Effect of catalyst structure on the rate of alkane oxidation over platinum

Chemical Printed

Engineering Science. tn Great Britam.

Vol.

45.

No.

8, pp.

2647-2651,

1990 0

EFFECT

R.F. HICKS,

OF CATALYST STRUCTURE ALKANE OXIDATION OVER H. QI, M.L. YOUNG,

Chemical

R.G. LEE,

ON THE RATE PLATINUM

000+2509/90 $3.00 + 0.00 1990 Pergamon Press plc

OF

W.J. HAN and A.B. KGGH

Engineering Department, University Los Angeles, CA 90024-1592

of California,

ABSTRACT The effect of catalyst structure on the intrinsic activity of platinum for methane and heptane oxidation has heen determined. The reactions were carried out in 5% excess oxygen and at low conversion. Depending on the support and method of catalyst preparation, the platinum may be distributed between a dispersed and a crystalline phase. These phases show characteristic absorbances at 2067 and 2085 cm-‘, respectively, in the infrared spectrum of adsorbed carbon monoxide. These phases catalyze methane oxidation at different rates. The turnover frequency for methane oxidation at 608 K is between 0.001 and 0.01 s-’ for the dispersed platinum and between 0.05 and 0.10 s-t for the crystalline platinum. The turnover frequency for heptane oxidation is insensitive to the platinum phase present, and varies from 0.003 to 0.011 s-t at 413 K. For platinum dispersions ranging from 6 to lOO%, the specific activities for methane and heptane oxidation are not significantly affected by crystallite size.

KEYWORDS Catalytic

Oxidation;

Methane;

Heprane;

Platinum

INTRODUCTION Platinum is used extensively to reduce hydrocarbon emissions from automobiles (Gandhi and Shelef, 1989) and from industrial processes (Spivey, 1987). Due to its high cost and increased worldwide consumption, much research has been devoted to improving the performance of platinum oxidation catalysts (Summers, 1989). A key factor affecting catalyst performance is the relationship between the inainsic reaction rate and the metal surface structure. Prior studies of catalytic hydrocarbon oxidation (Cullis and Willatt, 1983; Volter et al., 1987; Yao, 1980, 1984) have focused mainly on the reaction kinetics. In this paper, we describe our study of the effects of surface structure on the turnover frequencies for methane and heptane oxidation over platinum. EXPERIMENTAL Catalyst

Preparation

The methods of catalyst preparation are described*in detail elsewhere (Hicks et al., 1990). Five catalyst supports were used: A+O3 (A), Alpha Products AlOOH (160 m /g): A1203 (B), Alpha Products AlOOH calcined in air for 48 h?t 1323 K (60 m /g); AlzG(C), Degussa flame-synthesized aluminum oxide “C” calcined in air for 48 h at 1273 K (83 m /g); ZrOz(A), Degussa flame-synthesized zirconia (42 m’/g); and ZrOz(B), Zircar precipitated zirconia calcined in air for 48 h at 1323 K (3 m2/g). Platinum was deposited on these supports by ion exchange and incipient wetness impregnation with H2PtC16 and Pt(NH3)zClz. Afterwards the catalysts were calcined in air for 2 h between 773 and 973 K. The methods used to prepare the platinum catalysts are shown in Table 1. The platinum loadings were determined by inductively coupled plasma emission spectroscopy. Catalyst

Characterization

and Testing

The platinum dispersion of the fresh catalysts were determined by H2 titration of preadsorbed oxygen in a volumetric chemisorption apparatus3 assuming 1.5 Hz adsorbed per Pt-0, (Benson and Bogdart, 2965). Catalyst pellets, 32-60 mesh, were reduced in 200 cm /min hydrogen for 1 h at 573 K and cooled in 6.7~10~ N/m vacuum to 298 K. At 298 K, the adsorption isotherm was recorded between 10 and 150 Torr of Hz. The saturation portion of the curve was extrapolated to zero pressure to give the amount adsorbed. The structure of the platinum surface was examined by infrared spectroscopy of adsorbed carbon monoxide. A 13.mm-diameter wafer, weighing 0.1 g, was placed in a glass cell and treated as described above. At 298 K, the wafer was dosed with small amounts of carbon monoxide while recording the infrared spectrum. The spectra were compared at saturation coverage, where the band intensities stopped increasing rfyidly with each dose. These measurements were made on a Digilab FTS-40 spectrometer with a DTGS detector at 4 cm resolution and coadding 256 scans.

2647

R. F. HICKS et al.

2648 Table I.

Methods

Sample Key

of Catalyst

A1203

(c)

HzPtCla H2PtC16

A1203

(c)

H2PtC16

A1203

(B)

wNH3

Nz03 (A)

Z?::; aIncipient

Preparation Metal salt

Support

2

F13

E(FNH; 2

Deposition Technique

Calcination Temperature (“C)

0.80 0.84

Ion Exchange Ion Exchange

)4Q )4C12 6

Metal Loading (wt%)

Ion Exchange Impregnation* Ion Exchange Impregnation

700 600 500

4.90 0.78 0.50 0.30 0.50

wetness impregnation.

Steady state activities for methane and heptane oxidation were determined as follows. The sample was reduced in 50 cm3/rnin hydrogen for 1 h at 573 K, cooled to the reaction temperature in 50 cm3/min helium, then switched to the reaction mixture and allowed to react until a steady rate was observed. The reactions lasted from 4 to 8 h. The reaction conditions for methane oxidation were 4.7 mole% methane, 10.4 mole% oxygen, 84.9 mole% helium, 1.41~10~ N/m2 total e reaction conditions for heptane oxidation were 2.0 mole% pressure, 623 to 643 K and reaction times of 4 to 16 h. heptane, 23.0 mole% oxygen, 75 mole% helium, 1.63x1 ?IQ N/m2 total pressure, 373 to 433 K and reactions times of 6 to 15 h. Over 4.9% Pt/Alze, heptane oxidation, was stopped periodically to measure the amount of metal exposed by pulse adsorption of carbon monoxide, oxygen and hydrogen at 323 K. In addition, the amount of carbon deposited during heptane oxidation was measured by temperature programmed oxidation. The sample was heated in 140 cm’/min mixture of Reaction pro16 mole% oxygen and helium at 5 K/mitt to 773 while recording the amount of carbon dioxide produced. ducts were analyzed by gas chromatography, using a 1.83-m Carbosphere packed column and thermal conductivity detector. Carbon dioxide was the only product observed.

RESULTS Platinum

Surface

AND DISCUSSION

Structure

The infrared spectrum of adsorbed carbon monoxide on the platinum catalyst samples consists of either a narrow band near 2085 cm-’ or a broad band near 2067 cm-’ (Hicks et al., 1990). Sometimes a very small absorbance is also observed at 1850 cm-‘. Table 2 lists for each sample the frequency and full width at half maximum of the band above 2000 cm-‘. The narrow feature at 2085 cm-’ may be assigned to linearly bonded carbon monoxide on the (111) facets of platinum crystallites. Since the platinum crystallites are small, carbon monoxide adsorbed on the facets is analogous to small islands of carbon monoxide on Pt(ll1 single crystals. At low coverages on Pt( 11 l), carbon monoxide forms islands and exhibits an infrared band at 2085 cm- 11(Hayden et al., 1985). Bands due to adsorption on facet edges and other defect sites should not be evident in the infrared spectrum. Hayden and coworkers (1985) have shown that at high coverages on stepped Pt( 11 l), the vibrations of the terrace- and edge-bound carbonyls couple, so that only the terrace mode is observed. The results shown in Table 2 are consistent with this result. The dispersions of samples e-g vary from 30 to 100%. Over this range, the ratio of facet platinum atoms to edge and comer platinum atoms must increase substantially. Yet, the infrared spectra of adsorbed carbon monoxide on these samples are the same. This can be explained by vibrational coupling of the carbon monoxide on the edge and comer atoms to the carbon monoxide on the facets.

Table 2. Sample Key

Platinum Dispersion (%)

Effect of Catalyst

Structure

Infrared Bands For Adsorbed CO Frequency FWHM (cm-l ) (cm )

on the Rates of Alkane Oxidation Turnover Frequency (s-t) for C& Oxidation at 608 K Final Initiala

Turnover Frequency (s-l) for C7H16 Oxidation at 413 K Final Initial’

:

9”:

2070,2080shb 2068

::

0.001

0.005 0.001

0.&9

0.0035

2

11 6

2066,208Osh 2090,2069

38

0.02

0.01

0.013 0.009

0.0110 0.0045

; g

1Z 40

;1 Z;’

o.;o 0.07 0.09

o.;o 0.05 0.06

0.0024

0.003

=Initial value obtained bsh means shoulder.

2077 2083 2081 by linear extrapolation

of the rate at 2-14 h of reaction

back to zero time.

Rate

F13

of alkane

oxidation

over

platinum

2649

The assignment of the broad peak at 2067 cm-’ is under dispute. At low coverages on stepped Pt(l1 I), the carbon monoxide adsorbs at the edges, giving rise to an infrared band between 2065 and 2075 cm(Hayden et al.. 1985). This led Greenler and coworkers (1985) to conclude that the low frequency band observed on supported platinum catalysts is due to carbon monoxide bonding at comer and edge atoms on the crystallites. However, this interpretation ignores the effect of coverage on the infrared spectrum. The low frequency bands are observed at high coverages of carbon monoxide on the platinum crystallites, where vibrational coupling between terrace- and edge-bound carbonyls should eliminate the edge mode. An alternative interpretation, proposed by Rothschild et al. (1986). is that the 2067 cm-’ peak is due to a dispersed phase of platinum on alumina. Carbon monoxide molecules adsorbed on the dispersed platinum are isolated, and their vibrational frequency should correspond to the singleton mode. This is consistent with the single crystal results. On stepped Pt(l1 l), the vibrational frequency for an isolated carbon monoxide molecule at an edge site is 2065 cm-l. The existence of a dispersed phase of platinum has been established in several studies (Yao et al., 1979; and references therein). It is prepared by adsorption of HzPtCle onto alumina, oxidation between 573 and 773 K, and reduction at 573 K. ” species (Lieske et al., 1983). This species slowly decomposes during oxidaOxidation at 773 K produces a “Pt+40,Cl tion at 873 K and above. Comparison of tK e preparation method (Table 1) to the infrared data (Table 2) reveals a trend consistent with previous work. The dispersed phase of platinum is observed on samples a-d. These samples are prepared by ion exchange of H2PtC16 onto alumina. Samples a and b are oxidized at 773 K and exhibit high platinum dispersions. The infrared spectra of these catalysts are dominated by the low frequency band. For sample b, a weak shoulder at 2080 cm-’ is observed which indicates that some crystalline phase is present. Samples c and d are oxidized at 973 and 873 K and exhibit low platinum dispersions. The infrared spectrum of sample c consists of a peak at 2066 cm-’ and a strong shoulder at 2080 cm-‘. The infrared spectrum of sample d consists of equally intense peaks at 2090 and 2069 cm-‘. On both of these samples, the platinum is distributed between very large crystallites and a small amount of dispersed phase. Since all the platinum in the dispersed phase is exposed, while only a tiny fraction of the platinum in the crystallites is exposed, the 2067 cm-’ peak is relatively intense compared to the 2085 cm-’ peak. Samples e,fand g are prepared using either a zirconia support, or an alumina support in conjunction with the platinum amine salt. In these cases, the crystaliine phase of platinum is obtained. On samplef, Pt(NH3)4Clz is ion exchanged onto zirconia and a high dispersion results. Evidently, this sample contains exceedingly small crystallites of platinum. Methane

Oxidation

Turnover frequencies for methane oxidation over the platinum catalysts are shown in Table 2. The turnover frequencies are calculated by dividing the rate (moles COz/g s) by the initial amount of platinum exposed (moles P&/g). Except for sample b, the rates remain fairly constant throughout the reaction, and turnover frequencies extrapolated to zero time differ little from those measured at the end of the run. Infrared spectra of adsorbed carbon monoxide recorded after reaction exhibit similar frequencies and integrated intensities as those recorded before reaction (Hicks et al., 1990). These results suggest that it is valid to compute turnover frequencies based on the initial platinum dispersion. Over sample b, the rate of methane oxidation increases steadily throughout the 17 h run. Exposure to reaction conditions, also changes the distribution of peaks in the infrared spectrum of adsorbed carbon monoxide. The high frequency shoulder at 2080 cm-’ is much larger in the spectrum taken after reaction. On this sample, the increase in rate with time may be due to a gradual change in the platinum surface structure during reaction. Alternatively, the sample may contain chlorine, which is a known poison for methane oxidation (Cullis and Willatt, 1984). The increase in rate with time may be due to the slow removal of chlorine. Examination of the data in Table 2 reveals that the catalytic activity for methane oxidation varies by lOO-fold from the least active to the most active sample. There is a strong correlation of the turnover frequency with the infrared spectrum of adsorbed carbon monoxide. A low rate corresponds to the low frequency band, sample a, while a high rate corresponds to the high frequency band, samples e-g. Intermediate turnover frequencies are observed when both infrared bands are present, samples b and c. There is also a strong correlation of the apparent activation energies for methane oxidation with the infrared spectrum of adsorbed carbon monoxide. The activation energies are 165 Id/mole for sample a, 140 Id/mole for sample b, and 125 Id/mole for samples e-g. Based on the discussion of the infrared assignments above, the most likely explanation of these results is that the turnover frequency and the activation energy depend on the platinum phase present. Specifically, the turnover frequency and the activation energy on the dispersed phase are 100 times lower and 50% higher, respectively, than on the crystalline phase. The effect of crystallite size can bc examined independent of the type of platinum phase present. Samples e-g only conOn these samples, the platinum dispersion varies tain the crystalline phase as evidence by the infrared peak at 2080 cm-‘. from 3 1 to lOO%, whereas the turnover frequency varies from 0.05 to 0.10 se1 . This result suggests that crystallite size does not affect the activity of platinum for methane oxidation. In summary, the turnover frequency depends on the distribution of platinum between a dispersed and crystalline phase and not on the size of the metal crystallites. This observation may be contrasted to the usual concept of structure-sensitivity, where the turnover frequencies correlates with a change in the size of the metal particles (Boudart and Djega-Mariadassou, 1984). Heptane

Oxidation

On all the platinum catalysts, the rate of heptane oxidation falls rapidly during the first 2 h of reaction, but thereafter changes very slowly with time. Deactivation is accompanied by a large accumulation of coke on the catalyst. For example, the amount of carbon removed by temperature-programmed oxidation of sample d, after 8 h of reaction at 373 K, is 0.0036 mole/g. This is approximately 40 times more carbon than is present on the sample initially, and is 190 times greater than the moles of platinum exposed. Heptane dehydrogenation to coke occurs on the alumina support. During a 2 h exposure to the reaction mixture at 473 K, 0.0025 mole/g carbon are deposited on the alumina. The rate of coking on the support is comparable to the rate of coking on the platinum catalysts.

F13

R. F. HICKS et a[.

2650

Sample d was titrated with hydrogen, oxygen and carbon monoxide after reaction to ascertain whether carbon builds up on the platinum surface. The results of this experiment are summarized in Table 3. Before reaction, a platinum dispersion of After reaction, the platinum dispersion 10% is measured by hydrogen-oxygen titration and carbon monoxide adsorption. If it is pulsed with measured by hydrogen-oxygen titration depends on how the catalyst is first exposed to hydrogen. hydrogen (first sequence), repeated oxygen and hydrogen titrations yield a dispersion of 4%. Conversely, if hydrogen is flowed over the catalyst (second sequence), repeated oxygen and hyvgen titrations yield a dispersiog of 6%. Also, the moles/g compared to 2.07~10~ moles/g on the oxygen uptake immediately following the hydrogen flow is 5.51x10fresh catalyst. A large portion of this oxygen is converted into carbon dioxide. These results indicate that carbon p&ally covers the platinum surface after reaction. In a flow of hydrogen, the carbon is hydrogenated by spillover from the metal. Pulsing with oxygen removes some of the hydrogenated carbon from the surface. As shown in the last step of the second sequence, carbon monoxide displaces the adsorbed hydrogen and carbon, and gives a platinum dispersion equal to that of the fresh catalyst. The carbon on the platinum surface may not be a poison for heptane oxidation, but instead may be a The reactivity of the carbon with hydrogen and oxygen at 323 K and its easy displacement by carreaction intermediate. bon monoxide suggests that it is a reaction intermediate. Table 3. Titration When

of 4.9%

Pt/A1203

Amount Adprbed at 323 K (x10mole/g)

Titrant

Before reaction

after 275 min of Heptane Oxidation

3.E

02

After reaction 1 st sequence

3:63 2.76

H2 9

5.31 0.48

H2

1.59

-3 4

2

0.84 1.88 0.85

z 4

fl”wzg

5.;1 2.17 1.32 2.03 2.75

H2

H2 02 c”;;

Yhe

Adsorption

9 11

Et6

02

After reaction 2nd sequence

Platinum Dispersion* (%)

02 H2

stoichiometrics

are assumed

at 373 K

to be I.5 Hz, 0.75

::

6 7 1: 02 and 1 .O CO per Ph.

Turnover frequencies for heptane oxidation are presented in Table 2. They are equal to the rate (moles of COP/g s) divided by the initial amount of platinum exposed (moles PtJg). This calculation of the turnover frequency assumes that all the platinum surface atoms participate in heptane oxidation. Except for sample b, the initial and final turnover frequencies are within a factor of two of one another. Over sample b, the turnover frequency increases from 0.0009 to 0.0035 s-l A similar increase in the turnover frequency for methane oxidation is observed over this samfrom 2 to 12 h of reaction. ple. As discussed earlier, the change in oxidation rate may be ascribed to a gradual change in surface structure, or to a gradual removal of chlorine during the reaction. Examination of the data in Table 2 reveals that the tarnover frequency for heptane oxidation is relatively.insensitive to the structure of the catalyst. The turnover frequency varies from 0.003 s-l for samplefto 0.011 s-’ for sample d. Yet, these two samples have widely different dispersions, 100 versus 1 l%, and widely different distributions of adsorption sites, an The results presented here infrared band for adsorbed carbon monoxide at 2083 cm-’ versus two at 2090 and 2069 cm-‘. are in good agreement with the study of Yao (1980). He found that the turnover frequency for butane oxidation varies by 6-fold over a series of platinum catalysts with dispersions ranging from 4 to 87%. In summary, the activity of platinum for heptane oxidation does not depend on the crystallite size or on the distribution of platinum between dispersed and crystalline phases. CONCLUSIONS Turnover frequencies for methane oxidation depend on the distribution of platinum between two structures: a dispersed phase stabilized by alumina, and a crystalline phase. The latter may be as much as 100 times more active than the former. The turnover frequency for methane oxidation over the crystalline platinum does not vary with crystallite size. Turnover frequencies for heptane oxidation arc not affected by the platinum structure. ACKNOWLEDGEMENTS Funds were provided Center for Hazardous

by the Gas Research Institute (contract Substance Conuol at UCLA.

no. 5086-260-1247)

and the NSF Engineering

Research

Rate

F13

of alkane

oxidation

over

platinum

2651

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