KOPPERS INC. SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN I

Exhibit 10.51

KOPPERS INC.

SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN I

As Amended and Restated

Effective as of

January 1, 2009

TABLE OF CONTENTS

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TABLE OF CONTENTS

(continued)

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KOPPERS INC.

SUPPLEMENTAL EXECUTIVE RETIREMENT PLAN I

The purpose of the Koppers Inc. Supplemental Executive Retirement Plan I (formerly named the Koppers Industries, Inc. Retirement Income Restoration Plan) is to permit select members of management and highly compensated employees to supplement retirement benefits payable from the Retirement Plan for Koppers Inc. (the “Retirement Plan”).

ARTICLE I

TITLE AND EFFECTIVE DATE

Section 1.01 Title . This plan shall be known as the Koppers Inc. Supplemental Executive Retirement Plan I (hereinafter referred to as the “Plan”).

Section 1.02 Effective Date; Grandfathered Benefits . The original effective date of this Plan was January 1, 1991. The Plan, as hereby amended and restated, shall be effective as of January 1, 2009. The Plan as amended and restated applies only to a Participant’s Retirement Benefit that accrued on or after January 1, 2005, or Retirement Benefit that accrued prior to that date but was not fully vested on December 31, 2004. The Plan preceding this amendment and restatement applies to any Retirement Benefit that accrued and was vested prior to January 1, 2005 (“Grandfathered Benefits”); provided that any increase in the value of any subsidy with respect to Grandfathered Benefits payable upon retirement prior to the Retirement Plan’s Normal Retirement Date that accrues or increases as the result of service after December 31, 2004 shall not be treated as Grandfathered Benefits.

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ARTICLE II

DEFINITIONS AND CONSTRUCTION OF THE PLAN DOCUMENT

As used herein, the following words and phrases shall have the meanings specified below unless a different meaning is clearly required by the context:

Section 2.01 Actuarial Equivalent . “Actuarial Equivalent” shall mean a benefit having the same actuarial value as the benefit it replaces, determined using the same assumptions and methods as are used for determining an actuarial equivalent benefit under the Retirement Plan.

Section 2.02 Beneficiary . “Beneficiary” shall mean the person or persons or the estate of a Participant entitled to receive any benefits under this Plan.

Section 2.03 Board of Directors . “Board of Directors” shall mean the Board of Directors of the Company.

Section 2.04 Code . “Code” shall mean the Internal Revenue Code of 1986, as amended from time to time. Any reference to a particular Code section shall include any provision which modifies, replaces or supersedes it.

Section 2.05 Committee . “Committee” shall mean the Pension Committee of the Retirement Plan, which shall manage and administer the Plan.

Section 2.06 Company . “Company” shall mean Koppers Inc., its successors, and any organization into which or with which the Company may merge or consolidate or to which all or substantially all of its assets may be transferred. Where the context so requires, references to “Company” shall refer to Koppers Industries, Inc. as predecessor to Koppers Inc.

Section 2.07 Earliest Retirement Date . “Earliest Retirement Date” shall mean the earliest date as of which the Participant would be eligible to commence the receipt of his Retirement Plan Benefit, whether or not he elects to commence receipt of such Retirement Plan Benefit as of such date.

Section 2.08 Executive . “Executive” shall mean any member of management or highly compensated employee who is eligible to participate in the Retirement Plan.

Section 2.09 Normal Form . “Normal Form” shall mean the normal form of distribution of the Participant’s Retirement Benefit under the Retirement Plan (i.e., a single-life annuity for an unmarried Participant or a joint and 50% survivor annuity for a married Participant).

Section 2.10 Participant . “Participant” shall mean an Executive who is participating in the Plan.

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Section 2.11 Plan . “Plan” shall mean the Koppers Inc. Supplemental Executive Retirement Plan I, as described in this instrument, as amended from time to time.

Section 2.12 Plan Year . “Plan Year” shall mean the calendar year.

Section 2.13 Retirement . “Retirement” shall mean a Participant’s Separation from Service at a time when the Participant is eligible to commence immediately his Retirement Plan Benefit.

Section 2.14 Retirement Benefit . “Retirement Benefit” shall mean a Participant’s Retirement Benefit as provided under Section 4.01.

Section 2.15 Retirement Plan . “Retirement Plan” shall mean the Retirement Plan for Koppers Inc., as amended from time to time.

Section 2.16 Retirement Plan Benefit . “Retirement Plan Benefit” shall mean the accrued retirement benefit payable to the Participant under the Retirement Plan.

Section 2.17 Separation from Service . “Separation from Service” shall mean a Participant’s separation from service with the Company within the meaning of Code Section 409A. A Separation from Service occurs when the facts and circumstances indicate that the Company and the Participant reasonably anticipated that no further services would be performed after a certain date or that the level of services the Participant would perform after such date would permanently decrease to no more than 20% of the average level of services performed over the immediately preceding 36-month period (or, if shorter, the entire period of the Participant’s employment by the Company).

Section 2.18 Specified Employee . “Specified Employee” shall have the meaning set forth in Section 1.409A-1(i) of the Treasury Regulations issued under Code Section 409A.

Section 2.19 Termination of Employment . “Termination of Employment” shall mean a Participant’s Separation from Service as a regular employee of the Company, other than Retirement.

Titles of the Articles of this Plan are included for ease of reference only and are not to be used for the purpose of construing any portion or provision of this Plan document. Wherever the context so requires, masculine pronouns include the feminine and singular words shall include the plural.

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ARTICLE III

ELIGIBILITY

Section 3.01 Participation . Any individual who is a participant in the Retirement Plan and whose Annual Salary (as defined in the Retirement Plan) for any plan year beginning on or after January 1, 1989 exceeded the limitations imposed by Section 401(a)(17) of the Internal Revenue Code shall be eligible to be a Participant in this Plan; provided, that no Executive hired or rehired after December 31, 2006 became a Participant in this Plan after December 31, 2006, and the Retirement Benefit of any Participant in the Plan as of December 31, 2006 was frozen on such date and will not thereafter increase.

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ARTICLE IV

RETIREMENT BENEFITS

Section 4.01 Retirement Benefit . A Participant shall be entitled to a Retirement Benefit under this Plan if his Retirement Plan Benefit is less than such benefit would have been if the definition of “Annual Salary” in the Retirement Plan did not exclude, for years beginning with 1989 and ending with 2006, compensation in excess of the Section 401(a)(17) Limit. In such case, the Participant’s Retirement Benefit shall be equal to the difference between his actual Retirement Plan Benefit and the Retirement Plan Benefit to which he would have been entitled under the