КИНЕТИКА И КАТАЛИЗ, 2011, том 52, № 6, с. 856-863
UDC 541.123:546.762763-31
THE INFLUENCE OF ADDITION OF SILVER AND COPPER ON THE REDUCIBILITY OF CrAl3O6 SYSTEM
© 2011 T. P. Maniecki, P. Mierczynski*, W. Maniukiewicz, W. K. Jozwiak
Technical University of Lodz, Institute of General and Ecological Chemistry, Poland *E-mail: mierczyn25@wp.pl Received 20.01.2010
The influence of addition of silver and copper on the reduction behavior of binary oxide CrAl3O6 was investigated. The formation of copper chromite CuCr2O4 and silver chromate Ag2CrO4 during calcinations process was observed. The intermediate phase CrO was detected when copper and silver-copper systems were reduced at temperatures above 500°C. This intermediate is formed from Cr2O3, which is yielded by the initial
2_
reduction of &1О4 species.
Chromium oxide catalysts play an important role in many industrial processes and they are extensively used in many reactions such as oxidative dehydrogenation of isobutane [1—3], selective catalytic reduction of NOX with ammonia [4, 5] and polymerization [6, 7]. The formation of chromia, chromates and/or chromites is possible because chromium ions occur in a varying state of oxidation. It is well known that copper [8] and zinc [9] chromites are widely used in many industrial processes such as production of methanol, fatty ester and higher alcohols.
Many chromium compounds like CrO3, Cr(NO3)3 • • 9H2O, CrCl3 • 6H2O, Cr(OH)3, Cr2O3 are used as catalyst precursors. Chromia-supported catalysts are usually prepared from chromium trioxide. The surface of CrO3 catalysts treated in oxidative atmosphere contains
chromate-like species Cr2O4 . Specific active sites on chromia based catalysts are needed for certain catalytic reactions, e.g. water gas shift reaction. According to predictions and direct observations, chromium occurs on the surface of chromia-supported catalysts in different oxidation states ranging from Cr(II) to Cr(VI) with specific species depending upon Cr loading, preparation procedure and treatment conditions [10—14].
In this work, the silver-chromium, copper-chromium, chromium-aluminum dioxides and copper and/or silver-copper supported catalysts were prepared. The XRD in situ analysis in a stream of5% H2 + 95% Ar and the temperature-programmed reduction were used to inquire into the mechanism of reduction of CrAl3O6, copper and silver-copper supported catalysts. This work is aimed at the elucidation of the role played by silver and copper additives in the reduction of CrAl3O6 binary oxide.
Статья публикуется на английском языке в авторском варианте.
EXPERIMENTAL
Preparation of Catalysts
Catalysts were prepared according to the wet impregnation procedure. Chromium and aluminum nitrates were used to prepare the CrAl3O6 binary oxide. The mixture of chromium and aluminum hydroxides with an Al/Cr molar ratio of 3 was obtained by co-precipitation with ammonia, it was then filtered, washed out, dried and finally calcined at 400°C in air for 3 h. The chemical composition of the binary oxide can be given as CrAl3O6 (^BET = 157 m2/g, predominant pore size is around 2 nm). Copper and silver phase was introduced on support according to the wet impregnation procedure making use of an aqueous solution of the corresponding nitrate. The supported catalysts were then dried and finally calcined in air at 400 and 700°C. The metal loaded catalysts had compositions like 5, 10 or 20% Cu on a Al3CrO6 support, and 5% Ag, 1% Ag + + 20% Cu or 5% Ag + 20% Cu on a CrAl3O6 support. Silver-chromium and copper-chromium materials with the molar ratios Ag/Cr = 2 and Cu/Cr = 1 were prepared from the mixture of silver or copper nitrate and chromium(VI) oxide solutions by drying and calcining the mixture in air at 400°C for 3 h.
Characterization of Catalysts
Temperature programmed reduction (TPR-H2). The
TPR-H2 measurements were conducted using the automatic TPR system AMI-1 over the 25 to 900°C temperature range with the linear heating rate of 10°C/min. Samples (ca 0.1 g) were reduced in a stream of 40 cm3/min hydrogen (5% H2 + 95% Ar). Hydrogen consumption was monitored by a thermal conductivity detector.
XRD measurements. Approximately 150 mg of the sample crushed in agate mortar was loaded in the sam-
ple holder made of glass ceramics (Macor). The gas mixture 5% H2 + 95% Ar was used as a reducing reagent for the binary oxide support CrAl3O6. The sample was heated at a rate of 2°C/min. The high temperature wide-angle X-ray diffraction data were collected using a PANalytical X'Pert Pro diffractometer equipped with an Anton Paar XRK900 reactor chamber with 50°C steps from 50 to 850°C.
A PANalytical X'Celerator detector based on Real Time Multiple Strip technology capable of simultaneously measuring the intensities in the 29 range of10— 90° was used.
Powder X-ray diffraction patterns were obtained at room temperature using a PANalytical X'Pert Pro MPD diffractometer in Bragg—Brentano reflecting geometry. Nickel filtered CuZ"a radiation from a sealed tube was used. The samples were scanned at a rate of 0.0167° per step over the 5° < 29 < 90° range with a scan time of 27 s/step. Because raw diffraction data contain some noise, the background was subtracted during the analysis using Sonneveld and Visser algorithm [15] and the data were smoothed using cubic polynomial [16]. All calculations were done with X'Pert HighScore Plus computer program [17].
RESULTS AND DISCUSSION
Phase composition
XRD patterns for the reference samples (silver-chromium and copper-chromium binary oxides) along with the samples of supported catalysts (20% Cu/CrAl3O6, 5%Ag/CrAl3O6 and 5% + 20%Cu/CrAl3O6) are given in Figs. 1—4. The patterns were acquired by in situ measurements of the samples subjected to the temperature-programmed reduction. The in situ XRD technique was used to study the composition of the samples as a function of the reduction temperature.
XRD in situ analysis indicated that only the peaks corresponding to the Ag2CrO4 phase [18, 19] are observed for the reference silver-chromium binary oxide reduced at 25 and 140°C. The results demonstrate that this phase is stable and cannot be reduced in these conditions. In the X-ray pattern recorded at 170°C the silver chromate as the main phase and metallic silver as a minor phase can be detected. For the samples treated at 200°C the main peaks consistent with the metallic silver phase develop and a diffraction peak at 29 « 31° attributable to the silver chromate phase is also observed. Disappearance of the silver chromate phase at a reduction temperature of 230°C can be explained by reduction of Ag2CrO4 and formation of the Cr2O3 phase that is amorphous to X-rays. In the range from 230 to 500°C the reduction can occur by reactions like
A possible mechanism governing the reduction is suggested by the scheme given below [20, 21]. At the
initial step of the reduction process, surface Cr2O4 species are formed on the support. At the next step, two terminal oxygen atoms are removed, resulting in an intermediate structure with chromium bonded to the support via two oxygen atoms that gives Cr (II) O^ species. During the reduction water is formed as a product of the interaction with hydrogen implying the oxidation of Cr(II) to Cr(III). To explain the apparent discrepancy, we suggest that a hydroxyl group may be involved in the
reduction of the isolated tetrahedral CrO4 species by hydrogen (see the scheme).
w
Cr(VI)
O sO
H
I
O
+ 1.5H
2
,Cr(III) + 2H2O
O O O
2Ag2CrO4 + 5H2 ^ 4Ag + + Cr2O3 (amorphous) + 5H2O.
(I)
Scheme.
The presence of reflections due to metallic silver and crystalline a-chromia on the XRD pattern recorded at 500°C confirms our suggestion providing evidence in favour of the conclusion that the amorphous to X-rays phase of Cr2O3, which is not observed in the temperature range 230—500°C, transforms to crystalline a-Cr2O3 phase during the sintering process.
Based on the XRD in situ study of copper-chromium dioxide (the Cu/Cr molar ratio of 1) that served as a reference material, reflections from the copper chromite [22, 23, 24] phase are observed at the onset of the reduction process at temperatures ranging from 25 to 170°C. Therefore, the reference material at this temperature contains an unreducible copper chromite phase. An increase in the reduction temperature results in formation of a-Cr2O3 phase as indicated by corresponding XRD peaks discernable above 500°C. The source of this phase is believed to be a previously reduced copper chromite species:
CuCr2O4 + H2 ^ Cu + Cr2O3 + H2O. (II)
The reduction of copper chromite also occurs by a mechanism that is similar to this for silver chromate phase outlined above. However a further increase in the reduction temperature leads only to the growth of the crystallites in the sample.
The results of the detailed XRD in situ study of CrAl3O6 were reported in our earlier work [18]. X-ray data showed the presence of chromium-alumina dioxide and chromium(III) oxide phases in the sample which when reduced at temperatures >400°C yields the crystalline a-Cr2O3 phase.
The 20% Cu/CrAl3O6 supported catalysts showed reflections corresponding to the copper chromite, binary oxide, copper oxide, metal copper and a-Cr2O3. The phase composition of the catalysts depends on the reduction temperature. The CuCr2O4, CrAl3O6 and CuO phases detected up to 300°C confirm the results obtained for the reference materials. Increase in the re-
\ Ag ▼ Ag2CrO4 ^ a-Cr2O3
200°C
fT Ir TV I T I
w'^LwuiLJu* 170°C
140°C
25°C
~l I I I T
20 25 30 35 40 45 50 55 60 65 29, degrees
800°C
590°C
230°C
~1-1-1-1-1-T-r-1-r
20 25 30 35 40 45 50 55 60 65 29, degrees
Fig. 1. X-ray powder patterns recorded during reduction of Ag2CrO4 at temperatures from 25 to 800°C.
duction temperature changes the phase composition. Treatment with hydrogen at 300°C and higher temperatures leads to formation of the metallic copper from copper chromite [22—24] and copper ox
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