2564
The Journal of Physical Chemistry, Vol. 82,
No. 24,
Seiyama et al.
1978
Study of Metal Oxide Catalysts by Temperature Programmed Desorption. 4. Oxygen Adsorption on Various Metal Oxides Masakazu Iwamoto,t Yukihiro Yoda, Noboru Yamazoe, and Tetsuro Seiyama" Department of Materials Science and Technology, Faculty of Engineering, Kyushu University, Higashi-ku, Fukuoka 8 12, Japan (Received May 23, 1978) Publication costs assisted by Kyushu University
Oxygen adsorption properties of 16 metal oxides were investigated by means of a temperature programmed desorption (TPD) technique. Although oxygen adsorption phenomena were largely different depending on metal oxides, it was possible to classify the oxides as follows: (A) V205, MOO,, Bi203,WOs, and Bi203.2Mo03 which exhibit no oxygen desorption over the range 10-560 "C; (B) Cr203,MnOz, Fez03, Co304,NiO, and CuO which always give relatively large amounts of oxygen desorption; and (C) Ti02,ZnO, SnOz, A1203, and SiOz for which evacuation at high temperature followed by oxygen adsorption at relatively low temperature is required for oxygen desorption to appear over the range 10-400 "C, except the last two oxides. It is noted that group A oxides have layer structures except for BizOs,while group B consists of oxides with cations of d1-d9 electronic structures. Among the oxygen species adsorbed on group C oxides, the 02-ion was directly identified by means of ESR spectroscopy, and was assigned to specific desorption peaks of respective TPD spectra. The amounts of desorbed oxygen (V,,,) for group B and C oxides were only a few percent of the surface coverage, suggesting that the adsorption sites are some sorts of surface defects. There was a fairly good correlation between V,,, and the heat of oxide formation per g mol of 0 (-AHf"),and the amount of adsorbed oxygen tended to decrease with increasing -AHf". These results are discussed in relation to the catalytic properties of the oxides.
Introduction A survey of a large number of catalytic olefin oxidation reactions shows that catalytic activity of metal oxides can be classified into several patterns nearly independent of the molecules being For example, transition metal oxides tend to catalyze only deep oxidation while some other oxides catalyze selective oxidation. It is natural to ask if these patterns are related to the kinds of oxygen species involved, e.g., lattice oxygen and adsorbed oxygen. It is also of interest to know whether the oxygen species exist in discrete states or with a broad distribution of energies. In our previous papers, oxygen adsorption on nickel oxide,3 ferric oxide: and Cu(I1) ion-exchanged Y type zeolite5 has been studied by means of a temperature programmed desorption (TPD) techniquee6 The T P D technique has proved to be very useful for investigating oxygen adsorption on the solid surface. The types of oxygen adsorbates and their properties, such as binding energies, populations, and reactivities, have been elucidated. In the present paper, we have applied the same technique to oxygen adsorption on various metal oxides. Of surface oxygen species, one that is located a t the normal lattice position and is formally represented as 02is termed "lattice oxygen", and the others are termed "adsorbed oxygen". Interactions between oxygen and metal oxides have been studied fairly extensively by a number of workers with techniques such as adsorption kinetics, ESR, and thermodesorption. Several types of adsorbed oxygen have been proposed over various metal oxides, including 02-and 0- which have been confirmed by ESR.7 Recently, Halpern and Germains have examined the desorption of oxygen from first transition metal oxides by means of a flash desorption technique. However, the limitation of the method, Le., very rapid and nonlinear heating (ca. 30 "C/s on the average), prevented t,hem from investigating the phenomenon in sufficient detail. The t Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan.
0022-3654/78/2082-2564$0 1 .OO/O
purpose of the present study is to elucidate the formation and nature of several modes of oxygen adsorbates on the surface of 16 metal oxides. It also attempts to seek correlations between the oxygen adsorption properties and the catalytic properties of metal oxides.
Experimental Section Apparatus. The T P D apparatus used in this study was essentially the same as one reported in the previous s t ~ d y . ~ It consisted basically of two parts; one part in which sample conditioning and oxygen adsorption under specified conditions were undertaken in a conventional manner, and another part in which adsorbates on the sample were allowed to desorb thermally into a helium carrier under programmed heating. The concentration of the desorbed gas was monitored and recorded with a thermal conductivity detector. Materials. Helium gas of ultra-high-purity grade (> 99.995%) was supplied from Air Products and Chemicals. Before use, helium was passed through a liquid nitrogen cold trap in order to remove trace amounts of moisture. Oxygen gas was also dried before use by liquefaction and vaporization in the cold trap. Metal oxide samples were prepared by calcining metal salts or hydroxides, or were commercial materials of guaranteed grade. Catalysts were calcined in air a t 600 "C for 5 h before being mounted in the T P D cell, except for NiO which was obtained by decomposition of nickel carbonate in vacuo a t 600 "C for 17 h. Preparation methods of the 16 metal oxides are summarized in Table I with their BET surface area and color. All the samples were confirmed to be pure by X-ray powder diffraction. They were sieved to 20-60 mesh for use in T P D experiments. Methods and Procedures. Metal oxide, 1 g, was loaded into the T P D cell. For sample conditioning, every fresh sample was evacuated for more than 2 h a t 600 "C until no more water was condensed in a liquid nitrogen trap connected to the cell. The sample was then repeatedly 0 1978 American Chemical Society
Oxygen Adsorption on Various Metal Oxides
The Journal of Physical Chemistry, Vol. 82,
No. 24,
1978
2565
TABLE I: Preparation and Properties of Metal Oxides Used ~~
~~
oxide A1203
SiO, T i 0,
v*o, Cr203 MnO,
c0304
NiO CUO ZnO MOO, SnO, Bi,O,
wo,
Bi,O,.2MoO
surface area, m'lg
preparation methodsa
,
A1(NO3),.9H,O + Al(OH), commericial material (Guaranteed Grade) TiCl, + Ti( OH), NH4V0, commercial material (Guaranteed Grade) Mn(N0,),.6H20 --> Mn(OH), Fe(NO,),.SH,O -+ Fe(OH), CO(NO,),*~H,O -+ Co(OH), Ni(N0,),.6H20 -+ NiCO,b Cu(NO,), -+ Cu(OH), commercial material (Kadox 25)c (NH,),Mo,O,,~~H,O+ H,MoO,.H,O SnCI,.3H,O -+ H,SnO,,nH,O Bi(NO,),.5H,O -+ Bi(OH), commercial material (Guaranteed Grade) (NH4),Mo,0,,~4H,O + Bi(N0,),.5H20d
156.0 278.0 10.3 1.2 1.9 2.2 5.5 5.7 11.4 0.17 5.5 1.0 16.1 0.5 1.8 1.1
color white white white rust-brown dark green black reddish brown black yellowish green black white slightly yellow slightly gray yellow yellow yellow
0
type
anatase
0
type
NiO was prepared by decomposition of NiCO, in vacuo at a These materials were calcined in air at 600 'C for 5 h. The preparation method was mentioned in J. 600 "C for 1 7 h. Kadox 25 was supplied from New Jersey Zinc Co. Catal., 24, 76 (1972).
subjected to a series of routine TPD procedures composed of sample pretreatment, oxygen adsorption, and temperature programmed desorption. As pretreatment, each sample was exposed to 100 torr of oxygen for 1h followed by evacuation torr) for 1h at 600 "C. In succession, oxygen was adsorbed usually in either of the following two ways. Procedure I. On introduction of 100 torr of oxygen a t 600 "C, the sample was cooled a t a rate of ca. 10 "C/min to 10 "C, a t which temperature the system was evacuated for 15 min. Procedure II. After cooling to a desired temperature in vacuo, the sample was exposed to an oxygen atmosphere (100 torr) for 1 h and then evacuated for 15 min before further cooling to 10 "C within 30 min in vacuo. After these manipulations, a carrier gas (helium) was diverted to flow through the reactor a t a rate of 30 cm3/ min and the programmed heating was started. The heating rate was 20 OC/min unless specified otherwise. The thermally desorbed oxygen was recorded as a T P D chromatogram and its amount was evaluated from the area under the desorption peaks. The monitored gas was identified to be oxygen alone throughout the experiments by gas chromatography and mass spectroscopy. ESR spectra of oxygen adsorbed on metal oxides were recorded in the X-band region a t room temperature with a Hitachi 771 spectrometer equipped with a TElo2mode cavity. Before recording, oxide samples were treated in the same way as above in a 4-mm 0.d. Pyrex glass tube that was thin enough for insertion into the microwave cavity.
Results 1. Oxygen Desorption f r o m Various Oxides. T P D chromatograms of oxygen from 16 metal oxides showed a very wide variety. Some oxides exhibited no significant desorption, while other oxides showed complex desorption depending on the experimental conditions. For convenience, the results are described by classifying the oxides into three groups. (A) V,05,Moo3, BizO3, W03,and BizO3-2MoO3. These oxides gave no significant desorption in the range 10-560 "C, after oxygen preadsorption either by procedure I, or by procedure I1 a t various temperatures between 10 and 400 "C. The formation of 02-and 0- on partially reduced VzO5 supported on silica gel has been reported by Kazansky et al.9 and later by Yoshida et a1.l0 We also found that silica gel supported V205 (5 wt '70)which had been
- 15- --AI&
SI02 ~r203
> E v
*
- - NIO
05I-
W 0
0
200
400
6C
TEMPERATURE ("C) Flgure 1. TPD chromatograms of oxygen from various metal oxide samples after oxygen adsorption by procedure I.
partially reduced with hydrogen a t 500 "C exhibited a broad desorption peak ranging from 100 to 500 "C following oxygen preadsorption by procedure I, and that the corresponding oxygen adsorbates could be assigned mainly to 02-by ESR. Similar attempts were made for silica gel supported Moo3, Bi2O3, and Bi203.2Mo03(10 wt YO),but no oxygen desorption other than the one desorbing from the support itself was detected. (B) Cr2O3,MnO,, Fez03, Co304,NiO,and CuO. This group comprises transition metal oxides. T P D spectra after oxygen preadsorption by procedure I are shown in Figure 1. The number of T P D peaks as well as the amounts of desorbed oxygen vary with the kind of metal oxide. For the same metal oxide, however, desorption temperatures were little affected by sample preparation methods, though desorbed amounts changed considerably as described previously in the cases of Ni03 and Fe203.4 Similar facts were recently reported by Ekern and Ckandernall for oxygen desorption from silver filaments. It is inferred that the strength of adsorptive bonds is unique for each metal oxide, while the number of adsorption sites can be altered by the sample preparation method. T P D spectra from an oxide can be changed drastically when different conditions are adopted for oxygen preadsorption. Such changes have been reported in detail on Ni03 and Fe203.4 Figure 2 shows T P D spectra from M n 0 2 obtained after oxygen preadsorption by procedure
Seiyama et al. h
>
.g
15
AD TEMPPC)
210-3J 2-w
2In 8! 8
zE
TABLE 11: Desorption of Oxygen from Metal Oxides
s
600-10
-1
A
400
group A
150 5------- 10 054--
w 0-,
r\
a ,
,
/
-
