: the sample (0.1g) was oxidized
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in flow of oxygen at 300°C for 30 min; purged in He flow at the same temperature for 15 min,
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followed by reduction in 5%H2/He flow (20 ml min-1) at 300°C for 1 h, and then purged in He
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flow (30 ml min-1) at the same temperature for 30 min. The sample was cooled down to 100°C in
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He flow for 15 min at the same temperature for 0.5%N2O/He flow (20 ml min-1) pulse
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chemisorption. N2O pulses were injected in a series of 3-min intervals. The surface area of silver
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was determined considering the stoichiometry of the reaction (2). Scanning electron microscopy
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(SEM) images were obtained with a JEOL JSM-7600F microscope operated at 20 kV.
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Transmission electron micrographs were obtained using a JEOL 1200 EX II transmission
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electron microscope (TEM) operated with an acceleration voltage of 50 kV.
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2.3. Catalytic test, Catalytic tests were performed at atmospheric pressure in a tubular
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quartz reactor (‘6 mm i.d.), which was placed inside a programmable furnace in which the
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temperature was measured by a type K thermocouple located in a pocket in the center of the
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catalyst bed. Pure methanol was fed to a purpose-vaporizer by a Master Flex C/L variable-speed
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tubing pump (1 to 6 rpm) EW-95990-18. The carrier gas, containing oxygen for the partial 7 ACS Paragon Plus Environment
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oxidation experiments, was also fed to the vaporizer. The gas flow was adjusted by a Cole-
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Parmer compact mass flowmeter. All chemical pipes were heated to avoid condensation in the
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system. The feed and the product gas compositions were determined by online gas
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chromatography (GC), using a Bruker 450 GC equipped with three channels. The first is for
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hydrogen analysis using TCD. The gas separation was performed by HayeSep Q and 5 Å
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molecular sieves. Channel two is for analyzing nonflammable gases (viz., O2, N2, CO, CO2)
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using TCD, and separation was accomplished by HayeSep Q and MolSieve 13X columns
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connected in series. The third channel is for analyzing oxygenates (methanol, formic acid and
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formaldehyde), and separation was accomplished by HayeSep Q and Varian select
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formaldehyde. The product composition was measured at 15-min intervals, as determined by the
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duration of the GC analysis, with intermittent increments of the furnace temperature.
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Calibration enabled the quantitative analysis of methanol, hydrogen, oxygen, nitrogen, carbon
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monoxide and carbon dioxide. The rapid ignition of the partial-oxidation reaction complicates
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the time-consuming GC analysis. For all data reported, the reactant and product concentrations
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at the inlet and outlet of the reactor were estimated using a multi-channel computer program.
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The initial concentrations of methanol and oxygen were calculated before each catalytic run to
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verify their constant values with a 99% degree of confidence. The carbon and hydrogen mass
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balances with error were under 2%. For each run, three to five trials were performed. These
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multiple trials were used to estimate the 98% confidence intervals. Typically, 100 mg of the
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catalyst, ground and sieved to 212- 500 µm was used in all experiments. The catalyst was
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diluted with SiO2 to 10 wt% to prevent hot-spot formation in the bed. The catalyst activation
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was performed in situ by exposing the catalyst to 100 mL min-1 of 10% H2/N2, increasing the
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temperature to 250°C at 10°C min-1. This temperature was maintained for 1 h. Subsequently, the
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furnace temperature was lowered to ~100°C. The partial-oxidation experiments were performed
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under a total flow rate of 220 mL min-1 with an O2/methanol molar ratio of 0.5, balanced with
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nitrogen and the weight hourly space velocity; WHSV = 13.2 x 104 mL h-1 g-1 {WHSV= flow
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rate of feed gas (mL h-1) per weigh of catalyst (g)}. The catalytic reaction was performed from
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150-400°C to with seven evenly spaced temperature points, and each point was maintained for
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at least 1.5 h. The product selectivity (%) was defined as the product concentration x 100
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divided by the methanol conversion. The CH3OH conversion (%) = concentration of methanol
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consumed x100/ concentration of methanol in the feed. The product concentrations were
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calculated according to the following equation (1):
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CH3OH (g) + ½ O2
2 H2 + CO2.
(1)
3. RESULTS AND DISCUSSION
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3.1. Textural and structural characterization of the catalysts, The silver contents of the
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RE-doped ZnO-supported catalysts measured by atomic absorption spectroscopy (AAS) are
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summarized in the Table 1. It is clear that the rare earth oxide (viz, Gd, Dy)-doped catalysts
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had nearly the same silver content as the undoped AgZn catalyst. However, the AgLa20Zn and
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AgCe20Zn catalysts had relatively lower silver contents than that of the undoped AgZn
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catalyst. Furthermore, the AAS measurements showed that the measured rare earth metal
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content in each Ag-RE-Zn catalyst is approximately 20 wt %. On the other hand, the BET
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surface areas of the AgREZn samples are smaller than those of ZnO and AgZn.
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The XRD patterns of the nanoscale ZnO and RE-modified ZnO supports are shown in Figure
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1. All of the diffraction peaks of ZnO could be indexed to the hexagonal phase reported in the
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JCPDS file (36-1451). The obtained diffraction peaks of ZnO are intensive and narrow, 9 ACS Paragon Plus Environment
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indicating the well-crystallized nature of the support components. The calculated average particle
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size of ZnO was ca. 35.2 nm, calculated by the Debye-Scherrer formula.[20] The SEM images of
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ZnO (Supporting Information, Figure S1) showed that the sample is composed of discrete
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spherical particles with an average diameter of ca. 28.0 nm. The H2-TGA analysis revealed that
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the synthesized ZnO was inert to hydrogen reduction, as it did not show any reduction peak up to
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700°C (Supporting Information, Figure S2). Furthermore, the 2theta angle (degree) of the CeO2
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phase, i.e., 28.568°, 33.278° and 47.532° (JCPDS file, 81-0792), and of the Dy2O3 phase, i.e.,
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28.667° and 47.803° (JCPDS file, 19-0436), were detected in the diffraction patterns of Ce20Zn
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and Dy20Zn, respectively (Figure 1). However, for the Gd20Zn support, new peaks at the 2theta
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angle (degree) = 28.554°, 33.278°, and 47.526° were detected, which are most likely
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characteristic of a Gd2O3 phase (JCPDS file, 86-2477). It is of special interest to record that, in
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the case of the La20Zn support, only one weak, broad peak characteristic of a La2O3 phase was
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observed (Supporting Information, Figure S3). The absence of the other peaks characteristic of
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La2O3 may be linked to its amorphous nature and/or its high degree of dispersion within the
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solid-support composite system.
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In all XRD patterns of the Ag catalysts, except for those of the ZnO and rare earth oxide
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phases, a peak characteristic of the cubic Ag2O phase was detected at the 2theta angle (degree) =
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38.072° (JCPDS file, 65-3289), and a second peak that is characteristic of the hexagonal Ag2O
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phase (JCPDS file, 42-0874) was detected at the 2theat angle (degree) = 44.275° (Figure 2).
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The typical XRD peaks characteristic of the ZnO and Ag2O phases were observed in all
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patterns of AgCe1-xGdxZn (Figure 3). The fluorite CeO2 phase was also observed at the 2theta
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angle (degree) = 28.582°, 33.087°, 47.529°, 56.603° and 59.152°. We note that the development
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of C-type Gd2O3 in the CeO2 matrix cannot be clearly identified from the patterns due to the
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overlapping of their diffraction lines. Finally, the introduction of gadolinium into the cerium
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oxide lattice resulted in a shift in the diffraction peaks of CeO2 to higher 2theta values as the
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gadolinium content increased. This is indicative of the dissolution of Gd3+ into the cubic fluorite
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ceria lattice. Additional evidence for the formation of a Ce-Gd solid solution is that, with Gd2O3
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doping, the cell parameter “a” gradually increased from 5.40 (AgCe20Zn) to 5.44
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(Ag/Ce0.5Gd0.5Zn) (Table 1). Considering that the ionic radius of Gd3+ (0.105 nm) is larger than
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that of Ce4+ (0.097 nm), the incorporation of Gd3+ into the CeO2 lattice will result in lattice
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expansion. [21-23]
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As shown in Figure 4a, a reduction peak appeared at Tr = 205°C in the TPR curve of the
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AgZn catalyst. This peak indicated the presence of Ag2O in the catalyst.[24] No ZnO reduction
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peaks were observed in this study (up to 600°C). Reduction of the bulk ZnO to metallic Zn is
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thermodynamically feasible, but a temperature as high as 650°C is required.[25] Three reduction
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peaks were observed for the AgCe20Zn catalyst (Figure 4 b), in contrast to the results for AgZn.
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The first peak at 182°C is most likely attributable to the reduction of Ag2O but is shifted to a
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lower temperature by 23 degrees, which indicates that the addition of Ce to AgZn facilitates the
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catalyst reducibility. The second and third peaks at 450°C and 482°C can be ascribed to the
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reduction of the CeO2 surface-capping oxygen.[26] For the AgGd20Zn catalyst, the TPR profile
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(Figure 4 c) shows that the reduction peak corresponding to Ag2O is located at the same
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temperature as that of AgZn. In the reduction profiles of the AgLa20Zn and AgDy20Zn catalysts
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(Figure 4, d and e), only one peak is shown at 251°C. Compared to the results for AgZn, this
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peak can be attributed to the reduction of the Ag+ species because lanthanum oxide and
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dysprosium oxide are nonreducible. [27] The increase in the reduction temperature appears thus to
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be linked with the probable strong interaction involving Ag2O and the RE oxide (viz., La and
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Dy). On the other hand, the presence of Ag splits the reduction peak of the CeO2 species into two
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peaks located at 452°C and 485°C, respectively (Figure 4b), which are much lower than the peak
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observed at 556°C for the neat Ce20Zn support (Supporting Information, Figure S4). In fact,
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many researchers have shown that the presence of noble metals can improve the cerium oxide
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reducibility at lower temperatures, as assisted by hydrogen spillover.
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that, in the case of the AgCe20Zn system, Ag may catalyze H2-spillover on the surface, thereby
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accelerating the reduction of CeO2 surface oxygen.
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Figure 5 shows the H2-TPR profiles of Gd-doped AgCe20Zn catalysts with various atomic
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fractions of Gd. The profile of the unmodified AgCe20Zn catalyst is also included for
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comparison. In the case of the AgCe0.8Gd0.2Zn catalyst, the H2-TPR curve (Figure 5b) exhibited
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only one peak at 189°C. Referring to the H2-TPR profiles of AgZn and AgCe20Zn (Figure 4, a
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and b), this peak should be ascribed to the reduction of surface Ag2O, as facilitated by the
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presence of Ce; however, it became much weaker by Gd doping. Furthermore, the main peak at
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Tr = 189°C disappeared completely when the Gd atomic fraction increased from 0.2 to 0.5
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(Figure 5c). The absence of an Ag2O reduction peak can be attributed to the possibility that the
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Ag+ species in fresh AgCeGdZn catalysts had been completely reduced to metallic Ag by the Ce-
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Gd combination in the preparation, i.e., doping AgCe20Zn with Gd severely facilitates the
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reducibility of the Ag2O species.[10] In view of the TEM results (see Figure 6), the significant
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high reducibility of Ag2O in this catalyst can be attributed to the fact that the addition of Gd to
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AgCeZn decreased the size of Ag particles. Additionally, the presence of Gd (atom fraction=
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0.5) appears to have an effect on the location of the reduction peaks of the CeO2 and their
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intensity (Figure 5c). The two reduction peaks of CeO2 were shifted to higher temperatures, viz., 12 ACS Paragon Plus Environment
[28-30]
We can thus suggest
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523°C and 575°C. This may confirm the strong interaction between CeO2 and Gd2O3. The new
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reduction peak at 315°C appears to be most likely to be assigned to the reduction of the surface
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oxygen of the Ce-Gd solid solution (cf., XRD results).
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TEM micrographs of the AgZn, AgCe20Zn and AgCeGdZn catalysts are shown in Figure 6.
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In the TEM micrograph of the AgZn catalyst both silver and zinc particles gather into larger
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particles, with diameter larger than 10 nm (Figure 6a). However, in the TEM micrograph of
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AgCe20Zn catalyst (Figure 6b), the sintering of Ag particles is somewhat inhibited. Furthermore,
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the micrograph of AgCeGdZn catalyst (Figure 6c) shows smaller particles in highly dispersed
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state, without significant aggregations, and these particles are dispersed on the bulk of CeGdZn
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support with diameter smaller than 10 nm.
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3.2. Catalytic activity: partial oxidation of methanol (POM), The effect of La, Ce, Dy and
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Gd as promoters on the performance of the AgZn catalyst is shown in Table 2. It is evident that
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the activities of the samples under study decrease in the following order: AgCe20Zn>
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AgLa20Zn≈ AgDy20Zn> AgGd20Zn> AgZn. AgLa20Zn has the highest hydrogen selectivity
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among the studied catalysts, i.e., the selectivity is 97.8% but with only 32% methanol
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conversion. The optimal performance in methanol oxidation was achieved by AgCe20Zn, which
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exhibited a hydrogen selectivity of 90.8% with 95.2% methanol conversion. Furthermore, as
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shown in Table 2 most of supports are inactive however, Ce20Zn and Ce0.5Gd0.Zn supports
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exhibited very low activity with a high selectivity to hydrogen.
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Figure 7 shows the effect of the reaction temperature on the methanol conversion over RE-
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doped AgZn. The study was undertaken in the temperature range of 150 and 400°C using the
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O2/CH3OH ratio 0.5. In general, methanol conversion over most of catalysts increased with
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increasing reaction temperature up to 400°C. Obviously, the selectivity toward hydrogen (SH2) 13 ACS Paragon Plus Environment
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increased with increasing temperature and reached a maximum at 350°C followed by a
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significant decrease, especially in the case of the AgDy20Zn catalyst, as shown in Figure 8.
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Generally, a large variation occurred in the catalytic behavior for POM with temperature,
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depending on the type of promoting rare earth oxide used (Figure 8).
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It is well known that a short lifespan limits the practical applications of catalysts. Many
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catalysts have high initial activity but deactivate with reaction time.[31-33] The changes in the
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methanol oxidation performance over RE-doped catalysts with time on stream (TOS) are shown
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in Figures 9 and 10. From these figures, it is clear that AgGd20Zn is the most stable among the
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studied catalysts from the point of view of methanol conversion and H2-yield with TOS.
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One can thus conclude that because Ce is considered an excellent promoter of AgZn catalysts
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for the production of H2, Gd can be considered an effective stabilizing dopant for methanol
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conversion and the H2 yield with TOS. Based on these findings, further study was undertaken to
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investigate the effect of various Gd/Ce atomic ratios on the catalytic behavior of the AgZn
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catalyst system. Table 2 summarizes the results of methanol oxidation on Gd-doped AgCeZn
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with various atomic fractions at 350°C. The presence of Gd in the catalyst resulted in a
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significant decrease in the methanol conversion, not only at 350°C but also at various reaction
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temperatures (Figure 11), without a significant effect on the selectivity to H2 (Figure 12). In this
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catalyst series, the maximum performance was achieved by the Ag/Ce0.8Gd0.2Zn catalyst,
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(selectivity to H2 of 90% at a methanol conversion of 62.2%). More importantly, the stability
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study presented in Figure S5 (Supporting Information) and Figure 13 indicated that the addition
308
of Gd to the AgCeZn catalyst can greatly enhance its lifespan: no further decrease was observed
309
in the catalytic performance with TOS up to 20 h.
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It is clear that the addition of RE oxides (viz., Ce, Gd, La and Dy) to the Ag/ZnO catalyst has
311
a large beneficial effect on the selectivity to H2 in the partial oxidation of methanol. Mo et al.
312
reported for an Ag/ZnO catalyst that methanol undergoes dissociative adsorption to form
313
methoxy (CH3O) species over silver and that the C-H bond in the methoxy group is cleaved to
314
form a hydrogen atom.[10] Consequently, the hydrogen dissociated on silver crystallites is
315
oxidized to water by oxygen adsorbed onto the crystallites (Equation 2), with a loss of hydrogen
316
selectivity:
317
2Hads
+ Oads
H2O.
(2)
318
They claimed that, in the case of a Ce-doped catalyst, CeO2 served as a spillover sink to
319
collect hydrogen dissociated on silver crystallites, and ZnO acted as a porthole for the fast
320
desorption of hydrogen from the catalyst. In the present study, the H2-TPR results confirmed the
321
suggestion of hydrogen spillover from the Ag crystallites to the CeO2 particles where an
322
enhancement in the reducibility of the CeO2 species in the AgCe20Zn catalysts occurred.
323
Moreover, the good catalytic performance of AgCe20Zn may also result because the reducibility
324
of the Ag species on the surface of the AgZn catalyst was greatly enhanced by Ce doping, as
325
revealed by the H2-TPR results.
326
Although the observed enhancement in the activity of the AgCe20Zn catalysts can be
327
ascribed to a hydrogen-spillover phenomenon, this phenomenon still cannot interpret the superior
328
performance of the AgGd20Zn, AgLa20Zn and AgDy20Zn catalysts, as they are nonreducible
329
and/or they have a low affinity for hydrogen. Recently, Hereijgers et al. discussed the effect of
330
promoting an Au catalyst with La2O3 in POM to hydrogen. It was proposed that the acid/base
331
properties of the La2O3 play a crucial role in directing the reaction to hydrogen and suppressing
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332
CO formation.
Considering the analogy between Au and Ag, one can apply similar logic to
333
Au/La2O3 as that in the discussion by Hereijgers et al.
334
Lewis-acid sites of La2O3 in the AgLa20Zn system play a crucial role for stabilizing the methoxy
335
intermediate. It was also proposed that La2O3 as an amphoteric oxide adsorbed CO onto its basic
336
sites and activated it for hydrogenation to methanol on the neighboring acidic sites. Furthermore,
337
Hereijgers et al. [14] suggested that La2O3 facilitated the formation of O surface species, which
338
resulted in an increased activity of the CO oxidation.
[14]
; it can be proposed that the strong
339
The above results demonstrated that AgCeGdZn catalysts have a higher stability than
340
AgCeZn. Recently, it was also found that the addition of Gd to ceria had a positive effect on the
341
stability of Cu/CexGd1-xOy and Pt/CexGd1-xO2 catalysts during the steam reforming of methanol
342
and the catalytic decomposition of methane, respectively.[17,34] It was reported that a Ce-Gd solid
343
solution was created by the substitution of Ce4+ cations with Gd3+cations, and hence, more
344
oxygen vacancies were generated. The oxygen vacancies in the solid then provide a route to
345
increase the metal (Pt,Cu)-support interaction and inhibit metal sintering. Here, the XRD and
346
TEM results confirmed the formation of a Ce-Gd solid solution in the AgCeGdZn catalysts and
347
the presence of both Ce and Gd as modifiers increased the well dispersed silver species over
348
ZnO. Thus, the higher stability of Ag in the Ce-Gd-containing support catalysts may be due to
349
the formation of an Ag-O-Ce bond because this bond may act as an anchor, inhibiting Ag
350
sintering under oxidizing conditions at high temperature.
351
According, to the suggestions of Salazar-Villapando et al. and Trovarelli, .[34,35] it appears
352
that oxygen vacancies at the Ag-O-CeGd interface may also be responsible for improving the
353
mobility of oxygen in the bulk of Gd-doped AgCeZn catalysts, and the high resistance to carbon
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deposition might be caused by the transfer of active O from the carrier to an Ag atom, resulting
355
in carbon oxidation and improved accessibility of CH3OH to the active sites.
356
From the dispersion and activities (yield of H2) it is possible to evaluate relative turn-over
357
frequencies (TOF) for the silver catalysts studied here. TOF defined as mole of hydrogen per
358
exposed silver surface site (mole Ag in catalyst X dispersion) and second. Herein, TOF depends
359
on estimates of dispersion, which are calculated using experimentally N2O chemisorption (Table
360
3). It is clear that the selective POM to hydrogen is a structure-insensitive reaction on RE oxides
361
doped AgZn catalysts where, in the case of AgLa20Zn and AgDy20Zn catalysts TOF increased
362
upon doping AgZn with La and Dy; however, dispersion did not change. In addition, the higher
363
TOF of AgCe20Zn catalyst compared to AgZn catalyst cannot attributed only to increase of Ag
364
dispersion where dispersion did not increase significantly. Furthermore, addition of Gd to AgZn
365
leads to a significant increase in Ag dispersion; however, TOF decreased. It is not surprising that
366
there is no clear correlation between particle diameter and TOF. We plot TOF vs. particle size
367
for Ag particles for catalysts under study (Figure 14) and see no trend between TOF and the size
368
of the Ag particles. The TOFs results discussed above indicated clearly the low structure-
369
sensitivity of POM to hydrogen reaction on rare earths modified-Ag/ZnO catalysts.
370
conclusion confirmed our previous explanations for the higher selectivity of AgREZnO catalysts
371
for POM to hydrogen, which may be results of additional effects (viz., enhancement of hydrogen
372
spillover, increasing reducibility of Ag particles, acid/base properties of the rare earth oxide).
373
Interestingly, chemisorption results (Table 3) shows that the Ag dispersion increased with doping
374
AgCeZn catalyst with Gd which indicating that Ag particle dispersion must play a role in the
375
observed higher stability of AgCeGdZn catalysts. Table 3 shows that the average particle
376
diameter in most of AgCeGdZn catalysts is less than 1.5 nm. This particle size is very important; 17 ACS Paragon Plus Environment
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377
Farmer and Campbell[36] reported that silver atoms in nanoparticles less than 1.5 nm bind more
378
strongly to vacancy sites on unsaturated CeO1.8 than to stoichiometric sites and the sintering
379
rates of these particles on CeO1.8 will much slower than on CeO2.[36,37] Again, our finding that the
380
addition of Gd to AgCeZn catalyst decreased Ag average particles size less than 1.5 nm and
381
consequently increased its stability during POM is consistent with finding of Wang et al. and
382
Farmer et al.[36,37]
383
384
4. CONCLUSIONS
385
The addition of rare earth oxides (RE= La, Ce, and Gd) to ZnO-supported Ag catalyst
386
materials has a large positive effect on the selectivity toward hydrogen, and toward CO in the
387
partial oxidation of methanol. The trend in the H2 selectivity depends on both the degree of
388
reduction of Ag2O and the Ag/RE-ZnO interaction. The introduction of ceria into the ZnO-
389
supported catalysts is most beneficial for methanol oxidation to hydrogen over Ag/RE-ZnO
390
catalysts: 90.8% hydrogen selectivity with 95.2% methanol conversion was achieved over an
391
AgCe20Zn catalyst at 350°C with a weight hourly space velocity of = 13.2 x 104 mL h-1 g-1.
392
However, the AgCe20Zn catalyst showed a low stability during the reaction time on stream. The
393
addition of a slight amount of Gd to AgCeZn improved the catalyst stability for at least 24 h on
394
stream. Here, it is suggested that the formation of an Ag-O-CeGd bond caused the high stability
395
of Ag in the Ag/CeGdZn catalysts because this bond may act as an anchor, inhibiting the
396
sintering of Ag. The Ag/Ce0.9Gd0.1Zn-based catalyst can be regarded as a promising material for
397
H2 generation by the partial oxidation of methanol under severe reaction conditions because it
398
generated a lower amount of CO (~0.2%). 18 ACS Paragon Plus Environment
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ACKNOWLEDGMENTS
399
400 401 402 403
The financial support via research grant (PE-29-126) by King Abdul-Aziz City for Science and Technology is gratefully acknowledged. The principal investigator thanks Dr. Tiancun Xiao, inorganic chemistry laboratory, Oxford University for reviewing the language of a portion of this work.
404
405
Supporting Information Available
406 407 408
SEM Images, H2-TGA curve of the ZnO and XRD patterns of RE-ZnO supports in the 2-theta values range from 10-35°.This information is available free of charge via the Internet at http://pubs.acs.org/.
409 410
AUTHOR INFORMATION Corresponding Author
411 412
31982 Al-Hasa. Al-Hofof; P.O. Box 380, Saudi Arabia. Tel.: +966 (3) 5800000(1854), +96654788700, Fax: +966-(3)-5801243E-mail:
[email protected] 413 414
Permanent address: 11566 Cairo, Egypt;Tel.: +202-4831836- 108, Fax: +202 – 5255657. E-mail:
415
416 417
ASSOCIATED CONTENT
[email protected] REFRENCES
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Decomposition and Catalytic Partial Oxidation of Methane Over Pt/CexGd1 − xO2 and
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Pt/CexZr1 − xO2. Chem. Eng. J. 2011, 166, 738.
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(35) Trovarelli, A. Catalytic Properties of Ceria and CeO2-Containing Materials. Catal. Rev.Sci. and Eng. 1996, 38, 439. (36) Farmer, J. A.; Campbell C. T. Ceria maintains smaller metal catalyst particles by strong metal-support bonding. Science, 2010, 329, 923. (37) Wang, J. H.; Liu, M . L.; Lin, M. C. Oxygen reduction reactions in the SOFC cathode of. Ag/CeO2. Solid State ion. 2006, 177, 939.
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504 505
Table 1.
506 507
Overview of the structural properties of the investigated rare earth oxide-modified Ag-based catalyst materials, including Ag loading, surface area of the catalysts, Ag and cell parameters. catalyst
Ag[a]
SBET /m2g-1[b]
RE2O3[c]
(w/w%)
508 509
Cell parameter a/nm
ZnO
-
38.8
-
AgZn
4.44
33.3
-
AgLa20Zn
3.95
24.3
-
AgDy20Zn
4.41
24.9
-
AgCe20Zn
3.68
26.6
5.4031
AgGd20Zn
4.37
27.7
5.4506
Ag/Ce0.9Gd0.1Zn
4.12
16.7
5.4047
Ag/Ce0.8Gd0.2Zn
4.01
17.1
5.4193
Ag/Ce0.7Gd0.3Zn
4.07
17.4
5.4257
Ag/Ce0.5Gd0.5Zn
3.93
14.3
5.4362
[a] From AAS measurements, [b] from N2-adsorption data, and [c] from XRD data after Rietveld analysis.
510 511 512 513 514 515 516 517 518 519
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520
Table 2.
521 522
Performance of the rare earth metal-doped Ag/RE-ZnO catalysts at 350°C and after ca. 6 h on stream. Selectivity (%) Methanol conversion (%)
Hydrogen
CO
CO2
AgZn
6.4
87.8
6.8
5.4
AgLa20Zn
32.5
97.8
0.1
2.1
AgDy20Zn
33.2
88.1
3.3
8.6
AgCe20Zn
95.3
90.9
3.1
5.0
AgGd20Zn
31.9
85.2
5.3
8.5
Ag/Ce0.9Gd0.1Zn
32.9
96.1
0.2
3.7
Ag/Ce0.8Gd0.2Zn
62.2
90.0
1.2
8.9
Ag/ Ce0.7Gd0.3Zn
32.1
82.6
3.2
14.4
Ag/ Ce0.5Gd0.5Zn
41.2
92.7
4.3
3.2
0
-
-
-
Ce20Zn
4.1
85.9
2.8
11.3
Ce0.5Gd0.5Zn
2.7
87.5
2.7
9.8
Catalyst
Supports†
523
†
Supports = ZnO, La20Zn, Dy20Zn, Gd20Zn
524
525
526
527
528 529 25 ACS Paragon Plus Environment
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530 Table 3 531 Chemisorption data and turn over frequency calculated for catalysts under study.
Catalyst
Dispersion (%)
Particle size (nm)
Metal surface area/gmetal
TOF(sec-1) x 102
(m2/g) AgZn
37.7
3.12
183.2
8.4
AgLa20Zn
36.9
3.19
179.3
54.5
AgDy20Zn
36.1
3.26
175.61
45.9
AgCe20Zn
42.2
2.79
204.9
139.3
AgGd20Zn
50.1
2.36
243.39
31.0
Ag/Ce0.9Gd0.1Zn
88.5
1.33
429.57
21.7
Ag/Ce0.8Gd0.2Zn
91.5
1.29
444.4
38.1
Ag/ Ce0.7Gd0.3Zn
84.4
1.40
409.66
19.3
Ag/ Ce0.5Gd0.5Zn
76.1
1.55
369.35
31.9
532
533
534
535
536
537
538
539
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540 541 542
Figure 1. Powder X-ray diffraction patterns of ZnO and of La, Ce, Dy, and Gd-doped ZnO supports. Peaks marked by the symbols "o", "*", "□" and "+" indicate the peaks assigned to ZnO, CeO2, Dy2O3 and Gd2O3, respectively.
543 544
Figure 2. Powder X-ray diffraction patterns of AgZn and of La, Ce, Dy, Gd-doped ZnO supported silver catalysts.
545 546 547
Figure 3. Powder X-ray diffraction patterns of Ag/Ce1-xGdxZn catalysts. Peaks marked by the symbols "o", "*", "■" and "+" indicate the peaks assigned to ZnO, CeO2, Gd2O3 and Ag2O, respectively.
548 549
Figure 4. Temperature-programmed reduction profiles of AgZn and of La, Ce, Dy, Gd-doped ZnO-supported silver oxide catalysts.
550
Figure 5. Temperature-programmed reduction profiles of the Ag/Ce1-xGdxZn catalysts.
551
Figure 6. TEM micrographs of the catalysts: (a) AgZn, (b) AgCe20Zn and (d) AgCeGdZn.
552
Figure 7. Methanol conversion over an undoped AgZn catalyst and a doped Ag/(RE-ZnO)
553 554 555
catalyst as function of the temperature. Figure 8. Hydrogen selectivity over an undoped AgZn catalyst and a doped Ag/(RE/ZnO) catalyst as function of the temperature.
556 557
Figure 9. Time course of the methanol conversion over an undoped AgZn catalyst and a doped Ag/(REZnO) at 400°C.
558 559
Figure 10. Time course of the hydrogen yield over an undoped AgZn catalyst and a doped Ag/(RE-ZnO) catalyst at 400°C.
560
Figure 11. Methanol conversion over Gd-doped AgCeZn catalysts as function of the temperature.
561
Figure 12. Hydrogen selectivity over Gd-doped AgCeZn catalysts as a function of the temperature.
562
Figure 13. Time course of the hydrogen yield over Gd-doped AgCeZn catalysts at 400°C.
563
Figure 14. TOF vs. Ag particle size for catalysts under study.
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Figure 1
570 571 572 573 574 575 576 577 578 579 580 581 28 ACS Paragon Plus Environment
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582 583
Figure 2
584
585 586 587 588 589 590 591 592 593 594 595
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Figure 3
598
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Abstract Graphic
AgCe0.1Gd0.9Zn
0.2%
AgCeZn
3.1%
AgZn
6.8%
H2 selectivities of both Ag/ZnO and Ag/CeO2-ZnO catalysts deactivate with increased time on stream at 350°C. The addition of Gd to AgCeZn improves the H2 selectivity and the catalyst stability under severe reaction conditions. An AgCeGdZnO catalyst also exhibits low CO selectivity.
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