TiM - o-2. -li. Dioxymethylene species have been observed following formaldehyde adsorption .... V.A. Likholobov, V.N. Parmon and K.I. Zamarev, Russ. Chem.
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Surface Science 262 (1992) 113-127 North-Holland
W.Idriss,
KS. Kim 1 and M.A. Barteau
,:,:,,,,,
,,
surface scienk
,..:.:::::: M.:..: ....,,.,. ,.,..... :i.:.:.~,~:+: ~.“““.VA .‘.‘.i .. ..........._y.,.:.: peak-to-peak ratio of ca. 1.65-1.70, typical of a fully oxidized TiO, single-crystal surface (32-341. AES measurements were conducted by using a 2 keV electron beam at 2 mA emission current. The electron beam was defocused on the sample at a beam current of 0.1 PA in order to avoid surface reduction caused by electron beam damage. The TiO, sample was mounted by means of a sample holder made of tantalum foil (0.127 mm thick), spot-welded to 0.5 mm tantalum wires, The tantalum wires were spot-welded to two molybdenum rods of a rotatable sample manipulator. Cooling of the sample was obtained via conduction by contacting the sample holder assembly with a liquid nitrogen reservoir. The lowest temperature that could be reached with this sample mount in the VG ESCALAB was 250 K; a few spectra were obtained at lower sample temperature with the PHI model 550 system described previously [331. The sample temperature was monitored in all cases by means of a chromel-alumel thermocouple attached to the side of the sample using high-temperature cement (Aremco Ultra-Temp 516). AES and XPS measurements showed that no migration of the cement occurred during heat treatment of the sample throughout the experiments.
116
H. Id&s et af. / Formaldehyde oxidation and reduction on TiO,(oOl)
Formaldehyde was produced by thermal decomposition of paraformaldehyde (Aldrich), contained in a glass sample tube attached to the dosing manifold= Prior to use, parafo~aldehyde was degassed at ca. 350 K white pumping. The reactant vapors were dosed onto the single-crystal sample through variable leak valves equipped with stainless steel dosing needles. Dosing was conducted both at room temperature and at lower temperatures achieved by cooling with liquid nitrogen A typical XPS experiment consisted of an initial exposure of the sample using the same procedure as that for the TPD’ experiments. After pump-down, the sample was positioned under the X-ray source and the spectrum was collected. During XPS data collection the Ti(2p), 00s) and C(ls) regions were scanned over a 30 min period for each set. For experiments in which XPS spectra were cohected as a function of temperature, the first set in the series was collected as described above; the sample was then heated to the desired temperature, held at that temperature briefly (less than 5 s) and then allowed to cool to the original dosing temperature before additional spectra were collected. A heating rate of 1.2 K/s was used in TPD experiments and during the intermittent heating sequences in XPS experiments. All TPD and XPS data reported in this study were obtained following saturation exposures of formaldehyde.
3. Results
The surfaces of TiO, single crystals have been examined by several groups in order to determine surface structures, stabilities, the nature and concentration of defects, and the oxidation states of Ti cations. On a TiO,(flO) single crystal it was observed 135,361 that the ion-bombarded surface contained reduced Ti cations (in lower oxidation states than + 4). Similar results were obtained on sputtered TiO,(OOl) single-crystal surfaces [32,331. Upon annealing, oxygen diffusion from the bulk to the surface occurs I32.331 restoring the surface
Ti cations to their highest oxidation state (i-4). However while the (110) surface does not reconstruct 131,323 and is therefore considered a stable the~odynami~ structure, the Ti~~~OO~) surface rearranges [33,34] to give two stable faceted structures: a {Oll&faceted structure obtained by annealing the surface at ca. 7.50 K and a {114)faceted structure obtained by annealing the surface to 950 K. In the {Ollj-faceted structure, faceted structure, Ti cations are in the +4 oxidation state and are all fivefold coordinated, while in the ill4)-faceted structure Ti cations are four-, five-, and six-fold coordinated [32,33]. Ideaify, Ti cations in the bulk rutile structure are sixfold coordinated. Our investigation by angle-resolved XPS and UPS of the effects of different pretreatments on the degree of oxidation of the TiO,(OOl) surface will be discussed in future work f38], however we present in fig. 1 spectra of the Ti(2p) core levels, obtained after ion ~mbardment (fig. la), after annealing at an intermediate temperature (ca. 650 K) for 20 min (fig. lb), and for the fully oxidized surface, obtained by annealing the surface at 750 K for 20 min (fig. Ic). Fig. la presents the Ti(2p) core levels of a TiO~~~l) single crystat after argon ~mbardment for 1 h. This figure indicates that the surface was partially reduced. In agreement with earlier work on TiO,(OOl) and TiO,(llO) single crystals, several titanium oxidation states lower than t-4 are present in addition to the Ti” state that gives rise to the Ti(2p,/,) peak at 459.1 eV. The lower binding energy side of Ti(2p,& region exhibits severaf peaks which can be assigned to Ti3’ (457.6 eV>, Ti’* (456.1 eV) and Ti”+ at 454.9 eV (0 cx < 2). This last oxidation state is higher in binding energy than Tie (the TiUp,,,) line of Ti’ is at 454 eV [39,40] and the binding energy difference between Ti4+ and Tie is close to 5.2 eV 139,403) The peak at 454.9 eV can be assigned to Til+ cations, since it is higher in binding energy than Ti’ (4.53.9-454 eV) 1401and Iower than Ti” (456.1 eV) [39]. Annealing the surface to 650 K decreased dramaticahy the population of Ti cations in lower oxidation states than Ti4+ in the vicinity of the surface; this is shown in fig. lb by the decrease in intensity of the lower binding
117
H. Idriss et al. / Formaldehyde oxidation and reduction on Ti02Co01J
the Ti cations, and since this surface may exhibit a tendency to form different surface intermediates than the {OllI-faceted TiO,(Ol) surface (where Ti cations are only in the +4 oxidation state), we investigated the reactivity of formaldehyde by TPD on the sputtered surface, as well as on surfaces annealed at low temperatures, i.e., before reaching the fully oxidized surface. The following molecules were checked for but not observed as gaseous products in experiments on any surface: HCOOH, HCOOCH,, (CH,),O, CH,CHO and CH,CH,OH. 3.21. The Ar +-bombarded annealed surfaces
I-.
450 452
-.
454 456 458 Binding
* . -
’
.
460 462 464 466 468 * Energy
(eV)
Fig. 1. XPS Ti(2p) core level spectra of the TiO,(OOl) singlecrystal surfaces (experiments were performed in the VGESCALAB). (a) Sputtered; (b) annealed at 650 K for 20 min; (c) annealed at 750 K for 20 min.
energy side of the Ti(2p,,,) envelope and the increase in the signal from Ti4+ cations, indicating that Ti cations in lower oxidation states were oxidized to Ti4+. Fig. lc presents the Ti(2p) core levels of a surface annealed at 750 I(. Only one peak was observed for the Ti(Zp,,,) line, centered at 459.2-459.3 eV with a FWHM of ca. 1.2 eV. The peak position is characteristic of Ti4+ and the narrow FWHM clearly indicates that only one oxidation state, (+4), was present at the surface. Annealing at high temperatures produced no further changes in the spectrum. 3.2. Formaldehyde programmed
adsorption and iemperaturedecomposition
Since the ion-bombarded TiO,(OOl) surface was characterized by different oxidation states for
and low-temperature-
Two product channels were observed in the reaction of formaldehyde on the ion-bombarded and low-temperature annealed surfaces, as shown in fig. 2. Table 1 presents the products and fractional yields from fo~aldehyde TPD on these surfaces. Product yields were corrected for mass spectrometer sensitivity and quantified by the method described previously [%I. Products desorbing via the first channel, at ca. 370 K, were methanol and formaldehyde, accompanied by water and a small amount of hydrogen (not shown). The second set of peaks, at ca. 530-560 K, con-
I HCHO/TiOz
400
500
600
Temperature (K) Fig. 2. TPD spectra following adsorption of formaldehyde at room temperature on the sputtered surface of the TiO,(OOl) single crystal. Desorption spectra are corrected for mass spectrometer sensitivities.
118
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,(OOl)
sisted of methanol, formaldehyde, CO and CO,; trace amounts of water were also observed at this temperature. The fractional yields in each set of experiments were normalized to the total C(ls) peak area obtained by XPS; XPS spectra will be presented below. The highest observed total C(ls) peak area following adsorption of methanol at room temperature was on the sputtered surface (242 counts per second) and decreased to ca. 20% of that value (50 counts per second) for adsorption on the 550 K annealed surface. All peak areas were obtained after background subtraction Two points arise from the data in fig. 2 and table 1: (1) The amounts of formaldehyde and methanol desorbed at higher temperature (530560 K) were reduced by prior annealing of the surface to 450 K and became very small on the 550 K annealed surface. The ratio of methanol to formaldehyde desorbed at lower temperature (370-380 K) also decreased on the annealed surfaces relative to that observed on the sputtered surface; from 0.47 on the sputtered surface to 0.25 on the 550 K annealed surface. These results indicate that annealing the surface to 450 K and further to 550 K decreased its ability to reduce formaldehyde to methanol. (2) The extent of formaldehyde decomposition to CO and CO, during TPD decreased upon
Table 1 Product distribution Product
from formaldehyde
TPD on the sputtered
Peak temperature
(K)
prior annealing of the surface to higher temperatures. The total amount of CO and CO, desorbed from the sputtered surface was 46% of the total yield of carbon-containing species desorbed. This fraction decreased to 10.4% on the 450 K annealed surface and was at the trace level on the 550 K annealed surface. As will be shown below, these CO and CO, desorption peaks most likely result from surface carbon atoms deposited on the surface by complete decomposition of formaldehyde at lower temperature and not from formate decomposition (formates could be formed by formaldehyde oxidation). Formates were not observed in the XPS spectra after formaldehyde adsorption on these reduced surfaces, as discussed below. 3.2.2. The oxidized surfaces Prior annealing of the surface at still higher temperatures produced additional changes in the surface reactivity. As shown by figs. 3 and 4 and table 2, no desorption of formaldehyde or methanol occurred via the higher temperature channel on surfaces annealed at 700 K and above. XPS (fig. lb) showed that annealing the surface to 650 K resulted in the oxidation of the large majority of Ti cations in lower oxidation states than Ti4+, while annealing the surface to 750 K or above totally oxidized titanium suboxides to Ti4+. It is clear therefore, that the high-tempera-
surface
and on surfaces Relative Sputtered 15 7 26 6 34 12
HCHO MeOH HCHO MeOH co, co
365-380 365-380 530-560 530-560 530-560 530-560
Ratio MeOH/HCHO
365-380
- 0.47
Total C(ls) peak area
Room temp.
241
previously
annealed
to 450 and 550 K
yield ‘) 450 K
550 K
13.8 4 6.7 1.8 9.3 1.1
15.7 3.9 0.72 0.33 Traces Traces
0.29 90
0.25 50
a) The relative yield of a product n desorbed from a given surface was multiplied by the ratio of the XPS C(ls) peak area obtained after formaldehyde adsorption at room temperature on that surface to that of the highest observed C(ls) peak area (241 on the sputtered surface). (Example: the fractional yield of a product n from the 450 K TPD was multiplied by 90/241.) Thus, the entries in this table are relative to a total yield of carbon containing products from the sputtered surface = 100 (arbitrary units).
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,fOal)
119
HCHOITiO,
HCHO/TiO,
annealing temperature
annealing temperature 900 K
900 K
850 K
850 K
750 K
750 K
700 K
700 K
625 K
625 K
r\
500
600
300
500 K
400
500
600
700
Temperature (K)
Temperature (K)
Fig. 3. TPD spectra of m/e = 30 (X 1) following adsorption of fo~aidehyde at room temperature on the TiO,(OOl) single crystal at different prior anneating temperatures.
Fig. 4. TPD spectra of m/e = 31 (X 2) following adsorption of formaldehyde at room temperature on the TiO~(OOl) single crystal at different prior annealing temperatures.
ture reaction channel in fig. 2 and table 1 can be assigned to reactions occurring on reduced sites. No resolvable CO and CO, desorption peaks were observed for formaldehyde decomposition on oxidized surfaces. Similar results were obtained in methanol TPD experiments on this single crystal [43]. CO was formed in the course of methanol decomposition on the sputtered surface
but no CO desorption was observed from oxidized TiO,(OOl) surfaces [43]. As will be illustrated beIow by XPS, CO and CO, are probably formed by surface carbon oxidation. Thus it is reasonable to conclude that the total decomposition of adsorbed fo~aldehyde to carbon, hydrogen and oxygen atoms is suppressed by annealing, and thereby oxidizing, the reduced surfaces (fig. 1).
Table 2 Product distribution from formaldehyde TPD on the 700, 750, and 850 K annealed surfaces Product
Peak temperature (K)
&HO MeOH HCHO MeOH CO: co
365-380 36.5-380 530-560 530-560 530-560 530-560
17.6 5.2 0 0 0 0
Ratio MeOH/HCHO
365-380
* 0.30
Total Ctls) peak area
Room temp.
Relative yield a) 700 K
‘) As in table 1.
55
750 K
850 K
21.6 7.0 0 0 0 0
23.7 9.0 0 0 0 0
0.32 69
0.38 79
120
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,f’Oi?l)
methylene (paraformaldehyde) at low temperatures [l&41,42]. Similar results were obtained in this laboratory upon formaldehyde adsorption at 160 R on a ZnO @001~ single-crystal surface [14]; these are. shown for comparison in fig. 5b. The broad peak centered at 288.2 eV in the spectrum of formaldehyde on ZnQ was attributed to a condensed layer of formaldehyde. The FWHM af this peak was also larger than that expected for a single surface species. Thus, on the oxidized surface of TiO,@Ol) and on the Zn-polar face of zinc oxide, adsorbed fo~aidehyde gives a Cfls) peak at 288-288.2 eV, however, it is not possible to distinguish between molecular and polymeric species on these surfaces from XPS data. Before presenting the spectra resulting from formaldehyde adsorption on TiO,(OOl) near room 294 286 290 temperature, it is necessary to present the exact Binding Energy, G-V peak positions of other intermediates that may be Fig. 5. XPS c(lsI spectra for formaIdehyde adsorption. fa) On formed from fo~~dehyde, Methoxide and forthe oxidized surface of Ti0,@01) single crystai at 170 K. (b) mate species have been formed from formaldeOn Z.nOfOOOl)_Zn at 160 K (data from J.M. Vohs, PhD dissertation, University of Delaware, 1988). Both spectra were hyde on a variety of oxides, among them titania collected using a PNT 550 ESCA/Auger apparatus. powder 1181. On TiO,(OOl) single crystals methanol was dissociated to methoxy species at room temperature, giving a C(ls) peak centered at 2863 eV f431, while formic acid dissociated to formates, also at room temperature, to give a 3.3.1. F~~~~~~~y~~on the~~~~~-~~~~~~~~u~~~~ C(fs) peak at 289 eV 1441 with this instrument (fig. 6, a and b). It is also important to note that In order to identify the surface intermediates formed during HCHO decomposition, XPS exno methanol oxidation to formates was observed periments were conducted on the stoichiometric in these experiments, reproducing the previous surface, i.e., the surface annealed at 750 IS for 20 negative result [431. C(ls) spectra following formaldehyde adsorpmin and with ail Ti cations in the +4 oxidation tion on the (011) and {fl4)-faceted surfaces at state. Ad~rpt~on of formaldehyde at 170 K pro250-300 K are illustrated in fig. 6, curves c and d. duced a broad peak in the C&1 spectrum cenFrom the width of these peaks it is apparent that tered at 288 eV (fig, 5); ~jndividual XPS spectra at least three different carbon-containing species were referenced to the Ti(2p,,,) peak at 459.3 were present on the surface in each case and, in eV). The FWHM of the C(ls) peak was 2.7 eV, fact, faur peaks were required to fit these spectra larger than that expected for a single surface adequately. These four peaks, at 284.7, 286.3, species. This peak can be assigned primarily to a 288.1 and 289-5-289.7 eV following formaldehyde layer of molecular HCHO and to species derived adsorption on the (Oil)-faceted TiU~~OOl~ surfrom HCHU. Such species may include face, were assigned to surface carbon, methoxdio~ethylene~ since the formation of dioxyides, adsorbed formaldehyde and formate species methylene species has been reported for HCHO respectively. From fig. 6c, after subtraction of the adsorption on titania powders at 200-250 K background the ratio of the height of methoxide [l&31]. However the presence of polymerized peak to that of formate peak was ca. 4. It has formaldehyde cannot be ruled out, as adsorbed been suggested that methoxide and formate formaldehyde can polymerize to form polyoxy-
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,(OOl)
‘. ‘..’
.;.
.:
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d
..
:
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.:
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.-
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Barteau [45] for methyl formate adsorption on ZnO. These workers assigned a C(ls) peak at 290.9 eV following HCOOCH, adsorption to the carbonyl carbon of molecularly adsorbed methyl formate. Since good agreement was obtained for the binding energies of surface alkoxides and carboxylates between the present study on TiO, and those on ZnO [15,45] it is reasonable to expect by analogy that molecular methyl formate on TiO, would exhibit a C(ls) peak around 291 eV. For the C(ls) spectrum in curve cc), no peak was observed in this vicinity, indicative of the absence of molecularly adsorbed methyl formate. Adsorption of formaldehyde at room temperature on the (114}-faceted TiO,(OOl) surface (obtained by annealing the surface at 950 K for 20 min) is displayed in fig. 6d. A peak at 286.5 eV assigned to methoxides (this peak shifted about 0.2 eV from that observed on the {Olll-faceted surface), a shoulder on the lower binding energy side at about 284.7 eV attributed to surface carbon, and a peak at 289.5-289.7 eV attributed to formate species, are evident in the spectrum. The ratio of methoxide/formate species on this surface was ca. 2.5, somewhat lower than that observed on the (Oil)-faceted surface. This result suggests that the {114}-faceted surface is more selective for the Cannizzaro disproportionation of formaldehyde than is the {OllI-faceted surface. Moreover, as shown in fig. 6d, more surface carbon deposition occurred on the {114}-faceted surface; similar results were obtained for reactions of formic [44], acetic [33] and acrylic acids [46] on these two surfaces. No noticeable difference was observed between the C(ls) spectra of formaldehyde-dosed {Ollj-faceted and (114}-faceted surface upon further heating to higher temperatures. Fig. 7 presents the C(ls) spectra of the formaldehydedosed, {Olll-faceted surface as a function of surface temperature. Heating the surface to 450 K produced a spectrum, fig. 7b, which indicated that small amounts of methoxides were present along with surface carbon (at 284.7 eV). Temperature-programmed desorption studies after formaldehyde adsorption on this fully oxidized surface showed that there was only one channel for methanol and formaldehyde evolution. This
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: :.; 283
285
287
289
291
293
Binding Energy Fig. 6. XPS C(k) spectra of: (a) methanol-dosed surface at room temperature on the {Oil)-faceted surface structure; (b) formic-acid-dosed surface at room temperature on the {Oll)faceted surface structure; (c) formaldehyde-dosed surface at 2.50 K on the (Oil)-faceted surface structure; (d) formaldehyde-dosed surface on the (114)faceted surface structure at room temperature.
species are formed on TiO, powder [181 from formaldehyde by a Cannizzaro-type reaction, however, the Cannizzaro reaction would provide equal amounts of methoxides and formates, as observed, for example, on MgO(100) [161. In the present case, on the fully oxidized surface of a TiO,(OOl) single crystal, methoxides were about 4 times more abundant than formates, which indicates that the Cannizzaro reaction is not the only route to methoxides from formaldehyde. The formation of methyl formate was excluded on the basis of information reported by Vohs and
121
H. Idriss et al. / Formaldehyde oxidation and reduction
122
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fig. 8. Individual spectra were referenced to the Ti(2p,,,) peak at 459.1 eV for Ti4+ in sputtered TiO,. Fig. 8a displays the C(ls) spectrum following formaldehyde adso~tion at room temperature. One broad peak centered at 286.3 eV was observed. Two shoulders were also observed, one at Iower binding energy, around 285 eV, and the other at higher binding energy, around 288 eV; no peak or shoulder was observed in the C(ls) region of formate species at ca. 289.5-289.7 eV. The peak at 286.3 eV is in the same position as that observed after methanol adsorption on the fully oxidized surface (fig. 6a) and is therefore assigned to methoxides. The shoulder at 288 eV is due to the presence of formaldehyde; formaldehyde adsorption at 170 K on the {Oil}-faceted surface gave a peak centered at 288 eV (fig. 5).
a 283
285 287 289 Binding Energy
291
293
Fig. 7. XPS C(k) spectra for formaldehyde adsorption on the (Oil}-faceted surface structure of the TiO,(OOl) single crystal as a function of reaction temperature. (a) Formaldehyde dosed surface at room temperature; (b) surface of (a) heated to 450 K; (c) surface of(b) heated to 550 K; cd) surface of tc1 heated to 750 K.
: .,.:.:.,v.
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..
:
:
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..,.
dx
6
desorption channel gave rise to peaks with a maximum at ca. 370 IS. The XPS results are therefore consistent with those obtained by TPD (figs. 3 and 4). Further heating the surface to 550 K, fig. 7c, left only traces of surface carbon: this disappeared upon annealing the surface to 750 K, fig. 7d. 3.3.2. XPS experimentson the sputtered surface XPS experiments were also performed on a freshly ion-bombarded surface in order to study the effects of surface composition upon the reactions of HCHO on TiO~(OOl). This surface, as shown in fig. la, contains Ti cations in lower oxidation states than +4 in addition to some Ti4+, and is thus deficient in oxygen. A typical set of CM spectra for the HCHO-dosed surface as a function of surface temperature is displayed in
283
285 Binding
287
289
Energy
291
293
(eV)
Fig. 8. XPS Cfls) spectra for formaldehyde adsorbed on the ion bombarded surface of the TiO,(OOl) single crystal as a function of surface temperature. (a) Formaldehyde-dosed surface at room temperature; (b) surface of (a) heated to 450 K; (c) surface of(b) heated to 550 K, (d) surface of(c) annealed 5 min at 7.50K.
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,(OOl) 560, .-’ . .....“’ 2‘
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2,4 ;:.;.,+-_: to that of surface carbon (285 eV> at room temperature (fig. 8a) is close to two. In effect, this ratio agrees well with the net stoichiometry of formaldehyde disproportionation to methoxide plus C(a). Hydrogenation of adsorbed formaldehyde (HCHO(a)) to methoxide (CH,O(a)) requires one hydrogen, the complete decomposition of one molecule of formaldehyde gives two adsorbed hydrogen atoms (2H(a) and one adsorbed carbon atom (C(a)); thus two molecules of formaldehyde can be hydrogenated to two methoxide species for each carbon atom deposited by complete decomposition of a third molecule of formaldehyde. As a result of formaldehyde decomposition to adsorbed hydrogen (2H(a)), adsorbed carbon (C(a)) and lattice oxygen (O(l)), the sputtered, reduced surface becomes partially oxidized. This is illustrated in fig. 9. This figure displays the Ti(2p) lines of the sputtered surface before formaldehyde adsorption, curve a, and after adsorption, curve b. The lower binding energy side of the Ti(2p,,,) envelope centered at 459.1 eV decreased in intensity after formaldehyde adsorption, while the intensity of the Ti(2p,,,) line for Ti4+ cations increased, indicating that oxidation
123
of titanium suboxides at room temperature by reaction with formaldehyde took place. From fig. 9, after background subtraction, the increase of the Ti4+ intensity was ca. 10% (from 307 counts per second before formaldehyde adsorption to 337 counts per second after formaldehyde adsorption). Previous results in this laboratory indicated that titanium suboxides were also oxidized by adsorption of carboxylic acids (acetic [33], formic, and acrylic acids [46]). However, while carboxylic acids can function as oxidizing agents via C-O scission, net oxygen release from carboxylates does not necessarily required a reduced surface (see, e.g., ketene formation from acetic acid on TiO,(OOl) [33], Mg0(100) 1161 and on ZnO (OOOl)-Zn [47] single-crystal surfaces). The dissociation of the carbon-oxygen bond of aldehydes and ketones is much less common and requires cations in reduced states (see, e.g., carbony1 coupling on TiClJLiAlH, [481 and on reduced surfaces of TiO,(OOl) single crystals [49], and aldehyde decomposition on Ti+ and TiCl f ions in the gas phase [50]). The facile reduction of formaldehyde, accompanied by oxidation of the surface demonstrates the high reactivity of titanium suboxide surfaces toward oxygen-containing organic molecules. Heating the reduced surface to 450 K after formaldehyde adsorption caused the disappearance of the shoulder at higher binding energy (fig. 8b), indicating that the remaining formaldehyde that was not hydrogenated to methoxide nor decomposed to adsorbed carbon at room temperature either desorbed or reacted with the surface. The C(ls) peak of methoxide species decreased in intensity, and since there was no increase in surface carbon and no evidence for the presence of other surface species, it is reasonable to conclude that methoxide species were partly hydrogenated to methanol by 450 K. TPD experiments after formaldehyde adsorption on the sputtered surface, fig. 2, showed coincident desorption of formaldehyde and methanol at ca. 370 K and are consistent with these XPS results. Further heating the surface to 550 K decreased dramatically the C(1.s) peaks of both methoxides and surface carbon. Two small and separate peaks were observed (fig. 8~1, assigned to surface carbon (285
124
If. Id& et al. / Formaldehyde oxidation and reduction on Ti02f001f
eV) and methoxides (286.3 eV). Heating the surface to 700 K resulted in complete removal of all species from the surface except for traces of surface carbon; annealing the surface for 5 min at 750 K give a clean surface free from all adsorbates (fig. 8d). The absence of formate species at room temperature and also at higher temperatures on the sputtered surface indicated that methoxide formation on this surface is a direct hydrogenation process and not the result of Cannizzaro-type disproportionation of two formaldehyde molecules to one methoxide plus one formate species. It is likely that the reduced Ti”+ (x < +4) sites are responsible for this reduction. The population of these sites is decreased by annealing (figs. la and lb) and the methanol yield is correspondingly decreased.
from formaldehyde, and are responsible for the concurrent formation of methoxides and formates (fig. 6). Prior annealing of these surfaces to temperatures above 700 K increases the methanol/ formaldehyde desorption peak ratio at ca. 370 K (table 2). Formaldehyde adsorbs molecularly on TiO, at 200 K or below, and a portion desorbs at around 270 K without decomposition. Molecularly adsorbed formaldehyde (and aldehydes in general) are most likely coordinated to Lewis acid sites, in this case Ti4+ cations, through cr lone pair donation from the oxygen of the carbonyl [31,52] as follows: H
H
‘C’
II
\o/ Ti&
4. Discussion The above results can be summarized as follows: (1) The reduced surface, i.e., the sputtered surface, reduces formaldehyde to methanol; two sets of desorption peaks (at 370 K and 550 K (fig. 2)) were observed during TPD after formaldehyde adsorption at room temperature. The principal adsorbed species following formaldehyde adsorption were methoxides, as shown by XPS, fig. 8, (C(ls) peak centered at 286.3 eV). (2) The reduced surface decomposes a portion of the adsorbed fo~aldehyde to adsorbed atomic hydrogen (H(a)), lattice oxygen (O(l)) and surface carbon (C(a)). Fig. 8 shows the presence of a C(ls) peak at 284.7 eV corresponding to surface carbon at room temperature, fig. 9 shows an increase of the Ti(2p,,,) signal intensity for Ti4+ cations following formaldehyde adsorption, while fig. 2 shows surface carbon oxidation to CO and CO, during TPD after fo~aldehyde adsorption at room temperature on the reduced surfaces. (3) Oxidized surfaces, i.e., surfaces for which all Ti cations are in the +4 oxidation state, stabilize formaldehyde (fig. 5), suppress the higher temperature channel for methanol production
(1) o-2
The TiO~t~l) single crystal surface is reconstructed to a {Ofll-faceted structure by prior annealing to 750 K and the Ti4+ cations are fivefold coordinated to oxygen in this structure [30]. Each Ti cation would therefore constitute one Lewis acid site. Formaldehyde can then be adsorbed, most probably in an q1 configuration, on these Lewis acid sites as suggested by extended Hiickel calculations on Cr,O, l.221. As a result of this adsorption the carbon of the carbonyl becomes more electrophilic, favoring an attack from a nucleophilic surface oxygen ion to form dioxymethylene species as follows: H ‘c’
H I: 0
H \/
H
P\ 0 0 \ /
-li TiM - o-2 Dioxymethylene species have been observed following formaldehyde adsorption on TiO, (anatase) 1311,thoria 1311and CeO, [201 by FI’-IR as well as on ZnAl,O, by FT-IR and chemical trapping 1191.Busca et al. I.181suggested that the dioxymethylene formation is a consequence of the high electrophilicity of the carbonyl carbon and takes place via the reaction of formaldehyde with a cation-anion site pair at the surface.
125
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,fOOl)
Hydride transfer between two dioxymethylene species would give one methoxide and one formate and must occur to some extent below 300 K, as shown by fig. 6. This reaction corresponds to a net Cannizzaro disproportionation. In solution, these reactions involve direct hydride transfer [24,53] however this reaction may be surfacemediated on oxides [16]. It is important to note that Busca et al. [181 observed on titania powder previously heated at 723 K, that dioxymethylene began to disproportionate to methoxide and formate species at 270-300 K. These methoxides were hydrogenated to methanol which desorbed at 370 K in our temperature-programmed desorption experiments following formaldehyde adsorption at room temperature, fig. 4. It was not possible to resolve the products of formate decomposition in these TPD experiments, however the CM peak at 289.5-289.7 eV attributed to formate species (fig. 7) disappeared at 600 K, indicating that formates were totally decomposed or transformed at this temperature. For-mate species formed from formic acid on this surface were previously investigated by Kim and Barteau [44] and were reported to decompose at 560 K. Formate species were formed on both fully oxidized surfaces; the {Ollj-faceted structure, obtained by annealing at 750 K, fig. 6c, and the {114)-faceted structure, obtained by annealing the surface at 950 K, fig. 6d. This result indicates that the net Cannizzaro disproportionation of formaldehyde is not a structure-sensitive reaction. However, prior annealing of the surface at temperatures higher than 750 K continued to increase the AES O(510 eV)/Ti(380 eV> ratio, i.e., increased the overall oxygen concentration in the nearsurface region. Indeed, the conversion of formaldehyde to methanol increased as the prior annealing temperature was increased above 750 K. This observation is in agreement with the proposed reaction involving dioxymethylene as a key species for the disproportionation, since increasing the oxygen concentration means increasing the Lewis base sites which favor the formation of dioxymethylene species from adsorbed formaldehyde (reaction 2). On the reduced surface, formaldehyde was converted to methoxides at room temperature.
The reduced surface contains Ti3+ and Ti*+ and possibly Ti’ + (due to the formation of oxygen vacancies, Vo, after bombarding the surface with argon ions) in addition to Ti4+. Since the surface is deficient in oxygen, part of the formaldehyde is decomposed and used for oxygen restoration as follows. HCHO( a) + Vo + 2H( a) + C(a) + 0( 1))
(3)
where (1) denotes lattice oxygen. However, as a consequence of this oxygen restoration, hydrogen and carbon deposition occurs. The hydrogen produced by this decomposition is consumed by reduction of another molecule of formaldehyde to methoxide species: HCHO( a) + H(a) + CH,O(a)
.
(4)
The carbon remains on the surface (fig. 8) and is oxidized to CO and CO, at ca. 550 K (fig. 2) with a ratio on the sputtered surface (table 1) of co to co, of 3. From table 1, the fractional yield of surface carbon oxidized to CO and CO, on the sputtered surface was ca. 0.45, while that of the methanol and formaldehyde in the second desorption peak at ca. 550 K was ca. 0.32. Since the higher temperature desorption peak for formaldehyde results from methoxy dehydrogenation to formaldehyde, as demonstrated for methoxides formed by methanol adsorption 1381it is possible to attribute the total high-temperature methanol and formaldehyde yield to methoxide species present at room temperature. An estimate of the relative concentrations of surface species can be obtained from the XPS data. The peak at 286.3 eV (fig. 8) is 52% of the total C(ls) area while that of surface carbon (a shoulder at 284.8 eV) is equal to 23% and that of formaldehyde (a shoulder at 288 eV> is 25%. Thus methoxide species and surface carbon account for about 75% of the total carboncontaining surface species following adsorption of formaldehyde on the sputtered surface at room temperature. Indeed from TPD (fig. 2 and table 1) the products desorbed at ca. 550 K (CO, CO,, methanol and formaldehyde) have a total fractional yield of ca. 0.77 (0.34, 0.12, 0.06 and 0.26 for CO, CO,, methanol and formaldehyde re-
126
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,(OOl)
Prior annealing temperature (K) Fig IO. Corrected peak area ratio of methanol/formaldehyde desorbed at ca. 370 K during the TPD after formaldehyde adsorption at room temperature and the C(ls) peak height of methoxides formed after formaldehyde adsorption at room temperature on the Ti02(0011 single crystal as a function of prior anneaiing temperature.
spectively). Thus on the sputtered surface about 0.77 of adsorbed formaldehyde is converted to methoxide and surface carbon while 0.22-O-23 remains as adsorbed formaldehyde, Within 5% these results indicate that quantitative dctermina(ions from XPS and TPD are in a very close agreement_ Formaldehyde f20-25% of the overall adsorbed species) is probably adsorbed on Ti4* sites which may promote the reaction via a Cannizzaro type mechanism as on the fully oxidized surface, leading to the simultaneous desorption of formaldehyde and methanol at ca. 370 K (fig. 6). it is worth noting that curve fitting of the Ti(;?p,,,) peaks Gig. la) of the sputtered surface indicated that the Ti4” peak area of Ti(2p,,,,) cations at 459.1 eV was ca. 23% of the overall Ti(2p,,,) peak area. A summary of formaldehyde reactivity on TiO, surfaces is presented in fig. 10. The methanol/ formaldehyde ratio at ca. 370 K, corrected for mass speedometer sensitivity differences, as well as the C(ls) peak height of methoxy species formed upon adsorption of fo~aldehyd~ at room temperature, are presented as a function of prior annealing temperature. Less formaldehyde was converted to metbanol as the prior annealing temperature increased from 300 to 550 K? indicat-
ing that oxidation of the TiO,(OOl) surface formaldehyde. This is also evidenced by the decrease of the C(ls) peak height of the methoxides. Annealing the surface to 700 K and above resulted, again, in an increase of the methanol/ff peak area ratio and also of the concentration of methoxides formed from formaldehyde on the surface at room temperature. Since further annealing of the crystal causes further oxidation, and since XPS (fig. 6) showed that formate species are formed on the oxidized surface and that more formate species and surface carbon were observed on the fll43-faceted surface (annealed at 950 K) than on the {IlO)-faceted surface (annealed at 750 K), one can attribute this increase to a Cannizzaro type reaction. Methoxides are then hydrogenated to methanol by hydrogen atoms hberated in the course of the decomposition of other methoxide intermediates and formates.
5. Conclusions The reaction of formaldehyde on TiO,(OOl) single-crystal surfaces is sensitive to the oxidation state of the surface cations. Formaldehyde reacts on the reduced surface to give methanol peaks at ca. 370 K and 5.50 K in TPD experiments. The lower-temperature desorption peak remains even for formaldehyde adsorption an surfaces previously annealed at high temperatures, and is attributed to a Cannizzaro-type reaction occurring at Ti4 ’ cations. The h~gher~temperature desorption peak is probably due to reduction of fo~aldehyde to methanol at reduced, oxophilic Ti”’ (x < -1-4) sites. These sites decompose formaldehyde to adsorbed carbon, adsorbed hydrogen and lattice oxygen. These results indicate that formaldehyde can be decomposed not only by reduced metals but also by titanium suboxides. The fact that the Hugh-temperature reaction channel disappears on surfaces anneafed at 625 K and above indicates clearly that this reaction is associated with Ti cations in low oxidation states. Unlike the production of methoxides via the Cannizzaro reaction, direct reduction of formaldehyde does not involve the formation of formates.
H. Idriss et al. / Formaldehyde oxidation and reduction on TiO,(OOl)
Acknowledgement
We gratefully acknowledge the support of the National Science Foundation (Grants CBT8714416 and CTS-9100404) for this research.
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