The chemistry of dimethyltetrazine on Pt(111)

456

Surface Science 120 (1982) 456-466 North-Holland Publishing Company

THE CHEMISTRY David

DAHLGREN

Department Received

of Chemistry, 13 April

OF DIMETHYLTETRAZINE

ON Pt( 111)

and John C. HEMMINGER University of California, Irvine, California

1982; accepted

for publication

92717. USA

2 June 1982

AS part of a program to further document the chemistry of CN on transition metal surfaces we have studied the decomposition of dimethyltetrazine on Pt( 111). Products of the decomposition of dimethyltetrazine are Ha, N,, HCN, C,N, and small amounts of CH,CN. Most of the methyl groups (>90%) are totally dehydrogenated leaving residual carbon on the surface. At low coverage the initial decomposition is CH bond cleavage. At higher coverage direct production of molecular is observed as the initial decomposition mode. Pretreatment of the Pt with H,, N, at -30°C shifts the high coverage decomposition to higher temperatures. Changes in the decomposition with coverage is explained as due to a change in bonding geometry. We suggest that at low coverage the molecular plane is parallel to the surface with the methyl groups in proximity to the surface. while at high coverage the molecule bonds on edge possibly through two adjacent nitrogens.

1. Introduction A major goal of surface chemistry studies is to develop an understanding of the differences in chemistry of small functional groups on different metal surfaces. Much effort has been expended to document the difference in the chemistry of CO on several transition metal surfaces. Another simple functional group which exhibits diverse chemistry on metal surfaces is -CN-. Studies of CN containing compounds have been limited in most part to the surfaces of Pt and Ni. In particular, chemisorption studies of CH,CN and CH,NC have been reported on a variety of Pt and Ni surfaces [ 1,2]. Studies of HCN and C,N, have been reported utilizing Ni(1 ll), Pt(1 lo), Pt(lOO) and stepped Pt(s) 9( 111) X (111) surfaces [3-51. These four compounds all have CN groups with bond order of three. A striking difference appears in the chemistry of the CN triple bond on Pt and Ni surfaces. A major product pathway on Ni surfaces involves cleavage of the CN bond in all cases studied (with the exception of CH,CN on the smooth Ni( 111) surface). Comparatively. none of the Pt surfaces (even the highly stepped surface) exhibit any CN triple bond cleavage activity. A similar difference in activity is observed for CO on Pt and Ni. Ni is well known to be more active towards CO bond cleavage than Pt. The difference is much more dramatic in the case of the CN triple bond. In an 0039-6028/82/0000-0000/$02.75

0 1982 North-Holland

D. Dahlgren, J.C. Hemminger / Dimethyltetratine on Pi(ll I)

451

attempt to shed further light on this difference in chemistry of Pt and Ni, we have studied the decomposition of dimethyltetrazine (I) on a Pt( 111) surface. 7H3

This

Experimental

These experiments were carried out in an ultra high vacuum chamber (base pressure G 1 X lo-” Torr) equipped with electron optics for low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies. The chamber is also equipped with a UT1 100~ quadrupole mass spectrometer for monitoring gas exposures and for thermal desorption studies. The mass spectrometer is computer interfaced to allow multiple mass monitoring during a thermal desorption [6]. The temperature is also monitored by the computer via a thermocouple attached to the crystal. All thermal desorptions in this study utilized linear temperature programming. The dimethyltetrazine (DMTZ) used in these experiments was obtained from Dr. Jake Pacansky, IBM Research Laboratories, San Jose. Exposures of the crystal to DMTZ were carried out at the ambient or at 230 K with pressures from 5 X 10e9 to 5 X lop8 Torr (pressures are ion gauge readings uncorrected for sensitivity relative to N,). A small sample of the DMTZ was placed in a sample tube and then degassed by several freeze-pump-thaw cycles before introduction into the gas handling line. The vapor pressure of DMTZ is greater than 0.5 Torr at room temperature. The Pt( 111) crystal was cleaned by standard techniques of ion bombardment, annealing and oxygen treatments. Care was taken to avoid any contamination of the crystal by strongly bound oxygen which has been shown to affect the chemistry of some hydrocarbons on platinum [7,8].

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3. Results The temperature. Nor do any LEED structures develop upon slow heating to the onset of the first thermal desorption product. Our experiments show no evidence for molecular desorption of dimethyltetrazine following adsorption on a clean Pt(ll1) surface. The temperature programmed decomposition distribution including nitrogen (N2), hydrogen cyanide (HCN), cyanogen (C,N,), hydrogen (H,), (CH,CN) and a carbon residue left on the surface. These products are monitored in the thermal desorption experiments by monitoring masses 28, 27, 52, 2 and 41 respectively. spectrometer we can easily monitor all these masses in each TDS experiment. A schematic of the decomposition temperatures (degrees is shown in fig. 1. The thermal desorption spectra of the major products at several exposure levels are shown in figs. 2-5. The highest exposure level corresponds to a coverage of - 2 X lOI molecules per cm2 of surface from the carbon Auger signal [9]. The acetonitrile is a very minor product. The peak temperature of the CH,CN product does not appear to vary significantly from 200°C as the coverage changes. 3.1. Hydrogen The hydrogen thermal desorption (fig. 2) consists of two major peaks at 125’C and 205’C. Hydrogen adsorbed on clean Pt( 111) desorbs with a peak in the TDS in the

125”C,

205°C Hz

/ N2

CH3CN

HCN

NCCN

* Fig. 1. Schematic of the decomposition atures (degrees centrigrade).

C ad

of dimethyltetrazine

with peak thermal

desorption

temper-

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on Pt(l I I)

y3

NxcxNPt(lll) 101 KC/N

tli,

A

-25

25

I.25

Crystal

225

325

Temperature

425

(“Cl

525

-25

25

125

Crystal

225

325

425

Temperature

PC)

52 5

Fig. 2. Thermal dimethyltetrazine

desorption of H, (mass 2) following exposures to Pt( 111).Linear temperature ramp - 7’C/s.

of 1.2. 2.4, 3.6 and 6.0 L of

Fig. 3. Thermal dimethyltetrazine

desorption of N, (mass 28) following exposure to Pt( 111).Linear temperature ramp - 7’C/s.

of 1.2. 2.4. 4.2. and 6.0 L of

neighborhood of 75’C at low coverages. The low temperature peak we observe (125’C) saturates at exposures of - 3 L. At higher exposures the additional hydrogen contributes to the high temperature hydrogen peak at 205°C. 3.2. Nitrogen At low exposures the nitrogen desorption spectrum (fig. 3) is dominated by the peak at -210°C. The peak temperature is relatively insensitive to coverage. This temperature also corresponds to the temperature at which N atoms recombine and desorb from Pt( 111) [lo]. The low temperature nitrogen peak at 30°C which grows in at exposures above 3 L must correspond to direct production of molecular nitrogen followed by immediate desorption. Notice that the low temperature nitrogen peak (in fig. 3) starts to grow in at exposures greater than the saturation exposure level for the low temperature hydrogen peak.

460

D. Dahlgren, J.C. Hemminger

-25

25

125

225

Crystal Fig. 4. Thermal dimethyltetrazine

3.3. Hydrogen

325

425

Temperature

/ Dimeth.vltetrarine

525

on Pt(ll1)

625

(“Cl

desorption of HCN (mass 27) following exposure to Pt( 111).Linear temperature ramp - 7”C/s.

of 1.2. 2.4. 3.6 and 6.0 L of

cyanide

The HCN desorption temperature is essentially

(fig. 4) increases with exposure independent of exposure.

level and

the peak

3.4. Cyanogen The cyanogen thermal desorption peak is broad and shows a slight shift to higher temperature as the exposure level is increased. Thermal desorption following adsorption of C,N, on Pt( 111) results in a multiple peaked spectrum as shown in fig. 6. The cyanogen resulting from decomposition of DMTZ (fig. 5) resembles the high temperature peak in the cyanogen thermal desorption. Auger spectra show that the amount of carbon residue left on the surface corresponds to - 40% of that initially present. 3.5, Coadsorption

of hydrogen

and DMTZ

Coadsorption experiments were carried out with hydrogen and DMTZ and carbon monoxide and DMTZ. In a typical experiment the crystal was first

D. Dahlgren, J.C. Hemminger

/ Dimethyltetrarine

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f”3

N'C,

or pto kc/N

lCN)Z

I

I

LH,

t-

60L

!

-25

25

125

325

225

Crystal

425

525

Temperature

625

725

825

1°C)

of (CN), (mass 52) following exposure desorption to Pt( 111).Linear temperature ramp - 7”C/s.

Fig. 5. Thermal dimethyltetrazine

of 1.2, 2.4, 3.6 and 6.0 L of

exposed to 6-12 L hydrogen, then after pump out to base pressure the DMTZ exposure was carried out in the usual manner. Preexposure to hydrogen has little effect on the DMTZ chemistry at low coverages of DMTZ. However, at exposures of DMTZ in excess of 3 L preexposure of hydrogen to the surface has a dramatic effect. The low temperature nitrogen peak which

I

25

125

225

Crystal

325

425

Temperature

Fig. 6. Thermal desorption temperature ramp - 7OC/s.

mass

525

625

(“Cl

52 following

exposure

of 0.5 L cyanogen

to Pt( 111). Linear

D. Dahlgren, J. C. Hemminger

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/ Dmethyltetra:ine

on Pt(ill)

shows up at high DMTZ coverages is shifted to - 80°C. Fig. 7 shows the mass 28 data from a typical hydrogen preexposure experiment. The total amount of DMTZ on the surface is the same as shown by Auger data as well as integration of the mass 28 TDS data. No increase in the amount of hydrogenated products (HCN or CH,CN) was observed in any of the hydrogen preadsorption experiments. The additional hydrogen resulted in a peak in the mass 2 thermal desorption at 65°C. Indeed, preadsorption of D, gave exactly the same results with no incorporation of deuterium in the HCN or CH,CN. Postadsorption of hydrogen (adsorption of DMTZ followed by adsorption of Hz) resulted in no change from the hydrogen free experiment. Preadsorption or postadsorption of carbon monoxide also resulted in no change in the DMTZ chemistry. The CO was seen to desorb at temperatures of - 140°C as an additional peak in the mass 28 spectra with no corresponding peak in the mass 14 spectra. This thermal desorption temperature is close to that observed for CO adsorbed on clean Pt( 111).

4. Discussion It is clear from these experiments that under proper conditions a Pt surface is capable of cleaving aromatic CN bonds. Our experiments clearly indicate two coverage regimes in which the chemistry of DMTZ is significantly different

-25

25

I

I

125

225

Crystal

Fig. 7. Thermal dimethyltetrazine

1

325

Temperature

desorption exposure

425

525

(“Cl

of N, (mass 28) given a preexposure to Pt( 111). Linear temperature ramp

of 9.0 L H, followed -7”C/s.

by 6.0 L

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4. I. Low coverage regime

At low coverage (exposures less than 3 L) the initial decomposition mode is CH bond breaking. H, is observed as a product at temperatures as low as 50°C. However the low temperature peak in the hydrogen spectrum (fig. 2) occurs at a higher temperature than that at which hydrogen is observed from hydrogen adsorption on clean Pt( 111). Thus it is likely that CH bond breaking does not occur to a significant extent immediately on adsorption at or below room temperature. Initial CH bond breaking is certainly facile at temperatures above 50°C. Further CH bond breaking results in a second peak in the H, thermal desorption (fig. 2). The maximum of the second H, peak occurs at - 250°C. This additional CH bond breaking process is accompanied by N, desorption (fig. 3). At low coverages the first observation of N, results in a peak in the mass 28 spectra at - 210°C. The temperature of N atom recombination and desorption on Pt( 111) is - 21O’C [lo]. Thus, the N, we observe at low coverages may be the result of decomposition to N atoms at some lower temperature followed by recombination and desorption. HCN is the major hydrogen containing species observed other than H,. CH,CN is observed in extremely small quantities. The HCN is observed as a single peak at - 265°C. The HCN is most likely formed by CH bond breaking occurring on a surface with a low concentration of hydrogen (most of the hydrogen having desorbed at lower temperatures). The resulting hydrogen atom will encounter many CN groups on the surface before encountering another hydrogen atom. Hydrogenation of the CN to form HCN is then followed by desorption. This mechanism for HCN production is supported by coadsorption experiments of H, and C,N, on Pt( 111) which we have carried out [ 111. Thermal desorption following adsorption of H, and C,N, on Pt( 111) in substantial HCN production and desorption at 220°C. The temperature at which we observe HCN from DMTZ (265°C) is significantly above this temperature. Thus, the HCN produced from DMTZ is des.?rbed immediately after being formed. At temperatures above the desorption of HCN, the surface is covered with CN and residual carbon from dehydrogenation of the methyl groups. The form of CN on Pt surfaces at these high temperatures is a matter of speculation in the literature [3-51. The eventual desorption of C,N, is similar to the high temperature behavior observed following exposure of C,N, to a Pt(ll1) surface. The carbon residue left on the surface following decomposition of DMTZ corresponds to - 40% of the original carbon present by Auger spectroscopy. If both methyl groups on all adsorbed molecules were completely dehydrogenated a carbon residue of 50% would be expected. Small amounts of acetonitrile (CH,CN) are observed. A quantitative comparison is difficult however since the thermal desorptions are typically run to - 850°C and carbon dissolution into the crystal may be significant at these high tempera-

464

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J. C. Hernmmger

/ Dimethylterrmine

on Pt(111)

trtres. ft is ~315s likdy that df the caxbuns in rfif! arnmatic ring are rem5ved from the surface (as HCN, C2N, and CH,CN in order of importance), and that only a few percent of the methyl carbons ever leave the surface as CH,CN. In surnrn~y~ the low coverage decomposition of DMTZ on Ptft II) can be described by tow temperature initiation of the dehyd~o~cnation of the methyl groups followed by breakup of the aromatic ring to form adsorbed nitrogen atoms and CN groups which desorb as N, and HCN and C2N2 respectively.

4.2. High coverage regime

The mostdramatic change in the chemistry of DMTZ at higher coverages is the onset of a fow temperature peak in the N, desorption at N 30°C. This is well below the temperature for recombination and desorption of atomic nitrogen. The new decomposition path at high coverage must involve direct production of molecular N, which would immediately desorb at these temperatures. The additional adsorbed DMTZ molecuies also only contribute to the high temperature hydrogen desorption peak. The high temperature behavior of the high coverage experiments is the same as the low coverage experiments. A likely explanation of the coverage dependence of the chemistry would involve the molecule bonding to the surface with the molecular plane parallel to the surface at low coverages and on edge (possibly interacting through two adjacent nitrogens) at higher coverage. Such changes in bonding geometry with surface coverage appear to be very common for aromatic molecules bonded to Pt( I1 I) [ 121, At low coverage, with the molecular plane parallel to the surface, the methyl groups would be very close to the surface and subject to C-H bond cfeavage at low temp~ratures~ Once dehydrogenation of the methyl group has begun the molecule would be tightly bound to the surface and eventual totai decomposition of the aromatic system would be assured. At higher coverages with the molecules bonding on edge the methyl groups would be further from the surface and thus Iess susceptible to CH bond cleavage. A strong interaction with two adjacent Gtrogens in the ring would also facilitate direct production of molecular N, as is observed.

Preadsorption of hydrogen on the Pt(ll1) surface suppresses the initial decomposition of DMTZ at high coverages. Once the free hydrogen has been completely desorbed: the d~omposition of the DMTZ proceeds in an identicd manner to the hydrogen free high coverage experiments. Since the coverages of DMTZ are the same in the hydrogen free and hydrogen preadsorption experiments our results cannot be easily expIained by a simple site blocking model. Most fikely, #he preadsorbed hydrogen modifies the electron density of the Pt

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/ Dimethyltetrarine

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465

surface (the direction of the charge transfer in the Pt-H bond is a matter of discussion in the literature) and thus indirectly effects the initial interaction of the DMTZ with the Pt at high coverages. The ability to modify the chemistry of DMTZ by hydrogen preexposure is indicative of a weaker Pt-DMTZ interaction at high coverages. This is also consistent with both the edge bonded picture and the less drastic mode of initial decomposition at high coverage.

5. Summary Our experiments point to the following conclusions about the chemistry of dimethyltetrazine on Pt( 111). The mode of bonding of DMTZ to Pt( 111) is coverage dependent. At low coverages the molecule is most likely bonded with the molecular plane parallel to the surface, with the methyl groups in the proximity of the surface. At high coverages the initial decomposition involves direct production of molecular nitrogen and the molecule may be edge bonded, possibly through two adjacent nitrogens in the aromatic ring. This mode of bonding places the methyl groups further from the surface and less susceptible to dehydrogenation at low temperatures. The decomposition chemistry in the edge bonded (high coverage) mode is modified by hydrogen preadsorption. Decomposition of aromatic CN bonds definitely occurs on the Pt( 111) surface. In addition, the path of this decomposition changes dramatically with the bonding mode. This observation suggests that the chemistry of DMTZ on Pt may be sensitive to surface structure as well. Further experiments on other Pt surfaces will elucidate this point. In addition, further experiments on the hydrogen preadsorption effect we have observed may shed light on the electronic factors which control the chemistry of CN groups on Pt.

Acknowledgements This work has been supported in part by the Office of Naval Research, the Hooker Chemical Corporation Grant of Research Corporation.

References [I] J.C. Hemminger, E.L. Muetterties and G.A. Somorjai, J. Chem. Sot. 101 (1979) 62. [2] (a) C. Friend, E. Muetterties and J. Gland, J. Phys. Chem. 21 (1981) 3256: (b) C. Friend, J. Stein and E. Muetterties. J. Am. Chem. Sot. 103 (1981) 767. [3] F.P. Netzer, Surface Sci. 61 (1976) 343. [4] M.E. Bridge, R.A. Marbrow and R.M. Lambert. Surface [5] F.P. Netzer and R.A. Wille. Surface Sci. 74 (1978) 547.

Sci. 57 (1976) 415.

and

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[6] D. Dahlgren and J.C. Hemminger, to be published. [7] C.E. Smith, PhD Thesis, University of California, Berkeley (1967); Lawrence Berkeley Laboratory Manuscript LBL-8077. [8] C.E. Smith, J.P. Biberian and G.A. Somorjai, J. Catalysis 57 (1979) 426. [9] J.P. Biberian and G.A. Somorjai, Appl. Surface Sci. 2 (1979) 352. [lo] K. Schwaha and E. Bechtold, Surface Sci. 66 (1977) 383. [l l] A paper is in preparation describing the coadsorption chemistry of H, and C,N, on Pt( 111). Of importance here is the fact that we observe HCN to desorb at 220°C following coadsorption of H, and C,N, on Pt( 111). [ 121 (a) D. Dahlgren and J.C. Hemminger, Surface Sci. 109 (1981) L513; (b) L. Firment, PhD Thesis, University of California, Berkeley (1977).